6.6 BLOCK ENCODING AND PULSE POSITION MODULATION
In the previous systems, only binary encoding was considered, in which bits were sent one at a time over the optical link. An alternative is the use of block encoding in which bits are transmitted in blocks instead of one at a time. Optical block encoding is achieved by converting each block of b bits into one of M = 2b optical fields for transmission. Decoding of each block is achieved at the receiver by determining which of the M fields is being received during each block time.
One of the more popular forms of optical block encoding is by pulse position modulation (PPM). In PPM an optical pulse is placed in one of M adjacent time slots to represent the data block, as shown in Figure 6.14a. The M slots constitute a PPM frame, or word time Tf seconds. The location of the pulse in the frame, therefore, determines the data word. The system uses pulsed optics, and is, therefore, compatible with direct detection receivers. The pulse width is equal to the slot width Ts (if we assume ideal pulses and neglect slot overlaps due to possible pulse spreading). The laser need only produce pulses at the frame rate and, therefore, operates at the pulse repetition frequency (PRF) of 1/Tf = 1/MTs. Since the M-slot PPM frame can represent log2M bits, the PPM transmitter sends bits at the rate
given in bits/second. The PPM decoder must decide which ...