Optical Power Allocation Research Articles

Non-orthogonal multiple access in a radio over fiber link based on optical-domain power allocation.

Non-orthogonal multiple access (NOMA) in a radio over fiber (RoF) link based on optical-domain power allocation is proposed and demonstrated. The NOMA is implemented at the RoF transmitter where two spectrum-overlapped microwave vector signals with an identical power level are modulated on an optical carrier to generate two orthogonally polarized optical signals. By passing the optical signals through a polarization controller (PC) and a polarizer, the power levels of the two optical signals are controlled to achieve optical power allocation (OPA). The optical signals are then transmitted over a fiber to the receiver. Since the power levels of the two microwave vector signals applied to the modulator are identical and the power allocation is implemented in the optical domain, the nonlinearity due to the higher-power input microwave vector signal is reduced, leading to an increase in the dynamic range. At the receiver, the two optical signals are detected at a photodetector (PD). To demultiplex the two microwave vector signals, a digital signal processing (DSP) algorithm is developed. The proposed approach is evaluated experimentally. The results show that the transmission performance in terms of error vector magnitude (EVM) is improved thanks to the increased dynamic range.

Enhancement of calculation accuracy of the integrated photonic tensor flow processer by global optical power allocation.

We present a global optical power allocation architecture, which can enhance the calculation accuracy of the integrated photonic tensor flow processor (PTFP). By adjusting the optical power splitting ratio according to the weight value and loss of each calculating unit, this architecture can efficiently use optical power so that the signal-to-noise ratio of the PTFP is enhanced. In the case of considering the on-chip optical delay line and spectral loss, the calculation accuracy measured in the experiment is enhanced by more than 1 bit compared with the fixed optical power allocation architecture.

Low-Complexity Receiver for HACO-OFDM in Optical Wireless Communications

In this letter, a low-complexity receiver (Rx) is proposed for hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM). Owing to the special time-domain property of HACO-OFDM, overlaid asymmetrically clipped OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitoned (PAM-DMT) signals can be distinguished in the time domain to reduce its computational complexity. Besides, the near-optimal optical power allocation is further applied to optimize the proposed system performance. Theoretical analysis and simulation results demonstrate that, the proposed Rx can achieve nearly the same bit error rate (BER) performance as the BER-optimal iterative Rx but with an effective complexity reduction, thus demonstrating its application potential in high-speed optical communication systems.

Performance of Indoor VLC System Under Random Placement of LEDs With Nonimaging and Imaging Receiver

This article analyzes the performance of multiple-input-multiple-output indoor visible light communication (VLC) system by randomly deploying the light-emitting-diodes (LEDs) using Matern hardcore point process (MHCP). Furthermore, photodetectors (PDs) with two different field of view (FOVs) have been utilized for an imaging as well as nonimaging receiver structures. It is a widely known fact that in a conventional VLC system, signal-to-noise ratio (SNR) profile inside the room varies with respect to the LED placement and the PDs position. Consequently, the proposed work attempts to achieve uniform SNR across the room, by utilizing MHCP based LED placement at the transmitter, and a nonimaging receiver with four PDs using 1-FOV, 2-FOV, and imaging receiver configurations. Simulation results show that random deployment of LED using MHCP configuration results in a more uniform SNR profile inside the room as compared to regular LED deployment schemes. Furthermore, three different power allocation schemes for LEDs namely equal power, distance-based power, and an optimal power allocation are proposed. For the different power allocation schemes, the average SNR and the variance of the received optical power (OP) inside the room are derived and compared. In addition, the closed-form expression for the bit-error-rate (BER) probability is derived for the proposed MHCP configuration using <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> - <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> keying as a modulation scheme. Results show improvement in BER performance with the OP allocation in comparison to other power allocation schemes both for imaging and nonimaging receiver configuration.

