Digital Airwaves
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35C3 Refreshing Memories161 / 165
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Semiconductor memoryPlot (narrative)Digital signalTheorySoftwareDiagramLecture/Conference
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Phase transitionSymbol tableBitNoise (electronics)Computer animationXMLLecture/Conference
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Phase transitionNoise (electronics)Symbol tableLevel (video gaming)Order (biology)Goodness of fitPhysical systemData exchangeComputer animation
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TrigonometryImaginary numberPhase transitionEuclidean vectorShift operatorNumberCartesian productChemical polarityMultiplicationCartesian coordinate systemForm (programming)Imaginary numberConnectivity (graph theory)Execution unitComplex numberNumberCartesian coordinate systemPhase transitionDigital signal processingCategory of beingReal numberPower (physics)CASE <Informatik>Computer animation
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NumberCartesian productChemical polarityMultiplicationDigital filterAnalog-to-digital converterRight angleFourier transformFrequencyMereologyAdditionSoftware-defined radioSoftwareBitComputer animation
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Transcript: English(auto-generated)
00:18
I will give you a short introduction to software-defined radio, so some basics about this technology
00:24
and some modulation technology, which you also always need if you want to transmit something. So first of all, before we come to the software-defined radio, let's first have a look about what generally happens in a radio transmission.
00:42
So the parts you always need to get something over the air. So normally you have some input signal, you want to transmit an audio signal, radio for example, a video signal or just any data. Then you do some compression, mostly you do this if you have some digital stuff and
01:04
analog, you don't do this so much. You have some error correction, modulation and then the frequency assignment to the frequency you want to use for the transmission. Then you have a radio channel, sometimes you have mobility if you move, you have
01:25
a multipath propagation, you always have some noise added and often there are also like other signals in the air which also share the channel. And then at the other side it goes the other way around, you get the demodulation, error
01:40
correction if there are errors and the decompression and hopefully out comes your original audio or video signal or the data you had transmitted. So a bit to the frequency assignment, there are frequency plans, here you can see a frequency
02:03
plan of the US, they had a nice chart like this, here for example you can see the frequency band from 88 to 108 MHz, then some aeronautical services and other stuff at the other frequencies.
02:21
For Europe they have a really huge table, you can find it on the website of the ECHO, the European Communications Office, yeah it's quite large, but if you want to look what's probably on this frequency in the air you can have a look there.
02:45
So now let's start with a not software defined radio to get a bit more use to the principles what is happening there, here is for example an old AM receiver on this side.
03:00
So we get the signal in the air, the AM transmission, there are still some but they are actually switched off at the moment. Here now we have a super heterodyne receiver, it's called like this, so what we have, where is my mouse, here is my mouse, so we have here at the antenna, here is the antenna,
03:24
we have our signal S1, that's the signal we want to receive, then we have some filtering to get rid of all the other signals which are farther away, then we have our mixer here, so the LO frequency of this mixer, like the local oscillator frequency here is
03:49
always chosen in the way that the wanted signal always falls in the same intermediate frequency, with this you can have a very sharp filter here, the IF filter, so at
04:05
your IF filter output you only get the wanted signal, which then after the filtering again some amplification goes to the demodulator and in the case of AM now all your information
04:21
is actually in the amplitude of the signal, so for decoding and listening the easiest way would be just an envelope detector which could look like this, you have a diode which actually puts the negative part of the signal to the positive side and then here we just
04:42
use the LO path to get rid of the intermediate frequency which you can still hear and afterwards you can just listen to your audio signal. So in the case of Software-Defined Radio we stay to the RX front end in this example,
05:03
the TX path would be nearly similar the other way around, so again we have the antenna, antennas are also really important, always take a good adapted antenna to the frequency
05:20
you want to receive or the frequency you want to transmit because otherwise you won't get any signal out of the air or only a very low part of the signal, I gave a talk on antennas at 31st 3 C3, so if you are interested in antennas you can have a look
05:42
on media.ccc.de, then again we again have some filtering and amplifier and now we have an IQ mixer, here you can see it actually consists of two mixers and this local oscillator
06:02
signal is shifted by 90 degree to the lower part here of our signal, then again some filtering, amplification and then we get the analog to digital converters here to get our IQ signal
06:23
then to the computer for decoding and software. So we still have actually a big analog part here, so most of the front end is still an analog and the digital part actually is only this after the analog to digital converter in this case of a classical software defined radio front end.
