Chapter 2
Optical Spreading Codes
2.1 Introduction
In an optical code-division multiple access (OCDMA) system, the main purpose is to recognize the intended user in the presence of other users, since all users share the same channel. Another aim is to accommodate more possible subscribers in the system. In this case, the optical sequence codes with the best orthogonal characteristic should be employed. In other words, spreading code sequences are selected for the features of maximum auto-correlation and minimum cross-correlation in order to optimize the differentiation between the correct signal and the interference.
This chapter reviews fundamental spreading codes used in the application of OCDMA communications including coherent/incoherent, asynchronous/synchronous, temporal/spectral and one-dimensional/multidimensional schemes.
Basic bipolar codes including m-sequences, Gold sequences, Hadamard and Walsh-Hadamard codes will be studied in terms of construction and properties. The bipolar codes have been borrowed from wireless CDMA concept. Although, due to the differences in the signal carries of RF and lightwave as well as channel characteristics of air-interfaces in RF communications and fibre-optics in optical communications, unipolar codes are mostly acceptable in the OCDMA communications and networks. However, recent progresses in the optical coherent modulations and coding, bipolar codes are also employed in coherent OCDMA systems.
Optical orthogonal codes (OOC) as one ...
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