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Using relays in wireless networks can potentially lead to significant capacity increases. However, within an asynchronous multi-user communication setting, relaying might cause more interference in the network, and significant sum-rate deterioration may be observed. In this work the effect of cooperation in an interference limited, narrow-band wireless network is investigated. It is crucial to determine the optimal trade-off between the amount of throughput gain obtained via cooperation and the amount of interference introduced to the network. We quantify the amount of cooperation using the notion of a cooperative region for each active node. The nodes which lie in such a region are allowed to cooperate with the source. We adopt the decode-and-forward scheme at the relays and use the physical interference model to determine the probability that a relay node correctly decodes its corresponding source. Through numerical analysis and simulation, we study the optimal cooperative region size to maximize the network sum-rate and energy efficiency, based on network size, relay availability, node decoding threshold, and destination reception capability. It is shown that optimized system performance in terms of the network sum-rate and the power efficiency is significantly improved compared with cases where relay nodes are not exploited or where the cooperative region size is suboptimal.