The array is then called uniform if $|I_n| = I$ is the same for all elements. The phase terms $\angle I_n$ is chosen to decide the peak direction of the beam.
My question is: which current is $I_n$? I'm supposing that each cell is connected to a (physical or equivalent) current source. So, is $I_n$ the incident current (let's call it $a$) on the elements ports or the sum of incident and reflected current ($a-b$, where $b$ is the reflected current)?