1. Gaussian Minimum Shift Keying (GMSK): This is the primary modulation technique used for transmitting data in GSM. It's a type of continuous-phase frequency shift keying (CPFSK) modulation where the frequency shift is controlled by a Gaussian pulse shaping function. This results in a very compact signal spectrum and efficient use of bandwidth.
2. Differential Phase Shift Keying (DPSK): This modulation technique is used for transmitting the timing information needed for synchronization between the mobile phone and the base station. DPSK encodes data by changing the phase of the carrier signal, and the receiver detects the change in phase to recover the data.
3. Quadrature Phase Shift Keying (QPSK): This modulation technique is used for transmitting the control channels in GSM. It is a more efficient way of transmitting information compared to DPSK, as it allows for the transmission of two bits per symbol instead of one.
In summary, GSM uses GMSK for the primary data transmission, DPSK for timing information, and QPSK for control channels. This combination allows for efficient use of bandwidth and reliable transmission of data.
