IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 50, NO. 12, DECEMBER 2002 1971 Frame Time-Hopping Fiber-Optic Code-Division Multiple Access Using Generalized Optical Orthogonal Codes Amir R. Forouzan, Jawad A. Salehi, Member, IEEE, and Masoumeh Nasiri-Kenari, Member, IEEE Abstract—In this paper, a new spreading technique for intensity modulation direct detection fiber-optic code-division multiple-ac- cess (FO-CDMA) communication systems is proposed. This new spreading technique is based on generalized optical orthogonal codes (OOC) with large cardinality and minimal degradation in performance when compared with a more optimum system, namely, an optical CDMA system using OOC with autocorrelation and cross-correlation value bounded by one, i.e., OOC ( ). To obtain the performance of such systems, we use a recently introduced communication scheme, namely, frame time-hopping (FTH)-CDMA with random coding. It is discussed that systems with generalized OOC patterns and random time-hopping coding are close in structure and performance. Furthermore, the perfor- mance of such systems is near the performance of optical CDMA with optimum but low cardinality OOC ( ), which further renders the practicality of the proposed technique with very large cardinality. Two new receiver structures for FO-CDMA, namely, chip-level detector with optimum comparator threshold and cor- relation receiver with an electrical hard limiter, are also proposed. Performance analysis for a binary pulse position FTH-FO-CDMA network is considered for the correlation receiver, chip-level detector, correlation receiver with an optical hard limiter, op- timum receiver, and the two newly proposed receiver structures. The results also show that a chip-level detector with optimum comparator threshold is superior to a chip-level detector for received low signal powers, and predict that the performance of the correlation receiver with an electrical hard limiter is superior to all other considered receiver structures, e.g., requiring one third of transmission power to achieve a desired bit error rate when compared with other receiver structures. Index Terms—Chip-level detector, frame time-hopping (FTH), optical code-division multiple access (CDMA), optical hard limiter, optical orthogonal codes (OOC). I. INTRODUCTION O PTICAL code-division multiple access (CDMA) schemes based on intensity modulation/direct detection (IM/DD) not only utilize the potential of fiber-optic capacity but also remain compatible with today’s widely used IM/DD optical communications systems. Among the very early techniques that Paper approved by I. Andonovic, the Editor for Optical Networks and Devices of the IEEE Communications Society. Manuscript received October 25, 2001; revised January 15, 2002. This work was supported in part by the Iran Telecom Research Center (ITRC) under Contract 8031471. A. R. Forouzan is with the Department of Electrical and Computer Engi- neering, Faculty of Engineering, University of Tehran, Tehran, Iran (e-mail: forouzan@ece.ut.ac.ir). J. A. Salehi and M. Nasiri-Kenari are with the Electrical Engineering Department, Sharif University of Technology, Tehran, Iran (e-mail: jasalehi@sharif.edu; mnasiri@sharif.edu). Digital Object Identifier 10.1109/TCOMM.2002.806495 were introduced for IM/DD optical CDMA employed unipolar optical orthogonal codes (OOCs) with autocorrelation ( ) and cross-correlation ( ) value bounded by one [1], [2]. Though these classes of OOCs ( ) remain optimum from a multiaccess interference point of view, they suffer from low cardinality, which was shown to be on the order of (1) where is the number of OOCs, the code length, and the weight of the code. To improve upon the cardinality of the proposed OOCs, one must increase or relax the stringent re- quirement on the autocorrelation and cross-correlation value. Therefore, a tradeoff exists between the number of codes (users) and the error performance due to increased multiaccess interfer- ence. In this paper, our goal is to study and analyze the perfor- mance of less structured OOC codes that allow a large increase in the number of users without a large loss in its performance. In [3], the authors considered OOC codes with autocorrelation and cross-correlation value bounded by two. The cardinality of this code is (2) which is considerably higher than OOC with . For typical values of and [4], we can have, at most, 27 codewords with and 7924 codewords with . De- spite the complexity in analyzing the performance of the system in [3], it was shown that the degradation in performance for OOC codes is not severe, and, in fact, under cer- tain conditions such as for fixed number of users and code length , one can increase the weight of the OOC codes such that it will surpass in performance when compared with OOC optical CDMA systems. This leads us to believe that further relaxation on the cross correlation may not necessarily severely degrade the performance of an optical CDMA system while increasing dramatically the cardinality of the OOC codes. In [5], a general upper bound on the number of OOC codes was obtained, and it was shown to be (3) To generalize our scheme, we will consider , which implies that we consider all possible codes of length and weight (except the corresponding cyclic shifts) regardless 0090-6778/02$17.00 © 2002 IEEE