Underwater Acoustic OFDM: Past, Present, and Future Shengli Zhou Dept. of Electrical and Computer Engineering University of Connecticut http://uwsn.engr.uconn.edu WUWNET’11 Dec. 2, 2011 Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 1 / 26
Underwater Communications Cable Acoustic communications (ACOMM) Electromagnetic communications (Wireless Fibre Systems) Optical communications (Blue-Green Laser) Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 2 / 26
ACOMM Techniques Frequency shift keying (FSK) ◮ e.g., Teledyne Benthos, WHOI Micro-modem Direct sequence spread spectrum (DSSS) ◮ e.g., LinkQuest, DSPCOMM, Tritech Single carrier phase-shift-keying (PSK) transmissions ◮ e.g., WHOI Micro-modem, Benthos (additional processing card) Multicarrier modulation (in the form of OFDM) Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 3 / 26
OFDM: A Prevalent Choice for Broadband Wireless Systems DSL Modem WiFi (IEEE 802.11) WiMax (IEEE 802.16) 3GPP-LTE 4G and beyond Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 4 / 26
Number of Publications on OFDM 14 12 10 OCEANS Conference Papers Number of Publications IEEE/JASA Journal Papers 8 6 4 2 0 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 Year 1994 - 2005: sporadic effort and little progress 2006 - 2011: sustained effort and great progress Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 5 / 26
Outline OFDM basics: Pros and Cons Algorithm development Prototype development Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 6 / 26
Underwater Acoustic Channel Characteristics The sound propagates too slow! ◮ Long multipath ◮ Fast variation The SPACE’08 experiment, Martha’s Vineyard, depth 15 m 100 80 amplitude 60 40 20 0 2 4 6 8 10 12 delay [ms] Fast-varying multipath channel with a large delay spread Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 7 / 26
Block Transmission over LTI Channels Consider a linear time invariant (LTI) multipath channel N p � h ( t ) = A p δ ( τ − τ p ) p =1 Time domain waveform distortion; intersymbol interference (ISI) arises; complex channel equalizer needed y ( t ) = s ( t ) ∗ h ( t ) Frequency domain Y ( f ) = H ( f ) S ( f ) If s ( t ) is carefully constructed with no ISI in frequency domain S ( f ) | f = f m = s [ m ] Then no ISI at the receiver side Y ( f ) | f = f m = H ( f m ) s [ m ] Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 8 / 26
Basics of Orthogonal Frequency Division Multiplexing Frequency domain; f m − f k = ( m − k ) 1 T � � � S ( f ) = s [ k ] sinc ( f − f k ) T S ( f ) | f = f m = s [ m ] , k 1.2 s[k] s[k+1] s[k−1] 1 0.8 Frequency Fesponse 0.6 0.4 0.2 0 −0.2 −5 −4 −3 −2 −1 0 1 2 3 4 5 Frequency Time domain waveform; g ( t ) : rectangular pulse shaper f k = f c + k � s [ k ] e j 2 πf k t g ( t ) , s ( t ) = T k Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 9 / 26
Pros and Cons of OFDM Pros: ◮ Convert a dispersive channel to a set of parallel simple channels � K/ 2 − 1 � z m = H ( f m ) s [ m ] + n m m = − K/ 2 ◮ Receiver complexity does not depend on the channel delay spread! Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 10 / 26
Pros and Cons of OFDM Pros: ◮ Convert a dispersive channel to a set of parallel simple channels � K/ 2 − 1 � z m = H ( f m ) s [ m ] + n m m = − K/ 2 ◮ Receiver complexity does not depend on the channel delay spread! Cons: ◮ Poor performance on faded subchannels. ◮ Sensitive to the Doppler effect ⋆ Doppler shifts destroy the subcarrier orthogonality, and hence leads to intercarrier interference (ICI) ◮ Large peak -to-average power ratio (PAPR) Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 10 / 26
How to Drastically Enhance Performance in Fading Channels? 9 0 10 fading AWGN 8 fading AWGN −1 10 7 −2 6 10 average BER Capacity 5 −3 10 4 −4 10 3 2 −5 10 1 −6 10 0 0 5 10 15 20 25 −5 0 5 10 15 20 25 SNR(dB) SNR(dB) (a) BER vs SNR (b) Capacity vs SNR Fading channel drastically affects the uncoded performance Fading channel has the potential for reliable data transmission Solution: coded OFDM with strong codes, e.g., Turbo, LDPC codes that are capacity -achieving Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 11 / 26
