Showing papers in "IEEE Transactions on Vehicular Technology in 2005"

Ultrawideband propagation channels-theory, measurement, and modeling

Abstract: This paper presents an overview of ultrawideband (UWB) propagation channels. It first demonstrates how the frequency selectivity of propagation processes causes fundamental differences between UWB channels and "conventional" (narrowband) channels. The concept of pathloss has to be modified, and the well-known WSSUS model is not applicable anymore. The paper also describes deterministic and stochastic models for UWB channels, identifies the key parameters for the description of delay dispersion, attenuation, and directional characterization, and surveys the typical parameter values that have been measured. Measurement techniques and methods for extracting model parameters are also different in UWB channels; for example, the concepts of narrowband channel parameter estimation (e.g., maximum-likelihood estimation) have to be modified. Finally, channel models also have an important impact on the performance evaluation of various UWB systems.

Topological overview of hybrid electric and fuel cell vehicular power system architectures and configurations

Abstract: This paper discusses the operational characteristics of the topologies for hybrid electric vehicles (HEV), fuel cell vehicles (FCV), and more electric vehicles (MEV). A brief description of series hybrid, parallel hybrid, and fuel cell-based propulsion systems are presented. The paper also presents fuel cell propulsion applications, more specific to light-duty passenger cars as well as heavy-duty buses. Finally, some of the major fundamental issues that currently face these advanced vehicular technologies including the challenges for market penetration are highlighted.

Nonlinear observers for predicting state-of-charge and state-of-health of lead-acid batteries for hybrid-electric vehicles

Abstract: This paper describes the application of state-estimation techniques for the real-time prediction of the state-of-charge (SoC) and state-of-health (SoH) of lead-acid cells. Specifically, approaches based on the well-known Kalman Filter (KF) and Extended Kalman Filter (EKF), are presented, using a generic cell model, to provide correction for offset, drift, and long-term state divergence-an unfortunate feature of more traditional coulomb-counting techniques. The underlying dynamic behavior of each cell is modeled using two capacitors (bulk and surface) and three resistors (terminal, surface, and end), from which the SoC is determined from the voltage present on the bulk capacitor. Although the structure of the model has been previously reported for describing the characteristics of lithium-ion cells, here it is shown to also provide an alternative to commonly employed models of lead-acid cells when used in conjunction with a KF to estimate SoC and an EKF to predict state-of-health (SoH). Measurements using real-time road data are used to compare the performance of conventional integration-based methods for estimating SoC with those predicted from the presented state estimation schemes. Results show that the proposed methodologies are superior to more traditional techniques, with accuracy in determining the SoC within 2% being demonstrated. Moreover, by accounting for the nonlinearities present within the dynamic cell model, the application of an EKF is shown to provide verifiable indications of SoH of the cell pack.

Energy management strategies for vehicular electric power systems

Abstract: In the near future, a significant increase in electric power consumption in vehicles is expected. To limit the associated increase in fuel consumption and exhaust emissions, smart strategies for the generation, storage/retrieval, distribution, and consumption of electric power will be used. Inspired by the research on energy management for hybrid electric vehicles (HEVs), this paper presents an extensive study on controlling the vehicular electric power system to reduce the fuel use and emissions, by generating and storing electrical energy only at the most suitable moments. For this purpose, both off-line optimization methods using knowledge of the driving pattern and on-line implementable ones are developed and tested in a simulation environment. Results show a reduction in fuel use of 2%, even without a prediction of the driving cycle being used. Simultaneously, even larger reductions of the emissions are obtained. The strategies can also be applied to a mild HEV with an integrated starter alternator (ISA), without modifications, or to other types of HEVs with slight changes in the formulation.

