2.3 RF and Microwave Communication in the Troposphere
2.3.1 Limitations of RF and Microwave Communication in the Troposphere
The massive deployment of RF and microwave links for wireless communication around the world testifies to its popularity and preference as communication system designers’ choice.
Notwithstanding its popular use in electronic communication, it has quite a number of disadvantages: the first disadvantage is largely a drawback from the system (medium) itself.
Generally, all communication systems experience a type of system loss in the form of free space attenuation. Free space attenuation is the loss that accompanies the use of the free space or atmosphere (troposphere included) for radio and microwave propagation. It is often calculated as a loss between the transmitter and receiver and it is extensively included in system design.
The power received by a receiver from a transmitter is taken as [Seybold, 2005]:
( )
where PRX is the received power level in dB, PTX is the transmitted power level in dB, GRX and GTX
both represent the antenna gains at the receiver and transmitter respectively.
This expression is usually summarized in a logarithmic form by assuming that = 1 and deriving free space loss as the ratio of and . Thus,
Free Space Loss FSL (
) (
)
By summary, the above equation can be written in a much more canonical form as recommended by ITU-R known as the Friis transmission equation:
where is the distance in kilometers between the transmitter and receiver, is the frequency of transmission in MHz.
This expression indicates that signal attenuation and its consequent losses are both frequency dependent and distance dependent. From (2.6) and (2.7), it is noticed that radio transmission at
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higher frequencies such as those at microwave and millimeter bands are much more susceptible to free space loss at the receiving end.
The second disadvantage is based on signal losses, efficiency and availability, which are caused by the presence of hydrometeors and concentration of gaseous particles. Signal transmission in the troposphere at microwave frequencies above 10 GHz, hydrometeors and gaseous concentrations affect the link destructively resulting in attenuation, scattering, absorption and signal impairment as indicated by Banjo et al. [1986].
The most common of such destructive features are scintillation, which is notorious in satellite broadcast, while multipath fading and rain fading are common in terrestrial microwave communication [Banjo et al., 1986; Crane, 2003]. They are both due to the influence of hydrometeors. The mechanism behind this can be plainly observed when a link is set-up and closely monitored during transmission in the presence of the atmospheric conditions. Hydrometeors are believed to be responsible for most of the signal impairments especially in tropical, sub-tropical and equatorial climatic regions [Ajayi et al. 1996].
Hydrometeors can be classified into three major groups: clouds, solid precipitates and rain [Crane, 1996]. Clouds (fog and dew also included) may affect high frequency radio link resulting to an increase in the intensity of scintillation particularly in paths with cumulus clouds [Savvaris et al., 2004]. This is possible in links at high elevation in the atmosphere, which is generally prone to scattering effects from the water droplets in fogs and clouds. Solid precipitates on the other hand cause reflection and scattering of transmitted waves, which is enough to cause significant attenuation in the level of the propagated signal [Crane, 1996]. These precipitates include snow, hail, sleet and every frozen element of precipitation. They are products of near-zero and sub-zero temperature in the ambient atmospheric conditions, which are often features of geographic areas such as temperate regions, arid and tropical areas.
The most important of all these hydrometeors is rainfall. Rainfall is prevalent in almost every geographical location although the distribution often varies with latitude. Rainfall structure is characterized by its drop size, rate, volume density, shape, elevation, temperature, terminal speed and distribution all of which play a significant role in the determination and prediction of signal attenuation and degradation [Salema, 2003; Seybold, 2005]. The effects of rainfall are a potential source of problem in the design of radio and microwave links because of its degrading effect on
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communication. At frequencies above 6 GHz, rainfall could be destructive to signals and very destructive above 20 GHz based on the link distance and geographical location [Crane, 2003;
Seybold, 2005]. The path loss experienced from rainfall is known as rain loss. Rain losses often result in signal deterioration in the form of fading, to a more extreme form of path loss, known as rain fade [Seybold, 2005] which can result in link outage during signal transmission.
Dust particles in the atmosphere are known for disrupting the polarization mechanism in transmitted signals i.e. it causes depolarization and cross-polarization [Srivastava and Vishwakarma, 2003]. This also depends on frequency of transmission and visibility of the environment. The millimeter wave spectrum is adversely affected by dust particles, a feature of terrestrial communication in desert regions.
Gaseous components of the atmosphere that induce attenuation include oxygen, water vapour and carbon-dioxide. Their presence in the atmosphere result in a form of path loss called atmospheric loss; it is usually of more importance to signal transmission between the frequencies at 2 GHz and 100 GHz [Crane, 2003]. The attenuation ( due to gaseous concentration in the atmosphere is proportional to the specific attenuation. On a terrestrial path, this is calculated from ITU-R P.530- 13 [2009]:
where is the specific attenuation of the atmosphere in dB/km, d is the LoS distance in km, while,
and are the specific attenuations of oxygen and water vapour respectively.
Figure 2-2 shows the contribution of different atmospheric influences at different frequencies while Table 2-1 shows a summary of the types of path losses that accompanies propagation in wireless communication.
In summary, signal propagation in wireless communication (RF and microwave) in the troposphere is both frequency dependent and distance dependent. In addition, signal susceptibility at high frequencies is mainly associated with hydrometeors and atmospheric gases.
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Figure 2-2: Specific attenuation from the effects of oxygen and water vapour at microwave and millimeter wave frequencies [Crane, 2003]
Table 2-1: Path Losses in Wireless Communication systems [Seybold, 2005]
TYPE OF LOSSES LIKELY CAUSES
FREE SPACE LOSS Spatial loss due to transmission media
TRANSMITTER
POINTING ERROR Non-precise orientation of transmitter and receiver for line-of-sight transmission
RAIN LOSS High and low intensity rainfall rates during transmission
MULTIPATH LOSS Multiple reflections from hard surfaces during propagation
ATMOSPHERIC LOSS Presence of gaseous constituents in the atmosphere DIFFRACTION LOSS Variation in the refractivity gradient, ⁄ ,
between the transmitter and Receiver
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