Recent Trends in Multicarrier Underwater Acoustic Communications

Recent Trends in Multicarrier Underwater Acoustic Communications Prashant Kumar

Vinay Kumar Trivedi

Preetam Kumar

Department of Electrical Engineering Indian Institute of Technology Patna Patna-800013, India Email: [email protected]



Abstract—Underwater acoustic communication is essential in applications like remote control in the offshore oil industry, pollution monitoring in environmental systems, collection of scientific data recorded at ocean-bottom stations, disaster detection and early warning and underwater surveillance. Research on underwater wireless communication techniques plays a vital role in further exploring oceans and other marine environments. There has been an extensive growth in the volume of literature for underwater acoustic (UWA) communication but still it remains to be one of the most challenging areas of wireless communication. Over the years attention has turned on applying modified versions of multicarrier (MC) communication to underwater channel. This paper reviews the recent developments in the area of UWA communication related to multicarrier communication and particularly to orthogonal frequency division multiplexing (OFDM) with respect to applied, theoretical and simulation studies. An attempt has been made to present a compact yet exhaustive literature survey that will serve as a standard reference for researchers working in the area. Stress has been laid on the physical layer issues as it works as the basic foundation of any network. The focus areas of research activities have been identified and a summary of the ongoing activities and future trends has been presented. Keywords—Underwater acoustic communication; multicarrier communication; OFDM, alternative modulation strategies; diversity exploitation; recent trends.

I.

I NTRODUCTION

For hundreds of years electromagnetic waves have been extensively used for long range communications. But when the medium of propagation is water these waves find limited application and sound waves are used as the carrier. In contrast with terrestrial wireless radio communications, underwater wireless networks and communication channels can be considerably affected by aquatic environments, noise, constrained bandwidth and power resources. The first documented use of underwater acoustics (UWA) was in 1490, by Leonardo De Vinci, but this area of engineering received a major thrust only in the early 20th century when the tragic sinking of Titanic compelled the research community to explore the area of underwater communication. And during World Wars this area saw several new applications like underwater detection and underwater telephones. The introduction of things like underwater sensor network, submarine communications, military surveillance and remotelyoperated vehicles (ROV) opened a whole new area of research and development. Although most of the applied concepts have

been adopted from radio frequency (RF) applications, the major challenge has been on how to apply those successful concepts to the underwater channel, particularly at the physical layer. There has been a series of highly cited surveys related to UWA communication by some of the esteemed researchers. Baggeroer et al. have published a series of three highly cited extensive literature reviews [1]–[3]. Other important surveys include [4]–[8]. These articles serve the purpose of enhancing the knowledge and understanding of new researchers and also facilitate exchange of knowledge among different research groups across the globe. A closer look at the research papers published in recent times simply indicate that there has been a growing trend in adopting multicarrier (MC) communication for UWA channels. A recent addition to the collection is [9] which is a good reference book for researchers working in this area and discusses the pros and cons of orthogonal frequency division multiplexing (OFDM) applied to UWA channel with respect to simulation as well as experimentation. Following the last exhaustive survey [3] a lot of research has been carried out in the area of communication over underwater channels and there is a need to review the recent trends. This review paper aims at supplementing the underwater communication literature with a summary of all the recent advances and introduce the readers to the challenges and provide an insight into some of the open research problems which needs to be solved in the near future. The rest of the paper is organized as follows: Section II discusses the typical characteristics of underwater channel, section III examines the suitability of MC communication for UWA channel and explores some of the variants of OFDM applied to such channels. Section IV summarizes the diversity schemes applied to MC UWA communication. Section V reviews CDMA based MC techniques and multiuser issues in UWA communications and Section VI reviews channel estimation and equalization techniques for UWA MC communications. Finally we summarize the trends and identify some of the challenging problems that need the attention of the research community. II.

