Mobile ground-based terminals observe significant Doppler on the forward channel when communicating through low Earth orbit (LEO) satellites. This paper deals with the analytic derivation of the Doppler shift measured by a user on the surface of Earth on a signal transmitted by a circular orbit LEO satellite. Two simplifications are performed to obtain the analytical expression of the Doppler shift as a function of time. First, during the visibility duration of the satellite at a terminal, the trajectory of the satellite with respect to the Earth is approximated by a great circle arc. Second, the angular velocity of the satellite with respect to the user is assumed to be constant. Numerical results validate the approximations. Another result of our analysis is an expression for the visibility window duration of a satellite at a terminal as a function of the maximum elevation angle. An algorithm for estimating the parameters of the Doppler curve based on a couple of Doppler and Doppler-rate measurements is also presented.

Doppler characterization for LEO satellites

Recently, broadband maritime communication has attracted much attention due to the rapid development of blue economy. In addition to the conventional MF/HF/VHF bands, there has been increasing interests in the utilization of higher frequency bands to provide broadband data service to the sea area. To design efficient maritime communication systems, the first and a fundamental requirement is to develop a framework to understand the wireless channels. In an integrated air-ground-sea communications network, there are two major type of channels to be investigated, namely the air-to-sea channel (e.g., for communication links from the aircraft-based base stations or relays) and the near-sea-surface channel (for land-to-ship/ship-to-land or ship-to-ship communications). Due to the unique features of the maritime propagation environment such as sparse scattering, sea wave movement, and the ducting effect over the sea surface, the modeling of these maritime channel links differs from conventional terrestrial wireless channels in many aspects and, consequently, will result in significant impact on the transceiver design. In this survey, we highlight the most notable differences from the modeling perspective as well as the channel characteristics for the air-to-sea and near-sea-surface channel links, with more focus on the latter. After a thorough review of existing modeling approaches and measurement campaigns, we conclude that the sparse and the location-dependent properties constitute the most important and distinctive characteristics of the maritime wireless channels. As such, we further remark on the challenges and research topics for future development of maritime communications.

Wireless Channel Models for Maritime Communications

Frequency translation and applications of same are described herein, including wireless local area network (WLAN) applications such as IEEE Standard 802.11 WLANs. Such applications include, but are not limited to, frequency down-conversion, frequency up-conversion, enhanced signal reception, unified down-conversion and filtering, and combinations and applications of same.

Wireless local area network (WLAN) technology and applications including techniques of universal frequency translation

Methods for down converting a modulated carrier signal to a demodulated baseband signal are described herein The method requires that a first portion of energy is transferred from the modulated carrier signal, and stored at a first storage device when a first switch is on At least some of the energy stored in the first storage device is discharged when the first switch is off The method further comprises transferring a second portion of energy from the modulated carrier signal, storing at a second storage device the second portion of transferred energy when a second switch is on, and discharging at least some of the energy stored in the second storage device when the second switch is off A down-converted in-phase baseband signal portion is generated from the energy accumulated in the first storage device while both the charging and the discharging occurs, and a down-converted inverted in-phase baseband signal portion is generated from the energy accumulated in the second storage device while both the charging and the discharging occurs, and the two portions are combined with a first differential amplifier circuit to form a down-converted differential in-phase baseband signal

Method and system for down-converting an electromagnetic signal

In this paper, a large number of studies of the efiect of the foliage on single or lines of trees on modern wireless communication systems are reviewed. The paper is focused on the experimental works mainly done for commercial applications such as cellular communication and high speed point-to-point flxed link at the microwave and millimeter wave frequencies. For this review study, the development of the foliage loss prediction methods and the factors in∞uencing the tree-induced shadowing efiect are highlighted. In view of current research work in this area, some possible future works are proposed to improve the performance of modern wireless communication systems with the efiect of foliage.

