- Albert Einstein

Table of Contents:

• Introduction
• Basic Communication Theory
• Why do wireless systems need radiation?
• Why go wireless in the first place?
• EMF Basics
• Glossary of Basic Terms

Introduction:

Here we attempt to give a basic overview of the path or channel information travels in its journey from a source of information to a remote receiver of that information . Emphasis will be placed on the role of Radio frequency and Microwave radiation in this process.

(table of contents)

Basic Communication Theory

A communications system is responsible for the transmission of information from the sender to the recipient. At its simplest, the system contains:

• a transmission channel that is the physical link between the communicating parties.
• a modulator that takes the source signal and transforms it so that it is physically suitable for the transmission channel.
• a transmitter that actually introduces the modulated signal into the channel, usually amplifying the signal as it does so.
• a receiver that detects the transmitted signal on the channel and usually amplifies it (as it will have been attenuated by its journey through the channel).
• a demodulator that receives the original source signal from the received signal and passes it to the sink.

The transmission channel is the main issue concerning us here. Conventionally, it is the set of hard-wired cables that connect all the lines of the wireline phone companies. In Wireless systems, on the other hand, the cables are replaced by free space, but only at the cost of requiring the erection of antennas that allow the line of sight communication.

Why do wireless systems need radiation?

In 1875, when Alexander Graham Bell first invented telephony, he used a wire cable between the two nodes (i.e. two people with telephones). And so the standard conventional method of telephony has been the use of coaxial cables that connect any two nodes that need to communicate with each other.



One decade later, Heinrich Hertz produced the first man-made radio waves. Radio waves, like electricity and light, are forms of electromagnetic radiation-the energy is conveyed by 'waves' of magnetic and electrical fields. In a wire, these waves are induced and guided by an electrical current passing along and electrical conductor, but that is not the only way of propagating electromagnetic (EM) waves. By using a very strong electrical signal as a transmitting source, and electromagnetic wave can be made to spread far and wide through thin air. That is the principle of radio. The radio waves are produced by transmitters , which consist of a radio wave source connected to some form of antenna.

(table of contents)

(table of contents)

The recieving node can be a station with a large recieving antenna, or just a cellular phone or a page. All these recieving nodes perform different functions, but they all share the same essential parts as outlined in the block diagram of the recieving end. The difference comes mainly after the stage of the output signal, which can be transformed into sound (in cellulars), or data (PCS systems), or stays as an electric signal to be transmitted to a node further down the line (in repeater stations).

Why go wireless in the first place?

The maturity of radio frequency (RF) technology has permitted the use of electromagnetic radiation links as the major trunk channel for long distance communication. The use of microwave links has major advantages over cabling systems:

• Freedom from land acquisition rights. The acquisition of rights to lay cabling, repair cabling, and have permanent access to repeater stations is a major cost in the provision of cable communications. The use of radio links, that require only the acquisition of the transmitter/receiver station, removes this requirement. It also simplifies the maintenance and repair of the link.
• Ease of communication over difficult terrain. Some terrains make cable laying extremely difficult and expensive, even if the land acquisition cost is negligible.

The use of microwave links has a number of disadvantages, that mainly arise from the use of free-space communication:

• Bandwidth allocation is extremely limited. The competition for RF bandwidth from various competing users leads to very strict allocations of bandwidth. Unlike cabling systems, that can increase bandwidth by laying more cables, the radio frequency (RF) bandwidth allocation is finite and limitied. In practise, bandwidth allocations of 50MHz in the carrier range 300MHz to 1GHz are typical.
• Atmospheric effects. The use of free-space communication results in susceptibility to weather effects, particularly rain.
• Transmission path needs to be clear. Microwave communication requires line-of-sight, point-to-point communication. The frequency of repeater stations is determined by the terrain. Care must be taken in the system design to ensure freedom from obstacles. In addition, links must be kept free of future constructions that could obstruct the link.
• Interference and efficiency of propagation. The microwave system is open to RF interference. The various propagation models used to understand the effect of terrain and weather are many, and are used to aid the setup and properties of the antennas (height, gain, power etc...)
• Restrictive Costs. The cost of design, implementation and maintenance of microwave links is high. Many countries are not well equipped with good technical resources to provide efficient and continuous operation.

The modern urban environment presents a particular challenge, in that bandwidth allocation, RF interference, link obstruction and atmospheric pollution place maximum constraints on the system simultaneously. However, urban environments also have the highest land acquisition values too. Many modern cities have found it cost effective to build a single, very high tower to house an entire city's trunk communication microwave dishes. These towers are now a common feature of the modern urban landscape.

As the demand for bandwidth increases, microwave links will become increasingly unable to deliver. The use of increased carrier frequencies in the millimetre wave region would be advantageous. However, for technical reasons, no efficient method of producing large quantities of millimetre power have been found. This is a necessity, given the increase in atmospheric attenuation at millimetre wave frequencies.

(table of contents)

EMF stands for ElectroMagnetic Field. An electromagnetic field is:
"a field of force that is made up of associated electric and magnetic components, that results from the motion of an electric charge [current], and that possesses a definite amount of electromagnetic energy" - according to Webster's Third.

What this basically means is, EMFs are built of TWO "inseparable" components: ELECTRIC, AND MAGNETIC.

Electromagnetic fields can be "steady-state", like the North-South of a magnet, or they can oscillate in amplitude at any frequency, from nothing, up through the frequencies of electricity powerline fields at 50 and 60 Hz, through radio and communication frequencies, cellular telephones, microwave ovens, sattellite links, and on into visable light, and even further into x-rays and gamma radiation.(table of electromagnetic spectrum)

Every EMF field has a certain amount of energy stored in it. This energy is proportional to the square of the amplitude of the field. So the power delivered by an EMF field is the rate of energy emitted per unit time. The Power Flux, therefore, is the power per unit area, which is the relevant quantity referred to as 'Radiation'. (see Glossary)

 (table of contents)

Sources:

Clark M.P., Networks and Telecommunications , West Sussex, England: 1991

For a much more comprehensive version of the engineering behind Wireless Communication systems see:

http://www-dept.cs.ucl.ac.uk/staff/S.Bhatti/D51-notes/notes.html