Modems

Mapping Bits to a Constellation

Figure 2. Bit mapping for QPSK.

Before information can be transmitted using a digital modulation scheme, the information bits have to be mapped to the constellation. Each point in the constellation is known as a symbol, making the full collection of symbols a kind of alphabet of the modulation scheme. For most digital modulation schemes…

Tom Mon, 05/06/2019 - 09:57
What is a Constellation Diagram?

Figure 1. QPSK constellation.

A constellation diagram is a diagram that shows how the information in a digital modulation scheme such as phase shift keying (PSK) is represented in a modulated carrier. This article is a bit of an experiment; instead of introducing the constellation diagram by simply presenting it…

Tom Mon, 03/25/2019 - 12:54
What is an Analytic Signal?

Figure 1. Spectra of cosine (left) and complex exponential (right).

An analytic signal is a signal without negative frequency components. This might seem a strange concept if you’re not used to it, but analytic signals are used in signal processing, for example in modems for telecommunications. Let’s start from…

Tom Mon, 02/25/2019 - 09:56
Modems

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…

Tom Wed, 02/13/2019 - 10:52
How Does Phase-Shift Keying Modulation Work?

Figure 3. BPSK-modulated carrier.

After explaining how amplitude modulation and frequency modulation work, which are both analog modulation schemes, I’d now like to give an example of a digital modulation techique. A basic, but very relevant, example of a digital modulation scheme is…

Tom Sat, 01/12/2019 - 16:07
Why is Eb/N0 the natural figure of merit in digital communications?

In digital communications, the measure of the signal to noise ratio of a signal is Eb/N0. Why is that? To figure it out, let’s start from the classical SNR, which is the ratio of the average signal power S to the average noise power N, given by…

Tom Wed, 06/29/2016 - 18:54
How Does Frequency Modulation Work?

Figure 3. FM-modulated carrier.

After explaining how amplitude modulation works, the next step is Frequency Modulation (FM), which you might be more familiar with, since it is widely used for radio broadcasting. In practice, a carrier for FM broadcast radio would have…

Tom Sun, 11/16/2014 - 20:05