## 1 Introduction  
Optical wireless communication (OWC) stands as a pivotal technology that offers substantial opportunities to meet the demanding requirements of 6G and beyond and can serve as both access and cross-haul links; its versatile applications extend to device-to-device (D2D) communications and the Internet-of-Things (IoT) [1]. Two prominent OWC variants, visible light communication (VLC) and infrared OWC (IOWC), have showcased remarkable bit rates exceeding 20 Gbps for one optical wavelength [2], with VLC achieving up to 35 Gbps through wavelength division multiplexing (WDM) [3]. While VLC excels in scenarios requiring illumination, IOWC presents enhanced versatility and is particularly suitable for applications like LiFi uplinks and data centres. Integrating IOWCs with high-speed fibre communication systems has become increasingly significant, offering terabits-per-second (Tbps) indoor wireless access and interfacing seamlessly with fibre-to-the-home access networks [4]. Despite these advantages, the mobility of high-speed IOWC systems is constrained due to the use of narrow-divergence transmitters and receivers with narrow fields-of-view (FOVs). To address this limitation, implementing robust user tracking, localisation mechanisms, and optical beam steering is essential for the practical deployment of IOWC, thereby ensuring its effectiveness and widespread application [4; 5]. Vertical-cavity surface-emitting lasers (VCSELs) stand out as important light sources for intensity-modulated/direct-detection (IM/DD)-based OWC systems, primarily owing to their distinct advantages over other optical sources. Notably, VCSELs boast cost-effectiveness in terms of fabrication and require lower electrical power consumption compared to edge-emitting lasers [6]. Their superiority extends to offering higher modulation bandwidths and superior emission coherence in comparison to light-emitting diodes (LEDs). Leveraging their vertical emission and streamlined fabrication, VCSELs can be seamlessly integrated into two-dimensional (2D) arrays, facilitating spatial diversity and enabling the deployment of multiple-input-multiple-output (MIMO) systems [7]. The significance of VCSELs is underscored by experimental endeavours in gigabit OWC, where individual VCSEL sources operating at wavelengths of \(\sim\)650 nm [8; 9], \(\sim\)850 nm [10], and 1310 nm [2] have achieved bit rates of up to 25 Gbps using IM/DD. The pioneering fabrication of VCSELs within the 850-980 nm band marks a significant milestone in optical communications, and their continued dominance in the market is attributed to the alignment of these wavelengths with the high responsivity of cost-effective silicon photodetectors (PDs) [6]. However, the paramount consideration lies in ensuring the safety of the 850 nm system for the human eye, especially for protecting the retina. To meet the stringent class-1 laser safety requirements, compliance with the accessible emission limit (AEL) becomes imperative, and this is \(\sim\)\(-\)1.10 dBm, as specified by the International Electrotechnical Commission (IEC 60825-1) for a point source operating at 850 nm [11]. In the pursuit of optimising the information capacity of VCSEL-based OWC systems, extensive research has been dedicated to exploring spectral-efficient modulation schemes. Among these, multilevel pulse amplitude modulation (PAM-\(M\)) has emerged as a prominent contender. PAM involves transmitting symbols through pulses in a signal waveform, with the amplitudes of the pulses representing different symbols. A PAM transmission with \(M\) distinct amplitudes or \"levels\" represents \(\log_{2}M\) bits in each level, providing a robust mechanism for conveying information. PAM is the preferred choice for many commercial OWC systems due to its simplicity in handling real-valued symbols and its spectrum efficiencies that are comparable to other advanced modulation schemes such as orthogonal frequency division multiplexing (OFDM) [12].