Spectrum efficiency

Figure 1: Cellular sharing of spectrumFigure 1: Cellular sharing of spectrum

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.

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