CBRS LTE Technology for the Enterprise The Radio - Whitepaper · 2020. 12. 11. · the lte radio link and frame structure 4 lte-tdd frame structure: time-interleaved 13 uplink and

WHITE PAPER

CBRS LTE Technology for the Enterprise:The Radio

TABLE OF CONTENTS

2

3OVERVIEW

3INTRODUCTION TO CBRS

4OVERVIEW OF THIS PAPER

4THE LTE RADIO LINK AND FRAME STRUCTURE

13LTE-TDD FRAME STRUCTURE: TIME-INTERLEAVED UPLINK AND DOWNLINK SUBFRAMES

16SIGNAL POWER AND SIGNAL QUALITY MEASUREMENTS

21NEXT STEPS

22APPENDIX – RELEVANT 3GPP RELEASES FOR LTE

24APPENDIX – BIBLIOGRAPHY

3

WHITE PAPER CBRS LTE TECHNOLOGY FOR THE ENTERPRISE – THE RADIO

3

OVERVIEWThe launch of the CBRS band in 2019 and 2020 marks an opportunity for enterprises to build private networks using LTE technology that was previously only available for public cellular networks.

The new band covers 150 MHz of spectrum from 3550 to 3700 MHz. The US regulator has taken a new approach to spectrum sharing with CBRS, allowing unlicensed use but requiring users to be authorized by spectrum access systems to avoid interference with incumbent and high-priority, paid users.

While CBRS spectrum is not allocated to a particular technology, it is a natural fit for LTE (and later 5G), as it requires little modification to existing equipment. Mobile devices and IoT sensors are already available, as is radio and core network infrastructure.

As enterprises and other organizations start to build private LTE networks in the CBRS band to supplement Wi-Fi coverage, networking engineers who are already expert in LAN and WLAN face a steep learning curve to master the complexities of cellular technology in the enterprise.

This series of papers is intended for Wi-Fi engineers wishing to learn about CBRS equipment in enterprise networks. It explains those parts of LTE that are most relevant to CBRS equipment, and enables Wi-Fi engineers to successfully evaluate and operate enterprise CBRS networks.

This is the first of three papers in the series. Other papers cover ‘Signaling & Control’ and ‘Network Implementation & Design’.

INTRODUCTION TO CBRSA recent development in the wireless landscape, CBRS (Citizens Broadband Radio Service) is a US regulatory initiative for shared spectrum which allows new technology to be introduced into enterprise networks.

The FCC1 (Federal Communications Commission) laid the groundwork for this spectrum, 150 MHz from 3.55 to 3.70 GHz, from 2015 to 2019, and the first commercial networks were deployed in 2020. CBRS pioneers a three-tier licensing structure with opportunities for enterprises to purchase licenses allowing dedicated access to spectrum, or to operate wireless networks opportunistically in locations or RF channels where other licensees are not active. CBRS is intended to allow enterprises and organizations to set up private base stations for cellphones and other client devices.

High-powered outdoor transmitters can be operated, as well as low-powered indoor small cells, enabling both long-range and in-building communications. The sharing and allocation of spectrum is coordinated by national SAS (Spectrum Access System) servers, which allocate RF channels dynamically in order to avoid interference and protect incumbents. The FCC does not specify a wireless technology for CBRS, but the industry has adopted LTE (Long Term Evolution) standards to allow infrastructure equipment, devices and client modules used in the 4G cellular network to be adapted for use in the CBRS band.

However, traditional LTE infrastructure is typically optimized for the needs of cellular operators fielding very large networks, with skilled operations teams. Operators’ requirements for macro network equipment differ from enterprise needs where small, indoor radio units that can be wall-mounted and Ethernet-connected are preferred. Another drawback of traditional LTE infrastructure is the complex cellular core network software for management, control, and data functions. Enterprises do not have the scale of a national cellular operator; they require a small footprint, simpler structures and above all, simpler management interfaces.

Thus, it is unlikely that traditional cellular equipment vendors will find it easy to re-purpose their existing product lines for the emerging enterprise CBRS market. A contrasting approach can be found with startups built on 4G/5G expertise, better able to re-factor complex core software into simpler, cloud software and container-based platforms, and to interface these with small radio units more suited for enterprise use.

This paper concentrates on the technical aspects of LTE technology as it relates to CBRS networks. There is considerable complexity inherent in the standards, but many options in LTE standards do not apply to CBRS and are omitted. The paper also deals only with LTE, or 4G technology; although 5G equipment will eventually be adapted for CBRS, all networks installed through 2021 will be LTE-based.

1FCC 3.5 GHz band overview https://www.fcc.gov/wireless/bureau-divisions/mobility-division/35-ghz-band/35-ghz-band-overview

https://www.fcc.gov/wireless/bureau-divisions/mobility-division/35-ghz-band/35-ghz-band-overview


4

Technical terms and jargon are unavoidable in LTE. This paper simplifies where possible, sometimes at the expense of precisely accurate terminology. The first LTE terms to learn are ‘UE’ (User Equipment), the client device on the network, and ‘base station’ which is sometimes known in the market as CBSD (Citizens Broadband Radio Service Device), eNodeB (an LTE term) or even ‘access point’. Other important abbreviations are DL (Downlink, from base station to UE) and UL (Uplink, from UE to base station). LTE standards are developed by the 3GPP2 (Third-Generation Partnership Project) standards development organization.

