In this paper, we propose a novel downlink multiple access system with a multi-antenna transmitter and two single-antenna receivers, inspired by the underlying principles of hierarchical quadrature amplitude modulation (H-QAM) based multiple access (QAMA) and space-division multiple access (SDMA). In the proposed scheme, coded bits from two users are split and assigned to one shared symbol and two private symbols carried by different beams. Based on joint symbol mapping of H-QAM constellations and phase-aligned precoding at the transmitter, each receiver observes a different H-QAM constellation with Gray mapping, a unique parallax feature not shared by existing schemes. In addition to avoiding successive interference cancellation (SIC), each user independently demodulates its own bits on separate I and Q branches with calculations based on closed-form expressions. Hence the receiver complexity is on par with that of orthogonal multiple access (OMA), which is much lower than that in other competing alternatives such as non-orthogonal multiple access (NOMA) and rate-splitting multiple access (RSMA). We carry out system optimization and determine the achievable rate region. Numerical results show that the proposed system achieves superior performance compared with benchmark schemes employing SIC-free receivers, and further demonstrates competitive results across various performance metrics even relative to benchmark schemes relying on SIC-based receivers. The proposed scheme is attractive to 6G and IoT applications where the increased system complexity is at the transmitter while the receivers maintain low complexity.

QAMA, a recently proposed multiple access technology, utilizes hierarchical quadrature amplitude modulation (QAM) to achieve the same rate region as conventional multi-user superposition transmission (MUST) without requiring successive interference cancellation (SIC) at receivers. This is particularly advantageous for low-cost Internet of Things (IoT) devices. The key process involves adjusting the constellation to generate transmission modes that enable SIC-free operation and span the desired rate region. However, determining the optimal constellation currently requires either an exhaustive search over all Euclidean distances, which is computationally intensive, or storing a lookup codebook, which demands significant memory. To address this, a deep neural network (DNN)-based approach is proposed for QAMA constellation design. The approach employs a binary classification network to identify transmission modes under given user signal-to-noise ratios (SNRs), followed by a regression network to predict optimal Euclidean distances. Numerical results demonstrate that the DNN-based method significantly reduces computational complexity while maintaining performance comparable to exhaustive search.

Due to power attenuation, improving transmission efficiency in the radio-frequency (RF) band remains a significant challenge, which hinders advancements in various fields of the Internet of Things (IoT), such as wireless power transfer (WPT) and wireless communication. Array design and retro-directive beamforming (RD-BF) techniques offer simple and effective ways to enhance transmission efficiency. However, when the target is an array or in the near field, the RD-BF system (RD-BFS) cannot radiate more energy to the target due to phase irregularities in the target region, resulting in challenges in achieving higher efficiency. To address this issue, we propose the RF-based resonant beam system (RF-RBS), which adaptively optimizes phase and power distribution between transmitting and receiving arrays by leveraging the resonance mechanism to achieve higher transmission efficiency. We analyze the system structure and develop an analytical model to evaluate power flow and resonance establishment. Numerical analysis demonstrates that the proposed RF-RBS achieves self-alignment without beam control and provides higher transmission efficiency compared to RD-BFS, with improvements of up to 16%. This self-alignment capability allows the system to effectively transfer power and information across varying distances and offsets. The numerical results indicate the capability to transmit watt-level power and achieve 21 bps/Hz of downlink spectral efficiency in indoor settings, highlighting the advantages of RF-RBS in information and power transfer for mobile applications.

This two-part paper studies a point-to-point resonant beam communication (RBCom) system, where two separately deployed retroreflectors are adopted to generate the resonant beam between the transmitter and the receiver, and analyzes the transmission rate of the considered system under both the quasi-static and mobile scenarios. Part I of this paper focuses on the quasi-static scenario where the locations of the transmitter and the receiver are relatively fixed. Specifically, we propose a new information-bearing scheme which adopts a synchronization-based amplitude modulation method to mitigate the echo interference caused by the reflected resonant beam. With this scheme, we show that the quasi-static RBCom channel is equivalent to a Markov channel and can be further simplified as an amplitude-constrained additive white Gaussian noise channel. Moreover, we develop an algorithm that jointly employs the bisection and exhaustive search to maximize its capacity upper and lower bounds. Finally, numerical results validate our analysis. Part II of this paper discusses the performance of the RBCom system under the mobile scenario.

In federated learning (FL), the servers are generally seen as omnipotent to distribute the full models to all clients. This is not the case in wireless systems. The broadcast of models inevitably excludes some users with poor channel gain but valuable computation power and diverse local data. In this paper, taking the various computation and communication capabilities of users into account, we propose a federated learning scheme incorporating unstructured model pruning and non-orthogonal transmission. The pruning process divides the model into the core and the extended parts, with the ratio adaptive to the users’ achievable working rates. With non-orthogonal transmission, the strong users get both parts of the model while the weak get only the core part. Simulation results show that the proposed scheme involves more users than traditional wireless FL and improves the accuracy of tasks within the given communication rounds.

The rapid expansion of the Internet of Things (IoT) necessitates robust solutions for charging and communicating with a multitude of devices, making simultaneous wireless information and power transfer (SWIPT) technology increasingly vital. However, existing methods can hardly provide high charging power, great channel capacity, and flexible mobility at the same time. This manuscript introduces a millimeter-wave resonant beam system for SWIPT (mmRB-SWIPT), leveraging retro-directive antenna arrays to enable automatic beam alignment and enhanced transmission efficiency without additional controls. A dual-frequency design allows the system to operate in a frequency-division duplex mode, thereby resolving the echo interference issues encountered in prior resonant beam systems. Analytical models are developed to evaluate the system’s viability and performance, with numerical analysis indicating the capability to transmit watt-level power and achieve 4.8 bps/Hz of spectral efficiency in indoor settings.

This two-part paper focuses on the system design and performance analysis for a point-to-point resonant beam communication (RBCom) system under both the quasi-static and mobile scenarios. Part I of this paper proposes a synchronization-based information transmission scheme and derives the capacity upper and lower bounds for the quasi-static channel case. In Part II, we address the mobile scenario, where the receiver is in relative motion to the transmitter, and derive a mobile RBCom channel model that jointly considers the Doppler effect, channel variation, and echo interference. With the obtained channel model, we prove that the channel gain of the mobile RBCom decreases as the number of transmitted frames increases, and thus show that the considered mobile RBCom terminates after the transmitter sends a certain number of frames without frequency compensation. By deriving an upper bound on the number of successfully transmitted frames, we formulate the throughput maximization problem for the considered mobile RBCom system, and solve it via a sequential parametric convex approximation (SPCA) method. Finally, simulation results validate the analysis of our proposed method in some typical scenarios.

In this paper, we propose a nonorthogonal downlink multiple access scheme, where the base station assigns the bit streams to different users from an adjustable hierarchical quadrature-amplitude-modulation (QAM) constellation and all users deploy a simple non-iterative receiver with the same complexity as in an orthogonal multiple access (OMA) system. We analyze the achievable information rates of the proposed scheme, termed as QAMA, and show that it possesses a similar rate region as the hybrid OMA and MUST approach in the 5G standard, while the latter relies on successive interference cancellation (SIC) receivers and dynamic switching of two distinct modules. We propose a design procedure to determine a few transmission modes that closely approximate the rate region via a simple polygon. Thanks to the receiver simplicity, easy scalability to more than two users, and the near-optimal rate region, QAMA is an appealing candidate for 6G cellular and other advanced wireless systems.

This paper proposes a low-complexity groupwise detection scheme for massive multiple-input multiple-output (MIMO) systems based on two key features of massive MIMO channels. First, most inter-user correlations tend to be negligible as the number of antennas$M$at the base station (BS) increases, while some users might have a high correlation that is indistinguishable for linear projection-based detectors. We hence develop a variational Bayesian inference-based groupwise (VBI-G) detector based on a factorization approximation of the a posteriori probability of transmitted symbols, which sequentially performs nonlinear detection for small groups of highly correlated users without computations of large-scale matrices. Second, the variation of normalized inter-user correlation is insignificant over a wide range of frequency, thereby user grouping based on correlation coefficients can be performed on a wideband basis. Accordingly, we propose a low-complexity incremental greedy grouping algorithm for minimizing the Kullback-Leibler divergence of the unconditioned posterior probabilities with the constraint of a maximum group size. Theoretical analysis proves that for a BS equipped with a uniform linear array, the maximum group size grows logarithmically with the number of users$K$with high probability in the regime$M, K\rightarrow \infty $with$K/M < 1/2$under the worst line-of-sight propagation condition. Simulation results verify that the proposed VBI-G detector achieves superior performance with an extremely low computational overhead for massive MIMO systems.

Locating mobile devices precisely in indoor scenarios is a challenging task because of the signal diffraction and reflection in complicated environments. One vital cause deteriorating the localization performance is the inevitable power dissipation along the propagation path of localization signals. In this paper, we propose a high-accuracy localization scheme based on the resonant beam system (RBS) and the binocular vision, i.e., binocular based resonant beam localization (BRBL). The BRBL system utilizes the energy-concentrated and self-aligned transmission of RBS to realize high-efficiency signal propagation and self-positioning for the mobile target (MT). The binocular method is combined with RBS to obtain the three-dimensional (3-D) coordinates of the MT for the first time. To exhibit the localization mechanism, we first elaborate on the binocular localization model, including the resonant beam transmission analysis and the geometric derivation of the binocular method with RBS. Then, we establish the power model of RBS, and the signal and noise models of beam spot imaging, respectively, to analyse the performance of the BRBL system. Finally, the results of numerical analysis and comparison show that BRBL balances system complexity and localization precision effectively, i.e., providing cm-level localization accuracy with its simplicity and ease of implementation.

