Passive Phase Conjugation Research Articles

On the Possibility of using Vertical Receiving Arrays for Underwater Acoustic Communication on the Arctic Shelf

Numerical simulation is applied to analyze the possibilities of using vertical receiving arrays covering the entire waveguide for underwater acoustic communication on the shallow Arctic shelf. Binary phase shift keying with a carrier frequency of 750 Hz is selected for data transfer. It is shown that an algorithm for selecting acoustic signals corresponding to the first waveguide mode or an algorithm based on using passive-phase conjugation eliminates intersymbol interference. In case of spatial signal processing and waveguide depth of 30 m the error-free information transmission is possible when the signal-to-noise ratio is ≈15 dB less than that for a single receiver. It has been demonstrated these advantages to be present in different conditions on the Arctic shelf: with an acoustically soft or acoustically hard bottom, with surface waves, or with ice cover.

Covert underwater communication by camouflaging sea piling sounds

To achieve covert underwater acoustic communication, a unique technique of mimicking genuine sea-piling sounds as carrier to convey information is presented. It differs from traditional low signal to noise ratio covert underwater acoustic communication. The communication signal can be detected, but it could not be recognized from the perspective of camouflage to realize covert acoustic communication in the sea. According to the features of sea-piling sounds, a quasi-constant period based pulse position modulation scheme is proposed. First we put each piling sound with identical time interval in time domain and it is moved off the original position with a dedicated time delay which is calculated by modulated information. To make the communication signal akin to the piling sounds, the synchronization header is designed by time hopping pulse position modulation scheme with real piling sounds. At the receiver, basis pursuit algorithm under the theory of compressive sensing is used for channel estimation, and passive phase conjugation technique is used for channel equalization to eliminate the impacts of multipath channel. The feasibility and efficiency of the proposed method are verified by simulation and pool experiment.

An Enhanced Sharpness on Acoustic Focus Via Artificial Iterative Phase Conjugation Processing

For a low-frequency source in a homogenous medium, when the source is focused by phase conjugation (also referred to as the time reversal in the time domain), the focal spot is large because of the half-wavelength limit. In this paper, to reduce the focal spot size, artificial iterative phase-conjugated processing is proposed based on passive phase conjugation to focus a point-like source. As AIPCP operates in iteration mode using a computer, the iteration loops of each transducer are calculated independently, and then, AIPCP is achieved by summing the iteration outputs of all transducers together. Numerical simulations and experiments on an audible sound field are employed to illustrate that AIPCP generates a narrower focal spot than passive phase conjugation. Further analyses considered the theoretical change in focal spot size with the iteration number using Fraunhofer far-field approximation, and the result shows that the focal spot size decreases with increase in the iteration number. This finding is predicted in the near field and validated in the far field by simulations and experiments. Moreover, the half-wavelength limit is overcome at a distance from the sound source equal to the wavelength.

Spatially Multiplexed CDMA Multiuser Underwater Acoustic Communications

Direct-sequence (DS) code-division-multiple-access (CDMA) signaling allows multiple users to communicate simultaneously over the same bandwidth at the expense of a reduced rate. Due to severe cochannel interference (CoI) in multipath rich underwater channels, no more than 4 users have been demonstrated. Multiple-input-multiple-output (MIMO) communication also allows multiple users. Using parallel/sequential interference cancellation schemes, experimental results show similar limitations on the number of users. To extend the number of users beyond four (as required for certain applications), it is proposed to combine CDMA with MIMO, utilizing the spatial-temporal processing gain of passive-phase conjugation, and the approximate-orthogonality of the spreading codes to remove the (increased) CoI. The receiver algorithm uses simple matched filters without explicit channel-equalization and CoI cancellation. The algorithm performance is studied as a function of input-SNR, using simulated data based on experimentally measured channel impulse responses. M-sequences with 127, 255 and 511 chips are used. One finds that 255/511 chips are generally required to support 8 users, and 127/255 chips are needed to support 6 users for the channel studied. Multiuser communications are demonstrated with at-sea data for 8 users, with spreading codes of 511 chips, yielding zero bit errors. The data rate is ${\sim} 8$ bits/s/user, given 4 kHz bandwidth.

Experiential assessment of iteratively residual interference elimination in the passive phase conjugation for acoustic underwater communications

Underwater acoustic communications have found a growing interest because of its variety usages both in military and commercial applications. But, for ocean environment, the channel has an extended time varying multipath due to acoustic propagation. These phenomena cause pulse spreading and time varying inter-symbol interference (ISI). To overcome the interference caused by this harsh environment, equalizers with Passive Phase Conjugation (PPC) have been widely used in coherent communications. In this paper, we have proposed a combined scheme of PPC followed by an iteratively ISI cancelation algorithm to remove the residual ISI remained at the output of the PPC processor. By using a coarse estimation of the channel along with the ISI alleviated PPC output symbols; we can eliminate residual ISI from the received data signal. We have conducted extensive simulations and some experimental tests at the coastal environment of the Persian Gulf to investigate the performance of the proposed scheme. The advantages of the proposed combined scheme are using the channel probe signal for both frame synchronization and coarse estimation of the channel impulse response to using it in the iteratively ISI cancellation and the PPC processing block. The results showed the effectiveness of the combined method.

