Transmission schemes for visible light communications in multipath environments

Abstract

Discrete multitone transmission (DMT) is a useful approach to cope with the dispersive nature of VLC indoor channels. In such environments, the dispersion occurs either due to the LED in combination with its driver and or the multipath propagation channel. In this paper, we consider the impact of both types of dispersions on the bit error rate performance of a DMT based Li-Fi system. However, the main goal of this work is to compare the DMT scheme results with the transmission strategies employing frequency domain equalization. Here, we distinguish between pulse-amplitude modulation (PAM), partial-response coding, single-subcarrier (SSC) modulation, and "carrier-less amplitude and phase" (CAP) modulation. Keywords: visible light communication (VLC), Li-Fi, orthogonal frequency division multiplex (OFDM), discrete multitone transmission (DMT), block transmission, frequency domain equalization (FDE), wireless infrared (IR) transmission, polymer optical fiber (POF) communication, DC-balanced codes, partial-response coding INTRODUCTIONVisible light communication (VLC) uses the visible light portion of the electromagnetic spectrum to transmit data. It has become more and more attractive, since the LED lighting sources in homes, workplaces, factories or streets can additionally be used for wireless communication. Meanwhile, Li-Fi is an accepted term to describe this Wi-Fi analogy.In this work, we focus on the comparison of suitable transmission schemes for VLC based Li-Fi systems. Specifically, we consider only transmission schemes which ensure a DC-balance and thus a constant, data independent brightness, or schemes which can be easily combined with DC-balanced line codes. In order to take into account the bandwidth limitation caused by the LED in combination with its driver, we employ a Gaussian low-pass filter model at the transmitter. We do not claim that this model is generally valid, but the measurement results presented in [1] suggest rather a steep than a gradually roll-off beyond the upper cut-off frequency, which is said to be between 60 MHz and 180 MHz for a common high-power phosphorescent white-light LED with an optimized driver [1]. We also consider the effect of multipath propagation, if several spatially separated LEDs are used for illumination. Here we use simulated impulse responses, where high order diffuse reflections have been also taken into account.Discrete multitone transmission (DMT) is a promising approach to cope with the bandwidth limitation of the LED/LED-driver combo [2, 3]. Here we consider DC-biased DMT, since it inherently provides a DC-balance. DMT can be interpreted as a real valued variant of orthogonal frequency division multiplex (OFDM), where no quadrature up-conversion takes place after the inverse fast Fourier transform (IFFT) and digital-to-analog conversion at the transmitter. For VLC, it is impossible to modulate the field of a pair of sinusoidal high frequency (optical) carriers with a 90 • phase shift, since the LED-light source is strongly non-coherent. The optical field ap...

Silicon-Photomultiplier-Based Underwater Wireless Optical Communication Using Pulse-Amplitude Modulation

“…As expected, by increasing M , more signal-to-noise ratio (SNR) is required for signal detection, and the link span is accordingly decreased. We notice that due to the quantum-noise-limited Rx, we have a rather drastic decrease in the attainable Z by increasing M for a target BER (compared with [21]). Now let us consider the case of PAM transmission with SQRT processing.…”

“…Nevertheless, the use of (single-carrier) PAM can result in ISI at high data rates due to the BW limitation of the components, as shown in [8] for the case of on-off keying (OOK) modulation, necessitating channel equalization at the Rx. To deal with this, here we proposed the use of FDE at the Rx in order to boost the transmission rate far beyond the inherent BW of SiPM/LED [21], [22]. A key advantage of FDE is that it has a lower computational complexity as compared to time-domain equalization.…”

Carrierless amplitude and phase modulation in wireless visible light communication systems

“…Nonlinear equalizers using artificial neural networks have also been proposed and demonstrated in LED-based optical wireless links, achieving better link performance than conventional linear and Volterra equalizers [106]. Block transmission and FDE have also been proposed for VLC links [99,[107][108][109]. Blocks of received symbols are equalized as one in the frequency domain using a single-tap equalizer, while interblock interference is mitigated by inserting a cyclic prefix at the beginning of each block, similar to O-OFDM.…”

Advantage of CAP Signaling for VLC Systems Under Non-Linear LED Characteristics

“…The merits of frequency domain equalization (FDE) was recently shown in [7] where the channel equalization is performed in the frequency domain, allowing low complexity implementation of the Rx. This is done through adding a cyclic prefix (CP) to the frames of symbols at the Tx to avoid interframe interference; at the Rx, after CP removing, FDE is performed through taking a fast-Fourier transform (FFT) on the frame of received symbols, single-tap equalization, and an inverse FFT to come back to the time domain [8], [9].…”