From the Sputnik 'Beep' to messages from Pluto
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Chaos Communication Camp 201913 / 102
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00:00
BEEPMessage-PassingUnordnungMessage-PassingMinkowski-MetrikBetrag <Mathematik>Unrundheitt-TestRechter WinkelGrundraumComputerunterstützte ÜbersetzungDifferenzengleichungJSONXMLVorlesung/KonferenzBesprechung/Interview
00:43
GammafunktionDifferenteBEEPTransmissionskoeffizientUltimatumspielVorlesung/Konferenz
01:11
DruckverlaufTelekommunikationVakuumDruckverlaufBEEPDickeFrequenzKugelPunkt
01:46
PhasenumwandlungSymboltabelleDatentransferBitfehlerhäufigkeitÄquivalenzklasseLeistung <Physik>Ein-AusgabeFrequenzSpannweite <Stochastik>Vorzeichen <Mathematik>WellenlehreCodeMathematikInformationDichte <Physik>PunktspektrumRauschenBitPhasenumwandlungBandmatrixPunktSoundverarbeitungAutomatische HandlungsplanungAbstandDifferenteSymboltabelleWinkelVektorraumSichtenkonzeptKartesische KoordinatenEnergiedichteSinusfunktionFreier LadungsträgerRichtungTransmissionskoeffizientDivergente ReiheKurvenanpassungPaarvergleichDiagrammBitfehlerhäufigkeitInverser LimesLeistung <Physik>Physikalisches SystemUnendlichkeitNatürliche ZahlRandwertGruppenoperationPolstelleGeradeInternetworkingErhaltungssatzPlotterDatentransferAggregatzustandSpielkonsoleGewicht <Ausgleichsrechnung>WechselsprungElektronisches ForumBeweistheorieFormation <Mathematik>MultiplikationsoperatorSchwebungFeuchtigkeitArithmetisches MittelTropfenKonditionszahlFamilie <Mathematik>Computeranimation
08:02
FrequenzPunktLeistung <Physik>BitVerschlingungFluss <Mathematik>TransmissionskoeffizientEnergiedichteStreaming <Kommunikationstechnik>RichtungMereologieTropfenEin-AusgabeMusterspracheRauschenRegelkreisNavigierenFlächentheorieQuadratzahlEuler-WinkelSpezielle unitäre GruppeSpiegelung <Mathematik>InformationMultiplikationsoperatorComputerarchitekturWasserdampftafelPhysikalisches SystemFrequenzStochastische AbhängigkeitBenutzerfreundlichkeitPhysikalischer EffektEreignishorizontWhiteboardPartikelsystemWeb SiteSchnittmengeAbstandBetrag <Mathematik>SummierbarkeitWurm <Informatik>Workstation <Musikinstrument>Gewicht <Ausgleichsrechnung>CASE <Informatik>DatentransferSoundverarbeitungCharakteristisches PolynomResultanteTrennschärfe <Statistik>Normaler OperatorDatenstromUnrundheit
13:53
ZehnSpannweite <Stochastik>RauschenPhysikalisches SystemGewicht <Ausgleichsrechnung>SchlussregelFlächeninhaltThumbnailEnergiedichteSpezielle unitäre GruppeQuellcodeWeg <Topologie>Schwarzer StrahlerFrequenzWeb SiteRichtungPunktSummierbarkeitPunktspektrumVorlesung/Konferenz
15:56
EnergiedichteRauschenRichtungMonster-GruppeEin-AusgabeFlächeninhaltVorlesung/Konferenz
16:27
RechenwerkFAQDoppler-EffektFreier LadungsträgerLoopBitTUNIS <Programm>Formation <Mathematik>Ein-AusgabeMessage-PassingWeg <Topologie>FrequenzFreier LadungsträgerLeistung <Physik>Workstation <Musikinstrument>PunktspektrumSchwingungTransponderQuaderUmsetzung <Informatik>AdditionInformationDoppler-EffektMereologieBandmatrixDifferenteLoopMAPSweep-AlgorithmusSpannweite <Stochastik>RauschenMathematikCASE <Informatik>TelekommunikationStandardabweichungEnergiedichteVorzeichen <Mathematik>WellenlehreProzess <Informatik>Abgeschlossene MengeWeb SiteTermDatenfeldTablet PCPunktSchlussregelHeegaard-ZerlegungFunktion <Mathematik>SpielkonsoleWarteschlangeGruppenoperationDigitalsignalDomain <Netzwerk>TransmissionskoeffizientGamecontroller
21:37
