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Chirp-Based LPD/LPI Underwater Acoustic Communications with Code-Time-Frequency Multidimensional Spreading

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Chirp-Based LPD/LPI Underwater Acoustic Communications with Code-Time-Frequency Multidimensional Spreading Chirp-Based LPD/LPI Underwater Acoustic Communications with Code-Time-Frequency Multidimensional Spreading Emrecan Demirors and Tommaso Melodia Department of Electrical and Computer Engineering Northeastern University, Boston, MA 02115 E-mail:{edemirors, melodia}@ece.neu.edu ABSTRACT Yet, existing technology in this domain is for the most part Most underwater acoustic communication systems incorpo- based on transmitting well-recognized, easily detectable nar- rate well-recognized, easily detectable narrowband signals rowband signals modulated over low-frequency carriers at modulated over low-frequency carriers at high transmission high transmission powers, which ultimately limits the stealth- powers, which ultimately limits LPD/LPI performance of iness of the communication scheme with LPD/LPI (Low the communication scheme. While there have been promis- Probability of Detection/Low Probability of Interception) ing works concentrating on LPD/LPI performance, they are performance. A limited number of works have focused specif- for the most part based on direct-sequence spread spectrum ically on stealthy communication techniques. This body of (DSSS) techniques, which have been shown to be blindly work largely follows the successful and well-understood ap- detectable in previous work at relatively low SINR values. proach of adopting direct-sequence spread spectrum (DSSS) As a result, there is likely significant room to improve the techniques with either coherent or non-coherent modula- LPD/LPI performance of underwater acoustic communica- tions [3{5]. The main motive behind this approach is to take tion schemes. To this end, in this paper, we propose the pre- advantage of the processing gain that comes from spread- liminary design of a novel communication scheme based on spectrum encoding, which enables to carry out communica- transmitting chirp signals that are further spread over a mul- tions at relatively low signal levels and achieve high LPD/LPI tidimensional domain spanning code, time, and frequency. performance. [6,7] focus on the same goal with an alternative We evaluated the performance of the proposed scheme both approach that exploits frequency diversity instead of cod- with simulation and experimental studies. ing to achieve processing gain. While these works achieved promising results, there is clearly significant room to im- prove the LPD/LPI performance of the underwater commu- Keywords nication schemes. Low Probability of Detection, Low Probability of Intercep- To address this need, in this paper, we propose a novel tion, Robust acoustic transmission scheme for stealthy underwater com- munications based on transmitting chirp signals that are fur- ther spread over a multidimensional domain spanning code, 1. INTRODUCTION time, and frequency. The proposed scheme uses chirp-based As of today, mainstream approaches designed to achieve acoustic pulses with ultrasonic spectral content following an efficient underwater communications at the physical (PHY) frequency- and time-hopping pattern together with a su- layer of the communication protocol stack have mostly been perimposed spreading code. Therefore, it enables higher focused on designing spectrally efficient yet robust modula- LPD/LPI performance compared to schemes that consider tion schemes and receivers to operate on the limited band- only a single dimension, i.e., code or frequency and provides width available in the underwater acoustic channel [1, 2]. a hopping-coding pattern that is not easily recognizable or detectable. Moreover, chirp signals are ubiquitous in the un- Acknowledgement: The authors would like to thank Phil Cruver, CEO of Catalina Sea Ranch, for his logistical sup- derwater environment (e.g., dolphin clicks [8, 9]); therefore, port in the at-sea experiments discussed in this paper. This it is not easy for an adversary to detect the transmissions work is based upon work supported in part by the US Na- and associate them with a communication system. tional Science Foundation under grant CNS-1503609. We conducted a performance evaluation study for the pro- posed transmission scheme on a multi-scale simulator that evaluates underwater chirp-based communications at two Permission to make digital or hard copies of part or all of this work for personal or different levels, i.e., (i) at the wave level by modeling acous- classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation tic propagation in selected reference scenarios, (ii) at the on the first page. Copyrights for third-party components of this work must be honored. bit level by simulating in detail the proposed chirp-based For all other uses, contact the owner/author(s). transmission schemes. In addition, we performed an exper- WUWNET ’16 October 24-26, 2016, Shanghai, China iment evaluation of the proposed scheme by implementing c 2016 Copyright held by the owner/author(s). the proposed scheme on our custom software-defined acous- ACM ISBN 978-1-4503-4637-5/16/10. tic platform [10{12] and conducted field experiments in a DOI: http://dx.doi.org/10.1145/2999504.3001083 harbor in Los Angeles, CA. The remainder of this article is organized as follows. We first describe the proposed LPD/LPI transmission scheme in Section 2. Then, in Section 3 we provide both simulation and experimental performance evaluation Finally, we draw conclusions in Section 4. 