Radio Path Blocked by a Tree
The impact of vegetation attenuation on radio links in millimeter waveband could be treated in two manners: estimation of mean attenuation or estimation of short-term attenuation. It is important to note that the short-term component in particular is gaining great importance for the prospective MM-PMP systems, which provide real-time high quality services to the end customer. These services in particular require a signal of high quality at the reception site and all – even short-term – impairments are strictly unwanted, since they can easily knock out the service. The received signal might exhibit very high variations, when the signal propagates through a tree blowing in the wind.
The complexity of a short-term attenuation phenomenon, which is caused by the tree blocking LOS between the transmitter and the receiver, makes it almost impossible to describe without experimental measurements. However, interpretation of such results is not always simple and requires the performance of several different types of measurements. These are the reasons why our measurement campaign was performed in two phases:
- Field-test measurements
- Laboratory experiments
Field test experimentation was performed in order to confirm the existence of signal fading as a function of the wind speed loading the tree blocking LOS between the transmitter and the receiver. Getting the signal fading characteristics under the real environmental conditions in the millimeter waveband is a main contribution of this experiment. Different types of vegetation (bush and trees of different type) and its effects on the signal fading characteristics were investigated. Laboratory experimentation was designed and realized in order to understand the behavior of signal fading characteristics in more detailed manner. In a laboratory, different types of trees can be measured under exactly the same environmental conditions. It obviously allows much more reliable comparison of signal fading characteristics between obstacles and a better understanding of signal fading changes under different wind speed conditions.
New Short-Term Vegetation Attenuation Model
From the electromagnetic wave propagation point of view, all obstacles with a size comparable with the wavelength are important. In the case of a tree, leaves or needles still have a size comparable to the wavelength. The stronger the wind loading the tree the more the robust branches of the tree move. But obviously, even a very weak atmospheric flow causes movement of the tree. The non-stationary character of the tree or its parts causes fades to occur in the received signal. The basic idea of the model is that the obstacle (tree) is substituted by a set of elementary sources, representing the signal propagating through vegetation. In the model, leaves are represented by secondary elementary sources and holes are represented by dominant secondary elementary sources. The canopy region is represented by a complete 3-D lattice. The main target for a novel short-term attenuation model was to produce representative data for different wind speed conditions and tree types. This data should have similar statistical characteristics as the measured data has for a particular wind speed. A simple optimization process of the model was performed, in order to illustrate the capability of the new model to provide data with statistical characteristics (i.e. statistical distribution of short-term attenuation) similar to the measurement results. Results from the field-test experiment were used as reference data.
- Propagation of Electromagnetic Waves in Millimetre Waveband for Advanced Communication Systems, Czech Ministry of Education, Youth and Sports, grant no. OC 280.003, 2003-2005
- Performance and Reliability of Advanced Telecommunication Systems using Millimetre Wavelengths in the Czech Republic, Czech Science Foundation, grant no. GA102/01/0029, 2001-2003
- Propagation Impairment Mitigation for Millimetre Wave Radio Systems, Czech Ministry of Education, Youth and Sports, grant no. OC 280.001, 2001-2003
- Vegetation Influence on Signal Propagation in Millimeter Waveband, CTU Prague, CTU0206913, 2002
- Ledl, P. – Pechac, P. – Mazanek, M.: Time-Series Prediction of Attenuation Caused by Trees for Fixed Wireless Access Systems Operating in Millimeter Waveband. In Antennas & Propagation – Twelfth International Conference – ICAP 2003. IEE, 2003, p. 646-649. ISBN 0 85296 752 7.
- Ledl, P. – Pechac, P. – Mazanek, M.: Tree Attenuation Measurement Experiment in Millimeter Waveband. In COMITE 2003. 2003, p. 25-28. ISBN 80-86582-09-4.
- Ledl, P. – Pechac, P. – Mazanek, M.: Vegetation Effects Modeling for Terrestrial Broadband Radio Access Systems Operating in Millimeter Waveband. In ISAP i-02. Tokyo, 2002, p. 492-495.