Multipath Diversity Research Articles

High-performance deep-sea long-range underwater acoustic communication: Deconvolved conventional beamforming based approach

To address the challenges posed by large propagation loss and severe multipath delay spread, while aiming to improve communication rates in long-range underwater acoustic (UWA) communication, this paper introduces a novel approach: multi-beam (MB) Orthogonal Frequency Division Multiplexing (OFDM) based on deconvolved conventional beamforming (dCv). The proposed method establishes a signal processing framework for UWA channel receivers, wherein received signals corresponding to each angle beam are isolated and concentrated using the dCv technique. Subsequently, the separated signals from each angle undergo maximal-ratio combining (MRC) channel equalization followed by demodulation. Compared to conventional single-beam (SB) processing methods, the proposed approach capitalizes on multipath diversity gain achieved by combining MB outputs originating from various arrival angles. Moreover, employing dCv processing demonstrates superior performance compared to popularly employed beamforming techniques, especially when dealing with paths arriving from closely aligned angles. Additionally, the array beamforming utilized significantly enhances the signal-to-noise ratio (SNR) of each beam output, mitigating the elevated error rates in channel estimation associated with combining low SNR signals received from individual elements. Simulation results utilizing BELLHOP and experimental data from the South China Sea showcase notably improved bit error rate (BER) performance for the proposed method compared to SB equalization, MB-MRC equalization based on conventional beamforming (CBF), minimum variance distortionless response (MVDR), and worst-case performance optimization (WCPO), as well as standard MRC equalization techniques. The proposed receiver achieves error-free decoding results at a communication distance of 80 km with a data rate of 247 bps.

Complementary Coded Identical Code Cyclic Shift Multiple Access Under Asynchronous Frequency-Selective Fading Channels

A complementary coded code-division multiple-access (CC-CDMA) system is built on the direct sequence (DS) spread spectrum technique, which can suppress multipath interferences and multiple-access interferences due to the ideal correlation property of complementary codes (CCs). However, the DS spread spectrum technique will cause poor bandwidth efficiency; the CC-CDMA system suffers from serious interferences due to the ideal correlation of CCs may not be reconstructed at the receiver after passing through asynchronous frequency selective fading channels. In this paper, we propose a complementary coded identical code cyclic shift multiple access (CC-ICCSMA) system under asynchronous frequency selective fading channels, which allows multiple data streams to transmit in parallel by sharing one spreading code for improving the bandwidth efficiency. Considering the impact of interferences, we propose an ICCS matched filterbank to tackle the interferences, which can decode the data under all paths and achieve a high multipath diversity gain. Simulation results show that the proposed CC-ICCSMA system can achieve higher bandwidth efficiency than that of traditional CC-CDMA and can outperform the traditional DS-CDMA schemes in terms of bit-error rates and transmission rates.

Context-Aware Mobile Edge Resource Allocation in OFDMA Downlink System

Advanced sensing, data analysis, and communication techniques have promoted the emergence and tremendous development of the Intelligent Industrial Internet of Things (Intelligent IIoT). Intelligent IIoT-enabled 5G communication networks improve overall efficiency and open up a new market opportunity and economic growth era. In particular, for the ultra-reliable and low-latency communication (URLLC) scenario, The system requires a lightweight computing algorithm to ensure transmission reliability while providing rapid radio resource allocation. A proactive downlink system framework, supported by the reinforcement learning-based online model-free algorithm, is proposed to meet the upcoming challenge. The proactive task data transmission problem is decomposed into three sub-problems. With the help of anticipatory mobility management and virtual cell, the system gains high reliability through multipath diversity. The anchor node forwards the task data to multiple access points and manages radio resource allocation among and under the access points. The simulation justifies that the proposed framework handles the URLLC scenario well.

Channel Estimation and Multipath Diversity Reception for RIS-Empowered Broadband Wireless Systems Based on Cyclic-Prefixed Single-Carrier Transmission

