3.1 Spreading

In our long list of "data-hardening" techniques used in CDMA, the most significant comes as a direct result of spreading our bit stream's energy across the RF channel. In IS-95 (and IS-2000) we use three types of spreading codes: Long Codes, Short Codes, and Walsh Codes. These three can in turn be divided into two categories: the Pseudo Random Noise (PN) codes (both the Short and Long Codes fit into this first category), and orthogonal codes (Walsh codes). First, we will discuss the PN codes.

The primary property the PN codes must have to be useful is that they be "noiselike," i.e., as close to random as possible, with high auto correlation, while having very low cross correlation (see section 2.4). From the example of Rogoff's noise wheel, we learn that such "noiselike" functions need not be purely random, nor must they be infinitely long (although obviously they should be long enough to appear like a random noise sequence). To that end, we construct the digital version of PNs and use them much as was done in the Rogoff experiment. Only now, since they are digital, we find they are more easily constructed and manipulated. As with Rogoff's noise wheel, when we convolve our bit stream data with these PNs, we also find that we "spread" the energy in our bit stream across a much wider bandwidth. In effect, our bit stream "inherits" both the bandwidth characteristic of the PN codes, as well as their...