whats wrong in that program?

Question

itsik 2011-5-10

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im trying to write a matlab cod that find in random two varabels if its 00 --> 1 01 --> 1+i 10 --> -1 11 --> -1-i and the matrix ipMod gets the value in matched like if : ip= [0,1] then c= 1+i ? tried to make it with " if not equal" but i cant find my mistake thank u so much!! and this is the cod:

clear all
j=sqrt(-1)
nBit=2500
ip1 = rand(1,nBit) >0.5;
ip2=rand(1,nBit) >0.5;
ip=[ip1;ip2]
% QBPSK modulation
% bit00 --> 1
% bit01 --> 1+j
% bit10 -->-1
% bit11 -->-1-j
for q1=1:nBit
    q2=1:nBit
    if ip1(q1)~=1 & ip2(q2)~=1
        ipMod=1
    else
        if ip1(q1)~=1 & ip2(q2)~=0
            ipMod=1+j
        else
            if ip1(q1)~=0 & ip2(q2)~=1
                ipMod=-1
            else
                if ip1(q1)~=0 & ip2(q2)~=0
                    ipMod=-1-j
                end
            end
        end
    end
end

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Answer 1

Walter Roberson 2011-5-10

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You are not using a "for" loop over q2, so q2 is being set to a vector of values. Your "if" statements are then trying to process a vector of true and false conditions. The rule for "if" statements is that a vector (or matrix) of logical values is considered true if and only if all() of the vector (or matrix) is true. For example your statement

if ip1(q1)~=1 & ip2(q2)~=1

is interpreted as

if all(ip1(q1)~=1 & ip2(q2)~=1)

Glancing at your code, I think you want q2=q1

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Walter Roberson 2011-5-10

Also, you keep overwriting ipMod, so with your code only the final loop will have effect. You would want to store in to ipMod(q1).

By the way, the entire encoding loop can be replaced with:

ipMod = 1 - 2 * ip1 - 1i * ip2;

That's all that is needed, no "if" and no looping, just one statement.

itsik 2011-5-11

thanks!!!!

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Answer 2

Andrei Bobrov 2011-5-11

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more variant

nBit = 2500;
ipMod = zeros(1,nBit);
ip1 = rand(1,nBit) >0.5;
ip2 = rand(1,nBit) >0.5;
t1 = ip1==1;
t2 = ip2==1;
ipMod(t1 & t2) = -(1+1i);
ipMod(~t1 & t2) = 1+1i;
ipMod(t1 & ~t2) = -1;
ipMod(~t1 & ~t2) = 1;