Fade Depth Research Articles

Variability of ionospheric scintillation near the equatorial anomaly crest of the Indian zone

Abstract. Multistation observations of ionosphere scintillation at VHF (250 MHz) and GNSS L1 frequency from three locations – (i) Bokkhali (BOK) (geographic 21.6° N, 88.2° E, dip 31.48°, (ii) Raja Peary Mohan College Centre (RPMC) (geographic 22.66° N, 88.4° E, dip 33.5°) and (iii) Krishnath College Centre (KNC), Berhampore (geographic 24.1° N, 88.3° E, dip 35.9°) – at ~ 1° latitudinal separations near the northern crest of the equatorial ionization anomaly (EIA) of the Indian longitude sector are investigated in conjunction with total electron content (TEC) data and available ionosonde data near the magnetic equator to study fine structure in spatial and temporal variability patterns of scintillation occurrences. The observations are carried out in the autumnal equinoctial months of a high solar activity year (2011). In spite of smaller latitudinal/spatial separation among the observing stations, conspicuous differences are reflected in the onset time, duration, fade rate and fade depth of VHF scintillations as well as in spectral features. Scintillations are mostly associated with depletion in TEC around the anomaly crest and occurrence of ESF near the magnetic equator at an earlier time. Not only the strength of EIA, but also the locations of observing stations with respect to the post-sunset resurgence peak of EIA seem to play dominant role in dictating the severity of scintillation activity. A secondary enhancement in diurnal TEC in the post-sunset period seems to accentuate the irregularity activities near the anomaly crest, and a threshold value of the same may fruitfully be utilized for the prediction of scintillation around the locations. An idea regarding latitudinal extent of scintillation is developed by considering observations at L1 frequency from the GPS and GLONASS constellation of satellites. A critical value of h'F near the magnetic equator for the occurrence of simultaneous scintillation at the three centres is suggested. The observations are discussed considering electrodynamical aspect of equatorial irregularities.

A Practical Metric for Fading Environments

Wireless communications occurring in and about structures, vehicles, and other non-ideal environments are susceptible to channel fading due to multi-path conditions. For mobile communications, channels are typically modeled as Rician or, as a worst-case, Rayleigh. Emerging wireless systems, e.g., wireless sensor networks, have been shown to experience fading even more severe. We propose a metric, the 10% fade depth, by which large channel data sets can be readily distilled to characterize the fading environment be it Rician or hyper-Rayleigh. The metric is compared with the K-factor and applied to illustrate benefits of selection and polarization diversity.

Short-Term Fading Behavior in High-Speed Railway Cutting Scenario: Measurements, Analysis, and Statistical Models

Cuttings are widely used in high-speed railway (HSR) transportation to ensure the flatness of rails. The special structure of cuttings results in rich reflection and scattering, and creates dense multipath components. This paper presents a series of measurements of the propagation channel at 930 MHz conducted along the “Zhengzhou-Xi'an” HSR of China, to characterize the small-scale fading behavior of rail-cutting scenarios as a function of the geometry of cuttings, including crown width and bottom width. Raw data are collected in six cuttings (five cuttings are used for developing the model, while the other one is used for validation) in rural and suburban environments. We propose a set of effective methods to statistically model the spatial/temporal variations - including fade depth (FD), level crossing rate (LCR), average fade duration (AFD), and Ricean <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -factor - as a function of the structural parameters of cuttings. Akaike's Information Criterion (AIC)-based evaluation indicates that the Ricean distribution is the best to describe small-scale fading. In addition, the rich multipath and directionality of the transmitting antennas lead to a non-monotonous dependence of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -factor on the distance between transmitter and receiver. The autocovariance function of the deviation of the extracted <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -factors from the proposed model is presented and the coherence length is investigated. Our results show that even though a cutting is a scenario with severe fading, a “wide” cutting (i.e., with both wide crown and bottom widths) is conducive to the reduction of the severity of fading.

