Spreading sequence assignment in the downlink of OFCDM systems using multiple transmit antennas

In communication systems based on code-division multiple access, a proper assignment of spreading sequences can help in reducing the multiple access interference. In this paper, an optimized assignment of spreading sequences is proposed for downlink orthogonal frequency and code division multiplexing (OFCDM) systems applying transmit beamforming at the base station. Assignment involves a selection of the needed subset of spreading sequences and a distribution of these selected sequences according to the spatial signature of mobile terminals. A general assignment rule is exposed for any kind of sequence, which requires some computational effort. A very simple assignment rule is then proposed in the particular case of Walsh-Hadamard sequences. Simulation results over realistic transmission scenarios emphasize the necessity to optimize the assignment of spreading sequences as well as the efficiency of the proposed solution whatever the system load and the beamforming efficiency.