A low IF GMSK modem architecture with joint symbol timing recovery and frequency offset compensation

Abstract

This paper records our work in developing a GMSK modem with joint symbol timing recovery and frequency offset compensation capability. The received RF signal is first down converted to a low IF and then converted to complex baseband by a digital complex sampler. This baseband signal is then frequency discriminated and passed through a digital filter which in fact performs a fast Fourier transform (FFT). In other words, we apply an FFT algorithm on the frequency discriminated baseband signal in order to estimate symbol timing error and frequency offset simultaneously. The frequency offset can be estimated from the DC component of the FFT and the symbol timing error can be estimated from the phase angle of the FFT at a specified frequency which is equal to an integral multiple of half the bit rate. These two estimated parameters are then used for symbol timing recovery and frequency offset compensation. The bit error rate (BER) performance of our GMSK receiver architecture is assessed for an AWGN channel by computer simulation.