On the capacity of DS/CDMA cellular systems employing power control and adaptive transmission rate

The capacity of a CDMA network is an extremely important issue in terms of its economic viability. This capacity is strongly dependent on the control of various interference factors. The most important of these factors is that of power control. In a cellular network the received signal strength depends on three propagation loss factors: free space loss (a function of distance), shadowing, and multi-path propagation. If the mobile's received power at the base station with which it is communicating is constant, then the power control is said to be perfect. The multipath fading will have no effect on the system capacity when power control (PC) is perfect and the system has only one cell. In multiple cell systems, however, tracking the multipath fading of users in other cells increases the intracell interference and hence reduces the system capacity. In this thesis, we investigate the system capacity of DS/CDMA systems under perfect power control when the fading channel consists of 'M' equal strength resolvable Rayleigh paths. We investigate the effect of changing the number of the fading resolvable paths on the system capacity. The effect of soft handoff and the effect of limiting the maximum increase in power to compensate for multipath fading are also determined. If there is a line of sight between the mobile and the base station, less deep fades will be encountered and the system capacity will be higher. Thus, we find the system capacity for the case of Rician/Rayleigh fading channels. If we assume the users to be of data type, then during low fade periods, the transmission rate can be decreased instead of increasing the transmitted power. This results in lowering the user's transmission rate but reduces the interference. We show that such a scheme can result in a considerable increase in the system total average transmission rate. It is known that the required signal to interference ratio (SIR) is a function of the user's velocity where slow moving users require a lower SIR. By equalizing the received power from all users, slow users performance will be better than fast users. To guarantee that all users performance is acceptable, the performance of fast users is investigated when determining the system capacity. We propose a scheme where the required signal power from a user is determined according to its velocity. This lowers the powers of slow users and hence reduces interference which increases the capacity. The error in the received signal power is usually modeled as a lognormal variable with a standard deviation that is a function of the user velocity. We show a large difference in this standard deviation depending on whether or not the user's power is controlled by the base station where the interference is measured or by a different base station. We use the error statistics to investigate the system Erlang capacity and investigate the system performance when the transmission rate of data users is determined according to the number of users in the system. The effect of imperfect power control on such an adaptive rate scheme is also investigated.