Exact Outage Analysis of SIMO Relay-aided Underlay Communications with Limited Feedback

In a spectrum-sharing underlay environment, we carry out a comprehensive outage probability analysis for a secondary (unlicensed) system operating with a limited feedback from the primary (licensed) system. We focus on single-user single-input multiple-output (SIMO) secondary communications where the direct link is being assisted by a set of single-antenna decode-and-forward (DF) relay nodes acting in a half-duplex selective-and-incremental relaying mode. For practicality reasons, we consider that the secondary system transmitting nodes are only capable of acquiring a limited interference channel state information (I-CSI) of the underlying cognitive radio interference channels. Accordingly, the cumulative distribution functions (CDF) of the received signal-to-noise ratio (SNR) at the secondary receiving nodes are devised in recursive and tractable closed-form expressions. These statistics are then used to derive the exact end-to-end secondary system outage probability. Also, we investigate the asymptotic outage analysis of the secondary network, and derive the achievable diversity-and-multiplexing tradeoff (DMT). Finally, the analytical and simulation results are compared and interestingly shown to perfectly match while revealing that limited I-CSI acquisition by the secondary system is more favored than its spectrum-consuming counterpart.