Pulse Code-division Multiple-access System Research Articles

Code acquisition in coherent optical pulse CDMA systems utilizing coherent correlation demodulation

A code acquisition technique based on active optical correlation detection is introduced for coherent optical pulse code-division multiple-access (CDMA) systems implemented through coherent correlation demodulation. Passive matched filter acquisition, commonplace in most radio CDMA systems, does not translate directly into the optical domain since synchronization between the incoming optical signal and a local optical code of the intended user at the receiver has to take place in several picosecond duration. Parallel and serial acquisition schemes under the same search algorithm are described and the effect of optical device noise, partial autocorrelation, and multiuser interference on acquisition performance is theoretically analyzed. Numerical results are given for Gold codes. In general, the degree of parallelism inherent in code acquisition techniques for radio systems allows a tradeoff between acquisition speed and hardware cost; however, in optical acquisition schemes, increasing the degree of parallelism with the concomitant hardware cost penalty does not necessarily reduce the time to acquisition. This is due to optical power budget limitations brought about by the need to split the optical signal, although optical amplifier technology can improve the performance of these parallel search schemes. The expressions for the partial correlation and multiuser interference derived in the paper can also be utilized for the analysis of other radio and optical acquisition schemes.

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). 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.

Optimal performance of coherent optical pulse CDMA systems based on code and phase synchronisation and interference cancellation

An optimal multi-user receiver is proposed for asynchronous coherent optical pulse code division multiple access (CDMA) systems. The optical code sequences of all transmitted users are locally generated and correlated to the received signal via a proper code and phase synchronisation process. The resultant correlation signals are integrated over the bit duration and then discriminated to recover the information data in the electrical domain. Since each data bit of any intended user is interfered with by two bits from all other users, multi-stage interference cancellation is employed. By estimation of the random ingredients of all user signals, the multi-user interference is rebuilt and removed from the desired bit. The key advantage within the context of a narrow pulse optical implementation is that the proposed interference cancellation can be performed in the electrical domain. The system implementation is described and the validity of the cancellation technique is theoretically proved. A numerical evaluation for the system performance is provided, and issues with respect to the system application are discussed. The predicted performance of the system offers a theoretical basis for the consideration of optical pulse CDMA systems for use in high-capacity optical fibre networks.

Decision-directed PLL for coherent optical pulse CDMA systems in the presence of multiuser interference, laser phase noise, and shot noise

A digital decision-directed phase-locked loop (PLL) for use in optical pulse code-division multiple-access (CDMA) systems based on coherent correlation demodulation is proposed. PLL performance is affected by multiuser interference, laser phase noise and optical shot noise. The effect of these sources of interference and noise on PLL performance is evaluated based on a nonlinear model (the Fokker-Planck method) since a linear analysis yields large deviations between the analytical results and actual performance at low signal-to-noise ratios (SNRs). After describing the implementation of the PLL, the steady-state probability density function (pdf) of the phase estimator is derived. Numerical evaluation of the variance of the phase estimator is given for Gold codes. The linewidth requirements of the laser for an acceptable phase estimator variance and the value of the optimal loop bandwidth minimizing the impact of the interference and noise on the PLL are discussed.

Limits to the encoding and bounds on the performance of coherent ultrashort light pulse code-division multiple-access

The authors obtain lower and upper bounds on the peak intensity of a spectrally phase encoded ultrashort light pulse as a function of the length of spectral phase encoding. It is shown that using random phase codes for spectral encoding of ultrashort light pulses in a coherent ultrashort light pulse code-division multiple-access (CDMA) system is almost optimal. Furthermore, simple upper bounds and an asymptotic approximation on the bit error rate (BER) for a wide class of ultrashort light pulse CDMA systems are obtained. It is shown that, in spite of being simple, these upper bounds can be used to evaluate the performance of a coherent ultrashort light pulse CDMA system. >