Thomas Q. Wang

Analysis of An Optical Wireless Receiver Using a Hemispherical Lens With Application in MIMO Visible Light Communications

White lighting LEDs offer great potential for high speed communications, especially for indoor applications. However, for their widespread adoption, two important issues need to be addressed: the lack of diversity in multiple-input multiple output (MIMO) systems, and the small field of view of receivers. In this paper, we describe a design using a hemispherical lens in the receiver that solves these problems. By using classical optics, we derive exact expressions for the channel gain and the optical power density of the projected images. Simulation results of a typical indoor scenario show that the new system has a wide field of view, and provides adequate channel gain for angles of incidence as large as 70 degrees. We present the distribution of optical power on the imaging plane for various receiving positions and tilted receivers over a number of representative indoor scenarios. They show that the images of LEDs are clearly distinguishable. The results demonstrate the presence of low channel correlations between individual transmitters and receivers. Consequently, this confirms that the new technique is capable of providing significant diversity order for MIMO optical wireless applications.

Position Accuracy of Time-of-Arrival Based Ranging Using Visible Light With Application in Indoor Localization Systems

This paper analyzes an indoor positioning system that uses white lighting LEDs. Modulated signals transmitted by the LEDs are used as the basis of time-of-arrival-based distance estimation. The theoretical limits on the accuracy of estimation are calculated by deriving the Cramer-Rao bound for intensity modulated windowed sinusoidal signals. Calculations for a typical indoor scenario, assuming perfect synchronization between transmitter and receiver, but using realistic values for other parameters show that very accurate distance estimates are achievable, with typical errors being in the order of centimeters depending on the frequency and power of the sinusoidal signals, the distance from the LED and the properties of the LED and the photoreceiver.

MIMO Optical Wireless Communications Using ACO-OFDM and a Prism-Array Receiver

In this paper, we analyze the performance of a prism-array receiver in a multiple-input multiple-output (MIMO) intensity-modulated direct-detection optical wireless communication system. The elements of the MIMO channel matrix depend on the geometry of each prism and the orientation of the prism relative to the optical transmitter. An expression is derived for each channel gain as a function of the angle of the prism, the aspect ratio and area of the receiving surface of the prism, and the orientation and distance between the transmitter and receiver. Results are presented for the directionality and overall field of view (FOV) of a receiver comprising six prisms, showing that compact receivers can be designed, which combine well-conditioned channel matrices with wide FOVs. Simulation results are presented for a typical indoor visible light communication system where four light-emitting diode lights transmit information using asymmetrically clipped optical orthogonal frequency division multiplexing and linear equalizers are used in the receiver. It is shown that the overall bit error rate depends very strongly on the angle of the prisms and on the position of the receiver, but that a simple six-prism receiver can successfully decouple the four transmitted signals at typical receiver positions within the room.

Angular diversity for indoor MIMO optical wireless communications

In this paper, we present a new type of receiver for MIMO optical wireless communications. The new receiver consists of multiple receiving elements (REs), each made up of a photodetector (PD) placed under an aperture. The directionality of each RE is shown to depend on the relative position of the PD and the aperture. Receiving patterns are calculated for various MIMO receiver designs, demonstrating that significant angular diversity can be achieved within a compact receiver structure. Simulation results are presented for a number of typical indoor scenarios when the transmitted signals are modulated using asymmetrically clipped optical OFDM (ACO-OFDM). For the scenarios considered, the bit error rate is lowest in the middle of the room and highest in the corners. The results indicate that for typical parameter values, reliable communication can be achieved throughout the room.

Hemispherical lens based imaging receiver for MIMO optical wireless communications

White lighting LED based systems are emerging as an important form of high data rate communications, especially for indoor applications. Two limitations of existing systems are the small field of view of typical receivers and the poor performance of optical wireless MIMO due to lack of spatial diversity. In this paper we describe a novel design which overcomes these problems by using a hemispherical lens in the receiver. We show that the new system has a wide field of view and also provides significant spatial diversity for typical MIMO visible light scenarios. Numerical results are provided for a range of LED transmitters with different half power semi-angles. Our analysis shows that systems can be designed with adequate channel gain for angles of incidence as large as 70 degrees. The optical power density is also calculated to show the received optical power distributions for the case of four LED transmitters. The results indicate that the images of the LEDs are clearly separated. This reduces the channel correlations between individual transmitters and receivers and thus promises a significant diversity order for MIMO optical wireless systems.

