The answer is GMSK modulation. GMSK modulation is a refined form of PSK modulation schemes. Using PSK modulation the sudden phase changes in time domain causes a need of large bandwidth in the frequency domain. In order to mitigate that we use GMSK. For simplicity you can consider GMSK as an equivalent to a low pass filtering effect.
I am not totally sure about the bandwidth efficiency of GMSK. If any one can say the bandwidth efficiency, it can clear the doubt very well..