(B) 



M 



(A) MODULATING WAVEFORM. 



(B) PFM WAVEFORM TRANSMITTED OPTICALLY. 



(C) RECEIVED PFM WAVEFORM SHOWING EFFECTS OF 

 BANDLIMITING AND NOISE. COMPARATOR 

 SLICING LEVEL IS SUPERIMPOSED. 



(D) RECONSTRUCTED PULSE TRAIN FROM SLICER. 



(E) RECOVERED TRANSMITTED WAVEFORM AFTER FILTER. 



Figure 12. Waveforms encountered in the PFM system. 



carrier frequency divided by the pulse train center frequency. The frequency-expanded PFM 

 signal can efficiently take advantage of the wide overall bandwidth capability of fiber-optic 

 transmission. At the receiver, noise is added and the signal is effectively frequency-compressed 

 by the discriminator back to its original bandwidth. Because the signal components in the 

 PFM spectrum are correlated (ie, add up in the time domain to a time-varying pulse train) and 

 the noise components over the band are not, the signal components add coherently upon 

 detection and the noise power adds in a noncoherent manner, which yields a net processing 

 gain. This is entirely analogous to well known modulation formats such as FM and PCM, 

 which increase postdetection signal-to-noise ratio at the expense of predetection transmission 

 bandwidth. 



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