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A wave produced by the interaction of time-varying electric and magnetic fields. The propagation of electromagnetic wave is the fundamental limitation on performance of wireless communication. A signal propagating through the wireless channel is affected by phenomena such as scattering, reflection, refraction, and diffraction of the radiated energy. Due to the large number of different propagation mechanisms, a large number of different paths usually exist between the transmitter and receiver that the signal has propagated on. These phenomena give rise to fading and path-loss . The propagation models concentrates on predicting the average signal power strength at a particular distance from the transmitter. The aim of the propagation models is to present a combined method towards a set of tools that allow the system designer to evaluate the performance of the communication system characterized by variety type of fading channel models for different types of modulation or demodulation scheme . The connection of the line-of-sight radio links is either terrestrial or satellite . A sufficient clearance of obstructions in the terrestrial line-of-sight radio links is required; in order to ensure that the signal from the transmitting antenna is travelling in a straight line path to the receiving antenna . Terrestrial microwave line-of-sight radio links are used as a carrier in many applications such as :
Point-to-point systems for TV, telephone channels and data information.
Point-to-point link as a backbone of large networks for private or government uses.
Links between offices or buildings in urban areas.
Military applications such as fixed point-to-point and point-to-multipoint.
The allocated frequency band is divided into a number of separated channels, where each channel can be allocated to carry signals of specific application . Generally, In terrestrial microwave line-of-sight radio links, antennas are located on high towers, in order to avoid obstacles those lie in the path of radio wave propagation, such as hills and high buildings  . In, microwave line-of-sight radio links there is an area covers the signal path, known as first Fresnel zone. As shown in the Figure (1), this area is an ellipsoid boundary inside which, most of the signal power that reaches the receiving antenna . The first Fresnel zone should be kept clear in order to ensure signal propagation in a path clear from obstacles . The transmitting and receiving antennas must be adjusted in order to obtain a path clear from obstacles . The radius of Fresnel zone differs depending on the distance separating the transmitter from the receiver and the carrier frequency. Furthermore, propagating signals are affected by other factors, such as weather conditions (temperature, humidity, rainfall, fog, snow), natural terrain (lakes, mountains, seas) and earth's surface  .
Figure (1) Fresnel zone. d is the distance between the transmitter and the receiver, R is the radius of the Fresnel zone.
Due to all these factors, the transmitted signal may travel through different paths. As shown in Figure (2), the received signal is a mixture of several components, direct component that is travelled through the direct path and several non direct components those travelled through other non direct paths  . Depending on the path length through which the component was travelling, each component may differ from other components in the amount of delay and attenuation they suffered from .
Figure (2): Multipath propagation (Fresnel Zones for reflection) 
Where n =1, defining the first Fresnel ellipsoid and so forth. The Fresnel zone concept for the case of reflection is such that the path of reflection is between the source located at A, and an image of the antenna located at B . Fading has significant impacts on both analog and digital microwave systems. Many researches and recommendations have been done in order to overcome the impacts of fading on microwave radio links design, such as increasing the fade margin and changing antennas polarization. The mean path-loss depends on the transmission distance and the environment. The loss in free space is 20 dB/ decade, but in a more complex environment a loss of 40 dB/decade is common. Due to the multipath, a signal will arrive at the receiver by different paths. Since these paths have different lengths and angles of arrival, there will be a spread of the signal in time and direction. The spread in direction is referred to as angular spread, a fundamental means to describe the spatial properties of transmission channels. The power azimuth spectrum describes the spatial wave propagation properties found at a transmit or receive antenna .
