Optical Power Constraints Research Articles

Average Worst-Case PEP Optimality of Repetition Coding Among Rate-1 DC-Offset STBCs for MIMO Optical Intensity Channels

An optical wireless intensity-modulation direct-detection multiple-input multiple-output communication system is considered. The performance of $M$ -PAM rate-1 direct current offset space-time block codes is studied in terms of average worst-case pairwise error probability (WC-PEP) in quasi-static channels. It is shown that within this code class, the average WC-PEP is minimized by repetition coding (RC) under both electrical and optical individual power constraints, irrespective of channel statistics. This agrees with previously published results related to ON–OFF keying RC. This is further extended to sum power constraints, where it is shown that spatial beamforming minimizes the average WC-PEP within this code class, which simplifies to RC if the channel matrix has independent and indentically distributed columns and a sum electrical power constraint. Under a sum optical power constraint, this also holds true at high signal-to-noise ratio (SNR), but not at low SNR. Generally, the time dimension of this code class is redundant from an average WC-PEP perspective. Numerical results are provided to support the theoretical findings and to show that the average WC-PEP leads to a good approximation of the actual error probability at high SNR.

Joint Precoder and Equalizer Design for Multi-User Multi-Cell MIMO VLC Systems

In multi-user multi-cell multiple-input multiple-output (MIMO) visible light communication (VLC) systems, inter-cell interference (ICI) and inter-user interference (IUI) are the two key factors that could severely degrade the system performance. In this paper, for the first time, a novel joint precoder and equalizer design based on interference alignment is proposed to mitigate both IUI and ICI in multi-user multi-cell MIMO VLC systems under imperfect channel state information (CSI). The proposed approach aims to choose proper precoder and receiving equalizer to minimize the mean square error (MSE) under the unique optical power constraints in VLC. Furthermore, we consider optical channel estimation error in our formulated joint optimization problem when designing the joint precoder and receiving equalizer in multi-user multi-cell MIMO VLC systems. Numerical results show that the proposed design achieves a better performance under different users locations, channel estimation errors, and transmitter/receiver spacing, compared to the existing minimum mean square error (MMSE) and max-rate designs. Especially, when the transmit optical power is 12 W per light emitting diode (LED) lamp, our proposed design achieves a sum rate improvement of up to 18.2% and 28.7% as compared with the MMSE and max-rate designs, respectively. In addition, the proposed design can save about 1.4 and 4.3 W optical power per LED lamp as compared with the MMSE and max-rate designs, respectively, at a target bit error rate (BER) of 10 $^{-3}$ .

Protograph-Based EXIT Analysis and Optimization of LDPC Coded DCO-OFDM in VLC Systems

The direct current (DC)-biased optical orthogonal frequency division multiplexing (DCO-OFDM) with low-density parity-check coding is a popular scheme for visible light communication systems. Under the optical power constraint for dimming control, the protograph-based extrinsic information transfer (PEXIT) analysis is utilized to optimize the DC bias and signal power jointly by searching the lowest optical threshold. To reduce the search space, an approximate method is proposed to simplify the PEXIT procedure of the optimal solution. Both the optimal configuration and simplified method are in good agreement with the simulation results.

An Addition-Decomposable Relaying Protocol and Signal Design for Optical Wireless Communications

In this paper, we consider a kind of optical wireless communications cooperative relaying systems where the relay node not only forwards the source's signal, but also transmits its own signal simultaneously. We propose an addition-decomposable relaying (ADR) protocol for the relay node. In the protocol, the demodulated signal from the source and the signal from the relay are superposed with a signal design method called additively uniquely decomposable constellation pair (AUDCP). An extended Farey sequence is established to formulate the feasible power ratio region of the AUDCP signal. The error performance of the signal pair is dominated by two consecutive terms in the sequence. Making full use of the properties of the extended Farey sequence, we analyze the transmission performance of the protocol, and derive the closed-form dominant terms of symbol error rates (SERs) of the signal pair. In addition, under the average optical power constraint, we optimize power allocation for the cooperative system to minimize the average SER at the destination node. Simulation results demonstrate that the approximate SERs derived are close to the actual ones in high signal to noise ratio regimes, and the error performance of ADR outperforms that of conventional schemes.

