Using Fast Frequency Hopping Technique to Improve Reliability of Underwater Communication System

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Article Using Fast Frequency Hopping Technique to Improve Reliability of Underwater Communication System

Jan H. Schmidt Faculty of Electronics, Telecommunications and Informatics, Gda´nskUniversity of Technology, ul. Narutowicza 11/12, 80-233 Gda´nsk,Poland; [email protected]

Received: 30 December 2019; Accepted: 7 February 2020; Published: 10 February 2020

Abstract: Acoustic underwater communication systems designed to work reliably in shallow coastal waters must overcome major limitations such as multipath propagation and the Doppler effect. These restrictions are the reason for the complexity of receivers being built, whose task is to decode a symbol on the basis of the received signal. Additional complications are caused by the low propagation speed of the acoustic wave in the water and the relatively narrow bandwidth. Despite the continuous development of communication systems using coherent modulations, they are still not as reliable as is desirable for reliable data transmission applications. This article presents an acoustic underwater communication system that uses one of the varieties of the spread spectrum technique i.e., the fast frequency hopping technique (FFH). This technique takes advantage of binary frequency-shift keying (BFSK) with an incoherent detection method to ensure the implementation of a system whose main priority is reliable data transmission and secondary priority is the transmission rate. The compromised choice of parameters consisted of the selection between the narrow band of the hydroacoustic transducer and the maximum number of carrier frequency hops, which results from the need to take into account the effects of the Doppler effect. In turn, the number of hops and the symbol duration were selected adequately for the occurrence of multipath propagations of an acoustic wave. In addition, this article describes experimental communication tests carried out using a laboratory model of the FFH-BFSK data transmission system in the shallow water environment of Lake Wdzydze/Poland. The test results obtained for three channels of different lengths are discussed.

Keywords: shallow coastal waters; reliable underwater acoustic communication; fast frequency hopping technique; bit error rate

1. Introduction In the water environment, due to the fact that electromagnetic waves are strongly suppressed, acoustic waves are used to implement the underwater communication systems. Systems whose most important task is reliable operation in shallow coastal waters must overcome numerous limitations. One of the most important factors is multipath propagation. Its occurrence is the result of signal reflections from the water surface and the bottom (characteristic of horizontal channels) and any obstacles present in water reservoir. The acoustic wave may also be refracted and dispersed over the inhomogeneities of the medium. As a result, inter-symbol interference (ISI) is observed, which is undesirable in communication systems. Another important factor is the Doppler effect caused by the movement of the transmitter and/or receiver relative to each other. Although these adverse factors also occur in other communication systems, which are implemented using a different transmission medium, it is in the water environment and using acoustic waves where they become particularly emphasized. This is due to the low speed of propagation of the acoustic waves and the narrow available bandwidth, which directly results in limiting the throughput of the hydroacoustic channel [1]. Their seasonal

Appl. Sci. 2020, 10, 1172; doi:10.3390/app10031172 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 1172 2 of 12 variability is also observed, which is an additional difficulty in clearly defining the acoustic parameters of the underwater communication channels [2,3]. The already developed and commercially used hydroacoustic modems usually use a carrier frequency below 100 kHz, while for technological reasons, hydroacoustic transducers offer a bandwidth on the order of several kilohertz. All of these restrictions taken together are the reason for the complexity of the built receivers, whose task is to decode a symbol on the basis of the received signal. It is generally known that the underwater acoustic channel is one of the most demanding channels for the implementation of modern digital communication. This applies in particular to transmission through a horizontal channel occurring in shallow coastal waters. In wireless underwater communication, various modulation techniques are used, mainly frequency and phase modulation of the carrier wave [4–8]. They are used in both single-carrier and multi-carrier systems. The use of non-coherent modulations allows the undesirable effects of multipath propagation and the Doppler effect to be effectively overcome, especially in a horizontal underwater channel, where they provide transmission rates from several dozen to several hundred bits per second (bps) [9,10]. These modulations are most often used in devices requiring long-term operation from battery power sources, due to the simple design of the receiver. Coherent modulations are used for systems operating in the environment of deep waters as well as vertical channels. Coherent modulations use the available frequency band more efficiently, offering higher transmission rates, but require a more complex receiver that performs adaptive correction of the transmission channel characteristics [11,12]. Compared to non-coherent modulations, they require a signal with a higher signal-to-noise ratio for proper operation. In general, coherent modulation techniques remain less reliable than is desirable. Obtaining improving transmission reliability or higher transmission rates for this modulation method is possible as a result of transmission over many independent propagation paths, which is obtained by using multiple hydroacoustic antennas [13]. However, for compact applications, this means inconvenient hardware system complications. Another of the modulation techniques, the spread spectrum technique, is widely used in military ground communication systems due to its ability to work with a low signal-to-noise ratio and its good tolerance to multipath propagation and system jamming. In addition, the use of channel coding together with interleaving reduces the bit error rate. This article is a development of research carried out by the author over the past few years on reliable underwater communication systems for work in shallow coastal waters [14,15]. Its main subject is to specify the parameters for the FFH technique and to present the results of experimental tests on the lake. The compromised choice of parameters consisted of the selection between the narrow band of the hydroacoustic transducer and the maximum number of carrier frequency hops, which results from the need to take into account the effects of the Doppler effect. In turn, the number of hops and the symbol duration were selected adequately for the occurrence of multipath propagations of an acoustic wave.

