Showing papers in "IEEE Transactions on Vehicular Technology in 1999"

Capacity of Rayleigh fading channels under different adaptive transmission and diversity-combining techniques

Abstract: We study the Shannon capacity of adaptive transmission techniques in conjunction with diversity-combining. This capacity provides an upper bound on spectral efficiency using these techniques. We obtain closed-form solutions for the Rayleigh fading channel capacity under three adaptive policies: optimal power and rate adaptation, constant power with optimal rate adaptation, and channel inversion with fixed rate. Optimal power and rate adaptation yields a small increase in capacity over just rate adaptation, and this increase diminishes as the average received carrier-to-noise ratio (CNR) or the number of diversity branches increases. Channel inversion suffers the largest capacity penalty relative to the optimal technique, however, the penalty diminishes with increased diversity. Although diversity yields large capacity gains for all the techniques, the gain is most pronounced with channel inversion. For example, the capacity using channel inversion with two-branch diversity exceeds that of a single-branch system using optimal rate and power adaptation. Since channel inversion is the least complex scheme to implement, there is a tradeoff between complexity and capacity for the various adaptation methods and diversity-combining techniques.

ADVISOR 2.1: a user-friendly advanced powertrain simulation using a combined backward/forward approach

Abstract: ADVISOR 2.1 is the latest version of the National Renewable Energy Laboratory's advanced vehicle simulator. It was first developed in 1994 to support the US Department of Energy hybrid propulsion system program and is designed to be accurate, fast, flexible, easily sharable, and easy to use. This paper presents the model, focusing on its combination of forward- and backward-facing simulation approaches, and evaluates the model in terms of its design goals. ADVISOR predicts acceleration time to within 0.7% and energy use on the demanding US06 to within 0.6% for an underpowered series hybrid vehicle (0-100 km/h in 20 s). ADVISOR simulates vehicle performance on standard driving cycles between 2.6 and 8.0 times faster than a representative forward-facing vehicle model. Due in large part to ADVISOR's powerful graphical user interface and Web presence, over 800 users have downloaded ADVISOR from 45 different countries. Many of these users have contributed their own component data to the ADVISOR library.

Design and performance of multicarrier CDMA system in frequency-selective Rayleigh fading channels

Abstract: This paper presents the advantages and disadvantages of a multicarrier code-division multiple-access (MC-CDMA) system. The transmitter/receiver structure and, the bandwidth of transmitted signal spectrum are compared with those of a conventional direct-sequence (DS) CDMA system, and an MC-CDMA design method, how to determine the number of subcarriers and the length of guard interval is discussed. The bit error rate (BER) lower bounds for DS-CDMA and MC-CDMA systems are derived and their equivalence is theoretically demonstrated. Finally, the BER performance in downlink and uplink channels with frequency-selective Rayleigh fading is shown by computer simulation.

Area spectral efficiency of cellular mobile radio systems

Abstract: A general analytical framework quantifying the spectral efficiency of cellular systems with variable-rate transmission is introduced. This efficiency, the area spectral efficiency, defines the sum of the maximum average data rates per unit bandwidth per unit area supported by a cell's base station. Expressions for this efficiency as a function of the reuse distance for the worst and best case interference configurations are derived. Moreover, Monte Carlo simulations are developed to estimate the value of this efficiency for average interference conditions. Both fully loaded and partially loaded cellular systems are investigated. The effect of random user location is taken into account, and the impact of lognormal shadowing and Nakagami (1960) multipath fading is also studied.

On higher order statistics of the Nakagami-m distribution

Abstract: This paper proposes a fading model that leads to a formal, but simple method to obtain the exact formula of the Nakagami-m published in 1960 distribution for m=n/2, with n a nonzero integer number. Based on such a model, the joint distribution of the envelope and its time derivative are accomplished, and exact formulas for the level crossing rate (closed-form formula) and for the average fade duration are derived. Simulation curves and exact formulas are checked against each other and a very good agreement between them is attained. Although derived for discrete values of m (m being an integer multiple of 1/2), there are no mathematical constraints for these expressions to be used for any real value of m/spl ges/1/2.

