A near-optimum discriminator demodulator for binary FSK with wide tone spacing

Optical FSK communication systems often require large tone spacings to reduce bit-error-rate (BER) degradation from laser-linewidth-induced crosstalk. Until now, discriminator detection of FSK for such wide tone spacings has fallen short of matched filter performance because of the suboptimal choice of a prefilter. A near-optimum demodulator for 240 Mb/s, three-times minimum orthogonal CPFSK (continuous-phase frequency-shift keying) has been constructed, with a measured performance that is 0.5 dB from matched filter theory at 10/sup -9/ BER. The design can be scaled to other data rates and tone spacings. The demodulator incorporates a frequency tracking loop that has good performance at low signal levels and no data-pattern dependence. >