Hong-Yi Yu

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.

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.

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.

Block Precoding for Peak-Limited MISO Broadcast VLC: Constellation-Optimal Structure and Addition-Unique Designs

In this paper, we investigate the design of energy-efficient space-time modulation for peak-limited MISO broadcast visible light communication (VLC) systems by cooperatively managing the non-negative multiuser interference. We first characterize a constellation-optimal structure that maximizes the worst-case minimum Euclidean distance of all users for a general space-time modulation design. It turns out that the optimal space-time constellation can be constructed via the spatial repetition of the optimal multidimensional constellation in time dimension over ideal additive white Gaussian noise channels. Then, based on this structure, we specifically design two classes of energy-efficient time-dimensional constellations: 1) for the integer overall bit rate, we design the optimal linear precoded block design, which admits fast maximum likelihood demodulation algorithms. 2) for the non-integer case, we propose a nonlinear precoding scheme called block coded modulation, which sums the code word sets of the optimal linear design and a block channel code. In addition, we show that these two classes of designs are addition-unique, thus, generating an energy-efficient mapping from users’ data to the transmitted constellations. This property enables the efficient demodulation of the sum signal from a noisy received signal as well as the decoding of individual signal from the estimated sum signal. Extensive computer simulations indicate that our addition-unique designs have remarkable performance gains over the currently available zero-forcing, minimum mean square error and time-division multiple access methods for the multiuser multi-input-single-output VLC broadcast systems.

Energy-efficient space-time modulation for indoor MISO visible light communications.

We consider an indoor multi-input single-output (MISO) visible light communication (VLC) system without channel state information at the transmitter. For such a system, an energy-efficient time-collaborative modulation (TCM) constellation is first designed by minimizing a total optical power subject to a fixed minimum Euclidean distance. Then, a new space-time transmission scheme is proposed. Comprehensive computer simulations indicate that our proposed design always has better average error performance within illumination coverage area than the currently available schemes for this application.

Linear Receivers for Full-Diversity Training Space-Time Block Codes

A noncoherent flat-fading wireless communication system having multiple transmitter antennas and a single receiver antenna [multiple-input-single-output (MISO)] is considered in this paper, with focus on error performance analysis of a training space-time block code (STBC) for a square quadrature amplitude modulation (QAM) constellation using linear receivers. For such a system, a noncoherent training zero-forcing (ZF) receiver is proposed and proven to be able to extract full diversity from a training STBC, with the underlying coherent code enabling full diversity for the coherent ZF receiver. Comprehensive computer simulations verify the theoretic analysis provided in this paper.

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.