3.10 Forward-link Overview

At this point we probably have enough background to begin piecing together all these various components to construct a functional CDMA channel structure. For simplicity, we'll begin by looking at the "Forward Link." There are four main utility channels on a typical IS-95 CDMA Forward Link, and perhaps some 15 to 25 traffic channels, all of which reside on the same RF channel, but in their own unique portion of the code space on top of the RF channel via the orthogonality created by Walsh coding. Since the base station is stationary and provides the master timing reference to the mobiles, we do not have the coding orthogonality problems discussed in the previous section (as on the Reverse Link). This simplifies our Forward Link and allows us to construct coding channels by exploiting the orthogonality of the Walsh codes. We will briefly list each of these channels, while reserving more detailed discussion of messaging and protocols for the next chapter.

The first and simplest Forward Link channel is the Pilot Channel (see also section 3.8), which is little more than a continuous string of zeros and is directly spread by Walsh code 0. This spread signal is then fed into a combiner and summed with all the other channels (see below), and then convolved with the base station's Short Code (which both "tags" it as a signal from a particular base station and also helps provide additional isolation from all the other signals on the RF channel,...