SPACE TIME INTERFERENCE ALIGNMENT SCHEME FOR THE MIMO BC AND IC WITH DELAYED CSIT AND FINITE COHERENCE TIME

Most techniques designed for the the multi-input multipleoutput (MIMO) Broadcast Channel (BC) and MIMO Interference Channel (IC) require accurate current and instantaneous channel state information at the transmitter (CSIT). This is not a realistic assumption because of feedback delay. A novel approach by Lee and Heath, space-time interference alignment (STIA), proves that in the underdetermined (overloaded) multi-input single-output (MISO) BC with N-t transmit antennas and K = N-t + 1 users N-t (sum) Degrees of Freedom (DoF) are achievable if the feedback delay is not too big, thus disproving the conjecture that any delay in the feedback necessarily causes a DoF loss. However the feedback delay needs to remain less or equal to T-c/N-t+1, where T-c is the coherence time. We consider the MIMO BC and show that the use of multi-antenna receivers allows to achieve full (sum) DoF with bigger feedback delay, up to T-c/N-t/N-r+1. We also extend this result to the MIMO IC.