RechenwerkBrennen <Datenverarbeitung>Universal product codeProzess <Informatik>FehlermeldungEin-AusgabeFunktion <Mathematik>RauschenDivergente ReiheBitTransponderFormation <Mathematik>SymboltabelleMixed RealityTypentheorieWorkstation <Musikinstrument>Klassische PhysikDatenkompressionResultanteRelationentheorieNichtlinearer OperatorFrequenzInformationMathematikAnalogieschlussMultiplikationsoperatorInhalt <Mathematik>Orbit <Mathematik>Disjunktion <Logik>SchieberegisterFaltungscodeDatenstromFehlererkennungMusterspracheTeilbarkeitStreaming <Kommunikationstechnik>p-BlockLeistung <Physik>Element <Gruppentheorie>DifferentePhysikalisches SystemDigitale PhotographieWellenpaketTermWeb SiteSchwebungLochkarteAdditionVierzigVerschiebungsoperatorTropfenPunktStrahlensätzeFunktionalRechter WinkelProfil <Aerodynamik>Abstimmung <Frequenz>FlächentheorieGradientenverfahrenCodierung <Programmierung>QuaderAggregatzustandFaltungsoperatorComputeranimation
26:47
BitfehlerhäufigkeitCodierungstheorieStandardabweichungDateiformatWurm <Informatik>AggregatzustandCodierung <Programmierung>GruppenoperationFehlermeldungOffene MengeFaltungscodeStreaming <Kommunikationstechnik>SchieberegisterEnergiedichteKonfiguration <Informatik>BroadcastingverfahrenStandardabweichungSchnittmengeMinkowski-MetrikDatenaustauschDifferenteSchaltnetzWort <Informatik>SymboltabelleBitfehlerhäufigkeitSoundverarbeitungInformationFolge <Mathematik>AdditionGraphBitLeistung <Physik>Gewicht <Ausgleichsrechnung>CodeTypentheorieIntelWorkstation <Musikinstrument>Office-PaketTelekommunikationMaschinenschreibenVerschiebungsoperatorPunktCoxeter-GruppeComputeranimation
29:28
ZweiDatensatzMultiplikationsoperatorMP3Vorlesung/Konferenz
30:03
BroadcastingverfahrenWorkstation <Musikinstrument>Weg <Topologie>FrequenzNichtlinearer OperatorSweep-AlgorithmusTransmissionskoeffizientMathematikStreaming <Kommunikationstechnik>ProgrammierungEin-AusgabeTransaktionEnergiedichteDatentransferCASE <Informatik>Prozess <Informatik>MultiplikationsoperatorPhysikalisches SystemBesprechung/InterviewVorlesung/Konferenz
31:20
KommunikationsprotokollUmwandlungsenthalpieMinkowski-MetrikForcingBesprechung/Interview
31:52
p-BlockDichte <Physik>Turbo-CodeSchnittmengeCodierung <Programmierung>Gerade ZahlEntscheidungstheorieHorizontaleCodeDatenstromBitPunktStandardabweichungKommunikationsprotokollGlobale OptimierungKonfiguration <Informatik>InformationBroadcastingverfahrenPhysikalisches SystemVerschlingungOrdnung <Mathematik>Minkowski-MetrikInformationstheorieDienst <Informatik>RauschenMobiles InternetRuhmasseInternetworkingFokalpunktSystemaufrufTabelleMatrizenrechnungVorlesung/Konferenz
33:40
RauschenNatürliche ZahlPunktspektrumBesprechung/Interview
34:07
DifferentePropagatorBroadcastingverfahrenTopologiePunktspektrumVerschlingungProgrammierumgebungWorkstation <Musikinstrument>RauschenPunktPhysikalisches SystemWeißes RauschenLeistung <Physik>Gebäude <Mathematik>Vorlesung/Konferenz
34:54
UnrundheitBesprechung/InterviewJSONComputeranimation
35:23
Computeranimation
Transkript: Englisch(automatisch erzeugt)
00:15
messages from Pluto, signals from space 101. You know how they say that there is absolute silence in space, but for anyone listening on the right frequencies, there are still
00:25
plenty of signals to pick up, and our next speaker, Inko, is a PhD student in Stuttgart, and he's been working with satellites at the university, and he's going to tell us all about it, so I would like to welcome you, Inko, and I would like to ask you to welcome him with a warm round of applause and have a lot of fun with the
00:42
talk. Yeah, thank you. So, when Sputnik was launched and started transmitting its signal, most of the people I guess in this room have heard the characteristic beep, but what was
01:02
this beep actually? So, Sputnik actually had two different transmitters inside of it, that's why it has four antennas, two for each transmitter, and they were alternating an activity, so you could hear a beep either on one channel or on the other, and it was not just a beep, it was not just a signal, yes, we are here, we are up here, we can
01:23
reach you. It was actually encoding data, because Sputnik was a sphere, so you could more easily put it under pressure because electronics and vacuum was still a difficult topic back then. So, inside the beep, by changing the length of the beeps, so
01:44