Localization with mobile anchor points in wireless sensor networks

Abstract: Localization is one of the substantial issues in wireless sensor networks. Several approaches, including range-based and range-free, have been proposed to calculate positions for randomly deployed sensor nodes. With specific hardware, the range-based schemes typically achieve high accuracy based on either node-to-node distances or angles. On the other hand, the range-free mechanisms support coarse positioning accuracy with the less expense. This paper describes a range-free localization scheme using mobile anchor points. Each anchor point equipped with the GPS moves in the sensing field and broadcasts its current position periodically. The sensor nodes obtaining the information are able to compute their locations. With the scheme, no extra hardware or data communication is needed for the sensor nodes. Moreover, obstacles in the sensing fields can be tolerated. The localization mechanism has been implemented in the network simulator ns-2. The simulation results show that our scheme performed better than other range-free mechanisms.

Analysis of transmit antenna selection/maximal-ratio combining in Rayleigh fading channels

Abstract: In this paper, we investigate a multiple-input-multiple-output (MIMO) scheme combining transmit antenna selection and receiver maximal-ratio combining (the TAS/MRC scheme). In this scheme, a single transmit antenna, which maximizes the total received signal power at the receiver, is selected for uncoded transmission. The closed-form outage probability of the system with transmit antenna selection is presented. The bit error rate (BER) of the TAS/MRC scheme is derived for binary phase-shift keying (BPSK) in flat Rayleigh fading channels. The BER analysis demonstrates that the TAS/MRC scheme can achieve a full diversity order at high signal-to-noise ratios (SNRs), as if all the transmit antennas were used. The average SNR gain of the TAS/MRC is quantified and compared with those of uncoded receiver MRC and space-time block codes (STBCs). The analytical results are verified by simulation. It is shown that the TAS/MRC scheme outperforms some more complex space-time codes of the same spectral efficiency. The cost of the improved performance is a low-rate feedback channel. We also show that channel estimation errors based on pilot symbols have no impact on the diversity order over quasi-static fading channels.

Solutions to performance problems in VoIP over a 802.11 wireless LAN

Abstract: Voice over Internet Protocol (VoIP) over a wireless local area network (WLAN) is poised to become an important Internet application. However, two major technical problems that stand in the way are: 1) low VoIP capacity in WLAN and 2) unacceptable VoIP performance in the presence of coexisting traffic from other applications. With each VoIP stream typically requiring less than 10 kb/s, an 802.11b WLAN operated at 11 Mb/s could in principle support more than 500 VoIP sessions. In actuality, no more than a few sessions can be supported due to various protocol overheads (for GSM 6.10, it is about 12). This paper proposes and investigates a scheme that can improve the VoIP capacity by close to 100% without changing the standard 802.11 CSMA/CA protocol. In addition, we show that VoIP delay and loss performance in WLAN can be compromised severely in the presence of coexisting transmission-control protocol (TCP) traffic, even when the number of VoIP sessions is limited to half its potential capacity. A touted advantage of VoIP over traditional telephony is that it enables the creation of novel applications that integrate voice with data. The inability of VoIP and TCP traffic to coexist harmoniously over the WLAN poses a severe challenge to this vision. Fortunately, the problem can be largely solved by simple solutions that require only changes to the medium-access control (MAC) protocol at the access point. Specifically, in our proposed solutions, the MAC protocol at the wireless end stations does not need to be modified, making the solutions more readily deployable over the existing network infrastructure.

Intelligent energy management agent for a parallel hybrid vehicle-part I: system architecture and design of the driving situation identification process

Abstract: This two part paper proposes an intelligent energy management agent (IEMA) for parallel hybrid vehicles. IEMA incorporates a driving situation identification component whose role is to assess the driving environment, the driving style of the driver and the operating mode of the vehicle using long and short term statistical features of the drive cycle. This information is subsequently used by the torque distribution and charge sustenance components of IEMA to determine the power split strategy, which is shown to lead to enhanced fuel economy and reduced emissions. In Part I, the overall architecture of IEMA is presented and the driving situation identification process is described. It is specifically shown that a learning vector quantization (LVQ) network can effectively determine the driving condition using a limited duration of driving data. The overall performance of the system under a range of drive cycles is discussed in the second part of this paper.