Applying high-speed real-time wireless technologies in industrial applications has the great potential to reduce the deployment and maintenance costs compared to their wired counterparts. Wireless technologies enhance the mobility and reduce the communication jitter and delay for mobile industrial equipment, such as mobile collaborative robots. Unfortunately, most existing wireless solutions employed in industrial fields either cannot support the desired high-speed communications or cannot guarantee deterministic, real-time performance. A more recent wireless technology, RT-WiFi, achieves a good balance between high-speed data rates and deterministic communication performance. It is however developed on commercial-of-the-shelf (COTS) hardware, and takes considerable effort and hardware expertise to maintain and upgrade. To address these problems, this paper introduces the software-defined radio (SDR)-based RT-WiFi solution which we call SRT-WiFi. SRT-WiFi provides full-stack configurability for high-speed real-time wireless communications. We present the overall system architecture of SRT-WiFi and discuss its key functions which achieve better timing performance and solve the queue management and rate adaptation issues compared to COTS hardware-based RT-WiFi. To achieve effective network management with rate adaptation in multi-cluster SRT-WiFi, a novel scheduling problem is formulated and an effective algorithm is proposed to solve the problem. A multi-cluster SRT-WiFi testbed is developed to validate the design, and extensive experiments are performed to evaluate the performance at both device and system levels.

Smart factory automation has an ongoing trend to employ high-speed real-time wireless technologies to interconnect heterogeneous industrial assets to perform various sensing and control services, and support mobile equipment to conduct designated tasks in a collaborative fashion. Applications in automation industries usually have stringent requirements on both high data rates and deterministic real-time performance. Existing efforts, however, either cannot meet the performance requirements or are based on commercial-off-the-shelf (COTS) hardware and cannot provide full-stack configurability [1].

High communication speed and sufficient energy supply are the directions of technological development. Energy and information available anywhere and anytime have always been people’s good wishes. On this basis, the resonant beam system (RBS) has demonstrated its unique superiority in meeting the needs for energy and communication. The previous work has mostly focused on the analysis of charging performance of RBS and its steady-state characteristics. In order to analyze the communication performance of RBS more thoroughly, we propose a resonant beam charging and communication (RBCC) system and use the equivalent circuit analysis method to conduct transient analysis on it. The equivalent circuit reveals the dynamic establishment process of the resonant beam from scratch, which facilitates the analysis of the relaxation oscillation process and a deeper understanding of the energy transmission and communication performance. In addition, we explore the energy transmission and communication performance of the RBCC under different energy allocation strategies.

The increasing demands of power supply and data rate for mobile devices promote the research of simultaneous wireless information and power transfer (SWIPT). Optical SWIPT, as known as simultaneous light information and power transfer (SLIPT), has the potential for providing high-capacity communication and high-power wireless charging. However, SLIPT technologies based on light-emitting diodes have low efficiency due to energy dissipation over the air. Laser-based SLIPT technologies need strict positioning accuracy and scanning resolution, which may lead to the increase of costs and complexity. In this paper, we propose a mobile SLIPT scheme based on spatially separated laser resonator (SSLR) and intra-cavity second harmonic generation. The power and data are transferred via separated frequencies, while they share the same self-aligned resonant beam path, without the needs of receiver positioning and beam steering. We establish the analysis model of the resonant beam power and its second harmonic power. Numerical results show that the proposed system can achieve watt-level battery charging power and above 10-bit/s/Hz achievable rate at 6-m distance, which satisfies the requirements of most indoor mobile devices.

A massive multi-input-multi-output (MIMO) system can bring substantial improvement in spectral and energy efficiency for wireless communication systems. The high array gain and fine spatial resolution allow the utilization of relatively simple processing at the base station, such as the near-optimal regularized zero-forcing (RZF) precoding. Nevertheless, such precoding schemes require computing the inverse and multiplication of matrices, which leads to a large burden for baseband processing when the system dimension grows large. To take full advantage of the spatial sparsity of massive MIMO channels in the finite scattering environment, we propose a virtual channel precoding strategy that directly utilizes the sparse virtual channel representation (VCR) in precoding. Furthermore, we develop a virtual channel RZF precoding algorithm based on the pre-conditioned conjugate gradient method, namely PCG-VC-RZF, which can directly use the sparse virtual channel matrix while avoiding the complex computation of the Gram matrix. The computational complexity and transmission delay are shown to be reduced tremendously by the proposed algorithm when operating in a large-dimension system. We further provide a beam selection strategy for improving the performance of the proposed approach under a complexity constraint. The simulation results verify the efficiency of the proposed precoding scheme.

Optical wireless communication (OWC) meets the demands of the future six-generation mobile network (6G) as it operates at several hundreds of Terahertz and has the potential to enable data rate in the order of Tbps. However, most beam-steering OWC technologies require high-accuracy positioning and high-speed control. Resonant beam communication (RBCom), as one kind of non-positioning OWC technologies, has been proposed for high-rate mobile communications. The mobility of RBCom relies on its self-alignment characteristic where no positioning is required. In a previous study, an external-cavity second-harmonic-generation (SHG) RBCom system has been proposed for eliminating the echo interference inside the resonator. However, its energy conversion efficiency and complexity are of concern. In this paper, we propose an intra-cavity SHG RBCom system to simplify the system design and improve the energy conversion efficiency. We elaborate the system structure and establish an analytical model. Numerical results show that the energy consumption of the proposed intra-cavity design is reduced to reach the same level of channel capacity at the receiver compared with the external-cavity one.

High-power and high-rate simultaneous wireless information and power transfer (SWIPT) becomes more and more important with the development of Internet of Things technologies. Optical SWIPT, also known as simultaneous light information and power transfer (SLIPT), has unique advantages such as abundant spectrum resources and low propagation divergence, compared with radio-frequency (RF) SWIPT. However, optical SWIPT faces many challenges in beam steering and receiver positioning/tracking. Resonant beams generated by spatially separated laser resonators (SSLR) have many advantages, including high power, self-aligned mobility, and intrinsic safety. It has been proposed as the carrier of wireless charging and communication. Using resonant beams, mobile electronic devices can be remotely charged and supported with high-rate data transfer. In this paper, we propose a mobile optical SWIPT system based on asymmetric SSLR and present the system optimization procedure. We also determine the boundary of the charging power and communication rate, and discuss the trade-off between power transfer and information transfer. Numerical results show that both the charging power and the communication rate of the optimized asymmetric system are much higher than those of the symmetric system in the previous work.

In this article, we present an advanced coherent receiver for the IEEE 802.15.4 offset quadrature phase-shift keying (OQPSK) physical layer and provide an open-source implementation in the GNU Radio framework. Simulation and field test results show that the proposed receiver achieves about 11-dB power gain over an existing GNU Radio receiver, which treats OQPSK as minimum-shift-keying (MSK) for low-complexity processing. While suitable modules can be added to the MSK-based receiver for performance enhancement, the proposed receiver still maintains a 6-dB power gain. The proposed coherent receiver is attractive for IoT applications where a powerful software-defined-radio (SDR)-based gateway is deployed to interact with various sensors.

Simultaneous lightwave information and power transfer (SLIPT) has been regarded as a promising technology to deal with the ever-growing energy consumption and data-rate demands in the Internet of Things. We propose a resonant beam based SLIPT (RB-SLIPT) system, which deals with the conflict of high deliverable power and mobile receiver positioning with the existing SLIPT schemes. At first, we establish a mobile transmission channel model and depict the energy distribution in the channel. Then, we present a practical design and evaluate the energy/data transfer performance within the moving range of the RB-SLIPT. Numerical evaluation demonstrates that the RB-SLIPT can deliver more than 4W charging power and enable 3Gb/s achievable data rate with the moving range of 20° field of view (FOV) over 3m distance. Thus, RB-SLIPT can enable simultaneous high deliverable power and high data rate in mobile scenarios without tracking control.

Non‐orthogonal multiple access (NOMA) has recently attracted a lot of attention as a promising multiple access technology to be used in the fifth generation (5G) cellular networks. In this study, the authors investigate uplink NOMA transmissions in a cooperative cellular network in the presence of frequency selective multipath fading channels. Specifically, two users communicate with the base station simultaneously with existence of a half‐duplex relay employing the decode‐and‐forward scheme to assist the far user. At the transmitting nodes, the power allocation factors are decided based on the statistical channel state information (CSI) rather than perfect CSI, while at the receiving nodes, the decoding order between the users is pre‐specified and the effect of imperfect successive interference cancellation implementation is taken into account. Under Rayleigh fading channels, the authors derive the exact ergodic rate for the near user and the upper bound of the ergodic rate for the far user, and present an ad‐hoc approach to determine the power allocation factors when the target data rates are given. Simulation results show that NOMA consistently outperforms orthogonal multiple access, however, the performance of uplink NOMA depends heavily on the proper choices of the power allocation factors and the relay location.

