8.6 CODE DIVISION MULTIPLEXED NETWORKS
Code division multiplexing (CDM) [7, 9] is an alternative to WDM and TDM for multiple accessing, and its value is that it involves simple, nonsynchronous pulsed laser modulation for its operation. Each transmitting source is assigned a unique optical pulse sequence for its address. Digital data is encoded onto these assigned pulse sequences, and decoded at a receiver by a pulse sequence correlation. The system has the advantage of using simple pulsed lasers (without wavelength control) and standard wideband photodetectors (without narrow optical filters). All sources operate independently, and no clock is needed to align transmitters.
In OOK COM, each OOK bit is sent as the sequence of that transmitter or its absence, a shown in Figure 8.15a. The code sequence is produced at the encoder from the OOK bit pulse and can be generated either actively or passively. In the passive encoder (Fig. 8.15b) the sequence is generated by parallel fiberoptic delay lines, whose delays are adjusted to the pulse sequence separations, so that the summed output forms the sequence as the echoed version of the initial OOK pulse. In a active encoder, Figure 8.15c, the laser is directly modulated by the code sequence. All transmitters asynchronously superimpose their OOK pulse sequences for distribution by the network.
An individual receiver observes the sum of all such pulse transmissions, and recovers the data from a particular transmitter by using a matched pulse ...