Optical CDMA Networks: Principles, Analysis and Applications

Chapter 6

Coherent Temporal OCDMA Networks

6.1 Introduction

Recently, coherent time-spreading OCDMA employing either direct time-spreading using super structures such as fibre Bragg grating [1] and arrayed waveguide grating [2] or spectrally phase coding time-spreading by spatial lightwave modulator [3] has attracted a lot of attention because of its overall superior performance over incoherent schemes. Several architectures have been considered for the use of CDMA within optical fibre communication systems. The most common system uses direct-detection with bipolar codes such as Gold sequences [4], while unipolar prime code families, particularly the latest ones introduced in Chapter 2, have more flexible code-length and are partially orthogonal. To retain the advantage of {0, 1} codes as a power saving option, in this chapter we consider unipolar signalling and coherent binary phase shift keying (BPSK) modulation. The capacity of the system using prime codes is limited by the maximum achievable bit rate of the electronic circuitry that generates the pseudo-noise (PN) sequences. Here, we concentrate on a maximum attainable chip-rate of 10 Gchip/s and a desired bit rate of hundreds of Mbps. This imposes a limit on the length of the spreading sequences which must be in the order of hundreds of chips per bit.

In this chapter, the signal-to-noise ratio (SNR) of coherent homodyne and heterodyne OCDMA architectures employing the group-padded modified prime code (GPMPC), introduced in ...