114 research outputs found
Carrier Phase Estimation Through the Rotation Algorithm for 64-QAM Optical Systems
A novel low-complexity two-stage digital feedforward
carrier phase estimation algorithm based on the rotation
of constellation points to remove phase modulation for a 64-ary
quadrature amplitude modulation (QAM) system is proposed and
analyzed both experimentally and through numerical simulations.
The first stage is composed of a Viterbi and Viterbi (V&V) block,
based on either the standard quadrature phase shift keying (QPSK)
partitioning algorithm using only Class-1 symbols or a modified
QPSK partitioning scheme utilizing both Class-1 and outer most
triangle-edge (TE) symbols. The second stage applies the V&V algorithm
after the removal of phase modulation through rotation of
constellation points. Comparison of the proposed scheme with constellation
transformation, blind phase search (BPS) and BPS+MLE
(maximum likelihood estimation) algorithm is also shown. For an
OSNR penalty of 1 dB at bit error rate of 1e−2 , the proposed
scheme can tolerate a linewidth times symbol duration product
(Δν · Ts) equal to 3.7 × 1e−5 , making it possible to operate
32-GBd optical 64-QAM systems with current commercial tunable
laser
Carrier Synchronization in High Bit-Rate Optical Transmission Systems
In this dissertation, design of optical transmission systems with differential detection and coherent detection is briefly described. More over, algorithms for carrier synchronization and phase estimation with their implementation in high bit-rate optical transmission systems are proposed
Development of an acoustic communication link for micro underwater vehicles
PhD ThesisIn recent years there has been an increasing trend towards the use of
Micro Remotely Operated Vehicles (μROVs), such as the Videoray and
Seabotix LBV products, for a range of subsea applications, including
environmental monitoring, harbour security, military surveillance and
offshore inspection. A major operational limitation is the umbilical cable,
which is traditionally used to supply power and communications to the
vehicle. This tether has often been found to significantly restrict the
agility of the vehicle or in extreme cases, result in entanglement with
subsea structures.
This thesis addresses the challenges associated with developing a reliable
full-duplex wireless communications link aimed at tetherless operation
of a μROV. Previous research has demonstrated the ability to
support highly compressed video transmissions over several kilometres
through shallow water channels with large range-depth ratios. However,
the physical constraints of these platforms paired with the system cost
requirements pose significant additional challenges.
Firstly, the physical size/weight of transducers for the LF (8-16kHz)
and MF (16-32kHz) bands would significantly affect the dynamics of the
vehicle measuring less than 0.5m long. Therefore, this thesis explores the
challenges associated with moving the operating frequency up to around
50kHz centre, along with the opportunities for increased data rate and
tracking due to higher bandwidth.
The typical operating radius of μROVs is less than 200m, in water
< 100m deep, which gives rise to multipath channels characterised by
long timespread and relatively sparse arrivals. Hence, the system must
be optimised for performance in these conditions. The hardware costs of
large multi-element receiver arrays are prohibitive when compared to the
cost of the μROV platform. Additionally, the physical size of such arrays
complicates deployment from small surface vessels. Although some
recent developments in iterative equalisation and decoding structures
have enhanced the performance of single element receivers, they are not
found to be adequate in such channels. This work explores the optimum
cost/performance trade-off in a combination of a micro beamforming array
using a Bit Interleaved Coded Modulation with Iterative Decoding
(BICM-ID) receiver structure.
The highly dynamic nature of μROVs, with rapid acceleration/deceleration
and complex thruster/wake effects, are also a significant challenge to reliable
continuous communications. The thesis also explores how these effects
can best be mitigated via advanced Doppler correction techniques,
and adaptive coding and modulation via a simultaneous frequency multiplexed
down link. In order to fully explore continuous adaptation of
the transmitted signals, a real-time full-duplex communication system
was constructed in hardware, utilising low cost components and a highly
optimised PC based receiver structure. Rigorous testing, both in laboratory
conditions and through extensive field trials, have enabled the
author to explore the performance of the communication link on a vehicle
carrying out typical operations and presenting a wide range of channel,
noise, Doppler and transmission latency conditions. This has led to a
comprehensive set of design recommendations for a reliable and cost effective
link capable of continuous throughputs of >30 kbits/s
Optimising the efficiency of coherent optical packet switched networks
There is a continuing need to increase throughput in optical networks to satisfy the demands of internet applications. However, the non-linear Shannon capacity of
standard single mode fibre is being approached. Also, almost all of the power used in optical networks is used by electronic routers. One possible solution to deal with
both problems is to use optical packet switching. Optical packet switching uses fast switching tuneable lasers, which can change wavelength in the order of a several
nanoseconds, to dynamically vary wavelength assignments in a network, and thus achieve routing in the network without electronic routers. In addition, fast
wavelength assignment reduces waiting times, resulting in better utilization of network resources.
However, the frequency dynamics of the tuneable lasers after switching wavelengths increases the waiting times required to successfully transmit data packets. In this thesis, frequency and phase dynamics of a tuneable laser
transmitter, after a wavelength switching event, are initially characterised accurately using a novel technique. The effects that the frequency dynamics have on the transmission of coherent optical communication signals are mitigated using doubly differential decoding, a new approach proposed in this work for application in optical packet switched networks. This technique reduces the waiting times required to successfully transmit data after a wavelength switching event, and this enhances overall network efficiency and throughput. In addition, this work proposes and demonstrates the use of a least-mean squares algorithm to overcome polarisation demultiplexing issues which are present in these networks, which also decreases waiting times, increases network efficiency, and improves system robustness
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