/pdf/investigations-of-foliage-effect-on-modern-wireless-50qz3jrjra.pdf

Investigations of foliage effect on modern wireless communication systems: a review

This study is concerned with using modern spectral analysis techniques to derive estimates of multipath ray parameters (delays and amplitudes) from measurements of selective fading over a wide band at microwave frequencies. The applicability of the general category of autoregressive methods is addressed, and the specific method adopted, the Prony algorithm, is described. An illustrative example in which this is applied to a typical field measurement is followed by an investigation into the capabilities and limitations of the algorithm. A statistical analysis of the method has been carried out using a database of laboratory measurements. The Prony algorithm's ability to resolve delays to better than half the Fourier limit is shown, and its tendency to yield biased estimates is studied. >

An autoregressive approach to the identification of multipath ray parameters from field measurements

[1] The paper describes an extensive wideband channel sounding measurement campaign to investigate signal propagation through vegetation. The measurements have been conducted at three frequencies (1.3, 2 and 11.6 GHz) at sites with different measurement geometries and tree species. The data have been used to evaluate current narrowband empirical vegetation attenuation models and study the prevailing propagation mechanisms. Evaluation of the modified exponential decay (MED), maximum attenuation (MA) and nonzero gradient (NZG) models show that on a site by site basis, the NZG model gives the best prediction of excess attenuation due to vegetation. The MA model has been found to be the worst of the three models. The studies have shown that the measurement site used to obtain the NZG model parameter values given in International Telecommunication Union (ITU) [2001] is influenced by metal lampposts and passing traffic, and thus was based on corrupted data. The results show that the leaf state, measurement geometry and vegetation density are more important factors influencing signal attenuation than tree species or leaf shape. Generally, the 11.6 GHz signal was attenuated much more than the 1.3 and 2 GHz signals by vegetation in-leaf, but the differences in attenuation were not significant in the out-of-leaf state. A successful excess attenuation model due to vegetation must consider the measurement geometry and vegetation descriptive parameters as well as any contributions from ground reflection and/or diffraction over the top or round edges of the trees.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002RS002758

Radio wave propagation through vegetation: Factors influencing signal attenuation

The authors report a novel technique, the SVDP (singular value decomposition prony) algorithm, which has demonstrated its superiority in identifying the multipath rays in line-of-sight microwave links. The technique has been applied to field data and it has proved itself to be a very useful tool in analyzing the time-variant structure of the actual physical mechanism, i.e. the evolution of the delay and amplitude of the multipath rays. The authors present the results of analysis of field measurements made at 11 GHz over a wide bandwidth of 400 MHz on the French CNET at Lannion, using a 50 km link. The major applications of this technique are: the modelling of a multipath channel using the least possible number of rays with the objective of minimizing the RMS error; the analysis of the time-variant structure of a channel and hence the design of adaptive equalizers; and the generation of the statistics of the time-domain multipath parameters, particularly the delay and amplitude distributions. >

Analysis of the time variant structure of microwave line-of-sight multipath phenomena

The authors review the technique of phase noise stability measurement using complex demodulation and the approach to an unlocked spread spectrum communications probe. This system has recently been developed to characterize a transhorizon communications channel at 11.6 GHz under both tropospheric scatter and anomalous propagation conditions. The receiver has a bandwidth of 31.25 MHz but has no carrier recovery and therefore no absolute phase reference. However, provided that the phase jitter and the frequency drift are small enough during the channel measurement period, it is possible to obtain an accurate channel transmittance estimate. The system has the capability of implementing time-domain signal averaging "on the fly", thus permitting high-speed channel identification under low SNR regimes. As the averaging period is increased (to improve the SNR), low phase jitter in the oscillators becomes imperative. The SNR improvement achieved is thus controlled by the maximum averaging period, which is a direct reflection of the stability of the oscillators. >

Measurement of the phase noise characteristics of an unlocked communications channel identifier

A high speed, wideband channel probe is briefly described. The use of the probe on an experimental 158 km link across the English Channel is reported. Preliminary results depicting the time-variant and frequency-selective characteristics of the link are presented.

Wideband sounding of 11.6 GHz trans-horizon channel

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