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.

There will be articles on subjects such as the following (this list is like a table of contents, I’ll keep it up to date and add links when the articles are written).

  • Digital modulation techniques such as Phase Shift Keying (PSK).
  • The analytic representation of signals to simplify the mathematics, even though it changes the signals from real to complex.
  • Mapping bits to modulated waveforms.
  • Pulse shaping such as square-root raised cosine filtering to reduce intersymbol interference and to reduce the used bandwidth.
  • Adding noise to simulations to make them more realistic.
  • Loops to track the phase and frequency of the carrier and the symbols.
  • Demapping to go back from modulated waveforms to bits.

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?.

Next up after that is, I’m afraid, analytic signals. This is quite a mathematical subject, but it is needed to be able to properly introduce baseband processing in the context of modulation techniques such as PSK… Stay tuned!

Submitted by Tom Roelandts on 13 February 2019

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