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Difference between radio transmission technologies

  1. Nov 16, 2016 #1
    Hi All,

    I was wondering if someone could answer these questions for me please?

    Q1. If I stuck an antenna out, will it receive all the different signals e.g. Music Radio, TETRA, WiFi?
    Q2. What is the difference between these? I thought it was the frequency e.g. TETRA uses 300-400 MHz, WiFi uses 2.4 GHz and Music Radio uses whatever you set it.
    Q3. How does my computer know which one is WiFi? Is it by the frequency the signals are traveling at?
    Q4. If I place an antenna outside and there are different signals but I wanted to single out one of them (e.g. TETRA), will I have to listen for waves traveling at 300-400MHz and then do an FFT of that, a peak at any frequency between 300 to 400 MHz would mean there has been a transmission of data at that frequency? Will this apply to wifi too?

    I am not after learning about the encryption/decryption bit at the moment. I just want to know how you tune your equipment to identify different signals or what differentiates these technologies. I honestly don't really have much idea of what I am talking about but interested to learn so can you also please suggest a book to help me better understand what I am after? A book for beginners would be ideal.

    Thanks.
     
    Last edited: Nov 16, 2016
  2. jcsd
  3. Nov 16, 2016 #2

    Delta²

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    Q1. Yes it will receive a total signal that is the sum of these signals. But depending on the length and more generally on the geometric structure of the antenna some signals might have a stronger presence in the antenna than the others. For example in a 12cm length antenna the WiFi Signals will be stronger than the TETRA signals because 12cm is the wavelength that corresponds to the frequency of 2.4Ghz.
    Q2. The difference is the different frequency and the different modulation scheme mainly. For the various modulation schemes used in telecommunication technologies check Wikipedia, the most widely used nowdays is variations of the OFDM (Orthogonal Frequency Division Multiplexing) https://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing
    Q3. yes the computer analyses the signals that are in the 2,4Ghz +-0.2Ghz range to look for WiFi carriers.
    Q4. Yes in short that's how it is done ( I think fourier transform is used for modulation and inverse fourier transform for demodulation, not sure).
     
  4. Nov 16, 2016 #3

    analogdesign

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    Typically you wouldn't grab a large amount of spectrum and then do an FFT of it to find your signal. There are systems that do that (for example, some kinds of software-defined radios) but they tend to be inefficient.

    Usually you do analog filtering of some kind on the front end and reduce the bandwidth of your measurement that way. Very often it is done using an RF filter followed by mixer to pick out specific frequencies. The trend is to do more and more of the demodulation digitally but most systems still have extensive analog processing on the front end.

    Also, the acquisition and analysis of the wifi is done with custom logic and embedded software running on wifi transceiver chips. The host computer just sees it as a data port. Some communications (for example dial-up modems) have moved the signal acquisition and demodulation into software running on the host but the data rate of wifi is much too high to do that at present. I've had the privilege of working on both wifi (802.11b) and 56k modem chips earlier in my career and they are incredibly fascinating.
     
  5. Nov 17, 2016 #4
    Thanks for the replies guys.

    I read a bit more on this. From looking at modulation and the OFDM method I understand that the frequencies 100MHz(Music Radio), 300-400MHz(TETRA) and WiFi(2.4GHz) is not the frequency of the actual signal but a carrier frequency which is used to transport the signal through the air. The actual signal is mixed with the carrier frequency. Do I understand this correctly? If so how does one demodulate the signals? How do you take away the useful signal from Amplitude Modulated and Frequency Modulated signals?

    I looked at software defined radios too, some of the receivers that work with this can collect signals traveling from 100MHz to 2000MHz (Don't remember correctly) which means I won't be able to look at wifi signals but just for learning lets consider the two FM radio and TETRA signals. How would one demodulate these? If the software defined radio was set to listen to carrier frequencies of 100MHz, how would one go about demodulating/detecting it? Would it not be correct to say that if I saw a spike on the Inverse FT spectrum of the 100Mhz carrier signal then there has been a data transmission on the 100MHz carrier (I am expecting there will be significant spikes since music radios are on 24/7). Delta2 you said in short this is how it is done, then what am I missing? is there another step to the demodulation that I have missed out? Also do I really need an analogue filtering if I can work with digital signal defined radio?

