Showing papers in "IEEE Transactions on Vehicular Technology in 1998"

Subscriber location in CDMA cellular networks

Abstract: Subscriber radio location techniques are investigated for code-division multiple-access (CDMA) cellular networks. Two methods are considered for radio location: measured times of arrival (ToA) and angles of arrival (AoA). The ToA measurements are obtained from the code tracking loop in the CDMA receiver, and the AoA measurements at a base station (BS) are assumed to be made with an antenna array. The performance of the two methods is evaluated for both ranging and two-dimensional (2-D) location, while varying the propagation conditions and the number of BS's used for the location estimate.

WCDMA-the radio interface for future mobile multimedia communications

Abstract: This paper presents the wide-band code-division multiple-access (WCDMA) radio interface chosen by the ETSI as the basic radio-access technology for the universal mobile telecommunications system (UMTS). A detailed description of the physical layer of ETSI WCDMA is given together with an overview UMTS of the higher layers of the WCDMA radio interface. Finally, the WCDMA performance, based on results from the ETSI evaluation of UMTS radio interface candidates, is presented.

The diversity gain of transmit diversity in wireless systems with Rayleigh fading

Abstract: In this paper, we study the ability of transmit diversity to provide diversity benefit to a receiver in a Rayleigh fading environment. With transmit diversity, multiple antennas transmit delayed versions of a signal to create frequency-selective fading at a single antenna at the receiver, which uses equalization to obtain diversity gain against fading. We use Monte Carlo simulation to study transmit diversity for the case of independent Rayleigh fading from each transmit antenna to the receive antenna and maximum likelihood sequence estimation for equalization at the receiver. Our results show that transmit diversity with M transmit antennas provides a diversity gain within 0.1 dB of that with M receive antennas for any number of antennas. Thus, we can obtain the same diversity benefit at the remotes and base stations using multiple base-station antennas only.

On the statistical properties of deterministic simulation models for mobile fading channels

Abstract: Rice's (9144, 1945) sum of sinusoids can be used for an efficient approximation of colored Gaussian noise processes and is therefore of great importance to the software and hardware realization of mobile fading channel models. Although several methods can be found in the literature for the computation of the parameters characterizing a proper set of sinusoids, less is reported about the statistical properties of the resulting (deterministic) simulation model. In this paper, not only is the simulation model's amplitude and phase probability density function (PDF) investigated, but also higher order statistics [e.g., level-crossing rate (LCR) and average duration of fades (ADFs)]. It is shown that due to the deterministic nature of the simulation model, analytical expressions for the PDF of the amplitude and phase, autocorrelation function (ACF), crosscorrelation function (CCF), LCR, and ADFs can be derived. We also propose a new procedure for the determination of an optimal set of sinusoids, i.e., the method results for a given number of sinusoids in an optimal approximation of Gaussian, Rayleigh, and Rice processes with given Doppler power spectral density (PSD) properties. It is shown that the new method can easily be applied to the approximation of various other kinds of distribution functions, such as the Nakagami (1960) and Weibull distributions. Moreover, a quasi-optimal parameter computation method is presented.

FSA-based link assignment and routing in low-earth orbit satellite networks

Abstract: We propose a new framework for the link assignment (i.e., topological design) problem that arises from the use of intersatellite links (ISL's) in low-earth orbit (LEO) satellite networks. In the proposed framework, we model an LEO satellite network as a finite state automaton (FSA), where each state corresponds to an equal-length interval in the system period of the LEO satellite network. This FSA-based framework allows the link assignment problem in LEO satellite networks to be treated as a set of link assignment problems in fixed topology networks. Within this framework, we study various link assignment and routing schemes. In particular, both regular link assignment and link assignment optimized by simulated annealing are considered. For each link assignment, both static and dynamic routing schemes are considered. Our simulation results show that the optimized link assignment combined with static routing achieves the best performance in terms of both newly initiated call blocking probability and ongoing call blocking probability. The results also show that when the link assignment is the same, static routing gives better performance than dynamic routing since the latter requires a substantial amount of time to stabilize its routing table after a state transition.

