A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters Fabian Steinmetz, Jan Heitmann and Christian Renner 17th GI/ITG KuVS Fachgespräch "Sensornetze", Braunschweig September 13 th , 2018 Research Group smartPORT Research Group smartPORT TUHH TUHH Hamburg University of Technology Hamburg University of Technology
Motivation � Micro autonomous underwater vehicles ( µ AUVs) and underwater wireless sensor networks (UWSNs) � Possible applications � Localization of pollution � Tracking and inspection of ships � µ AUVs and UWSNs need a stable and low-power communication interface Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 1 1
Underwater Channel (I) � Acoustic wave ⇒ speed of sound c ≈ 1500 m / s � Reflections and scattering ⇒ frequency-selective and time-dependent channel � Shallow water scenarios ⇒ high impact of the surface reflection Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 2 2
Underwater Channel II Transmitted Noise 80 amplitude ( mV ) 60 � 1 s band-limited pseudo-random noise 40 � 10 kHz - 100 kHz 20 � Strong frequency dependency 0 25 50 75 100 frequency ( kHz ) 2 . 1 m Channel 5 . 1 m Channel 80 80 amplitude ( mV ) amplitude ( mV ) 60 60 40 40 20 20 0 0 25 50 75 100 25 50 75 100 frequency ( kHz ) frequency ( kHz ) Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 3 3
smartPORT Acoustic Underwater Modem � Small and low-power device � Contains of: � Mainboard with power supply and Cortex M4 microcontroller � Receiver with amplifiers, band-pass filter and AD-converter � Transmitter with DA-converter and power-amplifier � External hydrophone � Frequency Shift Keying (FSK) based synchronization and data transmission � Range up to 150 m and data rate 260 bit/s Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 4 4
FSK and Chirp Keying Up-Chirp 1 � Frequency Shift Keying (FSK) s ( t ) 0 � A bit is transmitted with a constant frequency � Non-coherent detection (currently implemented) -1 � Problem: synchronization 0 0 . 5 1 t/T → � Chirp Keying � Frequency sweep from f s to f e Down-Chirp ◮ f s < f e ⇒ up-chirp 1 ◮ f s > f e ⇒ down-chirp � Bandwidth B = | f e − f s | s ( t ) 0 � Spread the information over spectrum � Cross-correlation detection -1 � Good correlation properties 0 0 . 5 1 t/T → Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 5 5
FSK and Chirp Keying - Comparison FSK Non-Coherent FSK Cross-Correlation Chirp Cross-Correlation 2 2 2 1 1 1 0 0 0 -1 -1 -1 -2 -2 -2 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 t/T t/T t/T 1 1 1 0.5 0 0 0 -1 -1 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 t/T t/T t/T Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 6 6
FSK and Chirp Keying - Comparison FSK Non-Coherent FSK Cross-Correlation Chirp Cross-Correlation 2 2 2 1 1 1 0 0 0 -1 -1 -1 -2 -2 -2 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 t/T t/T t/T 1 1 1 0.5 0 0 0 -1 -1 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 t/T t/T t/T Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 6 6
FSK and Chirp Keying - Comparison FSK Non-Coherent FSK Cross-Correlation Chirp Cross-Correlation 2 2 2 1 1 1 0 0 0 -1 -1 -1 -2 -2 -2 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 t/T t/T t/T 1 1 1 0.5 0 0 0 -1 -1 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 t/T t/T t/T Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 6 6
FSK and Chirp Keying - Comparison FSK Non-Coherent FSK Cross-Correlation Chirp Cross-Correlation 2 2 2 1 1 1 0 0 0 -1 -1 -1 -2 -2 -2 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 t/T t/T t/T 1 1 1 0.5 0 0 0 -1 -1 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 t/T t/T t/T Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 6 6
Preamble Based Synchronization � Packet-based transmission � Preamble and starting frame delimiter (SFD) � Parameter Settings: before each data-packet � Orthogonal symbols with T = 2.5 ms � Preamble with alternating symbols � Chirp bandwidth B = 2.342 kHz � Up- and down-chirps � FSK with 400 Hz symbol spacing � Sinusoidal symbols with two frequencies � 16 preamble symbols + 4 SFD symbols � Previous evaluations: Good sync ⇒ correct data reception Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 7 7
Experimental Setup � Signal generation and processing with MATLAB � USB oscilloscope and waveform generator � Low sampling rate (200 kHz) � RX and TX circuits � Marina in Hamburg with LOS conditions � long wide-band chirp (10 kHz to 100 kHz, 100 ms) to obtain a ground-truth � 2.1 m and 5.1 m distance, 0.5 m depth Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 8 8
Synchronization Macroscope FSK Non-Coherent 90 25 frequency ( kHz ) relative freq. (%) 80 20 � 100 synchronizations per frequency band 70 15 over 5.1 m 60 10 � 40 kHz - 90 kHz in 2 kHz steps 50 5 40 0 − 500 − 250 0 250 500 synchronization error (µ s ) Chirp Cross-Correlation FSK Cross-Correlation 90 25 90 25 relative freq. (%) frequency ( kHz ) relative freq. (%) frequency ( kHz ) 80 20 80 20 70 15 70 15 60 10 60 10 50 5 50 5 40 0 40 0 − 500 − 250 0 250 500 − 500 − 250 0 250 500 synchronization error (µ s ) synchronization error (µ s ) Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 9 9
Synchronization Microscope (I) FSK Non-Coherent FSK Non-Coherent 10 100 relative freq. (%) � 1000 synchronizations over 2.1 m and 5.1 m 8 80 CDF (%) 6 60 � Fixed frequencies 4 40 � Chirp: 62.4 kHz - 64.8 kHz 2 20 � FSK: 62.4 kHz and 62.8 kHz 0 0 − 250 0 250 synchronization error (µ s ) Chirp Cross-Correlation Chirp Cross-Correlation FSK Cross-Correlation FSK Cross-Correlation 10 100 10 100 relative freq. (%) relative freq. (%) 8 80 8 80 CDF (%) CDF (%) 6 60 6 60 4 40 4 40 2 20 2 20 0 0 0 0 − 250 0 250 − 250 0 250 synchronization error (µ s ) synchronization error (µ s ) Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 10 10
Synchronization Microscope (II) FSK non-coh FSK xcorr Chirp xcorr 2.1 m Channel Inside 10 % margin 72.8 % 77.9 % 91.6 % Average sync. error 137.1 µs 122.2 µs 120.1 µs Standard deviation 77.2 µs 76.7 µs 73.6 µs 5.1 m Channel Inside 10 % margin 76.7 % 72.3 % 100 % Average sync. error 74.7 µs 62.3 µs 50.5 µs Standard deviation 87.7 µs 70.6 µs 48.7 µs Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 11 11
Conclusion and Outlook � A chirp-based synchronization enhances: � The number of received synchronizations 72 % ⇒ 100 % � The accuracy 71 µs ⇒ 49 µs (standard deviation) � The precision 62 µs ⇒ 51 µs (average sync. error) � Outlook: � smartPORT modem implementation � Chirps with a larger bandwidth � Research on chirp-based data transmission Fabian Steinmetz Fabian Steinmetz A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters A Case for Chirp Modulation for Low-Power Acoustic Communication in Shallow Waters 12 12
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