Microwave and Satellite Transmission in the Field of Wireless Communication

Tools to measure the environment have been with us for centuries; and throughout history, people have sought to understand their world, to see and hear beyond the range of their eyes and ears, and to touch the universe beyond their grasp. This leads people to search and discover new technologies. The rapid evolution of technology is leading to the creation of a seamless web of surveillance over much of the world. But only recently have technologies been developed to study the environment on a global scale. Traditional sensors such as thermometers, anemometers, and rain gauges do not provide a synoptic view of geophysical processes over large areas of the earth. Today, such views are obtained through electronic sensors mounted on aircraft, spacecraft, and satellites. These devices remotely sense geophysical processes by monitoring the reflection or emission of electromagnetic energy from natural media.

Wireless communication is not new. The existence of electromagnetic waves that travel at the speed of light was predicted by James Clerk Maxwell in 1864, and Heinrich Hertz proved Maxwell's theory with his experiments in the mid-1880s. The unit for measuring frequency hertz (Hz) is named after Heinrich Hertz. This discovery was the giant step in the communication world. In 1875, Alexander Graham Bell invented the telephone. Gugliclmo Marconi became the first person to send radio signals through the air in 1895 with a telegraphic device that coded information into a series of marks and spaces. The first applications of the wireless were for ship-to-ship and ship-to-shore communications. The passengers of the S.S. Republic were the first to be saved by a rescue using radio in 1909. Woodrow Wilson became the first U.S. President to make a radio broadcast. From 1920s to 1950s radio broadcasts, the one of the wireless communication means, were the dominant means of disseminating news and entertainment. In 1941, the 4-F, the world's first mobile telephone, is invented for World War II and in 1942 Gross developed the "Joan/Eleanor," a small wireless unit which was used during World War II for communications between ground and airborne troops. In 1947/1948 Gross developed the first personal radio. 1950 The pager is invented in 1950.

With the exponential growth in the use of wireless communications, the number of wireless services and subscribers has greatly expanded. This made our world smaller. Systems for mobile analog and digital cellular telephony have become commonplace. Next generation systems will provide enhanced communication services, such as data, electronic mail, and high-resolution digital video or even full multimedia communication. Every wireless system has to combat transmission and propagation effects that are nonexistent on a wired system. The role of radio has changed. Instead of being interested in covering large distances by radio, we emphasize the flexibility and comfort of (short-range) wireless access to the telecommunications infrastructure. The Internet was started by the United States Department of Defense (DOD) and was for defense. Now it became one of the most powerful tools in the field of education, business, and so many other fields. We cannot even think the communication without the Internet. The use of e-mail was unanticipated even by its designers. The Global Positioning System (GPS), too, was developed by the United States Department of Defense, for its tremendous application as a military locating utility by investing billions and billion of dollars. However, over the past several years, GPS has proven to be a useful tool in non-military mapping applications as well.

These new wireless technologies—satellite, radio, and supper wireless—offer more options. All these digital technologies provide comparable bandwidth, are competitively priced, and install faster than many existing wired solutions.

The success of modern remote sensing systems has encouraged the creation of more sophisticated sensors, which will soon gather an even wider range of information about the geophysics of our planet.

Our atmospheric remote sensing effort has grown rapidly in recent years as a result of our ability to create a new generation of millimeter-wave radar. Shorter wavelength laser radar (lidars), on the other hand, is often inadequate because electromagnetic energy at infrared and optical wavelengths cannot penetrate significantly into clouds. So we need millimeter-wave radiometers that can also measure atmospheric radiation. This could be the precursor of future satellite-mounted system making cloud measurements from space. Dual-channel radiometers operating near 20 and 30 GHz can measure total atmospheric water vapor and liquid from both ground-based and satellite-based sensors.

Microwave radar and radiometers are both used to remotely sense surface winds over the ocean. Measurements of these winds from a high altitudes is possible because surface roughness and foaming cause changes in microwave "signature" as a function of wind speed.

Apart from being used in other fields, microwave-wave frequencies are used in microwave and satellite transmissions. These transmissions consist of even very high frequency radio signals (3 GHz to 30 GHz) that are sent through air and space. Satellites used for spying can be even anywhere between 200 MHz and 325 GHz, mostly spread spectrum.

The radio frequency spectrum starts at around 10 kHz and ranges up to 300 GHz. In mobile radio communications we use the ranges of frequencies in the medium frequency (MF), high frequency (HF), and very high frequency (VHF) range.

Unlike broadcast radio signals, which are omnidirectional, microwave transmission is focused and unidirectional. Microwave transmission was developed from the same principles that guided the development of radio systems. Although these transmission media do not need cable, the transmission devices needed to utilize the medium are quite expensive.

There are two types of microwave transmissions:

1.     Terrestrial transmissions and

2.     Satellite transmissions.

Terrestrial microwave

The communication takes place from the ground to the ground through the electromagnetic waves within the earth atmosphere.

Operational Requirements:

Microwave generally has three requirements:

1.   A line of sight path

2.        High transmission towers

3.        Antennae

The line of sight is an undisturbed straight line between the transmitter and receiver. It compensates for the Earth's curvature. The distance between the two stations (towers) may be up to 30 miles (50 Km). To achieve longer transmission distances, microwave stations are placed in series. They should not be obstructed because microwaves must travel in unobstructed straight lines. Signals are received by one station, amplified, and then passed to the next relay station along the route. Most of the microwave equipment operates at the frequency of 18 GHz to 23 GHz.

The first microwave transmission occurred in 1933 across the English Channel, a distance of 12 miles (20 Km.). In 1947, the first commercial microwave network in the US was established between New York and Boston carrying television signals and multiplexed voice conversations. The use of microwave is expanding supporting more applications, including LAN bridging. It is one of the most agile and adaptable media available to handle data, voice and video for PCS, disaster recovery, local access bypass, and cellular telephone.

Satellite Transmission

A communications satellite is basically a microwave station placed in outer space. Satellites do not "bounce” the microwave signal. The advantage of satellite communication is that it can transmit data quickly over a long distance. Satellite communication services include up-link and down-link. Earth stations are always used with a satellite to send or receive data; the satellite serves as a relay station that receives signals from one station so called up-link and broadcasts it to the next station on earth called down-link at a different frequency. These two different frequencies are used so the data being transmitted from the earth station to the satellite does not interfere with the data going from the satellite to the earth station. Problems such as earth’s curvature, mountains, and other structures that block the line of sight of microwave transmissions make satellites an attractive alternative. The part of the satellite that actually transmits the signal is called transponder. When a transmission from earth is received by the transponder, the signal is amplified to improve its quality, and the frequency is changed. The data is then transmitted to earth by the transponder in a different frequency. The time taken by the satellite in converting the received signal to a sending signal and the time taken to travel from earth to a satellite and back to the earth is called propagation delay. Following data shows the propagation delay.

Terminal to satellite 22,300 miles

Satellite to computer 22,300 miles

Computer to satellite 22,300 miles