8.6. CODE-DIVISION MULTIPLEXING 389
Figure 8.32: Coding of binary data in CDM systems using a signature sequence in the form of a
7-chip code.
sion. Spectral spreading is accomplished by means of a unique code that is indepen-
dent of the signal itself. The decoder uses the same code for compressing the signal
spectrum and recovering the data. The spectrum-spreading code is called asignature
sequence. An advantage of the spread-spectrum method is that it is difficult to jam or
intercept the signal because of its coded nature. The CDM technique is thus especially
useful when security of the data is of concern.
Several methods can be used for data coding including direct-sequence encoding,
time hopping, and frequency hopping. Figure 8.32 shows an example of the direct-
sequence coding for optical CDM systems. Each bit of data is coded using a signature
sequence consisting of a large number, sayM, of shorter bits, called time “chips” bor-
rowing the terminology used for wireless (M=7 in the example shown). The effective
bit rate (or the chip rate) increases by the factor ofMbecause of coding. The signal
spectrum is spread over a much wider region related to the bandwidth of individual
chips. For example, the signal spectrum becomes broader by a factor of 64 ifM=64.
Of course, the same spectral bandwidth is used by many users distinguished on the
basis of different signature sequences assigned to them. The recovery of individual
signals sharing the same bandwidth requires that the signature sequences come from a
family of the orthogonal codes. The orthogonal nature of such codes ensures that each
signal can be decoded accurately at the receiver end [263]. Transmitters are allowed to
transmit messages at arbitrary times. The receiver recovers messages by decoding the
received signal using the same signature sequence that was used at the transmitter. The
decoding is accomplished using an optical correlation technique [254].
The encoders for direct-sequence coding typically use a delay-line scheme [249]
that looks superficially similar to that shown in Fig. 8.26 for multiplexing several
OTDM channels. The main difference is that a single modulator, placed after the
laser, imposes the data on the pulse train. The resulting pulse train is split into sev-
eral branches (equal to the number of code chips), and optical delay lines are used to
encode the channel. At the receiver end, the decoder consists of the delay lines in the
reverse order (matched-filter detection) such that it produces a peak in the correlation
output whenever the user’s code matches with a sequence of time chips in the received
signal. Chip patterns of other users also produce a peak through cross-correlation but
the amplitude of this peak is lower than the autocorrelation peak produced when the