Land Mobile Satellite Systems Research Articles

Land mobile satelliteKa-band shadowing statistics in built-up areas for different road orientations

There is a growing interest in land mobile satellite systems at Ka-band for broadband communications. An accurate characterization of shadowing effects is needed to help plan such new systems. Here, the analysis of a measurement campaign carried out in urban and suburban areas in which shadowing effects were recorded is presented. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 19: 381–384, 1998.

An empirical model for high elevation angle land-mobile satellite channels at urban environment

A land mobile satellite system (LMSS) propagation measurement campaign at 1.8 GHz was launched in Athens city center during June 1996 for high elevation angle (60/spl deg/, 70/spl deg/ and 80/spl deg/) channels. Four narrow streets with large building blocks and heavy traffic were tested. A new empirical model is extracted to calculate the link margin required to compensate for multipath fading in urban environments. A comparison with an existing model is presented. The results show a considerable increase in the fade depth compared to suburban and rural environments. In general, the fade depth decreases with increasing elevation angle, and the overall analysis indicates a strong dependence of the signal attenuation on the width of the streets, the average building height and the vegetation at the edges of the streets.

City center high-elevation angle propagation measurements at L band for land mobile satellite systems

A land mobile satellite system (LMSS) propagation experiment at 1.8 GHz was performed in Athens city center during June 1996 for high-elevation angle channels. The transmitter platform used was a helicopter flying on predetermined paths to simulate high-elevation angle (60/spl deg/, 70/spl deg/, and 80/spl deg/) communication. A land vehicle specially prepared to accommodate the receiver system was performing the channel recordings. Four narrow streets with large building blocks were selected as representative, and the statistical analysis presented includes cumulative distributions functions, level crossing rate (LCR), average duration of fades (AFD), and fade and nonfade duration distributions. This paper also attempts a comparison of the measured data statistics with statistical models and provides least-square fit formulas for characteristic measures. The results show considerable increase of the fade depth compared to suburban and rural environments. In general, the fade depth decreases with the increasing elevation angle, and the overall analysis indicates the strong dependence of the signal attenuation on the width of the streets, average building height, and vegetation at the edges of the streets.

Correlation model for shadow fading in land-mobilesatellite systems

A simple yet accurate correlation model for shadow fading in land-mobile satellite systems, derived from L and S-band channel recordings, is proposed. The model has been developed for heavily wooded and suburban environments and covers elevation angles of 60-80/spl deg/. It is demonstrated that in such environments the effective correlation distance of shadowing is in the order of a few tens of meters.

Physical model of shadowing probability for landmobile satellite propagation

A physical model of shadowing probability for a land-mobile satellite system is derived from a simple geometrical model. The resulting model exhibits physically reasonable behaviour, in good agreement with empirical measurements and allows the variation of shadowing states with both satellite elevation and azimuth to be predicted.

A MULTIPLE-CHANNEL FREE-ACCESS COLLISION RESOLUTION ALGORITHM BASED CONNECTION REQUEST PROCEDURE FOR LAND MOBILE SATELLITE NETWORKS

A new binary feedback multiple-channel free-access (MCFA) collision resolution algorithm (CRA) based random-access protocol (RAP) is devised to implement the connection request procedure for demand-assigned channel access protocols in land mobile satellite communications networks. The maximum stable throughput of this protocol is derived and is shown to be superior to a previously devised multiple-channel obvious blocked-access CRA based RAP as well as to slotted ALOHA. The throughput performance is also superior to the previously devised multiple-channel window blocked-access CRA based RAP for four or more channels. An exact analysis of the average packet delay for this MCFA CRA based RAP is given and shown to be in excellent agreement with computer simulation results. The protocol is shown to have smaller average delays than the best single-channel free access CRA based RAPs over a wide range of practically usable throughput rates with reasonable delays. This MCFA CRA based RAP was implemented on a channel request protocol test-bed simulator which incorporates the physical link characteristics of the MSAT-X land mobile satellite system. These test-bed simulation results also demonstrate the superiority of this protocol over slotted ALOHA for both error-free channel and mobile facing channel scenarios.

