Information contained in the slope of chirp signals can be employed for digital underwater acoustic communications across a broad range of applications

Kaminsky, Edit J.

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University of New Orleans

University of New Orleans ScholarWorks@UNO Electrical Engineering Faculty Publications Department of Electrical Engineering 2006 Chirp Signaling Offers Modulation Scheme for Underwater Communications Edit J. Kaminsky University of New Orleans, ejbourge@uno.edu Follow this and additional works at: https://scholarworks.uno.edu/ee_facpubs  Part of the Electrical and Electronics Commons Recommended Citation Kaminsky, E., “Chirp signaling offers modulation scheme for underwater communications", SPIE Newsroom, 10.1117/2.1200608.0357, (http://newsroom.spie.org/x4310.xml), 3 pp., 2006. This Article is brought to you for free and open access by the Department of Electrical Engineering at ScholarWorks@UNO. It has been accepted for inclusion in Electrical Engineering Faculty Publications by an authorized administrator of ScholarWorks@UNO. For more information, please contact scholarworks@uno.edu. SPIE Newsroom10.1117/2.1200608.0357Chirp signalling oﬀersmodulation scheme forunderwater communicationsEdit J. KaminskyInformation contained in the slope of chirp signals can be employed fordigital underwater acoustic communications across a broad range ofapplications.Wireless communication can be conducted in the underwateracoustic (UWA) channel through sonar signalling. We proposeusing the frequency-vs-time slope of sonar ‘chirps’ to transmitinformation in a robust scheme for a spread-spectrum underwa-ter communications system.Digital data can be represented by the slope of chirp signals. Inthe simplest binary case, an ‘up’ chirp (i.e., a signal with instan-taneous frequency that linearly increaseswith time) represents a‘1’ and a ‘down’ chirp represents a ‘0’. Higher-dimensional con-stellations can easily be obtained by using a larger number ofup/down slopes. The wide bandwidth should provide immu-nity from signal degradation in the channel.Underwater communicationsUsing a cable to establish communication between two remoteunderwater sites has several disadvantages. It is expensive toinstall and maintenance and repair are difficult, especially forcommunication in deep water. In addition, drag on the cable canbe a problem if the user is small and mobile. Sound propagatedthrough water is a better solution. However, the UWA channel isanunforgivingwirelessmedium that poses numerouschallengesto reliable, high data rate and long-distance communications.Four aspects of the UWA channel are of primary concern: am-bient noise, transmission loss due to geometrical spreading andabsorption, reverberation due to multiple paths, and Dopplerspreading due to relative motion. Each of these must be con-sidered in modeling the appropriate UWA channel.1,2 Multipathpropagation, for instance, could restrict communication in thechannel to non-coherent modulation schemes and low data ratetransmission.Figure 1. Binary chirp slope keying (CSK) has an advantage over bi-nary phase-shift-keying (PSK) in the Rayleigh channel, in terms ofprobability of error (PE) as a function of signal-to-noise ratio (SNR).In the past, underwater acoustic communications receivedmuch attention, mainly from the military, and the field wasclosely associated with submarine detection. In recent years,interest has shifted to more commercial applications. Today,various unmanned submersibles employ underwater commu-nications systems. These include robots, remotely operated un-derwater vehicles (ROVs), and unmanned underwater vehicles(UUVs), which are replacing divers in performing a variety ofoffshore tasks.As a spread-spectrum system, chirp slope keying (CSK) uti-lizes a data-modulated signal with its energy spread over abandwidth that is much greater than the rate of information be-ing transmitted.Chirp slope keyingChirp signals in digital communications were apparently origi-nally suggested by M. R. Winkler3 in 1962 and then abandoned.Continued on next page10.1117/2.1200608.0357 Page 2/3SPIE NewsroomFigure 2. Probability of error versus SNR for BCSK in the AWGNchannel with time and joint time-frequency receivers.The idea is to use a pair of linear chirps that have opposite chirprates for binary signaling. Binary chirp signals, or what Berni4called linear frequency sweeping (LFS), compared favorably tofrequency-shift-keying (FSK) and phase-shift-keying (PSK) incoherent