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In this paper, we propose a hybrid location-update scheme, which combines the time- and movement-based location-update approaches. In the proposed scheme, a mobile user updates its location after the following two events have occurred: First, the mobile crossed n cell boundaries, and then, a time interval T elapsed. We also examine the inverse scheme, where an update is performed after the n boundary crossings occur subsequent to the time interval T. We evaluate our scheme by applying it to various mobility models, such as exponential cell-residence times with a constant mobility pattern, exponential cell-residence times with a gamma-distributed mobility pattern, hyperexponential cell-residence times, and cell-residence times with a small coefficient of variation. From our numerical analysis and simulation results, we learn that the movement-based-only method (i.e., T = 0) performs well when the coefficient of variation of the cell-residence time is relatively small. However, when the coefficient of variation is large, the proposed hybrid scheme (i.e., T > 0, n > 0) outperforms each of the two approaches, namely, the time- and movement-based location-update schemes, when these schemes are individually applied. Index Terms--Hybrid-location update, mobile networks, movement-based location update, time-base location update.

For purposes of simulating contemporary communication systems, it is, in many cases, useful to apply error models for specific levels of abstraction. Such models should approximate the packet error behavior of a given system at a specific protocol layer, thus incorporating the possible detrimental effects of lower protocol layers. Packet error models can efficiently be realized using finite-state models; for example, there exists a wide range of studies on using Markov models to simulate communication channels. In this paper, we consider aggregated Markov processes, which are a subclass of hidden Markov models (HMMs). Artificial limitations are set on the state transition probabilities of the models to find efficient methods of parameter estimation. We apply these models to the simulation of the performance of Digital Video Broadcasting-Handheld (DVB-H). The parameters of the packet error models are approximated as functions of the time-variant received signal strength and speed of a mobile vehicular DVB-H receiver, and it is shown that useful results may be achieved with the described packet error models, particularly when simulating mobile reception in field conditions. Index Terms--Broadcasting, communication channels, Markov processes, mobile communication, simulation.

A deterministic model for propagation in outdoor urban environments based on the Fresnel-Kirchhoff integral is presented. Both two-dimensional (2-D) and three-dimensional (3-D) formulations considering reflection and diffraction are given. The theoretical predictions are compared with 2-D and 3-D measurements and good agreement is achieved. The 3-D experiments, conducted in an anechoic chamber with scaled models representing buildings, are described in detail and provide a benchmark for comparisons with other methods. Index Terms--Fresnel-Kirchhoff integral, measurements, radio propagation, urban environment, wireless communications.

The numerical dispersion relations of finite-difference time-domain (FDTD) methods have been analyzed extensively in lossless media. This paper investigates numerical dispersion and loss for Yee's FDTD in lossy media. It is shown that: the numerical velocity can be smaller or larger than the physical velocity; there is no 'magic time step size' in lossy media; and the numerical loss is smallest at the Courant limit. It is shown that the numerical loss is always larger than its physical value, and so Yee's FDTD over-estimates the absorption of electromagnetic energy in lossy media. The numerical velocity anisotropy can be positive or negative, but the numerical loss anisotropy is always positive. The anisotropies in the three-dimensional (3-D) case are usually larger than those in the 2-D case. Numerical experiments in 1-D are shown to agree with the theoretical prediction. Index Terms--Computational electromagnetics, finite-difference time-domain method (FDTD), numerical anisotropy, numerical dispersion, numerical loss.

A simple technique is proposed for modeling short normal-mode helical antennas using a commercial finite, difference time-domain (FDTD) code with a rectangular grid and a nominal extension of the wire. The approach allows affects on the input impedance and radiation performance of the helix to be examined and importantly does not require modification of the excitation subroutines. Normal-mode helical antennas for mobile communications use at 900 and 1800 MHz were designed using the proposed method and good agreement with measurements of impedance and near-magnetic field strength was found. The radiating performance of the helix was compared to that of a [lambda]/4 monopole and generally found to be inferior at 900 MHz due to only 19.2% efficiency in the presence of the head. At 1.8 GHz the two antenna types showed similar characteristics except in regard to bandwidth, 36.1% for the monopole and 7.8% for the helix, in the presence of the head. The modeled helix antennas produce spatial peak specific absorption rate (SAR) figures that are up to 27% greater at 900 MHz and up to 49% greater at 1.8 GHz than the corresponding monopole values due to the shorter antenna. Index Terms--Finite-difference time-domain (FDTD) methods, handheld wireless communication, helical antennas.

