doubts regarding ofdm stbc

hai all,
I am doing a project on OFDM - STBC , in which am using alamouti encoder in the first stage,i applied alamouti to the two datas a1 and a2 are consecutive data streams , i have created elements in alamouti encoder output matrix , that is a 4 data streams (a1 , a2 , conj(a1) , -conj(a2) );

next step is application of ofdm,, here i have alot of confusions, because i have 4 data streams , i need to apply IFFT to each stream,,so what should be the value of N , while am using N-point ifft command in matlab.. eg:- ifft(a1,N);

what should be the value of N, what does it means, in a normal case there will be a single data stream for applying ofdm,,but here i have 4..how should i forward my work,,can any one give a proper solution???....

akhilamalu said:

hai all,
I am doing a project on OFDM - STBC , in which am using alamouti encoder in the first stage,i applied alamouti to the two datas a1 and a2 are consecutive data streams , i have created elements in alamouti encoder output matrix , that is a 4 data streams (a1 , a2 , conj(a1) , -conj(a2) );

next step is application of ofdm,, here i have alot of confusions, because i have 4 data streams , i need to apply IFFT to each stream,,so what should be the value of N , while am using N-point ifft command in matlab.. eg:- ifft(a1,N);

what should be the value of N, what does it means, in a normal case there will be a single data stream for applying ofdm,,but here i have 4..how should i forward my work,,can any one give a proper solution???....

Click to expand...

Apply OFDM first then STBC. N is the number of subcarriers. It's usually a power of 2: 64, 256, 512, etc

David83 said:

Apply OFDM first then STBC. N is the number of sub carriers. It's usually a power of 2: 64, 256, 512, etc

Click to expand...

thank you sir, but in that paper the algorithm explained in such a way that , first we need to create the alamouti matrix(2x2), the 4 stream from two adjacent symbols. ofdm is suggested to do in the next step.
the paper is " Robust MIMO-OFDM System for Frequency-Selective Mobile Wireless Channels "

link : - https://ieeexplore.ieee.org/document/6872569/

can anybody help me to proceed this work ..:-|

I have a brief look at Section II, and as I expected, first apply N-point IFFT for two data sequences (a1 and a2) each of which contains N symbols. The result will be two OFDM symbols of N subcarriers (u1 and u2). The Alamouti coding is then applied to OFDM symbols, not data symbols.

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David83 said:

I have a brief look at Section II, and as I expected, first apply N-point IFFT for two data sequences (a1 and a2) each of which contains N symbols. The result will be two OFDM symbols of N subcarriers (u1 and u2). The Alamouti coding is then applied to OFDM symbols, not data symbols.

Click to expand...

I read it again, and it says that after forming the STBC from the two data sequences a1 and a2, namely u11=a1, u12=-a2^*, u21=a2 and u22=a1^*, you take IFFF for each sequence like x11=IFFT(u11,N) ... etc.

David83 said:

I have a brief look at Section II, and as I expected, first apply N-point IFFT for two data sequences (a1 and a2) each of which contains N symbols. The result will be two OFDM symbols of N subcarriers (u1 and u2). The Alamouti coding is then applied to OFDM symbols, not data symbols.

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I read it again, and it says that after forming the STBC from the two data sequences a1 and a2, namely u11=a1, u12=-a2^*, u21=a2 and u22=a1^*, you take IFFF for each sequence like x11=IFFT(u11,N) ... etc.

Click to expand...

yes sir , absolutely i have done the same ...

x1 = (randi([0 3],1,N)); % RANDOM SIGNAL
x2 = randi([0 3],1,N);

%QAM modulation and alamouti matrix formation
y1 = qammod(x1,M);
y2 = qammod(x2,M);
y2c=-conj(y2);
y1c=conj(y1);
% ofdm - ifft -for N=64; EACH STREAM HAVING 64 , SUBCARRIER POINTS
iffta1=(ifft(y1,N))';%A11
iffta2=(ifft(y2,N))';%A21
iffta3=(ifft(y2c,N))';%A12
iffta4=(ifft(y1c,N))';%A22

i got thi ifft output as column matrix
then i considered cp length 4, and i took the last 4 elements of each sequences and appended it to the first portion (sdding cyclic prefix ),,ie N+4 elements in each sequence ..
cplen=4;
cprefix1 =iffta1((N-cplen):N-1);%A11
iffta11=vertcat(cprefix1,iffta1);


cprefix2 =iffta2((N-cplen):N-1);%A21
iffta21=vertcat(cprefix2,iffta2);

cprefix3 =iffta3((N-cplen):N-1);%A12
iffta12=vertcat( cprefix3, iffta3);


cprefix4 =iffta4((N-cplen):N-1);%A22
iffta22=vertcat(cprefix4 ,iffta4);

need to know upto this point my concept is right or not ////