Large Delay Spread Research Articles

Sparse channel estimation for underwater acoustic OFDM systems with super-nested pilot design

Underwater acoustic channels are usually sparse and have large delay spread. In this paper, super-nested array structure in the field of array signal processing is borrowed to be the pilot design of underwater acoustic OFDM systems, in order to better estimate large delay spread channels with limited number of pilots. Specifically, by constructing the pilot subcarriers’ covariance matrix and the pilot position difference, the virtual pilot on the differential co-array are employed for sparse channel estimation. In order to reduce the error between the estimated pilot subcarriers’ covariance matrix and the ideal covariance matrix, the cross-correlation matrix of pilot subcarriers is estimated in advance for interference cancellation. Then the sparse iterative covariance estimation algorithm (SPICE) is adopted to further refine the covariance matrix and improve the channel estimation performance. Simulation, pool and sea experimental results show that the proposed method can effectively estimate the large delay spread sparse channels and improve the performance of underwater acoustic OFDM systems.

OTFS Channel Estimation and Detection for Channels with Very Large Delay Spread

OTFS Channel Estimation and Detection for Channels with Very Large Delay Spread

Spectral-Correlation Based Spectrum Sensing Under Large Delay Spread Channels

Correlation based detectors are very popular for spectrum sensing. In these detectors, temporal correlation is often employed to discriminate the signals of primary users from noises. However, large channel delay spread can significantly decrease the correlation of neighbouring samples of received signals, inducing severe performance degradation of temporal correlation based detectors. In this work, we show that the power spectral densities of different delayed versions of a primary signal have high correlation, despite unknown delays. Then, by exploiting the correlation of power spectral densities of received signals, we propose a spectral-correlation detector. To facilitate the practical application of the proposed detector, we also derive a theoretical expression for its decision threshold to achieve a given false alarm probability. Furthermore, in order to handle the case of channels with unknown delay profile, we employ the OR rule to combine the temporal- and spectral-correlation detectors, leading to a temporal-spectral-correlation detector. An expression for its decision threshold is also derived. Extensive numerical simulations are provided to verify the theoretical analysis and demonstrate the superior performance of the two proposed detectors compared to state-of-the-art temporal correlation based detectors.

Cross Z-Complementary Sets for Training Design in Spatial Modulation

Spatial modulation (SM) is one special type of multiple-input multiple-output (MIMO) technique whose advantage is that only one radio-frequency (RF) chain is needed. Recently, the so-called cross Z-complementary pair (CZCP) was proposed as the SM training sequence. Optimal channel estimation performance over frequency-selective channels can be achieved since the CZCPs have the specific zero correlation zone (ZCZ) for auto-correlation and cross-correlation sums. However, the ZCZ width of the CZCP is theoretically upper bounded by half sequence length. In this paper, the CZCP is extended to the cross Z-complementary set (CZCS) to have larger ZCZ width which can be used in SM to combat larger delay spread. Several generic constructions of CZCSs with large ZCZ widths and flexible lengths are proposed in this paper. Even the perfect CZCS, whose ZCZ width has the maximum value, can be obtained. Numerical simulations demonstrate that the proposed CZCS-based training sequence can tolerate large delay spreads to improve the channel estimation performance in SM.

Underwater acoustic communication using orthogonal signal division multiplexing with windowing

Underwater acoustic (UWA) communication is an essential technology that supports underwater surveys by transmitting images and movies wirelessly. However, UWA communication is still challenging due to the large delay and Doppler spreads in UWA channels. Doppler-resilient orthogonal signal division multiplexing (D-OSDM) under water has been found effective in such channels, but Doppler modeling errors prevent further improvement in communication quality. In this paper, we clarify the effect of Doppler modeling errors on the communication quality of D-OSDM and propose the use of a window function in D-OSDM to address the issues that such errors raise. We also evaluate the communication quality of D-OSDM using the window function through simulations and experiments. The experimental results show a 56% increase in the number of error-free data blocks, indicating that windowing can improve communication quality.

Frequency reversal-based inter-carrier interference mitigation for underwater acoustic OFDM communications

Orthogonal-frequency-division-multiplexing (OFDM) modulation has been intensively studied in underwater acoustic (UWA) communications recently for its ability to tackle large delay spread channels. Nevertheless, the inter-carrier interference (ICI) caused by Doppler spread of UWA channels degrades the system performance significantly. This paper proposes the concept of frequency reversal (FR) dual to time reversal (TR), as well as a feasible FR scheme. FR can compress the frequency spread caused by Doppler, and hence eliminate the ICI in OFDM communications. Furthermore, FR has similar advantages as TR: 1) low computational complexity and 2) robustness against complicated and changeable channels. Numerical simulation verifies the effectiveness of the proposed FR scheme as an ICI mitigation technique for UWA OFDM communications.

