Relay-Aided Interference Alignment for the Quasi-Static Interference Channel

In this paper, we first investigate the Degrees Of Freedom (DOF) for the M-user Interference Channel (IC) in static environments with the help of a MIMO relay. The relay stores the received signal during the first time-slot and sends a linearly-transformed version over the next time-slot. Using this scheme, it is shown that Interference Alignment can be done with much less complexity. Having very loose constraints on the channel structure, the proposed method calculates a proper choice for the relay gains such that the M-user IC is able to achieve a DOF of M/2. It is also proved that if the relay's output power scales as P/(log P)(t) where P is the power of the transmitters, there is no loss in the achieved DOF. This result is true for all positive and negative ranges of t. In other words, in an M-user IC with quasi-static channel gains, it is possible to achieve M/2 DOF through the use of a MIMO relay whose power grows at a rate slower than that of the transmitters. Similarly, a network of low-power users can benefit from our proposed method if the relay power scales faster than the power of the main transmitters. The results of this paper are valuable when being applied to a practical system. While it is very difficult to use Interference Alignment in M-user IC setup, adding a relay can make alignment more affordable by simplifying the transmitter/receiver structure.