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The Limits of General Purpose SDR devices

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The Limits of General Purpose SDR devices
Why an SDR board like a USRP or LimeSDR is not a cellular base station
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It's tempting to buy a SDR device like a LimeSDR or USRP family member in the expectation of operating any wireless communications system out there from pure software. In reality, however, the SDR board is really only one building block. Know the limitations and constraints of your SDR board and what you need around it to build a proper transceiver. For many years, there's an expectation that general purpose SDR devices like the Ettus USRP families, HackRF, bladeRF, LimeSDR, etc. can implement virtually any wireless system. While that is true in principle, it is equally important to understand the limitations and constraints. People with deep understanding of SDR and/or wireless communications systems will likely know all of those. However, SDRs are increasingly used by software developers and IT security experts. They often acquire an SDR board without understanding that this SDR board is only one building block, but by far not enough to e.g. operate a cellular base station. After investing a lot of time, some discover that they're unable to get it to work at all, or at the very least unable to get it to work reliably. This can easily lead to frustration on both the user side, as well as on the side of the authors of software used with those SDRs. The talk will particularly focus on using General Purpose SDRs in the context of cellular technologies from GSM to LTE. It will cover aspects such as band filters, channel filters, clock stability, harmonics as well as Rx and Tx power level calibration. The talk contains the essence of a decade of witnessing struggling SDR users (not only) with running Osmocom software with them. Let's share that with the next generation of SDR users, to prevent them falling into the same traps.
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Point (geometry) Axiom of choice Digital filter Game controller Dressing (medical) Oscillation Frequency Chain Computer configuration Phase transition Network socket Personal digital assistant Noise Loop (music) Scale (map) Execution unit Characteristic polynomial Public domain Term (mathematics) Computer Oscillation Digital-to-analog converter Arithmetic mean Frequency Sample (statistics) Computer configuration Integrated development environment Personal digital assistant Analog-to-digital converter Permanent Atomic number Normal (geometry)
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Computer virus Multiplication sign Execution unit Determinism Insertion loss Tracing (software) Mathematics Military operation Ontology Extension (kinesiology) Logic gate Workstation <Musikinstrument> Block (periodic table) Moment (mathematics) Sampling (statistics) Streaming media Bit Group theory Computer Digital-to-analog converter Product (business) Latent heat Frequency Sample (statistics) Chain Order (biology) IRIS-T System programming Mehrfachzugriff output Whiteboard Quicksort Directed graph Firmware Point (geometry) Frame problem Functional (mathematics) Duplex (telecommunications) Cellular automaton Division (mathematics) Device driver Entire function Theory Product (business) Chain Latent heat Propagator Causality Peripheral Whiteboard IRIS-T Software Computer hardware Mehrfachzugriff Lie group output Information Duplex (telecommunications) Interface (computing) Software-defined radio Division (mathematics) Stack (abstract data type) Device driver Timestamp Component-based software engineering Software Analog-to-digital converter Computer hardware Function (mathematics) Synchronization
Intel Context awareness Euler angles State of matter Scaling (geometry) Multiplication sign Real-time operating system Mereology Mathematics Component-based software engineering Different (Kate Ryan album) Buffer solution Befehlsprozessor Core dump Logic Aerodynamics Abstraction Logic gate Physical system Area Logical constant Software developer Sampling (statistics) Bit Group theory Control flow Computer Measurement Element (mathematics) Entire function Digital-to-analog converter Product (business) Latent heat Sample (statistics) Frequency Buffer solution System programming Website Stochastic kernel estimation Whiteboard Freeware Physical system Firmware Point (geometry) Ocean current Frame problem Game controller Open source Patch (Unix) Control flow Device driver Microcontroller Streaming media Heat transfer Entire function 2 (number) Product (business) Chain Peripheral Whiteboard Band matrix Software Computer hardware output Helix Analytic continuation Game controller Execution unit Interface (computing) Projective plane Field programmable gate array Software-defined radio Device driver Component-based software engineering Software Personal digital assistant Analog-to-digital converter Function (mathematics) Computer hardware Network topology Universe (mathematics) Synchronization Scheduling (computing) Abstraction