Non‐orthogonal multiple access has recently emerged as a promising multiple access technique for future wireless technology. In this study, the authors investigate superposition coding in a downlink cooperative cellular system based on the orthogonal frequency division multiplexing (OFDM) modulation in the presence of frequency selective multipath fading channels, a setup that has not been studied before. Specifically, the base station communicates with two paired mobile users simultaneously via the OFDM modulation with the help of a half‐duplex relay under either the decode‐and‐forward (DF) or amplify‐and‐forward (AF) scheme, where the power allocation between two mobile users depends on the statistical channel state information (CSI) rather than perfect CSI. They derive the ergodic rate regions under Rayleigh fading channels and present an ad‐hoc approach to determine the power splitting parameters when the target data rates are given. Simulation results based on the outage probabilities show that superposition coding consistently outperforms frequency division, however, the relative advantage of the DF and AF schemes in a superposition‐coded OFDM relay system depends on the system geometry.

Parallel combinatory multicarrier (PCMC) modulation is a generalisation of the legacy multicarrier frequency shift keying (FSK) scheme, where for each group of M subcarriers, more than one (say L ) subcarriers are chosen for simultaneous transmission. The PCMC scheme provides an effective way to increase the spectral efficiency while maintaining non‐coherent detection at the receiver. This study provides an in‐depth study of the PCMC scheme with emphasis on how to couple with binary or non‐binary channel coding. One favourable system configuration is identified, having parameters ( L , M ) = (4,8), which increases the spectral efficiency by 50% relative to the most efficient FSK scheme from the FSK family. Coupled with non‐binary low‐density parity‐check (LDPC) coding over the Galois field GF(64), the PCMC scheme with ( L , M ) = (4,8) is shown to have robust performance in multipath fading channels.

Time synchronization and localization are basic services in a sensor network system. Although they often depend on each other, they are usually tackled independently. In this work, we investigate the time synchronization and localization problems in underwater sensor networks, where more challenges are introduced because of the unique characteristics of the water environment. These challenges include long propagation delay and transmission delay, low bandwidth, energy constraint, mobility, etc. We propose a joint solution for localization and time synchronization, in which the stratification effect of underwater medium is considered, so that the bias in the range estimates caused by assuming sound waves travel in straight lines in water environments is compensated. By combining time synchronization and localization, the accuracy of both are improved jointly. Additionally, an advanced tracking algorithm interactive multiple model (IMM) is adopted to improve the accuracy of localization in the mobile case. Furthermore, by combining both services, the number of required exchanged messages is significantly reduced, which saves on energy consumption. Simulation results show that both services are improved and benefit from this scheme.

Preestablished secret keys are often used to encrypt and decrypt data in communication systems, which are not secure once the keys are compromised. One desirable method is to generate secret keys dynamically using correlated channel measurements. We explore this concept in underwater acoustic (UWA) channels, and present a protocol that can generate secret keys dynamically based on the channel frequency response (CFR) in orthogonal frequency-division multiplexing systems. The multi-bit quantization is carried out on the amplitude of each tone, and the Bose-Chaudhuri-Hocquenghem codes are used for information reconciliation. Part of the protocol was implemented in lake tests, and multiple data sets were collected. The lake test results verify that the amplitude of CFR can be used as a randomness source for key generation in UWA channels, and we also find the low correlation between the mutual channels of the legitimate users. Based on the lake test results, we incorporate two modules into the protocol for performance improvement. The first module employs the adaptively weighted probing signaling to increase the channel correlation, and the second module of block-sliced key verification is used to deal with channel dynamics and increase the key agreement probability. The simulation results demonstrate the improved performance of the enhanced protocol.

Most existing studies on Delay/Disruption Tolerant Networks (DTNs) consider pair-wise node encountering that assumes nodes only meet in pairs. In many mobile wireless networks, a group of nodes, instead of only a pair of nodes, may meet each other. In this paper, we study how to effectively transmit a set of packets from a source to a destination in such group meeting scenarios. The optimization goal is to minimize the delay for the packets to reach the destination while limiting the energy consumption. We first assume that node encountering is known beforehand, and develop an algorithm to obtain the minimum delay. We then develop two practical network coding based schemes. Both schemes use a token technique to limit the total number of transmissions, and only incur signaling at the beginning of a group meeting. One scheme requires nodes in a group to exchange their encoding matrices with each other, while the other only requires exchanging rank information. Simulation results demonstrate that both schemes achieve delays close to the minimum delay for moderate number of tokens. They present different tradeoffs in the number of transmissions and the signaling overhead.

Tomographic synthetic aperture radar imaging (TomoSAR) is a new SAR imaging modality that extends the synthetic aperture principle into the elevation direction. In TomoSAR, the elevation resolution depends on the length of elevation aperture and the baseline distribution. When the length of elevation aperture is fixed, the number of baselines is important for the elevation reconstruction. Therefore, improving the elevation imaging quality with the finite amount of baselines is worth researching. This letter proposes a novel missing data compensation approach in TomoSAR via matrix completion (MC), which is a technique of the low-rank matrix recovery from a subset of the matrix elements. In the proposed method, we first exploit MC to estimate the 2-D focused image data of unknown baselines, which are located on the uniform data grid without changing the length of elevation aperture. Based on the recovered data, we then recover the elevation reflectivity function by the spectral analysis (SA) method. Compared with the conventional SA technique, the proposed approach can achieve much higher elevation image quality, due to more available data offered by MC. The effectiveness of the proposed method is validated with the experimental results based on simulated and real airborne data.

Abstract Recent surges in the development of underwater acoustic networks (UANs) have lead to a rapid acceptance of this technology in scientific, commercial, and military applications. However, limited work has been performed on developing secure communication mechanisms and techniques to protect these networks. Security mechanisms are wildly studied in terrestrial networks, and various defense mechanisms have been developed as safeguards. Because of the difference in communication mediums and physical environments, the existing solutions for terrestrial networks cannot be directly applied for UANs. In this paper, we study the effects of denial‐of‐service jamming attacks on UANs using real‐world field tests. We develop our own jammer hardware and signals in order to analyze the characteristics of different jamming attack models on a network. Our tests are performed on multiple commercial brand acoustic modems and an orthogonal frequency division multiplexing modem prototype. We show that UANs can be easily jammed using carefully timed attacks, which are energy efficient. Copyright © 2012 John Wiley & Sons, Ltd.

Due to frequency selectivity, long propagation delay caused by low speed of sound, and fast channel variation, multiple-access protocols that work well in terrestrial networks may not perform as well in underwater environment. We take a physical-layer approach and investigate frequency-selective underwater channels with uncoordinated multiple bursty transmissions. We first derive the statistical distribution of the interference as seen by a typical receiver, considering burstiness of interference transmissions, packet length, and spatial distributions of the interferers. We then derive expressions for the probability density functions of the frequency-dependent signal-to-interference-and-noise ratio (SINR). Expressions for the outage probability of a typical link between two nodes are obtained in the frequency-selective scenarios and the flat-fading special cases. The outage probabilities depend on the bursty transmission probability and the number of interfering transmitters. Previous expressions in the literature on outage probability for flat fading channels and interference-limited systems can be recovered as special cases of our results. Analysis on the throughput of network is provided. Optimization over the number of nodes and bursty transmission probability is performed to maximize the network throughput. Simulation results are presented, which validate the theoretical results and illustrate typical features of the interference in an underwater network.

Finding the position of a device within an underwater network, known as localization, has been critical for the advancement of autonomous underwater vehicle (AUV) research. Many of the current methods of underwater localization rely on a modem's ability to accurately estimate the time of flight for a given message transmission. This ability is also utilized by a modem's built-in ranging function. To advance the research in underwater localization, a comparison study was done on the ranging function of three commercially available underwater acoustic modems, all of which utilize different modulation schemes. A pair of AquaSeNT (OFDM), Benthos (MFSK), and LinkQuest (DSSS) acoustic modems were tested at several positions in both a pool and lake environment. It was found that the AquaSeNT was the most robust, as in it always calculated a distance, at the expense of precision. The precision of the AquaSeNT was effected by a consistent 8-m bias. The LinkQuest was found to be the most accurate and precise, but had issues communicating in short channels (large delay spread), and would occasionally produce erroneous measurements. The Benthos had a balance between communication success as well as precision and accuracy.

Underwater images are useful for researching the characteristics of the environment and its inhabitants. Using the underwater acoustic modems along with an autonomous underwater vehicle (AUV), images can be transmitted from a camera mounted on the AUV to a modem. A real-time implementation was done to transmit and receive images underwater using the AquaSENT acoustic modems. An image was extracted from a streaming web cam, resized, and split into individual packets which were capable of being transmitted by a modem. Once these packets were received by another modem, they were reconstructed to display the original image. The packet sizes that were transmitted were varied in a controlled test bed with shallow water. The preliminary results show that packets were generally fully received, and the transmitted image was identical to the received image.

