What is Optical CDMA?
CDMA (Code Division Multiple Access) has been extensively studied in the context of microwave communications as it allows users to access any shred channel randomly at any arbitrary time. Its use in optical fiber networks has attracted considerable attention since 1985. In long-haul optical fiber transmission links and networks, the information consists of a multiplexed aggregate data stream originating from many individual subscribers and normally is sent in a well-timed synchronous format.
O-CDMA (Optical CDMA) communication systems do not require any time or frequency management. It can operate asynchronously without centralized control and it does not suffer from packet collisions. As a result, O-CDMA systems have lower latencies than TDMA or In a O-CDMA system each bit is divided up into N time periods, called chips. By sending short optical pulse during some chip intervals, but no others, an optical signature sequence or codeword can be created. Each user on the O-CDMA system has a unique signature sequence. The encoder of the each transmitter represents each 1bit by sending signature sequence, however a binary o bit is not encoded and is represented using all-zero sequence. Since each bit is represented by a pattern of lit and unlit chips, the bandwidth of the data stream is increased. The O-CDMA is encoded data is then send to an ‘N x N’ known as star coupler (in local area network ) or ‘1xN’ coupler(in an access network) and broadcast to all nodes. The crosstalk between different users sharing the common fiber channel, known as MAI (Multiple Access Interface) is usually the dominant source of bit errors in an O-CDMA system.
The main difference of O-CDMA systems from wireless CDMA is the code structure. Optical systems are mainly intensity modulated and hence the chips in the O-CDMA system are alternating ‘1’ s and ‘0’ s instead of ‘-1’ s and ‘+1’ s. In O-CDMA the overlap of optical pulses results in the addition of optical power. Optimum CDMA codes have been found assuming bipolar signals which can take on positive and negative values, though optical signal can also be processed coherently to provide bipolar signals. Recently practical optical -fiber systems use direct detection and can therefore process only unipolar signals consisting of “1” s and “0” s. CDMA codes consisting entirely of ‘1’ s and ‘0’ s are referred to as optical codes and several variants have been recently proposed in many literature.
An important class of CDMA optical codes is so called set of OOCs (Orthogonal Optical Codes). OOC is a family of (0,1) sequences with good auto and cross correlation properties. The (1,0) sequences of OOC are called its codewords. In other words, the cross -correlation of two distinct CDMA codes must take a value as low as possible.
Future optical network need to support multimedia service. Codes needed to provide multimedia services have been investigated in much literature. Researches are still going on to develop a better O-CDMA code which will support the multimedia services. In O-CDMA, Bandwidth is shared, it is optical power from other users on the same wavelength channels that lead to the beat and short noise. There are also cost barriers. The encoding broad band and decoding hardware is expensive for O-CDMA. For generation a large number of wavelengths the broadband LED is the cheapest option, but the light generated may not have a high enough intensity for O-CDMA applications both the laser diode array and the EDFA options have the required power but are currently expensive. Performance of O-CDMA communications is clearly dependent on the MUI (Multiple User Interface) the type of modulation used and the receiver topology. The number of users is dependent on the code length and weight but these parameters should be carefully chosen since sometimes the system performance worst if we increase any of them.