Based on block-orthogonal frequency division multiple access (block-OFDMA) and distributed space-time coding, we provide a novel design for a multi-user uplink communication system with fixed wireless relay stations. The relays can decode the information symbols received from the mobile users in the system and forward this information to the base station. We do not favor a system that enforces cooperation between users and relays at all times. We propose an adaptive scheme for a Rayleigh fading environment in which users and relays cooperate depending on the average power of the channels in the network. The proposed adaptive cooperative scheme increases the performance and the flexibility of the OFDMA uplink cellular system.