A Comparative Study of Unipolar OFDM Schemes in Gaussian Optical Intensity Channel

We study the information rates of unipolar orthogonal frequency division multiplexing (OFDM) in discrete-time optical intensity channels (OIC) with Gaussian noise under average optical power constraint. Several single-, double-, and multi-component unipolar OFDM schemes are considered under the assumption that independent and identically distributed. Gaussian or complex Gaussian codebook ensemble and nearest neighbor decoding (minimum Euclidean distance decoding) are used. We obtain an array of information rate result. These results validate existing signal-to-noise-and-distortion-ratio-based rate analysis, establish the equivalence of information rates of certain schemes, and demonstrate the evident benefits of using component-multiplexing at high signal-to-noise-ratio (SNR). For double- and multi-component schemes, the component power allocation strategies that maximize the information rates are investigated. In particular, by utilizing a power allocation strategy, we prove that several multi-component schemes approach the high SNR capacity of the discrete-time Gaussian OIC under average power constraint to within 0.07 bits.

Adaptive LACO-OFDM With Variable Layer for Visible Light Communication

In this paper, the adaptive layered asymmetrically clipped optical orthogonal frequency division multiplexing (LACO-OFDM) with variable layer is investigated. The channel capacities of the adaptive LACO-OFDM with different layers are formulated to determine the optimal layer amount under different optical signal-to-noise ratio scenarios with electrical or optical power constraint, respectively. Computer simulations verify the validity of the theoretical analysis for the proposed layer assignment scheme.

Optimal power allocation for downlink two-user non-orthogonal multiple access in visible light communication

In this work, we consider employing the NOMA (Non-Orthogonal Multiple Access) technique in downlink VLC (Visible Light Communication) for performance enhancement. In particular, focusing on a typical NOMA scenario with two users, we optimize the power allocation strategies under both sum-rate maximization and max-min fairness criteria, where practical optical power and QoS (Quality of Service) constraints are included. As our main contribution, we achieve optimal power allocation solutions in semi-closed forms via mathematical analysis, which, to the best of our knowledge, have not been reported in literature. Simulation results demonstrate that NOMA can provide remarkable performance gains over OMA (Orthogonal Multiple Access) in the context of VLC downlinks.

Interference in IM/DD Optical Wireless Communication Networks

The emerging field of optical wireless communications (OWC) offers exceptional promise as a technology for next-generation wireless networks. High data rate capabilities and ultra-dense access point deployment will allow OWC to supplement traditional RF technologies and remove congestion from the crowded RF spectrum. When implementing OWC via intensity modulation with direct detection (IM/DD), peak optical power emission constrains the instantaneous optical power. Average optical power is also constrained by eye safety regulations and illumination requirements of infrared and visible light communication (VLC) systems, respectively. These constraints differ from the conventional electrical power constraint of RF and wireline systems. Accordingly, performance metrics such as signal-to-noise ratio (SNR) and signal-to-interference-plus-noise ratio (SINR) have been redefined in relation to the optical channel constraints in order to provide fair comparison across OWC implementations. In densely deployed networks, interference has a significant effect on system performance. Two key properties simplify the analysis of RF networks: 1) the relationship between electrical power and interference variance is modulation agnostic and 2) many interferers are typically assumed. The former allows SINR to be defined in terms of the channel constraint and the latter allows the aggregate disturbance from interference-plus-noise to be modeled as an additive Gaussian random component. In OWC networks, the optical power constraints relate to interference variance in a modulation specific manner, and the highly directional optical channel creates instances where a small set of interferers dominate the aggregate disturbance. In this work, we first derive bounds on the variance of OWC interference under the constraints of an OWC channel. We then evaluate the accuracy of assuming that interference follows a Gaussian distribution. Finally, we show results of a study in a testbed environment with 15 VLC-enabled LED luminaires in order to empirically evaluate OWC interference characteristics.