In this paper, a cyclic-prefixed single-carrier (CPSC) transmission scheme with phase shift keying (PSK) signaling is presented for broadband wireless communications systems empowered by a reconfigurable intelligent surface (RIS). In the proposed CPSC-RIS, the RIS is configured according to the transmitted PSK symbols such that different cyclically delayed versions of the incident signal are created by the RIS to achieve multipath diversity. A practical and efficient channel estimator is developed for CPSC-RIS and the mean square error of the channel estimation is expressed in closed-form. We analyze the bit error rate (BER) performance of CPSC-RIS over frequency-selective Nakagami- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> fading channels. An upper bound on the BER is derived by assuming maximum-likelihood detection. Furthermore, by applying the concept of index modulation (IM), we propose an extension of CPSC-RIS, termed CPSC-RIS-IM, which enhances the spectral efficiency. In addition to conventional constellation information of PSK symbols, CPSC-RIS-IM uses the full permutations of cyclic delays caused by the RIS to carry information. A sub-optimal receiver is designed for CPSC-RIS-IM to aim at low computational complexity. Our simulation results in terms of BER corroborate the performance analysis and the superiority of CPSC-RIS(-IM) over the conventional CPSC without an RIS and orthogonal frequency division multiplexing with an RIS.

Enhanced diversity scheme for orthogonal frequency division multiplexing systems over doubly selective fading channels

AbstractFrequency‐time selective fading degrades the performance of communication systems, but it also provides an opportunity to collect multipath diversity and Doppler diversity. In this paper, an oversampled grouped‐linear‐constellation‐precoding (GLCP) scheme is proposed for orthogonal frequency division multiplexing (OFDM) systems to extract the diversity inherent in doubly selective fading channels. In the proposed scheme, GLCP is employed to precode the information symbols of OFDM at the transmitter, and the precoded OFDM symbols are oversampled in the time domain at the receiver. A full‐groups maximum likelihood (ML) decoder and a successive group decoder to decode the oversampled OFDM symbols with diversity gains are developed. The successive group decoder combines the group ML decoding algorithm with a block‐decision‐feedback‐equalization (BDFE) algorithm. The proposed scheme can take advantage of both the GLCP scheme and the oversampling scheme to achieve multipath diversity gains and Doppler diversity gains in doubly selective fading channels. Simulation results show that the proposed scheme can significantly improve the performance of OFDM systems in the frequency selective fading channel and the doubly selective fading channel in terms of bit error ratio, indicating that the diversity gains can be greatly enhanced by the proposed scheme in doubly selective fading channels.

Low-Latency Service Chaining With Predefined NSH-Based Multipath Across Multiple Datacenters

Service Function Chaining (SFC) provides a method of forwarding traffic flows through one or more service functions (SFs). For service providers, chaining SFs across multiple datacenters to deliver end-to-end services not only provides better utilization of computing resources of datacenters, but also achieves scalability and fault tolerance. However, most telecommunication applications are sensitive to latency, which tends to degrade due to both virtualization and the long distances among datacenters. In this paper, we extend the network service header (NSH) protocol and propose a multipath chaining with the partially-ordered NSH (MCPON) mechanism to achieve low-latency, partially-ordered service function chaining. MCPON adopts a proactive multipath installation for commonly-used service function paths (SFP, a sequence of requisite SFs) to eliminate reactive path decision delays and to reduce end-to-end service latency. To increase multipath diversity for better load balancing, we modify the original NSH encapsulation design so that the multiple paths selected for an SFP are not limited to having the same execution orders of some non-order-constrained SFs. MCPON also utilizes an entry-saving forwarding table design which enables forwarding entries to be shared among different SFC requests. Our evaluations show that proactive k-path computation for an SFC of length l at a scale of n SFFs saves time complexity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm{ O}}(kl^{3}n^{3})$ </tex-math></inline-formula> , and multipath service chaining reduces latency by 33–68% compared to single-path service chaining in our simulation scenarios.

Time and Frequency Synchronization for OTFS

In this letter, we propose timing offset (TO) and carrier frequency offset (CFO) estimators for orthogonal time frequency space modulation (OTFS). The proposed estimators do not require any additional training overheads as they deploy the same pilot signal that is used for channel estimation. Hence, no additional training overhead is required for synchronization, thanks to the periodic properties of the pilot signal in the delay-time domain. Our proposed TO estimator finds the start of each OTFS block by searching for a periodic sequence in delay and time dimensions. The CFO is then estimated by finding the mean of the angles of a two-dimensional correlation function at the best timing instant. As a novel aspect of these estimators, the multipath diversity of the channel is exploited to achieve a high estimation accuracy. Finally, we show the efficacy of our proposed synchronization techniques through extensive simulations.

Blind beamforming-based strong interference suppression in underwater acoustic direct-sequence code-division multiple-access systems.

