Crosstalk interference in FSK coherent optical systems

In this work, the impairment due to crosstalk interference in a FSK coherent optical system is investigated. The crosstalk can arise from both co-channel interference due to the dual-filter receiver structure, and inter-channel interference due to the frequency division multiplexing technique in multichannel systems. Here, we perform the analysis of a multichannel system, taking into account simultaneously the two contributions to interference. An approximation of the crosstalk contributions seems necessary in order to carry out the analysis. The joint moment characterization of the filtered phase noise and of the interference is suitable to get accurate results     

Crosstalk performance of integrated optical cross-connects

Crosstalk performance of monolithically integrated multiwavelength optical cross-connects (OXC's) depends strongly on their architecture. In this paper, a semiquantitative analysis of crosstalk in 11 different architectures is presented. Two architectures are analyzed numerically in more detail and the results of the analysis show good agreement with previously reported experimental results     

A coherent photonic wavelength-division switching system forbroad-band networks

A coherent photonic wavelength-division (WD) switching system, utilizing a coherent wavelength switch (λ switch), is proposed. In the proposed coherent λ switch, the tunable wavelength filter function is accomplished using coherent optical detection with a wavelength tunable local oscillator. The coherent photonic WD switching system has the following features; (1) low crosstalk switching for dense WDM signal, and (2) large line capacity capability. Design considerations show that 32 wavelength division channels can be available with a coherent λ switch. It is also shown that a broadband metropolitan-area-network with over 1000 line capacity is possible, using a multistage connection in the coherent λ switches. The switching function of the coherent λ switch is demonstrated in a two-channel wavelength-synchronized switching experiment, using 8-GHz-spaced, 280-Mb/s optical FSK signals     

Crosstalk penalities due to coherent Rayleigh noise inbidirectional optical communication systems

The statistical properties, i.e. power spectral densities and probability density functions, of the noise due to Rayleigh backscattered light and due to interference of backscattered and reflected light are derived from the autocorrelation function of the optical source field. The crosstalk penalties due to Rayleigh backscattering in single- and double-source bidirectional systems are calculated. It is shown that it is possible to realize a bidirectional system with commercially available connectors in which Rayleigh backscattering is the dominant crosstalk contribution. The measured crosstalk penalties and bit-error probabilities in a double-source bidirectional system confirm the theoretical results     

Fully programmable ring-resonator-based integrated photonic circuit for phase coherent applications

A novel ring-resonator-based integrated photonic chip with ultrafine frequency resolution, providing programmable, stable, and accurate optical-phase control is demonstrated. The ability to manipulate the optical phase of the individual frequency components of a signal is a powerful tool for optical communications, signal processing, and RF photonics applications. As a demonstration of the power of these components, we report their use as programmable spectral-phase encoders (SPEs) and decoders for wavelength-division-multiplexing (WDM)-compatible optical code-division multiple access (OCDMA). Most important for the application here, the high resolution of these ring-resonator circuits makes possible the independent control of the optical phase of the individual tightly spaced frequency lines of a mode-locked laser (MLL). This unique approach allows us to limit the coded signal's spectral bandwidth, thereby allowing for high spectral efficiency (compared to other OCDMA systems) and compatibility with existing WDM systems with a rapidly reconfigurable set of codes. A four-user OCDMA system using polarization multiplexing is shown to operate at data rates of 2.5 Gb/s within a 40-GHz transparent optical window with a bit error rate (BER) better than 10/sup -9/ and a spectral efficiency of 25%.     

Crosstalk in a two channel coherent fiber optic FSK system using asemiconductor laser amplifier

Bit-error-rate measurements on a two-channel optical transmission system using a low-reflectivity laser amplifier are presented. The signal lasers are directly frequency-modulated or amplitude-modulated at 1 Gb/s. The crosstalk penalties in an FSK-FSK system are very small and are mainly attributed to the residual small amplitude modulation of the lasers resulting from the direct frequency modulation. The crosstalk in an ASK-FSK system is considerably larger     

分布式全相参雷达相干参数估计及相参性能分析

针对一般结构的分布式全相参雷达,首先推导了包括时延差及"和相位差"的相干参数估计的克拉美—罗界(CRB)闭式解;然后考虑时延和相位补偿误差同时存在时,分析了全相参模式下的输出信噪比增益(oSNRg),基于相干参数估计的CRB,给出了oSNRg的上界的数值解。结论表明,"和相位差"的CRB与信号载频及有效带宽均无关;oSNRg随着发射天线数的增加而提高;而增加接收天线数,能否提高oSNRg取决于输入信噪比大小,较大时则oSNRg随之提高,较小时反而随之下降。最后仿真实验验证了研究结论的正确性。     

