On the capacity of TDMA and CDMA for broadband wireless packet access

Studies of the capacity of cellular systems, stated in terms of the admissible number of remote users, have generally been limited to voice telephony. We address the problem of comparing the interference-limited performance of CDMA and TDMA systems in a packet switched environment. The objective is to determine whether the capacity advantages claimed for circuit-switched CDMA still apply in a packet-switched environment, where the natural time diversity of bursty transmission may be a significant factor. Under a set of specific assumptions about the wireless environment (including path loss, shadow fading, multipath delay spread, co-channel interference, power control, coding), we evaluate the number of users which can be admitted to the system while maintaining some desired quality-of-service level. Four different classes of users with different characteristics and requirements are considered. The system capacity is found to significantly depend on the QoS objectives, which might be stated in terms of availability of some specified signal to interference level, packet loss rate, or mean tolerable delay. The main finding is that strict requirements imposed on the radio access level tend to favor CDMA, whereas if some form of packet recovery at the higher layers is allowed (implying a relaxed set of requirements on the radio interface), then a somewhat higher capacity may be achieved by TDMA.