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Issue 4 • Date May 1997

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Displaying Results 1 - 16 of 16
  • Guest Editorial Computer-aided Modeling, Analysis, And Design Of Communication Links

    Publication Year: 1997 , Page(s): 593 - 596
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    Freely Available from IEEE
  • Error-rate patterns for the modeling of optically amplified transmission systems

    Publication Year: 1997 , Page(s): 707 - 716
    Cited by:  Papers (9)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (292 KB)  

    High-speed, photonically amplified digital links have their performance limited by a complex combination of intersymbol interference, laser nonlinearities, and signal-dependent noise. Determination of the worst case patterns cannot rely on the classical eye diagram anymore, but requires more sophisticated computational metrics, which are discussed. A new approach to performance evaluation of these systems is based on error-rate patterns that have been proposed before as a generalization of the classical eye-diagram concept in an experimental environment. We discuss the extension of this approach to the computer-aided modeling, analysis, and design (CAMAD) of these communication links. View full abstract»

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  • Karhunen-Loeve expansion of the WSSUS channel output and its application to efficient simulation

    Publication Year: 1997 , Page(s): 640 - 646
    Cited by:  Papers (35)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (284 KB)  

    This paper derives a Karhunen-Loeve (K-L) expansion of the time-varying output of a multipath Rayleigh fading wide-sense-stationary uncorrelated-scattering (WSSUS) channel. It is shown that under the same mean-squared error condition, the number of terms required by the truncated K-L expansion is less than that of the series expansion obtained by using the discrete-path approximation of the channel so that simulation using the K-L expansion is more efficient. This computational advantage becomes more significant as higher simulation accuracy is required. The derived K-L expansion is applied to develop an efficient simulation technique for digital transmission over a multipath Rayleigh fading WSSUS channel using an optimum receiver. We show that the proposed technique requires shorter computation time than two other known simulation techniques View full abstract»

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  • Quick simulation: a review of importance sampling techniques in communications systems

    Publication Year: 1997 , Page(s): 597 - 613
    Cited by:  Papers (107)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (484 KB)  

    Importance sampling (IS) is a simulation technique which aims to reduce the variance (or other cost function) of a given simulation estimator. In communication systems, this usually, but not always, means attempting to reduce the variance of the bit error rate (BER) estimator. By reducing the variance, IS estimators can achieve a given precision from shorter simulation runs; hence the term “quick simulation.” The idea behind IS is that certain values of the input random variables in a simulation have more impact on the parameter being estimated than others. If these “important” values are emphasized by sampling more frequently, then the estimator variance can be reduced. Hence, the basic methodology in IS is to choose a distribution which encourages the important values. This use of a “biased” distribution will, of course, result in a biased estimator if applied directly in the simulation. However, there is a simple procedure whereby the simulation outputs are weighted to correct for the use of the biased distribution, and this ensures that the new IS estimator is unbiased. Hence, the “art” of designing quick simulations via IS is entirely dependent on the choice of biased distribution. Over the last 50 years, IS techniques have flourished, but it is only in the last decade that coherent design methods have emerged. The outcome of these developments is that at the expense of increasing technical content, modern techniques can offer substantial run-time saving for a very broad range of problems. We present a comprehensive history and survey of IS methods. In addition, we offer a guide to the strengths and weaknesses of the techniques, and hence indicate which techniques are suitable for various types of communications systems. We stress that simple approaches can still yield useful savings, and so the simulation practitioner as well as the technical researcher should consider IS as a possible simulation tool View full abstract»

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  • Gradient estimation for stochastic optimization of optical code-division multiple-access systems .I. Generalized sensitivity analysis

    Publication Year: 1997 , Page(s): 731 - 741
    Cited by:  Papers (4)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (396 KB)  

    For optimizing the performance of optical code-division multiple-access (CDMA) systems, there is a need for determining the sensitivity of the bit-error rate (BER) of the system to various system parameters. Asymptotic approximations and bounds, used for system bit-error probabilities, seldom capture the sensitivities of the system performance. We develop single-run gradient estimation methods for such optical CDMA systems using a discrete-event dynamic systems (DEDS) approach. Specifically, computer-aided techniques such as infinitesimal perturbation analysis (IPA) and likelihood ratio (LR) methods are used for analyzing the sensitivity of the average BER to a wide class of system parameters. It is shown that the above formulation is equally applicable to time-encoded and frequency-encoded systems. Further, the estimates derived are unbiased, and also optimality of the variance of these estimates is shown via the theory of common random variates and importance sampling techniques View full abstract»

