Direct - Sequence Encoding.

The new components needed for CDM systems are the encoders and decoders located at the transmitter and receiver ends , respectively. The encoder spreads the signal spectrum over a much wider region than the minimum bandwidth necessary for transmission. Spectral spreading is accomplishing by means of a unique code that is independent 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 is called a Signature Sequence. An advantage of the spread spectrum method is that it is difficult ti 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 . The direct sequence coding for optical CDM systems. Each bit of data is coded using A signature  sequence consisting of a large number , say M of shorter bits, called time"chips'. borrowing the terminology used for wireless. The effective bit rate increases by the factor of M because of coding. 

The signal spectrum is spread over a much wider region related to the bandwidth of individual chips.The resulting pulse train is split into several branches 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 such that it produced a peak in the correlation output whenever the user s code matches with a sequences of time chips in the received signal.Chip patterns of this peak is lower than the autocorrelation peak produced when the chip pattern matches precisely. The CDM pulse trains consisting of 0 and 1 chips suffer from two problems;

1.First , only unipolar codes can be used simply because optical intensity or power cannot be negative. The number of such codes in a family of orthogonal codes is often not very large until the code length is increased to beyond 100 chips.

2.Second , the crosscorrelation function of the unipolar codes is relatively high, making the probability of an error also large.

Both of these problems can be solved if the optical phase is used for coding in place of the amplitude.

Such schemes are being pursued and are called coherent CDMA techniques. An advantages of coherent CDM is that many families of bipolar orthogonal codes , can be employed in the optical domain. When a CW laser source is used in combination with a phase modulator . another CW laser is required at the receiver for coherent detection.