###### Abstract  
Line-of-sight (LOS) indoor optical wireless communications (OWC) enable a high data rate transmission while potentially suffering from optical channel obstructions. Additional LOS links using diversity techniques can tackle the received signal performance degradation, where channel gains often differ in multiple LOS channels. In this paper, a novel active transmitter detection scheme in spatial modulation (SM) is proposed to be incorporated with signal space diversity (SSD) technique to enable an increased OWC system throughput with an improved bit-error-rate (BER). This transmitter detection scheme is composed of a signal pre-distortion technique at the transmitter and a power-based statistical detection method at the receiver, which can address the problem of power-based transmitter detection in SM using carrierless amplitude and phase modulation waveforms with numerous signal levels. Experimental results show that, with the proposed transmitter detection scheme, SSD can be effectively provided with ~0.61 dB signal-to-noise-ratio (SNR) improvement. Additionally, an improved data rate ~7.5 Gbit/s is expected due to effective transmitter detection in SM. The SSD performances at different constellation rotation angles and under different channel gain distributions are also investigated, respectively. The proposed scheme provides a practical solution to implement power-based SM and thus aids the SSD realization for improving system performance. os 202  
links are often observed, multi-transmitter deployment can be further leveraged to boost the system throughput in addition to providing diversity [9]. Spatial multiplexing (SMux) and spatial modulation (SM) are such data rate boost techniques that perform better under channel gain imbalance [10]. Compared to SMux, SM is reported to be more robust in combating the highly correlated channels that typically occur in the LOS indoor optical wireless links, with lower complexity RF chain configurations [11]. Even so, high channel correlation still degrades the accurate detection of the active transmitter in SM. To address this issue, various channel coding schemes have been proposed [12; 13; 14], where the \"encoder-decoder\" configuration increases the computation complexity of the system. On the other hand, precoding design applied to SM at the transmitter is another effective approach to enhance the robustness of active transmitter detection in highly correlated channels, with partial or full channel state information (CSI) pre-known [15; 16]. However, SM itself does not achieve transmit diversity for enhancing link robustness [11]. The signal constellation dimension has been further explored to offer diversity [17; 18] and simultaneously take advantage of the simplified RF channel activation in SM without introducing extra antenna matrix projection [19]. It is reported that a simple signal space diversity (SSD) technique using phase rotation can be integrated with SM [20] to enhance diversity through the complex Rayleigh fading channels. For the typical intensity modulation/direct detection (IM/DD)-based indoor OWC transmission, however, a different channel condition with real-valued channel gains is expected [21]. Therefore, this SSD technique has been further modified and comprehensively investigated in our previous work to show a distinct diversity performance [22; 23]. Note that, all the above SSD investigations were based on the assumption of perfect detection of the active transmitter in SM. On the other hand, the performance of active transmitter detection can also be significantly affected by the high channel correlation in practical scenarios, which further affects the recovery of carrierless amplitude and phase (CAP) modulation signals involving many signal levels. To address the active transmitter detection issue in SM incorporating SSD, we have theoretically proposed a joint maximum-a-posteriori (MAP) estimation algorithm [24].