Multichannel Decision Feedback Equalizer Research Articles

A sparse adaptive decision feedback equalization approach for the high data rate vertical underwater acoustic communication in deep sea

In underwater acoustic communication (UWAC) environments, multichannel decision feedback equalizer (MDFE) techniques combined with single carrier modulation schemes are widely employed to suppress multipath interference while adapting to time-varying channel response. While MDFE is a powerful technique, the estimation of the numerous filter taps is generally complex problem, and the numerically complexities can lead to the instability of solutions including the occurrence of the burst errors. In this study, we propose a method that introduces L1-norm regularization into the adaptive algorithm of MDFE, considering a sparse channel response environment in the vertical UWAC channel in deep-sea areas. We applied the proposed method to simulation and experimental data, comparing it with conventional methods. As a result, the proposed method demonstrated a clear improvement in bit error rate performance, obtaining sparse filter coefficient estimates and effectively suppressing burst errors.

90-m/560-Mbps underwater wireless optical communication utilizing subband multiple-mode full permutation CAP combined with an SNR-weighted detector and multi-channel DFE.

In this paper, a joint signal processing scheme including a subband multiple-mode full permutation carrierless amplitude phase modulation (SMMP-CAP), signal-to-noise ratio weighted detector (SNR-WD), and multi-channel decision feedback equalizer (MC-DFE) is proposed to mitigate the bandwidth limitation of a high-speed long-reach underwater wireless optical communication (UWOC) system. Referring to the trellis coded modulation (TCM) subset division strategy, 16 quadrature amplitude modulation (QAM) mapping set is divided into four 4-QAM mapping subsets by SMMP-CAP scheme. An SNR-WD and an MC-DFE are employed to enhance the demodulation effect of this system in a fading channel. In a laboratory experiment, the minimal required received optical powers (ROPs) for data rates of 480 Mbps, 600 Mbps, and 720 Mbps, at hard-decision forward error correction (HD-FEC) threshold of 3.80 × 10-3, are -32.7 dBm, -31.3 dBm, and -25.5 dBm, respectively. Moreover, the proposed system successfully achieves a data rate of 560 Mbps in a swimming pool with a transmission distance up to 90 m and a total attenuation measured to be 54.64 dB. To the best of our knowledge, this is the first time to demonstrate a high-speed, long-distance UWOC system by employing an SMMP-CAP scheme.

Lake Experimentation of in-Band Full-Duplex Underwater Acoustic Communications With a Receiving Array

In-band full-duplex (IBFD) communications is a promising technique to achieve spectral-efficient communication in the underwater environment. To achieve IBFD underwater communications, suppression of self-interference (SI) is a prerequisite. In this paper, SI suppression using physical separation and digital SI cancellation is investigated under different transmitter-receiver geometries. An iterative IBFD receiver combining joint channel estimation, SI cancellation, and multichannel decision feedback equalizer is developed. Due to the potential deterioration of the channel estimation accuracy, a digital SI cancellation control mechanism is used to regulate the suppression process. The feasibility of coherent IBFD underwater acoustic communications was tested in a lake environment. The two SI suppression methods, the SI cancellation control mechanism, and the receiver performance were evaluated using the experimental measurements. A bit error rate (BER) in the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-2}$ </tex-math></inline-formula> was achieved at the range of 80 m for IBFD communications.

Lake experimentation of in-band full-duplex underwater acoustic communications

In-band full-duplex (IBFD) communications is a promising technique to achieve spectral-efficient communication in the underwater environment. Characteristics of self-interference and methods to suppress the self-interference are crucial to attaining IBFD acoustic communications. In this effort, we analyze the characteristic of the self-interference for the underwater acoustic channel through experimental measurements. Self-interference reduction using physical separation and digital cancellation is investigated under different transmitter-receiver geometries. An iterative IBFD receiver combining joint channel estimation, SI cancellation, and multichannel decision feedback equalizer is developed. The feasibility of coherent IBFD underwater acoustic communications was tested in a lake environment. The self-interference suppression methods and the receiver performance were evaluated using the experimental measurements. IBFD communications was demonstrated at 80 m. With 10 dB local self-interference source level reduction, IBFD communications was extended to the 170 m range. [Work supported by the National Science Foundation.]