either using the one frequency more or the other, they encoded if the pressure dropped under 0.35 bars, or if the temperature went outside the range where they were comfortable with. So, let's take a step back and look at the problem
02:00
more generally, how can you encode data onto a radio frequency wave? If you just sent a sine wave, you basically are sending no information, so you have to make changes to it. So, one kind of changes you can make to a sine wave is changing its amplitude, so, if you want to have a different symbol, you use a
02:21
different amplitude. Another way to encode data is to change the phase of the signal. So, if you look here, in the diagram, you can see that, on the boundary of the symbol, the phase jumps a certain amount, and then jumps back, so we have a change here, and we can encode data with it. But, of course, you
02:42
can also combine both techniques to do amplitude and phase modulation at the same time, and modulation is the name of this technique to imprint information onto the RF carrier. So, now, if we want to have a more, yeah, more
03:02
complicated kind of modulation, usually, what we use is a constellation diagram. So, in a constellation diagram, you can see on the 2D plane usually points, and these points encode the information of amplitude via the distance from the origin, and phase via the angle to the vector from the X axis.
03:24
And now we can just define our valid symbols inside this constellation diagram, and assign bit values to them. So, for example, here you can see we have four valid points, so we have four different symbols, and we can assign two bits to each of them. And now, if we want to transmit a series of bits, we
03:45
just use different valid points from our constellation diagram. So, as you have probably already guessed, so if we want to transmit more information, more bits with the same symbol, we just have to use a constellation diagram which has more valid points. So, for example, here, 32 points in 32 APSK with five bits
04:04
per symbol, so why don't we just use infinite points in the constellation diagram? Here we have to take a look at what happens when we actually transmit the information. So, when you do the transmission, we can pretty accurately hit these valid points in our constellation diagram, but then when
04:22
we receive them, we see that they are not actually there where we were expecting them. So, they're not always hit the same point, so, under some conditions, we can't validly identify the correct symbol any more. The effect we are seeing here is noise. So, noise is changing up the
04:42
point in the constellation, and, at some point, we can't identify them any more. So, what else can we do to increase our information which we transmit? As I already said, if you just send a sine wave, basically, you're not transmitting any information, and, if you look at the spectrum of your transmission, you're just seeing a very, very thin line,
05:03
basically infinitesimal small, but now, when we start making changes, doing modulation, transmitting information, we are seeing the spectrum widening up, and, actually, the more symbols we transmit, the more frequently we change our symbol, the wider the frequency
05:22
range, and, of course, the more we plot gets, so we're using more bandwidth, and that also means other people can use the same bandwidth, so we have a limited resource here, so we can't also not just go infinite with our symbol rate. Another problem we have here is that, if we want to accurately demodulate, so we cover the information, identify the IQ
05:42
constellation points, we have to take into account this whole frequency range, and, normally, what you have is in, let's say, natural noise situations, that you have a noise floor which is approximately equal across your band, so you just have, over the
06:01
