Frequently Asked Questions - What is the electromagnetic spectrum?
There is no simple definition of spectrum. From a technical viewpoint, spectrum is the range of electromagnetic waves that can be used to transmit information. From a practical point of view, however, the effective management of spectrum embraces not only electromagnetic waves, but the technology used to transmit those waves, as well as the economic and political aspects of what is now a valuable national resource.
Our understanding of spectrum has changed a great deal since Marconi first spanned the Atlantic with his “wireless telegraph message”. In 1902, he used the whole spectrum available at the time to send a few bits per second over thousands of square kilometres.
The spark transmitter used for Marconi’s telegraph occupied all electromagnetic frequencies available to existing receivers. As a result, no one could use radio for communication within a 3500 km radius of the transmitting station in England.
If other users wanted to send messages in that area, they needed to coordinate their transmissions in different “time slots” in order to share the medium. This technique became known as Time Division Multiple Access or TDMA.
Users located further than 3500 km from Marconi’s transmitter could use the spectrum because the power of radio waves decreases as we move farther away from the transmitter. Reusing the spectrum in different geographical areas is called Space Division Multiple Access or SDMA.
Marconi was later able to build a transmitter that could restrict emissions to just a range of frequencies, and a receiver that could be “tuned” to a particular frequency range. This allowed many users to transmit in the same area (space) and at the same time. This process of assigning different frequencies to different users became known as Frequency Division Multiple Access or FDMA. With FDMA, radio became a practical means of communication, and the only technology capable of reaching ships in open seas.
National agencies were created to coordinate allocation of frequencies to different users. Since radio waves are not stopped by national borders, however, international agreements were also needed. The international organization that had been created to regulate the transmission of telegrams among different countries was commissioned to allocate use of electromagnetic spectrum. Today, the International Telecommunications Union (ITU), is the oldest United Nations agency, and issues recommendations for frequency use to 193 United Nations (UN) members.
The use of spectrum for military applications raised a new issue. “Jamming” refers to intentional interference in spectrum to impede communication . To make jamming more difficult, a new technique was developed in which the information to be transmitted was combined with a special mathematical code. Only receivers with knowledge of that particular code could interpret the information. The coded signal was transmitted at low power but using a very wide interval of frequencies. This technique was later adapted to civilian applications is called Code Division Multiple Access or CDMA. Today CDMA is used extensively in modern communications systems.
In summary, spectrum can be shared among many users by assigning different time slots, different frequency intervals, different regions of space, or different codes. A combination of these methods is used in the latest cellular systems.
Besides issues of national sovereignty defence, very strong economic and political interests play a determinant role in the management of spectrum, largely due to the rapidly increasing economic value of spectrum. Spectrum management strategies also need to be constantly updated to stay in tune with advances in communication technologies.
Through ever advancing modulation and coding techniques, telecommunications engineers are discovering more and more efficient ways to transmit information using time, frequency and space diversity. Their goal is to increase “spectrum efficiency”, defined as the number of bits per second (bit/s) that can be transmitted in each Hertz (Hz) of spectrum per square kilometre of area.
For example, the first attempts to provide mobile telephone services used a powerful transmitter conveniently located to give coverage to a whole city. This transmitter (called a Base Station in this context), divided the allocated frequency band into a number, say 30, channels such that only 30 conversations could be held simultaneously in the whole city. As a consequence, the service was very expensive and only the extremely wealthy could afford it.
This situation prevailed for many years until advances in electronic technology allowed implementation of a scheme to take advantage of “space diversity”. Instead of using a single powerful transmitter to cover the whole city, the area to be serviced was divided into many “cells”, each one served by a low power transmitter. Cells that are sufficiently far apart can use the same channels without interference. This is known as “frequency reuse”.
With the cellular scheme, the first 10 channels use frequency band 1, the second 10 channels frequency band 2 and the remaining 10 channels frequency band 3. This is shown in figure 1, in which the colours correspond to different frequency bands. Notice that the colours repeat only at distances far enough to avoid interference. If we divide the city into, for example, 50 cells, we can now have 10X50 = 500 simultaneous users in the same city instead of 30. Therefore, by adding cells of smaller dimensions (specified by lower transmission power) we can increase the number of available channels until we reach a limit imposed by the interference.
The example above shows that clever use of existing resources can dramatically increase efficiency.
Although the main use of spectrum is for communication purposes, there are also other uses such as cooking food in microwave ovens, medical applications, garage door openers and so on. Some frequency bands are allocated for these purposes in what is known as the Industrial, Scientific and Medical (ISM) bands. This spectrum usage is normally for short distance applications.
A breakthrough occurred in 1985 when the Federal Commission of Communications (FCC), the agency that oversees the spectrum in the U.S., allowed use of this spectrum for communications applications, provided that the transmission power was kept to a very low level to minimize interference. People could freely use these “unlicensed” bands without applying for a license, provided that the equipment used had been certified by an authorized laboratory that ensured compliance with interference mitigation measures. It is a mistake to imagine that unlicensed spectrum is a complete free-for-all, that it is unregulated. It is precisely the detailed technical regulation of unlicensed devices that makes it possible for them to co-exist together. Probably most significant factor in this respect is the fact that unlicensed devices are regulated to have comparatively low power outputs to limit their ability to interfere with each other.
As the ISM or unlicensed bands were opened to data communication, several manufactures began taking advantage of this opportunity to offer equipment that could communicate among computers without the need for cables. Wireless data networks covering significant geographic areas were built.
Note that open spectrum used in unlicensed bands cannot prevent interference issues, especially in very crowded areas. Nevertheless, open spectrum has proven a success for short distance applications in cities and for long distance applications as well in rural areas.
It is therefore advisable to investigate new forms of spectrum allocation, taking into consideration the needs of many stakeholders and strike a balance among them. Recent advances in technology make a dynamic spectrum allocation mechanism a feasible alternative.
As an analogy, the current method of spectrum allocation is similar to a railway system, the railroads can be idle a considerable amount of time. The dynamic spectrum allocation is akin to the highway system that can be used at all times by different users.
The turning point, however, came in 1997 when the Institute of Electrical and Electronics Engineers (IEEE) approved the 802.11 Standard, the basis of what is now known as WiFi. The existence of a standard that guaranteed the interoperability of equipment produced by different manufacturers fuelled an impressive growth of the market, which in turn drove competition and led to a dramatic decrease in the cost of devices. In particular, the portion of the ISM band between 2400 and 2483 MHz is currently available in most of the world without need for a license and is widely used by laptops, tablets, smart phones and even photographic cameras.
The role that unlicensed spectrum played in the enormous success of WiFi high speed Internet access cannot be overstated. Airports, hotels and cafes all over the world offer WiFi Internet access on their premises, and low cost wireless community networks have been built both in rural areas and in cities covering considerable geographic areas – all thanks to the availability of unlicensed spectrum.
Mobile phone operators, which have to pay dearly for spectrum licenses, were initially quite hostile to this apparently unfair competition. But with rocketing data usages thanks to a burgeoning smartphone industry, they soon realized that off-loading the traffic to WiFi was in their best interest, because it relieved the traffic in their distribution network (known as backhaul). Now mobile phone operators encourage their customers to use WiFi wherever it is available and to use the more expensive cellular service only when out of range of any WiFi Access Point.
This demonstrates the value of unlicensed spectrum even to traditional telecommunications operators who often have lobbied against it.