Secrecy-Oriented Transmitter Optimization for Visible Light Communication Systems

Securing visible light communication (VLC) systems with physical-layer technologies has drawn increasing attention. In this paper, we leverage both transmit beamforming and jamming techniques to enhance communication secrecy for a multiple-input single-output (MISO) VLC system with presence of multiple eavesdroppers. The transmit beamformer and jamming precoder are jointly optimized in the sense that the signal-to-noise ratio (SNR) of the legitimate user is maximized subject to maximum SNR constraints imposed on eavesdroppers, as well as light-emitting diode (LED) optical power constraints. Despite the fact that the corresponding optimization problem has a nonconvex fractional form, we are able to determine its locally optimal solution via Charnes–Cooper transformation and minorization-maximization algorithm. Furthermore, we study the extension to a more complicated scenario where perfect channel state information at the transmitter (CSIT) is not available. The superiority of the proposed algorithms is finally verified via simulations.

Bandlimited Optical Intensity Modulation Under Average and Peak Power Constraints

Bandlimited optical intensity channels, arising in applications such as visible light communications, require that all signals satisfy a bandwidth constraint as well as average, peak, and non-negativity amplitude constraints. In this paper, a 2-D signal space for bandlimited optical intensity channels is presented. A novel feature of this model is that the non-negativity and peak constraints are relaxed, and the signal space parameterizes the probability of the negative or peak amplitude excursions in the output. Although the intensity channel only supports non-negative amplitudes, the impact of clipping on system performance is shown to be negligible if the likelihood of the negative excursion is small. A tractable approximation using finite series is used to accurately compute the probability of clipping under average and peak optical power constraints. The optical power and spectral efficiencies using hexagonal lattice constellations are computed. Schemes designed in this paper have higher average and peak optical power gains than $M$ -ary pulse amplitude modulation (PAM) using previously established techniques for spectral efficiencies greater than 2.5 and 3.5 bits/s/Hz, respectively. For high spectral efficiency, e.g., greater than 6 bits/s/Hz, the proposed scheme attains a more than 2-dB average and peak optical power gain over 16-PAM using the previous approaches.

Leakage-based precoding for MU-MIMO VLC systems under optical power constraint

In this paper, we investigate a multiuser multiple-input multiple-output (MU-MIMO) system for indoor visible light communication (VLC), in which precoding is conducted under optical power constraint rather than electrical power constraint. Leakage-based precoding designed by maximizing signal-to-leakage-plus-noise ratio (SLNR) is adopted to suppress the multiuser interference under optical power constraint and power allocation is proposed to maximize the throughput of the system. Simulations demonstrate the performance gain of optimal power allocation and indicate that the leakage-based precoding scheme outperforms zero forcing counterpart when the channel is highly correlated and still works well when the number of transmitters is less than that of receivers.

Diversity-optimal power loading for intensity modulated MIMO optical wireless communications.

In this paper, we consider the design of space code for an intensity modulated direct detection multi-input-multi-output optical wireless communication (IM/DD MIMO-OWC) system, in which channel coefficients are independent and non-identically log-normal distributed, with variances and means known at the transmitter and channel state information available at the receiver. Utilizing the existing space code design criterion for IM/DD MIMO-OWC with a maximum likelihood (ML) detector, we design a diversity-optimal space code (DOSC) that maximizes both large-scale diversity and small-scale diversity gains and prove that the spatial repetition code (RC) with a diversity-optimized power allocation is diversity-optimal among all the high dimensional nonnegative space code schemes under a commonly used optical power constraint. In addition, we show that one of significant advantages of the DOSC is to allow low-complexity ML detection. Simulation results indicate that in high signal-to-noise ratio (SNR) regimes, our proposed DOSC significantly outperforms RC, which is the best space code currently available for such system.

Optimal Constellation Design for Indoor 2x2 MIMO Visible Light Communications

Owing to the line-of-sight characteristics in indoor visible light communication (VLC) systems, different geometrical locations of transmitters and receivers provide distinct channel correlations, thus bringing various impacts on the conventional multiple-input multiple-output (MIMO) techniques. In this letter, by taking full advantage of the channel state information (CSI) available at both the transmitter and the receiver, we present an optimal constellation for the 2 × 2 MIMO-VLC system, where the signal vectors are selected to maximize the minimum Euclidean distance under a total optical power constraint. Simulation results verify both the accuracy and the validity of our analytical derivation, and show its significant error performance gains over repetition coding (RC) and space shift keying (SSK) for mobile receivers, respectively.

