In one of the comments to the previous articles, the question was asked whether it is possible to determine the type of modulation by the type of signal. The idea of looking at the main types of modulation seemed pretty interesting.
Let's try to figure out, without formulas and as simple as possible, how you can transfer data from point "A" to point "B".
OOK (On-Off Keying)
The simplest type of digital coding. Just turn on / off the transmitter in accordance with the binary signal:
On the spectrum, such a signal looks something like this, there are quite a few of them at ~ 433 MHz:
The transmitter circuit is very simple, therefore it is actively used in wireless remotes, radio buttons and other devices at a price of $ 1-2. There is usually no encryption here, the frequency and bit sequence are hard-wired, anyone can transmit and receive a signal, so I would not put such a remote control on the garage door where the Lamborgini stands, but for a bedside lamp it will do (this a lamp bought in the nearest MediaMarkt has been working for me for 3 years, there have been no false alarms, the principle of "elusive Joe" in action).
It is interesting to note that historically this is probably one of the earliest modes of radio transmission. If you turn on / off the transmitter with a key and receive the signal by ear or on paper tape, we get the good old Morse code.
Amplitude modulation (AM)
We will probably be able to see AM for a long time - modulation is used both in broadcasting stations and in transmitters of the 118-137 MHz air band. A distinctive feature of AM is that the spectrum is symmetrical about the center frequency. "By eye" you can even roughly understand what is being transmitted, speech or music. Screenshot from the Websdr Twente online receiver:
Historically, AM was one of the first methods of receiving and transmitting speech - the well-known "school" detector receiver circuit was extremely simple, and did not even require batteries for reception - high-impedance headphones had enough radio wave energy to work. It is curious that such receivers were mass-produced in the USSR right up to the 60s:
Komsomolets detector receiver (c) Wikipedia
Apparently, there were certain problems with the availability of both receivers and power sources in the outback, so the detector receiver remained relevant for a long time.
Frequency Modulation (FM)
The well-known FM broadcasting works in frequency modulation. It is interesting to note that in the transmitter of an FM station, not only sound is encoded - a complex signal is transmitted, including mono and stereo channels, pilot tone, RDS, etc. In order not to be confused with "normal" FM, engineers usually call this modulation WFM (Wide FM). In the HDSDR program it is easy to see the spectrum of the radio station after decoding:
On the signal (bottom right) it is easy to see the pilot tone at a frequency of 19 kHz, RDS, mono and stereo FM broadcast channels. Unlike WFM, baby monitors, walkie-talkies and other similar devices use "narrow" FM (NFM, Narrow FM) modulation, where only sound is transmitted.
Frequency modulation is actively used for digital signals, in this case switching of two frequencies can be used to transmit a binary code. An example is the signal of the German station Pinneberg , the presence of two frequencies is clearly visible on the spectrum:
Pinneberg transmits weather reports to ships on long, medium and short waves. Frequencies, in principle, can be more than 2x. An example of such a signal is the FT8 amateur radio :
With the FT8, radio amateurs can exchange short messages over a distance of several thousand kilometers with only a few watts of power.
It is interesting that modulation can also be combined - for example, in aviation, the system is used ACARS transmitting text messages. The digital FM signal is transmitted through the AM transmitter. Why is it so hard? Probably, a ready-made transmitter is used, to the input of which a digital circuit was simply connected to form an FM signal. Legacy in its purest form, but it's probably cheaper than replacing millions of transmitters in airports and aircraft around the world.
Phase modulation (PSK)
In addition to the frequency, we can also change the phase of the signal, which gives us phase modulation. Such signals can be reliably received over long distances, and are used in particular in satellite communications. Of the amateur radio protocols, PSK31 can be noted , which at one time was very popular.
With the help of PSK31 you can exchange information in the form of "text chat" by connecting the transceiver to a computer. There can be more than 2 phases, for example 4, 18 or 16, it all depends on the speed and communication channel.
It is possible to change both the phase and the amplitude at the same time, which gives us even greater speed, but requires more complex encoding and decoding. QAM is an example of such a signal. This signal is most easily visualized on the phase plane:
Image (c) QAM Wikipedia
QAM modulation is used in data transmission in the LTE standard and in digital television DVB-T.
Orthogonal frequency-division multiplexing (OFDM)
One of the modern modulation methods is OFDM . Its essence lies in the fact that individual bits of the signal can be transmitted in parallel, representing the signal as independently operating frequency channels (subcarriers), each of which transmits its own separate bit. There are certain mathematical rules to ensure that the channels do not cross and can be decoded. DRM
can be cited as an example , signals of this format can be seen on broadcast bands, the difference between AM and DRM is clearly visible on the spectrum: This is a 10 kHz digital signal, in which 206 carriers are transmitted in parallel with an interval of 47 Hz. DRM standard ( Digital Radio Mondiale
) is used for medium and short wave digital radio transmission, please do not confuse with other Digital Rights Management standard.
OFDM is also used in WiFi (802.11a), the signal structure is more complicated there, those who wish can study the PDF on their own.
Code-division multiple access (CDMA)
Another way of broadband transmission is data sharing. Data for multiple recipients can be combined into one signal using a special function (eg Walsh code ) that guarantees both forward and backward conversion. One of the key factors in both OFDM and CDMA is the so-called "orthogonality", the received signals must not be "mixed" in order to extract the original data from the resulting signal.
CDMA coding is used in 3G mobile networks. A good example of parsing CDMA with pen and paper can be found here , I recommend to those interested to look.
Conclusion
All that is given above is, of course, a very brief explanation "on the fingers", in reality, the description of only one decoder can take several times more text, and it is unlikely that many people need it here - Habr is still not a scientific journal. However, I hope the readers still have a general impression. If the audience is interested (which will be determined by the text estimates :), one of the signals can be analyzed in more detail.
In conclusion, it is interesting to note that various coding schemes are not just some kind of mathematical abstraction - all this is actively used, including for military purposes (for example, the STANAG protocol of NATO modems). This screenshot was taken while writing text using the Websdr online receiver :
As you can see, despite the presence of the Internet in almost any inhabited place on the planet, the ability to transfer data directly, anonymously and without intermediaries, is very relevant - each line on the graph is a communication channel that is working right now (and yes, attentive readers can even notice signals here Morse code despite 2020).