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  • Comparisons of a computer-based propagation prediction tool with experimental data collected in urban microcellular environments

    Publication Year: 1997 , Page(s): 677 - 684
    Cited by:  Papers (90)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (184 KB)  

    Engineers designing and installing outdoor and indoor wireless communications systems need effective and practical tools to help them determine base station antenna locations for adequate signal coverage. Computer-based radio propagation prediction tools are now often used in designing these systems. We assess the performance of such a propagation tool based on ray-tracing and advanced computational methods. We have compared its predictions with outdoor experimental data collected in Manhattan and Boston (at 900 MHz and 2 GHz). The comparisons show that the computer-based propagation tool can predict signal strengths in these environments with very good accuracy. The prediction errors are within 6 dB in both mean and standard deviation. This shows that simulations, rather than costly field measurements, can lead to accurate determination of the coverage area for a given system design View full abstract»

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  • A stochastic importance sampling methodology for the efficient simulation of adaptive systems in frequency nonselective Rayleigh fading channels

    Publication Year: 1997 , Page(s): 614 - 625
    Cited by:  Papers (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (552 KB)  

    We present an IS stochastic technique for the efficient simulation of adaptive systems which employ diversity in the presence of frequency nonselective slow Rayleigh fading and additive, white, Gaussian noise. The computational efficiency is achieved using techniques based on importance sampling (IS). We utilize a stochastic gradient descent (SGD) algorithm to determine the near-optimal IS parameters that characterize the dominant fading process. After accounting for the overhead of the optimization algorithm, average speed-up factors of up to six orders of magnitude [over conventional Monte Carlo (MC)] were attained for error probabilities as low as 10-11 for a fourth-order diversity model View full abstract»

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  • A novel approach to modeling of OQPSK-type digital transmission over nonlinear radio channels

    Publication Year: 1997 , Page(s): 647 - 655
    Cited by:  Papers (13)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (360 KB)  

    Offset quaternary PSK-type (OQPSK) digital modulation schemes are often recommended for radio transmission systems employing grossly nonlinear amplifiers, and have found widespread use in land mobile and/or satellite communications. Most of the recommended OQPSK-type schemes-also called MSK-type schemes-jointly exhibit the following features: a constant envelope or, at least, a low envelope fluctuation; a compact spectrum after a power-efficient, saturated amplifier, and a high detection efficiency obtained with simple, low-cost receivers. We introduce a novel approach to computer-aided modeling of OQPSK-type digital transmission over nonlinear radio channels. By taking into account the specific nature of OQPSK-type modulations, when using the Volterra approach as proposed by Benedetto et al. (1987), we conclude that two signal representations (“parallel” and “serial”) can be achieved which are similar to the conventional Laurent (1986) representation of binary CPM signals with h=1/2. With the proposed approach, the OQPSK-type signals have an invariant structure along the transmission path (which is supposed to include, at least, one nonlinearity) in the sense that they are all made of several linear components of the same type. Some examples are given, including signal representation and power spectrum results. The proposed unified signal representations can be very useful for design and performance evaluation of radio communication systems (mobile and/or satellite applications). They combine flexibility, accuracy, and simplicity, and allow high computational efficiencies View full abstract»

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  • A time-domain optical transmission system simulation package accounting for nonlinear and polarization-related effects in fiber

    Publication Year: 1997 , Page(s): 751 - 765
    Cited by:  Papers (30)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (504 KB)  

    The fast-paced evolution of long-haul and high-bit-rate terrestrial and submarine optical transmission links requires powerful analysis tools that take into account all the relevant phenomena in the fiber. To provide such a tool, we developed a time-domain optical system simulation package, integrated in the TOPSIM simulation environment. The fiber simulation module makes use of the vector form of the propagation equations to account for the quasi-degenerate two-mode (the two polarizations) medium propagation characteristics. This way, all polarization-related effects and their interplay with the other linear and nonlinear phenomena in the fiber can be accurately modeled. In particular, the fiber third-order susceptivity, responsible for all major nonlinear effects, is expressed in its actual vector form, so that nonlinear polarization mode coupling could be accounted for. Conventional birefringence and PMD are generated using appropriate random models. A novel feature of the simulator is that it uses time-domain digital filters to simulate dispersion effects, as opposed to the usual FFT-based algorithms. This approach leads to more efficient computing for a wide range of bandwidth and dispersion values. We present the fiber simulation module in detail. As an example of the use of the simulation package, the analysis of a long-haul two-channel transoceanic WDM transmission system is presented View full abstract»