Performance Analysis of Underwater Acoustic Communications in Barrow Strait

The Arctic Ocean and its various shallow passages are rapidly changing due to global warming. The development of a large-scale wireless network that collects and distributes oceanographic data remotely would be an extremely valuable asset to researchers. This concept, however, is completely dependent upon the ability to acoustically transmit digital information under water over long ranges, which is not currently available. In this work, we investigate the performance of four transceivers that are based on frequency-hopping binary frequency-shift keying and multilevel phase-shift keying (M-PSK) signaling in the 300 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 400-Hz acoustic band. The transceivers use low-rate coding that provides bit rates from 1.8 to 13.3 b/s (bits per second). Their bit error rates are computed based on real data recorded in Barrow Strait (NU). During the 2019 experimental activities, the transmitter was towed by a supporting vessel at an average speed of 4 kn. The receiver, a vertical hydrophone array, was at distance between 14 and 33 km. Additionally, the received signals experienced extended multipath propagation and strong in-band impulsive interferences. For a single-hydrophone receiver, our postprocessing analysis shows reliable bit rates up to 6.7 b/s. When processing uses the array and performs multichannel decision feedback equalization (DFE), the maximum designed rate, i.e., 13.3 b/s, is confirmed. Further analysis reveals that the experimental links could support higher data rates. Our results demonstrate that 2-PSK signaling combined with single-channel DFE and 4-PSK signaling combined with multichannel DFE could reliably achieve 100 and 200 b/s, respectively.

Two Dimensional Adaptive Multichannel Decision Feedback Equalization for OTFS System

Orthogonal time-frequency-space (OTFS) modulation has shown significant error performance advantages over orthogonal frequency division multiplexing (OFDM) in time-varying channels. However, the symbols in OTFS frames still suffer from inter-Doppler interference (IDI) and inter-symbol interference (ISI). In this letter, we propose a two dimensional (2D) adaptive multichannel decision feedback equalizer (DFE) to combat the interference. The proposed receiver constructs the input signal and feedback signal of the DFE based on the characteristic of OTFS modulation. Meanwhile, by exploiting the sparsity feature of the channels in delay-Doppler domain, the iterative improved proportionate normalized least mean squares (IPNLMS) algorithm is employed to achieve fast convergence and fast tracking. The simulation results show that the proposed equalizer could achieve satisfied BER performance with lower complexity.

The relationship of time-reversal and multi-channel decision feedback equalization in underwater acoustic communication

The relationship between the performance of the passive time-reversal (PTR) communication technique and multi-channel decision feedback equalization for moving-source single-input/multiple-output (SIMO) communication problems in the multipath-rich shallow sea environment is investigated. Both methods are applied to a synthetic dataset of an underwater acoustic moving-source model, generated by the normal mode method, to evaluate utilization of the wavefield energy as an adaptive diversity-combining method. The results show that PTR followed by decision feedback equalization (PTR-DFE) is the more robust technique in a moving-source environment with moderate source speed. In particular, PTR-DFE can utilize most of the energy in moderate-speed conditions. It is also discussed how the performance of each method can be limited in the moving-source SIMO problem. The results indicate that the performance of PTR-DFE can be limited by the convergence property and filter tap length limitations of the DFE in problems with rapidly moving sources.

Underwater acoustic communication to an unmanned underwater vehicle with a compact vector sensor array

Although underwater acoustic (UWA) communication based on vector sensors has been intensively investigated in past years, most of these works focus on theoretical analysis and simulation, and only very limited research on the practical application of vector sensors to underwater platforms (i.e., UUVs) is available. Recently, a data transmission experiment between a surface vessel and a mobile UUV was conducted in Qiandao Lake, Zhejiang Province, China. During this experiment, a compact vector sensor array with four elements was installed on the UUV. This paper proposes a two-stage data processing scheme for the experimental data. The fractional Fourier transform (FrFT) is first used to estimate the delay-Doppler parameter of UWA channels. The delay estimation can help determine the number of equalizer taps, while the Doppler estimation can help determine the resampling frequency in the passband. Then, a new receiver called the vector multichannel decision feedback equalizer (VM-DFE) is proposed to deal with the inter-symbol interference (ISI) introduced by UWA channels. The experimental results show that the proposed two-stage data processing scheme realizes satisfactory performance and achieves a 1.8 dB gain compared with a pressure sensor-based equalizer.

