21 research outputs found
Code Design for Visible Light Communications Under Illumination Constraints
Visible light communication (VLC) uses the same LEDs which are an efficient source of illumination
to transmit information concurrently using optical direct-detection. As a result of modulating
the LED to convey information, there may be a perceived change in the light perception
which besides being annoying, may produce physiological consequences under prolonged exposure.
The aim of this research is to propose code design methodologies for controlling the effects
of light intensity flickering, brightness control, and color shifts due to the modulation, encoding
information bits in organized optical symbol sequences, and improving the coding gain by the use
of the Viterbi algorithm.
In order to mitigate the effect of intensity flickering presented in On-Off Keying modulation,
five codes are designed with two proposed algorithms using finite-state machines (FSMs) for constraining
the runs of zeros or ones. The codes are compared with the codes proposed in the IEEE
802.15.7 standard on VLC (Manchester code, 4B6B code, and the 8B10B code) in terms of flicker
mitigation using the perceived flicker index (PFI) (a mathematical measure of flicker introduced in
this study) and error-rate performance. The designed codes show asymptotic coding gains between
1:25 and 6 dB with a low sacrifice in PFI.
To avoid color shifts in color-shift keying (CSK) modulation, four codes were designed from
optimally CSK constellations and two classes of codes where one class is based on FSMs and
the other on trellis-coded modulation (TCM) according to the desired color perception constraint.
The designed codes show asymptotic coding gains between 1:5 to 3:5 dB with respect to uncoded
transmission.
For brightness control, variable-weight multipulse pulse-position modulation (VW-MPPM) is
introduced as an alternative for increasing the spectral efficiency by the selection of multipulse
pulse-position modulation symbols of diverse weight to attain the desired dimming level. Combining
VM-MPPM symbols with Huffman codes and TCM, two designed codes are compared with
MPPM codes for dimming level of 0:67 and 0:40 showing an asymptotic coding gain of 0:94 and
1:29 dB, respectively.
Finally, we show the trade-offs between coding gain improvement and their effects on light
perception
Implementation of Offset Pulse Position Modulation
Optical fibre systems have played a key role in making possible the extraordinary growth in world-wide communications that has occurred in the last 25 years, and are vital in enabling the proliferating use of the Internet. Its high bandwidth capabilities, low attenuation characteristics, low cost, and immunity from the many disturbances that can afflict electrical wires and wireless communication links make it ideal for gigabit transmission and a major building block in the telecommunication infrastructure.
The main concern of this thesis is a full and detailed investigation and implementation of the Offset Pulse Position Modulation (Offset PPM) communication system. Novel work is carried out for applying Offset PPM over an optical communication channel theoretically and experimentally to examine the system performance.
An Offset PPM encoder and decoder were implemented to code Pulse Code Modulation (PCM) format into Offset PPM format and to decode back the Offset PPM to PCM. The first paradigm of implementation was implemented using electronic components.
A further investigation took place on the Offset PPM associated output. Computer programming and simulation using the VHSIC Hardware Description Language (VHDL) of this PPM code was considered and comparison with previous theoretical results presented. The received Offset PPM signal returned back to its original input PCM form without errors.
Successful VHDL and Field Programmable Gate Array (FPGA) implementation using Altera Quartus II of Offset PPM encoder and decoder as a single system has been presented in the study. An FPGA embedded Bit Error Rate (BER) test device has also been implemented for sensitivity measurements purposes and all the designs have been tested successfully with back-to-back testing. Results show that Offset PPM is an advantageous PPM code for optic communication. Furthermore, the system has achieved a very high data rate of 50 Mb/s without an optical communication set.
An optical communication system (transmitter/receiver) over POF was developed and the Offset PPM scheme was investigated through this optical channel. Results show that the Offset PPM sequence transferred through the optic system without being altered. In addition, this implementation is optimised PPM coding; the system is working perfectly with up to 10 Mb/s with 10-12 BER based on the limitations of the optical communication set.
All the results and analyses indicate that Offset PPM is an ideal alternative to be considered for highly dispersive optical channels, and performance evaluation for higher bandwidths also favourably compares to existing coding schemes
Optical wireless data transfer for rotor detection and diagnostics
A special application of optical wireless data transfer, namely on-line monitoring
and diagnostic of rotors in turbines and engines, has been considered in this thesis. In
this application, to maintain line of sight, i.e. data transfer, between a sensor placed on a
rotating component inside the turbine and a monitoring point placed in a fixed position
outside the turbine, a periodic fast fading channel is generated, which gives the
transceivers more flexibility regarding their mounting location. The communication in
such a channel is affected by the intermittency and variation of the signal power, which
produces a unique channel condition that influences the performance of the optical
transceiver.
