Feedback-Aided Pilot Placement for OFDM Relay Links with Subcarrier Pairing

The advantage of using channel feedback information to determine the optimal pilot placement is examined for amplify-and-forward OFDM relay links. This work extends upon the previously proposed feedback-aided pilot placement scheme to systems that employ subcarrier pairing at the relay. With channel feedback, the pilot subcarriers can be properly chosen to reduce the channel estimation error and increase the effective signal-to-noise ratio (SNR) of data subcarriers. When subcarrier pairing is employed, the relay reselects the subcarriers used to forward the source´s data based on the effective SNR of each subcarrier, which in order is affected by the pilot placement policy. Here, the minimum effective SNR observed by all subcarriers´ data is utilized as the performance measure since it is what dominates the symbol-error-rate (SER) at high SNR. For a given pilot placement, the best subcarrier pairing scheme is found to be the inverse mapping of the source-to-relay (SR) and relay-to-destination (RD) channels based on the quality of their effective SNR. A lower bound is derived for the gain achievable with any feedback-aided pilot placement scheme. Due to the complexity of finding the optimal pilot placement, a more efficient iterative pilot relocation (IPR) scheme is then proposed. In this scheme, the subcarrier with the worst equivalent channel is used to replace one of the originally selected pilot locations in each iteration. The efficacy of the proposed policy is demonstrated through numerical simulations.