Query: interference underwater acoustic

title: Digital Self Interference Cancellation for Full Duplex Underwater Acoustic Systems
abstract: Underwater acoustic (UWA) communication suffers from the limited available bandwidth for data transmission. Full duplex (FD) communication has demonstrated the ability of achieving high spectral efficiency in terrestrial radio communications. There is a significant potential in adopting the benefits of FD in UWA systems. The major obstacle in FD communications is the severe self interference (SI) introduced by the near end transmitted signal. For FD UWA communications, the low signal frequency allows high resolution analogue to digital converter (ADCs) to be used. With higher performance ADCs, it might be possible to achieve higher digital SI cancellation performance than that in FD radio systems. In this brief, we present experimental results of digital SI cancellation in FD UWA system, based on the use of the low complexity recursive least squares (RLS) adaptive filter with dichotomous coordinate descent iterations. The experimental results demonstrate that up to 46 dB of SI is cancelled when we use the transmitted digital data as the regressor in the adaptive filter. To improve the SI cancellation performance without introducing high complexity operation, we use the digitalized power amplifier (PA) output as the regressor to deal with the non linear distortions caused by the PA in the transmitted chain. With this technique, as high a level as 69 dB of digital SI cancellation is achieved.

title: Self Adaptive Resource Allocation in Underwater Acoustic Interference Channel: A Reinforcement Learning Approach
abstract: Since underwater acoustic channels are shared by multiple heterogeneous entities and can suffer from severe interference, underwater acoustic communication networks (UACNs) are faced with the challenge of mitigating interference and improving communication quality by implementing distributed resource allocation approaches. In this article, we introduce the concept of reinforced learning in intelligent control to the UACNs by treating the nodes as intelligent agents and the node networks as multiagent networks. By partitioning the state space and the action space, we formulate a reward function and a search strategy and propose a distributed resource allocation algorithm based on cooperative <inline formula> <tex math notation="LaTeX"$Q$ /tex math>/inline formula> Learning. In addition, we verify the convergence of the proposed algorithm. Finally, simulation results in two different underwater application scenarios show that the proposed algorithm outperforms the existing algorithms in improving the network transmission capacity, and can reduce the overhead of resource allocation by using cooperative <inline formula> <tex math notation="LaTeX"$Q$ /tex math>/inline formula> Learning.

title: Cellular Clustering Based Interference Aware Data Transmission Protocol for Underwater Acoustic Sensor Networks
abstract: Recently, the development of three dimensional interference aware data transmission methods has attracted the attention of scholars due to the increased interest in exploiting and studying the underwater acoustic sensor networks (UASNs) In this paper, an interference aware data transmission protocol based on cellular clustering architecture is proposed. The protocol involves two steps. The first one is an inter cell time division multiple access (TDMA) scheduling, which reduces acoustic interference by restricting simultaneous data transmission via adjacent routing paths; and the second one is an intra cell hierarchical routing, which targets efficient data collection in the submarine and reliable data transmission from the seabed to the surface. Moreover, a novel Ekman spiral based low cost location prediction method and a void hole recovery scheme are adopted in each step to support the practicability of proposed protocol. Theoretical analysis and simulations indicate that the proposed protocol has advantages related to the quality of service (QoS) of UASNs because the signal interference is significantly mitigated.

title: The Effect of Acoustic Shell Coupling on Near End Self Interference Signal of In Band Full Duplex Underwater Acoustic Communication Modem
abstract: In Band Full Duplex (IBFD) Underwater Acoustic (UWA) communication technology plays a major role in enhancing the performance of Underwater acoustic sensor networks (UWSN) Self Interference (SI) is one of the main inherent challenges affecting the performance of IBFD UWA communication. To reconstruct the SI signal and counteract the SI effect, this is important to estimate the short range channel through which the SI signal passes. Inaccurate estimation will result in the performance degradation of IBFD UWA communication. From the perspective of engineering implementation, we consider that the UWA communication modem shell has a significant influence on the short range SI channel, which will limit the efficiency of self interference cancellation in the analog domain to some degree. Therefore we utilize a simplified model to simulate the influence of the structure of the IBFD UWA communication modem on the receiving end. This paper studies the effect of acoustic shell coupling on near end self interference signal of IBFD UWA communication modem. Some suggestions on the design of shell structure of IBFD UWA communication modem are given.

title: Frequency domain equalization with interference rejection combining for single carrier multiple input multiple output underwater acoustic communications.
abstract: This paper describes a three step frequency domain equalization scheme for multiple input multiple output (MIMO) underwater acoustic communication. First, an iterative least squares channel estimation method is developed to enhance the accuracy of channel estimation in MIMO communication. The interference rejection combining method is then adopted to suppress co channel interference based on the estimated MIMO channels. This technique exploits the correlation between the interference received through different channels. Finally, a decision feedback equalizer embedded with a digital phase lock loop is cascaded before the final determination of equalized symbols to compensate the phase rotation. Experimental results show that the bit error rates of the proposed scheme can be several orders of magnitude lower than those of conventional frequency domain equalization schemes.