06:49
So IQ data are pretty cool, so they contain actually the raw signal that is coming out of the air, so you could also record the raw signal, it's fastly getting huge
07:06
and for example do then the demo modulation later. If you put those IQ signals on a coordinate plane, which you can see here on the right side, you can see all of the phase shift between
07:20
of 90 degree between the I, which is the in phase component and the Q, which is the quadrature component of the signal. If we assign some numbers, we can also combine them with a vector, we can use putagoras for example to get the amplitude
07:46
of the resulting vector, we can do some trigonometry to get the angle, and actually those two parameters like the angle and the amplitude
08:00
are the main parameters you can put information in. So in the example before, like the AM modulation, you only use actually the amplitude of the signal, in contrast to this an FM modulation for example has a constant amplitude and all the information is put to the phase or the frequency.
08:24
So no matter what kind of modulation is used, this IQ data actually contain all the necessary information. A nice example of a modulation which is often used nowadays and that also uses both of those parameters is the QAM modulation.
08:42
Okay, I already told this. The QAM modulation here for example is a constellation diagram out of the program KNU radio. It's a bit shifted, everything doesn't matter. So here again we have our in phase component on the x-axis and the quadrature component
09:04
on the vertical axis. With the 4 QAM we have 4 symbols, so we can put in 2 bits per symbol. A 16 QAM for example you can put in 4 bits per symbol. If we go further,
09:22
64 QAM we can put in 6 bits per symbol. This for example is used in DVBT or DAB like broadcasting systems or in Wi-Fi 802.11n uses up to 64 QAM, LTE
09:44
also uses up to 64 QAM. When we go further, 802.11ac uses 256 QAM, so even more dots you can put in 8 bits per symbol and so does LTE-Advanced.
10:06
The more data you want to transmit, the more symbols you need. 802.11ac uses up to 1024 QAM with 10 bits per symbol and so does the successor of 4G like the 5G KNU radio
10:26
also uses up to 1024 QAM. It becomes interesting when we add some noise. So you always, as I told you, always got the channel, you always got noise. This is what
10:43
happens if we add some noise to the 64 QAM, you could still estimate where the original symbol would be. This becomes even more difficult if we go to the 1024 QAM.
11:01
That's also why those broadband systems always use an adaptive modulation like within the first data exchange. They communicate about the quality of the signal and only if you get a really good signal level at the receiver you choose the highest order modulation, otherwise
11:22
it is ramped down to lower orders. So these high order modulations only work with really good signal levels. So let's go back to the IQ data. Those IQ data are closely related to complex
11:42
numbers. So to get a complex number let's add some imaginary unit j. So we get our complex number actually as c equals i plus j multiplied by q which are again our in-phase and quadrature
12:02
component. So complex numbers, you can write them in the cartesian form which I showed. The mostly often used form is actually the polar form where we add Euler's number. So it becomes like c equals a multiplied by E, Euler's number, to the power of j phi which is our phase here
12:29
again. So in this case like our real axis, the in-phase axis here becomes our real axis
12:41
and the q-axis becomes our imaginary axis. This property of this polar form which is often needed in digital signal processing is the multiplication. Like if you multiply two polar
13:06
formed complex numbers, this ends up in an addition of the the elevated parts here. And this is often used for example in Fourier transformations or if you mix signals to get them
13:22
from one frequency to the other. More on this later. It looks quite complex but it's really worth using it at the end. So the first step in the software-defined radio is then to get the right parts of the signal through the front end because if you don't get your IQ data actually
13:45
properly afterwards decoding it in software becomes very very difficult or even impossible. So let's have a look at the different parts of our software-defined receiver.
