PSK modulation techniques are in general better than FSK modulations in terms of bandwidth requirements or in other words PSK modulation schemes offer better bits/s/hz efficiency. However FSK modulation schemes are more power efficient i,e, offers better BER performance at a given S/N (signal-to-noise ratio). FSK modulation schemes also perform better in terms of adjacent channel interference in non-linear power amplifications. That is the reason why GMSK (which is a variant of FSK) has been used in GSM system.