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In this paper we investigate the practical design for the multiple-antennacognitive radio (CR) networks sharing the geographically used or unusedspectrum. We consider a single cell network formed by the primary users (PU),which are half-duplex two-hop relay channels and the secondary users (SU) aresingle user additive white Gaussian noise channels. In addition, thecoexistence constraint which requires PUs' coding schemes and rates unchangedwith the emergence of SU, should be satisfied. The contribution of this paperare twofold. First, we explicitly design the scheme to pair the SUs to theexisting PUs in a single cell network. Second, we jointly design the nonlinearprecoder, relay beamformer, and the transmitter and receiver beamformers tominimize the sum mean square error of the SU system. In the first part, wederive an approximate relation between the relay ratio, chordal distance andstrengths of the vector channels, and the transmit powers. Based on thisrelation, we are able to solve the optimal pairing between SUs and PUsefficiently. In the second part, considering the feasibility of implementation,we exploit the Tomlinson-Harashima precoding instead of the dirty paper codingto mitigate the interference at the SU receiver, which is known sideinformation at the SU transmitter. To complete the design, we first approximatethe optimization problem as a convex one. Then we propose an iterativealgorithm to solve it with CVX. This joint design exploits all the degrees ofdesign. To the best of our knowledge, both the two parts have never beenconsidered in the literature. Numerical results show that the proposed pairingscheme outperforms the greedy and random pairing with low complexity. Numericalresults also show that even if all the channel matrices are full rank, underwhich the simple zero forcing scheme is infeasible, the proposed scheme canstill work well.

Practical Design for Multiple-Antenna Cognitive Radio Networks with Coexistence Constraint