Performance Analysis of Unipolar Code in Different Phase Modulations of Coherent Optical CDMA

This paper examines the bit-error rate (BER) for an optical direct-detection code-division multiple-access (OCDMA) system employing a novel prime code family, hereby referred to as 'Double-Padded Modified Prime Code (DPMPC)' for the spreading and de-spreading operations in addition to coherent homodyne dual-balanced detection. As a coherent modulation, binary phase-shift keying (BPSK) format is deployed where the phase is modulated by either an external phase-modulator or injection-locking methods. The phase limitation and the performance for both phase-modulations including multiple access interferences (MAI) in a shot-noise limited regime are studied and moderate penalty associated with the limited phase excursion is revealed. with bipolar codes such as Gold sequences; whereas, unipolar codes like prime code families, particularly the proposed one (section II), have more flexible code-length and are almost orthogonal. To retain the advantage of 0/1 codes as a power saving option, we consider in this study unipolar signaling along with the employing of binary phase shift keying (BPSK) as a coherent modulation. The capacity of the system using prime codes is limited by the maximum achievable bit-rate of the electronic circuitry generating the pseudo-noise (PN) sequences. We settle on a maximum attainable chip-rate of 10Gchip/s and a desired bit-rate of hundreds of Mbps, leading to a limit in the length of the spreading sequences on the order of hundreds of chips per bit. In this study, the analysis as the signal-to-noise ratios (SNR) of OCDMA systems and corresponding BERs using a new prime code family and two phase modulation methods with coherent detection are investigated. In this paper the analysis starts with introducing new prime code family named double-padded modified prime code (DPMPC) in the following section. Coherent OCDMA architecture is discussed in section III where followed by system analysis of external phase modulator with dual-balanced detector. Section V compares the results and examinations of external phase modulator with injection-locking methods and finally reveals the performance penalty imposed on the OCDMA system as a result of the limited phase excursion of ±0.42π (4). The remarks come in the concluding section.