Macrocell Environments Research Articles

Ricean $K$-Factors in Narrow-Band Fixed Wireless Channels: Theory, Experiments, and Statistical Models

Fixed wireless channels in suburban macrocells are subject to fading due to scattering by moving objects such as windblown trees and foliage in the environment. When, as is often the case, the fading follows a Ricean distribution, the first-order statistics of fading are completely described by the corresponding average path gain and Ricean <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -factor. Because such fading has important implications for the design of both narrow-band and wideband multipoint communication systems that are deployed in such environments, it must be well characterized. We conducted a set of 1.9-GHz experiments in suburban macrocell environments to generate a collective database from which we could construct a simple model for the probability distribution of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> as experienced by fixed wireless users. Specifically, we find <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> to be lognormal, with the median being a simple function of season, antenna height, antenna beamwidth, and distance and with a standard deviation of 8 dB. We also present plausible physical arguments to explain these observations, elaborate on the variability of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> with time, frequency, and location, and show the strong influence of wind conditions on <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> .

An empirical model for dual-diversity reception over fixed wireless channels in suburban macrocell environments

Fixed wireless multipoint communication systems are increasingly being deployed in suburban macrocell environments, either to provide voice and data services to residences (wideband systems) or to supply automated meter reading and related supervisory control and data acquisition services (narrowband systems). Here, we show that a particularly simple yet complete first-order statistical characterization of two-branch diversity reception over fixed wireless channels can be given in terms of just five channel parameters: the average path gains and Ricean K-factors for each branch and the complex envelope correlation coefficient between the time-varying parts of the two path gains. We further show that all five parameters can be estimated from amplitude-only measurements of path gain vs. time, subject only to the assumption that the time-varying component of the path gain is complex Gaussian. We present representative sets of model parameters based upon data collected in typical suburban macrocell environments over time, location, and/or frequency using polarization diversity antennas. Because each of the model parameters, or its logarithm, is close to Gaussian, the set may be cast as a five-element vector of jointly random Gaussian processes. Despite obvious physical differences between the environments, the results show a remarkable consistency.

Characterization of time variation on 1.9 GHz fixed wireless channels in suburban macrocell environments

We observed temporal fading on 1.9 GHz fixed wireless channels during short-term measurements at 107 different locations in a suburban macrocell environment characterized by flat terrain and heavy foliage in order to determine how the rate of fading varies with average wind speed and distance. For each location, we estimated: (1) the Ricean K-factor using a moment-based estimator and (2) an equivalent Doppler frequency which is related to the maximum Doppler frequency by a factor that depends upon the shape of the Doppler spectrum. We did so by fitting the measured level crossing rate (LCR) and average fade duration (AFD) distributions to expressions normally justified for mobile wireless links using a method recently proposed by Feick, Valenzuela and Ahumada (2007). As has been observed at other sites, the Ricean K-factor decreased with both average wind speed and distance. However, we found that the equivalent Doppler frequency was effectively uncorrelated with wind speed and noticeably increased with distance. Similar measurements at other sites will be required to determine the extent to which these trends are affected by foliage density and tower height.

Improving a Cluster Based Directional Channel Model in Realistic Macro-Cell Environment

In this paper a realistic directional channel model that is an extension of the COST 273 channel model is presented. The model uses a cluster of scatterers and visibility region generation based strategy with increased realism, due to the introduction of terrain and clutter information. New approaches for path-loss prediction and line of sight modeling are considered, affecting the cluster path gain model implementation. The new model was implemented using terrain, clutter, street and user mobility information for the city of Lisbon, Portugal. Some of the model’s outputs are presented, mainly path loss and small/large-scale fading statistics.

Deterministic and Statistical-Based Channel Models in the MIMO Link Evaluation

This letter studies the performance of a multiple-input-multiple-output (MIMO) link in a macrocell environment, with emphasis on the effects of the antenna parameters as applied in different channel models. Two types of channel models are used in this evaluation. The first is a deterministic electromagnetic-based channel model that physically describes the antennas within the channel environment, capturing their detailed effects on the link performance. The second is a standardized statistical-based model where the 3GPP spatial channel model (SCM) is investigated. The antenna effects in this channel are roughly included through the radiation patterns of isolated dipoles. The differences between the channel model types as applied to the antenna parameters are discussed. Our analyses show that understanding the assumptions made in the different channel models is important in the evaluation of a system, especially in multiple-antenna systems.

