Template for Papers ECOC 2015

Feed-forward pre-equalization is investigated to extend the transmission capability of μLEDbased links, providing better receiver sensitivities up to 5dB compared with post-equalization. Errorfree 2Gb/s free-space VLC over 0.6m is demonstrated using a PAM-4 modulated blue μLED and an APD receiver. Introduction Visible light communication (VLC) links for intraroom data communications have been investigated intensively recently as light-emitting diodes (LEDs) can provide high-speed data transmission as well as illumination. Such LEDbased optical wireless systems are a potential solution to the spectrum crunch in radio frequency systems as they can make use of hundreds of THz of un-regulated bandwidth 1 . Moreover, VLC systems demonstrate important advantages, namely high security, costefficiency and energy-efficiency 1 . Low-cost VLC links are typically based on the use of intensity modulation and direct detection (IM/DD) schemes with the major challenge for higher speed transmission being the limited modulation bandwidth of the LEDs. Therefore various techniques have been investigated to compensate this LED bandwidth limitation and therefore improve the achievable data rate. Modulation schemes with high spectral efficiency, such as orthogonal frequency division multiplexing (OFDM) and pulse amplitude modulation (PAM), have also been studied 2 . High data rate multiple-input multiple-output (MIMO) optical wireless communications have also been proposed to achieve higher capacity 3 . Optical spatial modulation (OSM) can also demonstrate an enhanced link rate 4 . Equalization has been investigated to extend the link bandwidth and improve the data rate. Multiple-resonant equalization has been proposed and 80 Mb/s data transmission has been achieved with a bit-error-rate (BER) ˂10 -6 using a pre-equalized white LED 5 . VLC postequalization circuits, which reshape the channel response, have also been studied, achieving non-return-to-zero (NRZ) data transmission up to 340 Mbit/s 6 . Moreover, simulation studies on an adaptive equalization system using decision feedback equalizer have shown the potential to achieve 1 Gb/s data transmission using 4 feedforward taps and 2 decision feedback taps 7 . In this work, feed-forward equalization (FFE) is proposed for use at the VLC transmitter in conjunction with a PAM modulation scheme in order to achieve high data rates of > 1 Gb/s in free-space VLC links. Simulation results demonstrate that such pre-equalization provides up to 5 dB better receiver sensitivity compared with post-equalization as the receiver noise is not enhanced in the former case. Moreover, experimental results demonstrate that simple 2tap and 3-tap feed-forward equalization is able to remove the inter-symbol-interference (ISI) caused by the limited link bandwidth of a line of sight (LOS) VLC system, greatly improving the link performance. Micro-pixelated LEDs (μLEDs) are used as they exhibit higher modulation bandwidth than conventional large-size LEDs 8 . Furthermore, four-level PAM (PAM-4) is used at the transmitter, as it exhibits double the spectral efficiency of NRZ modulation. An avalanche photodiode (APD) is used at the receiver since APDs have been shown to provide an additional link power budget 9 . Using a 3-tap feed-forward pre-equalizer, error free 2 Gb/s transmission is achieved over a 0.6 m free-space VLC link. The results prove that feed-forward pre-equalization with only a few taps can improve the μLEDbased link performance greatly, providing a cost-effective solution for high speed VLC links. Feed-forward equalization and simulation results Equalization has been used in radio communication systems to compensate for ISI. Fig. 1: Schematic of the feed-forward equalizer. C1 +