0018-9545 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TVT.2019.2890794, IEEE Transactions on Vehicular Technology Subcarrier-Index Modulated Multicarrier Space-Time Shift Keying: Achievable Rate, Performance and Design Guidelines Mohammad Ismat Kadir Abstract—Multicarrier (MC) space-time shift keying (STSK) relying on the concept of subcarrier-index modulated orthogonal frequency division multiplexing (SIM-OFDM) is peoposed. In conventional MC-STSK system, the performance erosion of STSK in dispersive channels is mitigated by employing OFDM. The SIM-OFDM aided MC-STSK activates a specified limited number of subcarriers for transmitting the STSK codewords. The indices of the activated subcarriers provide additional multiplexing gain. A new receiver architecture, which uses the single-stream maximum-likelihood (ML) detector of STSK and the subcarrier activation principle of SIM-OFDM, is designed. A soft decision logarithmic likelihood ratio (LLR) based detector is also conceived. Helpful design guidelines are provided for the system. The achievable rate, complexity and performance of the system are analytically quantified and verified with the aid of numerical simulations. The system is capable of overcoming the impairments imposed by wideband channels, is benefited from index modulation as well as from STSK. The scheme exhibits an improved error performance compared to the classic OFDM- based STSK as a result of the more reliable detection of the subcarrier index bits. Index Terms—Space-time shift keying (STSK), orthogonal frequency division multiplexing (OFDM), subcarrier index mod- ulation, multiple-input multiple-output (MIMO). I. I NTRODUCTION T HE concept of subcarrier index modulation (SIM) aided orthogonal frequency division multiplexing (OFDM) [1]– [4] has emerged as a beneficial technique. The idea is to acti- vate a subset of OFDM subcarriers rather than activating all the subcarriers available. The indices of the subcarriers activated facilitate the transmission of extra source information. The pioneering study on SIM-OFDM was carried out in [1] and was followed by [2]–[6]. A detailed transceiver architecture of the SIM-OFDM system was proposed in [2] and extended to the multiple-input multiple-output (MIMO) scenario in [3]. On the other hand, space-time shift keying (STSK) [7], [8] attracted substantial research interests as a beneficial MIMO scheme. STSK subsumes the previous MIMO concepts of spatial multiplexing (SMUX) [9], space-time block code (STBC) [10], linear dispersion code (LDC) [11], [12], spatial modulation (SM) [13] and space shift keying (SSK) [14]. The motivation of STSK came from the flexible diversity- multiplexing tradeoff (DMT) [15] provided by LDC as well Copyright (c) 2015 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org. Mohammad Ismat Kadir is with the Electronics and Communication Engineering Department of Khulna University, Khulna-9208, Bangladesh. He was a Visiting Fellow with the School of ECS, University of Southampton, SO17 1BJ, UK from 2016 to 2017 (e-mail: ismat.kadir@ece.ku.ac.bd). The financial support of the Commonwealth Scholarship Commission’s Academic Fellowship awarded to M. I. Kadir is gratefully acknowledged. as from the low-complexity design of SM and SSK. While SMUX is capable of providing spatial multiplexing gain and STBC is conceived for providing transmit diversity gain, LDC is capable of striking a flexible DMT, albeit at the cost of a substantially-increased decoding complexity. As a design alternative, SM/SSK activates a single antenna element (AE) and the index of the AE activated can provide additional mul- tiplexing gain. SM/SSK imposes a significantly low decoding complexity. Against these conflicting design ideas, STSK was proposed for striking a flexible DMT, whilst the decoding complexity is significantly reduced [8]. The basic idea behind this reduction in complexity is that STSK activates a single LDC-style dispersion matrix (DM) at every signaling interval. Thus both SM/SSK and STSK may be regarded as the ‘index modulation (IM)’ schemes. SM/SSK [14], [16], [17] employs the AE index as an information-bearing entity and may be called the ‘spatial IM’ scheme, whereas STSK uses the DM in- dex for conveying additional information and may be referred to as the ‘DM IM’ scheme. As a benefit of the employment of the DMs, STSK [7], [8] can disperse the information bits to both the spatial and time dimensions. Consequently, STSK is capable of achieving both the multiplexing gain and the spatial as well as time domain diversity gain. Motivated by the additional multiplexing gain provided by the AE index of SM and by the DM index of STSK, a gener- alized SM (GSM) [18] and a generalized STSK (GSTSK) [19] scheme were also proposed for increasing the achievable spectral efficiency. In GSM, a number of transmit AEs rather than a single AE are selectively activated at a time. In GSTSK, a number of DMs are activated simultaneously for the transmission of more information bits over the spatial- and time domain. The GSM and the GSTSK scheme, however, attain this increased spectral efficiency at the cost of a modest increase in complexity as well as the inter-element interference (IEI). Thus the GSM and the GSTSK schemes provide a fair compromise between the attainable spectral efficiency and complexity as well as IEI with its SMUX counterpart. In line with the developments on the SM, GSM, STSK and the SIM- OFDM, generalized space- and frequency index modulation (GSFIM) [20] and space-time frequency index modulation (STFIM) [21] were also proposed. Although SM, STSK, GSM and GSTSK provide satisfac- tory performance in frequency-flat channels, the performance degrades in frequency-selective wireless channels. To mitigate the performance degradation of STSK in frequency-selective channels, a range of multicarrier (MC) STSK [8], [22] schemes were proposed. These schemes rely on MC technique for slowing down the transmission rate over each subcarrier. As a result, the individual subchannels experience frequency-