A Phase — Noise Insensitive Modulation Scheme for CDMA Optical Local Area Networks

Abstract

Optical Local Area Networks have been widely investigated and are now diffusely used in those applications which require very high bit-rate values. With the aim to exploit completely the large bandwidth offered by single-mode fibers, so increasing the number of connectable users, various multiple access schemes have been proposed, based on time, frequency or code division. Among them, our present interest is focused on the last technique, whose employment in the framework of optical systems still presents some open problems. In particular, it is known that the CDMA scheme is based on the auto- and mutual-correlation properties of the code sequences utilized. The best results are obtained with bipolar codes, such as the Gold code, but this implies the adoption of coherent modulation formats, whose performance may be corrupted by the presence of phase noise due to the transmitter and the local lasers. To overcome this drawback, in this paper we propose a hybrid technique, obtained as a combination of phase and polarization modulations, which reveals to be insensitive to the phase noise. After a brief description of the proposed technique, the system performance, in terms of probability error as a function of the code length and the number of users, is derived and compared with a traditional 2-PSK modulation.