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Logical constant NP-hard View (database) Multiplication sign Correspondence (mathematics) Execution unit Workstation <Musikinstrument> Public domain Function (mathematics) Mereology Proper map Food energy Data transmission Measurement Mathematics Envelope (mathematics) Network socket Personal digital assistant Aerodynamics Information Data conversion Distortion (mathematics) Physical system Workstation <Musikinstrument> Adaptive behavior Digitizing Sound effect Range (statistics) Bit Digital signal Food energy Term (mathematics) Flow separation Measurement Filtration Arithmetic mean Frequency Order (biology) Chain System programming Linearization output Right angle Energy level Whiteboard Resultant Point (geometry) Filter <Stochastik> Spectrum (functional analysis) Digital filter Game controller Open source Cellular automaton Limit (category theory) Mass Power (physics) Frequency Propagator Term (mathematics) Musical ensemble Harmonic analysis Energy level output Dynamic range Absolute value Form (programming) Scale (map) Execution unit Scaling (geometry) Variety (linguistics) Weight Projective plane Software-defined radio Limit (category theory) Power (physics) Transmitter Software Function (mathematics) Analog-to-digital converter Mixed reality Normed vector space Transmissionskoeffizient Musical ensemble Table (information) Spectrum (functional analysis) Distortion (mathematics)
Spectrum (functional analysis) Digital filter Execution unit Variety (linguistics) Causality Personal digital assistant Statement (computer science) Cuboid Software-defined radio Limit (category theory) Physical system Measurement
Axiom of choice Multiplication Multiplication sign Electronic mailing list Device driver Mereology Coprocessor Proper map Entire function Transmitter Frequency Moore's law Software Network socket Chain Transmissionskoeffizient Right angle Extension (kinesiology) Arithmetic progression Spectrum (functional analysis) Form (programming) Computer architecture
Computer virus Filter <Stochastik> Musical ensemble Mobile app Open source Multiplication sign Range (statistics) Group theory Set (mathematics) Proper map Product (business) Frequency Goodness of fit Component-based software engineering Internetworking Hacker (term) Computer configuration Term (mathematics) Videoconferencing Software testing Default (computer science) Dependent and independent variables Distribution (mathematics) Duplex (telecommunications) Nichtkommutative Jordan-Algebra Projective plane Fitness function Bit Proof theory Software Factory (trading post) Website Whiteboard Thomas Bayes Spacetime
Finite element method
it's a. a even know what the problematic he was fixing the l.t.e. networks here at the event so hopefully it's over.
working know now yet slight little bit of background about myself i've been working on free and open source software and open source hardware for more than twenty years now i use to tool in its kind of work for that's a long time ago now and also to the bit of enforcement of the general public licence but that during the last decade.
eight i'm primarily involved in various projects around the open source mobile communications and as part of that work and we see lots of people who'll also want to play of course with the author malcolm stack or without the software that works the ever suffered find radio. and we see lots of problems and i thought i have tried to make a summary talk on what the limitations are and what the problems are and to make sure people don't always fall into the same traps so first of all suffer to find radio well and lots of people associate that term prime. i really with certain devices that are used in the hacker and the icy security and research communities. but in reality actually almost all radios today are self-pity find radios and then that's why i'm talking about so-called general purpose softer to find radio is where the non general purpose s.t.r. devices for example your mobile phones mean the internally use the concept of self-pity find radio but they don't.
expose the capabilities of that concept to the user and to you it doesn't matter whether it's a bit limited in hardware and software or in some combination thereof it's a black box and you cannot really modified in any reasonable way and most of this software to find reading devices that are not general purpose such us phones or d.v.d. receivers on. as i said virtually any radio. forms those essential signaling crossing steps and software that's why it's called self-pity find radio but that happens inside you speak or stupidly inside if p.t.a. c n n most often with proprietary but stream of from and and then on the other hand we have to so-called and or i called in general purpose as the are bortz where you basically have. of some device that doesn't do much more than the up and down conversion from radio frequency and the envelope which of the conversion using a p.c. or back in hardware and then you get a span samples into your p.c. or whatever general purpose computer and typically its eighty six arm devices the. these days and you do all this offer processing and in the general purpose offer on general purpose of you without the use of the species or anything like that. so having set the terminology if we look at that number of key projects related to general purposes suffered a fine radio while i was started with in two thousand one window radio was founded.
launched by eric lawson at the time was a very generous ground to a donation via john gilmore tool create a softness you'd which is very popular even until today even more so today tool use general purpose suffer to find radio devices in two thousand and four met at a swiss year. with us at the camp launched the first u.s. r.p. device the universal self the radio peripheral. and then you can see in the last to ten years or seven years is still lots of new devices are software that enables devices has been published many people will have heard about r.t.l. s.t.r. devices which is basically use b d v d don't know which can be used to not do the actual.