Absolute Value Layered ACO-OFDM for Intensity-Modulated Optical Wireless Channels

Enhanced unipolar orthogonal frequency division multiplexing (eU-OFDM) and layered asymmetrically clipped optical OFDM (LACO-OFDM) are spectrally efficient modulation techniques for intensity modulated systems which layer multiple non-negative signals. In this paper, we propose absolute value layered asymmetrically clipped optical OFDM (ALACO-OFDM), which further improves the spectral efficiency while using a smaller number of layers and no explicit direct current (DC) bias. In ALACO-OFDM, asymmetrically clipped optical OFDM (ACO-OFDM) signals are sent at the first $L$ layers and absolute value optical OFDM (AVO-OFDM) is used for the remaining subcarriers and transmitted simultaneously. Analysis indicates that ALACO- achieves higher spectral efficiency than eU- or LACO-OFDM while using a smaller number of layers. Bounds on achievable information rates of ALACO-OFDM and related layered ACO-OFDM techniques are also developed. Two optical power allocation schemes over the layers of ALACO-OFDM are developed with the objective of optimizing uncoded transmission performance and the achievable information rate respectively. Additionally, a theoretical bound on the uncoded BER of ALACO-OFDM is derived. Monte Carlo simulation results indicate ALACO-OFDM with the optimum power allocation achieves significant uncoded BER performance gains compared to its counterparts at the same spectral efficiency while having a smaller peak-to-average power ratio.

Design of a Cyclic Shifted LACO-OFDM for Optical Wireless Communication

This paper proposes a cyclic shifted layer asymmetrically clipped optical orthogonal frequency division multiplexing system (CS-LACO-OFDM) for optical wireless communications. The transmitted signal in CS-LACO-OFDM is generated by combining the signal of the first layer and signals of the remaining $L$ -1 layers with cyclic shift equivalents, where $L$ is the number of the CS-LACO-OFDM layers. In particular, the sets of cyclic shift value in CS-LACO-OFDM are modulated on the odd subcarriers of the first layer with complex-valued symbols. Information on cyclic shift sets can be easily detected with the help of modulated symbols conveyed in the first layer without increasing the receiver complexity. Simulation results show that the CS-LACO-OFDM has a similar peak-to-average power ratio performance as LACO-OFDM with separate selective mapping scheme when the number of candidate signals is the same but at remarkably low computational complexity. Furthermore, the average bit error rate of CS-LACO-OFDM over all layers is minimized by the proposed optimal optical power allocation.

Advanced ADO-OFDM With Adaptive Subcarrier Assignment and Optimized Power Allocation

Visible light communication (VLC) is emerging as a promising technology thanks to the superior advantages of light-emitting diodes. In this letter, an advanced asymmetrically clipped dc biased optical orthogonal frequency division multiplexing (AADO-OFDM) scheme with adaptive subcarrier assignment is proposed to accommodate the demands of multi-users. Additionally, the optimization of optical power allocation for AADO-OFDM under the constrained total optical power is investigated and formulated to maximize the tight lower bound of the total channel capacity. Simulation results show that compared with the schemes without optimal power allocation, the proposed scheme can maximize the lower bound of channel capacity.

Energy-Efficient Optimization Design for the Multi-Color LED Based Visible Light Communication Systems under Illumination Constraints

Visible light communications (VLCs) utilizing multi-color light-emitting diodes (LEDs) can achieve a high modulation bandwidth and high-quality illumination compared with phosphor-converted LEDs, which have attracted much attention. However, the spectrum overlapping of different colors may cause the crosstalk problem, which should be considered in the practical multi-color LED-based VLC systems. Due to the ever-increasing energy consumption, the interest in an energy-saving communication technique has further increased. In this paper, in order to maximize energy efficiency, an optimization problem of the optical power allocation scheme is formulated for the multi-color LED-based VLC systems under the necessary communication requirements and illumination constraints with luminance, chromaticity, and signal to interference plus noise ratio (SINR) constraints. Simulation results indicate that the proposed optimal power allocation scheme can reduce energy consumption while satisfying the illumination and communication requirements.