In an underwater acoustic direct-sequence code-division multiple-access (DS-CDMA) system, the performance of the conventional receiver is severely limited by the strong multiple-access interferences, which is due to the near-far effect. In this paper, a time-frequency-time with eigendecomposition-based blind beamformer (TFT-EBB) method is proposed for a distant user to suppress strong interference from a nearby user in DS-CDMA underwater acoustic communications. When the interference level is moderately higher than that of the desired signal, the TFT-EBB method can estimate the composite steering vector of all multipath components of the weak desired user signal via the eigendecomposition of the array covariance matrix at each frequency bin. Then the proposed energy detection-based RAKE receiver is cascaded to achieve multipath diversity, in which it requires less prior information compared with the conventional RAKE receiver. The proposed method is evaluated by using both simulated and field experimental data, which verifies its effectiveness.

Efficient Channel Equalization and Symbol Detection for MIMO OTFS Systems

The application of multiple-input multiple-output (MIMO) over orthogonal time frequency space (OTFS) modulation is envisioned to provide high-data-rate wireless transmission in high-mobility environments. However, in these communication scenarios, the multiple-dimensional interference, which can generate from space, delay and Doppler domains, challenges the channel equalization and symbol detection at the MIMO-OTFS receiver. To tackle this issue, we propose a time-space domain channel equalizer, relying on the mathematical least squares minimum residual algorithm, to remove the channel distortion on data symbols. The proposed channel equalizer adopts a recursion method to achieve symbol estimates, which can realize fast convergence by leveraging the sparsity of MIMO-OTFS channel matrix. Instead of directly remapping the equalized OTFS symbols into data bits, we develop an enhanced data detection (EDD) scheme to iteratively demodulate the superposed multi-antenna signal. The EDD can not only realize the linear-complexity interference cancellation, but also efficiently reap the spatial and multi-path diversities of MIMO-OTFS channel. The simulations show the proposed channel equalization and EDD algorithms enable the MIMO-OTFS receiver to robustly demodulate multi-stream 256-ary quadrature amplitude modulation symbols, under a maximum velocity of 550 km/h at 5.9 GHz carrier frequency.

Precoded IM-OFDM-SS for Underwater Acoustic Communication

The nature of the multipath channel and the peak-to-average power ratio (PAPR) are regarded as the main challenges restricting the design of a multicarrier reliable underwater acoustic (UWA) communication. This paper proposes a new scheme, precoded index modulation orthogonal frequency division modulation spread spectrum (IM-OFDM-SS), for UWA communication. The precoded IM-OFDM-SS is proposed to increase the transmission efficiency and exploit the spreading and multipath diversities and, at the same time, reduce the PAPR to achieve a reliable communication system. Two different precoders, discrete Hartley transform (DHT) and discrete cosine transform (DCT), are utilized in the proposed scheme and compared with the conventional IM-OFDM-SS scheme. Simulation and real experimental results demonstrate the outperformance of the proposed precoded IM-OFDM-SS in comparison to the conventional benchmarks in terms of PAPR and bit error rate (BER) performance.

X-Transform Time-Domain Synchronous IM-OFDM-SS for Underwater Acoustic Communication

Time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) is presented to enhance the spectral and energy efficiency over the conventional cyclic prefix OFDM and zero padding OFDM, but the interbloc interference (IBI) between the data block and adjacent pilot symbols deteriorates its performance. The IBI is considered as the main TDS-OFDM drawback in the harsh long-delay underwater acoustic (UWA) multipath channel. This article proposes a new scheme called X-transform time-domain synchronous index modulation (IM) OFDM to eliminate the TDS-OFDM IBI, explore the multipath diversity, and reduce the peak-to-average power ratio (PAPR). Particularly, X-transform, which merges the discrete Hartley transform and discrete Fourier transform in a very low-complexity unitary matrix, and IM techniques have been employed to remove the effects of IBI existing in underwater acoustic (UWA) TDS-OFDM. Moreover, compressive sensing has been adopted to guarantee a reliable UWA channel estimation based on a guard interval IBI-free region. The proposed scheme is evaluated over simulated and experimental UWA channels. The computer simulation and the real experiment show the outperformance of the proposed scheme in terms of bit error rate, computational complexity, PAPR, energy efficiency, and spectral efficiency over its conventional benchmarks.