The 3GPP is a standards body for the cellular industry, similar to the role that the IEEE3 (Institute of Electrical and Electronics Engineers) plays for the enterprise network industry. Ethernet and Wi-Fi are IEEE standards, whereas LTE and 5G are 3GPP standards. 3GPP cellular standards are known as “Releases” and are numbered. For example, the first 4G standard was initially introduced in 3GPP Release 8 in 2008. The 5G standards were initially introduced in Release 15 (2018) and are being extended in Releases 16 (2020) and 17 (due late 2022).

The CBRS Alliance4 plays a similar role in the enterprise cellular ecosystem as the Wi-Fi Alliance. The Alliance has adopted ‘OnGo’ as the brand name of their certification for CBRS products.

The Wireless Innovation Forum5 (WinnForum) produces standards and certifications related to CBRS, including base station and professional installer certifications.

OVERVIEW OF THIS PAPERThis series of papers is organized to give enterprise networking engineers a view of the technology underlying CBRS networks.

This, the first paper, deals with the radio link. The LTE radio has parallels in Wi-Fi technology, and where relevant, introductions compare Wi-Fi technology with LTE to help those with a Wi-Fi background.

The ordering of topics is unconventional. Most texts start from the lowest layers and work up, but here we jump around in a sequence where each section provides context for the next.

First, the difference between FDD and TDD explains why CBRS differs from most public cellular LTE networks, and contrasts how it deals with ping-pong communication in a single band with the Wi-Fi approach.

Then, after covering channel widths and a simple view of framing, we return to the lowest levels and investigate OFDM, symbols, and modulation, which is again similar to Wi-Fi. The symbol is placed in a frame, and several pages introduce the Physical Resource Block, a fundamental unit in LTE engineering but somewhat foreign to the Wi-Fi world, and then build Resource Blocks into the resource grid, another widely-used construct in LTE. The allocation of Resource Blocks in the resource grid is by OFDMA, related to the Wi-Fi 6 OFDMA feature.

Following OFDMA, the paper returns to the TDD frame structure, showing how downlink and uplink subframes are allocated, and introducing the special subframe at transitions from downlink to uplink. This, along with the resource block concept introduced earlier, allows calculation of representative user data rates for different configurations of CBRS.

Finally, the paper spends some pages on an important subject for network engineers, the measurements of signal strength and signal quality used in LTE. Wi-Fi engineers may find these measurements a little difficult to grasp, as they are somewhat artificial concepts, but it is worthwhile investing some time to understand their derivation.

THE LTE RADIO LINK AND FRAME STRUCTUREThe LTE air-interface is best understood with reference to its frame structure, as the centrally scheduled and controlled architecture defines many functions all the way down to the physical layer.

This section lays out the basic structure of a symbol, the OFDMA bandwidth allocation structure and LTE framing.

Introduction for Wi-Fi experts

A Wi-Fi network (up to Wi-Fi 6) is organized as a decentralized system of loosely coupled nodes. When a node has data to send, it senses for clear air then starts to transmit. If the air is occupied, it applies a random backoff timer and tries again, using the CSMA/CA (carrier sense multiple access with collision avoidance) protocol. Airtime hogging is controlled by limiting transmit opportunity durations. It receives by listening to every packet header, but only fully decoding packets that match its address. The access point is a first among equals, but it respects the same contention rules as other nodes. And all transmissions take place in the same RF channel. The LTE protocols turn all these assumptions on their head.

23GPP https://www.3gpp.org/3IEEE Standards Association https://standards.ieee.org/4CBRS Alliance https://www.cbrsalliance.org/5WinnForum CBRS activity https://cbrs.wirelessinnovation.org/

https://www.3gpp.org/

https://standards.ieee.org/

https://www.cbrsalliance.org/

https://cbrs.wirelessinnovation.org/

5


Figure 1. Wi-Fi 6 OFDM symbol duration and subcarriers

Traditional LTE transmission used FDD (Frequency Division Duplex), with uplink and downlink operating in separated RF channels. This allowed continuous bidirectional transmissions and was well suited to symmetrical traffic, like voice, where uplink and downlink had identical bandwidth requirements.

The LTE-FDD structure over paired spectrum is still used in public cellular networks, although CBRS spectrum is not paired, and requires the ping-pong TDD (Time Division Duplex) protocol used by Wi-Fi, albeit not node-by-node. However, much of the LTE literature assumes FDD operation so it is briefly described below.

Figure 2. Wi-Fi Single User Transmission

The second important difference between Wi-Fi and cellular protocols is centralized transmission control. In LTE, the base station instructs each UE when and how it can transmit, and when to receive. This structure has been adopted as an option in Wi-Fi 6, but it was not previously implemented in Wi-Fi equipment.

Centralized control allows the base station to identify precise time and frequency coordinates for each transmission, but these coordinates require a frame of reference, hence LTE uses a repeated framing structure. The base station transmits reference signals that allow all UEs to synchronize to its frame and to index timeslots and symbol intervals, so all transmissions in LTE are synchronous with the base station.

In order to maintain tight synchronization, reference symbols are distributed throughout the frame, both in time and frequency. As with Wi-Fi, LTE uses an OFDM (Orthogonal Frequency Division Multiplexing) set of subcarriers in the frequency domain, so a particular symbol is indexed by its time and subcarrier offsets. Also, as in Wi-Fi, the subcarrier spacing is fixed, so numbers scale linearly with RF channel width.


6

Figure 3. Wi-Fi 6 OFDMA compared with single-user OFDM in Wi-Fi 5

Several Wi-Fi concepts introduced in Wi-Fi 6 have long been used in cellular protocols. The OFDMA structure, where the access point assigns groups of sub-carriers per-client is similar to LTE, as is the ability to schedule transmissions in time, and to control uplink transmissions in addition to the downlink. Meanwhile, OFDM transmission has been used in Wi-Fi since the late 1990s and was adopted by 3GPP for LTE in 2008. The modulation types – QPSK (Quadrature Phase-Shift Keying), QAM (Quadrature Amplitude Modulation) – are common to both systems.

