3 Issues in RFIC Design: Noise, Linearity, and SignalsNote that with FSK the bit error probability is related to the symbol error prob-ability by: / 21BsMPPM=×- (2.103)Therefore the probability of bit error is given by: ,2ssBooEMEPMQNNæöæö £×ç÷ç÷èøèø (2.104)2.4.3 Minimum Shift Keying (MSK)A related modulation that can be thought of as either phase or frequency modula-tion is called minimum shift keying (MSK). It is a very simple form of modulation that, during each bit period, either advances the phase of the carrier by 90° to indi-cate a one, or retards the phase by 90° to indicate a zero. Thus, an MSK signal can be represented by: MSKRF( )cos( )2StAtd t tπωφæö= ×+ ×× +ç÷èø (2.105)where d(t) has a value of ±1 to indicate the value of a bit that was transmitted. In order to generate this phase change, the frequency must be instantaneously higher or lower than the carrier frequency over the bit period. Since the phase must change by p/2 in a period of TB (the bit rate), the frequency must be / 2 124BBffTππD =×= (2.106)Figure 2.20 Probability of (a) symbol error versus Eb/No and (b) bit error versus Es/No for FSK.