A generalized suboptimum unequally spaced channel allocationtechnique. II. In coherent WDM systems

For pt.I see ibid., vol.46, no.8, p.1027-37 (1998). Four-wave mixing (FWM) in dispersion-shifted optical fiber is a major problem associated with low optical input power levels in optical wavelength-division multiplexed (WDM) systems. To reduce the crosstalk caused by FWM, a generalized suboptimum unequally spaced channel allocation (S-BISCA) technique has been proposed. While the S-USCA technique reduces the PWM power substantially, it also reduces the minimum channel spacing compared to conventional equal channel spacing (ECS) systems when the same number of carrier channels are accommodated in a fixed optical bandwidth. This results in more interchannel interference (ICI) when employing the S-USCA scheme. The power penalty of the ECS and the S-USCA systems caused by crosstalk and frequency drift are investigated and compared in this paper. The superior system performance region, where S-USCA systems out perform ECS systems, is also quantified. For 20-channel systems using an amplitude-shift keying (ASK) heterodyne detection scheme, for instance, results show that the S-USCA technique pays less power penalty up to bit rates of 5.5, 7.5, and 9.5 Gb/s, when all channels have identical states of polarization and the launched input power per channel P_in, equals to -6, -3, and 0 dBm, respectively     

WDM systems with unequally spaced channels

Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A method is discussed to find non-uniform channel separations for which no four-wave mixing product is superimposed on any of the transmitted channels, therefore suppressing FWM crosstalk. The residual crosstalk, due to channel power depletion only, is analytically evaluated for intensity-modulated repeaterless wavelength-division-multiplexed (WDM) systems and compared to experimental results. The theory includes the effect of the channel depletion on the amplitude of each phase-matched FWM wave. The probability of error is evaluated including the statistics of the pattern dependent channel depletion. The BER curve computed for an 8-channel WDM system is found to be in good agreement with experimental results. In the experiment, repeaterless transmission of eight 10 Gb/s WDM channels over 137 km (11 Tb/s-km) of dispersion-shifted fiber was demonstrated and error-free operation was achieved over a wide range of input powers using unequally spaced channels. The same system with equally spaced channels could not achieve a probability of error lower than 10^-6. The use of unequal channel spacing allowed fiber input power to be increased by as much as 7 dB, which could be translated into a fivefold increase of the bit rate per channel (and therefore of the system capacity), or to an increase in the system length of about 30 km     

Fast frequency allocation in WDM systems with unequally spaced channels

A new method for allocating the unequally space channels (USCs) in WDM systems to minimise the crosstalk caused by four-wave mixing is presented. The proposed method has the advantages of easy computer implementation, fast generation of USC sets, and flexible change of system parameters. The results show that a larger-size set can be easily constructed from the smaller-size set to facilitate engineering applications. The optimal/suboptimal USC sets are also obtained.     

Performance enhancement of IM/DD optical wireless systems

Due to the fast development of telecommunications and the great demand of bandwidth, optical wireless communication (OWC) has become a promising trend. OWC offers flexible networks for both indoor and outdoor communications providing high security, high speed and a license free spectrum. Data transmission over indoor OWC systems with diffused channel model at high speed will extend the channel impulse response over several symbol periods with many reflections causing inter-symbol interference. This interference distorts the data symbols being transmitted and affects the signal recovery at the receiver side. This paper studies error control coding for OWC systems over additive white Gaussian noise and ceiling bounce channel models. It considers Hamming coding and channel equalization for unipolar optical orthogonal frequency division multiplexing (OFDM) system. The objective is to mitigate the channel distortion, and hence achieve reliable transmission and correct detection at the receiver side.     

Efficient coding techniques for ADO-OFDM in IM/DD systems

Optical wireless communication (OWC) is an age-long technology, which is based on optical data transmission through free space, and it can be implemented in both indoor and outdoor applications. Asymmetrically clipped DC-biased optical orthogonal frequency division multiplexing (ADO-OFDM) is a modulation scheme that can be utilized in indoor OWC systems. It is based on transmitting DC-biased optical OFDM (DCO-OFDM) for even-frequency subcarriers and asymmetrically clipped optical OFDM (ACO-OFDM) for odd-frequency subcarriers. The ADO-OFDM exhibits better optical power performance than those of the conventional ACO-OFDM and DCO-OFDM. The main disadvantage of ADO-OFDM is the clipping noise, which mainly affects the even subcarriers. So, in this paper, convolutional and turbo coding techniques are investigated to improve the bit error rate performance of the ADO-OFDM over Additive white Gaussian noise and diffuse channels. Simulation results show that the proposed coded ADO-OFDM using convolutional and turbo coding techniques achieves significantly lower BERs compared to that of the uncoded ADO-OFDM. In addition, turbo-coded ADO-OFDM gives the best BER performance.     

