Optical CDMA Networks: Principles, Analysis and Applications

Chapter 7

Hybrid Temporal Coherent and Incoherent OCDMA Networks

7.1 Introduction

In a conventional OCDMA, each time-slot is divided into chips consisting of (0,1)-sequences (i.e. the spreading code-length) addressed to each user. The data is modulated and assigned through optical pulses at certain chips of each allocated time-slot in either on-off keying (OOK) [1] or pulse position modulation (PPM) formats [2].

The modulated signal is then transmitted after being multiplied by the spreading code in the OCDMA encoder via optical tapped-delay lines (OTDL), i.e. the output optical pulse in the first chip of a time-slot is spread in the time domain to several chips corresponding to 1s of the spreading codes. The optical pulse sequences transmitted from users are multiplexed in the star passive optical network (SPON) couplers as an infrastructure reference and then transmitted over fibre to the destination (FTTx). At the receiver, in order to obtain the intended signal from the received signal, de-spreading is performed in a de-correlator which consists of an OTDL with inverse tap coefficients. The optical pulses are merged at the last chip in a slot, and the desired data is extracted in the demodulator based on the modulation scheme.

As mentioned, when the number of simultaneous active users increases, the effect of channel interference also inherently increases in incoherent OCDMA. On the other hand, multiple access interference (MAI) cancellation techniques at the receivers have ...