Principle of Microwave Radio Communications
A Tower of Wireless Antennas
Types of Microwave Systems
- Intrastate or feeder service microwave systems - generally categorized as short haul since they are used to carry information for relatively short distances, such as between cities within the same state.
- Long haul microwave systems - used to carry information for long distances.
Microwaves are electromagnetic waves with a frequency greater than 1 GHz (1,000,000 Hz). Microwave signals, due to their inherently high frequencies, have relatively short wavelengths, hence the name "micro" waves. The wavelengths of microwave frequencies fall between 1 cm and 60 cm; slightly longer than the infrared energy. Operating in the microwave region solves many problems of overcrowding in the radio spectrum. It also introduces additional benefits but also causes some unique problems. Working with equipment that operates in this region requires special knowledge and skills considerably different from those needed for conventional electronic equipment.
For a typical microwave radio link, information originates and terminates at the terminal stations, while repeaters simply relay the information to the next downlink microwave station. Stations must be placed in a way that the terrain such as mountains, buildings and lakes, do not interfere with the transmission of signals. Geographic location of stations must be carefully selected in such a way that natural and man-made barriers do not interfere with propagation between stations.
Microwave communications requires the line-of-sight or space wave propagation method. There are some instances where barriers are inevitable which cause obstructions between the transmitter and receiver. This kind of problem is best resolved by repeaters.
It is a device used to re-radiate the intercepted microwave energy without the use of additional electronic power. It also has the ability to redirect intercepted microwave radars to the other direction.
It is a receiver and a transmitter placed back to back or in tandem with microwave repeaters. There are two types of active repeater namely: baseband and heterodyne or IF.
In baseband repeaters, the received radio frequency (RF) carrier is down-converted to an intermediate frequency (IF), amplified, filtered, and then demodulated to baseband. In a heterodyne repeater, the received RF carrier is down-converted to an IF, amplified, reshaped, up-converted to RF, and then retransmitted. The baseband signal is unaltered by the repeater because the signal is never demodulated below IF.
The microwave systems use LOS transmission, thus a direct signal path must exist between the transmit and receive antennas. When the signal path undergoes a sever degradation, a service interruption will occur. The radio path losses vary with atmospheric conditions that can cause corresponding reductions in the received signal strength. This reduction in signal strength is temporary and is referred to as radio fade.
The purpose of using diversity is to increase the reliability of the system by increasing its ability. There is more than one transmission path or method of transmission available between a transmitter and a receiver in diversity. Depending on the type of combiner in use, the output signal-to-noise ratio is improved as compared to any single path.
Frequency diversity is simply modulating two different RF carrier frequencies with the same IF intelligence, then transmitting both RF signals to a given destination. It utilizes the phenomenon that the period of fading differs for carrier frequencies separated by 2-5%. This system employs two transmitters and two receivers. Frequency diversity arrangements provide simple equipment redundance. Its disadvantage is that it doubles the amount of necessary frequency spectrum and equipment.
In space diversity, the output of a transmitter is fed to two or more antennas that are physically separated by an appreciable number of wavelengths. At the receiving end, there may be more than one antenna providing the input signal to the receiver. It has been observed that multipath fading will not occur simultaneously at both antennas.
In polarization diversity, a single RF carrier is propagated with two different electromagnetic polarizations (either vertical or horizontal). Electromagnetic waves of different polarizations do not necessarily experience the same transmission impairments. This type of diversity is used in conjunction with space diversity. One transmit/receive antenna pair is vertically polarized, and the other is horizontally polarized. It is also possible to use frequency, polarization and space diversity simultaneously.
It is specialized form of diversity that consists of a standard frequency-diversity path where the two transmitter/receiver pairs at one end of the path are separated from each other and connected to different antennas that are vertically separated as in space diversity. This arrangement provides a space-diversity effect in both directions: in one direction because the receivers are vertically spaced and in the other direction because the transmitters are vertically spaced.
Advantages of Microwave Radio
- Distances between switching centers are less.
- Radio systems do not require a right-of way acquisition between stations.
- Due to their high operating frequencies, microwave systems can carry large quantities of information.
- It requires small antennas
- Minimum crosstalk exists between voice channels.
- Few repeaters are necessary for amplification
- Increased reliability and less maintenance are important factors.
Disadvantages of Microwave Radio
- Measuring techniques are difficult to perfect and implement at microwave frequencies.
- It is more difficult to analyze and design circuits at microwave frequencies.
- Transient time is more critical.
- It is necessary to use specialized components
- Microwave frequencies propagate in straight line, which further limits their use to LOS applications.
Is it worthy to operate system in microwave region, knowing that it requires complex and expensive equipment?
The microwave frequency spectrum is used for telephone communications. Many long-distance telephone systems use microwave relay links for carrying telephone calls. With multiplexing techniques, thousands of two-way communications are modulated on a single carrier and then relayed from one station to another over long distances.
Radar (Radio Detection and Ranging) also operates in the microwave region. It is a method of detecting the presence of a distant object and determining its distance and direction. Radar systems transmit a high-frequency signal which is then deflected from the distant object. The reflected signal is picked up by the radar unit and compared to the transmitted signal. The time difference between the two gives the distance to the object.
Television stations and networks use microwave relay links to transmit TV signals over long distances rather than rely on coax cables.
A growing application for microwave communications is space communications. Communications with satellites, deep-space probes, and other spacecraft is usually done by microwave transmission. This is due to the reason that microwave signals are not reflected or absorbed by the ionosphere as are many lower-frequency signals.
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