the digital video broadcast the the modulation in hardware but simply stream you samples into the p c m and he used to manage to receive signals on companies like new and have launched the plate our effort of a kick start of michael osman very well known in this community launched a hectare of one. two thousand fourteen and for example the two thousand seventeen fairly recently lie micro has launched a lime s.t.r. device the first one and crowds of light and so on so we have lots of projects in that area some proprietary some open hardware and free software projects and the related to his have been. more every little more accessible which is great because it means everyone can play with the technology and very low cost devices like r.t.l. s.t.r. allow you to be key to only receive an where some more higher cost devices have received and transmit capabilities and a lot of people from i see security or. your software or networking background have looked at the s.d.r. devices in various different ways and and leading curve is still rather steve unfortunately if you don't have any are far and electronics background this quite a number of free open source projects around the weather related to an unrelated. google radio and unfortunately not many of them are very easy to use and there's still many many many different protocol systems technologies radio technologies which are without any or this highly incomplete free and open source software so that sort of the state to school and now we will look at actually the un in.
the details of what those limitations are that i've put at the title so yeah it's great that all of this excessive so as i said in reality unfortunately some people have misconceptions about the complexity involved and if your think of if somebody thinks that buying at random s.t.r.. selling some free open source software it will enable you to run a soul of a station for example then well it's unfortunately not that simple i wish you were but in reality it's not and it's particularly not the case if you're interested in selling technology such as she is empty furious you m.p.'s l.t.e. five g. new radio or any of that and. why is that the case well as many different topics i'm going to start with the topic around clocks have several different topics related to clocks. the seller standards all have very strict requirements on the precision of the carrier clock so if and a base station is to transmit on a certain frequency that has to transmit on that frequency of to have very high degree of accuracy typical requirements in this area about thirty parts per billion. and that's unfortunately about a factor of thousand off the accuracy that to the normal crystal oscillator so than on the crystals have that you find in an expensive devices so unama crystal may have twenty ppm parts per million various the requirement actually say something like thirty or fifty parts per billion. million. some devices have so-called to seek solace that's temperature compensated cross lost the latest which then have around two hundred eighty p.p.b. stability which is nice to much closer to what the specs says but still is a factor of ten or something like that off and and having a clock that's.
often means all kinds of strange effects on the most commonly known one is that phones may not detect your network even though it's broadcasting order may not reliable detect a network or they may even drop off the network though the last last part is less likely and this is the number one problem that we see even people play with so little. knowledge the year with s.t.r. as they see some unreliable behavior they cannot make it work at all and the number one question is well do you have a proper o'clock and if the answer is no then basically it's sad to say but one can stop even trying to assist our to help at that point because there's so many problems that can follow as a result of that and so that's. the accuracy of the clock at the next point is stability of the clock out an because the calibrating the clock once to be precise to be calibrated is nice but then the clocks drift that to temperature to aging all kinds of effects and that means you would need to calibrate the clock against a proper reference.
they're quite frequently and because it depends on how stable the clock is from their point of calibration to a point in the future how quickly it drifts from that calibration point and in receive only use cases where you just receiving some signal you can basically do all that with some fancy mass and post-processing you can you have to trek. the carrier clock anyway and you have to make the window of a wider to follow that that's fine i'm but it also only works because the transmitters typically are required to be highly precise so the receiver clock can drift but the transmitter at least is known at a fixed frequency but as soon as you start to transmit particularly in the cellar area you need is hyper. decision and clock as a position and stability another aspect of clocks is a phase noise which translates into twitter in the digital domain. and it's a problem because said the face and voice and can distort basically any part of the signal and is some devices that even a problem where an you'll have a very small no no proper on all decent appear they look filtering after the. reference clock that you feed into a device and then basically you a sample offsets when you sample a signal from the analog to digital domain it's required that your sample times are in precisely equal distant points in time so that the digital representation of to signal actually represents the analogue signal but if your sample times i. our deterring around then you get completely bogus the results and then our kinds of the mess in your signal so there's some devices that we have seen that are more exposed to this such as the lightest the army which doesn't really have an external o'clock include any way you have to have sold or it and change or his sister and put the.
i think a u f l socket on and so on so it's maybe not the intended use case and options do what kind of options do you have to fix this well you can go for socolow six also doesn't have a nice dress lost a litre of an iced means basically you put across the in some enclosure.
