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Two-way relaying, which enables bidirectional simultaneous data transmission between two nodes, is an efficient means to reduce the spectral efficiency loss observed in conventional half-duplex relaying schemes. In this paper, we consider a multiuser cellular two-way relaying scenario with several mobile stations (MSs) at one end of the bidirectional link and a single base station (BS) serving all MSs at the other end. Both the BS and the MSs exchange private messages simultaneously via a single relay node, i.e., concurrent uplink and downlink, in only two time slots, independent of the number of MSs. In the downlink, while the relay separates different MSs spatially, e.g., using either zero-forcing beamforming or zero-forcing dirty paper coding, it benefits from XOR precoding followed by self-interference cancellation, in order to separate messages within a message pair to be exchanged between the BS and each MS. The corresponding sum rate optimization problem is solved with an iterative algorithm based on semidefinite programming.