Several constructions of two-dimensional (2-D) codes have been proposed to overcome the drawbacks of nonlinear effects in large spread sequences of one-dimensional (1-D) unipolar codes in fiber-optic code-division multiple-access (FO-CDMA) networks. Wavelength-time (W/T) encoding of the 2-D codes is practical in FO-CDMA networks. W/T codes reported so far can be classified mainly into two types: 1) hybrid codes, where one type of sequence is crossed with another to improve the cardinality and correlation properties and 2) conversion of 1-D sequences to 2-D codes to reduce the "timelike" property. This paper describes the basic principles of a new family of wavelength/time multiple-pulses-per-row (W/T MPR) codes, for incoherent FO-CDMA networks, which have good cardinality, spectral efficiency, and minimal cross correlation values. In addition, an expression for the upper bound on the cardinality of W/T MPR codes is derived. Another feature of the W/T MPR codes is that the aspect ratio can be varied by a tradeoff between wavelength and temporal lengths. The correlation properties of W/T MPR codes are verified by simulation using Matlab. For given wavelength time dimensions, various W/T codes, whose cardinalities are known, are compared, and it is shown that the W/T MPR family of codes have better cardinality and spectral efficiency than the other W/T codes. Performance analysis of the W/T MPR codes and their limiting cases is carried out for various parameter variations such as the dimensions of wavelength, time, and weight of the code.

We report the design of a new family of twodimensional codes for fiber-optic CDMA networks. These newly designed temporal/spatial single-pulse-per-row (T/S SPR) codes have out-of-phase autocorrelation zero and cross correlation equal to one. Optical orthogonal codes (OOC's) have the lowest out-of-phase autocorrelation and cross-correlation values (both equal to one) among the one-dimensional codes. We compare the performance of our codes to the OOC's for a given probability of error. Experimentally we verify the autocorrelation and crosscorrelation properties of our codes and Park et al. [6] codes. I. INTRODUCTION I N local area networks (LAN's) the traffic being bursty, asynchronous multiplexing schemes are more efficient than synchronous multiplexing schemes. Fiber-optic code division multiple access (FO-CDMA) is one such scheme which is well suited for high speed LAN's. In FO-CDMA the optical signal is spread over a frequency band much wider than the minimum bandwidth required to send the information. The spreading of the signal is achieved by encoding it with a suitable sequence. To exploit the potential of FO-CDMA, optical signal processing is essential. Using fiber-optic tapped delay lines (FO-TDL) it is possible to optically encode/decode the chosen sequences. Conventional bipolar codes used in electronic CDMA are not suitable for FO-CDMA networks using incoherent signal processing. Although coherent signal processing in FO-CDMA is possible in principle, it is not practical because of the difficulty in maintaining the correct phase at the high frequency of the optical carrier. Hence unipolar {0, 1} codes which have low out-of-phase autocorrelation and cross-correlation values are used for asynchronous FO-CDMA networks. Unipolar codes maintain low out-of-phase autocorrelation and cross correlation by minimizing the number of coincidences of ones rather than by cancellation as in bipolar codes. Hence, the number of ones in a unipolar code is less than the number of zeros. The primary task of the receiver or decoder in a FO-CDMA network is to recover the signal in the presence of other interfering signals. Hence codes suitable for FO-CDMA systems should have the following properties [1]:

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