14:00
After the antenna filtering and amplifier we have this IQ mixer. So to keep it a bit more simple for now we just skip the IQ part and have a look what a mixer in general is doing. To get the signal from the transmitted frequency to the IF to the intermediate frequency it is
14:26
multiplied with an LO signal and then filtered. This multiplication actually ends up here in an addition here this higher part and in a subtraction of the two frequencies we put in
14:42
here. And with the filter we actually get rid of the higher part here. So the mixer defines the frequency range the SDR front end is working on. For example there are those quite cheap RTL SDR USB sticks which were originally made for DVB-T reception.
15:07
They work for example from 24 megahertz up to 1766 megahertz. Then there's the hacker F which is also an often used SDR front end works from 1 megahertz up to 6 gigahertz. And the radio
15:28
batch from the CCC camp 2015 works from 50 megahertz up to 4 gigahertz. As I told the mixer here is a bit simplified. Here is for example the mixer chipset of
15:46
the HackRF. Here you can see the IQ mixing part here. Next step then after again some filtering and amplification is the analog to digital converter.
16:05
We get the analog signal in here and what the computer actually needs are samples of the signal. So they have to be taken at dedicated times T here and so we get the sampling rate 1 divided by T. This sampling rate must comply with the Nyquist-Channon
16:25
sampling theorem otherwise your signal can't be reconstructed properly. You get effects like aliasing where you have frequencies that actually are not there but are created are caused by the under sampling of the signal.
16:46
And for complying the this Nyquist-Channon theorem like the the bandwidth of your signal of the signal you want to digitize has to be smaller than 1 divided by 2T.
17:02
Here an example of an DAB plus signal. DAB plus is nice because of course it always has a bandwidth of 1.5 megahertz. It has quite sharp edges because it uses an OFDM modulation. This here was received with an
17:26
RTL-FDR DAB DVB-T stick with the software GQR-X which has a maximum sampling rate of 3.2 megahertz. So let's check for Nyquist. So we have our bandwidth of 1.5 megahertz. We have the
17:55
bandwidth of 1.5 megahertz is smaller than 1.6 megahertz. So great we can receive a DAB plus signal with a DAB receiver.
18:09
You might ask now this is also for the DVB-T reception which has a bandwidth of 8 megahertz. So you would need a sampling rate of 60 megahertz to receive or to digitize this.
18:25
That's actually a nice example of the usage of STR in comparison to dedicated chipsets. DVB-T here doesn't use the STR mode of this chipset but it has a dedicated DVB-T chipset
18:43
in here. So chipset development is quite expensive but if there is a mass market and for it can be produced very cheap. So actually the STR mode was probably added for the DAB reception.
19:04
Also with the growing bandwidth the power consumption of the STR mode becomes quite high because you have always to digitize the whole bandwidth of your signal. So if it comes for example to LTE with 20 or 40 megahertz bandwidth this becomes quite relevant.
19:29
Okay we can get the DAB signal here. The next relevant parameter here is the resolution of the ADC. With a three-bit resolution for example you would get eight discrete values
19:45
from your signal. With the 8-bit resolution you get 256 values. With 60-bit you get a lot of values. And those parts of the step here you can see for
20:03
example the 3-bit resolution and the 16-bit resolution of a sign signal. And all those parts of the steps of the 3-bit resolution actually end up in noise which is called quantization noise.
20:22
Here for example you see the spectral view of the signal. The first one with a 6-bit resolution you can see the noise floor here at minus 68 dB and below with the 8-bit resolution the noise floor falls down by 12 dB so we get a noise floor down at minus 80 dB.
