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
Mobile Communication 3 Markus LanerSeminar, SS 2008 0325687
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
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Contents
• Introduction
• History of Resource Reuse• Proposals and Specification for LTE• Inter-cell Interference• Soft Frequency Reuse• Conclusion
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History of Resource Reuse
GSM
- digital because higher SIR supported- clusters of cells- frequency reuse factor between 3 and 9- not very efficient
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History of Resource Reuse
UMTS
- 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
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Contents
• Introduction• History of Resource Reuse
• Proposals and Specification for LTE• Inter-cell Interference• Soft Frequency Reuse• Conclusion
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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
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Contents
• Introduction• History of Resource Reuse• Proposals and Specification for LTE
• Inter-cell Interference• Soft Frequency Reuse• Conclusion
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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
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Inter-cell Interference
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
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Inter-cell Interference
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
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Inter-cell Interference
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
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Inter-cell Interference
beam forming at eNodeBBSs track their active subscribers with the antenna beam
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Inter-cell Interference
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
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Contents
• Introduction• History of Resource Reuse• Proposals and Specification for LTE• Inter-cell Interference
• Soft Frequency Reuse• Conclusion
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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
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Soft Frequency Reuse
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)
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Soft Frequency Reuse
Impact of ICI on capacity, reuse 1
y-axisCell Throughput:whole cell[bits/second]
x-axisarrival rate:poisson distriubuted[erlang / number of servers]
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Soft Frequency Reuse
Impact of ICI on capacity: reuse 1/3
Throughput per cell-edge userThroughput of the whole cell
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Soft Frequency Reuse
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
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Soft Frequency Reuse
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
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Soft Frequency Reuse
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
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Contents
• Introduction• History of Resource Reuse• Proposals and Specification for LTE• Inter-cell Interference• Soft Frequency Reuse
• Conclusion
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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
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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)
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The End
Thank you,
Questions ?
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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
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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
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Appendix
References, Books:- Dahlman, Parkvall, Sköld, Beming, 3G Evolution – HSPA and LTE
for Mobile Broadband, Academic Press, 2007- Molisch, Wireless Communications, Wiley & Sons, 2005