GSM Physical Layer Modulation
GSM uses Gaussian-Fitered Minimum Shift Keying (GMSK) as it's modulation scheme
GMSK is a special type of digital FM modulation. Ones and zeroes are represented by shifting the RF carrier by plus or minus 67.708 kHz. Modulation techniques that use two frequencies to represent ones and zeroes are called frequency shift keying (FSK). In the case of GSM, the data rate of 270.833 kbps is chosen to be exactly four times the RF frequency shift. This has the effect of minimizing the modulation spectrum and improving channel efficiency. FSK modulation where the bit rate is exactly four times the frequency shift is called minimum shift keying (MSK). In GSM, the modulation spectrum is further reduced by applying a gaussian pre-modulation filter. This slows down the rapid frequency transitions, which would otherwise spread energy into adjacent channels.