dvb-t Orthogonal frequency-division multiplexing

hi every one i am happy to join this forum
please i have two question
1-
in dvb-t / Mapper: the digital bit sequence is mapped into a base band modulated sequence of complex symbols. There are valid modulation schemes: QPSK, 16-QAM, 64-QAM,256-QAM . what is the frequency of this modulation ? .....Mhz

2- OFDM Modulation: the sequence of blocks is modulated according to the OFDM technique, using 2048, 4096, or 8192 carriers (2k, 4k, 8k mode, respectively). how can generate this carrier with same amplitude and deference frequency ?

sorry may be my My question is not clear .how Inverse Fast Fourier Transform (IFFT) performs on the 2k, 4k or 8k carriers ? how this carriers generated?

1. If you input data sample period is T, then after splitting to N channels you get symbol rate = 1/(NT). So this is the frequency. The term 'frequency of modulation' I dont understand.

2. These carriers are not generated. Their origin is DFT. Look at the disrete fourier transform formula: When you multiply your input data sample x with DFT matrix element, e^(i*2*pi*n*k/N), its according to euler's formula the same as performing a quadrature modulation - x*cos(2*pi*n*k/N) + x*i*sin(2*pi*n*k/N).

After applying an FFT to the vector of length N and sample rate fs, you get the spectrum samples from 0 to fs with the step fs/N. These are your subcarriers.

when you split your input dataflow into multiple low rate channels and then map the data to channel symbols, you get a true frequency representation of the data - spectrum samples which are spaced by fs/N. But you should provide your transmitter (DAC) output with time domain samples. To get a time domain representation from frequecy domain you should make the IFFT.

thank to eng. mityan for his reply .

*The term 'frequency of modulation' . i mean carrier frequency of QAM " how choice of this frequency ?".
*in dvb-t some transmitters have IF stage . how choice this IF frequency ?
*in dvb-t which stage determine the center frequency of channel ?

The answer on 1st question about QAM carrier search in my previous post.
About IF and center freq I dont know, just read the literature. But actually DVB_T RF passband signal comes out from direct shift from 0 to channel, not by doubling the real baseband, because it's already complex after symbol mapping. The center frequency means nothing

hi every one
sorry may be i am not understand the answer of eng. mityan about my first question
.
what is the frequency of the carrier ? if it differ for different cases 4 ,16,64,256 QAM that is used in ofdm for dvb-t ?
if the frequency carrier depend on sample rate of bit stream ?
please if there is paper to explain this point

This is not the OFDM diagram. Please check out: https://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

The carrier frequencies of different OFDM systems are given.

But if you want to know the particular frequencies of all subcarriers, the table in the above link contains the values of subcarrier spacing, so in baseband representation these frequencies will be (0...fft_length-1)*spacing.
For more information refer to document ETSI EN 300 744.

Engineer Mityan, Can you help me with the OFDM code? actually i am confused with the parameters. Thank you

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I have a Matlab code for OFDM but that is introducing so many parameters for slot structure. I hope you can help me as you have a sound grip on the topic.

Post the code, public or private, and we cope with it together. I am not a participant of any OFDM project, so some specific points (PAPR and sync) have to be studied further, but the rest is quite clear.

Engr Mityan Thank you so much for your positive response. Can you give me any email address where i can post the code?