# Millimeter Wave Attenuation Due to Wind and Heavy Rain in a Tropical Region
## 1 Introduction  
Fixed wireless links using millimeter waves (mm-waves) and terahertz (THz) have been considered for use in both fifth-generation (5G) and sixth-generation (6G) wireless systems, for example, in the applications of point-to-point links in the backhaul and access networks [1; 2; 3; 4; 5]. The advantage of mm-waves and THz frequencies compared with microwaves is their wider bandwidths allowing for high-speed data transmissions. The frequencies in the 71-76 GHz and 81-86 GHz bands have been considered for commercial usage in the APT-AWG-REP-81 report [6]. Although, the path loss of mm-waves and the THz Fixed Wireless System (FWS) is larger due to their shorter wavelength and gaseous attenuations, the loss can be compensated by increasing the antenna gains, resulting in a narrow beam. Thus, not only the received signal strength of the FWS suffers from droplet attenuation, but the beam misalignment caused by a strong wind also causes fluctuation in the received signal strength in these high frequency bands. In a reverse manner, the variation of signal strengths in the FWS networks can be applied to sense weather conditions. The existing dense mm-wave links may be used to provide high resolution and real-time weather information that is unattainable using weather radars [7; 8; 9]. A comprehensive list of research works in rainfall measurements based on commercial microwave links was summarized in a recent review paper [10]. Wireless channel modeling may be achieved via machine learning techniques [11; 12]. In a line-of-sight case, however, the attenuation is conventionally determined from physical phenomena including wave scattering, atmospheric absorption, and from empirical data such as ITU-R models. The process to estimate the attenuation involves, firstly, the calculation of specific attenuation in dB/km using the ITU-R P.838 model [13]. Then the attenuation for a specific distance is found by multiplying the specific attenuation by the effective path length, which is the product of the path length and the distance factor according to the ITU-R P.530 recommendation [14]. Recently, various works have been conducted to verify the standard models of rain attenuation given via ITU-R in several microwave, mm-wave and THz bands. Experimental studies found that no single model is suitable for all climate regions [15; 16]. For example, the measurements in tropical Malaysia in 26 GHz and 32.6 GHz frequencies showed that the estimated attenuation using the ITU-R model underestimated the higher rain rate [17; 18; 19]. In the E band, there have been a few studies in tropical regions, such as long term measurements at 73 GHz and 83 GHz in Malaysia [20; 21] and 71 GHz and 81 GHz in eastern China [9]; however, only rain attenuation was considered in the previous studies. In other works, the effect of rain on short-range fixed links have been investigated in the 25.84 GHz (K band) and 77.52 GHz (E band) bands in the United Kingdom, which has a temperate climate [22; 23]. It was found that the updated ITU-R P.530-18 model provides the rain attenuation estimates that more closely match the experimental data when the path length is under 200 m than the previous version of the standard; however, the data were limited to rain rates under 30 mm/h. In the higher frequency bands, there have been experimental studies at 120 GHz [24] and 300 GHz [25; 26] frequencies, but all in temperate climates. Only recently, there has been a study to evaluate the FWS link budget from both rain and wind effects both in the mm-wave region (theoretically and experimentally, in Japan) and the THz region (theoretically) [27]. Their results formed the basis on which the model in the APT-AWG-REP-81 report was developed [6]. The data for the wind-induced attenuation are still limited, especially when the wind effect combines with the rain effect simultaneously. In this paper, we studied the concurrent rain and wind effects, which is the first study of this kind for E-band FWS links in a tropical location.