Existing indoor positioning methods for visible-light communication systems require large database, powerful signal processing units, additional sensors such as gyroscopes, or the receiver must be placed towards a certain angle. These assumptions limit the applications of the indoor positioning systems in low-cost scenarios. In this work, we propose a novel positioning framework based on the angle differences of arrival (ADOA) in 3-dimensional coordinate systems, which can be used in receivers with image sensors or photo diodes. The proposed ADOA positioning does not require the receiver to be placed toward a certain angle and no additional sensor is required. Two positioning algorithms are proposed: one is based on the method of exhaustion (MEX), and the other is based on the least squares method (LSM). The MEX algorithm is analytically proved to be the optimal, while the LSM algorithm has much lower complexity. Both upper and lower bounds are derived for the average discrepancy between the exact position and the estimated position. These performance bounds can facilitate the design of light-emitting diode array. Experimental results show that the MEX algorithm can achieve average error of 3.20 cm with time cost of 0.36 sec, and the LSM algorithm can achieve average error of 14.66 cm with time cost of 0.001 sec.

Since optical links are easily blocked by obstacles in the environment, they have been considered difficult to directly perform wireless communications. However, reconfigurable intelligent surfaces (RISs), as a new type of digital coding meta-materials, can significantly improve the coverage of optical communication by establishing new links. We propose a controllable multi-branch wireless optical communication system based on optical RISs. By setting up multiple optical RISs in the environment, multiple artificial channels are built to improve the system performance and to reduce the outage probability. In this paper, we investigate three factors affecting channel coefficients, including beam jitter, RIS jitter, and obstruction probability. We derive closed-form probability density function of channel coefficients, asymptotic average bit-error rate (BER), and the outage probability for systems with multiple branches. Our analysis reveals that the probability density function contains an impulse function, which causes irreducible error rate and outage probability floors. Based on our numerical results, the BER and outage probability floor of the multi-branch system are significantly reduced compared with the single direct path system. Therefore, the optical RIS assisted multi-branch wireless communication is a promising solution against obstacles of optical channel.

This paper investigates estimation of direction of arrivals (DOAs) and time of arrivals (TOAs) for a multi-user orthogonal frequency-division multiplexing (OFDM) system that is equipped with large-scale arrays and interfered by carrier frequency offsets (CFOs). We propose a novel scheme for joint TOA and DOA estimation with CFO compensation by taking advantages of large-scale receive antenna arrays. The pilot is designed to balance the pilot length and the estimation performance. The proposed CFO estimation scheme is with lower complexity. After analyzing the resolution of joint DOA and TOA estimation, we find a criterion to choose an appropriate smoothing window for the best resolution and provide insights in sub-carrier selection strategy. We also prove that the large-scale array can asymptotically achieve perfect resolution of both TOA and DOA estimation. We reveal the pseudo peak (PP) elimination property of large-scale arrays and exploit it to develop a novel path number estimation algorithm accordingly. Based on our numerical results, the proposed scheme can efficiently eliminate the CFO effect, the optimized smoothing window attains much better performance, and the path number estimation algorithm performs better than the traditional minimum description length (MDL) approach, especially in the low signal-to-noise ratio (SNR) regime.

Traditional visible light positioning (VLP) systems estimate receivers’ coordinates based on the assumption that light-emitting diode (LED) coordinates are known accurately, which is not always practical. Even with accurate laser range finders, the structural changes of the buildings due to temperature, humidity, and material aging might affect the LED coordinates unpredictably. Besides, when the number of LEDs increases, it is time-consuming to build a database of LED coordinates. In this paper, we propose a novel system exploiting two optical angle-of-arrival (AOA) estimators to localize the LEDs, and the AOA estimators have fixed relative positions. Each AOA estimator has four photodiodes (PDs) towards different orientations to estimate the incident light direction. Considering the additive noises imposed on the PDs, we derive a closed-form error expression of the LED coordinate estimator. Analytical and simulation results show that the two AOA estimators should be set symmetrically with respect to the room center to reduce the error of the LED coordinates database. Moreover, an experimental platform is built with two fixed AOA estimators and one moving LED in order to verify the approach. The measured average localization error is 4.46 cm and the time of one localization is about 23.4 ms.

Prior asymptotic performance analyses are based on the series expansion of the moment-generating function (MGF) or the probability density function (PDF) of channel coefficients. However, these techniques fail for lognormal fading channels because the Taylor series of the PDF of a lognormal random variable is zero at the origin and the MGF does not have an explicit form. Although lognormal fading model has been widely applied in wireless communications and free-space optical communications, few analytical tools are available to provide elegant performance expressions for correlated lognormal channels. In this work, we propose a novel framework to analyze the asymptotic outage probabilities of selection combining (SC), equal-gain combining (EGC) and maximum-ratio combining (MRC) over equally correlated lognormal fading channels. Based on these closed-form results, we show: i) the outage probability of EGC or MRC becomes an infinitely small quantity compared to that of SC at high signal-to-noise ratio (SNR); ii) channel correlation can cause an infinite performance loss at high SNR; iii) negatively correlated lognormal channels can outperform the independent lognormal channels. The analyses reveal insights into the long-standing problem of asymptotic performance analyses over correlated lognormal channels, and circumvent the time-consuming Monte Carlo simulation and numerical integration.