and you heat that's to a defined temperature inside of the enclosure and you have from insulation to the outside of the enclosure so the point being that if your room temperature new environment temperature changes the crystal inside stays at their constant or almost constant temperature and thereby you can com you can basically avoid all the temperature related. the rest of us the latest and good all seek souls sufficient to calibrate them once every six to twelve months and then you can run with stable frequencies and that's a nice but still you need to recalibrate and yet so option that's a relatively inexpensive for the many people use is that she's g.p.s. discipline oscillate. this g.p.s. discipline oscillator basically has an awesome later that can be in all six or of easy to see if so that's a voltage control temperature compensated crystal oscillator which is disciplined or steered by the clock reference that is derived from the g.p.s. signal you can do that also with other satellite navigation systems they also. will receive us that can do it with galileo ago when i saw something galileo had some trouble this year but most commonly it's a g b s t o that people use and that's extremely stable but of course requires you to have permanent g.p.s. antenna as which may be inconvenient if you don't you're not in your we know or something like that. and it also requires permanent devious coverage and what if people are jamming aid or young tennis broken norm depends on your environment and your circumstances whether that's a choice the more exotic choices are then it would be awesome latest which a very stable much longer stable than all seek solace you don't need any external reference this there.
rather large and power hungry devices and you can find them relatively inexpensive second hand on e bay or ham radio circles about yates bollock he could power consuming large not really something that you would want to carry around a fancy option our so-called chip scale atomic clocks they exist but there too. expensive for most use cases even professionals fia soul not really something for the general actor playing with s.t.r. it's also important that are some for amateurs of that the reference clock must the full feeling order to be compatible with your device and.
the most important part is the frequency homes actually all important but the frequency is and sometimes the problematic especially if it was to have different s.t.r. us like i have someone to ten megahertz reference others want to study point seventy two megahertz reference some of us want to forty megawatts reference i'm sure that other reference frequencies that they. and an end so you cannot just by a singer g.p.s. the old that operates a single frequency but then you may need some other devices that connects five hundred p. and else can actually generate the frequency that you need from the frequency that your g.p.s. to your generates another question is whether you have need to sign or square waif and that. fans on the clock in put off the device and some can most typically if the device except to sign away from and you have a square of its rather easy you just need a low cost food and then the sideways everything that remains but it's the other way around and then it's problematic so if the device once a square weight important you only have an oscar later provide sign away. then it you need a special device to convert for from side to square with as little to address possible because that again the children across your problems and last but not least of all touch must match. also something that you have to keep in mind almost no actually all as the arse that i have worked with require you to actively select the external reference frequency input it's not sufficient to just connected but it needs to be selected and that's election to be some parameters that might not be easily exposed and you might have to teach whatever application that you use. going to pass this parameter dollars through the driver chain into the hardware so that actually the external references selected and used son devices provide the luxury of having an elite the next to the input to indicate to that this is the clock include now active and but unfortunately it's not yet industry standard to do so on one important factor. i am also to consider if you want to work percent of the technologies almost all of them require full duplex operation and full duplex means that receive and transmit must be active at the same time similar dangerously in different frequencies and that yeah well it's sort of obvious but it means that for example you cannot use the heck are.
if as edgy his m.b.a. station or as an l.t.e. base station because it's not a full duplex device it's only capable of eyes are receiving or transmitting and some people say oh why cannot i combine two of them. yeah his theory but then we have some other requirements that we will look into later which require that both of them are synchronized to a large extent and yes in theory you can do that but i think by the time you're done with making that successful you won't have any time to actually work with what you originally wanted to do it.
and one other important requirement in when it comes to s.t.r. us is a time stamp com ons on time stamped of transmittal receive them and and that means that it's not sufficient that you can just you know you create some samples on your p.c. and ustream. them out over the radio but she actually one tool precisely define at which particular point in time a given sample or which represents the start of a burst is transmitted over the radio and vice versa if you receive signals it's very important to know the precise time at which a certain sample or the first sample of a block off. samples has been received and because that is important information for example in g.m. an l.t.e. that information determines among other things from which phone that is actually transmitted because there is time division multiple access happening so it's important to have this functionality that and despite the fact that many people. we want to play with seller technologies and as the iris it's unfortunate are still not a standard feature of a new products are introduced so quite often when new devices come on the market they elect this functionality so you cannot use them at all of us cellular technology. and that need support basically across the chain from the s.d.r. hardware to the gate where whatever runs on the f. p.g.a. that's in the device if there is warmed to the firm very off the device the hosts software and the driver sec everywhere this time stamps need to be passed around to back and force in both receive in transmit side and you also need. the sink between our recent weeks. and which means that the you need to relate be able to relate the clocking of the receiving transmit samples and you need to know that a certain moment in time represents the receive signal at the antenna input and not to receive input at your a t c. so you have your antenna important and you have some loss employee fires of whatever kinds of blocks even p.c.b. traces which all causes delays in the signal propagation. so you actually have a delay between when your a g c s r u d a c. is out putting an analog signal until it appears that the antenna interface and vice versa a delay between when the received signal hits the antenna and until it is actually digitized by eighty c. and those delays need to be measured and you need to know them. and your software needs to know about them because the suffer needs to compensate for those analog delays and and these values are different from every board not every unit of a given the virus but let's say if you have a u s r p two or u s r p one or a use for p.b. two hundred or a lie as the are each of those boards. products has a different value that needs to be measured in and which the software needs to know to compensate for this analog delay that's in the signal change and if you attach it to additional external devices and actually that also needs to be compensated for in order to be a compliant with the reef. quiet specifications. and sometimes that's sort of a bit of a problem because new products try to emulate all driver interface is so let's say a new product comes on the market is compatible with the driver infrastructure that existing products had by then of course the compensation value or this is still a value that you use is not known yet so.