Line-of-sight microwave radio transmission (LOS)
In this type of signal transmission the receiving and transmitting antennas are generally mounted on a far above the ground tower so that to keep away from the obstructions in the path of signal propagation. Creating reliable radio frequency line-of-sight system generally requires path clearances greater than those required in achieving visual (LOS). The amount of additional clearance depends on the particular frequency at which the system operates  . In LOS systems it is possible to increase the signal-to-noise ratio by increasing the directivity of the antennas, because the received signal increases with the gain of both the transmit and receive antennas . Usually, the LOS works between 109 to 1012 Hz. Meantime; it has a limitation in range after the distance beyond 50km. This can be considered a disadvantage. To solve this problem, it requires series of relay stations to repeat transmitting the original signal to reach its destination. The original signal will suffer from many factors that affect on the signal strength that may causes a weak distorted received signal, or in worst cast it may causes a complete outage. Hence, the relay can solve part of the problem .
One of the main sources of impairment in the wireless channel is the occurrence of fading. Fading can be classified as long term fading and short term fading. Long term fading is due to shadowing and the distance between the source and destination. It is also referred to as path loss. Long term fading can be controlled by techniques like power control. Short term fading is due to the multipath propagation of the transmitted signal due to reflections from various objects  . When the delay differences between the multipath components are small as compared to the symbol interval, these components can add constructively or destructively at the receiver depending upon the carrier frequency and delay differences. Multipath fading can be controlled by techniques like diversity and channel coding .
3.1 Fading classification
The variations can be roughly divided into two types: The first is Large-scale fading, due to path loss of signal as a function of distance and shadowing by large objects such as buildings and hills. This occurs as the mobile moves through a distance of the order of the cell size, and is typically frequency independent . And the second is Small-scale fading, due to the constructive and destructive interference of the multiple signal paths between the transmitter and receiver. This occurs at the spatial scale of the order of the carrier wavelength, and is frequency dependent there are two types of small-scale fading Depends on multipath time delay spread and based on Doppler spread. The multipath time delay spread which contains :
BW of signal < BW of channel
Delay spread < Symbol period
Frequency Selective (non-flat) Fading
BW of signal > BW of channel
Delay spread > symbol period
And the second type based on Doppler spread which contains:
Low Doppler spread
Coherence time > Symbol period
Channel variations slower than baseband signal variations
High Doppler spread
Coherence time < symbol period (time selective fading)
Channel variations faster than baseband signal variations
After the classification above, Multi path time delay class will be discussed as in the following:
Frequency non-selective (also called flat fading) Channel
If the bandwidth of the transmitted signal is small compared with, then all frequency components of the signal would roughly undergo the same degree of fading. We notice that because of the reciprocal relationship between Tm and the one between bandwidth and symbol duration, in a frequency non-selective channel, the symbol duration is large compared with Tm. In this case, delays between different paths are relatively small with respect to the symbol duration . Occurs in different forms; such as ducting which will be explained later and rain attenuation.
Frequency selective channel
On the other hand, if the bandwidth of the transmitted signal is large compared with, then different frequency components of the signal (that differ by more than would undergo different degrees of fading. Due to the reciprocal relationships, the symbol duration is small compared with Tm. Delays between different paths can be relatively large with respect to the symbol duration. . we conclude that the Frequency selective fading may occur in two forms; atmospheric multipath fading and ground reflection multipath fading :
ATMOSPHERIC MULTIPATH FADING
Due to the importance of ducting in multipath fading, it will be discussed in the following few lines. Ducting behavior differs because the atmosphere layers are different from each other. In special cases the microwave beam is not trapped, but deflected. It can be seen from produces fading. If the two waves received in complete anti phase, a drop in the received power long for few seconds may happen . Atmospheric multipath fading usually happens during night time in hot, humid and wind-free conditions . In general, frequency selective fading is fast fading, the average duration of a 40 dB fade is about 4 seconds and the average duration of a 20 dB fade is 40 seconds .
GROUND REFLECTION MULTIPATH FADING
Ground reflection due to trees, buildings or rocks those lie in the propagation path of the signal, can cause multipath fading . The signal reaches the receiver, reflected from different items and travelled through multiple paths. In practice it is found that there is a relation between the microwave link length and the number of the indirect paths through which the signal has travelled . As the microwave link length increases, the number of the non-direct paths through which the signal may be travelled increases. For example, microwave links longer than 40 Km are found to be more susceptible to multipath fading than shorter microwave links. In addition, the occurrence of multipath fading depends on the geographical characteristics of the region over which the microwave link operates  . When atmospheric multipath and ground reflection multipath fading occur at the same time, the fade depth may reach 40 dB .
