Low-SNR Operation of FSK Demodulators

Abstract

This thesis introduces the concept of phase rotation cancellation (PRC) as a previously unknown error mechanism in narrow-band FSK systems, and proposes a new FSK demodulation algorithm to resolve PRC errors. Phase rotation cancellation is defined as an event where the signal phasor rotation in the IQ plane is temporarily cancelled by the rotation of quadrature noise. The PRC errors are the dominant error mechanism in narrow-band FSK due to the small dynamic range at the output of a conventional FSK demodulator. In addition, any pre-modulation filtering further reduces the dynamic range and increases the probability for PRC errors to occur. Based on the discovered characteristics of PRC errors, a novel FSK demodulation algorithm is developed that greatly improves detection accuracy in FSK systems. Furthermore, the novel demodulation algorithm allows for sub-datarate receiver bandwidth operation, which offers improved receiver sensitivity and blocker performance. It is shown that the new demodulation algorithm eliminates nearly all PRC errors, leaving only errors due to clicks. In a GFSK system with BT = 0.5 and h = 0.5, an improvement in receiver sensitivity of up to 4.4 dB is achieved with the new demodulation algorithm.