Measurements and Analysis of Propagation Channels in High-Speed Railway Viaducts

This paper reports (i) a set of measurements of the wireless propagation channel at 930 MHz, conducted along the "Zhengzhou-Xian" high-speed railway of China in various railway viaduct scenarios, and (ii) an analysis and modeling of the small-scale and large-scale channel parameters based on those measurements. The environment can be categorized into four cases, covering viaducts with different heights and in different suburban environments. Small values of fade depth, level crossing rates, and average fade duration are observed. Akaike's Information Criterion (AIC)-based evaluation indicates that the Ricean distribution is the best to describe small-scale amplitude fading. An analysis of the envelope autocovariance function shows that the coherence distance is less than 10 cm. The Ricean K-factor is modeled as a piecewise-linear function of distance. Moreover, a breakpoint path loss model is developed and shadow fading is investigated using the same break point as for the distance-dependent K-factor model. The Suzuki distribution is found to offer a good fit for the composite multipath/shadowing channels. We find that the viaduct height H, together with the number of surrounding scatterers, significantly affects the small- and large-scale channel parameters. These results are applicable to both normal-speed and high-speed railways, and will be useful in the modeling of railway viaduct channels and the design of railway wireless communication systems.

STATISTICAL ESTIMATION OF FADE DEPTH AND OUTAGE PROBABILITY DUE TO MULTIPATH PROPAGATION IN SOUTHERN AFRICA

This study builds on the earlier work by Odedina and Afullo on Multipath fading in Durban. Their work was based on multipath measurements in Durban over a 6.73km Line-of-Sight (LOS) link. This submission uses the geoclimatic factor approach and ITU-R recommendations P530-14 to obtain the multipath fading occurrence in flve cities in South Africa, including Durban. Three-year radiosonde data is used in estimating the percentage of time that a certain fade depth is exceeded and hence outage probability due to atmospheric multipath propagation, assuming the given fade depth leads to the received signal falling below the squelch level. We employ the Inverse Distance Square technique to estimate point refractivity gradient not exceeded for 1% of the time in the lowest 65m above the ground for flve locations within South Africa. Standard error of the mean and confldence interval for both annual averages and seasonal averages of point refractivity gradient is calculated to re∞ect possible deviation in the given readings. These values of point refractivity gradient obtained are used in determining the geoclimatic factor K. The results presented show monthly, seasonal and annual variation of both point refractivity gradient and geoclimatic factor K. The results conflrm that the geoclimatic factor K is region based. The percentage of time a given fade depth is exceeded for a single frequency increases rapidly with increasing path length. This is due to the fact that as the path length increases so do the multiple re∞ections leading to multipath propagation, which can result in either signal enhancement or multipath fading. A comparison of fade depth and outage probabilities is made with the earlier work in Durban and Rwanda in Central Africa.

AN ALTERNATIVE TECHNIQUE FOR ESTIMATING THE K-FACTOR FROM THE PHASE OF THE ELECTROMAGNETIC FIELD WITHIN A REVERBERATING CHAMBER

In this paper, an alternative technique for estimating the Rice factor, K, is applied to the phase of electromagnetic fleld within a reverberating chamber (RC) for classifying the fading depth on the coherent components in the emulated line-of-sight (LOS) environments. The estimator is time-efiective and general, and can be applied for any angle of arrival (AoA) of the received fleld and for any time varying propagation channel as a complementary method to the classical estimators for evaluating K above all when small but consistent coherent components are present. Measurements accomplished at the RC of the Universitµa di Napoli Parthenope (formerly Istituto Universitario Navale, IUN) conflrm the goodness of the proposed technique.

Dynamic Propagation Channel Characterization and Modeling for Human Body Communication

This paper presents the first characterization and modeling of dynamic propagation channels for human body communication (HBC). In-situ experiments were performed using customized transceivers in an anechoic chamber. Three HBC propagation channels, i.e., from right leg to left leg, from right hand to left hand and from right hand to left leg, were investigated under thirty-three motion scenarios. Snapshots of data (2,800,000) were acquired from five volunteers. Various path gains caused by different locations and movements were quantified and the statistical distributions were estimated. In general, for a given reference threshold è = −10 dB, the maximum average level crossing rate of the HBC was approximately 1.99 Hz, the maximum average fade time was 59.4 ms, and the percentage of bad channel duration time was less than 4.16%. The HBC exhibited a fade depth of −4 dB at 90% complementary cumulative probability. The statistical parameters were observed to be centered for each propagation channel. Subsequently a Fritchman model was implemented to estimate the burst characteristics of the on-body fading. It was concluded that the HBC is motion-insensitive, which is sufficient for reliable communication link during motions, and therefore it has great potential for body sensor/area networks.