UWA C HANNEL C HARACTERISTICS

Due to the multipath signal propagation and time variability of the medium, acoustic communication has poor link quality and irregular fading, which severely limits the capacity. Reflections from the ocean surface and the bottom are the main cause for multipath propagation. The UWA channel suffer

from frequency-dependent attenuation and has low propagation speed of sound, around 1500 m/s, which make UWA communication quite difficult and necessitates devoted system design. Acoustic signals are preferred because high-frequency radio signals attenuate fast and optical signals generally scatter in the ocean channel. The acoustic channel supports a low data rate which is only several tens of kilobits per second for ranges under 1 km, and it has even lower data rates at longer distances [10]. Since nodes are battery operated, lowering the transmission power may extend network life time but at the cost of increased bit error rate (BER), as signal-to-noiseratio (SNR) might not be high enough to ensure satisfactory information transmission. Fig. 1.

A. Channel Modelling Channel modelling plays an important role in the efficient design of reliable and high data rate systems. The purpose of channel modelling is to aid in evaluation of signal processing algorithms in an attempt to enhance the success of field experiments. Signal fading can be Rayleigh, Rician, or K-distributed, while other channels may just as well be characterized as being deterministic [10]. In addition to signal, noise comes from a wide variety of sources with different statistics, such as precipitation, marine mammals, sonar systems, and offshore civil works. The noise may sometimes be coloured and Gaussian in the presence of breaking waves, or impulsive and white due to cavitating ship propellers or snapping shrimps. Underwater tests have shown that the channel often exhibit poor behaviour than a Rayleigh distributed channel modelling. Spatial and temporal variations have significant impact on the BER performance of the ocean channels. Shallow acoustic channel has been modelled as a four to five path Rayleigh channel [11]–[13]. A time variant and correlated fading factor in the signals amplitude or power accounts for the multiplicative fluctuation of each individual path. Additive white Gaussian noise (AWGN) for the system is combined at the receiver. The channel impulse response can be expressed by h(t, τ ) =

M X

αi (t)δ(t − τi (t))

(1)

i=1

where M represents the number of rays used in the multipath channel model. αi (t) and τi (t) are the complex-valued time varying channel magnitudes and the propagation delay of the kth path [13] and [14]. In [15] a typical channel power delay profile obtained from experimentation has been discussed considering UWA communication between a single source and a destination terminal. The distance between the transmitter and the receiver is 3 km and the receiver is at a depth of 75 m. The carrier frequency is taken to be 15 kHz. Figure 1 shows the channel path delays and magnitudes of the 5 non-zero taps commonly used for simulation. This channel profile has been used by many researchers to demonstrate the effectiveness of their proposed schemes with the help of computer simulation. In [16] the authors describe the statistical modelling for shallow water acoustic communications channels. Wideband singlecarrier and multi-carrier probe signals have been employed to measure the time-varying channel response, and to estimate its statistical properties with a set of measured channel responses, obtained from the signals recorded during the Kauai acoustic communications (KAM08) experiment.

III.

Channel impulse response [15]

M ULTICARRIER C OMMUNICATION FOR U NDERWATER C HANNELS

Multicarrier communication is an alternative to overcome the excess delay spread inherited in underwater acoustic channels. MC modulation in the form of OFDM has been actively pursued for UWA communication in the recent years. OFDM transmits signals over multiple orthogonal sub-carriers simultaneously and performs robustly in harsh multi-path environments achieving reasonable spectral efficiency and supporting high-data rate. The convergence of high-capability, lowpower digital signal processors and advancements in acoustic channel modelling and communication theory has allowed several OFDM based commercial and scientific underwater applications to emerge [9], [17]. The single-tap equalization enjoyed by MC methods relies on orthogonality among the carriers. In the presence of Doppler spread, orthogonality no longer holds, and inter-carrier interference (ICI) results. A way of addressing this challenge is to apply a sparse Fourier basis expansion to model the channel in the frequency domain and allow for a small amount of ICI [18]. Joint channel estimation and data detection may be done using a turbo-style receiver. A banded OFDM approach [19] can also be used to further reduce the complexity of channel estimation at the receiver. In [20] the authors demonstrate the unique feature of OFDM, that one signal design can be easily scaled to fit into different transmission bandwidths with negligible modification in the receiver. The proposed scheme was tested with data collected from experiments at AUV Fest, Panama City, FL, June 2007, and at the Buzzards Bay, MA, Aug. 2007. QPSK modulation was used over a range of bandwidths from 3 kHz to 50 kHz, leading to data rates from 1.5 kbps to 25 kbps after rate 1/2 coding. 16-QAM modulation was also tested over bandwidths from 12 kHz to 50 kHz, leading to data rates from 12 kbps to 50 kbps. The flexibility of OFDM under different system setups was verified for very low BER. Some of the recent trends in UWA multicarrier communication is enlisted below: The orthogonality feature of conventional OFDM is also provided by inverse DCT (IDCT)-DCT structure with the added advantages of reduced implementation area and increased computational speed as only real calculations is required. DCT based OFDM for UWA Communication was introduced in [21] and it was shown that proposed system provided higher peak-to-average power ratio (PAPR) reduction and achieved

