  Radio frequency (RF) chain circuits play a major role in digital receiver
architectures, allowing passband communication signals to be processed in
baseband. When operating at high frequencies, these circuits tend to be costly.
This increased cost imposes a major limitation on future multiple-input
multiple-output (MIMO) communication technologies. A common approach to
mitigate the increased cost is to utilize hybrid architectures, in which the
received signal is combined in analog into a lower dimension, thus reducing the
number of RF chains. In this work we study the design and hardware
implementation of hybrid architectures via minimizing channel estimation error.
We first derive the optimal solution for complex-gain combiners and propose an
alternating optimization algorithm for phase-shifter combiners. We then present
a hardware prototype implementing analog combining for RF chain reduction. The
prototype consists of a specially designed configurable combining board as well
as a dedicated experimental setup. Our hardware prototype allows evaluating the
effect of analog combining in MIMO systems using actual communication signals.
The experimental study, which focuses on channel estimation accuracy in MIMO
channels, demonstrates that using the proposed prototype, the achievable
channel estimation performance is within a small gap in a statistical sense
from that obtained using a costly receiver in which each antenna is connected
to a dedicated RF chain.
