BER performance evaluation for CPFSK phase and polarizationdiversity coherent optical receivers

The following methods of continuous phase frequency shift keying (CPFSK) signal demodulation are compared for phase and polarization diversity receivers: single filter, dual filter, delay and multiply. Bit error rate (BER) values are obtained in each case showing that for negligible laser linewidths the delay demodulation method outperforms the dual-filter method by approximately 3 dB, and the single-filter method by 6 dB. Since the noise bandwidth for minimum-shift keying (MSK) and delay demodulation is approximately twice as small as for the other methods, one should add another 3 dB to get the gain for that modulation/demodulation method. The influences of nonzero laser linewidth, noise correlation, and the nonGaussian character of the probability density functions of the noise at the sampler are taken into account     

LDPC码在相干光接收机中性能的研究

针对LDPC码在光纤通信系统中的应用,提出将相位估计算法与LDPC译码算法相结合,研究表明将M次幂载波相位估计算法输出的调制信息估计值进行软解调后,能有效地与LDPC软译码算法相结合。     

Effects of laser phase drift on coherent optical CDMA

Code division multiple access (CDMA) has been proposed for use in fiber high speed point-to-point systems. Previous research into CDMA has centered on completely coherent detection, that is, when the phases of all users in the system are known or tracked. While coherent detection is a reasonable assumption in many radio frequency systems, semiconductor lasers suffer from serious phase variation that makes tracking of all phases difficult to accomplish. This paper examines the effects of phase drift on several multiuser detectors for binary phase shift keying (BPSK), as well as two detectors for binary frequency shift keying (BFSK). It is shown that when the desired user's phase can be tracked, BPSK match filtering offers better bit error rates than BFSK, and closed form solutions for the asymptotic error probabilities are derived. When there is no phase tracking, it is shown that the probability of error of the multiuser detectors approaches one half. The analysis is based on a proof of the asymptotic normality of the interference caused by even a single undesired user. All asymptotic results are for infinitely long spreading codes that can be modeled as a random sequence of plus and minus ones. Whether or not the desired user's phase is tracked, all receivers examined suffer from the near-far problem     

相对相位噪声对相干光通信系统性能影响

在使用前向抽运喇曼放大器的相干光正交频分复用传输系统中,由于喇曼抽运的强度噪声和交叉相位调制之间相互作用产生相对相位噪声,导致系统性能大幅下降。为了研究此问题,采用数值分析方法进行了理论分析与实验验证,分析了不同的调制格式下,相对相位噪声对相干光正交频分复用系统的影响,并且比较了正交频分复用多载波系统与单载波系统在相同条件下的系统性能,取得了由相对相位噪声所导致的系统损伤程度数据。结果表明,抽运和信号之间相对较大的离散系数有助于抑制相对相位噪声引起的损伤,高阶调制信号比低阶信号对相对相位噪声耐受性低;频谱相同效率时,正交幅度调制格式比相移键控耐受性更好。     

Performance of coherent optical CPFSK-DD with intersymbolinterference, noise correlation, and laser phase noise

An exact probability of error expression is presented for a narrow-deviation binary CPFSK coherent optical receiver utilizing differential detection (CPFSK-DD). The result is given in terms of the Marcum Q-function and takes into account the non-Gaussian noise statistics at the decision moment, intersymbol interference, noise correlation, and laser phase noise. Numerical results indicate a local oscillator shot-noise-limited receiver sensitivity of 23.4 photons/b when using a modulation index of 0.67 in combination with an IF filter having a sixth-order Butterworth magnitude response and a normalized 3-dB bandwidth of 1.09. For a given IF filter and IF linewidth, it is found that the modulation index and the IF filter bandwidth should be jointly optimized in order to achieve the best overall receiver sensitivity. When the IF linewidth times the differential delay product is 0.34 %, modulation index and the optimum normalized 3-dB bandwidth are found to be 0.72 and 1.1, respectively. By using Monte Carlo simulation, it is demonstrated that adding laser phase noise at the demodulation stage of the analysis is a valid simplifying assumption for a wide range of practical design parameters     

Statistical characterization of filtered phase noise in optical receivers

This article presents an accurate numerical approach to statistically characterize filtered phase noise, usually encountered in the analysis of heterodyne optical fiber communication receivers. Filtered phase noise is generated by simulation. Generated samples are used to estimate the probability density functions and moment generating functions of phase noise random variables. The proposed approach is valid for large as well as small phase noise. Furthermore, it comes close to providing analytical expressions for the computed statistical measures.     