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  • Efficient modeling and evaluation of fiber-fed microcellular networks in a land mobile channel using a GMSK modem scheme

    Publication Year: 1997 , Page(s): 694 - 706
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (444 KB)  

    Fiber feeders have been proposed for simplifying the base station in microcells, and thus make feasible the use of the microcellular architecture to overcome the barrier posed by the finite available spectra. We provide an analytical model to evaluate the exact error performance of the system, which accounts for the statistical effects in the mobile channel and the nonlinearity of the laser diode. The nonlinearity of the optical source coupled with the fading nature of the wireless channel mandates a solution based on simulation. However, the computer run time for the simulation is prohibitive, virtually rendering the model useless. We propose a computer-amenable “simplified model” which reduces the computation time immensely and also provides reasonably tight bounds. The BER performance for various channel conditions is evaluated using both models, and comparative studies are made on fiber-optic and non-fiber-optic systems. The tradeoff between the system performance and system capacity is discussed. Results obtained justify the use of fiber-optic feeders for future microcellular systems from a BER-performance point of view View full abstract»

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  • Performance of digital DECT radio links based on semianalytical methods

    Publication Year: 1997 , Page(s): 667 - 676
    Cited by:  Papers (7)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (388 KB)  

    We report a very efficient semianalytical approach for the performance evaluation of differential detection schemes for GMSK signals of the DECT standard. Precisely, for a given channel, the performance is determined by means of an analytical procedure which includes the saddlepoint approximation. We consider both static channels (with impulse response generated by the simulation program SIRCIM) and two-ray Rayleigh and log-normal fading channels. As a departure from previous works, our receiver includes an all-digital part after the analog differential detection scheme. The digital part includes: (1) a block for the estimation of both the optimum sampling phase and the nonlinear channel coefficients (by making use of the DECT training sequence), (2) a one-tap decision feedback (DF) equalizer, and (3) a block for the evaluation of the approximate optimum bias level (γ e) in the threshold detector. Both the DF equalizer coefficient and γe are based on the nonlinear channel coefficients estimate. For channels with a normalized delay spread up to 0.2, the use of the optimum threshold together with the DF equalizer permits a gain of about 2 dB at BER=10-6 with respect to a receiver without equalization and a zero-level decision threshold. In addition, we discover that, in indoor environments, the 2-bit GMSK detector performs roughly the same as the 1-bit detector. The threshold optimization is also effective in the presence of channels affected by fading. To support this statement, we report the performance of the 1-bit differential detection scheme combined with antenna selection diversity in the presence of a two-ray log-normal and Rayleigh fading channel View full abstract»

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  • Gradient estimation for stochastic optimization of optical code-division multiple-access systems. II. Adaptive detection

    Publication Year: 1997 , Page(s): 742 - 750
    Cited by:  Papers (2)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (300 KB)  

    For pt.I see ibid., vol.15, no.4, p.731-41 (1997). We develop infinitesimal perturbation analysis (IPA)-based stochastic gradient algorithms for deriving optimum detectors with the average probability of bit error being the objective function that is minimized. Specifically, we develop both a class of linear as well as nonlinear (threshold) detectors. In the linear scheme, the receiver despreads the received optical signal with a sequence that minimizes the average bit-error rate. In the case of the threshold detector, the detection threshold for the photoelectron count is optimized to achieved minimum average bit-error rate. These algorithms use maximum likelihood estimates of the multiple access interference based on observations of the photoelectron counts during each bit interval, and alleviate the disadvantage of previously proposed schemes that require explicit knowledge of the interference statistics. Computer-aided implementations of the detectors derived are shown to outperform the correlation detector. Sequential implementations of the adaptive detectors that require no preamble are also developed, and make them very viable detectors for systems subject to temporal variations View full abstract»

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  • From electromagnetics to system performance: a new method for the error-rate prediction of atmospheric communications links

    Publication Year: 1997 , Page(s): 656 - 666
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (164 KB)  