Efficient Nakagami-m fading channel Simulation

Abstract: An efficient method for generating correlated Nakagami-m fading envelope samples is presented; this method is applicable for arbitrary values of the fading parameter m. The new method is compared to other methods used to generate Nakagami-m random variates. An accurate approximation to the inverse Nakagami-m cumulative distribution function, valid for all values of m, is derived. Uncertainties regarding the autocorrelation of the Nakagami-m fading process are discussed. The fading envelope autocorrelation is determined by simulation and asymptotic analysis.

Observability of error States in GPS/INS integration

Abstract: Observability properties of errors in an integrated navigation system are studied with a control-theoretic approach in this paper. A navigation system with a low-grade inertial measurement unit and an accurate single-antenna Global Positioning System (GPS) measurement system is considered for observability analysis. Uncertainties in attitude, gyro bias, and GPS antenna lever arm were shown to determine unobservable errors in the position, velocity, and accelerometer bias. It was proved that all the errors can be made observable by maneuvering. Acceleration changes improve the estimates of attitude and gyro bias. Changes in angular velocity enhance the lever arm estimate. However, both the motions of translation and constant angular velocity have no influence on the estimation of the lever arm. A covariance simulation with an extended Kalman filter was performed to confirm the observability analysis.

MIMO systems with adaptive modulation

Abstract: Adaptive modulation (AM) schemes in multiple input multiple output (MIMO) systems with a perfect or imperfect channel state information (CSI) at both the transmitter and receiver were investigated. Under an average transmit power and instantaneous bit error rate (BER) constraint, the transmit parameters including the subchannel transmit power and/or spectral efficiency are optimally adapted in the spatial and/or temporal domain to maximize the average spectral efficiency (ASE). Two categories, the continuous rate and discrete rate, of adaptive systems were considered, where the derived asymptotic closed form expressions for the former provided much insight into the latter. Analytical and numerical results showed that a full multiplexing gain was achieved in variable rate variable power (VRVP) systems and variable rate (VR) systems. Variable power (VP) systems with unequal numbers of transmit and receive antennas also achieved the full multiplexing gain, unlike VP systems with equal number of transmit and receive antennas. The effect of CSI imperfection on the ASE and BER was evaluated for VR systems and closed form expressions for the ASE and BER were obtained. They prove to be a useful tool to assess the system performance without taking time consuming AM MIMO system simulations.

Intelligent energy management agent for a parallel hybrid vehicle-part II: torque distribution, charge sustenance strategies, and performance results

Abstract: This paper represents the second part of a two-part paper on development of an intelligent energy management agent (IEMA) for parallel hybrid vehicles. In this part, energy management strategies for the torque distribution and charge sustenance tasks are established and implemented. Driving situation awareness-based fuzzy rule bases are developed to make intelligent decisions on the power split function. A charge sustenance strategy is developed in parallel to maintain adequate reserves of energy in the storage device for supporting an extended range of driving. Simulation study is conducted for the proposed IEMA and performance results are analyzed to evaluate its viability as a possible solution to and an extendable framework for energy management for parallel hybrid electric vehicles.

Wideband channel sounder with measurements and model for the 60 GHz indoor radio channel

Abstract: A wideband channel sounder and measurement results for the short range indoor 60 GHz channel are presented. The channel sounder is based on a 1 gigasamples/s dual channel arbitrary waveform generator and A/D converter/software demodulator, which synthesize and detect a baseband PN sequence with 500 MHz bandwidth. A heterodyne transmitter and receiver translate the baseband PN sequence to and from the 60 GHz band. Ten channel measurements taken across the 59 GHz to 64 GHz range are concatenated to provide a continuous channel measurement covering 5 GHz of bandwidth, resulting in 0.2 ns time domain channel impulse response resolution. The dynamic range and maximum sensitivity performance of the channel sounder are discussed in detail. Comparisons of results with a vector network analyzer based system are shown to verify the accuracy of the sounder. In an extensive measurement campaign with vertically polarized omnidirectional antennas, several different rooms (offices, labs, conference rooms and others) in four different buildings have been investigated. Over 700 channel measurements are the basis for a comprehensive characterization of the short range 60 GHz indoor radio channel with omnidirectional antennas. Finally, a simple stochastic static multipath channel model is derived from the measurement results.

