Random Access - lmk.lnt.de · It conveys the actual random access procedure ... • Contention free Random Access can be use in areas where low latency is required, such as handover

University of Erlangen-NürnbergChair of Mobile Communications

Random AccessSeminar LTE: Der Mobilfunk der Zukunft

Almamy Touray

December 2, 2009

Outline

1. Introduction: 3GPP-LTE2. Brief Overview of Random Access (RA) Process in LTE3. System Model of RA 4. What situations require RA?5. RA Procedures

– Contention based– Contention free-based

6. Physical Random Access Channel (PRACH) Design– PRACH Structure– Multiplexing of PRACH with PUSCH and PUCCH

7. PRACH Implementation– Functional Structure of PRACH preamble Transmitter– eNodeB PRACH Receiver– PRACH Receiver options

8. Summary9. References

2 Almamy Touray: Random Access2

Introduction: 3GPP Long Term Evolution (LTE)

• 3GPP (3rd Generation Partnership Project) LTE • 4th generation of radio technologies designed to increase the

capacity and speed of mobile telephone networks.• LTE is a set of enhancements to Universal Mobile Telecommunications

Systems (UMTS).• LTE is be based on Single Carrier OFDMA in uplink and OFDM in

downlink unlike UMTS that uses WCDMA (Wideband Code Division Multiple Access )in the uplink and WCDMA in the downlink


Brief Overview of Random Access Channel

• Used to signal a connection request to the network• Uplink channel in mobile communication.• Contention-based where several users might access the

same resource.• Plays a fundamental role as an interface between non-

synchronised UEs and the orthogonal transmission scheme of the uplink radio.

• Once uplink synchronisation is achieved for a UE, the eNodeB can schedule orthogonal uplink transmission resources for it


System Model of Random Access

System description of Random Access


What situations require Random Access?

1. UE in RRC_CONNECTED state, but not uplink-synchronized. needing to send new uplink data or control information (e.g. an event-triggered measurement report).

2. UE in RRC_CONNECTED state, but not uplink- synchronized, needing to receive new downlink data, and, therefore, to transmit corresponding ACK/NACK in the uplink,

3. UE in RRC_CONNECTED state, handing over from its current serving cell to a target cell,

4. A transmission from RRC_IDLE state to RRC_CONNECTED, for example for initial access or tracking area updates,

5. Recovering from radio link failure.


RACH in UMTS versus RACH in LTE

• In both schemes, the primary role of RACH is for initial network access• No user data in LTE RACH. This is exclusively sent on the Physical

Uplink Shared CHannel (PUSCH).• LTE RACH is used to achieving uplink time synchronization for UEs.• We have low overhead in LTE RACH compared to WCDMA.

– This is due to the fact that the CP of uplink transmission in LTE RACH only needs to allow for round-trip delay estimation instead of the timing of individual channel taps.

– No power ramping, which can be a major course for latency and interference.


Random Access Process

Message sequence chart for the Random Access procedure

1. eNodeB broadcasts information regarding random access, 2. multiple UEs transmit randomly selected random access code. 3. eNode conducts multi-user detection process and allocates resources to detected UEs.4. each UEs transmits detailed information using allocated resources.5. eNodeB confirms to each UE


RACH Design

• Contention-based (implying an inherent risk of collision): This is a situation where several UEs may access the same resource and hence the possibility of collision between them.

• Contention free: The eNodeB assigns distinct preamble to each UE.


10 Almamy Touray: Random Access

Contention-based

Contention-based Random Access procedure

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Contention-based


• Preamble Transmission– UE selects one of the 64-Ncf available PRACH contention-based signatures, where Ncf is the number of

signatures reserved by eNodeB for contention-free RACH.– UE selects a signature from the subgroup corresponding to the size of transmission resource needed for the

appropriate RACH use case.– The eNodeB can control the number of signatures in each subgroup according to the observed loads in each

group• Random Access Response

– T he Random Access Response (RAR) is sent by the eNodeB on the Physical Downlink Shared Channel (PDSCH), and addressed with an ID, the Random Access Radio Network Temporary Identifier (RA-RNTI), identifying the time-frequency slot in which the preamble was detected.

– If multiple UEs had collided by selecting the same signature in the same preamble time-frequency resource, they would both receive the same RAR.

• Layer 2/Layer 3 (L2/L3) Message– This message is the first scheduled uplink transmission on the PUSCH and makes use of Hybrid Automatic

Repeat Request (HARQ), also conveys UE identifier etc. It conveys the actual random access procedure message.

• Contention resolution– This the eNodeB uses this option step to end the PRACH procedure.

Contention resolution: The UE behaviour upon reception of contention resolution message, therefore, has three possibilities:

1. The UE correctly decodes the message and detects its own identity: it sends back positive acknowledgement ACK

2. The UE correctly decodes the message and discovers that it contains another UE identity (contention resolution): it sends nothing back (Discontinuous Transmission, DTX)

3. The UE fails to decode the message: it sends nothing back (DTX)


Contention-based

Collision Detection


Contention-free

Contention-free Random Access procedure

Contention-free Random Access can be use in areas where low latency is required, such as handover and resumption of downlink traffic for UE. By allocating dedicated signatures to the UE on a peer-need basis


Why Contention free-based?