channels. In non-coherent channels LFS was consid-ered less appealing because of the requirement for a phase re-covery system.Our simulation results show considerable improvement inperformance in the signal-to-noise (SNR) ranges of interest inperformance when binary CSK is used instead of binary PSK inthe Rayleigh fading channel.5,6A linear complex chirp signal may be modeled as in (1):sc(t) = A exp[j2π(f0 t+µ2t2)]. (1)The real part of sc(t) is used as the transmitted signal, withthe frequency slope µ indicating the transmitted bit. In termsof maximum and minimum instantaneous frequencies, fmax andfmin , and the signaling interval T, an ‘up’ chirp is represented asin (2) while a ‘down’ chirp is represented by (3):su(t) = A cos[2πt(fmin +fmax − fmin2Tt)], (2)sd(t) = A cos[2πt(fmax +fmax − fmin2Tt)]. (3)The binary information stream {Bi} selects the chirp pulses{pi (t)} to be transmitted at each signaling interval as follows:pi(t) ={su(t), Bi = 1sd(t), Bi = 0. (4)ResultsMonte Carlo simulations were performed in the additive whiteGaussian noise (AWGN) and the Rayleigh fading channels.Figure 1 shows the advantage of CSK over PSK in the noisyRayleigh fading model with a ratio of bandwidth to informationrate of 100.Clearly, for SNR above about 14.5dB, chirp slope keying per-forms better than standard phase shift keying in the simulatedUWA environment.The results shown in Figure 1 represent the performancefor the standard coherent correlation receiver with perfectphase recovery. Joint time-frequency techniques may be usedto gain performance advantages at the expense of computa-tional complexity. This is exemplified in Figure 2, which givesresults for CSK in the AWGN channel both for the stan-dard time-domain correlation detector and for a joint time-frequency detector,6 which uses the Wigner distribution7 andthe Radon transform. The Wigner distribution is used to ob-tain the frequency vs. time image spectrum of the received sig-nal. The Radon transform, by projection of the image inten-sity along a radial line oriented at a specific angle, can thendetermine the slope, thereby finishing the demodulation pro-cess.Conclusion and future workOur brief discussion of chirp slope keying (CSK) suggests it isa promising alternative to ordinary modulation schemes for un-derwater acoustical communications. Accurate modeling of theUWA channel is necessary in order to accurately predict systemperformance. Further work in this area is needed. Investigationof joint-time frequency receivers also may prove useful, espe-cially if efficient algorithms are designed to reduce the computa-tional complexity.Author InformationEdit J. KaminskyElectrical EngineeringUniversity of New OrleansNew Orleans, LAhttp://fs.uno.edu/ejbourgehttp://ece.engr.uno.edu/Dr. Edit J. Kaminsky Bourgeois is an Associate Professor and theAssociate Chair of the Department of Electrical Engineering atthe University of New Orleans. She has received numerous ser-vice and teaching awards and is a member of the honor societiesTau Beta Pi and Eta Kappa Nu. She is an active volunteer in theIEEE.Her research interests includedigital communications, un-derwater acoustics, fuzzy systems, neural networks, sonar sys-tems, and signal processing. In addition, Dr. Kaminsky has pre-Continued on next page10.1117/2.1200608.0357 Page 3/3SPIE Newsroomsented papers in the areas of communications and sonar signalprocessing at several SPIE conferences.References1. M. Stojanovic, Underwater Acoustic Communications, IEEE Electro Int’l, pp. 435–440, June 1995.2. A. B. Baggeroer, Acoustic Telemetry –An Overview, IEEE J. Oceanic Eng. OE-9 (4),pp. 229–235, 1984.3. M. R. Winkler, Chirp Signals for Communications,WESCON Convention Record14 (2), 1962.4. A. J. Berni andW. D.Greeg,On theUtility of Chirp Modulation for Digital Signaling,IEEE Trans. on Commun. 21 (6), pp. 748–751, 1973.5. E. J. Kaminsky and L. Simanjuntak, Chirp Slope Keying for Underwater Communi-cations, Proc. SPIE 5778, pp. 894–905,2005.6. E. Kaminsky and M. Barbu, Receiver structures for underwater acoustical communi-cations using chirp slope keying, Proc. SPIE 6201 (37), 2006.7. L. Cohen, Time-frequency distributions - A review, Proc. IEEE 77 (7), pp. 941–981,1989.c© 2006 SPIE—The International Society for Optical Engineering

Chirp Signaling Offers Modulation Scheme for Underwater Communications

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Chirp Signaling Offers Modulation Scheme for Underwater Communications

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