Foliage attenuation in urban tropical vegetation at millimeter wave frequency bands

Abstract

Millimeter wave (mmWave) bands offer greater bandwidth for the 5th Generation (5G) communication system in order to achieve higher data rates. Understanding the mmWave channel is a fundamental requirement to develop the future 5G systems. Therefore, extensive field measurements with respect to the behavior in realistic channels must be carried out to characterize these bands. To date, little knowledge is established on the foliage attenuation of mmWave bands in tropical environment. Existing measurements have been carried out mostly in the temperate region where the vegetation has different physical characteristics compared to those in tropical region. Thus, this research aims to measure and characterise the foliage attenuation in urban tropical environment. The site for real time data collection is located within Universiti Teknologi Malaysia Kuala Lumpur campus where vegetation geometries are observed as a single tree or a row of trees within small cell radius up to 200 m. Both the deployed direct Line-of-Sight (LOS) and Non-Line-of-Sight (NLOS) links operate at millimeter frequencies particularly at 6, 10, 18, 20, 28 and 38 GHz. The measurement system is arranged based on typical narrowband setup under full foliage environment. The received signal strength (RSS) is collected throughout the experiment in foliated environment and compared to the free space measurement. A signal generator is configured to transmit pure continuous wave through a steerable directional horn antenna. The RSS values are captured on a portable spectrum analyzer. In general, the measurement results show that the most significant foliage attenuation is caused by the NLOS link through the trunk followed by the branches and tree-top. Average foliage attenuation observed to be highest at 38 GHz between 18.1 dB to 30.6 dB and lowest at 6 GHz between 11.3 dB to 22.9 dB for NLOS slant paths obstructed by a single tree. Meanwhile a single tree obstruction at horizontal path induces foliage attenuation of 44.28 dB at 20 GHz by Eugenia tree, whereas the lowest attenuation of 22.35 dB at 6 GHz is attributed by weeping bottlebrush tree. On the other hand, the highest foliage attenuation induced by a line of trees occurs to be 49.86 dB at 28 GHz. Other important factors such as measurement geometry and vegetation density are observed. For instance, the foliage attenuation is higher at denser foliage and larger foliage depth. In general, the existing empirical models underestimate the tropical foliage measurements. The inaccuracies of these models could be due to the fact that the size, types and density of trees in tropical region is different from temperate region. It is found that the overall trend shows that foliage attenuation is more severe at higher mmWave frequencies at least by 21 dB as compared to the lower ones.