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PROPRIETARY RIGHTS STATEMENT

This document contains information, which is proprietary to the Flex5Gware Consortium.

Research and Innovation Action

Flex5Gware Flexible and efficient hardware/software platforms for

5G network elements and devices

H2020 Grant Agreement Number: 671563

WP3 – 5G Mixed-signal technologies

D3.1 - Mixed-signal strategies for 5G: requirements and concepts

Contractual Delivery Date: 31 December 2015

Actual Delivery Date: 26 December 2015

Responsible Beneficiary: CEA

Contributing Beneficiaries: ALUD, CEA, IMC, IMINDS, SEQ, VTT

Dissemination Level: Public

Version: 1.0

PROPRIETARY RIGHTS STATEMENT

This document contains information, which is proprietary to the Flex5Gware Consortium.

This page is left blank intentionally

H2020 Grant Agreement Number: 671563 Document ID: WP3 / D3.1

Dissemination Level: Public Page 3

Document Information

Document ID: D3.1

Version Date: 24/12/2015

Total Number of Pages: 65

Abstract: This deliverable sets the requirements for mixed-signal

technologies (WP3) in Flex5Gware. Its goal is to describe some

of the most promising mixed-signal concepts identified for 5G.

These concepts will be further developed during the course of

the project. A set of associated key performance indicators for

each mixed-signal technology are derived in order to drive the

research and concepts towards the main goals set by the

project.

Keywords: Mixed Signal, RF, FTTA, PAPR reduction, DPD, Full-Duplex,

RF DAC, Envelope Tracking, Multiband RF signal generation,

Compact multichain transmitter, Low resolution ADC.

Authors

Full Name Beneficiary / Organisation

e-mail Role

Thomas Bohn ALUD [email protected] Contributor

Xin Yu ALUD [email protected] Contributor

Vincent Berg CEA [email protected] Editor / Contributor

Patrick Rosson CEA [email protected] Contributor

Leonardo Gomes Baltar IMC [email protected] Contributor

Kilian Roth IMC [email protected] Contributor

Johan Bauwelinck IMINDS [email protected] Contributor

Efstathios Katranaras SEQ [email protected] Contributor

Haesik Kim VTT [email protected] Contributor

Tapio Rautio VTT [email protected] Contributor

Atso Hekkala VTT [email protected] Contributor



Reviewers

Full Name Beneficiary / Organisation

e-mail Date

Johan Bauwelinck IMINDS [email protected] 07/12/2015

Haesik Kim VTT [email protected] 07/12/2015

Kilian Roth IMC [email protected] 07/12/2015

Miquel Payaró CTTC [email protected] 15/12/2015

Pablo Serrano UC3M [email protected] 18/12/2015

Fredrik Tillman EAB [email protected] 23/12/2015

Version history

Version Date Comments

1.0 23/12/2015 First Revision



Executive Summary

The overall objective of Flex5Gware is to deliver highly reconfigurable hardware (HW) platforms together with HW agnostic software (SW) platforms. Flex5Gware is targeting both network elements and devices and takes into account increased capacity, reduced energy footprint, as well as scalability and modularity, to enable a smooth transition from 4G mobile wireless systems to 5G. Flex5Gware therefore investigates a number of technical aspects related to HW platforms and associated SW modules with the following goals:

To improve the energy and spectrum efficiency.

To improve the modularity and flexibility.

The Flex5Gware consortium has identified four groups of technologies that are necessary to reach these goals: RF front-end and antennas (WP2), Mixed-signal technologies (WP3), Digital Front-end and HW/SW function split (WP4) and finally SW modules and functions (WP5). The goal of WP3 is to investigate mixed-signal technologies with the aim to improve overall achievable capacity, reduce power consumption and increase HW versatility and reconfigurability. Investigations are considered for both parts of the 5G system, user equipment and network elements.

Flex5Gware identified the following areas of investigation where advances in mixed-signal technologies are to be considered, researched and developed:

• Multiband radiofrequency signal generation • Compact multichain transmitter • Low resolution Analog-to-Digital converters geared towards massive Multiple-Input

Multiple Output applications in the mmWave frequencies • Envelope tracking for RF power amplifiers • Peak-to-average power ratio reduction and predistortion techniques • New high bandwidth antenna links based on new fibre-to-the-antenna transceiver

subsystem technology • Full-duplex operation

For each identified key building block, the main challenges and the approach envisaged by Flex5Gware have been detailed. Key Performance Indicators (KPIs) have then been derived according to the Proof-of-Concept environments of WP6 associated with the use case families of WP1. The KPIs introduced in WP1 have been used to derive the main requirements of each mixed-signal key building block. An analysis of the KPIs of each building block of WP3 highlighted that the main motivations of innovation in this workpackage are matching the main goals of energy efficiency, spectrum efficiency, modularity and flexibility set by the Flex5Gware project.



Table of Contents

1. Introduction .........................................................................................................9

2. Mixed-signal Techniques and Strategies identified for 5G ........................... 11

2.1 Introduction to Mixed-signal Techniques ..........................................................11 2.2 Techniques identified by Flex5Gware ...............................................................16

2.2.1 Multiband RF signal generation .........................................................................16 2.2.2 Massive Multiple Input Multiple Output architectures .........................................19 2.2.3 Envelope Tracking for RF Power Amplifiers ......................................................23 2.2.4 PAPR reduction and digital predistortion techniques .........................................28 2.2.5 High-bandwidth antenna links based on new fibre-to- the-antenna transceiver subsystem technology ..................................................................................................33 2.2.6 Full duplex techniques ......................................................................................35

3. Mixed-Signal Techniques Requirements of Flex5Gware ............................... 45

3.1 Overview of Flex5Gware use case families .......................................................45 3.2 Mapping of the mixed-signal technologies to the Flex5Gware use cases ......49

3.2.1 Multiband RF signal generation .........................................................................49 3.2.2 Massive Multiple Input Multiple Output architectures .........................................50 3.2.3 Envelope Tracking for RF Power Amplifiers ......................................................51 3.2.4 PAPR reduction and digital predistortion techniques .........................................52 3.2.5 High-bandwidth antenna links based on new fibre-to-the antenna transceiver subsystem technology ..................................................................................................53 3.2.6 Full duplex operation .........................................................................................54

3.3 Key Performance Indicators covered by the Mixed-signal Innovations ..........56

4. Conclusion ........................................................................................................ 58

5. List of Accronyms and Abbreviations ............................................................ 59

6. References ......................................................................................................... 61



List of Figures

Figure 1-1: Structure of the HW and SW key building blocks provided by Flex5Gware ......... 9 Figure 2-1: Schematic of a dual band transceiver sharing RF components ..........................12 Figure 2-2: Different receiver architectures for mmWave MIMO operation............................13 Figure 2-3: Envelope tracking principle system ....................................................................14 Figure 2-4: Principles of full duplex operation .......................................................................15 Figure 2-5: Schematic of a 3-band transceiver .....................................................................16 Figure 2-6: RF signal spectrum and envelope for single band operation ..............................17 Figure 2-7: RF signal spectrum and envelope for dual band operation .................................18 Figure 2-8: Highly compact multichain transmitter ................................................................19 Figure 2-9: analogue/hybrid beamforming receiver front-end................................................21 Figure 2-10: 1-bit quantized digital receiver beamforming ....................................................21 Figure 2-11: Fixed supply. ....................................................................................................23 Figure 2-12: Average power tracking ....................................................................................24 Figure 2-13: Envelope tracking .............................................................................................24 Figure 2-14: Power efficiency of commercial PAs. ................................................................25 Figure 2-15: Example test setup for envelope tracking characterisation. ..............................27 Figure 2-16: Comparison of conventional OFDM waveform and FBMC waveform ..............30 Figure 2-17: ETTUS X310 and B200 SDR devices in VTT 5G Lab .......................................31 Figure 2-18: User Interface (Real-time LTE-based system) ..................................................31 Figure 2-19: DAS system based on a Fibre-To-The-X network [Feb2014] ............................33 Figure 2-20: Initial architecture .............................................................................................34 Figure 2-21: In-band Full-duplex SOTA architecture [Rik2014] .............................................35 Figure 2-22: Full duplex system ............................................................................................36 Figure 2-23: Self-interference cancellation versus antenna distance [Dup2014b] .................37 Figure 2-24: Multi antenna cancellation solution [Dup2014b] ................................................37 Figure 2-25: Self-interference cancellation versus antenna distance and polarisation [Dup2014b] ...........................................................................................................................37 Figure 2-26: Self-interference for two and one antennas [Huu2014] .....................................38 Figure 2-27: Two stage analogue SIC [Dup2014b] ...............................................................38 Figure 2-28: Hybrid transformer to cancel self-interference [Dup2014b] [Rik2014] ...............39 Figure 2-29: Analogue and digital cancellations of FD receiver from [Bha2013] ...................41 Figure 2-30: Stanford architecture overview [Bha2013] ........................................................41 Figure 2-31: Relative phase error [Per2015] .........................................................................42 Figure 2-32: Flex5Gware FD architecture overview ..............................................................42 Figure 2-33: Architecture of AD9361 ....................................................................................43 Figure 2-34: Flex5Gware RF front-end architecture ..............................................................44 Figure 3-1: Use cases and associated proofs of concepts envisaged by Flex5Gware. .........46 Figure 3-2: Simplified TDD structure proposed by [San2014] ...............................................55 Figure 3-3: Interference generated in half duplex and full-duplex for scenario of [San2014] .55



List of Tables

Table 2-1: Comparison of conventional PAPR reduction techniques. ...................................29 Table 2-2: Summary of SotA demonstrators for full-duplex ...................................................40 Table 3-1: List of Flex5Gware consolidated KPIs derived in WP1 ........................................46 Table 3-2: KPIs expected to be covered by Multiband RF signal generation ........................49 Table 3-3: KPIs expected to be covered by Multi Chain Transmitters ...................................50 Table 3-4: KPI expected to be covered by Low Resolution ADC ..........................................51 Table 3-5: KPI expected to be covered by Envelope Tracking ..............................................52 Table 3-6: KPI expected to be covered by PAPR Techniques ..............................................53 Table 3-7: KPI expected to be covered by FFTA ..................................................................54 Table 3-8: Average performance gain of full-duplex as a function of interference cancellation .............................................................................................................................................56 Table 3-9: KPI expected to be covered by mixed-signal technologies ..................................56 Table 3-10: Summary of identified KPI for WP3 ...................................................................57



1. Introduction The overall objective of Flex5Gware is to deliver highly reconfigurable hardware (HW) platforms together with HW agnostic software (SW) platforms. The project is targeting both network elements and devices and takes into account increased capacity, reduced energy footprint, as well as scalability and modularity, to enable a smooth transition from 4G mobile wireless systems to 5G. Flex5Gware therefore investigates a number of technical aspects related to HW platforms and associated SW modules. From its early stage of conception, Flex5Gware proposed to focus on the following goals:


To improve the modularity and flexibility. The project decided to follow what is often referred to as a bottom up approach to reach its main objective. The approach consists of adopting a holistic approach with the evaluation of key HW and SW building blocks for the future generation of cellular networks (5G): i.e. to perform research and demonstrate implementations on key building blocks of 5G.

Figure 1-1: Structure of the HW and SW key building blocks provided by Flex5Gware

The Flex5Gware consortium has identified four groups of technologies (Figure 1-1) that are necessary to reach these goals: RF front-ends and antennas (WP2), Mixed-signal technologies (WP3), Digital Front-ends and HW/SW function splits (WP4) and finally SW modules and functions (WP5). The goal of WP3 is to investigate mixed-signal technologies with the aim to improve overall achievable capacity, reduce power consumption and increase HW versatility and reconfigurability. Investigations will be considered for both parts of the 5G system, user equipment and network element. The main objectives of this work package include:

To study broadband analogue-to-digital / digital-to-analogue converters for multiple band operation and large bandwidth notably for the millimetre wave (mmWave)



frequency range. Impact on 5G platforms will be explored to optimize performance and power consumption trade-offs.

To study new power consumption reduction techniques based on joint analogue and digital processing. Envelope tracking schemes will, for instance, be evaluated for 5G waveforms and compared to Long Term Evolution (LTE) and LTE-Advance (LTE-A). Peak-to-Average Power Ratio (PAPR) reduction and predistortion techniques, combined with Digital Signal Processing (DSP) techniques, are considered in the 5G scenario context. Furthermore, function split between analogue and digital signal processing in 5G network element transceivers will be studied. Complexity and power consumption of potential analogue algorithm implementations will be explored and compared to digital implementations.

To investigate wide-bandwidth antenna links based on new fibre-to-the antenna transceiver subsystem technology, designed to ease deployments and increase versatility of basestations. Large bandwidth (up to 1 GHz) and frequency up to 10 GHz will be investigated.

To study and validate impact on 5G platforms of full duplex operations. Performance, complexity, flexibility and agility trade-offs will be analysed. New techniques adapted to the 5G scenario will be proposed and evaluated. Increase of capacity results will serve as basis for air interface design recommendation.

This deliverable aims at setting the requirements for mixed-signal technologies (WP3) in Flex5Gware. Its goal is to describe some of the most promising mixed-signal concepts identified for 5G. These concepts will be further developed during the course of the project. A set of associated key performance indicators (KPIs) for each mixed-signal technology is then derived in order to drive the research and concepts towards the main goals set by the project. Deliverable D3.1 is structured as follows. Section 2 gives an introduction to the mixed-signal technologies identified by Flex5Gware in order to give an overview of the main innovations and concepts that will be developed in WP3. Then, the identified mixed-signal technologies are further described. In section 3, they are eventually mapped to the use case families set in WP1 of Flex5Gware in order to identify KPIs for each technology. These KPIs will be used to set the main requirements of each key building block and drive the development of the mixed-signal technology and research along the achievement of WP3. Section 4 concludes the deliverable.



