randi generating random complex numbers from 4-QAM

I am trying to generate the transmitted symbol x randomly from the set of constellation points seen below from a 4-QAM constellation diagram. This is in respect to the input-output model y = h · x + w for a total of 10^4 independent realizations. I went to the Matlab site and found the following code:

a = randi([-5,5],10,1,"like",1i)

I tweaked it to represent the points on my 4-QAM diagram as so

a = randi([-2,2],10e4,1,"like",1i)

However when I try to run it I get the following error:

Error using randi

Complex or sparse input following 'like' is not supported.

Error in Untitled (line 2)

a = randi([-2,2],10,1,"like",1i)

Perhaps I am overcomplicating this but I can use some assistance in updating my code to produce a random generation of complex numbers in respect to the constellation points on the 4-QAM diagram seen below. Thank you in advance.

Answer 1

[the cyclist]

Your code works fine for me, and here:

a = randi([-2,2],10e4,1,"like",1i);

plot(real(a),imag(a),"o")

I'm not sure what to suggest, other than restarting MATLAB. What do you get from

which randi -all

[Jose Iglesias]

I submitted this issue to the Mathworks technical support. I am using version R2020a so maybe that is the issue. I tried to run which randi -all with no luck. Is there a different way to generate random complex integers you can recommend? Thank you for your response.

[Walter Roberson]

I notice the user has marked R2020a; perhaps it was not supported then.

(I had no idea it was supported at all!)

[the cyclist]

I think you are correct. I looked for this in the version history of randi, but missed it! Looks like this was introduced in R2022a.

EDIT: The Mathematics section of the release notes (for R2022a) corroborates this.

Answer 2

[the cyclist]

My other answer focused on "Why didn't this syntax work?" (and concluded that you don't have a recent enough version).

This is answer is focused "How can I do what I need?"

There are two straightforward solutions:

(1) Generate the real and imaginary parts separately, and add them. For example.

N = 11;

re = 4*randi(2,N,1) - 6;
im = 4*randi(2,N,1) - 6;

c = re + i*im

c = 
   2.0000 + 2.0000i
  -2.0000 + 2.0000i
  -2.0000 + 2.0000i
   2.0000 - 2.0000i
   2.0000 + 2.0000i
  -2.0000 - 2.0000i
   2.0000 - 2.0000i
   2.0000 + 2.0000i
  -2.0000 + 2.0000i
  -2.0000 + 2.0000i
  -2.0000 - 2.0000i

(2) Generate a random value from 1 to 4 (because you have 4 distinct points), and use that as an index into the complex values you want.

N = 11;
c_vals = [-2-2*i, -2+2*i, 2-2*i,2+2*i];
idx = randi(4,N,1);
c = c_vals(idx)'

c = 
  -2.0000 + 2.0000i
   2.0000 + 2.0000i
   2.0000 + 2.0000i
   2.0000 - 2.0000i
   2.0000 - 2.0000i
  -2.0000 + 2.0000i
  -2.0000 + 2.0000i
  -2.0000 - 2.0000i
  -2.0000 - 2.0000i
   2.0000 + 2.0000i
   2.0000 + 2.0000i

[Jose Iglesias]

Thank you for your input. I ended up upgrading to R2023a and it ran the original code a = randi([-5,5],10,1,"like",1i) with no issues.

[the cyclist]

Glad to hear you have a solution