Effects of Rician faded and log-normal shadowed signals on spectrumefficiency in microcellular radio

An assessment of spectrum efficiency for a microcellular land mobile radio system is presented by considering the desired signal as (fast) Rician fading with (slow) log-normal shadowing and cochannel interfering signals as uncorrelated (fast) Rayleigh fading superimposed over (slow) log-normal shadowing. Spectrum efficiency is defined in terms of reuse distance, i.e., cluster size, traffic intensity, bandwidth of the system, and area of a cell by considering cochannel interference probability. The expression for cochannel interference probability is derived using appropriate path-loss law for microcells for four different cases: Rician plus log-normal desired signal and Rayleigh plus log-normal interfering signals; Rician desired signal and Rayleigh fading plus log-normal shadowing interfering signals; Rician desired signal and Rayleigh interfering signals; and both desired and interfering signals as Rician fading. The performance of a microcellular system is compared with that of a conventional macrocellular system     

Microwave path-loss modeling in urban line-of-sight environments

To model the path-loss characteristics in microwave urban line-of-sight (LOS) propagation, we performed measurements in metropolitan Tokyo at frequencies of 3.35, 8.45, and 15.75 GHz. The actual breakpoint distance was shorter than the theoretical one because of the influence of vehicles, pedestrians, and other objects on the road. This effect has been referred to as effective road height. We found that the breakpoint disappeared when the mobile-antenna height approached the effective road height. Based on these breakpoint characteristics, we propose new upper- and lower-bound formulas for LOS path loss in urban microcellular environments. They will be useful for the system and cell design of fourth-generation multimedia mobile communication systems using microwave bands.     

A unified framework for the study of wireless packet access in thepresence of slow propagation impairments

The paper introduces a unified analytical framework which allows accurate study of multiple access performance in wireless packet systems. The method is based on the analysis of the interference statistics and takes into account slowly varying shadowing and best base station assignment, as well as the effect of the nonuniform traffic distribution induced transmission failures. As an example of application of the technique, two protocols are considered: (1) the classic slotted ALOHA, for which a comprehensive study in a realistic wireless environment has not yet been presented, despite the many analyses published in the past; and (2) the capture division packet access (CDPA). An important conclusion of our study is that if power control and best cell site assignment are jointly used, the performance loss due to slow shadowing is almost negligible     

Connectivity Requirements for Self-Organizing Traffic Information Systems

To facilitate the dissemination of a time-critical information in a vehicular ad hoc network (VANET), immediate network connectivity is needed. In other words, a time-critical message from a source should be able to propagate and reach all of the vehicles on the road segment without any delay due to the unavailability of the vehicles to forward the message. Clearly, this requires the road segment to have a certain number of vehicles equipped with communication devices. It is the task of a system designer to determine the minimum number of vehicles (i.e., the minimum penetration) necessary to form a connected network as well as the critical transmission range required to provide such connectivity. In this paper, we present an analytical framework for determining the connectivity requirements in distributing the traffic information in a self-organizing vehicular network. One- and two-way street scenarios are considered. In addition to the conventional perspective on connectivity, where the characteristics of the wireless channel are often neglected, our analysis offers a new view by taking important physical-layer parameters, such as fading, propagation path loss, transmit power, and transmission data rate, into consideration.      

Advanced LOS path-loss model in microcellular mobile communications

This paper proposes an advanced line-of-sight path-loss model incorporating characteristics of an actual urban environment. Accurate results are obtained by using a two-path model with shadowing caused by obstacles that would appear in an actual urban environment and rising reflection area caused by vehicular traffic. By applying the model in an urban environment, microcellular path loss can be predicted from ultra-high-frequency to microwave bands     

Path Loss Modeling for Vehicular System Performance and Communication Protocols Evaluation