2. Mixed-signal Techniques and Strategies identified for 5G 2.1 Introduction to Mixed-signal Techniques

The advances in very large scale integration (VLSI) have significantly helped the almost exclusive use of digital communication in modern communication systems. Any modern development in satellite, fibre and cellular communication systems is now known as “all digital”. However, for radio-wave and optical transmission, the transmission channel (i.e. the propagation of the electromagnetic wave) is inherently an analogue phenomenon: the propagation of an analogue signal over a medium (air or fibre). Analogue signals are continuous in both time and value while digital signals are discrete in time and value. We define as mixed-signal technique, a technique that is applied at both sides of the analogue/digital interface. With the adoption of digital modulation techniques, analogue-to-digital and digital-to-analogue converters (ADC and DAC), the development of mixed-signal technologies started. Then as wireless communication systems continuously improved spectral efficiency, its performance was affected by the limitations of the analogue front end (AFE): any AFE imperfection produces issues that directly affect the transmitter and the receiver. Compensation for AFE imperfections by means of digital front-end processing became more generalized to mitigate problems introduced by DC offset, I/Q imbalance and non-linearities of the power amplifier for example. A survey of some compensation techniques are proposed in [Gar2011]. Current cellular systems use the benefits introduced by mixed-signal techniques to improve integration, flexibility and reduce power consumption of the radio transceiver. Flex5Gware identified the following areas of investigations where advances in mixed-signal technologies should be considered for 5G:

Current and future generations of cellular systems consider multiple bands with multiple carrier frequencies and already include carrier aggregation. Support for the generation of multiband RF is considered.

Massive Multiple Input Multiple Output (M-MIMO) techniques are not possible without improvements and compromises at the transmitter. Compact multichain transmitters are investigated for transmission. Furthermore, low resolution ADCs are to be researched to enable M-MIMO notably for mmWave applications at the receiver side.

Power consumption should also be considered with envelope tracking for RF amplifiers adapted to 5G constraints. Peak-to-average power ratio reduction and predistortion techniques are to be investigated in the new context of 5G.

The generalization of the adoption of distributed antenna systems combined with the multiplication of frequency bands will require new high bandwidth antenna links based on new fibre-to-the-antenna transceiver subsystem technology

The demand for higher spectral efficiency leads also to consider full-duplex operation for 5G.

The challenges of the identified mixed-signal key building blocks are here briefly introduced. Advances proposed by Flex5Gware for each key building block are then further developed. Multiband RF signal generation Today’s multiband transceivers often share digital components, but consist of separate analogue RF generation circuits for each band. Only a few multiband transceivers share power amplifier (PA) or low noise Amplifier (LNA) as is shown in Figure 2-1. Such a transceiver still needs dedicated conversion paths for each signal band, namely one DAC and mixer for the transmit band and one mixer and ADC for the receive band.



Figure 2-1: Schematic of a dual band transceiver sharing RF components

Such architecture results in highly sophisticated RF implementations as e.g. for the generation of the RF carrier frequencies, many local oscillators (LOs) are working simultaneously and close to each other, inducing crosstalk and interference to both, the own and the other radio band. Therefore, this approach provides only limited freedom in terms of supported bands. Furthermore, LOs are costly and energy consuming components. Compact Multichain Transmitter Some radio access techniques require simultaneous operation of multiple radios and cost, complexity and power consumption of the RF transceiver is therefore often increased. Cost complexity and power consumption are, however, key requirements of wireless communication systems. M-MIMO has been identified as a key technology to enable the next generation of wireless communication systems. By having at least tens of transmitters at the base station, one can exploit the multi transmission channel behaviour and perform constructive wave interference at the user equipment (UE) side. One of the critical issues of M-MIMO implementation comes from the large number of transceivers. This results in high hardware costs and induces integration challenges for multi-antenna arrays. The target of the Multichain transmitter (TX) is to develop a compact and cost effective device, which supports multiple low power TX chains (at least 8), including digital to analogue conversion and RF signal generation. Low Resolution Analog-to-Digital (ADC) receivers for mmWave MIMO Current LTE systems have a limited number of antennas at the base station and at the user equipment. Since the bandwidth is also not too wide, the power consumption of a RF receiver chain with high resolution ADCs at each antenna is feasible (Figure 2-2). Future mmWave mobile broadband systems are envisioned to have a larger bandwidth and an increased number of antennas [Boc2014]. This combination makes a full digital beamforming system hardly feasible in terms of power consumption. The ADC can be considered as a major bottleneck [Mur2015]. 802.11ad (aka. WiGig) is the first commercially available consumer grade system operating in the mmWave spectrum. To prevent high power consumption, WiGig has been based on

DigitalSignalProcessing

*

PA+*2.650GHz

2.145GHz

TX S1

TX S2

DAC

DAC

LNA

*

*2.530GHz

1.955GHz

RX S1

RX S2

ADC

ADC



analogue beamforming (Figure 2-2). This has the major disadvantage that the system is highly dependent on the calibration and the alignment of the TX and receiver (RX) beam. Digital beamforming with low resolution ADCs has the potential to maintain the full digital flexibility without a prohibitive high power consumption (Figure 2-2). A disadvantage is a limited performance in the high SNR regime [Sin2009].

Digital Beamforming used in LTE

Analog Beamforming used in 802.11ad (WiGig)

digital baseband

Mr

RF-chain high

resolution

RF-chain high

resolution

digital baseband

Mr

RF-chain low

resolution

RF-chain low

resolution

RF-chain high

resolution

digital

baseband

Low resolution beamforming

Figure 2-2: Different receiver architectures for mmWave MIMO operation

Envelope Tracking for RF power amplifiers The increasing PAPR in today’s and emerging mobile systems as well as the need for broadband operation have a significant impact on the RF transmitter efficiency. Envelope tracking (ET) is becoming a popular technology which addresses both challenges and optimizes PA power consumption at high output power levels when transmitting at high data rates and/or with advanced modulation schemes. The principle behind ET is to use an envelope signal path and continuously adjust the power supply voltage for any instantaneous output power requirements in order to ensure PA operation at peak efficiency (see Figure 2-3).



Figure 2-3: Envelope tracking principle system

ET comprises a number of circuit blocks that must be carefully designed and well calibrated to support the targeted waveform. As 5G systems are expected to have more advanced and complex waveform shapes and also operate at higher carrier frequencies, the benefits of ET need to be re-examined and new requirements on the performance of the systems’ building blocks need to be studied in detail. PAPR reduction and digital predistortion techniques The main disadvantage of multicarrier techniques is the high PAPR. As it has been pointed out in the previous paragraph, PAPR is critical for the PA efficiency. Thus, many PAPR reduction techniques have been developed. They can be classified by data predistortion and signal predistortion. Basically, the data predistortion manipulates the transmitted data and includes selected mapping (SLM), Partial transmit sequences (PTS), Block coding, Tone Reservation and Injection. The data predistortion techniques provide us with significant performance improvement, but the computational complexity is very high especially with a high order modulation and larger number of subcarriers. On the other hand, the signal predistortion modifies the transmitted signal: it includes clipping and filtering. These techniques are straightforward but they cause in-band signal distortion and out-of-band emission. Some clipping and filtering techniques compensate in-band signal distortion in the analogue domain. They all have pros and cons and there is no optimal solution yet. In addition, new waveforms (such as Filter Bank Multi-Carrier (FBMC), Faster-than-Nyquist (FTN), Non-orthogonal asynchronous waveforms, Generalized Frequency Division Multiplexing, and Universal Filtered Multi-carrier) are promising techniques for the radio waveform in forthcoming 5G. However, they are based on a multicarrier technique and still suffer from high PAPR. Thus, one of key 5G research challenges is to develop a new PAPR reduction technique for these multicarrier systems. High Bandwidth Antenna links based on new fibre-to-the-antenna transceiver subsystem technology Fibre-to-the-antenna (FTTA) links are a promising solution to connect large amounts of distributed antennas to a central base station, where complex signal processing and networking functions can be grouped and managed efficiently, providing on-demand capacity wherever and whenever it is needed. FTTA links can be tailored to different Flex5Gware use cases, such as crowded venues, smart cities and 50+ Mbps everywhere. As operators will evolve to 5G, distributed antenna systems (DAS) deployments will require more and more



optical links to scale up the capacity and coverage. Compact optical FTTA transceivers could also support M-MIMO with the benefit of increased data rate for a higher number of users. Now, to reduce the transceiver (TRx) complexity, power consumption, and latency introduced by signal processing functions, our focus on FTTA is to transport cellular signals optically, but in their native format, avoiding format conversions and avoiding remote digital signal processing, so that ideally, only RF front-end circuits are needed, which can be closely integrated with the remote antennas. Such analogue FTTA links can support various modulation schemes and frequency bands, making them versatile and increasing flexibility for DAS deployments. Full-Duplex Techniques A general trend in wireless communication systems evolution, and more particularly in radio resource usage, is to find solutions to improve spectral efficiency and flexibility in spectrum use. Traditional access modes consider time (TDD), frequency (FDD) or code division access. Orthogonality between transmission and reception is guaranteed by design. Full-duplex (FD) radio transmission breaks the orthogonal division access between transmission and reception: the same carrier frequency is simultaneously used for both transmission and reception at the wireless transceiver.

Figure 2-4: Principles of full duplex operation

FD transmission provides significant improvements to wireless communication systems operation. An increased link capacity is provided as the same frequency resource is used for transmission and reception at the same time. Furthermore, FD operation could give more flexibility than traditional FDD access to the spectrum. As the transceiver only requires a single (frequency/time slot) parameter in the case of FD rather than an uplink and a downlink frequency in the case of FDD access, more options may be provided for dynamic spectrum access. It gives therefore the technology an advantage for dynamic spectrum access as only one resource should be dynamically addressed instead of two as is the case for FDD and TDD. Security of transmission is slightly improved as signal interception is made significantly more challenging. For these reasons, FD has been identified as a potential technology component for 5G networks.



2.2 Techniques identified by Flex5Gware

2.2.1 Multiband RF signal generation Starting with advanced 4G techniques such as carrier aggregation, co-ordinated simultaneous operation in different radio bands becomes necessary. 5G systems are expected to be even more demanding on RF front-ends in terms of aggregated bandwidth as well as covered radio bands. The joint generation of Multiband RF signals with a single commonly used RF line-up seems to be a promising approach compared to multiple separate RF generation circuits. Such Multiband RF front-end hardware provides the following features:

- Frequency agile over the range of multiple single band radios - Reconfigurable frequency bands respectively to a set of frequency bands - Flexible split of the RF power amongst the supported bands - Enabling co-ordinated operation by its construction.

The following figure shows schematically an example of a 3-band transceiver. Signals S1, S2 and S3 are baseband mobile communication signals, dedicated to 3 different radio bands.

Figure 2-5: Schematic of a 3-band transceiver

The work will concentrate on multiband transmitters including the associated observation receiver (feedback receiver), needed for digital algorithms to mitigate analogue imperfections (PA non-linearity and gain variance, RF carrier leakage etc.). The hardware issues to be solved lay in the fact that the frequency spacing between the radio bands supported by such transceiver varies from hundreds of MHz up to the GHz-range. Treating the multiband signal as one signal leads to a mostly empty broadband signal of some GHz wherein only some tens of MHz are used as aggregated carrier bandwidth, causing unmanageable high data rates and wasting computational effort to a large extent. One of the technical challenge of these transceiver architectures is therefore to find a fair compromise between the computational effort introduced by the high data rate and the actual useful bandwidth of the input signals.

DigitalSignalProcessing

PA

TX S1

TX S2RFDAC

~TX S3

RX/FBRFADC

RX S1

RX S2

RX S3



With the rapid progress of converter technologies RF DACs and RF ADCs are becoming promising candidates for multiband radio applications. A laboratory set-up for a 3-band transmitter is planned using these technologies. The 3 supported radio bands shall be positioned between 2.5 and 4.2 GHz aligned to the multiband RF front end defined in WP2. The considered set of radio bands is positioned around the following central frequencies 2.6, 2.8 and 3.5 GHz. With a combined total data rate of around 1.5 Gbit/s, the 3 data input channels (TX S1 to TX S3 in Figure 2-5), an aggregated signal BW of about 600 MHz shall be supported. For the transmitter-related digital signal processing, techniques have to be developed which are working at the individual input signals and which take into account the effects caused by sharing hardware components. The two most important algorithms in this context are the reduction of the peak to average power ratio (PAPR) and the PA linearization by digital predistortion (DPD). One issue for these algorithms is that, for single band applications, they work on the baseband signal as the baseband signal directly translates to the envelope of the RF signal amplified by the PA. For single band applications, the carrier frequency itself does not influence the algorithms. For multiband transmitters, one has multiple input signals as well as multiple RF carrier frequencies, which lead to a RF signal envelope depending on all the input signals and all the carrier frequencies. Therefore, for multiband applications, the carrier frequencies have to be taken into account by the algorithms. The following figures illustrate exemplarily this issue. Figure 2-6 shows, on the left hand side, a single band LTE signal spectrum and on the right hand side the same signal in time domain. The envelope of the RF signal translates to the magnitude of the baseband signal. Figure 2-7 illustrates a dual band case. The spectrum plot on the left hand side shows two different LTE carriers with a carrier frequency distance of around 600 MHz. The time domain plot on the right hand side reflects the higher dynamic of the signal envelope and the beat signal which depends on the carrier frequency distance.

Figure 2-6: RF signal spectrum and envelope for single band operation



Figure 2-7: RF signal spectrum and envelope for dual band operation

Another important issue, not only for the PAPR-reduction, is the Radio Access Technology (RAT) dependency. The RAT specifications set spectral masks and Error Vector Magnitude (EVM) limits which have to be taken into account by the algorithms. For example, in 5G systems using Universal Filtered-OFDM (UF-OFDM), a slightly higher PAPR compared to 4G systems is expected. As long as the spectral requirements for 5G are still in a definition phase, 4G requirements will be used as the base line.



2.2.2 Massive Multiple Input Multiple Output architectures 2.2.2.1 Compact Multichain transmitter The following figure shows an example of a highly integrated compact multi chain transmitter. Since future transmit schemes like M-MIMO are based on a large number of transmitters for a single transmitter site, compact solutions for multiple transmit chains become essential in order to provide cost efficient solutions with low power consumption, with reduced calibration effort and form factor.