A statistical basis for lognormal shadowing effects in multipath fading channels

Abstract: Empirical justifications for the lognormal, Rayleigh and Suzuki (1977) probability density functions in multipath fading channels are examined by quantifying the rates of convergence of the central limit theorem (CLT) for the addition and multiplication of random variables. The accuracy of modeling the distribution of rays which experience multiple reflections/diffractions between transmitter and receiver as lognormal is quantified. In addition, it is shown that the vector sum of lognormal rays, such as in a narrow-band signal envelope, may best be approximated as being either Rayleigh, lognormal or Suzuki distributed depending on the fading channel conditions. These conditions are defined statistically.

A hybrid analytical-simulation procedure for performance evaluation in M-QAM-OFDM schemes in presence of nonlinear distortions

Abstract: Presents a hybrid analytical-simulation procedure for performance evaluation in M-ary quadrature amplitude modulation (M-QAM) orthogonal frequency-division multiplexing (OFDM) digital radio systems in the presence of nonlinear distortions caused by high-power amplifiers (HPAs). The present analysis is carried out considering an additive white Gaussian noise (AWGN) transmission channel. It is shown that, in the case of an OFDM system with a large number of subcarriers, the distortion on the received symbol caused by the amplifier can be modeled, with good approximation, as a "Gaussian nonlinear noise" added to the received symbol. This important result allows a hybrid analytical-simulation approach to solve the problem of performance evaluation. In practice, the simulation aspect is only used to estimate means and variances of the "nonlinear noise". Such estimated parameters are subsequently used to evaluate analytically the system bit-error rate (BER) using an expression, which takes into account both AWGN and "nonlinear noise" effects. The advantage of the proposed method lies in the strongly reduced computational time. In fact, an accurate estimate of the "nonlinear noise" parameters requires only few iterations when compared with a classical semianalytical approach. This is especially true when low BER values (<10/sup -4/) have to be estimated. The proposed procedure is applied to evaluate M-QAM-OFDM performance in the presence of nonlinear distortions caused by traveling-wave tube (TWT) and solid-state-power (SSP) amplifiers.

Fixed channel assignment in cellular radio networks using a modified genetic algorithm

Abstract: With the limited frequency spectrum and an increasing demand for cellular communication services, the problem of channel assignment becomes increasingly important. However, finding a conflict-free channel assignment with the minimum channel span is NP hard. Therefore, we formulate the problem by assuming a given channel span. Our objective is to obtain a conflict-free channel assignment among the cells, which satisfies both the electromagnetic compatibility (EMC) constraints and traffic demand requirements. We propose an approach based on a modified genetic algorithm (GA). The approach consists of a genetic-fix algorithm that generates and manipulates individuals with fixed size (i.e., in binary representation, the number of ones is fixed) and a minimum-separation encoding scheme that eliminates redundant zeros in the solution representation. Using these two strategies, the search space can be reduced substantially. Simulations on the first four benchmark problems showed that this algorithm could achieve at least 80%, if not 100%, convergence to solutions within reasonable time. In the fifth benchmark problem, our algorithm found better solutions with shorter channel span than any existing algorithms. Such significant results indicate that our approach is indeed a good method for solving the channel-assignment problem.

A study of a land mobile satellite channel model with asymmetrical Doppler power spectrum and lognormally distributed line-of-sight component

Abstract: In satellite to mobile communications, there is generally, in addition to the diffuse multipath component, a strong line-of-sight (LOS) component present. The spectral and statistical properties of both components are influenced by shadowing caused by obstacles such as trees, houses, or small buildings. In this paper, an analytical model for a land mobile satellite channel is analyzed that takes into consideration various kinds of shadowing situations. For the LOS component, it is assumed that, first, the received frequency shows a Doppler shift. Second, the amplitude variations coincide with a lognormal distribution. Moreover, it is also assumed that shadowing influences the diffuse scattered component, with the consequence that its Doppler power spectrum gets an asymmetrical shape. The resulting spectral and statistical properties of the analytical model are investigated. Exact solutions for the probability density function (pdf) of the amplitude and phase are given, and approximative solutions for the level-crossing rate (LCR) and average duration of fades (ADFs) are derived. Finally, an efficient deterministic simulation model is presented that enables the implementation of the proposed analytical model on a digital computer.

