The unfairness problem among TCP connections has been proved to be very severe in the IEEE 802.11-based wireless ad hoc networks because the hidden station problem still exists and the binary exponential backoff algorithm always favors the latest successful station. In this paper, a novel protocol, neighbor-medium-aware MAC (NEMA-MAC), is proposed to improve the TCP fairness. By adding a medium (channel) state field in the head of the traditional IEEE 802.11 MAC frame, the NEMA-MAC protocol provides a communication mechanism to resolve the hidden station problem. In addition, when a collision occurs, the new backoff algorithm makes the senders cooperatively adjust the contention window according to their local and neighbors' channel usage indexes. The simulation results show that TCP sessions can acquire satisfying fairness and increase the throughput in the NEMA-MAC-based multihop ad hoc networks.

In this article, we present a scattered pilot aided channel estimation and tracking technique for MIMO-OFDM systems. First we extend Li's estimation algorithm to scattered pilot-aided case. Then a scattered pilot-aided tracking scheme is developed to track the time-varying MIMO channel. The algorithms we present reduce the system overhead and channel estimation complexity to a low level with acceptable performance degradation. Simulations of a MIMO-OFDM system with four-transmit and four-receive antennas show that the new algorithm has better performance than Li's algorithm in time-varying MIMO channels.

This paper proposes a SNR estimation scheme based on Gibbs sampler. This scheme can estimate SNR using a very short received sequence, and does not require any prior information of the transmitted symbol. Compared with the existing estimators, the performance of this method is better when real SNR is larger than 5 dB in both single path channel and multi-path channel.