On $2 \times 2$ MIMO Gaussian Channels with a Small Discrete-Time Peak-Power Constraint

Author:

Alex Dytso, Luca Barletta, Gerhard Kramer

Keyword:

Computer Science, Information Theory, Information Theory (cs.IT)

journal:

--

date:

2024-01-30 00:00:00

Abstract

A multi-input multi-output (MIMO) Gaussian channel with two transmit antennas and two receive antennas is studied that is subject to an input peak-power constraint. The capacity and the capacity-achieving input distribution are unknown in general. The problem is shown to be equivalent to a channel with an identity matrix but where the input lies inside and on an ellipse with principal axis length $r_p$ and minor axis length $r_m$. If $r_p \le \sqrt{2}$, then the capacity-achieving input has support on the ellipse. A sufficient condition is derived under which a two-point distribution is optimal. Finally, if $r_m < r_p \le \sqrt{2}$, then the capacity-achieving distribution is discrete.