Performance of coherent optical receivers

Coherent optical communications, an area of research that shows great promise for future high-bandwidth and long-haul applications, is reviewed. Coherent optical receivers, which add light to the received signal as part of the detection process, have numerous advantages over direct-detection receivers, most notably increased sensitivity and increased selectivity, at the cost of increased complexity. The performance of coherent optical receivers under shot-noise-limited conditions is reviewed for a variety of modulation and demodulation formats. In addition, laser phase noise is discussed, and its effect on receiver performance is analyzed     

The effect of laser linewidth on coherent optical receivers with nonsynchronous demodulation

Recent theoretical analysis of the effect of using lasers with significant linewidths in coherent optical fiber transmission systems has shown how the design of the optical receiver, particularly the IF stage, affects the sensitivity. This paper reviews this theory for two classes of coherent systems-those making a differential phase measurement, and those not using phase information-where the requirements on laser linewidth are not stringent. We discuss the factors that affect the performance of systems with significant laser phase noise. Wherever possible, we compare the theoretical results with published system experiments. We show that our theory fits the experimental data well and indicates why experimental results have fallen short of the theoretical limits.     

Analysis of optical amplifier noise in coherent opticalcommunication systems with optical image rejection receivers

A detailed theoretical analysis of optical amplifier noise in coherent optical communication systems with heterodyne receivers is presented. The analysis quantifies in particular how optical image rejection receiver configurations reduce the influence of optical amplifier noise on system performance. Two types of optical image rejection receivers are investigated: a novel, all-optical configuration and the conventional, microwave-based configuration. The analysis shows that local oscillator-spontaneous emission beat noise (LO-SP), signal-spontaneous emission beat noise (S-SP), and spontaneous-spontaneous beat noise (SP-SP) can all be reduced by 3 dB, thereby doubling the dynamic range of the optical amplifier. A 2.5-dB improvement in dynamic range has been demonstrated experimentally with the all-optical image rejection configuration. The implications of the increased dynamic range thus obtained are also discussed from a systems point of view     

Impact of laser intensity noise on ASK two-port optical homodyne receivers

The letter describes initial experimental results obtained with a multiport optical homodyne receiver employing a DFB laser. The receiver performance is found to be limited by the intensity noise of the local oscillator rather than by the phase noise, even when the product of the IF linewidth and the bit duration is as large as 0.56. A relative intensity noise level of at least ? 140dB/Hz will be required for a satisfactory receiver performance with ? 15dBm local oscillator power.     

The phase of a vector perturbed by Gaussian noise anddifferentially coherent receivers

Some results are presented regarding the asymptotic distribution of the phase of a vector perturbed by Gaussian noise. It is shown that for large signal-to-noise ratio, the asymptotic distribution of the phase is of the Tikhonov type. This framework is then used for the synthesis of differentially coherent receiver structures, one for M -ary phase-shift keying (MPSK) and the other for minimum-shift keying (MSK). The first structure bridges the performance gap between coherent and differentially coherent demodulation of MPSK. The MSK receiver uses matched filtering with differential demodulation     

The impact of laser phase noise on the coherent subcarriermultiplexing system

In coherent optical subcarrier multiplexing (CSCM) systems, the laser phase noise may cause signal spectrum broadening and hence, causes significant deterioration in the system performance. The impact of phase noise on the CSCM system is analyzed in terms of carrier-to-noise ratio, intermodulation distortion, and adjacent channel crosstalk. The optimal modulation index and carrier to noise ratio are also presented. Some numerical results are outlined     

Moment characterization of phase noise in coherent optical systems

The statistical characterization of the phase noise introduced by a semiconductor laser in a coherent optical transmission system is a key problem in the system performance evaluation. The authors consider the moment characterization, of the complex random process. Starting from the implicit representation of the probability density function through the Fokker-Planck equation, the authors obtain closed form analytical expressions for the moments of the filtered phase noise both in stationary and nonstationary conditions. Then the use of the moments for the computation of probability densities through orthogonal polynomial series expansion and maximum entropy approach is considered in application examples     

Performance of ADC resolution-limited optical coherent receivers with DA-ML carrier phase estimation

We investigate the performance of decision-aided maximum likelihood (DA-ML) carrier phase estimation (CPE) algorithm in analog-to-digital converter (ADC) resolution-limited optical coherent receivers. The signal model of ADC resolution-limited receiver is established, and its bit error rate with DA-ML CPE in M-ary phase-shift keying (MPSK) system is analyzed. Simulation results show that \(N+2\) (N bits per symbol)-bit resolution is acceptable and \(N+3\) bit is sufficient for DA-ML receiver in MPSK \((M=4, 8, 16)\) system, and for 16-ary quadrature amplitude modulation system, 6-bit resolution is reasonable. Simulation and theoretical analyses both indicate that the optimal ADC quantization range equals to the average amplitude of received symbols which carry the information symbols with the highest energy on the constellation.     