A simplified dispersion limit formula for IM/DD systems and itscomparison with experimental results

A simplified formula for the dispersion limit of single-mode-fiber intensity modulation/direct detection (IM/DD) systems is derived for arbitrary given normalized pulse width U, eye opening penalty X and source linewidth enhancement factor α. From the comparison with published theoretical analyses, computer simulation and experimental data, its validity and the limits of existing formulae are shown. Using this simple and general formula, one can easily obtain the dispersion limit of various fiber types under different working conditions in IM/DD systems     

Cumulative waveform distortion in cascaded optical amplifierrepeaters for multigigabit IM/DD systems

An analysis was conducted of a cumulative pattern-dependent waveform distortion in cascaded semiconductor laser and Er³⁺-doped fiber amplifiers. At 2.5 Gb/s, cumulative waveform distortion limits the number of cascaded amplifiers to about 20 for the semiconductor amplifiers. The Er³⁺-doped fiber amplifier is relatively unaffected-over 100 stages can be cascaded. The Er³⁺ amplifier is seen to be the better choice for long-haul multigigabit systems     

A nonlinear equalization for a PAM4 IM/DD system using MZM

The nonlinear distortion caused by the Mach-Zehnder modulator(MZM) is one of the main limiting factors for the improvement of the transmission performance of the intensity modulation and direct detection(IM/DD) optical communication system. In order to solve the problem, an improved Volterra nonlinear equalization(VNLE) method is proposed. Compared with the traditional VNLE that uses the least mean square(LMS) to calculate the tap coefficients, the improved VNLE uses the least square(LS) method ...     

Limitations in long haul IM/DD optical fibre systems caused by chromatic dispersion and nonlinear Kerr effect

The power penalty induced by dispersion, Kerr nonlinearity and attenuation is numerically investigated for long haul IM/DD systems with cascaded optical amplifiers. The authors demonstrate that the eye opening is critically dependant on the input mean power.<>     

A PAPR reduction technique using Hadamard transform combined with clipping and filtering based on DCT/IDCT for IM/DD optical OFDM systems

In Intensity Modulator/Direct Detection (IM/DD) optical OFDM systems, the high peak-to-power average ratio (PAPR) will cause signal impairments through the nonlinearity of modulator and fiber. In this paper, a joint PAPR reduction technique based on Hadamard transformation and clipping and filtering using DCT/IDCT transform has been proposed for mitigating the impairments in IM/DD optical OFDM system. We then experimentally evaluated the effect of PAPR reduction on the bit error rate (BER) performance and the results show the effectiveness of the proposed technique. At a bit error rate (BER) of 1 × 10⁻³, the receiver sensitivity of the proposed 2.5 Gb/s IM/DD optical OFDM system after 100-km standard single-mode fiber transmission has been improved by 0.8 dB, 1.3 dB and 3.1 dB for a launch power of 6.4 dBm, 8 dBm and 10 dBm respectively when compared with the classical system.     

TDD-MIMO系统中由I/Q不平衡引起的信道非互易性补偿方法

分析了I/Q不平衡引起的信道互易性丧失对系统容量的影响,提出了一种基于信道状态测量的补偿算法。该算法中基站(BS)和移动台(MS)各自对接收信道进行测量而得到上下行链路的信道状态信息(CSI),然后将2条链路的CSI汇集到BS处,BS计算出分别用于BS和MS的校准矩阵。仿真结果表明所提算法能够有效地对I/Q不平衡进行校准,使信道互易性得到保持,从而使系统容量的损失得以弥补。     

Convolutionally coded multicarrier DS-CDMA systems in a multipathfading channel. I. Performance analysis

This paper presents a multicarrier asynchronous direct-sequence code-division multiple-access (DS-CDMA) system wherein the output of a convolutional encoder modulates multiple band-limited DS-CDMA waveforms, which are transmitted in parallel at different carrier frequencies. The receiver detects and combines signals for the desired user and feeds a soft-decision Viterbi decoder. The performance of this system is compared to that of a conventional single-carrier DS-CDMA system with a RAKE receiver, assuming a slowly varying frequency-selective Rayleigh fading channel and assuming the presence of additive white Gaussian noise and multiple-access interference. Results will demonstrate similar performance at roughly equal receiver complexity     

A cost-effective ultra-dense WDM PON system with speed of 12.5 Gbit/s and channel spacing of 12.5 GHz

In this paper, an experimental scheme for cordwood color division multiplexing（CDM） visible light communication（VLC） system is proposed. The principle and structure of the experimental scheme are described. Right angle prisms and band-pass filter are used to make the unit of the optical collector and splitter. We can add or subtract the unit as we need. The cordwood CDM-VLC system can also be effectively used as a model to accomplish color shift keying and color division duplexing. The experimental system provides a new way for researching VLC.     

