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OFDM modulation about 128 point IFFT

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aifi

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OFDM modulation

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?
 

Re: OFDM modulation

yes,but you must add another 28 zeros in order to calculate 128 points IFFT,after that you can join guard interval and cyclic prefix .
 

Re: OFDM modulation

CP is not counted in 128. It is appended after IFFT operation.
 

OFDM modulation

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)]
 

Re: OFDM modulation

try to use this command V=Asin[2*pi*ft + @(t)]
 

Re: OFDM modulation

from 0 to 14 null ,100 data & from 114 to 128 null. CP add after ifft.
 

Re: OFDM modulation

The CP is added after IFFT and the Serial and Parallel conversion. That means, the CP protect the OFDM frame, not the Modulated symbol.

Regards,

Carlos
 

Re: OFDM modulation

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...

Regrds
Naveed
 

where I have to insert the pilot tones in 128 ifft size? I think 100 is for data and pilot subcarriers.
 

Hi,

- 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).

- Cyclic prefix is added after IFFT function.

- Pilot subcarriers uses some of data sub carries

Regards

Chaker
 
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?
 

Hi,

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.

I hope it is helpful

Regards
 
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);
 

Hi,

Sorry, i have small knowledge about the interleaver but i advise to look to the Wimax standard because it use the same interleaver.

It said that Ncbps can be 192 or 384 or .. So you must see interleaver box in different configuration for Wimax digital communication system.

I will check for more information.
 
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,
 

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