2. Features of a Reliable Underwater Communication System Spread spectrum techniques were originally developed for applications in military systems due to their low probability of intercept and high resistance to jamming signals, i.e., they make it difficult for the opponent to eavesdrop on transmissions and unauthorized sources transmitting false information—interfering with communication through interference with the receiver system. In addition, it is possible to share the frequency band with many types of conventional modulations with or without minimal interference. This technique is based on the statement: The more the noise dominates the signal, the wider the band the signal must occupy to receive it correctly. It results from Shannon’s theorem on channel bandwidth with a specified bandwidth B and signal-to-noise ratio (SNR) [16]. This technique significantly expands the bandwidth used, while allowing you to work with a much smaller ratio of signal strength to noise power, which is done by converting the narrowband information signal to a signal with a much wider band [17,18]. Appl. Sci. 2020, 10, 1172 3 of 12

Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 12 There are many varieties of spread spectrum technique, among which the most typical are [19]: • DirectDirect SequenceSequence Spread Spectrum technique (DSSS) [[20],20], •• Frequency Hopping Spread Spectrum technique (FHSS), Frequency Hopping Spread Spectrum technique (FHSS), •• Time Hopping Spread Spectrum technique (THSS), Time Hopping Spread Spectrum technique (THSS), •• Chirp Spread Spectrum technique (CSS) [21]. Chirp Spread Spectrum technique (CSS) [21]. • One of these varieties is considered in the article, i.e., FHSS, as a technique that will ensure reliableOne operation of these of varieties an underwater is considered communication in the article, system. i.e., FHSS,The next as part a technique of the chapter that will is devoted ensure reliableto discussing operation the ofprinciple an underwater of the system communication with this technique, system. The and next then part the of implementation the chapter is devoted of the toreceiver discussing and the principleassumptions of theof systemthis technique with this in technique,the case of and counteracting then the implementation the limitation ofof thethe receivercommunication and the channel. assumptions of this technique in the case of counteracting the limitation of the communicationCommunication channel. systems utilizing a frequency hopping technique most often use frequency-shiftCommunication keying systems (FSK)utilizing with an a incoherent frequency hoppingdetection technique method most[19]. oftenA diagram use frequency-shift of a typical keyingsystem (FSK)with the with details an incoherent of the modulator detection and method demodulator [19]. A diagram is shown of ain typical Figure system 1. with the details of the modulator and demodulator is shown in Figure1.

FHSS Modulator

Pseudo-random Transmitter Frequency sequence synthesiser generator

Channel Frequency Input data Interleaver FSK Modulator coder hopping channel

Channel Demodulator Frequency Underwater Output data Deinterleaver decoder FSK dehopping communication

Frequency synthesiser

Pseudo-random Pseudo-random sequence sequence Receiver synchroniser generator FHSS Demodulator

Figure 1. Scheme of data transmission system with frequency hopping technique with details of the modulatorFigure 1. Scheme and demodulator. of data transmission system with frequency hopping technique with details of the modulator and demodulator. In the FHSS technique, all available channel bandwidth are divided into adjacent frequency subchannels.In the FHSS Thecarrier technique, frequency all available between channel the subchannels bandwidth is switched are divided using ainto selected adjacent code frequency sequence, whichsubchannels. is known The in carrier both the frequency transmitter between and the the receiver. subchannels The L -lengthis switched code using sequence a selected controls code the frequencysequence, synthesizerwhich is known in each in symbolboth the interval transmitterTS. Inand the the transmitter, receiver. theThe sentL-length symbol code is subjectedsequence tocontrols channel the coding frequency and interleaving,synthesizer in and each then symbol goes interval to the FSK TS. In modulator, the transmitter, which the sends sent the symbol symbol is withsubjected duration to channelTS, and coding gives the and signal interleaving, the appropriate and then frequency goes to the for theFSK subchannel modulator, depending which sends on thethe FSKsymbol modulation with duration order used.TS, and The gives signal the thus signal generated the appropriate is then placed frequency in the appropriate for the subchannel frequency subchanneldepending byon meansthe FSK of amodulation frequency conversionorder used. system The signal for a time thus equal generated to the timeis then of singleplacedhop intime the TappropriateH (or dwell timefrequency). Then, subc afterhannel amplification, by means such of a signalfrequency goes conver throughsion the system hydroacoustic for a time transducer equal to tothe the time underwater of single hop channel. time T OnH (or the dwell receiving time). Then, side, theafter amplified amplification, signal such in the a preamplifiersignal goes through goes to thethe receiver.hydroacoustic In the transducer receiver, the to signalthe un isderwater reduced channel. to the baseband On the receivin in whichg side, FSK the demodulation amplified signal occurs. in Thethe preamplifier synchronization goes signal to the necessary receiver. toInmaintain the receiv theer, synchronizationthe signal is reduced of the to code the sequencebaseband generator in which isFSK obtained demodulation from the occurs. received The signal synchronization by the code sequencesignal necessary synchronizer to maintain system the [19 synchronization]. of the code sequence generator is obtained from the received signal by the code sequence synchronizer system [19]. Appl. Sci. 2020, 10, 1172 4 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 12

WhenWhen the the carrier carrier frequency frequency changes changes every every few bitsfew ofbi datats of stream, data stream, it is a system it is a with systemslow with frequency slow hoppingfrequency technique hopping(SFH). technique (SFH).

2.1.2.1. FastFast FrequencyFrequency HoppingHopping TechniqueTechnique

WhenWhen TTSS//TTHH > 1,1, i.e., i.e., the the change change of of carrier carrier frequency frequency occurs occurs repeatedly repeatedly during during the the duration duration of ofthe the TST dataS data symbol, symbol, then then it is it a issystem a system with with the fast the frequencyfast frequency hopping hopping technique technique (FFH).(FFH). Figure Figure 2 shows2 showsa graphic a graphic operation operation diagram diagram of the of thefast fast frequenc frequencyy hopping hopping technique technique for for an an example example of aa codecode sequencesequence pattern. pattern.

