# Effect of Temperature on Channel Compensation in Optical Camera Communication
## 2 Related Work
The authors remarked the importance of a comprehensive design of the internal thermal structure and the appropriate selection of the packaging material to extend the lifetime and preserve the performance of the LED [22]. Regarding the effect of the driving current, [PERSON] et al. analyzed the optimal working conditions of a low-power SMD LED concerning the luminous flux and its efficacy [23]. The authors compared how those properties were affected by the injection current and the ambient temperature, concluding that the latter affected more that LED's substrate. [PERSON] et al. carried out a study analyzing the effect of rapid thermal cycling on luminous flux and luminous efficiency of high-power LED, demonstrating that the effect could significantly impair the LED properties and interface microstructures [24]. Channel compensation is a capital processing stage in any system in which there is inter-channel interference. In VLC-based systems, MIMO and CSK (and WDM) are the two main techniques that can require compensation. MIMO VLC systems have attracted the attention of the scientific and industrial communities in recent years due to their potential use in real environments using multi-LED lamps and the current industrial trend based on the application of Orthogonal Frequency Division Multiplexing (OFDM) [25; 26]. According to this trend, [PERSON] et al. studied adaptive bit-loading and power-allocation schemes for indoor MIMO-VLC systems [27]. The authors proposed the use of 2 \(\times\) 2 receiver arrays with angular diversity. The simulation results, which included a realistic estimation of the impulse responses, show that elevation angles close to 45\({}^{\circ}\) can optimize the BER performance after channel compensation. However, the transmission-side setup included four independent LED lamps separated by a large distance, and no jitter or synchronization issues were mentioned in the paper. This last aspect is essential when the spatial diversity emitter covers a significant deployment area, as highlighted by [PERSON] et al. in [28]. A similar work based on the same angular diversity concept was presented by [PERSON], [PERSON], and [PERSON] in [29]. In this case, the authors validated a cubic receiver under laboratory conditions, demonstrating the feasibility of the \(\pi/2\)-diversity. [PERSON] and [PERSON] in [30] performed a comprehensive comparison between spatial multiplexing and repetitive coding MIMO techniques for Carrierless Amplitude and Phase (CAP) modulation. The authors demonstrated that repetitive coding is a better option for highly-correlated channels due to the ill-conditioned nature of the channel matrix (similar channel gains for each channel). However, this depends on the input SNR. [PERSON] et al. proposed the use of an RGB OCC link in [31]. The authors demonstrated spectral efficiencies up to 3 bits/s/Hz in a 60 m link in their work. Although the paper states that MIMO techniques were employed, a single-LED link was essayed. Regarding RGB channel compensation, a ZF scheme (as in [10]) was used with no output SNR penalty (typical issue of ZF) thanks to the use of imaging optics at the receiver side. [PERSON] and [PERSON] studied different compensation techniques for MIMO-VLC systems [32]. The authors analyzed the performance of ZF, ZF with successive interference cancellation (ZF-SIC), and Minimum Mean Squared Error (MMSE) equalizers. As expected, the best performance was obtained for the MMSE equalizer, which considers the channel matrix's noisy nature. Nonetheless, the covariance matrix must be known a priori or estimated before the compensation stage. In order to overcome this limitation, [PERSON] et al. proposed using a convolutional autoencoder to denoise the channel matrix [33]. Nevertheless, this proposal was based only on simulation results and no experimental outcomes were provided.