As Underwater Acoustic Networks (UANs) become more commonly utilized, it is crucial to research security against interference and jamming as the networks are based in an exposed environment. We have identified some of the key communication vulnerabilities across underwater networks using basic jamming techniques. Three commercial modems from the United States were analyzed in our tests, particularly LinkQuest, AquaSeNT and Benthos modems, which use DSSS, OFDM and MFSK transmission schemes. We observed that each modem has a preamble that contains a packet that is vulnerable to a shorter, more efficient targeted jam. Each modem is also vulnerable to single tone jamming at certain frequencies in the bandwidth, and can be reliably but less efficiently jammed with frequency sweeps.

Time synchronization plays a critical role in distributed network systems. In this paper, we investigate the time synchronization problem in the context of underwater sensor networks (UWSNs). Although many time-synchronization protocols have been proposed for terrestrial wireless sensor networks, none of them can be directly applied to UWSNs. This is because most of these protocols do not consider long propagation delays and sensor node mobility, which are important attributes in UWSNs. In addition, UWSNs usually have high requirements in energy efficiency. To solve these new challenges, innovative time synchronization solutions are demanded. In this paper, we propose a pairwise, cross-layer, time-synchronization scheme for mobile underwater sensor networks, called DA-Sync. The scheme proposes a framework to estimate the doppler shift caused by mobility, more precisely through accounting the impact of the skew. To refine the relative velocity estimation, and consequently to enhance the synchronization accuracy, the Kalman filter is employed. Further, the clock skew and offset are calibrated by two runs of linear regression. Simulation results show that DA-Sync outperforms the existing synchronization schemes in both accuracy and energy efficiency.

ABSTRACT Multi‐input multi‐output orthogonal frequency‐division multiplexing (MIMO‐OFDM) has been actively studied for high data rate communications over the bandwidth‐limited underwater acoustic (UWA) channels. Unlike existing receivers that treat the intercarrier interference (ICI) as additive noise, in this paper, the proposed receiver considers ICI explicitly together with the co‐channel interference (CCI) due to parallel transmissions in MIMO‐OFDM. Using a recently developed progressive receiver framework, the proposed receiver starts with low‐complexity ICI‐ignorant processing and then progresses to ICI‐aware processing with increasing ICI levels. The key components of the proposed receiver include the following: (1) compressed sensing‐based sparse channel estimation, (2) soft‐input soft‐output minimum mean square error/Markov chain Monte Carlo detector for interference mitigation, and (3) soft nonbinary low‐density parity check decoding. In addition to simulation, we use real data from the Surface Processes and Acoustic Communications Experiment 2008 (SPACE08) and the Mobile Acoustic Communications Experiment 2010 (MACE10) to verify the system performance, where the transmitter in SPACE08 was stationary and that in MACE10 was slowly moving. Simulation and experimental results show that explicitly addressing ICI and CCI significantly improves the performance of MIMO‐OFDM in UWA systems. Copyright © 2012 John Wiley & Sons, Ltd.

This paper investigates moving target localization by the biased time-of-arrivals (TOAs), where an extracted TOA is biased by the unknown Doppler of the target. The phenomenon applies to radar and sonar systems that employ Doppler tolerant waveforms. While in principle those would allow the estimation of both position and velocity using position-only measurements, we unfortunately find that the Fisher information matrix is ill-conditioned. However, a Quasi-maximum likelihood estimator based on the misspecified model is suggested, and its performance is analyzed.

Physical-layer network coding (PLNC) allows two remote terminals to exchange information in two time slots via a half-duplex relay. In this paper, we consider PLNC in underwater acoustic (UWA) two-way relay networks, with particular attention on the recovery of the network-coded codeword at the relay. Considering multiple paths in UWA channels with large delay spreads and temporal variations, orthogonal-frequency division-multiplexing (OFDM) is used as an underlying modulation. Distinct from existing works which mainly focus on the flat-fading channel and the non-iterative PLNC decoding, we investigate three iterative receivers at the relay in doubly-selective UWA channels. To estimate the network-coded codeword, the three receivers respectively adopt i) a conventional decoding scheme to recover the codewords from two terminals independently; ii) a decoding scheme to recover the network-coded codeword directly, and iii) a joint decoding scheme to recover a composite codeword formed by the codewords from the two terminals. In the time-varying channel, the intercarrier interference (ICI) in the received signal has been explicitly addressed in three receivers. Extensive simulations and experimental results reveal that the decoding scheme for direct recovery of the network-coded codeword cannot work well in the multipath channel, and that with a lower complexity the performance of the iterative separate-decoding scheme can catch up with that of the joint decoding scheme.

Recently significant progress has been made on point-to-point underwater acoustic communications, and the interest has grown on the application of those techniques in multiuser communication settings, where the asynchronous nature of multiuser communication poses a grand challenge. This paper develops a time-asynchronous multiuser reception approach for orthogonal frequency-division multiplexing (OFDM) transmissions in underwater acoustic channels. The received data burst is segmented and apportioned to multiple processing units in an overlapped fashion, where the length of the processing unit depends on the maximum asynchronism among users on the OFDM block level. Interference cancellation is adopted to reduce the interblock interference between overlapped processing units. Within each processing unit, the residual inter-block interference from multiple users is aggregated as one external interference which can be parameterized. Multiuser channel estimation, data detection, and interference mitigation are then carried out in an iterative fashion. Simulation and emulated experimental results demonstrate the robustness of the proposed receiver with signal asynchronism among multiple users in both time-invariant and time-varying environments. It is observed that the receiver decoding performance degrades as the channel time variation and the maximum relative delay among users increase.

A multiple-input multiple-output (MIMO) radar can improve system performance with waveform and spatial diversities. Mathematically, the multiple independent waveforms increase the dimension of signal space, so optimal transmission power allocation deserves investigation. The majority of the literature prefers to omit the effect of propagation attenuation, and considers the receiving gain vectors to be independently and identically distributed (i.i.d.) in power allocation. In this paper, we integrate the propagation losses into MIMO radar signal model, and investigate the power allocation problems under three popular criteria: maximizing the mutual information, minimizing the minimum mean square errors, and maximizing the echo energy. As their objective functions are either convex or concave, the optimal strategies are theoretically guaranteed.

The interaction between a smart target and a smart MIMO radar is investigated from a game theory perspective. Since the target and the radar form an adversarial system, their interaction is modeled as a two-person zero-sum game. The mutual information criterion is used to formulate the utility functions. The unilateral, hierarchical, and symmetric games are studied, and the equilibria solutions are derived.

Time synchronization and localization are basic services in a sensor network system. Although they often depend on each other, they are usually tackled independently. In this work, we investigate time synchronization and localization problems in underwater sensor networks. We propose a joint solution for localization and time synchronization, in which the stratification effect of underwater medium is considered, so that the bias in the range estimates caused by assuming sound waves travel in straight lines in water environments is compensated. By combining time synchronization and localization, the accuracy of both are improved jointly. Additionally, an advanced tracking algorithm IMM (interactive multiple model) is adopted to improve the accuracy of localization in the mobile case. Furthermore, by combining both services, the number of required exchanged messages is significantly reduced, which saves on energy consumption. Simulation results show that both services are improved and benefit from this scheme.

Finding the minimum distance of low-density-parity-check (LDPC) codes is an NP-hard problem. Different from all existing works that focus on binary LDPC codes, we in this paper aim to compute the minimum distance of nonbinary LDPC codes, motivated by the fact that operating in a large Galois field provides one important degree of freedom to achieve both good waterfall and error-floor performance. Our method is based on the existing nearest nonzero codeword search (NNCS) method, but several modifications are incorporated for nonbinary LDPC codes, including the modified error impulse pattern, the dithering method, and the nonbinary decoder. Numerical results on the estimated minimum distances show that a code's minimum distance can be increased by careful selection of nonzero elements of the parity check matrix, or by increasing the mean column weight, or by increasing the size of the Galois field. These results support observations that have been made based on simulated performance in the literature. Finally, we provide an upper bound on the minimum distance for nonbinary quasi-cyclic LDPC codes.

A new decoder architecture for nonbinary low-density paritycheck (LDPC) codes is presented in this paper to reduce the hardware operational complexity in VLSI implementations. The low decoding complexity is achieved by employing adaptive message control (AMC) that dynamically trims the message length of belief information to reduce the amount of memory accesses and arithmetic operations. To implement the proposed AMC, we develop the architecture of a horizontal sequential nonbinary LDPC decoder. Key components in the architecture have been designed with the consideration of variable message lengths to leverage the benefit of the proposed AMC. Simulation results demonstrate that the proposed nonbinary LDPC decoder architecture can significantly reduce hardware operations and power consumption as compared with existing work with negligible performance degradation.

In this paper, we propose a practical scheme, Non-Binary Joint Network-Channel Coding (NB-JNCC), for reliable multi-path multi-hop communication in arbitrary large-scale wireless networks. NB-JNCC seamlessly couples channel coding and network coding to effectively combat the detrimental effect of fading of wireless channels. Specifically, NB-JNCC combines non-binary irregular low-density parity-check (LDPC) channel coding and random linear network coding through iterative joint decoding, which helps to fully exploit the spatial diversity and redundancy residing in both channel codes and network codes. In addition, since it operates over a high order Galois field, NB-JNCC can be directly combined with high order modulation without the need of any bit-to-symbol conversion nor its inverse. Through both analysis and simulation, we demonstrate the significant performance improvement of NB-JNCC over other schemes.

