Digital Communications with Emphasis on Data Modems

5FREQUENCY SHIFT KEYING (FSK) MODULATION, DEMODULATION, AND PERFORMANCE

5.1 INTRODUCTION

Frequency shift keying (FSK) modulation is a robust modulation, in that, phase coherence is not required to achieve near optimum performance. In this chapter, the focus is on binary FSK (BFSK) and several implementations are discussed in the context of known or estimated frequency and phase information regarding the received signal. In particular, the performance of coherent and noncoherent detection of BFSK is characterized in terms of known frequency and phase; known frequency and unknown phase; unknown frequency and phase. These knowledge‐based characteristics of the received signal dictate the use of coherent detection or noncoherent detection and when the frequency and/or phase are unknown noncoherent detection must be used. The theoretical loss of coherently detected BFSK relative to antipodal signaling is 3 dB for all bit‐error conditions and, with known frequency and unknown phase, the performance loss of noncoherent BFSK detection is degraded by about 0.9 dB at P_be = 10⁻⁵ relative to coherent BFSK detection. On the other hand, with unknown frequency and phase, the loss is a function of the range of the frequency uncertainty with losses as high as 17 dB at P_be = 10⁻⁵ relative to coherent detection for a frequency uncertainty range of 10⁴ times the bit rate. The necessity to estimate and track bit timing for optimum sampling of the detection filtering is required with all implementations. ...