RSS-based visible light positioning based on channel state information

As a result of the influence of indoor multipath effects, visible light positioning (VLP) technology based on the received signal strength (RSS) has difficulty providing stable and highly accurate position estimation. In order to cope with this challenge, a trilateration-based positioning method using channel state information (CSI) is proposed. Two algorithms that can estimate the number of channel paths are studied, which are the prerequisites for the obtaining of an accurate ratio of received power of the line-of-sight (LOS) path to total received power. Unlike traditional trilateration positioning methods where total received power is used, the proposed method introduces CSI as a means of estimating the received power of the LOS path, which it uses for ranging. By reducing interference from the reflected paths in the received power, the proposed method has high-precision and high-reliability positioning capabilities. The positioning performance of the proposed method in an indoor environment with four LED bulbs and a single photodetector (PD) is evaluated through simulation. The simulation results demonstrate that the proposed method provides an average accuracy of 6.1 cm and a maximum positioning error of 17.7 cm in a cubic space with a size of 4 m×4 m×3 m. Compared to the existing least-squares (LS) method that uses total received power, the new method achieves approximately 83% improvement in the mean error of positioning and 81% in root mean square error (RMSE).

Testbed for Experimental Characterization of Indoor Visible Light Communication Channels

In this paper, we describe an experimental testbed designed to evaluate indoor visible light communications (VLC) in realistic scenarios. The system is based on a mockup where the location and orientation of the optical receiver can be modified with precision for a static configuration of walls and ceiling lamp arrangements. The system utilizes a timing synchronization method, which is based on evaluating the training sequence periods used for channel response estimation, which enables robust frame synchronization. In addition, an adaptive rate orthogonal frequency-division multiplexing (OFDM) scheme is used to assess the VLC performance throughout the receiver plane emulating a real communication. The preliminary results obtained with this prototype, considering a multiple-input single-output (MISO) scenario, demonstrate that reflection on walls yields a significant increase in data rates, which can be additionally improved if appropriate orientation of the receiver is implemented. However, vertical orientation upward of the optical receiver still constitutes a simple solution but efficient enough. Moreover, a good agreement between simulation and experimental results is observed, which confirms the suitability of the mockup as an experimental testbed for practical evaluation of indoor VLC systems, where system performance for different lamp arrangements and receiver designs, including multi-user communications, can be studied.

The frequency domain versus time domain methods for processing of intracranial pressure (ICP) signals

Two methods for analyzing intracranial pressure (ICP) waveforms were compared. The frequency domain (FD) method converts the signal from the time domain to the frequency domain by a fast Fourier transform (FFT), while the time domain (TD) method calculates peak-to-peak value of the pulse waveform directly from the time samples. First, the ICP signal was regenerated from the first harmonic of the FFT and compared against the time domain raw ICP signal. We found that the FD method may underestimate pulse amplitude if there is heart rate variability or a high harmonic distortion. Second, to explore the significance in a larger data set, differences between FD- and TD-derived pulse amplitudes were determined for a total of 50,978 6-s time windows of 79 head injury patients. The mean difference in pulse pressure amplitude was 2.9 mmHg for the 50,978 6-s time windows. Differences between TD- and FD-derived pulse amplitudes were >or= 2.0 mmHg in 58.8% of the 50,978 time windows. In about 33% of time windows FD amplitudes were <2 mmHg when TD amplitudes were >or= 4 mmHg, and vice versa. Hence, the TD method is superior to the FD method for calculation of pulse amplitudes. Nevertheless, in this material both the TD and FD methods revealed significantly elevated pulse amplitudes in head injury patients with bad outcome (i.e. Glasgow Outcome Score 1-3).