Intensity noise in ASK coherent lightwave receivers

The effect of local oscillator intensity noise on the performance of two and three-branch ASK homodyne receivers and single-branch ASK heterodyne receivers is investigated and an optimum local oscillator power is found. At optimum local oscillator power, both the three-branch and heterodyne receivers are found to have a somewhat better sensitivity than the two-branch receiver. If the local oscillator power is high than the optimum value, the three-branch receiver is significantly less sensitive to intensity noise than the other two receivers     

Digital adaptive phase noise reduction in coherent optical links

Coherent optical links enable high-density constellations and, consequently, a higher throughput. However, the phase noise associated with the transmitter and the receiver lasers is a challenging issue in coherent lightwave systems. The authors present an approach that relies on using digital signal processing techniques to compensate for the laser phase-noise effects. The proposed approach exploits the digital processing power to address the problems arising from optical imperfections. The authors present an adaptive filtering scheme that reduces the effect of the laser phase noise and, consequently, relaxes the laser linewidth requirement. The proposed approach shows how the signal processing techniques can be exploited to compensate for the optical impairments by utilizing the continuing scale down in size and power in very large scale integration (VLSI) technology.     

Analysis methods for optical heterodyne DPSK receivers corrupted bylaser phase noise

Two methods are presented for analyzing the effects of phase noise on the performance of an optical heterodyne binary differential-(DPSK) system. The first method utilizes a perturbation solution for filtered phase noise. By comparing the results of this analysis with simulated results, it is shown that the perturbation solution is accurate for laser linewidths up to at least 10% of the bit rate. Using this analysis, the accuracy of the widely used approximation, whereby the effects of filtering on the magnitude of the phase-noise corrupted signal are neglected, is verified. The author's second method is based on moments of random variables. As the level of phase noise in a practical DPSK system must be small, an improved formulation for the moments of the filtered phase noise is derived. It removes the major cause of this numerical instability. A maximum-entropy probability density function estimation technique is applied to the problem of analyzing the performance of a DPSK receiver. By comparing results with those obtained using the perturbation analysis, it is found that the moment-based method is effectively limited to relatively large error probabilities     

Influence of extinction ratio on performance of optical receivers incorporating laser preamplifiers

Reports the performance degradation of optical receivers incorporating semiconductor laser amplifiers (SLAs) caused by the nonzero extinction ratio of the input optical signal. The resulting sensitivity penalty dependencies on bit rate, SLA gain and facet reflectivity are also investigated. The results clearly show that the new receiver can be more affected by a finite extinction ratio compared to a conventional pin or APD receiver. Using a bandpass optical filter to reduce amplifier spontaneous emission noise will increase the extinction ratio penalty.<>     

Phase diversity techniques for coherent optical receivers

In the present state of the art, coherent optical receivers most often operate in the heterodyne mode. Here a photodiode-amplifier combination having bandwidth greater than twice the bit rate (B) is needed: indeed bandwidths considerably greater than2Bare preferably employed to ease design of the bandpass filter needed for noise limitation, and to avoid demodulator penalties in some modulation schemes. For the high bit rate systems now coming into service (560 Mbit/s-2.4 Gbit/s), the optical receiver design requirements become more stringent for coherent heterodyne operation. The various modes of "zero IF" operation, however, require only baseband receiver module bandwidth. The options available are either homodyne (phase locked) operation, or phase diversity (multiport) techniques. In this paper, we compare these options, and show that phase diversity techniques are capable of good performance for high bit rate coherent receivers. In phase diversity operation, not only is phase locking avoided, but also the necessary frequency locking does not have high stability requirements. Furthermore, there are advantages in operating with a small frequency offset from zero (of the order of 1 percent of the bit rate). An experimental receiver has been operated at 320 and 680 Mbit/s, demodulating both amplitude shift keying (ASK) and differential phase shift keying (DPSK). Operation with FSK is also possible. Sensitivities so far achieved of -47.5 dBm (320-Mbit/s ASK) and -42 dBm (680- Mbit/s ASK) with limited local oscillator power are capable of substantial improvement when higher power local oscillators and lower noise receive modules become available. Demodulation of DPSK at 320 Mbit/s has also been achieved and shows a measured receiver sensitivity improvement of over 4 dB over ASK at the same bit rate and local oscillator power. These practical results show clearly that phase diversity is a very realistic option for high bit rate systems.     

基于延时自零差光相干接收机的激光器相位噪声测试系统

在相干光通信系统中,激光器相位噪声是影响接收机灵敏度的重要因素。针对相干光通信中的激光光源相位噪声测试提出并研究了表征激光器相位噪声的3个关键指标,分别是电场的功率谱密度、相位误差方差和FM噪声谱线,建立了基于延时自零差相干接收技术的窄线宽激光器相位噪声测试系统,实现了系统仿真,并对一窄线宽激光器进行了相位噪声测试,相比传统的自外差线宽测量技术,此方法在满足测试分辨率要求的同时能够更全面表征激光器相位噪声特性。