In this letter, we introduce a full-duplex protocol for simultaneous transmission between the uplink and the downlink of cellular networks. The protocol takes advantage of the inactive antenna(s) in multiple-input-multiple-output (MIMO) systems with a single active radio frequency (RF) front-end. More precisely, for the downlink transmissions, we make use of spatial modulation (SM), and for the uplink, we make use of the coordinate-interleaved orthogonal design (CIOD)-based space-time block code (STBC). We provide accurate mathematical expressions for evaluating the error-performances and the achievable diversity order at the base station (BS) and at the mobile terminal (MT) in the presence of self-interference. Our results demonstrate clearly the potential of SM and CIOD for full-duplex operation.