Space division multiple access for cellular CDMA

Space division multiple access (SDMA) will form an important part of the new wideband code division multiple access (W-CDMA) standard that will realise the Universal Mobile Telephone System (UMTS). This paper addresses a few issues of importance when SDMA techniques are used in a cellular CDMA system. Firstly a theoretical analysis of a SDMA/CDMA system is performed. The analysis is focused on a single cell, multipath Rayleigh fading scenario. As system performance measure, the bit error rate (BER) is used as the criterion to investigate the influence of user location and number of antennas. An important parameter in a SDMA system is the antenna array element spacing. In our analysis a uniform linear array (ULA) is considered and a measure is defined to determine the optimal antenna element spacing in a CDMA cellular environment. Normally the mobile users in a cell are assumed to be uniformly distributed in cellular performance calculations. To reflect a more realistic situation, we propose a novel probability density function for the nonuniform distribution of the mobile users in the cell. It is shown that multipath, even with antenna arrays, reduces the system performance substantially