. The distance of the signal point from the origin tells the carrier amplitude that is the same for all symbols in our examples in Figure 4.8. To get an idea about the spectral requirements of digital phase modula- tion, we can consider a single BPSK carrier burst representing for example a single 0-bit. Its spectral width depends on the duration of the symbol, which Transmission equals T in Figure 4.2. Symbol rate or modulation rate 1/T is expressed in bauds. Then most of the spectrum resides in the frequency range from fc - 1/T to fc + 1/T as shown in Figure 4.2. Binary 1 differs only by the carrier phase and the amplitude spectrum is the same. If we double the data rate of BPSK we have to cut symbol duration T to half, which doubles the required bandwidth. On the other hand, we can double the bit rate without increasing the bandwidth by using QPSK, in which each symbol carries two bits instead of one as shown in Figure 4.8. If symbol duration remains the same, the spectral width remains the same as well. We could increase the data rate further by using eight different carrier phases as in 8-PSK in Figure 4.9. If the modulation rate is the same for BPSK in Figure 4.8 and for 8-PSK in Figure 4.9, both methods occupy the same frequency band but the bit rate of 8-PSK is three times that of BPSK. The cost we pay for this increased data rate is lower noise tolerance. If the trans- mission power of both systems is the same, the distance of signal points from origin is the same in Figures 4.8 and 4.9. Then the 8-PSK signals are much closer to other signals than are those in BPSK, and much lower noise or interference can cause errors in the receiver. The 8-PSK is used in cellular networks to increase the data rate in low-interference environments. If inter- ference increases, modulation is changed to binary modulation, which toler- ates higher interference. Use of more phases than in 8-PSK is usually not feasible because of reduced noise tolerance. Instead we can combine AM and PM as shown in Figure 4.9 to become 16-QA