TS = 9TH

f9 3 f8 5 f7 9 f6 7 f5 2 f4 4 f3 8

Carrier frequency f2 6 f1 1 1 t

0 TH 2TH 3TH 4TH 5TH 6TH 7TH 8TH 9TH 10TH Time - symbol transmission with MFSK modulation

Figure 2. Operation diagram of the fast frequency hopping technique for an example code sequence Figure 2. Operation diagram of the fast frequency hopping technique for an example code sequence pattern with a length of 9 (TS = 9 TH). pattern with a length of 9 (TS = 9··TH). As mentioned, non-coherent FSK frequency modulation is usually used in this method. The use of coherentAs mentioned, modulations non-coherent would be diFSKﬃcult frequency due to themodulation phase coherence is usually at used the time in this in which method. the The carrier use frequencyof coherent is changedmodulations according would to be the difficult pattern determineddue to the phase by the coherence code sequence. at the time in which the carrierIn thefrequency FFH technique, is changed the hopaccording frequency to th ise equalpattern to fdeterminedH = 1/TH. In by the the case code of non-coherent sequence. detection, to ensureIn the the FFH required technique, orthogonality the hop betweenfrequency the is tones equal of to the fH FSK= 1/ modulationTH. In the case signal, of non-coherent a minimum distancedetection, between to ensure them the being required a multiple orthogonality of fH should between be used. the Thistones property of the FSK ensures modulation that no crosstalk signal, a occursminimum with distance the remaining between hop them frequency being a bands. multiple of fH should be used. This property ensures that no crosstalkAn important occurs parameter with the remaining characteristic hop frequency of the spread bands. spectrum techniques, the NSS spreading factor,An for important the FFH technique parameter corresponds characteristic to the of lengththe spread of the spectrum code sequence techniques,L and isthe equal NSS tospreadingTS/TH. factor,The for form the ofFFH the technique transmitted corresponds low-pass FFH-FSKto the length signal of canthe code be described sequence by L theand Formula is equal (1)to T [22S/T],H. The form of the transmitted low-pass FFH-FSK signal can be described by the Formula (1) [22], r 2ES sn(t) = 2E cos[2π( fl + fm) t + ϕn] (1) ()= TS S []π ()+ + ϕ sn t cos 2 fl fm t n (1) TS where: where: " # " # l 1 l (n 1) + − − TS t (n 1) + TS, 1 n N, 1 l L, 0 m M 1, (2) ()− − + Ll 1 ≤ ≤≤ ≤ ()− − +L l  ≤ ≤ ≤ ≤ ≤ ≤≤ ≤ ≤ ≤ ≤ ≤ − − n 1 TS t n 1 TS , 1 n N, 1 l L, 0 m M 1, (2)  L   L while ES/TS stands for the power of the transmitted symbol signal, TS is the duration of the symbol signal, l is the hop number, f is the intermediate frequency for the selected band of the l-th frequency hop, while ES/TS stands for thel power of the transmitted symbol signal, TS is the duration of the symbol f is the m-th signiﬁcant frequency, ϕ is an unknown random phase associated with the transmitted msignal, l is the hop number, fl is the intermediaten frequency for the selected band of the l-th frequency symbol, n is the number of the transmitted symbol, M is the modulation order of FSK, and N is the hop, fm is the m-th significant frequency, ϕn is an unknown random phase associated with the numbertransmitted of all symbol, symbols nto is send.the number of the transmitted symbol, M is the modulation order of FSK, and N is the number of all symbols to send. Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12