In this paper, a statistical model for the land mobile satellite (LMS) channel is presented. The model is capable of describing both narrow- and wide-band conditions. Other relevant characteristic is that it can be used to study links with geostationary as well as nongeostationary satellites. The model is of the generative type, i.e., it is capable of producing time series of a large number of signal features: amplitudes, phases, instantaneous power-delay profiles, Doppler spectra, etc. Model parameters extracted from a comprehensive experimental data bank are also provided for a number of environments and elevation angles at L-, S-, and Ka-Bands. Index Terms--Land mobile satellite (LMS), propagation, statistical modeling.

Rapid time variations of the mobile communication channel have a dramatic effect on the performance of multicarrier modulation. This paper models the Doppler spread and computes its effect on the bit error rate (BER) for multicarrier code division multiple access (MC-CDMA) transmission and compares it to orthogonal frequency division multiplexing (OFDM). Also, we evaluate the transmission capacity per subcarrier to quantify the potential of MC-CDMA and (coded-) OFDM. We focus on linear receivers, in particular those using the minimum mean-square error (MMSE) criterion. Our channel and system models allow the computation of analytical performance results. Simulations verify some commonly used, yet critical assumptions. Index Terms--Doppler effect, fading channels, intercarrier interference, multicarrier code division multiple access (MC-CDMA), multicarrier modulation, orthogonal frequency division multiplexing (OFDM), Rayleigh fading.

Three mathematical models for the design of cellular networks are presented.

The level-crossing rate, average duration of fades and cumulative distribution of deterministic simulation models for Rice fading channels were analyzed. Exact and simple approximative formulas were derived for these parameters. Various procedures for the design of the parameters of the deterministic simulation model were used to present the numerical results for the derived formulas. The results of comparisons with simulation models are also presented.

A novel algorithm is proposed for the spatial interpolation of a uniform linear array originally sampled at less than the spatial Nyquist rate in the presence of noise. The multichannel narrowband interpolation method is based on a forward-backward linear prediction technique and interpolates N - 1 virtual sensor outputs based on the outputs of N real sensors. Simulation results show the effectiveness and usefulness of the proposed algorithm as applied to code division multiple access mobile communications.

A useful model for general time-varying channels is a finite state Markov chain. In this paper, maximum likelihood sequence estimation (MLSE) for signals over finite state Markov channels (FSMC's) is studied. Also studied is the maximum a posteriori (MAP) channel state estimation. When coded signals with interleaving are transmitted, the channel estimates can be used to make soft-decision decoding. The error performance of the proposed sequence and channel state estimation schemes are evaluated through computer simulations. The effect of channel modeling error is also discussed.

A new approach to the modeling of wireless propagation in buildings is introduced. We treat the scattering by walls and local clutter probabilistically through either a relaxation-time approximation in a Boltzmann equation or by using a diffusion equation. The result is a range of models in which one can vary the tradeoff between the complexity of the building description and the accuracy of the prediction. The two limits of this range are ray tracing at the most accurate end and a simple decay law at the most simple. By comparing results for two of these new models with measurements, we conclude that a reasonably accurate description of propagation can be obtained with a relatively simple model. The most effective way to use the models is by combining them with a few measurements through a sampling technique. Index Terms - Indoor, propagation, statistical model, wireless.

Due to the limited bandwidth of wireless networks, an efficient medium-access control protocol is essential to meet the growing demand of wireless access. Most multiple-access protocols require contentions (collisions) in the process of acquiring the transmission medium. While collisions cannot be avoided, successive collisions that consist of the same group of active stations are totally unnecessary. Successive collisions not only waste bandwidth, but also raise the concern of saturation in the channel. In this paper, we solve the problem of repetitive contentions involving the same set of stations by using the theory of finite projective planes. Due to the property of single-point intersection for an arbitrary pair of sets in the finite projective plane, we can minimize the number of unnecessary collisions. Protocol finite projective plane-based medium access control (FMAC) is highly flexible and has many features including adaptation for a mobile environment, support for priority assignment and handoffs in cellular networks, and extension of asynchronous transfer mode (ATM) services to mobile users. A performance evaluation shows that the throughput of the system is higher than that of slotted ALOHA. By dynamically adjusting the retransmission probability and the order of the finite projective plane, protocol FMAC can be stabilized. Index Terms - MAC, multiple access, wireless ATM, wireless networks.

