We derive the exact maximal-ratio combining (MRC) performance of dual-hop cooperative diversity systems with L-antenna destination reception in Nakagami-m fading channels, where a single-antenna relay operates in fixed-gain (semi-blind) amplify-and-forward (AF) mode. We also show that if m0, m1, and m2 are the respective Nakagami parameters for the source-to-destination, source-to-relay, and relayto-destination links, the fixed-gain AF cooperation with MRC achieves the diversity order of m0L + min {m1, m2L}.