The necessary system bandwidth of a transmission system using the FFH-FSK technique is Appl. Sci. 2020, 10, 1172 5 of 12 determined by the Equation (3): ()+ = L M 1 [] The necessary system bandwidthB ofFFH a− transmissionFSK systemHz using the FFH-FSK technique is(3) T determined by the Equation (3): H L (M + 1) The FFH system performs theBFFH transmissionFSK = and reception[Hz] of symbols based on diversity(3) − T techniques [23]. In general, diversity techniques contributesH to ensure redundancy. Some of the receivedThe FFHsymbols system can performsbe treated theas more transmission reliable than and others, reception and ofbased symbols on them, based the onright diversity decision techniquesabout the [received23]. In general,symbol diversitycan be made. techniques The receipt contributes of each to of ensure the redundant redundancy. symbols Some must of the be receivedindependent symbols in order can be for treated the diversity as more technique reliable than used others, to meet and its basedobjectives. on them, the right decision aboutThe the receiveddiversity symboltechnique can used be made.in the FFH The technique receipt of is each a hybrid of the of redundant time and frequency symbols mustdiversity, be independentwhich can be in orderseen in for Figure the diversity 2. Typically, technique time used diversity to meet accomplishes its objectives. repeated transmitting and receivingThe diversity of the same technique symbol used over in time. the FFH However, technique the isindependent a hybrid of timereception and frequencyof the symbol diversity, in the whichfadedcan channel be seen requires in Figure that 2each. Typically, symbol timetransmission diversity with accomplishes Ts duration repeated be time separated transmitting by a and time receivinggreater than of the the same coherence symbol time over Tc time.of the However,channel. In the turn, independent frequency receptiondiversity typically of the symbol performs in the the fadedtransmission channel and requires reception that each of the symbol same symbol transmission on multiple with T carriers duration frequencies be time separatedat the same by time, a time but greateradjacent than carrier the coherencefrequencies time mustTc beof separated the channel. by a In band turn, larger frequency than the diversity coherence typically bandwidth performs Bc of thethe transmission channel. However, and reception the time of the diversity same symbol and that on multipleimplemented carrier in frequencies the FFH technique at the same operate time, butsequentially, adjacent carrier because frequencies the redundant must symbols be separated are received by a band individually larger than at the a specific coherence time bandwidth and use the Bcavailableof the channel. carrier However,frequencies. the For time a diversityfixed bandwidth and that per implemented signal, the inorder the FFHof diversity technique L increases operate sequentially,with the amount because of theredundancy. redundant Simi symbolslarly,are the received average individually signal energy at a(· specificL) also timeincreases, and use but the the availabletransmission carrier rate frequencies. (/L) decreases. For a fixed bandwidth per signal, the order of diversity L increases with the amountWith ofregard redundancy. to the stream Similarly, of transmitted the average symbol signals, energy the diversification ( L) also increases, used butin the the FFH transmission technique · ratecan ( /beL) decreases.interpreted in a simplified way as the use of repetition coding [24,25]. Using hard-decision decodingWith regard in the toreceiver, the stream a decision of transmitted is made symbols, about which the diversification symbol was usedreceived in the for FFH each technique hop with canduration be interpreted TH. If we inassume a simplified that the way example as the probability use of repetition of receiving coding an[24 erroneous,25]. Using symbol hard-decision is Ps = 0.3 decodingfor a single in theTH hopping receiver, time, a decision which is is made equivalent about to which a single symbol symbol was transmission received for ( eachL = 1), hop then with the durationprobabilityTH. of If wereceiving assume an that erroneous the example symbol probability for the odd of receivingvalues of symbol an erroneous repetition symbol (=values is Ps = of0.3 the fororder a single of diversity)TH hopping can be time, determined which is using equivalent Formula to a(4) single [25]. symbolThe probability transmission is presented (L = 1), in then Figure the 3 probabilityfor several of dozen receiving consecutive an erroneous values symbol of L repetitions for the odd and values it follows of symbol that as repetition the repetitions (=values increase, of the orderthe P ofb value diversity) decreases. can be At determined this stage usingof considerations, Formula (4) [regarding25]. The probability the quality is of presented data transmission, in Figure3 it foris severalreasonable dozen to suppose consecutive that valuesthe FFH of techniqueL repetitions will and provide it follows the desired that as P theb value repetitions with the increase, optimal thenumberPb value of L decreases. replicates. At this stage of considerations, regarding the quality of data transmission, it is reasonable to suppose that the FFH technique will provide the desired P value with the optimal L b  L i L−i number of L replicates. P ()L =   P (1− P ) S L   !  S S (4) X L+1 i i= L  i L i P (L) = 2 P (1 P ) − (4) S i S − S i= L+1 2

FigureFigure 3. 3.The The value value of of the the probability probability error error of of the the received received symbol symbol for for di differentﬀerent repetition repetition values values (=(=orderorder of of diversity). diversity). Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12

2.2. Implemented Receiver The structure of the FFH-FSK receiver implemented in the considered data transmission system is shown in Figure 4. Initially, the received signal r(t) is subjected to analogue-to-digital conversion, and then the selected band signal is reduced to the baseband of the specified FFH-FSK width. The next operations lead to a discrete signal spectrum, which is used to determine the power density spectrum of the discrete signal for subsequent TH durations. The obtained spectrum covers the entire FFH-FSK band, i.e., it covers all possible branches corresponding to the significant frequencies of

Appl.FSK Sci.modulation2020, 10, 1172 for each of the L possible hops, hence the number of all branches analyzed is 6L of· M 12. Next, the receiver extracts the values of the spectral lines Ym,k and determines the decision values Zm according to the selected reception diversity algorithm (m = 0, ..., M − 1; k = 1, ..., L), in pursuance of 2.2.the Implementedassumptions Receiver of soft-decision decoding. The basic condition for the proper functioning of the FFH techniqueThe structure receiver of is the itsFFH-FSK synchronization receiver with implemented the frame in of the the considered transmitted data symbol, transmission and then system the isalgorithm shown in is Figure zeroed4. and Initially, all decision the received variables signal Zm arer(t) worked is subjected out. to analogue-to-digital conversion, and thenIn the the first selected stage, band for the signal next is L reduced hop times to the TH basebandthat occur of for the transmissions speciﬁed FFH-FSK of a single width. symbol, The next the operationsvalues of the lead Ym,k to bands a discrete are extracted signal spectrum, for each which of the is M used FSKto branches determine and the L of power possible density hops, spectrum creating a matrix with dimensions (M + L) · L. In the second stage, based on the obtained matrix, the of the discrete signal for subsequent TH durations. The obtained spectrum covers the entire FFH-FSK band,operation i.e., it of covers focusing all possible the spread branches spectrum corresponding symbol signal to the is signiﬁcant performed frequencies by selecting of FSKthe modulationvalue of the forspectral each oflines the fromL possible M-th hops,FSK branches hence the for number the next of L all hop branches times T analyzedH. Transmissions is L M. of Next, a single the receiver symbol are L hops, and the next hop k is in accordance with the hop pattern determined· based on the fixed extracts the values of the spectral lines Ym,k and determines the decision values Zm according to the selectedcode sequence. reception As diversity a result algorithmof these actions, (m = 0, a... matrix, M of1; kvalues= 1, ... W,m,kL), with in pursuance dimensions of the M assumptions· L is created, − ofwhich soft-decision is used decoding. by the reception The basic conditiondiversity foralgorithm the proper to functioning determine of the the FFHdecision technique value receiver of Zm, isindependently its synchronization for each with branch the frame of m. ofThe the obtained transmitted decision symbol, values and of then Zm allow the algorithm the final isdecision zeroed to and be made, i.e., to assign the appropriate FSK symbol to the obtained decision values Zm. all decision variables Zm are worked out.

