er a 2-bit digital channel with as much noise as signal. In our radio jargon, consider a binary phase shift keying (BPSK) channel plus additive white gaussian noise (AWGN) with a 0-dB signal-to-noise (S/N) ratio. The transmitter sends a +1.0 or -1.0 for each bit, and the noise adds a random bell-shaped normal distribution with a standard de- viation of 1.0, as shown in Fig. 8.1. The solid bell-shaped curve on the right side is the distribution of received signal plus noise when the transmitter sends a +1 bit, and the dotted bell-shaped curve on the left side is the distribution for a bit. The receiver considers any received signal on the right side of the center line to be a +1 and any re- ceived signal on the left side to be a 1. The receiver receives a bit error when the solid curve is on the left of the center line or when the dotted curve is on the right. According to our college statistics course, a normal distribution is more than 1 stan- dard deviation high 16 percent of the time, more than 1 standard deviation low 16 per- cent of the time, and in the fat middle part of the curve the other 68 percent of the time. This gives us a bit error rate (BER) of 16 percent. 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. The CDMA Principle The CDMA Principle Probability Density Voltage Figure 8.1 BPSK with a 0-dB S/N ratio. If we do not want to live with a 16 percent BER, then we could boost the signal power by 7 dB (a factor of 5) so that we have an S/N ratio of 7 dB. A factor of 5 in power is a factor of V5 (2.236) in amplitude (voltage). This gives us the picture in Fig. 8.2, +V5 or - V5 for each bit with the same noise contribution. The peak of each curve is now further from the 0-V centerline. As before, we have a bit error when the solid curve is on the left of the centerline or when the dotted curve is on the right. The same college stat