Time synchronization plays a critical role in distributed network systems. In this paper, we investigate the time synchronization problem in the context of underwater sensor networks (UWSNs). We propose a pairwise, cross-layer, time synchronization scheme for mobile underwater sensor networks, called TSMU. Facilitated by the Kalman Filter, the proposed method greatly improves the dynamic propagation delay estimation by exploring the Doppler effect. Simulation results show that TSMU outperforms existing synchronization schemes in both accuracy and energy efficiency.

Bearings-only localization with light-of-sight (LOS) propagation is well understood. This paper concentrates on bearing-only localization with non-line-of-sight (NLOS) measurements, where the target radiation arrives at the sensor after a specular reflection. The wrinkle is that the reflecting surface is circular (inner side of a circle), and is assumed known. Since the target-sensor geometry has multiple configurations, the maximum likelihood (ML) solution may not exist. However, if a concentric opaque circle (such as the earth) exists within the reflecting one, the propagation path is unique; a grid search based ML is available for such a circumstance. Since ML is computationally consuming, two suboptimal algorithms based on small angle approximation are developed. Their performances are numerically compared.

A new decoding algorithm, referred to as Min-Sum with Adaptive Message Control (AMC-MS), is proposed to reduce the decoding complexity of nonbinary LDPC codes. The proposed algorithm adaptively trims the message length of belief information to reduce the amount of arithmetic operations. Exploiting the fact that during the decoding iteration, the distribution of belief information will become more concentrated around the correct element in the case of convergence, the messages can be truncated accordingly by considering only a few entries with large likelihood. Simulation results with a GF(16) nonbinary LDPC code indicate that the proposed approach can reduce arithmetic operations by up to 65% compared with non-truncated cases. Compared with the state-of-the-art extended MS (EMS) decoding, the proposed AMC-MS algorithm can reduce the computation by up to 50%, thereby enabling low-complexity decoding of nonbinary LDPC codes.

Although time-domain oversampling of the received baseband signal is common for single-carrier transmissions, the counterpart of frequency-domain oversampling is rarely used for multicarrier transmissions. In this paper, we explore frequency-domain oversampling to improve the system performance of zero-padded OFDM transmissions over underwater acoustic channels with large Doppler spread. We use a signal design that enables separate sparse channel estimation and data detection, rendering a low complexity receiver. Based on both simulation and experimental results, we observe that the receiver with frequency-domain oversampling outperforms the conventional one considerably in channels with moderate and large Doppler spreads, and the gain increases as the Doppler spread increases. Although a raised-cosine pulse-shaping window can be used to improve the system performance relative to a rectangular window at the expense of data rate reduction, the performance gain is much less than that brought by frequency-domain oversampling in the considered OFDM system for Doppler spread channels.

As is clear from the ambiguity function (AF) uncertainty principle and the underlying matched filtering operation, pulse compression (PC) radars cannot accurately and simultaneously measure both the time delay and Doppler shift of moving targets. On the other hand, compressed sensing (CS) radar would seem to be emerging as a means to do just that, jointly to estimate the delay and Doppler shift, through convex optimization instead of matched filtering, such that PC resolution limits no longer apply. However, it turns out that the AF is closely related to the CS “dictionary coherence coefficient,” which affects the optimization recoverability. In this paper we formalize this relationship.

In this paper, we construct nonbinary quasi-cyclic low-density parity-check (QC-LDPC) codes whose parity check matrices consist of an array of square sub-matrices which are either zero matrices or circulant permutation matrices. We propose a novel method to design the shift offset values of the circulant permutation sub-matrices, so that the code length can vary while maintaining a large girth. Extensive Monte Carlo simulations demonstrate that the obtained codes of a wide range of rates (from 1/2 to 8/9) with length from 1000 to 10000 bits have very good performance over both AWGN and Rayleigh fading channels. Furthermore, the proposed method is extended to design multiple nonbinary QC-LDPC codes simultaneously where each individual code can achieve large girth with variable lengths. The proposed codes are appealing to practical adaptive systems where the block length and code rate need to be adaptively adjusted depending on traffic characteristics and channel conditions.

In this paper, we study nonbinary regular LDPC cycle codes whose parity check matrix H has fixed column weight j = 2 and fixed row weight d. Through graph analysis, we show that the parity check matrix H of a regular cycle code can be put into an equivalent structure in the form of concatenation of row-permuted block-diagonal matrices if d is even, or, if d is odd and the code's associated graph contains at least one spanning subgraph that consists of disjoint edges. This equivalent structure of H enables: i) parallel processing in lineartime encoding; ii) considerable resource reduction on the code storage for encoding and decoding; and iii) parallel processing in sequential belief-propagation decoding, which increases the throughput without compromising performance or complexity. On the code's structure design, we propose a novel design methodology based on the equivalent structure of H. Finally, we present various numerical results on the code performance and the decoding complexity.

In this paper, we present a novel method to construct nonbinary irregular LDPC codes whose parity check matrix has only column weights of 2 and t, where t ¿ 3. The constructed codes can be encoded in linear time and in a parallel fashion. Also, they can achieve near-Shannon-limit performance over both AWGN and Rayleigh fading channels with a moderate field size. Analysis based on nonbinary EXIT charts is presented. Codes constructed by the proposed method with block lengths ranging from 1, 000 bits to 10, 000 bits are simulated. Simulation results show that codes of rate 1/2 and length 10, 000 bits can achieve block-error-rate (BLER) of 10-5within 1.1 dB and 1.3 dB away from the Shannon limits of the AWGN and Rayleigh channels, respectively. In an AWGN channel, codes of rate r = 8/9 and length around 4,000 bits can achieve BLER of 10-5within 1.1 dB from the Shannon limit, while codes of rate r = 15/16 and length around 9,000 bits can achieve BLER of 10-5within 1.0 dB from the Shannon limit. We conjecture that to further lower the error floor (e.g., to a BLER of 10-10), a column weight no less than 3 is preferred and may be necessary, especially for codes with high rate and over Galois fields of small to moderate sizes.

In this paper, we propose a practical scheme, called Non-Binary Joint Network-Channel Decoding (NB-JNCD) for reliable communication in wireless networks. It seamlessly couples channel coding and network coding, and can effectively combat the detrimental effect of fading of wireless channels, especially in large networks. On a high order Galois field, NB-JNCD combines non-binary LDPC channel coding and random linear network coding through iterative joint decoding, which helps fully exploit the spatial diversity and redundancy residing in both codes. Furthermore, the scheme can unify non-binary source coding and high order modulation without the need of any bit-to-symbol conversion and its inverse. Through analysis and simulation, we demonstrate the significant performance improvement of NB-JNCD against other schemes.

Using group theory, we analyze cycle GF(2p) codes that use Cayley graphs as their associated graphs. First, we show that through row and column permutations the parity check matrix H can be put in a concatenation form of row-permuted block-diagonal matrices. Encoding utilizing this form can be performed in linear time and in parallel. Second, we derive a rule to determine the nonzero entries of H and present determinate and semi-determinate codes. Our simulations show that the determinate and semi-determinate codes have better performance than codes with randomly generated nonzero entries for GF(16) and GF(64), and have similar performance for GF(256). The constructed determinate and semi-determinate codes over GF(64) and GF(256) can outperform the binary irregular counterparts of the same block lengths. One distinct advantage for determinate and semi-determinate codes is that they greatly reduce the storage cost of H for decoding. The results in this correspondence are appealing for the implementation of efficient encoders and decoders for this class of promising LDPC codes, especially when the block length is large.

In this paper, we study non-binary regular LDPC cycle codes whose parity check matrix has fixed column weight 2 and fixed row weight d. We prove that the parity check matrix of any regular cycle code can be put into a concatenation form of row-permuted block-diagonal matrices after row and column permutations if d is even, or, if d is odd and the code's associated graph contains at least one spanning subgraph that consists of disjoint edges. Utilizing this structure enables parallel processing in linear-time encoding, and parallel processing in sequential belief-propagation decoding, which increases the throughput without compromising performance or complexity. Numerical results are presented to compare the code performance and the decoding complexity.

Multicarrier modulation in the form of OFDM has been actively pursued for underwater acoustic communication recently. In this paper, we present a desirable property of OFDM that one signal design can be easily scaled to fit into different transmission bandwidths with negligible changes on the receiver. We have tested the proposed design with data collected from experiments at AUV Fest, Panama City, FL, June 2007, and at the Buzzards Bay, MA, Aug. 2007. With QPSK modulation, we have used different bandwidths from 3 kHz to 50 kHz, leading to data rates from 1.5 kbps to 25 kbps after rate 1/2 coding. With 16-QAM modulation, we have used different bandwidths from 12 kHz to 50 kHz, leading to data rates from 12 kbps to 50 kbps. Excellent BER performance has been achieved, which confirms the flexibility of OFDM under different system setups.

Recently, multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM) has been shown feasible for underwater acoustic communications via effective algorithms to handle the channel time-variability. In this paper, we propose to use nonbinary low density parity check (LDPC) codes to address two other main issues in OFDM: (i) plain (or uncoded) OFDM has poor performance in fading channels, and (ii) OFDM transmission has high peak to average power ratio (PAPR). We develop new methods to construct nonbinary regular and irregular LDPC codes that achieve excellent performance, match well with the underlying modulation, and can be encoded in linear time and in a parallel fashion. Based on the fact that the generator matrix of LDPC codes has high density, we further show how to reduce the PAPR considerably with minimal overhead. Experimental results confirm the excellent performance of the proposed nonbinary LDPC codes in multicarrier underwater acoustic communications.

