Code Flexibility of 2-D Time-Spreading Wavelength-Hopping In OCDMA Systems

The potential performance and successful experimental demonstrations of two-dimensional time-spreading wavelength-hopping (TW) optical code-division multiple access systems have motivated a large volume of research into the development of new TW codes. The choice of the code is crucial in network design since the code not only affects the overall system performance but also sets limits on the physical implementation of the encoding-decoding process. In this paper, ldquoflexibilityrdquo of TW codes is investigated, providing clarity on the tradeoff between key code factors, specifically the number of available wavelengths and time chips. Network performance is evaluated by taking into consideration multiple access interference and beat noise at both the physical and link layers. Furthermore, a general formula describing the impact of the extinction ratio (ER) on the bit-error probability is explored. The results indicate the importance of code flexibility and ER in system performance optimization.

I. Introduction

The rapid growth in the demand for higher system capacity to support a range of new services opens opportunities for new multiplexing techniques that maximize the benefit of the available bandwidth within the deployed optical fiber infrastructure. Optical code-division multiple access (OCDMA) is one promising candidate capable of supporting bursty and variable data-rate traffic in an asynchronous manner and is well suited to fulfill the needs of local area and access networks [1]–[3]. Among the different types of reported OCDMA systems, the two-dimensional (2-D) time-spreading wavelength-hopping (TW) scheme offers desirable advantages, particularly in terms of access to a large number of users. In TW schemes, each data pulse is spread into many chips constituting the code length; some/all chips are then transmitted at different wavelengths according to a certain algorithm. In general, several parameters impact system performance, particularly the code properties. A comparison between several symmetric and asymmetric 2-D TW codes has been presented [4], highlighting the superiority of some symmetric codes and introducing a desired feature, referred to as “flexibility.” The term flexibility reflects the degree of independence of time chips, or equivalently the code length, on the number of wavelengths, or code weight. If these two properties are independent, the code then offers a high degree of flexibility. A lack of code flexibility reflects dependence between time and wavelengths.