Showing papers in "IEEE Transactions on Vehicular Technology in 1997"

A new path-gain/delay-spread propagation model for digital cellular channels

Abstract: We derive a statistical model for the distribution of RMS delay spread (/spl tau//sub rms/) within a cellular environment, including the effects of base-to-mobile distance, environment type (urban, suburban, rural, and mountainous areas), and the correlation between delay spread and shadow fading. We begin with intuitive arguments that /spl tau//sub rms/ should be lognormally distributed at any given distance d; that the median of this distribution should grow as some (weak) power of d and that the variation about the median should be negatively correlated with shadow fading gain. We then present empirical evidence, drawn from a wide array of published reports, which gives strong support to these conjectures. Finally, we combine our findings with the widely used model for path gain in a cellular environment. The result is a compact statistical model for the joint distribution of path gain and delay spread. The model lends itself readily to Monte Carlo simulation and is useful for performance studies of cellular systems with bandwidths up to tens of kilohertz.

ARQ error control for fading mobile radio channels

Abstract: In this paper, we study the correlation properties of the fading mobile radio channel Based on these studies, we model the channel as a one-step Markov process whose transition probabilities are a function of the channel characteristics Then we present the throughput performance of the Go-Back-N and selective-repeat automatic repeat request (ARQ) protocols with timer control, using the Markov model for both forward and feedback channels This approximation is found to be very good, as confirmed by simulation results

Estimating position and velocity of mobiles in a cellular radio network

Abstract: Determining the position and velocity of mobiles is an important issue for hierarchical cellular networks since the efficient allocation of mobiles to large or microcells depends on its present velocity. We suggest a method of tracing a mobile by evaluating subsequent signal-strength measurements to different base stations. The required data are available in the Global System for Mobile (GSM) system. The basic idea resembles multidimensional scaling (MDS), a well-recognized method in statistical data analysis. Furthermore, the raw data are smoothed by a linear regression setup that simultaneously yields an elegant, smoothed estimator of the mobile's speed. The method is extensively tested for data generated by the simulation tool GOOSE.

Two-dimensional ray-tracing modeling for propagation prediction in microcellular environments

Abstract: The application of diffraction theory and geometrical optics for modeling the propagation in microcellular urban environments is investigated. The model simplifies the reality by considering only a two-dimensional (2-D) geometry, where the building walls are modeled by segments and the buildings are considered infinitely high. The model can handle arbitrary layouts of buildings according to an efficient algorithm briefly described in this paper. Comparisons between several predictions and measurements in two different cities in Switzerland are presented. A detailed investigation of the two main parameters considered in the predictions, i.e., the reflection and diffraction coefficients, is also provided. The study shows that the ray tracing model is appropriate for path-loss coverage prediction even in complex environments.

Analysis of M-ary phase-shift keying with diversity reception for land-mobile satellite channels

Abstract: An analytical technique well suited to numerical analysis is presented for computing the average bit-error rate (BER) and outage probability of M-ary phase-shift keying (PSK) in the land-mobile satellite channel (LMSC) with microdiversity reception. Closed-form expressions are found for L-branch microdiversity using both selection diversity combining (SDC) and maximal ratio combining (MRC). These expressions are extended to include both M-ary coherent PSK (M-PSK) and differential PSK [M-differential PSK (DPSK)]. Following previous empirical studies, the LMSC is modeled as a weighted sum of Rice and Suzuki distributions. Numerical results are provided illustrating the achievable performance of both M-PSK and M-DPSK with diversity reception. Using measured channel parameters, the performance in various mobile environments for various satellite elevation angles is also found.

The effect of quadrature modulator and demodulator errors on adaptive digital predistorters for amplifier linearization

Abstract: With the increasing importance of spectral efficiency in mobile communications, the power amplifier linearity has become a critical design issue for nonconstant envelope modulation. Imperfections in quadrature modulators and demodulators-gain and phase imbalance and DC offset-have a crippling effect on amplifier linearization circuits, a fact that has previously been noted experimentally. This paper is the first analysis of these effects on an adaptive predistorter. The primary result is an expression that provides an explicit tradeoff among intermodulation power, accuracy of the quadrature modulator and demodulator, and speed of adaptation. Another useful result is a simple and easily measured error figure for quadrature modulators and demodulators.

