Yi-Jun Zhu

Space-Collaborative Constellation Designs for MIMO Indoor Visible Light Communications

In conventional multi-input multioutput (MIMO) visible light communication (VLC) techniques such as repetition coding (RC), spatial multiplexing (SMP), and spatial modulation (SM), the transmitted symbols through different light-emitting diodes located different space are usually generated from unipolar pulse amplitude modulation constellations independently. In this letter, we propose a novel constellation design for MIMO indoor VLC systems, in which the two symbols from two spaces collaborate so closely that the average optical power is minimized under the constraint that the minimum Euclidean distance is fixed. Therefore, this new constellation is called space-collaborative constellation (CC). Computer simulations show that the CC always has better error performance than SM and SMP, and also has better error performance than RC for most of good channels as well as for some really bad channels.

Full large-scale diversity space codes for MIMO optical wireless communications

In this paper, we consider a multiple-input-multiple-output optical wireless communication (MIMO-OWC) system suffering from log-normal fading. In this scenario, a general criterion for the design of full large-scale diversity space code (FLDSC) with the maximum likelihood (ML) detector is developed. Based on our criterion, FLDSC is attained if and only if all the entries of the space coding matrix are positive. Particularly for 2×2 MIMO-OWC with unipolar pulse amplitude modulation (PAM), a closed-form linear FLDSC satisfying this criterion is attained by smartly taking advantage of some available properties as well as by developing some new interesting properties on Farey sequences in number theory to rigorously attack the continuous and discrete variables mixed max-min problem. In fact, this specific design not only proves that a repetition code (RC) is the best linear FLDSC, but also uncovers a significant difference between MIMO radio frequency (RF) communications and MIMO-OWC that space-only transmission is sufficient for a full diversity achievement. Computer simulations demonstrate that FLDSC substantially outperforms spatial multiplexing with the same total optical power and spectral efficiency and the latter obtains only the small-scale diversity gain.

Optimal Constellation Design for Indoor 2×2 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.

On the Optimality of Spatial Repetition Coding for MIMO Optical Wireless Communications

As a spatial diversity transmission scheme, repetition code (RC) is conjectured to be optimal in the sense of error performance for an intensity modulated direct detection multi-input-multi-output optical wireless communication (IM/DD MIMO-OWC) over log-normal fading channels. Despite the fact that all the experimental evidences thus far have strongly demonstrated that this hypothesis is indeed true, its mathematical proof remains a long-standing open problem mainly due to the lack of an explicit signal design criterion like MIMO radio frequency communications. In this letter, subject to two commonly used power constraints, we prove the optimality of RC under a much weaker condition in the sense of maximizing both large-scale and small-scale diversity gains for any space signalling using the recently established pair-wise error probability design criterion for a maximum likelihood (ML) detector.

Signal-Cooperative Multilayer-Modulated VLC Systems for Automotive Applications

In visible light communication (VLC) systems for automotive applications, different kinds of traffic data with intended priority are usually demanded for transmission to meet an important requirement for both driving safety and the simplicity of a receiver, mainly due to a time urgency issue. For this need, a concept called an additively uniquely decomposable constellation (signal amplitude set) group (AUDCG) is proposed in this paper to transmit multilayer data for VLC through constellation cooperation. Then, an optimal AUDCG is designed by minimizing the average optical power subject to a fixed minimum Euclidean distance. One of the significant advantages of this optimal design is fast demodulation of the sum signal from a noisy received signal, as well as fast decoding of individual signal from the estimated sum signal. Another important advantage is that this design allows each user constellation to be flexibly assigned to meet different priority requirements. Computer simulations indicate that our proposed design has better error performance than the currently available time-orthogonal transmission scheme for this application.