An ultra-wideband transceiver architecture for low power, low rate, wireless systems

Abstract: This paper presents the system architecture, modeling, and design constraints for a baseband, integrated, CMOS, impulse ultra-wideband transceiver targeting very low power consumption on the order of 1 mW. Intended for a sensor network application, the radio supports low communication rates (/spl sim/100 kpbs) and ranging capabilities over short distances (/spl sim/10 m). Based on a "mostly digital" architecture, the analog complexity is reduced by moving the A/D convertor as close to the antenna as is reasonable. Pulses are generated from simple digital switches, overlaying the signal energy on the lower FCC UWB band (0-960 MHz). Reception is achieved using baseband gain blocks feeding a time-interleaved bank of low resolution A/D converters. A window of energy is captured in time and fed to the digital backend for processing. To save power and area, the digital backend implements only a pulse template correlation filter block overlaid with an additional spreading code. As a pulse template is used, no specific channel estimation or interference cancellation is assumed. The system performance is quantified for this case and implementation tradeoffs are explored with a strong focus on reducing power consumption. In particular, the issues of modulation choice, clock generation, gain and noise figure, ADC resolution, and digital signal processing requirements will be discussed.

Downlink and uplink resource allocation in IEEE 802.11 wireless LANs

Abstract: Wireless local area networks (WLANs) based on the IEEE 802.11 standard are becoming increasingly popular and widely deployed. It is likely that WLAN will become an important complementary technology for future cellular systems and will typically be used to provide hotspot coverage. In this paper, the complementary use of WLANs in conjunction with mobile cellular networks is studied. We identify the fairness problem between uplink and downlink traffic flows in the IEEE 802.11 distributed coordination function and then propose an easy solution that can be implemented at the access point (AP) in the MAC layer without modification of the standard for stations (STAs). This solution aims at providing a controllable resource-allocation method between uplink and downlink traffic flows and adapting the parameters according to the dynamic traffic load changes. The proposed solution also enhances the system utilization by reducing the probability of frame collision.

Dynamic simulation for analysis of hybrid electric vehicle system and subsystem interactions, including power electronics

Abstract: Simulation tools for hybrid electric vehicles (HEVs) can be classified into steady-state and dynamic models, according to their purpose. Tools with steady-state models are useful for system-level analysis. The information gained is helpful for assessing long-term behavior of the vehicle. Tools that utilize dynamic models give in-depth information about the short-term behavior of sublevel components. In this paper, a dynamic model of a hybrid electric vehicle that includes fuel cells, batteries, ultracapacitors, and induction machine drives is presented. Simulation results of vehicle configurations with a battery, a fuel cell-battery combination and a fuel cell-ultracapacitor combination are discussed. The focus of the model is a detailed assessment of different subsystem components, particularly component losses.

Iterative learning control of antilock braking of electric and hybrid vehicles

Abstract: Hybrid electric vehicles (HEVs) use multiple sources of power for propulsion which provides great ease and flexibility to achieve advanced controllability and additional driving performance. In this paper, the electric motor in HEV and electric vehicle (EV) propulsion systems is used to achieve antilock braking performance without a conventional antilock braking system (ABS). The paper illustrates that the antilock braking of HEV can be easily achieved using iterative learning control for various road conditions. A vehicle model, a slip ratio model, and a vehicle speed observer were developed to control the antilock performance of HEV during braking. Through iterative learning process, the motor torque is optimized to keep the tire slip ratio corresponding to the peak traction coefficient during braking. Simulations were performed on a compact size vehicle to validate the proposed control method. The control algorithm proposed in this paper may also be used for the ABS control of conventional vehicles.

