MU-MIMO boosts the capacity of wireless LANs by transmitting data streams to multiple stations (STAs) concurrently, thus scaling up the achievable data rate by a factor equal to the number of antennas on the Access Point (AP). For an AP to use this technique, it must first estimate the Channel State Information at the Transmitter (CSIT) between each of its transmit antennas and each receiving antenna through a method termed channel sounding. The estimated CSIT is then used to compute precoding weights for the multi-stream transmitter. CSIT can also be used for resource allocation, such as user grouping and inter-cell interference mitigation. However, the explicit channel sounding mechanism introduced by standards such as IEEE 802.11ac incurs an overhead for CSIT estimation which increases with the number of transmit antennas at the AP and the number of aggregate STA antennas. This overhead can significantly mitigate the performance gains of MU-MIMO.
We demonstrate elimination of explicit channel sounding altogether via purely opportunistic channel sounding in which CSIT is implicitly estimated from each received uplink transmission, whether a data or control frame. The opportunistic policy eliminates CSIT sounding overhead if the channel remains sufficiently unchanged between uplink transmissions. Our scalable protocol enables a network to achieve the performance gains of a multi-antenna system.