Examining Microwave Line Of Sight Technology Engineering Essay
Microwave Line-of-Sight (LOS) radio links are point-to-point links with microwave carriers in the frequency range 1-40 GHz. Microwave frequencies are used in high data rate telecommunications over long distances  .
In line-of-sight radio links, the space is the transmission medium and the signals are propagated in a line-of-sight. The length of the link may lie in the range from 10 to 100 Km, and repeaters are required each 40 to 50 Km to preserve the signal power level. Also microwave repeaters may be used when it is impossible to establish a line-of-sight connection between two nodes due to obstacles obstruct the line-of-sight path    .
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 . Fig. (1) presents a terrestrial line-of-sight radio link model.
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Fig (1): A typical terrestrial Line-of-sight link model
The transmitted signal may be analog or digital. An analog microwave system uses analog techniques for its modulation, while in digital microwave system an analog carrier is used with one of the digital modulation techniques. Also spread-spectrum and time-sharing techniques can be used in microwave line-of-sight links  .
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 illustrated in Fig. (2), 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.
Fig (2): Fresnel zone
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  .
Due to all these factors, the transmitted signal may travel through different paths. As shown in Fig. (3), 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 .
The vector addition or subtraction of those components will cause a variation in the received signal level. This phenomenon is known as fading  .
Fig (3): Multipath propagation in LOS transmission systems
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 line-of-sight radio links, such as increasing the fade margin and changing antennas polarization.
From the coherence bandwidth Bc we can classify the fading in to two main categories flat fading, which is also called frequency independent or non-selective fading, and frequency selective fading . The coherent bandwidth Bc is the bandwidth over which the signal varies by about 10% .
The non-selective fading can be recognized from the frequency selective fading by comparing the signal bandwidth B and the coherence bandwidth Bc. If the coherence bandwidth Bc was greater than the signal bandwidth B, then the channel has suffered from frequency-non selective or flat fading. While if the coherence bandwidth Bc was smaller than the signal bandwidth B, then the signal has suffered from frequency selective fading .
In the next sections we discuss the two fading categories and their impact on terrestrial microwave line-of-sight radio links.
Flat fading occurs in different forms; such as ducting and rain attenuation .
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  .
As presented in Fig. (4), ducting causes microwaves to trap up and down in a duct, travelling far from the receiver.
Fig (4): Ducting Phenomenon
The effect of rain on microwave radio propagation could not be neglect able, especially at the higher frequencies . When microwaves signals propagated through a medium containing rain, attenuation results from scattering and absorption by rain drops .
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The amount of attenuation the microwaves may experience is a function of many variables such as the size and shape of drops, the rainfall intensity and the operating frequency. The amount of attenuation increases with frequency and rainfall intensity .
At 6 GHz, microwaves attenuate due to light rain by an amount of 0.01 dB/Km. This attenuation increases to 1 dB/Km under very heavy rain conditions. For hops of 40 Km length attenuation of 1 dB/Km will cause degradation in the transmission system quality .
At frequency bands above 10 GHz, heavy rain is the dominant factor that causes fading . For example, at 12 GHz the amount of attenuation may reach 10 dB/Km, where is under very severe rain fall, transmission breaks may occur for a period of time .
Generally, rain outage in microwave paths depends on the operating frequency, the polarization and the rain rate .
Another form of flat fading happen when the beam bends from its direct path, upward or downward .
The ratio between the radius of the earth and another value known as the effective earth radius is known as the K factor, which is equal to 4/3 under normal atmospheric conditions. The effective earth radius is the radius that allows the microwave beam to be drown as a line-of-sight in a given atmospheric conditions .
Bending happens due to changes in temperature, humidity and density in the higher layers of the atmosphere. When these conditions change, the beam may bend upward or downward, depending on the K factor.
Fig (5): a. downward bending b. upward bending
(a) When the K factor is greater than 4/3 a downward bending happens where the beam bent downward, Fig. (5-a), on the other hand when the K factor is less than 4/3 then an upward bending happens, where the beam bends upward, Fig. (5-b) .
FREQUENCT SELECTIVE FADING
Frequency selective fading may occur in two forms; atmospheric multipath fading and ground reflection multipath fading .
ATMOSPHERIC MULTIPATH FADING
As we mentioned before, ducting happens due to an existence of atmospheric layers of different densities. Ducting leads to make the microwave beam trapping in an atmospheric waveguide, called duct .