Yearly variability of rain attenuation over a communication link at 13 GHz during monsoon months over Indian eastern region

In this paper, results on yearly variability of measured attenuation at 13 GHz under rainy conditions over a microwave communication link path situated between Sarojini College and Airport of Dum Dum area in Kolkata have been presented. The link belongs to an user agency and suffers a large attenuation due to heavy rain. The variation microwave amplitude was monitored and rain rate measurements were made on regular basis simultaneously by a fast response rain gauge in the monsoon months during 2007 and 2008. The observed microwave radio signal was characterized with a steady signal level ~−42 to −43 dBm with fade depth ~1 to 2 dB under clear sky situation. The results on attenuation have been studied in relation to simultaneous rain rate measurements. Yearly variability of rain attenuation has also been highlighted. These results are also useful for designing and installation of future microwave communication links both for terrestrial and satellite paths in this part of India, where rain characteristics are similar to Indian eastern coastal station. The probability distribution of attenuation and rain rates has been deduced from measured observations.

Operational condition for direct single-mode-fiber coupled free-space optical terminal under strong atmospheric turbulence

The author describes the key design feature of a newly developed compact free-space optical terminal, which can achieve robust direct single-mode-fiber-coupling with advanced bidirectional beacon tracking system and excellent operational performance under various adverse weather conditions, such as strong atmospheric turbulences and rain/snow falls. The author found that there is a good correlation between the scintillation index of beacon/signal receiving intensities and the signal fading depth. Therefore, the signal link quality can be predicted from the beacon scintillation index using the classical Rytov theory. According to the outdoor experimental results, it can be concluded that the scintillation index of the beacon beam should be less than 0.1 for stable and reliable communication links.

Performance analysis of WLAN physical layers using Markov channel model

Orthogonal Frequency Division Multiplexing (OFDM) and multiple antennas are being used in various standards of Wireless Local Area Network (WLAN) to enhance data rates and to achieve higher reliability in fading channel conditions. Channel modeling plays a crucial role, especially in wireless environment, in evaluating the performance of physical layer in fading environment. The paper presents a Finite State Markov (FSM) channel model, for computation of fading channel performance measures. Frequency and duration analysis is proposed to evaluate the performance of WLAN physical layers for OFDM, OFDM with Selection Combining (SC) and OFDM with Maximum Ratio Combining (MRC) diversity techniques. Level Crossing Rate (LCR) and Average Fade Duration (AFD) are computed with the analysis of FSM model. Bit Error Rate (BER) is then calculated with the help of LCR and AFD. LCR, AFD and BER performances are compared for three physical layers with varying fade depths and for different values of Signal to Noise Ratio (SNR). Results show that OFDM with MRC gives minimum value of BER for fade depths more than 0dB, while BER is minimum for OFDM with SC for fade depths below 0dB. OFDM with diversity always outperforms than OFDM without diversity for all values of fade depths.

The Fading Characteristic of the GSM-R Network’s Field Intensity under Raining Condition

The signal strength of the GSM-R network’s field intensity attenuates under raining condition due to the raindrop’s absorption and scattering. Moreover, the fading characteristic is influenced greatly by the scattering effect of the rainfall. Without regard to the landform and other factors, this paper uses Mie Theory to research the receiving level’s attenuation coefficient at different rainfall rates. And the related knowledge of electromagnetic waves through random medium is used to analyze the receiving level’s fading characteristic under raining condition. Analytical results indicate that the fading distribution model of the receiving level in GSM-R network is changed, except for the strength’s attenuation. Moreover, the fading depth grows under raining condition as a result of the scattered power caused by the raindrop. The analysis in this paper is proved by the measured data along the test section of Beijing-Tianjin line, and the research results provide the theoretical basis for GSM-R network’s anti-fading measures in the severe weather.

