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
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.
