Capacity of the MIMO interference channel to within a constant gap

The capacity region of the 2-user multi-input multi-output (MIMO) Gaussian interference channel (IC) is characterized to within a constant gap that is independent of the signal-to-noise ratio (SNR) and all channel parameters. The general MIMO IC is considered with an arbitrary number of antennas at each node. For a class of MIMO ICs characterized by the relation between the numbers of antennas at the different nodes this gap is strictly smaller than the gap in a previous result obtained by Telatar and Tse. For instance, the gap for the SIMO IC with single antenna transmitters and N-antenna receivers obtained here is 1 bit, instead of N bits. Moreover, a simple (or universal) and an explicit achievable coding scheme are given here that have the constant-gap-to-capacity property. Consequently, explicit upper and lower bounds to the capacity region are obtained. A reciprocity result is also proved which is that the capacity of the reciprocal MIMO IC is within a constant gap of the capacity region of the forward MIMO IC.