Frequency Bands between Licensed and Unlicensed

48 from various directions. This kind of method is known as ‘space diversity’. Using multiple antennas can solve the problem since different components of a signal are subjected to different phase shift, time delay and attenuation. An antenna may encounter severe fading and be unable to gather a signal efficiently but the usage of many antennas will boost the chance of gathering a clearer version of the said signal. Outdoor transmission faces more problems since the signals have to clear large physical ob- jects for example trees and buildings. As for the visual line-of- sight, even in situations whereby a person can look from one position of an antenna to another, it does not necessarily mean a radio line-of-sight also occurs particularly in scenarios of long distance communication. Radio waves actually require some space to reach the receiver and the wave will not be able to “squeeze” pass a tiny hole drilled in the wall. It actually needs a clearance of the Fresnel zone which is a long ellipsoid stretched between two antennas. The first Fresnel zone refers to the sphe- roid space enclosed within the orbit of the path when the dis- crepancies between the straight line directly drawn between the two antennas and the indirect path that crosses a single point at the edge of the Fresnel zone, with half the wavelength. The area is a spheroid space required for the wave to be transmitted to- wards the receiving antenna centered along the direct straight line path between the antennas. For instance if the signal fre- quency is 30 GHz then by applying the formula Equation 2.6: If the speed of radio wave transmitted through free space is roughly 3 × ͳͲ ଼ ms, the wavelength λ would be 3 × ͳͲ ଼ 30 × ݔͳͲ ଽ = 0.01 m or 1cm. Therefore, half wavelength is 5 mm. The wave will reach the receiver by a direct straight line path and within a spheroid area of 5 mm. In order to achieve propa- gation in free space at least 60 of the first Fresnel zone should 2.6 49 be clear of any physical obstacle. Besides that, in order to esti- mate the path loss or attenuation, the terrain pro file around the spheroid area has to be taken into account and this can be car- ried out using well-established models such as the Longley-Rice Model Hufford, 1999, phillips et al 2011 whereby the medi- an of the transmission loss can be projected using the geometry of the terrain pro file and the refractivity of the troposphere. An Urban Factor UF can be used to calculate any additional at- tenuation caused by urban clutter encircling the receiving an- tenna. This model can be efficiently sued as an irregular terrain model ITS but the effects of buildings and foliage are not tak- en into consideration. In order to optimize the propagating path for long distance communication which exceeds 5 to 8 kilome- ters, the curvature of the earth has to be taken into account. The transmission loss varies according to how much power finally arrives at the receiving antenna. Attenuation has to be given due consideration as the signal will finally become too weak to be picked up by the receiver. At the same time, the reliability and range of wireless telecommunication systems are effected by weather conditions for example snow, rain or fog. In tropical areas consistent heavy rain exceed 100mmhr and lasts for hours. Hence, the impact of rain on attenuation has to be con- sidered as the dBkm. Measurement of attenuation shows the power loss in dB per kilometer of distance travelled. There are several factors that has an actual impact especially the rainfall and the frequency of the carrier. Generally, the higher the fre- quency and or the heavier the rain is; more power will be lost per kilometer. Nevertheless, attenuation induced by rain is not a main issue if the rainfall is below 20mmhr if the r system is operating under 10 GHz. Figure 2.8 shows the comparison be- tween the attenuation for 10 GHz and 50 GHz. It is important to take note that the signals of horizontal polarization will experi- ence a higher degree of attenuation than vertical polarization under a similar situation. Figure 2.9 also shows that the differ- ence between two polarizations also increases as the frequency andor rainfall increases. A heavy rainfall on radio propagation