Abstract: Due to its large bandwidth and the complexity of indoor environment, the UWB channel is frequency selective, which affects the data rate and reliability of UWB communi- cation. This paper proposes an amplify-forward relay scheme to mitigate this reliability problem. The relay can be used to acquire capacity gain and diversity gain just like MIMO. The first investigation focuses on the relay-node selection criterion and combination ration for capacity gain. Capacity gain depends on the correlation of frequency responses between direct link and relay link in addition to relay link SNR. Increased similarity of the two frequency responses increases capacity gain. Capacity gain’s best relay and combination ratio are found by taking the derivative of the capacity equation. Solution of the quadratic derivative equation of ratio finds the optimal ratios for every potential relay node. The second investigation focuses on relay- node selection criterion and combination ratio for diversity gain respectively.Higher diversity gain corresponds to less-deep-faded subchannels of combined signal. Reduced computation divides the problem into two steps: The first step selects the relay node according to the criterion that is dependent on the average of SNR(f) and variance of SNR(f) . The second step solves the combination ratio-optimization problem for optimal ratio. Numerical simulation shows that our scheme is effective in pursuit of capacity gain and of diversity gain.

Key words: Cooperative communication, amplify-and-forward, relay selection, combination ratio, OFDM-UWB