TABLE I. Tap 1 24 3 4 5 6

A TYPICAL CHANNEL DELAY PROFILE [22] Relative delay (µs) 0 400 1000 1600 2700 4800

Average power (dB) −9.3 0.0 −3.6 −8.9 −12.3 −19.7

demodulation for UWA communication was considered in [27]. Numerical results demonstrated the superiority of HC schemes with PFFT demodulation over the single carrier and OFDM schemes with PFFT demodulation at a very moderate complexity. B. Multicarrier Communication with Doppler Compensation

better noise immunity and hence better BER performance than standard OFDM, while maintaining a low implementation cost. A. Adaptive Modulation Techniques Three different MC schemes: OFDM, the filtered multitone (FMT) modulation and the staggered multitone (SMT) modulation were discussed in [22]. Simulation results have been used to compare the merits and demerits of these schemes in practical UWA channel. It was shown that under the long delay underwater environments, OFDM may not be as efficient as in terrestrial situations. Both FMT and SMT do not need a cyclic prefix (CP) which leads to less power loss than OFDM. However, FMT suffers from the low spectral efficiency and SMT performs worse due to multipath. The paper considers a particular channel delay profile (TABLE I) for simulation studies. An adaptive modulation and coding (AMC) system with a finite number of transmission modes in the context of UWA OFDM is proposed in [23]. Effective SNR computed after channel estimation and channel decoding is considered as a new performance metric for mode switching which is used to predict the system performance more consistently than the input SNR and the pilot SNR. Tests have been conducted to maximize the transmission rate with given transmission power. In [24] the authors propose the use of filterbank multicarrier (FBMC) technique for UWA communications. A novel cost function for optimization of the filterbank prototype filter, to achieve a robust performance in doubly dispersive channels, has been proposed. A design algorithm that optimizes the proposed cost function is also developed. In [25] the authors explored the design aspect of adaptive modulation based on OFDM for UWA communication and studied the performance using real time at-sea experiment with a design criteria to maximize the system throughput under a target BER. The effectiveness of the scheme is demonstrated using computer simulation and real channel measurements recorded in shallow water off the west coast of Kauai in 2008 and 2011. In [26] an adaptive underwater acoustic modem with high bandwidth and power efficiency was proposed. Key features included interleave-division multiplexing (IDM) in time and frequency domain (and optionally in the spatial domain) in conjunction with generalized (non-orthogonal) multi-carrier (MC) transmission. At the receiver side, joint inter-symbol interference (ISI) and adjacent channel interference (ACI) cancellation were performed, in addition to iterative decoding. Adaptation (with respect to variable data rates, variable signal power, variable signal bandwidth, variable range, variable channel conditions, variable service requirements, etc.) was shown to be supported to a great extend. The modem was implemented by means of software-defined radio (SDR). A wide range of potential applications including communication system for autonomous underwater vehicles (AUV) was targeted. Weighted type fractional Fourier transform based hybrid carrier (HC) modulation system with partial FFT (PFFT)

