Optical Communications, 2nd Edition

6.9 CLOCK SYNCHRONIZATION WITH PULSED OPTICS

We have shown that pulsed digital communication requires accurate timing to properly set the photodetector integrations used in the decoding. Since performance degrades rather quickly as timing errors increase, receiver decoding clocks must be accurate to within a fraction of the pulse time. In this section, we consider the circuitry usually used for pulsed timing in OOK, Manchester, and PPM systems.

Digital clocking is achieved in a parallel subsystem to the decoder, as was shown in Figure 6.22. The slot clocking objective can be described by redrawing Figure 6.22 as in Figure 6.28. The transmitted laser data pulse arrives at the receiver as shown, its location is time dependent on the laser pulsing and propagation delays. The receiver decoder requires a pulse clock (or slot clock in PPM) that is time coherent with the pulse edges to set the start and stop times of the decoder integration. This clock can be represented by a periodic square wave, as shown, whose leading edges time the decoder integrations. This clock must remain time coherent with the arriving pulse edges in spite of variations in the laser pulsing time and propagation delays, which may drift in time from pulse to pulse. This clock tracking can be accomplished by attempting to measure the time offset Δ between an arriving pulse and the clock edges every pulse or frame time and using the measured error to update (shift) the local clock.

Figure 6.27. PWE degradation in ...