  We use linear control theory to construct an output feedback controller for
the attenuation of small-amplitude Tollmien-Schlichting (TS) wavepackets in a
flat-plate boundary layer.We distribute evenly in the spanwise direction up to
72 localized objects near the wall (18 disturbances sources, 18 actuators, 18
estimation sensors and 18 objective sensors). In a fully three-dimensional
configuration,the interconnection between inputs and outputs becomes quickly
unfeasible when the number of actuators and sensors increases in the spanswise
direction. The objective of this work is to understand how an efficient
controller may be designed by connecting only a subset of the actuators to
sensors, thereby reducing the complexity of the controller, without comprising
the efficiency. We find that using a semi-decentralized approach - where small
control units consisting of 3 estimation sensors connected to 3 actuators are
replicated 6 times along the spanwise direction - results only in a 11%
reduction of control performance. Our results reveal that the best performance
is obtained for a control unit which (i) is sufficiently wide to account for
the full spanwise scale of the wavepacket when it reaches the actuators and
(ii) is designed to account for the perturbations which are coming from the
lateral sides (crosstalk) of the estimation sensors. We have also found that
the influence of crosstalk is not as essential as the spreading effect.
