Degrees of Freedom of the MIMO Interference Channel With Cooperation and Cognition

In this paper, we explore the benefits, from the perspective of degrees of freedom (DOF), of user cooperation and cognitive message sharing for a two-user multiple-input multiple-output (MIMO) Gaussian interference channel with M ₁, M ₂ antennas at transmitters and N ₁, N ₂ antennas at receivers. For the case of user cooperation (including cooperation at transmitters only, at receivers only, and at transmitters as well as receivers), the sum DOF is min{M ₁+M ₂, N ₁+N ₂, max(M ₁, N ₂), max(M ₂, N ₁)} , which is the same as the sum DOF of the channel without cooperation. For the case of cognitive message sharing, the sum DOF is min{M ₁+M ₂, N ₁+N ₂, (1-1_T2)((1-1_R2) max(M ₁, N ₂) + 1_R2 (M ₁+N ₂)) + 1_T2(M ₁+M ₂), (1-1 _T1)((1-1_R1) middotmax(M ₂, N ₁) + 1_R1 (M ₂+N ₁)) + 1_T1(M ₁+M ₂)} where 1_Ti = 1 (0) when transmitter i is (is not) a cognitive transmitter and 1Ri is defined in the same fashion. Our results show that while both techniques may increase the sum capacity of the MIMO interference channel, only cognitive message sharing can increase the sum DOF. We also find that it may be more beneficial for a user to have a cognitive transmitter than to have a cognitive receiver.