title: A New Acoustic Channel Interference Model for 3 D Underwater Acoustic Sensor Networks and Throughput Analysis
abstract: Internet of Underwater Things (IoUT) is regarded as the network of interconnected smart underwater objects that enables marine monitoring and exploration. Underwater acoustic sensor networks (UASNs) is an ideal network infrastructure to support IoUT. Usually, network level analysis requires a simplified model for channel interference. Existing research on UASNs usually applies channel interference models from terrestrial wireless networks. Its effectiveness is questionable. In this article, we investigate two specific features of acoustic channels and propose a new channel interference model. First, based on the fact that acoustic signals have inconsistent transmission range in vertical and horizontal directions, we propose an ellipsoid sensing model and derive the probability that a communication pair will have an effective link. Second, we investigate the receiving time inconsistency in time slotted acoustic networks and derive the analytical probability of collision free transmission. Finally, a new protocol model is proposed to describe the acoustic channel interference. Its theoretical throughput, in terms of delivery rate, is also presented. Numerical examples illustrate the effectiveness of the theoretical analysis.

title: Self Interference Channel Characterization in Underwater Acoustic In Band Full Duplex Communications Using OFDM
abstract: Due to the limited available bandwidth and dynamic channel, data rates are extremely limited in underwater acoustic (UWA) communications. Addressing this concern, in band full duplex (IBFD) has the potential to double the efficiency in a given bandwidth. In an IBFD scheme, transmission and reception are performed simultaneously in the same frequency band. However, in UWA IBFD, because of reflections from the surface and bottom and the inhomogeneity of the water, a significant part of the transmitted signal returns back to the IBFD receiver. This signal contaminates the desired signal from the remote end and is known as the self interference (SI) With an estimate of the self interference channel impulse response (SCIR) a receiver can estimate and eliminate the SI. A better understanding of the statistical characteristics of the SCIR is necessary for an accurate SI cancellation. In this article, we use an orthogonal frequency division multiplexing (OFDM) signal to characterize the SCIR in a lake water experiment. To verify the results, SCIR estimation is performed by using estimators in both the frequency and time domains. We show that, in our experiment, regardless of the depth of hydrophone, the direct path of SCIR is strong, stable and easily tracked; however, the reflection paths are weaker and rapidly time varying making SI cancellation challenging. Among the reflections, the first bounce from the water surface is the prevalent path with a short coherence time around 70 ms.

title: FD UWA: Full Duplex Underwater Acoustic Comms via Self Interference Cancellation in Space
abstract: Traditionally, underwater acoustic communications is half duplex, i.e. the hydrophones and transducers operate in non overlapping time slots/frequency bands in one direction at a time or frequency. To double the spectral efficiency and allow simultaneous transmission and reception in Full Duplex mode (FD) a Self Interference Cancellation (SIC) technique in space is introduced and deployed. Specifically, a novel underwater acoustic system is proposed to perform FD SIC efficiently via an integrated design combining underwater Acoustic Vector Sensor (AVS) and Phased Array Transducer (PAT) to realize spatial SIC. The energy focusing function of the Beamformer (BF) helps PAT avoid self and mutual spatial interference. The AVS keeps updating the direction of arrival information to let BF adjust the steering angle via an adaptive protocol. The proposal is evaluated and verified via simulations in realistic underwater acoustic channels and is able to achieve 59 dB SIC at 80 kHz steering angle at 5 deg and at least 37 dB within the steering angle region before the input of digital SIC. This indicates that the design is a promising solution for the chosen angle region to perform spatial SIC as well as to prevent the grating lobe interference. The design is being experimentally validated using the data collected from an underwater testbed and implemented on an Field Programmable Gate Array (FPGA) board that provides energy efficiency and real time processing capabilities.

title: Two Stage Self Interference Cancellation for Full Duplex Underwater Acoustic Systems
abstract: Underwater acoustic (UWA) communication suffers from the limited available bandwidth of acoustic links. We consider in band full duplex (FD) communication in UWA channels, which has the potential of doubling the channel capacity. The key challenge of FD operation is the strong self interference (SI) introduced by the near end transmission. For a time invariant UWA channel, the SI can be efficiently cancelled using an adaptive filter. In practice, the time varying reflections particularly introduced by the moving sea surface limit the cancellation performance. In this paper, we propose a two stage SI cancellation scheme with a transceiver structure comprising one projector and two hydrophones. In the first stage, the stable SI path from each hydrophone is cancelled using a digital canceller based on the recursive least squares (RLS) algorithm with dichotomous coordinate descent (DCD) iterations. In the second stage, the scheme tries to cancel the residual SI using an RLS DCD based adaptive beamformer. The beamformer combines the residual signals at the hydrophones in such a way to minimize the final residual SI. The cancellation performance of the proposed scheme is investigated using the Waymark simulator and with lake experiments. As demonstrated by the simulation results, the SI can be efficiently cancelled with the proposed two stage SI cancellation scheme. The experimental results also demonstrate the capability of the adaptive beamformer in reducing the residual SI after digital cancellation.

title: In Band Full Duplex Interference for Underwater Acoustic Communication Systems
abstract: This work presents an adaptive self interference cancellation (SIC) method for in band full duplex underwater acoustic (IBFD UWA) systems along with a model for the self interference (SI) for shallow water acoustic channels. The proposed system utilizes orthogonal frequency division multiplexing with quadrature phase shift keying modulation to exchange information between two nodes operating in IBFD mode. The proposed adaptive SIC scheme employs the normalized least mean square (NLMS) algorithm to suppress the SI signal and avoid saturating the local analog to digital (ADC) converter. Unlike existing research works, we investigate the effect of imperfect detection of the signal of interest on the ability of the SIC to diminish the SI signal. We provide experimental results to support the SI model developed and simulation results to demonstrate the ability of the proposed adaptive SIC scheme to mitigate the SI signal to approximately the level of the ambient noise.