Offset and Power Optimization for DCO-OFDM in Visible Light Communication Systems

Direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) is used in visible light communications (VLC) for high rate transmission. Due to the unipolarity of optical signals, a direct current (DC) offset along with proper clipping is adopted in DCO-OFDM. A high DC-offset causes a waste of power and a low DC-offset causes a severe clipping distortion. Thus, the DC-offset and information-carrying power shall be jointly optimized to balance clipping distortion and power consumption. We consider the offset and power optimization problem under the optical and/or electrical power constraints over flat and dispersive channels. In the flat case, we analytically characterize the optimal solution, based on which several important insights are provided. We also investigate the relation and impact of the two constraints, and propose efficient algorithms to obtain the optimal solution. In the dispersive case, we identify the scope of the optimal solution, and provide efficient algorithms to jointly optimize the offset and power. The optimality and superiority of the proposed methods are validated by numerical results.

Rate Maximization for Downlink Multiuser Visible Light Communications

We are interested in optimizing transmit beamforming for a visible light communication multiuser system such that the achievable sum rate is maximized. We first derive a closed-form sum rate expression via information-theoretic tools. Then, by further including light-emitting diode optical power constraints, we formulate the rate maximization problem in a mathematical optimization form. To address this difficult non-convex problem, we establish an iterative algorithm that exploits sequential parametric convex approximation. Compared with the existing zero-forcing beamforming strategies, the proposed approach does not restrict the co-channel interference to be zero, and thus achieves a higher sum rate, as is validated by simulations.

Multiuser MISO Transceiver Design for Indoor Downlink Visible Light Communication Under Per-LED Optical Power Constraints

Light-emitting diode (LED)-based visible light communication (VLC), combining illumination and communication, is a promising technique for providing high-speed, low-cost indoor wireless services. In indoor environments, multiple LEDs routinely used as lighting sources may also be concomitantly invoked to support wireless services for multiple users, thus forming a multiuser multiple-input-single-output (MU-MISO) system. Since the user terminals detect all the light rays impinging from multiple LEDs, inter-user interference may severely degrade the attainable system performance. Hence, we conceive a transceiver design for indoor VLC MU-MISO systems to suppress the multiuser interference (MUI). In contrast to classic radio-frequency (RF) communication, in VLC, the signals transmitted from the LEDs are restricted by optical constraints, such as the real-valued nonnegativity of the optical signal, the maximum permissible optical intensity, and the constant brightness requirements of the LEDs. Given these practical constraints, we design the optimal transceiver relying on the objective function of minimizing the maximum mean square error (MMSE) between the legitimate transmitted and received signals of the users and show that it can be readily found by solving a convex second-order cone program. Then, we also propose a simplified transceiver design by incorporating zero-forcing (ZF) transmit precoding (TPC) and show that the TPC coefficients can be efficiently found by solving a linear program. The performance of both the optimal and the simplified transceiver is characterized by comprehensive numerical results under diverse practical VLC system setups.

Polar-Based OFDM and SC-FDE Links Toward Energy-Efficient Gbps Transmission Under IM-DD Optical System Constraints [Invited

