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A single source-destination pair communicating via a layer of parallel relay nodes under quasi-static slow fading environment is investigated. One existing transmission protocol is considered, namely, the combination of the distributed version of the half symbol-rate complex constellation orthogonal space-time block codes (OSTBC) with adaptive amplify-and-forward (AAF) relaying strategy. We call this transmission protocol as distributed orthogonal space-time block coded adaptive amplify-and-forward (DOSTBC-AAF). To improve the performance of DOSTBC-AAF, a distributed on-off power control (OOPC) rule applied to the relays is analytically derived and is proved to achieve full diversity order. The outage performance of DOSTBC-AAF with and without power control is evaluated. Our simulation results show that DOSTBC-AAF with all relays transmitting at full power (FP) achieves no diversity gain, whereas DOSTBC-AAF with OOPC achieves full diversity order. Correspondingly, at high signal-to-noise ratio (SNR), the diversity-multiplexing tradeoff (DMT) achieved by DOSTBC-AAF (OOPC) is analytically derived and is numerically shown to outperform DOSTBC-AAF (FP) significantly.