To investigate the channel condition and the error rate of the periodic fast fading
channel with signal fluctuation, a model is developed to simulate the optical channel by
considering the variation of signal power as a result of the change in the relative
position of the photodiode with respect to the Lambertian radiation pattern of the LED,
in a simplified linear geometry. The error rate is estimated using the Saddlepoint
approximation on a specific threshold strategy. The results show that the channel can
afford the sensor data transmission and the performance can be improved by modifying
several parameters, such as geometrical distance, transmitter power and load resistor.
Compared to a normal channel, a higher load resistor on the photodiode front end has
the advantage of decreasing the noise level and increasing the data capacity in the fast
fading channel. The analysis of the automatic gain control amplifier indicates that a
higher load resistor needs a lower loop gain and from the model of the Transimpedance
amplifier (TIA), the bandwidth extension from the amplifier is more significant for a
higher resistor.
In addition to the theoretical model, an experimental setup is built to emulate the
channel in practice. The degree of similarity between the experimental setup and the
theoretical model of the channel is estimated from the comparison of the generated communication windows. Since it has been found that differences exist in the duration
of the communication window and the variation of the signal power, scaling factors to
ensure their compatibility have been derived. Transceiver hardware which
implemented the modelled functionality has been developed and a protocol to
establish the communication with the required error rate has been proposed. Using the
hardware implementation, a detection method for both rising and falling edges of the
signal pulses and a threshold strategy have been demonstrated. The device power
consumption is also estimated.
What is more, the electromagnetic environment of a squirrel cage motor is
simulated using the finite element method to investigate the interference and the
possibility of providing power to the IR communication devices using power
scavenging.
In the conclusion, the key findings of the thesis are summarised. A solution is
proposed for sensor data transfer using an optical channel for rotor monitoring
applications, which involves the design of the IR transceiver, the implementation of
the developed protocol and the power consumption estimation
Multidimensional Optimized Optical Modulation Formats
This chapter overviews the relatively large body of work (experimental and theoretical) on modulation formats for optical coherent links. It first gives basic definitions and performance metrics for modulation formats that are common in the literature. Then, the chapter discusses optimization of modulation formats in coded systems. It distinguishes between three cases, depending on the type of decoder employed, which pose quite different requirements on the choice of modulation format. The three cases are soft-decision decoding, hard-decision decoding, and iterative decoding, which loosely correspond to weak, medium, and strong coding, respectively. The chapter also discusses the realizations of the transmitter and transmission link properties and the receiver algorithms, including DSP and decoding. It further explains how to simply determine the transmitted symbol from the received 4D vector, without resorting to a full search of the Euclidean distances to all points in the whole constellation
Modulation formats for multi-core fiber transmission
©2014 Optical Society of America We investigate high dimensional modulation formats for multi-core fibers (MCFs) and spatial superchannels. We show that the low skew variations between MCF cores maybe exploited to generate 'multi-core' formats that offer significant advantages over independently transmitting conventional 4-dimensional formats in each core. We describe how pulse position modulation formats may be transposed to the spatial domain and then investigate a family of modulation formats referred to as core-coding, one of which has the same power and spectral efficiency as polarization switched quaternary phase shift keying but with half of the optical power, potentially improving non-linear tolerance for long distance transmission, albeit at the cost of implementation challenges. Finally, we investigate the application of set-partitioning to multi-core formats using a single-parity check bit transmitted in one quadrature of one polarization in one of the cores and polarization-division multiplexing quadrature phase shift keying data in all remaining cores. We observe that for high core counts, an advantage of almost 3 dB in asymptotic power efficiency may be obtained with negligible impact on spectral efficiency, which translates into experimentally measured reduction in the required optical signal-to-noise ratio of up to 1.8 dB at a bit-error-rate of 10-5 and the same data-rate, and additional transmission reach of up to 20%
Mobile Ad-Hoc Networks
Being infrastructure-less and without central administration control, wireless ad-hoc networking is playing a more and more important role in extending the coverage of traditional wireless infrastructure (cellular networks, wireless LAN, etc). This book includes state-of-the-art techniques and solutions for wireless ad-hoc networks. It focuses on the following topics in ad-hoc networks: quality-of-service and video communication, routing protocol and cross-layer design. A few interesting problems about security and delay-tolerant networks are also discussed. This book is targeted to provide network engineers and researchers with design guidelines for large scale wireless ad hoc networks