In this paper, we propose a novel method for detection, synchronization and Doppler scale estimation for underwater acoustic communication using orthogonal frequency division multiplex (OFDM) waveforms. This new method involves transmitting two identical OFDM symbols together with a cyclic prefix, while the receiver uses a bank of parallel self-correlators. Each correlator is matched to a different Doppler scaling factor with respect to the waveform dilation or compression. We characterize the receiver operating characteristic in terms of probability of false alarm and probability of detection. We also analyze the impact of Doppler scale estimation accuracy on the data transmission performance. These analytical results provide guidelines for the selection of the detection threshold and Doppler scale resolution. In addition to computer-based simulations, we have tested the proposed method with real data from an experiment at Buzzards Bay, MA, Dec. 15, 2006. Using only one preamble, the proposed method achieves similar performance on the Doppler scale estimation and the bit error rate as an existing method that uses two linearly-frequencymodulated (LFM) waveforms, one as a preamble and the other as a postamble, around each data burst transmission. Compared with the LFM based method, the proposed method works with a constant detection threshold independent of the noise level and is suited to handle the presence of dense multipath channels. More importantly, the proposed approach does not need to buffer the whole data packet before data demodulation, which facilitates future development of online realtime receivers for multicarrier underwater acoustic communications.

In this paper we develop a fine synchronization algorithm for multiband OFDM transmission in the presence of frequency selective channels. This algorithm is based on maximum a posteriori (MAP) joint timing and channel estimation that incorporates channel statistical information, leading to considerable performance enhancement relative to existing maximum likelihood (ML) approaches. We carry out a thorough performance analysis of the fine timing algorithm, and link the diversity concept widely used in data communications to the timing performance. We show that the probability of the timing offset equal to or larger than Δ taps has a diversity order of NBmin(Δ,L) in Rayleigh fading channels, where NBis the number of subbands and L is the number of channel taps. This result reveals that the timing estimate is very much concentrated around the true timing as the signal to noise ratio (SNR) increases. Our simulations confirm the theoretical analysis, and also demonstrate the robustness of the proposed timing algorithm against model mismatches in a realistic UWB indoor channel.

To achieve reliable packet transmission over a wireless link without feedback, we propose a layered coding approach that uses error-correction coding within each packet and erasure-correction coding across the packets. This layered approach is also applicable to an end-to-end data transport over a network where a wireless link is the performance bottleneck. We investigate how to optimally combine the strengths of error- and erasure-correction coding to optimize the system performance with a given resource constraint, or to maximize the resource utilization efficiency subject to a prescribed performance. Our results determine the optimum tradeoff in splitting redundancy between error-correction coding and erasure-correction codes, which depends on the fading statistics and the average signal to noise ratio (SNR) of the wireless channel. For severe fading channels, such as Rayleigh fading channels, the tradeoff leans towards more redundancy on erasure-correction coding across packets, and less so on error-correction coding within each packet. For channels with better fading conditions, more redundancy can be spent on error-correction coding. The analysis has been extended to a limiting case with a large number of packets, and a scenario where only discrete rates are available via a finite number of transmission modes.

Cooperative communication with single relay selection is a simple but effective communication scheme for energy-constrained networks. In this paper, we propose a novel selective single-relay cooperative scheme, combining selective-relay cooperative communication with physical-layer power control. Based on the MAC-layer RTS-CTS signaling, a set of potential relays compute individually the required transmission power to participate in the cooperative communication, and compete within a window of fixed length. The "best" relay is selected in a distributed fashion with minimum signaling overhead. We derive power-control solutions corresponding to two policies on relay selection: one is to minimize the energy consumption per data packet, and the other is to maximize the network lifetime. Our numerical and simulation results confirm that the proposed scheme achieves significant energy savings and prolongs the network lifetime considerably.

Abstract This paper reviews the physical fundamentals and engineering implementations for efficient information exchange via wireless communication using physical waves as the carrier among nodes in an underwater sensor network (UWSN). The physical waves under discussion include sound, radio, and light. We first present the fundamental physics of different waves; then we discuss and compare the pros and cons for adopting different communication carriers (acoustic, radio, and optical) based on the fundamental first principles of physics and engineering practice. The discussions are mainly targeted at underwater sensor networks (UWSNs) with densely deployed nodes. Based on the comparison study, we make recommendations for the selection of communication carriers for UWSNs with engineering countermeasures that can possibly enhance the communication efficiency in specified underwater environments. Copyright © 2008 John Wiley & Sons, Ltd.

In this paper, we propose a new fairness criterion based on QoS satisfaction of users. Unlike the existing throughput fairness criterion suitable for non-realtime traffic and the existing delay fairness criterion tailored for real-time traffic, the proposed QoS satisfaction criterion handles heterogeneous traffics in an unified fashion. Based on the new fairness criterion, we propose a cross-layer resource allocation scheme for downlink multiuser OFDM with heterogeneous traffics, where user scheduling is followed by subcarrier-bit allocation. The resource allocation depends on not only the channel state information (CSI) and QoS requirements of users, but also random packet arrivals and queue states. Simulations confirm significant performance improvement and user QoS satisfaction fairness by the proposed scheme.

We develop optimal probability loading for parallel binary channels, subject to a constraint on the total probability of sending ones. The distinctions from the waterfilling power loading for parallel Gaussian channels, particularly the latter's "dropping" of poor-quality channels, are highlighted. The only binary-input binary-output channel that is never dropped is the Z-channel.

In the above titled paper (ibid., vol. 51, no. 5, pp. 1900-1907, May 05), changes were made to equations on p. 1905.

We investigate an adaptive MIMO-OFDM system with a feedback link that can only convey a finite number of bits. We consider three different transmitter configurations: i) beamforming applied per OFDM subcarrier, ii) precoded spatial multiplexing applied per subcarrier, and iii) precoded orthogonal space time block coding applied per subcarrier. Depending on the channel realization, the receiver selects the optimal beamforming vector or precoding matrix from a finite-size codebook on each subcarrier, and informs the transmitter through finite-rate feedback. Exploiting the fact that the channel responses across OFDM subcarriers are correlated, we propose two methods to reduce the amount of feedback. One is recursive feedback encoding that selects the optimal beamforming/precoding choices sequentially across the subcarriers, and adopts a smaller-size time-varying codebook per subcarrier depending on prior decisions. The other is trellis-based feedback encoding that selects the optimal decisions for all subcarriers at once along a trellis structure via the Viterbi algorithm. Our methods are applicable to different transmitter configurations in a unified fashion. Simulation results demonstrate that the trellis-based approach outperforms the recursive method as well as an existing interpolation-based alternative at high signal-to-noise-ratio, as the latter suffers from "diversity loss"

We consider a MIMO-OFDM system with transmit beamforming applied on each OFDM subcarrier, where each beamforming vector is drawn from a codebook with finite size. Depending on the channel realization, the receiver decides the optimal beamforming vector on each subcarrier, and informs the transmitter through a rate-limited feedback link. Exploiting the fact that the channel responses across OFDM subcarriers are correlated, we propose two methods to reduce the amount of needed feedback. One is recursive feedback encoding that selects the optimal beamforming vectors sequentially across the subcarriers, and adopts a smaller-size time-varying codebook per subcarrier depending on prior decisions. The other is trellis-based feedback encoding that selects the optimal beamforming vectors for all subcarriers at once along a trellis structure via the A. The trellis-based feedback encoding outperforms the recursive feedback encoding at the expense of encoding complexity at the receiver. Simulation results demonstrate that our trellis-based approach outperforms an existing interpolation-based alternative, as the latter incurs diversity loss at high SNR.

Consider a codebook containing N unit-norm complex vectors in a K-dimensional space. In a number of applications, the codebook that minimizes the maximal cross-correlation amplitude (I/sub max/) is often desirable. Relying on tools from combinatorial design theory, we construct analytically optimal codebooks meeting, in certain cases, Welch's lower bound. When analytical constructions are not available, we develop an efficient numerical search method based on Lloyd's algorithm, which leads to considerable improvement on the achieved I/sub max/ over existing alternatives. We also derive a composite lower bound on the minimum achievable I/sub max/ that is effective for any N.

Providing guaranteed quality-of-service (QoS) for multimedia applications over wireless fading channels is challenging. To this end, we develop a cross-layer design for multiuser scheduling at the data link layer, with each user employing adaptive modulation and coding (AMC) at the physical layer. By classifying users into: QoS-guaranteed and best-effort users, the proposed scheduler enables prescribed QoS guarantees and efficient bandwidth utilization simultaneously. Furthermore, our cross-layer scheduler enjoys low-complexity implementation and analysis, provides service isolation and scalability, decouples delay from dynamically-scheduled bandwidth, and is backward compatible with existing separate-layer designs. Accuracy of the performance analysis is verified by simulations and pertinent robustness issues are briefly discussed. Numerical examples illustrate the steady-state statistical performance for a single and multiple users, as well as the asymptotic behavior for a large number of users.

