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 cooperative beamforming 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 70 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 (≤ 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 μs, causing that the symbol SNR decreases as large as 20 dB.

1 Introduction

Decentralization nature of WiFi networks contributes to the pervasive application of WiFi but also makes WiFi inteference-limited. With the popularization of smart-phones and tablets, more and more applications rely on WiFi to deliver media-rich contents, resulting in the exponentially growing of wireless traffic demand. Intuitively, increasing the number of WiFi access points (APs) will accordingly increase the network capacity. However, the capacity of WiFi networks is mainly limited by interferences. Therefore, deploying more APs does not guarantee the improvement of the total network throughput; even worse, it may decrease the network throughput because these transmitters may contend with each others and cause mutual interferences.