PACKET CDMA VERSUS DYNAMIC TDMA FOR MULTIPLE-ACCESS IN AN INTEGRATED VOICE DATA PCN

This paper presents a comparative evaluation of dynamic TDMA and spread-spectrum packet CDMA approaches to multiple access in an integrated voice/data PCN environment. After briefly outlining a cellular packet-switching architecture for voice/data PCN systems, dynamic TDMA and packet CDMA protocols appropriate for such traffic scenarios are described. Simulation-based network models which have been developed for performance evaluation of these competing access techniques are then outlined. These models are exercised with example integrated voice/data traffic models to obtain comparative system performance measures such as channel utilization, voice blocking probability, and data delay. Operating points based on typical performance constraints such as voice blocking probability less-than-or-equal-to 0.01 (for TDMA), voice packet loss rate less-than-or-equal-to 10(-3) (for CDMA), and data delay less-than-or-equal-to 250 ms are obtained. The results demonstrate the net operating bandwidth efficiencies in the range of 0.1 bits/Hz/cell achieved by conventional TDMA (with a fixed reuse factor of 7 and no power control); corresponding values for direct-sequence CDMA (with perfect power control) are in the region of 0.2-0.4 bits/s/Hz/cell for channel propagation constant (gamma) varying between 2.5 and 4.0. Overall, it is shown that while CDMA typically does provide a 2:1 or greater capacity advantage over TDMA, the gains are sensitive to the assumed propagation model and are generally associated with higher data delay for long messages generated by file transfer and multimedia applications.