Adaptive modulation has the potential to increase system throughput considerably by adapting transmission parameters to the time-varying channel characteristics. Crucial to adaptive systems is the requirement of a feedback channel, that is often capable of carrying only a limited number of bits. Under such a bandwidth-constrained feedback link, we aim to optimize a multiantenna system based on transmit beamforming and adaptive modulation, where the transmit power, the signal constellation, the beamforming direction, and the feedback strategy, are designed jointly. Our proposed nested iterative approach leads to an approximate, yet practical, solution. Simulation results demonstrate considerable improvement in transmission rate, as the number of feedback bits increases.

Consider a codebook containing N unit-norm complex vectors in a K-dimensional space. In a number of applications, the codebook that minimizes the maximal cross-correlation amplitude (I/sub max/) is often desirable. Relying on tools from combinatorial number theory, we construct analytically optimal codebooks meeting, in certain cases, the Welch lower bound. When analytical constructions are not available, we develop an efficient numerical search method based on a generalized Lloyd algorithm, which leads to considerable improvement on the achieved I/sub max/ over existing alternatives. We also derive a composite lower bound on the minimum achievable I/sub max/ that is effective for any codebook size N.

Assuming there are always sufficient data waiting to be transmitted, adaptive modulation and coding (AMC) schemes at the physical layer have been traditionally designed separately from higher layers. However, this assumption is not always valid when queuing effects are taken into account at the data link layer. In this paper, we analyze the joint effects of finite-length queuing and AMC for transmissions over wireless links. We present a general analytical procedure, and derive the packet loss rate, the average throughput, and the average spectral efficiency (ASE) of AMC. Guided by our performance analysis, we introduce a cross-layer design, which optimizes the target packet error rate of AMC at the physical layer, to minimize thpacket loss rate and maximize the average throughput, when combined with a finite-length queue at the data link layer. Numerical results illustrate the dependence of system performance on various parameters, and quantify the performance gain due to cross-layer optimization. Our focus is on the single user case, but we also discuss briefly possible applications to multiuser scenarios.

Providing guaranteed quality of service (QoS) with efficient bandwidth utilization over wireless fading channels is challenging. In this paper, we derive the throughput, the packet loss rate and the average delay of an end-to-end wireless link, where the transmitter relies on adaptive modulation and coding (AMC) at the physical layer while accounting for finite-length buffer effects at the data link layer. Guided by our cross-layer performance analysis, we develop a simple procedure to determine the minimal required bandwidth, which guarantees the prescribed QoS over the wireless link.

We study a wireless access system with adaptive modulation and coding (AMC) at the physical layer, finite-length queuing at the data link layer and a TCP protocol at the transport layer. We analyze the end-to-end TCP performance via a fixed-point procedure that effectively couples TCP with the AMC-based wireless link. Guided by the performance analysis, we present a simple cross-layer design, which optimizes the target packet error rate in AMC at the physical layer, so that the TCP throughput at the transport layer is maximized.

Wired-wireless multimedia networks require diverse quality-of-service (QoS) support. To this end, it is essential to rely on QoS metrics pertinent to wireless links. In this paper, we develop a cross-layer model for adaptive wireless links, which enables derivation of the desired QoS metrics analytically from the typical wireless parameters across the hardware-radio layer, the physical layer and the data link layer. We illustrate the advantages of our model: generality, simplicity, scalability and backward compatibility. Finally, we outline its applications to power control, TCP, UDP and bandwidth scheduling in wireless networks.

In the presence of frequency-selective multipath fading channels, code-division multiple access (CDMA) suffers from multiuser interference (MUI) and intersymbol interference (ISI); but when properly designed, it enjoys multipath diversity. Orthogonal frequency-division multiple access (OFDMA) is MUI-free, but it does not enable the available channel diversity without employing error-control coding. On the other hand, coded OFDMA may achieve lower diversity than a CDMA system employing the same error-control codes. In this paper, we merge the advantages of OFDMA and CDMA to minimize MUI effects, and also enable the maximum available diversity for every user. In our group orthogonal multicarrier CDMA (GO-MC-CDMA) scheme, groups of users share a set of subcarriers. By judiciously choosing group subcarriers, we guarantee that every user transmits with maximum diversity. MUI is only present among users in the same group, and is suppressed via multiuser detection, which becomes practically feasible because we assign a small number of users per group. Performance is analyzed, and simulations are carried out to illustrate the merits of GO-MC-CDMA relative to existing alternatives.

We developed a cross-layer design which combines adaptive modulation and coding at the physical layer with a truncated automatic repeat request protocol at the data link layer, in order to maximize spectral efficiency under prescribed delay and error performance constraints. We derive the achieved spectral efficiency in closed-form for transmissions over Nakagami-m block fading channels. Numerical results reveal that retransmissions at the data link layer relieve stringent error control requirements at the physical layer, and thereby enable considerable spectral efficiency gain. This gain is comparable with that offered by diversity, provided that the maximum number of transmissions per packet equals the diversity order. Diminishing returns on spectral efficiency, that result when increasing the maximum number of retransmissions, suggest that a small number of retransmissions offers a desirable delay-throughput tradeoff, in practice.

We combine error-control coding with linear precoding (LP) for flat-fading channels, as well as for wireless orthogonal frequency-division multiplexing transmissions through frequency-selective fading channels. The performance is analyzed and compared with the corresponding error-control-coded system without precoding. By wedding LP with conventional error-control coding, the diversity order becomes equal to the error-control code's minimum Hamming distance times the precoder size. We also derive a low-complexity turbo-decoding algorithm for joint coded-precoded transmissions. We analyze the decoding complexity and compare it with an error-control-coded system without LP. Extensive simulations with convolutional and turbo codes for HiperLan/2 channels support the analysis and demonstrate superior performance of the proposed system.

Adaptive modulation improves the system throughput considerably by matching transmitter parameters to time-varying wireless fading channels. Crucial to adaptive modulation is the quality of channel state information at the transmitter. In this paper, we first present a channel predictor based on pilot symbol assisted modulation for multiple-input multiple-output Rayleigh fading channels. We then analyze the impact of the channel prediction error on the bit error rate performance of a transmit-beamformer with adaptive modulation that treats the predicted channels as perfect. Our numerical results reveal the critical value of the normalized prediction error, below which the predicted channels can be treated as perfect by the adaptive modulator; otherwise, explicit consideration of the channel imperfection must be accounted for at the transmitter.

Adaptive modulation has the potential to increase the system throughput significantly by matching transmitter parameters to time-varying channel conditions. However, adaptive modulation schemes that rely on perfect channel state information (CSI) are sensitive to CSI imperfections induced by estimation errors and feedback delays. In this paper, we design adaptive modulation schemes for multiantenna transmissions based on partial CSI, that models the spatial fading channels as Gaussian random variables with nonzero mean and white covariance, conditioned on feedback information. Based on a two-dimensional beamformer, our proposed transmitter optimally adapts the basis beams, the power allocation between two beams, and the signal constellation, to maximize the transmission rate, while maintaining a target bit-error rate. Adaptive trellis-coded multiantenna modulation is also investigated. Numerical results demonstrate the rate improvement, and illustrate an interesting tradeoff that emerges between feedback quality and hardware complexity.

A so-termed chip-interleaved block-spread (CIBS) code division multiple access (CDMA) system has been introduced for cellular applications in the presence of frequency selective multipath channels. In both uplink and downlink operation, CIBS-CDMA achieves multiuser-interference (MUI) free reception within each cell. This paper focuses on the cellular downlink configuration and compares CIBS-CDMA against the conventional direct-sequence (DS) CDMA system, which relies on a chip equalizer to restore code orthogonality and, subsequently, suppresses MUI by despreading. We provide a unifying framework for both systems and investigate their performance in the presence of intercell interference and soft-handoff operation. Extensive comparisons from load, performance, complexity, and flexibility perspectives illustrate the merits, along with the disadvantages, of CIBS-CDMA over DS-CDMA, and reveal its potential for future wireless systems.

We consider the problem of embedded image transmission with feedback of channel state information (CSI) and retransmission request. Adaptive modulation is employed to improve the spectral efficiency. Constrained retransmission is employed to provide reliable packet transmission with limited delay. Performance analysis and numerical results over Nakagami block fading channels are provided for the transmission system with statistical or instantaneous CSI at the transmitter. The results show that CSI at the transmitter is very important for adaptive modulation/coding over fading channels.

Adaptive modulation improves system throughput considerably by matching transmitter parameters to time-varying wireless fading channels. Crucial to adaptive modulation is the quality of channel state information (CSI) at the transmitter. We consider a channel predictor based on pilot symbol assisted modulation (PSAM) for multi-input multi-output (MIMO) Rayleigh fading channels. We analyze the impact of the channel prediction error on the bit error rate (BER) performance of an adaptive system assuming perfect CSI. Our numerical results reveal the critical value of the normalized prediction error, below which the predicted channels can be treated as perfect by the adaptive modulator; otherwise, explicit consideration of the channel imperfection must be accounted for at the transmitter.

