Spread space-spectrum multiple access

In this paper; a new multiple access scheme suitable for the forward link transmission in a multiuser MIMO system is investigated. Unlike traditional MIMO multiple access schemes, which rely on orthogonal-temporal channels (e.g. TDMA), the proposed scheme, namely the spread space-spectrum multiple access (SSSMA), exploits the space-domain for multiuser multiplexing. As its name suggests, SSSMA utilizes the available degrees of freedom offered by the spread-spectrum and those by the multiple transmit antennas to perform multiple access. At the base-station transmitter, each coded data stream corresponding to a unique user-channel is modulated with a user-specific two-dimensional spreading sequence and added together with other channels' modulated signals. At each user's receiver, the multiple-access-interference (MAI) generated from the same base station is mitigated through iterative multiuser detection and decoding. We focus on the performance of SSSMA in two different environments: (1) local point-to-multipoint network such as central access LAN; and (2) a power-controlled cellular system. It is shown that not only does the SSSMA offer near-theoretic-capacity performance in both environments, it is able to exploit a new form of diversity, namely the space-interferers diversity, in an ideal power-controlled network while other MIMO multiple access schemes fail to do so.