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Wideband Monopulse Spatial Filtering for Large Array Receivers for Reverberant Underwater Communication Channels. Karl Nieman Applied Research Laboratories: The University of Texas at Austin Kenneth Perrine, Terry Henderson, Keith Lent, Terry Brudner, and Brian Evans IEEE OCEANS 9/22/10.
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Wideband Monopulse Spatial Filtering for Large Array Receivers for Reverberant Underwater Communication Channels Karl Nieman Applied Research Laboratories: The University of Texas at Austin Kenneth Perrine, Terry Henderson, Keith Lent, Terry Brudner, and Brian Evans IEEE OCEANS 9/22/10
Outline • Underwater Acoustic Communications (ACOMMS) • Vertical Array ACOMMS Receiver • Wideband Monopulse Beamforming • Multi-Channel Equalization • Multipath Simulation • Experimental Results from Lake Travis 9/22/10 K. Nieman, IEEEOCEANS’10 2
ACOMMS with Large Array Receiver Buoys Array Receiver Divers Communicator(s) Seafloor Instruments UUVs 9/22/10 K. Nieman, IEEEOCEANS’10 3
ACOMMS vs. RF Wireless Comms • Acoustic propagation speed slower by 200,000X • More severe effects of time-varying boundaries and Doppler • Absorption-limited bandwidth • In 10°C seawater: • 1 kHz: ~0.06 dB/km • 100 kHz: ~33.4 dB/km USEABLE BAND (at 1 km) 9/22/10 K. Nieman, IEEEOCEANS’10 4
Shallow Water ACOMMS Channel Impairments • Severe Doppler may be present • Substantial vertical multipath due to boundary reflections • Rapidly-varying channel • Coherence time is only marginally longer than impulse response duration: absolute amplitude Shallow water impulse response of LFM sweep over 1 s. 9/22/10 K. Nieman, IEEEOCEANS’10 5
ACOMMS Signal Processing • Compensating for channel impairments: • Detect, synchronize and compensate for Doppler • Adaptively equalize • Adaptive equalizer size dilemma: • Small to maintain stability while tracking quickly-varying paths • Large to fully deconvolve long channel response • Often, all acoustic paths cannot be coherently combined, leaving residual incoherent paths that degrade performance. • Performance is then reverberation limited rather than background-noise limited 9/22/10 K. Nieman, IEEEOCEANS’10 6
Our configuration: Adaptive Decision Feedback Equalizer (DFE) 11 half-symbol spaced forward taps 5 decision feedback taps Hybrid LS-LMS algorithm with adaptation step size 0.01 4096 M-QAM data symbols per packet Problem: Performance is limited in shallow water Solution: Substitute a large array of closely-spaced elements for omni receiver Use vertical multi-channel processing to combat multipath Single-Channel ACOMMS Receiver wet dry Frame detection/ synchroni-zation Bulk Doppler detection/ correction Single channel adaptive DFE omni receiver 9/22/10 K. Nieman, IEEEOCEANS’10 7
We compare four vertical processing options: K=N option: Pass all Noutputs to DFE K=1 option: Form single vertical beam s0(t) for DFE K=2 option: Form two vertical wideband monopulse beams for DFE K=3 option: Form three vertical wideband monopulse beams for DFE K=N has been addressed by many investigators But often with large vertical element separation Large N can threaten equalizer success K=1,2, or 3 can exploit beamformer capabilities already needed for other purposes ACOMMS Receiver Using Array wet dry Horizontal beamform/select Vertical processing options Frame detection/ synchroni-zation Bulk Doppler detection/ correction K-channel adaptive DFE N layers 9/22/10 K. Nieman, IEEEOCEANS’10 8
K=1 option: Form single vertical beam output s0(t) Steer to reject bad multipath (e.g. boundary reverberation) K=2 option: Use wideband monopulse processing to get 2nd beam output s1(t) such that s1(t) ≈ αs0(t) where K=3 option: Include a 3rd beam output s2(t) such that s2(t) ≈ αs1(t) Explanation of Vertical Processing for K =1,2,3 sum weight delay Spatial response Nelements beamwidth (frequency independent) 9/22/10 K. Nieman, IEEEOCEANS’10 9
Wideband Monopulse Wideband Null Steering • Simple linear combination of s0(t) and s1(t) gives single wideband null: • Linear combination of s0(t),s1(t),and s2(t)gives two indepedently-steerable wideband nulls • Equalizer can form these linear combinations (or any others), in the K=2 and K=3 cases, to best minimize MSE • In fact, it does • Monopulse method thus reduces the number of input channels to the equalizer yet allows a spatial null(s) to be steered toward localized interferences 0.17s0(t) + s1(t) linear combination 9/22/10 K. Nieman, IEEEOCEANS’10 10
