Pankil M. Butala

Metameric modulation for diffuse visible light communications with constant ambient lighting

Advances in solid-state lighting are renewing interest in the adoption of the visible spectrum for optical wireless communications. Under the luminaire-as-transmitter model, wireless communication is achieved by modulating LED(s) that must simultaneously meet the illumination mission. Illumination requirements include maintaining energy efficiency, constant color and intensity control whereas communications requirements are speed and BER goals. In this paper we explore the perceptual qualities of visible light from LED luminaires to render color. We then propose a novel modulation scheme for visible light communications which can maintain constant perceived ambient lighting. By using D>;3 LEDs, multiple lighting states that are indistinguishable to humans but are distinguishable to an electronic receiver can be achieved. Changes between these states are detected as intensity modulation in different wavelength bands.

SVD-VLC: A novel capacity maximizing VLC MIMO system architecture under illumination constraints

Multiple-input multiple-output (MIMO) systems using multiple light emitting diode (LED) sources and photodiode (PD) detectors are attractive for visible light communication (VLC) as they offer a capacity gain proportional to the number of parallel single-input single-output (SISO) channels. MIMO VLC systems exploit the high signal-to-noise ratio (SNR) of a SISO channel offered due to typical illumination requirements to overcome the capacity constraints due to limited modulation bandwidth of LEDs. In this work, a modified singular value decomposition VLC (SVD-VLC) MIMO system is proposed. This system maximizes the data rate while maintaining the target illumination and allowing the channel matrix to vary in order to support mobility in a practical indoor VLC deployment. The upper bound on capacity of the proposed SVD-VLC MIMO system is calculated assuming an imaging receiver. The relationship between the proposed system performance and system parameters total power constraint, lens aperture and random receiver locations are described.

Performance of optical spatial modulation and spatial multiplexing with imaging receiver

Spatial modulation (SM) and spatial multiplexing (SMP) are two multiple-input multiple-output (MIMO) techniques for transmitting data over an indoor optical wireless channel. Receivers for SM and SMP can be of the non-imaging type, in which case the channel matrix coefficients can be highly correlated, or of the imaging type, which can reduce the degree of correlation and improve overall system performance. In this work, we propose a new framework to analyze the performance of imaging MIMO systems. This framework is applied to characterize the performance of SM and SMP under both imaging and non-imaging receivers. Results of our analysis indicate that imaging receivers can provide significant signal-to-noise ratio (SNR) improvements up to 45dB under SM and SMP as compared to the use of non-imaging receivers. Finally, the application of the proposed analysis framework indicates specific design principles to optimize imaging receiver parameters.

Trace-Orthogonal PPM-Space Time Block Coding Under Rate Constraints for Visible Light Communication

Visible light communications (VLC) represents a new frontier of communications allowing high data-rate Internet access, specially in indoor environments, where the use of light emitting diodes (LEDs) is growing as a viable alternative to traditional illumination. As a result, LED output intensity can be varied faster than human eye can perceive, thus guaranteeing simultaneous wireless communications and illumination. One of the key challenges is the limited modulation bandwidth of sources that is typically around several MHz. The use of multiple input and multiple output (MIMO) techniques in optical wireless system helps to increase the capacity of the system and thus improve the system performance. In this paper, we investigate the use of an optical MIMO technique jointly with pulse position modulation (PPM) in order to improve the data rates without reducing the reliability of the link. PPM is known to be signal-to-noise ratio efficient modulation format, while it is bandwidth inefficient so the use of MIMO can compensate that drawback with reasonable complexity. Furthermore, an offline tool for VLC system planning, including error probability and transmission rate, has been proposed in order to solve the tradeoff between transmission rate and error rate. Finally, several numerical results and performance comparisons are reported.

Monitoring walking and cycling of middle-aged to older community dwellers using wireless wearable accelerometers

Changes in gait parameters have been shown to be an important indicator of several age-related cognitive and physical declines of older adults. In this paper we propose a method to monitor and analyze walking and cycling activities based on a triaxial accelerometer worn on one ankle. We use an algorithm that can (1) distinguish between static and dynamic functional activities, (2) detect walking and cycling events, (3) identify gait parameters, including step frequency, number of steps, number of walking periods, and total walking duration per day, and (4) evaluate cycling parameters, including cycling frequency, number of cycling periods, and total cycling duration. Our algorithm is evaluated against the triaxial accelerometer data obtained from a group of 297 middle-aged to older adults wearing an activity monitor on the right ankle for approximately one week while performing unconstrained daily activities in the home and community setting. The correlation coefficients between each of detected gait and cycling parameters on two weekdays are all statistically significant, ranging from 0.668 to 0.873. These results demonstrate good test-retest reliability of our method in monitoring walking and cycling activities and analyzing gait and cycling parameters. This algorithm is efficient and causal in time and thus implementable for real-time monitoring and feedback.

Multi-wavelength visible light communication system design

Visible light communication (VLC) is achieved by modulation of one or more spectral components in the visible spectrum (≈380-780 um). The use of this range provides an opportunity to exploit an otherwise untapped medium that is used in human lighting. Most VLC systems constructed to date focus on using a broad visible band generated by phosphor-converted light emitting diodes, or by filtering to isolate the blue component from these sources. Multi-wavelength systems consider additional wavelength bands that are combined to produce the desired communications capacity and lighting output. This color combining, or mixing, realizes desired color temperature and intensity and represents a form of wavelength-division multiplexing. This paper investigates the relationships between the colors comprising the lighting source for a range of lighting states, the spectral separation of communication channels, the relative intensities required to realize lighting states, how modulation can be most effectively mapped to the available color channels, and the design of an optical filtering approach to maximize signal to noise ratio while minimizing crosstalk at the receiver. Simulation results based on a three colored VLC system are discussed using orthogonal frequency division multiplexing for each color. It is shown that the system is the most power efficient at 6250 K correlated color temperature, with transmitter spectral spread of 5 nm and filter transmittance width of 40 nm.

Performance of Color Shift Keying under Non-Linear System Model and Illumination Constraints

The IEEE 802.15.7 standard defines specifications for short-range optical wireless communication (OWC) using visible light. The standard specifies color shift keying (CSK) as the preferred modulation scheme for indoor OWC while simultaneously providing illumination. In light of illumination requirements, human eyes optical perception introduces unique non-linearities in the CSK signaling chain. It is shown that these non-linearities introduce performance penalties of more than 15 dB, 10 dB, and 5 dB for M = 4, 8, and 16 CSK respectively. A new metric called luminous- signal to-noise ratio (LSNR) is also introduced to fairly compare performance of any two OWC signaling schemes operating at a user defined illumination intensity level and is used to compare performance of M-ary CSK implemented with different colored sources. Within the same context, and at a target bit error rate of ≤ 10^-3, simulation results indicate that clipping negative receiver output signal does not have an impact on M-ary CSK performance.