Differential Phase-shift Keying Systems Research Articles

Phase shift keyed systems based on a gain switched laser transmitter

Return-to-Zero (RZ) and Non-Return-to-Zero (NRZ) Differential Phase Shift Keyed (DPSK) systems require cheap and optimal transmitters for widespread implementation. The authors report on a gain switched Discrete Mode (DM) laser that can be employed as a cost efficient transmitter in a 10.7 Gb/s RZ DPSK system and compare its performance to that of a gain switched Distributed Feed-Back (DFB) laser. Experimental results show that the gain switched DM laser readily provides error free performance and a receiver sensitivity of -33.1 dBm in the 10.7 Gbit/s RZ DPSK system. The standard DFB laser on the other hand displays an error floor at 10(-1) in the same RZ DPSK system. The difference in performance, between the two types of gain switched transmitters, is analysed by investigating their linewidths. We also demonstrate, for the first time, the generation of a highly coherent gain switched pulse train which displays a spectral comb of approximately 13 sidebands spaced by the 10.7 GHz modulation frequency. The filtered side-bands are then employed as narrow linewidth Continuous Wave (CW) sources in a 10.7 Gb/s NRZ DPSK system.

Tolerance of laser frequency offset in optical minimum-shift keying transmission systems

A mismatch between laser frequency and delay interferometer (DI) phase is found to be the most critical impairment for the receiver performance in a practical phase-modulated (PM) system. This paper investigates the receiver performance degradation caused by frequency offset between optical signal and DI in a 10 Gb/s minimum-shift keying (MSK) system, and compares it with the conventional PM formats, optical differential-phase-shift keying (DPSK) and differential-quadrature-phase-shift keying (DQPSK), which are nonreturn-to-zero (NRZ) and 50% duty cycle return-to-zero (RZ). Results show that the MSK system is about double times and six times more robust to frequency offset than the DPSK and DQPSK systems operating at the same bit rate, respectively.

On the Statistics of Intrachannel Four-Wave Mixing in Phase-Modulated Optical Communication Systems

We have analytically derived the correlation functions of intrachannel four-wave mixing (IFWM)-induced phase and amplitude noises in phase-modulated optical communication systems. The phase and amplitude noises are correlated with each other for binary phase-shift keying (PSK) systems but uncorrelated for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary PSK systems with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> > 2. We have also derived analytical approximations to the probability distribution of IFWM-induced phase noise for PSK and differential PSK systems. Furthermore, we have studied the performance of an optimal linear phase-noise predictor derived from the IFWM-induced phase-noise autocorrelation function. This yields a performance improvement of 1.8 dB when IFWM-induced phase noise is the dominant impairment, and an improvement of 0.8 to 1.2 dB in the presence of amplified spontaneous emission noise and nonlinear phase noise in typical terrestrial links.

Simultaneous pulse carving misalignment and π-phase-shift monitoring of a return-to-zero differential phase-shift keying system using a coherent hybrid

A technique for monitoring both the pulse carving misalignment and pi phase shift in a return-to-zero (RZ) differential phase-shift keying (DPSK) system based on a phase modulator is proposed and demonstrated. By monitoring the mean-power variation of a hybrid local oscillation lightwave with the modulated signal lightwave without any deliberated control, the technique can provide the information on pulse carving misalignment and phase modulation index simultaneously, as the power variation reflects the asymmetric distribution of RZ-DPSK in a signal constellation diagram. It is characterized as transparent to signal bit rate, central wavelength, and absolute power level. Both the simulation and experiment results show that more than +/- 15% bit duration misalignment and +/- 5% phase shift deviated from pi can be detected.

Error rates of DPSK systems with MIMO EGC diversity reception over Rayleigh fading channels

This paper analyzes the average bit error probability (BEP) of the differential binary and quaternary phase-shift keying (DBPSK and DQPSK respectively) with multiple-input multiple-output (MIMO) systems employing postdetection equal gain combining (MIMO EGC) diversity reception over Rayleigh fading channels. Finite closed-form expressions for the average BEP of DBPSK and DQPSK are presented. Two approaches are introduced to analyze the error rate of DQPSK. The proposed structure for the differential phase-shift keying (DPSK) with MIMO EGC provides a reduced-complexity and low-cost receiver for MIMO systems compared to the coherent phase-shift keying system (PSK) with MIMO employing maximal ratio combining (MIMO MRC) diversity reception. Finally, a useful procedure for computing the associated Legendre functions of the second kind with half-odd-integer order and arbitrarily degree is presented.