HandoverThere are two types of handover:• Intra-RAT, which is within one radio access technology (i.e. LTE -to-

LTE from one eNodeB to another)• Inter-RAT, between radio access technologies e.g.: between LTE and

GSM or 3G WCDMA, WIMAX or even wireless LAN

• Helps to reduce latency


Contention based versus Contention free-based

• Contention-based can be used by any accessing UE in need of an uplink connection

• Contention free Random Access can be use in areas where low latency is required, such as handover and downlink data arrival events.

• In both procedures, the Random Access preamble is transmitted by the accessing UEs.

There exists 64 Random Access preambles allocated for each cell of an eNodeB, and each eNodeB dynamically configures two disjoint sets of preambles to be used by the two Random Access procedures seperately.


Physical Random Access Channel (PRACH) Design

PRACH multiplexing with PUSCH and PUCCH

PRACH: Physical Random Access ChannelPUCCH: Physical Uplink Control ChannelPUSCH: Physical Uplink Shared Channel

The PRACH, PUCCH, and PUSCH are orthogonal to one another and as such, preambles interfering with user data will not occur as oppose to UMTS


PRACH Structure

• two preambles at the eNodeB received with different timings due to propagation delay• We used the GT(Guard Time) is to absorb the propagation delay• The receiver can then sample the waveform at the optimum time• The maximum cell radius is given by the CP (Cyclic Prefix) length• If it is not long enough, then it will not counteract the multipath reflection delay spread.• On the other hand, if it is too long, then it will reduce the data throughput capacity.



PRACH Structure

Field durations and achievable cell radius of the PRACH preamble formats

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PRACH Implementation

Functional Structure of PRACH preamble transmitter

UE Transmitter:• The PRACH preamble can be generated at the system sampling rate, by means of a large IDFT as

illustrated above• the DFT block is optional as the sequence can be mapped directly in the frequency domain at the IDFT

input


PRACH Implementation


Disadvantage of this methodIt does not require any time-domain filtering at baseband, but leads to large IDFT sizes (up to 24, 576 for a 20 MHz spectrum allocation), which are cumbersome to implement in practice.

Solution• generating preamble using a smaller IDFT, actually an IFFT• shifting the preamble to the received frequency location through time-domain

up-sampling and filtering. • This results to hybrid frequency/time-domain generation as shown in the figure

below.

Hybrid frequency/time-domain PRACH generation

Hybrid frequency/time domain PRACH generation


eNodeB PRACH Receiver

PRACH receiver options



Front-End• In both the frequency-domain and the hybrid time/Frequency domain

approaches, we have the removal of the CP, which always take place at the front-end of the system sampling rate, the PDP computation and signature detection.

• In the full-frequency-domain approach, the DFT computation cannot start until the complete sequence is stored in memory, which increases delay.



• On the contrary, the hybrid time-frequency domain method first extracts the relevant PRACH signal through a time-domain frequency shift and down-sampling and anti-aliasing filter.

• The use of down-sampling and the anti-aliasing filter are for generating PRACH time samples suitable for FFT or simple DFT computation at a sampling rate which is an integer fraction of the system sampling rate.

• Unlike the full-frequency-domain approach, the hybrid time/frequency-domain computation can start as soon as the first samples have been received, which helps to reduce latency


Summary

• RA provides uplink synchronization necessary for scheduling of UEs• RACH bandwidth in LTE is minimal compared to WCDMA used in

UMTS• RACH in LTE fits into the orthogonal time-frequency structure of the

uplink compared to UMTS which uses WCDMA• Random Access in LTE can be contention-free • Contention- free allows for handover and other scenarios which require

low latency, which is are key issues in mobile communications• We use the Zadoff-Chu (ZC) sequence as RA preamble for LTE

networks• The PRACH preamble enables the eNodeB to estimate UE

transmission timing


References

• J. G. Proakis, Digital Communications. New York, McGraw-Hill, 1995• W.W Smith and J.M.Smith, Handbook of Real-time Fast Fourier

Transforms, New York:Wiley Inter-Science, 1995• M.Shafi, S. Ogose and T. Hattori, Wireless Communications in the 21st

Century, New York: Wiley Inter-Science, 2002• Panasonic and NTT DoCoMo, R1-062175: Random Acess Burst

Design for E-UTRA, www.3gpp.org 3GPP TSG RAN WGI, meeting 46, Tallinn, Estonia, August 2006

• R.l. Frank, s. a. Zadoff and R. Heimiller, Phase Shift Pulse Coses with good Periodic Correlation Properties, IRE IEEE Trans. on Information Theory, Vol. 7


References

• D. C. Chu, Polyphase Codes with Good Periodic Correlation Properties. IEEE Trans. on Information Theory, Vol. 18

• ETSIEN 300 910, Radio Transmission and Reception (Release 1999), www.etsi.org.

• Huawei, R1-072325: Multiple Values of Cyclic Shift Increment NCS, www.3gpp.org, 3GPP TSG RAN WGI

• Panasonic, R1-071517: RACH Sequence Allocation for Efficient Matched Filter Implementation


Questions and Comments

Thank you so much for your audience!





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Random Access - lmk.lnt.de · It conveys the actual random access procedure ... • Contention free Random Access can be use in areas where low latency is required, such as handover