Figure 2-8: Highly compact multichain transmitter

One of the essential topics addressed in Flex5Gware is to elaborate solutions with high flexibility regarding carrier frequency and the number of transmit chains. In order to minimize power consumption due to multiple interfaces between distinct devices (e.g. ADCs, DACs, digital processing, analogue components) a multi-chip solution shall be avoided. The target is to head for a solution, where the core transmitter part can be implemented using only one technology. The PA as well as the filtering shall not be integrated, as there is no promising concept, which will provide sufficient performance and flexibility for these components from the current state-of-the-art. A highly challenging issue that derives from the high integration of multiple transmit chains is crosstalk between signal chains. Investigations to characterize the relevant crosstalk will be researched. Furthermore concepts to avoid crosstalk shall be evaluated. If avoiding crosstalk is not possible in a reasonable way, concepts and algorithms shall be evaluated in order to mitigate crosstalk. The main focus will be on the investigation of a multiple transmit chain solution. Nevertheless multiple receiver chain solutions shall be evaluated as well. Depending on the success on the receiver aspects, this topic may be investigated more deeply. The target is to provide simulation results and a hardware implemented multichain transmitter prototype for proof of concept in WP6. The target specification can be summarized as follows:

High integration of 8-16 transceivers in one monolithic device

High degree of reconfigurability, flexibility (carrier frequency, bandwidth….)

Carrier frequency 800 MHz…3.8 GHz

Bandwidth ≥ 5 MHz

ACLR ≥ 45 dBc

The compact multichain transmitter shall minimize design complexity regarding interfaces and calibration. Furthermore, it should decrease device mismatch and device variation already by construction.

RF out

RF out

RF out

RF out

Multi-Chain TXM=8 TX

Filter

Filter

Filter

Filter

Dig. BB in

Dig. BB in

Dig. BB in

Dig. BB in



2.2.2.2 Low Resolution ADC receivers for mmWave MIMO For 5G, higher carrier frequencies are considered [Boc2014]. These frequencies are in the range of 6 to 100 GHz. Generally they are referred to as mmWave even though the frequency range includes the lower centimetre wave range. The major advantage of utilizing this frequency range comes from the availability of large frequency bandwidth. To attain a fair link budget, the effective antenna aperture of a mmWave system must be comparable to current systems operating at carrier frequencies below 6 GHz. Since the antenna gain, and therefore the directivity, increases with the aperture, an array is the only solution to attain a high effective aperture while maintaining an omnidirectional coverage. Large bandwidth antenna arrays are a huge challenge for the hardware implementation, especially considering the power consumption. At the moment analogue or hybrid beamforming are considered as possible solutions to reduce the power consumption. This solution consists of Mr antennas and phase shifters, MRFE analogue combiners and radio front-end chains. The radio front-ends contain all additional analogue components to convert the signal into the digital baseband domain. For a large bandwidth, the ADC consumes a considerable amount of power [Mur2015], therefore the analogue-to-digital conversion can be considered as the bottleneck for large antenna arrays with a sizeable bandwidth [Sin2009]. The reported ADC designs in [Mur2015] show that above a sampling frequency of 200 to 400 MHz, the power consumption increases in a quadratic manner with the sampling frequency. The power consumption is also exponential with the effective resolution of the ADC. Thus digital beamforming has prohibitive high power consumption for mmWave systems at the receiver of the mobile and base stations. Analogue or hybrid beamforming highly depends on the calibration procedure. Testing is a major cost factor during the manufacturing of current mmWave products [Nik2015]. Another major disadvantage is the dependency on the alignment of the TX and RX beams at the base stations and UEs. If a high antenna gain is needed, the corresponding beamwidth is very small. This makes the acquisition and constant alignment of the beams in a changing environment very challenging [Bar2015], [Rap2014] and [Moj2015]. Therefore a solution that offers the full flexibility of MIMO with reduced power consumption would be to use a radio front-end behind each antenna with a low resolution ADC [Nos2006], [Alk2014] and [Mez2007]. In the extreme case, that would mean utilizing a 1-bit ADC for the inphase and quadrature component of the signal. This has the advantage that an AGC is not needed and can be replaced by a much simpler limiting amplifier. Because the 1-bit quantization represents a major non-linear processing step at the end of the receiver chain, this also reduces the requirements on the rest of the receiver chain and could potentially improve the power consumption without any further compromises in terms of performance. In this case only digital beamforming needs to be done. Since such a system would utilize a large number of antennas, the overall signal processing would be more elaborate. The many signal processing tasks have to be performed per antenna and can be executed in parallel. The low resolution approach has the following advantages and disadvantages compared to analogue/hybrid beamforming. Advantages:

Flexibility of MIMO operation

Less dependent on calibration compared to analogue/hybrid beamforming based solutions

No beam acquisition is required therefore it enables faster link setup time

Low Baseband processing complexity per antenna Disadvantages:

Performance in high SNR regime is limited by the quantization [Sin2009] and [Moj2015]

Many digital signal processing tasks are not fully mature



If the system is only operating at a low SNR per antenna, the advantages should prevail. The overall architecture of analogue/hybrid beamforming is shown in Figure 2-9. Here, the signals from Mr antennas are phase shifted and combined by MRFE signal combiners followed by a RF-chain and a high resolution A/D conversion. In contrast, the architecture with 1-bit A/D conversion for I and Q has Mr RF chains and purely digital beamforming as shown in Figure 2-10. Therefore we have access to the digital signal at each antenna.

RF-

chain

RF-

chain

digital

basebandMRFE

Mr

ADC

ADC

Figure 2-9: analogue/hybrid beamforming receiver front-end

RF-chain

RF-

chain

digital

basebandMr

low resolution A/D

Figure 2-10: 1-bit quantized digital receiver beamforming

The target of this work is to prove the feasibility of key signal processing components of the receiver system with 1-bit ADC. Up to now, the topics of synchronisation, multipath channel estimation and equalization have been identified. The synchronisation is always the first task a receiver has to perform before the data processing can start. This does also include cell detection of the cellular network, which is usually referred to as cell search. Up to now the evaluation of the channel estimation for such a system did not cover all possible cases, especially for channels with time dispersion, i.e. multipath propagation, the evaluation still shows major gaps. It is important to understand which level of accuracy is required for the channel estimation, especially with relation to the equalization. This naturally brings us to the



question which type of equalization schemes and modulation formats are possible to be considered for this system. The performance of these building blocks is especially interesting when compared to a system based on analogue/hybrid beamforming. The power consumption of a mmWave radio front-end with 1-bit quantization can be compared to the performance of the analogue beamforming front-end. Starting with this, we can compare the channel capacity of full resolution MIMO, 1-bit quantized MIMO and analogue/hybrid beamforming with equal power consumption.



2.2.3 Envelope Tracking for RF Power Amplifiers For battery powered devices, high RF transmitter efficiency has always been a desirable design objective and even for base stations low overall power consumption and heat emission is a key performance parameter. The efficiency of the RF PA, usually considered as the ratio of RF output power to DC input power, has an impact on the efficiency of the overall system. The main operating characteristics of a PA are linearity, signal gain, power output and efficiency. Two main factors define the PA efficiency:

1. Operating mode In linear operating mode, the whole input signal is within the linear range of the device and output voltage is always between lower and upper limit. Theoretical maximum PA efficiency in this mode is 50 % but much lower levels are achieved in real systems. When the signal swing is increased beyond the linear range, the PA output will start saturating. In this compressed mode, the instantaneous amplitude of signal peaks will be reduced but the overall power efficiency typically improves.

2. Waveform shape Modulation without an amplitude component (e.g. FM) leads to a steady waveform shape with a near-constant signal magnitude. In this case the PA can operate well into compression and achieve high efficiency. This will cause harmonics of the carrier frequency but these can be easily filtered out by post-PA low-pass filters. On the other hand, when modulation with an amplitude component is applied to a carrier (e.g. QAM), the signal is distorted if it is passed through an amplifier that is run in compression. The situation becomes worse when the input signal PAPR is high; the PA has to be able to accommodate the high peak levels while still only running at a low average power level.

In conventional PA design, fixed supply is used causing power to be wasted when a device transmits below its maximum output power.

Figure 2-11: Fixed supply.

In 2G PAs, where the output signal has a near-constant amplitude, it has been possible to adjust the fixed supply to power the PA while achieving high PA efficiency. However, this has not been the case for 3G and 4G devices where variable envelope signals are the norm. In that case, the supply voltage needs to be dynamically adjusted in order to improve efficiency. Early implementations used average power tracking (APT) where the PA supply voltage is adjusted based on the average power over a certain period (for example over one 1 millisecond subframe in LTE). However, with the DC tracking approach, instantaneous peaks are currently not handled leaving significant energy waste.



Figure 2-12: Average power tracking

Several alternative architectures have been proposed that allow running the PAs in compression whilst achieving good transmit linearity. For example, Doherty amplifiers consist of two separate devices, one main device providing most of the signal power and a peaking amplifier element. The latter operates in an assistive way only for handling peak power levels. The main disadvantage of the Doherty approach is that it generally requires more complex circuitry and is costly to manufacture. However, this approach is often used in base stations. More recently, ET has been introduced in commercial PAs and LTE handsets. The principle behind ET technology is to constantly operate the PA in compression, i.e. constantly adjusting the power supply voltage to ensure PA operation at peak efficiency for any instantaneous output power requirements. For this, the ET technique employs a system for tracking the RF envelope and updating the power supply voltage at I/Q sub-sample rate. The result is that very limited amount of unnecessary power is dissipated.

Figure 2-13: Envelope tracking

LTE SC-FDMA uplink waveforms are generally designed to achieve low PAPR. Despite that, ET already proves to be extremely beneficial. The system power efficiency (i.e. considering also the power supply components consumption) of commercial PAs has been measured as a function of output power by one partner of WP3 (SEQ) and is given in Figure 2-14.



Figure 2-14: Power efficiency of commercial PAs.

The red curve shows power efficiency of an amplifier on a fixed supply voltage (“Direct Battery”). The best case efficiency is achieved for maximum output signal and is equal to around 40 % which still ensures sufficient amplifier linearity. At lower power levels, the current drawn by the amplifier reduces roughly in line with the square root of the RF power level while the supply voltage stays the same. Therefore, the DC input power reduces more slowly than the RF output power which results in lower efficiency. The yellow curve shows how some efficiency can be recovered by switching the PA into a different gain mode. The green curve employs APT where the supply voltage is reduced below maximum power. This can easily double the power efficiency. Finally, the blue curve shows the behaviour under ET. The amplifier is run in compression throughout the whole power range and the supply voltage closely follows the instantaneous signal magnitude which improves efficiency to beyond 60 %. Therefore, it is not a surprise that an increasing number of high-end LTE handsets are using ET in order to reduce power consumption and increase their battery life (e.g. 12 phones were already launched with ET by 2014 [Nuj2014]). The effect of ET is that it also typically leads to improved PA linearity. In fact, when designing PAs for envelope tracking or when selecting a mapping function between output power and PA supply voltage, there is a trade-off between power efficiency and amplifier linearity. In some cases, linearity of the PA may be optimised at the expense of some efficiency, for example to minimize out-of-band emissions or in-band signal distortion. Quite often, ET brings a simultaneous improvement to both, power efficiency and linearity. 5G systems are expected to have more advanced and complex waveform shapes. For example, higher order QAM will increase the variance in envelope waveform statistics rendering non-instantaneous tracking techniques more inefficient. Considering the introduction of fragmented or larger carrier access increased levels of PAPR could be expected. This will increase the benefit obtainable with ET. To reduce pollution of spectrum for other users it is likely that 5G systems will demand lower in-band and out-of-band emissions. This translates to higher linearity requirements on the PA. Again, ET seems ideally suited for improving linearity. Furthermore, the upcoming aggregation of multiple



(licensed or even unlicensed) carriers in uplink, not only will lead to increased PAPR of transmit RF signal, but also will increase the number of simultaneously running transmitters and therefore system power consumption. A key challenge introduced in the 5G context for practical ET implementations has to do with the expected operation at high-GHz carriers. Regarding > 6 GHz carriers, mmWave commercial PAs are already available; for example the already mentioned 60 GHz WiGig (802.11ad) and also Wireless HDMI (WPAN) applications. However, we still need to understand how such PAs should be modelled and whether existing commercial designs can be readily adapted to work with a variable supply voltage. Using data from vendors of 60 GHz PAs, PA models can be matched against measured performance. The model must accurately reflect the gain of the PA as a function of both input power and supply voltage. This will capture what is known as AM/AM distortion. Also, the phase delay experienced by the signal through the amplifier must be expressed as a function of input power and supply voltage (AM/PM distortion). Furthermore, wider signal bandwidth is already identified as a key challenge for ET power supplies (the devices that generate the dynamic PA supply voltage from a battery voltage). A wider system bandwidth makes the architecture more susceptible to small gain and timing misalignments between the RF signal path and the envelope path. The misalignments work against the linearity improvements that envelope tracking normally promises. For that reason a tight integration of PA with the ET power supply may be needed as suggested in [Nuj2014]. In the context of 5G, the sensitivity of envelope tracking to timing and gain misalignments can be studied and compared to the sensitivity of today’s LTE transmitters using SC-FDMA waveforms and 20 MHz channel bandwidth. Other effects that will be studied include the effects of small gain and delay ripples on in-band signal quality. Figure 2-15 shows a screenshot of a KeySight SystemVue (an advanced design system tool for analog circuits) test setup for characterizing ET. The test setup contains a model of a transmitter (including impairments such as imperfect filter amplitude and delay responses, carrier leakage and IQ image), a calculation of the envelope signal (including quantization effects), a model of a PA and the envelope tracking power supply (adding noise and further small distortion). The PA output spectrum can be analysed in terms of unwanted in-band or out-of-band spectral components and signal quality. Power efficiency can also be measured as a function of RF output power. Such architecture could be considered to investigate the promises of envelope tracking, starting from LTE-A and later on moving to 5G system setups.



Figure 2-15: Example test setup for envelope tracking characterisation.