Nonlinear spacing policies for automated heavy-duty vehicles

Abstract: In the longitudinal control problem for automated heavy-duty vehicles, an important control objective is string stability, which ensures that errors decrease as they propagate upstream through the platoon. It is well known that when vehicles operate autonomously, string stability can he achieved by using speed-dependent spacing with constant time headway. However, this results in large steady-state intervehicle spacings, hence, decreased traffic throughput. This disadvantage is even more pronounced in heavy-duty vehicles, which require larger time headways due to their low actuation-to-weight ratio. In this paper, we develop two new nonlinear spacing policies-variable time headway and variable separation error gait-which all but eliminate this undesirable side effect. The first policy significantly reduces the transient errors and allows us to use much smaller spacings in autonomous platoon operation, while the second one results in smoother and more robust longitudinal control. Furthermore, the two can be combined to yield even better robustness, as is shown through our qualitative analysis.

A new empirical model for indoor propagation prediction

Abstract: In this paper, a new empirical model for indoor propagation prediction is presented. The inspiration for our model is to enhance existing empirical models for indoor propagation prediction by incorporating additional phenomena suggested by electromagnetic techniques such as the uniform theory of diffraction (UTD), but still retain the straightforwardness of the empirical approach. The advantage is that computation time for indoor propagation prediction is low without greatly compromising prediction accuracy. Comparisons of our predicted results to measurements indicate that improvements in accuracy over conventional empirical models are achieved.

Effect of soft and softer handoffs on CDMA system capacity

Abstract: The effect of soft and softer handoffs on code-division multiple-access (CDMA) system capacity is evaluated for unsectorized and sectorized hexagonal cells according to an average bit energy-to-interference power spectral density, which corresponds to a bit-error rate (BER) of 10/sup -3/. The effect of imperfect sectorization on sectorization efficiency is also considered. On the reverse link, there is no capacity loss as no extra channels are needed to perform soft handoff, while the macrodiversity provided by soft handoff can improve the reverse-link quality and extend the cell coverage. On the forward link, when soft handoff is employed in unsectorized cells, the capacity loss due to two traffic channels assigned to a user in the handoff zone is 0.2% or 1.1% for a voice activity factor of 3/8 or 1/2, respectively. As the forward-link capacity is higher than that of the reverse link, this small capacity loss does not affect the system capacity. For sectorized cells having three sectors per cell, there are overlapping coverage areas between sectors, where mobiles in these areas are subjected to an increase in cochannel interference. For an overlapping angle of 5/spl deg/, the sectorization efficiency is 0.96 and 0.7 for the reverse-link and forward-link systems, respectively. When soft and softer handoffs are employed, the forward-link sectorization efficiency is improved to 0.97. We find the application of soft and softer handoff improves not only the forward-link capacity, but also the signal-to-interference ratio (SIR) for mobiles near the cell and sector boundaries.

Symbol rate and modulation level-controlled adaptive modulation/TDMA/TDD system for high-bit-rate wireless data transmission

Abstract: This paper proposes a time-division multiple-access/time-division duplex (TDMA/TDD)-based symbol rate and modulation level-controlled adaptive modulation system for high-bit-rate data transmission. The proposed system controls both the symbol rate and modulation level for the next transmission time slot according to the estimated carrier power to noise spectral density ratio (C/N/sub 0/) and delay spread for the time slot to achieve higher bit rate and higher transmission quality as well as higher delay-spread immunity. It is demonstrated by computer simulation and laboratory experiments that the proposed system can achieve a higher average bit rate with higher transmission quality in comparison with the fixed-rate quaternary phase-shift keying (QPSK) system and modulation level-controlled adaptive modulation system in both flat Rayleigh and frequency-selective fading environments. The simulated and experimental results also show that the proposed adaptive modulation techniques can be applied to 1-2-Mb/s indoor and outdoor microcellular systems with its delay spread of up to 250 ns and its terminal mobility of up to pedestrian speed without employing any special antifrequency-selective fading techniques, such as the adaptive equalizer and space diversity.

