Abstract
Microwave links are terrestrial hops providing backhaul to all types of systems whether wired or wireless. They are in operation since 1950s but with the growth of digital wireless, all of them adopted digital modulation techniques with power control. The chapter starts out by asking basic questions on what these links are and how are they designed. Cursory calculations are made on link budget to verify the link budget indicated in Fig. 14.6 where path loss/gain is illustrated.
Answer to some questions result in more details about microwave link design and parameters used to provide a reliable link. Other than the RF signal, factors such as wind load on the antenna and the tower it stands on, are considered, and reviewed. Important considerations in microwave links include Fresnel Zone, Earth’s curvature, polarization, terrain, climate, and frequency band. Other considerations that can affect operation of microwave dished are wind load, snow, and ice. Some of them are addressed by active power control available in all digital microwave links.
There are reviews of regulations and guidelines that support and extend the use of smaller dishes and higher frequency bands. Historic change is introduction of millimeter band for point-to-point links. These bands directly support the Gigabit throughput rates expected by backhaul networks. All microwave links provide low latency (< 1 ms) unmatched by any wired network. Adaptive Modulation and Coding (ACM) is one that supports active use by regulators and operators since it allows a compromise solution. The compromise in terms of variable throughput rate due to weather conditions, allows smaller antennas (simpler towers, including option of mounting on roof tops), lower power limits over the air allows more links to co-exist. The chapter concludes highlighting growth of microwave links in Asia and shows why over half of world’s cellular systems use them.
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Abbreviations
- Millimeter wave:
-
Refer to RF signals in the 30–300 GHz range, in which an unlicensed band of 60 GHz and licensed band of 80 GHz are used for millimeter wave links
- Microwave link:
-
Refers to a pair of towers with parabolic or other antennas mounted. The antennas on both of them together act as a pair. One transmits and the other one receives and vice versa. The tower in an urban area or near the control center is designated primary, and the other tower could be in the wilderness.
- Fresnel zone:
-
Named after physicist Augustin-Jean Fresnel, is one of a series of confocal prolate ellipsoidal regions of space between and around a transmitter and a receiver. The primary wave will travel in a relative straight line from the transmitter to the receiver. Other deflections can occur from any object located within that zone.
- Frequency division multiplex (FDM):
-
A method of separating the transmit and receive frequency bands so that such simultaneous communication becomes possible. Transmit frequency F1 and receive frequency F2 are kept considerably apart to ensure that there will be no interference between them
- U-NII bands:
-
Unlicensed-National Information Infrastructure bands are defined in the USA by the Federal Communications Commission and the allocated bands are in the 5–6 GHz range
- Microwave licensed frequency bands:
-
The bands widely licensed by regulatory agencies globally are in the regions of 6 GHz, 11 GHz, 18 GHz, 23 GHz, and 80 GHz. Well-established microwave link products are provided in these bands, by most vendors
- TE mode—transverse electric mode:
-
This waveguide mode is dependent upon the transverse electric waves; sometimes also called H waves, characterized by the electric vector (E) being always perpendicular to the direction of propagation
- TM mode:
-
Transverse magnetic waves, also called E waves, are characterized by the magnetic vector (H vector) being always perpendicular to the direction of propagation
- TEM mode:
-
The transverse electro-magnetic wave cannot be propagated within a waveguide, but it can be propagated within a coaxial cable. It is the mode that is commonly used within coaxial and open wire feeders. The TEM wave is characterized by the fact that both the electric vector (E vector) and the magnetic vector (H vector) are perpendicular to the direction of propagation
- Cross-polarization or XPD:
-
Cross-polarization is the polarization orthogonal to the desired polarization. For example, if the fields from an antenna are meant to be horizontally polarized, the cross-polarization in this case is vertical polarization. If the polarization is right hand circularly polarized (RHCP), the cross-polarization is left hand circularly polarized (LHCP)
- Orthogonal mode transducer (OMT):
-
Orthogonal mode transducer (OMT) is a passive microwave component that separates a signal into two linear orthogonal polarized signals received from a common port or, vice versa, it combines two such signals from vertical and horizontal ports into a common port. The OMT supports circularly, elliptically, and linearly polarized waveforms
- Earth’s curvature and K-factor:
-
Earth’s bulge is a term used in telecommunications that refers to the circular segment of the Earth profile that blocks off long-distance communications. Since the geometric line of sight passes at varying heights over the Earth, the propagating radio wave encounters slightly different propagation conditions over the path. The usual effect of the declining pressure of the atmosphere with height is to bend radio waves down towards the surface of the Earth, effectively increasing the Earth’s radius, and the distance to the radio horizon, by a factor of around 4⁄3. This K-factor can change from its average value depending on the weather
- Fade margin:
-
It is an allowance provided during telecommunication link design that accounts for sufficient system gain or sensitivity to accommodate expected fading. The objective is to ensure that the required quality of service is maintained
- Scintillation effect:
-
Scintillations are rapid fluctuations in the phase and amplitude of an electro-magnetic wave. They occur due to local rapid variations in the refractive index of the medium through which the wave is traversing. Scintillation is mainly caused by the local variation of the ionospheric electron density
- Automatic transmit power control (ATPC):
-
It is a feature in microwave systems that automatically increases the transmit power during “Fade” conditions such as heavy rainfall. ATPC can be used with adaptive code modulation (ACM) to maximize link uptime, stability, and availability. When the “fade” conditions (like snow or rainfall) are over, the ATPC system reduces the transmit power again. This reduces the stress on the microwave power amplifiers, which in turn reduces power consumption, heat generation, and increases expected lifetime (MTBF or mean time between failure)
- QAM or quadrature amplitude modulation:
-
It refers to a family of digital modulation methods widely used in modern telecommunications to transmit information. QAM consists of two digital bit streams that change (or modulate) the amplitudes of two carrier waves using amplitude-shift keying (ASK). The two carrier waves of the same frequency are out of phase with each other by 90°, known as orthogonality or quadrature. The transmitted signal is created by adding the two carrier waves together. At the receiver, the two waves can be coherently separated (demodulated) using their orthogonality property
- Low latency:
-
The term applies to the time taken by the data stream to go from one point to another. Microwave links, in general, have the lowest latency (between 100 and 1 ms) that meets the 5G requirement for URLLC
- Antenna wind load:
-
Wind load applies to antennas of all shapes and sizes that are mounted outdoors. It can be reduced by reducing the “frontal area” using mesh design or similar design methods. The generic formula for calculating wind load is F = A x P x Cd, where F is the wind load, A is the surface area of the antenna (usually given in square feet), P is the wind pressure (calculated from another formula), and Cd is the drag coefficient
- Remote tower:
-
Among the two towers used for microwave links, usually one is in a populated where the traffic begins/ends. Other tower or towers can be in complete wilderness, either in agricultural land, forest, mountain top, etc. Such towers are described as the “remote tower” which means access and maintenance of such towers requires planning in terms of getting there, how it is operated, etc.
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Raghunandan, K. (2022). Microwave and Millimeter-Wave Links. In: Introduction to Wireless Communications and Networks. Textbooks in Telecommunication Engineering. Springer, Cham. https://doi.org/10.1007/978-3-030-92188-0_14
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