All interfering radio power is the same. This is an important modeling point in our concept of CDMA, perhaps the most important point: Interference from other CDMA calls is the same as additive white gaussian noise (AWGN) at the same power level. In fact, orthogonal codes and joint detection (JD) are ways for interferers to be far less in- trusive than AWGN with the same radio power. And multipath in the radio environ- ment can create intersymbol interference that can make it worse. We have perfect power control. These equations presume that the CDMA system has set its power levels to their exact optimal levels. Of course, the power-control process is based on radio measurements, which can have measurement errors, and power con- trol is a dynamic process that adapts to changing radio signal paths. As subscribers start calls, end calls, and move in and out of radio shadows, the power levels adjust to the changing radio environment. 28.2 Applying the Equations to Both Directions We can address some of these assumptions, as well as some other issues, and adjust our equations and our model to understand CDMA capacity. In this section we will look at changes to our model that are relevant to both directions. In the next two sections we will look at issues where changes unique to each direction need to be made. 28.2.1 Variation in data rates The most noticeable difference between the equations as presented earlier and real CDMA is the distribution of channel data rates. Voice channels are adjusted continu- ously by VAD as the subscriber's voice allows, wideband CDMA (W-CDMA) uses AMR to select a voice data rate depending on radio conditions, and there are circuit-switched and packet data services on the CDMA carrier. VAD reduces the average voice call utilization of the CDMA resource. If speakers are silent half the time, then the primary effect of VAD is to double the spreading gain of Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com Copyright © 2004 The McGraw-Hill Companie