Why the frequency spectrum of my OFDM output signal has a series of spike?

Question

I'm trying to understand OFDM modulation using Matlab Simulink and I've built the following system

I sent to the input of the OFDM block a frame of 15 samples that comes from the 16-QAM modulator. The OFDM modulator also receives a frame of 15 pilot symbols that I have chosen (see below)

Now, My OFDM block has the following settings:

My question is, why the frequency spectrum of the output signal of the OFDM block has huge spikes?

I have supposed to be the short FFT length compared to the number of pilot subcarriers but, I'm not sure about that

Thanks

Answer 1

I believe you will find that the spikes occur at multiples of your symbol rate. eg: if symbols have length 1ms, then you will have spikes every 1kHz in the Freq domain. By symbol length I mean when the symbol has a disontinuity, abrupt change , between symbols. This is expected from sampling theory. I also see it in GNURadio. I had the same problem bc I was inputting incorrect complex values into the IFFT.I was inputting +1 & 0 as input data. This created much more discontinuities than necc and hence spikes. Once I realised this I corrected it. What you are doing with the IFFT is creating a series of BPSK or QPSK signals and adding them all together. if you input complex data into et IFFT, then you are actually entering 2 bits at a time, as 2 bits represent 4 possible phases , hence QPSK. But you also need to enter the data as a bipolar value eg +1 & -1. So you input complex values must be a pair of +1 & -1 values, a complex pair. Also, you must ensure all your unused components frequencies are set to 0, by entering 0 in those positions of your data. This is what I did to get the non spiky output.