Opportunistic scheduling algorithms in downlink centralized wireless networks.

Abstract

As wireless spectrum efficiency is becoming increasingly important with the growing demands for wideband wireless service scheduling algorithm plays an important role in the design of advanced wireless networks. Opportunistic scheduling algorithms for wireless communication networks under different QoS constraints have gained popularity in recent years since they have potentials of achieving higher system performance. In this dissertation firstly we formulate the framework of opportunistic scheduling algorithms. Then we propose three new opportunistic scheduling schemes under different QoS criteria and situations (single channel or multiple channel). 1. Temporal fairness opportunistic scheduling algorithm in the short term. We replicate the temporal fairness opportunistic scheduling algorithm in the long term. From simulation results we find that this algorithm improves the system performance and complies with the temporal fairness constraint in the long term. However, the disadvantage of this algorithm is that it is unfair from the beginning of simulation to 10000 time slot on system resource (time slots) allocation - we say it is unfair in the short term. With such a scheme, it is possible that some users with bad channel conditions would starve for a long time (more than a few seconds) , which is undesirable to certain users (say, real-time users). So we propose the new scheme called temporal fairness opportunistic scheduling algorithm in the short term to satisfy users ' requirements of system resource in both short term and long term. Our simulation results show that the new scheme performs well with respect to both temporal fairness constraint and system performance improvement. 2. Delay-concerned opportunistic scheduling algorithm. While most work has been done on opportunistic scheduling algorithm under fairness constraints on user level, we consider users' packet delay in opportunistic scheduling. Firstly we examine the packet delay performance under the long term temporal fairness opportunistic scheduling (TFOL) algorithm. We also simulate the earliest deadline-first (EDF) scheduling algorithm in the wireless environment. We find that the disadvantage of opportunistic scheduling algorithm is that it is unfair in packet delay distribution because it results in a bias for users with good channel conditions in packet delay to improve system performance. Under EDF algorithm, packet delay of users with different channel conditions is almost the same but the problem is that it is worse than the opportunistic scheduling algorithm. So we propose another new scheme which considers both users' channel conditions and packet delay. Simulation results show that the new scheme works well with respect to both system performance improvement and the balance of packet delay distribution. 3. Utilitarian fairness scheduling algorithm in multiple wireless channel networks. Existing studies have so far focused on the design of scheduling algorithm in the single wireless communication network under the fairness constraint. A common assumption of existing designs is that only a single user can access the channel at a given time slot. However, spread spectrum techniques are increasingly being deployed to allow multiple data users to transmit simultaneously on a relatively small number of separate high-rate channels. Not much work has been done on the scheduling algorithm in the multiple wireless channel networks. Furthermore in wire-line network, when a certain amount of resource is assigned to a user, it guarantees that the user gets some amount of performance, but in wireless network this point is different because channel conditions are different among users. Hence, in wireless channel the user's performance does not directly depend on its allocation of system resource. Finally the opportunistic scheduling mechanism for wireless communication networks is gaining popularity because it utilizes the "multi-user diversity" to maximize the system performance. So, considering these three points in the fourth section, we propose utilitarian fairness scheduling algorithm in multiple wireless channel networks. Utilitarian fairness is to guarantee that every user can get its performance requirement which is pre-defined. The proposed criterion fits in with wireless networks. We also use the opportunistic scheduling mechanism to maximize system performance under the utilitarian fairness constraint. Simulation results show that the new scheme works well in both utilitarian fairness and utilitarian efficiency of system resource in the multiple wireless channel situation.