Evaluation of personal communications dual-antenna handset diversity performance

Abstract: In personal wireless communications systems, multipath propagation has a significant effect on system design and performance Signal strength fading caused by destructive interference between multiple replicas of the signal of interest arriving at the receiver over different paths often is the limiting factor in system range/fidelity Antenna diversity is one technique that can be used to help overcome multipath fading This paper presents a description of experiments, data processing, and results used to evaluate the diversity performance of three candidate dual-antenna handset configurations: two side-mounted planar-inverted F antennas (PIFAs), a back-mounted PIFA with a top-mounted helix, a top-mounted PIFA, and a "flip" monopole In particular, the indoor industrial, scientific, and medical (ISM) band (902-928 MHz) propagation channel was of interest These experiments did not include operator proximity effects, and in these tests, the dual-antenna handset remained stationary while the transmitter was moved along predetermined indoor paths The issue of data normalization for extraction of fast fading behavior from measured data is addressed, with results showing its effect on observed correlation presented Also, measured indoor fading distributions are presented and seen to fit the Rician and Rayleigh models well From the diversity results presented, it is seen that the three proposed dual-antenna handsets yield sufficient decorrelation to warrant consideration for use in diversity systems

Line-of-sight angle of arrival estimation in the outdoor multipath environment

Abstract: Angle of arrival (AOA) estimation utilizing the MUltiple Signal Identification and Classification (MUSIC) algorithm is investigated for land vehicle location systems. A "virtual" antenna array is used to overcome problems inherent to MUSIC when operating in a highly correlated signal environment. Based on assumptions regarding the multipath radio environment, triangulation is possible by estimating the AOA of the line-of-sight (LOS) component in the received signal. Computer simulations are used to analyze the ability of techniques incorporating MUSIC to achieve this. Since triangulation is achieved using a narrowband signal, no new spectrum is required. Instead, the current cellular telephone bands as well as the existing basestation hardware could be exploited resulting in a cost-efficient and easily implemented system. A further cost reduction may be realized by processing received signal data while travelling along a nonlinear trajectory. In this way, an unambiguous angular spectrum is obtained with one antenna.

UMTS universal mobile telecommunications system: development of standards for the third generation

Abstract: The standards for the universal mobile telecommunications system (UMTS) are being developed by the European Telecommunications Standardization Institute (ETSI), UMTS is a set of standards aimed at the global market. It will be a real third-generation global system for mobile communications, adding new features and introducing relevant technological innovations while evolving from both a global system for mobile communications (GSM) and integrated services digital networks (ISDNs), UMTS standardization is a market-driven process, and UMT standards are based on the carefully selected market requirements developed in close cooperation with the UMTS Forum and GSM Memorandum of Understanding Association (MoU), UMTS belongs to the IMT-2000 family and, while supporting existing services, will be capable of offering new and revolutionary services including multimedia and access to the Internet, offering a speed of 2 Mb/s for a single user at a radio-access network. This paper attempts to give an objective view of the UMTS standardization in Europe.

Performance of closed-loop power control in DS-CDMA cellular systems

Abstract: In situations where the round-trip delay between the mobile and the base stations is smaller than the correlation time of the channel, power control schemes using feedback from the base station can effectively compensate for the fast fading due to multipath. We study several closed-loop power control (CLPC) algorithms by analysis and detailed simulation. We introduce a new loglinear model for analyzing the received power correlation statistics of a CLPC scheme. The model provides analytical expressions for the temporal correlation of the power controlled channel parameterized by the update rate, loop delay, and vehicle speed. The received power correlation statistics quantify the ability of closed-loop power control to compensate for the time-varying channel. To study more complex update strategies, detailed simulations that estimate the channel bit-error performance are carried out. Simulation results are combined with coding bounds to obtain quasi-analytic estimates of the reverse link capacity in a direct-sequence code-division multiple-access (DS-CDMA) cellular system. The quasi-analytic approach quantifies the performance improvements due to effective power control in both single-cell and multicell DS-CDMA systems operating over both frequency-nonselective and frequency-selective fading channels. The effect of nonstationary base stations on the system performance is also presented.

