On the capacity of cellular CDMA and TDMA over nondispersive channels

We investigate the interuser interference in cellular code-division multiple access (CDMA) and time-division multiple access (TDMA) systems and compare the two multiple access techniques at different levels of user density per cell. The study includes the interference both on the downstream channel (from base station to users) and on the upstream channel (from users to base station). The purpose is to compare the two multiple access techniques when the total bandwidth occupancy and the maximum number of users per cell are the same. The results indicate that on a nondispersive radio channel with a path loss that is proportional to the fourth power of the distance, the interuser interference is approximately 10 dB higher in CDMA_ The implication of this result, which holds for both the downstream and the upstream channels, is that for the same total bandwidth and number of users per cell, TDMA gives substantially superior bit error rate (BER) performance. This is confirmed by our numerical results evaluated for both additive white Gaussian noise (AWGN) channels and memoryless flat Rayleigh fading channels. Alternatively, CDMA needs approximately 10 times more bandwidth to achieve the same signal-to-interference (S/I) ratio, a result which may seem surprising, because the fact that CDMA employs a frequency reuse factor of I is often given as an evidence that this technique can support a higher number of users than TDMA in a given geographic area and frequency bandwidth.