Ultra-wideband transmitted reference systems

Abstract: This paper derives optimal receiver structures for an ultra-wideband transmitted reference (UWB TR) system in multipath environments, based on the average likelihood ratio test (ALRT) with Rayleigh or lognormal path strength models. Several suboptimal receivers are obtained by either applying an approximation to the log-likelihood function without any specific channel statistical models or by approximating two ALRT optimal receiver structures. It is shown that the generalized likelihood ratio test optimal receiver is one of the suboptimal receiver structures in the ALRT sense. Average bit error probabilities of ALRT receivers are evaluated. Results show that ALRT optimal and suboptimal receivers derived from Rayleigh and lognormal models can perform equally well in each other's environments. This paper also investigates ad hoc cross-correlation receivers in detail, and discusses the equivalence between cross-correlation receivers and one theoretically derived ALRT suboptimal receiver. Results show that the noise /spl times/ noise term in a cross-correlation receiver can be modeled quite accurately by a Gaussian random variable when the noise time/spl times/bandwidth product is large, and cross-correlation receivers are suboptimal structures which have worse performance than ALRT receivers.

An ultrawideband system architecture for tag based wireless sensor networks

Abstract: With the latest improvements in device size, power consumption, and communications, sensor networks are becoming increasingly more popular. There has also been a great increase in the popularity of commercial applications based on ultrawideband (UWB). Impulse radio (IR) based UWB technology utilizes noise-like signal, has potentially low complexity and low cost, is resistant to severe multipath, and has very good time domain resolution allowing for location and tracking applications. In this paper, the architecture and performance of a noncoherent low complexity UWB impulse radio based transceiver designed for low data rate, low cost sensor network applications is presented. The UWB-IR transmitter is based on a delay locked loop (DLL) and UWB monocycle pulse generator. The UWB-IR receiver utilises a noncoherent, energy detection based approach, which makes it largely independent of the shape of the transmit waveform and robust to multipath channels. The test circuits are designed for 0.35 /spl mu/m SiGe BiCMOS technology. This paper presents system simulations results as well as the performance of key functional blocks of the designed UWB application specific integrated circuit (ASIC) transceiver architecture. The simulated power consumption of UWB-IR transceiver circuits is 136 mW with 100% duty cycle with a 3.3 V power supply.

General ICI self-cancellation scheme for OFDM systems

Abstract: One of the challenges in designing orthogonal frequency-division multiplexing (OFDM) systems is their inherent sensitivity to any frequency shift in the signal. A frequency offset between the local oscillators at the transmitter and receiver causes a single frequency shift in the signal, while a time-varying channel can cause a spread of frequency shifts known as the Doppler spread. Frequency shifts ruin the orthogonality of OFDM subcarriers and cause intercarrier interference (ICI); therefore, quickly diminishing the performance of the system. ICI self-cancellation schemes have been proposed to reduce the sensitivity of OFDM systems to frequency shifts. These schemes use signal processing and frequency domain coding to reduce the amount of ICI generated as a result of frequency shifts, with little additional computational complexity. These methods can be used as an alternative to the fine frequency-offset estimation methods to battle oscillator frequency offset or simply be used as an ICI mitigation technique when the system is operating over time-varying channels. We propose a general ICI self-cancellation scheme that can be implemented through windowing at the transmitter and receiver. We show that the previously proposed self-cancellation schemes are equivalent to special cases of this method. Through theoretical analysis of the signal-to-interference ratio and bit-error rate and the use of Monte Carlo simulations, we demonstrate that the proposed system considerably outperforms the existing systems in the presence of frequency offset or time variations in the channel. We consider both coherent and noncoherent systems.

Stray current control in DC mass transit systems

Abstract: Stray current control is essential in direct current (DC) mass transit systems where the rail insulation is not of sufficient quality to prevent a corrosion risk to the rails, supporting and third-party infrastructure. This paper details the principles behind the need for stray current control and examines the relationship between the stray current collection system design and its efficiency. The use of floating return rails is shown to provide a reduction in stray current level in comparison to a grounded system, significantly reducing the corrosion level of the traction system running rails. An increase in conductivity of the stray current collection system or a reduction in the soil resistivity surrounding the traction system is shown to decrease the corrosion risk to the supporting and third party infrastructure.