A Matlab-based modeling and simulation package for electric and hybrid electric vehicle design

Abstract: This paper discusses a simulation and modeling package developed at Texas A&M University, V-Elph 2.01. V-Elph facilitates in-depth studies of electric vehicle (EV) and hybrid EV (HEV) configurations or energy management strategies through visual programming by creating components as hierarchical subsystems that can be used interchangeably as embedded systems. V-Elph is composed of detailed models of four major types of components: electric motors, internal combustion engines, batteries, and support components that can be integrated to model and simulate drive trains having all electric, series hybrid, and parallel hybrid configurations. V-Elph was written in the Matlab/Simulink graphical simulation language and is portable to most computer platforms. This paper also discusses the methodology for designing vehicle drive trains using the V-Elph package. An EV, a series HEV, a parallel HEV, and a conventional internal combustion engine (ICE) driven drive train have been designed using the simulation package. Simulation results such as fuel consumption, vehicle emissions, and complexity are compared and discussed for each vehicle.

Location tracking of mobiles in cellular radio networks

Abstract: Some useful services in cellular radio networks and also a class of handover algorithms require knowledge of the present position and velocity of mobiles. This paper deals with a method to track mobiles by on-line monitoring of field strength data of surrounding base stations at successive time points. Such data is available in present global system for mobile communication (GSM) systems each 0.48 s and also in code-division multiple-access (CDMA) systems for transmission control. Because of strong random fluctuations of the signals, appropriate smoothing is the key point of the procedure. We develop a locally linear prediction model of successive positions as a basis for Kalman filtering. This approach turns out to be extremely successful, achieving average mislocations of 70 m in simulated test runs. Further improvement is possible by using external geographical information.

Effective bandwidth-based admission control for multiservice CDMA cellular networks

Abstract: We develop product form traffic models for single- and multiple-cell code-division multiple-access (CDMA) networks with multiple classes of mobile subscribers. The key feature of this development is the specification of a flexible call admission control procedure that details the numbers of mobiles of each class in each cell that the system operator should allow in order to maintain an acceptable quality of service. Effective bandwidth techniques from the analysis of statistical multiplexing at an asynchronous transfer mode (ATM) based broadband integrated services digital network (ISDN) link are used to give performance guarantees that overcome the variability in interference levels characteristic of CDMA cellular networks. The result is an admissible region bounded by a finite number of hyperplanes and a simple and efficient call admission policy. The CDMA mobile network, operating within the admissible region described, has a very similar form to a circuit-switched network operating with fixed routing. This similarity allows the existing traffic modeling techniques and network management strategies for general loss networks to be applied to CDMA mobile cellular networks. In particular, with standard assumptions on the call arrival processes and holding times, the stationary state distribution has a product form on the truncated state space defined by the call admission strategy.

Radio propagation measurements and prediction using three-dimensional ray tracing in urban environments at 908 MHz and 1.9 GHz

Abstract: There is an explosive growth in the market of wireless communications services in urban areas. New regulatory environments as well as competition in the communications industry require that these systems be deployed quickly and at low cost. Computer-based radio propagation prediction tools are strong candidates for this goal. We introduce an outdoor radio propagation prediction tool using a ray tracing technique in two dimensions (2-D) and three dimensions (3-D). We have compared the predicted and measured results in various propagation environments. Comparisons indicate that 2-D is adequate for a low transmitter while 3-D is recommended for a high transmitter whose height is comparable with or higher than surrounding buildings. In most locations, the computer tool predicts the correct propagation loss with a mean error of less than 7 dB and a standard deviation of less than 8 dB.

A Q-learning-based dynamic channel assignment technique for mobile communication systems

Abstract: This paper deals with the problem of channel assignment in mobile communication systems. In particular, we propose an alternative approach to solving the dynamic channel assignment (DCA) problem through a form of real-time reinforcement learning known as Q learning. Instead of relying on a known teacher, the system is designed to learn an optimal assignment policy by directly interacting with the mobile communication environment. The performance of the Q-learning-based DCA was examined by extensive simulation studies on a 49-cell mobile communication system under various conditions including homogeneous and inhomogeneous traffic distributions, time-varying traffic patterns, and channel failures. Comparative studies with the fixed channel assignment (FCA) scheme and one of the best dynamic channel assignment strategies (MAXAVAIL) have revealed that the proposed approach is able to perform better than the FCA in various situations and is capable of achieving a similar performance to that achieved by MAXAVAIL, but with a significantly reduced computational complexity.

