Alphan Sahin

Hybrid 3-D Localization for Visible Light Communication Systems

In this study, we investigate the hybrid utilization of angle-of-arrival (AOA) and received signal strength (RSS) information in visible light communication (VLC) systems for 3-D localization. We show that AOA-based localization method allows the receiver to locate itself via a least squares estimator by exploiting the directionality of light-emitting diodes (LEDs). We then prove that when the RSS information is taken into account, the positioning accuracy of AOA-based localization can be improved further using a weighted least squares solution. On the other hand, when the radiation patterns of LEDs are explicitly considered in the estimation, RSS-based localization yields highly accurate results. In order to deal with the system of non-linear equations for RSS-based localization, we develop an analytical learning rule based on the Newton-Raphson method. The non-convex structure is addressed by initializing the learning rule based on 1) location estimates, and 2) a newly developed method, which we refer as a random report and cluster algorithm. As a benchmark, we also derive the analytical expression of the Cramér-Rao lower bound for RSS-based localization, which captures any deployment scenario positioning in 3-D geometry. Finally, we demonstrate the effectiveness of the proposed solutions for a wide range of LED characteristics and orientations through extensive computer simulations.

Multi-Element Transmitter Design and Performance Evaluation for Visible Light Communication

Visible light communication (VLC) systems have significant potential for enabling broadband data rates by relying on existing lighting infrastructure. In this paper, we investigate the performance of multi-element VLC transmitter architecture for different transmitter configurations. In particular, we consider multi- element transmitters with three, seven, and nineteen light emitting diodes (LEDs). For a fixed number of LEDs installed in a room, we evaluate the signal-to-interference-plus-noise ratio (SINR) distributions across the room considering different transmitter geometries and a multipath VLC environment. Illumination distributions in the room are also generated for different configurations, and performance trade-offs for different transmitter architectures are investigated. Impact of transmit power at the LEDs and field of view at VLC receivers on the SINR performance are explored. Simulation results show that transmitter configuration with three LED gives the best performance for simultaneously achieving good illumination and SINR distribution under different circumstances.

Accuracy of AOA-Based and RSS-Based 3D Localization for Visible Light Communications

In this study, we investigate angle-of-arrival (AOA) and received signal strength (RSS) based localization methods for visible light communication (VLC) systems. We show that while AOA-based localization allows the receiver to locate itself via a least squares estimator by exploiting the directionality of light-emitting diodes (LEDs), RSS-based approach takes Lambertian pattern of LEDs into account and better deals with further improving the localization accuracy via a nonlinear least squares (NLS) estimator. In order to reduce the complexity of the NLS estimator, we develop an analytical learning rule based on the Newton-Raphson method and use the result of AOA-based localization as an initial point for the learning rule. As a benchmark, we also derive generic analytical expressions of the Cramer-Rao lower bound (CRLB) for RSS-based localization.

Suppressing alignment: An approach for out-of-band interference reduction in OFDM systems

In this work, we introduce a novel approach, called suppressing alignment, to reduce the out-of-band (OOB) interference of orthogonal frequency division multiplexing (OFDM) systems. Suppressing alignment exploits the unavoidable redundancy provided by the cyclic prefix (CP) and the wireless communications channel to generate an OOB interference suppressing signal at the OFDM transmitter. However, after passing through the wireless channel, the suppressing signal is aligned with the CP duration at the OFDM receiver, essentially causing no interference to the data portion of the OFDM symbol. The proposed approach reduces the OOB interference by tens of decibels and does not require any change in the receiver structure of legacy OFDM.

Multi-Element VLC Networks: LED Assignment, Power Control, and Optimum Combining

Visible light communications (VLCs) are a promising technology to address the spectrum crunch problem in radio frequency networks. A major advantage of VLC networks is that they can use the existing lighting infrastructure in indoor environments, which may have large number of LEDs for illumination. While LEDs used for lighting typically have limited bandwidth, presence of many LEDs can be exploited for indoor VLC networks, to serve each user by multiple LEDs for improving link quality and throughput. In this paper, LEDs are grouped and assigned to the users based on received signal strength from each LED, for which different solutions are proposed to achieve maximum throughput, proportional fairness, and quality of service. Additionally, power optimization of LEDs for a given assignment is investigated, and the Jacobian and Hessian matrices of the corresponding optimization problem are derived. Moreover, for multi-element receivers with LED grouping at the transmitter, an improved optimal combining method is proposed. This method suppresses interference caused by simultaneous data transfer of LEDs and improves the overall signal-to-interference-plus-noise-ratio by 2–5 dB. Lastly, an efficient calculation of channel response is presented to simulate multipath VLC channel with low computational complexity.

Diversity combining and piezoelectric beam steering for multi-element VLC networks

Visible light communications (VLC) is an emerging wireless communication technology that can serve high speed data rates by using existing lighting infrastructure without congesting the radio spectrum. This paper studies the effect of various multi-element VLC transmitter and receiver configurations on the link quality. With this purpose, the signal-to-interference-plus-noise-ratio (SINR) distribution is investigated via extensive simulations considering a multipath environment with different transceiver configurations and diversity combining techniques. Additionally, we propose a beam steering technique using the piezo-electric based tilting of LEDs to improve the SINR, which tilts the LEDs with piezo-electric actuators towards the location of the receiver. Simulation results show that significant gains can be obtained on SINR performance by using optimized transceiver configurations and diversity combining at the receiver. Moreover, even with small titling angles at the transmitter, up to 12 dB SINR gain is obtained using beam steering.

Joint Time-Frequency Alignment for PAPR and OOBE Suppression of OFDM-Based Waveforms

Orthogonal frequency division multiplexing (OFDM) waveform has been adopted by many wireless standards due to its numerous advantages. However, OFDM symbols have two critical drawbacks: high out-of-band emission (OOBE) and high peak-to-average power ratio (PAPR). Cyclic prefix (CP) alignment is one of the promising methods that jointly suppresses the OOBE and PAPR of OFDM symbols without introducing extra receiver complexity. However, its suppression performance remains limited in practical scenarios as it only exploits the degrees of freedom (DoFs) provided by the CP part of the OFDM symbols. In this letter, we propose a novel approach which jointly exploits the time domain resources, i.e., CP, and the frequency domain resources that are not effectively used by the receiver, i.e., guard tones and the subcarriers faded by the multipath channel. Thus, the available DoFs are substantially increased and more efficiently utilized for alignment purpose. It is shown that the OOBE and PAPR suppression performance is significantly enhanced with the proposed method as compared with the original approach.