  Dual-function radar-communications (DFRC) systems implement radar and
communication functionalities on a single platform. Jointly designing these
subsystems can lead to substantial gains in performance as well as size, cost,
and power consumption. In this paper, we propose a DFRC system, which utilizes
generalized spatial modulation (GSM) to realize coexisting radar and
communications waveforms. Our proposed GSM-based scheme, referred to as spatial
modulation based communication-radar (SpaCoR) system, allocates antenna
elements among the subsystems based on the transmitted message, thus achieving
increased communication rates by embedding additional data bits in the antenna
selection. We formulate the resulting signal models, and present a dedicated
radar processing scheme. To evaluate the radar performance, we characterize the
statistical properties of the transmit beam pattern. Then, we present a
hardware prototype of the proposed DFRC system, demonstrating the feasibility
of the scheme. Our results show that the proposed GSM system achieves improved
communication performance compared to techniques utilizing fixed allocations
operating at the same data rate. For the radar subsystem, our experiments show
that the spatial agility induced by the GSM transmission improves the angular
resolution and reduces the sidelobe level in the transmit beam pattern compared
to using fixed antenna allocations.