Underwater communications in time-varying sparse channels using passive-phase conjugation

Passive-phase conjugation (PPC) achieves the pulse compression for the time delayed arrivals. This property has been used in underwater acoustic communications. In a time-varying channel, the temporal focusing degrades with time evolution. The block-based approach is implemented to avoid the impact of channel variations. The basic idea is that the channel is assumed constant within each block of a short time interval, and the channel is updated using detected symbols in the previous block. Using real data collected in a recent sea trial, this paper investigates: (1) the communications using PPC processing, where the block-based approach is used, (2) the matching pursuit (MP) algorithm exploiting the channel sparseness. It is found that the MP algorithm improves performance of the communications using PPC processing.

Experimental demonstration of spread spectrum communication over long range multipath channels

This paper presents a spread spectrum scheme, which is designed for long range underwater acoustic communications of low signal-to-noise ratios. This scheme uses two maximum length sequences, which are overlapped in time. One is used as a time reference, and the other applies code cyclic shift keying (CCSK) to carry information. Compared with conventional spread spectrum techniques, CCSK achieves a higher spectral efficiency, and this property is of significance in spread spectrum communications. With the help of a time reference, performance impairment from timing errors for CCSK is reduced, as each CCSK symbol has its own time reference. For this scheme, three receiver structures are presented, and they are: (1) correlation receiver, (2) passive-phase conjugation receiver, and (3) time reversal receiver structures. A recent sea experiment was carried out in a range dependent channel over a distance of 10km. By real data processing, performance of the three receiver structures are compared and discussed. The presented results demonstrate the feasibility of this spread spectrum scheme.

Experimental Assessment of Different Receiver Structures for Underwater Acoustic Communications over Multipath Channels

Underwater communication channels are often complicated, and in particular multipath propagation may cause intersymbol interference (ISI). This paper addresses how to remove ISI, and evaluates the performance of three different receiver structures and their implementations. Using real data collected in a high-frequency (10–14 kHz) field experiment, the receiver structures are evaluated by off-line data processing. The three structures are multichannel decision feedback equalizer (DFE), passive time reversal receiver (passive-phase conjugation (PPC) with a single channel DFE), and the joint PPC with multichannel DFE. In sparse channels, dominant arrivals represent the channel information, and the matching pursuit (MP) algorithm which exploits the channel sparseness has been investigated for PPC processing. In the assessment, it is found that: (1) it is advantageous to obtain spatial gain using the adaptive multichannel combining scheme; and (2) the MP algorithm improves the performance of communications using PPC processing.

Passive Phase Conjugation Applied to an OFDM-Based Terrestrial Communication System

The demand for high-data-rate radio systems that can robustly operate in multipath environments presents challenging technical issues. In this paper, we propose a passive phase conjugation (PPC)-based orthogonal frequency-division multiplexing (OFDM) system. Compared with the conventional OFDM system, the proposed system achieves lower computational complexity since the need for conventional channel estimation and equalization can be eliminated. The performance gain of the proposed technique is shown to be particularly advantageous in systems with multiple receiver array elements. Numerical results are provided to illustrate the performance of the system when operating through a multipath radio channel. We also derive a closed-form formula that can be used to predict the performance of the proposed scheme over multipath channels with a uniform power delay profile. Our analysis validates the simulation results and provides insights into the behavior of wireless centric OFDM-PPC systems.

Joint passive-phase conjugation with adaptive multichannel combining for coherent underwater acoustic communications

This paper presents a receiver structure which exploits spatial diversity by adaptive multichannel combining, which improves the performance of passive time reversal communications realized by passive-phase conjugation (PPC). PPC processing achieves pulse compression for the time delayed arrivals at the receiver, and this property is used for coherent communications to reduce the computational load. The presented structure takes advantage of pulse compression and performs adaptive multichannel combining, where the number of taps for adaptive multichannel processing is significantly reduced in order to decrease the computational load. With a previous output mean square error (MSE), the adaptive combining minimizes current output MSE, where spatial diversity is exploited by the adaptive combining. This structure improves performance of the passive time reversal approach, even though the taps for combining span one symbol interval. The performance improvement is demonstrated by a set of real data collected in a recent sea experiment, which was conducted in a range dependent acoustic channel over a range of 4 km.