Estimating local mean signal strength of indoor multipath propagation

Abstract: We explore techniques for the measurement of local mean signal strength at 900 MHz and 2 GHz. In particular, we characterize the impact of transmitter and receiver antenna rotation on the estimated local mean. Then, we explore the collection of high resolution data while moving along a linear trajectory and using linear averaging techniques to estimate the local mean. With this information, the best measurement techniques can be chosen depending on the required speed versus accuracy tradeoff. Finally, we use a ray tracing propagation model to evaluate different methods of calculating the local mean signal strength for indoor environments.

A UTD propagation model in urban microcellular environments

Abstract: This paper presents a three-dimensional (3-D) propagation model for path-loss prediction in a typical urban site, based on geometrical optics (GO) and uniform theory of diffraction (UTD). The model takes into account numerous rays that undergo reflections from the ground and wall surfaces and diffraction from the corners or rooftops of buildings. The exact location of the reflection and diffraction points is essential in order to calculate the polarization components of the reflected and diffracted fields and their trajectories. This is accomplished by local ray-fixed coordinate systems in combination with appropriate dyadic reflection and diffraction coefficients. Finally, a vector addition of the received fields is carried out to obtain the total received field strength and, subsequently, the path loss along a predetermined route. The model computes the contributions of various categories of rays, as selected, in a flexible manner. Several results-path loss versus distance and power-delay profile-are given, and comparisons with measured data are presented.

The capacity of downlink fading channels with variable rate and power

Abstract: We obtain the Shannon capacity region of the down-link (broadcast) channel in fading and additive white Gaussian noise (AWGN) for time-division, frequency-division, and code-division. For all of these techniques, the maximum capacity is achieved when the transmitter varies the data rate sent to each user as their channels vary. This optimal scheme requires channel estimates at the transmitter; dynamic allocation of timeslots, bandwidth, or codes; and variable-rate and power transmission. For both AWGN and fading channels, nonorthogonal code-division with successive decoding has the largest capacity region, while time-division, frequency-division, and orthogonal code-division have the same smaller region. However, when all users have the same average received power, the capacity region for all these techniques is the same. In addition, the optimal nonorthogonal code is a multiresolution code which does not increase the signal bandwidth. Spread-spectrum code-division with successive interference cancellation has a similar rate region as this optimal technique, however, the region is reduced due to bandwidth expansion. We also examine the capacity region of nonorthogonal code-division without interference cancellation and of orthogonal code-division when multipath corrupts the code orthogonality. Our results can be used to bound the spectral efficiency of the downlink channel using time-division, frequency-division, and code-division, both with and without multiuser detection.

A simplified analytical model for predicting path loss in urban and suburban environments

Abstract: An analytical propagation model has recently been developed to predict radio signal attenuation in urban and suburban environments. This analytical model explicates the path loss as a result of signal reduction due to free space wavefront spreading, multiple diffraction past rows of buildings, and building shadowing. It is applicable for cellular mobile services as well as personal communications services (PCS) in both macro- and microcellular environments. Good accuracy was found for this analytical model by comparing the predictions with numerous measurements made in various propagation environments. However, since the analytical model involves multiple-dimension integration to calculate the signal attenuation due to multiple diffraction past rows of buildings, the model in its original format does not lend itself to easy implementation into a radio system planning tool. A simplified version of the analytical model is developed in this paper, which can be used for three different propagation scenarios with base-station antenna above, below, and near the average rooftop level.

Mobility management for personal communications systems

Abstract: Using a combination of empirical data and theoretical models, this paper develops a model of user behavior for a personal communications system environment. This model is used to analyze a mobility management strategy that combines automatic updates by the users-either when they make significant moves or when they go extended periods without network interaction; multiple hysteresis in the form of dynamic registration areas and delayed updates; and a focused paging strategy that minimizes the number of pages given a constraint on the time allowed to a page. Over a range of system and user parameters, the total paging and update traffic can be kept below 1 per 2000 user seconds. The impact on the user's handset is less than ten brief updates per day. The total traffic is only a factor of three more than the minimum, immobile users case.