Vehicular communications are receiving considerable attention due to the introduction of the intelligent transportation system (ITS) concept, enabling smart and intelligent driving technologies and applications. To design, evaluate and optimize ITS applications and services oriented to improve vehicular safety, but also non-safety applications based on wireless systems, the knowledge of the propagation channel is vital. In particular, the mean path loss is one of the most important parameters used in the link budget, being a measure of the channel quality and limiting the maximum allowed distance between the transmitter (Tx) and the receiver (Rx). From a narrowband vehicular-to-vehicular (V2V) channel measurement campaign carried out at 5.9 GHz in three different urban environments characterized by high traffic density, this paper analyzes the path loss in terms of the Tx-Rx separation distance and fading statistics. Based on a linear slope model, values for the path loss exponent and the standard deviation of shadowing are reported. We have evaluated the packet error rate (PER) and the maximum achievable Tx-Rx separation distance for a PER threshold level of 10 % according to the digital short-range communications (DSRC) specifications. The results reported here can be incorporated in an easy way to vehicular networks (VANETs) simulators in order to develop, evaluate and validate new protocols and systems architecture configurations under realistic propagation conditions.     

Reuse efficiency in urban microcellular networks

We study how efficiently radio channel resources can be reused in microcellular networks. Our focus is on dense urban networks in which cell-site antennas are located well below the rooftops of surrounding buildings. Compared with conventional cellular networks, this kind of microcellular system introduces a markedly different propagation environment, characterized by a mix of strong line-of-sight (LOS) propagation and much weaker non-LOS propagation. We address a number of important issues associated with site layout and reuse planning in such microcellular networks (rectilinear street environments) and present numerical results showing the influence of frequency, cell size, link quality requirements, shadow-fading statistics, and power control on the reuse efficiency. For 900-MHz systems, orthogonal multiple-access schemes (e.g., time division), and a typical set of system parameters, we estimate that the reuse efficiency in urban microcellular networks could be up to 50% higher than that in conventional cellular networks with three-sector antennas     

Intercell Radio Interference Studies in Broadband Wireless Access Networks

Capacity has become of primary importance in broadband wireless access (BWA) networks due to the ever-increasing demand for multimedia services and the possibility of providing wireless Internet, leading to their standardization by IEEE (802.16 WirelessMAN) and ETSI (BRAN HIPERACCESS). The major factor limiting capacity in such systems is interference originating from adjacent cells, namely intercell interference. This paper presents a general analysis of intercell interference for a spectrally efficient BWA cellular configuration. It examines the statistical properties of the carrier-to-interference ratio in both downstream and upstream channels. The focus is on the spatial inhomogeneity of rain attenuation over multiple paths, which is the dominant fading mechanism in the frequency range above 20 GHz, especially when two-layered [i.e., line-of-sight (LOS) and non-LOS] architectures are involved. Besides attenuation from precipitation, various architectural and propagation aspects of local multipoint distribution service systems are investigated through simulations, and worst-case interference scenarios are identified     

Network issues for wireless communications

This article specifically focuses on wireless personal communications, i.e., wireless access, that provides either terminal or personal mobility. In particular, we discuss some important issues in networking, traffic, and performance. Although within radio and networking aspects there are significant commonalities between traditional cellular mobile communications and wireless personal communications, there exist distinct differences due to radio propagation and fading effects, interference environment, smaller cell sizes, type and pattern of mobility, and call delivery. Indeed, with respect to networking issues, a large set of system choices, characteristics of traffic to be carried, and important parameters have to be considered. These include the problems involved in selecting an appropriate multiaccess technology to efficiently handle the required subscriber service profile across a multiplicity of systems to complete a call. To present a meaningful discussion of these issues, we address in some detail radio resource assignment, mobility management, call control, and traffic aspect, which have significant impact on the network performance     

Concatenated orthogonal/PN spreading sequences and theirapplication to cellular DS-CDMA systems with integrated traffic