Spatial diversity in multichannel processing for underwater acoustic communications

This paper discusses spatial diversity for underwater communications and presents a passive-phase conjugation (PPC) based multichannel equalizer. Multichannel processing involves taking advantage of multiple receivers distributed in space to take advantage of spatial diversity. The multichannel decision feedback equalizer (DFE) uses multiple nonlinear adaptive filters to remove intersymbol interference (ISI), and its complexity increases with time spread plus the number of receiving channels. Passive time reversal realized by the passive-phase conjugation (PPC) approach refocuses time delayed arrivals at the receiving array, where spatial diversity is obtained by the multichannel combining. The property of refocusing can be used to mitigate ISI for underwater communications, and only one single channel equalizer is required to remove residual ISI. PPC achieves pulse compression for time delayed arrivals, where the number of taps is significantly reduced for a PPC based equalizer. Based on temporal diversity obtained by pulse compression, the proposed structure improves the performance of time reversal communications by adaptive diversity combining. Three receiver structures are evaluated by processing real data collected in an experiment, which was conducted in a range-dependent acoustic channel over a range of 4 km. Results have demonstrated superior performance of the proposed receiver structure.

Experimental assessment of a multicarrier underwater acoustic communication system

This paper presents a multicarrier communication system which transmits information on independent subcarriers to achieve an increased data rate. For this system, a passive-phase conjugation (PPC) based receiver structure is assessed by processing data collected in sea trials. Based on temporal diversity (pulse compression) exploited by PPC processing, an adaptive multichannel decision feedback equalizer is used to remove intersymbol interference, where spatial diversity is exploited by adaptive multichannel combination. The digital phase-locked loop (DPLL) technique is implemented for carrier-phase tracking. In the scenario of low input signal-to-noise ratios (SNRs), the receiver structure achieved superior performance using a common DPLL. In a depth-fluctuated environment, two sea experiments were conducted over ranges of 2 km and 4 km, respectively, and this communication system was assessed with a data rate of 4 kbps. In terms of mean square error, output SNR and bit error rate, this receiver structure has demonstrated its performance for the multicarrier communication system.

Coherent underwater communication using passive time reversal over multipath channels

Passive time reversal exploits underwater acoustic channels’ spatial and temporal diversity. It can refocus multipath propagated signal at the receiver and can be realized simply by the passive phase conjugation (PPC) method. By the temporal focusing, time delay spread caused by multipath propagation is mitigated for spectral efficient coherent communications. However, the PPC method is unable to eliminate multipath and is limited by channel variations. An adaptive equalizer is therefore needed to compensate residual multipath after refocusing and to track channel temporal variations. Spatial diversity is obtained by using a vertical receiving array. In this paper we used 4-hydrophone array and demonstrated that the adaptive decision feedback equalization in conjunction with PPC significantly decreased the bit error rate.

An efficient spread spectrum pulse position modulation scheme for point-to-point underwater acoustic communication

The underwater acoustic channel is a complicated and time-varying multipath channel, and many equalization algorithms have been researched and developed to overcome the difficulties for underwater acoustic communication. Unfortunately, many algorithms are computational intensive and prone to lose convergence due to their sensitiveness to different channel configurations. In this paper, a pulse position modulation (PPM) scheme is proposed, and it uses two M-sequences of low cross-correlation to transfer information, which are modulated on two orthogonal carriers. One is used as a reference sequence, and the other is shifted relative to the reference. Information is carried by the starting time difference between the two sequences in each symbol. Comparing with conventional direct-sequence spread spectrum technique, the proposed scheme is more spectral efficient. Two receiver designs are given, one takes advantages of M-sequences’ auto-correlation properties, and the other is motivated by passive phase conjugation (PPC) to take advantages of the channel. Combined with M-sequence, PPC performance is augmented without a receiving array to cover the water column, and it is far less complex than adaptive equalizers for receivers. Results from lake field trials are analyzed, and they verify potential applications of this PPM scheme.

Passive phase conjugation processing to forward scattering waves by target in shallow water

This paper describes the detection of an underwater target using a passive phase conjugation processing. It is assumed that a sound source and an array are disposed in shallow water, and a target exists between them. The traveling wave from the sound source and the scattered wave from the target are received by the array. However, they are received almost simultaneously. In addition, because the level of the traveling wave is considerably larger than that of the scattered wave, the detection of the scattered wave is difficult. Then, the traveling wave components are removed from the signals received in the array. And, the passive phase conjugation processing is given to the signals. The signal to which the passive phase conjugation is processed is similar with the signal radiated from the target. The signal radiated from the target relates to the sound wave that enters at the position of the target from the sound source for the small target. The sound wave that enters at the position of the target is uniquely decided depending on the condition of the propagation environment and the sound source. Therefore, the result that the passive phase conjugation is processed contains the position information of the target.