DEVELOPMENT OF AN ANALYTIC MODEL THAT CHARACTERIZES THE FADE DEPTH OF IEEE 802.15.4a UWB CHANNELS

This article presents the development of an analytic model of variation of power or energy in function of the band width of the IEEE 802.15.4a UWB channel. This analytic model characterizes the fade depth of the channel UWB IEEE 802.15.4a, also shows that the power in the frequency domain of the UWB channel follows a Gamma statistical distribution. The overall results are obtained by simulation in Matlab, validated by Kolmogorov-Smirnov test and calculating the mean square error. Finally the theoretical results are compared with measurement campaigns done by other authors.

An Empirical Path Loss Model and Fading Analysis for High-Speed Railway Viaduct Scenarios

Based on the narrowband 930-MHz measurements taken along the “Zhengzhou-Xi'an” high-speed railway in China, an empirical path loss model is proposed. It is applicable to high-speed railway viaduct scenarios, considering the influences of viaduct height H and base station antenna relative height h, which are not well-covered by existing large-scale models. The path loss exponents are investigated, based on which the influence of viaduct on propagation is discussed. The fading depth up to 15.96 dB and the Ricean K -factor with mean value of 3.79 dB are obtained.

Accuracy of temporal fade margin prediction for fixed wireless links exposed to urban traffic

The authors describe an empirical study to determine the accuracy of Ricean and Nakagami-m distributions for the prediction of fade margins, for 90 and 99% of temporal availability in fixed wireless links. They compare the results obtained assuming an envelope distribution (based on Ricean and Nakagami-m models) and those observed from measured data. Their results suggest that fade depths, for individual users predicted using a Ricean distribution, have a relatively small risk of being exceeded in practice. In contrast, a Nakagami distribution is somewhat ‘optimistic’ regarding the extra power that will in practice be required by a specific user. They also present a more systems-oriented approach, where instead of evaluating the modelling accuracy for a single terminal, they consider a collection of users in a spatial region. This is relevant for sensor networks or relay-assisted infrastructures. They found that the larger than predicted temporal fades encountered in some areas will to a large extent be compensated by those that are in practice smaller than expected. Thus, in this case, the average margin that needs to be considered is similar to that predicted by either distribution.

First- and second-order statistical characterizations of the dynamic body area propagation channel of various bandwidths

Comprehensive statistical characterizations of the dynamic narrowband on-body area and on-body to off-body area channels are presented. These characterizations are based on real-time measurements of the time domain channel response at carrier frequencies near the 900- and 2,400-MHz industrial, scientific, and medical bands and at a carrier frequency near the 402-MHz medical implant communications band. We consider varying amounts of body movement, numerous transmit–receive pair locations on the human body, and various bandwidths. We also consider long periods, i.e., hours of everyday activity (predominantly indoor scenarios), for on-body channel characterization. Various adult human test subjects are used. It is shown, by applying the Akaike information criterion, that the Weibull and Gamma distributions generally fit agglomerates of received signal amplitude data and that in various individual cases the Lognormal distribution provides a good fit. We also characterize fade duration and fade depth with direct matching to second-order temporal statistics. These first- and second-order characterizations have important utility in the design and evaluation of body area communications systems.

Clear-Air Signal Level Measurement for Microwave Line-of-Sight Link Application in South Africa

This paper presents an analysis of the 19.5 GHz clear-air signal level measurement variations over a line of sight link set up between two campuses of University of KwaZulu-Natal in Durban, South Africa. The measurement over the link spans a period of eleven months. An analytical expression for fade depth exceedance is proposed for KwaZulu-Natal, in South Africa. The link outage probability is compared with the results obtained from two ITU-R models as per Recommendations P.530-8 and P.530-12, Morita's model for Japan, and the model of Vigants used in the United States. The Morita model over sea and coastal regions gives an outage probability of 0.060%, compared to 0.037% obtained from the measurements, with the two ITU-R models also giving close values of 0.027% and 0.024%, respectively. However, Vigants' model is found to be the least appropriate for outage estimation in South Africa.

