[Signal Processing] In an assignment A(I) = B, the number of elements in B and I must be the same.

The following code gives the error, "In an assignment A(I) = B, the number of elements in B and I must be the same." due to the last line of code.

numSamples = 10;                    % Number of samples
pulseShape = ones([0,numSamples]);  % Generate a series of impulses
numBits = 1e6;                      % Number of input bits
bits = randi([0 1],numBits,1);      % Generate a million random binary bits
bits(bits==0) = -1;                 % Modulate signal to BPSK
waveformBPSK = zeros([0,numBits .* numSamples]); % Creates signal spectrum
% Convolutes input pulses with spectrum at every sampled value; convert all
% bits at sampled values to be the corresponding input bit value.
waveformBPSK(1:numSamples:end) = bits;

In general, I would like to convert the zero values of the vector waveformBPSK at the values of numSamples to its corresponding bit values (generated randomly by bits).

It looks to me if the problem is because the program sees that bits is still a vector of 1 million values and cannot convert each value of the waveformBPSK to the bits vector. However, I would like to convert each zero value of the vector waveformBPSK at the sampled value of numSamples to each individual value of bits.

Cheers!

Answer 1

[Star Strider]

What would we do without accumarray?

Replace the last line in your code with these two:

z = accumarray( [1:numSamples:size(waveformBPSK,2)]', bits, [], @(x) x);
waveformBPSK = z;

They produce a column vector, so transpose to row if necessary.

[Tien T]

YES! This works perfectly, thank you!

Could you briefly explain some details of the code you provided?

1. Why must the dimensions of the waveformBPSK be 2?
2. What exactly are the variables x and @(x)?

[Star Strider]

My pleasure!

1. The dimensions of waveformBPSK are not 2, but I wanted to specify that the length of the ‘index’ argument to accumarray went to the end of waveformBPSK, the number of its columns. The second argument to the size function specifies the dimension to measure. The row dimension is 1, the column dimension is 2. So ‘size(waveformBPSK,2)’ tells MATLAB to define the end of the array of indices as the column-length of waveformBPSK. (It replaces ‘end’ in your declaration of waveformBPSK, because ‘end’ does not always work to define the end of a vector.)
2. The variables x and @(x) define an anonymous function ‘@(x)x’ that takes the values of ‘bits’ and maps them to the indices specified by the first argument without modification. See the documentation on Anonymous Functions and accumarray for details. (The documentation explains them better than I can.) Experiment with Anonymous Functions and accumarray when you have time. They are extraordinarily useful.

[Tien T]

Very much appreciated!

I also realized that my original code was not working as expected due to the fact that bits is an array of 1000000x1 while waveformBPSK(1:numSamples:end) is an array of 1x1000000. The error can be avoided if the original initialization for bits is:

bits = randi([0 1],1,numBits);

Thanks again, Star!

[Star Strider]

My pleasure!