Reverse link capacity of synchronous CDMA systems

CDMA is the most common multiple access scheme in third generation cellular systems. The gain from a CDMA system is increased considerably if the spreading codes are orthogonal. The reverse link (mobiles to base station) is a multipoint-to-point transmission. Hence, maintaining orthogonality between the mobile transmissions requires that the signals be received at the same time at the base station. This is not easy to achieve due to the changes in the propagation conditions between the different mobiles and the base station. In order to receive the signals from different mobiles at the same time, the base station has to send commands, known as time alignment (TA) commands, to each terminal to adjust its transmission time. We report on some channel measurements made in a dense urban mobile environment (London, UK) and conclude that synchronous transmission is also practical in these and other mobile environments with some modification to the time tracking algorithm. Then we show that S-CDMA can provide up to 60% increase in the system capacity.