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Explain QPSK and ODFM

Discussion in 'Radio and Wireless' started by epsolutions, Dec 1, 2019.

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  1. epsolutions


    Sep 7, 2019
    In the context of 4G, can someone please tell me, in as simple terms as possible, how a QPSK signal modulates the OFDM sub-carriers?

    What form does an OFDM "symbol" take?

    I have read a lot online, but cannot visualize what the transmitted signal would look like.
  2. maximyudayev


    Feb 1, 2020
    OFDM is a multiplexing technique where it merges orthogonal carriers (viewed from spectrum, zero crossing of one signal is located at the same frequency as the peak of the other signal, neighboring signal - hence it is named “orthogonal”).

    The way each split part of the initial bit stream is modulated is dependent on the specification (i.e 1024-QAM for 802.11ax, or QPSK for another standard). After each subcarrier is modulated, all of the separate signals are superimposed back into one, the OFDM signal, which looks like gibberish analog signal.

    Hope it helps
  3. Nanren888


    Nov 8, 2015
    Imaging that I have a whole pile of carriers, all running separately.
    Modulate each carrier with something, say QPSK.
    Now I tell you that I chose the carriers and the modulation so that the spectra of each will not interfer with the others.
    Add all the modulated carrier signals together.
    Actually usually prodcuded in the frequency domain and made into a tijme signal with a DFT of some sort, but the signals are the same.
    Then add some subtle things that keep the peak of the signal under control and provie some sequences that make synchronisation easier and you have it.
    Visualise what it looks like? A mess of lots of carriers all at the same time.
    The bit rates, symbol rates on each carrier tend to be adaptive to the channel that that carrier is seeing, so good channels will be given more data, higher rates, higher constellations. Poor channels will get less data.
    The narrow bandwidth of any chanel tends to mean that delays are less important: you don't have to worry so much about delay spread.
    So an OFDM symbol, each carrier sees a normal constellation point. Carriers see them at different rates, so I guess an OFDM symbol is a collection of QPSK symbols distributed over a lot of different carriers.
    As I mentioned, I suspect real systems likely create the signals in the frequency domain, a frame, packet at a time, so maybe the symbol is the whole packet?
    Someone who actually knows about 4G might provide a better explanation, or fix some of my guesses.
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