Michael Einhaus, ComNets, RWTH Aachen University

Dynamic Resource Allocation in Dynamic Resource Allocation in OFDMA SystemsOFDMA Systems

Michael Einhaus

Chair of Communication Networks RWTH Aachen University, Germany

13. ComNets-Workshop, Mobil- und TelekommunikationMarch 31st, 2006, Aachen, Germany

13. ComNets-Workshop 200613. ComNets-Workshop 2006

2Michael Einhaus, ComNets, RWTH Aachen University

OverviewOverview

• Introduction• OFDMA• Medium Access Control• Multi User Diversity• Interference Awareness• Conclusion

3Michael Einhaus, ComNets, RWTH Aachen University

IntroductionIntroduction

• Handling of ISI• Spectral efficiency• Exploitation of diversity• Refined resource granularity• High flexibility

Why OFDMA? (Orthogonal Frequency Division Multiple Access)

Challenges• Synchronization• Handling of diversity and flexibility

4Michael Einhaus, ComNets, RWTH Aachen University

OFDMA Subchannel StructureOFDMA Subchannel Structure

clu

ster

edsu

bch

ann

eld

istr

ibu

ted

su

bch

ann

el

WIM

AX

WIG

WA

MW

INN

ER

Clustered subchannel interference/fading avoidance, exploitation of diversity, increased signaling overhead

Distributed subchannel averaging

5Michael Einhaus, ComNets, RWTH Aachen University

MAC Frame StructureMAC Frame Structure

• Centrally controlled (IEEE 802.16)

• 2-dimensional resources

• Scheduling within resource subsets

• Fast and slow adaptation

time slot (8 OFDMA symbols)

32

su

bc

ha

nn

els

signaling overhead

resources

544 s (40 x 13.6 s)

resource subset

6Michael Einhaus, ComNets, RWTH Aachen University

OFDMA Subchannel DiversityOFDMA Subchannel Diversity

• V = 50km/h fDoppler~230Hz• Τrms = 250ns• Δf = 78.125kHz (80MHz/1024)

• Subchannel diversity depends on subcarrier grouping

• Diversity exploitation requires sophisticated scheduling algorithms

• Diversity exploitation increases signaling overhead

7Michael Einhaus, ComNets, RWTH Aachen University

Basic OFDMA SchedulerBasic OFDMA Scheduler

• Perfect channel knowledge

• Uniform power allocation(no water-filling)

• Adaptive PHY-mode selection

resource

32

su

bc

ha

nn

els

time slot

8Michael Einhaus, ComNets, RWTH Aachen University

Exploitation of Multi User DiversityExploitation of Multi User Diversity

• Single cell scenario• 32 mobile terminals• Downlink traffic (Poisson)• 100 byte packets• Different OFDMA schemes

– Clustered subchannels (MUD)– Clustered subchannels (random)– Distributed subchannels

(averaging)

• Pathloss coefficient : = 2.5• TX Power : 30 dBm• QPSK¾, 16QAM¾, 64QAM¾

2000 kbit/s200 m

150 m

9Michael Einhaus, ComNets, RWTH Aachen University

SINR Estimation / PHY Mode SelectionSINR Estimation / PHY Mode Selection

iiii IFNLPSINR • SINR estimation is

required for optimal resource and PHY mode selection

• Afflicted with uncertainty– Fading– Interference

• Variance depends on OFDMA scheme

10Michael Einhaus, ComNets, RWTH Aachen University

Interference Aware Resource AllocationInterference Aware Resource Allocation

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IIEI

• Perfect channel knowledge

• Interference estimation

= 0

• 2 cells (D = 500m, R = 150m)

• 8 mobile terminals

MT 1

MT 1(SINR0 = 15 dB)

11Michael Einhaus, ComNets, RWTH Aachen University

Interference Aware Resource Allocation (II)Interference Aware Resource Allocation (II)

12Michael Einhaus, ComNets, RWTH Aachen University

Conclusion and OutlookConclusion and Outlook

• OFDMA has the potential to provide high spectral efficiency

• Exploitation of diversity implies problems in the estimation of interference

• The design of an OFDMA system comprises a large set of parameters and options

Conclusion

Open Issues• Averaging vs. avoidance• Adaptation to channel or interference• Joint scheduling of base stations• Uplink

13Michael Einhaus, ComNets, RWTH Aachen University

Thank you for your attention !

Michael Einhausein@comnets.rwth-aachen.de

Any questions?