FDD and TDD operation

As if to underline the contrast between Wi-Fi and LTE, the term “frame” has different meanings in each. In enterprise networks, “frame” is commonly used to refer to a layer 2 Protocol Data Unit including the header and footer used in its transmission. On Ethernet or Wi-Fi networks frames are asynchronous and can occur at any time. However, in an LTE system the airtime is rigidly structured into repeating structures called “frames” that are always the same duration. LTE frames are decomposed into many smaller elements that occur in the same positions with each repetition.

This section covers FDD and TDD operation, RF channels, the OFDMA frame structure, symbols, scheduling blocks and data rates.

Figure 4. LTE FDD and TDD options

7


LTE systems traditionally use the FDD transmission mode. In FDD, two RF channels are assigned to each cell. One is used for the base station to transmit to UEs on the downlink, the other for UEs to transmit and the base station to receive on the uplink. One big advantage of FDD is continuous duplex transmission: provided the two RF channels are separated in frequency, both the base station and UEs can transmit and receive simultaneously, providing efficient use of the medium. However, FDD is structured for balanced (usually equal) uplink and downlink traffic: if, on aggregate, UEs have more to receive than to transmit, then the downlink will reach overload while the aggregate uplink is not fully occupied. Thus, FDD can give high spectral efficiency, but only for balanced uplink-downlink traffic.

The alternative to FDD is TDD mode. Here, the base station and UEs use the same RF channel. It is not yet possible to build a radio that can simultaneously transmit and receive on a single channel, so downlink and uplink must take turns to transmit. This requires some coordination and guard intervals between different transmissions, so it can have more dead-time overhead than FDD, but it can be flexible for unbalanced downlink-uplink traffic such as data traffic, and it works better where a narrow spectrum band precludes frequency separation of DL and UL channels. All CBRS transmission is TDD rather than FDD.

Channelization and baseline assumptions for calculations

LTE frame structures are defined within channel bandwidths of 1.4, 3, 5, 10, 15, and 20 MHz. This paper refers to LTE-FDD channel pairs as, for example, 10x10 MHz where DL and UL are each 10 MHz wide.

LTE-TDD channels have a single bandwidth value, and CBRS is defined in 10 MHz channels. In any location, a priority access user can license up to 40 MHz of bandwidth, while up to 150 MHz, subject to preemption, may be available to general authorized users. Current CBRS equipment is able to fill 40 MHz by using a 20 MHz channel on each of two radios in a physical base station unit, then using Carrier Aggregation (see later) to combine those into a single base station transmission covering 40 MHz.

This paper includes many tables and charts with calculations. For continuity and easy comparison, these use 10x10 or 10 MHz channels. Also, 2x2 MIMO (Multiple Input, Multiple Output, see later) is generally assumed for the downlink and 1x1 MIMO (strictly SISO, Single Input, Single Output) for the uplink, as that is the current state of the art for CBRS equipment, and although 256-QAM (Quadrature-Amplitude Modulation) modulation is available as an option, current CBRS equipment operates at a maximum of 64-QAM.

LTE engineers often combine downlink and uplink rates in a statement of system maximum rates, e.g. 194 Mbps DL + 27 Mbps UL would combine for 221 Mbps aggregate. Whenever figures are quoted, assumptions should be checked.

Approximate conversions from the base values used in this paper are easy: a 20 MHz channel supports around twice the rate of 10 MHz, and 2x2 MIMO twice the rate of 1x1.

Figure 5. LTE frame structure


8

LTE transmissions are structured on a strict timetable, set by the base station. An LTE ‘frame’ is 10 msec long. It is divided into 10 subframes, each containing 2 slots. Thus, each of the 20 slots in a frame has a duration of 0.5 msec. In FDD mode, all slots in one RF channel (conventionally the higher frequency) are for downlink transmissions from the base station, while the paired channel carries uplink transmissions from UEs. In TDD mode, for CBRS, slots are assigned for uplink or downlink transmissions, according to a fixed format, and some slots are allocated for control traffic and synchronization purposes. More on the CBRS LTE-TDD frame structure later.

OFDM and symbols

The symbol is the basic unit of transmission of OFDM (Orthogonal Frequency Division Multiplexing). As will become clear over the next few pages, understanding symbols is central to understanding the LTE air interface structure. Each symbol contains a snippet of a sinewave, at a particular amplitude (power) and phase (offset) that distinguishes it from other symbols. The number of distinct amplitude and phase options governs the amount of information that can be transmitted in a symbol. For example, if a symbol had one of two amplitudes and one of two phases, it would represent one of 4 states, or 2 binary bits. If the equivalent were 4 amplitudes and 4 phases, the symbol would represent one of 16 states, or 4 binary bits.

It’s obviously better – more efficient – to use as many discrete amplitude and phase steps as possible, but we are limited by the presence of noise, which can shift the perceived states by the time they reach the receiver, and cause errors due to incorrect decoding. These errors can be detected and corrected by sending extra, redundant, information in the transmission or by retransmitting, but redundant information subtracts from overall bandwidth efficiency.

So, for any given signal-to-noise ratio at the receiver, there is an optimal number of amplitude and phase steps. LTE uses QPSK for low-rate modulation, and 16-QAM, 64-QAM and 256-QAM for encoding increasing bits per symbol. QPSK has two bits per symbol, 16-QAM four, 64-QAM six and 256-QAM eight bits per symbol.

