Multi-group multi-way relaying with reduced number of relay antennas

In this paper, multi-group multi-way relaying is considered. There are L groups with K nodes in each group. Each node wants to share d data streams with all the other nodes in its group. A single MIMO relay assists the communications. The relay does not have enough antennas to spatially separate the data streams. However, the relay assists in performing interference alignment at the receivers. In order to find the interference alignment solution, we generalize the concept of signal and channel alignment developed for the MIMO Y channel and the two-way relay channel to group signal alignment and group channel alignment. In comparison to conventional multi-group multi-way relaying schemes [1, 2], where at least R ≥ LKd - d antennas are required, in our proposed scheme, exploiting the multiple antennas at the nodes, only R ≥ LKd - Ld antennas are needed. The number of antennas required at the nodes to achieve this is also derived. It is shown that the proposed interference alignment based scheme achieves more degrees of freedom than the reference schemes without interference alignment.