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This paper proposes a distributed quasi-orthogonal space-time block code (DQOSTBC) over frequency selective fading channels for amplify-and-forward cooperative wireless relay networks. The proposed DQOSTBC is designed to achieve maximal data rate, spatial diversity gain and full decoupling for a relay network with four active relays. These goals are obtained by precoding the received signals at each relay to form the quasi-orthogonal space-time block codes (QOSTBCs) as developed for multiple-input multiple-output systems. To avoid the intersymbol interference caused by the dispersive nature of frequency selective fading, the frequency domain equalizer is exploited in our design for low-complexity receiver. In addition, the simulation results prove that our design provides 4 dB improvement in terms of bit error rate as a benefit of applying the QOSTBC to cooperative networks.