In this paper, we investigate the application of the concatenated orthogonal/PN spreading scheme for a cellular direct sequence code-division multiple access (DS-CDMA) system with integrated traffic. The performance of the system is evaluated in terms of user capacity. In order to incorporate traffic with a wide range of source rates, line rates (adjusted data rates before spreading) have to be selected for transmission. For traffic with source rates higher than the line rate of concern, we propose the use of concatenated orthogonal/PN spreading sequences to subdivide a high rate stream into parallel line rate streams. Therefore, in this paper, we first analyze the properties of the concatenated orthogonal/PN spreading sequences. The results are used to evaluate their performance for homogenous voice traffic in various cellular mobile environments with multipath fading, lognormal shadowing, and path loss. Our results show that the proposed spreading scheme offers a significant improvement in the forward link capacity as compared to using the conventional nonconcatenated long PN sequence, especially if the multipath fading is Rician (e.g., microcellular and indoor picocellular systems). Incorporating the notion of line rate, we then evaluate the performance of a system with integrated voice and video traffic. Special emphasis is placed on the effect of line rate selection on the overall capacity which leads to the optimal selection of line rates     

Performance of microcellular mobile radio in a cochannelinterference, natural, and man-made noise environment

A model is developed for performance analysis of a microcellular digital mobile radio system with Rayleigh-faded cochannel interference, Gaussian noise, and narrowband impulsive noise (Middleton's Class A noise) using the differential phase shift keying (DPSK) modulation technique. The desired signal is assumed to be Rician faded. The effects of selection diversity on the performance are investigated and compared to the nondiversity case. The performance is measured in terms of bit error probability and spectrum efficiency. The influence of reuse distance (i.e. cluster size), traffic intensity (i.e. blocking probability), impulsive index, Rician parameter, and turning point of the dual path loss law characteristic of microcells on the performance parameters is investigated in detail     

A Propagation-Measurement-Based Evaluation of Channel Characteristics and Models Pertinent to the Expansion of Mobile Radio Systems to Frequencies Beyond 2 GHz

This paper concerns the measurement-based comparison of urban microcellular mobile radio channel characteristics at 1.9 GHz and a higher frequency, i.e., 5.8 GHz, where future wireless systems could operate. Characteristics that are reported include transmission loss, root-mean-square delay spread, frequency correlation, and envelope fading statistics. Conclusions are drawn concerning whether significant differences in the performance of mobile radio links operating under the same signal-to-noise ratio conditions in the two bands would be expected in the area where the measurements were made     

Millimeter-Wave Propagation Channel Characterization for Short-Range Wireless Communications

This paper presents and analyzes the results of millimeter-wave 60-GHz frequency range propagation channel measurements that are performed in various indoor environments for continuous-route and direction-of-arrival (DOA) measurement campaigns. The statistical parameters of the propagation channel, such as the number of paths, the RMS delay spread, the path loss, and the shadowing, are inspected. Moreover, the interdependencies of different characteristics of the multipath channel are also investigated. A linear relationship between the number of paths and the delay spread is found, negative cross correlation between the shadow fading and the delay spread can be established, and an upper bound exponential model of the delay spread and the path loss is developed to estimate the worst case of the RMS delay spread at given path loss. Based on the DOA measurements that are carried out in a room [line of sight (LOS)] and in a corridor with both LOS and nonline-of-sight (NLOS) scenarios, radio-wave propagation mechanisms are studied. It is found that considering the direct wave and the first-order reflected waves from smooth surfaces is sufficient in the LOS cases. Transmission loss is very high; however, diffraction is found to be a significant propagation mechanism in NLOS propagation environments. The results can be used for the design of 60-GHz radio systems in short-range wireless applications.     

Performance analysis of microcellular mobile radio systems with shadowed cochannel interferers

A study is presented of microcellular mobile radio systems where the desired signal has Rician statistics and cochannel interferers experience lognormal shadowing as well as Rayleigh fading. This implies a Rician/Rayleigh-plus-lognormal microcell interference model. The probability density function of the signal-to-interference ratio is derived and used to evaluate the performance of microcellular systems in terms of the outage probability.<>     

A Mathematical Theory of Network Interference and Its Applications

In this paper, we introduce a mathematical framework for the characterization of network interference in wireless systems. We consider a network in which the interferers are scattered according to a spatial Poisson process and are operating asynchronously in a wireless environment subject to path loss, shadowing, and multipath fading. We start by determining the statistical distribution of the aggregate network interference. We then investigate four applications of the proposed model: 1) interference in cognitive radio networks; 2) interference in wireless packet networks; 3) spectrum of the aggregate radio-frequency emission of wireless networks; and 4) coexistence between ultrawideband and narrowband systems. Our framework accounts for all the essential physical parameters that affect network interference, such as the wireless propagation effects, the transmission technology, the spatial density of interferers, and the transmitted power of the interferers.      