the value for another board is used and that of course is wrong at that point until somebody actually said stalin in measures that delay and up sense a patch for the software and so on which brings us to develop a vendor drivers were if you're buying a general purpose s.t.r. what you're buying a on obtaining must not be buying but anyway you getting not just her. i'm very proud of course lots of software and software that runs on your host p.c. software runs in the us p.g.a. in the gate where in the control over their it's an microcontroller or a soft core on the device and all that software unfortunately often at least initially has a quality that may might be much worse than the hot air quality. t. and then lots of vendors have their own to actually every vendor i would say have their own different driver second a p eyes to talk to the s.d.r. us and the a.p. eyes often are not stable across releases soul every couple of months as something breaks subtly in the a.p. eyes on which doesn't make it as things easy a soul. that means that software in general needs to support all those different drivers and follow up with all the changes that the introduced in the interface is it's not particularly nice and unfortunately there's also non-technical reasons.
this case i think one of the well known examples is that the e.u. h.d. drive a stick which was the universe low post driver use our people strive ordinarily his main time to entrust labs which is not part of native interest than once and they don't they won't murcia drivers for competitors so if somebody else produce a product and. this is an open source project u.h.t. right and but the official tree they will not emerge drivers for other the products from other vendors and which i think is unheard of if you look at other software projects i mean i've said i have a beautiful in its kind of background and i've never heard that somebody who is maintaining a given current subsystem and made. might be on a into its payroll is refusing to much drivers said that have produced by and dior made for him to the hardware so i think it's quite a bit of a strange an attitude in the context of free software here of course somebody could just come around and four kids and merge almost driver spot to yeah they are. works but everybody has their own fork and then you have additional a us like so p s t r which try to unify all these different a.p. eyes but then the state gets deeper and you have another layoff abstraction and the layoffs obstruction might make it more difficult to control some aspects of the hardware so yes it's good but it all has its the. some positive and negative size and sometimes one thinks the world could be much simpler and another in all areas of software development and i think there has been more standardized nation on common a.p. eyes and and common interfaces rather than everyone rolling their own and i'm all these compatibility issues. another important topic. is related to a so-called overruns in and around its soul. in the end what you have is a when you run an s.d.r. is the transmit site expects and continuous stream of samples to be transmitted and the receive site generates a continuous stream of samples that i received and your entire system and hardware software peripherals whatever needs to be capable of. sustaining that at any time so every time one component of whether it's the software hardware component is not capable of consuming and producing the samples in time you get dropouts leading to buffer and around spot for overruns you may know that from no audio related work maybe not today with today's computers but if you. been around you know ten fifteen twenty years ago and you were playing with digital audio on the seas then you may know some of those problems that you get overruns in under runs and the speed is not sufficient tool to keep up with the real time requirements of such abuse cases well that was odd your with let's say forty four kilo hurts of a sampling frequency now we're talking about. it's the are we talking about mega samples so we're talking about millions of samples that need to be processed saw the requirements are much stricter and much more. the demanding and then all of these different devices they all have a first because they're not really designed for you know continue streaming of transfer and those staffers then again a a buffer containing a hundred samples means your stuff gets delayed by hundred sample so he introduced.