2.2.4 PAPR reduction and digital predistortion techniques Even if the PAPR concept has already been introduced before qualitatively, formally, the PAPR is defined as the ratio of the maximum to the average power as follows:

𝑃𝐴𝑃𝑅 = max

0≤𝑛≤𝑁|𝑥𝑛|2

𝐸(|𝑥𝑛|2)

where E() denotes expectation, N is the number of subcarriers in a multicarrier modulation and xn is the signal. This equation is one of the important metrics and represents a peak level of e.g. one OFDM symbol. The complementary cumulative distribution function (CCDF) is widely used to estimate the performance of PAPR reduction techniques [Bau1996].

Pr(𝑃𝐴𝑃𝑅 > 𝑌) = 1 − (1 − 𝑒−𝑌)𝑁 𝑌 > 0 where Y denotes a certain threshold and Pr(PAPR > Y) is the CCDF of the PAPR. This expression fits well when the number of the subcarriers (N) is relatively small. However, it doesn’t fit well when an OFDM has a large N. Therefore, a new CCDF expression as an empirical approximation is used as follows [Bau1996]:

Pr(𝑃𝐴𝑃𝑅 > 𝑌) = 1 − (1 − 𝑒−𝑌)2.8 𝑁 𝑌 > 0 . When we consider U statistically independent transmit groups, the CCDF expression can be represented as follows [Bau1996]:

Pr(𝑃𝐴𝑃𝑅 > 𝑌) = (1 − (1 − 𝑒−𝑌)𝑁 )𝑈 𝑌 > 0 . This expression refers to the probability that the signal is above a given threshold Y within U OFDM symbols. We can find numerous PAPR techniques for OFDM/OFDMA. One of these techniques, clipping and filtering [Li1998] [Och2000] can reduce PAPR but at the same time distorts the original OFDM symbol and creates a bit error rate (BER) degradation. The basic idea of a coding scheme [Jon1994] [Fan1999] for PAPR reduction is to reduce the same phase probability of each subcarrier in the OFDM symbol. However, the main disadvantage of this scheme is that the performance is dependent on the coding rate. Namely, a lower coding rate brings a better PAPR reduction, but the data rate is reduced. The basic idea of tone reservation and tone injection [Tel1999] [Yoo2006] is to reserve or inject several tones for PAPR reduction in the transmitter. These tones are not used for data transmission. The selective mapping technique and the partial transmit sequences technique [Yan2006] [Bax2007] can find an OFDM symbol with a low PAPR through selecting one OFDM symbol from many candidates. However, complexity is high due to multiple inverse fast Fourier transforms and additional information is required. The interleaving technique [Mal2011] is similar to the selective mapping technique. This technique uses many interleavers and selects an OFDM symbol from the candidates. As we briefly reviewed PAPR reduction techniques, all solutions have pros and cons and there is not a clear winner. In Table 2-1, the described PAPR reduction techniques are compared.



Table 2-1: Comparison of conventional PAPR reduction techniques.

Power

increase BER

degradation Data rate reduction

Complexity

Clipping and filtering

N Y N Low

Block coding N N Y Low

Interleaving N Y Y Low

Tone reservation

Y N Y High

Tone Injection Y N N High

Partial transmit

sequences N N Y High

Selective mapping

N N Y High

The 4G system adopts OFDM/OFDMA techniques and faces several limitations: high sensitivity to frequency offset and phase noise, spectrum shaping problem, and long symbol duration. The 5G system requires advanced waveforms in order to meet 5G requirements such as support for fragmented spectrum, suitability for short bursts, robustness to frequency/timing offset, low cost, and low latency depending on the use cases defined in WP1. There are several 5G candidate waveforms: Filter-Bank Multi-Carrier (FBMC), Non-orthogonal asynchronous waveforms, Generalized Frequency Division Multiplexing, and Universal Filtered Multi-carrier. They all are based on filtered multicarrier techniques. Among them, the FMBC technique provides many advantages:

1. Flexible and efficient spectrum access, 2. Efficient separation of multiple users (this is a key feature for HetNet coexistence), 3. Asynchronous transmission.

The digital filters of the FBMC allow to significantly reduce sidelobes. Thus, we can guarantee the separation of multiple users and multiple users could have different transmission parameters (e.g. subcarrier locations and/or bandwidth). In addition, it can leave spectral holes so that it is suitable for combining with cognitive radio techniques. We compare a conventional OFDM waveform with an FBMC waveform using computer simulation. Figure 2-16 illustrates the comparison of power spectral density of conventional OFDM waveform with FBMC for three blocks of 16 subcarriers.



Figure 2-16: Comparison of conventional OFDM waveform and FBMC waveform

As we can observe in Figure 2-16, the sidelobes of FBMC waveforms are significantly reduced. The main disadvantages of FBMC are the higher implementation complexity due to the digital filter design and a high PAPR (PAPR is also a problem for FBMC because it is based on a multicarrier technique). In particular, the high PAPR degrades the efficiency of a PA and produces out-of-band and in-band interference. Unfortunately, conventional PAPR reduction techniques cannot be directly applied to FBMC due to the overlapping nature of FBMC symbols. Some FBMC PAPR reduction techniques [Skrz2006, Lu2012, Qu2013] require very high complexity in order to solve the optimization problem. The FBMC PAPR reduction technique presented in [Kol2012] is based on an iterative technique, requires high complexity and introduces large amount of latency. Therefore, achieving low complexity is one of key design parameters for FBMC PAPR reduction techniques. The PAPR reduction study will focus on both algorithm design and implementation. The developed algorithm will be demonstrated in PoC#3 of Flex5Gware.

1. Digital Predistortion (DPD) techniques are used to compensate for the nonlinear behaviour of the PA. The goal of the DPD is to improve the adjacent channel power ratio (ACPR) and BER performances. These have to be considered simultaneously with the energy efficiency considerations. DPD research challenges in 5G can be summarized as follows: High speed DPD algorithms for wide band systems: DPD algorithms must typically be applied at a sample rate reaching at least 3 or 5 times the baseband signal bandwidth. In 5G systems, ultra-wide bandwidth due to very high data rate is anticipated.

2. DPD algorithms for multiband communication: when using carrier aggregation, two or more RF signals may concurrently be transmitted through one PA. In addition to intermodulation distortion, also cross-band modulation effects can occur. Conventional DPD techniques may not give good enough performance [Liu2013, Abdelhafiz2015].

In addition, the combined processing work of PAPR reduction and DPD techniques provides more comprehensive solutions for the energy efficiency studies. The developed algorithms will be demonstrated on VTT’s 5G SDR platform shown in Figure 2-17 and Figure 2-18. The VTT 5G SDR platform includes a pair of Ettus Research URSP X310 SDR platforms with wideband RF daughter boards. URSP X310 has multiple high-speed interfaces, a flexible clocking architecture and a large user-programmable Xilinx Kintex 7 FPGA for transceiver implementation. The VTT 5G SDR platform utilises an LTE Framework from National Instruments. A real-time, modifiable LTE PHY downlink is running partly on FPGA platforms, partly on a host-PC. The developed PAPR reduction algorithm

-1 -0.5 0 0.5 1 1.5-60

-50

-40

-30

-20

-10

0OFDM

-1 -0.5 0 0.5 1 1.5-90

-80

-70

-60

-50

-40

-30

-20

-10

0FBMC



and DPD algorithm can be implemented in Matlab or in a FPGA. The implementation scenario is (1) Waveform generation (OFDM, FBMC, etc.), (2) PAPR reduction implementation in Matlab (Offline processing) and DPD technique implementation in FPGA (Real-time signal processing), (3) Connection to external PA (Conventional PA, GaN HEMT HPA, etc.), (4) Emulated ratio channel and device under test/measurement, (5) Received signal measurement. The performance and complexity of the PAPR reduction and DPD technique will be investigated with and without both techniques. The energy efficiency will be evaluated by measuring the PAPR value and the power consumption.

Figure 2-17: ETTUS X310 and B200 SDR devices in VTT 5G Lab

Figure 2-18: User Interface (Real-time LTE-based system)



PAPR reduction, PA predistortion and baseband algorithms with joint analogue and digital design Energy consumption of a transceiver functionality is highly dependent on the implementation method. Software implementation of a functionality running on general purpose processor or on digital signal processor could consume one hundred times more energy than a digital hardware module optimised for the specific functionality. Sometimes, an analogue implementation can be an order of magnitude more energy efficient than its corresponding digital implementations [Sar2012]. In addition to energy efficiency, better area efficiency can also be achieved. When a high dynamic range is required, the difference between analogue and digital implementation is reduced. Algorithm studies will be concentrated on PAPR reduction and digital predistortion (DPD) of the PA. DPD is typically implemented using digital logic and applied to improve RF front-end functionalities. Analogue PA predistortion has been used instead of digital predistortion because of cost minimisation. However, so far, results have not been promising for wideband modulated signals. Some PAPR reduction techniques have clear benefits in the digital baseband processing domain, but some methods could also be applied to the RF front-end. Flex5Gware will study if there are potential hybrid analogue-digital PAPR reduction methods. Analogue implementations of transceiver baseband algorithms have been studied and already proven efficient under certain circumstances. For example, analogue implementations of channel decoding [Miy2014], FFT [Leh2010], MIMO [Rou2015] and beamforming are presented in the literature. [Leh2010] proposes a discrete-time FFT processor for ultrawideband OFDM transceivers. IFFT, FFT and equalisation functions are processing discrete-time analogue signals. The location of the analogue-digital interface is the main difference compared to the traditional OFDM transceiver. Signal dynamics are reduced after FFT, which allows using low-resolution analogue-to-digital conversion and improvement in energy efficiency can be achieved. We will study, based on literature, whether there are feasible analogue algorithm implementations that should be taken into account in the design of energy efficient network element HW architecture. This is related to the work on WP4 targeting to optimum digital HW architecture for the baseband processing in the 5G network elements.



2.2.5 High-bandwidth antenna links based on new fibre-to- the-antenna transceiver subsystem technology To meet the ever increasing demand for wireless capacity and coverage, small cells and distributed antenna systems (DAS) will be essential for future 5G roll-outs. As the number and the density of remote radio units is expected to increase drastically, the complexity of each remote radio unit becomes an important consideration for cost and power consumption. Fibre-to-the-antenna (FTTA) links are a promising solution to connect large amounts of distributed antennas to a central base station, where complex signal processing and networking functions can be grouped and managed efficiently, providing on-demand capacity wherever and whenever it is needed.

Figure 2-19: DAS system based on a Fibre-To-The-X network [Feb2014]

DAS are, for example, heavily used in crowded venues such as sports stadiums, where kilometres of optical fibre are used to connect hundreds of antennas to reliably deliver high capacity WiFi and 4G services to thousands of people. For example, Verizon equipped Denver Broncos' stadium with 210 antennas connected by 7 miles of optical fibre. Another interesting example is the US Super Bowl, “Superbowl Goes 4G”, discussed in Dailywireless, January 2012. As operators will evolve to 5G, DAS deployments will require more and more optical links to scale up the capacity and coverage. Another important 5G use case for FTTA links is considering future schemes like M-MIMO, which have already been discussed in this document and which are based on a large number of transmitters and/or receivers. Compact solutions for multiple transmit chains become essential in order to provide cost efficient solutions with low power consumption and reduced form factor. The main advantages of optical fibres, compared to copper cabling, are abundant bandwidth, very low attenuation (e.g. 0.2-0.4 dB/km1 at 1550 nm), low weight, low crosstalk, flexibility, insensitive to electromagnetic interference and low cost, only 6 eurocents per meter. For this reason, optical fibres are often used for 4G backhauling [FTTH2013]. However, with increasing peak bit rates and increasing user densities, traditional front-hauling solutions to connect remote antenna units are not the most efficient [Cha2015]. As speeds continue to increase greater portions of the total link will mitigate copper limitations by fibre-optic solutions.

1 As a reference, a 100 m Cat6 gigabit ethernet cable has a loss of around 35 dB at 250 MHz



Present optical front-hauling solutions are mainly based on links with digital on-off amplitude shift keying modulation e.g. using the Common Public Radio Interface (CPRI). Although the specified CPRI data rates (e.g. 9.83 Gbit/s) are still relatively easy to handle by fibre-optic transceivers, it is clear that the additional signal conversions (digital-RF) and the associated protocol stack introduce complexity and delays. Now, to reduce the TRx complexity, power consumption, and latency introduced by signal processing functions, our focus on FTTA is to transport cellular signals optically, but in their native format, avoiding format conversions and avoiding remote digital signal processing, so that ideally, only RF front-end circuits are needed, which can be closely integrated with the remote antennas. Such analogue FTTA links can support various modulation schemes and frequency bands, making them versatile and increasing flexibility for DAS deployments. As such, we will investigate optical-wireless architectures, opto-electronic integration and the functional split between base station and remote antenna units for the most demanding 5G applications. Besides low-loss transport of RF or mmWave signals, optical technologies also provide additional functions for broadcasting, routing, frequency conversion (IF-over-fibre), optical beamforming, resilience and switching, but these aspects will be not be investigated in detail in Flex5Gware.

Figure 2-20: Initial architecture

Although the idea of using optical links is not new for RF signals, commercial remote antenna solutions are expensive, bulky and power-hungry [opticalzonu], while academic radio-over-fibre demonstrations are typically based on brute force techniques, with expensive components and expensive equipment to realize high bitrates. Our approach, in line with the spirit of Flex5Gware, will focus on more practical approaches with state-of-the-art components, considering aspects such as cost, complexity and power consumption. Our investigations will start from low-cost, but fast components emerging from research or industry. Depending on the bandwidth/linearity requirements we will consider the direct modulation of VCSELs or external modulation using electro-absorption modulators or Mach Zehnder modulators. Prototype devices may be dies or packaged. The design of PCBs or substrates for experiments will for example include RF interfacing, biasing, thermal control, fibre alignment, low-noise linear amplifiers and RF filtering. Predistortion of the transmit signal with an arbitrary waveform generator may be considered to mitigate analogue imperfections, in synergy with the other partners in Flex5Gware that are working on predistortion.

