Optimal Timing at the Relay in OFDM Based Two Way Relay Systems
In amplify and forward (AF) two way relay networks (TWRN), two sources are allowed to transmit simultaneously in the multiple access time slot. Signal misalignment at the relay occurs due to the inaccurate time slot synchronization and different propagation delay. In broadband wireless communications, the misalignment could cover several symbol intervals. In orthogonal frequency division multiplexing (OFDM) based TWRN, such signal misalignment raises question on how to choose the fast Fourier transform (FFT) window at the relay node in order to minimize the interference which will be forwarded to the sources. In this paper, the timing issue of the received superimposed signal at the relay for both sources in OFDM based AFTWRN is investigated. The optimal timing that minimizes the total interference power at the relay is studied. The imperfect timing induced interference power at each timing point is derived. An efficient estimator is proposed for the relay to decide where to establish the FFT window boundary in order to introduce the minimum interference plus noise power based on the superimposed training block from the two source nodes. The estimator is a sliding window estimator measuring the total interference plus noise power at each timing position, and the timing position which minimizes the metric function is the optimal position to establish the FFT window. Finally, the performance of the proposed estimator is evaluated by computer simulation in the presence of different amount of signal misalignment.
ofdm two way relay network synchronization amplify-and-forward