User-cooperation has been studied extensively in the literature. This mechanism achieves many of the same gains that can be had by using multi-antenna (MIMO) communications in applications where the size of a node is limited. Existing analysis assumes that the system is scheduled; both the relay knows to listen when a packet is being sent. We present analysis of a class of random-access cooperative systems and show that the same performance as the scheduled systems can be achieved as long as certain requirements are met in the packet detection scheme of the relay. In particular, we find that a static decision threshold on energy detection results in a cooperative network that performs asymptotically no better than a simple point-to-point network with no relay at all. However, a decision threshold that dynamically shifts with average SNR allows the system to achieve full spatial diversity.
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