Propagation Modeling and Simulation For Land Mobile Satellite Systems

Land Mobile Satellite Systems (LMSS) provide voice and data communications to mobile users via a geosynchronouss satellite. LMSS is an advanced communications technology which combines satellite and mobile communications techniques. Propagation considerations are very important for a reliable LMSS. Trees shadowing and multipath propagation effects due to scattering from the surrounding terrain are the dominant effects. These effects cause fading at reception. For link design, fading level or excess path loss must be known. A LMSS propagation model is developed using statistical distribution functions of shadowing and multipath propagation. The model parameters are obtained from experimental data. Propagation measurements at the L band have been made to determine the model parametters. Fading distribution and fading depth are obtained using the model and simulation for different LMSS propagation environments.

Evaluation of handover mechanisms in shadowed low earth orbit land mobile satellite systems

AbstractThis paper presents a quantitative analysis by computer simulation of the active set update (ASU) handover algorithm for a shadowed low earth orbit (LEO) land mobile satellite (LMS) environment. As a precursor to the handover analysis, the mutual visibility statistics for a 66 satellite polar and 48 satellite rosette‐type constellation are presented. These results show the statistical nature of the levels of satellite diversity and mobile‐to‐satellite elevation angles (to the highest satellite) within each network and also indicate the influence of the channel characteristics on the handover strategy. A two‐state Markov modulated channel model is assumed in the handover analysis, and this enables the assessment of increased levels of power and time hysteresis on the quality of service and network signalling load in a shadowed land mobile satellite environment. In particular, attention is given to the different modes of ASU operation for hard handover, switch diversity and soft handover.

Open-loop power control error in a land mobile satellite system

In order to combat large scale shadowing and distance losses in a land mobile satellite system, an adaptive power control (APC) scheme is essential. Such a scheme, implemented on the uplink ensures that all users' signals arrive at the base station with equal average power as they move within the satellite spot beam-an important requirement in a CDMA system. Because of the lengthy round-trip delay on a satellite link, closed-loop power control systems are only of marginal benefit. Therefore, an open-loop APC scheme is proposed to counteract the effects of shadowing and distance loss. A fairly general channel model, consisting of log-normal shadowing and Rician fading, is assumed. This can be applied to a specific two-state land mobile satellite channel model, involving shadowed intervals with Rayleigh fading and unshadowed intervals with Rician fading. It is found that the power control error can be approximated by a log-normally distributed random variable. To quantify the performance of the APC, the standard deviation of the power control error in decibels is analyzed as a function of the specular power-to-scatter power ratio, the measurement time and the vehicle velocity. To illustrate the usefulness of the results, we analyze the effect of the power control error on the system capacity of a CDMA mobile satellite link. >

Covariance matrix properties and multiple symbol/soft decision detection in slow flat Rician fading

For a small number of symbols N and slow flat fading channels, it is shown that covariance matrices encountered in practice have two nonnegligible eigenvalues, the first much larger than the second, with a symmetric eigenvector associated with the first eigenvalue, and a skew symmetric eigenvector associated with the second eigenvalue. The first eigenvector is well approximated by a conditional mean, and the second eigenvector represents a small drift about the mean. The eigenvalues and eigenvectors of the slow flat fading channel covariance matrix are shown to be strongly related to those of a certain conditional covariance matrix. The maximum likelihood (ML) rules for block hard decision and symbol-by-symbol hard decision, and a rule for soft decision detection of M-DPSK, all using multiple symbol information, are obtained for the Rician channel as a function of N. The eigenvalue-eigenvector results lead to practical implementations of all rules. For small to moderate N, it is shown that a simple open-loop algorithm, of complexity N log N, attains the performance of the ML decision rules for an E/sub s//N/sub 0/ range of interest for several land mobile satellite systems. The ML decision rules are seen to give rapidly diminishing returns as N increases, showing that simple noncoherent techniques can have very effective performance for the Rician fading channel. Lastly, several conclusions are drawn about the asymptotic channel behavior, including the Rayleigh channel. The work is directly applicable to the Australian and North American land mobile satellite systems. >

