of jitter is not significant in data transmission. 29.1 Bit Error Rate (BER) and Eb/No Let us start with a binary phase shift keying (BPSK) modulated channel with additive white gaussian noise (AWGN) as we did in Sec. 8.2. The BPSK transmitter sends a pos- itive value for +1 and a negative value for -1, and the AWGN adds a normally dis- tributed random component. The BPSK receiver sees a +1 if the sum is positive and -1 if the sum is negative. The bit will be in error when the AWGN component adds enough error to the signal to push it onto the wrong side of the zero-voltage line, as shown in Figs. 8.1 and 8.2. What is the probability that this will happen? The amplitude of the desired signal is the square root of the signal power, and the standard deviation of the noise will be the 1The E/No requirement is a physical result of the BER requirement set in the quality of service (QoS) defined in the data transmission standard. 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 Capacity and Quality Principles square root of the noise power. This means that the bit values will be + Eb/No and - VE/No times the noise standard deviation. The bit error rate is the tail of the nor- mal curve that falls on the wrong side of the zero-amplitude line. It is given by the for- mula in Eq. (29.1): BER = 1erfe(No) (29.1) [The function erfc(x) is defined in Eq. (26.4) in Sec. 26.3. erfc(x) is the probability that a normally distributed random variable will be x or more standard deviations from its mean.] In the case of quadraphase phase shift keying (QPSK) with AWGN interference, the receiver takes the transmitted two-dimensional, complex signal-plus-noise value and finds the closest point on the QPSK constellation, as shown in Fig. 29.1. The in-phase and quadrature dimensions can be treated completely independently in computing the B