Y0,k r(t) Analogue-to- Digital Converter

YM-1,k r(n)

Direct Digital Converter W0,k Z0 symbol Determination of Reception Focusing the power density ,…,L diversity Decision system the spectrum 1

spectrum k= algorithm

WM-1,k ZM-1 Windowing

Y0,L

Digital Fourier Transform YM-1,L

Sequence code Sequence code synchronizer generator

Figure 4. Structure of the implemented FFH-FSK receiver.

Figure 4. Structure of the implemented FFH-FSK receiver. In the first stage, for the next L hop times TH that occur for transmissions of a single symbol, the values of the Y bands are extracted for each of the M FSK branches and L of possible hops, The square-lawm,k combining algorithm was used as the reception diversity algorithm in the FFH creating a matrix with dimensions (M + L) L. In the second stage, based on the obtained matrix, technique, which sums L values of Wm,k from· each MFSK branch, according to Formula (5). It the operation of focusing the spread spectrum symbol signal is performed by selecting the value of the assumes that independent transmission channels have the same gain [23]. spectral lines from M-th FSK branches for the next L hop times TH. Transmissions of a single symbol are L hops, and the next hop k is in accordance with the hop pattern determined based on the fixed code sequence. As a result of these actions, a matrix of values W with dimensions M L is created, which m,k · is used by the reception diversity algorithm to determine the decision value of Zm, independently for each branch of m. The obtained decision values of Zm allow the final decision to be made, i.e., to assign the appropriate FSK symbol to the obtained decision values Zm. Appl. Sci. 2020, 10, 1172 7 of 12

The square-law combining algorithm was used as the reception diversity algorithm in the FFH technique, which sums L values of Wm,k from each MFSK branch, according to Formula (5). It assumes that independent transmission channels have the same gain [23].

XL 2 Zm = Wm,k (5) k=1

2.3. Counteracting the Effects of multipath Propagation and the Doppler Effect To meet the condition of implementing frequency diversity, the occurrence of independent fading in the frequency-selective channel should be assumed, when the frequency spacing between the two signals is greater than the coherence bandwidth Bc of the channel (∆fFFH-FSK = 1/TH > Bc)[3]. Therefore, the planning of frequency hops in conjunction with the design of the appropriate hop pattern resulting from the generated code sequence is important for the FFH system under consideration. However, thoughtful design of the hop pattern must take into account that the length of the code sequence, and thus the number of possible hops, is limited by the specificity of the hydroacoustic channel and the useful bandwidth of available transducers, typically several kHz. This fact also significantly limits the design of a more complex pattern design. In general, in order to assume the frequency non-selective channel (also known as flat fading channel), it is required to use spacing between neighbouring hop frequencies smaller than the coherence bandwidth Bc of the channel (∆fFFH-FSK < Bc). In contrast, the relatively broad bandwidth of the FFH system can be modelled as a group of independent narrowband frequency non-selective channels using a stochastic process with slow Rician or Rayleigh fading [23]. The term slow fading means that the fading is constant during a single hop and then changes in signal amplitudes caused by the presence of fading are invariable for a single hop. For the number L of frequency hopping per transmission and reception of a single symbol, the effective duration of the symbol is defined as Ts = (L 1) T , − · H and hence in the time domain, the above assumption about the frequency non-selectivity channel can be written as Ts = (L 1) T > Tm. − · H For TH > Tm the channel has flat fading and, therefore, allows the impact of multipath effects to be neglected, while for TH < Tm the channel is frequency-selective and the FFH technique aims to remove the effect of multipath effects on the direct path signal. Because the FFH system receiver after a reception time equal to the hop time TH jumps to the new hop frequency when the multipath components begin to reach the receiver, the arriving multipath components occupy a different band from the currently received signal, so the effect of unwanted interference should be insignificant. During reception equal to the hop time TH, at least one non-fading signal associated with the direct path is processed. The condition for meeting the requirements for the implementation of time diversity in the case of a channel with fast fading is to ensure that the time interval Tcc between the transmission of subsequent symbol signals with the duration of TH at the same hop frequency is longer than the coherence time Tc of the channel. This means mutual independence in the time domain for two hops at the same frequency. At the same time, the time interval Tcc can be interpreted as the time at which each of the hopping frequencies used is unused, which allows the multipath components present in the hopping band to be suppressed. This assumption applies provided that the maximum delay spread Tm is shorter than the discussed period Tcc. To ensure the occurrence of slow fading, which is more convenient as opposed to fast fading, the hop time TH must be shorter than the coherence time Tc of the channel (TH < Tc), thus the condition ∆fFFH-FSK > Bd must be fulfilled by analogy. To prevent the influence of the Doppler effect, which occurs as a Doppler deviation, the bandwidth reserved for a particular signal should be determined with a specified significant frequency. Therefore, the condition Bd << Bs must be met, where Bd is the maximum Doppler spread and Bs is the band of the transmitted symbol. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 12 of the Sonar Systems Department/Gdańsk University of Technology. Figure 5 shows the general configuration of the measuring stations. Each of the transmitted signals was generated in the MATLAB environment, and the conversion of this signal to analogue form was performed using the digital-to-analogue converter available in the National Instruments USB-6363 recording and generating device. The signal was then amplified and transmitted through an underwater channel using a HTL-10 underwater telephone [15]. Its design is based on the assumptions of programmable radio (SDR—Software-Defined Radio). This assumption makes it possible to use it for research purposes with various signals in the bands from 1 kHz to 60 kHz. The HTL-10, which I am a co-author of the hardware and software part, has been successfully used by the Polish Navy on its helicopters for several years. It uses hydroacoustic antenna of the OKA-2M dipping sonar that is designed to search for submarines because the helicopter devices allow it to descend into the water using a several-hundred-meter-long cable when the helicopter is hovering. The transmitting measuring station was placed on board a motor boat. The same measuring equipmentAppl. Sci. 2020 ,was10, 1172 used on the receiving side, but it was configured differently. The receiving8 of 12 measuring station was placed in a measuring container placed on a floating platform moored to a bridge with a fixed position during each of the measuring sessions. The received signal was sampled and3. Results archived. of UnderwaterThe transmitting Experimental transducer Tests was lowered from the motor boat to a depth of hTx = 10 m regardlessIn order of the to determine depth of the the water quality reservoir, of data transmission, which for all experimental transmitting testspositions were conductedof the measuring on 4–5 stationsMay 2017, was on from Lake 20 Wdzydze to 30 m. The/Poland receiv nearing transducer the headquarters was lowered of the Hydroacousticto a depth of hRx Research = 4 m, in a Station place whereof the Sonarthe water Systems depth Department was equal/GdańskUniversity to 7 m. In subsequent of Technology. sessions, the Figure distance5 shows d between the general the transmittingconfiguration and of thereceiving measuring stations stations. changed and amounted to 340, 550 and 1035 m, respectively.