An extended Suzuki model for land mobile satellite channels and its statistical properties

Abstract: This paper deals with the statistical characterization of a stochastic process which is a product of a Rice and lognormal process. Thereby, we consider the more general case where the two Gaussian noise processes describing the Rice process are correlated. The resulting process are named as extended Suzuki process, which can be used as a suitable statistical model for describing the fading behavior of large classes of frequency nonselective land mobile satellite channels. In particular, the statistical properties (e.g., probability density function (pdf) of amplitude and phase, level-crossing rate, and average duration of fades) of the Rice process with cross-correlated components as well as of the proposed extended Suzuki process are investigated. Moreover, all statistical model parameters are optimized numerically to fit the cumulative distribution function and the level-crossing rate of the underlying analytical model to measured data collected in different environments. Finally, an efficient simulation model is presented which is in excellent conformity with the proposed analytical model.

The performance of polarization diversity schemes at a base station in small/micro cells at 1800 MHz

Abstract: The aim of this paper is to evaluate experimentally the relationships between cross-polarization discrimination (XPD), signal cross correlation, and polarization diversity gain with horizontally/vertically (HV) polarized reception at the base-station (BS) end at 1800 MHz. The performance of the horizontal/vertical polarization diversity scheme was also compared with a diversity scheme with /spl plusmn/45/spl deg/ slanted polarizations and horizontal space diversity at 1800 MHz in a personal communication system (PCS) mobile network. A measurement campaign was conducted in small/micro cells in different types of areas, taking into account the influence of mobile antenna inclination. According to the measurements, XPD values for horizontal/vertical polarizations vary between 5-15 dB, depending on the environment. Furthermore, XPD values depend highly on the radio propagation path between the BS and mobile station (MS) due to line-of-sight (LOS) and nonline-of-sight (NLOS) situations. Signal cross correlations of horizontal and vertical polarizations in both LOS and NLOS situations were clearly below 0.7, which is the generally accepted value to have a reasonable improvement at the receiving end with diversity. Finally, the results showed that almost equal diversity gain and system performance in a PCS network at 1800 MHz can be achieved in small/micro cells in different environments with /spl plusmn/45/spl deg/ slanted polarizations at the BS end when comparing results with horizontal space diversity. The performance of horizontal/vertical polarization diversity scheme was approximately 1 dB worse than horizontal space diversity.

Characterization of UHF radio propagation channels in tunnel environments for microcellular and personal communications

Abstract: Narrowband and wideband propagation measurements have been conducted in five tunnels. The effects of pedestrians, vehicles, and curvature on propagation are included in the characterization. The narrowband propagation is characterized in terms of power distance law, slow fading, and fast fading statistics: wideband propagation, rms delay spread, and its statistics. The results show that the power distance law is insensitive to the location of the transmit antenna in the cross section of a tunnel, but not to the antenna insertion loss. The lognormal distribution basically fits the slow variation of the received signals, whereas the Rician distribution, the fast fading in the straight empty sections of the tunnels. In the curved tunnel sections, the Rayleigh distribution does not fit the fast fading variation as is expected. Tunnel radio propagation channels are dependent strongly upon frequency. Higher frequency signals exhibit more severe fluctuations and larger rms delay spread. The rms delay spreads are found to be generally less than 25 and 103 ns for the emptied and occupied tunnel conditions, respectively. One can conclude that the tunnel channels have a broad coherent bandwidth and can support the data rate up to 1 Mb per second without equalization.

A generalized QS-CDMA system and the design of new spreading codes

Abstract: A generalized quasi-synchronous code-division multiple-access (QS-CDMA) system for digital mobile radio communications is proposed. In a QS-CDMA system, the relative time delay between the signals of different users is random and restricted in a certain time range, that is, the signals are quasi-synchronous. The analysis shows that the multiple-access interference (MAI) of the QS-CDMA system is determined by the cross-correlation between spreading codes around the origin. To minimize the MAI of the QS-CDMA system, we design a new set of spreading codes. The performance is evaluated according to the criteria of the bit error rate (BER). Analytic results of the BER are obtained by using two methods: Gaussian approximation and characteristic function approaches, which are checked by modified Monte Carlo computer simulations known as "importance sampling." The results indicate that the performance of the QS-CDMA system using the spreading codes we construct is much improved.