Figure 6. Symbols carry data

Symbols cannot be transmitted back-to-back over the air because of the multipath effect, where reflections between the transmitter and receiver result in several copies of the radio signal arriving at the receiver, with delays proportional to the reflected echo paths. To make sure the echoes have died away between symbols, a cyclic prefix is attached to the beginning of each symbol.

The longer the distances involved, the more the multipath echoes are delayed and the longer the prefix must be. Of the two cyclic prefix options in LTE, CBRS systems use the short 4.7 usec prefix that allows for a path difference of approximately 1 km between the direct signal and multipaths, allowing CBRS to fit 7 symbols into each 0.5 msec slot. Each symbol, including its prefix, lasts 71.4 usec (the first symbol in a slot has a longer 5.2 usec prefix to fill a slot with an exact number of LTE time units).

9


The following table gives a simple analysis of the raw spectral efficiency of LTE if symbols were transmitted continuously and every symbol contained user data. It shows that in a 10 MHz channel, there are 600 usable subcarriers (with 15 kHz spacing, and some reserved for guard bands). The symbol duration without the prefix is 66.7 usec, giving a raw rate of 15000 symbols/sec/subcarrier, but with cyclic prefixes the usable number is 14000. Another way to calculate is to consider that, after the cyclic prefix there are 7 symbols per slot of 0.5 msec, so the symbol rate (per subcarrier) is 7 x 2000 = 14000 symbols/second/subcarrier.

The example below shows how to calculate a raw bit rate from subcarrier and symbol counts, and a consequent spectral efficiency figure, a measure of how many bps can be carried per Hz of RF bandwidth.

SIMPLE ANALYSIS OF AGGREGATE DATA RATE OF LTE SIGNALS, IGNORING OVERHEAD

Channel width MHz 10

MIMO spatial streams 2

Subcarriers 600

Symbols per second per subcarrier 14000

Bits per symbol (64-QAM) 6

Mbps, aggregate 100.8

Spectral efficiency (bits/sec/Hz/stream) 10.08

This analysis ignores all overhead above the physical layer and assumes a high rate of modulation so the figure of 100.8 Mbps represents an upper limit for the downlink of an LTE-FDD network operating in a paired 10 MHz channel with 2x2 MIMO, or approximately for a LTE-TDD network in 10 MHz with 2x2 MIMO, aggregating downlink and uplink.

Physical Resource Blocks

LTE engineers have developed a standard format for assignment of bandwidth: this section introduces that format.

The smallest bandwidth unit in LTE is the PRE (Physical Resource Element or just Resource Element for short) which is one symbol on one subcarrier. Having introduced ‘PRE’, we will not use it further in this paper, reverting to ‘symbol’ for simplicity.

The most commonly used bandwidth unit, and the minimum unit dynamically allocated to user traffic is the PRB (Physical Resource Block). A PRB is defined as a slot in the time dimension (containing 7 symbols with the short cyclic prefix) and 12 subcarriers per 200 kHz of channel bandwidth in the

frequency dimension (20 kHz of bandwidth per PRB is lost to guard intervals). Therefore, a PRB carries 84 symbols of data. The technique for allocating user bandwidth to many individual subcarriers across frequency and time dimensions is known as OFDMA (Orthogonal Frequency Division Multiple Access) and contributes to the flexibility and bandwidth efficiency of LTE.

(OFDM and OFDMA are often confused. The former is a technique for modulating subcarriers independently with different data words, in a way that avoids cross-subcarrier interference. Each subcarrier-time fragment is an OFDM symbol. The latter is a scheme to allocate the data words, across frequency and time, to different sources and destinations on the radio link. It may be helpful to think of OFDM as a layer 1 concept, and OFDMA as layer 2.)


10

In LTE and CBRS systems, downlink traffic is transmitted by the base station using straightforward OFDMA, where a symbol is generated per-subcarrier as explained above. However, the uplink uses a different, but similar, protocol known as SC-FDMA (Single Carrier Frequency Division Multiple Access). This is primarily because OFDMA signals have a high PAPR (Peak to Average Power Ratio), and RF power amplifiers that can handle high PAPR with good linearity, essential for low error-rates, are both expensive and power-hungry.

These are unattractive characteristics for mass-produced, low-cost devices so LTE adopted SC-FDMA for UEs. This paper will skip the detail for SC-FDMA: in brief, it adds extra steps to the modulation function that reduce the PAPR but for other purposes it does not make a difference to the explanations in this paper. Indeed, SC-FDMA is sometimes called ‘linearly precoded OFDMA’.

The diagram below shows how 84 symbols occupy a single PRB.

Figure 7. Physical resource blocks

The many ways of measuring time and frequency in LTE can be confusing. The table below summarizes the commonly used terms and how they relate.

TIME AND FREQUENCY MEASURES IN LTE TERMINOLOGY

Time measures msec per second symbols Frequency measures Hz Subcarriers/PRBsymbol 0.0667 14000 1 symbol 15 kHz 1

slot 0.5 2000 7 subcarrier 15 kHz 1

resource block 0.5 2000 7 resource block 200 kHz 12

subframe 1 1000 14 subcarrier count 10 MHz 50 PRB

frame 10 100 140 subcarrier count 20 MHz 100 PRB

subcarrier count 40 MHz 200 PRB

1 symbol is 66.7 usec in time x 15 kHz in frequency 1 resource block (PRB) is 0.5 msec in time x 200 kHz in frequency

11


One could legitimately ask why the concept of PRBs is needed. Why create a new unit of seemingly arbitrary size? One good reason for using PRBs is that allocating user bandwidth in chunks reduces the signaling overhead, since allocation maps are sent before each subframe and using PRBs reduces the granularity of bandwidth by 84 times. Contrariwise, if the bandwidth granularity gets too large then bandwidth will be wasted as the unfilled remainder of a PRB is padded: a PRB is considered a reasonable compromise.