Laptop Laptop

MATLAB MATLAB

Digital-to-Analogue converter Analogue-to-Digital converter

NI USB-6363 NI USB-6363

Underwater telephone Underwater telephone

HTL-10 HTL-10

surface

Receive transducer hRx = 4m

hTx = 10m bottom

Transmitting transducer

20-30m

d= 340m, 550m, 1035m

Figure 5. Conﬁguration of measuring stations.

Each of the transmitted signals was generated in the MATLAB environment, and the conversion of this signal to analogue form was performed using the digital-to-analogue converter available in the National Instruments USB-6363 recording and generating device. The signal was then amplified and transmitted through an underwater channel using a HTL-10 underwater telephone [15]. Its design is based on the assumptions of programmable radio (SDR—Software-Defined Radio). This assumption makes it possible to use it for research purposes with various signals in the bands from 1 kHz to 60 kHz. The HTL-10, which I am a co-author of the hardware and software part, has been successfully used by the Polish Navy on its helicopters for several years. It uses hydroacoustic antenna of the OKA-2M dipping sonar that is designed to search for submarines because the helicopter devices allow it to descend into the water using a several-hundred-meter-long cable when the helicopter is hovering. The transmitting measuring station was placed on board a motor boat. The same measuring equipment was used on the receiving side, but it was configured differently. The receiving measuring Appl. Sci. 2020, 10, 1172 9 of 12 station was placed in a measuring container placed on a floating platform moored to a bridge with a fixed position during each of the measuring sessions. The received signal was sampled and archived. The transmitting transducer was lowered from the motor boat to a depth of hTx = 10 m regardless of the depth of the water reservoir, which for all transmitting positions of the measuring stations was Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 12 from 20 to 30 m. The receiving transducer was lowered to a depth of hRx = 4 m, in a place where the water depth was equal to 7 m. In subsequent sessions, the distance d between the transmitting and Figure 5. Configuration of measuring stations. receiving stations changed and amounted to 340, 550 and 1035 m, respectively. OnOn the the receiving receiving side, side, the the analogue analogue signal signal was was subjected subjected to to initial initial analogue analogue processing processing by by the the analogueanalogue processing processing unit unit of of the the underwater underwater telephone telephone HTL-10. HTL-10. The The obtained obtained signal signal is is digitized digitized by by an analogue-to-digital converter with a sampling frequency of fs = 200 kHz. Then, the digital receiver an analogue-to-digital converter with a sampling frequency of fs = 200 kHz. Then, the digital receiver usesuses the the digital digital down down converter converter technique technique (DDC) (DDC) to to directly directly convert convert the the real real passband passband signal signal around around thethe selected selected carrier carrier frequency frequency into into a a complex complex baseband baseband signal signal around around zero zero frequency frequency [26 [26].]. In In the the conversionconversion process, process, the the receiver receiver uses uses optimized optimized algorithms algorithms for for digital digital signal signal processing. processing. BeforeBefore performing performing the the series series of of measurements, measurements, the the sound sound velocity velocity distribution distribution profile profile was was measuredmeasured using using a a sound sound velocityvelocity distributiondistribution meter.meter. The measured measured profile profile and and the the determination determination of ofthe the acoustic acoustic wave wave propagation propagation paths paths based based on it mademade itit possiblepossible toto estimate estimate the the expected expected range range obtainableobtainable of of the the communication communication system, system, which which is is presented presented in in Figure Figure6. 6. Propagation Propagation paths paths for for the the hydroacoustichydroacoustic transducer transducer submerged submerged to to a a depth depth of of 10 10 m m were were determined. determined. On On the the presented presented sound sound velocityvelocity distribution distribution profile profile in in Figure Figure6a, 6a, the the sound soun velocityd velocity gradient gradient is negative, is negative, hence hence the drawingthe drawing of propagationof propagation path path clearly clearly indicates indicates that that sound sound rays rays are refractedare refracted towards towards the bottom. the bottom.

(a) (b)

FigureFigure 6. 6.(a ()a Measured) Measured sound sound velocity velocity distribution distribution proﬁle; profile; (b ()b Estimated) Estimated system system range. range.