Cross-ice acoustic communication by ice-mounted geophones: An initial experimental demonstration

Recently, cross-ice acoustic communication experiment was conducted in shallow water with the water column covered by fifty centimeters thick ice. During this experiment, single carrier communication signals with a center frequency 750 Hz and bandwidth 500 Hz were transmitted from a flextensional transducer to a receiving array with five ice-mounted geophones uniformly spaced by 0.5 m. This paper firstly analyzes the estimated cross-ice acoustic channels in terms of amplitude and phase characteristic, temporal coherence and spatial coherence. The communication performance is then presented based on multi-channel decision feedback equalizer (M-DFE). According to the experimental results, it can be concluded that it is feasible to realize cross-ice data transmissions using ice-mounted geophones at short ranges.

Experimental demonstration of phase-coherent underwater acoustic communications using a compact array

High-rate phase coherent acoustic communication systems generally require large arrays to obtain spatial diversity. Large arrays may not be suitable for small underwater platforms such as underwater unmanned vehicles (UUVs). In this paper, acoustic communication utilizing a compact array with closely spaced elements was demonstrated using the data collected during the August 2010 lake experiment. In addition, a bidirectional multichannel decision feedback equalizer (DFE) was developed to enhance the performance of communication systems. The recorded data from three communication ranges of 25 m, 920 m, and 1 300 m were processed to verify the system performance. The results showed an improvement in output symbol signal-to-noise ratio (OSNR) of approximately 3.4 dB, and 4.9 dB for bidirectional two-channel DFE and four-channel DFE over a single-channel DFE, respectively.

Multichannel equalization method based on vector time-reversal and decision feedback equalizer technology

A kind of underwater communication receiver combined with Vector Time-Reversal algorithm and multichannel decision feedback equalizer (VTR-MDFE) is studied, which can address the problem of inter-symbol interference and waveform distortion, resulting from time-varying multipath channel. Impacts of environmental noise can be suppressed by use of vector sensor’s combined directivity and Corresponding vector signal processing technology. Meanwhile, error rate is reduced while system capacity is enhanced by time-reversal algorithm’s anti-multipath ability. The performance of VTR-MDFE are proved by simulated results in the communication channel provided by a ray-based acoustic model, for different ocean conditions and source-receiver geometries.

Experimental results of adaptive multichannel decision feedback equalization in shallow channel

A kind of underwater communication receiver combined with mean-squared error adaptive error adaptive algorithm and multichannel decision feedback equalization (AMDFE) is studied, which can address the problem of underwater communications over a time-varying multipath channel in the presence of worse intersymbol interference and waveform distortion. Its performances in different range and sound profile have been analyzed by simulated shallow water channels in the earlier paper. In this paper, the performances of AMDFE in different underwater channels with different parameters are mainly demonstrated by the experiments on the lake and sea. The anti-multipath ability and parameters selection regulation of AMDFE in both near and long range are proved by real data in shallow water.

Performance of adaptive multichannel decision-feedback equalization in the simulated underwater acoustic channel

Adaptive multichannel decision feedback equalization [M. Stojanovic, J. Catipovic, and J. G. Proakis, J. Acoust. Soc. Am. 94, 1621–1631 (1993)] is widely adopted to address the severe inter-symbol interference encountered in the underwater acoustic communication channel. In this presentation, its performance will be reported in the simulated communication channel provided by a ray-based acoustic model, for different ocean conditions and source-receiver geometries. The ray model uses the Rayleigh parameter to prescribe the sea surface effects on the acoustic signal. It also supports different types of sediment. The ray model output has been compared with the experimental data and shows comparable results in transmission loss. We will also compare against the performance of multichannel decision feedback equalization supported by existing ray models, for example, BELLHOP.

