ER of QPSK. The energy per bit Eb is divided equally between the two dimensions, and the noise power is also divided equally, so the formula for BER is exactly the same for QPSK as it is for BPSK. The graph of BER as a function of Eb/No in the BPSK and QPSK cases is shown in Fig. 29.2. Figure 29.1 The receiver finds the closest point on the constellation. Downloaded from Digital Engineering Library @ McGraw-Hil (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 Eb/Nb (dB) Figure 29.2 Bit error rate as a function of Eb/No. For more complex constellations such as 8-ary or 16QAM, the calculations are also more complex. Since the points on the constellation are closer together, each constella- tion symbol requires more energy than QPSK relative to the noise level. However, there are more bits in each symbol, so the energy per bit Eb relative to the noise level remains about the same as QPSK. The principle of code division multiple access (CDMA) pro- cessing gain is that interference from other CDMA channels can be treated the same as AWGN. The performance of a data service is not the raw BER but the BER after forward er- ror correction (FEC). This depends on the method of FEC, as shown in Fig. 29.3. The reduction in BER for typical convolutional coding (K = 7) is shown for two-to-one coding (rate 1/2) and for three-to-one coding (rate 1/3). The rate 1/3 coding requires 3 coded bits for every encoded bit, whereas the rate 1/2 coding only requires 2. The rate 1/3K = 41 line represents a very complex coder, computationally intense but theoretically possible. Downloaded from Digital Engineering Library @ McGraw-Hil (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 Uncoded Rate