# Optical Wireless Communications Using Signal Space Diversity with Spatial Modulation
## 1 Introduction  
The COVID-19 pandemic has accelerated future workplace transformation. A surge of wireless data volume and connection speed has been seen in each residence [1] due to the multiple smart device connections and high-speed broadband applications such as real-time high-definition video streaming for remote working, education, and entertainment, etc. Optical wireless communication (OWC) provides a promising solution to accommodate such changes in wireless traffic, as it can offer scalable, higher-bandwidth transmission without conventional radio-frequency (RF) spectrum regulation and RF interference [2]. In addition, its simple installation via the broadband fiber-to-the-premises distribution networks is also preferred in indoor applications [3]. The prevalent line-of-sight (LOS) link configuration in the OWC system enables ultra-high-speed wireless transmission. However, this LOS link is subjected to optical channel obstructions caused by small opaque objects or mobile end-users, thus resulting in link performance degradation or even link outages [4, 5]. To enhance the LOS link robustness in full compliance with the eye safety regulation, spatial diversity techniques have been investigated in the intensity modulation/direct detection (IM/DD)-based indoor OWC systems using multiple-input-single-output (MISO) or multiple-input-multiple-output (MIMO) configurations [6, 7]. In the classical approach to offer spatial diversity in the OWC system, additional spatial resources are deployed to mainly provide redundant links. However, to fully take advantage of these multiple spatial resources, it is worth further exploring spatial diversity techniques that can be incorporated with the data rate boost techniques to achieve the dual benefits of improved BER performance and increased system throughput in the OWC system. The feasible data rate boost techniques should be available for the spatial diversity links with differences in channel gain across spatial channels to deal with the LOS optical beam obstruction. Spatial multiplexing (SMux) is one of such data rate boost techniques that can utilize the channel gain difference. However, this technique is implemented with a vital prerequisite of a very low channel correlation [8]. As multiple spatial channels in the indoor OWC system are often highly correlated, extra approaches must be adopted to facilitate signal demultiplexing from multiple spatial channels at the receiver, such as imaging optics applied as the optical demultiplexer [9], the superposed signal constellation modulation for multi-channel signal recognition [10], or other multiple access techniques incorporated for signal demultiplexing [11]. These extra approaches in SMux complicate the implementation of spatial diversity links in the OWC system, especially for the MISO system where signal decoupling must be performed at the \"single\" receiver. On the other hand, spatial modulation (SM) is another viable data rate boost technique utilizing channel gain imbalance. Compared with SMux, SM has a lower spectral efficiency. However, SM is more robust against highly correlated multi-channels for the indoor OWC transmission. When spatial diversity is incorporated with SM, receiver diversity can be readily realized in the MIMO configuration for outdoor FSO links [12] and indoor OWC links using angle diversity receivers [13]. However, for indoor OWC applications, angle diversity receivers are expensive to deploy due to the complicated structures. Instead, it is much simpler for a single PD-based receiver to achieve a portable and compact end-user with lower power consumption. Thus, transmitter diversity in the MISO configuration is a preferred option. For the viable transmitter diversity techniques that can be incorporated with SM, a closed-loop feedback-aided transmit aperture selection approach has been introduced for outdoor FSO communications [14]. However, an uplink must be established to offer feedback.