Improved AUV Autonomy Provided by an Underwater ... - CiteSeerX

Proceedings of the Sixteenth (2006) International Offshore and Polar Engineering Conference San Francisco, California, USA, May 28-June 2, 2006 Copyright © 2006 by The International Society of Offshore and Polar Engineers ISBN 1-880653-66-4 (Set); ISSN 1098-6189 (Set)

Improved AUV Autonomy Provided by an Underwater Acoustic Link Joël Trubuil♦ Gerard Lapierre♣ Joël Labat♦ Nicolas Beuzelin♣ André Goalic♦ & Christophe Laot♦ ♦

♣

ENST-Bretagne, Brest Cedex, France G.E.S.M.A. (Groupe d’Etudes Sous-Marines de l’Atlantique), Naval Brest, France

ABSTRACT : GESMA (Groupe d’Etudes Sous-Marines de l’Atlantique), initiated a project named TRIDENT (TRansmission d’Images et de Données EN Temps réel) a few years ago. The project’s objective is to develop a high data rate link allowing the transmission of different information (text, images...). A real-time receiver platform based on a spatiotemporal equalizer developed by ENST Bretagne, was carried out to reduce the various perturbations brought by the underwater acoustic channel (UWA). Different results show the robustness of the TRIDENT system for transmission in a strongly disturbed channel. Blind adaptive equalizers have been mainly dedicated to continuous data stream applications. Combined with an iterative procedure for short bursts, this spatio-temporal equalizer can resolve the challenge of recovering almost all the data. Nevertheless, some improvements were needed to achieve a solution compatible with potential future users assumptions. This paper aims to present some improvements and develop some technical aspects concerning the integration of such a device into a more complex system as an AUV could stand for. Key words : Underwater acoustic communication; adaptive equalization; blind equalization; AUV; real-time platform.

I. INTRODUCTION Since a few years GESMA, in collaboration with ENST Bretagne and SERCEL, is developing a robust acoustic link to improve autonomy of mine hunting vehicles. To deal with multipath and noise, a real time prototype was designed by ENST Bretagne. This prototype named TRIDENT (Trubuil and al., 2002) is a real time acoustic link designed to transmit images, text and data. An acoustic transmission usually is corrupted by different impacts brought about by the underwater channel. One can note multipath propagation, Doppler effect and noise. From another point of view, carrier frequency and available

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bandwidth are much lower than those existing in other communication channels. To mitigate these different effects and optimize spectral efficiency, a spatio-temporal equalizer introduced by J. Labat and al. (1998; 2001) was chosen. This equalizer is the core of our receiver platform. GESMA platform allows real-time transmission of information at a data rate up to 20 kbps in a horizontal configuration without periodic and training sequences. After, to obtain an operational product, improvements were made to the initial transmission scheme. These evolutions concern the need for iterative equalization (Lapierre and al., 2003) and frame design. All these modifications are intended to increase the acoustic transmission reliability needed by users. This paper aims to provide an overview of the GESMA project. At the beginning, we present the design and previous results of the TRIDENT system. Then, we describe the spatio-temporal equalizer and justify the choice of the SOC-MI-DFE (Self Optimized Configuration Multiple Input Decision Feedback Equalizer) on real data experiments. After, this paper will develop the needed improvements for the acoustic modem integration onboard the REDERMOR II AUV (Autonomous Underwater Vehicle) (Tourmelin and Lemaire, 2001). Then, we focus on the transmission protocol used and acoustic compatibility. The last part will present some preliminary results reached during our last sea-trials in June 2005. Images from a sonar acquisition can be directly transmitted from the vehicle to the surface vessel in order to detect possible mine presence.

II. BACKGROUND Multipath propagation and noise are responsible of most of the effects on underwater acoustic communication. These physical impacts produce a time dispersion and variability of the received signal. To cope with all these perturbations induced by such harsh channels, equalizers are necessary for coherent communication if there is severe spread of transmitted signal in time. In addition, for non-stationary channels, equalizers need to be adaptive. Most of the time, transmitted symbols are organized in frames built with a probe to synchronize the

