# A Partitioned IRS-Aided Transmit SM Scheme for Wireless Communication
## 1 Introduction  
The six generation (6G) wireless communication requires significant innovation and progress of communication paradigm to meet the enormous demands for spectrum efficiency (SE) and energy efficiency (EE) [1, 2]. Spatial modulation (SM) [3], essentially as a spatial multiplexing technology for wireless communication, has been emerging in recent years due to its high SE and EE. Unlike conventional signal transmission, SM conveys not only the modulated symbols, but also the index of the activated antenna as extra information. Moreover, inter-antenna synchronization (IAS) and inter-channel interference (ICI) are avoided due to the single-antenna activation. Later, SM was simplified as space shift keying (SSK) [4] in which only the activated antenna indices were conveyed to reduce the complexity of the system and detection. Furthermore, SM and SSK were respectively extended to generalized spatial modulation (GSM) [5] and generalized space shift keying(GSSK) [6], where multiple antennas were activated simultaneously, thereby improving the diversity gain or SE for wireless communication systems. On the other hand, intelligent reflecting surface (IRS), consisting of a large array of passive elements, is regarded as an intelligent device that can change electromagnetic waves by adjusting the phase of the incident signal, aiming to maximize the signal-to-noise ratio (SNR) of the receiver (Rx) [7; 8]. Moreover, IRS signal reflection does not require a specialized RF chain for signal processing. Due to its advantages in high transmission reliability and EE, IRS is widely recognized as one of the most promising technologies for 6G [9]. In light of the significance of SM and IRS, exploring the potential of IRS-aided SM systems to achieve high reliability, SE, and EE, has been widely researched. Refs. [10; 11; 12; 13; 14; 15] researched IRS-aided received SM or its evolution forms such as SSK, GSSK, and GSM in single-input multiple-output (SIMO) systems. The theoretical average bit error probability (ABER) and simulation analyses were provided, demonstrating the significant superiority of combining IRS and SM. Ref. [16] proposed IRS-assisted GSM with phase offset scheme, which enhanced the IRS beamforming and widened the activated antenna combination channel differences to improve the system bit error rate (BER) performance. Ref. [17] proposed a novel transmissive reconfigurable intelligent surface (TRIS) transmitter-enabled SM multiple-input multiple-output (MIMO) system where the specific column elements of the TRIS panel are activated per time slot. Refs. [18; 19; 20; 21; 22] focused on the research of IRS-aided transmit SSK or GSSK in multiple-input single-output (MISO) systems. Moreover, IRS partitioning improved the distinctions between transmission channels and effectively improved the BER performance in the partitioned IRS-aided transmit SSK (PIRS-TSSK) scheme [20]. Then, it was extended to multi-antenna activated scenario and a partitioned IRS-aided transmit SSSK (PIRS-TSSSK) scheme and a partitioned IRS-aided flexible GSSK (PIRS-FGSSK) scheme were proposed to further improve SE and antenna utilization [22]. It is worth noting that although [22] improves antenna utilization compared to [20], it does not transmit symbols, which still limits the further enhancement of SE to some extent. In summary, in the existing studies mentioned above, the transmitter (Tx) employs SSK or GSSK, which only conveys antenna index information without carrying symbols. This can be considered as an absence of symbolic information transmission, which constrains the wireless communication system's SE. In particular, at high SE requirements, the number of transmit antennas required is large, thus leading to more groups of IRSs and fewer reflecting elements per group; then, the beamforming effect will deteriorate and the BER performance will decrease. To address this issue, this paper proposes a partitioned IRS-aided transmit spatial modulation (PIRS-TSM) scheme.