Wireless communication signal propagation

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Ideally, wireless signals travel in a straight line from the transmitter to the intended receiver. This type of propagation is called Line Of Sight,Wireless signals are based on the principle of LOS, which uses very little energy and can receive very clear signals. However, because air is an unguided medium and the path between the transmitter and the receiver is not very clear, wireless signals do not usually travel in a straight line. When an obstacle blocks the signal's path, the signal may go around the object, be absorbed by the object, or undergo any of the following phenomena: reflection, diffraction, or scattering. The geometry of the object determines which of these three phenomena will occur. (1) Reflection, Diffraction, and Scattering Reflection in wireless signal transmission is no different than reflection from other electromagnetic waves, such as light or sound. The wave encounters an obstacle and reflects—or bounces back—toward its source. For objects larger than the average wavelength of the signal, the wireless signal will bounce back. For example, consider a microwave oven. Because the average wavelength of microwaves is less than 1 mm, once the microwaves are emitted, they will reflect off the interior walls of the microwave oven (which are usually at least 15 cm long). Exactly which objects will cause wireless signals to reflect depends on the wavelength of the signal. In wireless LANs, signals with wavelengths between 1 and 10 meters may be used, so these objects include walls, ceilings, and the ground. In "diffraction," a wireless signal breaks down into secondary waves when it encounters an obstacle. The secondary waves continue to propagate in the direction in which they broke down. If you could see diffracted radio signals, you would see that they bend around the obstacle. Objects with sharp edges -- including walls and corners of tables -- cause diffraction. "Scattering" is when a signal spreads out or reflects in many different directions. Scattering occurs when a wireless signal encounters an object that is smaller than the signal's wavelength. Scattering is also related to the roughness of the surface the wireless signal encounters. The rougher the surface, the more easily the signal will scatter when it encounters it. Outdoors, trees and road signs can cause mobile phone signals to scatter. In addition, environmental conditions (such as fog, rain, and snow) may also cause reflection, scattering, and diffraction. (2) Multipath Signals Due to the effects of reflection, diffraction, and scattering, wireless signals can reach their destination along many different paths. Such signals are called "multipath signals." The creation of multipath signals does not depend on how the signals are sent out. They may radiate with equal intensity in many directions from the source, or they may radiate primarily in one direction from the source. However, once the signals are sent out, they will travel along many paths due to reflection, diffraction, and scattering. Figure 6 shows the multipath signals caused by these three types of signals. The multipath nature of wireless signals is both an advantage and a disadvantage. On the one hand, because the signals reflect off obstacles, they are more likely to reach their destination. In an environment such as an office building, wireless services rely on signals reflecting off walls, ceilings, floors, and furniture to eventually reach their destination. The disadvantage of multipath signal transmission is that due to its different paths, the multipath signal propagates at different distances between the transmitter and the receiver. Therefore, multiple instances of the same signal will arrive at the receiver at different times, causing fading and delay. 5. Narrowband, Wideband and Spread Spectrum Signals Transmission technologies differ based on how much of the radio spectrum their signals use. An important distinction is whether wireless uses narrowband or wideband signal transmission. In "narrowband", the transmitter concentrates the signal energy on a single frequency or a very small frequency range. In contrast to narrowband, "wideband" refers to a signal transmission method that uses a relatively wide band of the radio spectrum. Using multiple frequencies to transmit a signal is called spread spectrum technology. In other words, the signal never stays on one frequency range continuously during transmission. One result of spreading the signal over a wider frequency band is that each of its frequencies requires less power than a narrowband signal. This distribution of signal strength makes spread spectrum signals less likely to interfere with narrowband signals transmitted on the same frequency band. [/Another consequence of distributing signals over multiple frequencies is improved security. Because the signals are distributed according to a sequence known only to authorized transmitters and receivers, it is more difficult for unauthorized receivers to capture and decode the signals. A specific implementation of spread spectrum is Frequency Hopping Spread Spectrum (FHSS). In FHSS transmission, the signal hops between several different frequencies in a frequency band in the same synchronization pattern known to the receiver and transmitter of the channel. Another type of spread spectrum signal is called Direct Sequence Spread Spectrum (DSSS). In DSSS, the bits of the signal are spread across the entire frequency band at the same time. Each bit is encoded so that the receiver can reconstruct the original signal when it receives the bits. 6. Fixed and Mobile Every wireless communication falls into one of two categories: fixed or mobile. In a "fixed" wireless system, the location of the transmitter and receiver does not change. The transmitting antenna directs its energy toward the receiver antenna, so more energy is available for the signal. For situations where long distances or difficult terrain must be covered, fixed wireless connections are more economical than laying cables. However, not all communications are suitable for fixed wireless. For example, mobile users cannot use services that require them to remain in one location to receive a signal. Instead, mobile phones, paging, wireless LANs, and many other services use "mobile" wireless systems. In a mobile wireless system, the receiver can be located anywhere within a specific range of the transmitter. This allows the receiver to move from one location to another while continuing to receive the signal.