20:50
What we also see here is actually here are some examples. The RTL-STR has two 8-bit ADCs. The HackRF and the radio have a dual 8-bit receive ADCs and as they are also for
21:09
transmitting purposes they have a dual 8-bit, a 10-bit transmit DAC so the other way around to get your digital signal in the analog domain again. The RTL-STR is only for receiving purposes.
21:28
What we also see here is on the right side we get our signal in the time domain, on the left side we get the frequency domain. So how do we get the frequency view of our signal?
21:44
Here for example in the form of a spectral view and down here is this with the nice colors. This part is called a waterfall diagram. Here we have in the spectrum view
22:05
we see the level of our signal components over the frequency and the waterfall diagram then shows the different levels and different colors plotted over the time here.
22:22
So how do we get the frequency view of our signal? Actually we use a Fourier transformation to convert the time domain signal into the frequency domain. Wikipedia actually had a nice animation about this in public domain. So we
22:45
have a square wave signal which is a linear combination of sines of different frequencies here in blue and the component frequencies of these signs then are spread across the frequency
23:01
spectrum and they are represented here as peaks in the frequency domain. So mathematically this looks like this. Here we get the different components, the sine components of our square wave signal. For the sake of simplicity we just skip the harmonics here. Just take the
23:24
sine signal, calculate the Fourier transformation which is an integral of our of our function. The sine signal here multiplied by e to the power of minus j 2p f t and
23:41
integrated over t. We use again the polar form here which then ends up in a multiplication of this component and the integral of this multiplication
24:01
then ends up in delta impulses at a frequency here of a and minus a. And we still have half of an inverse imaginary unit here. If we have a look at the Fourier transform of a complex
24:22
constant wave signal this actually simplifies to one delta impulse here at a frequency of a. For practical purposes, computational purposes, we use a DFT like a discrete Fourier transformation
24:45
so the integral ends up in a summation of the signal components and actually normally we use a fast Fourier transformation which you also see in all the
25:00
software which is actually an algorithm to efficiently calculate a DFT. So let's have a view again at the DAB signal here with the GQRX software. We have the water file Vivo and because it's a bit small now here it's actually quite seen it's a bit bigger so on the left side we
25:26
have an FFT size of 32,768 and on the right side an FFT size of 512 and actually with the FFT length you define afterwards the resolution of the bandwidth of
25:45
the spectrum. So you can see here it's much more coarser than with a higher radio resolution bandwidth here on the left side. Then the sliders down here
26:03
you can find those sliders and stuff here in the FTT settings of GQRX. If you want to have a look at the software the sliders here down I also have them a bit bigger here you can
26:21
define the reference level so if you have a very low signal you have to put it a bit down and the also the range like the range you see your signal so if you have a high dynamic signal you need a large range to see all the parts of the signal if you have a very very low signal
26:43
power you need to switch it down to a smaller range to actually see anything of your signal. So the possibility is actually to efficiently calculate an FFT or IFFT like the
27:03
inverse Fourier transformation also gives the possibility to avoid a use of multi-carrier modulation methods as OFDM here, orthogonal frequency division multiplex. Nowadays this is often used in mobile communication systems such as LTE
27:23
due to its resistance to the effects of the propagation channel. For example multi-pass propagation often causes destructive interferences so some of your carriers here actually are in destructive interference parts so they are
27:49
actually attenuated a lot and if you distribute your information over several carriers you still have the chance to receive some of the carriers and then you can afterwards
28:06
use some error correction mechanisms to repair actually the data and get something out of the data. And so here the FFT or in the TX case and the transmission case an inverse FFT is used
28:23
actually to distribute the for example the QAM data to the different frequencies to the different carriers. Then it's again the regular IQ mixer and in the case of the
28:46
reception we use the FFT to get the symbols the QAM symbols for example out of our different carriers. Here again you see I like DAB again the DAB signal here we have DAB
29:11
uses 1536 sub-carriers and the number of sub-carriers here actually is also always a compromise of how close your sub-carriers are which defines how much
29:26
shifts in case of mobile reception your system is capable to scope with and on the other hand it defines how long your signal is in the air so the more carrier you have the longer
29:44
your signal is and that has an effect on how much delays your signal can scope with. Additionally, often there is a guard interval added to the symbol to scope with more
30:02
delays for example DAB is a broadcasting system with a capability of single frequency networks so you can run different transmitters on the same frequency with the same program but especially in the overlapping areas this results in very large delays so that's why
30:23
the broadcasting system has very much carriers. LTE in contrast only has in the downlink with a 10 megahertz bandwidth 601 carriers in the uplink 600 and 800.11ac for example
30:46
with 40 megahertz bandwidth has 128 carriers. So now let's come back from this quite complex world of software-defined radio to the real world. So what SDR actually brings are quite
31:06
cheap and flexible solutions of formerly very expensive technology that's why it's actually often used in academia or also for prototyping purposes but there's also a quite big community
31:24
developing open source software for software-defined radio. I want to show you now like two examples where those SDR technologies facilitated community-driven projects. One is digital radio which goes digital in Switzerland or community radio goes digital in Switzerland.