clayton seas and the aggregate number of offers you have all over yourself and harvesting determines what kind of systems you can implement because some of them may have rather but rather a strong requirements on what the maximum latency must be between receiving something in transmitting something that depends on whatever you have received before. so you cannot implement three g. base station for example of a real one and with an s.d.r. that's attached over us be because he was p.'s just the latency is way too high you can never you can never do the timings and even for a t e it's actually rather amazing what projects like s.r.s. l.t.e. are doing that they can. and do this within the string and requirements of l.t.e. despite wasting so much time of moving data overuse be back and forth. one thing that can help a little bit is using real times crisis get your priorities get our are on linux and and you also have to pay a lot of attention tool a dynamic problem management systems which may change your see if you speed and frequency and so on and run time so better disable all these things if you want to have a reliable operation. and with the and s.t.r. based system and also we see problems from bias is sometimes where you know let's say you exception external screen and then some system management called on its eighty six fox tries to do something in meanwhile year dropping millions of samples right or you put your system.
in talking simply attach a power supply and then something happens in the deep the downturn system management mode that interrupts the normal process think and again some samples are gone and you get over runs in and around and another topic to look into his the that transmit gain settings because well. thinking as you are stressed and eighty c. and attack is too simplistic of course you have all kinds of animal components next to it and that's the uptown convert to and you have maybe some filters or whatever and you have to gain stages so you in the transit side you convert from digital to analog and then you have different gain stages and all those gains stages. you can configure normally am from software and and you need to configure all those gains station is that properly in no way that they operate with a linear correct risks and they don't distort the signal of because if you over half are too high signal at the input off the next stage you will get clip.
being all kinds of weird effects and that the stores to signal introduces harmonics introduce out of brand interference that didn't stop to war brought cost and it is so you need to basically twiddle all these settings again for each and every s.t.r. device that. you want to support from software and basically to all those settings manually until the signal looks best and you hit the sweet spot where the signal looks best and with some devices you may not get to the point of to signal ever looking the way how a professional measurement device for the given telecommunication system would actually. practices signal to look like. and then of course all of those settings of out again stages they depend on the frequency and or the band in which you operate so used to and all those parameters and figure them out correctly and then you change the frequency and you start again to fiddle with all those patterns in order to get on of them set up properly. a it which brings us to other things on the transmit side which is the power level so normal general purpose as the our device typically outputs only what's your own t.v. and maybe forty b.n. ten b.n. and that's very little power which is good if you trust to lap test.
on a desk but if you actually want to do real operation and then of course you need some kind of amplification means simply needs to be amplified so while you go on e bay and you find lots of inexpensive follow up the fires for the brand that you're looking at and so why is that not a good idea it's well the output signal of us tr. already contains normally quite a significant amount of harmonics that's basically your twice the price four times and so on the frequency of that year transmitting and if you just put an amplifier behind that the amplifier will transmit also and will amplify although also all those harmonics and so you'll normally want to put proper filtering before and. if necessary also after the amplifier so you then have u.s.t.r. and already three edition of blocks on your desk and the clock and and so on. one and then actually it's more complicated than that because an amplifier is not in general and live their part meaning it does not employee fi any signal the same way like another signal a part of your signal and then it am particularly the more affordable amplify us. not so linear and that trade offs between power consumption linearity so you can build highly linear amplifiers that consumes tons of power or which then means you need to cool them and so on or you can have less linear amplifiers and particularly signals that have a high peak average picture average power or a show that means to change the amplitude. while transmitting am it gets distorted a lot by nonlinear it is in the amplifier and if you look at qom maschler that base stations that actually implement all the systems you have what's called digital adapted for pre distortion to compensate for the nonlinear and he's so basically you output and signalled you put it in your amplify and. then you digitize that again from the analog to digital domain you measure what's the difference between my intended signal and the actual signal that the output of the amplifier and then you do all kinds of mass to predict stuart you actually did as so you actually digital signal before you convert to analog and feed into the amplifier and you do that as a continuous loop because all of that have.