Space Codes for MIMO Optical Wireless Communications: Error Performance Criterion and Code Construction

In this paper, we consider a multiple-input-multiple-output optical wireless communication (MIMO-OWC) system in the presence of log-normal fading. In this scenario, a general criterion for the design of full-diversity space code (FDSC) with the maximum likelihood detector is developed. This criterion reveals that in a high signal-to-noise ratio regime, MIMO-OWC offers both large-scale diversity gain, governing the exponential decay of the error curve, and small-scale diversity gain, producing traditional power-law decay. Particularly for a two by two MIMO-OWC system with unipolar pulse amplitude modulation, a closed-form solution to the design problem of a linear FDSC optimizing both diversity gains is attained by taking advantage of the available properties on the successive terms of Farey sequences in number theory as well as by developing new properties on the disjoint intervals formed by the Farey sequence terms to attack the continuous and discrete variables mixed max–min design problem. In fact, this specific design not only proves that a repetition code is the optimal linear FDSC optimizing both the diversity gains but also uncovers a significant difference between MIMO radio frequency communications and MIMO-OWC that space dimension alone is sufficient for a full large-scale diversity achievement. Computer simulations demonstrate that FDSC substantially outperforms uncoded spatial multiplexing with the same total optical power and spectral efficiency, and the latter provides only the small-scale diversity gain.

A Double-Layer VLC System With Low-Complexity ML Detection and Binary Constellation Designs

We consider a double-layer visible light communication system where transmitted data is split into two layers with different priorities and variant transmission rates. High-priority data is assigned to the upper layer with low transmission rate, while low-priority data is assigned to the lower layer with higher transmission rate. For this scenario, we develop a low-complexity maximum likelihood detection algorithm. In addition, several binary constellation designs corresponding to different priority orders are presented. Computer simulations show that maximum likelihood detection detection significantly improves the error-rate performance, compared to sum detection, without compromising the computational complexity of the receiver.

Experimental study on SPAD-based VLC systems with an LED status indicator

According to our experimental observation, the effect of dead time caused by the quenching circuit of single photon avalanche diode (SPAD) devices could affect the system performance significantly. Hence, we propose a corresponding synchronization scheme and a fast blind union detection (FBUD) algorithm for the received photon-counting pulse waveform by considering the effect of dead time. To verify our proposed theory under the long distance turbulence fading channel, we demonstrate a long distance SPAD-based visible light communication system with a red light-emitting diode status indicator in the practical circuit board. The experimental results indicate that the proposed FBUD algorithm outperforms the conventional dead-time free Poisson multiple-symbol detection method under different conditions. Meanwhile, the bit-to-error ratio gaps of the two algorithms will become more and more larger with the increasing of the data rate.

Blind Detection for SPAD-Based Underwater VLC System Under P–G Mixed Noise Model

There is Poisson slot noise generated by the single photon avalanche diode (SPAD) and Gaussian thermal noise generated by the transimpedance amplifier, respectively, if an analog SPAD detector is applied in the visible light communication (VLC) system. In this letter, for the long distance SPAD-based underwater VLC system, a fast blind multiple-symbol detection (MSD) algorithm is developed under the pure Poisson noise model. And then, a blind MSD algorithm based on generalized maximum-likelihood rule under the Poisson–Gaussian (P–G) mixed noise model is proposed. A generalized Anscombe transform (GAT) is employed to make the P-G mixed noise model as an equivalent additive white Gaussian noise model, then an equivalent minimum Euclidean distance criterion is presented to formulate the optimization problem. Simulation results show that our blind MSD scheme can be identical to the performance under a GAT-based detector with perfect channel state information.

Training Receivers for Repetition-Coded MISO Outdoor Visible Light Communications

In this paper, we consider a multiple-input single-output (MISO) outdoor visible light communications (VLC) system in which the channel state information (CSI) at the receiver side is unknown and where the channel coefficients are independent and lognormal distributed. For such a system, we propose to take advantage of the repetition coding scheme in the space dimension and to add a fixed nonzero pilot bit to the head of the block to ensure the reliable blind estimation of channel coefficients. Then, a symbol-by-symbol maximum-likelihood (ML) receiver with linear complexity for the repetition-coded MISO outdoor VLC is developed, and the bit error performance is derived by integrating the conditional error function on the dominant decision region. It is proved that the training ML receiver can assure full-diversity achievement. In addition, optimal power allocation between the pilot bit and data bits is proven to be a concave problem, and an optimal power allocation scheme is presented. Simulation results demonstrate that the training receivers can achieve full diversity and that the optimal power allocation scheme can improve the error performance significantly.