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نوع فایل : Word
تعداد صفحات : 96
Contents
1. Introduction 1
1.1 Motivation and problem description 1
1.2 Literature Review 3
1.3 Thesis outline 5
1.4 Contributions 7
2. Underwater Acoustic Communication and Channel Model 8
2.1 Channel Model 8
2.2 The Baseband channel 11
2.3 FSK Signal modulation and demodulations: 18
3. Receiver and Transmitter Diversity Techniques: 21
3.1 Antenna Diversity 22
3.2 Temporal Diversity 23
3.3 Branch Combining 24
3.4 Diversity gain and failure 25
3.5 Conventional multiple antenna systems 27
3.6 Parallel stream MIMO 28
4. Review of Adaptive Equalizers 32
4.1 Linear equalization 36
4.1.1 Peak distortion criterion 38
4.1.2 Mean square error criterion 40
4.1.3 Fractionally spaced equalizer 42
4.2 Decision feedback equalization 43
4.2.1 Mean square error optimization criterion 46
4.3 Filter coefficient update methods 48
4.3.1 LMS 48
4.3.2 RLS 51
4.4 Selected experimental results 53
5 Diversity Equalization 57
5.1 Baseband communication system model 57
5.2 Joint channel equalizer (JCE) 63
5.3 Diversity with separate optimization using an independent channel equalizer (ICE) 77
6 Conclusions and Topics for Future Research 83
6.2 Further research directions 84
References 86
ABSTRACT
Currently available commercial underwater acoustic modems have been optimized for long distance communications, on the order of tens to hundreds of kilometers, trading transmission rate for increased range, reliability and robustness. However, new under-water applications are arising which do not require such long distance links. One example is the use of small fleets of closely spaced Autonomous Underwater Vehicles (AUVs) for tasks such as mine detection where the vehicle spacing is more likely to be measured in hundreds of meters at most.
This thesis investigates ways to improve transmission rates over this much shorter link. In particular, it considers the use of an additional hydrophone, currently used for navigation, to achieve diversity at the receiver. We propose an adaptive receiver structure that is capable of reliable asynchronous communication with improved efficiency.
Existing underwater acoustic equalization studies are limited to optimizing the mini-mum Mean-Square Error (MSE) jointly among all spatial diversity channels, called the Joint Channel Equalizer (JCE). In this study we propose a new sub-optimal equalizer that separately optimizes the diversity channels. We have called this the Independent Channel Equalizer (ICE). It ultimately results in a higher MSE but the system is more robust to step changes. This is beneficial to allow rapid re-establishment of communications. Re-sults are presented both in terms of the MSE and the probability of Bit Error Rate (BER). The latter is important, as it is the ultimate measure for a digital communication system.
1. Introduction 1.1 Motivation and problem description
As technology progresses our ability to utilize and operate within the underwater environment is advancing. Once confined to merely operating on the surface we are now rap-idly exploring the depths of the oceans for military, commercial and scientific purposes.
The use of Autonomous Underwater Vehicles (AUVs) is showing great potential among many new areas of underwater research. Possible applications for this technology include dangerous and/or routine tasks such as minesweeping and reconnaissance, neither of which is suitable for human operators. However, there are many challenges that need to be addressed. These include the autonomous control of a fleet of AUVs and consequently the underwater communications required for this.
In [1,2] the problem of control algorithms for the formation flying of AUVs is considered. As part of the solution a pair of hydrophones is used to provide relative headings to aid in maintaining the formation. The presence of the two hydrophones also present possible opportunities to improve communications between AUVs, using techniques such as receiver diversity, transmitter diversity or Multiple-Input-Multiple-Output (MIMO) communications.
In this application the hydrophones are located at the bow and stern of the AUV. Due to the physical dimensions of the AUV the separation is approximately one meter. This is much closer than typical diversity receiver separations [3]. However, other researchers have shown that receiver separations as low as 0.35 meters can provide additional sources of information in the underwater acoustic channel [4].
The fading and multi-path underwater acoustic channel has always been a great impediment to building reliable underwater communication systems. Many complex physical phenomena cause the propagating acoustic wave intensity and phase to vary temporally and spatially. Thus, one advantage of spatial diversity equalization is its ability to improve the limited signal-to-noise ratio at the receiver through coherent combining. The multi-path spread in these channels is largely caused by reflections from ocean boundaries and refraction due to sound speed variations as a function of depth. Since the degradation in the signal is caused by multi-path intersymbol interference (ISI), simply increasing the signal-to-noise-ratio will not alleviate the problem.
It is well known that coherent equalization techniques improve the bandwidth efficiency of the communication system, thus increasing the data rate [6]. Equalization techniques are well understood in radio communications [5]-[7]. A variety of equalization techniques are available, such as zero-forcing equalization, MMSE equalization, and block equalization. Decision Feedback Equalization (DFE) can be considered an effective technique because it can help to eliminate causal ISI in addition to compensating for the channel [5].
When the communication channel is unknown to the designer, adaptive equalization techniques can be used to first extract the channel response from a training sequence and then compensate the channel distortion in the incoming data symbols [7]. In this thesis the problems above are considered and a sub-optimal decision feedback adaptive equalizer with spatial diversity has been proposed and the results are compared to the optimal equalizer.
1.2 Literature Review
A general overview of the current state of underwater communications is presented in [8]. The underwater channel and its characteristics are discussed and examples of a number of communication systems of various ranges are given. It also outlines some of the current research topics, namely, receiver complexity reduction, interference cancellation and multi-user communication, system self-optimization, modulation and coding and mobile underwater communication.
Among current research a common approach to improving communication rate and reliability is to use multiple diversity channels and a jointly optimized receiver structure. Researchers have shown that for the underwater acoustic channels receiver separations as low as 0.35 meters can provide valid diversity channels [4]. In [3] a joint channel equalizer is formulated where the equalizer is optimized across all diversity channels with the receivers spaced apart in depth by between 9.4 and 55.2 meters. The separation between transmitter and receiver is 8 nautical miles. This separation is significantly greater than any expected separation within the AUV fleet. The receiver was found to have excellent performance. However, it was computationally complex and thus a sub-optimal design was presented with much reduced complexity. This sub-optimal equalizer consisted of a set of single channel equalizers followed by a MMSE combiner. Satisfactory performance was found with a DFE equalizer composed of 25 feed-forward taps and 15 feedback taps.