this depends on temperature and aging and all kinds of other net nice effects and i have heard i mean i'm not involved in in those kind of projects but effort several times that at least a couple of years ago several times that the the pre distortion actually is more computationally complex than the system that the implement the four. this place so there's a lot of mass that goes into this prehistoric shunned. in order to have proper signal at the end of the amplifier so yes you can and use an amplifier for a very simple way forms for constant envelope things like to use them for example but if you want to do every year something like that you need very the near the fire yeah and then the next topic is that the level. somehow i normally should be calibrated on because well and s.t.r. doesn't give you a defined output right you have your analog digital converter and you cannot put from that's a zero to whatever four thousand ninety six if it's the twelfth get it back and that's your basically a scale but you don't really know them to what that corresponds. in terms of an absolute output power at the end of view of the board on a socket. and then again if you change the frequency all put never be different so basically on the digital domain you do everything the same but then you change the frequency and your output power level changes because again they're not linearity is in all these parts and unlike base station are dry the your profession old radio hard. for that purpose built for a certain purpose the general purpose as the are born son not calibrated on don't come with calibration tables the vendors could do that but it's a lot of effort to calibrate those units and shipped elevation table so they don't so you don't have any absolute value and the absence of known absolute values means you don't actually know how much you transmit at any given point. one in time which means that you create problems if you really want to or operate let's say a seller network with that because you if you don't know how much how strong you transmit you may create interference you can agree to proper network planning and so on and i have to catch up a bit in terms of time and so on to receive side we also have harmonics not just on the transmission. it's i am and that basically means failure s.t.r. front it isn't open the mixer. and let's say you want to receive nine hundred megahertz but it an intentionally will also receive one thousand eight hundred megahertz because that's you know twice the amount and that's the the first time on it so. you need to put filtering on the analog side otherwise you will receive all kinds of things that you don't want to receive so you need an analog band fitted between the antenna and the arts input. ideally that bansal should only cover the band of interest and should filter out everything else outside of the band. next topic which is related to that is the dynamic range on the receive side because again yes you are front and is a wide open receiver and any energy that is present at the input of the receiver with its near a fire will pass at least to the first stage and then a that's the first employee fire and the receive chain.
and then i'm a propagate even to the mix for input depending on the detail to lay out of the device with a chip in and possibly even further down to the a.t.c. and any energy that is outside the signal that you interested in is just going to war. to create a problem for you but equally if it's a strong signal because a strong signal means that it might clip and clipping means distortion and distortions means you won't receive anything useful and in the end then basically your all that the entire received gain chain must be to want to accommodate the highest. it's a signal that you receive a strong signal to receive even if it's a signal you're completely not interested in let's say you want to receive d.s.m. but there's an f.m. station nearby an f.m. transmitter you might have that f.m. signal in there unless you have filtering before yesterday and advice at the input otherwise that might separate una and basically you you lose all the sensitivity and. all the dynamic range. and then also on the receive side you want to know absolute power levels in some of the system's specify to use receive us that a calibrated to tell you exactly what exactly of ross lee plus minus two d. b. is actually a very large tolerance so the measurement devise ride it meaning that. receive signals received a certain value and that receive signals level in turn really been put to power control lopes which tells the phone how much to him increase or decrease that transmits strength receiver to be received at the proper level but if you receive level it's not calibrated and distrust some random. guess what are some arbitrary value that's not calibrated to anything like substance such amount of the bees a full scale but how do i converted to an absolute value you cannot unless you calibrate the device so again you have trouble their own which brings me to the end it's all. i'm sorry to rant about so many at tropics i'm not saying that people should not play with s.t.r. to the contrary it's very exciting technology and everybody should be working with us the arse but you have to understand the limitations of the system you working with if you want to have proper results and it's not as easy as. it's just a getting a general purpose as the our device from whatever when her. and then playing with software if you don't have other accessories like it's a proper clock source or spectrum analyze it actually look at what the output during an or some other tools and like the filters and so on loan also amplifiers may be powerfully fire us the antenna asked litter is attenuated is so it's much more likely to. that you will end up collecting an entire is to offer devices and accessories a so their s.v.r. board may be the start but it's unlikely to be the end off a working with the s.d.r. devices so be prepared that there is other things that you may need to obtain beyond the s.d.r. board and.
i said it's important to understand those limitations and to make sure that at least the most common our sauces i excluded before your trying to the box systems that have seven more layers of stacked on top of them and something doesn't work and you have no idea what's actually the cause. if you lay a zero it's not even behaving properly so yeah that was basically what i wanted to present on i hope it wasn't too boring. and i hope there will be some questions or challenges or contradicting statements or whatever else so thank you. you to.
i'm very burst. do we have any questions from the audience and i'm going to do this again here we are remember one sentence no comments just a question. so i assume that the cellular phones don't have very accurate clocks so if the transmitter what basically why does the transmitter need an accurate clock if the receiver doesn't because if the if the transmitter is is off would.
