Optical Communications, 2nd Edition

6.3 MANCHESTER PULSED SIGNALS

The basic disadvantage of OOK signaling is that key receiver parameter values, such as power levels, must be known, or measured, to optimally set the threshold. A pulse format that avoids this difficulty uses pulse-to-pulse comparisons for decoding. A way to do this is to send the optical pulse in one of two adjacent time intervals and to then compare the integrated output current over each interval. The bit is decoded according to which integration produces the higher value, and no threshold need be selected. This format is referred to as Manchester coding.

Figure 6.8. Manchester signaling. (a) Pulse encoding format. (b) Receiver and decoder.

The encoding operation is shown in Figure 6.8a. A “1” bit is sent as a pulse in the first half of the bit interal, and a “0” bit as a pulse in the second half. The decoder (Figure 6.8b) separately integrates the detector output over the two half-bit intervals and compares for bit decoding. The system still uses pulse signaling, but the pulse time is one half the bit time. The decoder integrators, therefore, have shorter time intervals (higher bandwidths) than for the OOK system.

The bit error probability is now the probability that the bit half-interval containing the pulse does not produce the higher integrator output. This again depends on the type of optical channel.

6.3.1 Shot-Noise-Limited-Ideal-Gain Poisson ...