How to Deal With the Doppler Effect? ICI is inevitable! Need signal processing algorithms to address ICI explicitly Signal processing� tailored to� OFDM� underwater� demodulation� channels� One example: Progressive receiver [JSTSP’2011] There are other alternative approaches Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 12 / 26
Progressive Receiver Adapt the receiver to channel conditions automatically without any a priori information Achieves both low complexity and robust performance over time -varying UWA channels H 0 H 1 H 2 H 3 . . . . . . . . . . . . . . . . . . . . . . . . z s n . . . = . . . + . . . . . . . . . . . . Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 13 / 26
Receiver Structure Pre-processing; set D = 0 1. The system model keeps z = H D s + n being updated Channel estimation Increase the span of ICI in equalization model Noise variance estimation Increase the maximum Increase D ; provide soft possible Doppler spread information ICI equalization in channel estimation 2. Soft information from the Nonbinary LDPC decoding channel decoder is utilized 3. No extra pilot -overhead No success or needed D = D max Yes Output decisions Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 14 / 26
Block Success Percentage: SPACE08 Success percentage vs. number of phones Success percentage vs. number of phones 1 1 D = 3 0.9 0.9 D = 3 D = 2 0.8 0.8 D = 2 D = 1 0.7 0.7 D = 1 D = 0 0.6 0.6 D = 0 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 S1 (60 m) S5 (1000 m) Averaged over Julian dates 295 -302 With 4 phones: 90% (D = 0), 95% (D = 1), and up to 98.8% (D = 3) Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 15 / 26
How to Alleviate the PAPR Impact? 0 10 Multicarrier −1 10 without Pr(PAPR>Thresh) PAPR control Single−Carrier Multicarrier with PAPR control −2 10 QPSK 8−QAM 16−QAM −3 10 6 8 10 12 14 16 Thresh [dB] QPSK: The gap between OFDM and single -carrier is about 6dB QAMs: The gap between OFDM and single-carrier is about 4dB Design considerations on power amplifier and transducer: ◮ Peak or average power constrained? Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 16 / 26
Summary OFDM is an elegant scheme It has a clear advantage for short -range long dispersive multipath channels. It is an appealing technique for shallow-water high-data-rate acoustic applications Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 17 / 26
WUWNet’07 Demo, Sept. 2007 Link A to B Link B to A Single -input single-output (SISO) OFDM in-air demonstration The data rate is 3.1 kb/s, with QPSK modulation, rate 1/2 LDPC coding, and bandwidth of 5.5 kHz Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 18 / 26
WUWNet’08 Demo, Sept. 2008 Multi -input multi-output ( 2 × 2 ) OFDM in-air demonstration The data rate is 6.2 kb/s, with QPSK modulation, rate 1/2 LDPC coding, and bandwidth of 5.5 kHz Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 19 / 26
WUWNet’09 and ’10 Demo, Nov. 2009 & Nov. 2010 Aqua -fModem Prototype: With keyboard input and LCD display Floating-point TMS320C6713 DSP board; running @ 225 MHz Fixed-point TMS320C6416 DSP board; running @ 1 GHz Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 20 / 26
WUWNET’11 Demo: A Network of Modems One -hop network, RTS/CTS based MAC protocol Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 21 / 26
Current / Future Issues (1) Multi -input multi-output (MIMO) ◮ Co-located: Increase the data rate via spatial modulation h 11 x 1 y 1 h 12 x 2 y 2 . . . . . . x Nt y Nr h NtNr ◮ Distributed MIMO, asynchronous MIMO User 3 User 1 User 2 v 3 = 0 v 1 v 2 Recivers Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 22 / 26
Current / Future Issues (2) Interference (Sonar, impulse interference, multiuser interference) Timedomain_for_637_phone_10 600 400 200 0 −200 −400 −600 0 0.5 1 1.5 2 2.5 6 x 10 ◮ Interference mitigation / avoidance / alignment / management Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 23 / 26
Current / Future Issues (3) Networking issues ◮ MAC ◮ Routing ◮ Reliable data transfer ◮ Applications How to efficiently interact with higher layers? Joint optimization (cross -layer design)? Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 24 / 26
Thank you! Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 25 / 26
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