Yu Song Meng

Empirical Near Ground Path Loss Modeling in a Forest at VHF and UHF Bands

Near ground radio wave propagation is examined in a tropical plantation experimentally at VHF and UHF bands. The propagation loss with forest depth is empirically analyzed using an integrated model combining both the foliage induced effect and the ground effect. Several well-known empirical foliage models are compared and analyzed. It is observed that the fitted ITU-R model taking into account the ground reflection, can provide a close approximation to the path loss in a tropical palm plantation. However, the accuracy of this model becomes poor when lateral wave dominates in the VHF band. A modification to the ITU-R model is then proposed taking into consideration the lateral wave effect. The modified model is verified using measured and published data, and found to have higher accuracy for large foliage depth in the VHF band as compared to the existing empirical foliage models.

INVESTIGATIONS OF FOLIAGE EFFECT ON MODERN WIRELESS COMMUNICATION SYSTEMS: A REVIEW

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.

STUDY OF PROPAGATION LOSS PREDICTION IN FOREST ENVIRONMENT

A comprehensive review of radio wave attenuation in forest environments is presented in this paper. The classic analytical methods of propagation loss modeling and prediction are described flrst. This provides information on the physical processes that the radio waves undergo while propagating through a forest. The focus of this paper is on the review and summary of the experimental work done in this area and the development of empirical propagation loss prediction models. The propagation loss variation due to external factors such as antenna height-gain, depolarization, humidity efiect etc. are examined and discussed individually. In view of current research work done in this area, some possible future work is proposed to improve the performance of radio links in forest environment.

Measurements and Characterizations of Air-to-Ground Channel Over Sea Surface at C-Band With Low Airborne Altitudes

This paper presents an experimental study of air-to-ground channels over sea surface at the C-band (5.7 GHz) with low airborne altitudes (0.37-1.83 km) through wideband channel measurements. In this paper, the multipath statistics and the propagation loss at different airborne altitudes are estimated and analyzed. It is observed that about 95% (86%) of the measured channel responses can be represented by the 3-ray (2-ray) multipath model. As the airborne altitude decreases, there is a higher probability for the appearance of multipath components. Moreover, it is found that the evaporation duct and elevated duct over the sea surface are the two important factors that can significantly affect the over-water air-to-ground communication link. These ducts can also decrease the rate of radio-wave attenuation, i.e., a decrease in path-loss exponent n in the log-distance path-loss models.

The Effects of Tropical Weather on Radio-Wave Propagation Over Foliage Channel

This paper investigates the dynamic property of a tropical forested channel due to the weather effect on very high frequency (VHF) and ultrahigh frequency (UHF) radio-wave propagation. In this paper, continuous-wave (CW) envelope fading waveforms are recorded over a period of 50 s with static antennas. This paper focuses on the analysis of the combined effect of wind and rain, which is often encountered in a tropical forest. The induced temporal effects are discussed and compared with theoretical models. It is found that the distribution of temporal fading components resembles a Rician distribution function. Its Rician K factor gradually decreases as the strength of either wind or rain increases due to the movement of the forest components.

PATH LOSS MODELING FOR NEAR-GROUND VHF RADIO-WAVE PROPAGATION THROUGH FORESTS WITH TREE-CANOPY REFLECTION EFFECT

This paper presents a new methodology to model the near- ground short-range propagation loss in forested areas at the VHF and the UHF band. The path loss modeling is performed by an integration of the foliage induced efiect and the efiect from the radio- wave re∞ection/re∞ections. The analysis shows that the re∞ection from the dense tree canopy is important for short-range propagation at the VHF band and therefore, cannot be ignored. When taking into consideration the contribution from the possible tree-canopy re∞ection, the modeled path loss is reduced by more than 15dB in the VHF band. A good agreement of the modeled path loss with the measured loss in tropical forested areas is achieved.

FURTHER STUDY OF RAINFALL EFFECT ON VHF FORESTED RADIO-WAVE PROPAGATION WITH FOUR- LAYERED MODEL

In this paper, rainfall efiect on the VHF radio-wave propagation in a tropical forest is further studied in details. Theoretical study and experimental investigations are performed with the help of a four-layered model for forested environment. It is found that the lateral wave traveling along the air-canopy interface, the direct waves, and the ground re∞ected waves are the main modes for VHF radio-wave propagation in forest. The rainfall can afiect these propagating waves to difierent extents. Especially, due to the increase in the dielectric permittivity of the wet canopy layer by rain water, the time of arrival of the direct wave traveling through the canopy layer can be delayed signiflcantly. Finally, the dielectric permittivity for the wet canopy layer under difierent rain events is evaluated empirically.

Investigation of Rainfall Effect on Forested Radio Wave Propagation

In this paper, the rainfall is found to have distinct effect on various parts of the propagating components for the VHF radio wave propagation in a tropical forest. By the studies of the measured power delay profiles under different weather conditions, it is observed that the lateral wave is the dominant mode of the VHF propagation and is not affected by the presence of rain as it is similar to free space propagation over the tree tops, while the significant multipath components which propagate through the forest medium is heavily influenced. This is again verified through the estimated delay spread statistics.

Generic Model and Case Studies of Microwave Power Sensor Calibration Using Direct Comparison Transfer

This paper proposes a generic model for analyzing the microwave power sensor calibration by direct comparison transfer. The model is derived using a signal flow graph together with nontouching loop rule analyses. The proposed model is investigated through several case studies which are commonly encountered and provides a theoretical basis for practical applications. Finally, realizations of the derived calibration equations for some practical cases are also discussed.

Comparison of radio-sounding profiles for cloud attenuation analysis in the tropical region

In this paper, comparisons of radio-sounding profiles with different resolutions are performed for prediction of cloud attenuation in the tropical region. The Salonen and Uppala model which was reported to have an excellent prediction capability for cloud attenuation is used. One-month data is processed for calculating the height of cloud base, the 0°C isothermal level, and the cloud attenuation. The results show that the free radiosonde data with low resolution is sufficient for the cloud attenuation prediction.