frequency, a certain noise band, and the more bandwidth you use into your application, the more you have to also take in the noise within the band, so that means, if you have a high symbol rate, you're changing lots of symbols, you have a high bandwidth, so you're automatically also getting more noise power into your system. So here from the left side,
06:21
where you have the symbols and the noise floor, on the right side, you can see what you can get if you do a bandpass filtering, and you can obviously see you get more noise power if you have a wider signal. So, how to look at this, how to compare different constellation diagrams? Usually, you compare the bit error rate here on the
06:42
vertical axis, so the probability that a certain bit is validly transmitted over something that is called the energy per bit over the noise density. So, what does this mean? As I said, you can vary the symbol rate, you get more noise, but you also get more information, you
07:00
can have a different amount of spectral efficiency, so bits per symbol, and to get a more general view of it, you just compare how many energy you can put into each bit. So usually, you get a curve like here, so the bit error rate drops the more energy you put into your signal, so
07:21
the more energy you have per bit, and the more complex the constellation is, the closer the points in the constellation get to each other, the higher the bit error rate is for a certain EBN0, for a certain amount of energy you put into the bit. Because they're closer together, they're more easily disturbed. So, what can you do to
07:43
increase this energy per bit over the noise floor? So, this is actually at the receiver, so we can just try to focus the signal more into direction we want to receive the signal. So, for a receiver which is sufficiently far away from the transmitter, it actually
08:01
doesn't matter if the transmitter transmits all the energy towards the transceiver, or a higher energy but all around it. So, what we are typically looking at is the equivalent isotropic radiated power. So, the power we are seeing if we are assuming that the transmitter is transmitting into every direction, so omnidirectional,
08:23
and this is basically just the amount of power we put into the antenna times the gain of the antenna. So, if we have an antenna which has a very, very focused beam, so we have lots of energy towards our receiver, we don't need that much input power to get the same result at the receiver. But there is a conundrum we have here. If
08:45
we increase the gain, so the amount of power directed to our receiver, we also have to take into account that our beam width, so the amount of focusing is increased. That means, if let's say we have an attitude
09:04
control system on our satellite, we try to find our ground station, we have to try harder, because, if we just point the satellite a little bit away from our receiver, suddenly we have a huge drop in received signal strength, and so we can't really get the same link
09:22
energy which we had before. So, this is a general thing you have to take into account if you have a more focused beam, you have a smaller beam width, but a higher gain, so more energy on the receiving side, and vice versa. But also, if you increase the frequency for, let's say, the same general architecture of your
09:42
dish, you also increase the focusing of your antenna. So, if you have, let's say, 10 gigahertz instead of 2 gigahertz, you have to try harder to point your antenna. So, what kinds of antennas we typically have on satellite systems? One kind of antenna is the
10:00
quasi-omni-directional antenna system which is used for, let's say, sending commands to the satellite when it's close to earth or so, and also when there is an anomaly on board, the satellite usually tries to gain as much solar power as it can, so it points towards the sun with its solar airways, and it might not really point to an earth station any more.
