Wednesday, November 27, 2019

Optical Code Division Multiple Access (OCDMA)

CDMA in the optical domain is termed as  OCDMA ,i.e; it is a technology which supports multiple simultaneous transmission in the same time slot and same frequency to realize multiplexing transmission and multiple access by coding in the optical domain. It can operate asynchronously without any centralized control and doesn't suffer packet collision. OCDMA encoded data from multiple users is send to an N*N coupler (LAN) or 1*N coupler (access network) and broadcast  via fiber optic cable to multiple nodes where it is correlated(compared to stored replica of itself) to find out the matching receiver and compared to a threshold level to recover the original data

Block diagram of an OCDMA system 

           OOC algorithms, On-Off keying (OOK), Pulse Position Modulation (PPM),  Wavelength/ Time (W/T) matrix codes can be used throughout the application of the system. In OCDMA each bit is divided into N time periods called chips. By sending short optical pulse during some chip interval an optical signature or codeword can be created. Each bit is represented by a pattern of lit and unlit chips (but binary 0 is not encoded and is represented using all zero sequence). The bandwidth of data stream can be thus increased. Optical systems are intensity modulated, chips are 1's and 0's whereas in CDMA it can be 1's and -1's.
             The drawback of this system that affect its QoS is Multiple Access Interference (MAI) that occurs due to the cross talk between different users sharing the same channel. Its a type of interference caused by multiple cellular users using the same frequency allocation at same time. MAI can be reduced by Optical Parallel Interference Cancellation (OPIC) technique or by using Optical Hard Limiter(OHL) or Optical Normaliser at the front end of the receiver.

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