Analytical Approach to Model the Fade Depth and the Fade Margin in UWB Channels

In this paper, the variations of the ultrawideband (UWB) channel power as a function of the channel bandwidth are investigated. An analytical approach to characterize the fade depth and the fade margin due to small-scale fading in indoor environments is proposed. The approach is based on the IEEE 802.15.4a ultrawideband channel model and the assumption that the channel power can be modeled by a Gamma distribution. This analytical approach is checked by comparison with results derived through Monte Carlo simulations. The results show that the fade depth and the fade margin are closely related to the channel bandwidth. This analytical approach can be used to design and implement UWB communications systems.

Field Measurement on Simple Vehicle-Mounted Antenna System Using a Geostationary Satellite

This paper presents a field measurement of a simple antenna system mounted on a vehicle by utilizing a geostationary test satellite called Engineering Test Satellite VIII (ETS-VIII). Our developed antenna system is compact, lightweight, and promising for low-cost production. The antenna system is constructed by a 16-cm patch array antenna, which has simple satellite tracking that is controlled by a control unit as the vehicle's bearing is updated from a navigation system in real time. A Global Positioning System (GPS) module is utilized for the navigation system to provide accurate information of the vehicle's position and bearing during traveling. A control unit is provided as antenna-beam control and measured-data acquisition. We thoroughly examine the developed system in field measurements under open field areas and blockage areas in order to evaluate the propagation characteristics caused by utility poles, pedestrian overpasses, and vegetation-covered roads. In this measurement, the received signal power and the average bit error rate (BER) are simultaneously retrieved. Steadily received levels and average BER are satisfactorily attained during satellite tracking in open field areas. Moreover, the fade characteristics and average BER performance are also investigated during signal blockage. The results show that different environments give different degrees of attenuation, which affects the BER performance in terms of fade depth. Ultimately, our proposed antenna system can contribute to the design of future cost-effective mobile satellite communications.

Depth and Rate of Fading on Fixed Wireless Channels Between 200 MHz and 2 GHz in Suburban Macrocell Environments

Various bands between 200 MHz and 2 GHz have recently been reallocated to multipoint fixed wireless services. The links in such systems are usually obstructed by buildings and foliage and are susceptible to fading caused by windblown trees and foliage. To date, there have been relatively few efforts to characterize either the depth of fading in bands below 1.9 GHz or the rate of fading in any of these bands. We transmitted CW signals in the 220, 850 and 1900 MHz bands from a transmitter located 80 m above ground level in a typical suburban macrocell environment and collected time-series of received signal strength at distances between 1 and 4 km from the site. We reduced the data to show how the depth and rate of fading depend on the frequency band, time-averaged wind speed and distance in such an environment. Our most significant finding is that the rate of signal fading is very similar in all three bands. In particular, it is not proportional to carrier frequency, as a simplistic model involving moving scatterers might suggest. These results will provide useful guidance to those who seek to simulate, or develop detailed physical models of, fade dynamics in such environments.

Two Algorithms of Power Loading for MCM System in Nakagami Fading Channel

We consider two non-iterative algorithms of adaptive power loading for multicarrier modulation (MCM) system, The first one minimizes the average power of the system transmitter and ensures the preset average bit-error rate, while the second reduces the average transmitting power subject to the given values of demanded bit-error rate and of the outage probability. The algorithms may be used for power-efficient management of the up-link in cellular communication, where mobile terminals use rechargeable batteries, or of the downlink in satellite communication with solar power source of a transponder. We present performance analysis of the adaptive MCM systems supported by computer simulation for the case of the m-Nakagami fading and additive white Gaussian noise in the forward and backward channels. Evaluation of the power gain of the proposed strategies and its comparison with uniform power loading shows that the gain depends on the fading depth and average signal to noise ratio in the system sub-channels.