Propagation in wide tunnels at 2 GHz: a statistical analysis

Abstract: This paper describes the main statistical characteristics of the propagation channel in a tunnel of large transverse dimensions, with the carrier frequency being near 2 GHz. Theoretical and experimental results on the fading distribution as well as those deduced from a wide-band analysis are presented.

Performance of switched-beam smart antennas for cellular radio systems

Abstract: The performance and feasibility of switched-beam smart antennas for cellular radio systems is investigated. Switched-beam smart antenna systems are shown to either increase the capacity or extend the radio coverage by increasing the carrier-to-interference ratio (CIR). The trunking efficiency degradation caused by narrow-beam sectoring can be recovered or even improved by using a variety of trunkpool techniques. One drawback of switched-beam smart antennas is that the improvement in reverse-channel performance is not uniform over the entire cell area, and this presents a limiting factor on the achievable gains, practical considerations such as power control and the limited deployment of smart antenna cells are also addressed.

Performance analysis for a guaranteed handover service in an LEO constellation with a "satellite-fixed cell" system

Abstract: It is anticipated that the satellite component of the future universal mobile telecommunications system (UMTS) will be based (partly or totally) on non-geostationary (nonGEO) constellations of satellites to serve mixed populations of users, each category being treated through different contracts stipulating different quality of service (QoS). In particular, we envisage a high-quality premium service which guarantees the success of each handover procedure, called guaranteed handover (GH) service, and a low-cost lower quality service called regular service, where handover failures are accepted provided that the probability of a call being unsuccessful does not exceed a given value. This paper proposes a strategy which eliminates forced call terminations due to handover failures, thus allowing the GH service. This procedure applies to low Earth orbit (LEO) constellations using the satellite-fixed cell technique. An analytical model has been derived to calculate QoS parameters for a mixed population of GH and regular users. Providing both GH service to some users and regular service to other users requires an increased satellite capacity with respect to the case where all the users are served with the regular service; this capacity increase has been evaluated as a function of the percentage of GH users, the traffic load per cell, and the considered satellite mobility environment. The GH approach has been validated through the comparison with another scheme which envisages the queuing of handover requests for privileged users.

Multidimensional PRMA with prioritized Bayesian broadcast-a MAC strategy for multiservice traffic over UMTS

Abstract: Multidimensional packet reservation multiple access is proposed as a medium-access control (MAC) strategy for the uplink channel of the UTRA (UMTS terrestrial radio access) time-division/code-division multiple-access (TD/CDMA) mode to benefit from efficient statistical multiplexing on the large common pool of available resources (i.e., slots defined both in time and code domain). A prioritized Bayesian broadcast algorithm is derived to stabilize multidimensional packet reservation multiple access (MD PRMA) and to allow for access delay discrimination of four different access classes. Access delay spread can be selected, and trading voice-dropping ratio against data-access delay is possible. To control multiple-access interference, Bayesian broadcast can be combined with load-based access control. The performance of both frequency-division duplex (FDD) and time-division duplex (TDD) mode is evaluated, the latter particularly relevant for TD/CDMA. For mixed voice, Worldwide Web (WWW) browsing, and e-mail traffic, the UMTS WWW model is used, while the e-mail traffic model is derived here.

A window-based image processing technique for quantitative and qualitative analysis of road traffic parameters

Abstract: Traffic information is an important tool in the planning, maintenance, and control of any modern transport system. Of special interest to traffic engineers are parameters of traffic flow such as volume, speed, type of vehicle, queue parameters, traffic movements at junctions, etc. Various algorithms, mainly based on background differencing techniques, have been applied for this purpose. Since background-based algorithms are very sensitive to ambient lighting conditions, they have not yielded the expectative results. In this paper, we describe a novel approach to measure traffic parameters. This approach is based on applying edge-detection techniques to the key regions or windows. This method of measuring road traffic parameters eliminates the need of a background frame, which is an essential, but unreliable technique for background-based image-detection methods. A dynamic threshold selection technique has also been introduced to select the threshold value automatically. The image process algorithm has been applied to measure basic traffic parameters such a traffic volume, types of vehicles, as well as the complex traffic parameters such as queue parameters and movements of vehicles at a traffic junction.

