On Ergodic Sum Capacity of Fading Cognitive Multiple-Access and Broadcast Channels

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  • 27 June 2008
Abstract
This paper studies the information-theoretic limits of a cognitive radio (CR) network under spectrum sharing with an existing primary radio network for wireless communications. In particular, the fading cognitive multiple-access channel (C-MAC) is first studied, where multiple CRs transmit to the base station (BS) under both individual transmit-power constraints and a set of interference-power constraints each applied at one of the primary user receivers. For the fading primary and secondary user channels, we consider the long-term (LT) or the short-term (ST) transmit-power constraint over the fading states at each secondary transmitter, combined with the LT or ST interference-power constraint at each primary 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) scheme in the secondary 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. The fading cognitive broadcast channel (C-BC) that models the downlink transmissions in the secondary network is then studied under the LT/ST transmit-power constraint at the BS jointly with the LT/ST interference-power constraint at each of the primary receivers. It is shown that TDMA is indeed optimal for achieving the ergodic sum capacity of the fading C-BC for all combinations of transmit-power and interference-power constraints. Numerical results show that the proposed dynamic power and rate allocation schemes achieve substantial throughput gains compared to some fixed resource allocation schemes in fading CR networks.

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