On the teletraffic capacity of CDMA cellular networks

Abstract: The aim of this paper is to contribute to the understanding of the teletraffic behavior of code-division multiple-access (CDMA) cellular networks. In particular, we examine a technique to assess the reverse link traffic capacity and its sensitivity to various propagation and system parameters. We begin by discussing methods of characterizing interference from other users in the network. These methods are extremely important in the development of the traffic models. We begin with a review of several existing approaches to the problem of handling other-cell interference before presenting a novel characterization of the interference in the form of an analytic expression for the interference distribution function in the deterministic propagation environment. We then look at extending the capacity analyses that assume a fixed and equal number of users in every cell to handle the random nature of call arrivals and departures. The simplest way to do this is by modeling each cell of the network as an independent M/G/x/spl infin/ queue. This allows us to replace the deterministic number of users in each cell by an independent Poisson random variable for each cell. The resulting compound Poisson sums have some very nice properties that allow us to calculate an outage probability by analyzing a single random sum. This leads to a very efficient technique for assessing the reverse link traffic capacity of CDMA cellular networks.

Using front and back information for tight vehicle following maneuvers

Abstract: During manual driving, most human drivers often use information about the speed and position of the preceding and following vehicles in order to adjust the position and speed of their vehicles. The purpose of this paper is to design an autonomous intelligent cruise control (AICC) which mimics this human driving behavior. The proposed AICC law uses relative speed and spacing information from the preceding and following vehicles in order to choose the proper control action for smooth vehicle following and for maintaining a desired intervehicle spacing specified by the driver. The vehicle stability and platoon stability (in the case of multiple vehicles) in both directions (backward and forward) are guaranteed by the proposed AICC law. Furthermore, platoon stability is guaranteed for a speed-dependent desired spacing (time headway approach) as well as for a constant desired spacing at all speeds.

Maximal-ratio combining over Nakagami fading channels with an arbitrary branch covariance matrix

Abstract: The error probability of maximal ratio combiners (MRCs) in a correlated Nakagami environment with an arbitrary branch covariance matrix is not available in the literature although some work has been done for two special cases with constant and exponential correlations. Correlation structures of this type, though of theoretical interest, may not match to practical situations, even for an antenna array of a totally symmetrical configuration. In this paper, we tackle the general problem by virtue of characteristic functions, avoiding the difficulty of explicitly obtaining the probability density function (PDF) for the signal-to-noise ratio (SNR) at the MRC output. We derive a simple closed-form solution for arbitrarily correlated channels with an integral fading parameter and a solution in the form of a one-fold integral for a fading parameter of nonintegral values. Simple algorithms have also been developed for their efficient implementation. The formulas are then used to analyze two possible antenna configurations for a base station, ending up with some findings of interest to system design.

A low-complexity frame synchronization and frequency offset compensation scheme for OFDM systems over fading channels

Abstract: This paper presents a fast low-complexity synchronization scheme for orthogonal frequency division multiplexing (OFDM) systems over fading channels. By utilizing the guard interval in OFDM signals, the frame synchronization and the frequency offset estimation are considered simultaneously. The implementation can be simplified by only using the sign bits of the in-phase and the quadrature components of the received OFDM signal for frame synchronization and frequency offset compensation. A frequency-offset independent frame synchronization algorithm is derived, and a low-complexity frequency offset estimator based on the synchronized correlator output is presented in this paper. Due to the subcarrier ambiguity in the guard-interval-based (GIB) frequency detector, the maximum correctable frequency range is limited to /spl plusmn/1/2 of the subcarrier spacing. In this paper, we also present a new frequency acquisition scheme that can solve the subcarrier ambiguity problem and extend the frequency acquisition range to nearly a half of the useful OFDM signal bandwidth.

