Achievable Degrees of Freedom for the Two-Cell Two-Hop MIMO Interference Channel With Half-Duplex Relays

We consider the two-cell two-hop multiple-input-multiple-output interference channel with half-duplex relays, where each source group having M single antenna users communicates with the corresponding destination with M antennas via two relays, each having M antennas. For such a channel, by exploiting three time slots, the previously known achievable degrees of freedom (DoF) is 2M/3 regardless of whether the half-duplex relays have global channel state information (CSI) for the first hop. In this paper, we show that using n ≥ 3 time slots, the achievable DoF is (n - 1)M/n, which is higher than the previous result of 2M/3 DoF for the case of n ≥ 4. The achievability is shown by a new relaying protocol, which combines the alternate transmission strategy with an interference cancellation technique. A major implication of the derived result is that a normalized DoF of one can be achieved asymptotically without requiring global CSI at the source and relay nodes.