Degrees of Freedom of the K User M × N MIMO Interference Channel

We provide inner bound and outer bound for the total number of degrees of freedom of theKuser multiple-input multiple-output (MIMO) Gaussian interference channel withMantennas at each transmitter andNantennas at each receiver if the channel coefficients are time-varying and drawn from a continuous distribution. The bounds are tight when the ratio [(max(M,N))/(min(M,N))]=Ris equal to an integer. For this case, we show that the total number of degrees of freedom is equal to min(M,N)KifK≤Rand min(M,N)[(R)/(R+1)]KifK>R. Achievability is based on interference alignment. We also provide examples where using interference alignment combined with zero forcing can achieve more degrees of freedom than merely zero forcing for some MIMO interference channels with constant channel coefficients.