This paper extends at system level the analysis of on-board nonlinearity effects covered by a companion paper [1]. A satellite personal communication network (S-PCN) like Globalstar [3] utilizing bent-pipe transponders and active phased-array antennas in conjunction with code-division multiple access (CDMA) is considered. The present analysis incorporates the effects of payload nonlinearity, variable-rate vocoders, and multibeam interference. It is shown that nonlinearity effects on the system capacity are generally negligible except for extremely loaded beam conditions which are rare events.

Good estimates of local mean signal power level are needed by wireless cellular systems for use in channel access, power control and handoff. Many systems today use weighted sample average estimators which are not optimal in Rayleigh fading environments. The optimal local mean signal level estimator for the Rayleigh fading environment will be derived and compared with the sample average estimator. Index Terms - Fading, parameter estimation.

The design of future wireless communication systems employing microcells will require a flexible tool for determining the dependence of the coverage area on base-station antenna height and position. In this paper, the accuracy of a ray-based algorithm versus slope intercept models for propagation prediction in residential environments is investigated by comparison with measurements. The measurements were taken in two areas of Trenton, NJ, where building heights ranged from one to three stories. For base-station antennas at or above rooftop levels, the algorithm is shown to give good agreement with the measurements. For these high antennas, the average and standard deviations of the difference between the site-specific prediction and measurement are less than 1 and 5 dB, respectively. For base-station antennas below the rooftops, additional ray paths are required in order to achieve similar prediction accuracy.

We present a throughput analysis for the memory automatic repeat request (ARQ) schemes with self-decodable frame retransmissions, taking into account frame header and frame acknowledgment failure probability. A lower bound on the throughput is obtained for a very general class of schemes which encompasses many of those proposed in the literature. Numerical results are given for several example systems, and suitable header and acknowledgment forward error correction (FEC) encodings are determined. Index Terms - ARQ, copy combining, error correction coding, header failure, memory ARQ, throughput.

We propose an algorithm for joint carrier phase and timing estimation with offset quadriphase modulation (OQPSK) signaling. The derivation is based on maximum-likelihood arguments and leads to a feedforward structure that operates on signal samples taken at bit rate. Phase and timing algorithms operate in parallel - not sequentially - and provide their estimates in a fixed time. The synchronizer is readily implemented in digital form and is particularly suitable for burst mode transmissions. Its performance is analytically assessed and verified by simulations. Comparisons are made with other methods described in the literature. Index Terms - Digital communications, offset quadriphase modulation, synchronization.

This paper studies personal communications services (PCS's) channel allocation assuming the Erlang call holding time distribution (a generalization of the exponential distribution) to investigate the effect of the variance of the call holding times on the call completion probability. Our analysis indicates that the call completion probability decreases as the variance of the call holding times decreases. This effect becomes more pronounced as the variance of the cell residence times decreases. Index Terms - Channel allocation, Erlang distribution, handover, personal communication services.

This paper presents a new upperbound on the pairwise error probability of MPSK sequences for the Rayleigh fading channel when channel state information (CSI) is unavailable. This bound is derived by adding weight factors in computing symbol metrics. Simulation results show that the weight factors which optimize the upperbound likely optimize the error rate as well. Multilevel coded MPSK schemes for the Rayleigh fading channel are also devised. Results show that the added weight factors improve the error performances of these schemes in the case that CSI is unavailable. Index Terms - Coded modulation, fading channel, multilevel coding.

This paper proposes a fading model that leads to a formal, but simple method to obtain the exact formula of the Nakagami-m 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 [greater than or equal to] 1/2. Index Terms - Average fade duration, fading statistics, level crossing rate, Nakagami-m distribution.

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. Index Terms - CDMA, digital communication, diversity methods, fading channels, spread-spectrum communication, time-division multiplexing.

An effective site-specific hybrid model is developed to predict the path loss and the effective radio coverage on single floors. The hybrid model, combined with a two-dimensional (2-D) model and an easy-to-use direct-transmitted ray (DTR) model, is incorporated with the building blueprint in usage. Both effects of the interior walls and the metallic objects such as whiteboards, bookcases, and standing air conditioners on radio propagation are considered. It is found that inclusion of metallic objects will enhance the performance of the model. Index Terms - Field strength measurement, indoor propagation modeling, radio coverage.

