Coherent optical pulse CDMA systems based on coherent correlation detection

Passive optical matched-filtered detection (MFD) has been employed in many proposed optical pulse code division multiple access (CDMA) system implementations, driving the development of unipolar pseudo-orthogonal codes (incoherent). In this paper, coherent optical pulse CDMA systems based on coherent correlation detection (CCD) through homodyne correlation detection (HCD) and self-HCD directly in the optical domain is proposed. With HCD, optical sequences from a pulsed laser, modulated by the data and encoded by an optical tapped delay-line encoder, are multiplexed in an optical fiber network. At the receiver, the optical code sequence of the intended user is locally generated. Through proper code and carrier phase synchronization, the local optical code is multiplied with the received signal chip by chip via an optical correlator consisting of a 3-dB coupler and a balanced detector. Thresholding is performed in the electrical domain after integration of the optical correlator output over one bit interval. The self-HCD approach utilizes two bipolar code sequences multiplexed alternately in time, obviating the need for the generation of a local code at the receiver. The received signal is divided at the receiver, decoded by two encoders (matched to those at the transmitters), and correlated via the optical correlator. The removal for the need of the local oscillator avoids the stringent implementation issues with HCD such as optical frequency stability and carrier phase noise. Following a description of the two implementations, system performances are theoretically analyzed and a comparison of the several approaches given.