A 4-wavelength optical multi/demultiplexer for WDM subscriber loop systems using analog baseband video transmission

This paper describes the design principles and performance of optical multi/demultiplexers (MUX/DEMUX's) in wavelength-division multiplexing (WDM) subscriber-loop systems over a 50- μm core diameter, graded-index (GI), multimode fiber, which employ analog baseband video transmission using laser diodes (LD's). In this WDM arrangement, requirements for MUX/DEMUX's are: 1) low insertion loss; 2) no signal degradation caused by optical interchannel crosstalk; 3) only a small amount of analog baseband signal degradation caused by the use of MUX/DEMUX; 4) a small size and simple structure capable of multiplexing three or four wavelengths; and 5) good stability. The newly developed 4-wavelength MUX/DEMUX satisfies the preceding requirements and is suitable for application to WDM subscriber-loop systems using analog baseband signals along with digital signals.     

A pilot-based dynamic channel assignment scheme for wireless access TDMA/FDMA systems

We propose a control architecture for implementing a dynamic channel assignment (DCA) algorithm which optimizes two-way channel quality in a TDMA portable radio system. Computer simulations are used to evaluate the performance of this DCA method. A common control frequency, which is frame-synchronized among base stations, provides (1) beacons for portables to locate base stations and obtain DCA information, (2) broadcast channels for system and alerting information, and (3) pilot signals to permit portables to evaluate downlink interference. This allows low-complexity radio ports and portables to mutually select channels to avoid interference and avoid creating excessive interference. Results from computer simulations demonstrate the good spectrum efficiency of this method and its potential for handling nonuniform traffic demand. This work is targeted toward understanding the implications to local exchange networks of wireless system alternatives that could provide access to those networks.A preliminary work with the same title was presented at the Second International Conference on Universal Personal Communications, October 12–15, 1993, Ottawa, Canada.     

EIA/NRSC DAR systems subjective tests. I. Audio codec quality

The method, results and conclusions of subjective tests of the basic audio quality of nine digital audio radio (DAR) systems are presented. These systems were proposed to the EIA/NRSC committees for standardization in the USA. The subjective audio tests were carried out at the Communications Research Centre in Ottawa, Canada for the Electronics Industries Association (EIA). Clear comparative results were obtained. The discussion focuses on the subjective merits of the various systems in relation to bit rate considerations     

Performance analysis of both hybrid and frequency-hoppedphase-coherent spread-spectrum systems. I. A hybrid DS/FH system

The authors present and analyze a model for both hybrid and frequency-hopped spread-spectrum systems in which a fully digital coherent receiver is used to demodulate the data. A receiver for a hybrid DS/FH (direct-sequence/frequency-hopped) system using a digital delay-lock loop is considered. In the absence of frequency uncertainty, it is shown that the tracking error can be modeled as an ergodic Markov chain with a finite-state set, and the probability density function of the steady-state tracking error is evaluated. When there is a frequency uncertainty, the dynamics of the resulting nonstationary phase error can be obtained, and an expression to evaluate the probability distribution of the first time at which the phase error hits predetermined boundary values is derived. Bit error rate performance is determined in the presence of both additive white Gaussian noise and various types of interference, and the performance is compared to that of noncoherent FSK systems     

电力线OFDM/OQAM通信系统信道相位预处理均衡算法

针对电力线通信（PLC）正交频分复用（OFDM）系统存在频率掩码和频谱资源利用率低等问题,提出将正交频分复用/偏移正交幅度调制（OFDM/OQAM）技术应用到PLC系统中。该技术通过选用频率选择性较好的滤波器来抑制带外干扰,且不需要循环前缀,但是OFDM/OQAM系统在PLC频域复数信道下会产生严重的自干扰,传统的均衡算法并不能有效地消除OFDM/OQAM系统的固有干扰。针对这种情况提出了一种基于信道相位预处理的均衡算法。该算法的思想是让接收信号乘以一个信道相位调整因子,使等效信道的虚部尽量小,以此来减小接收信号中的干扰分量。仿真结果表明,与传统的均衡算法相比,所提算法在误比特率为1.0×10－6时可以获得0.5dB的性能提升。     

A new design of optical add/drop filters and multi-channel filters based on hexagonal PhCRR for WDM systems

Photonic Network Communications - In this research, using photonic crystal dielectric rods with a triangular lattice constant, a photonic crystal ring resonator (PhCRR) has been designed in order...     

A socio-technical model for deploying expert systems. I. Thegeneral theory

This paper is addressed to an audience of managers and senior analysts involved with planning and controlling the knowledge engineering life cycle. A general framework for understanding the sociotechnical implications of expert systems is provided. It is argued that expert systems comprise two distinct but entwined subsystems. There is a technical component that concerns the task domain and the knowledge engineering process. There is a social dimension of how users and managers relate to the system and how the system fits with the organization. The authors believe that understanding the associations between these factors is essential in evaluating the efficiency and effectiveness of expert systems