Channel-Adaptive Frequency-Domain Relay Processing in Multicarrier Multihop Transmission Systems

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 outgoing link. In the frequency domain, such an amplification essentially is an 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 outgoing links. The end-to-end achievable rate optimization problem is formulated and shown to lack the desired concavity property and thus is not amenable to Karush-Kuhn-Tucker approaches like water-filling. A heuristic power allocation and relay processing algorithm is proposed, which adaptively selects and matches a subset of the subcarriers for each link, according to the realization of the link frequency responses. Numerical results show that the heuristic algorithm achieves considerable performance gains, compared to conventional amplify-and-forward relay processing without adaptive power allocation