Resource Allocation and Reuse in LTE

Mobile Communication 2 Markus LanerSeminar, SS 2008 0325687

Contents

• Introduction• History of Resource Reuse• Proposals and Specification for LTE• Inter-cell Interference• Soft Frequency Reuse• Conclusion

Introduction

Resource reuse to enhance capacity

- use expensive spectrum acquired by providers most efficiently

- minimize number of BS but maximize number of users- leads to interference limited systems

Contents

• Introduction

• History of Resource Reuse• Proposals and Specification for LTE• Inter-cell Interference• Soft Frequency Reuse• Conclusion

History of Resource Reuse

- digital because higher SIR supported- clusters of cells- frequency reuse factor between 3 and 9- not very efficient

History of Resource Reuse

- real freq reuse 1 through spreading and scrambling- no more freq planning- Macro diversity => connections to 1 or more BS

Scrambling A

Spreading 1,2,3

Scrambling B

Spreading 1,2,3

Contents

• Introduction• History of Resource Reuse

• Proposals and Specification for LTE• Inter-cell Interference• Soft Frequency Reuse• Conclusion

Proposals and Specifications

• no specifications defined jet ( necessary? )

• frequency reuse 1 proposed

• hard handover, no macro diversity• many proposals to combat ICI (3GPP TR 25.814)

– ICI cancellation– ICI co-ordination– ICI randomization

Contents

• Introduction• History of Resource Reuse• Proposals and Specification for LTE

• Inter-cell Interference• Soft Frequency Reuse• Conclusion

Inter-cell Interference

ICI cancellation

multiple antennas can be used to suppress interferer

- demodulation and subtraction (Blast)- IRC (Interference rejection combining) instead of MRC (Maximum ratio

combining)

- optimum combining

- uplink: multiple BS can be seen as giant MIMO system

drawback: extremely high complexity

ICI randomization

similar to the method used in UMTS- cell specific scrambling- cell specific interleaving (IDMA) (interleave-division multiple access)

the UE can identify the transmitting BS via different patterns, interference is spread over multiple channels

ICI co-ordinationmost promising methods for LTE- soft frequency reuse (SFR)

reuse 1 -> cell-centre users (CCU)reuse 3 -> cell-edge users (CEU)

- co-ordination between BSsTLA (two level allocation scheme) (see graphic)

UFR (universal frequency reuse) -> staticDMA (dynamic major-group allocation scheme) -> dynamic

adaptive modulation and coding (AMC)

users with different SIR get different modulation and coding schemes assigned

- modulation schemes up to 64-QAM- Turbo coding with different code rates

drawback: wasting of resources

beam forming at eNodeBBSs track their active subscribers with the antenna beam

Muting-> UE tracks the scheduling information of neighboring cells-> UE recognizes when co-channel interferer are active-> in this case the UE is ‘mute’, doesn’t transmit

drawback: wasting of resources

Contents

• Introduction• History of Resource Reuse• Proposals and Specification for LTE• Inter-cell Interference

• Soft Frequency Reuse• Conclusion

Soft Frequency Reuse

TLA (two level allocation scheme)

CRA (cell-level resource allocation)

- static, major- & minor-group

URA (user-level resource allocation)

- division in CEU (high priority) and CCU by geometry factor

benefits:- CQI (channel quality information) has to be available at BS

anyway, can be used for partitioning users- low complexity- better cell-edge performance

drawbacks:- signaling overhead- reuse partitioning is suboptimal from information

theoretical point of view- overall loss of throughput (see next slides)

Impact of ICI on capacity, reuse 1

y-axisCell Throughput:whole cell[bits/second]

x-axisarrival rate:poisson distriubuted[erlang / number of servers]

Impact of ICI on capacity: reuse 1/3

Throughput per cell-edge userThroughput of the whole cell

Types of traffic

Narrow-band services (calls) will perform better than wide-band services (FTP)

reason: narrow-band services allow more flexibility in resource allocation

Served traffic of user X

circles mark the link utilization factor:0.2 / 0.4 / 0.6 / 0.8 / 1

UFR (Ki Tae Kim, 2007)(universal frequency reuse)

- special patterns (sequences) for frequency allocation coordinated with neighboring cells

- lower interference for small LF (loading factor)

- static configuration, low complexity

Loading factor

three different algorithms for three different cells C0, C1, C2

DMA (Fan, Qian, Zheng, Wang, 2007)

(dynamic major-group allocation scheme)

dynamic allocation of major group sub carriers depending on the number of CEU (sectors per site)

- benefits depending on major- to minor-group power-ratio

- high complexity

Contents

• Introduction• History of Resource Reuse• Proposals and Specification for LTE• Inter-cell Interference• Soft Frequency Reuse

• Conclusion

Conclusion

Specifications / Suggestions: - reuse 1- no macro-diversity- hard handover

ICI overcoming: - cancellation: too complex

- mitigation / coordination: will be supported in LTE

- randomization: no real improvement

- beam forming at eNodeB: will be supported in LTE

- AMC (adaptive modulation and coding): will be supported in LTE

Conclusion

Soft Frequency Reuse:- also called reuse 1/3- partitioning of users in CEU and CCU through geometry factor- good performance for CEU- low complexity- overall loss of throughput- further improvements through different scheduling / resource

assignment methods possible (UFR, DMA)

The End

Thank you,

Questions ?

Appendix

References, Specifications:- 3GPP TR 25.892, Feasibility Study for OFDM for UTRAN

enhancement, 2004-06- 3GPP TR 25.814, Physical layer aspects for eUTRA, 2006-09- 3GPP R1-050764, Ericsson, Inter-cell Interference Handling for

eUTRA, 2005-08-29, London- 3GPP R1-050763, Ericsson, Muting – Further Discussion and

Results, 2005-08-29, London- 3GPP R1-060864, Texas Instruments, Overview of Resource

Management Techniques for Interference Mitigation in eUTRA, 2006-03-27, Athens

Appendix

References, Papers:- Bin Fan, A Dynamic Resource Allocation Scheme Based on Soft

Frequency Reuse for OFDMA Systems, 2007- Elayoubi, On frequency allocation in 3G LTE systems, 2006- Fan, An Inter-Cell Interference Coordination Technique Based on

User's Ratio and Multi-Level Frequency Allocations, 2007- Jorguseski, Downlink Resource Allocation in Beyond 3G OFDMA

Cellular Systems, 2007- Ki Tae Kim, A Universal Frequency Reuse System in Mobile Cellular

Environment, 2007- Simonsson, Frequency Reuse and Inter-cell Interference Co-

ordination in eUTRA, 2007

Appendix

References, Books:- Dahlman, Parkvall, Sköld, Beming, 3G Evolution – HSPA and LTE

for Mobile Broadband, Academic Press, 2007- Molisch, Wireless Communications, Wiley & Sons, 2005

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