Effects of fading
3.2.1 Envelope and phase fluctuations
When the signal suffer from fading during transmission both the phase and envelope vary over time, so that effect strongly degrade the performance of communication systems therefore different analysis are taken to compensate those effects at the receiver during the coherent demodulation, however for non-coherent modulation the phase variation is not needed at the receiver therefore the effect of fading to phase variation is not affect the system performance  .
The decision of communication system receiver is based on the observation of the received signal over two or more symbol time period, therefore when the symbol time duration is smaller than the channel coherent time tc. (tc is the period of time after which the correlation function of two samples of the channel response taken at the same frequency but different time instants drops below a certain predetermined threshold) that leads to burst error because of the variation of the fading channel from one symbol interval to the next symbol.
Average outage duration
The average outage duration(AOD) is also represent the average fade duration and this is an important performance criteria for proper selection of the parameters of the communication system such as transmission symbol rate, time slot duration and packet length depending on the amount of the fading in which the transmitted signal is experienced.
Free space loss
Although the atmosphere and terrain affect on path loss for low frequencies, this loss increases with increasing both the frequency and distance. This is known as free space loss and is defined as the loss created between two isotropic antennas in free space, where there are no ground influences or obstructions. Spreading energy primarily causes the loss as the wave front travels through the space .the distance between the transmitter and the receiver as shown in the following formula:
Free space loss (db) = 32.45 + 20 log F (MHz) + 20 Log d(Km)
Ducting phenomenon happens due to changes in the temperature of the lower layers of the troposphere. Usually, ducting happens over wide areas of water, where temperature and humidity inversions happen  . Ducting causes microwaves to trap up and down in a duct, travelling far from the receiver. As presented in Figure (3), ducting causes microwaves to trap up and down in a duct, travelling far from the receiver.
Figure (3) Ducting
Techniques for fading combat
For each fading mechanism numbers of specific steps must be taken in order to combat the effects of that fading mechanism.
Experiments showed that microwave links outage due to rain increases rapidly with frequency and path length. In practice it is found that :
Outage time due to rain can be reduced by using antennas of big size, reducing path lengths and increasing the fade margins .
Reducing the frequency used to be below 10 GHz helps in eliminating the attenuation caused by rain.
Microwave links implement vertical polarization is less susceptible to be attenuated by rain fall than microwave links those implement horizontal polarization.
Also in practice it is found that microwave hops those operate over wide areas of water or over desert regions, showed that reducing the link length to 35 Km or less helps in avoiding ground reflection multipath fading . during the wave travelling through a transmission channel it experience distortion because of fading therefore there are three techniques(diversity, channel coding and equalization)to mitigate fading and improve the received signal quality, however these techniques are varied in cost, complexity and effectiveness in practice.these techniques as follow:
The equalization technique is any signal operation processing that is used to minimize intersymbol interference created by multipath within time dispersive channel in frequency selective fading. The equalizer at the receiver side compensate for delay characteristics and average range of expected channel amplitude, if the modulation (B.w)is more than the coherent (B.w)of the radio channel then the modulation pulses spread in time and result intersymbol interference.
Channel coding technique
This technique a redundant data bits is added to the transmitted message, this process is achieved by channel coder before the modulation. The disadvantage of the channel coding is the expansion of the require bandwidth for the transmission because of the redundant added bits .