that be equivalent to the receiver been off so in short the transmitter must be accurate so the phones don't have to be accurate and because the phones calibrates the receiver or calibrate their internal clock to a reference signal that is specifically crafted in the transmitted way from. from the base station so they can calibrate their clock to what the transmitter sending now you may or may or may not know that but not the problem is you know all part of your question why is it an important the transmitters actor well the problem is that transmit is not the only transmitter on the on in your coverage and on its transmitting so you have multiple of them and the phone clocks. its internal calibrates its internal clock against the first signal it sees and that for example means if you have a base station on your lap desk which has the wrong clock but you have a real network was a proper clock lego commercial network around you switch your phone on it sees the by coincidence of by whatever he sees the commercial network first. it locks its eight calibrates its internal clock to something the right from the commercial network and it will no longer see any other transmitter that doesn't have an accurate clock and than they ever expected that affect the you operate everything the signals they are you may even see it in a spectrum and allies but you phones don't even see the list the network because it's. not at the frequency that they can understand. the question of bone computational power i'm asking because i don't i aware of that pluto s.t.r. little cheap device with a zinc processor. if you've got the clock figured out the analog chain in front of it you think it would be computationally feasible to do l.t. you with such a device or this that way of. but the clock i just heard i always complained about at their the progress we are not having an extent of looking for but i just heard like ten minutes and four before this talk that there is actually won the test and expose it on the and on a socket so if you open it there is likely a socket are some something they can put a reference looking to and several reasons why. my right now you cannot do l.t.e. with the pluto s.t.i. or and the fright world i would say personally the first reason why doesn't work is because the entire driver infrastructure that this device uses doesn't that do not do any form of time standing neither on tees are it's so i think it's primarily a bad choice of soft architecture that constrain. is it in that way maybe other restrictions but that alone is a reason but i think it can be fixed.
i used to request with even if you'd be right there is a request from the intimate to we have a question from the internet i'm sorry but no i know a your question please just one sentence was no comment well that's very difficult to comment very good talk things through a very. complete as well and also very a dense information but i'd what i missed didn't miss a group the late.
feuding style and then asked and he is that the war yeah that's correct and for sure it can be expanded in other topics that thanks for that it may be. we can do a workshop with something on those topics that somebody is interest that what rich steps would be required to build the foot duplex capable i start off to radio bachus. out. it to be honest i cannot respond immediately i would have to check the schematics again i think at the very least i mean that you would have to start of clocking them off the same reference. and then yeah well yeah i don't want to try to make an answer without looking at the facts but i think i think people have shown that things like this can be done as a proof of concept about yeah it's it's a lot of effort and a lot of factory and yeah i mean you can just use a proper device instead. i'm rather than spending a lot of time solving a problem that other people have already sold but yeah that's my opinion. hi old town do you have any recommendation work to find the filters because i remember i certainly bay but they are rather rare or expensive any recommendation. and it depends a bit on what kind of brands if you work with seller band's most often you can actually find the fittest that are used in the phones themselves they are so physical stick their fitness offered us and and a you can typically a find them on e bay another component distributes the problem is to come in such small packages. that you need and rather how can i say a good as him the soldiering skills and then also sometimes the problem is that they might not have fifty all matching but some you know hundred on symmetrical something like that sold some balloon are some whatever might be needed to use it in the fifty own system and for the transmit site if you have real power. more than the only real option at the frequencies in the cellar dormant is to use cavity a fitness or cavity duplex theirs. and yeah there are some sources but i don't want to make advertisements year and this talk on contact me privately. two more questions the two more only one question when cent of it so just thank you for the great introduction so you from experience i know that everybody and clocks have terrorists a spaceship the. that you would ever try really the and clocks and if you did so what you do to clean them up at to be honest i don't use or a video clock i bought one once because it was so cheap. use that so far so yeah i have a g.p.s. disciplined haven't all six although that's his g.p.s. disciplined so it can run for quite some time even without a lock and that's what i use in my set of an app some ten megahertz distribution in my lap by a quick see a virus so that i can attach the devices to the reference. different locations different tests. a thank you for good talk to you said calibration of the board's is a hard problem are they are good enough cents for each kind of what that we have and it's not is it may be a business opportunity to help people to calibrate they asked the eyes. ok so the question is about what kind of calibration are we talking it is it's there. the deed that there is even transmit a signal strength i don't really know what the year whether it would be sufficient to create let's say one set of default calibration and then assume that more or less most of the products are in in the same range i don't know what the spread is for those as the ares i think only the skinny on. the manufacturers would know if they ever checked. about what the spread is i think i wouldn't think of in terms of business opportunity but i think there's a large opportunity for an open source project that can help many s.t.r. uses his to basically design software and described hire a set of using which you can do all these calibrations in a fairly autumn added way with any s.t.r. virtually as the. device under test and then that could be something that hackers spaces for example could operate to help other people calibrate the devices. . if you have one know ok that is thank you everybody jordan.
you've. a you.