The effects of PMD and PDL on the performance of OOK and DPSK systems

Using multiple importance sampling (MIS) technique, the optical communication system performance affected by polarization-mode dispersion (PMD) and polarization-dependent loss (PDL) is simulated numerically. The power penalties and system outage probabilities are obtained for optical communication systems with on-off keying (OOK) or differential phase shift keying (DPSK) modulation. The simulation results show that DPSK always performs better than OOK. When the system has moderate PDL (about 1.5 dB), it is necessary to compensate effectively the fiber PMD in order to enhance the system tolerance to the PMD and PDL. However, the efficiency of PMD compensation will degrade rapidly when the PMD of the whole optical link drops into the low value.

Simultaneous differential phase-shift keying payload and subcarrier-multiplexed label generation using a single modulator and its application to dispersion monitoring

We present a new simultaneous differential phase-shift keying (DPSK) payload and subcarrier-multiplexed (SCM) label generation technique using a single dual-electrode Mach-Zehnder modulator (DE-MZM). We conduct simulations to verify the feasibility of the new SCM labeling scheme and examine the mutual interference between the DPSK payload and the SCM label. By selecting the subcarrier at half of the payload bit-rate frequency, we can significantly suppress the SCM label induced-power penalty to the DPSK payload. Moreover, we apply the SCM label generated by the proposed labeling technique to chromatic dispersion monitoring for DPSK systems. Simulation results show that by detecting radio frequency power of both the clock tone and the SCM label, the monitoring range is greatly improved.

Nonlinear Phase Noise in Optical-Fiber-Communication Systems

Gordon and Mollenauer, in their famous paper published in 1990, laid out how the interplay between the nonlinear Kerr effect in optical fibers and the amplified spontaneous-emission (ASE) noise from the optical-amplifiers results in enhanced levels of noise and degrades the performance of modulation schemes that encode information in, particularly, the phase of the optical carrier. This phenomenon has been termed as nonlinear phase noise in the literature. In this paper, we first present a comparative and critical review of previous techniques that have been proposed for the analysis of nonlinear phase noise by forming a classification framework that reveals some key underlying features. We then present a unifying theory and a comprehensive methodology and computational techniques for the analysis and characterization of nonlinear phase noise and its impact on system performance by building on and extending previous work that we identify as most favorable and systematic. In our treatment, we consider a multichannel multispan optically amplified dense wavelength-division multiplexed system and develop general techniques for the analysis of the intricate interplay among Kerr nonlinearity, chromatic dispersion, and ASE noise, and for computing the bit-error-ratio performance of differential phase-shift-keying (DPSK) systems. By means of the extensive results we present, we demonstrate and argue that correlated noise behavior plays a most significant role in understanding nonlinear phase noise and its impact on DPSK system performance.

Pilot-tone-based WDM monitoring technique for DPSK systems

To evaluate the effectiveness of the pilot-tone-based monitoring technique in differential phase-shift keying (DPSK) systems, we measure the mutual interactions between the pilot tones and DPSK signals and compare the results with those of on-off keying (OOK) signals. The results show that, by using the DPSK format instead of OOK, it is possible to increase the number of wavelength-division-multiplexed channels that can be monitored simultaneously by using pilot tones without using optical demultiplexers by more than one hundred times

Convergence of phase noise in DPSK transmission systems by novel phase noise averagers

This investigation proposes a novel all-optical phase noise averager to reduce residual phase noise in the differential phase-shift keying (DPSK) transmission system with phase-preserving amplitude regenerators. The proposed phase noise averager is based on a phase-sensitive amplifier but does not require an extra phase-locking optical pump beam. It can increase the correlation between the phase noises of neighboring bits and greatly reduce the differential phase noise in the transmission system. Independently of the cascaded spans, analytical analysis demonstrates that, in the DPSK system with repeated averagers, the total differential phase noise will be less than that before the first averager. Theoretical analysis and numerical simulation are carried out and confirm the significant improvement of DPSK signals using the proposed novel phase noise averagers.

5120-km RZ-DPSK transmission over G.652 fiber at 10 Gb/s without optical dispersion compensation

We report the longest unregenerated reach achieved for a 10-Gb/s return-to-zero differential phase-shift keying system on standard dispersion fiber (G.652) with no optical dispersion compensation. A total of 82.433 ns/nm of chromatic dispersion compensation was achieved.

Analysis of intrachannel impairments in differential phase-shift keying transmission systems

The evolution of the phase in a return-to-zero differential phase-shift keying system is studied by the generalized momentum method. The results show that phase fluctuation in the case of dispersion-managed solitons is much smaller than that in quasi-linear systems for a given launch power.

BER Estimation of Optical WDM RZ-DPSK Systems Through the Differential Phase&amp;lt;tex&amp;gt;$Q$&amp;lt;/tex&amp;gt;

The optical return-to-zero differential phase-shift keying system is analyzed to determine the accuracy of the differential phase Q method in estimating the bit-error rate (BER). It is found that it consistently underestimates the BER though it successfully predicts the qualitative behavior of single-channel and wavelength-division-multiplexed systems for back-to-back and point-to-point configurations. A simple modification reduced the underestimation and produced accurate results.

