OFDM-MAC algorithms and their impact on TCP performance in next generation mobile networks

Abstract

With the increasing adoption of wireless technology, it is reasonable to expect an increase in the demand for supporting both real time multimedia and high rate reliable data services. Next generation wireless systems employ Orthogonal Frequency Division Multiplexing (OFDM) at the physical layer owing to the high data rate transmissions possible without an increase in bandwidth. Toward improving the performance of these systems, we look at the design of resource allocation algorithms at the medium access layer and their impact on higher layers. While TCP performance has been extensively studied with respect to interaction with link layer ARQ, little attention has been given to the interaction of TCP with the MAC layer. In this work, we explore cross layer interactions in an OFDM based wireless system, specifically focusing on channel aware resource allocation strategies at the MAC layer and their impact on TCP congestion control. Both efficiency oriented and fairness oriented MAC resource allocation strategies were designed for evaluating the performance of TCP. The former schemes exploit channel diversity to maximize system throughput, while the latter aim to provide fair resource allocation over sufficiently long time durations. First, we evaluate the resource allocation schemes from a TCP goodput standpoint and show that the class of MAC algorithms that incorporate a fairness metric and consider the backlog outperform the channel diversity exploiting schemes. Second, we introduce a cross layer strategy of using the congestion window instead of backlog at the base station resource scheduler. We compare the schemes previously designed-with and without cross layer feedback-and show that the pure cross layer strategy of using the congestion window is not warranted, as the MAC algorithms considering the queue length at the base station produced good results.