31:49
Like digitizing local community radios has actually long been a problem. Community radios are a non-profit making media produced by a local community and serving
32:00
a local community. There's also one here in Leipzig which are also doing a program from the congress here. I think they are actually starting now for I think for three hours today. It's called fairy dust FM so if you want to listen you can look at the at the wiki
32:21
where to receive them. They mostly do not have a huge budget for running a radio. The development was facilitated by low threshold cheap transmitters so FM transmitters are really cheap now or they can be built. With DAB digital audio broadcast the possibilities
32:48
of running your own cheap transmitter became quite difficult for a long time. DAB was developed by the big broadcasting corporations like BBC or the German public media
33:01
and it's actually adapted to their needs. You can put in a lot of programs and multiplexes. You can run huge single frequency networks. There's a national SFN in Germany for example. Local community radios so does local commercial radios need more flexible cheap radio
33:23
transmission. You might argue that digital radio isn't relevant anymore but actually there are countries that start to switch off FM and only streaming through the internet is also
33:41
not an appropriate solution. So what happened some years ago was that people started to write open source DAB SDR software to build up quite cheap DAB transmitters. You can find it the software here on open digital radio. They have this nice pink green with a transmission
34:02
power as a logo. And in Switzerland the FM switch off is set to 2024 so it's quite coming closer and a lot of communities are already on the digital airwaves there with the solution of
34:23
of software defined radio based transmitter technologies. The UK is also on the way to switch off FM and there the Ofcom actually recently started a survey about the demand for small scale DAB also based on this SDR solution which makes it affordable to community radios.
34:48
Another example is community driven cellular telephony. In remote areas for example in Mexico and probably in a lot of more countries often there
35:03
is no cellular network connection at all as it's just not a good business for mobile broadband providers. If you have only a few hundred clients to use it or customers who pay for it.
35:21
I was some years ago in the south of Mexico for an article about the first community-driven cellular network which was also built on open source SDR technology like open BSC and open BTS which made it then quite affordable for the communities there.
35:42
Today this association telecommunicaciones indigenas communitarias has a license to run autonomous telephone networks in different parts of Mexico as Chiapas, Veracruz and Puebla and now nowadays they are already running nearly 20 cellular networks there and they also
36:04
do a lot of trainings and write a lot of manuals so if you want to learn how to run your own GSM networks there actually only you can have a look on their site.
36:23
So these are only two examples of projects where SDR facilitated low budget communication. So you might ask if you now want to have a look on SDR yourself where to start.
36:42
So for radio reception this cheap RTL SDR USB sticks are your friend. They cost around 10 to 20 euros depending on where you get it and there's software like this GQRX which I already had a lot of examples in my slides
37:05
which runs on Linux and Mac. Here is an example of GQRX for FM reception for example. It has also an built-in FM decoder so you can really listen to FM radio. They are also
37:22
AM decoder and some others also. You can also dump the IQ data with this GQRX for decoding it later. There's also software for Windows like SDR Sharp or HD-SDR or Win-SDR.
