unication system, even under the assumption of white Gaussian noise interference. When inter- ference is not modeled as white Gaussian noise, analysis is even more diffi- Spread-Spectrum CDMA for PCS and PCN cult. For this reason, very few expressions for bit error probability are given for interference that is not white Gaussian noise. For coherent BPSK, the bit error probability against white Gaussian noise is well known and is given by Pb = = Q(V2E6/No) (3.4) where Q(x) is the Gaussian integral function (3.5) For this ideal noise-limited case, we have the following required values of Eb/No: 4.32 dB, for Pb=10-2 = for Pb=10-3 (3.6) for Pb=10-4 9.59 dB, for Pb=10-5. By using differentially coherent detection of DPBSK (assuming, again, that the limiting factor is white Gaussian noise), the bit error probability is given by the simple expression (3.7) For this idealized case, the required Eb/No is as follows: 5.92 dB, for Pb=10-2 Eb7.93dB,forPb=10-3 (3.8) No9.3 dB,forPb= 10-4 10.34 dB, for Ph=10-5. Note that the difference between coherent detection and differentially coherent detection is less than 2 dB. The above bit error probability expressions also apply to spread-spectrum communication systems of the direct-sequence type when the basic modu- lation is BPSK and the interference is white Gaussian noise. For our spread- spectrum system, however, the performance limitation is due to interference from other similar spread-spectrum radios. In general, applications are characterized by a high density of users with mobile units roaming in an area with many microcells. This situation is ide- alized in Figure 3.8. In the S-CDMA system design, during the outbound frame, each mobile unit will be receiving signals from its assigned base sta- tion, as shown in Figure 3.6. It could also be receiving interference from neighboring base stations. For the inbound case, all the mobile units will be sending interfering signals to neighboring base stations. However, thanks to our use of orthogonal codes in the S-CDMA system, ther