  Transmitter (TX) cooperation at various levels has been shown to increase the
sum throughput of multiuser multiple-input multiple-output (MIMO) systems. In
this paper we consider a K-user MIMO system where TXs have only global channel
state knowledge. It has been theoretically shown that interference alignment
(IA) achieves the K/2 degrees of freedom of this K-user MIMO interference
channel. However, results on IA and all proposed transceiver techniques for
this channel up to date, assume conventional antenna arrays at the transceivers
with multiple radio-frequency (RF) chains, each connected to a different
antenna element. To reduce the consequent hardware burden and power dissipation
imposed by such arrays, we propose in this paper the utilization of compact
single-RF electronically steerable parasitic (passive) array radiators (ESPARs)
at the cooperating TXs. A signal model capable of capturing the characteristics
of the considered antenna arrays is first described and then a general
precoding design methodology for the tunable parasitic loads at the TXs' ESPARs
is introduced. Specific precoding techniques and an indicative ESPAR design are
presented for a 3-user 2x2 MIMO system with one ESPAR TX, and the obtained
performance evaluation results show that the gains of TX cooperation are still
feasible.