Doppler spreading is introduced by relative motion between the transmitter and the receiver or by the water motion in the channel. If the motion is slowly varying like ship motion, such shifts can be compensated; however random motion manifests a continuous spreading [1]. It becomes essential to estimate the Doppler shift and compensate it for all UWA communication applications. In [28] the UWA channel was compared with the conventional RF channels. Several design considerations applicable to the physical layer design of UWA communication system was described based on field measurements. The unique problems in synchronization under high Doppler spread were described highlighting the key differences in the way Doppler affects the RF and the UWA channels. OFDM receiver design for UWA channels with user- and/or path specific Doppler scaling distortions was considered in [29]. The robustness of the proposed receivers were demonstrated via simulation as well as emulation. In [30] the authors proposed a class of methods for compensating the Doppler distortions of the UWA channel for differentially coherent detection of OFDM signals. These methods were based on multiple FFT demodulation, and were implemented as partial (P), shaped (S), fractional (F), and Taylor (T) series expansion FFT demodulation. These methods replaced the conventional FFT demodulation with a few FFTs and a combiner. Frequency-domain oversampling was explored in [31], to improve the system performance of zero-padded (ZP) OFDM transmissions over UWA channels with large Doppler spread. The use of a signal design that enables separate sparse channel estimation and data detection, which reduces equalization complexity was proposed. Based on both simulation and experimental results, it was demonstrated that the receiver with frequency-domain oversampling outperforms the conventional one considerably, where the gain increases as the Doppler spread increases. In [32] the authors treat the channel as having a common Doppler scaling factor on all propagation paths, and proposed a two-step approach to mitigate the Doppler effect: 1) nonuniform Doppler compensation via resampling that converts a “wideband” problem into a “narrowband” problem and 2) high-resolution uniform compensation of the residual Doppler. The receiver was based on block-by-block processing, and shown to be suitable for fast-varying channels. The method proposed was tested in two shallow-water experiments. Over a bandwidth of 12 kHz, the data rates were 7.0, 8.6, 9.7 kb/s with QPSK modulation and rate 2/3 convolutional coding, when the numbers of subcarriers were 512, 1024, and 2048, respectively. Good performance was achieved even when the transmitter and the receiver were moving at a relative speed of up to 10 kn, where the Doppler shifts were greater than the OFDM subcarrier spacing. The authors introduce some future research topics, including shortening methods for channels whose delay spread is longer than the guard interval, extension of resampling to generalized time-varying filtering for channels with different Doppler scaling factors on different paths, and MIMO techniques.

C. PAPR reduction Techniques for UWA channels MC communication for UWA faces two typical issues: i) plain OFDM has poor performance in the presence of channel fading, and ii) OFDM transmission has a high PAPR. In [33] the authors proposed the application of nonbinary low density parity check (LDPC) codes to address these issues. New methods were proposed to construct nonbinary irregular LDPC codes that achieved excellent performance, which suits with the underlying modulation, and could be encoded in linear time and in parallel. Simulation and experimental results confirmed the excellent performance of the proposed nonbinary LDPC codes. Based on the property that the generator matrix of LDPC codes has high density, a method to reduce the PAPR considerably with minimal overhead was also demonstrated. Extensive simulations together with a summary on field test results were presented. The problem of high PAPR problem and its reduction in underwater MC acoustic communications was considered in [34] using data-independent selection of a pre-specified set of out-of-band adjacent tones and a datadependent out-of-band tone selection using the matchingpursuit algorithm and then iteratively updating the tones coefficients was proposed. It was shown that the proposed scheme resulted in low PAPR. Energy compaction property of DCT works well in reducing the PAPR in UWA channel. In [35] it was shown that DCT spread OFDM has lower computational complexity compared to DFT spread OFDM. The BER performance was evaluated for both the schemes. It was shown that DCT spread OFDM has a better performance than DFT spread OFDM in UWA channel. In [36] spreading by Walsh-Hadamard (WH) codes, discrete Fourier transform (DFT), discrete cosine transform (DCT) and carrier interferometry (CI) codes were compared for UWA communication using OFDM. The spreading technique not only helped overcome frequency-selective and multipath fading but also provided reduced PAPR. CI-SOFDM was shown to provide very good BER. To further improve the BER performance a two level orthogonal spreading by WH followed by CI-SOFDM was also evaluated for underwater channel. D. Point-to-Point vs Cooperative Communication Cooperative communication techniques have been successfully utilized in RF communication systems and UWA is gradually adopting it. Applications like underwater sensor network based early warning systems [37] and cooperative search and survey using autonomous underwater vehicles (AUVs) demand efficient cooperative communication whereas applications like diver-to-diver communication, underwater measurement and control prefer point-to-point communication [38]. In [39] the authors presented a contemporary overview of UWA communication (UWAC) and investigated physical layer aspects on cooperative transmission techniques for future UWAC systems. Simulation results demonstrated the superiority of cooperative UWAC systems over their point-to-point counterparts. Physical-layer network coding (PLNC) allows two remote terminals to exchange information in two time slots via a half-duplex relay. In [40] the authors considered PLNC in UWA two-way relay networks, with particular attention on the recovery of the network-coded codeword at the relay. The authors investigated three iterative receivers at the relay in doubly-selective UWA channels. In the time-varying channel, the inter-carrier interference (ICI) in the received signal had

