Channel-aware multi-user uplink transmission scheme for SIMO-OFDM systems

The problem of medium access control （MAC） in wireless single-input multiple-output-orthogonal frequency division multiplexing （SIMO-OFDM） systems is addressed. Traditional random access protocols have low overheads and inferior performance. Centralized methods have superior performance and high overheads. To achieve the tradeoff between overhead and performance, we propose a channelaware uplink transmission （CaUT） scheme for SIMO-OFDM systems. In CaUT, users transmit requestto-send （RTS） at some subcarriers whose channel gains are above a predetermined threshold. Using the channel state information provided by RTS, access point performs user selection with receive beamforming to decide which users can access and then broadcasts the selection results via clear-to-send （CTS） to users. We present a distributed power control scheme by using a simple fixed modulation mode. We optimize the modulation order and channel gain thresholds to maximize the separable packets subject to the bit-error-rate （BER） and temporal fairness requirements and the individual average transmit power constraints. The performance of CaUT scheme is analyzed analytically and evaluated by simulations. Simulation results show that CaUT can achieve more significant throughput performance than traditional random access protocols.

Cooperative multiple access channels: Achievable rates and optimal resource allocation

We investigate the multiple access channels （MAC） where sources can cooperate via half-duplex relaying and refer to it as cooperative MAC channels （CMAC）. Assuming perfect channel state information （CSI） at the transmitters and the receivers, we determine the bounds on the achievable rate region of a Gaussian CMAC channel and an inner bound on the outage capacity region of a fading CMAC channel. Based on superposition modulation, a half-duplex cooperative relay scheme with optimal resource allocation is proposed to achieve the bounds of capacity region. Analytical results and simulation results show that the achievable rate region of a Gaussian CMAC channel is larger than that of a Gaussian MAC channel with direct transmission （DT） schemes. But they have the same achievable sum rate. Moreover, the proposed scheme can provide higher outage capacity region than DT schemes in a fading MAC channel due to the fact that sources can share the resources with each other to reduce outages.