10:21
So, in this case, we still want to be able to transmit data to the satellite telecommands, or receive information about the satellite status, so we have to have an antenna system which is approximately equally good at transmitting into every direction, so, for higher frequencies, you typically need more than one antenna to get this quasi-omni-directional
10:42
characteristics, so we typically have two into opposite directions. Then we have payload, most often payload, or, yes, communication data, streams which often use high-gain antennas, so they have a more focused beam, get a higher EBN zero, can transmit
11:00
more bits without getting into trouble with the noise floor, but have to point their antenna, so here we have the normal operation, so we assume that the satellite is able to point its antenna, and we can use the benefits of having a high gain for the same power, so having a higher the same power, so having a higher gain for the receive strength. Then there are some, let's say,
11:22
more interesting antenna patterns, let's say you have navigational satellite, GPS, Galileo, something like that, and you basically want to illuminate a whole side of the earth, but some parts, because, as we all know, the earth is round, so some parts of the earth is farther away than you, than the
11:41
points directly beneath you, if you point towards the earth centre, so you basically would need a little bit more gain towards the far sides of the earth, so the earth limb, and so you here have antenna patterns which are called ESO flux patterns which don't actually have the highest gain in the bore side, so directly vertical from
12:03
the antenna surface, but more to the sides, so it has a little dim in the middle, but this is good, because now you have about the same receive strength on the whole planetary surface with the surface. But yes, that's one side, so we now have optimised our transmission, now the
12:22
signal actually has to get to our ground station, and there are lots of things which can attenuate your signal. First of all, and usually most of all, it's just the distance, so if you are transmitting radiation, electromagnetic radiation, the power drops off with the square of the distance, because you
12:40
just spread the energy over a larger and larger surface, which goes up with the square of the distance, so you get much, much lower receive strength if you move farther away, or the satellite is farther away, but then, on top of that, you get other effects, for example, if you have a relatively low frequency, let's say
13:02
sub-1 gigahertz, ionospheric attenuation can become significant, so you get a drop in the signal, or even reflection if you just hit the upper parts of the atmosphere which are ionised, so, yes, charged particles, and then everything in the atmosphere, so, rain is usually a huge problem, if you have a high frequency, sometimes
13:23
maybe you experience it if you have a small TV, set TV dish, then if you have rain or snow or so, you get a significant drop, maybe even so far that you get signal loss. But yes, in general, every water vapour in the atmosphere is a problem, and rain is an extreme case here.
13:41
If you, let's say, are not that good with selecting your frequency, you could also hit just absorption spike within the gases in the atmosphere, so, for example, oxygen has an absorption spike in the tens of gigahertz range where you just really can't get much signal through because just the oxygen in the air is
14:02
attenuating the signal. So, now we basically know how our signal gets there, what is the noise, how is the noise determined. Much of the noise we see in satellite systems are actually generated also outside of the receiver itself. So, as a rule of thumb,
14:21
you can say that everything that absorbs energy at a certain frequency also emits very well at that frequency. So, again, we have our old enemy rain here, which, if you have a rain shower and you try to receive through it, you suddenly get a much higher noise floor which adds insult to injury
14:40
regarding the attenuation before. Another major source of noise in our, let's say, solar system is the sun. So, the sun is not only powerful, as we all had to learn in the last couple of days, in the
15:02
visible spectrum but also in the radio spectrum. So, if your satellite is passing in front of the sun and you have to point, or would have to point your antenna towards it, suddenly you get a lot, lot of signal which can swamp out your signal from the satellite, and you can't track it any more. So, this is an actual problem with satellite systems. But then, if you get
15:22
better, you have very good receiving technology at some point, you hit, let's say, upper limit, because, as you know, there is a cosmic microwave background which is behaving like a black body radiating at about two to three Kelvin, and this is also a certain noise floor which you basically can't get under because you
15:40
will always see it no matter where you look. And then, for certain antennas, if you try to be very efficient, you of course try to use the whole dish you have paid for, then it can be that the dish has certain side lobes and also back lobes, so it's not only that the dish has the main beam focused on the satellite, but also some other direction
16:02
you might get reflected energy into your dish, and then the earth surface becomes significant. So, if you have a dish which tries to use much of its area, you often have bigger side lobes, and you get a problem if you're pointing to low elevations because suddenly the earth surface radiates into your antenna and increases your noise floor.
16:22
So, maybe we can just take a look in this example satellite, so what I'm showing here is the uplink path, so let's say from ground to satellite, maybe to transmit telecommands, so what has the satellite to do, or things like that, and let's see how the signal is received. So, at first, on
16:42
the input antenna, we have a bandpass filter which is relatively coarse, it's basically just there to split apart the parts of the frequency we use to receive data from the ground from the parts we use to transmit to the ground, because otherwise, as soon as you would switch on your transmitter, you would just swamp your receiver.