A Generalized Model for the Spatial Characteristics of the Cellular Mobile Channel

In this paper, we present an intensive study of the spatial characteristics of the cellular mobile channel for picocell, microcell, and macrocell environments. We review the previous physical channel models and make appropriate comments/corrections wherever needed. We find that almost all physical channel models proposed so far are specific to particular cellular environments and that no general model exists in the literature. Thus, we propose a generalized physical channel model, referred to as the eccentro-scattering model, and derive the expressions for the probability density function (pdf) in an angle of arrival (AoA) of the multipath signals at a base station (BS) for the picocell, microcell, and macrocell environments using the two most commonly used scatterers' distributions, i.e., uniform and Gaussian. The derived formulas, in closed form, can be further used in designing beamwidth and channel tracking algorithms and assessing the performance of smart antennas. We discuss the relations between the results obtained for different cellular environments in the uniform and the Gaussian scattering and investigate the effects of the standard deviation of the scatter density and size of the scattering disc on the pdf of AoA at the BS. Previous work on the AoA statistics considering either the shape of the scattering regions or the distribution of scatterers within that region can more easily be reproduced using the provided general formula. We prove that assuming the scatterers to be either uniformly or Gaussian distributed in sparsely populated areas gives almost the same distribution of AoA of multipaths at the BS.

Macrocell multiple-input multiple-output system analysis

We develop a semi-deterministic semi-stochastic channel model for the multiple-input multiple-output (MIMO) system under the macrocell environment with local-to-mobile and local-to-base scatterers. We show that employing closely-spaced antennas (e.g., phased array) at the base station is capable of achieving diversity via the local-to-base scatterers, which avoids impractical large aperture requirement for the spatial diversity at the base station. We evaluate the system performance in terms of ergodic capacity, average pairwise error probability (PEP), and signal-to-noise ratio (SNR); derive closed-form expressions for lower and upper bounds on the capacity and PEP; and show that the capacity, multiplexing and diversity gains are limited by the number of multipaths around the base station. The base-station array affects the lower bound on the capacity and the upper bound on the error probability through the same metric; thus, optimal design of the base station array based on this metric will optimize the two different information theoretic measures simultaneously. The fading correlation matrix also appears in the two bounds in the same form. To improve the performance of the macrocell MIMO system, we propose using artificial scatterers and discuss optimal design issues. Numerical examples demonstrate the accuracy of our analytical results and tightness of performance bounds.

Field trial of space-time equalizer and delay diversity transmission in uplink for TDMA mobile communication

Space-time (ST) equalizer and delay diversity transmission (DDT) were field tested in uplink for a time-division multiple-access mobile communication system. The ST equalizer used a cascade connection of constrained array processors and branch-metric-combining (BMC) maximum-likelihood sequence estimation which was designed to provide both space- and path-diversity gains from first-arrival and one-symbol-delay paths while suppressing excessive intersymbol interference. DDT with two antennas was used for the mobile transmitter to ensure sufficient path diversity at the ST equalizer in a small-delay-spread environment. Test results showed that the ST equalizer bit error rate (BER) was significantly better than an ST equalizer without BMC and a single array processor for both micro- and macrocell environments. Furthermore, the ST equalizer BER with DDT was better than that without DDT for the microcell environment.

Random-Access Control Mechanisms Using Adaptive Traffic Load in ALOHA and CSMA Strategies for EDGE

In this paper, three random access control mechanisms based on the well-known Slotted ALOHA, NP-CSMA, and 1P-CSMA protocols are presented. The basic idea is to limit the number of transmissions and retransmissions at high traffic loads in order to minimize collisions while keeping system stability. A new medium-access control protocol called Adaptive Traffic Load (ATL) is proposed. With ATL, all users are assigned authorization of transmission probabilities that vary according to the prevailing average traffic conditions in the system. ATL ensures that the system throughput is kept constant at its maximum value regardless of the traffic load. A mathematical analysis to calculate the probability density function of the access delay in the ATL protocol under the assumption of infinite user population is also presented. Mean access delay follows increases exponentially with respect to the traffic load when conventional random access protocols are used. However, it follows a linear function with respect to the traffic load when ATL is used. The average traffic load of the system is an input of the ATL protocol in order to assign access authorization probabilities to all users attempting to access the network. A simple algorithm for traffic load estimation based on the probability of finding empty slots in the system within an estimation period is proposed in this paper to asses the average traffic load. In the numerical evaluations of ATL, the Enhanced Data for GSM Evolution (EDGE) system is considered as a case study. For high arrival rates, channel utilization can be low in EDGE even if the system has sufficient capacity to serve incoming data users. A mathematical analysis of ATL-Slotted ALOHA as well as ATL-CSMA is presented. In the case of ATL-CSMA, system throughput varies according to the cell size. Hence, the ATL protocol is evaluated in picocell, microcell and macrocell environments as recommended by ITU-R. Also, the performance of EDGE is evaluated in terms of average data rate and packet delay for both S-ALOHA and ATL S-ALOHA considering long range dependent traffic type.

Models of breast cancer growth and investigations of adjuvant surgical oophorectomy.