37:43
Always keep in mind that listening to non-public broadcasts is forbidden. The next level then would be GNU radio. I already showed in between the talk plots from GNU radio like the constellation plots of QAM modulation. GNU radio actually offers a very
38:07
large framework for software-defined radio functions also to build your own applications. There are sources, for example here is a source where you can connect your RTL SDR USB stick,
38:26
define here the sampling rate, the frequency and different other stuff here. Then you have a lot of functions here for example the FM demodulation, you have a spectrum here the FFT sync, different resamplers and then you have different things
38:47
here. You can connect it to your sound card with the audio sync and in this case listen to FM radio. You can also define a sync to connect your HackRF to transmit something.
39:06
You can also write your own functions so it's quite easy in this graphical front-end, the GNU radio companion to add own functions. There are many tutorials also in the internet
39:24
and very active community and it's also very often used in academia. So if you are perhaps studying or planning to study there are very often projects around GNU radio which you can work on if you're interested. There's also a lot of different
39:43
SDR hardware available. So the HackRF I already mentioned, the radio badge from the CCC camp, so if you don't have one you can ask around perhaps someone still have one lying around. There are more expensive ones which then have for example better resolutions, the ADCs have
40:06
better resolutions. There is the USRP family which is much more expensive but you can do a lot more with this and it's also very often used in academia. I also
40:23
knew it from my time I worked at the university. So further information, if you are now becoming really interesting, there are lots of massive open online courses. For example I saw one from the University of Madrid but in English.
40:42
So there are video tutorials for example from the makers of the HackRF at their website. There are also nice free available books on SDR by analog devices for example. If you look for SDR for engineers and if you are now here, there is an SDR challenge at the congress.
41:10
They have a table in hall three in the wastelands there. If we have a look at the small print, there are various different SDR challenges from quite easy to difficult.
41:25
There's a game server to claim your flag in a team and if you don't have an SDR you can borrow one like this RTL SDR sticks for a deposit and there are also if you don't like all this
41:48
feel free to ask questions if you want.
42:33
The European Convention of Human Rights has an article about being free to conduct journalism
42:41
and there was a claim that journalism includes just listening to the entire FM spectrum. Yeah the FM spectrum is public so there's no problem but there are other services like that are not encrypted because in former times this technology just wasn't available or affordable
43:03
for normal persons. So nowadays you have much more possibilities to receive other frequencies for example quite easily which are not public and so it's forbidden to listen to them actually. Yeah but by what? Is there a law?
43:23
The law or I'm not a lawyer so I don't know exactly what law it is. Okay any other questions? Does the internet have questions by now? If you have a question by the way just walk to a microphone.
43:46
The internet doesn't have any questions but MCR of Open Digital Radio would like to thank you for speaking of them. Okay that's not the question. Sorry what I didn't get it. No questions.
44:01
Okay okay great so. Well that's a kick from them. Thank you all for your attention. Oh sorry microphone number two. Yeah it's not a question either it's just a clarification of the legal situation. So basically you're allowed to listen to non-public broadcasts or non-public radio traffic
44:25
for example like aeronautical but you're not allowed to record it and to publish the information that you gathered. Okay thanks.
44:41
So theoretically sitting at home and listening to yeah I mean the tower talking to the pilots or whatever or even to police is allowed you're just not allowed to well basically make a profit from it. That's the legal situation in Germany I don't know how it looks
45:03
in other parts of Europe. Since we're violating the protocol of Q&A anyway by not asking questions. I am a lawyer and it varies from member state to member state. You could question that as attention if the European Convention of Human Rights or not but it really varies from member state to member state.
45:22
Well in that case. Now I really would like to have a genuine question. Something that starts with a sentence ends with a question mark. Do we have any takers?
45:44
Oh in that case thank you so much for your attention. Bye.