Performance of Electrical Equalizers in Optically Amplified OOK and DPSK Systems

We study the performance of feed-forward equalizers (FFEs) and decision-feedback equalizers (DFEs) in compensating chromatic dispersion and first-order polarization-mode dispersion (PMD) in optically amplified nonreturn-to-zero on-off keying (NRZ-OOK) and NRZ differential phase-shift-keying (DPSK) systems at 10 Gb/s. We use Monte Carlo simulation of the least-mean-square algorithm to estimate equalizer tap weights. Once stationary tap weights are obtained, we compute the bit-error ratio using an analytical method whose accuracy has been verified by simulation. We find that FFE and DFE offer the greatest improvement when combating first-order PMD with NRZ-OOK, reducing power penalties by about 45%. With NRZ-DPSK, the equalizers offer quite limited improvement.

Tolerance to In-Band Coherent Crosstalk of Differential Phase-Shift-Keyed Signal With Balanced Detection and FEC

It is found experimentally that the differential phase-shift-keyed (DPSK) signal with balanced detection has /spl sim/6 dB higher tolerance to in-band coherent crosstalk than on-off-keyed (OOK) signal. We attribute the improved crosstalk tolerance of the DPSK signal to its 6-dB lowered peak signal power as compared to the OOK signal for equal bit-error rate in optical signal-to-noise-ratio-limited systems. The impact of signal coherence on the crosstalk tolerance in a 10-Gb/s DPSK system adopting a standard Reed-Solomon forward-error correction is also investigated.

Error Probability of DPSK Signals With Cross-Phase Modulation Induced Nonlinear Phase Noise

The error probability is derived analytically for differential phase-shift keying (DPSK) signals contaminated by both self- and cross-phase modulation (SPM and XPM)-induced nonlinear phase noise. XPM-induced nonlinear phase noise is modeled as Gaussian distributed phase noise. When fiber dispersion is compensated perfectly in each fiber span, XPM-induced nonlinear phase is summed coherently span after span and is the dominant nonlinear phase noise for typical wavelength-division-multiplexed (WDM) DPSK systems. For systems without or with XPM-suppressed dispersion compensation, SPM-induced nonlinear phase noise is usually the dominant nonlinear phase noise. With longer walkoff length, for the same mean nonlinear phase shift, 10-Gb/s systems are more sensitive to XPM-induced nonlinear phase noise than 40-Gb/s systems.

Numerical simulation of the SPM penalty in a 10-Gb/s RZ-DPSK system

The impact of self-phase modulation-induced nonlinear phase noise in a 10-Gb/s return-to-zero differential phase-shift keying system is studied by numerical simulation. We show that the simple differential phase Q method based on the Gaussian approximation for the phase noise provides a relatively good estimate of the nonlinear penalty.

Performance degradation of phase-modulated systems due to nonlinear phase noise

The error probability is calculated for phase-modulated systems with nonlinear phase noise. Using the assumption that the phase of amplifier noise and nonlinear phase noise are independent of each other, the error probability and penalty are calculated for both phase-shift keying (PSK) and differential phase-shift keying (DPSK) systems. The mean nonlinear phase shift must be less than about 1.00 and 0.63 rad for a penalty less than 1 dB for PSK and DPSK systems, respectively.

Generation and transmission of millimeter-wave data-modulated optical signals using an optical injection phase-lock loop

Generation and transmission of millimeter-wave data-modulated optical signals is presented using an optical injection phase-lock loop (OIPLL). Millimeter-wave signal generation is demonstrated with wide locking range, 30-GHz low phase noise level, -93 dBc/Hz, and a wide frequency tuning range, 4-60 GHz generation demonstrated using optical injection locking only, verified by using OIPLL in the 26-40 GHz range. The OIPLL is also used to transmit error-free 140-Mb/s amplitude shift keying and 68-Mb/s differential phase-shift keying (DPSK) modulated millimeter-wave signals over up to 65 km of uncompensated standard singlemode fiber. The DPSK system uses reference frequency modulation, eliminating the need for optical amplification.

Efficient simulation of coded MC-DS-CDMA systems

We use computer simulations to compare the performance improvements associated with coding on a Gaussianized binary phase-shift keying (BPSK) multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) system and a nonGaussianized differential phase-shift keying (DPSK)-MC-DS-CDMA system. The effects of coding on the Gaussianized system approach the performance of a coded BPSK system operating in a Gaussian channel while the non-Gaussianized system performance is approximately 4 dB from the coded DPSK system operating in a Gaussian channel.