Two Antennas are Better than One: A Measurement Study of 802.11n

We present a measurement study of how well 802.11n works in practice. Our goal is to understand the performance impact of the multiple antenna (spatial) techniques that are the key new capability of 802.11n. We experiment with a 10 node indoor testbed equipped with commodity Intel 802.11n NICs that have three antennas, i.e., 3x3 MIMO, and lowlevel instrumentation to let us to measure RF channel conditions. We find 802.11n to be highly effective at improving link rates. Two or three antennas improves the median rate by 2× and 2.2×, respectively. This realizes most of the predicted MIMO gains. However, we find that spatial diversity is key to realize these gains beyond 2 streams, with the 2x3 configuration outperforming the 3x3 configuration for at least three-quarters of our links. By analyzing the RF channel conditions, we conclude that additional diversity produces especially large gains because it mitigates frequencyselective fading. We believe this is the first measurement study of 802.11n with commodity wireless NICs that relates high-level performance to low-level channel effects.