How Radio Waves Transmit Data?
By: Dmitrii Eliuseev
Tags: Electronics, Signal Processing, Digital, Programming, Radio

Wireless technologies are now everywhere around us. But how does it really work? I will review different types of modulation and signal transmission modes, from the simplest to the most complicated.

Let’s try to figure out, without formulas and as simply as possible, how we can transfer data from point “A” to point “B”.

OOK (On-Off Keying)

The simplest form of digital coding. Just turn on / off the radio transmitter in accordance with the binary signal:

OOK Signal Waveform

On the spectrum, such a signal looks something like this, there are quite a few of them at ~ 434 MHz frequency:

OOK Signal Spectrum

The transmitter circuitry is very simple, therefore it is actively used in wireless remotes, radio-controlled buttons and other similar $ 1–2 devices. There is usually no encryption, the frequency and bit sequence are hard-coded, anyone can transmit and receive this signal. I would not put such a remote control on the garage door where the Lamborghini is kept, but for a bedside lamp, it is not a problem (such a lamp, I bought in the MediaMarkt, is working for 3 years and nobody tried to hack it, the “Elusive Joe Principle” in action).

It is interesting to note that historically the OOK is probably the earliest radio transmission mode. If we turn on and off the transmitter with a keyer and receive the signal by ear or using a paper tape, we get the good old Morse code.

Amplitude modulation

We will probably be able to see and listen to AM signals for a long time — modulation is used both in broadcasting and on the 118–137 MHz airband. A distinctive feature of AM is that the spectrum is symmetrical about the center frequency. Visually we can even roughly estimate what is being transmitted, voice or music:

AM Stations

Historically, AM was one of the first ways to receive and transmit speech — the crystal detector radio receiver was extremely simple, and did not even require batteries for work, the energy was taken from the radiowaves itself.

Single-sideband modulation

Single sideband modulation is a refinement case of amplitude modulation. As mentioned above, the AM signal spectrum is symmetrical about the center. But it is possible to transmit only “one half” of the signal, which provides a longer range with the same transmitter power:

Single Side Band Modulation, https://en.wikipedia.org/wiki/Single-sideband_modulation

As we can see from the picture, we can tune to the upper or lower sideband of a signal, such a mode in the receiver or transmitter is respectively designated USB or LSB.

Single-sideband modulation mode is used to transmit voice messages, weather forecasts, and it is also used by radio amateurs. It is also important for the radio reception in general because in the USB or LSB mode the signal is actually transferred from the radio frequency to the low frequency without distortion — this allows to receive various types of digital signals, discussed below. It is important to keep this in mind when choosing a radio receiver — digital modes (FSK, PSK, etc.) can be received and decoded only in the single-sideband modulation mode, a conventional consumer receiver supporting only “standard” AM will not be able to receive such signals.

Frequency modulation

The well-known FM broadcasting works in frequency modulation. It is interesting to note that in the FM station transmitter, not only sound is encoded — a complex signal is transmitted, that includes mono and stereo channels, pilot tone and RDS. Not to be confused with “standard” FM, the hardware manufacturers usually name this modulation WFM (Wide FM). Using the HDSDR software, it is easy to see the spectrum of the FM-station after decoding:

FM Station, HDSDR

On the demodulated signal (bottom right) it’s easy to see the pilot tone at 19 kHz, RDS, mono and stereo channels. Unlike WFM, wireless baby monitors, walkie-talkies and other similar devices use “Narrow FM” (NFM) modulation, where the only sound is transmitted.

Frequency modulation is also used for digital signals, in this case, switching between two frequencies can be used to transmit a binary code. An example is the signal of the Pinneberg station in Germany, the presence of two frequencies is clearly visible on the spectrum:

RTTY Signal

Pinneberg transmits weather forecasts to ships via long, medium and short waves. Frequencies number in the modulation itself, in principle, can be more than two. An example of such a signal is an FT8 amateur radio transmission:

FT8 Signal

Using the FT8, radio amateurs can exchange short messages over a distance of several thousand kilometers with only a few watts of power.

Interestingly, the signal modulation can be combined — for example, in aviation, the ACARS system is used, which can transmit and receive text messages. The digital FM signal is transmitted via the AM transmitter. Why? Probably, the AM transmitter is used, to the input of which a digital circuit was simply connected to send an FM signal. It’s just probably cheaper than replacing millions of transmitters in airports and aircraft around the world.

Phase modulation

In addition to the frequency, we can also change the signal phase, which gives us phase modulation. Such signals can be received over long distance and are used in particular in satellite communications. As one of the amateur radio digital protocols, the PSK31 can be noted.

PSK31 Signal Waveform

With the help of PSK31, information can be sent in the form of a “text chat” by connecting the transceiver to a computer. There can be also more than 2 phases in a signal, for example, 4, 8 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. The simplest way to visualize it is the phase plane:

QAM modulation is used for data transmission in the LTE and digital television DVB-T.

Orthogonal frequency-division multiplexing (OFDM)

One of the modern modulation methods is OFDM. Its idea is in the fact that individual bits of the signal can be transmitted in parallel, by 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 are “orthogonal”, do not influence each other and can be decoded.

DRM can be cited as an example, signals of this format can be received on broadcasting bands, the difference between AM and DRM is clearly visible on the spectrum:

DRM Signal

It is a 10 kHz wide digital signal with 206 carriers transmitted in parallel at 47 Hz intervals. DRM (Digital Radio Mondiale) standard is used to broadcast digital radio on medium and short waves (it should not be mixed with another DRM — Digital Rights Management standard).

OFDM is also used in WiFi (802.11a) and other digital protocols.

Code-division multiple access (CDMA)

Another way of broadband transmission is to combine different messages in one data block. Data for multiple recipients can be combined into one signal using a special function (eg Walsh code) that guarantees both forward and backward data conversion. One of the key factors in both OFDM and CDMA is the so-called “orthogonality” — the signals must not be “mixed” together, that allows extracting data back from the received signal.

CDMA encoding is used in 3G networks. A good example of parsing codes using pen and paper can be found here.

Conclusion

All that is given above is, of course, a very short explanation, in reality, the description of only one decoder can take several times more text than this entire article, and it is unlikely that many people need it here — Medium is not a scientific magazine. However, I hope the readers still got a general impression.

Finally, it is interesting to note that various signals encoding schemes are not just some kind of mathematical abstraction — all this is actively used, also for military purposes (for example, the STANAG protocol of NATO modems). This screenshot was taken while writing the text using the Websdr online receiver:

8.4–8.6 MHz Radio Spectrum

As we can see, despite the presence of the Internet and satellite connection in almost any inhabited place of the Earth, the ability to transfer data directly and anonymously is still relevant — each line on the spectrum is an active communication channel that is working right now (and by the way, attentive readers can even notice the Morse signals here despite the year 2020).