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QoS Aware Adaptive Subcarrier Allocation in OFDMA Systems

Explore the optimization of resource allocation in OFDMA systems for broadband wireless access, balancing QoS requirements and system performance with adaptive modulation. Various algorithms are proposed and simulated to improve subcarrier and bit allocation efficiency in a multiuser environment.

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QoS Aware Adaptive Subcarrier Allocation in OFDMA Systems

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  1. QoS Aware Adaptive Subcarrier Allocation in OFDMA Systems Mustafa Ergen & Sinem Coleri {ergen,csinem}@eecs.berkeley.edu University of California Berkeley

  2. Introduction • Motivation • Orthogonal Frequency Division Multiple Access(OFDMA) • OFDMA System • Resource Allocation Problem • Algorithms • Optimal • Suboptimal • Simulation • Conclusion

  3. Motivation • Broadband Wireless Access • Ex: IEEE 802.16, Wireless MAN • OFDM • Eliminates InterSymbol Interference • OFDMA

  4. OFDM Diagram

  5. OFDM-TDMA Time Subcarrier User 1 … … User 2 User 3 Multiuser OFDM • OFDM-TDMA • OFDM-FDMA • OFDMA OFDM-FDMA Time Subcarrier OFDMA Time Subcarrier

  6. Resource Allocation Goals: • Dynamic subcarrier selection • Improve system performance with adaptive modulation • More bits transmitted in large channel gain carriers • Provide QoS • Rate and BER

  7. Resource Allocation Assumptions: • Base station knows the channel • Base station informs the mobiles for allocation subcarrier user Base Station

  8. System oCoS=Ptotal for downlink oCoS=Pu for uplink Application Network rQoS=[rR,rBER] oQoS=[oR,oBER,oCoS] Resource Allocation [User x Subcarrier] Physical Layer

  9. OFDMA

  10. QoS RATE: [12 6 6 8 ] BER: [1e-2 1e-2 1e-4 1e-4] Subcarrier User Resource Allocation Channel Resource Allocation 64-QAM 16-QAM 4-QAM

  11. Notation

  12. Subcarrier User Subcarrier Subcarrier User User User Optimal Pc1 Subcarrier • Integer Programming Pc2 Pc3

  13. Motivation for Sub-optimal Algorithms • IP is complex • Allocation should be done within the coherence time • Time increases exponentially with the number of constraints

  14. Current Suboptimal Algorithms 2-step: • Subcarrier Allocation • Assume the data rate for all subcarriers • Assume modulation rate is fixed • Assign the subcarriers • Bit Loading • Greedy approach to assign the bits of user

  15. Subcarrier Subcarrier User User User Current Suboptimal Algorithms • Subcarrier Allocation • Hungarian algorithm • Optimal, very complex • LP approximation to IP problem • Close to optimal • Bit Loading Subcarrier

  16. Problems in Current Suboptimal Algorithms • Subcarrier assignment and bit loading are separated • Users with bad channels may need higher number of subcarriers • Not iterative subcarrier assignment

  17. Iterative Algorithm Iterative algorithm based on • Assignment of bits according to highest modulation • Finding the best places • Distributing the assigned bits to other subcarriers or to non-assigned subcarriers • Exchanging the subcarriers among user pairs for power reduction.

  18. Iterative Algorithm • Fair Selection(FS) • Greedy Release(GR) • Horizontal Swaping(HS) • Vertical Swaping(VS)

  19. Start FAIR SELECTION Ptotal<Pmax Modulation-- GREEDY RELEASE Iterative Algorithm HORIZONTAL SWAP VERTICAL SWAP ASSIGNMENT ITERATION

  20. Simulation Environment Build the OFDMA system • Modulations:4-QAM,16-QAM,64-QAM • Independent Rayleigh fading channel to each user • Number of subcarriers =128 • Nodes are perfectly synchronized

  21. CDF of total transmit power without Pmax constraint

  22. CDF of total transmit power with Pmax constraint

  23. Average bit SNR vs. RMS delay spread As RMS delay spread increases, the fading variation increases hence higher gains are obtained by adaptive allocation

  24. Average bit SNR vs. number of users As the number of users increases, the probability of obtaining good channel at a subcarrier increases

  25. Instantaneous Average bit SNR vs Time Iterative Algorithm improves its Average Bit SNR by the time.

  26. Conclusion • OFDMA • Broadband Wireless Access • Resource Allocation • Channel Information • QoS Requirement • Optimal Algorithms • complex • Iterative Algorithms

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