Research On Cooperative Communication For The Future Mobile Communication Systems

Cooperative communication, in which single-antenna users share their antennas cooperatively to create a virtual multiple-input multiple-output (MIMO) system, leads to much higher data rate and more reliable services over a larger coverage. In the multiple-relay co-operative communication systems, how to utilize the appropriate relay nodes is the key point to fully exploit the cooperative diversity. In or-der to further improve cooperative diversity, this dissertation mainly focus on the cooperative beamforming scheme, relay selection scheme, and jointly combines cooperative communication with orthogonal fre-quency division multiplexing (OFDM) and MIMO techniques. The main results are summarized as follows:(1) Cooperative beamforming has previously been proven to be an efficient way to improve the cooperative diversity. This method gen-erally requires all relay nodes to participate in beamforming, which can be seen as "all participate" cooperative beamforming. However, not all relay nodes have constructive impacts on the end-to-end per-formance. Based on this observation, we propose a new cooperative scheme which only selects those "appropriate" relay nodes to perform cooperative beamforming. Such relay nodes can be determined with only mean channel gains, which reduces the complexity of obtaining global instantaneous channel state information (CSI). This scheme guarantees that energy is only allocated to the "appropriate" relay nodes, and accordingly provide superior diversity. We also prove that power allocation between source and selected relay nodes is a convex problem, which can be resolved with lower computational complexity. Simulation results demonstrate that our scheme achieves an essential improvement in terms of end-to-end performance for either optimal or limited feedback scenario, as well as high energy-efficiency for the energy-constrained networks.(2) Selection semi-blind amplify-and-forward (AF) relaying carries out transmit beamforming (TB) at source transmission and maximum ratio combining (MRC) at destination reception. We investigated the outage performance for multiple-relay cooperative networks with mul-tiple antennas deployed at source and destination nodes. Based on the Kolmogorov-Smirnov test, we first analyzed the impact of configura-tion of destination antennas and Nakagami parameter on the outage performance under arbitrary Nakagami-m fading channels. Results reveal that increasing the number of destination antennas is not nec-essary for an improvement of outage performance with any Nakagami parameter. Inspired by this fact, an approximation is proposed to the optimal selection. Simulation results demonstrate that the approxi-mation achieves a comparable performance to the optimal selection with lower implementation complexity.(3) The closed-form expression of outage probability for MIMO cooperative system with selective OFDMA relaying is presented un-der arbitrary Nakagami-m fading channels for both CSI-assisted AF relaying and semi-blind AF relaying. It can be observed that the number of subcarriers and relays has opposite impacts on the outage performance. That is, increasing subcarriers will result in degradation of the performance, whereas increasing the number of relays will lead to the improvement of outage performance. The impact of number of relays on the outage performance is first investigated based on the Kolmogrov-Smirnov test, which reveals that increasing relays is not always necessary for the improvement of outage performance. Exten-sive simulations validate the theoretical analysis, and thus this method can be used to determine the appropriate number of relay nodes in the system.(4) A selection scheme is proposed for the cooperative transmission of MIMO regenerative cooperative system. Base on the fact, capacity of the MIMO system depends on the power gain of the MIMO channel. This scheme first makes singular value decomposition (SVD) of the tow hop channels and selects single relay node to maximize the minimum-case channel power gain among all relay nodes. A distributed selection procedure is provided based on the "distributed timer" and thus makes selection scheme appropriate for the practical implementation. Based on the derived outage probability expression, a simple power allocation method is also developed for the sum power constrained network. The-oretical analysis and simulation results demonstrate that this scheme achieves a significant improvement in terms of outage performance. The distributed selection scheme is proven to have a good balance between the system performance and the implementation complexity. Therefore, it can be a promising choice for practical utilization.