Fig (6): Atmospheric multipath propagation in LOS transmission
In special cases the microwave beam is not trapped, but deflected. It can be seen from Fig. (6), that the microwave energy may be received by the receiving antenna through paths other than the direct path. Almost the received waves reach out of phase, which 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 .
FADING MITIGATION TECHNIQUES
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.
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Reducing the frequency used to be below 10 GHz helps in eliminating the attenuation caused by rain.
Microwave links implement vertical polarization are 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 . In addition there are several techniques used to mitigate the fading effects on terrestrial microwave line-of-sight radio links; such as diversity, spread spectrum and forward error correction.
Generally, the theory of using diversity techniques is to transmit the signal power in two ways, which are different from each other  .
Here bellow we describe the two main widely used diversity methods used, which are:
Fig (7): (a): Frequency diversity technique (b): Space diversity technique
In frequency diversity, Fig.(7-a), the signal is transmitted through the same path over two different frequencies f1 and f2, where the difference between the two frequencies is equal to or greater than 100 MHz, in order to avoid interference. 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  .
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 .
When transmitting the signal from left to right, the signal is transmitted from single antenna over a frequency f1 and received by two separated antennas. In similar way, when transmitting the signal from right to left, the signal is transmitted over a different frequency f2 from single antenna to be received by two separated antennas  .
In each state a combiner is used to combine the signals of the receivers to give a single signal . Otherwise a selector may be used instead of the combiner in order to select the signal with the lowest bit error ratio (BER) . Fig. (7-b) presents the space diversity technique.
As we mentioned earlier, 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. Space diversity is also known as antenna diversity  .
SPREAD SPECTRUM MICROWAVE SYSTEMS
Spread spectrum is a digital coding technique, in which the data are protected by increasing the transmitted bandwidth and reducing the power density.
The two main types of spread spectrum systems are, Direct-Sequence Spread Spectrum (DSSS) and Frequency-Hopping Spread Spectrum (FHSS) .
DIRECT-SEQUENCE SPREAD SPECTRUM (DSSS)
In DSSS, a predefined sequence, called the spreading code is used to divide the signal into chunks of data. In the receiver side, these chunks are used to reconstruct the original signal, where the receiver must be employing a matching encoding technique .
FREQUENCY-HOPPING SPREAD SPECTRUM (FHSS)
In FHSS, spreading can be done by hopping the signal in a random manner to a different frequency several times per second . The transmitted signal is modulated with M carrier frequencies, where one carrier frequency is used at a time . In both spread spectrum techniques, a pseudorandom sequence is used. For DSSS the signal is mixed with the pseudorandom sequence, while for FHSS the pseudorandom sequence is used to change the signal frequency several times .
The notion of using spread spectrum techniques for mitigating frequency-selective and multipath fading is the fact that spreading the energy of the transmitted signal over a wide range of frequencies increases its capability to resist interference .
FORWARD ERROR CORRECTION
FEC can be defined as an error control method that adds a systematic redundancy at the transmitting side such that the errors caused by the medium during the signal propagation, can be corrected at the receiver, where a decoding algorithm is employed. FEC is used for mitigating the effects of fading in the digital microwave line-of-sight radio links . In FEC the information bits are encoded by an encoder at the transmitter side, modulated and transmitted over a transmission medium. At the receiver side a demodulation process is performed then a decoder is used to recover the original information .
ITU-R MODELS FOR TERESTTIAL MICROWAVE LINE-OF-SIGHT RADIO LINKS PLANNING
A standard model is derived by the radio communications sector of the international telecommunications union (ITU-R), to calculate the amount of fading a terrestrial microwave line-of-sight radio link may experience. The standard model used is ITU-R P.530, named (propagation data and prediction methods required for the design of terrestrial line-of-sight systems)  .
In this standard model methods have been developed that help in predicting the most important parameters affecting the planning of terrestrial microwave line-of-sight radio links. In 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 .
ITU-R P.530 model is a long and detailed standard, which provides some equations that enable the radio engineers to predict the percentage time the link will be unavailable for the worst month specified. The attenuation found is then converted to be per annual.
Another standard model, ITU-R P.838, 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  . In ITU-R P.838, predicting the attenuation due to rain depends on a rainfall rate exceeding 0.01% of the time . For a high availability microwave links, rain attenuation must be computed accurately using the ITU-R models.
In this paper 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. Also the ITU-R models that help in predicting the amount of attenuation that microwave links may suffer from, has been presented.
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