Precoding-Aided Secure Time-Domain Index Modulation

In this letter, we propose a generalized precoding-aided secure time-domain index modulation (PSTD-IM) scheme for enhancing the rate and secrecy of the system. Unlike in the conventional time-domain index modulation (TD-IM), where a single (or a set of) Nyquist pulse is activated over multiple symbol periods at the transmitter, in this letter, the pulse (or the set of pulses) is activated at the legitimate receiver, thereby securing the information transfer. In addition, we project artificial noise (AN) onto the null space of the legitimate user and degrade the signal-to-noise-ratio (SNR) at the eavesdropper. In fact, we propose the above-discussed generalized PSTD-IM for a multi-user downlink scenario, where each user is expected to receive an independent TD-IM symbol. We analyze the secrecy rate and derive a closed-form expression for an upper bound on the average bit error rate (ABER) of the proposed scheme. Further, we show that the proposed scheme achieves transmit diversity as opposed to the existing spatial modulation schemes.