CDMA (Code Division Multiple Access) has been extensively studied in the context of microwave communications, as it allows users to access any randomly fragmented channel at any arbitrary time. Its use in fiber optic networks has attracted considerable attention since 1985. In long-distance fiber optic transmission links and networks, information consists of a multiplexed aggregate data stream originating from many individual subscribers and typically sent in a convenient synchronous format.
O-CDMA (optical CDMA) communication systems do not require any time or frequency management. It can operate asynchronously without centralized control and is free of packet collisions. As a result, O-CDMA systems have lower latencies than TDMA o In an O-CDMA system, each bit is divided into N time periods, called chips. By sending short optical pulses during some chip intervals, but not others, an optical signature sequence or keyword can be created. Each user of the O-CDMA system has a unique signature sequence. The encoder of each transmitter represents each 1 bit by sending a signature sequence, however, one bit or binary is not encoded and is represented using a sequence of zeros. Since each bit is represented by a pattern of chips on and off, the bandwidth of the data stream is increased. The O-CDMA encoded data is sent to an ‘N x N’ coupler known as a star coupler (in a local area network) or ‘1xN’ coupler (in an access network) and is transmitted to all nodes. Crosstalk between different users sharing the common fiber channel, known as MAI (Multiple Access Interface), is often the dominant source of bit errors in an O-CDMA system.
The main difference between O-CDMA and wireless CDMA systems is the structure of the code. Optical systems are mainly intensity modulated and therefore the chips in the O-CDMA system alternate ‘1’ and ‘0’ instead of ‘-1’ and ‘+1’. In O-CDMA, the overlapping of optical pulses results in the addition of optical power. Optimal CDMA codes have been found assuming bipolar signals that can take positive and negative values, although the optical signal can also be coherently processed to provide bipolar signals. Recently practical fiber optic systems use direct detection and can therefore only process unipolar signals consisting of “1” and “0”. CDMA codes consisting entirely of ‘1’ and ‘0’ are called optical codes and several variants have recently been proposed in much literature.
An important class of CDMA optical codes is the so-called set of OOCs (orthogonal optical codes). OOC is a family of (0,1) sequences with good automatic and cross-correlation properties. The OOC (1.0) sequences are called keywords. In other words, the cross-correlation of two different CDMA codes should take as low a value as possible.
The future optical network needs to support the multimedia service. The codes necessary to provide multimedia services have been investigated in much literature. Research is still underway to develop a better O-CDMA code to support multimedia services. In O-CDMA, the bandwidth is shared, it is the optical power of other users on the same wavelength channels that generates the pacing and brief noise. There are also cost barriers. Wideband encoding and decoding hardware is expensive for O-CDMA. To generate a large number of wavelengths, broadband LED is the most economical option, but the light generated may not be high enough in intensity for O-CDMA applications. Both the laser diode array and EDFA options have the required power, but are currently expensive. The performance of O-CDMA communications clearly depends on the MUI (multiple user interface), the type of modulation used and the receiver topology. The number of users depends on the length and weight of the code, but these parameters must be chosen carefully, since sometimes the system performance worsens if we increase any of them.