17:00
After that, we have here a low noise amplifier which is used to increase the power you have on your incoming signal. It has to also travel through all the attenuation, the free space, so it's relatively low, and you have to boost it, so you can use it in the further stages when you make changes for it, and the first change you are doing to
17:21
it is this here, this is a mixer, just in case somebody hasn't dabbled with RF electronics or so, what does the mixer do? Basically, you can input signals to a mixer, often two, let's say an information-carrying signal, and one basically a sine wave, and what comes out is the difference and
17:42
the addition of the two frequencies, so typically, out of such a simple mixer you get the carrier itself, okay, now here down there should be the original baseband signal, and then plus and minus the frequency here of this information wave, and now, if you just need one of them, you can filter it,
18:01
and then have your original signal on a different frequency, so what you're doing here basically is just getting the 7, 8 gigahertz signal you got from the ground and mixing it down so you can more easily decode it, so afterwards you have this bandwidth filter again
18:21
filtering out unneeded parts of your mixing process, and you have another down mixer, so you have a two-stage down conversions here, so now afterwards you have changed the frequency your signal occupies from your very high couple of gigahertz range down to
18:41
basically zero hertz and a couple of megahertz, or even kilohertz, then you have another gain stage before you go into your analog-digital conversion and do more changes and detection in the digital domain, what is interesting here is that based here on your controller, it also determines how much
19:00
energy it sees on its input and also within the small, small band pass of your input filter here where the carrier actually is, so where the signal is transmitting on, and you can use this to form a tracking loop and adjust the frequency it puts into the mixer here, so it can adjust for the
19:23
let's say wrong frequency on a receiver because it just tunes until it still sees in the middle of its receiving band, so this look in practice, so typically, you have a ground station which is trying to transmit on a certain frequency, but then due to
19:40
for example doppler shift or an agent oscillator on your spacecraft or whatever, the spacecraft sees this signal on a different frequency, and now if it has just a small box where it can decode the signal, of course, this wasn't, it wouldn't work, so what the ground station does in the beginning of a pass is it sweeps, so it just changes the signal around, and at some
20:02
point, if it sweeps far enough, the signal is actually within the box of the input filter on the spacecraft, and then this tracking loop activates, and from there on, the spacecraft receiver always tries to keep the signal within the centre of its box, so now even if
20:21
the sweep continues or stops, it has still tracked the signal and can now, if the ground station starts to transmit information, it can extract the information always at the same frequency offset. So now a different example for a different kind of satellite, we have a broadcasting satellite here, so what you would
20:41
maybe receive if you have a satellite dish for TV at home, so typically what we have here is again a bandpass filter for input-output splitting, then an LMA, so a low noise amplifier to just boost the signal a little bit so you can work with it, and then everything, the whole band with all the TV stations is mixed so it's
21:01
offset in frequency by a certain amount. Here you don't have a closed-loop tracking, it's just always the same amount that's offset. So now the ground station would have to account for that if the oscillator ages or so and changes its frequency, it would have to be changed on the ground station side. What you then have is an input multiplexer which basically splits
21:22
the spectrum you get into it into certain transponder ranges, so a transponder is let's say a channel on your satellite and within this channel let's say 36 megahertz was the standard especially for satellites which were once designed to transmit analogue TV. These portions are split
21:41
within the input multiplexer and each of them are fed through one of these amplifier elements with a bandpass filter afterwards, so they are independently boosted amplified and now these amplifiers are power amplifiers so what you get out of it is a very strong signal which is able to travel back the 36,000
22:02
kilometres to the earth's surface and being received by your satellite antenna. Then we have the output multiplexer which combines all these different transponder bands again to one single channel. You have the isolation via bandpass filter. Remember now we have a different frequency because of the mixing here
22:21
so these frequencies here are different. They can't re-enter your satellite receiving side and now they're radiated out of your antenna dish or array whatever. As you might have noticed here we don't really have much digital electronic here so the classical broadcast satellite is also called the bandpipe satellite. It just
22:42
takes everything that comes from the earth, mixes it, amplifies it and sends it back. It doesn't concern itself with what kind of signal is used. Let's say if a TV station wants to change its video encoder from MPEG-2 to MPEG-4 it doesn't care. Even it would just start to send radio or switch from analogue to digital TV.