2. CHIRP-BASED LPD/LPI SCHEME Based on the considerations and needs, we design and propose a novel, robust LPD/LPI transmission scheme that uses chirp-based acoustic pulses with ultrasonic spectral con- tent following a frequency- and time-hopping pattern to- gether with a superimposed spreading code. The scheme is designed to enable higher LPD/LPI performance com- pared to state-of-the-art schemes that consider only a single Figure 1: Example of an ongoing transmission us- dimension (i.e., code, time, or frequency) and provides a ing frequency-hopping frame length, time-hopping hopping-coding pattern that (i) is not easily recognizable or frame length, and spreading code length equal to detectable by an adversary; (ii) can be robustly detected in 5, 5, 3, respectively. Frequency-hopping sequence adverse channels by a friendly receiver that is aware of the FH = f2; 0; 3g, time-hopping sequence TH = f1; 3; 3g, frequency-time hopping pattern, as well as of the spreading and spreading code sequence SC = f1; −1; −1g. Send- code used. ing information bit 1. Basics. The proposed scheme is designed based on the principle of transmitting a chirp signal with a frequency- and chips organized in frames of duration T = N ·T where N time-hopping pattern following pseudo-random sequences, f h c h is the number of chips per frame. and with a superimposed spreading code. Chirp-based trans- The system transmits one chirp signal in one chip per mission, frequency- and time-hopping patterns, and spread- frame on one sub-band, and determines in which chip and spectrum encoding enables a communication scheme with sub-band to transmit based on a time hopping sequence (THS) high LPI/LPD performance, receiver performance that is and a frequency hopping sequence (FHS), respectively. Both robust and resilient against severe channel effects (i.e., multi- time and frequency hopping sequences are based on pseudo- path, scattering, and Doppler), and hardly identifiable char- random sequences generated by seeding random number gen- acteristics that are not easily associated with a specific sys- erators. Moreover, we introduce a channel coding scheme tem employing them. to improve the receiver performance against channel non- Why Chirp Transmissions? Chirp modulation or lin- idealities. Various channel coding solutions have been pre- ear frequency modulation (LFM) was first used in [13]. Since sented in [22{25] with different performance levels and com- then, chirp signals have been used as a communication tech- putational complexity. In the proposed scheme, we repre- nology that can enable low data rate, robust, low-power sent each information bit with psuedo-orthogonal spreading (LPD/LPI) wireless communications on simple-design, low- codes because of (i) limited computational complexity and cost transceivers in different applications including indoor (ii) inherent resilience to multipath [26{28]. wireless communications [14], multiuser applications [15,16], A chirp signal is characterized by a time-varying instanta- and WLAN [17] and WPAN [18] applications. Chirp sig- neous frequency, which changes in time from an initial value nals also have been proposed in the UWA communications f to a final value f . In the time domain, the signal can be literature as highly reliable but low data rate alternatives 0 1 expressed as [10, 19, 20]. The characteristics of chirp transmissions appear to ide- A cos(2πf t + πµt2) 0 ≤ t ≤ T; ally address the requirements of an LPI/LPD scheme. First, c(t) = 0 (1) their high processing gain (time-bandwidth product) and re- 0 otherwise; silience against severe channel effects, i.e., multipath, scat- tering, Doppler effect, enables high LPD/LPI performance where A is the amplitude of the chirp, f is the initial since robust reception performance under low signal-to-noise 0 chirp frequency, µ = f1−f0 is the chirp frequency-variation (SNR) conditions reduces the need for high transmission T rate, while T represents the chirp period. We refer to a power [21]. Second, the wideband nature of chirp signals chirp with parameter µ > 0 as an up-chirp; otherwise, we results in high LPD/LPI performance since the low power call it a down-chirp. Up and down chirp signals are almost spectral density reduces the probability of detection and in- orthogonal to each other. The total bandwidth of the chirp tercepts. Third, chirp signals are ubiquitous in the underwa- signal can be obtained as B = f − f . ter environment (e.g., dolphin clicks [8, 9]). Therefore, they 1 0 The train of chirps may be modulated based on binary or- cannot be easily associated with a specific communication thogonal keying (BOK) by leveraging the quasi-orthogonality system.
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