Crosstalk in a two-channel coherent fibre-optic ASK system using anoptical amplifier and non-negligible linewidth lasers

Presents measurements of crosstalk penalties in a two-channel ASK system using 1.55 μm DFB lasers and a semiconductor laser amplifier. The amplifier was operated well below gain saturation. No additional receiver sensitivity penalty due to the amplifier was observed for single channel operation. However, the additional crosstalk in the amplifier required at 10% increase of the channel spacing compared to the spacing in the nonamplifier system     

Accurate performance evaluation of weakly coherent optical systems

An accurate performance evaluation approach which uses a closed-form exact analytical expression of the phase noise moments is presented. This enables one to derive a high-order Gaussian quadrature rule for the integrations needed to take into account the phase noise in the computation of error probability. A systematic comparison with results obtained through a Monte Carlo simulation shows that the approach is more accurate than previous methods. The analysis is performed on ASK and FSK heterodyne receivers with integrate-and-dump filtering, envelope detection, and optimized postdetection low-pass filtering. The feasibility of ASK and FSK heterodyne systems at bit rates comparable to the spectral line bandwidth of the laser sources is confirmed. The theory applied seems to be adequate to attack other problems, such as the evaluation of the effects of crosstalk between the FSK filters or among frequency division multiplexed channels     

A framework for analyzing the user membership in cellular CDMAnetworks

A theoretical framework is developed to analyze the user membership in cellular networks. The main interest is on code-division multiple-access (CDMA) networks, although the validity of the proposed approach is not restricted to these networks. The membership of a user terminal to a certain base station (BS) is established if the local mean attenuation to this BS is the lowest one among all surrounding BSs. The estimate of local mean attenuation on the radio links from a mobile terminal to each of the surrounding BSs is carried out through proper filtering of the sequences of received signal strength measurements taken on broadcast signals at regular time intervals. In CDMA networks, pilot channels can be considered, while broadcast control channels can be generically considered in narrow-band systems. A decision is made each time on whether the terminal is to be assigned to its “current” BS or if it would be better served by a different BS. Two performance measures are considered: the probability that the best BS is not selected and the average number of membership switchings per time unit. A channel model with log-normal shadowing and fast fading superimposed is considered. Analytical expressions for the above-mentioned performance measures are derived in a general system scenario. From numerical results, it is seen that a proper choice of filter parameters is required to achieve the lowest values for both performance measures. Moreover, these values are not negligible when the mobile terminal is close to a boundary cell region. The obtained results are considered to be of interest in handover and power control algorithms     

Impact of cell size on capacity and handoff in deployed CDMAnetworks

A study into the impact of cell size parameter on the performance of a CDMA network is presented. The findings suggest optimum cell sizes for maximum CDMA capacity and reduced handoff overhead     

Partially coherent DS-SS performance in frequency selectivemultipath fading

The bit-error rate (BER) performance of a direct sequence spread spectrum (DS-SS) signal, operating over a multipath Rayleigh fading channel, is investigated when corrupted by phase noise as well as additive white Gaussian noise (AWGN). The phase noise arises from phase locked loop (PLL) dynamics and results in imperfect receiver phase estimates whereby the phase errors assume Tikhonov densities. The phase estimates are used by a multipath-combining RAKE receiver for demodulation. Approximate upper-bounds on the bit error probability are obtained and evaluated for different combinations of channel parameters and for various values of the average loop signal-to-noise ratio (SNR). Results indicate that for a PLL with loop SNR 10 dB above the system E _b/η₀, the degradation is less than 3 dB, and for a loop SNR of 20 dB above E_b/η₀, the degradation is less than 1 dB     

Ultimate performance of heterodyne/coherent optical fiber communications

Research and development of heterodyne/homodyne and coherent optical fiber communications have expanded rapidly because of the expectation for repeater separation elongation and superwide-band frequency-division multiplexing. In this paper, recent progress in heterodyne/coherent optical fiber communications is reviewed with emphasis on theoretical and experimental studies pursuing its ultimate performance. The topics discussed are: the recent and future trend in the system design, the transmitter power limit due to fiber nonlinearity and the resulting repeater separation limit, the laser noise problem, the fiber polarization control and polarization diversity, the possibility of narrow frequency-separation frequency-division multiplexing, and finally, a comparison of heterodyne/coherent and photon-counting communications.     

