Space-time block coded spatial modulation scheme enhanced by employing an intersymbol phase and power allocation

Turkish Journal of Electrical Engineering & Computer Sciences http://journals.tubitak.gov.tr/elektrik/ Turk J Elec Eng & Comp Sci (2017) 25: 3713 – 3724 c TÜBİTAK ⃝ doi:10.3906/elk-1606-144 Research Article Space-time block coded spatial modulation scheme enhanced by employing an intersymbol phase and power allocation Ahmet Faruk COŞKUN∗ Informatics and Information Security Research Center, Scientific and Technological Research Council of Turkey, Kocaeli, Turkey Received: 10.06.2016 • Accepted/Published Online: 13.03.2017 • Final Version: 05.10.2017 Abstract: This paper provides a brief and insightful examination of the advantages achieved by employing combined phase offsetting and power allocation mechanisms in the space-time block coded (STBC) transmission of spatial modulation schemes with cyclic structure (CSM). In order to achieve increased coding gain distances (CGDs) between transmitted code words (CWs), the power allocation mechanism is cascaded to the constellation mapping process of a STBC-CSM scheme, and the optimal power and phase weights for the conveyed information symbols, together with the phases for each CW, are searched by a heuristic algorithm based on differential evolution that yields the best CGD available for the employed CWs. The outcomes of the optimization study, which is focused on maximizing the minimum CGD between all possible CW combinations for different numbers of antennas and modulation schemes, are then used to investigate and exhibit the average enhancements in the communications reliability. Key words: Space-time block codes, spatial modulation, phase and power allocation 1. Introduction New wireless communications standards have increasingly required downlink and uplink transmission rates and enhanced spectral efficiencies [1]. Multiantenna transmission and/or reception techniques have been shown to provide significant improvements in either spectral efficiency (i.e. spatial multiplexing) [2] or communications reliability (i.e. enhanced coding gain and diversity orders) [3]. Early designs of spatial multiplexing techniques such as the vertical Bell lab-layered space-time coding (V-BLAST) [2] had a strict constraint about the number of receiving antennas required to achieve considerable error performances; additionally, it suffered from the receiver’s decoding complexity. Further research studies proposed the spatial modulation (SM) scheme and introduced its simple transmitter and receiver structures that would cause no additional burden to state-of-theart encoding and decoding techniques [4]. By conveying extra information via the index of the transmit antenna employed among a multiantenna transceiver end, the SM scheme has provided enhanced spectral efficiencies with the usage of the existing maximum-likelihood (ML)-based or other low-complexity decoding routines [4,5]. The disability of the sole SM scheme at the point of providing transmit diversity was later resolved by combined schemes such as space-time block coded (STBC)-SM [6] that aim to boost the spectral efficiency of the transmission while maintaining a restricted part of the transmit diversity available. The literature consists of several works that have focused on the dual-diversity orthogonal space-time block coding scheme (i.e. Alamouti ∗ Correspondence: 3713 COŞKUN/Turk J Elec Eng & Comp Sci STBC [3]), whose spectral efficiency is enhanced by information conveyed by the orthogonal space-time block coded (OSTBC) code word (CW) index. However, it is clear that STBC-SM sacrifices a considerable portion of the available spectral efficiency that would be achieved by a sole SM scheme. Further research has intended to introduce different designs of the combined STBC-SM scheme that would be superior in terms of spectral efficiency and/or communications reliability [7–9]. By introducing the concept of spatial constellation matrices, a high-rate STBC-SM (H-STBC-SM) scheme designed for 4 and 6 transmit antennas was introduced in [7]. Owing to its extended CW set (twice the number of CWs of STBC-SM for the same number of transmit antennas), the H-STBC-SM provided an increase in overall spectral efficiency. Nevertheless, it suffered from degraded error performance when compared to STBC-SM. Another high-rated combination of the STBC and SM schemes, the Complex Interleaved Orthogonal Design SM with the High Degree of Spatial Modulation (CIOD-SM-H), was shown in [8] to provide the same spectral efficiency as the H-STBC-SM, but it exhibited no better error performance than the STBC-SM. The authors of [9] proposed a high-rate spatial modulation scheme by modifying the transmitted CWs due to a cyclic structure (i.e. STBC-CSM). The proposed STBCCSM scheme was shown to increase spectral efficiency with the help of the extended code set when compared to the pioneering design in [6], resulting in higher spectral efficiencies even when employing fewer transmit antennas. Moreover, the STBC-CSM scheme has achieved considerable coding gain distances (CGDs) between the available CWs that pave the way for operating at lower bit error rates (BERs) when compared to the H-STBC-SM and CIOD-SM-H schemes and at nearly the same BERs as the STBC-SM. This paper proposes a modified transmitter design based on the high-rate scheme (STBC-CSM) introduced in [9]. With the help of the extensive examination presented in this paper, the modified STBC-CSM design, combined with intersymbol power allocation (ISPA), is shown to achieve lower BERs in Rayleigh-fading environments as a result of the enhanced CGDs between available CWs. The remainder of this paper is organized as follows. In Section 2, the proposed STBC-CSM scheme employing ISPA at the transmitter is introduced. Section 3 exhibits the optimization process of the proposed scheme in terms of phase and power allocation coefficients related to the CWs and information symbols. Simulation results on the average BER performance of the STBC-CSM with ISPA are provided in Section 4 in order to demonstrate the enhancement in communications reliability. Finally, Section 5 concludes the paper by highlighting the main results. 2. Enhanced STBC-CSM with ISPA The proposed scheme (STBC-CSM with ISPA) is derived from the high-rate STBC-CSM scheme introduced in [9] with the help of a simple and efficient modification on the signal constellations of the information symbols that are inserted into the transmitted CWs. The STBC-CSM scheme promised to stretch the Euclidean distance between the information-bearing x1 and x2 (i.e. increased CGDs between CWs) by employing two different signal constellations, the second of which is rotated by an angle ϕ with respect to the first. With the help of this design, the minimum CGDs were evaluated and exhibited in [9] for different modulation types and a number of transmit antennas. The study in this paper also relies on the basic CW generation mechanism with cyclic structure introduced in [9]. Additionally, by applying power allocation to symbols x1 (...truncated)