Another reason is to permit LTE to scale to a wide variety of channel bandwidths – from 1.4 MHz to 20 MHz or more. CBRS supports a minimum of 5 MHz bandwidth, though 10 MHz and 20 MHz increments will be most common. As shown in the table above, PRBs offer the system operator a capacity metric that scales independently of the actual radio configuration.

The base station allocates different combinations of PRBs for transmission to and from different UEs. Each PRB is an opportunity to send 84 OFDM symbols, each representing 2 bits for QPSK modulation, 4 bits for 16-QAM, 6 bits for 64-QAM, or 8 bits for 256-QAM. Since one PRB per slot through a complete frame corresponds to 84 x 20 = 1680 symbols per frame or 168,000 symbols per second, the user data rate associated with that stream for 64-QAM would be 168,000 x 6 = 1.008 Mbps.

We can construct a massive grid showing all Physical Resource Blocks and symbols in a frame. The diagram below is for an LTE-FDD network in a 1.4 MHz channel for simplicity. The 1.4 MHz channel is 6 PRBs wide, while a 10 MHz channel is 50 PRBs wide, and a 20 MHz channel 100 PRBs wide in the frequency domain.

Figure 8. The resource grid: Physical Resource Blocks symbol format in LTE frames

To calculate how many PRBs are supported by an LTE-FDD system operating in 10 MHz, we take the time dimension in slots (2000 slots per second) and the frequency dimension in resource blocks (10 MHz / 200 kHz per PRB = 50 PRBs).


12

This system supports 2000 x 50 = 100000 PRBs / sec.

In symbols this rate is 100000 x 84 = 8400000 symbols / sec

To convert to a bit rate at 256-QAM multiply by 8 bits per symbol for 67.2 Mbps

If 2x2 MIMO is active double the rate to 134.4 Mbps

The table below relates resource blocks to symbol rates to bit rates with a more practical calculation that takes overhead into account. The ‘practical’ rates translate to what might be seen on public cellular networks under very good conditions.

APPROXIMATE PRACTICAL MAXIMUM DATA RATES IN LTE-FDD

Channel band-width MHz

Total resource blocks per slot @ 200 kHz

Slots per second @ 0.5 msec

Sym-bols per resource block

Symbols per second

Mbps @ 256-QAM with no overhead

Practical Mbps @ QPSK

Practical Mbps @ 16-QAM



Paired 10 50 2000 84 8400000 134.4 27 54 81 108

Paired 20 100 2000 84 16800000 268.8 54 108 161 215

Paired 40 200 2000 84 33600000 537.6 108 215 323 430Practical rates assume 54% coding, signaling & control channel overhead, 2x2 MIMO Spectrum occupied by e.g. dual 10 MHz FDD = 20 MHz; bit rates calculated one-direction

OFDMA in LTE and the scheduler

As introduced earlier, LTE uses a multiplexing structure called OFDMA (Orthogonal Frequency Division Multiple Access) to pack data into its RF channel.

OFDMA is easy to understand once PRBs have been digested; it is the ability to assign a certain block of user data to a block in the frequency-time matrix shown above. A PRB assignment could look like the diagram below.

Figure 9. OFDMA in LTE

13


Some UEs may need to send or receive periodic traffic, others may be bursty. Traffic is fitted into the frequency-time grid in the most efficient way. One aspect of OFDM is that communication to and from different UEs is subject to frequency-dependent patterns of degradation: selection of PRBs depends on which subcarriers are faded at any particular time.

Such a system relies on centralized control, and in LTE the base station controls the PRB assignment for OFDMA, for both downlink and uplink. LTE uses the concept of TTI (Transmission Time Interval) to define the span of a scheduler control interval. In LTE the TTI is equal to one subframe, and the scheduler uses the current subframe to transmit the schedule for the next one, signaling to UEs how they should receive and transmit in their allocated PRBs. More on schedulers later.

LTE-TDD FRAME STRUCTURE: TIME-INTERLEAVED UPLINK AND DOWNLINK SUBFRAMESWhen LTE is used in TDD mode for CBRS, the overall frame, subframe and slot structure is unchanged. But since both uplink and downlink transmissions use the same RF channel, they must be interleaved in time.

Introduction for Wi-Fi experts

In Wi-Fi, all nodes communicate on a common RF channel. Any node – whether access point or client device – can seize the medium and transmit when it senses no competing transmissions. This decentralized protocol is well-suited for situations with bursty, unpredictable traffic patterns: it allows packet-by-packet decisions on which node can transmit, in an unscheduled TDD protocol.

LTE has more structure. The baseline FDD mode assumes continuous DL and UL transmission, each in a different RF channel. In TDD mode, applicable to CBRS, the frame is divided into downlink and uplink segments: rather than packet-by-packet, this is fractional-frame ping-pong with multiple DL-only subframes followed by multiple UL-only subframes. The DL-UL split is an immutable structure, set when the network is first configured, and defines a bandwidth split of, for example, 50:50% or 60:40% downlink to uplink subframes. The base station can assign downlink and uplink Resource Blocks to any UE, but it cannot dynamically expand or contract the aggregate downlink or uplink bandwidth.

Whereas Wi-Fi interleaves uplink and downlink traffic on a packet-by-packet basis, LTE can only go to subframe-by-subframe.

