# A 40 Mb/s VLC System Reusing an Existing Large LED Panel in an Indoor Office Environment
## 1 Introduction
Nevertheless, not much attention has been paid to the viability and complexity of implementing VLC using the existing lighting infrastructure following the standardised specifications. The current commercial products normally require the complete replacement of the existing illumination systems with new luminaires supporting VLC. It is therefore desirable to make the most of the existing illumination systems to save the investment and reduce the deployment cost. In this paper, we investigate the possibilities and challenges in upgrading the existing LED-based luminaries with VLC functionality by developing a demonstration system in a practical indoor environment. To the best of the authors' knowledge, there exist limited reports on reusing the existing commercial LED luminaries for simultaneous VLC communication and illumination in the literature. In [21], a commercial LED table lamp was upgraded to a VLC system providing a 10 Base-T Ethernet connection via an optical wireless transmission of 12.5 Mb/s Manchester coded on-off-keying (OOK) signals. The achievable system capacity is not fully exploited due to the transmission speed limitation of the Ethernet connection. Additionally, challenges and issues in implementing such a practical system based on existing luminaries are not covered in [21]. Most of the indoor VLC systems that are reported in the literature are based on dedicated designs using LEDs over a short distance in a controlled laboratory environment,focusing on achieving high data rates. Recently reported experimental systems have been demonstrating transmission speed in the order of Gb/s with precisely aligned point-to-point links that are based on single-colour LEDs (scLEDs) [22; 23; 24]. The use of the scLEDs significantly contributes to the throughput in Gb/s, since they offer a higher modulation bandwidth than the phosphor-converted white LEDs (pcwLEDs) with the absence of the slow response phosphor. However, the existing illuminance products have been widely adopted pcwLEDs due to their low cost, thus imposing hurdles to achieving comparable date rates. As shown later, the LED panel that was adopted in our demonstration system consists of 240 pcwLEDs and it has a 3-dB bandwidth of only 1.4 MHz. Although the bandwidth of pcwLEDs can be increased to between 10 and 20 MHz by using blue filters at the receiver side to mitigate the slow temporal response of the phosphor, this improvement in bandwidth is paid for at the cost of power loss or signal-to-noise ratio (SNR) penalty due to the phosphor-converted portion of the received light spectrum being filtered out [25]. It is unnecessary to use blue filters if multi-carrier modulation schemes, especially with bit-loading, are applied because multi-carrier modulation can utilise all of the signal power in the full received light spectrum to achieve higher SNR and, therefore, higher system capacity, or data rates [26]. Similarly, analogue pre-equalisers [27; 28] are designed to compensate for the frequency response of the LED to extend the normalised 3-dB bandwidth by sacrificing the modulation depth, which leads to the same issue as using blue filters [29]. Hence, we adopted a multi-band carrierless amplitude and phase modulation (CAP) [30; 31; 32] with bit-loading to combat the limited bandwidth problem due to its simple implementation and low peak-to-average power ratio (PAPR) [33]. In addition, those high-speed experimental VLC systems often require bulky optics to realise precise alignment between transmitters and optical receivers, which is unreasonable for practical VLC applications in the real world. An optical receiver with a large field of view (FoV) and sensitivity is a key enabling factor for the deployment of VLC applications. We have designed and built a compact optical receiver with a half angle of \(\pm 35^{\circ}\), which highly relaxes the alignment requirement, in order to demonstrate the usability of our system in real-world scenarios. Another key challenge that is faced in our system is the flicker caused by the low-cost LED driver.