Recent studies have investigated the use of orthogonal frequency division multiplexing (OFDM) and single carrier frequency-domain equalization (SC-FDE) for gigabyte per second (Gpbs) short-range optical wired and wireless access networks based on direct intensitymodulation with direct detection (IM-DD). OFDM systems have an inherent high peak-to-average power ratio (PAPR). SC-FDE systems have lower PAPR and thus outperform comparable OFDM systems in terms of bit-error performance under various practical considerations, mainly the limited dynamic range of operation at the transmitter. In RF-based OFDM and SC-FDE systems, the output signal is bipolar, complex, and cannot be transmitted in IM-DD systems. Therefore, several power efficient schemes, however spectrally inefficient, have been proposed to make positive OFDM and SC-FDE signals, e.g., asymmetrically clipped optical OFDM (ACO-OFDM) and repetition and clipping optical SCFDE (RCO-SCFDE). To increase the data rate, novel OFDM and SC-FDE signal formats, called polar OFDM (P-OFDM) and polar SC-FDE (P-SC-FDE), based on a polar representation of complex symbols, are proposed. The proposed format offers twice as much spectral efficiency as state-of-the art unipolar OFDM and SC-FDE formats. Moreover, using a power allocation approach on the time-domain positive samples, the PAPR is further reduced, and the numerical evaluation of the bit-error performance under optical source power and dynamic range constraints demonstrates superior results toward Gpbs transmission.

Quantum noise in optical tweezers

Quantum enhanced sensitivity in optical tweezers based particle tracking was recently demonstrated. This has provided the necessary tool for quantum metrology to play an important role in biological measurements. Here we introduce the basic theory relevant to such optical tweezers experiments, and overview the significance of sub-shot noise limited sensitivity to practical experiments. In particular, biophysical experiments are subject to optical power constraints, which therefore limits the absolute sensitivity which is classically achievable. Quantum enhanced particle tracking can overcome this limit, and is therefore likely to play an important role in such biophysical experiments in the near future.

Information Rate of OFDM-Based Optical Wireless Communication Systems With Nonlinear Distortion

In this paper, a piecewise polynomial function is proposed as a generalized model for the nonlinear transfer characteristic of the transmitter for optical wireless communications (OWC). The two general multicarrier modulation formats for OWC based on orthogonal frequency-division multiplexing (OFDM), direct-current-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM), are studied. The nonlinear distortion of the electrical signal-to-noise ratio (SNR) at the receiver is derived in closed form, and it is verified by means of a Monte Carlo simulation. This flexible and accurate model allows for the application of pre-distortion and linearization of the dynamic range of the transmitter between points of minimum and maximum radiated optical power. Through scaling and DC-biasing the transmitted signal is optimally conditioned in accord with the optical power constraints of the transmitter front-end, i.e., minimum, average and maximum radiated optical power. The mutual information of the optimized optical OFDM (O-OFDM) schemes is presented as a measure of the capacity of these OWC systems under an average electrical power constraint. When the additional DC bias power is neglected, DCO-OFDM is shown to achieve the Shannon capacity when the optimization is employed, while ACO-OFDM exhibits a 3-dB gap which grows with higher information rate targets. When the DC bias power is counted towards the signal power, DCO-OFDM outperforms ACO-OFDM for the majority of average optical power levels with the increase of the information rate target or the dynamic range. The results can be considered as a lower bound on the O-OFDM system capacity.

EVM and achievable data rate analysis of clipped OFDM signals in visible light communication

Orthogonal frequency division multiplexing (OFDM) has been considered for visible light communication (VLC); thanks to its ability to boost data rates as well as its robustness against frequency-selective fading channels. A major disadvantage of OFDM is the large dynamic range of its time-domain waveforms, making OFDM vulnerable to nonlinearity of light emitting diodes. DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are two popular OFDM techniques developed for the VLC. In this article, we will analyze the performance of the DCO-OFDM and ACO-OFDM signals in terms of error vector magnitude (EVM), signal-to-distortion ratio (SDR), and achievable data rates under both average optical power and dynamic optical power constraints. EVM is a commonly used metric to characterize distortions. We will describe an approach to numerically calculate the EVM for DCO-OFDM and ACO-OFDM. We will derive the optimum biasing ratio in the sense of minimizing EVM for DCO-OFDM. In addition, we will formulate the EVM minimization problem as a convex linear optimization problem and obtain an EVM lower bound against which to compare the DCO-OFDM and ACO-OFDM techniques. We will prove that the ACO-OFDM can achieve the lower bound. Average optical power and dynamic optical power are two main constraints in VLC. We will derive the achievable data rates under these two constraints for both additive white Gaussian noise channel and frequency-selective channel. We will compare the performance of DCO-OFDM and ACO-OFDM under different power constraint scenarios.