## 1 Introduction
These PHY layers are mainly intended for use in systems with diffused light sources and video displays with kbps data rates, as well as using complex modulation schemes of spatial two-phase shift keying (S2-PSK), dimmable spatial eight-PSK, undersampled frequency shift on-off keying (UPSOOK), variable transparent amplitude-shape-color, etc. However, the standard clauses on the PHYs V and VI are mainly on the demodulation schemes, and their real applications are still being revised. In [13], a data rate of 126.72 kbps was achieved, using 192 data-carrying LEDs modulated using color intensity modulation (CIM) and a 330 frames per second (fps) global-shutter (GS) camera-based Rx, which is expensive and not commonly used, over a link span of 1.4 m. However, the Tx was set to the refresh rate (i.e., transmission frequency) of 82.5 Hz, which is still lower than the maximum allowed flickering time period of 5 ms (200 Hz) [14]. A red, green, and blue (RGB) LED-based rolling-shutter (RS) OCC (RS-OCC) utilizing a combination of UPSOOK, wavelength-division multiplexing, and MIMO offering improved space efficiency of 3 bits/Hz/LED was reported in [15]. In [16], a multilevel intensity modulation (IM) RS-based camera detection link with a data throughput of 10 kbps over a transmission range of 2 m was reported in [15; 16]. Whereas, a beacon jointed packet reconstruction scheme for mobile-phone-based VLC with commercial white phosphor LEDs and a 60 fps RS camera achieved the net data rate of 10.3 kbps (172 bits/frame observed over a large LED surface) over a transmission distance of 20 cm [17]. In [18], COTS LEDs with raptor code (with linear time encoding and decoding, thus reduced computational complexity and decoding overhead) have been investigated in RS-OCC. Non-line-of-sight MIMO links using Luxeon LEDs based on diffused reflections and space and time division multiple access, as well as the equal gain combining technique, was reported in [19]. which achieved flicker-free transmission up to a 10 m link span. However, as listed in the survey in [20], for indoor OCC based IoT applications, low data rates ranging from 15 to 896 bps for transmitting short messages in device-to-device communications over transmission spans of 25 cm to 1 m is more than sufficient. Over the past few years, multiple neopixel boards have been used in electronic devices such as screen displays in home automation, advertising, televisions, human interfaces, etc. [21]. These devices can be used as part of the Tx to provide IoT based MIMO-OCC links in smart environments. High-throughput links such as visual MIMO systems for screen-camera communications link were proposed in [22], where the impact of non-linear channel equalization, non-binary channel coding, probabilistic shaping, and non-linear precoding for high-order modulation schemes and respective applications such as inter/intra vehicle communications, near field communication, and augmented reality were investigated. In [22], it was shown that the reliability and throughput of the optical communication links can be improved using various channel coding techniques based on nonbinary low-density parity-check codes [23], polar and turbo codes [24], and advanced modulation schemes, such as chaos and LoRa [25]. As previously mentioned, short-range and low data rates IoT links in indoor environments such as device-to-device communications with short messages, indoor positioning, navigation, small identification information, and communications through advertisements need to be considered [26]. However, the major challenge in implementing such systems is the requirement for flicker-free transmission at lower data rates (i.e., most cases in IoT applications). Therefore, in this paper, we propose a low data rate RS-based MIMO-OCC scheme with grouped LED Tx for indoor IoT environments, which is flicker-free.