s. All rights reserved. Any use is subject to the Terms of Use as given at the website. CDMA Principles for Multicellular Systems Capacity and Quality Principles voice calls. This has the effect of doubling the maximum I/S ratio the voice CDMA chan- nel can allow, the value of a. We have to be a little bit careful because the equations were based on a constant channel, and VAD creates a varying channel, so there is a sta- tistical distribution of interference, which is a variation factor that needs to taken into account. As long as each voice call is a reasonably small fraction of the CDMA carrier, we are probably safe ignoring that extra variation. AMR is not the same as VAD because it depends on the radio environment rather than on subscriber's voice. This means that we can figure the system will be at its low- est AMR bit rate when the radio conditions are most hostile. In short, we can base our choice of a on the lowest bit rate available in AMR. The presence of other CDMA subscriber traffic, circuit-switched and packet data, can be thought of as adding users to the equations with a multiplication factor. If a sub- scriber is using circuit-switched Integrated Services Digital Network (ISDN) data ser- vice with 20 times the average voice data rate and double the Eb/No requirement, then that channel is the equivalent of 40 voice users. As long as each packet data user is us- ing a reasonably small fraction of the total CDMA resource, less than one-third, then we should be able to incorporate their average usage into the equations. However, we do have to keep an eye open for the much more variable usage pattern of a packet data user. 28.2.2 Power control The power-control process has two effects not directly addressed in the CDMA equa- tions as presented earlier. First, there are power-control errors, inaccuracies in the open-loop and closed-loop power-control processes. Second, there is the overhead factor of the power-control data stream. The best way to model the power-control errors is simply to accept that 