Capture effects of wireless CSMA/CA protocols in Rayleigh and shadow fading channels

Abstract: We investigate the capture effects for a wireless local area network (LAN) system in the presence of multipath, shadowing, and near-far effects The performance of wireless media access control (MAC) protocols with Rayleigh fading, shadowing, and capture effect are analyzed We consider carrier-sense multiple-access/collision-avoidance (CSMA/CA) protocols as the wireless MAC protocols, since CSMA/CA protocols are based on the standard for wireless LAN's IEEE 80211 We analyze and compare the channel throughput and packet delay for three types of CSMA/CA protocols: basic CSMA/CA, stop-and-wait automatic repeat request (SW ARQ) CSMA/CA, and four-way handshake (4-WH) CSMA/CA We calculate the capture probability of an access point (AP) in a channel with Rayleigh fading, shadowing, and near-far effects, and we derive the throughput and packet delay for the various protocols We have found that the performance of CSMA/CA in a radio channel model is 50% less than in an error-free channel model in low-traffic load, while the throughput and packet delay of CSMA/CA in a radio channel model show better performance than in an error-free channel model in high-traffic load We also found that the 4-WH CSMA/CA protocol is superior to the other CSMA/CA protocols in high-traffic load

CAE tools for quasi-static modeling and optimization of hybrid powertrains

Abstract: Hybrid vehicles offer larger flexibility than conventional powertrains and, therefore, opportunities for improved fuel economy, but they need systematic design and optimization procedures to realize that potential. Especially choosing the best system structure, parametrization, and supervisory control algorithms is not trivial. This paper presents a tool which supports these tasks and which is based on a somewhat unusual system description. With this approach, fast simulations over entire test cycles are achieved on standard computer platforms. To demonstrate the benefits of the proposed tool, three case studies are shown, one including experimental data.

Performance of coherent receivers with hybrid SC/MRC over Nakagami-m fading channels

Abstract: A performance analysis of two hybrid selective combining/maximal ratio combining (SC/MRC) diversity receivers over Nakagami-m (1960) fading channels with a flat multipath intensity profile is presented and numerically compared with that of the conventional SC and MRC schemes. Numerical results for particular cases of interest show that the bit error rate (BER) degradation arising from the use of hybrid SC/MRC instead of MRC is independent of the average signal-to-noise ratio (SNR) regardless of the severity of the fading and that MRC provides a higher rate of improvement than the hybrid SC/MRC as the severity of fading decreases.

Performance analysis of a coded OFDM system in time-varying multipath Rayleigh fading channels

Abstract: We analyze the combined influence of the interchannel and intersymbol interferences, which result from the time variation and delay spread of mobile channels, on the performance of an orthogonal frequency-division multiplexing (OFDM) system. Both analysis and simulation results are presented for uncoded 16-QAM. We also investigate the performance of a Reed-Solomon (RS) coded 16-QAM OFDM system when the number of subcarriers varies: it is observed that there is an optimum number of subcarriers that minimizes the post decoding symbol error probability of the RS code for each channel state.

PreRAKE diversity combining in time-division duplex CDMA mobile communications

Abstract: A preRAKE system is proposed for the time-division duplex (TDD) code-division multiple-access (CDMA) systems for portable communications. Since the up and down links are time slots on the same carrier frequency, they have the same channel impulse response during a short period of time. Instead of building a RAKE receiver in the portable unit, the base station (BS) can preRAKE the signal before transmission in the down link using the channel impulse response estimated from the up link. When the preRAKED signal is convolved with the channel impulse response, the function of the RAKE receiver is automatically performed. The mobile or portable unit uses a conventional receiver and still achieves the diversity gain of a RAKE receiver.

The coverage-capacity tradeoff in cellular CDMA systems

Abstract: In cellular CDMA systems that employ single-user detectors, in-cell interference limits the coverage of the cell. Thus for a given upper limit on transmit power, the coverage of a cell is inversely proportional to the number of users in it. This tradeoff between coverage and number of users is explicitly characterized here. Our analysis may be used in cellular planning to set hard limits on the number of users admitted into the cell in order to meet coverage requirements. Furthermore, our approach allows us to arrive at a precise definition for the pole capacity of a cell, which serves as an upper bound on the number of users a cell can support as the coverage shrinks to zero. We also present a technique to calculate cell coverage as a function of carried traffic, for any given admission policy.