    Thanks.
     
    Last edited: Nov 17, 2016
  6. Nov 17, 2016 #5

    analogdesign

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    You understand correctly, in RF communications the carrier is somehow combined with the message. It sounds like you could be well-served reading up on AM and FM transmission. The wikipedia pages for both are good and cover the essential concepts. AM is easy to understand, you take you low-frequency message signal and superimpose it on the carrier. To demodulate it all you have to do is filter out the carrier. That is why the original radios were all AM and why you can demodulate and AM signal will just a diode and a capacitor.

    FM is more complex but the idea is that the frequency of the carrier is modulated by the message signal. So, for example, a radio signal would vary its signal from 100 MHz to 100 Mz +/- 20 kHz or so to include the music. The challenge here is that now you need circuitry that responds to the rate-of-change (or derivative) of the carrier signal, so it is not so simple (but it is still pretty simple).

    OFDM (what wifi uses) is a lot more complex. In essence it modulates multiple signal streams onto orthogonal bases that can be transmitted simultaneously without interfering with each other. That probably makes no sense yet, so I would start with the basics of AM and FM.

    The basic idea is that demodulating a signal is kind of the opposite of modulating it in the first place so each kind of modulation will need to be demodulated in a different way. Many modulation schemes are too subtle to demod just looking at the spectrum but you could demodulate FM in principle that way (but you'd need a pretty fast FFT).
     
  7. Nov 17, 2016 #6
    I just came across PSK (Phase-Shift Keying) but haven't read it properly. Is PSK used for Frequency Modulation of FM radio? if so will I have to do the reverse/opposite of PSK to take away the carrier signal instead of Inverse FT? By fast FFT do you mean I will need a high processing power? What if I was using this method on TETRA instead which I imagine will not have a lot of communication all the time so will I still need high processing power to see a change on the spectrum?

    I will read up on AM and FM shortly. :smile:
     
  8. Nov 17, 2016 #7

    analogdesign

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    PSK is a digital modulation technique, while the FM modulation for music radio stations is (for the vast majority) is analog. "Digital Radio" like Sirius/XM uses entirely different transmission schemes. At any rate, demodulating PSK is more complicated than just an Inverse FFT. You can get a lot of info about it by googling (when you're ready... I don't think it would be very useful to read about it yet).

    I don't know much about TETRA but if you want to demodulate slowly you're still going to have to have a fast digitizer and a large, fast buffer memory. It sounds pretty complex. I've never seen someone try to do that but it should work in principle.

    By fast FFT I did mean a lot of processing power. Once you start to learn about the different modulation technologies you'll see that all of them have more elegant ways to finesse that data out rather than going for a brute-force FFT method. Often they use FFTs but not in the way you're describing.

    So I would start small, and learn how AM and FM radio stations work and how receivers work. All this was known long ago but the technology is more understandable. Then you'll have the tools you need to make sense of more modern techniques.

    As an example, if you're interested in wireline techniques, the methods used for a 300 or 1200 baud modem are simple to understand. The methods used in a 56k modem are mind-boggling. So, start small!
     
  9. Nov 17, 2016 #8

    sophiecentaur

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    You have asked far too many questions at once for anyone to give you a satisfactory answer to any of them. What you need to do is to start with the basic ideas of carrying signals on an RF carrier wave and how it can be done, using simple systems like AM, then FM and so on. Start there and work upwards. Wiki is more than capable of giving you the information you need but it is not a structured program of instruction- which is what you need. Get hold of a Text Book or Amateur Radio publication about the basics of communications Google and find something that suits you. The posts on this thread are interesting and 'not wrong' but, whether or not they can help you, is another thing because the thread is not structured enough. That's the nature of PF.
     
  10. Nov 19, 2016 #9
    Thanks for the replies. I am reading on AM and FM (FSK) etc and will come back when I have proper questions to ask. Cheers.
     
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