Exact Closed-Form Expressions for the Outage Probability and Ergodic Capacity of Decode-and-Forward Opportunistic Relaying

Exact statistics of the local signal-to-noise ratios (SNRs) of the best relay in decode-and-forward (DF) opportunistic relaying (ORe) are derived. It is observed that although the different links are assumed to suffer independent fadings, the best-relay local SNRs are dependent. Both joint and marginal statistics are determined for the general case of nonidentical SNR distributions, and a source-relay-symmetric (S-R-sym.), relay-destination-symmetric (R-D-sym.) case. Both general fading and Rayleigh fading cases are considered. Using the statistics derived, exact, closed-form expressions for the outage probability and ergodic capacity of DF ORe are calculated in the S-R-sym., RD-sym., Rayleigh fading case. The exact results for the outage probability show almost linearly increasing diversity order with the number of relays. The exact results for the ergodic capacity show a multiplexing gain almost equaling one half and a power gain increasing with the number of relays that exhibits diminishing returns.