hi...
i have something to ask everyone...if i want to design an OFDM transmitter for UWB application,so i need 128 point IFFT. For IFFT,100 points are for data ,the rest are for guard interval and cyclic prefix. that means we need 100 QPSK signals to pump out as IFFT's input data , is it true?
as i know, the output of QPSK is sinusoidal wave,so all output will pump into serial to parallel converter, is it possible to design serial to parallel converter using verilog? because, from my basic understanding, the output from the parallel converter will exactly zero or one rather than sinusoidal wave? the following is a formula for QPSK :
V=Asin[2*pi*ft + @(t)]
hi..
see Eric Cosby's matlab program (but it is QAM based).. its good.. i have added cyclic extension to it because he didn't do it.. but right now i don't have it.. the ifft/fft size selection is the requirement of the no. of carriers to be used.. u have to adjust the guard signal accordingly...
- You must add 28 null values to your 100 values mapped with QPSK to perform 128 IFFT. If you insert them at the end [100 QPSK mapped values 28 null values], your spectrum will began at DC frequency. If you want to shift your signal to a carrier frequency (fc), you must multiply the result of IFFT by exp(2*pi*j*t*fc) : IFFT([100 QPSK mapped values 28 null values]) * exp(2*pi*j*t*fc).
thank you very much for quick answer but the thing I am wondering that is there any importance of using the qpsk in mapping for example if i want to use mQAM what will be difference? and second question the location of the pilots are important or not? and the last when i see pilot insertion examples some of the pilots are multiplied with -1 gain, what is the reason behind that?
1- The OFDM modulation present BER performances of sub-carrier modulation (called mapping) with multiplying the baudrate with the number of DATA subcarries.
If you change the Mapping from qpsk to M-qam, you will change the BER performances (see M-PSK vs M-QAM BER comparison in Digital Communication books ).
2- Pilot subcarriers are previously known and transmit known symbols.These subcarriers are generally used in Header or Preamble to make estimation on channel and perform synchronization.
So it is better to spread them along your spectrum to have channel information in different frequency part.
In some system, one pilot subcarrier can be used in data symbol to provide a phase reference for Differential-PSK mapping. It must be the first subcarrier.
3- pilot subcarrier can be mapped differently from other data subcarrier. Gain with -1 could be a phase values of 180° in BPSK mapping.
thank you very much yeah it was so helpfull. If you dont mind can i ask you one question what is the reason beyond that matlab code. they said it is done for the interleaver in order to achieve distribution for subcarriers. but when i try it for two ofdma symbol it gives me error. And the bit number that generates from the bernoulli how i can predict the number amount that will be genereated??
k=0:Ncbps-1;
mk=(Ncbps/12 )*mod(k,12)+floor(k/12);
s=ceil(Ncbps/2);
jk=s*floor(mk/s)+mod(mk+Ncbps-floor(12*mk/Ncbps),s);
[x,int_idx]=sort(jk);
thank you very much mohammed the problem is that even the standards is not clear about the specifications it is like a draft they delete sth and add sth every day it is hard to understand what exactly needed for the implementation.
Hi ,
what's mean this kind of subcarrier mapping :
The logical frequency subcarriers 1 to 61 are mapped to the same numbered IFFT inputs while the logical frequency subcarriers –61 to –1(How can i mapping the complex Data into logical frequency -60 to -1: what's means the logical frequency -61 to -1 ) are mapped into IFFT inputs 67 to 127 respectively. The rest of the inputs, 62 to 66 and the 0 (DC) input, are set to zero?
regards,