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receiver, a training sequence to allow equalizer convergence and finally true data. The training sequence has to be periodically repeated in order to satisfy the worst case conditions. This strategy decreases the maximum reachable throughput data rate. Blind approach equalization increases throughput data rate because it is not necessary to use preamble or another training sequence. This method has proved to be very efficient on severe channels, such as UWA ones (Stojanovic and al., 1994). The TRIDENT acoustic system is a high data rate acoustic link based on blind spatio-temporal equalizer called SOC-MI-DFE (Labat and al., 1998; 2001). This equalizer uses input signals sampled on several sensors coming from the same emission source. This space diversity provides a better SNR (Signal to Noise Ratio) compared to a mono-sensor version. The SOCMI-DFE is able to run according to two modes (fig 1.): a convergence or starting mode, in which the equalizer uses a self-controlled way, and a tracking mode where it is controlled by its own decisions. More details about adaptation criteria can be found in Labat and al. (1998; 2001). Switching between the modes are carried out in an automatic and reversible way by comparison of the MSE (Mean Square Error) with a threshold J0 (-6dB in our application). The power of this device is thus explained by these changing modes which are differentiated both on structural and algorithmic levels. s 1 [k] +

u 1 [k]

-

Convergence mode

B 1(z)

A(z) v[k]

e s n [k] +

u n[k]

-

MSE < Jo

dˆ [k]

w[k]

-j. θ[k]

B n(z)

MSE > Jo

A(z)

s 1 [k] B 1(z)

v[k]

y[k] +

dˆ [k]

w[k]

validated and its performance evaluated. The transmitter can be fixed on a buoy, or for mobility tests, positioned on a ROV (Remotely Operated Vehicle). Communications up to 500, 1000 and 2000 meters were thus carried out in horizontal and vertical transmissions. The TRIDENT system can use two carrier frequencies (20 kHz or 35 kHz) with a QPSK modulation (Quadrature Phase Shift Keying). Several data rate can be tested. The data are transmitted with a bit rate ranging from 8.75 kbps to 23.3 kbps (35 kHz carrier frequency) or 6.7 kbps to 40 kbps (20 kHz carrier frequency). Spectral efficiency is very good regards to the data rate over carrier frequency ratio. Up to now, information is transmitted without channel coding. In reception, acoustic signals are received on 4 hydrophones laid vertically. Hydrophones interspaces is nearly 20 cm (around 5 wavelengths) (fig. 6). A first example shows the evolution of the estimated MSE (difference between decision device input and output) (fig. 2) of the SOC-MI-DFE for 1 sensor (either with sensor 1 or sensor 3), 2 and 4 sensors. Recorded pictures are transmitted during 4 minutes at a bit rate of 14 kbps. The distance between transmitter and receiver was about 1000m and the depth around 22m (shallow water environment). This transmission is successfully demodulated proving once again receiver robustness. Even if estimated MSE are relatively low whatever the number of sensors used, one can note that performances of the SOC-MI-DFE with one sensor can greatly fluctuate according to the position of the sensor. It clearly shows the impact of acoustic channel in terms of fading even for two quite closed sensors. The gain brought by 4 sensors can reach more than 7 dB. It shows that spatial reception diversity increases the transmission quality. The estimated MSE is also more stable which means that the composite channel between the source and the 4 hydrophones antenna is easier to equalize than each of the 4 single acoustic channel taken separately.

e -j. θ[k] A(z)

s n [k] B n(z)

Tracking mode

fig 1. SOC-MI-DFE (convergence and tracking modes) Each input signal from hydrophone is synchronously sampled. As a consequence, demodulation can be performed using digital processing. Demodulator output gives an even number of sample by symbol duration. This key point must be highlighted for everyone who aims to develop a continuous data transmission scheme. Timing recovery is achieved by the GARDNER algorithm (Gardner, 1986) and then equalization can be performed. Real time only is achieved if treatments duration is lower than the symbol duration. The receiver platform is based on an acquisition board plugged into a personal computer (PC). Board architecture is based on a Texas Instruments (TI) TMS320C6201 Digital Signal Processor (DSP). Sea-trials were carried out in various areas around Brest bay in 2002 and 2003 to evaluate the acoustic transmission reliability. The real time process of the TRIDENT system has been

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fig. 2 : Evolution of the MSE for 1, 2 and 4 sensors

However, the performance obtained by the spatio-temporal equalizer SOC-MI-DFE may be not suitable. For example, convergence time required may be too long to be compatible