Analysis of availability improvement in LMSS by means of satellite diversity based on three-state propagation channel model

Abstract: Aiming at future multimedia land mobile-satellite services (LMSS) consisting of a large number of nongeostationary Earth-orbit satellites, we present an LMSS propagation channel model for assessing the effect of a satellite diversity scheme so that high service availability and high signal quality are assured. We classify general fading environments for LMSS into three states. By taking the occurrence probability of each state into account, a new fading channel model is developed. The validity of the model is identified by comparing its predicted values in terms of the cumulative distribution function (CDF) with measured data available so far. Then, based on this model, we calculate the satellite diversity effect assuming that the area is illuminated simultaneously by at least two satellites moving in low Earth orbits (LEO) over urban and suburban environments. In addition, state transition characteristics based on a Markov model are presented.

Performance analysis of coherent DS-CDMA systems in a Nakagami fading channel with arbitrary parameters

Abstract: A new expression for the bit error rate (BER) of an asynchronous direct-sequence code-division multiple-access (DS-CDMA) system with coherent BPSK demodulation in a multipath Nakagami (1960) fading channel is derived. The analysis assumes an arbitrary number of independent, but nonidentical resolvable paths combined by the RAKE receiver. The results obtained show that in such systems, the effect of nonidentical fading on system performance cannot be ignored.

Interference cancelling equalizer (ICE) for mobile radio communication

Abstract: This paper proposes an adaptive interference cancelling equalizer (ICE), which not only equalizes intersymbol interference (ISI), but also cancels cochannel interference (CCI) in the received signal in Rayleigh-fading environments, ICE is an adaptive multiuser detector for the frequency-selective fading environment commonly experienced by mobile communication channels. ICE employs a novel detection scheme: recursive least-squares maximum-likelihood sequence estimation (RLS-MLSE), which simultaneously estimates time-varying channel parameters and transmitted signal sequences. Diversity reception is used to enhance the signal detection performance of ICE. A computer simulation of a 40-kb/s QPSK time-division multiple-access (TDMA) cellular mobile radio system demonstrates the possibility of improving system capacity with ICE. Simulations of ICE with and without diversity are carried out under various fading conditions. For the maximum Doppler frequency of 40 Hz, ICE can attain an average bit-error rate (BER) of 10/sup -2/ under a single CCI carrier-to-interference ratio (CIR) of /spl sim/14 dB. Moreover, ICE for two independent CCI signals can attain the average BER of 1.5/spl times/10/sup -2/ with average CIR/spl ges/-10 dB.

3-D mobile radio channel statistics

Abstract: In the case of three-dimensional (3-D) scattering, which is appropriate to the calculation of signal statistics for personal communications, it is shown that the basic Doppler spectrum of the received signal is uniform over the full range of Doppler frequencies centered on the carrier. The corresponding temporal and spatial autocorrelations are, therefore, sine functions.

Doppler spectrum in mobile-to-mobile communications in the presence of three-dimensional multipath scattering

Abstract: This paper provides analytical expressions for the time-autocorrelation function and the Doppler spectrum in a mobile radio channel in the presence of three-dimensional (3-D) multipath scattering. Mathematical modeling of the transmission channel is based on the assumption of wide-sense stationary (WSS) scattering. This paper deals with the case of a mobile-to-mobile radio link and derives formulas for the mobile-to-radiobase link as a special case. The Doppler spectrum examples provided are typical when assuming mobile terminal(s) located in densely built-up urban areas and simple dipole terminal antenna(s). Furthermore, the Doppler spectrum for transmission between a nongeostationary satellite and a mobile terminal located on the ground is analyzed. If different antenna patterns are to be considered and detailed knowledge is available on the multipath distribution, realistic Doppler spectra may be calculated using the approach presented in this paper.

