Ergodic and outage capacities of relaying channels in spectrum-sharing constrained systems

This paper investigates the capacity of multiple relay channels in different fading and shadowing environments under spectrum-sharing constraints. We consider that a secondary user (SU) is allowed to share the spectrum band with a primary user (PU) provided that the SU’s transmit power remains below an interference power threshold set by the PU. Considering a scenario where the SU’s transmitter and receiver cannot communicate directly, a relay node, chosen among a set of K terminals, helps transmitting data from the SU’s transmitter to the destination. The SU’s transmitter and chosen relay node adapt their corresponding transmission parameters so as to satisfy the interference-power constraint at the PU’s receiver. We derive closed-form expressions for the ergodic capacity of the SU’s channel in Rayleigh fading, Nakagami−m fading and Lognormal shadowing environments. We further obtain the outage capacity assuming the aforementioned environments and the above-mentioned spectrum-sharing limitations. Numerical results are provided to reinforce our theoretical derivations. Index Terms Cognitive radio, relay systems, spectrum sharing, ergodic capacity, outage capacity, opportunistic relaying, fading, shadowing. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada under Research Grant RGPIN 222907. Part of this work has been presented at IEEE International Conference on Telecommunications, Doha, Qatar, Apr. 2010.