Scheduling in WiMAX based wireless networks

Abstract

This thesis describes two transmission scheduling algorithms and a hybrid ARQ transmission handling algorithm designed for WiMAX based wireless networks. WiMAX networks are based on Orthogonal Frequency Division Multiple Access (OFDMA) technique. In OFDMA based wireless networks, the bandwidth is divided into narrow sub-divisions, each of which is called a sub-carrier. Sets of these sub-carriers are allocated to users. The first algorithm is based on graph theoretic techniques and is employed to schedule user transmissions by assigning selective OFDMA subcarriers. This algorithm allocates resources based on the instantaneous channel properties like signal to interference and noise ratios experienced by users. The second scheduling algorithm deals with transmission scheduling in WiMAX based hybrid wireless networks. An important application of WiMAX technology is to serve as backbone to connect to the internet for 802.11 networks. The proposed scheduler aims at minimizing the delay experienced by 802.16 users due to increase in the number of 802.16 users. The design and implementation of these algorithms is discussed and the results are summarized in the subsequent chapters. Hybrid Automatic Repeat Request (HARQ) is a technology used to enhance the correctness in wireless transmissions. HARQ efficiently uses both forward error correction and error detection techniques in order to provide better throughput performance in poor signal quality conditions. The effectiveness of this technology in Orthogonal Frequency Division Multiple Access (OFDMA) based wireless networks is studied. The proposed HARQ algorithm assigns sub-carriers for retransmissions and schedules them in time. The algorithm is tested for certain applications such as video, VoIP that pose stringent limits on delay and jitter. These applications have critical delay requirements especially on retransmitted packets, as delayed out-of-sequence packets are generally ignored. In the proposed algorithm, a timestamp is attached to the retransmission request and its value is determined based on delay requirements. The OFDMA scheduler allocates sub-carriers and time offsets to these packets based on channel properties and the timestamp value. In the case when the timestamp value is less than the estimated propagation time, the retransmission is aborted. Therefore, the traffic load introduced by unnecessary retransmissions is reduced and the throughput performance of the system can be improved. This thesis report explains the importance of effective transmission scheduling in WiMAX networks with the help of these three cases.