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I have a time domain signal based on LTE standards. Carrier spacing = 15KHz, Symbols per slot = 6, extended Cyclic Prefix = 512 samples FFT size = 2048, No. of sub carriers = 1200, channel BW = 20MHz, Samp Freq = 2048*15KHz = 30.72 MHz, Total Bits = 14400 (Symbols * slots * bits used to MOD * total subcarriers).

It is QPSK modulated (using qammod ("IntegerSymbols",2^bits,binary) in matlab); This QAM modulated data is IFFT'ed after zero padding on the sides and DC carrier insertion is done and then CP is added (512 samples to a 2048 sample bin).

This signal now is windowed (Point wise multiplied by a window function(only in the TRANSMITTER side) (FOR DIFFERENT REASONS) in time domain) "BY A HAMMING WINDOW FUNTION" Keeping all the overlap add constraint in mind.

On the receiver side, no noise is added, no specific channel is modeled. Simple reception. On the receiver side, CP is removed and then the stream is FFT'ed and only the central info carrying subcarriers are taken. This stream at this point is still modulated. However when I try to plot a "scatter plot figure" of the QAM modulated data from the txer side and the FFT'ed data in the receiver, I get a pattern as shown in the figure above which seems to be some sort of quantized thing. I am not sure what it is or what is it caused by. Could you please help me on it? On decoding Error/BER is 0 but EVM is -18dB.enter image description here

P.S: I've asked this ques before but I reckon the details weren't clear. So fingers crossed this time.

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  • $\begingroup$ hey, I've answered your question more than two months ago and never got any feedback – no comment, no up- or downvote or an acception as answer. Did you just happen to have forgotten to do that? $\endgroup$ – Marcus Müller Mar 15 at 19:11
  • $\begingroup$ sorry logged in after long :) $\endgroup$ – samz12 Apr 8 at 10:01
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That's the picture you'd get usually if you had a single-carrier system with a slight IQ imbalance. (but that would usually be kind of slanted; but we don't know what your OFDM receiver does, so we can't tell you.)

In a multi-carrier system like OFDM, I'd look into whether I accidentally added inter-carrier-interference (ICI), for example by having an uncorrected fine frequency offset. Or, and that's not that uncommon for simulations, by having a control loop trying to achieve frequency synchronization, but goes wrong. (But that would usually be less "nice and orderly looking". However, we don't know what your OFDM receiver does, so we can't tell you.)

Since this is in simulation: Test. Send a stream of constant symbols, from only one subcarrier. That way, you can see where the effect comes from. Iterate. Debug.

You're the engineer sitting in front of that problem – go and investigate!

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