On ergodic sum capacity of fading cognitive multiple-access channel

This paper studies the information-theoretic limits of a cognitive radio (CR) network under spectrum sharing with an existing primary radio (PR) network for wireless communications. In particular, the fading cognitive multiple-access channel (C-MAC) is studied, where multiple CR users transmit to the base station (BS) of the CR network under both individual transmit power constraints and a set of interference-power constraints each applied at one of the PR receivers. For the fading PR and CR channels, we consider the long-term (LT) or the short-term (ST) transmit-power constraint over the fading states at each CR transmitter, combined with the LT interference-power constraint at each PR receiver. In each case, we characterize the ergodic sum capacity of the corresponding fading C-MAC, which is defined as the maximum sum-rate of CR users averaged over all the fading sates. Furthermore, we derive in each case the conditions for the optimality of the time-division-multiple-access (TDMA) in the CR network, i.e., when it is optimal to schedule at most one CR user for transmission at each time to achieve the ergodic sum capacity of the fading C-MAC.