Teletraffic performance evaluation of microcellular personal communication networks (PCN's) with prioritized handoff procedures

Abstract: Evaluates four handoff priority-oriented channel allocation schemes. These give priority to handoff calls by reserving channels exclusively for handoff calls. The measurement-based handover channel adaptive reassignment scheme (MHAR-A) exclusively uses reserved handover channels for newly originated calls if a certain criterion is satisfied. All four schemes studied differ from the conventional guard channel-based handover priority-oriented channel allocation scheme. To study the schemes, a personal communication network (PCN) based on city street microcells is considered. A teletraffic simulation model accommodating a fast moving vehicle is developed, and the performance parameters are obtained. The performances of all four schemes are compared with the nonpriority scheme and the conventional guard channel-based handover priority-oriented channel allocation scheme. It was found that some of the channel allocation algorithms studied improved the teletraffic capacity over the nonpriority and the conventional guard case. Also, the probability of new call blocking and carried traffic was improved for three of the schemes when compared to the conventional guard scheme. The MHAR-A scheme did not perform up to expectation. Nevertheless, it can be used to finely control the communication service quality equivalent to the control obtained by varying the number of handoff channels in a fraction of one. Increasing the number of reserved handover channels in fraction of one can never be achieved in the conventional guard channel-based handover priority-oriented channel allocation scheme.

Application of electrically peaking hybrid (ELPH) propulsion system to a full-size passenger car with simulated design verification

Abstract: An electrically peaking hybrid electric (ELPH) propulsion system is being developed that has a parallel configuration. A small engine is used to supply power approximately equal to the average load power. The operation of the engine is managed by a vehicle controller and an engine controller such that the engine always operates with nearly full load-the optimal fuel economy operation. An induction motor is used to supply the peaking power required by the electrically peaking load. The motor can also absorb the excess power of the engine while the load power is less than the peak. This power, along with the regenerative braking power, can be used to charge the batteries on board to maintain the battery state-of-charge (SOC) at a reasonable level. With the electrically peaking principle, two control strategies for the drive train have been developed. One is called maximum battery SOC control strategy, by which the engine and electric motor are controlled so that the battery SOC is maintained at its top level as much as possible. This control strategy may be used in urban driving in which accelerating and decelerating driving is common and high-battery SOC is absolutely important for normal driving. The other control strategy is called engine turn-on and turn-off control by which the engine is controlled to operate in a turn-on and turn-off manner. This control strategy may be used in highway driving. Based on the ELPH principle and the drive train control strategies, a drive train for a full-size five-seat passenger car has been designed and verified using the V-ELPH computer simulation package.

Interference-based channel assignment for DS-CDMA cellular systems

Abstract: Link capacity is defined as the number of channels available in a link. In direct-sequence code-division multiple-access (DS-CDMA) cellular systems, this is limited by the interference present in the link. The interference is affected by many environmental factors, and, thus, the link capacity of the systems varies with the environment. Due to the varying link capacity, static channel assignment (SCA) based on fixed link capacity is not fully using the link capacity. This paper proposes a more efficient channel assignment based on the interference received at the base station (BS). In the proposed algorithm, a channel is assigned if the corresponding interference margin is less than the allowed interference, and, thus, channels are assigned adaptively to dynamically varying link capacity. Using the proposed algorithm yields more channels than using SCA in such an environment changes with nonhomogeneous traffic load or varying radio path loss. The algorithm also improves service grade by reserving channels for handoff calls.

Multipath resolving with frequency dependence for wide-band wireless channel modeling

Abstract: Multiple ray paths are resolved using high-resolution digital signal processing algorithms. The Cramer-Rao (CR) bound is used as a benchmark where a combination of the singular value decomposition method and the eigen-matrix pencil method is proven to be most successful. The conventional complex channel model for wireless propagation is extended to include the frequency-dependent feature of rays which can be used to classify the ray arrivals and provide physical insight of the channel. A novel complex-time model is used to approximate the suggested model. This approach is important to various applications such as equalizers, RAKE receivers, etc., in wireless communication systems. Five key features (noise immunity, robustness, resolution, accuracy, and physical insight) of the proposed algorithm are studied using numerical examples.

