Capacity-Optimized Pre-Equalizer Design for Visible Light Communication Systems

TL;DR

This work tackles capacity optimization for IMDD VLC subject to LED bandwidth limitations by deriving a closed-form channel-capacity model that explicitly depends on analog pre-equalizer parameters. It then formulates and solves a capacity-maximization problem using zero-pole matching to obtain analytical optimal pole placements under different channel attenuations, and validates the approach with simulations showing substantial capacity gains over bandwidth-centric designs. The results demonstrate that, in good channels, symmetric pole placement can maximize capacity, while in severe channels a first-order equalizer suffices, providing a practical design framework and guidelines for capacity-aware VLC front-ends. Overall, the proposed capacity-optimized equalizer design offers a principled path to extend VLC data rates without incurring prohibitive SNR penalties, and supports switchable configurations to adapt to changing channel conditions.

Abstract

Since commercial LEDs are primarily designed for illumination rather than data transmission, their modulation bandwidth is inherently limited to a few MHz. This becomes a major bottleneck in the implementation of visible light communication (VLC) systems necessiating the design of pre-equalizers. While state-of-the-art equalizer designs primarily focus on the data rate increasing through bandwidth expansion, they often overlook the accompanying degradation in signal-to-noise ratio (SNR). Achieving effective bandwidth extension without introducing excessive SNR penalties remains a significant challenge, since the channel capacity is a non-linear function of both parameters. In this paper, we present a fundamental analysis of how the parameters of the LED and pre-equalization circuits influence the channel capacity in intensity modulation and direct detection (IMDD)-based VLC systems. We derive a closed-form expression for channel capacity model that is an explicitly function of analog pre-equalizer circuit parameters. Building upon the derived capacity expression, we propose a systematic design methodology for analog pre-equalizers that effectively balances bandwidth and SNR, thereby maximizing the overall channel capacity across a wide range of channel attenuations. We present extensive numerical results to validate the effectiveness of the proposed design and demonstrate the improvements over conventional bandwidth-optimized pre-equalizer designs.

Figures (11)

• Figure 1: Schematic of the optical wireless transmission link. The system is modeled as a cascaded two-port network, where each module contributes to the overall transmission characteristics.
• Figure 2: Second-order equivalent circuit for an LED.
• Figure 3: The architecture of the general second-order equalization circuit.
• Figure 4: The frequency response of the LED equivalent circuit, the equalizer circuit, and the equalized optical link, illustrating how the equalizer compensates for the LED’s poles and shifts the poles of the cascaded link to higher frequencies.
• Figure 5: The relationship between the equalizer’s poles and zeros and the circuit parameters.