GMSK Demodulator Using Costas Loop for Software-Defined Radio

DSP based implementation of Gaussian minimum shift keying (GMSK) demodulator using Costas loop has been described. The demodulator consists of a Costas loop for carrier recovery, data recovery, and phase detection. For low data rates, we have tested the scheme on a TIpsilas DSP TMS-320C6713. These are implemented using MATLAB and SIMULINK tools and tested on TIpsilas DSP using code composure studio (CCS). Carrier has been recovered using a loop of center-frequency locking scheme as in binary phase shift keying (BPSK) Costas-loop. Demodulator performance for AWGN channel for different data rates is shown. Performance is measured for bandwidth time product (BT = 0.3 and 0.5) with data rate between 8 kbps to 256 kbps. Bit synchronization and image coding and decoding are designed according to the need of demodulator. The scheme may find its use in software-defined radio (SDR) applications.