been explicitly addressed in the three receivers. Extensive simulations and experimental results revealed that the decoding scheme for direct recovery of the network-coded codeword cannot work well in the multipath channel, and that with a lower complexity the performance of the iterative separatedecoding scheme could catch up with that of the joint decoding scheme. OFDM modulated dynamic code cooperation scheme for underwater relay networks where OFDM accommodates multipath fading with large delay spread was proposed in [41]. IV.

D IVERSITY S CHEMES FOR M ULTICARRIER UWA C OMMUNICATION

As most of the techniques used in RF communication has been gradually adopted by UWA communication, spatial diversity schemes were also introduced for UWA communication. MIMO techniques have been proposed for UWA communication and have yielded reliable systems [42]. For simple systems space-frequency and space-time transmit diversity schemes have been proposed [43]. In [44] the authors investigated MIMO OFDM communications that can effectively increase the data rate in band-limited underwater channels. The performance of MIMO OFDM communications was presented using data collected from the KAM11 experiment conducted in shallow water, west of Kauai, Hawaii, which involved multiple transmit and receive arrays with a 10 kHz bandwidth (20 − 30 kHz) and modulations (QPSK and 16-QAM) at range up to 3 km. In particular, the performance of the 2IMO and 4IMO cases with two modulations (QPSK and 16-QAM) were analyzed using three different detectors: ZF detector, MMSE equalization, and MMSE-SINR-OSIC. In addition, the impact of diversity combining on performance with LDPC decoding was also presented. Error-free performance was achieved using 2048-QPSK packets at a data rate of 21 kb/s. In [45] the authors proposed space-frequency block coding, applied over the carriers of an OFDM system for obtaining transmit diversity in an underwater acoustic channel. Results demonstrated an average mean squared error gain of about 2 dB as compared to the single-transmitter case and an order of magnitude decrease in the bit error rate when the number of carriers was chosen optimally. In [46] the advantage of the differentially coherent SFBC detection over the conventional, coherent SFBC detection which suffers from imperfect channel estimation and use differential space frequency block codes (SFBCs) with OFDM over underwater acoustic channels were discussed. System performance was demonstrated using real data transmitted in the 12 − 26 kHz acoustic band from a vehicle moving at 0.5 − 2 m/s and received over a 100 m shallow water channel, using 4-QAM and a varying number of carriers ranging from 128 to 2048. In [47] the authors evaluated the performance of WalshHadamard (WH) code spread OFDM in terms of BER improvement and PAPR reduction. Space-time and spacefrequency diversity were used to further improve the performance of WH-OFDM in underwater acoustic channel. MIMO techniques are currently extensively considered in UWA communications to overcome the bandwidth limitation of undersea channel. Associated with OFDM modulation, MIMO techniques provide substantial gain in spectral efficiency and fair robustness against frequency fading while keeping simple equalizer structure. In [48] the authors studied accurately the gain provided by MIMO-OFDM paradigm over Shallow

Water Acoustic (SWA) channel by taking into account channel characteristics as well as OFDM parameters. V.