Comparative assessment of hybrid electric and fuel cell vehicles based on comprehensive well-to-wheels efficiency analysis

Abstract: One of the major issues surrounding the research and development work involving hybrid electric and fuel cell vehicles (HEVs and FCVs) is their overall efficiency of converting the input fuel into actual work at the wheels of the vehicle. The main idea behind efficiency comparisons between HEVs and FCVs is the analysis of their respective well-to-tank (WTT) and tank-to-wheels (TTW) efficiencies, the product of which reveals the well-to-wheels (WTW) efficiency, which is one of the deciding factors for technology acceptance. This paper primarily aims at presenting critical comparative issues with regards to the overall efficiencies of the most popularly proposed HEV and FCV topologies. Finally, the overall efficiency analysis performed in this paper will lay down the foundation for a concrete conclusive comparison between advanced vehicular topologies of the future.

Utilizing multiuser diversity for efficient support of quality of service over a fading channel

Abstract: We consider the problem of quality of service (QoS) provisioning for K users sharing a downlink time-slotted fading channel. We develop simple and efficient schemes for admission control, resource allocation, and scheduling, which can yield substantial capacity gain. The efficiency is achieved by virtue of recently identified multiuser diversity. A unique feature of our work is explicit provisioning of statistical QoS, which is characterized by a data rate, delay bound, and delay-bound violation probability triplet. The results show that compared with a fixed-slot assignment scheme, our approach can substantially increase the statistical delay-constrained capacity of a fading channel (i.e., the maximum data rate achievable with the delay-bound violation probability satisfied), when delay requirements are not very tight, while yet guaranteeing QoS at any delay requirement.

On the coexistence of power-controlled ultrawide-band systems with UMTS, GPS, DCS1800, and fixed wireless systems

Abstract: Ultrawide bandwidth (UWB) wireless technology will play a key role in short-range wireless connectivity supporting very high bit rates availability, low power consumption, and location capabilities. UWB can be conveniently deployed in the design of wireless local and personal area networks, providing advanced integrated multimedia services to nomadic users within hot-spot areas. The very large bandwidth required by the UWB signals cannot be exclusively allocated; thus, UWB band overlaps with the bands allocated to many other narrow-band systems. Therefore, the assessment of the interference caused by UWB devices on already-existing systems is of primary importance to ensure coexistence and, therefore, to guarantee acceptance of UWB technology worldwide. We study the coexistence issues between an UWB-based system and universal mobile telecommunication systems, global positioning systems, DCS1800, and fixed wireless access systems and point-to-point (PP) links terminals. UWB interference is evaluated accounting for the UWB signal model, a realistic UWB master/slave system architecture with power-controlled terminals. Furthermore, we analyze the dependence of the UWB interference from the UWB signal parameters and demonstrate through computer simulation that, in all practical cases, a UWB system can coexist with the selected victim terminals without causing any dangerous interference.

Evaluation of motor characteristics for hybrid electric vehicles using the hardware-in-the-loop concept

Abstract: If the concept of Hardware-in-the-Loop (HIL) is applied to component testing, characteristic of component of hybrid electric vehicle in real vehicle environment can be evaluated without actually installing that component in real vehicle. In this paper, when commercially available test motor is adopted as a drivetrain of hybrid vehicle, we need to figure out which drive train configuration would be best for specific purpose. The characteristic of the motor when it is installed in the vehicle at different drive train and driving mode can be simulated and actual characteristic can be measured. Also both results can be compared. For the hardware characteristic measurement, test facility which consists of vehicle simulator and dynamometer is required. In this case, vehicle controller in the vehicle simulator is used as a vehicle controller and dynamometer is used to simulate vehicle dynamics. Two drive train types, 4-motor series, and 2-motor parallel type are proposed. Vehicle speed tracks driving cycle speed command well in both simulation and HIL implementation.

