sically converts the binary input stream into two par- allel, half rate signal v1(t) and vq(t) (i.e. the in-phase and quadrature signals). These sig- nals taking values +A/N2 or -A/N2 in any symbol interval, are fed to two balanced modulators with input carriers or relative phase 0° and 90°, respectively. Then the QPSK signal could be given by (2.13) If we assume Ts is the time interval and vi = +A/N2 and VQ = -A/N2, it can be shown that the output s(t) is (2.14) cos(wct) 70 MHz Binary NRZ oscillator Input DATA rb = bit/s Demultiplexer 90 deg Output Signal sin(wct) QPSK signal vector diagram vo(t) (b) QPSK Modulation Figure 2.3 QPSK configuration, after [3]. Assuming a coherent demodulator, the latter includes a quadrature detector consisting of two balanced multipliers with carrier inputs in phase quadrature, followed by root- Nyquist filter in the output I and Q arms. Then, the resultant I and Q signals are sam- pled at the centre of each symbol to produce the demodulator output I and Q signals, which in turn are delivered to the decoder [3]. Generally, an M-PSK modulator produces symbols with one of M phase values spaced 2n/M apart. Then each signal is demodulated correctly at the receiver when the phase within /M radians of the correct phase at the demodulator sampling instant. If noise is present, evaluation of the probability of error requires a calculation of the probability The UMTS Network and Radio Access Technology that the received phase lies outside the angular segment within radians of the trans- mitted symbol at the sampling instant. Therefore, the probability that a demodulator error occurs can be referred to as the sym- bol error probability Ps. In the context of the M-ary modulation scheme with M = 2b bits, each symbol represents b bits. The most probable symbol errors are then those that choose an incorrect symbol adjacent to the correct one. When using Gray coding, only one bit error results from a symbol error. Thereupon, the bit error probability Pb is re- lated to the symbol error probabili