Foliage attenuation measurement at millimeter wave frequencies in tropical vegetation

One of the key challenges in realizing the vision of millimeter wave (mmWave) cellular networks is the severity of signal attenuation due to foliage. The foliage attenuation in tropical region differs from the ones in temperate region. Tropical vegetation has broad leaves whereas leaves of vegetation in temperate region are generally needle-like. It is predicted that the foliage attenuation could be higher in tropical region where the foliage is relatively comparable or bigger than the wavelength plus the humidity condition. Little knowledge is established on the foliage attenuation in tropical region. Previously, extensive measurements has been carried out mostly in the temperate region. Existing literature has focused primarily on the 28 and 38 GHz Local Multipoint Distribution Service (LMDS). Recently, field measurements on tropical foliage attenuation in the mmWave band have been carried out in the emerging of WiFi technology, wireless sensor network (WSN) and broadband fixed wireless access between operating frequency of 2.4 and 5.8 GHz. In this experiment, propagation through canopy was adopted and the tree was densely foliated. This paper presents a preliminary report of an on-going research work investigating the effect of a single tree on propagation signals at millimeter frequencies of 2 to 18 GHz and 26.5 to 40 GHz. The foliage attenuation is shown to be dependent of operating frequencies. The results of the experiment show that the presence of a single tree in a link can cause the propagation signal to suffer foliage attenuation loss. The fitted foliage attenuation curves reveal that the foliage attenuation decreases when the frequency increases. For instance, the foliage loss can be as high as 24 dB or 9.69 dB/m in excess of free space at 9 GHz. It is found that the Weissberger and ITU-R provide the optimistic estimate whereas COST 235 gives the most pessimistic estimate of the measurement data. It is also revealed that FITU-R represents the highest rate of attenuation increment as a function of frequency.