Microwave Technology?

 Microwave Technology?

Microwave is a line-of-sight wireless communication technology that uses high frequency beams of radio waves to provide high speed wireless connections that can send and receive voice, video, and data information.

Microwave links are are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it.

Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

The higher part of the radio electromagnetic spectrum with frequencies are above 30 GHz and below 100 GHz, are called “millimeter waves” because their wavelengths are conveniently measured in millimeters, and their wavelengths range from 10 mm down to 3.0 mm. Radio waves in this band are usually strongly attenuated by the Earthly atmosphere and particles contained in it, especially during wet weather. Also, in wide band of frequencies around 60 GHz, the radio waves are strongly attenuated by molecular oxygen in the atmosphere. The electronic technologies needed in the millimeter wave band are also much more complex and harder to manufacture than those of the microwave band, hence cost of Millimeter Wave Radios are generally higher.

History of Microwave Communication:

James Clerk Maxwell, using his famous “Maxwell’s equations,” predicted the existence of invisible electromagnetic waves, of which microwaves are a part, in 1865. In 1888, Heinrich Hertz became the first to demonstrate the existence of such waves by building an apparatus that produced and detected microwaves in the ultra high frequency region. Hertz recognized that the results of his experiment validated Maxwell’s prediction, but he did not see any practical applications for these invisible waves. Later work by others led to the invention of wireless communications, based on microwaves. Contributors to this work included Nikola Tesla, Guglielmo Marconi, Samuel Morse, Sir William Thomson (later Lord Kelvin), Oliver Heaviside, Lord Rayleigh, and Oliver Lodge.

Microwave Link over English Channel, 1931

In 1931 a US-French consortium demonstrated an experimental microwave relay link across the English Channel using 10 foot (3m) dishes, one of the earliest microwave communication systems. Telephony, telegraph and facsimile data was transmitted over the 1.7 GHz beams 40 miles between Dover, UK and Calais, France. However it could not compete with cheap undersea cable rates, and a planned commercial system was never built.
During the 1950s the AT&T Long Lines system of microwave relay links grew to carry the majority of US long distance telephone traffic, as well as intercontinental television network signals. The prototype was called TDX and was tested with a connection between New York City and Murray Hill, the location of Bell Laboratories in 1946. The TDX system was set up between New York and Boston in 1947.

Modern Commercial Microwave Links:

Microwave Communication Tower

A microwave link is a communications system that uses a beam of radio waves in the microwave frequency range to transmit video, audio, or data between two locations, which can be from just a few feet or meters to several miles or kilometers apart. Examples of Commercial Microwave links  Modern Microwave Links can carry up to 400Mbps in a 56MHz channel using 256QAM modulation and IP header compression techniques.  Operating Distances for microwave links are determined by antenna size (gain), frequency band, and link capacity.  The availability of clear Line of Sight is crucial for Microwave links for which the Earth’s curvature has to be allowed

Microwave Link 400Mbps

Microwave links are commonly used by television broadcasters to transmit programmes across a country, for instance, or from an outside broadcast back to a studio. Mobile units can be camera mounted, allowing cameras the freedom to move around without trailing cables. These are often seen on the touchlines of sports fields on Steadicam systems.

Planning of microwave links

Microwave links have to be planned considering the following parameters:

• Required distance (km/miles) and capacity (Mbps)
• Desired Availability target (%) for the link
• Availability of Clear Line of Sight (LOS) between end nodes
• Towers or masts if required to achieve clear LOS
• Allowed frequency bands specific to region/country
• Environmental constraints, including rain fade
• Cost of licenses for required frequency bands

Microwave Frequency Bands

Microwave signals are often divided into three categories:

ultra high frequency (UHF) (0.3-3 GHz);
super high frequency (SHF) (3-30 GHz); and
extremely high frequency (EHF) (30-300 GHz).
In addition, microwave frequency bands are designated by specific letters. The designations by the Radio Society of Great Britain are given below.
Microwave frequency bands
Designation Frequency range
L band 1 to 2 GHz
S band 2 to 4 GHz
C band 4 to 8 GHz
X band 8 to 12 GHz
Ku band 12 to 18 GHz
K band 18 to 26.5 GHz
Ka band 26.5 to 40 GHz
Q band 30 to 50 GHz
U band 40 to 60 GHz
V band 50 to 75 GHz
E band 60 to 90 GHz
W band 75 to 110 GHz
F band 90 to 140 GHz
D band 110 to 170 GHz

The term “P band” is sometimes used for ultra high frequencies below the L-band. For other definitions, see Letter Designations of Microwave Bands

Lower Microwave frequencies are used for longer links, and regions with higher rain fade.  Conversely, Higher frequencies are used for shorter links and regions with lower rain fade.

Diversity in Microwave Links

Example of a 1+0 Unprotected Microwave Link

In terrestrial microwave links, a diversity scheme refers to a method for improving the reliability of a message signal by using two or more communication channels with different characteristics. Diversity plays an important role in combatting fading and co-channel interference and avoiding error bursts. It is based on the fact that individual channels experience different levels of fading and interference. Multiple versions of the same signal may be transmitted and/or received and combined in the receiver. Alternatively, a redundant forward error correction code may be added and different parts of the message transmitted over different channels. Diversity techniques may exploit the multipath propagation, resulting in a diversity gain, often measured indecibels.

The following classes of diversity schemes are typical in Terrestrial Microwave Links:

• Unprotected:  Microwave links where there is no diversity or protection are classified as Unprotected and also as 1+0.  There is one set of equipment installed, and no diversity or backup
• Hot Standby: Two sets of microwave equipment (ODUs, or active radios) are installed generally connected to the same antenna, tuned to the same frequency channel.  One is “powered down” or in standby mode, generally with the receiver active but transmitter muted.  If the active unit fails, it is powered down and the standby unit is activated.  Hot Standby is abbreviated as HSB, and is often used in 1+1 configurations (one active, one standby).
• Frequency diversity: The signal is transmitted using several frequency channels or spread over a wide spectrum that is affected by frequency-selective fading. Microwave radio links often use several active radio channels plus one protection channel for automatic use by any faded channel. This is known as N+1 protection
• Space diversity: The signal is transmitted over several different propagation paths. In the case of wired transmission, this can be achieved by transmitting via multiple wires. In the case of wireless transmission, it can be achieved by antenna diversity using multiple transmitter antennas (transmit diversity) and/or multiple receiving antennas (reception diversity).
• Polarization diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization. A diversity combining technique is applied on the receiver side.