Performance of Optical Receivers Using Photodetectors With Different Fields of View in a MIMO ACO-OFDM System

In this paper, we analyze the performance of optical receivers using photodetectors (PDs) with two different fields of view (FOVs) in a multiple-input multiple-output optical wireless communication system which uses intensity modulation and direct detection. The novel aspect is that the PDs in the receiver do not all have the same FOV. It is shown that the use of PDs with different FOVs leads to an invertible channel matrix even when the PDs are closely spaced. Simulations for a typical indoor visible light communications scenario where LED lights are used as data transmitters show that the signal-to-noise ratios at the equalizer outputs are much higher than for a receiver of the same dimension where all the PDs have the same FOV. Good performance can be achieved with PDs located in a 3.5 cm by 3.5 cm area. Finally, the overall bit error rate (BER) is calculated for systems using asymmetrically clipped optical OFDM as the modulation scheme. Results are presented for both zero forcing and minimum mean square error equalizers. It is shown that the BER varies with the receiver position, with higher values in the center and the corners of the room.

Cramer-Rao bound for indoor visible light positioning using an aperture-based angular-diversity receiver

In this paper, we investigate the problem of indoor positioning using visible light systems. The directional detector array we use is comprised of a number of receiving elements, each consisting of an aperture and a photo diode, which are arranged to offer good angular diversity, and can be implemented within a compact receiver structure [1]. The receiving elements receive the light from a number of white LEDs, which are typically attached to the ceiling, and which act as anchors. In order to get an indication of the received signal strengths of the different LEDs, we average the received signals over time. The relative signal strengths in the different receiving elements do not only provide information on the distance between the LEDs and the detector array, but also about the angle-of-arrival of the light. By combining the information of the receiving elements, the position of the detector can be estimated. In order to assess the accuracy of positioning algorithms based on this approach, we derive the Cramer-Rao lower bound on the position accuracy. Assuming the white LEDs transmit an optical power of 1 W, and the time averaging is done over 1 millisecond, an accuracy of the order of a centimetre can be achieved.

Performance of indoor MIMO optical wireless system using linear receiver with prism array

In this paper, we analyze the performance of an indoor MIMO optical wireless system with a linear receiver. The receiver uses an array of prisms to form channel matrices that can achieve angular diversity within a compact receiver structure. The transmission of light through a prism onto a photodetecting surface is analyzed showing that the channel gain depends on the orientation of the prisms. Thus, by arranging the prisms carefully, channel matrices with full rank can be formed in a multiple LED scenario, which facilitates the use of low complexity linear de-multiplexing algorithms, including zero-forcing (ZF) and minimum mean square error (MMSE). We present bit error rate (BER) results in a typical indoor scenario. It is shown that the BER varies with the position of the receiver, with relatively high values for the receiver located at the center and near the corners of the room.

MIMO Optical Wireless Receiver Using Photodetectors with Different Fields of View

This paper describes a new form of receiver for multiple-input multiple-output (MIMO) optical wireless communications. The new receiver uses a number of photodetectors (PDs), all facing in the same direction but with different fields of view (FOVs). This enables a compact planar receiver structure. The performance of the receiver is analyzed for an indoor visible light communications system where LED lights are used as data transmitters. It is shown that the MIMO channel matrix has full rank for typical receiver positions and that for a given number of PDs, the new receiver provides much greater diversity than a conventional receiver of the same overall dimensions in which all of the PDs have the same FOV. Finally the overall bit error rate (BER) is presented for systems using asymmetrically clipped optical OFDM (ACO-OFDM) as the modulation scheme. Results are presented for both zero forcing (ZF) and minimum mean square error (MMSE) equalization in the receiver. It is shown that the BER varies with receiver position, with relatively high values in the center and the corners of the room.

Theoretical Lower Bound for Indoor Visible Light Positioning Using Received Signal Strength Measurements and an Aperture-Based Receiver

Indoor visible light positioning (VLP) using signals transmitted by lighting LEDs is a topic attracting increasing interest within the research community. In the recent years, VLP techniques using a range of receiver structures and positioning algorithms have been described. In this paper, we analyze the performance of a VLP system, which uses an aperture-based receiver and measurements of received signal strength. An aperture-based receiver has a number of receiving elements, each consisting of a photodiode and an associated aperture. It has been shown that receivers of this form can be designed which are compact and provide both a wide overall field-of-view and good angular diversity. As a result, they can efficiently extract position-related information from light transmitted by nondirectional LEDs. In our approach, we correlate the signals at the outputs of the photodiodes with a set of reference signals. The resulting observations include information on the received signal strength as well as the angle-of-arrival, and are used to directly estimate the receivers position. In order to assess the performance of positioning algorithms based on this approach, we derive the Cramer–Rao lower bound on the position estimate. We show that the Cramer–Rao bound depends on the selected reference signal, and that subcentimetre to centimetre accuracy can be obtained, using only a limited number of nondirectional LEDs.