CDMA BASED TECHNIQUES FOR UWA C OMMUNICATION

Code division multiple access (CDMA) techniques have recently been utilised to achieve reliable multiuser communication in asynchronous shallow water acoustic networks. In direct-sequence (DS)-CDMA, spread data are transmitted at a single carrier frequency. In contrast, in multi-carrier (MC)-CDMA a set of carrier frequencies is employed to achieve frequency diversity. In [49] the authors consider this type of communication technique. The performance of MCCDMA for short length complex spreading sequences, which demonstrate both good cross-correlation and auto-correlation properties, was evaluated by means of computer simulations. The proposed adaptive receiver architectures integrated the functions of despreading, multi-access and narrowband interference cancellation, equalisation, and phase-carrier tracking. The effectiveness of the system in a rapidly time-varying, frequency selective channel was demonstrated. Simulation scenarios were realised with respect to subcarrier numbers, delay spread, Doppler spread, nearfar effect and different combinations of receiver structure, indicating the reliability of the communication link in a low SNR in shallow water network. In the case of long multipath delay, the multicarrier scheme offers a significant advantage over the single-carrier DS-CDMA scheme. In [50] the same authors proposed adaptive receiver architectures, based on the minimisation of the mean square error (MSE), integrated the functions of multi-access interference cancellation, equalisation and phase-carrier tracking. The performance of DS and MC-CDMA was evaluated and compared under different simulation scenarios with respect to the multipath delay, Doppler broadening and number of users, indicating the effectiveness of each system in an underwater environment. Analytical derivation of the BER performance of the MMSE MC receiver in a Rayleigh-fading environment was also presented. In the DS-CDMA scheme the receiver operates in the time domain, while in the MC-CDMA system the equalisation is performed in the frequency domain. The demonstrated results of each scenario were compared and the DS-CDMA system demonstrated an improved performance compared to the MC-CDMA system for low SNR values. However, for SNR values higher than 20 dB, the MC-CDMA scheme outperformed the DS-CDMA system due to its frequency diversity. Further it is shown that MC-CDMA does not exhibit an irreducible error floor and there was no significant performance decay when longer multipath spread existed. In conventional OFDM the incoming serial data stream is split into N parallel streams, following which an N (N being the total number of subcarriers being used) point IFFT operation is performed resulting in a complex baseband OFDM signal To recover the symbols at the receiver, FFT operation is performed. Spectral voids are always present in frequency selective multipath channels like the UWA channel and can rigorously distort or annul out the OFDM carriers that are in their neighbourhood, resulting in a loss of the symbols carried by those subcarriers. This ultimately results in a high BER and bandwidth wastage because of retransmission of lost data. For long propagation delay UWA channel retransmission is not

appreciated. Use of forward error correction (FEC) technique is quite feasible. Another approach is to scatter the groups of data symbols over the entire bandwidth. This technique is called code spread (CS) OFDM. CSOFDM or simply SOFDM enjoys the spreading gain of code division multiple access (CDMA) along with multicarrier transmission [35], [36], [51], [52]. Such a system is shown in figure 2. The data symbols are scattered over the complete bandwidth such that every individual subcarrier bears a linear combination of all the data symbols. Even if some subcarriers are faded intensely, then also there is a possibility to recover the total transmitted symbols. This adds to the properties of OFDM in which symbol is lost if the subcarrier modulating it is severely faded during transmission. At the receiver spread symbols are simply separated by the use of linear matrix operation. In [53] the

Fig. 2.