CDMA cellular systems performance with fading, shadowing, and imperfect power control

Abstract: This paper addresses capacity estimation for cellular code-division multiple-access (CDMA) systems, assuming the IS-95 standard as a reference. Extending a previous analytical method (Viterbi et al., 1994), we obtain a sequence of bounds on capacity, and then we introduce an accurate approximation to reduce computation complexity. The analysis accounts for interference internal and external to the reference cell, fading, shadowing, and imperfect power control. Outage probability is expressed in terms of the characteristic functions (cf's) of the interference and imperfect power control random variables (RV's). The interference contributions are computed on the basis of a Poisson distribution for the number of users in a lognormally shadowed channel. Results are provided for different channel conditions and are validated against Monte Carlo simulations. A comparison against previously published CDMA capacity estimates is carried out, aimed at clarifying some controversial issues. It is also confirmed that large system capacity is achievable under tight power control.

Frame error rates for convolutional codes on fading channels and the concept of effective E/sub b//N/sub 0/

Abstract: Estimation of Viterbi decoder performance over channels with time-varying received signal levels is the subject of this paper. This work is motivated by a desire to obtain good estimates of the frame error rate (FER) for convolutional codes with bit-level interleaving over fading channels subject to practical power control algorithms. The convolutional code performance is quantified through the FER and effective E/sub b//N/sub 0/. where the latter is defined as the E/sub b//N/sub 0/ on an additive white Gaussian noise (AWGN) channel that results in the same FER. Given a received vector of (time-varying) E/sub b//N/sub 0/ values, we compute analytic estimates for the probability of frame error and the effective E/sub b//N/sub 0/ for a Viterbi decoder and interleaver combination. In particular, we validate our analysis using the R=1/3 convolutional code and interleaver used on the IS-95 CDMA reverse channel. Comparisons with simulations show that even for E/sub b//N/sub 0/ vectors with very large variations, our proposed estimates are good to within 0.2 dB for the effective E/sub b//N/sub 0/, giving FER estimates within a factor of two-five of the simulations.

Transient and steady-state short-circuit currents in rectifiers for DC traction supply

Abstract: This paper presents some significant extension and analytical formalization of the knowledge on transient and steady-state short-circuit currents in DC traction systems. Electrical supply substations (ESSs) in modern metrorail and railway DC systems are equipped with 6- and 12-pulse diode bridge rectifiers. Large existing plants are equipped with 6-pulse converters, but the 12-pulse option is preferred in the design of new plants in the parallel configuration. Such a converter presents a good compromise between number and voltage/current rating of semiconductor devices, complexity and rating of transformers, DC-side voltage ripple, and AC-side current harmonic content. In this paper, the author considers the features of 6- and 12-pulse conversion units in terms of transient and steady-state short-circuit currents on zero fault impedance, accurate determination of the transient DC short-circuit current is important for correct rating and tuning of protection devices and power equipment (high-speed circuit breakers, isolator switches, switchgears, DC-busbars, etc.). An analytical solution of the short-circuit behavior is given for zero fault impedance currents: phase currents of the transformer, transient short-circuit DC current, approximate expression of the DC short-circuit current, and accurate expression of the steady-state DC short-circuit current. The general calculation method used in the paper is extensible to any conversion unit with a double secondary transformer.

Analysis of a CDMA soft-handoff algorithm

Abstract: This paper analyzes the performance of a soft-handoff algorithm of the type that has been proposed in IS-95 for a code-division multiple access (CDMA) system. While the analysis is stimulated by IS-95, the focus is more on developing applicable analysis methodology rather than on specifically modeling the present IS-95 algorithm. In the soft-handoff scheme, multiple base stations are involved in the communication to and from one mobile station to improve the handoff performance on the boundaries between two base stations by providing channel diversity. On the other hand, there are additional resources used during soft-handoff. There is, thus, a tradeoff between diversity advantage and resource utilization. The analysis provides quantification for this tradeoff. The analysis is validated with simulation. The analytical results can be used to gain insight and help select the appropriate handoff thresholds.