Phase synchronization in a satellite‐mobile channel

AbstractThis paper studies several aspects relevant to the design of phase synchronization circuits for land mobile satellite systems (LMSS). First, the results of a propagation experiment are presented, with special focus on the effects of vehicle mobility on the signal phase. It is found that the changes of speed and direction cause frequency variations that can be modelled as linear. Bounds for the frequency derivative are given for different percentages of time. The use of third‐order PLLs is considered, as they are optimum for tracking a frequency ramp embedded in white Gaussian noise. The model of third‐order PLL employed is presented, as it is an original model, developed by the authors. Then the performances of second‐ and third‐order PLLs are compared. Theoretical, as well as simulation, results are given. It is shown that third‐order loops give better performance in the presence of frequency variations of the kind observed in this link, but they suffer from a higher sensitivity to oscillators' phase noise. Several conclusions regarding the use of third‐order loops in future systems are presented.

Signal statistics obtained from a LMSS experiment in Europe with the MARECS satellite

A land mobile satellite system (LMSS) propagation experiment at 1.5 GHz has been carried out in Europe in the framework of ESA's PROSAT program using the MARECS satellite as the signal source. Rural, suburban and urban environments were investigated in Spain, France and Sweden in order to analyze the propagation factors responsible for degradations in LMSS. A second series of tests were performed in Spain using a locomotive as mobile platform. The results obtained show that shadowing is the main impairment in LMSS, achieving availabilities of 80-90% for the unfaded signal which decrease to 30% for the extreme cases of urban environments, dense woods, or suburban environments in low elevation angles, or railroads with a large number of tunnels. >

Further results on LMSS experiment in Kyoto City

In February of 1990, land mobile satellite experiments were conducted in Kyoto City, Japan, using the Engineering Test Satellite V (ETS-V). The received signal strength was recorded, as was the speed of the mobile as it traversed various downtown streets. The setting was that of a built-up urban area with medium and high rise buildings, electric utility poles, and trees lining the streets. These caused obstruction to the satellite line of sight (LOS). The signal level fluctuation due to diffraction of radio waves by the urban structures is analyzed. The results obtained from the measurements and theoretical calculation agree. The spatial frequencies due to diffraction are found to vary between 1.8 Hz to 5.3 Hz. It is concluded that this variation of received signal level will not adversely affect the receiver performance. Based on recorded signal strength, the performance of a digital land mobile satellite system (LMSS), under the above mentioned environment, is analyzed and the results presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Modeling and simulation of mobile satellite propagation

Mobile satellite systems are subject to severe fading due to blockage of the line-of-sight (LOS) path by roadside vegetation. A thorough understanding of the fading effects is necessary for the design of a reliable land-mobile satellite system. Analytical and empirical models are presented for predicting fade statistics for vegetative shadowing of mobile satellite terminals. A software simulator for generating simulated fade data is also presented. A physical model relating physical path parameters to propagation model parameters is presented, and results using the model are shown.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Narrowband channel statistics from multiband propagation measurements applicable to high elevation angle land-mobile satellite systems

Results from a multiband propagation experiment for a high elevation angle land-mobile satellite (LMS) channel are presented. A small helicopter was used to fly the transmitter module, transmitting frequencies in the L, S, and Ku bands. A vehicle equipped with the corresponding receivers and data acquisition system was followed by the helicopter on the selected routes. The measurement campaign was undertaken in two phases in September 1991 and in April 1992. The results show considerable decrease in signal attenuation when the path elevation angle is high due to reduced shadowing. In general, the attenuation increases with the increasing radio frequency. Foliage density has also been found to influence the fade levels, especially at higher path elevation angles. The overall analysis of the propagation data indicates that the fading character of the LMS channel is significantly dependent on the surrounding environment.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Statistical observations of data transmission over land mobile satellite channels

