Adaptive modulation is used in the new generation of wireless systems to increase the system throughput and significantly improve spectral efficiency by matching parameters of the physical layer to the time-varying fading channels. Most high-rate applications for such wireless systems rely on the reliable service provided by TCP protocol. The effect of adaptive modulation on TCP throughput is investigated. A semi-Markov chain model for TCP congestion/flow control behavior and a multi-state Markov chain model for Rayleigh fading channels are used together to derive the steady state throughput of TCP Tahoe and Reno. The theoretical prediction based on our analysis is consistent with simulation results using the network simulator NS2. The analytical and simulation results triggered the idea of cross-layer TCP protocol design for single-user scenarios. The fading parameters of wireless channels detected in the physical layer can be used to dynamically tune the parameters (such as packet length and advertised receiver window size) of the TCP protocol in the transport layer so that TCP throughput is improved.
For multi-user scenarios, we study how multi-user diversity can be used to improve the aggregate TCP throughput of base stations in fading channels. Since TCP performance involves complex interactions among layers of the networking protocol stack, the cross-layer design approach is adopted to tackle the problem. The performance improvement is achieved through channel-aware packet scheduling algorithms and active delay of TCP ACK packets in the buffer. Based on the adaptive modulation information from the physical layer, the advertised receive window size of TCP ACK packets is dynamically changed to accommodate the rate changes resulting from adaptive modulation. Our simulation results show that the new cross-layer approach increases TCP throughput.