Study on point-to-point mobile underwater acoustic communication

Point-to-point mobile underwater acoustic (UWA) communication is one of the key technologies for UWA networks which consists of mobile and stationary nodes. The mobile communication UWA channels are analyzed, whose eigen rays have different Doppler shifts which enhances the complexity of intersymbol interference (ISI) caused by the multipath channel, that goes against the UWA communication. The channel equalization using single sensor passive phase conjugation mirror (PPCM) is proposed to compensate for the Doppler shift in real time and to focus the multipath signals. The application of single sensor PPC to communication system based on Pattern time delay shift coding scheme is carried out in lake trials. The results of lake trials validated the capability of PPCM mitigating ISI and the robustness of the communication system.

Sound beam scanning in sediment layer using phase conjugated pseudo sound source

When an object that is buried underwater in a sediment layer is detected from the diagonal, detection is often hampered by reflection from the bottom surface. Stated in different terms, the sound-wave energy that reaches the object is small because of reflection and refraction that occurs at the bottom surface when a sound wave is applied to the object in the sediment for diagonal detection. To alleviate that problem, a pseudo sound source can be set up in the sediment layer. The sound wave radiated from the pseudo sound source is then received by a time-reversal array. The sound wave can be injected into the sediment layer by reversing the received signal with time, and radiating it again from the time-reversal array. The beam can hit the target through movement of this pseudo sound source along the bottom surface. The sound wave that hits the target is reflected and returns in water. The sound wave is subsequently reflected many times at the surface and bottom, and is diffused over time. The passive-phase conjugate processing is then given to the received diffusing signal to reduce this diffusion. The pulse is compressed by this processing, thereby obtaining the target position.

Point-to-point underwater acoustic communications using spread-spectrum passive phase conjugation

The ocean is often a complex multipath channel and progress has been made in developing equalization algorithms to overcome this. Unfortunately, many of these algorithms are computationally demanding and not as power-efficient as one would like; in many applications it may be better to trade bit rate for longer operational life. In 2000 the U.S. Navy was developing an underwater wireless acoustic network called Seaweb, for which a number of modulation schemes were being tested in a series of SignalEx experiments. This paper discusses two modulation schemes and associated receiver algorithms that were developed and tested for Seaweb applications. These receiver designs take advantage of time reversal (phase conjugation) and properties of spread spectrum sequences known as Gold sequences. Furthermore, they are much less complex than receivers using adaptive equalizers. This paper will present results of testing these signaling and receiver concepts during two experiments at sea.

Correlation-Based Decision-Feedback Equalizer for Underwater Acoustic Communications

The purpose of this paper is to develop a decision-feedback equalizer (DFE) using a fixed set of parameters applicable to most shallow oceans with minimal user supervision (i.e., a turn key system). This work is motivated by the superior performance [bit error rate (BER)] of the multichannel DFE compared with other methods, such as passive-phase conjugation (PPC), at the same time noting its sensitivity to different acoustic environments. The approach is to couple PPC, utilizing its adaptability to different environments, with a single-channel DFE. This coupling forms an optimal processor for acoustic communications in theory, but it has never been implemented in practice. By coupling with DFE, the method achieves the same spatial diversity as conventional multichannel DFE, without requiring a large number of receivers as does PPC. The correlation-based DFE in terms of the autocorrelation functions of the channel impulse responses summed over the receiver channels (the Q function) is derived. This paper shows in terms of waveguide physics, further supported by real data, the many desirable features of the Q function that suggest, given adequate sampling of the water column, a general applicability of the correlation-based equalizer to different environments, irrespective of the sound speed profiles, bottom properties, and source-receiver ranges/depths. This property can be expected to hold approximately for a small number of receivers with spatial diversity. This paper demonstrates the robustness of the new equalizer with moving source data despite the range change (which modifies the impulse response) and symbol phase change due to time-varying Doppler

Examination of bit error characteristics due to a drifting source in passive phase conjugation underwater acoustic communications

Recent experimental work in acoustic communications using passive phase conjugation has shown that the demodulation error depends on the relative drift rate between the transmitter and receiver [Rouseff et al., IEEE J. Ocean Eng. 26, 821–831 (2001)]. The observed effect involves the mismatch between the initial impulse response and the subsequent response after the source or receiver has changed locations. The result can also be interpreted in terms of the spot size of the retro-focused field. In the present work, the effect of relative drift between source and receiver is studied by numerical simulations. The communications bit error rate is quantified as a function of drift rate, carrier frequency, array geometry and source-receiver range. [This work was supported by Pukyong National University Research Abroad Fund in 2003.]