Design and analysis of transmitter diversity using intentional frequency offset for wireless communications

Abstract: Coded modulation (usually with interleaving) is used in fading channel communications to achieve a good error performance. The major benefit from using coded modulation in fading channels is achieved if each code symbol of a codeword (or coded sequence) suffers statistically different fading (preferably independent fading). However, in many applications of mobile communications (e.g., in a metropolitan environment), a low vehicle speed (and hence, a small Doppler spread, f/sub D/) is very common. With a small Doppler spread, ideal or close-to-ideal interleaving is no longer feasible and all code symbols of a codeword would suffer highly correlated fading especially in stationary fading (f/sub D//spl ap/0). Coded modulations will thus suffer seriously degraded performance. Previous performance analyses based on ideal interleaving are not accurate when a small Doppler spread is encountered and the much used union bound error probability analysis is loose for small Doppler spreads. To rectify this situation, this paper presents an improved performance analysis of coded modulations with correlated fading and pilot-symbol-assisted modulation (PSAM). Transmitter diversity can generate the necessary time-varying fading to maintain the effectiveness of a coded signaling scheme which this paper examines in detail using an intentional frequency offset between antennas. This work found that proper selections of the intentional frequency offset and interleaving depth can lead to good performance with traditional coded modulations (if enough antennas are used) using essentially the same simple demodulation structure as used in the traditional single-antenna PSAM.

Three-dimensional modeling of 900-MHz and 2.44-GHz radio propagation in corridors

Abstract: A three-dimensional (3-D) propagation model, combined with a patched-wall model, has been developed to predict radio loss in a corridor environment. The ray-tracing technique is used and combined with the ray-fixed coordinate system to simplify the computations of transmission, reflection, and diffraction coefficients in 3-D space. The computed path loss is compared with the measured value of 900-MHz and 2.44-GHz radio propagation along a hallway and gives a reasonable agreement. It is also found that the fields transmitted through the interior walls give the major contribution to the received field when the radio path lies around a corner.

Architecture design, frequency planning, and performance analysis for a microcell/macrocell overlaying system

Abstract: An innovative hierarchical microcell/macrocell architecture is presented. By applying the concept of cluster planning, the proposed sectoring arrangement can provide good shielding between microcells and macrocells. As a result, underlaid microcells can reuse the same frequencies as overlaying macrocells without decreasing the macrocell system capacity. With the proposed method, microcells not only can be gradually deployed, but they can be extensively installed to provide complete coverage and increase capacity throughout the service area. With these flexibilities, the proposed method allows existing macrocellular systems to evolve smoothly into a hierarchical microcell/macrocell architecture.

Vehicle-to-vehicle communications for AVCS platooning

Abstract: Vehicle-to-vehicle radio links suffer from multipath fading and interference from other vehicles. We discuss the impact of these effects on communication networks supporting an intelligent transportation system (ITS), in particular, automated vehicle control systems (AVCSs). A statistical model for this channel is considered, and the performance of the network involving many links is evaluated. We compare the performance of time division multiple access (TDMA), direct-sequence code division multiple access (DS-CDMA), and frequency hopping with TDMA in this environment. The reliability of the radio link is investigated by specifying the radio spectrum occupation for a given required reliability of the radio link.

Performance evaluation of a cellular base station multibeam antenna

Abstract: Experimental test results are used to determine the performance that can be achieved from a multibeam antenna array, with fixed-beam azimuths, relative to a traditional dual-diversity three-sector antenna configuration. The performance tradeoffs between the hysterisis level, switching time, and gain improvement for a multibeam antenna are also examined. The multibeam antenna uses selection combining to switch the signals from the two strongest directional beams to the base station's main and diversity receivers. To assess the impact of beamwidth on overall system performance, the following two multibeam antennas were tested: a 12-beam 30/spl deg/ beamwidth array and a 24-beam 15/spl deg/ beamwidth array. Both multibeam antennas were field-tested in typical cellular base station sites located in heavy urban and light urban environments. Altogether, the system performance is evaluated by investigating three fundamental aspects of multibeam antenna behavior. First, the relative powers of the signals measured in each directional beam of the multibeam antenna are characterized. Then, beam separation statistics for the strongest two signals are examined. Gain improvements achievable with a multibeam antenna compared to the traditional sector configuration are determined in the second phase of the analysis. Results indicate that in excess of 5 dB of gain enhancement can be achieved with a 24-beam base station antenna in a cellular mobile radio environment. Finally, the effects of hysterisis level and switching time are characterized based on gain reductions relative to a reference case with no hysterisis and a 0.5-s switching decision time. Useful approximations are developed for the gain effects associated with varying hysterisis levels and switching times. The resulting design curves and empirical rules allow engineers to quantify multibeam antenna performance while making appropriate tradeoffs for parameter selection.