A prioritized handoff dynamic channel allocation strategy for PCS

Abstract: An analytical method is developed to calculate the blocking probability (p/sub b/), the probability of handoff failure (p/sub h/), the forced termination probability (p/sub ft/), and the probability that a call is not completed (p/sub nc/) for the no priority (NPS) and reserved channel (RCS) schemes for handoff, using fixed channel allocation (FCA) in a microcellular system. Based only on the knowledge of the new call arrival rate, a method of assessing the handoff arrival rate for any kind of traffic is derived. The analytical method is valid for uniform and nonuniform traffic distributions and is verified by computer simulations. An extension (generalization) to the nonuniform compact pattern allocation algorithm is presented as an application of this analysis. Based on this extended concept, a modified version of a dynamic channel allocation strategy (DCA) called compact pattern with maximized channel borrowing (CPMCB) is presented. With modifications, it is shown that CPMCB is a self-adaptive prioritized handoff DCA strategy with enhanced performance that can be exploited in a personal communications service (PCS) environment leading either to a reduction in infrastructure or to an increase in capacity and grade of service. The effect of user mobility on the grade of service is also considered using CPMCB.

Handoff ordering using signal prediction priority queuing in personal communication systems

Abstract: Handoffs in a mobile cellular communications environment will become an increasingly important issue as cell sizes shrink to accommodate an increasingly large demand for services. A new handoff ordering method is proposed which can be used to provide rapid handovers with a smaller percentage of dropped calls than other methods. Signal prediction priority queuing (SPPQ) is a generic queuing method which can be adapted to almost any handover technique for the benefit of decreased dropped calls. In typical personal communication systems (PCSs) environment, it is shown that for realistic call-blocking probabilities (2%-6%), SPPQ leads to about 15% fewer dropped calls compared to first-in-first-out (FIFO) queuing. This benefit comes at the expense of a slight increase (<1%) in blocked call percentage. The proposed SPPQ scheme is also compared to a previously developed method called measurement-based priority scheme (MBPS), and, based on worst case scenarios, it is shown to outperform MBPS as well.

A new strategy for the application of genetic algorithms to the channel-assignment problem

Abstract: The channel-assignment problem in cellular radio networks is known to belong to the class of NP-complete optimization problems. So far, this problem has been solved by heuristic assignment strategies or by the application of combinatorial optimization tools like simulated annealing or neural networks. We propose a new powerful approach to the channel-assignment problem by combining the two mentioned groups of solution techniques. The results obtained by the application to a well-known benchmark problem reveal that this absolutely new strategy clearly outperforms the already existing algorithms.

Development of a linear alternator-engine for hybrid electric vehicle applications

Abstract: This paper examines the design and operation of a generation system that utilizes a linear crankless internal combustion engine in conjunction with a linear alternator. This system directly utilizes the linear motion of the piston to drive the alternator rather than first converting to rotary motion. The result is a more compact, reliable, and efficient unit as the system has only one moving part, making the system ideal for use in series hybrid electric vehicles. This paper describes the overall system design as well as the subsystems including the engine and alternator. A dynamic simulation is then presented which utilizes the model developed to determine the output characteristics of the system. The prototype system was successfully tested, and experimental results are also included.

Capacities of single-code and multicode DS-CDMA systems accommodating multiclass services

Abstract: This paper is concerned with capacities of direct-sequence (DS) code-division multiple-access (CDMA) systems accommodating multiclass services with different transmission rates and bit error rates (BER's). The capacities are expressed by inequalities to which the number of accepted calls of each class should conform. Those inequalities are necessary and sufficient conditions satisfying the requirement of the bit energy-to-interference power spectral density ratio. The optimal received power is derived to cause the least interference to other signals while maintaining the acceptable bit energy-to-interference power spectral density ratio. It is shown that if the maximally receivable power of a call of each class is identical in the multicode system and the single-code system using a variable processing gain, the capacities of both systems are also identical in nonfading channels. However, in multipath fading channels, the multicode system is shown to be better than the single-code system in terms of capacity. Capacity is also derived in a dynamic multiple-cell environment.

Nonlinear optimization for freeway control using variable-speed signaling

Abstract: A traffic control problem with a dynamic macroscopic model is considered by means of simulation. An optimal control problem is stated for variable-speed signaling in order to improve traffic behavior near congestion. A traffic state estimator based on the extended Kalman filter is designed to generate real-time estimates of the traffic density and, by means of these, to activate speed signaling. The variable-speed signaling control law is closed loop and is set by minimizing (or maximizing) a performance criterion. The optimization procedure is based on Powell's method, and its off-line execution is computationally tractable on low-cost computers too. Simulation results demonstrate the efficacy of the proposed approach for preventing and reducing congestion.