Feedback-aided pilot placement for amplify-and-forward OFDM relay links

This paper examines the use of channel feedback to determine the optimal pilot placement for channel estimation of amplify-and-forward OFDM relay links. With channel information from the previous symbol period, new pilot locations can be determined so that, on the one hand, channel estimation errors are controlled at a low level and, on the other hand, reliable subcarrier channels are made available for data transmission. The minimum effective SNR across subcarriers is a factor that dominates the symbol-error-rate (SER) at high SNR and, thus, is utilized as the performance measure in this work. First, a lower bound is derived for the achievable SNR gain, which is defined as the ratio between the effective SNR achieved with feedback and that without feedback (i.e., the equal-spaced pilot-placement scheme). The analytic results show that, with a fixed number of subcarriers, the feedback gain will grow as the number of pilot tones increases. Due to the complexity required in finding the optimal solution, a suboptimal but efficient algorithm, called the iterative pilot relocation (IPR) scheme, is proposed, where iterative exchanges between old pilot locations and the location of the worst data subcarrier are done to improve the minimum effective SNR. The efficacy of the proposed IPR scheme is demonstrated through numerical simulations.