Central base station

DSP

ADC

Baseband

RF-to-optical

DSP

ADC

Baseband

RF-to-optical

DSP

DAC - ADC

Baseband

RF

E/O

E/O

E/O

E/O

E/O

E/O

RF optical

RFO/E

RFO/E

Remote

antenna unit

RF up/down

Remote

antenna unit

RF up/down



2.2.6 Full duplex techniques In-band full-duplex operation in wireless communications has been considered so far an unsolvable problem. In the book [Gol2005], Prof. Goldsmith wrote: “It is generally not possible for radios to receive and transmit on the same frequency band because of the interference that results“. The main challenge of a full-duplex device is to deal with the inherent self-interference. The main advantage of a full duplex system is that the time and frequency resources do not need to be distributed between uplink (UL) and downlink (DL). They can be used for both at the same time [Boc2014], thus allowing a considerable increase in bandwidth efficiency. For the moment, the isolation between the transmitter and receiver was the dominant solution to this problem. For example, full-duplex relays are based on separate TX and RX antennas. The antennas are separated in space or have a different polarization. In the last decade this has evolved to systems that employ analogue and digital cancellation in addition to the isolation [Sab2014]. For a small cell, the power difference between TX and RX is smaller compared to macro cell basestations. Therefore they are probably going to be the first scenarios where full-duplex is to be adopted. The main challenge and critical success factor of full-duplex is to receive a lower power signal that has been significantly attenuated by the path loss while cancelling a powerful signal (the signal coming from the local transmitter). To resolve this issue, the transceiver must cancel the linear contribution of the interference, but also the non-linear interference contribution introduced by the RF datapath.

Figure 2-21: In-band Full-duplex SOTA architecture [Rik2014]

To achieve an acceptable level of self-interference cancellation (SIC) (linear and non-linear), several techniques need to be combined at different levels of the transceiver chain from the antenna to the digital baseband. Some of these techniques originated from the continuous wave radar technologies. The three different areas of SIC are identified as (Figure 2-21):

Isolation at the antenna interface

Analogue SIC in the RF front-end

Digital SIC in the digital baseband Antenna isolation is complemented by a first RF stage cancellation to limit the dynamic requirements on the receiver. A second stage, analogue RF cancellation, may then limit the dynamics of the signal to avoid saturation at the analogue-to-digital conversion. Both active and passive approaches can be considered in the analogue domain. Isolation techniques can rely on different antenna polarizations to isolate the transmitted from the received signal. Alternatively, both TX and RX antennas can be placed far apart [Dup2014b]. In this case the antenna would then be replaced by two separate antennas, one for TX and one for RX. If the same antenna is used a circulator can provide an isolation of about 15 to 20 dB [Dup2014b].



Active techniques are based on regenerating the echo signal and subtracting it in the receiver path before the LNA. The main purpose of cancelling the unwanted signal before the Low Noise Amplifier (LNA) is to keep it operating in the linear zone. This echo signal could be generated by an analogue filter based on delay lines/phase shifters and attenuation or digitally with an additional transmitter chain [Huu2005], [Dup2014a] and [BHA2013]. The analogue generation has the major advantage that the TX non-linearities are taken into account because the transmit signal itself is used for generation of the cancellation signal. However, this has the major disadvantage of a more complicated calibration and adaptation of the self-interference cancellation filter. It is also not possible to take other non-linearities into account. These other non-linearities could, for example, be introduced by the interference going through the circulator. A second option for generating the cancellation signal would be an auxiliary transmitter. This has different advantages and drawbacks. A major disadvantage is that the TX non-linearities probably won’t match for both primary and the auxiliary transmitter. But with this architecture it is possible to generate the cancellation signal with the full flexibility of digital processing [Dup2014a]. Flex5Gware considers the evaluation of the architecture shown in Figure 2-22. In the next paragraphs the different types of self-interference cancellation techniques are described. Only combining all the techniques that will be detailed below, the overall interference rejection will be sufficient to enable full duplex operation.

analog signal

shaping

digital signal

shaping

SIC

control

transmitter

RF chain 2DAC

transmit

signal

receiver

digital signal

shaping

receiver

RF chainADC

transmitter

RF chain 1DAC

to ctrl info

to ctrl info

from ctrl info

antenna interface

optional blocks

transmitter blocks

receiver blocks

full duplex blocks

- -

Figure 2-22: Full duplex system

Antenna solutions A number of different antenna set-ups are considered. The level of isolation is of course dependent of the considered solution.

Two separate antennas: The cancellation is directly correlated with the distance between transmitter and receiver. The self-interference cancellation expected at around 2 GHz is given in Figure 2-23. This solution is interesting if the size of the transceiver is not restricted to a very small volume (compared to the wavelength).



Figure 2-23: Self-interference cancellation versus antenna distance [Dup2014b]

Exploit a basic concept with multiple antennas: in Figure 2-24, two antennas are fed with signals of opposite phase. The receive antenna is located between both transmit antennas in order to receive a positive signal interference and its inverse. Ideally the sum should be null.

Figure 2-24: Multi antenna cancellation solution [Dup2014b]

Use of the antenna polarisation: the use of two orthogonal polarisations can help the system to reduce the self-interference. Figure 2-25 shows the simulated performance when combined distance and polarisation diversity are used. The results are compared to the effect of distance only between transmitter and receiver. Simulations have been done at around 2 GHz.

Figure 2-25: Self-interference cancellation versus antenna distance and polarisation [Dup2014b]

It is important to notice that the use of two different antennas (i.e. one to transmit and one to receive) induces two different antenna patterns and two different channels. With only one antenna, the antenna gain in one direction is the same for transmitting and for receiving. This assumption is interesting in point-to-point links when close loop channel estimation is



necessary. When two antennas are used, the gain can be different in the same direction. This is therefore interesting in point-to-multipoint scenarios. RF and IF solutions Two cases are considered depending whether the transceiver exhibits two antennas or only one antenna. When two antennas are considered, antenna isolation may be performed as previously mentioned.

Figure 2-26: Self-interference for two and one antennas [Huu2014]

In the second case, when only one antenna is used, a circulator is mandatory to allow transmission and reception at the same time, on the same frequency. Unfortunately, the circulator does not provide perfect isolation and therefore causes some of the transmitted signal to leak to the receiver. This is often referred to as leakage. Therefore part of the transmitted signal is directly fed to the receiver. Moreover, the antenna is not perfectly matched and part of the transmitted power is reflected by the antenna towards the circulator. The circulator transmits then this reflection towards the receiver. This last effect is usually the main contributor of self-interference. In both cases, the signal is fed through the receiver using two different paths: one through the transmitter (coupling or leakage) and one through the receive antenna (reception or reflection). Figure 2-27 presents a 2-stage analogue SIC architecture.

Figure 2-27: Two stage analogue SIC [Dup2014b]

This architecture allows to reduce the power at the input of the LNA. There are different ways to cancel the self-interference. For example, Figure 2-28 shows a solution based on the architecture of a hybrid transformer. A reconfigurable network tries to balance the impedance seen from the antenna and from the network.

Leakage

Reflection

1er stage



Figure 2-28: Hybrid transformer to cancel self-interference [Dup2014b] [Rik2014]

Digital solutions For the digital cancellation, a multitude of different algorithms are possible. The main objective is to cancel the residual linear and especially the non-linear self-interference. Since the digital transmit signal is perfectly known, this estimation can rely on a constant stream of known reference signals, thus enables a constant tracking of the remaining interference. Usually this is implemented using an adaptive filter [Kor2015]. Different adaptation schemes such as adaptive approximation of the MMSE filter (aka. Wiener filter) or other linear and non-linear adaptation algorithms are possible. Since at the digital receiver the interfering signal will contain the non-linear distortions of the transmitter and receiver, it is very important to take them into account. System Performance Analysis The overall architecture of a system with an auxiliary transmitter is shown in Figure 2-22 including the optional blocks. In principle we could split the SIC control into two control loops. The first one is generating the signal for the auxiliary transmitter. This control of the adaptation needs to have accurate access to the actual transmit and receive powers. Up to now it is unclear if non-linear interference can be cancelled and what the interference cancellation level of this architecture is going to be. The interference cancellation capability greatly depends on the hardware constraints, but it is not clear which components are going to be the bottleneck for the performance. The second loop controls the digital only adaptive filter [Kor2015]. The task of this filter is shaping the signal to model the residual linear and non-linear components of the interfering signal. Afterwards the signal is subtracted from the total received signal. After the whole self-interference cancellation processing chain only the desired clean receive signal with a minimal additional interference should remain. At the end of the processing chain a classical receiver for the desired waveform can be used. The Duplo project [DUPLO] already identified and classified a few demonstrations of full duplex. These demonstrations have been summarized here again for reference and compared with latest published demonstration described in [Kor2015].



Table 2-2: Summary of SotA demonstrators for full-duplex

Reference Solution Performance

[Cho2011] Separate RX and TX antennas (20cm). RF cancellation with balun. Digital BB interference cancellation

73dB rejection in RF and digital BB estimated 40 dB in antenna isolation (measurement bandwidth: 10 MHz) Total 113 dB

[Dua2012] Separate TX and RX antennas (20cm distance) RF cancellation with additional RF chain Digital BB interference cancellation

Antenna separation (AS) 41 dB AS + RF + Digital BB 78 dB (measurement bandwidth: 625 kHz) Total 119 dB

[Eve2013] Separate TX and RX antennas (50 cm distance, with 90° beamwidth and different tilting), optional use of cross-polarized antennas Active RF and BB cancellation

Antenna only w/o cross-pol. 60 dB Antenna only with cross-pol. 70 dB Antenna (w/o cross-pol.)+ RF + BB 86 dB Antenna (with cross-pol.)+ RF + BB 95 dB (measurement bandwidth: 20 MHz) Total 86dB to 95dB

[Bha2013] Single antenna + circulator Adaptive analogue RF canceller Digital BB interference cancellation

Circulator + analogue cancellation 62 dB Digital BB cancellation 48 dB (measurement bandwidth: 80 MHz, in 2.4 GHz band)

Total 110 dB

[Kor2015] Single antenna + circulator Full analogue tracking loop for the RF cancellation Digital BB SIC including nonlinearities

Circulator + analogue cancellation 56 dB Digital BB cancellation 46 dB (measurement bandwidth: 20 MHz, in 2.4 GHz band)

Total 102 dB

The setup proposed by [Dua2012] although promising, considered a relatively small bandwidth of 625 kHz and benefits from 41 dB of antenna separation. A team of Stanford [Bha2013] proposed the most accomplished solution with only one antenna used for both transmission and reception (circulator option). This option is used as reference architecture for further analysis. The target of the Stanford team was to reach 110 dB interference cancellation performance over a bandwidth of 80 MHz and using a single antenna. Wireless Local area Network (WLAN) applications have been considered as the main scenario. The transmitted power is therefore limited to 20 dBm whereas the noise level at the input of the receiver is equal to -90 dBm once integrated over the 80 MHz. This includes thermal noise and the noise figure of the receiver. The cancellation requirements of the different modules are given in Figure 2-29. The figure also shows the transmitted noise introduced by the DAC as well as the potential harmonics generated by the nonlinearities of the transmission.



Figure 2-29: Analogue and digital cancellations of FD receiver from [Bha2013]

To reach such an ambitious objective, the authors of [Bah2013] designed a smart broadband analogue canceller controlled by digital signal processing. It consists of an analogue finite impulse response (FIR) filter of 16 fixed delay lines. These correspond of the 16 taps of the FIR. On each delay line a digitally controlled attenuator is inserted. The attenuator selected is a 31.5 dB attenuator controlled by 7 bits (attenuation step of 0.25dB). This solution has the advantage of taking into account the transmitted noise and the non-linear effects of the transmission. Nevertheless, it is a relatively expensive and cumbersome solution.

Figure 2-30: Stanford architecture overview [Bha2013]

It is expected that the analogue filter demands an important calibration phase in order to select the best weight for each delay line. The relative magnitude error induced by the attenuator is usually restricted to the attenuator resolution. The relative phase error introduced by the attenuator is given in Figure 2-31 as a function of the frequency. When the frequency is increased the phase error is significantly affected. This has as a consequence at best to introduce frequency dependent linear effects or in the most likely scenario, to further amplify non-linear effects.



Figure 2-31: Relative phase error [Per2015]

However, in order to achieve a level of attenuation of 40 dB cancellation by this analogue cancellation operator, a very high level of accuracy in both phase and amplitude on all delay lines must be assumed. We estimated that using this type of analogue FIR set-up, 0.5° and 0.1 dB over the used bandwidth should be guaranteed. In order to reduce the complexity of the analogue filtering, Flex5Gware proposes to evaluate the use of a mirror transmitter designed to cancel the self-interference in the RF. The analogue filter is replaced by a copy of the first transmitter combined with a single tap filter. The single tap filter is digitally controlled (Figure 2-32). The solution investigated in this project is based on the following architecture:

(a) (b)

Figure 2-32: Flex5Gware FD architecture overview

As the current generation of standards assumes support for MIMO, 2x2 MIMO software programmable RF components are widely available. A 2x2 MIMO transceiver supporting both frequency division duplexing and time division duplexing can be transformed into a single-input single-output transceiver supporting in-band full-duplex. This degree of flexibility as well as the requirements will be justified in the scenario mapping. To generate the RF signals and to achieve the RF frequency down-conversion, the following RF chip provided by Analog Devices AD9361 has been selected for a further evaluation. The architecture of Analog Devices AD9361 is presented in Figure 2-33.



Figure 2-33: Architecture of AD9361

This component supports a 2x2 transceiver working between 70 MHz and 6 GHz. It also includes ADC/DAC converters (12 bits). It is interfaced to the digital domain by a high speed CMOS LVDS input for the data and a standard SPI bus interface for the control. Analog-to-digital conversion and programmable direct upconversions are provided, however the component does not include any RF filtering after the direct conversion. A preliminary architecture of the RF front-end interfacing the AD9361 is given in Figure 2-34. RF band filters have been added to avoid cross-coupling of the transmitted image and non-linearities into the receive signal. A main transmitter is connected to an antenna isolator. The transmitted signal is also coupled and a simple attenuation adjuster is added. This signal is combined with a mirrored version of the transmitted signal and used for cancellation. The combined received signal is then filtered. Both couplers should be selected to have low insertion loss.