This paper considers a new approach to the problem of adaptive separation of digitally modulated signals arriving at an antenna array. The basic idea is to decouple the estimation procedure, treating one signal at a time to be the signal of interest. The method uses an iterative scheme similar to the well-documented iterative least squares with projection (ILSP) algorithm, exploiting the finite alphabet (FA) property of the signals to obtain signal estimates. The proposed algorithm is analyzed in terms of consistency and uniqueness properties, and a statistical performance analysis is carried out. The analysis together with simulation results indicate that a significant saving in computational complexity is accomplished relative to ILSP, while essentially having similar performance. Index Terms - Adaptive arrays, array signal processing, interchannel interference, interference suppression.

Revised versions of the mutually referenced equalizer and the subspace intersection methods were proposed. The two methods are used as algorithms for multichannel blind equalization, and their enhanced variants both optimize a new blind criterion called maximum coherence. The subspace intersection solution is derived by maximizing the power of the sum of all outputs of the equalizer, given that the sum of the powers is kept constant.

The future (third-generation) mobile communications system will provide not only voice and low-speed data services, but also video and high-speed data services. It will eventually have the capability for wireless multimedia communications. As a radio-access technique for such a system, the code-division multiple access (CDMA) has been considered in many potential third-generation systems including CODIT [12] and the proposal in [14]. In this paper, we model those CDMA systems as the spectrally overlaid multiband CDMA (SOM-CDMA) system. In an SOM-CDMA system, several CDMA (sub)systems with different spreading bandwidths are spectrally overlaid with each other. The reverse-link capacity of an SOM-CDMA system is calculated, and some numerical results are presented with discussion. As a result, we demonstrate how much capacity the SOM-CDMA has and how we can achieve an optimal allocation of frequency and power between (sub)systems. The results in this paper can be used as a guideline in designing the radio network of SOM-CDMA systems as well as a means to assess the performance of given systems. The results can be applied to any CDMA system with an SOM-CDMA concept, including CODIT and the proposal in [14]. Index Terms - Code-division multiple access (CDMA), mobile networks, network capacity, spectrally overlaid multiband CDMA, wide-band CDMA, wireless multimedia communications.

In this work, we perform an analysis of a channel for the UHF wave propagation in the city street. The street is modeled as a planar multislit waveguide with screens and slits distributed by a Poisson law. Statistical propagation characteristics in such a waveguide can be expressed in terms of multiple ray fields approaching the observer along a direct ray and the rays reflected by the waveguide walls. The corresponding average field and intensity distributions can be transformed into the sums of mode-like solutions using the Poisson summation formula. Numerical examples are presented and compared with the experimental data. Index Terms - UHF radio propagation, urban areas.

A theoretical model is proposed to compute the path loss in a vegetated residential environment, with particular application to mobile radio systems. As in the past, the row of houses or blocks of buildings are viewed as diffracting cylinders lying on the earth and the canopy of the trees are located adjacent to and above the houses/buildings. In this approach, a row of houses or buildings is represented by an absorbing screen and the adjacent canopy of trees by a partially absorbing phase screen. The phase-screen properties are found by finding the mean field in the canopy of the tree. Physical optics (PO) is then used to evaluate the diffracting field at the receiver level by using a multiple Kirchhoff-Huygens integration for each absorbing/phase half-screen combination. Index Terms - Propagation, urban areas, vegetation.

Cellular systems have hitherto been modeled mostly by graphs for the purpose of channel assignment. However, hypergraph modeling of cellular systems offers a significant advantage over graph modeling in terms of the total traffic carried by the system. For example, we shall show that a 37-cell system when modeled by a hypergraph carries around 30% more traffic than when modeled by a graph. We study the performance of channelized cellular systems modeled by hypergraphs in comparison with those modeled by graphs. For this purpose, we have evaluated the capacities of these cellular networks defined in [3]. Evaluation of the capacity necessitates generation of maximal independent sets of hypergraphs. We describe some new algorithms that we have developed for this purpose.