This section explains the LTE-TDD frame structure, the special subframe, and data rate calculations for LTE-TDD.

LTE-TDD frame configurations

In the 3GPP specifications, a TDD frame has 7 options, allowing for different combinations from 2 to 8 downlink subframes, with the balance for uplink and usually two ‘special’ subframes. CBRS equipment supports options 1 and 2 only, and the choice is made when a network is first installed, with all base stations in the network using the same TDD frame structure.

Figure 10. LTE TDD frame options for CBRS

Configuration 1 supports 4 downlink and 4 uplink subframes per frame, for traffic with an equal balance.

Configuration 2 supports 6 downlink and 2 uplink subframes per frame, for networks where there is considerably more downlink than uplink traffic load.


14

LTE-TDD Special Subframe

The special subframe is required for the transition from DL to UL subframes, but not vice-versa. It allows synchronization to be re-established, and delays and multipath effects accounted for.

Following the special subframe, UEs transmit independently on the uplink, but must accurately adhere to their allocated timeslots to avoid interfering with other UE transmissions, and to allow accurate decoding by the base station.

Figure 11. LTE TDD special subframe structure

As seen above, CBRS frame configurations define two special subframes per LTE-TDD frame.

Each special subframe comprises three sections.

The downlink pilot timeslot contains the P-SS (Primary Synchronization Signal) and has space for some user data in the downlink shared channel (PDSCH).

The guard period is ‘dead’ time during the switch from downlink to uplink transmission to allow for propagation time effects due to distance: the larger the cell radius, the longer the guard period must be.

The uplink pilot timeslot can contain UL random access traffic (PRACH) and reference signals (SRS) but not user traffic.

LTE-TDD defines 9 possible combinations of DwPTS, UpPTS and GP. Configuration 7 is used in CBRS equipment, as it offers a large number of symbols available for user data, balanced against a cell radius limit (from the length of the guard period) of ~20 km, adequate for all anticipated CBRS applications.

Resource Grid for CBRS (LTE-TDD) frames

A full resource grid for even a 5 MHz channel is too complex to show here, but the following diagrams depict the patterns for representative CBRS networks. The horizontal scale, in time units, always has 10 subframes (20 slots) in a full frame; for a TDD configuration with configuration 1 and special subframe of type 7, a common configuration for CBRS, all downlink subframes start with signaling symbols. For PRBs close to the mid-channel point, designated symbol locations are used for signaling and control channels.

As the channel bandwidth scales, PRBs are added at the periphery of the channel.

15


Figure 12. Resource grid pattern for CBRS (LTE-TDD), at 5, 10, 20 MHz channels

CBRS channels will most often use 10 and 20 MHz channels, and can combine channels using Channel Aggregation.

Oscilloscope plot of CBRS frames

The graph below, an oscilloscope plot of power over time, illustrates TDD operation in a CBRS network. The plot covers 5 full frames, each with distinct DL, UL and Special subframes, in configuration 7 as shown above. Two traces are superimposed.

When there is no user traffic (blue trace), the only activity is DL RBs driven by the base station. Gaps in the trace, where power drops to the noise level, mark UL subframes. After a UE connects and starts transmitting and receiving traffic, both UL and DL subframes have increased power.

Figure 13. CBRS spectrum occupancy


16

Data rates in LTE-TDD

With knowledge of the LTE-TDD frame structure, the data-rate table used above to calculate LTE-FDD data rates can be modified to predict user data rates in CBRS networks.

The main difference is that two subframes per frame are special subframes, with reduced capacity to carry user traffic.

APPROXIMATE DOWNLINK DATA RATES IN LTE-TDD CBRS CONFIGURATIONS

Channel Bandwidth MHz

TDD frame option configura-tion

Total PRBs for FDD

Downlink subframes for TDD

Downlink PRBs per frame

Symbols per second

Mbps @ QPSK

Mbps @ 16-QAM

Mbps @ 64-QAM

10 1 2

50 50

4 6

20 30

33600005040000

11 16

22 32

32 48

20 1 2

100 100

4 6

40 60

672000010080000

22 32

43 65

65 97

40 1 2

200 200

4 6

80 120

1344000020160000

43 65

86 129

129 194

2x2 MIMO

APPROXIMATE UPLINK DATA RATES IN LTE-TDD CBRS CONFIGURATIONS

Channel Bandwidth MHz

TDD frame option configura-tion

Total PRBs for FDD

Uplink subframes for TDD

Uplink PRBs per frame

Symbols per second

Mbps @ QPSK

Mbps @ 16-QAM

Mbps @ 64-QAM

10 1 2

50 50

4 2

20 10

33600001680000

5 2

9 5

14 7

20 1 2

100 100

4 2

40 20

67200003360000

9 5

18 9

27 14

40 1 2

200 200

4 2

80 40

134400006720000

18 9

36 18

54 27

1x1 MIMO

The figures above are a close match to observed values in CBRS networks. They are maximum values, assuming only one UE per base station, and also represent the maximum capacity of a single base station. The data rates add, e.g. an uplink connection and a downlink connection with the data rates above can coexist in time.

SIGNAL POWER AND SIGNAL QUALITY MEASUREMENTSSince this first paper in the series on LTE deals with the lowest layers of the radio link, it is appropriate to add a section on signal power and quality measurements.

Wi-Fi measures signal power, noise power and SNR across the channel, while LTE uses specific reference signals for these measurements.

The measurements are similar and the values comparable, but the nomenclature (RSSI, SINR in Wi-Fi; RSRP and RSRQ in LTE) differs.

