OFDM simulation questions

[siaukla]

Hie, I am currently doing the simulation of OFDM with different modulation techniques. I have read the tutorial 22: OFDM on the website and would like to seek some help in writing the code for simulations.

In the tutorial, under the topic 'Example of OFDM using 4 sub-carriers', let's say I have a data which has been modulated using the psk modulation and it consist of real and imaginary values.

I am able to do the serial to parallel conversion but I do not understand the steps after it. How am I going to write the matlab code for the carrier thing? I have a value of, 1 + 1j, 1 - 1j and so on...

Please provide some help to me as I am a beginner is this field. Thanks in advance...

Regards,
siaukla

[hunter006]

Hi siaukla,

I'm currently doing a project in OFDM and am almost finished with my simulation (specifically OFDM (802.11a compliant) simulation). Note that while I used a 64 long IFFT, there are 48 data carriers in IEEE 802.11a OFDM, so consequently my data sequence is 48 long, not 64 long.

The steps I used were:
1. Set up data (put the data in to the columns, and make it a format that Matlab recognises for the modulation process - used reshape() and num2str() along with bin2dec to make things easier) - resulted in a 48 long array of integers
2. Modulate data (QAM, PSK - qammod/pskmod) - resulted in 48 long array of complex #'s
3. Send the data through the IFFT block (resulted in a 64 long array in the time domain)
4. Send the data for post-IFFT processing - guard interval, etc.
5. DAC and upconverter

Since you are going from the data to the carriers, I am assuming that you have just generated the symbols, and would now like to put them through some process that results in an output sequence. If the signal is just being analysed in the digital domain, the DAC and upconverter can be skipped, however the specifics of the two are the DAC is convolution and filtering, and the upconverter is simply multiplying the entire signal by exp(pi*carrier_f)

What you need to do is as follows: for each of the generated symbols, put them in to an array that you can perform an IFFT on. The specifics of the array are up to you, but for faster execution I suggest using a 2^N length array. For mine, 802.11a specifies it's a 64-bin IFFT. For demonstration purposes, I will use a 4 bin IFT.

After you've done that, you process it via IFFT by passing it directly in to the IFFT module of matlab. The resulting output will be in the time domain. Post process the data and then you have your transmission signal.

The tough part will be the receiver. My hints to you are that you will need to do (mostly) the reverse process of what is done by the transmitter, as well as a few other "extra" things, such as implementing a slicer to deal with noisy signals (a slicer takes in values within a range and locks it in to the value it's supposd to be - much like the bottom plot of the help file under "Modulating a Random Signal", it locks the green dots in to the black ones), phase correction, and so on. Matlab is helpful in that the pskdemod/qamdemod automatically slices, so you don't HAVE to worry about the slicing, but how do you counter phase errors? In IEEE 802.11a some of the carriers are sacrificed to be pilot carriers, and those are used to correct phase, but there is a lot of interest in correction without the use of pilot carriers. I leave that up to you...

My final hint is to work things out using 4-QAM/4-PSK first. It's very easy to process and very easy to double check your results with what you expect. I hope this was helpful to you.

[hunter006]

moo = randint(16,1); % Random binary input, 16 bits long
IFFT_input = qammod(bin2dec(num2str(reshape(moo,[],4))),16); % 4 long complex values
tx_time_dom = ifft(IFFT_input);
% post processing

Your code should roughly reflect something like this.

[waqar]

I m new to Matlab. I m receiving the error "Error: Function definitions are not permitted at the prompt or in scripts." while i m execut a program.


Help me if u can.


Best Regards.
Waqar

[nbnadoda]

i am new in matlab .can anybody send me codes for basic ofdm transmitter and receiver

[fthckr]

I think this code is very useful and you should read OFDM tutorial from complex to real. Because it was written for beginners.

% OFDM System Version 0.10
% Created on September 10, 2004 by Surujlal Dasrath
% ------------
...% Description:
% ------------
% Use function RANDSRC to generate random information symbols taken from
% source alphabet of M symbols. A fixed number of data points will be
% used. No training or noise will be added to this system.
% soc.li/GLIaMyO
% NOTE: This is not exactly OFDM since no Cyclic Prefixing was used.
% =======================================================================
%
% ------------------------
% A: Transmitter End
% ------------------------
% 1. Generate random serial data with M symbols (RANDSRC will do this)
% 2. Perform modulation (4-PSK = QPSK for now)
% 3. Serial to parallel
% 4. IFFT
% 5. Parallel to serial
clear, clc, close all
% 1. Generate data 00_0 01_1 10_2 11_3
M = 4;
no_of_data_points = 64;
input = randsrc(1,no_of_data_points, 0:M-1)
figure(1)
stem(input); grid on; xlabel('Data'); ylabel('Amplitude');title('Input')

% 2. Modulation 0_1 1_i 2_-1 3_-i
msg_tx = pskmod(input, M)
scatterplot(msg_tx);

% 3. Serial to Parallel
parallel_mod_data = reshape(msg_tx,8,8)

% 4. 64-point IFFT
ofdm_msg = ifft(parallel_mod_data)

% 5. Parallel to serial
ofdm_msg_tx = reshape(ofdm_msg, 1,64)
figure(3)
plot(real(ofdm_msg_tx)); grid on;title('Real part of OFDM signal')
figure(4)
plot(imag(ofdm_msg_tx)); grid on;title('Imaginary part of OFDM signal')

% ---------------------
% B: Receiver End
% ---------------------

% 1. Serial to Parallel
ofdm_msg_rx = reshape(ofdm_msg_tx, 8,8)

% 2. 64-point FFT
msg_rx = fft(ofdm_msg_rx)

% 3. Parallel to serial
msg_rx_ser = reshape(msg_rx, 1, 64)

% 4. Demodulation
msg_demod = pskdemod(msg_rx_ser, M)
figure(5)
stem(msg_demod); grid on;title('Output')