Analytical Formulae for Path Loss Prediction in Urban Street Grid Microcellular Environments

Rapid growth in the demand of wireless communications makes the use of microcells essential. Ray tracing models have shown good path loss prediction accuracy in microcellular environments. However, these techniques have great computational and memory requirements. The purpose of this paper is to derive analytical propagation path loss formulae for microcellular communications in urban street grid environments. The formulae are derived based on the geometrical optics and diffraction theory and they take into account multiple reflections along street walls and diffractions around street corners with ground reflections neglected; i.e., a two-dimensional approach. Comparisons between our theoretical results and published measurements in Tokyo and New York Cities show good agreement.     

Markov analysis of the PRMA protocol for local wireless networks

PRMA (packet reservation multiple access) is a reservation-ALOHA access protocol specifically designed for wireless microcellular networks that handle both real-time and non-real-time traffic. We present a thorough analysis of this protocol, considering real-time traffic only, based on a suitable Markov model. The size of the model is such that it can be directly used for an exact quantitative analysis of the system. In particular, we are able to analyze the packet dropping process, by evaluating both average and distribution measures. The latter are particularly useful to characterize the degradation caused to real-time traffic (e.g., voice) by the loss of consecutive packets. Besides, we also derive from the Markov model a qualitative analysis of the system stability, based on the equilibrium point analysis (EPA) technique. By this technique, we characterize the system stability and analyze the effect on it of several system parameters (e.g., load, permission probability). This revised version was published online in June 2006 with corrections to the Cover Date.     

Teletraffic performance of highway microcells with overlaymacrocell

The teletraffic performance of a highway microcellular digital mobile radio system having an oversailing macrocell that spans many microcells is presented. The microcellular cluster is composed of concatenated segments of the highway where each segment is a microcell, typically 500-2000 m in length, with the base stations located at lamp-post elevations. A narrowband time-division-multiple-access arrangement supporting ten channels per carrier and one carrier per base station is used. The teletraffic analysis assumes there are n-up and n-down lanes, and that the vehicular speeds conform to a truncated Gaussian distribution whose mean speed is 100 or 50 km/h when the vehicular traffic is free-flowing or in traffic-congested conditions, respectively     

Call admission control strategy for system throughput maximization considering both call- and packet-level QoSs

This letter presents a call admission control (CAC) strategy for system throughput maximization in wireless uplink systems. This strategy considers not only the call-level quality of service (QoS) (i.e., blocking probability) but also the packet-level QoS (i.e., outage probability). Using the statistical co-channel interference (CCI) model and state diagram, the outage probability and the blocking probability are investigated as a function of the relative traffic load. We formulate the CAC strategy problem based on relative traffic load, and suggest a solution. The numerical results show that maximum system throughput can be achieved by controlling the relative traffic load. In addition, we illustrate the region where system throughput is constrained by call- and packet-level QoSs. This examination shows that the call and packet-level QoSs must be considered together to achieve maximum system throughput.     

Routing in Sparse Vehicular Ad Hoc Wireless Networks

A vehicular ad hoc network (VANET) may exhibit a bipolar behavior, i.e., the network can either be fully connected or sparsely connected depending on the time of day or on the market penetration rate of the wireless communication devices. In this paper, we use empirical vehicle traffic data measured on 1-80 freeway in California to develop a comprehensive analytical framework to study the disconnected network phenomenon and its network characteristics. These characteristics shed light on the key routing performance metrics of interest in disconnected VANETs, such as the average time taken to propagate a packet to disconnected nodes (i.e., the re-healing time). Our results show that, depending on the sparsity of vehicles or the market penetration rate of cars using Dedicated Short Range Communication (DSRC) technology, the network re-healing time can vary from a few seconds to several minutes. This suggests that, for vehicular safety applications, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times. In addition, the developed analytical framework and its predictions provide valuable insights into the VANET routing performance in the disconnected network regime.