DCAP: Improving the Capacity of WiFi Networks with Distributed Cooperative Access Points

This paper presents the Distributed Cooperative Access Points (DCAP) system that can simultaneously serve multiple clients using <italic>cooperative beamforming</italic> to increase the capacity of WiFi-type wireless networks. The distributed APs are connected by Ethernet and driven by independent low-cost local oscillators. To facilitate cooperative beamforming, we address three major challenges: the phase synchronization, the channel state information (CSI) measurement, and the user selection. Specifically, we develop 1) a cooperative tracking scheme to track signal phase drifts at symbol level without adding extra hardware complexity; 2) an incremental CSI estimation mechanism that removes the per-frame CSI measurement overhead of previous approaches; and 3) a simple random user selection algorithm that scales the network capacity linearly and delivers over <inline-formula><tex-math notation="LaTeX">$70$</tex-math> <alternatives><inline-graphic xlink:href="yang-ieq1-2709743.gif"/></alternatives></inline-formula> percent performance compared to the optimal but complex greedy algorithm. We implement DCAP on the Sora software radio platform and evaluate it in a wireless network with nine nodes. Experimental results show that the cooperative beamforming is feasible in practice, and our cooperative phase tracking can ensure strict phase alignment (<inline-formula> <tex-math notation="LaTeX">$\leq$</tex-math><alternatives><inline-graphic xlink:href="yang-ieq2-2709743.gif"/> </alternatives></inline-formula> 0.03 radian) among APs during the entire beamforming period (1.2 ms). Otherwise, without tracking, phases may drift by 0.3 radian over merely 600 <inline-formula><tex-math notation="LaTeX">$\mu$ </tex-math><alternatives><inline-graphic xlink:href="yang-ieq3-2709743.gif"/></alternatives></inline-formula>s, causing that the symbol SNR decreases as large as 20 dB.