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with burst communication systems. Transmission by burst is often required when acoustic compatibility is needed. In our project, acoustic modem must work with other acoustic devices as positioning system, Loch Doppler, front or side scan sonar… The acoustic bandwidth is quite low (lower than 100 kHz) and short time pulses often used induced a large frequency range. This means that acoustic compatibility is very hard to achieve. As a consequence, one has to synchronize all existing devices using time sharing. The time left for acoustic transmission is then limited. Continuous data transmission is then no longer possible. As a consequence, the blind equalizer must be periodically turned into its convergence structure at the beginning of each frame. The fact that the asymptotic performances of linear equalizer (structure of the convergence mode) are less good than that of DFE (structure of the tracking mode) could lead to degradation on the performance link. One solution developed during 2004 and 2005 consisted in studying feasibility of iterative equalization (Lapierre and al., 2003). The received signal is equalized with an iterative process. At first, it is classically equalized. Then, the received signal is processed one time again with the new filter taps. This iterative process can be done as many times as real time constraints are respected. Iterative processing withdraws remaining errors after that first process. It could be considered as single pass equalization carried out on a long frame constituted by the succession of little frames. Figure 3 illustrates the impact of iterative equalization on real signals. Estimated MSE is drawn on the left side. Output constellations are placed on the right side. The bluer line concerns single pass equalization and the red line represents results of two passes equalization.

There are many acoustic devices onboard this vehicle. The REDERMOR II is controlled and commanded by a low rate acoustic link. The data bit rate of this modem is from 20 to 300 bps with a 35 kHz carrier frequency. Telemetry and recorded data are recovered by the Trident system. It allows images transmission from a front or side scan sonar or a CCD camera. Two systems are used to evaluate the AUV position: a Nautronix beacon working with a 15-18 kHz bandwidth and a Generic Acoustic Processing System (GAPS) with a 18 kHz interrogation frequency and a 25-30 kHz bandwidth response. All of these equipments use its own frequency bandwidth and acoustic compatibility must be assumed. Figure 5 shows the frequency allocations of the REDERMOR II during a sea trial. In parallel, an Acoustic Doppler Current Profiler (ADCP) deployed by another ship was used to measure water currents. The ADCP transmit “pings” at a 12 kHz frequency with an harmonic at 36 kHz. The figure below is really interesting as it shows that some filtering assumptions need to be taken into account in the future. Finally, the acoustic signal used for communication purpose is then damaged by the impact of underwater channel and other sources which modify noise characteristics.

Low data rate Expected High data rate

2 pass 1 pass

MSE (dB)

fig. 4 : REDERMOR II

GAPS response Acomms Symboles

fig. 3 : Evolution of the MSE and output equalizer constellations for single (blue) and iterative (red) treatments.

GAPS request

III. REDERMOR II CONFIGURATION

Nautronix Beacon

The REDERMOR II (Tourmelin and Lemaire, 2001) is an experimental platform for sea trials developed to test different concepts of mine hunting missions. Figure 4 shows the AUV deployment. This AUV can be remotely-operated or managed in an autonomous way with or without acoustic link. The vehicle length is 6m, the diameter is approximately 1m and the weight is about 3 tons. The maximum depth reached by the REDERMOR II is 200m.

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ADCP Cage (12+36 kHz)

fig. 5 : Frequency allocations with respect to several users The French Navy ship THETIS is used to deploy the vehicle for sea trials. During the mission, the AUV mean altitude is 15m above sea bottom and its speed is approximately 2.5

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knots. For the high and low data rate links, the hydrophones are deployed at 7m depth (about 3m below the ship's draught) with a floating chain. It implies low speed of the ship. The two existing acoustic links used specific transducers but the same carrier frequency. It implied a needed time sharing. That is why low and high data rate acoustic transmissions are exchanged periodically. A typical frame duration is about 220s among which the first 180s are reserved for image transmission and the last 40s for control and command applications. Nevertheless, sonar acquisition take place for the duration of each frame. This means that the bit rate of the acoustic link must be increased. THETIS

Hydrophones antenna

REDERMOR II

fig. 6 : Sea trials configuration

The first result concerns a MLBS (Maximum Length Binary Sequence) transmission. The data bit rate is 12 kbps with a QPSK modulation and a 35 kHz carrier frequency. SOC-MIDFE parameters are as follows: Transversal filters B: 17 taps, recursive filter A: 80 taps regards to the impulse response duration. Figure 8 shows three subplots. The first picture in the top of the figure shows the time-frequency representation of recorded signals over 5s and 48 kHz. One can easily see the signal used for communication about 35 kHz, the ADCP signal at 12 kHz and some interferers such as GAPS or Nautronix signals. These short time jamming signals will directly impact on transmission performances. The middle part of the figure depicts the MSE evolution during 5s (more than 60 000 bits). The upper line stands for the MSE for single process equalization although the lower one stands for an iterative equalization with two passes. When sudden perturbation occurs, one can see that MSE is increasing due to the variation on the noise structure. Most of conventional equalizers need in that case some training sequences. The Trident system is always able to take these impacts into account and prevent this device to evolve towards a pathological situation. The equalizer convergence mode is used when the reliability in estimated data is not sufficient. This transmission shows the interest of space diversity. The possibility to switch from one structure to another according to the channel severity proves equalizer robustness. The last part of the figure 7 gives some views on signals constellations with input s (sensor 1) and output equalizer (w) taken between 3 and 3.5s. Output constellation proves the processing gain brought by equalization process. Please refer to the figure 1 for location of signal s, u, v and w.