    An original and general method is proposed to predict the impact of electromagnetic effects due to the environment on the error performance of microwave communications systems. The method is based on the concept of a process-oriented model which consists of a simplified representation of the physical phenomenon responsible for transmission impairments and simulates its influence on the signals received by any given communications link. The method yields a thorough characterization of the system impairments in both the time domain, by calculating the fluctuations of the received signal during the evolution of the physical process, and the frequency domain, by inferring the communication channel transfer function from the frequency dependence of the received waveform. In addition, by introducing the simulated channel transfer function as a filter in a communications system simulation software, the method evaluates the degradation induced in the system performance. Three applications as different as scintillation due to turbulence in clouds, depolarization by rain and ice crystals, and interference in the presence of diffraction on obstacles demonstrate the ability of the method to deal with actual radio-communications system impairments View full abstract»

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  • Fast simulation of DS/CDMA with and without coding in multipath fading channels

    Publication Year: 1997 , Page(s): 626 - 639
    Cited by:  Papers (21)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (536 KB)  

    Code-division multiple access (CDMA) has found widespread applications in various applications such as advanced wireless technologies and digital cellular communications. We consider the error-rate performance of asynchronous direct-sequence CDMA in a wireless radio communications environment that is characterized by Rayleigh multipath fading channels. Because of the system complexity, the performance of CDMA systems in such an environment is difficult to assess analytically. Monte Carlo simulations can offer an alternative performance evaluation tool. Unfortunately, in practice, such an alternative is often limited by an excessive computational burden. Importance sampling is a variance reduction technique which can significantly reduce the computational cost required by Monte Carlo simulations. This paper describes the basic principles behind this powerful technique. In particular, it illustrates its use as a computationally efficient and accurate tool for computer-aided design and analysis of uncoded as well as coded CDMA in wireless communications View full abstract»

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  • Computer modeling and simulation of the Optoelectronic Technology Consortium (OETC) optical bus

    Publication Year: 1997 , Page(s): 717 - 730
    Cited by:  Papers (11)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (316 KB)  

    The Optoelectronic Technology Consortium (OETC) 16 Gbit/s, 32-channel parallel fiber optic bus introduces new requirements for modeling and simulation of an optical bus which were not present for previous single-channel optical link systems. These requirements include the simulation of the statistical variation in component parameters, timing skew, and crosstalk. Due to the required simulation of statistical variation in component parameters, a time efficient simulation approach such as the quasi-analytical simulation methodology is vital. In addition, the adoption of a block-oriented simulation environment facilitates mixed-level simulation which minimizes the computational requirements of the bus simulation while achieving the required accuracy in the simulation of all bus components. Simulation results indicate that while noise is not a limiting factor in the performance of the optical bus, timing skew, and device uniformity across the channels of the bus are the most important factors for satisfactory performance. Experimental measurements confirm these simulation results View full abstract»

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  • On a complete simulation model for the design of high-speed digital radios

    Publication Year: 1997 , Page(s): 685 - 693
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (304 KB)  

    This paper describes a complete digital radio system model which takes into account the effects of most degradations due to channel conditions and equipment imperfections. System parameters which are taken into consideration include, but are not limited to, the following: modulator imbalance, filter frequency responses, power amplifier nonlinearities, carrier and symbol timing recovery loops, and synthesizer phase noise. The parameters for each module in the radio can be varied, and the end-to-end performance computed. Furthermore, a novel semianalytic method is developed for the purpose of speeding up the simulation leading to the calculation of the bit error rate (BER) versus Eb/No for the radio model. This novel technique, when compared with classical semianalytic methods, provides 1 dB improvement in the accuracy of the simulation results. The model accurately predicts the radio performance as measured by BER versus E b/No, dispersive fade margin, transmitted frequency spectrum, and transient acquisition responses. Simulation results for 16 QAM and OQPSK systems were compared to measurements on two physical radios. The accuracy of the simulation results was found to be within 0.1 dB in Eb/No at a BER of 10-6 without the RF portions and between 0.2 and 0.45 dB for a complete radio system View full abstract»

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Aims & Scope

IEEE Journal on Selected Areas in Communications focuses on all telecommunications, including telephone, telegraphy, facsimile, and point-to-point television, by electromagnetic propagation.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
Muriel Médard
MIT