LTE provides specific definitions for these measurements, and they are of use to engineers, as test equipment, including commercial smartphones with diagnostic apps give values for RSRP (Reference Signal Received Power), RSSI (Received Signal Strength Indicator) and RSRQ (Reference Signal Received Quality).

17


Figure 14. Resource Block locations for Reference Signals

These measurements are made by the UE on specific symbols, the RS (Reference Signals) from antenna 0 (with a multi-antenna base station) that occur in known time locations in every PRB. The subcarriers used change with configured Cell ID according to a pattern defined in the standards.

RSRP, RSSI and RSRQ are conceptually simple, but the values are confusing because the measurements are defined across different bases, leading to scaling factors. The concepts are shown below.

Figure 15. RSRP, RSRQ and RSSI in LTE


18

RSRP measures the useful signal power of reference symbols. RSSI measures the total power in reference symbol resource blocks. RSRQ is the ratio of the two (or difference in dB terms).

The notes below provide more detail on each measurement.

RS symbols are transmitted across the band, and the RSRP measurement is defined as the average power of an RS symbol. Thus, RSRP is invariant across channel width.

Meanwhile, RSSI measures the total power received in RS symbol elements. As this includes interference and noise, RSSI, if it were measured on the same basis as RSRP, would always be higher for a given network and UE.

The following table converts from RSSI Received Signal Strength Indicator) to RSRP for 10 and 20 MHz channel widths, assuming no noise or interference in the RS symbols. Because RSRP is measured per-subcarrier (or symbol) and RSSI is over the channel bandwidth, there is a constant scaling factor of log10(12 * PRBs per channel width) due to 12 subcarriers per PRB.

RSSI TO RSRP CONVERSION

RSSI dBm

10 MHz RSRP dBm

20 MHz RSRP dBm

0 -27.8 -30.8

-40 -67.8 -70.8

-45 -72.8 -75.8

-50 -77.8 -80.8

-55 -82.8 -85.8

-60 -87.8 -90.8

-65 -92.8 -95.8

-70 -97.8 -100.8

-75 -102.8 -105.8

-80 -107.8 -110.8

-85 -112.8 -115.8

-90 -117.8 -120.8

-95 -122.8 -125.8

-100 -127.8 -130.8

RSRP is measured in dBm and typical values in a CBRS network might range from -75 to -120 dBm at the edge of coverage. RSRP measurements are used by the UE in the cell selection process when in the idle state, and are reported to the base station as part of the handover procedure.

19


RSRP (dBm)

SNR no NF SNR with NF

-70 62.2 55.2

-75 57.2 50.2

-80 52.2 45.2

-85 47.2 40.2

-90 42.2 35.2

-95 37.2 30.2

-100 32.2 25.2

-105 27.2 20.2

-110 22.2 15.2

-115 17.2 10.2

-120 12.2 5.2

-125 7.2 0.2

-130 2.2 -4.8

-135 -2.8 -9.8

-140 -7.8 -14.8

Figure 17. RSRP related to SNR with only thermal noise

The chart above shows the relationship between SNR due to thermal noise and RSRP for a 15 kHz subcarrier in LTE. Many LTE references calculate this and subtract the receiver noise figure (~7 dB) to indicate a reference signal at the receiver.

The third measurement, RSRQ targets signal quality. This takes the ratio of RSRP, the signal power in the reference signal, divided by the RSSI, the total measured power normalized so it is calculated across the same reference signals.

Since the total measured power includes interference from noise and from other LTE transmitters, the ratio expressed as dB is always negative. The range of reported RSRQ values is from -3 to -19.5 dB. RSRQ as a measure is similar to SINR or C/I (Carrier to Interference).

Formally, RSRQ = N * (RSRP / RSSI) where N is the number of PRBs in a channel width. RSRQ is invariant over channel width, therefore the N terms cancel.

GENERALLY ACCEPTED TYPICAL VALUES FOR LTE SIGNALS

Conditions RSRP dBm RSRQ dB SINR dBExcellent above -80 above -10 above 20

Good -80 to -90 -10 to -15 13 to 20

Fair -90 to -100 -15 to -20 0 to 13

Poor below -100 below -20 below 0

Thus, LTE engineers use RSRP and RSRQ as measures of signal strength and signal quality. RSSI is a hidden value but is easily computed from the other two.

The diagram below is a partial screenshot from a smartphone app, showing 4 base station signals (LTE but not CBRS) with associated signal values, taken in an area of poor but usable coverage. The underlines indicate the base station the UE is registered to.

Figure 18. RSRP, RSRQ and RSSI in LTE: values from a phone app


20

The following section uses spectrum analyzer plots to demonstrate the difference between Wi-Fi and LTE spectrum occupancy.

The Wi-Fi example shows a 20 dB difference between an idle access point, transmitting beacons but with no user traffic, and one with a full traffic load. This corresponds to a 1% channel occupancy for periodic beacons and other management traffic.

Figure 20. Wi-Fi spectrum occupancy

This 20 dB figure contrasts with the graph below where CBRS power in the channel varies much less with user data activity.

(The plots also illustrate that Wi-Fi does not use a DC subcarrier in the middle of the band, while LTE-CBRS does.)

Figure 21. CBRS spectrum occupancy

21


NEXT STEPSThe concepts here - OFDM symbols, framing, OFDMA, power measurements – are both familiar and interesting to most of us who work with Wi-Fi and other wireless protocols. (Unfortunately, the next paper in the series is harder to get through, but the final one is more accessible.)

