Spatial multiple access holds the promise to boost the capacity of
wireless networks when an access point has multiple antennas. Due
to the asynchronous and uncontrolled nature of wireless LANs,
conventional MIMO technology does not work efficiently when
concurrent transmissions from multiple stations are uncoordinated.
In this paper, we present the design and implementation of a crosslayer
system, called SAM, that addresses the challenges of enabling
spatial multiple access for multiple devices in a random access network
like WLAN. SAM uses a chain-decoding technique to reliably
recover the channel parameters for each device, and iteratively
decode concurrent frames with misaligned symbol timings
and frequency offsets. We propose a new MAC protocol, called
CCMA, to enable concurrent transmissions by different mobile stations
while remaining backward compatible with 802.11. Finally,
we implement the PHY and MAC layer of SAM using the Sora
high-performance software radio platform. Our evaluation results
under real wireless conditions show that SAM can improve network
uplink throughput by 70% with two antennas over 802.11.