ions. The Chernoff bound derived here applies to the outbound channel in the S-CDMA system examined in Section 3.6.3, a bound that depends only on the assumption that data bits on all radios in the network are independent binary random variables. There is no assumption about noise or equivalent noise models. Inbound Channel We now consider the receiver at a base station that is coherently demodu- lating a DS/BPSK signal from a mobile unit. This mobile unit uses a unique spreading codeword to transmit digital voice with TDD bursts to the base station. We assume that all mobile units in active communication with the same base station are synchronized in time, use orthogonal codewords, and, therefore, do not interfere with each other at the base station. Thus, as shown in Figure 3.7, each base station receives interference only from the mobile units of neighboring base stations. This base station samples the output of a matched filter corresponding to the mobile unit's codeword. Here, interference is caused by mobile units Appendix 3B. Error Bounds for Interference-Limited Channels transmitting in other microcells. The interference is given by (3B.19) where Ck is the cross-correlation term for the interfering codewords with the codeword of the mobile unit transmitting to the base station. The random data bits modulating these interfering signals are given by ak, while the ran- dom phase terms are given by All these terms are assumed to be inde- pendent random variables, with phase random variables having uniform probability density over all phases. Recall that from the Chernoff bound we have the term (3B.20) where (3B.21) is the modified Bessel function of order 0. Since k! > 1, we have the sim- ple bound = exx2 (3B.22) Using this inequality in the previous equation results in (3B.23) As in the previous section, we define J as the sum of the squares of all the interfering cross-correlation terms. Here, it is defined as Jo = Cc (3B.24) Commercial Applications The bit error probability is bounded by (3B.25) Again, cho