Rice's sum of sinusoids can be used for an efficient approximation of colored Gaussian noise processes and is therefore of great importance to the software and hardware realization of mobile fading channel models. Although several methods can be found in the literature for the computation of the parameters characterizing a proper set of sinusoids, only less is reported about the statistical properties of the resulting (deterministic) simulation model. This is the topic of the present paper, whereby not only the simulation model's amplitude and phase probability density function (pdf) will be investigated, but also higher order statistics [e.g., level-crossing rate (lcr) and average duration of fades (adf's)]. It will be shown that due to the deterministic nature of the simulation model, analytical expressions for the pdf of amplitude and phase, autocorrelation function (acf), cross-correlation function (ccf), lcr, and adf's can be derived. We also propose a new procedure for the determination of an optimal set of sinusoids, i.e., the method results for a given number of sinusoids in an optimal approximation of Gaussian, Rayleigh, and Rice processes with given Doppler power spectral density (psd) properties. It will be shown that the new method can easily be applied to the approximation of various other kinds of distribution functions, such as the Nakagami and Weibull distributions. Moreover, a quasi-optimal parameter computation method is presented. Index Terms - Deterministic simulation models, nonfrequency-selective fading channels, Rician channels, statistics.

The wide-sense stationary-uncorrelated scattering (WSSUS) channel model is a commonly employed model for the multipath channel experienced in mobile communications. The second-order statistics of these channels are described by the delay cross-power density [[Phi].sub.h]([Tau]; [Delta]t) or by its [Delta]t-Fourier transform, the scattering function [S.sub.h]([Tau]; [Lambda]). This paper presents an analysis of the delay cross-power density and scattering functions for mobile communications channels. We assume an arbitrary spatially uncorrelated scattering (US) field with arbitrary propagation-loss factors. Our first result is a general integral expression for [[Phi].sub.h]([Tau]; [Delta]t) that holds with both transmitter and receiver being mobile. We then derive more detailed results for the case of a stationary base station. We derive an infinite Bessel series for [[Phi].sub.h]([Tau]: [Delta]t) and a closed-form expression for [S.sub.h]([Tau]: [Lambda]). These results generalize the well-known classical approximation for the time-correlation function [Mathematical Expression Omitted], which corresponds to the zeroth term of our Bessel series.

The use of microcell to increase the capacity of cellular mobile communication, especially in the dense urban areas, is an attractive possibility. A number of experimental and theoretical studies have been undertaken regarding propagation in the urban microcellular system. A microcell prediction model based on building thickness is developed to predict the statistical properties of the cell site. The model is validated based on several drive tests performed in different countries/cities with different transmitter heights, frequencies, cell-site parameters, and mobile environments. The first finding is that this empirical-based microcell model performed well in all these areas with a combination of various parameters. Second, the terrain does have a major impact on microcell prediction. This paper addresses the sufficient requirement of having only a two-dimensional (2-D) microcell prediction model. The impact of terrain contour on the microcell prediction is not included in this paper and will be published later. Index Terms - Building, microcell, prediction, propagation, ray tracing, terrain.

Spatial diversity is an attractive technology to cope with the fading channel encountered in mobile communications. This paper presents novel closed analytical expressions of the bit-error rate (BER) achievable in a coherent binary phase-shift-keying (CBPSK) direct-sequence code-division multiple-access (DS/CDMA) system for any power delay profile and for either postdetection selection or maximal ratio combining (MRC). In particular, expressions for the cutoff rate [R.sub.o] and for its related parameter D are also formulated in order to assess the system performance under the consideration of some channel coding schemes. Finally, an exemplary study is carried out in order to illustrate the behavior of a realistic space-diversity code-division multiple-access (CDMA) system according to the analytical expressions that have been derived. Index Terms - Channel coding, code-division multiple access, fading channel, mobile communications, spatial diversity.

The probability of error performance of a direct-sequence code-division multiple-access (DS/CDMA) system employing noncoherent M-ary orthogonal signaling in a Nakagami multipath fading channel is analyzed. A RAKE receiver structure with square-law demodulation is used at the receiver. The multiple-access interferences are modeled as Gaussian and expressions derived for the exact probability of error. The performance is also evaluated in terms of the number of users that can be supported by the system at a given probability of error. The effect of correlated fading on system performance is also investigated by considering two correlation models, which can be characterized by a single correlation coefficient [Rho]. In the first model, the correlation coefficient between any two diversity branches is constant. In the second model, it is assumed that the correlation coefficient between any two diversity branches decreases exponentially as the separation between them increases. For both models, it is found that the presence of correlation deteriorates system performance. The use of larger signal alphabets than binary modulation in conjunction with diversity reception provides a considerable performance improvement even in the presence of correlated fading. Index Terms Correlated Nakagami fading, DS-CDMA system, Mary noncoherent orthogonal modulation.

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

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. In this paper, 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. Index Terms - Cellular mobile system, C/I balancing, distributed power control, instability detection, power control.

