Channel-adaptive frequency-domain relay processing in multicarrier multihop transmission

Conventionally, a memoryless analog repeater at the relay of a multihop transmission system amplifies the signal received from its incoming link, and retransmits the amplified signal to its outcoming link. In the frequency domain, such an amplification essentially is ideal bandpass filtering, treating all the frequency components uniformly. For multicarrier systems like orthogonal frequency division multiplexing (OFDM) over frequency-selective channels, such a frequency-flat amplification is inadequate to exploit the benefits of adaptive processing at the relay. This paper analyzes the potential performance gain of non-uniform frequency-domain relay amplification, in which the gain coefficients for subcarriers are adapted from the frequency responses of both the incoming and outcoming links. An end-to-end achievable rate optimization problem is formulated. A simple heuristic power allocation algorithm is proposed. Numerical results indicate that the heuristic algorithm achieves considerable performance gains compared to conventional amplify-and-forward relay processing.