s in the case of FM. Figure 4.7 shows an example where the phase of the carrier is increased with the strength of the message. When message returns to zero there is a sudden phase change when the carrier returns to its nominal phase. In digital binary PM, which is called binary phase shift keying (BPSK), the phase of the carrier is varied according to whether the digital signal is 1 or 0. Figure 4.8 shows an example of BPSK where the digital sequence of 0011011 is transmitted. In binary phase modulation we need only two car- rier phases, which are chosen to be 0° for binary 0 and 180° for binary 1 in Figure 4.8. Message modulator Phase modulated Carrier wave carrier wave Figure 4.7 Principle of PM. Introduction to Telecommunications Network Engineering Constellation diagram of BPSK Carrier Constellation diagram of QPSK +180° +180° -180° +180° -180° Figure 4.8 Digital PM. Often we use more than these two phases of the carrier in digital modu- lation. When four carrier phases are used, each phase transmits the value of two binary bits and we talk about quadrature phase shift keying (QPSK). Fig- ure 4.8 illustrates an example of QPSK. An original carrier wave and the modulated one are drawn in the figure. At a point in time a pair of bits is taken from the incoming bit stream (110001101111..) of the modulator and the carrier phase is shifted according to the value of these two bits until the next two bits are received. One easily understandable way to describe digital phase modulation is by means of a constellation diagram, as shown in Figure 4.8. In the constella- tion diagram, the I axis refers to the in-phase carrier wave and Q stands for the quadrature carrier, that is, the carrier in 90° phase shift. Each signal point in the diagram represents one possible transmitted "symbol" or waveform that represents binary values of one or two bits in the examples of Figure 4.8. We can see easily from the constellation diagram for QPSK that, for exam- ple, the bit combination 01 is sent as a carrier with a +90° phase shift