Figure 2-34: Flex5Gware RF front-end architecture



3. Mixed-Signal Techniques Requirements of Flex5Gware 3.1 Overview of Flex5Gware use case families

A set of use case families has been chosen in WP1 together with associated key performance indicators (KPIs). The main outcome of the work is to drive the work carried out in the technical WPs with KPIs derived from the use case families of WP1. Indeed, the use case families have not only been chosen to illustrate some key scenarios envisaged in 5G, but also to better cover the main objectives that have been set by the future 5G cellular networks. WP1 defined three use case families according to the Next Generation Mobile Network (NGMN) report, and identified 7 relevant use cases. The three use case families are: Broadband access in dense areas, Broadband access everywhere and, finally, massive Internet of things [D1.1]. The use case families are not meant to be exhaustive with full coverage of all technical aspects related to 5G. The main scope of the use case families is instead to show the needed flexibility and illustrate the wide span of different requirements on 5G hardware and software platforms. Broadband access in dense areas captures the growing demand of high data rates in urban and crowded placed with a multitude of users demanding high quality of peak rate services. The use case family is divided into two main use cases: crowded venues and dynamic hotspots. Crowded venues use case represents a situation where many users are temporarily located in an area where a single cell is already deployed. Dynamic hotspots consider a use case that momentarily handles large crowds of people for occasional periods in time. Broadband access everywhere focuses on achieving a quality of service consistency and how to get enough performance close to cell borders in scarcely populated areas where a smaller grid size cannot always be used. The use case family is divided into two main use cases: 50+ Mbps everywhere and Mobile broadband in vehicles. Finally, Massive Internet of things considers not only use cases where the number of connections will be demanding, but also the wide range of characteristics and expected service levels that are the consequence of among all IoT products. The use case family is divided into three main use cases: Smart cities, performance equipment and V2X communication for enhanced driving. The Smart cities use case addresses the massive deployment of urban IoT installations to enhance the quality of life, the performance equipment use case targets high end products in terms of capabilities, such as data rate, processing power, and user interface, finally, V2X communication for enhanced driving considers machine type traffic expected to be generated by vehicle type applications (e.g.: collision avoidance, autonomous driving, vehicle platooning). Flex5Gware proposed a bottom up approach for hardware and software development. The project adopts a holistic approach performing research and implementations on key building blocks and considers the ability of these functions to co-operate and provide versatile, flexible, reconfigurable, efficient operations for HW/SW platforms as required by future 5G networks. Numerous proofs-of-concepts (PoC) will be used to demonstrate the main research advances of Flex5Gware. The PoCs and use case families have been associated in a view to drive research results and an overview is given in Figure 3-1.



Figure 3-1: Use cases and associated proofs of concepts envisaged by Flex5Gware.

The PoCs that include mixed-signal technology innovations developed within Flex5Gware have been circled in red. The PoC performance will then be demonstrated and evaluated in WP6. The Flex5Gware scenarios (or use case families) and use cases are used to qualitatively and quantitatively define the high level KPIs that are relevant to each technology under investigation [D1.1]. The KPIs have been listed and the definition of the acronyms used in [D1.1] has been recalled in Table 3-1 for reference.

Table 3-1: List of Flex5Gware consolidated KPIs derived in WP1

KPI Acronym Description and related Flex5Gware approach

Flexibility / versatility / reconfigurability

FVR

The definition of this KPI has a quite broad coverage (which will be particularized in each relevant use case), but it is related to the 5G requirement of an increase on the number of functions that a certain HW or SW module can perform. Within this context, the terms flexibility, versatility and reconfigurability have rather similar meanings and can even sometimes be used interchangeably. However, whenever a very precise meaning is necessary, we will refer to flexibility to denote the ability to cope with variable circumstances; versatility for having varied uses or serving many functions competently; and, reconfigurability for the ability to self-rearrange elements or settings (e.g., of a certain HW or SW module). Thus, within a fine level of detail, the term flexibility is the more general one and incorporates both versatility and reconfigurability as particular cases. For example, Flex5Gware will deliver new approaches on versatile multi-band transceiver implementations that will result in RF base station key elements enabling operation bandwidths of 1 GHz (gain factor of 10 with respect to current technologies).



Cost CST

The cost KPI is defined as the expenditure of resources, such as time, materials or labor, for the attainment of a certain HW or SW module. Flex5Gware contributions will have a significant impact on the cost reduction with respect of the state-of-the-art of 5G handheld devices and network elements (e.g., base stations) via cost reductions mostly based on their HW components. The impact of decreasing the HW platform cost is especially remarkable in 5G where the number of communicating nodes is expected to be unprecedentedly large. Flex5Gware will conduct research and demonstration on low cost architectures and components ranging from the use of advanced semiconductor materials to low-cost on-chip frequency generation for mmWave band radios.

Energy efficiency NRG

This KPI is related to the energy consumption reduction of terminal devices and network elements and to the improvement of performance while keeping the energy consumption at the same level. The most common metric that is used to characterize this KPI is the reduction in the consumed Joules per delivered bit. This KPI can be one-to-one mapped to the 5G-PPP KPI S2. Thus, Flex5Gware approach towards that KPI is already described in Error! Reference source not found..

Resilience and continuity

RES

The resilience and continuity KPI refers to the probability that a certain amount of data is successfully delivered to its destination within a given time frame. This KPI is especially relevant in wireless environments due to, e.g., the rapidly changing nature of the propagation conditions. Within the Flex5Gware project, this KPI can be one-to-one mapped to the 5G-PPP KPI P4. Thus, Flex5Gware approach towards that KPI is already described in Error! Reference source not found..

Mobile data volume

• Aggregated data rate

• Coverage / ubiquitous access

MDV

This KPI deals with the aggregated cell capacity in both the uplink and the downlink within a given geographical area. Moreover, when particularized to a single cell and its coverage area, it can also be related to cell edge performance (expressed in terms of guaranteed minimum data rates at the cell edge). Given its definition of aggregated data rate divided by a given area, this KPI is compound. Thus, Flex5Gware contributions toward this KPI can either be aimed at increasing the aggregated data rate (e.g., via increasing the user data rate or the number of users, see the UDR and NoU KPIs) or also by reducing the coverage area by increasing the cell access points via, e.g., small cell deployment. In this latter example, Flex5Gware will contribute with the demonstration of multi-tenant, low-energy and low-cost small cell base stations based on virtualized scalable software.



Number of users / connected devices

NoU

The number of users / connected devices KPI refers to the number of devices that are connected to the network via one or multiple access points while satisfying a certain quality metric that can be related to some other Flex5Gware KPI (e.g., user data rate, latency, etc.). Although this KPI has clearly an impact on 5G-PPP KPI S4 (Error! Reference source not found.), the Flex5Gware contribution to the number of users KPI can be directly linked to the 5G-PPP KPI P3. Thus, Flex5Gware approach towards that KPI is already described linked to P3 in Error! Reference source not found..

Bandwidth

• Radio bandwidth

• Operation bandwidth

BW

This KPI is related to the bandwidth supported by both network nodes and UE/sensors/actuators. As it has been pointed out before, the radio bandwidth (RBW) describes the full RF bandwidth received or transmitted by a radio unit, whereas the operational bandwidth (OBW) refers to the sum of all used channels inside the radio bandwidth.Flex5Gware addresses aspects related to this KPI by, e.g., multi-band operation, fast A/D converters that operate at huge bandwidths, and the enabling of operation at mmWave bands.

Latency LAT

This KPI relates to the network latency (round trip time) and to the link latency, which is measured as the time between a packet being available at the transmitter and the availability of this packet at the receiver (which takes into account, e.g., constraints and delays imposed by the HW). Flex5Gware will provide solutions for 5G communication platforms so that the latency can be reduced via the development of HW architectures that will support flexible waveforms. In addition, the dynamic HW/SW function split together with the Analogue/Digital signal processing trade-off considered in Flex5Gware will also contribute to reducing latency via choosing configurations that trade energy consumption with latency. Other contributions of Flex5Gware designed to reduce latency are represented by the development of very fast and multiband A/D and D/A converters, which give additional degrees of freedom at system level to reduce latency when the bandwidth is increased.

User data rate UDR

This KPI relates to the achieved end user (e.g., handheld for human traffic, device for MTC, etc.) data rates (both UL and DL) in different forms: peak, average or minimum guarantee. For example, Flex5Gware will contribute to increase the typical data rate by: i) increasing the user data rate per spectrum unit (e.g., via full duplex operation, the HW support for 5G waveforms like FBMC, and faster FEC decoding architectures), ii) increasing the user bandwidth (via, e.g., multi-band operation, fast A/D converters that operate at huge



bandwidths, and the enabling of operation at mmWave bands), and iii) reducing the experienced interference (e.g., through dynamic basestation coordination and/or massive MIMO transmissions).

Integration / size / footprint

ISF

This KPI is related mostly to the HW footprint related to its size/volume, but also on the SW design implications on the digital HW (e.g., via the need of additional integrated memory due to the size of the SW programs). In Flex5Gware, this KPI is addressed e.g., via co-integration of an on-chip frequency generation system in monolithic radio SoCs (system on chips), where signal integrity is important or also via the co-integration of power amplifier and antennas in the range of 20 to 40 GHz in nanometre CMOS technologies for small-cell electronics.

3.2 Mapping of the mixed-signal technologies to the Flex5Gware use cases

3.2.1 Multiband RF signal generation The multiband transmit-chain allows the realization of multiband transceivers with the concurrent operation in several radio bands below 6 GHz and a significantly increased operating bandwidth. As a key building block applicable for different radio bands and power classes, it supports all use cases of the broadband families as they are 50+ Mbps everywhere, Mobile broadband in vehicles, Crowded venues, and Dynamic hotspots by providing increased data rate as well as better connectivity to the users.

Table 3-2: KPIs expected to be covered by Multiband RF signal generation

KPI Covered by the technology

Comment Requirement

BW

Increased radio bandwidth due to broad band analogue radio design Increased operation bandwidth in each of the addressable bands

Serving 3 bands with one single RF chain, placed arbitrarily between 450 MHz and 4.2 GHz Providing up to 6x20 MHz signal bandwidth and the necessary oversampling for PA linearization

FVR

Reconfigurable carrier frequencies within the supported bands Minimizing the number of frequency band specific hardware components

High degree of reconfigurability High degree of design re-usability

MDV

The spectrum efficiency and thus the achievable data volume are defined by deployment and transceiver implementation, basically as multi antenna

A high mobile data volume is enabled by the provided aggregated bandwidth. The values are defined by the achievable spectrum efficiency.



systems.

3.2.2 Massive Multiple Input Multiple Output architectures 3.2.2.1 Compact Multi chain transmitter The multichain transmitter enables the realization of MIMO transceivers as well as M-MIMO transceivers depending on the number of connected antennas and applied precoding algorithms. Thus it supports the increase of spectral efficiency leading to the benefit of increased data rate for a higher number of users, what can be exploited by the broadband use cases Crowded venues, Dynamic hotspots, and 50+ Mbps everywhere.

Table 3-3: KPIs expected to be covered by Multi Chain Transmitters


Comment Requirement

MDV

M-MIMO can drastically increase the data throughput by spatial spectrum reuse. It requires a dedicated signal processing controlling an array of RF transceivers. The Multichain transmitter is a component to realize the RF signal generation of M-MIMO systems with a reasonable number of transmitters of high integration and low cost.

Compact and cost efficient components facilitate the implementation of M-MIMO systems. This enables a significantly increase of spectral efficiency and data volume.

NoU

Increasing the number of transmitters for a more accurate spatial spectrum reuse, allows serving more users with higher data rate.

The facilitation of M-MIMO systems implies the exploitation of spatial multiplexing supporting an increased number of users.

NRG

High degree of integration minimizes power dissipation and hence less energy per transmitted bit.

Replacing conventional hardware for data and frequency conversion by a novel compact multichain approach implies increasing the integration density and hence improving power efficiency per transmission chain.

3.2.2.2 Low Resolution ADC receivers for mmWave MIMO A receiver with low resolution ADC can be seen as a solution to parts of the problem in the two use-case families: Broadband access in dense urban areas and Broadband access everywhere. For the first use case family, it could contribute to both Crowded venues and Dynamic hotspot use cases and for Broadband access everywhere, to the 50+ Mbps everywhere use case. The main advantage of the technology is the full digital signal processing flexibility while maintaining a power efficient operation. Especially at the UE side this technology can support a large bandwidth with a sizeable antenna array. Compared to



the analogue beamforming based solution it is also more resilient to misaligned beams. In the case of analogue beamforming this would potentially break the connection.

Table 3-4: KPI expected to be covered by Low Resolution ADC

KPI Covered by

the technology Comment Requirement

FVR

Low resolution ADC systems have the

advantage to have the full digital MIMO

flexibility compared to an

analogue/hybrid beamforming based

receiver architecture.

Simulation based demonstration

that different MIMO schemes like

spatial multiplexing or STBC are

possible.

NRG

A major part of the power consumption

of a mmWave analogue front-end is the

ADC. Reducing the resolution improves

the power consumption by a significant

amount.

Comparable throughput to full

resolution MIMO and

analogue/hybrid beamforming at

lower power consumption of the

analogue front-end. This can only

hold true in the low SNR regime.

BW

A low resolution A/D conversion

consumes considerably less power

consumption than a high resolution A/D

conversion. Therefore such a system

would support a larger bandwidth

without an increased power

consumption

Compared to other receiver

architectures the supported

bandwidth for a given power

budget per antenna should be

larger.