Adaptive multichannel decision feedback equalization using subarray processing

The adaptive multichannel Decision Feedback Equalizer (DFE) has been shown to be an effective algorithm for enabling reliable high rate acoustic communications in complex and time-varying underwater environments. The choice of the number of channels used in an equalizer presents a performance trade-off. The minimal achievable error that can be realized by the equalizer decreases as the number of channels increases. However, an increase in the number of channels increases the number of filter weights that need to be adapted. Thus, the computational complexity of least squares and Kalman type adaptation algorithms is proportional to the number of channels squared. In addition, the averaging interval required by an adaptation algorithm in order to achieve good performance grows linearly with the number of parameters. Thus, an increase in the number of M-DFE channels can reduce the rate of channel fluctuation that can successfully tracked by the M-DFE. The partitioning of an array into sub-arrays which are each independently equalized before combining their outputs can both improve performance and reduce complexity in processing real-world signals. The choice of the size and sensor locations for the subarrays is analyzed. The resulting adaptive subarray multichannel DFE algorithm is compared to other multichannel equalization algorithms.

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.

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.

Subspace dimension estimation for equalization of underwater acoustic channels.

The use of an array of receiving elements has several advantages such as increased signal to noise ratio due to array gain and the ability form beams, which reject interference and enhance a source. One common way to integrate an array receiver into a communication system is through the use of a multichannel decision feedback equalizer (DFE), which adaptively combines the signals from the receive elements and mitigates channel effects. However, the computational complexity of a multichannel DFE is too high for implementation into real‐time systems. One solution is to use a beamformer to reduce the number of channels the DFE needs to combine and thus reducing the overall complexity. While there has been some research showing the optimal number of beams if the channel is known, it is still an open question how to determine this information from a received signal. This research examines approaches for determining the number of beams to use so that the performance of a reduced complexity beamformer‐DFE system is at least as good as a system, which uses multichannel DFE with all of the sensors as inputs. Results will be presented from experimental data comparing the performance of the systems in different ocean environments.

Coherent acoustic communication in a tidal estuary with busy shipping traffic

High-rate acoustic communication experiments were conducted in a dynamic estuarine environment. Two current profilers deployed in a shipping lane were interfaced with acoustic modems, which modulated and transmitted the sensor readings every 200 s over a period of four days. QPSK modulation was employed at a raw data rate of 8 kbits on a 12-kHz carrier. Two 16-element hydrophone arrays, one horizontal and one vertical, were deployed near the shore. A multichannel decision-feedback equalizer was used to demodulate the modem signals received on both arrays. Long-term statistical analysis reveals the effects of the tidal cycle, subsea unit location, attenuation by the wake of passing vessels, and high levels of ship-generated noise on the fidelity of the communication links. The use of receiver arrays enables vast improvement in the overall reliability of data delivery compared with a single-receiver system, with performance depending strongly on array orientation. The vertical array offers the best performance overall, although the horizontal array proves more robust against shipping noise. Spatial coherence estimates, variation of array aperture, and inspection of array angular responses point to adaptive beamforming and coherent combining as the chief mechanisms of array gain.

Adaptive multichannel decision feedback equalization for volterra type nonlinear communication channels

A model-based approach for the decision feedback equalization of Volterra type nonlinear communication channels is proposed such that the linear model-based decision feedback equalization can be considered as a special case of the proposed approach. In designing the decision feedback equalizer, the nonlinear decision feedback equalization problem is visualized as a linear, multichannel equalization problem. A complete modified Gram–Schmidt orthogonalization of the input vector is achieved by using modified sequential processing multichannel lattice stages. The elements of the multichannel desired signal vector are then estimated from the new orthogonal set by using only scalar operations. The probability of error performance of the proposed equalizer is improved by the estimation of the elements of the desired signal vector through a sigmoid activation function so that a polynomial perceptron equalizer is realized. The comparative computational complexity calculations and performance results of the proposed decision feedback equalizer are also provided.