A Real-Time Wideband Subband LMS Algorithm for Full-Duplex Communications

In-band full-duplex (IBFD) communications has the potential to nearly double the spectrum efficiency of existing 5G communications. Digital signal processing to mitigate self-interference is one way to realize this full-duplex capability. However, the aggregated wideband nature of 5G coupled with the potential for large delay spreads and short coherence times makes the realistic implementation of such an IBFD processor challenging. In this paper, we address the architectural considerations and describe the practical implementation of a real-time IBFD digital processor and its performance when operating on wideband data collected in a highly dynamic environment.

Mobile underwater acoustic communication with orthogonal signal division multiplexing under inter-carrier interference larger than a guardband

Underwater acoustic (UWA) communication is a key technology providing wireless communication, but using UWA channels is challenging due to the existence of large Doppler and delay spreads. UWA channels also have long but sparse reverberation tails. To address these issues simultaneously, we have proposed Doppler-resilient orthogonal signal division multiplexing using compressed sensing (D-OSDM-CS). The optimization of compressed sensing (CS) under a large Doppler spread remains difficult since noise induced by inter-carrier interference (ICI) is present in the received signal. Herein, we mathematically clarify the effect of ICI-induced noise on the received signal, and we propose an advanced D-OSDM-CS method that can measure ICI-induced noise and optimize the regularization parameter of CS. We also evaluate the performance of the advanced D-OSDM-CS in simulations and experiments. The obtained results suggest that the advanced D-OSDM-CS achieves better communication quality than the existing D-OSDM-CS under a large-Doppler-spread environment.

A CP Reduction Scheme Based on Symbol Repetition for Narrow-Band IoT Systems

In this article, we propose a novel waveform with low cyclic prefix (CP) overhead for narrow-band Internet-of-things (NB-IoT) systems. Compared with the classical orthogonal frequency-division multiplexing (OFDM), the proposed waveform employs the concept of symbol repetition on multiple successive OFDM symbols (SR-OFDMs) and only one CP is occupied by multiple SR-OFDM symbols; hence, the CP overhead is reduced significantly, improving the spectral and energy efficiencies in NB-IoT systems. It is proved that the proposed SR-OFDM can effectively fight against the multipath fading channels by a simple single-tap equalization, avoiding the mutual interference among symbols. In addition, it is shown that the proposed SR-OFDM and classical OFDM have the same size of the inverse discrete Fourier transform (IDFT) at the transmitter, which indicates the same subcarrier spacing. Especially, it is demonstrated that the signals of multiple symbols are separated at the receiver, and the demodulation of each symbol can be performed independently, which is helpful to reduce the complexity of symbol demodulation. To evaluate the proposed SR-OFDM, simulations have been done by considering the Stanford university interim (SUI) channels, which have large channel delay spread and exhibit high frequency selectivity.

Flexible OFDM Transceiver for Underwater Acoustic Channel: Modeling, Implementation and Parameter Tuning

Acoustic signals are a first choice in underwater communication since sound waves face very low attenuation in water compared to radio frequency (RF). However, the tendency of receiver to detect multipath signals (due to signals bouncing off surface or the bottom) induces large delay spreads and thus strong channel frequency selectivity. Additionally, rapid spatial and temporal variations in underwater acoustic (UWA) channel makes it hostile for communication systems based on single carrier modulation. Multicarrier modulation techniques, on the other hand, can greatly improve bandwidth utilization and help deal with time dispersal effects. Orthogonal frequency division multiplexing (OFDM) is a proven multicarrier communication system having capabilities to cope with frequency selectivity and delay spreads effectively. OFDM has started to get attention for being a simpler alternative to high complexity and high maintenance single carrier systems in UWA communication systems. This work proposes a Matlab model of an OFDM transceiver along with UWA channel characterization based on Rician shadowed fading model as it perfectly characterizes the way in which a shallow UWA channel behaves. Eventually, the proposed design allows implementation of various OFDM modulation methods and to perform Monte Carlo simulations for bit error rate comparisons together with the ability to tune multiple UWA channel parameters.

On Receive Window Design for UF-OFDM Signals Over Multipath Channels

We study the impact of receive windowing on the quality of signal reception for the multicarrier waveform “universal filtered orthogonal frequency division multiplexing” (UF-OFDM), which shows a great potential in asynchronous transmissions. Due to the absence of a cyclic prefix (CP), UF-OFDM signals are more vulnerable to multipath channels. We show that the receive window can be designed to suppress the intersymbol interference (ISI) and intercarrier interference (ICI) induced by multipath propagation. The SINR performance of UF-OFDM signals is evaluated, both analytically and by simulations, for various multipath channel models. About 3 dB SINR gain can be achieved by simply extending the length of the receive window for channels with relatively large root mean square delay spreads.

