Abstract
Visible light communications (VLC) technology permits the exploitation of light-emitting diode (LED) luminaries for simultaneous illumination and broadband wireless communication. Optical orthogonal frequency-division multiplexing (O-OFDM) is a promising modulation technique for VLC systems, in which the real-valued O-OFDM baseband signal is used to modulate the instantaneous power of the optical carrier to achieve gigabit data rates. However, a major design challenge that limits the commercialization of VLC is how to incorporate the industry-preferred pulse-width modulation (PWM) light dimming technique while maintaining a broadband and reliable communication link. In this work, a novel signal format, reverse polarity O-OFDM (RPO-OFDM), is proposed to combine the fast O-OFDM communication signal with the relatively slow PWM dimming signal, where both signals contribute to the effective LED brightness. The advantages of using RPO-OFDM include, (1) the data rate is not limited by the frequency of the PWM signal, (2) the LED dynamic range is fully utilized to minimize the nonlinear distortion of the O-OFDM communication signal, and (3) the bit-error performance is sustained over a large fraction of the luminaire dimming range. In addition, RPO-OFDM offers a practical approach to utilize off-the-shelf LED drivers. We show results of numerical simulations to study the trade-offs between the PWM duty cycle, average electrical O-OFDM signal power, radiated optical flux as well as human perceived light.
Highlights
Advances in solid-state lighting (SSL) are enabling light- emitting diode (LED) to be the primary illumination source of the future [1]
In Visible light communications (VLC) systems, the communication signal is modulated onto the instantaneous power of the optical carrier and the optical detector generates a current proportional to the received instantaneous power, or intensity modulation with direct detection (IM/DD) [5]
Our numerical simulations show that the proposed RPO-OFDM and dimming/communication signals combination algorithm demonstrate the viability of achieving both dimming and high data rate goals across a wide range of intensity settings while preserving compliance with industry-standard dimming techniques
Summary
Advances in solid-state lighting (SSL) are enabling LEDs to be the primary illumination source of the future [1]. The practical communication is only feasible when the PWM signal is at least twice the frequency assigned to the largest O-OFDM sub-carrier frequency (i.e., inter-carrier interference (ICI) for slower PWM rates) This constraint diminishes the feasibility of industry compatible dimmed broadband VLC link as PWM frequencies of off-the-shelf LED drivers are in KHz. In contrast, our work proposes a novel approach (1) to utilize the entire PWM cycle for data transmission instead of limiting the transmission to be only during the ”on-state” so that the data throughput is not limited by the PWM frequency, (2) to use the full LED dynamic range of operation to minimize the nonlinear distortion (mainly clipping) of the O-OFDM signal and DC biasing is not required and the linear range of operation is extended, and (3) to maintain a high link capacity for a wide dimming range, and the signal-to-noise ratio (SNR) is independent on the dimming level within that wide range.
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