3.2.3 Envelope Tracking for RF Power Amplifiers The ET technique could potentially allow facing the challenge of high PAPR on PA operation at UE side for 5G systems. Thus, it can be seen as a solution for energy efficiency on two use case families Massive IoT and Broadband access everywhere by enabling significant increase of UE RF transmitter efficiency. For the Massive IoT use case family, it could contribute to Performance equipment while for Broadband access everywhere the contribution could be to 50+ Mbps everywhere. Power-added efficiency (PAE - i.e. the ratio between the difference of the RF output power and the RF input power with the DC input power) of the RF PA is the most appropriate metric to measure the performance of envelope tracking solutions. The efficiency of the RF PA has an impact on the efficiency of the overall system. Relatively poor PAE of the RF PA is one of the primary factors that decrease the battery life in today’s user devices. It also affects the power supply requirements, RF amplifier design and cost and the complexity of the product design, for example when heat-sinks or heat spreaders have to be added. Note that it is also possible to use ET for tuning PA parameters to provide more linearity instead of efficiency. In that case, out-of-band and in-band transmissions can be reduced and increased number of users can be supported. In general, the envelope tracking efficiency is expected to outperform any non-continuous adjustment schemes for PA supply. The signal path (i.e. digital I,Q data path fed to a pair of



DACs, filtered and quadrature-mixed to RF before being fed through RF driver and PA stages) is common to both conventional and envelope tracking systems (see Figure 2-1). The efficiency of ET comes from the envelope tracking path. In that case, the shaping table transfer function (determining the correct PA supply voltage for a given instantaneous power) is key to transmitter performance; to this end, commercial handset PAs are characterised over a range of supply voltages in order to operate the PA always with a high degree of compression and optimise efficiency. Thus, at the envelope path, the instantaneous power levels are calculated on each I,Q sub‐sample pair, the envelope is shaped according to the feedback from signal path, a DAC and a filter provide the nominal envelope waveform to the power supply modulator which, in turn, translates the envelope waveform into a high‐current, dynamic voltage supply for the PA. To harness the maximum potential from the ET technology, it is imperative that the envelope and signal paths are closely aligned in timing and magnitude to preserve the integrity of the transmitted RF signal. This can be generally be achieved via delay, gain and offset adjustments in the envelope path. As already illustrated in Figure 2-14, current commercial PAs can reach on maximum output signal PAE of around 60 % (taking into account also the power dissipated by ET circuit) compared with the 40-45 % PAE of less sophisticated solutions. Therefore, a 25 % power consumption reduction in PA supply is achievable in current systems. The study will explore if the bandwidth, modulation scheme and carrier frequency parameters applicable to 5G systems will allow for ET solutions of similar complexity (and cost) as today’s ET solutions for LTE systems and whether the same benefits can be anticipated.

Table 3-5: KPI expected to be covered by Envelope Tracking


Comment Requirement

NRG

ET should contribute to the power consumption reduction of the transmitter by improving PA efficiency.

At least 25 % PA power consumption reduction is expected from the technology.

NoU

ET can be used to reduce out-of-band and in-band emissions, therefore, increase the number of users than can be simultaneously supported.

ET operation should allow improved amplifier linearity so that in-band and out-of-band emissions are reduced by 6 dB. This in turn allows for a four-fold increase in number of users in a cell.

3.2.4 PAPR reduction and digital predistortion techniques PAPR reduction technique and DPD technique will be able to improve energy efficiency and reduce cost (operational cost). Thus, it is mapped with the following use cases: 50+ Mbps everywhere, Smart cities and Performance equipment. The KPIs considered for this development are listed below with the supported use cases



Table 3-6: KPI expected to be covered by PAPR Techniques


Comment Requirement

NRG

Typical PAPR of 4G LTE is 7~8 dB. The values depend on IFFT/FFT size, modulation schemes, etc. DPD technique is designed for suppression of distortions (measured by e.g. EVM, BER; ACPR).

Below 7~8 dB PAPR

CST

Operational expenditure (OPEX) includes site rental cost, electricity, maintenance, backhaul cost, and so on. The PAPR reduction technique is highly related to the operational cost (electricity). In addition, the DPD technique helps to achieve cost reductions by improving PA characteristics.

Proportional to 𝜂 (PA efficiency, =0.5/PAPR).

3.2.5 High-bandwidth antenna links based on new fibre-to-the antenna transceiver subsystem technology FTTA links can be tailored to different scenarios. The advantage of FTTA links will be most pronounced for the use cases of Crowded venues, Smart cities and 50+ Mbps everywhere. Today, Crowded venues already deploy hundreds of antennas and kilometres of optical fibres to connect the antennas. As operators will evolve to 5G, DAS deployments will require more and more optical links to scale up the capacity and coverage. In Smart cities, Fibre-To-The-X (FTTX) networks will become the dominant access infrastructure. The available FTTX infrastructure can be used in parallel, through wavelength division multiplexing, to serve DAS, front-hauling or backhauling of mobile communications. As the main advantage of optical fibre is abundant capacity, optical fibre is well suited to deliver 50+Mbps Everywhere. FTTA links can be developed for high bandwidths (e.g. 0.1-1 GHz) or for concurrent operation in several radio bands. Compact optical FTTA transceivers could also support M-MIMO with the benefit of increased data rate for a higher number of users. The initial specifications to explore FTTA solutions will target a frequency range up to 10 GHz, instantaneous bandwidth of up to 1 GHz, a latency of maximum 100 ns, a reach of 0.1-10 km and a power consumption below 250 mW.



Table 3-7: KPI expected to be covered by FFTA


Comment Requirement

FVR

FTTA links are flexible in reach, frequency band and bandwidth. Compact FTTA solutions can be conceived for M-MIMO.

Our FTTA developments are targeting 100 m to 10 km distances, bandwidths up to 1 GHz and a frequency range up to 10 GHz.

MDV

FTTA links are a promising solution to connect large amounts of distributed antennas to provide higher data rates with better coverage.

To support increasing data rates, the FTTA developments are targeting high bandwidths up to 1 GHz and frequency bands up to 10 GHz. Complexity will be minimized to allow for compact solutions that can be closely integrated with antennas.

BW

Fibre-optic components with tens of GHz of bandwidth are commonly available. As such fibre-optic solutions will help to transport high-bandwidth or multiband 5G signals over considerable distances, beyond the capabilities of copper.

Our FTTA developments are targeting an instantaneous bandwidth of up to 1 GHz, as a trade-off between system requirements and component costs.

LAT

Today’s front-hauling solutions typically transport the wireless signals in other signal formats. Signal conversions inevitably introduce additional latency.

Our target is to achieve a maximum latency of 100 ns introduced by the RF-optical conversions.

3.2.6 Full duplex operation Flex5Gware considers full duplex operation in the use case family Broadband access in dense areas (Crowded venues and Dynamic hotspots). These use case families have also been considered by the FP7 project Metis [Met2014]. [Met2014] identified that cell sizes in cellular communications are ever and ever shrinking (from kilometres to less than hundreds of meters). This phenomenon has been driven to increase the capacity of the cell. As full-duplex operation aims at doubling the capacity and as the requirement on the performance of full-duplex for this scenario are slightly less demanding, this use case family seem to be particularly suited to full-duplex. Signal-to-interference ratio or signal-to-noise-and-interference ratio (SINR) is the most appropriate metric to measure the performance of full-duplex systems due to the strong self-interference caused by the coupling of the transmitted signal to its own receiver. Since self-interference cannot be perfectly cancelled, the SINR at the input of the receiver’s detector is lower when using full-duplex transmission than with half-duplex. The SINR requirements for which full-duplex operation is beneficial should be evaluated and specified. The shrinking of cell sizes has also meant that the power difference between a receiver and transmitter is reduced. Furthermore, the advance of non-orthogonal multicarrier based transmissions could make the receiver less susceptible to the remaining interference. Small cells are also very appropriate network architectures when considering the scenarios of dynamic hotspot and crowded venues envisaged in Flex5Gware.



Recently, [San2014] investigated system implications of a small cell scenario for full-duplex applications in a similar context. The authors presented a hybrid scheduler that defaults to half-duplex TDD operation but can assign full duplex timeslots when it is advantageous to do so. The considered scenario consists of a small cell with legacy half duplex user equipment and a base station that can operate in full duplex or half duplex mode. A simplified structure that does not take into account special timeslots or guard periods for TX/Rx switching is introduced.

Figure 3-2: Simplified TDD structure proposed by [San2014]

The difference between the half duplex and the full duplex mode is further illustrated in Figure 3-3. When user equipments are in normal TDD half-duplex mode, interference between UE1 and UE2 is prevented by the orthogonality of the channel access in time. When the FD scenario is considered, both UEs are scheduled in the same timeslot, potentially doubling the total throughput of the cell. Two sources of interference have to be considered for the operation to be effective: first, at the base station, the interference generated by the signal transmitted to UE1 impacts the performance of the reception of the signal received from UE2, then, at the interference from UE2 to UE1 which in this scenario is heavily dependent of the two UE locations.

Figure 3-3: Interference generated in half duplex and full-duplex for scenario of [San2014]

The base station transceiver is operating in full-duplex mode whereas both UEs are operating in half duplex mode.



For this scenario to be effective for a given user, downlink propagation channel from BS to UE1 must be comparable to the propagation of the interference channel from UE2 to UE1. Furthermore, favourable propagation between UEs and base station reduces the requirements of self-interference cancellation at the BS. A scheduler adapted to this FD scenario has been devised in [San2014]. The scheduler is designed to optimise the logarithmic sum of the average rates of all the UEs. The data rate of every user is estimated from the interference to noise ratio seen by the base station/the user. Assuming a random distribution of the users, [San2014] derived the following average performance gains of full duplex as a function of the level of interference cancellation. Performance gains of full duplex operation versus half duplex operation considering this scenario are given in the table here below:

Table 3-8: Average performance gain of full-duplex as a function of interference cancellation

Performance of interference cancellation

55 dB 65 dB 75 dB 85 db

Downlink 2.0 % 21 % 56 % 69 %

Uplink 0.4 % 4.9 % 33 % 81 %

This conclusion proves that even for relatively simple scenarios, full-duplex operation is beneficial when interference cancellation is larger than 55 dB. At 75 dB of cancellation at the basestation, the performance gain is almost equal to 50 %. These results give a level of requirement that could be used to set the minimum performance target for full-duplex operation.

Table 3-9: KPI expected to be covered by mixed-signal technologies


Comment Requirement

UDR Full duplex operation as considered in Flex5Gware should contribute to increase the typical data rate per spectrum unit.

At least 50 % average data rate increase is expected from the technology

FVR

Flexibility may also be added at the system level as switching between TDD and FD should be relatively straightforward. Flexibility is also improved when dynamic spectrum access is considered

Switching between half duplex (SISO or MIMO) and full duplex (SISO) Switching central frequency for the data link

3.3 Key Performance Indicators covered by the Mixed-signal Innovations

Table 3-10 gives a qualitative summary of the main KPIs covered by WP3. WP3 adopted the approach of evaluating key mixed-signal building blocks for 5G. The approach shows a good level of coverage of the main KPIs identified by WP1.



Furthermore, an important part of the focus of research for mixed-signal technologies will be geared towards flexibility, versatility and reconfigurability (FVR) as well as energy efficiency (NRG). These are the goals that have been set at the early stage of Flex5Gware. Furthermore, all the other KPIs are covered by mixed-signal technologies except for integration / size / footprint (ISF) and Resilience and continuity (RES). Mobile data volume, bandwidth and number of users per cell are also important objectives that will be addressed. Integration / size / footprint (ISF) is not considered as the concepts will be developed using mainly commercial-off-the-shelves components to prove feasibility. Resilience and continuity (RES) although not explicitly considered as a key drivers for WP3. They are however implicitly considered by some of the developments. Resilience and continuity as understood by Flex5Gware is driven at the system level by the SW platforms that will have the capabilities to select the most appropriate RAT among the offered possibilities. Clearly support for multiple-RATs is considered in WP3; however the focus of the building block is more geared towards bandwidth, flexibility and data volume.

Table 3-10: Summary of identified KPI for WP3

Technology FVR CST NRG RES MDV NoU BW LAT UDR ISF

Multiband RF signal generation

Compact Multichain Transmitter

Low Resolution ADC receivers for M-MIMO

Envelope Tracking for RF Power Amplifier

PAPR Reduction and Digital predistortion

High-Bandwidth antenna links based on FFTA

Full-Duplex operation

Legend: FVR: Flexibility / versatility / reconfigurability CST: Cost NRG: Energy Efficiency RES: Resilience and continuity MDV: Mobile data volume

NoU: Number of users / connected devices BW: Bandwidth LAT: Latency UDR: User data rate ISF: Integration / size / footprint



4. Conclusion Deliverable D3.1 aimed at setting the initial requirements for mixed-signal technologies (WP3) in Flex5Gware. Its goal was to describe some of the most promising mixed-signal concepts identified for 5G and which are going to be developed during the course of Flex5Gware in WP3. Flex5Gware identified the following areas of investigations where advances in mixed-signal technologies are to be considered, researched and developed:

• Multiband RF signal generation • Compact multichain transmitter • Low resolution ADCs geared towards M-MIMO applications in the mmWave

frequencies • Envelope tracking for RF amplifiers for RF PAs • Peak-to-average power ratio reduction and predistortion techniques • New high bandwidth antenna links based on new fibre-to-the-antenna transceiver

subsystem technology • Full-duplex operation in the context of 5G.

For each identified key building block, the main challenges and the approach envisaged by Flex5Gware have been detailed. KPIs have then been derived according to the PoC environments of WP6 associated with the use case families of WP1. The KPIs introduced in WP1 have been used to derive the main requirements of each mixed-signal key building block. An analysis of the KPIs of each building block of WP3 highlighted that the main motivations of innovation in this workpackage are matching the main goals set by the Flex5Gware project. These are:


To improve the modularity and flexibility.



