We present JMB, a joint multiuser beamforming system, that enables independent access points (APs) to beamform their signals and communicate with their clients on the same channel as if they were one large MIMO transmitter. The key enabling technology behind JMB is a new low-overhead technique for synchronizing the phase of multiple transmitters in a distributed manner. The design allows a wireless LAN to scale its throughput by continually adding more APs on the same channel. JMB is implemented and tested with both software radio clients and off-the-shelf 802.11n cards, and evaluated in a dense congested deployment resembling a conference room. Results from a 10-AP software-radio testbed show a linear increase in network throughput with a median gain of 8.1–9.4×. Our results also demonstrate that JMB's joint multiuser beamforming can provide throughput gains with unmodified 802.11n cards.
Wireless spectrum is limited; wireless demands can, however, grow unlimited. Busy Wi-Fi networks, for instance, in conference rooms, hotels, and enterprises are unable to keep up with user demands,10, 24 even causing high-profile failures like the wireless network collapse during the Steve Jobs iPhone 4 keynote. Cellular networks are in a similar predicament, with their demands forecast to exceed available capacity within the next few years.20 This is not for lack of improvement in the performance of wireless devices. Indeed, individual devices have improved dramatically in recent years through innovations like multi-antenna systems, better hardware, and lower receiver noise. The problem, however, is that there is a mismatch between the way user demands scale and network throughput scales; user demands scale with the number of devices in the network but network throughput does not. Unless network throughput also scales with the number of devices, wireless networks will always find it hard to keep up with their demands, and the projected demands will keep exceeding the projected capacity.
In this paper, we present a system that enables a network to scale its throughput with the number of transmitting devices. We focus on the scenario of typical busy wireless environments such as multiple users in a conference room, enterprise, hotel, etc. We enable a wireless LAN to keep increasing its total throughput by continuously adding more access points (APs) on the same channel.