Selection Combining-based Packet Combining Schemes for Multi-path Diversity System

In multi-path diversity system, the receiver receives several signals bearing the same information, through distinct and independent fading channels. The packets traversing through independent and distinct paths would undergo different degree of fading. Thus, even if the copy of the packet is distorted badly in one path, the copy received from other paths might be lesser distorted; so by combining the copies of the packet, the correct copy of the packet could be retrieved. In this paper, selection combining-based packet combining scheme for multi-path diversity system have been proposed. In the proposed scheme, a packet is transmitted to the receiver via $$m$$ different paths, where $$m \ge 2$$ . At the receiver, erroneously received copies of the packet are combined to recover the correct copy of the packet. The proposed selection combining-based packet combining scheme is analyzed and studied using simulation on MatLab. It was observed experimentally that proposed scheme has superior performance than that of conventional packet combining schemes. The superior performance of the proposed scheme with selection combining scheme can be attributed to diversity achieved using multi-path transmission.

Symbiotic Radio: Cognitive Backscattering Communications for Future Wireless Networks

The heterogenous wireless services and exponentially growing traffic call for novel spectrum- and energy-efficient wireless communication technologies. Recently, a new technique, called symbiotic radio (SR), is proposed to exploit the benefits and address the drawbacks of cognitive radio (CR) and ambient backscattering communications (AmBC), leading to mutualism spectrum sharing and highly reliable backscattering communications. In particular, the secondary transmitter (STx) in SR transmits messages to the secondary receiver (SRx) over the RF signals originating from the primary transmitter (PTx) based on cognitive backscattering communications, thus the secondary system shares not only the radio spectrum, but also the power, and infrastructure with the primary system. In return, the secondary transmission provides beneficial multipath diversity to the primary system, therefore the two systems form mutualism spectrum sharing. More importantly, joint decoding is exploited at SRx to achieve highly reliable backscattering communications. In this article, to exploit the full potential of SR, we provide a systematic view for SR and address three fundamental tasks in SR: (1) enhancing the backscattering link via active load; (2) achieving highly reliable communications through joint decoding; and (3) capturing PTx’s RF signals using reconfigurable intelligent surfaces. Emerging applications, design challenges and open research problems will also be discussed.

Reducing web latency with coding-based fast multi-path loss recovery

TCP latency is critical to the performance of Web services. However, packet loss greatly impairs the TCP performance due to its poor loss recovery mechanisms. Recent work FUSO addressed this problem by leveraging multi-path diversity for proactive loss recovery, i.e., using “good” paths to proactively retransmit the potentially lost packet on “bad” paths before they are retransmitted after duplicate ACKs or timeout. Nevertheless, since it has no clue about which packet is (or will be) lost, FUSO simply proactively retransmits the oldest unACKed packet whenever there is a chance for proactive loss recovery. Through analysis and comprehensive experiments, we show that although FUSO behaves well in data center networks, which it is originally designed for, in the Internet scenario, such simple proactive retransmission of the oldest unACKed packet is not accurate enough to recover the lost packets, which causes performance penalty. To address the problem, this paper presents CoFUSO, a Coding-Based Fast Multi-Path Loss Recovery. Different from FUSO, when there is a chance for proactive loss recovery, CoFUSO generates a coding packet that codes all (or multiple) unACKed packets together. As such, CoFUSO can always proactively retransmit the “right” lost packet, since the receiver side can decode the lost packet by combining the coding packet with other received packets. We implement CoFUSO in Linux kernel with $$\sim$$ 2K lines of code. Testbed and simulation results show that, under lossy condition, CoFUSO can greatly improve the average and 99th percentile flow completion time (FCT) by $$\sim$$ 12% and $$\sim$$ 59% in the testbed, and up to $$\sim$$ 16.9% and $$\sim$$ 54.5% in the simulation, respectively.

Coded OFDM-IM with iterative detection☆

Coded Orthogonal frequency division multiplexing with index modulation (OFDM-IM) has been proposed, where channel coding and soft decision are employed to exploit the multipath diversity for the information conveyed by the constellation symbols. However, the performance of coded OFDM-IM is dependent on the detection performance of subcarrier activation pattern (SAP) and the diversity order of the SAPs requires further investigation. In this paper, a lower-bound of LLR detector is derived to show that the maximum achievable diversity order is the minimum Hamming distance between the SAPs for the LLR detector. Based on the analyses, iterative detection is proposed to improve the performance of coded OFDM-IM, where information-assisted LLR detector is proposed to improve the detection performance of SAP by using the soft information of the constellation symbols. Simulation results are provided to corroborate the diversity analyses and show the advantage of the coded OFDM-IM with iterative detection.