System performance of coherent transmission over cascaded in-linefiber amplifiers

The limitations of cascaded in-line amplifier systems using coherent modulation-demodulation schemes are examined by evaluating the product of the data rate and the transmission distance. The linear amplified spontaneous emission (ASE) accumulation is shown to make the maximum value of the data rate-distance product increase proportionally with the ratio of the amplifier output signal power to the noise figure. It is also shown that the Kerr-nonlinearity-induced phase noise limits the product of the data rate and the third power of the distance, the maximum value of which is inversely proportional to both the amplifier output signal power and the noise figure. The fiber dispersion is known to limit the product of the distance and the square of the data rate by causing waveform distortion. By taking these three relations into account, it is concluded that coherent signal transmission has a maximum in-line amplifier system length of 10³-10⁴ km in the gigabit-per-second range. Among these three factors, the nonlinearity-induced phase noise has the greatest impact     

Tunable photonic bandpass filter with high-frequency response performance

A continuously tunable RF/microwave photonic bandpass filter that simultaneously achieves high skirt selectivity and high stopband attenuation is presented. It is based on controlling the signal polarisation state in a reflective amplified recirculating delay line loop with a polariser at the output. A technique to overcome the phase noise limitation in the structure is also presented. Experimental results demonstrate continuous tuning of the high-performance photonic bandpass filter and a continuously tunable photonic bandpass filter with a low-noise performance.     

A new correlator receiver architecture for noncoherent optical CDMAnetworks with bipolar capacity

A new correlator receiver architecture based on a modified version of unipolar-bipolar correlation is proposed for noncoherent optical fiber code-division multiple-access (CDMA) networks. For this architecture, the receiver average bit-error rate (BER) performance is numerically evaluated as a function of the received optical power for noncoherent transmission and direct detection with the number of simultaneous users as a parameter. The BER performance is also evaluated by a closed-form formula that is developed in this paper. Comparison of the results from the latter with numerical results show that, the formula provides a good approximation to the system performance. Furthermore, the closed form solution suggests that the system can achieve the same capacity as a CDMA system using coherent detection     

两种光子晶体滤波器性能的比较

利用传输矩阵法,通过分别计算由高折射率介质与金属叠加而成的多层膜和由高、低折射率介质交替排列、中间含有间隔的复式膜的透射率,比较了与这两种膜对应的多层膜滤波器和复式膜滤波器的滤波特性。结果表明:多层膜滤波器的角度宽容性较好,但单色性和透射性较差,此类滤波器适合于入射角变化大,但对单色性和透射性要求不高的情况。而复式膜滤波器的性能恰好和多层膜滤波器的相反,特别适合小角度入射的情况。     

基于相干光OFDM的WDM系统传输性能分析

首先对相干光OFDMWDM系统理论模型和基本原理进行了研究,然后基于Opt i syst em仿真软件,对相干光OFDMWDM传输系统进行了仿真分析,并研究了该系统的传输性能,发现基于相干检测的光OFDM技术和WDM技术结合,可以有效抑制光纤色散和非线性影响,提高系统的性能。将此系统与传统的NRZ WDM光传输系统进行比较,发现系统性能较NRZ系统有很大的提高。     

Enhanced performance of phase-conjugated OFDM subcarriers using digital coherent superposition

We have recently demonstrated the improved phase noise tolerance in coherent optical OFDM by digital coherent superposition of optical OFDM subcarriers, which are phase-conjugated pairs with Hermitian symmetry. In this approach, we realized DCS of OFDM subcarriers in single polarization (or channel), denoted as SP-DCS-OFDM. In this paper, we show that another approach is in dual polarizations, or DP-DCS-OFDM. According to the concept of phase-conjugated twin wave, both approaches can also mitigate the fiber nonlinearity to the first order. We prove that both approaches can cancel the inter-carrier interference (ICI) due to phase noise to the second order, and we derive the reduced ICI power to the fourth order using the Wiener phase noise model. In simulation, we show the enhanced tolerance to the laser phase noise and fiber nonlinearity.     

相干多径环境下自适应阵列的性能改进

针对传统自适应波束形成算法无法抑制相干干扰的问题,引入了一种预处理技术--自适应加权空间平滑(Weighted Spatial Smoothing,WSS)算法,结合线性约束最小方差准则,可以有效地抑制相干干扰.同时,针对WSS引起的孔径损失,提出了一种对子阵进行相位补偿,利用全阵做波束形成的方法.理论分析和仿真结果表明,该方法对相干干扰的抑制能力明显优于常规均匀空间平滑(Uniform Spatial Smoothing,USS)算法,而且避免了孔径损失.