 1/3 K = 7 10 -4 Rate 1/2 Rate 1/3 K = 41 Eb/Nb (dB) Figure 29.3 Bit error rate with forward error correction. 29.2 Eb/No, Rate, and Capacity The number of channels a CDMA carrier can carry is the spreading gain divided by E/No. The spreading gain is simply the chip rate divided by the data rate. We can sum- marize this in Eq. (29.2): a = Eb/No W/R (29.2) where a = maximum number of interferers W = chip rate R = data rate The capacity of the CDMA carrier for this data service is a + 1. If we want to find a, then the CDMA carrier determines the chip rate, and the data service defines the bit rate. The power-control process of the data service channel will adjust the radio power until the BER requirement is satisfied. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. CDMA Data Capacity Principles CDMA Data Capacity Principles The bit rate is easy; the CDMA carrier has so many bits per second available under given radio conditions and Eb/No requirements. Everything else being equal, a CDMA carrier can serve twice as many users at 32 kbps as it can at 64 kbps. We want to find the value of Eb/No that corresponds to the data service BER so that we can use Eq. (29.2) to find the CDMA carrier capacity for a data service. For mixed services, we can think in terms of each channel's share, 1/a, of the CDMA carrier pie in Eq. (29.3): (29.3) This allows us to mix and match services, even services at the same bit rate that re- quire different Eb/No, such as RS1 and RS2 voice calls. And it allows us to mix and match voice calls and a variety of data services at different rates and different BER and Eb/No requirements. The connection between BER and Eb/No is clear when the signal waveform is clean, where the connection is a single, stable radio path. As described in Sec. 28.3.4, the presence of multipath degrades data service capacity just as it degrades voic