  Interference alignment (IA) has been shown to achieve the maximum achievable
degrees of freedom in the interference channel. This results in sum rate
scaling linearly with the number of users in the high signal-to-noise-ratio
(SNR) regime. Linear scaling is achieved by precoding transmitted signals to
align interference subspaces at the receivers, given channel knowledge of all
transmit-receive pairs, effectively reducing the number of discernible
interferers. The theory of IA was derived under assumptions about the richness
of scattering in the propagation channel; practical channels do not guarantee
such ideal characteristics. This paper presents the first experimental study of
IA in measured multiple-input multiple-output orthogonal frequency-division
multiplexing (MIMO-OFDM) interference channels. Our measurement campaign
includes a variety of indoor and outdoor measurement scenarios at The
University of Texas at Austin. We show that IA achieves the claimed scaling
factors, or degrees of freedom, in several measured channel settings for a 3
user, 2 antennas per node setup. In addition to verifying the claimed
performance, we characterize the effect of Kronecker spatial correlation on sum
rate and present two other correlation measures, which we show are more tightly
related to the achieved sum rate.
