What is a modem? How does it work?

A modem is a modulator / demodulator which is a two part system.

What?

In any satcom system the reason for broadcasting via satellite is that you want to get information from one part of the world to another. So how do we get our TV picture from our camera into the antenna?

The transmit path up to the satellite uses a modulator to take you TV data and modulate it onto a carrier signal. This is the same process as any radio transmission. Once modulated your data is hitching a ride on the carrier signal, this signal is transmitted to the satellite and back to earth again.

At the destination antenna, your signal is then received at a demodulator. The demodulator works just as any radio receiver, the TV data hops off the carrier signal and is then demodulated. The data can then be fed into a TV and watched.

This principle is the same for any modulator and demodulator. These two quite separate devices are usually packaged together to make a Modem. They can however, be used on their own, for example, just like your home Satellite TV receiver. This is a demodulator only and separates the TV data from the carrier signal for you to watch on your TV.

So what is QPSK?

QPSK stands for Quadrature Phased Shift Keying, keying is a method of communication and is probably a word left over from the old morse code method. There are many modulation techniques of which QPSK is just one. It increases the accuracy of data at higher data rates by having 4 states.

This is acheived by using a sine wave which is phased shifted (delayed) by 90 degrees, 180 degrees and 270 degrees. With 0 degrees being state 1, 90 degrees being state 2 and so on. The diagram above shows 4 ellipses one in each quadrant. These vary in shape with the data that is modulated onto each phase. Four times the amount of data can therefore be transmitted with this technique or four times the accuracy.

Modulation techniques is such a technical area I am struggling to keep things simple here, so a new article will cover the techniques in more depth.

The most useful sounding technique is CDMA or Code Division Multiple Access. This technique spreads the data across a wide area of frequency such that there is very little or no detectable carrier. The data is coded and can be recovered from the noise by decoding the signal providing the correct range of frequencies or bandwidth is used.

CDMA also allows multiple signals to be stacked upon each other at the same frequency. This then crams in more signals in the same frequency range requiring less space. The system is old but it is only recently being exploited to it's full potential and is being used in everything from the latest mobile phone technology to vehicle telematics applications.

Another technique is FDMA, Frequency Division Multiple Access, and is most commonly used with satellites. Quite simply, each access has its own frequency. The power of each carrier is balanced across the whole satellite channel and with DAMA, Demand Assignment Multiple Access, the frequencies of each carrier are controlled by a central control center. Each transmitter is commanded to a frequency by the use of a common control channel. This is implemented in our every day lives through the management of our cellular phone network. We don't notice but our phones are continually changing frequency and power level as we speak.