Cellular Terrestrial Broadcast—Physical Layer Evolution From 3GPP Release 9 to Release 16

We present the latest developments in the physical layer evolution for the transmission of Multimedia Broadcast Multicast Service (MBMS) over fifth generation (5G) cellular networks. We first provide an overview of how the MBMS physical layer has evolved during the initial releases of the Third Generation Partnership Project (3GPP) specifications and provide motivations and insights for key developments that have helped mature MBMS into a competitive broadcasting solution. We provide an overview of dedicated MBMS carriers for Multimedia Broadcast multicast service Single Frequency Network (MBSFN) transmission as well as Orthogonal Frequency Division Multiplexing (OFDM) numerologies with longer cyclic prefixes to support MBSFN transmission over large geographical areas with large distances between cooperating cell sites. Next, we describe the most recent enhancements to the MBMS physical layer, addressing advanced use cases in the recently completed release of the 3GPP specifications (Release 16). First, we describe the physical layer design for enabling MBSFN-based MBMS services for rooftop receivers with inter-site distances between cooperating cell sites as large as 125 km. In this realm, we demonstrate the need for an OFDM numerology with a 300-microsecond cyclic prefix to communicate effectively over channels with very large delay spreads. Next, we explain how support for high mobility up to 250 kmph was added to the standards by using an OFDM numerology that gracefully trades off inter-symbol interference due to large delay spreads and inter-carrier interference due to large doppler spreads to achieve better performance than existing numerologies at high speeds. Finally, we describe the enhancements that were made to the Cell Acquisition Subframe (CAS)—specifically, the introduction of larger aggregation levels for the Physical Downlink Control Channel (PDCCH) and the support of Physical Broadcast Channel (PBCH) repetitions—that significantly increase the coverage for the control and system information associated with MBMS that are carried by these subframes.

Improvement of communication quality using compressed sensing in underwater acoustic communication system with orthogonal signal division multiplexing

We propose the use of compressed sensing for an underwater acoustic (UWA) communication system with orthogonal signal division multiplexing (OSDM) to achieve reliable communication in UWA channels with large delay and Doppler spreads. However, OSDM does not fully exploit the characteristics of UWA channels, which have long but sparse reverberation tails. Hence, we propose an OSDM channel estimator using compressed sensing and evaluate its performance in simulations and experiments. The simulations revealed that the optimal gain determining the performance of the channel estimator depends on standard deviation of noise σ, and OSDM using compressed sensing outperforms normal OSDM. We also evaluate OSDM performance using compressed sensing in field testing in the coastal area of Suruga Bay, Japan. The experimental results suggest that compressed sensing can boost OSDM performance; the number of OSDM error blocks using compressed sensing is half that in normal OSDM. Thus, OSDM using compressed sensing provides reliable communication.

Underwater Acoustic Communication Using OFDM Technique in Simulink Environment

The paper contains modeling and simulation of Orthogonal Frequency Division Multiplexing (OFDM) technique for under water acoustic communication. Speed of sound inside water is 1500 m/s which causes large delay spread due to multipath propagation and also fading effect can be observed which gives birth to interference. Orthogonal frequency division multiplexing reduces long delay spread due to low data rate of subcarriers and hence Inter Symbol Interference (ISI) can be eliminated. Also the orthogonality of subcarriers reduces Inter Carrier Interference (ICI). The objective of the paper is to improve the performance of OFDM in underwater environment by the use of interleaver. Also the effect of coding is observed. The model was simulated in MATLAB Simulink and the results are presented.

A Frequency Offset Transmit Reference System in Dense Multipath Environments: Propagation Effects and Design Considerations

Frequency offset transmit reference (FoTR)—a noncoherent spread spectrum technique—is considered in dense multipath fading environments, in the context of wideband communication. The interplay between system parameters and propagation effects is investigated. Analytical relations between key design parameters and the channel delay spread are derived, which provide a framework for determining reasonable system parameters that optimize the performance. An approximated closed-form expression for the outage probability is also obtained, which is shown to be fairly accurate for low outage probabilities. It is shown that FoTR suffers significantly from noise-enhancement, but, is nonetheless, robust against frequency-selective fading. A large value of frequency offset can significantly deteriorate the performance, particularly in environments with large channel delay spreads. This restricts the maximum data rate as well as the multiple-access capability of the scheme. Despite the limitations, FoTR can be a useful communication scheme for low data rate sensor networks deployed in dense multipath environments, particularly where the design demands a simple receiver and a low outage probability.

Blind Fractionally Spaced Channel Equalization for Shallow Water PPM Digital Communications Links.

