I’m working on a series of articles on modems. It’ll be a loose collection of posts, and not a true multi-part article such as the one on tomography.

Modem technology is not widely known and/or understood, even by a technical audience. However, in our modern day world of smartphones and all sorts of electronical gadgets, modems are everywhere. Your smartphone alone has several; there’s the main cellular data modem, but also Wi-Fi and Bluetooth have their own modems. Even if you put your phone in flight mode, which disables all transmitters, the GPS receiver might still be active, since GPS communication is one way (a GPS module does not have a transmitter).

The word modem is a portmanteau of modulator and demodulator. The basic concept behind modems is modulation. I’ll only consider digital modulation, as used to send and receive bits and bytes over some connection. What modulation does is changing some characteristic of a carrier wave to enable it to transfer information. In digital modems, this is typically the phase and the amplitude of the signal. In analog modulation, as used by broadcast radio, this can also be amplitude modulation, but more typically is frequency modulation. Frequency modulation is used less often in digital modems, but it can be.

I’ve already mentioned some of the reasons for modulation to be necessary in the beginning of How Does Amplitude Modulation Work?. Unmodulated signals all occupy the same frequency range, i.e., they are all baseband signals. This means that they cannot share a medium (the air or a cable). Modulation solves this problem, and it can also shift the frequencies of the signal to a higher range, so that smaller antennas can be used.

As a first example of a digital modulation technique, to whet your appetite, have a look at the the article How Does Phase-Shift Keying (PSK) Modulation Work?.

The next article in the series that is already finished is What is an Analytic Signal?, with a (brief) introduction to analytic signals. This is quite a mathematical subject, but it is needed to be able to properly describe how modems work.

What is a Constellation Diagram? uses the analytic form of the PSK waveforms to introduce the constellation diagram as a concise representation of the modulated information. The constellation diagram is used for digital modulation schemes in general, and not only for PSK.

Mapping Bits to a Constellation is then the next step.

If you are doing simulations, you’ll want to know How to Add Noise to Set a Digital Communications Signal to a Given Es/N0.

There are more modem related articles in the pipeline, so stay tuned…

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Submitted on 13 February 2019