| Summary: | Spectral, and energy-efficient, communication is essential for the current and future wireless communication generations. Therefore a novel OFDM with double quadrature signed index shift keying (OFDM-DQSISK) is proposed. Double levels of quadrature signed index shift keying (ISK) system is proposed to enhance the spectral efficiency of OFDM with index modulation (OFDM-IM) system. In the proposed DQSISK level one, the variable <inline-formula> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> and its quadrature <inline-formula> <tex-math notation="LaTeX">$j\alpha $ </tex-math></inline-formula> are exploited to differentiate between the indices of the active in-phase and quadrature-phase subcarriers, respectively. While the signed versions, i.e., <inline-formula> <tex-math notation="LaTeX">$-\alpha $ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$-j\alpha $ </tex-math></inline-formula>, are exploited in the DQSISK level two to distinguish between the active in-phase and quadrature-phase subcarrier indices. In both levels, all possible subcarrier combinations, including the case of inactive subcarriers, are exploited in the proposed DQSISK. Hence, the spectral efficiency of the DQSISK is improved as it linearly increases with the subcarrier grouping size. Also, the value of <inline-formula> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> is optimized over the DQSISK structure using the Grey Wolf Optimizer (GWO) to minimize the proposed scheme’s peak average power ratio (PAPR). Furthermore, the variable <inline-formula> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> is replaced with a standard M-ary Plus amplitude modulation (PAM) to introduce a higher SE version of the DQSISK. The simulation results illustrate the proposed scheme’s superiority, as it attains higher SE and low PAPR compared to OFDM and the up-to-date OFDM-IM schemes.
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