Degrees of Freedom for the Constant MIMO Interference Channel With CoMP Transmission

The multiple-input-multiple-output (MIMO) interference channel (IC) is considered with constant channel coefficients, N transmit and receive antennas at each user, and coordinated multipoint (CoMP) transmission, in which each message is jointly transmitted by Mt successive transmitters. It is shown that the degrees of freedom with CoMP transmission using a two-stage scheme involving both zero forcing and interference alignment can be significantly greater than without CoMP transmission. In particular, all K users can achieve d degrees of freedom almost surely if d ≤ ⌊(2N)/(K - M_t + 1)⌋ and M_t ≤ K - 2. Additionally, the first M_t - 1 users can achieve d₁ degrees of freedom, and the last K - M_t + 1 users can achieve d₂ degrees of freedom almost surely if d₂ ≤ ⌊(2N)/(K -M_t+1)⌋, M_t ≤ K - 2, and d₁ + (K - M_t + 1) d₂ ≤ 2N. Next, the analysis of the constant MIMO IC with CoMP transmission is applied to the development of iterative algorithms to design transmit and receive vectors analogous to those without CoMP transmission. The degrees of freedom analysis is shown through simulations to provide a good guideline for choosing how many beams each user should send for the iterative algorithms. Furthermore, it is seen in the numerical results that CoMP transmission can result in greatly improved convergence speed for the iterative interference alignment algorithms.