This work considers the physical-layer security of a wiretap channel, where a transmitter communicates with a receiver using the precoding-aided spatial modulation (PSM) in the presence of an unauthorized passive eavesdropper. We first analyze the security capability of the PSM and show that the PSM has the property of low-probability-of-interception (LPI). Based on the observations, we then design a secret PSM (SPSM), which represents a generalization of the PSM. We derive the upper-bounds for the bit error rate (BER) of both the desired receiver and the eavesdropper when assuming communications over Rayleigh fading channels. Both the numerical results evaluated from the BER upper-bounds and the simulation results are provided to demonstrate the secrecy performance of the SPSM. Our studies show that the SPSM can significantly enhance the security of the PSM, in addition to inheriting all its merits.
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