MSE (dB)

Sea trials were carried out in June 2005 in the bay of DOUARNENEZ. Receiver performance of the high data rate acoustic link is evaluated by MSE measurements at the output equalizer and various constellations picked at different places (input or decision device). In reception, the 4 hydrophones are approximately located at 20cm from each other. Figure 7 depicts the evolution of the planned route. Pink circle give the AUV location and red cross the ship position. The range between THETIS and REDERMOR II is about 770m and its speed is about 2.5 knots.

Frequency (hz)

IV. SEA TRIALS RESULTS

s

u Time (s) v

w

fig. 8 : MSE evolution regards to some channel perturbations The second result is about MLBS (Maximum Length Binary Sequence) transmission with a 12 kbps data bit rate and a 35 kHz carrier frequency. The AUV trajectory is the same but the

fig. 7 : REDERMOR II trajectory during a mission

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4

MSE (dB)

Frequency (hz)

distance between emitter and receiver is about 1000m. 4s of the communication were evaluated. The upper part of the figure 9 depicts time-frequency representation of recorded signals. There are fewer interferers than in the previous example. The lower part of the figure depicts the MSE evolution as it is described above. It is interesting to note that the first pass on the equalization process needs a time convergence greatest than 2s. Estimated data will not be reliable during this time. It could be explained by a bad choice on taps value during initialization process. If one considers the taps value achieved at the end of the first process (after 4s) and uses these values as the new ones for the second pass, the time convergence is drastically reduced. The equalizer begins directly in its tracking mode preventing all errors in the estimated data even at the beginning of the frame. All occurs as no convergence time was needed.

REFERENCES Trubuil, J, Lapierre, G, Le Gall, T, and Labat, J (2002). "Realtime high data rate acoustic link based on spatio temporal blind equalization: the TRIDENT acoustic system," Proc. OCEANS, Biloxi, Vol.4, pp 2438-2443. Labat, J, Macchi, O, and Laot, C (1998). "Adaptive decision feedback equalization : can you skip the training period ?," IEEE Trans on Comm, vol. 46, N°7, pp 921-930. Labat, J, and Laot, C (2001). "Blind adaptive multiple input decision feedback equalizer with a self optimized configuration ," IEEE Trans on Comm, Vol. 49, N°4. Lapierre, G, Labat, J, and Trubuil, J (2003). "Iterative equalization for underwater acoustic channel – Potentiality for the TRIDENT system," Proc. OCEANS, San Diego, pp 1547-1553. Toumelin, N, and Lemaire, J (2001). "New capabilities of the REDERMOR unmanned underwater vehicle," Proc. OCEANS, Honolulu, Vol.2, pp 1032-1035. Stojanovic, M, Catipovic, J, and Proakis, J (1994). "Phase Coherent Digital Communications for Underwater Acoustic Channels," IEEE Journal of Oceanic Engineering, vol. 19, No.1, pp 100-111. Gardner, F (1986). "A BPSK, QPSK timing-error detector for sampled receivers," IEEE Trans on Comm., Flight COM34, N°5.

Time (s) fig. 9 : Contribution of the iterative equalization Nevertheless, all spikes in the MSE evolution could not be avoided due to the presence of interferers.

V. CONCLUSION AND PERSPECTIVES This paper presents the high data rate acoustic link named TRIDENT, developed by GESMA, ENST Bretagne and SERCEL. After some recalls of the TRIDENT system, test configuration and sea trials are described in order to validate the system onboard the REDERMOR II AUV. According to the results obtained for sonar image transmission, one can note the robustness of the spatio-temporal equalizer. For strongly non-stationary channels, the space diversity and iterative equalization allow significant improvement in receiver performance. The first tests in an operational context show that real time transmission of information (images...) is feasible even with harsh channels such as the underwater acoustic channel and presence of multiple interferers. In this context, the contribution of a channel coding can improve the transmission robustness and protect the data transmitted from remaining errors. Synchronization, time sharing between all acoustic devices will also be a key point in the future to achieve potential users requirements.

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