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A cooperative MAC protocol with virtual-antenna array support in a multi-AP WLAN system

In this paper we propose a cooperative-aware medium access control (MAC) protocol in the newly emerged multiple access point (MAP) WLAN system, where each user can associate with multiple APs. Leveraging the multiple association feature, the spectrum efficient virtual-antenna array (VA) cooperative strategy can be utilized. However, the APs selected for VA transmission cannot serve packet transmission individually, which leads to spatial reuse inefficiency. To balance the tradeoff between spatial reuse efficiency and cooperative gain, an interference model that describes interference among cooperative transmissions should be first established. In this paper, a virtual link model is proposed, where each virtual link is composed of one end user, one combination of VA APs and one data rate to represent one cooperative transmission. Based on this model, a multi-cell virtual link scheduling problem is formulated to achieve optimal system performance. Combined with local clique searching procedure and physical carrier sensing adaptation scheme, the dual decomposition of the scheduling problem can distributively achieve the near optimal performance. Finally a VA-based cooperative MAC (V-MAC) protocol is proposed to implement the cooperative scheduling scheme. Simulation result shows V-MAC significantly improves the system throughput meanwhile guarantees the system fairness.