litary users in battle against hostile interfer- ence can protect civilian users in shopping malls against friendly interference. The spread-spectrum system can protect its users against other users of the same channel, thus making capacity gains over conventional modulation. There are two major differences in capacity planning for conventional reuse [fre- quency division multiple access (FDMA) and TDMA] and spread spectrum (CDMA). First, the conventional reuse channel set is fixed and determined by the number of radios installed at each base station. Call quality is determined by channel reuse in other cells and not by how many calls are using the same-cell channels. In spread spec- trum, capacity is determined not by allocation of channels but by radio conditions in the carrier used by all the calls. Second, the primary interferers to a conventional reuse call are in other cells using the same radio carrier. In spread spectrum, the same-cell, same-carrier calls are the primary interferers, with other-cell users being of secondary importance. Each call has to have a certain share, a percentage, of the total power of the carrier at its receiver. If the call has a sufficiently high percentage of the received power, then it will be able to distinguish the signal from the other calls in the same frequency band. This notion of a required fraction of the signal is an important and strange feature of spread spectrum. Making the pizza pie bigger does not serve more people if every diner insists on eating one-eighth of it. Similarly, if each call requires 3 percent of the total signal, then the thirty-fourth caller is going to have a tough time of it. Under noisy con- ditions, that same 3 percent limit may only support 15 or 20 calls, and the base station has to know when to turn away more calls before all calls run out of signal quality. 8.2 Processing Gain The basis of CDMA is its processing gain, sometimes called spreading gain. This is the multiple-access part of CDMA. To see what processing gain is all about, consid