Analysis of composite walls and their effects on short-path propagation modeling

Abstract: For short-propagation paths, correctly representing reflections of electromagnetic energy from surfaces is critical for accurate signal-level predictions. In this paper, the method of homogenization is used to determine the effective material properties of composite materials commonly used in construction. The reflection coefficients for block walls and other types of materials calculated with these homogenized effective material properties are presented. The importance of accurately representing the reflections for signal-level prediction models is also investigated. It is shown that a 5-10-dB error in received signal strength (RSS) can occur if the composite walls are not handled appropriately. Such accurate predictions of signal propagation over a short distance is applicable to microcellular personal communications services deployments in urban canyons as well as indoor wireless private branch exchanges and local-area networks.

A locally optimal handoff algorithm for cellular communications

Abstract: The design of handoff algorithms for cellular communication systems based on mobile signal strength measurements is considered. The design problem is posed as an optimization to obtain the best tradeoff between the expected number of service failures and expected number of handoffs, where a service failure is defined to be the event that the signal strength falls below a level required for satisfactory service to the subscriber. Based on dynamic programming arguments, an optimal solution is obtained, which, though impractical, can be used as a benchmark in the comparison of suboptimal schemes. A simple locally optimal handoff algorithm is derived from the optimal solution. Like the standard hysteresis algorithm, the locally optimal algorithm is characterized by a single threshold. A systematic method for the comparison of various handoff algorithms that are akin to the receiver operating characteristic (ROC) curves of radar detection is presented. Simulation results show that the locally optimal algorithm outperforms the hysteresis algorithm, especially in situations where accurate prediction of signal strength is possible. A straightforward technique for adapting the locally optimal algorithm to changing environments is suggested. That natural adaptability is the algorithm's principle advantage over current approaches.

Measurement of building penetration into medium buildings at 900 and 1500 MHz

Abstract: The propagation loss into ten medium-sized buildings in Schaumburg, IL, has been measured. At 900 MHz, the mean penetration loss in the lower enclosed floors at or near ground level was found to be 10.8 dB with a standard deviation of 5.8 dB. At 1500 MHz, penetration loss was found to be 10.2 dB, and the standard deviation was 5.6 dB. Data was also taken up to 12 floors (with a higher concentration of data on the first five floors) to show higher elevation trends in the penetration loss. The measured building penetration loss was combined with data from other references, and the slope of a best fit curve as a function of frequency is found to be -7.9 dB per decade.

Cellular radio channel assignment using a modified Hopfield network

Abstract: The channel-assignment problem is important in mobile telephone communication. Since the usable range of the frequency spectrum is limited, the optimal channel-assignment problem has become increasingly important. A new channel-assignment algorithm using a modified Hopfield (1985, 1986) neural network is proposed. The channel-assignment problem is formulated as an energy-minimization problem that is implemented by a modified discrete Hopfield network. Also, a new technique to escape the local minima is introduced. In this algorithm, an energy function is derived, and the appropriate interconnection weights between the neurons are specified. The interconnection weights between the neurons are designed in such a way that each neuron receives inhibitory support if the constraint conditions are violated and receives excitatory support if the constraint conditions are satisfied. To escape the local minima, if the number of assigned channels are less than the required channel numbers (RCNs), one or more channels are assigned in addition to already assigned channels such that the total number of assigned channels is the same as the required number of channels in the cell even though the energy is increased. Various initialization techniques, which use the specific characteristics of frequency-assignment problems in cellular radio networks, such as cosite constraint (CSC), adjacent channel constraint (ACC), and cochannel constraint (CCC), and updating methods are investigated. In the previously proposed neural-network approach, some frequencies are fixed to accelerate the convergence time. In our algorithms, no frequency is fixed before the frequency-assignment procedure. This new algorithm, together with the proposed initialization and updating techniques and without fixing frequencies in any cells, has better performance results than the results reported previously utilizing fixed frequencies in certain cells.