5. List of Accronyms and Abbreviations

4G Fourth generation of cellular networks

5G Fifth generation of cellular networks

ACLR Adjacent Channel Leakage Ratio

ACPR Adjacent Channel Power Ratio

ADC Analogue to Digital Converter

AFE Analogue Front End

APT Average Power Tracking

BER Bit Error Rate

CMOS Complementary Metal Oxide Semi-conductor

CPRI Common Public Radio Interface

DAC Digital to Analogue Converter

DAS Distributed Antenna Systems

DC Direct Current

DL Downlink

DPD Digital Predistortion

DSP Digital Signal Processing

ET Envelope Tracking

EVM Error Vector Magnitude

FBMC Filter Bank Multi-Carrier

FD Full Duplex

FDD Frequency Division Multiplexing

FFT Fast Fourier Transform

FFTA Fibre-to-the-antenna

FIR Finite Impulse Response (Filter)

FM Frequency Modulation

FPGA Field Programmable Gate Array

FTN Faster than Nyquist

HW Hardware

KPI Key Performance Indicator

LNA Low Noise Amplifier

LO Local Oscillator

LTE Long Term Evolution

LTE-A Long Term Evolution – Advanced

LVDS Low Voltage Differential Signalling

M-MIMO Massive Multiple Input Multiple Output

mmWave Milimeterwave (frequency)

OFDM Orthogonal Frequency Division Multiplexing

PA Power Amplifier

PAPR Peak-to-Average Ratio

PCB Printed Circuit Board

PHY Physical Layer

PoC Proof of Concept

PTS Partial Transmit Sequences

QAM Quadrature Amplitude Modulation

RAT Radio Access Technology

RF Radiofrequency

RX Receiver

SC-FDMA Single Carrier Frequency Division Multiple Access



SDR Software Defined Radio

SIC Signal Interference Cancellation

SLM Selected Mapping

SW Software

TDD Time Division Multiplexing

TRx Transceiver

TX Transmitter

UF-OFDM Universal Filtered Orthogonal Frequency Multiplexing Modulation

UL Uplink

VCSEL Vertical-Cavity Surface-Emitting Laser

VLSI Very Large Scale Integration

WLAN Wireless Local Area Network



6. References [Alk2014] Alkhateeb, A.; Jianhua Mo; Gonzalez-Prelcic, N.; Heath, R.W., "MIMO

Precoding and Combining Solutions for Millimeter-Wave Systems," in Communications Magazine, IEEE , vol.52, no.12, pp.122-131, December 2014

[Bar2015] Barati Nt., C.; Hosseini, S.; Rangan, S.; Liu, P.; Korakis, T.; Panwar, S.; Rappaport, T., "Directional Cell Discovery in Millimeter Wave Cellular Networks," in Wireless Communications, IEEE Transactions on , vol.PP, no.99, 2015

[Bau1996] R.W. Bauml, R.F:H. Fischer and J.B. Huber, “Reducing the peak-to-average power ratio of multicarrier modulation by selected mapping,” Electronics letters, vol.32, no.22, pp. 2056-2057, Oct. 1996.

[Bax2007] R.J. Baxley and G.T.Zhou, “Comparing selected mapping and partial transmit sequence for PAR reduction,” IEEE Trans. Broadcasting, vol. 53, no. 4, pp.797-803, Dec. 2007.

[Bha2013] D.Bharadia, E.McMilin, S.Katti, ”Full Duplex Radios”, SIGCOMM’13, Aug 12-16, 2013, Hong Kong, China.

[Boc2014] Boccardi, F; Heath, R.W.; Lozano, A.; Marzetta, T.L.; Popovski, P., "Five disruptive technology directions for 5G," in Communications Magazine, IEEE , vol.52, no.2, pp.74-80, February 2014

[Cha2015] Pizzinat, P. Chanclou, T. Diallo, F. Saliou.: ‘Things You should Know About Fronthaul’, in Lightwave Technology, Journal of , vol.33, no.5, pp.1077-1083, March1, 1 2015

[Cho2011] J. Choi, T. Kim, D. Bharadia, S. Seth, K. Srinivasan, P. Levis, S. Katti, P. Sinha, "Practical, Real-time, Full Duplex Wireless," International Conference on Mobile Computing and Networking, Sept. 2011.

[D11] Flex5Gware Deliverable D1.1, “WP1 – 5G Architecture requirements, specifications, and use cases”, H2020 Grant Agreement Number: 671563, Dec. 2015.

[Dua2012] M.Duarte, C.Dick, A.Sabharwal,”Experiment-Driven Characterization of Full-Duplex Wireless Systems’, IEEE Tr. On Wireless Communications, Vol.11, NO.12, Dec 2012,pp.4296-4307.

[Dup2014a] DUPLO Deliverable D2.3.1, “Transceiver circuits simulation, implementation and measurement”, 2014

[Dup2014b] DUPLO Deliverable D2.1, “Design and Measurement report for RF and antenna solutions for self-interference cancellation”, 2014

[Dup2015] DUPLO Deliverable D3.1, “Enhanced Solutions for Full-duplex Transceiver and Systems”, 2015

[DUPLO] FP7 DUPLO Project, contract number: CNECT-ICT-316369 , available at http://www.fp7-duplo.eu/



[Eve2013] E.Everett, A. Sahai, A. Sabharwal,”Passive Self-Interference

Suppression for Full-Duplex Infrastructure Nodes”, IEEE Transactions on Wireless Communication, October 2013.

[Fan1999] P.Y.Fan and X.G.Xia, “Block coded modulation for the reduction of the peak to average power ratio in OFDM Systems,” IEEE Trans. Cons. Elect, vol. 45, no. 4, pp.1025-1029, Nov. 1999.

[Feb2014] T. LeFebvre, “Understanding Distributed Antenna Systems (DASs)”, Electronic Design, Jan 13, 2014

[FTTH2013] FTTHCouncil, ‘Working Together: the Synergies of Fibre and Wireless Networks’, 2013

[Gar2011] Garcia-Hernandez, M.; Prieto-Guerrero, A.; Laguna-Sanchez, G.A., "Survey on Compensation for Analog Front End Imperfections by Means of Adaptive Digital Front End for On-Chip OFDM Wireless Transmitters," in Electronics, Robotics and Automotive Mechanics Conference (CERMA), 2011 IEEE , vol., no., pp.343-348, 15-18 Nov. 2011

[Gol2005] Goldsmith, A., “Wireless Communcation”, Cambridge University Press, 2005

[Huu2005] Huusari, T.; Yang-Seok Choi; Liikkanen, P.; Korpi, D.; Talwar, S.; Valkama, M., "Wideband Self-Adaptive RF Cancellation Circuit for Full-Duplex Radio: Operating Principle and Measurements," in Vehicular Technology Conference (VTC Spring), 2015 IEEE 81st , vol., no., pp.1-7, 11-14 May 2015

[Huu2014] Timo Huusari, “Analog RF Cancellation of Self Interference in Full-Diplex Transceivers” Master’s Thesis 13 August 2014

[Jon1994] A.E.Jones, T.A.Wilkinson and S.K. Barton, “Block coding scheme for reduction of peak-to-average envelope power ratio of multicarrier transmission systems,” IET Elect. Let., vol. 30, no. 8, pp.2098-2099, Dec. 1994.

[Kol2012] Z. Kollar, L. Varga, and K. Czimer, “Clipping-based iterative papr reduction techniques for FBMC,” in Proceedings of 17th International OFDM Workshop 2012 (InOWo’12); OFDM 2012, Aug. 2012, pp. 1–7.

[Kor2015] D. Korpi, Y.-S. Choi, T. Huusari, L. Anttila, S. Talwar, and M. Valkama, “Adaptive nonlinear digital self-interference cancellation for mobile inband full-duplex radio: Algorithms and RF measurements”, IEEE Global Communications Conference (GLOBECOM), Dec. 2015

[Leh2010] Lehne, M. and Raman, S., “A Discrete-Time FFT Processor for Ultrawideband OFDM Wireless Transceivers: Architecture and Behavioral Modeling”, IEEE Trans. on Circuits and Syst. I: Reg. Papers, vol. 57, pp. 3011 - 3022, Nov. 2010.

[Li1998] X.Li and L. Cimini, “Effects of clipping and filtering on the performance of OFDM,” IEEE Comm. Let., vol. 2, no. 4, pp. 131-133,May 1998.



[Liu2013] You-Jiang Liu; Wenhua Chen; Jie Zhou; Bang-Hua Zhou; Ghannouchi,

F., "Digital Predistortion for Concurrent Dual-Band Transmitters Using 2-D Modified Memory Polynomials," in IEEE Transactions on Microwave Theory and Techniques, vol.61, no.1, pp.281-290, Jan. 2013

[Lu2012] S. Lu, D. Qu, and Y. He, “Sliding window tone reservation technique for the peak-to-average power ratio reduction of FBMC-OQAM signals,” IEEE Wireless Commun. Lett., vol. 1, no. 4, pp. 268–271, Aug. 2012.

[Mal2011] P. Malathi and P.T. Vanathi, “Peak to Average Power ratio (PAPR) Reduction Techniques for OFDM-MIMO System,” International Journal of Computer Science Issues, Special Issue, vol. 1, issue 1, pp.92- 96, Nov. 2011.

[Mez2007] Mezghani, A.; Nossek, J.A., "On Ultra-Wideband MIMO Systems with 1-bit Quantized Outputs: Performance Analysis and Input Optimization," in Information Theory, 2007. ISIT 2007. IEEE International Symposium on , vol., no., pp.1286-1289, 24-29 June 2007

[Met2014] Deliverable D2.2 Novel radio link concepts and state of the art analysis, October 31, 2014.

[Miy2014] Miyashita, D., Yamaki, R., Hashiyoshi, K., Kobayashi, H., Kousai, S., Oowaki, Y.; and Unekawa, Y., “An LDPC Decoder With Time-Domain Analog and Digital Mixed-Signal Processing,” IEEE Journal of Solid-State Circuits, vol. 49, pp. 73 - 83, Jan. 2014.

[Moj2015] Jianhua Mo; Heath, R.W., "Capacity Analysis of One-Bit Quantized MIMO Systems With Transmitter Channel State Information," in Signal Processing, IEEE Transactions on , vol.63, no.20, pp.5498-5512, Oct.15, 2015

[Mur2015] B. Murmann, "ADC Performance Survey 1997-2015," [Online]. Available: http://web.stanford.edu/~murmann/adcsurvey.html.

[Nik2015] Niknejad, Ali. 2015. “Going the Distance with CMOs: mm-Waves and Beyond.” Brooklyn 5G Summit.

[Nos2006]

Nossek, J. A.; Ivrlac, M. T., “Capacity and coding for quantized MIMO Systems”, International Wireless Communications and Mobile Computing Conference, 2006

[Nuj2014]

Jeremy Hendy (Nujira), “Envelope Tracking: Fuel injection for the RF Front End”, presented in Advance Techniques for Radio Transmitters (Radio Technology SIG event from Cambridge Wireless), Sep. 2014.

[Och2000] H. Ochiai and H. Imai, “Performance of the deliberate clipping with

adaptive symbol selection for strictly band-limited OFDM systems,” IEEE Jou. Select. Areas Comm., vol.18, no. 11, pp.2270-2277, Nov. 2000.

[opticalzonu] http://www.opticalzonu.com/

http://web.stanford.edu/~murmann/adcsurvey.html

http://www.opticalzonu.com/



[Per2015] Perigrine Semiconductor, Product specification 50ohms RF Digital Attenuator 7bits 31.75 dB 9KHz - 8 GHz

[Qu2013] D. Qu, S. Lu, and T. Jiang, “Multi-block joint optimization for the peakto-average power ratio reduction of FBMC-OQAM signals,” IEEE Trans.Signal Process., vol. 61, no. 7, pp. 1605–1613, Apr. 2013.

[Rap2014] Rappaport, T.; Heath, R. W.; Daniels, R. C.; Murdock, J. N., “Millimeter Wave Wireless Communicatioon, Prentice Hall, 2014

[Rik2014] Kari Rikkinen, University of Oulu, DUPLO, A new full-duplex radio transmission paradigm, Pre-FIA Workshop, 17 March 2014 Athens

[Rou2015] Roufarshbaf, H., Madhow, U., Rodwell, M. and Rajagopal, S., “Efficient analogue multiband channelization for bandwidth scaling in mm-wave systems,” in IEEE ICC, Jun. 2015, pp. 1316 – 1321.

[Sab2014] Sabharwal, A.; Schniter, P.; Dongning Guo; Bliss, D.W.; Rangarajan, S.; Wichman, R., "In-Band Full-Duplex Wireless: Challenges and Opportunities," in Selected Areas in Communications, IEEE Journal on , vol.32, no.9, pp.1637-1652, Sept. 2014

[San2014] Sanjay Goyal et al., Improving Small Cell Capacity with Common-Carrier Full Duplex Radios, IEEE ICC 2014 - Wireless Communications Symposium

[Sar2012] R. Sarpeshkar, “Universal principles for ultra low power and energy efficient design,” IEEE Transactions on Circuits and Systems II: Express Briefs, vol. 59, pp. 193 – 198, no. 4, 2012.

[Sin2009] Singh, J.; Dabeer, O.; Madhow, U., "On the limits of communication with low-precision analogue-to-digital conversion at the receiver," in Communications, IEEE Transactions on , vol.57, no.12, pp.3629-3639, December 2009

[Skr2006] A. Skrzypczak, J.-P. Javaudin, and P. Siohan, “Reduction of the peak-to-average power ratio for the OFDM/OQAM modulation,” in 2006. VTC2006-Spring. IEEE 63rd Vehicular Technology Conference, vol. 4, May 2006, pp. 2018–2022.

[Tel1999] J. Tellado, “Peak to Average Power Ratio Reduction for Multicarrier Modulation,” PhD thesis, Stanford University, 1999.

[Yan2006] L. Yang, R.S. Chen, Y.M. Siu and K.K. Soo, “PAPR reduction of an OFDM signal by use of PTS with low computational complexity,” IEEE Trans. Brod. Vol. 52, no. 1, pp.83-86, Mar. 2006.

[Yoo2006] S.S. Yoo, S. Yoon, S.Y. Kim and I. Song, “A novel PAPR reduction scheme for OFDM systems: Selective mapping of partial tones,” IEEE Trans. Cons Elect, vol. 52, no. 1, pp.40-43, Feb. 2006.



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Research and Innovation Action - Home - Flex5Gware · Fredrik Tillman EAB [email protected] 23 ... List of Flex5Gware consolidated KPIs derived ... waveforms and compared