radio signals are not very efficient. It came as a surprise, therefore, when Qualcomm first made the claim that its spread-spectrum digital cellular phone system resulted in higher overall capacity than any other system. Yet its claim was true. This spread-spectrum introduced a new para- digm for designing multicell voice networks using CDMA [15, 16]. It is based on the fact that the main criterion for efficiency, or capacity, should be this: FOR A GIVEN TOTAL BANDWIDTH, CAPACITY IS THE MAXIMUM NUMBER OF ACTIVE USERS IN A HIGH-DENSITY NETWORK OF CELLS. Commercial Applications Most earlier works focused on the capacity of a multiple-access system in terms of the number of users per bandwidth or the number of users per iso- lated cell in a star network. The key point is that in a multicellular system illustrated in Figure 3.3, spatial isolation and voice activation are critical for achieving a high overall capacity as defined above. Spatial isolation owing to the typical terrestrial ultra-high-frequency (UHF) propagation attenua- tion with the distance to the fourth power, together with the added protec- tion of spread-spectrum techniques, allows for 100 percent frequency of reuse among all cells in a network [15]. This 100 percent reuse is the reason that, in a well-designed spread-spectrum radio network, the overall capac- ity can be much greater than that with conventional narrowband radio mul- tiple-access techniques. Note that this is a more global capacity criterion. Capacity does not con- sider the maximum number of users per bandwidth or the maximum num- ber of users per cell, but is more practically based on the total number of users in the total area covered by multiple cells. Normally, evaluating sys- tems on the basis of users per bandwidth, users per cell, and users per area Figure 3.3. High-density network of microcells. For a given bandwidth and size of each microcell, what is the total number of active channels? A New Paradigm for Designing Radio Networks would give the same result. With spread-spec