System Model for Spread OFDM [52]

authors proposed a basic MC-CMDA via carrier interferometry Codes(CI codes) in an Underwater Acoustic(UWA) Channel. Compared with OFDM in UWA communication, MC-CDMA via CI codes can allow a number of users simultaneously and asynchronously accessing the UWA channel. A trade-off relation exists between diversity gain and user capacity as well as between spread spectrum gain and system efficiency. Both simulation and initial tank experiment proved the feasibility in UWA channels. A. Issues related to Multiuser UWA Communications A time-synchronous multiuser reception approach for OFDM transmission in UWA channels was developed in [54]. Interference cancellation was adopted to reduce the inter-block interference (IBI) between overlapped processing units. Simulation and emulation results demonstrated the robustness of the proposed receiver with signal asynchronism among multiple users in both time invariant and time varying environments. A technique to combat the asynchronism issues in employing OFDM-based transmissions at the source node by preceding every OFDM block with an extremely long CP which reduces the transmission rates dramatically was proposed in [55]. A distributed OFDM system with multiple quasi-synchronous users, where different users may transmit different numbers of parallel data streams was proposed in [56]. VI. C HANNEL E STIMATION AND E QUALIZATION TECHNIQUES FOR M ULTICARRIER UWA C OMMUNICATION In [57] the authors demonstrated various channel estimators that exploited the channel sparsity in a MC UWA system, including subspace algorithms from the array precessing literature, namely multiple signal classifier (MUSIC) and estimation of signal parameters via rotational invariance technique (ESPRIT), and the recent compressed sensing algorithms in form

of Orthogonal Matching Pursuit (OMP) and Basis Pursuit (BP). It was shown that subspace methods could tolerate small to moderate Doppler effects, and outperform the least square (LS) approach when the channel was sparse. At the same time compressed sensing algorithms uniformly outperformed the LS and subspace methods and could handle channels with significant Doppler spread. In [58] the same authors discussed sparse channel estimation for time-varying UWA channels using MC transmission. In case when pilots and data symbols were not separated at the receiver, it was shown that subcarriers corresponding to data symbols could be used to observe the ICI caused by the neighbouring pilot subcarriers. The primary challenges in underwater communications are very large delay and Doppler spread, which complicate the application of “optimal” methods of communication. The traditional approach expects the symbol duration of the transmitted signal to be larger than channel delay spread. This results in a low rate “sampling” of the channel impulse response (once per symbol in each sub-channel) and a sub-channel bandwidth that is less than the coherence bandwidth of the physical propagation channel. In [59] the authors proposed an alternative approach to use of shorter pulses. This resulted in subchannels that have delay spreads of greater than one symbol but which are small enough in terms of symbols of delay spread to enable the use of soft decision multi-path decoders (e.g., the maximum-likelihood sequence detection (MLSD) algorithm). This resulted in a more robust and reliable system in doubly spread channels. The proposed system had far better performance than the traditional decision feedback equalizer (DFE) in a channel with severe frequency selective fading, and its energy efficiency was 3 dB better than a system which uses a pilot signal. A UWA communication system using orthogonal signaldivision multiplexing (OSDM), a scheme that measures the multipath profile without an adaptation or interpolation process, to achieve stable communication in doubly spread channels was proposed in [60]. It was suggested that OSDM could provide a highly reliable communication environment for UWA communication with multipath and Doppler spread, such as in shallow water. In [61] the authors considered OFDM transmission in fast varying UWA channels and demonstrated that reliable communication could be achieved without any guard interval and with a superimposed pilot. Such OFDM transmission had a high spectral efficiency but incurs severe ISI and ICI and interference from superimposed pilots. A singleinput-multiple-output (SIMO) OFDM system for UWA communication with an aim of improving band width efficiency by reducing the amount of overheads was discussed in [62]. The proposed design was analysed and tested at a sea trial. A. Time Reversal and Passive-phase conjugation Methods for UWA Communications Time reversal is a feedback wave focusing technique that can be used to transparently compensate for multipath distortion in digital communications over several types of physical propagation media, such as radio and acoustic channels. In [63] the authors proposed time-reversed OFDM communication for underwater channels. The paper discussed issues related to protocol design and data modulation/demodulation when MC signals were used. It was shown that focusing information can be derived at the transmitter array by pre-filtering a single

