rary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering Concepts Key Radio Concepts Quiet Some Noise Too Much Noise Figure 1.12 Quiet and noisy 16QAM received constellations. The factor of 2 gain from 1-bit BPSK to 2-bit QPSK is a one-time efficiency gain be- cause BPSK simply ignores half the information opportunity. The gain in bit rate going up to 3-bit 8-ary or 4-bit 16QAM is not free. The effect of radio noise is to wiggle the points on the constellation picture, and more noise wiggles them further, as shown in Fig. 1.12. As the constellation points get closer together in more complicated modulation schemes, the noise component must be lower for the same bit-accuracy rate. A denser constellation gets its higher bit rate at the expense of a tougher S/N requirement. There are other versions of QPSK, variations on the same four-point concept. There is offset QPSK (OQPSK), shown in Fig. 1.13, that displaces the four points by 45 degrees to 1 +i,1-i,-1+i,and - -1-i. There is differential QPSK (DQPSK), a noncoherent modulation scheme in which the four phase angles are taken relative to the previous symbol. The GSM system uses gaussian minimum phase-shift keying (GMSK), a ver- sion of QPSK in which the carrier amplitude rises and falls in a gaussian-shaped impulse response to minimize radio transmission out of the desired frequency band. In general, coherent modulation schemes are somewhat more resistant to noise than noncoherent schemes for the same bit rate. In a world with perfect phase alignment, perfect separation of in-phase and quadrature components, we would expect 3 dB bet- ter performance. Actual coherent systems tend to be about 2 dB better than similar noncoherent modulation schemes. OQPSK Figure 1.13 QPSK amd OQPSK constellations. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subje