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.

Modeling and analysis tools are essential for design and evaluation of complex systems. This is particularly true for cellular systems, where, for instance, a variety of handoff, channel allocation, and data-transmission algorithms have been proposed. In this paper, the capabilities of Petri nets (PN's) are used as a novel approach in the analysis of handoff, dynamic channel allocation (DCA), and cellular digital packet data resource management problems. The generalized stochastic PN (GSPN) models are obtained and analyzed as continuous-time Markov chains (MC's) derived from the reachability graphs. Solution of the MC results in performance indicators, which show the impacts of different algorithms on the system behavior. Index Terms - Cellular data, cellular systems, dynamic channel assignment, handoff, modeling, Petri nets.

A very simple but general scheme for calculating irreducible bit error rate (BER) (namely, BER floor) due to intersymbol interference in frequency-selective Nakagami-Rice fading environments has been developed. The scheme, which we call the equivalent transmission-path model, plays a role in connecting wave propagation with digital transmission characteristics in a general manner. Through computer simulations assuming various types of delay profiles, we first identify key parameters of Nakagami-Rice fading dominating principally the occurrence of irreducible errors, and we develop a simple method for the calculation of irreducible BER utilizing the nature of the key parameters. Then we examine the accuracy of the scheme for various types of phase-shift keying (PSK) transmission systems. Finally, based on the scheme, we show calculation examples of BER floor characteristics in line-of-sight fading environments. Index Terms - BER floor, BER map, intersymbol interference, key parameters, Nakagami-Rice fading.

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 ground and wall surfaces and diffraction from corners or rooftops of buildings. The exact location of 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. Index Terms - Path loss, power-delay profile, propagation modeling, uniform theory of diffraction.

In this paper, a model is proposed to estimate the signal statistics for urban-area base stations with highly elevated antennas. Based on the model, the envelope correlation of the received signal is estimated in the case of hybrid space-frequency diversity reception at the base. It is shown that the spatial correlation of the complex signals is determined by the mean angle of arrival and the width of the incoming beam. Useful closed-form approximations for inline and broadside antenna orientations are obtained. Comparisons between the theoretical model and available experimental data provide adequate justification for the model. It is shown that the lowest spatial correlations among all the possible angular orientations are obtained when the mean direction of incoming signals is perpendicular to the axis of the base antennas. For this case, an antenna separation of 26[Lambda] is required to achieve an envelope correlation of 0.7 when the width of the incoming beam is 0.7 [degrees], and this separation reduces to 9[Lambda] for a width of 3 [degrees]. The results indicate that the required spacings can be further reduced by using frequency diversity in conjunction with space diversity. Generalization of the model to incorporate local scattering effects is also proposed. Index Terms - Diversity, mobile communications, mobile propagation.

Time-division multiple access (TDMA)-based systems have been adopted as the first generation digital cellular standards in North America, Europe, and Japan. The increasing demand for capacity motivates the study of alternate systems and enhancements to the adopted TDMA standards. In particular, CDMA techniques using direct sequence and slow frequency hopping have been suggested. In this paper we analyze the capacity of a slow-frequency-hopped TDMA-based cellular system via calculation of outage probability. Expressions for outage probability as a function of voice activity, power control, and antenna diversity are provided for mobile-to-base transmission. These expressions are used to evaluate system capacity (users/MHz/cell) based on simulating mobile locations within 37 hexagonal cells. The sensitivity to power control error, voice activity factor, and soft capacity of the system is investigated.

Suboptimal parallel schemes for the acquisition of spreading sequences in chip-asynchronous spread-spectrum systems are considered. These acquisition schemes estimate the unknown delay of the received signal with respect to a locally generated spreading code. Two schemes are presented which are considerably easier to implement than the optimal estimator. An analytical expression is given for the error probability of the simpler of the two schemes, and it is shown that the average error probability of this scheme decreases exponentially with the signal-to-noise ratio. Numerical results show that the performance of both suboptimal estimators is comparable to that of the optimal estimator.

In a noncellular or large cell-size mobile radio communication system, log-normal shadowing as well as Rayleigh fading becomes the predominant source of system degradation. This paper proposes an efficient stop and wait automatic repeat request (SAW-ARQ) protocol with adaptive packet length to provide reliable mobile data packet transmission. The adaptive SAW-ARQ protocol controls the transmitting packet length according to the time-varying channel condition estimated with the number of ACK (acknowledgment packet) and NACK (negative-acknowledgment packet). Computer simulation results show that the proposed protocol can achieve high throughput and reduce the number of retransmission effectively for slow and fast Rayleigh fading/log-normal shadowing conditions.