In telecommunications, Diversity is one of the techniques to combat channel fading. Diversity makes use of more than one independently faded version of the transmitted signal to improve the overall reception. This is improving the wireless link at relative low cost. This is because if several copies of the original signal are sent through different paths, they encounter different channel characteristic and therefore the probability that all the paths will experience deep fading at the same instant is greatly reduced. Here bellow we describe the three main widely used diversity methods used, which are  :
It is also known as antenna diversity, there is more than one receiving antenna at different positions (heights). The antenna output can be combined through an adjustable delay . In space diversity technique, the same information is sent through two different paths, thus two separate transmitting (or receiving) antennas located at different heights are used. Where a particular space between the two antennas is required. Two separate transmitting antennas may be used in this technique, where the transmitter energy will be divided between the two antennas. Space diversity technique requires either the transmitting or the receiving antenna to be vertically polarized, while the other is horizontality polarized  . The correlation coefficient is depending on three factors, the polarization angle, the cross polarization discrimination and the offset angle from the mean beam direction of the diversity antenna. The correlation coefficient becomes higher when the offset angle becomes larger, and the correlation coefficient decreases as the polarization angle increase .
Figure ( ) Concept of space diversity .
The frequency diversity is a common technique which is used in (LOS) links that transmits several channel in frequency division multiplex, the frequency diversity transmits information in different carrier frequency in order to achieve that frequencies separation by more than the channel coherent bandwidth in order not experience the same fading. The idea of using two different frequencies is the fact that, each frequency behavior differs from the other for a given conditions, for this reason one frequency may arrive without error, while the other may be corrupted  . Because of using two frequencies, this technique requires two transmitters and two receivers, therefore the system cost is high, moreover, the limited microwave spectrum and obtaining a license for restricting the use of two different frequencies will make it difficult to use this technique  .
6.3.3 Time diversity
A simple concept is to repeat every signal, or on request. Diversity based on time is usual with digital modulation systems. Time diversity technique involve transmits information at period of time spacing which is exceed the coherence time of the channel, so the receiver will receive multiple repetitions of the transmitted signal with independent fading conditions.
6.3.4 Polarization Diversity
It is one of the diversity a method is used in the mitigation requiring two separate antennas with different polarizations for reception and transmission. This means transmitting same frequency with different polarization. This is can be considered as another solution, because using two polarizations will reduce that attenuation  .
Number of paths for a packet to transit between two points Path diversity is a means to use multiple diverse or partially diverse paths to transport packets from a source to a destination. The reason for using path diversity is to provide the possibility for fault tolerance  .
6.3.6 Angle Diversity
This type provides different angles of feeder (two or more) perpendicular on the reflector at the receiver site. The multipath fading occurs due to different arrival angles at the receiver site. This way can discriminate between two received signals immediately to select the unfading signal .
It is standard models about propagation data and prediction methods require for the design of terrestrial line of sight system, these models is used to calculate the amount of fading that the signal experience through the propagation . There are two model As explained below:
ITU-R P.530 model
Fading mechanisms are divided in to clear air and precipitation. Clear air mechanism is caused from atmospheric layering. While precipitation mechanism is caused by rain .it provides the following information: attenuation due to atmospheric gases, fading due multipath arising from surface reflection, diffraction fading due to obstruction of the path by terrain obstacles, fading due to atmospheric multipath.
ITU-R P.838 model
Named (specific attenuation model for rain form use in prediction methods), used for predicting rain attenuation. It is proposed in this model to calculate the rain attenuation from the knowledge of the rain rate. A recommended procedure is given to make the calculations  .
The fading is a common phenomena and constraint on the performance of wireless communication system and the fading may vary from one time to another and depending on the environment conditions however statistical modeling and characterization of different effect result an accurate models and range of relatively simple for fading channels which is depend a specific prorogation environment. In this report terrestrial microwave line-of-sight radio links systems has been presented. Fading phenomenon, the different mechanisms of fading and their effects on fixed microwave line-of-sight radio links. It is require using one of the technique to combat the fading effect such as diversity or equalization technique. In order to design a reliable microwave radio link it is required to maintaining adequate clearance for at least 60% invisible Fresnel zone and take other considerations such as the power and the frequency of the transmitted signal . Also the ITU-R models that help in predicting the amount of attenuation that microwave links may suffer from, has been presented.