rate some interference, as shown in Figure 3.4, SO the introduction of each additional active radio raises the overall level of interference to the base station receivers receiving CDMA signals from mobile transmitters. Each mobile radio can introduce a unique level of inter- ference, reflecting its cell-site power level, timing synchronization relative to other signals at the cell site, and specific code cross-correlation with other CDMA signals. The number of CDMA channels allowed in the star network depends on the level of total interference that can be tolerated. Unlike the time- dimension-limited TDMA system, the CDMA system is limited by inter- ference, and consequently the quality of the radio design plays a key role in its overall capacity. Certainly, a well-designed radio will have a required bit error probability with a higher level of interference than a poorly designed radio. Forward error-correction coding techniques can also increase the threshold of tolerable interference and, thus, increase overall CDMA capacity. Spread-spectrum CDMA systems, therefore, place a greater premium on good overall mobile radio design and overall system design than conventional narrowband FDMA and TDMA multiple-access radio systems. Returning to Figure 3.4, suppose that at the cell site the signal level of all mobile users is the same and interference between radios is modeled as Gaussian noise. This is the case with A-CDMA, in which we assume that each radio has a required bit error probability that defines a required energy-per-bit-to-noise ratio given by Eb/No. This Eb/No defines the thresh- old shown in Figure 3.4. Given these assumptions, the relationship between the number of users M, the processing gain PG, and the required E/ is given by (3.1) which is a special case of (5.2) in Part 5, Chapter 2. For a given bit error probability, the required Eb/No depends on how well the radio is designed and how much error-correction coding is used. The ideal Shannon limit in white Gaussian noise shows that error-free commun