23:02
It doesn't matter because the satellites are pretty dumb, at least the broadcast satellites. But this is an advantage because here you can make changes on the ground side and in 15 years or so the satellite is in orbit it's still able to perform its function. On the other hand remember the noise problem. The
23:22
satellite here already sees on its antenna side noise coming from for example the earth surface atmosphere and so on. So the signal is already hampered by a certain noise floor when it's being received at the satellite. And then it is amplified, the noise is also amplified, everything is sent back. So now you pick up
23:41
additional noise on your way back to the earth and of course on your earth-based small antenna. So this system is not ideally ideal if you want to use a very low noise system because it just amplifies also the noise it receives. The other thing would be a regenerative satellite which actually tries to decode the
24:01
data it sees, does error correction, re-modulates the data and sends it back. So I already mentioned forward error correction. So forward error correction we have to do to prevent that the noise we are getting is actually completely destroying a satellite, because unlike let's say
24:21
IPTCP, we can't usually ask for a retransmission. Of course there are things where you can, but let's say for a TV satellite, you can't ask the satellite, everyone can't ask the satellite to just retransmit the last frame, so you try to make it very robust and that you do via forward error correction. So what kind of
24:42
forward error correction could you do? For example one type of forward error correction, you just take your bit stream or your data stream, you chop it up in blocks, each block gets additional data, so we add some redundancy to the signal, and then it's put together again and transmitted. Another thing
25:01
you could do would just take the input data stream and produce a completely different output data stream, and one example for this I will now show you. It's called convolutional coding, so it's a convolutional coding. You have here on the left the input, your
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data stream, and it's fed into a series of shift registers so every time a bit enters here, everything goes one to the right, and then your output is actually not your input bit stream, but a series of two symbols for each bit, each of them a result of a series of
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exclusive or operations here, depending not only on the input bit, but also on the state, so in the shift registers. And what this does is basically the information on a bit is smeared over a series of consecutive bits, so if you then get a problem in your reception, so you maybe
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lose data, you can still infer the content of your bit from the other bits you received correctly. But on the other side, for each bit you input here, you get two bits out, so you actually double the amount of bits you have to transmit, and to counteract that,
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one technique would be a so-called puncturing, where you have a fixed pattern of bits you just omit. So you can say, okay, now I have this factor 2 due to my convolutional coding, but I now just drop, let's say, every third bit or so, and the receiver knows this, so it can just assume, okay, this is 0.5, let's
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say, and then fix it in the decoding process. How can such a decoding process look? One example would be here, Vertiby decoding. This is using, I won't completely explain it here, but using the fact that within your shift register and an incoming bit, you only have certain allowed state transitions
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between the shift register. So, for example, if you have a shift register state which is all zero, you can just go into exactly two different states with the next bit, which is still all zero, or one, one, and the others are zero, and from this, you get exactly two possible two-bit combinations
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which are emitted from your convolutional encoder. So, in the other way around, if you receive a symbol, you can make a guess, and the more symbols you get, the better guess it is, what kind of path the signal takes through your allowed state transitions. So, in the end, after a certain amount, you are fairly certain that you know which kind of sequence
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you received. So, what the effect is on your bit error rate graph is basically that you get the kind of coding gain through any kind of forward error correction. So, when you look at the bit error rate graph without coding, and with coding, you basically get additional energy within your
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signal which you don't have to really put into. So, if you are further interested in different kinds of space communication, two things you might be able to go, one is the CCSDS standard sets which is basically a committee which
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exclusively deals with standards in space data exchange and communication, so there you can find information on the nitty-gritty details of how bits are encoded on satellite telemetry and telecommands, so let's say housekeeping data, temperatures, battery, voltages, and so on from satellites, or telecommands,
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so how to command the satellites. Quick word of warning, actually, it's more like an option sheet for your communication, so typically for each mission, they select a certain amount of, I want to have this forward error correction, this bit rate, and this modulation type, so it's not all, let's say directly
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compatible, you have to have some kind of additional knowledge to decode satellite telemetry and telecommands stream. The other thing is the ETSI standards for DVB-S2-S2-X, which is basically the broadcasting standard for satellite TV or set IP if it goes over a geostationary satellite, so all these
