Achievable Sum Rate and Degrees of Freedom of Opportunistic Interference Alignment in MIMO Interfering Broadcast Channels

In this paper, the sum rate of opportunistic interference alignment (OIA) is analyzed in multiple-input-multiple-output interfering broadcast channels. The alignment metric upon which users are scheduled is based on the chordal distance between certain interfering subspaces at each receiver, and the closed-form expressions for the rates of the scheduled users are derived. Furthermore, we show that for a system in which each user hasN receive antennas and the jth base station transmits d(j) data streams, where Sigma(I)(j=1) d(j) = N+1 and N >= 2, the rate for each user can be approximated by the mean of a Gumbel random variable. Further analysis reveals that if the number of users in cell i scales as rho(alpha), where rho is the normalized transmit power and alpha is an element of [0, 1], then cell i can achieve alpha d(i) degrees of freedom. The simulation results confirm the validity of the theoretical analysis and the accuracy of the approximation. Thus, the sum rate analysis provided herein is an effective performance evaluation method for multi-cell OIA.