Voice transmission 32 kbps ADPCM digital voice Data transmission 4.8 kbps signaling channel Duplex voice channels per carrier Time frame 10 msec Time slot per channel 0.417 msec (including guard space) Number of time slots per frame Burst bit rate 1.152 Mbps Output power 250-mW burst peak Commercial Applications Note that with 24 time slots, the guard times needed between slots account for 23.3 percent of the total time frame. Each voice channel with TDD, over- head, and time gaps uses 96 kbps of the 1156-kbps burst rate. This DECT design permits 12 voice users in every 1.728 MHz of band- width. By normalizing to 1 MHz, we have 6.94 active voice users per mega- hertz of bandwidth. In a large network of densely packed microcells, the available total bandwidth per cell will be reduced by a factor of 1/2 because adjacent microcells cannot share the same frequency band. The frequency- of-reuse limitation of conventional narrowband radios can reduce overall capacity for the area covered by all microcells by as much as one-seventh. Thus, this DECT system used in a high density of microcells averages only one active channel per megahertz per cell. In the above S-CDMA example, we see that the bandwidth is roughly the same, but the potential number of active voice channels per cell is much larger for signal-to-interference ratios as high as 15 dB. This can be seen in the number of users allowed in the simulation results shown in Figures 3.9 and 3.10, in which 100 percent frequency of reuse is allowed. 3.7 HIGHER-CAPACITY OPTIMAL RECEIVERS Up to this point, we have assumed that the receivers were designed for inter- ference like noise and that a signal-to-interference threshold limited per- formance. In fact, interference experienced in the CDMA system is quite different from noise, and this fact can be exploited to achieve better per- formance and to increase capacity beyond the dimensional limit defined by the number of orthogonal codewords. In Part 5, Chapter 2, spread-spectrum multiple-access concepts were dis- cussed.