Abstract:
This paper presents a performance analysis approach for OCDMA networks, which takes into account both the physical layer characteristics and random media access schemes. Analysis results demonstrate its effectiveness in characterizing the OCDMA network dynamics. ©2005 Optical Society of America OCIS codes: (060.4250) Networks; (060.4510) Optical communications
1. Introduction
Conventional optical local access networks typically use wavelength-division multiplexing (WDM) and timedivision multiplexing (TDM) techniques which require wavelength and time domain processing. With the success of commercial wireless CDMA (Code Division Multiple Access) networks, optical CDMA (OCDMA) networks [1- 6] have been envisaged to provide flexible bandwidth access to many subscribers through optical encoded transmissions. The OCDMA technology promises many attractive features not available in WDM or TDM networks, including data-format-independent physical layer security, decentralized network control, and increased flexibility in the granularity of provisioned bandwidth. Much of research efforts [1-5] on OCDMA so far have being focused on the physical layer of the network. With the rapid progress of optical enabling technologies, there is a growing demand for a clear understanding of the OCDMA performance beyond the physical layer. In an OCDMA network with random media access schemes, several intrinsic features at both physical layer and the media access control (MAC) layer have adverse impacts on network performance. For instance, due to the inherent multiple access interference and the background physical noise, some transmitted packets will arrive at their destinations with uncorrectable bit-errors and suffer from this packet corruption effect. At the MAC layer, channel collision may occur when two or more source nodes transmit on the same code channel during a packet transmission time. Thus, in order to fully understand OCDMA network dynamics, it is essential to develop a performance analysis methodology that considers not only the characteristics of the physical layer, but also the media access schemes at the higher layer. This paper proposes a performance analysis approach to support the arbitrary OCDMA schemes, and obtains a steady-state throughput estimation for OCDMA networks with random access schemes. In this approach, the physical layer performance is fully summarized in a bit error rate (BER) function, and the performance analysis takes into account the effects of both packet corruption and channel collision. In particular, we analyze a representative OCDMA scheme with various system parameters and demonstrate the effectiveness of this approach.
2. System description and analysis approach Fig. 1 shows an OCDMA network based on a star coupler, where multiple nodes share the optical bandwidth by using optical codes. In this multiple-channel system, each channel corresponds to an OCDMA code. At packet level, the network operates in a time-slotted mode. The length of a data packet is L bits and it corresponds to a slot duration. At the beginning of a time slot, the source will transmit its packet with probability ρ. Each node has the same traffic rate and the destination is uniformly distributed. At bit level, we will consider two cases: the bit-synchronous mode means that optical pulses from multiple users are aligned with each other in the time domain when arriving at the star coupler, while the bit-asynchronous case has no such an alignment requirement.. A star-coupler-based OCDMA network. The network consists of N nodes. Here, N is the cardinality of the adopted optical code set. Each node is equipped with a tunable transmitter and a fixed receiver (TTFR), and each node is assigned a unique code for packet reception when it starts up. The main performance metric in this work is throughput, defined as the average number of successful received packets by destinations in one time slot. Analysis of throughput entails a consideration of packet corruption and channel collision, and raises two questions related to their effects.

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