Random access signaling for network MIMO uplink

Increasing popularity of mobile devices and upload-intensive applications is rapidly driving the uplink traffic demand in wireless LANs. Network MIMO (netMIMO) can potentially meet the demand by enabling concurrent uplink transmissions to an AP cluster (APC) comprised of multiple access points. NetMIMO's PHY-layer communication algorithms have been well explored, but the MAC-level signaling procedure remains an open issue: prior to uplink transmission, a group of clients must gain channel access, and ensure synchronization and channel orthogonality with each other. But such signaling is fundamentally challenging, because netMIMO clients tend to be widely distributed and may not even sense each other. In this paper, we introduce the first signaling protocol, called NURA, to meet the challenge. NURA clients employ a novel medium-access-signaling mechanism to realize group-based random access and synchronization, without disturbing ongoing uplink transmissions. The APC leverages a lightweight user-admission mechanism to group users with orthogonal channels (and hence high uplink capacity), without requiring costly channel-state feedback from all users. We have implemented NURA on a software-radio based netMIMO platform. Our experiments show that NURA is feasible, efficient, and can readily serve as the a priori signaling mechanism for distributed asynchronous netMIMO clients.