Spreading code assignment techniques for MIMO-CDMA systems

Abstract

A MIMO-CDMA system with Nt transmit antennas can transmit Nt symbols on a given signaling interval, and those N t symbols are referred to as a message vector. In conventional MIMO-CDMA systems, a message group is spread by using orthogonal direct sequence spreading codes. In this thesis, less known and recently introduced the parity-bit-selected spreading and the permutation spreading techniques are used for MIMO-CDMA system. This thesis develops the closed form theoretical expressions for parity-bit-selected spreading and permutation spreading techniques. We also introduce new design strategies for permutation spreading technique. In the MIMO-CDMA system employing parity bit selected spreading, rather than assigning each transmit antenna with different spreading sequences, the message vector is input to a systematic block encoder whose output parity bits select the spreading sequence to be used. In other words, with a given message vector, all the transmit antennas would use the same spreading sequence. In the MIMO-CDMA system using permutation spreading, rather than using the parity bits to select one spreading sequence, the parity bits are used to select Nt different spreading sequences from a set of spreading sequences. A unique permutation of spreading sequences is assigned for different parity bits. Compared to MIMO-CDMA systems employing conventional spreading method, the simulation results show that, in frequency-nonselective Rayleigh fading channel, parity bit selected spreading improves the system performance, while permutation spreading can further improve the system performance in both single-user and multi-user environments.