Results of Simulation and In-Water Tests at Lake Travis 9/22/10 K. Nieman, IEEEOCEANS’10 11
Shallow Water Simulation Ray trace from omni-directional source to 6-element vertical line array for different shallow water channels (ranges of 25-700 m) Isotropic sound speed 3 paths (direct, surface bounce, and bottom bounce) source array • Performance metric used:output signal-to-noise-ratio (OSNR) • Inverse of mean squared error at DFE output Ray trace of propagation paths through isotropic medium. 9/22/10 K. Nieman, IEEEOCEANS’10 12
No Doppler Simulation K=1 option: K=2 option: K=3 option: K=N option: , , , OSNR at EQ output for stationary 6-element line array processed using four different techniques. 9/22/10 K. Nieman, IEEEOCEANS’10 13
5 m/s Doppler Simulation K=1 option: K=2 option: K=3 option: K=N option: , , , OSNR at EQ output for moving 6-element line array processed using four different techniques. 9/22/10 K. Nieman, IEEEOCEANS’10 14
Evidence of Null-Steering in Equalized Channel equalizer length path 1 (direct) path 2 (surface) path 3 (bottom) K=1 LFM matched-filter of EQ-forward filtered output for (a) s0(t), (b) s0(t) + s1(t), and (c) s0(t) + s1(t) + s2(t). Orange and green shaded region shows EQ time span for forward and feedback filter taps, respectively. K=2 K=3 9/16/10 K. Nieman, IEEEOCEANS’10 15
Experimental Data from Lake Travis Two sets of data collected at ARL:UT’s Lake Travis Test Station (LTTS) in 2009 using array receiver + mobile, omni source Overhead view of Lake Travis Test Station with overlaid bathymetric map. 9/16/10 K. Nieman, IEEEOCEANS’10 16
Test A: 6-Element Vertical Line Array Evaluated gain in OSNR of K=2 re: K=1 vs. range Just as in simulation, gains are highly dependent on range (multipath structure), and decrease with increasing range OSNR gain of s1(t) + s0(t) vs. s0(t) for 155 16QAM packets. Results sorted by range (red dots, right vertical axis). 9/22/10 K. Nieman, IEEEOCEANS’10 17
Test B: 8-Element Vertical Line Array Gain vs. single-element receiver for each technique K=2,3 have similar mean to K=N w/ less variance Also, 1-2 orders of magnitude less complexity in DFE alone K=N K=3 K=2 K=1 OSNR gain and ±σ over single element receiver for four processing methods for 96 packets at various rates. 9/22/10 K. Nieman, IEEEOCEANS’10 18
Closing Remarks Conclusions K=2,3 can reduce the size/complexity of the equalizer w/ similar performance to K=N in shallow water Smaller equalizer can be stable w/ increased adaptation rate Outperforms straight beamforming (K=1) in all data Caveats Observed gains dependent on: multipath environment (e.g. range, depths) platform motion May not do as well w/ uniform interference 9/22/10 K. Nieman, IEEEOCEANS’10 19
Questions? References: [1] – R. E. Francois and G. R. Garrison, “Sound absorption based on ocean measurements – 2. boric acid contribution and equation for total absorption,” J. Acoust. Soc. Am., vol. 72, no. 6, pp. 1879-1890, 1982. [2] – K. A. Perrine, K. F. Nieman, K. H. Lent, T. L. Henderson, T. J. Brudner, B. L. Evans, “Doppler estimation and correction for shallow underwater acoustic communications,” Proc. Asilomar Conf. on Signals, Systems, and Computers, 2010. [3] – K. F. Nieman, K. A. Perrine, K. H. Lent, T. L. Henderson, T. J. Brudner, B. L. Evans, “Multi-stage and sparse equalizer design for communication systems in reverberant underwater channels,” Proc. IEEE Int. Workshop on Signal Processing Systems, 2010. [4] – P. J. Beaujean and L. R. LeBlanc, “Adaptive array processing for high speed acoustic communication in shallow water,” IEEE J. of Oceanic Engineering, vol. 29, no. 3, pp. 807-823, 2004. [5] – T. L. Henderson, “Matched beam theory for unambiguous broadband direction finding,” J. Acoust. Soc. Am., vol. 78, no. 2, pp. 563-574, 1985. Also, our group has released underwater acoustic data recorded at Lake Travis for public use: http://users.ece.utexas.edu/~bevans/projects/underwater/datasets/index.html 9/22/10 K. Nieman, IEEEOCEANS’10 20
Bad Doppler Example The phase of a QPSK signal after linear Doppler correction. 9/22/10 K. Nieman, IEEEOCEANS’10 21
Data Packet Data packet with durations in symbol periods
Wideband Monopulse ACOMMS Receiver For this study, multi-channel decision-feedback EQ configured with: 11 half-symbol-spaced forward taps per channel 5 symbol-spaced decision-feedback taps LMS adaptation rate of 0.01 data packet 9/22/10 K. Nieman, IEEEOCEANS’10 23