Broadband Wireless Access BWA

46 medicine application will be more accessible and reliable to be utilized by people with different needs.

2.3 Operating Outdoor

The impact of electrical noise emitted by the gadgets nearby may be very prominent as the signal strength will weaken in relation to distance travelled attenuation, Transmitted signals can be lost or corrupted if the noise is very severe resulting in the data to be useless. At the same time, beside the noise and attenuation, distortion of the signal can be a problem as data travels through metal conductors. Distortion happens in many ways and is subjected to the kind of obstacles that lie along the signal path. Normally, the shape of the signal is distorted for instance when a square wave can no longer maintain its smooth pulse. Even though, the problem of signal propagation does happen indoors, there are more factors which are beyond con- trol in the outdoor environment which makes more signals de- grade severely. The yardstick that measures the signal loss in a transmission link is the loss which is predicted to happen in a free space meaning the loss which happens along a path which is free of everything that might reflect or absorb signal energy. If a radio wave which is being transmitted hit a physical obstacle it will be subjected to the phenomena as in Figure 2.7: - i. Diffraction: a signal will split into secondary waves. Diffraction will happen if a propagating signals strikes a surface which sharp edges. The waves emitted by the surface will be present in space and some fraction of the waves may penetrate behind the obstacle and create a power loss. A phenomenon of waves bending around the obstacle will occur. 47 ii. Reflection: a signal will be re flected back to the source, which is the transmitting antenna, using the same prin- ciple as a mirror reflects light. Reflection is cause by a propagating wave which is much larger than the wave- length of the carrier that hits a physical object. iii. Scattering: a signal will re flect with different parts spreading in various directions if the signal becomes dif- fused upon encountering an obstacle. Scattering is dif- ferent from diffraction. Scattering occurs when the propagating wave hits an object that is smaller compared to its wavelength for example air pollutant particles, dust, rough surfaces and other irregularities in the chan- nel. As the signal scatters in multiple directions, it will provide extra energy as recognized by the receiver. Therefore the signal received will be more substantial than those affected by diffraction and re flection. Figure 2.7 Factors in the spread of the wireless signal degrades different results FONG et al 2011 These will cause a lost in signal strength which is termed as fad- ing. This impact can be overcome by utilizing numerous anten- nas to pick up different parts of the same signal which arrives 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