UWB delay profile models for residential and commercial indoor environments

Abstract: We present models for the ultrawideband (UWB) channel delay profile in indoor environments, based on the processing of two large sets of measured data. Both measurement sets are for a center frequency of 5 GHz, but the bandwidths are very different-1.25 GHz and 6 GHz. We model both line-of-sight (LOS) and nonline-of-sight (NLOS) paths, and do so for both single-family homes and commercial buildings. Also, we consider both the profile at a receiver point, which we call the multipath intensity profile (MIP), and the locally spatially averaged profile, which we call the power delay profile (PDP). For both cases, we find that the profile for NLOS paths can be modeled as a decaying exponential times a noise-like variation with lognormal statistics and that, for LOS paths, the profile has the same form plus a strong component at the minimum delay. The model consists of statistical descriptions of the parameters of these functions, including the effects of transmit-receive (T--R) distance and of variations from building to building. We show simulation results for a few cases to demonstrate that the model accurately predicts key properties of the measured channels, such as the distribution of rms delay spread.

Timing acquisition in ultra-wideband communication systems

Abstract: The goal of this paper is to highlight the significance of the timing acquisition problem in ultra-wideband (UWB) communication systems and discuss efficient solutions to the problem. We discuss how the distinguishing features of UWB communication systems, such as their wide bandwidth and low transmission power constraints, are responsible for making the acquisition of UWB signals a difficult task. A survey of the current approaches to UWB signal acquisition is also given. In addition, we discuss some of the issues and challenges in UWB signal acquisition which may not have received sufficient attention in existing literature.

On the performance of wide-bandwidth signal acquisition in dense multipath channels

Abstract: This paper investigates important properties of acquisition receivers that employ commonly used serial-search strategies. In particular, we focus on the properties of the mean acquisition time (MAT) for wide bandwidth signals in dense multipath channels. We show that a lower bound of the MAT over all possible search strategies is the solution to an integer programming problem with a convex objective function. We also give an upper bound expression for the MAT over all possible search strategies. We demonstrate that the MAT of the fixed-step serial search (FSSS) does not depend on the timing delay of the first resolvable path, thereby simplifying the evaluation of the MAT of the FSSS. The results in this paper can be applied to design and analysis of fast acquisition systems in various wideband scenarios.

Location management in cellular networks: classification of the most important paradigms, realistic Simulation framework, and relative performance analysis

Abstract: This paper actualizes the classification of location management methods published up to now and presents results of a related extensive performance comparison of the most important paradigms for location management in cellular networks. First, a universal structure of a performance analysis framework for location management methods is defined and analyzed. Then, both a user mobility model and a specific simulation environment claiming to be as realistic as possible are suggested and implemented. Finally, the simulation framework obtained is used for a systematic comparative performance analysis of a representative sample of the most important location management schemes. The overall conclusion is that a location-area based location management method designed around a profile or history-based direction information offers the absolute best performance, in terms of location management cost, compared to all other alternative approaches. The key difference from previous works in literature is clearly underlined concerning both mobility modeling and novel location management schemes.

OFDM power loading using limited feedback

Abstract: Orthogonal frequency division multiplexing (OFDM) is a practical broadband signaling technique for use in multipath fading channels. Over the past ten years, research has shown that power loading, where the power allocations on the OFDM frequency tones are jointly optimized, can improve error rate or capacity performance. The implementation of power loading, however, is dependent on the presence of complete forward link channel knowledge at the transmitter. In systems using frequency division duplexing (FDD), this assumption is unrealistic. In this paper, we propose power loading for OFDM symbols using a limited number of feedback bits sent from the receiver to the transmitter. The power loading vector is designed at the receiver, which is assumed to have perfect knowledge of the forward link channel, and conveyed back to the transmitter over a limited rate feedback channel. To allow for the vector to be represented by a small number of bits, the power loading vector is restricted to lie in a finite set, or codebook, of power loading vectors. This codebook is designed offline and known a priori to both the transmitter and receiver. We present two power allocation selection algorithms that optimize the probability of symbol error and capacity, respectively. Simulation results show that the proposed limited feedback techniques provide performance close to full channel knowledge power loading.