Environment-adaptation mobile radio propagation prediction using radial basis function neural networks

Abstract: This paper investigates the application of a radial basis function (RBF) neural network to the prediction of field strength based on topographical and morphographical data. The RBF neural network is a two-layer localized receptive field network whose output nodes from a combination of radial activation functions computed by the hidden layer nodes. Appropriate centers and connection weights in the RBF network lead to a network that is capable of forming the best approximation to any continuous nonlinear mapping up to an arbitrary resolution. Such an approximation introduces best nonlinear approximation capability into the prediction model in order to accurately predict propagation loss over an arbitrary environment based on adaptive learning from measurement data. The adaptive learning employs hybrid competitive and recursive least squares algorithms. The unsupervised competitive algorithm adjusts the centers while the recursive least squares (RLS) algorithm estimates the connection weights. Because these two learning rules are both linear, rapid convergence is guaranteed. This hybrid algorithm significantly enhances the real-time or adaptive capability of the RBF-based prediction model. The applications to Okumura's (1968) data are included to demonstrate the effectiveness of the RBF neural network approach.

Hidden Markov models for burst error characterization in indoor radio channels

Abstract: Many digital communication channels exhibit statistical dependencies among errors. The design of error control schemes for such channels and their performance evaluation is simplified if appropriate generative models of the overall communication link are available. This paper presents a new class of generative models based on the interconnection of hidden Markov submodels parameterized by the Baum-Welch algorithm. The method has some resemblance to the well-studied problem of speech recognition of isolated words; however, in our approach, instead of dealing with words, one deals with error bursts, and the final goal is to generate bursts rather than to recognize words. The proposed model is particularly suitable for simulating error profiles with long bursts, as is often the case in indoor radio channels, where the error-free gaps inside a burst are heavily nonrenewal. The merits of the method are corroborated by applying the technique to two particular examples of indoor code-division multiple-access (CDMA) radio links.

Sequential packing algorithm for channel assignment under cochannel and adjacent-channel interference constraint

Abstract: Generally, the channel-assignment problem (CAP) for mobile cellular systems is solved by graph-coloring algorithms. These algorithms, though sometimes can yield an optimal solution, do not supply any information on whether an optimal solution has been found or bow far away it is from the optimum. In view of these undesirable features, two relevant results are presented. First, a lower bound for the minimum number of channels required to satisfy a given call-traffic demand is derived. This lower bound is tighter than the existing ones under certain conditions and can be used as a supplement for other approximate algorithms. Second, we propose an efficient heuristic algorithm to solve this problem. Although the CAP is nondeterministic polynomial (NP) complete in general, our algorithm provides an optimal solution for a special class of network topologies. For the general case, promising results are obtained, and numerical examples show that our algorithm has a better performance than many existing algorithms.

Efficient distributed power control for cellular mobile systems

Abstract: Various distributed carrier-to-interference (C/I) balancing procedures have been developed to cope with implementational drawbacks of centralized power-control schemes. However, it would be inevitable for these distributed schemes to lose some system performance compared to a centralized one since they control their power level based on only the local C/I measurements. An efficient distributed power-control (DPC) scheme is proposed to maximize the system performance. A distributed C/I balancing algorithm is suggested, which converges fast and is robust to C/I estimation errors. A simple instability detection rule is also suggested to accelerate the balancing phase of DPC schemes. Numerical results indicate that our algorithm achieves improvements in terms of the outage probability as well as the algorithm speed. Robustness to the C/I measurement errors is also explored.

Genetic algorithms applied to cellular call admission: local policies

Abstract: It is well known that if a stochastic service system (such as a cellular network) is shared by users with different characteristics (such as differing handoff rates or call holding times), the overall system performance can be improved by denial of service requests even when the success capacity exists. Such selective denial of service based on the system state is defined as the call admission. A previous paper suggested the use of genetic algorithms (GAs) to find near-optimal call admission policies for cellular networks. In this paper, we define local call admission policies that make admission decisions based on partial state information. We search for the best local call admission policies for one-dimensional (1-D) cellular networks using genetic algorithms and show that the performance of the best local policies is comparable to optima for small systems. We test our algorithm on larger systems and show that the local policies found outperform the maximum packing and best handoff reservation policies for the systems we have considered. We find that the local policies suggested by the genetic algorithm search in these cases are double threshold policies. We then find the best double threshold policies by exhaustive search for both 1-D and Manhattan model cellular networks and show that they almost always outperform the best trunk reservation policies for these systems.