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Scheduling and Optimization. Criteria and Algorithms for Scheduling of Packet Data over Wireless Channels. Nilo Casimiro Ericsson, Signals & Systems, Uppsala University. Outline. Introduction, background Scheduling for spectral efficiency Latest scheduling insights

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Scheduling and Optimization


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    1. Scheduling and Optimization Criteria and Algorithms for Scheduling of Packet Data over Wireless Channels Nilo Casimiro Ericsson, Signals & Systems, Uppsala University

    2. Outline • Introduction, background • Scheduling for spectral efficiency • Latest scheduling insights • No need for complex optimization • Provide an average throughput • Adaptive criteria – simulation results • Conclusion

    3. Packet data over fading channels Avoid fading dips!

    4. Scheduling of OFDM bins • Perform scheduling based on predicted average SNR in time-frequency bins •  • For each bin let the “best” user transmit; use adaptive modulation and ARQ 1 4 3 5 2 user freq time

    5. What the scheduler does:

    6. Scheduling algorithms • Simple “linear” maximization • Best First • Maximum Allocation • Robin Hood • “Exact” buffer-matching • Controlled Steepest Descent • Exhaustive search

    7. Complexity (25 bins) two-step one-step + swap one-step

    8. But, is the criterion right at all? • Buffer content minimimization at each scheduling instant seems short-sighted • Search algorithms allocate resources to match buffer content as exactly as possible • Sum-of-squares criteria • Uncertain predictions… • ”Academic” interest, off course • Instead: Maintain a (constant?) average (over time) throughput for each active stream • Based on maximized “linear” criteria • If necessary: re-allocations from over-provisioned streams

    9. Traffic adaptive criteria • Previously in Robin Hood (Coarse adaptivity) • Three features compared in some order: • Modulation, Priority, SNR • If two have equal Modulation => compare Priority, etc… • Can change order to (adaptation to traffic situation)Priority, Modulation, SNR • New: Quantize features into (e.g.) Modulation 3 bits m1,m2,m3 Priority 2 bits p1,p2 SNR 2 bits s1,s2 (explain!) • The new feature: m1,m2,m3,p1,p2,s1,s2 • But also: m1,p1,p2,m2,m3,s1,s2

    10. Adaptive criteria example • 3 bits for Modulation (0-7) • 2 bits for Priority (0-3) • 0 bits for SNR (omitted) • mmmpp • mppmm • User 1: • M = 6 (64QAM), mmm = 1102 • P = 1 (medium low), pp = 012 • A) mmmpp = 110012= 2510 • B) mppmm = 101012 = 1910 • User 2: • M = 5 (32QAM), mmm = 1012 • P = 2 (medium high),pp = 102 • A) mmmpp = 101102= 2210 • B) mppmm = 110012 = 2510 > <

    11. Simulation of scheduler • 25 OFDM bins per schedule • 5 MHz carrier @ 1900 MHz • Time-frequency bin size: 0.667 ms x 200 kHz • 108 payload symbols per bin • 12 users • 8 modulation levels (3 bits) • 0-7 (“quiet”-128QAM) • SNR thresholds: [ 6.5 10 14 18 22.5 26 30 ] dB • (why not 1-8?) • 4 priority levels (2 bits) • 0-3 • Random SNR for each user and bin • 100 schedule simulations per criteria setup

    12. 12 users, 4 priorities: 3 users of each priority Same SNR distribution for all: N(10,10) Maximum modulation: 7 (128QAM) Simulation 1: Throughput per user Criteria:mmmpp Criteria:mmppm Criteria:mppmm Criteria:ppmmm Criteria:mmpmp Criteria:mpmpm Criteria:pmpmm Criteria:mpmmp Criteria:pmmpm Criteria:pmmmp (A) (B) Total throughput

    13. 12 users, 4 priorities: 3 users of each priority 4 different SNR distributions: N({15,12,9,6},5) Highest priority for worst SNR Simulation 2: Throughput per user Criteria:mmmpp Criteria:mmppm Criteria:mppmm Criteria:ppmmm Criteria:mmpmp Criteria:mpmpm Criteria:pmpmm Criteria:mpmmp Criteria:pmmpm Criteria:pmmmp (A) (B) Total throughput

    14. Conclusion • For practical scheduler: abandon complex search algorithms • Too many uncertainties (channel prediction, buffer usage) • Scheduling can handle also distant users with worse conditions than near users • Work with “priorities” • Upgrade the importance of “priority” • Probably, average throughput target will also help distant users • Over-provisioned near users will give resources to under-provisioned distant users

    15. 12 users, 4 priorities: 3 users of each priority 3 different SNR distributions: N({5,10,15},5) Maximum modulation: 7 (128QAM) Simulation 3: Throughput per user Criteria:mmmpp Criteria:mmppm Criteria:mppmm Criteria:ppmmm Criteria:mmpmp Criteria:mpmpm Criteria:pmpmm Criteria:mpmmp Criteria:pmmpm Criteria:pmmmp (A) (B) Total throughput