Null-Space Expansion Applications to MIMO with Distributed Antennas

This research focuses on multiple-input, multiple-output (MIMO) signal separation in environments with changing channels, such as mobile communications. When signals are separated by combining weights that are fixed in a packet or until the next reference symbol, the orthogonality between interference signals and combining weights becomes broken due to the movement of the receiver, resulting in interference leakage. To mitigate such degradation, a null-space expansion technique is employed, assuming that there are extra antennas at the receiver side. This technique improves channel time-varying immunity by using extra receiving antennas to form a null-space that suppresses interference among streams in MIMO or inter-user interference (IUI) in multi-user MIMO. In this paper, the performance advantage of the null-space expansion technique is revealed through in-lab and field measurement campaigns.