Hopefully, you will keep this paper as a reference, and return to it when confronted with questions or problems thrown up by practical issues in your network.It is intended to present enough information to understand how the protocols work, but not to overwhelm. For more detailed explanations, the bibliography at the end of this paper includes a number of books that we in Aruba have found useful, but beyond this list there are many texts on LTE and 3GPP standards.

In time, CBRS equipment will move from LTE and 4G to the 5G standards, and much of this information will need to be revisited. The functional blocks and protocols may change, but the 3GPP’s approach to the problems of moving data across a fundamentally unreliable wireless channel will remain, an interesting counterbalance to the philosophy behind Wi-Fi – even as the two continue to converge.


22

APPENDIX – RELEVANT 3GPP RELEASES FOR LTE3GPP publishes a new ‘release’ every 12 – 24 months. Engineers often refer to them by number, for example ‘R11’ refers to the capabilities released in June 2013.

Figure 22. 3GPP releases for LTE and 5G

The chart above shows dates and performance levels for the LTE releases. While R15 and R16 mark the crossover to 5G, most CBRS equipment, and many public cellular networks and UEs, are some way behind the latest specifications. (Performance figures are intended to show capabilities of high-spec equipment under excellent conditions, some way short of theoretical best-case performance but optimistic for real-world cellular networks.)

The CBRS Alliance publishes specifications that define how to deploy an ‘OnGo’ LTE private network, as described in this paper (other configurations are also covered: neutral host network, and hybrid private and neutral host network). These specifications relate to 3GPP releases, OnGo release 3

aligns with 3GPP R15, and release 4 with 3GPP R16.

CBRS equipment is not always capable of the performance of current public cellular networks. It generally complies with R12 features, but many functions are more limited than the values in the table below (e.g. uplink/downlink max rates for CBRS base stations are 200/50 Mbps in a 40 MHz channel, and MIMO is limited to 2 antennas).

Similar to base station capabilities, UE capabilities are defined by Category. A Category lists a number of performance-related functions that the UE must have, although some parameters have ranges.

23


Release Date frozen

Commer-cial Name

Main Features

UE Category

Cat DL/UL

Max DL Mbps

Max UL Mbps

DL CCs

DL MIMO antennas

DL/UL QAM

Phone example

Date

R8 Dec 2008

‘LTE’ Initial LTE 1 10 5 1 1 64/16

2 50 25 1 2 64/16

3 100 50 1 2 64/16 iPhone5 09/12

4 150 50 1 2 64/16 iPhone6 09/14

5 300 75 1 4 64/64

R9 Dec 2009

Self- organizing networks

R10 Jun 2011

‘LTE- Advanced’

Carrier Aggregation

6 300 50 2 4 64/16 Galaxy S5 04/14

7 300 100 2 4 64/16

8 3000 1500 5 8 64/64

R11 Jun 2013

Coordinated Multipoint, Ma-chine-to-ma-chine

9 450 50 3 4 64/16 Galaxy S7 03/16

10 450 100 3 4 64/16

11 600 50 4 4 64/16

12 600 100 4 4 64/16 iPhone7 09/16

R12 Sep 2014

Dual Connectivity, MIMO enhancements, Small cells

15/13 800 150 32 8 256/256 Pixel

iPhone X

10/16

09/1816/ 1000 50 8

R13 Mar 2016

LTE- Advanced Pro’

More CA,LAA DL,More antennas,Lower latency

18/15 1200 225 16 256/64iPhone 11 09/19

19;/ 1600 16 256

R14 Sep 2017

Low rates for IoT, Small cell features,Vehicles,eLAA

/16 1000 105 32 256 Galaxy S8 Galaxy S10

01/17 01/19/18 1200 210

/20 2000 315

© Copyright 2020 Hewlett Packard Enterprise Development LP. The information contained herein is subject to change without notice. The only warranties for Hewlett Packard Enterprise products and services are set forth in the express warranty statements accompanying such products and services. Nothing herein should be construed as constituting an additional warranty. Hewlett Packard Enterprise shall not be liable for technical or editorial errors or omissions contained herein.

WP_CBRSLTETechnologyfortheEnterprise–TheRadio_RVK_120920

Contact Us Share

24


APPENDIX – BIBLIOGRAPHYFor the reader who wishes to learn more about LTE, CBRS and 5G, here is a list of technical textbooks on various aspects of 4G and 5G systems.

An Introduction to LTE: LTE, LTE-Advanced, SAE and 4G Mobile Communications, Christopher Cox, 360pp

System wide overview of Evolved Packet System from the core to the air interface. In particular, Cox explains LTE MAC layer, framing, error recovery and channelization.

Indoor Radio Planning: A Practical Guide for 2G, 3G and 4G, Morten Tolstrup, 560pp

This is one of the only dedicated textbooks on RF design & capacity planning for indoor small cells. It complements the other two books which take an end-to-end system approach but do not really consider layer 1 design.

5G NR: Architecture, Technology, Implementation, and Operation of 3GPP New Radio Standards, Sassan Ahmadi, 900pp

Reviews all aspects of the 5G system from core to air interface, including virtualization and disaggregation from core to RAN.

Three-Tier Shared Spectrum, Shared Infrastructure, and a Path to 5G, Preston Marshall

This provides a complete historical view of the origins, history and policy objectives of the three-tier architecture that became CBRS. It also considers other spectrum sharing models, including the 2-tier LSA model that exists in Europe.

Femtocells: Technologies and Deployment, Jie Zhang

An overview of small cell technology.

Documents

CBRS LTE Technology for the Enterprise The Radio - Whitepaper · 2020. 12. 11. · the lte radio link and frame structure 4 lte-tdd frame structure: time-interleaved 13 uplink and