This paper investigates a secure transmission in the multiple-input multiple-output (MIMO) cognitive wiretap networks, where a secondary transmitter (Alice) sends data to a secondary receiver (Bob) in the presence of an eavesdropper (Eve). In order to solve the problems of inter channel interference and inter antenna synchronization encountered by traditional MIMO technologies, the antenna transmission scheme is adopted at the transmitter. As a comparison, we design two different antenna

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... ssion schemes, namely transmit antenna selection maximal-ratio combining (TAS-MRC) scheme and differential spatial modulation maximal-ratio combining (DSM-MRC) scheme, respectively. Moreover, due to outdated channel state information (CSI) of the interference link from Alice to the primary user (PU), we propose power control mechanism to protect the quality of service (QoS) of PU. Furthermore, the closed-form for the secrecy outage probability and the secrecy throughput with TAS-MRC and DSM-MRC schemes are derived to evaluate the secrecy performance, respectively. What's more, we explore the security diversity gain and coding gain based on the asymptotic secrecy outage probability. As the results, it demonstrates that DSM-MRC requires less CSI and is convenient for modulation and demodulation, but sacrifices some secrecy performance gains between two proposed schemes. INDEX TERMS Physical layer security, cognitive radio, differential spatial modulation, secrecy outage probability, secrecy throughput. 5780 VOLUME 9, 2021 YONG CHEN received the B.S. degree in communications engineering from Xiamen University, Xiamen, China, in 2018. He is currently pursuing the M.S. degree in information and communications engineering with the Army Engineering University of PLA, Nanjing, China. His research interests include cognitive radio systems, NOMA systems, and physical layer security. TAO ZHANG received the B.S. degree in communications engineering and the Ph.D. degree in communications and information systems from the College