N = 1536;          % non-power of 2 FFT length used in LTE
N1 = 512; N2 = 3;  % 512 x 3 = 1536
x = randn(N,1);    % some input signal

% generate N1 x N2 matrix of input data, reading them row wise
X = reshape(x,[N2,N1])';

% DFT over each column
X = fft(X);

% Multiplication with twiddle factors
X = X.*exp(-1i*2*pi/N*(0:N1-1)'*(0:N2-1));

% DFT over each row
X = (fft(X.')).';

% read out column wise
X = X(:);