In the considered system, the fast frequency hopping technique assumes the use of BFSK In the considered system, the fast frequency hopping technique assumes the use of BFSK modulation due to the availability of a narrow system frequency band of approximately 5000 Hz. modulation due to the availability of a narrow system frequency band of approximately 5000 Hz. In In this technique, hops are made between available subbands during the transmission of a single symbol. this technique, hops are made between available subbands during the transmission of a single The jumps are made according to the assumed pattern, and their maximum number in the tests was symbol. The jumps are made according to the assumed pattern, and their maximum number in the set at L = 9. The transmissions of subsequent symbols with the hop duration TH = {4 ms, 16 ms, 64 ms} tests was set at L = 9. The transmissions of subsequent symbols with the hop duration TH = {4 ms, 16 were subjected to experimental tests. The recorded transmissions of individual symbols were used ms, 64 ms} were subjected to experimental tests. The recorded transmissions of individual symbols to determine the bit error rate of reception. A sampled signal with a time equal to the duration of were used to determine the bit error rate of reception. A sampled signal with a time equal to the a single hop is subjected to analysis based on a periodogram. Tests of the FFH technique with BFSK duration of a single hop is subjected to analysis based on a periodogram. Tests of the FFH technique modulation were carried out for all channel types, i.e., d = 340 m, d = 550 m and d = 1035 m using the with BFSK modulation were carried out for all channel types, i.e., d = 340 m, d = 550 m and d = 1035 m following carrier frequency hopping pattern ([f 1 f 2 f 3 f 4 f 5 f 6 f 7 f 8 f 9 f 1 f 2 f 3 f 4 ... ]). For the d = 550 m using the following carrier frequency hopping pattern ([f1 f2 f3 f4 f5 f6 f7 f8 f9 f1 f2 f3 f4 ...]). For the d = 550 channel, data transmission was performed using pseudo-random hops ([f 1 f 5 f 9 f 4 f 8 f 2 f 6 f 3 f 7 f 1 f 5 m channel, data transmission was performed using pseudo-random hops ([f1 f5 f9 f4 f8 f2 f6 f3 f7 f1 f5 f9 f4 f f ... ]). Experimental studies were carried out under slightly different meteorological conditions, 9…]).4 Experimental studies were carried out under slightly different meteorological conditions, i.e., i.e., for d = 340 m and d = 1035 m, the sea state was 1, while for d = 550 m the sea state was 3. The sea for d = 340 m and d = 1035 m, the sea state was 1, while for d = 550 m the sea state was 3. The sea state state was determined according to the Douglas sea scale, where the sea state indicates the state of the was determined according to the Douglas sea scale, where the sea state indicates the state of the sea sea surface depending on the height of the wave. For sea state 1, wave height is 0.0–0.1 m and this surface depending on the height of the wave. For sea state 1, wave height is 0.0–0.1 m and this state is referred to as calm-rippled. In turn, for sea state 3, wave height is 0.5–1.5 m and it is defined as slight. During measurements, the HTL-10 was used, which has limited transmission power adjustment capabilities, which made it impossible to perform a comparative analysis of the received bit error rate values for different SNR values of the received signal. For BFSK modulation, a distance between significant frequencies of ∆f = 250 Hz was determined, which allows the maximum Doppler deviation of 125 Hz to be taken into account. The distance used requires the use of a subband with a width of 2·∆f = 500 Hz. The system bandwidth, determined on the basis of Formula (3), for a Appl. Sci. 2020, 10, 1172 10 of 12 state is referred to as calm-rippled. In turn, for sea state 3, wave height is 0.5–1.5 m and it is defined as slight. During measurements, the HTL-10 was used, which has limited transmission power adjustment capabilities, which made it impossible to perform a comparative analysis of the received bit error rate values for different SNR values of the received signal. For BFSK modulation, a distance between significant frequencies of ∆f = 250 Hz was determined, which allows the maximum Doppler deviation of 125 Hz to be taken into account. The distance used requires the use of a subband with a width of 2 ∆f = 500 Hz. The system bandwidth, determined on the basis of Formula (3), for a maximum hop · count of L = 9 is 4500 Hz. The measurements were carried out using the carrier frequency fc = 30 kHz. As a result of the analysis of the recorded data transmission signals with the FFH-BFSK technique, in the d = 340 m channel no transmission errors were found for the considered order of diversity L = {3, 5, 7, 9}. Of all of the tested channels, this channel was characterized by the shortest maximum delay spread Tm of ~2 ms. Tables1 and2, which contain the results of FFH technique tests for d = 550 m and d = 1035 m channels, show that only the use of TH = 4 ms hop time was associated with transmission errors. Errors were observed for the first channel case when the diversity order was set at L = 3 and L = 5 and for the second channel at the L = 3 diversity order. The increase in the order of diversity, as well as the increase in the hop time, caused the transmission to also be error-free in the d = 550 m channel, the most difficult of the tested channels. The maximum delay spread Tm for the first channel was ~7 ms, and for the second one ~9 ms. The differences were probably in the reinforcements of individual multipath components. The above tests were performed for the FFH technique with a standard with successive hops. For longer hop times TH = {16 ms, 64 ms}, no errors occurred.

Table 1. Experimental tests results for the d = 550 m channel (subsequent jumps, TH = 4 ms, EH/N0 = 31 dB).

Bit Error Rate Order of Diversity L Transmission Rate [bps] BER 3 83.3 0.024 5 50 0.019 7 35.7 <0.001 9 27.7 <0.001

Table 2. Experimental tests results for the d = 1035 m channel (subseq. jumps, TH = 4 ms, EH/N0 = 31 dB).