Performance studies of MIMO based DCO-OFDM in underwater wireless optical communication systems

The research in underwater communication system is focused towards wireless networks in view of recent developments in wireless technology and increasing need for deep sea data mining. The underwater wireless optical communication (UWOC) have more advantages for short-range wireless links due to very high bandwidth and data rate than acoustic communication. Although UWOC offers high capacity links at low latencies, they suffer from limited communication range due to various distinctive characteristics of underwater channel. A major degrading effect associated with the underwater channel is turbulence-induced fading. To mitigate this fading and to extend the viable communication range, spatial multipath diversity techniques can be adopted over UWOC links. The proposed work evaluates the performance of a multicarrier modulation scheme, i.e., MIMO based DC biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) for underwater wireless optical channel. The simulation of MIMO-DC biased optical OFDM technique employing spatial diversity in underwater optical communication is a novel approach towards the determination of an optimal modulation technique for underwater optical wireless channels. The performance is evaluated based on the various parameters such as Signal to Interference plus Noise Ratio (SINR), Throughput and Bit Error Rate (BER). A simple experiment is conducted to demonstrate the BER performance assessment for different link distance of 0.45 m and 1.5 m with different transmitter and receiver configurations (SIMO and MIMO) for both clear water and coastal water using MIMO DCO OFDM technique.

Multipath Diversity for OFDM Based Visible Light Communication Systems Through Fractional Sampling

In this paper, we propose fractional sampling for optical orthogonal frequency division multiplexing (O-OFDM) in order to achieve diversity gain for indoor visible light communication systems. Fractional sampling (FS) is a diversity technique that generates different replicas of the received OFDM symbol by sampling the received signal higher than the baud rate. Indoor optical wireless channels exhibit frequency selectivity and FS increases the reliability of the transmission by exploiting the multipath diversity. Our numerical results show that for a realistic indoor channel model, proposed FS receiver provides significant performance improvement over conventional OFDM receiver.

THE M-ARY SECURITY DIFFERENTIAL CHAOTIC FREQUENCY SHIFT SYSTEM BASED ON THE MIMO MODEL

To improve the data security and efficiency of the multi-input multi-output (MIMO)-based differential chaotic shift keying (DCSK) system under the wireless multipath fading channel, a new non-coherent modulation solution, i.e., the M-ary security differential chaotic shift keying (MS-DCSK), is proposed in this study. The scrambling matrix and shift matrix are used to construct a scrambling matrix group in this system, enabling the scrambling matrix to enhance the security of chaotic signals while carrying the M-ary bit information. By doing so, the security, energy efficiency, and spectral efficiency of the system are improved. In addition, by taking advantage of the quasi-orthogonal characteristics of chaotic sequences, the system receiver can effectively utilize the multipath diversity and space diversity of the multi-antenna transmitter and receiver system, without requiring the channel state information (CSI).

Space-time diversity signal processing employed by W-CDMA systems

Transmit diversity under transmission of signals from the base station to mobile station using the code-division multiple-access (CDMA) system allows us obtaining the performance gains close to the mobile station receiver diversity without complexity of the mobile station receiver antenna array. The multipath diversity can be achieved using the pre-RAKE precoding at the transmitter that could be employed and there is no need to install the RAKE receiver at the mobile station. We investigate several transmitter diversity techniques employed by wideband CDMA (W-CDMA) systems. We also study a possibility to combine transmitter diversity and precoding that achieves the gain of maximum ratio combining of all space and frequency diversity branches jointly with long-term prediction algorithm. It was demonstrated that the performance of the space-time pre-RAKE method approaches the performance of the maximum ratio combining for all space and frequency diversity branches. It is shown that all closed loop methods depend on the long range prediction to approximate the ideal performance in the fast fading environment.

Implementation of Seven Finger RAKE Receiver using MRC Technique

The radio receiver that counters the effects of the multipath fading is known as a rake receiver. Rake receiver used to combine the multipath signals by number of ‘sub receivers’ known as ‘fingers’. Rake receiver uses a multipath diversity principle; it rakes the energy from the multipath propagated signal components. This paper gives the review on implementation of MRC (Maximal Ratio Combining) for the seven fingers rake receiver. It gives the same information to receiver as transmitter, it eliminates the multipath fading and the time offset estimation to seven fingers of rake receiver. In the simulation results we observed that both the signals that is input and output signal resembles the same with the help of MRC.