Code division multiple access (CDMA) has been proposed for personal communications networks for both terrestrial and satellite links. In this paper, we analyze the performance of the downlink (i.e., the base-to-mobile link) of a low earth orbiting (LEO) mobile satellite channel. An important characteristic of this fading channel is that the desired signal and the multiple access interference from all spot beams of the corresponding satellite fade simultaneously. In this respect, the satellite downlink is also different from its terrestrial counterpart. A two-state fading channel model is considered. In the nonshadowed state, the signal envelope is characterized by Rician statistics and in the shadowed state by the Rayleigh statistics. We analyze the probability of error performance when coherent detection, diversity, spectrum sharing by two service providers, and forward error correction are employed.

Power control (PC) is an important issue in a direct sequence-code division multiple access (DS-CDMA) cellular mobile radio system. Higher link performance and greater system capacity cannot be achieved unless an appropriate PC mechanism is employed. In previous research, a delta-modulation (DM) realization of strength-based and SIR-based PC mechanisms for uplink communication has been studied by simulation. In order to obtain higher PC trackability, in this paper we study a pulse-code-modulation (PCM) realization of the above two PC mechanisms for the uplink of a DS-CDMA cellular mobile radio system. The simulation results presented here indicate that PC mechanisms with PCM realization for the uplink can achieve a lower outage probability and thus higher link performance than PC mechanisms with DM realization. We also obtain optimal design parameters such as the stepsize and the control mode for the two PCM PC mechanisms. In addition, we compare the two PCM PC mechanisms in terms of their outage probability and stability and find that the strength-based mechanism has a higher outage probability but greater stability than the SIR-based mechanism.

In this paper we detail the development of a near-far (NF) resistant sliding window decorrelating algorithm (SLWA) that overcomes the near-far problem (NFP) pertaining to the conventional linear correlation receiver (CLCR) and alleviates some of the limitations of the standard decorrelator [1]. The SLWA architecture is extended to incorporate differentially coherent multipath (RAKE) diversity combining techniques and hence achieves simultaneous rejection of multiuser and multipath interference. This paper also presents a novel algorithm for the updating of decorrelator coefficients using a fully parallel architecture. We present a mathematical model for the performance of a sliding window scheme, the main contribution of which is the analysis of finite sequence length multipath decorrelation under the practical limitation of incomplete RAKE combining. In addition to numerical results, we present results pertaining to capacity improvements achieved over the CLCR, derived from a simulation of a multiuser direct-sequence code-division multiple-access (DS-CDMA) mobile radio system.

Variable rate speech coding is now recognized as an important system component for high-capacity cellular networks because it exploits speech statistics to reduce the average bit rate, which results in reduced interference and increased capacity. Once a variable rate capability is available, an additional capacity enhancement can be achieved by introducing network control of the user bit rate in response to changing traffic levels. In this paper, we introduce the concept of network control of rate and propose a particular network-control method for code-division multiple access (CDMA) systems. Based on an M/M/[infinity]//M queueing model applied to a cell under heavy traffic conditions and a new performance measure called averaged speech quality, we obtain simulation results to demonstrate how network control of rate can achieve improved speech quality or increased capacity for a given quality objective.

The proliferation of mobile, portable, and personal communication systems will bring a variety of offered services. Practical systems that are envisioned must support different types of calls. These may include voice only, mixed voice and data, high-speed data, low-speed data, image transmission, and an array of intelligent network services. In addition there may be a mixture of platforms (such as persons, autos, buses, trains, boats, and planes) having a range of mobility characteristics. In such environments, the bandwidth and/or resources needed for different call sessions will not be identical. As a result, calls will generally encounter different blocking and hand-off constraints. These effects are in addition to differences in blocking and forced (call) termination probabilities that are attributable to differing platform mobilities and (resource) channel quotas. Cellular systems with mixed platforms that support calls with differing resource requirements are considered. Loss-type systems and hybrid delay-loss systems are treated. In each case, priority access to resources for hand-off calls is allowable. We identify a suitable state characterization and framework for performance analysis that enables numerical computation of theoretical performance results. Example performance characteristics are obtained. These show carried traffic, blocking probability, and forced termination probability for each platform type and for each call type.