Bit Error Rate Order of Diversity L Transmission Rate [bps] BER 3 83.3 0.022 5 50 <0.001 7 35.7 <0.001 9 27.7 <0.001

Using the pattern with pseudo-random jumps in the FFH technique for the d = 550 m channel, the results were obtained. As for the successive hopping case, transmissions with errors were obtained only for the duration of the hop TH = 4 ms and only for the order of diversification L = 3. The use of the pattern with successive hops and the pattern with pseudo-random hops gave similar results of the quality of data transmission expressed as BER, which could be expected due to the small number of possible hops. Figure7 presents examples of decision values corresponding to a logical ‘0’ and a logical ‘1’ obtained for subsequent hops falling on the transmission of a single symbol ‘0’ in the case of transmission through the d = 340 m and d = 550 m channel. The figure confirms the fact that the channels were characterized by different multipath propagations of acoustic wave. Appl.Appl. Sci. Sci.2020 2020, ,10 10,, 1172 x FOR PEER REVIEW 1111 of of 12 12

(a) (b)

FigureFigure 7. 7.Examples Examples of decisionof decision values values corresponding corresponding to logical to ‘0’logical and logical‘0′ and ‘1’ logical obtained ‘1′ for obtained successive for hopssuccessive per transmission hops per transmission of a single symbol of a single ‘0’ for symbol diﬀerent ‘0 channels:′ for different (a) for channels:d = 340m (achannel;) for d = (b340) for m dchannel;= 550 m ( channel.b) for d = 550 m channel.

SummarizingSummarizing the the obtained obtained results results of the of experimentalthe experimental tests carried tests carried out on aout lake, on the a FFHlake, technique the FFH conﬁrmstechnique high confirms eﬃciency high when efficiency working inwhen shallow work coastaling in waters. shallow The coastal commercial waters. modems The designedcommercial to workmodems in horizontal designed channels to work that in use horizontal other modulation channels techniques that use other allow obtainingmodulation similar techniques transmission allow ratesobtaining of several similar dozen transmission to hundreds rates of of bits several per second dozen andto hundreds require theof bits use pe ofr strongsecond channel and require coding. the Testsuse of gave strong an answerchannel oncoding. the nature Tests gave of the an errors answer occurring. on the nature During of the the errors analysis occurring. of the FFH-BFSKDuring the techniqueanalysis of with the LFFH-BFSK= {2, ... , 9},technique no burst with errors L were= {2, ..., observed, 9}, no burst only errors random were errors observed, were observed. only random errors were observed. 4. Conclusions

4. ConclusionsThis article is devoted to an underwater acoustic communication system, which has been set to have specific requirements resulting from working in the difficult propagation conditions that occur in This article is devoted to an underwater acoustic communication system, which has been set to shallow coastal waters. The specificity of the work environment of the underwater communication have specific requirements resulting from working in the difficult propagation conditions that occur system results in numerous limitations of digital data transmission. The transmitted signal undergoes in shallow coastal waters. The specificity of the work environment of the underwater strong multipath propagation and the influence of the Doppler effect. These phenomena and the communication system results in numerous limitations of digital data transmission. The transmitted narrow frequency band of the system, resulting from the low operating frequency (from several to signal undergoes strong multipath propagation and the influence of the Doppler effect. These several dozen kHz) and design restrictions of ultrasonic transducers, result in relatively low data phenomena and the narrow frequency band of the system, resulting from the low operating transmission rates being achieved. In turn, transmission losses and noise are the main factors that limit frequency (from several to several dozen kHz) and design restrictions of ultrasonic transducers, the range of the system and determine the choice of operating frequency. An additional significant result in relatively low data transmission rates being achieved. In turn, transmission losses and noise limitation is the low speed of acoustic wave propagation, which results in a strong delay in the reception are the main factors that limit the range of the system and determine the choice of operating of transmitted signals, which in two-way communication limits the effective transmission rate. frequency. An additional significant limitation is the low speed of acoustic wave propagation, which An underwater communication system using the fast frequency hopping technique has been results in a strong delay in the reception of transmitted signals, which in two-way communication described in detail. The influence of acoustic wave propagation conditions in shallow coastal limits the effective transmission rate. waters on the quality of data transmission in the considered communication system was examined. An underwater communication system using the fast frequency hopping technique has been The results of experimental studies confirmed the thesis that increasing the order of diversity L allowed described in detail. The influence of acoustic wave propagation conditions in shallow coastal waters an improvement of detection conditions to be achieved and thus a reduction in the transmission bit on the quality of data transmission in the considered communication system was examined. The error rate. It can also be stated that the FFH technique is effective in a channel with fading despite results of experimental studies confirmed the thesis that increasing the order of diversity L allowed the fact that the available operating bandwidth is relatively narrow for a communication system. an improvement of detection conditions to be achieved and thus a reduction in the transmission bit Unfortunately, an increase in the order of diversity L reduces the system’s transmission rate. error rate. It can also be stated that the FFH technique is effective in a channel with fading despite Funding:the fact Thisthat researchthe available received operating no external bandwidth funding. is relatively narrow for a communication system. Acknowledgments:Unfortunately, an increaseSpecial thanks in the toorder Iwona of diversity Kochańskaand L reduces Krzysztof the system’s Liedtke transmission for technical rate. support in experimental tests. Funding: This research received no external funding. Conflicts of Interest: The author declares no conflict of interest in this work. Acknowledgments: Special thanks to Iwona Kochańska and Krzysztof Liedtke for technical support in Referencesexperimental tests.

1.Conflicts Urick, of R.J. Interest:Principles The of author Underwater declares Sound no forconfli Engineersct of interest; McGraw: in this New work. York, NY, USA, 1983.

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