It is shown how Rician fading limits the performance of the HDLC protocol. It is observed that for accurate reception, a packet must be transmitted entirely during an interfade interval. This means that packets ought to be sufficiently short so that most of them fall in the interfade time intervals. This leads to a high transmission efficiency. Consequently, the HDLC protocol is more efficient with shorter packets. However, a smaller packet size increases the overhead fraction of the packet and thereby reduces the effective throughput. In addition, the HDLC peak efficiency and optimum packet size increase with increasing fade margin and decrease with increasing vehicle velocity. Among other effects, the bit rate of the land-mobile satellite system channel is the most critical for optimum packet size and HDLC efficiency. Thus, a low bit rate implies long packet duration for a moderate packet size in bytes. >

Land mobile satellite systems

The general advantages and potential operating characteristics of the mobile satellite service (MSS) are described, and distinctions are made between radio telephone, which is interconnected to the public switched telephone network, and private mobile radio systems. Mobile satellite service offers voice, data, position location, and paging services, interconnection to the public switched telephone network, and the possibility of private networks. Performance and cost characteristics are given along with summaries of market needs and market demands. The space and ground systems of the MSS are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Error correction/masking for digital voice transmission over the land mobile satellite system

The development of a 2400-b/s speech digitizer which provides an acceptable level of intelligibility and quality over land mobile satellite channels is described. Performance tests over simulated channels in the UHF band (800 MHz) are presented. The voice digitizer is a linear prediction (LPC) vocoder which uses a channel error correction and concealment procedure tailored to error statistics for a minimum-shift keyed (MSK) downlink to a moving vehicle. The error-handling technique is based on perceptual criteria and utilizes the parametric nature of LPC representation of speech. A single-error-correcting, single-burst-detecting (28, 20) fire code is shown to be the best choice for the application. The intelligibility of the vocoder is measured and compared to the standard LPC-10 algorithm. The major remaining sources of speech quality degradation due to channel errors are determined and ranked.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fade measurements at L-band and UHF in mountainous terrain for land mobile satellite systems

Fading results related to land mobile satellite communications at L-band (1502 MHz) and UHF (870 MHz) are described. These results were derived from an experiment performed in a series of canyon passes in the Boulder, Colorado region of the USA. The experimental configuration involved a helicopter as the source platform, which maintained a relatively fixed geometry with a mobile van containing the receiver and data-acquisition system. An unobstructed line of sight between the radiating sources and the receiving van was, for the most part, also maintained. In this configuration, the dominant mechanism causing signal fading (or enhancement) is a result of multipath. The resulting fade distributions demonstrated that at the 1% and 5% levels, 5.5 and 2.6 dB fades were on the average exceeded at L-band and 4.8 and 2.4 dB at UHF, respectively, for a path elevation angle of 45 degrees . The canyon results as compared with previous roadside-tree-shadowing results demonstrate that the deciding factor dictating fade margin for future land mobile satellite systems is tree shadowing rather than fades caused by multipath.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Roadside tree attenuation measurements at UHF for land mobile satellite systems

Tree attenuation results at 870 MHz are described for experiments conducted in October 1985 and March 1986 in Central Maryland. These experiments employed a helicopter as a source platform and a van with receiver and data acquisition instrumentation. Tree attenuation results were obtained for the cases in which the van was stationary and in motion. The experiments were performed for the purpose of providing the designers of planned land mobile satellite systems with important elements in the determination of link parameter requirements; namely, the expected fading statistics due to roadside trees for both mobile and stationary vehicles. Single tree attenuation results gave worst case median fades as high as 15 dB although roadside tree values were noted to produce fades in excess of 20 dB for small percentages of time. The cumulative fade distributions and their relative contributions as a function of path elevation angle, right side versus left side driving, and different road types are derived from the field measurements. Upon comparing the attenuations from bare deciduous trees (March 1986) with those due to trees in full foliage (October 1985), the increase in dB attenuations were, in general, less than 25 percent for the dynamic cases, and less than 40 percent for the worst case static configuration. This result demonstrates thai the dominant fading is caused by the wooded tree branches as opposed to the leaves on these branches. The tail end of the observed fade distributions was observed to follow lognormal distributions with respect to dB attenuation.