Clinical observations of the natural history of breast cancer and its response to a variety of therapeutic interventions have contributed to changing concepts about the growth and metastatic spread of this disease. Increased attention has been given to tumor cell dormancy and the occurrence of greatly delayed metastatic disease development, which has been important to rethinking therapy. Although gene profiling of breast tumors recently has highlighted the importance of individual tumor characteristics in patients' prognosis, considerable data also support the concept of breast cancer as a problem of macro- and microenvironmental regulatory imbalance and dynamic chaos. Observations of unexpectedly large survival benefits from adjuvant surgical oophorectomy done in the luteal phase of the menstrual cycle in premenopausal women are consistent with an interpretation that extratumoral interactions in the host environment are important in prognosis. These observations also suggest that a treatment paradigm shift from an exclusive focus on cell kill and specific tumor cell molecular targets to one focused also on broad host regulatory control may be useful. Clinical trials and laboratory mechanistic investigations based on these data and observations can determine the potential impact of therapeutic interventions targeting host system macro and micro tumor cell environments.

3-D double-directional radio channel characterization for urban macrocellular applications

We measured the spatial properties of the three-dimensional (3D) double-directional radio channel in urban macrocell environments separately at both ends of the link. In this paper, we study propagation conditions pertaining to reception and transmission at the mobile terminal, measured using a wideband channel sounder and a dual-polarized spherical antenna array. We were able to refine the results of the measurements conducted at the base station, and extend the study to full double-directional 3D channels. Individual propagation paths could be identified precisely, in some cases even considerable scattering from lampposts was observed. Our results show that over-rooftop-dominated propagation often occurs via building roofs with LOS to the base station antenna, acting as strong secondary signal sources. Based on measurements along continuous routes we demonstrate that the dominant propagation mechanisms can vary considerably when the mobile moves in the environment. We also present typical directional properties of the 3D radio channel at the mobile terminal in urban macrocell environments characterized by street canyons, showing how the angular distribution of energy is correlated with the excess delay.

Forward-link soft-handoff in CDMA with multiple-antenna selection and fast joint power control

We consider forward-link soft-handoff with multiple antenna selection and fast joint power control at high data rates in a cellular code-division multiple-access network, where signals are directed to a mobile station (MS) from antennas located at the same or different base stations. The total power transmitted to any mobile is divided among the active antennas selected according to the momentary channel conditions so as to maximize the signal-interference ratio at each MS. Multiple-antenna selection is used to mitigate the effects of both short- and long-term fading, and achieve the best soft-handoff with respect to system capacity and complexity. To achieve capacity gains with soft-handoff, we derive optimum handoff thresholds corresponding to the optimum handoff region in different cell environments. Numerical results demonstrate that under high Doppler spread and large handoff-delay conditions, the proposed soft-handoff employing two transmit antennas and the optimum handoff threshold achieves a significant gain in microcell environments, but not in macrocell environments.

Double characterisations of power angular spectrum in macro-cell environment

A propagation experiment using a uniform linear array at the base station in rural and suburban areas is presented. Wideband parameters, cluster behaviour and angular parameters at the base and mobile stations are analysed. Statistical results are provided and typical situations presented.

Hybrid TDOA/AOA mobile user location for wideband CDMA cellular systems

This paper proposes a mobile user location scheme for wideband code-division multiple-access (CDMA) wireless communication systems. To achieve high location accuracy and low cost of the mobile receiver, the location scheme combines the time difference of arrival (TDOA) measurements from the forward link pilot signals with the angle of arrival (AOA) measurement from the reverse link pilot signal. High chip rates in wideband CDMA systems facilitate accurate TDOA measurements, and a smart antenna used at the home base station (BS) can provide accurate AOA measurement in a macrocell environment. A two-step least square location estimator is developed based on a linear form of the AOA equation in the small error region. Numerical results demonstrate that the proposed hybrid TDOA/AOA location scheme gives a much higher location accuracy than TDOA only location, when the number of base stations is small and/or when the TDOA measurements have a relatively poor accuracy.

Analysis of smart antenna outage in UTRA FDD networks

The outage occurring from beamformer-based smart antennas is analysed for an antenna array deployed in an urban macro-cell environment. Spatio-temporal channel data obtained from a small, urban cell and from within the UTRA frequency allocation has enabled beamforming performance to be determined from the measured uplink and downlink frequency bands. The analysis shows a sub-optimum uplink beamformer performance is observed for 8% of occasions and a 25% downlink beamforming outage for an eight-element uniformly spaced linear array.

Link and System Level Performance of Multiuser Detection CDMA Uplink

The link level performance and the cellular system capacity in the uplink direction of a CDMA cellular system utilising multiuser detection base station receivers is analysed by simulation. In the receiver, parallel multistage multiuser detection is employed together with two-antenna diversity reception and fast closed-loop power control. A system level simulator is built to utilise the link level simulation results and to show the increase in cellular capacity obtained by using multiuser detection. The capacity is studied in urban micro and macro-cell environments utilising the channel models developed in the European CODIT project. The modelling of the environment specific and CDMA specific features is considered in the system simulator. The system level simulator is calibrated with analytical capacity calculations.