172 research outputs found
Self-interference cancellation in underwater acoustic communications systems using orthogonal pilots in IBFD
This paper proposes a Self-interference (SI) cancellation system model of Underwater acoustic (UWA) communication for in-band full-duplex (IBFD) technology. The SI channel is separated from the Far channel by exploiting a concurrently orthogonal pilot channel estimation technique using two orthogonal frequency-division multiplexing (OFDM) blocks to establish orthogonality between them based on a unitary matrix. Compared to the half-duplex channel estimator, the mean squared error (MSE) and the bit error rate (BER) provided strong evidence for the efficiency of the proposed SI cancellation. Since full-duplex systems are more efficient than half-duplex ones, the proposed approach might be seen as a viable option for them. The proposed method proved effective when used with a fixed full-duplex (FD) position and FD shifting of up to 4°. Different channel lengths and distances are adopted to evaluate the proposed method. Initial findings indicate that MSE for the SI channel minimum mean-square error (MMSE) estimator at 20 dB is 0.118 · 10−3, for fixed FD. In addition, this paper presents a geometry channel model for the Far channel in the IBFD underwater communication system that describes the propagation delay of the multipath reflection. The simulation results for the multipath propagation delay spread are similar to the traditional results, with the delay spread of the suggested model reaching (79 ms), which is close to the Bellhop simulator result (78 ms)
Self-Interference Cancellation for Full-Duplex Underwater Acoustic Systems
This work develops and investigates self-interference (SI) cancellation (SIC) techniques for full-duplex (FD) underwater acoustic (UWA) systems. To enable the FD operation in UWA systems, a high level of SIC is required. The main approach used in this work is the digital cancellation based on adaptive filtering. A general structure of the digital canceller is proposed which addresses key factors affecting the SIC performance, including the power amplifier and pre-amplifier nonlinearities, up- and down-sampling effects. With the proposed structure, the SI can be effectively cancelled in time-invariant channels by classical recursive least-square (RLS) adaptive filters, e.g., the sliding-window RLS (SRLS), but the SIC performance degrades in time-varying channels. A new SRLS adaptive filter based on parabolic interpolation of the channel time variations is proposed, which improves the SIC performance at the expense of the high complexity. To reduce the complexity, while providing the high SIC, a new family of interpolating adaptive filters which combine the SRLS adaptive algorithm with Legendre polynomials (SRLS-L) is proposed. A sparse adaptive filter is further proposed to exploit the sparsity in the expansion coefficients of the Legendre polynomials. For interpolating adaptive filtering algorithms, the mean squared error is unsuitable for measuring the SIC performance due to the overfitting. Therefore, a new evaluation metric, SIC factor, is proposed. The SIC performance of the proposed adaptive filters is investigated and compared with that of the classical SRLS algorithm by simulation, water tank and lake experiments. Results indicate that the proposed adaptive filters significantly improve the SIC performance in time-varying scenarios, especially with high-order sparse SRLS-L adaptive filter. Furthermore, SIC schemes with multiple antennas are investigated to explore the possibility of achieving extra amount of SIC in acoustic domain and cancelling the fast-varying surface reflections by adaptive beamforming
Channel Estimators for Full-Duplex Communication using Orthogonal Pilot Sequences
Full-duplex communication is desirable to maximize the spectral efficiency, despite the challenges it puts forth. The key challenge inhibiting the operation of radios in full-duplex mode is self-interference. In this paper, we propose a pilot-based channel estimation to estimate both self-interference and communication channels simultaneously at both ends of a full-duplex link using orthogonal sequences. The Cramer-Rao Lower Bound for estimators of both the channels was determined and compared with the half-duplex channel estimator. We performed simulations varying sequence length and channel taps and studied the performance of the estimators. We also studied the effect of synchronization between the sequences on the performance of the estimators. Thus, providing a solution to balance the trade-off between the accuracy of the channel estimation and the overhead added to the transmissions for full-duplex communication
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
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