Publication: Development of systems based on visible light communications for high added value applications
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2023-10
Defense date
2023-12-01
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Abstract
The discovery of the blue Light-emitting diodes (LEDs) in the last decade of the last century changed the way we as a society approach the need for artificial lighting, as the combination of the blue light emitted by the LED with a yellow phosphor layer reactive to the blue wavelength generates white light. White LEDs are more efficient than incandescent bulbs and last longer, which is a benefit not only for the end user but for everyone as the energy used for lighting could be significantly reduced and therefore the impact on the environment. LEDs not only improve the efficiency of existing light bulbs, but because they are based on semiconductors, their switching speed is several orders of magnitude higher than other types of illumination technologies.
On the other hand, the radio frequency spectrum is becoming saturated due to
the incredible rate at which more and more devices are being connected wirelessly.
The growing market of Internet of things (IoT) devices and the concept of the
smart home, where everything can be connected and controlled from the palm of
your hand, has limited the available bandwidth for data exchange with the Internet.
However, because wireless communications can penetrate walls and travel tens of
meters, even in low-power devices, our devices are not the only ones fighting for a
space to transmit information, but we share the available channel with all of our
neighbors’ devices. This ability of signals to cross physical borders also threatens
our data security, as the data may be accessible to third parties.
Considering these two ideas, Visible light communication (VLC) is gaining a lot
of interest in recent years, as it can provide a secure connection limited to the light
spot where it is transmitted. In this way, LED bulbs will not only cover our need
for lighting but also support part of the communication, helping to free up the radio
frequency channel. VLCs also offers alternatives in places with high electromagnetic
interference, and the bandwidth is fully available for each transmitter. VLC has
also been recognized as a viable technology for indoor positioning, as the system can
know where it is, based on transmitter localization (LED lamp). This is particularly
useful because it can achieve centimeter accuracy, which is impossible with other
positioning systems such as satellite navigation.
Therefore, this thesis work aims to design and develop a working VLC system
that supports multiple end applications. The intended system must be small and inexpensive to be attractive for future adaptations. With this we try to gain knowledge
about the inherent challenges that VLC systems have and, with it, help VLC
technology to go one step further to be useful and adaptable to our daily lives.
First, we realized that LEDs are a main element in any VLC system, and
therefore the more knowledge we have about it, the better performance we can
achieve with this type of solution. VLC uses Intensity modulation/Direct detection
(IM/DD) to transmit information, and to do so, the intensity of the LED is
varied over time by modulating the current injected into the device. For this reason,
the knowledge of the electrical equivalent of the LEDs is a useful tool for the
correct design of VLC systems. In this thesis work, we have proposed a method to
electrically characterize high-power white LEDs and, with it, create an equivalent
electrical circuit useful for simulation and driver design.
Then, we study the different modulation formats used in wireless communication
channels and the modulation formats that can only be implemented with VLC,
such as Color shift keying (CSK). In this aspect, we have noticed that even if
the modulation formats are mature and well known, VLC channels present some
peculiarities that need to be explored more deeply in order to adapt the modulations
to this type of channel. In the case of this thesis, we explore the implementation of an
Orthogonal frequency division multiplexing (OFDM) modulation in a VLC channel,
and with it, we found a novel way to perform the time synchronization and phase
correction in a pilot-based OFDM. The requirement that the transmitted signal be
real was overcome by using the hermitian symmetry property of the Inverse fast
Fourier transform (IFFT).
In summary, we were able to develop a working VLC system in all stages, hardware,
firmware, and software, reaching transmission speeds up to 234 kbps. The
system works with a custom Android application designed for this prototype. The
received light information is transformed into bits at the receiver and then sent
through Bluetooth low energy (BLE) to the Android device. The application processes
the received data and displays useful information to the user. In this prototype,
the information is used for indoor localization, but it is not limited to that.
The system also supports the addition of Augmented reality (AR) glasses to enhance
the user experience. The transmitter prototype is an LED bulb, which can
be connected directly to the power grid with a GU-10 or GUZ-10 socket. The VLC
receiver measures 34.5x46.6x17.6mm, and it is powered by a CR2032 battery.
Description
Mención Internacional en el título de doctor
Keywords
Light emitting diodes, Orthogonal frequency-division multiplexing (OFDM), Visible light communication, Optical wireless communications, Modulation bandwidth, Orthogonal frequency division multiplexing