On the capacity of cellular CDMA and TDMA over nondispersive channels

We investigate the interuser interference in cellular code-division multipl e access (CDMA) and time-division multiple access (TDMA) systems and compar e the two multiple access techniques at different levels of user density pe r cell. The study includes the interference both on the downstream channel (from base station to users) and on the upstream channel (from users to bas e station). The purpose is to compare the two multiple access techniques wh en the total bandwidth occupancy and the maximum number of users per cell a re the same. The results indicate that on a nondispersive radio channel wit h a path loss that is proportional to the fourth power of the distance, the interuser interference is approximately 10 dB higher in CDMA_ The implicat ion of this result, which holds for both the downstream and the upstream ch annels, 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 Gauss ian noise (AWGN) channels and memoryless flat Rayleigh fading channels. Alt ernatively, CDMA needs approximately 10 times more bandwidth to achieve the same signal-to-interference (S/I) ratio, a result which may seem surprisin g, because the fact that CDMA employs a frequency reuse factor of I is ofte n given as an evidence that this technique can support a higher number of u sers than TDMA in a given geographic area and frequency bandwidth.