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Abstract:
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We investigate in detail two multiuser opportunistic scheduling problems in centralized wireless systems : the scheduling of "delay -sensitive" flows with packet delay requirements of a few tens to few hundreds of milliseconds over the air interface , and the scheduling of "best -effort" flows with the objective of minimizing mean file transfer delay .
Schedulers for delay -sensitive flows are characterized by a fundamental tradeoff between "maximizing total service rate by being opportunistic" and "balancing unequal queues (or delays ) across users" . In choosing how to realize this tradeoff in schedulers , our key premise is that "robustness" should be a primary design objective alongside performance . Different performance objectives - - mean packet delay , the tail of worst user's queue distribution , or that of the overall queue distribution - - result in remarkably different scheduling policies . Different design objectives and resulting schedulers are also not equally robust , which is important due to the uncertainty and variability in both the wireless environment and the traffic . The proposed class of schedulers offers low packet delays , less sensitivity to the scheduler parameters and channel characteristics , and a more graceful degradation of service in terms of the fraction of users meeting their delay requirements under transient overloads , when compared with other well -known schedulers .
Schedulers for best -effort flows are characterized by a fundamental tradeoff between "maximizing the total service rate" and "prioritizing flows with short residual sizes" . We characterize two regimes based on the "degree" of opportunistic gain present in the system . In the first regime - - where the opportunistic capacity of the system increases sharply with the number of users - - the use of residual flow -size information in scheduling will 'not' result in a significant reduction in flow -level delays . Whereas , in the second regime - - where the opportunistic capacity increases slowly with the number of users - - using flow -size information alongside channel state information 'may' result in a significant reduction . We then propose a class of schedulers which offers good performance in either regime , in terms of mean file transfer delays as well as probability of blocking for systems that enforce flow admission control .
This thesis provides a comprehensive theoretical study of these fundamental tradeoffs for opportunistic schedulers , as well as an exploration of some of the practical ramifications to engineering wireless systems . |