Iterative receiver for ADO-OFDM with near-optimal optical power allocation

Visible light communication (VLC) systems using orthogonal frequency division multiplexing (OFDM) are attracting increasing interests due to its inherent benefits such as high spectral efficiency, resistance to frequency-selective channels and so on. In this paper, a novel iterative receiver is proposed for asymmetrically clipped DC biased optical OFDM (ADO-OFDM), where asymmetrically clipped optical OFDM (ACO-OFDM) and DC biased OFDM (DCO-OFDM) signals are transmitted simultaneously. In our proposed iterative receiver, ACO-OFDM and DCO-OFDM time-domain signals are distinguished firstly. Then pairwise clipping, negative clipping and pairwise averaging are utilized in the iterative receiver to reduce the effect of noise and interference. In addition, an optimal solution to the optical power allocation factor for ACO-OFDM and DCO-OFDM signals is derived. Furthermore, to reduce the computational complexity, an approximation of the optimal solution is obtained. Both theoretical analysis and simulation results indicate that the approximate solution is near-optimal, and only a few detection iterations are required for the iterative receiver.

Multi-User Sum-Rate Optimization for Visible Light Communications With Lighting Constraints

In visible light communication (VLC) systems, white light emitting diodes (LEDs) are used as illumination sources and transmitters simultaneously. Compared to the phosphor-converted LEDs, multi-chip LEDs have higher modulation bandwidth. Consequently, the multi-chip based VLC systems have great potential for high data rate transmission. Since each chip of the multi-chip LEDs can be modulated independently, parallel communication channels are viable for information transmission. In this paper, in order to maximize the multi-user sum-rate for the multi-chip based multi-input single-output VLC systems, an electrical and optical power allocation scheme is proposed in consideration of the luminance, chromaticity, amplitude and bit error rate constraints. From the perspective of human color vision, the chromaticity constraint is defined within a MacAdam ellipse. As a result, the degree of freedom can be achieved by relaxing the chromaticity constraint from a fixed color point to an elliptic region. Numerical results demonstrate that with the increase of the total luminous flux, the maximum sum-rates present an open-down parabolic tendency due to the limited dynamic range of LEDs. Higher data rate can be achieved under higher correlated color temperature (CCT) for the variation of light components. In addition, the simulation results indicate that the shapes of the chromaticity constrained region (either ellipse or quadrangle) have little impact on the multi-user sum-rate at the same CCT.

Optical power allocation for adaptive transmissions in wavelength-division multiplexing free space optical networks

Attracting increasing attention in recent years, the Free Space Optics (FSO) technology has been recognized as a cost-effective wireless access technology for multi-Gigabit rate wireless networks. Radio on Free Space Optics (RoFSO) provides a new approach to support various bandwidth-intensive wireless services in an optical wireless link. In an RoFSO system using wavelength-division multiplexing (WDM), it is possible to concurrently transmit multiple data streams consisting of various wireless services at very high rate. In this paper, we investigate the problem of optical power allocation under power budget and eye safety constraints for adaptive WDM transmission in RoFSO networks. We develop power allocation schemes for adaptive WDM transmissions to combat the effect of weather turbulence on RoFSO links. Simulation results show that WDM RoFSO can support high data rates even over long distance or under bad weather conditions with an adequate system design.

Transmission capacity maximization for LED array-assisted multiuser VLC systems

In this paper, we present a light-emitting diode (LED) array-assisted visible light communication (VLC) system. Considering the requirements of illumination, the human eye safety, and the energy saving, we optimize the optical power allocation (PA) to maximize the transmission capacity of the overall system. Specifically, we discuss two typical scenarios where the LED array in each room serves multiple users either simultaneously or in a round-robin manner. The problem corresponding to the first case turns out to be nonconvex whose globally optimal solution is hard to obtain. To address this difficult problem, we first adopt the famous gradient projection algorithm to obtain a locally optimal solution, then we employ powerful convex relaxation techniques, and achieve a high-quality solution. While for the round-robin case, we are able to derive an optimal solution in closed form by virtue of Karush-Kuhn-Tucker conditions. Interestingly, the derived closedform solution follows a water-filling pattern with both square root and two-side clipping operations, which differs from the conventional water-filling-like PA in radiofrequency channels. Numerical results depict the performance advantage provided by the proposed PA strategies under various settings.