The Interference Channel Revisited: Aligning Interference by Adjusting Antenna Separation

It is shown that, in a line-of-sight setting, a receiver equipped with two antennas may approximately null an arbitrary large number of spatial directions to any desired degree, while maintaining the interference-free signal-to-noise ratio, by judiciously adjusting the distance between the antenna elements, provided that the spacing can be made sufficiently large. The main theoretical result builds on ergodic theory. The practicality of the scheme at moderate signal-to-noise values is demonstrated for a scenario where each transmitter is equipped with a single antenna and each receiver has two receive chains and where the desired spacing between antenna elements is achieved by selecting the appropriate antennas from a large enough linear antenna array. We further extend the proposed scheme to show that interference can be nearly eliminated also in specular multipath channels as well as multiple-input multiple-output interference channels where a single additional receive antenna suffices to approximately align all interferers into a one-dimensional subspace. To demonstrate the performance of the scheme, we show significant gains for interference channels with four as well as six users, at low to moderate signal-to-noise ratios (0\text{ - }20 dB). The robustness of the proposed technique to small channel estimation errors is also explored.