observed broadband channel probe, thus streamlining the design of communication protocols. A receiver architecture based on MIMO decision-feedback equalization was proposed when few transmit elements were used, creating significant residual ISI. The technique was applied to time-reversed OFDM transmission, and simulation results suggested that residual ISI could be reduced to a point were conventional demodulation based on cyclic prefixes and Fourier analysis becomes feasible with moderate hardware complexity at the transmitter. OFDM decoding using a MIMO DFE was proposed as an alternative to prefix-based demodulation, which experiences significant degradation when the mirror is too sparse. Even with imperfect focusing, time-reversed communication makes sense because the equalizer may be far less complex than in arbitrary ocean channels. The application of MC communication with timereversal mirror (TRM) as a signal-focusing tool for UWA channel was proposed in [64]. A channel estimation algorithm specially designed for TRM OFDM was presented. TRM with OFDM was shown to improve system BER by a factor of 5 as compared to an OFDM system without TRM. Pilot-tones were used with correlation measurements to accurately estimate the OFDM channel. And it was shown that time reversal mirror as a tool could significantly improve the performance of an underwater OFDM system, alleviating BER by a factor of 5-10 depending on the transmit array size. The effect of a strong FEC code and sync algorithms can complement a TRM-OFDM system in building reasonably high throughput telemetry systems. Underwater acoustic channels are often of extended time delay spread, time-varying, and sparse. Passivephase conjugation (PPC) achieves the pulse compression (temporal focusing) for the time delayed arrivals. This property has been used in underwater acoustic communications. In [65] the authors investigated: (1) the communications using PPC processing, where the block-based approach was used; and (2) the matching pursuit (MP) algorithm exploiting the channel sparseness. It was shown that the MP algorithm improved performance of the communications using PPC processing. VII.

S UMMARY AND C ONCLUSION

This paper reviewed the recent development in multicarrier underwater acoustic communications with an emphasis on physical layer. Ongoing research in this area include analysis, simulation, emulation and actual sea-trials. It has been observed that many successful concepts of terrestrial radio communication have been adopted for underwater communications and modified accordingly. Focus areas of research have been: a) Underwater channel modelling: There is a strong need for a set of commonly accepted underwater channel models (like the COST and WINNER channel models in cellular radio) for the purpose of a fair comparison of different modem designs. b) Empirical model for the noise of the acoustic underwater channel: Although noise has temporal and spatial variations but still a standard empirical model is always desirable for system design. c) Adaptive modulation techniques: Conventional OFDM may not be the best multicarrier modulation technique for underwater acoustic communications and hence different variants of OFDM have been explored. These variants of OFDM offer merits like low complexity, low PAPR and high resilience to frequency selective UWA channels.

d) Doppler compensation: When comparing RF and UWA communications, Doppler compensation becomes a very important issue and deserves significance in system design. e) Spatial Diversity and MIMO: Underwater channel is very harsh and it is very difficult to achieve high SNR value at the receiver. Techniques like transmit diversity, receive diversity and MIMO prove to be very helpful in multicarrier UWA communication. f) Channel estimation and equalization: Performance improvement of multicarrier underwater acoustic system greatly depends on channel estimators that exploit channel sparsity in the time and/or Doppler domain. Time reversal and passive phase conjugation methods are other promising candidates in this domain. g) Hybrid schemes: The literature review also indicate the importance of hybrid schemes like MC-CDMA, spread OFDM, etc. which can be very easily adopted for underwater communications. The prime objective of this paper is to encourage research efforts to lay down fundamental basis for the development of new multicarrier communication techniques for efficient underwater communication. An attempt has been made to present a compact yet exhaustive literature survey that will serve as standard reference for researchers working in this area.

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ACKNOWLEDGMENT This work has been supported by the Centre of Excellence on Advanced System Engineering, IIT Patna. vide project no. R&DINT/EE/CAS/PED/2014-15/90

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