Underwater acoustic digital communications suffer from inter-symbol interference deriving from signal distortions caused by the channel propagation. Facing such kind of impairment becomes particularly challenging when dealing with shallow water scenarios characterized by short channel coherence time and large delay spread caused by time-varying multipath effects. Channel equalization operated on the received signal represents a crucial issue in order to mitigate the effect of inter-symbol interference and improve the link reliability. In this direction, this contribution presents a preliminary performance analysis of acoustic digital links adopting pulse position modulation in severe multipath scenarios. First, we show how the spectral redundancy offered by pulse position modulated signals can be fruitfully exploited when using fractional sampling at the receiver side, which is an interesting approach rarely addressed by the current literature. In this context, a novel blind equalization scheme is devised. Specifically, the equalizer is blindly designed according to a suitably modified Bussgang scheme in which the zero-memory nonlinearity is replaced by a M-memory nonlinearity, M being the pulse position modulation order. Numerical results not only confirm the feasibility of the technique described here, but also assess the quality of its performance. An extension to a very interesting complex case is also provided.

Propagation Path Loss Modeling and Outdoor Coverage Measurements Review in Millimeter Wave Bands for 5G Cellular Communications

The global bandwidth inadequacy facing wireless carriers has motivated the exploration of the underutilized millimeter wave (mm-wave) frequency spectrum for future broadband cellular communication networks, and mmWave band is one of the promising candidates due to wide spectrum. This paper presents propagation path loss and outdoor coverage and link budget measurements for frequencies above 6 GHz (mm-wave bands) using directional horn antennas at the transmitter and omnidirectional antennas at the receiver. This work presents measurements showing the propagation time delay spread and path loss as a function of separation distance for different frequencies and antenna pointing angles for many types of real-world environments. The data presented here show that at 28 GHz, 38 GHz and 60 GHz, unobstructed Line of Site (LOS) channels obey free space propagation path loss while non-LOS (NLOS) channels have large multipath delay spreads and can utilize many different pointing angles to provide propagation links. At 60 GHz, there is more path loss and smaller delay spreads. Power delay profiles PDPs were measured at every individual pointing angle for each TX and RX location, and integrating each of the PDPs to obtain received power as a function of pointing angle. The result shows that the mean RMS delay spread varies between 7.2 ns and 74.4 ns for 60 GHz and 28 GHz respectively in NLOS scenario.

Improved Eigenfilter Design Method for Channel Shortening Equalizer in TH-UWB

The large delay spread of ultra-wideband (UWB) channels causes intersymbol interference and therefore increases the complexity of a UWB Rake receiver. To resolve these problems, we present a new method for the design of channel shortening equalizers (CSE) in time-hopping UWB systems. Our proposed system is based on an eigenfilter method using a new objective function. In contrast to existing methods based essentially on the optimization of the CSE following one single criterion, our design allows using complementary criteria. The proposed system aims at minimizing most of the energy of the shortened channel impulse response outside the desired delay-spread window and the spectral distortion related to the filtering function. Furthermore, it simultaneously aims at reducing the multiple access interference and noise power. Indeed, the proposed CSE reduces the number of effective channel taps before the correlation receiver, which renders its architecture less complex.

Underwater acoustic communication using Doppler-resilient orthogonal signal division multiplexing in a harbor environment

Underwater acoustic (UWA) channels are one of the historical mobile ultrawideband channels characterized by large delay and Doppler spreads, but reliable UWA communication remains challenging. Here we performed an initial demonstration of the Doppler-resilient orthogonal signal division multiplexing (D-OSDM) technique in an actual sea environment. D-OSDM spreads data symbols in both time and frequency with guardbands to exploit the time and frequency diversity of UWA channels. The experiment was performed in a challenging scenario: the transmitter was fixed on a floating pier, and the receiver was mounted on a moving remote-controlled boat. The harbor UWA channel had a delay spread of 50 ms and a Doppler spread of up to 4.5 Hz, in the presence of additive Gaussian noise, and the combination of two Rayleigh fading models (a two-path model without Doppler spread and a multi-path model with Doppler spread) was able to successfully model the actual environment. Our results also confirmed that a UWA communication link using D-OSDM will deliver excellent reliability even for a harbor UWA channel with a mobile receiver; D-OSDM achieves better communication quality compared to other communication schemes in both simulations and experiments.

Underwater sound channel in the northeastern East China Sea

In May 2015 a shallow-water acoustic variability experiment (SAVEX15) was conducted in the northeastern East China Sea. Surprisingly, an underwater sound channel that is typical for deep water was discovered in this shallow water waveguide (∼100 m deep) with the channel axis at around 40 m. For a broad-band source (0.5–2.0 kHz) deployed close to the channel axis, channel impulse responses observed by a vertical array exhibited a complex arrival structure with a large delay spread (e.g., 160 milliseconds). Most arrivals are found to be surface and bottom reflected, while a pair of high-intensity refracted arrivals are embedded in the early reflected arrivals. Broadband simulations based on a normal mode propagation model show good agreement with data.