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things you can find in these standards. And with this, I'm at the end of my presentation, so thank you for your attention, and I'm open for questions. Thank you very much for the interesting talk. We still have plenty of time, so if you have any
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questions that you would ask our speaker, please move to our microphone angel who is standing there as a living microphone stand. You just walk over there and you ask your question, and we'd be very happy if anyone would feel ready to ask something, because as I mentioned, we still have plenty of time. I'm hoping that this is a question that is moving
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here in the second row. Yes. Hi, thanks for the talk. I have a question regarding the sweeping for the, for locking to the frequency. You said that the sweep is done on the transmitter side. Is this common? Why is this not done on the
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receiving side? Because the receiver would know when the, when it loses track of the frequency. Yeah, actually it's usually done because communication with a satellite and the sweeping is typically done for operating a satellite, so telemetry, telecommand, not so much for, let's say, broadcasting satellites. So it's initially,
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it's initiated by the ground station. So typically, the ground station has the much more complex systems, so it can more easily react to changes in the, in the, let's say, transmission stream. So it starts the transaction basically by starting the sweeping process, and then what often happens is that the satellite maybe wasn't
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even transmitting before, but then when it sees this energy on its input band, it also starts transmitting itself. So just in case you didn't program it to transmit in time. So it's just done on the receiving side because that's more easily controlled by the ground operation crew. Thank you very much.
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Do we have any further questions? Don't be shy. It's not every day that you have a specialist for satellites in the room that you can ask pretty much anything. Nobody? Yes, please do move to the microphone. Thank you. Oh, and there's someone else. Okay, so, but you first
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because I saw you first and then you. Thank you. Do you know if there is a specific protocol for space-to-space communication or whole protocol are space-to-ground or ground-to-space? Yeah, typically,
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for example, CCSDS standards has this whole option sheets of communication and if you don't want to use the options for space-to-ground communication, for space-to-space communication, there are also some standards there you can look at. So for example, for advanced orbital systems or inter-satellite links.
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The next question from this microphone, please. What special encodings would you do if you have to transmit something really far away like to the New Horizons probe or to Mars? So there you probably would use a kind of code which is very close to the theoretical optimum.
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So from information theory, you have a theoretical optimum you can get for a certain signal-to-noise ratio and there you probably use turbo codes, for example. So I didn't present it here, but turbo codes are basically a set of encodings where you have on the encoding side two encoders working in tandem,
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so each producing a data stream then on the decoder side you can use this to infer even more data by doing soft decisions on the encoder. Or another example would be which is more let's say from the broadcasting side where they selected I think also because the patent was lapsed at the point was low density parity check matrixes,
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but there you have the problem that you need very huge blocks. So for example for satellite TV typically the block size is about 65k bits you have to transmit as a whole to get this high gain in the encoding side. So it's probably not optimal if you just want to send a couple of bits to your probe.
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Thank you very much. Is there a further question? No. Is there a question from the internet? Also no. This is your last chance. Any other questions? Yes. Hi. So what's the nature of the noise? Is it like across the spectrum? Is there any ways
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that you can like make the signal more resilient to noise or interference? Yeah. So I didn't talk about interference because typically if you have let's say a professional GAN stations you build it in the valley so you don't get that much interference from let's say Wi-Fi access points or so. Actually if you are
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designing a satellite system often you assume if you are building it for let's say broadcasting or point-to-point links that it's about white noise so just equal across the spectrum. Of course if you do something else let's say a GPS-like system so radio navigational satellites you have to account
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for different kinds of noise or interference for example in urban environments you get the multipath propagation when you get a signal directly from the satellite and another one reflected from the Skype scraper next to you and then you need different techniques to accommodate for that. Thank you. Further questions? I'm under the impression
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that we are running out of questions so thank you very much for all your questions and please give another warm round of applause to our speaker. Thank you very much for answering all the questions.