Many codes have been proposed for optical CDMA system as discussed in Svetislav, Mari, Zoran, Kosti, and Titlebaum (1993), Salehi (1989), Liu and Tsao (2002), Maric, Moreno, and Corrada (1996), Wei and Ghafouri-Shiraz (2002), and Prucnal, Santoro, and Ting (1986). Optical code division multiple access (OCDMA) has been recognized as one of the most important technologies for supporting many users in shared media simultaneous, and in some cases can increase the transmission capacity of an optical fiber. OCDMA is an exciting developments in short haul optical networking because it can support both wide and narrow bandwidth applications on the same network, it connects large number of asynchronous users with low latency and jitter, and permits quality of service guarantees to be managed at the physical layer, offers robust signal security and has simplified network topologies. However, for improperly designed codes, the maximum number of simultaneous users and the performance of the system can be seriously limited by the multiple access interference (MAI) or crosstalk from other users. Another issue in OCDMA is how the coding is implemented. The beginning idea of OCDMA was restricted in time domain, in which the encoding/decoding could not been fully utilized in optical domain. Therefore a new coding in OCDMA has been introduced based on spectral encoding (Kavehrad & Zaccarin, 1995; Pearce & Aazhang, 1994; Smith, Blaikie, & Taylor, 1998; Wei & Ghafouri-Shiraz, 2002). The system, called Optical Spectrum CDMA, or OS-CDMA, has the advantage of using inexpensive optical sources, and simple direct detection receivers. In this article with an emphasis on the Spectral Amplitude Coding scheme, a new code known as Khazani-Syed (KS) code is introduced.
The unique concept of KS-codes is due to the arrangement of two subcodes in their code family, in a way that the resulting cross-correlation among two users is always less or equal to one (Al Junid, Ali, Ramli, & Abdullah, 2004; Aljunid, Zan, Anas, & Abdullah, 2004; Aljunid, Samad, Othman, Hisham, Kasiman, & Abdullah, 2005). Before probing any further, let us define the four important parameters used to characterize the KS-code. They are code weight, W, number of mapping, M, number of users, K, and code length, N.
The properties of KS-code can be best depicted in a Venn diagram as shown in Figure 1. Let Ci and Cj, be two different codes which belong to two different users, with . Both codes are from the same family of KS-code, C. P(A) and P(B) are defined as the probability of existence of sub-codes A and B, respectively. Both codes, Ci and Cj would have one of three sub-codes combination probabilities; either (1) or (2) or (3). From figure 1, it is stated that in KS-code, the event of is defined as in-phase CC where the frequency of occurs at most one. In another word, the maximum CC, (i.e. the in-phase CC of [1 1 0] and [0 1 1] is equal to one).
Key Terms in this Chapter
UUCP: This stands for Unix to Unix Copy, which is a Unix utility program and protocol that allows one Unix system to send files to another via a serial line which may be a cable going directly from one machine’s serial port to another’s or may involve a modem at each end of a telephone line.
AOL: A U.S. online service provider based in Vienna, Virginia, AOL claims to be the largest and fastest-growing provider of online services in the world, with the most active subscriber base. AOL offers its three million subscribers electronic mail, interactive newspapers and magazines, conferencing, software libraries, computing support, and online classes, among other services.
FTP: This stands for File Transfer Protocol, which is protocol that allows users to copy files between their local system and any system that they can reach on the network.
GPS: This stands for Global Positioning System. GPS was developed by the U.S. Department of Defense to allow the military to accurately determine their precise location anywhere in the world. GPS uses a collection of 24 satellites positioned in orbit to allow a person who has the proper equipment to automatically have their position triangulated to determine their location.
ARPANET: The first multisite, packet-switched network, ARPAnet was designed to support the Advanced Research Projects Agency (ARPA) for the transferring of files and resource sharing.
USENET: Usenet is a world-wide distributed discussion system. It consists of a set of “newsgroups” with names that are classified hierarchically by subject. “Articles” or “messages” are “posted” to these newsgroups by people on computers with the appropriate software; these articles are then broadcast to other interconnected computer systems via a wide variety of networks. Some newsgroups are “moderated”; in these newsgroups, the articles are first sent to a moderator for approval before appearing in the newsgroup. Usenet is available on a wide variety of computer systems and networks, but the bulk of modern Usenet traffic is transported over either the Internet or UUCP.
PDA: This stands for Personal Digital Assistant. A PDA is a small digital device that is used to store information such as phone numbers, addresses, schedules, calendars, and so forth. A PDA may also be referred to as a handheld device or as a Palm. The Palm Pilot was one of the original PDA’s and is now joined by others such as Palm Tungsten, HP IPaq, Palm Zire, and the Toshiba Pocket PC.
Telnet: A terminal emulation program for TCP/IP networks such as the Internet, the Telnet program runs on the client computer and connects it to a host on the network. Commands can be then entered through the Telnet program, and they will be executed as if the user was entering them directly on the server console.