Adaptive modulation has the potential to increase the system throughput significantly by matching transmitter parameters to time-varying channel conditions. However, adaptive modulation assuming perfect channel state information (CSI) is sensitive to CSI imperfections induced by estimation errors and feedback delays. In this paper, we design adaptive modulation schemes for multi-antenna transmission based on partial CSI, that models the spatial fading channels as Gaussian random variables with non-zero mean and white covariance, conditioned on feedback information. Based on a two-directional beamformer, our proposed transmitter optimally adapts the beam directions, the power allocation between two beams, and the signal constellation, to maximize the transmission rate while maintaining a target bit error rate (BER). Numerical results demonstrate the rate improvement, and illustrate an interesting tradeoff that emerges between feedback quality and hardware complexity.

Orthogonal frequency division multiple access (OFDMA) gains increasing attention for broadband, high data rate wireless/wireline communications. In this paper, we propose a novel unitary precoded (UP) OFDMA scheme for uplink applications, which increases the system bandwidth efficiency, while preserving constant modulus transmissions. Theoretical analysis for the proposed UP-OFDMA with channel coding quantifies the performance improvement introduced by unitary precoding. It provides guidelines for practical system designs, and reveals performance gap between the proposed system and the single user bound. Simulation results confirm that the proposed system improves performance considerably relative to conventional OFDMA.

Orthogonal frequency-division multiple access (OFDMA) gains increasing attention for broadband, high data rate wireless communications. We develop a novel unitary precoded (UP) OFDMA scheme that is particularly appealing for the uplink, because it offers high bandwidth efficiency, and constant modulus transmissions for each user. Theoretical analysis of UP-OFDMA with channel coding shows the performance improvement introduced by unitary precoding. It provides useful guidelines for practical system designs, and also quantifies the performance of UP-OFDMA relative to the single-user bound. Simulations confirm that UP-OFDMA improves performance considerably relative to conventional OFDMA.

Suppression of multiuser interference (MUI) and mitigation of time- and frequency-selective (doubly selective) channel effects constitute major challenges in the design of third-generation wireless mobile systems. Relying on a basis expansion model (BEM) for doubly selective channels, we develop a channel-independent block spreading scheme that preserves mutual orthogonality among single-cell users at the receiver. This alleviates the need for complex multiuser detection, and enables separation of the desired user by a simple code-matched channel-independent block despreading scheme that is maximum-likelihood (ML) optimal under the BEM plus white Gaussian noise assumption on the channel. In addition, each user achieves the maximum delay-Doppler diversity for Gaussian distributed BEM coefficients. Issues like links with existing multiuser transceivers, existence, user efficiency, special cases, backward compatibility with direct-sequence code-division multiple access (DS-CDMA), and error control coding, are briefly discussed.

We study space-time block coding for single-carrier block transmissions over frequency-selective multipath fading channels. We propose novel transmission schemes that achieve a maximum diversity of order N/sub t/N/sub r/(L+1) in rich scattering environments, where N/sub t/ (N/sub r/) is the number of transmit (receive) antennas, and L is the order of the finite impulse response (FIR) channels. We show that linear receiver processing collects full antenna diversity, while the overall complexity remains comparable to that of single-antenna transmissions over frequency-selective channels. We develop transmissions enabling maximum-likelihood optimal decoding based on Viterbi's ( 1998) algorithm, as well as turbo decoding. With single receive and two transmit antennas, the proposed transmission format is capacity achieving. Simulation results demonstrate that joint exploitation of space-multipath diversity leads to significantly improved performance in the presence of frequency-selective fading channels.

Optimal transmitter designs obeying the water-filling principle are well-documented, and widely applied, when the propagation channel is deterministically known and regularly updated at the transmitter. Because channel state information (CSI) may be costly or impossible to acquire in rapidly varying wireless environments, we develop in this paper statistical water-filling approaches for stationary random fading channels. These approaches require only knowledge of the channel correlations that do not necessitate frequent updates, and can be easily acquired. Applied to a multiple transmit-antenna paradigm, our optimal transmitter design turns out to be an eigen-beamformer with multiple beams pointing to orthogonal directions along the eigenvectors of the channel's correlation matrix, and with proper power loading across the beams. The optimality pertains to minimizing a tight bound on the symbol error rate. The resulting loaded eigen-beamforming outperforms not only the equal-power allocation across all antennas, but also the conventional beamformer that transmits the available power along the strongest direction. Coupled with orthogonal space-time block codes, two-dimensional (2-D) eigen-beamforming emerges as a more attractive choice than conventional one-dimensional (1-D) beamforming with uniformly better performance, without rate reduction, and without complexity increase.

We rely on a Basis Expansion Model (BEM) for the channel, to design three Space-Time-Doppler (SID) codecs that enable maximum diversity gains, when block transmissions undergo time-selective fading effects. Within the constraints of each maximum-diversity design, it is also possible to achieve maximum coding gain, at least when the BEM parameters are i.i.d. The theoretical results are corroborated by simulation results and the BEM is validated.

Recently, a so-termed Chip-Interleaved Block-Spread (CIBS) CDMA system has been introduced, which enables Multi-User Interference (MUI) free reception. This transceiver can be used in the uplink as well as in the downlink. In this paper, we focus on the downlink and compare the downlink CIBS-CDMA system employing an MUI-free receiver with the conventional downlink Direct-Sequence (DS) CDMA system employing a chip equalizer receiver. The analysis, which is validated by simulation results, reveals that CIBS-CDMA with an MUI-free receiver has a number of advantages over DS-CDMA with a chip equalizer receiver.

Optimal transmitter designs obeying the water-filling principle are well-documented, and widely applied when the propagation channel is deterministically known, and regularly updated at the transmitter. Because channel state information is impossible to be known perfectly at the transmitter in practical wireless systems, we develop in this paper optimal transmitter design based on the knowledge of mean values of the underlying channels. Applied to a multiple transmit-antenna paradigm, our optimal transmitter design turns out to be an eigen-beamformer with multiple beams pointing to orthogonal directions along the eigenvectors of the channel correlation matrix conditioned on channel mean, and with proper power loading across beams. The optimality pertains to minimizing a tight bound on the symbol error rate. Coupled with orthogonal space time block codes, two-directional eigen-beamforming emerges as a more attractive choice than conventional one-directional beamforming with uniformly improved performance, without rate reduction.

Optimal transmitter designs obeying the water-filling principle are well-documented, and widely applied when the propagation channel is deterministically known and regularly updated at the transmitter. Because channel state information may be costly or impossible to acquire in rapidly varying wireless environments, we develop in this paper statistical water-filling approaches for stationary random fading channels. The resulting optimal designs require only knowledge of the channel's second order statistics that do not require frequent updates, and can be easily acquired. Optimality refers to minimizing a tight bound on the symbol error rate. Applied to a multiple transmit-antenna paradigm, the optimal precoder turns out to be a generalized eigen-beamformer with multiple beams pointing to orthogonal directions along the eigenvectors of the channel's covariance matrix, and with proper power loading across the beams. Coupled with orthogonal space time block codes, two-directional eigen-beamforming emerges as a more attractive choice than conventional one-directional beamforming, with uniformly better performance, and without rate reduction or complexity increase.

This paper addresses the design of a novel scheme that combines error control coding (EC) and unitary precoding (UP) in order to obtain high diversity gains in OFDM systems with low complexity. The overall diversity of the proposed system is shown to be the product of the individual diversities achievable by the error control coding and by the unitary precoding, while the complexity is just a linear multiple of the sum of their individual complexities. In a practical HiperLan/2 setup, the proposed system achieves a gain of 4 dB, (3.3 dB), at a bit error rate (BER) of 10/sup -3/, relative to conventional OFDM systems that only deploy convolutional (turbo) coding.

A novel multiuser-interference (MUI)-free code division multiple access (CDMA) transceiver for frequency-selective multipath channels is developed. Relying on chip-interleaving and zero padded transmissions, orthogonality among different users' spreading codes is maintained at the receiver even after frequency-selective propagation. As a result, deterministic multiuser separation with low-complexity code-matched filtering becomes possible without loss of maximum likelihood optimality. In addition to MUI-free reception, the proposed system guarantees channel-irrespective symbol detection and achieves high bandwidth efficiency by increasing the symbol block size. Filling the zero-gaps with known symbols allows for perfectly constant modulus transmissions. Important variants of the proposed transceivers are derived to include cyclic prefixed transmissions and various redundant or nonredundant precoding alternatives. (Semi-) blind channel estimation algorithms are also discussed. Simulation results demonstrate improved performance of the proposed system relative to competing alternatives.

We compare single user digital multi-carrier spread spectrum (MC-SS) modulation with direct sequence (DS) SS (with a modified implementation) in the presence of narrowband interference (NBI) and multipath fading. We derive closed-form expressions for the symbol error probability for both the linear MMSE receiver as well as the conventional matched-filter receiver under different scenarios: additive white Gaussian noise (AWGN) channel with NBI, multipath channel with or without NBI. We show that DS-SS can achieve the same performance as MC-SS if the spreading code is carefully designed to have perfect periodic autocorrelation function (PACF). On the other hand, MC-SS is more robust to narrowband interference and multipath fading than is DS-SS with the widely used spreading codes that do not possess perfect PACE. Our analysis reveals that the performance improvement of MC-SS is precisely due to the implicit construction of an equivalent spreading code having nonconstant amplitude but possessing perfect periodic autocorrelation.

Suppression of multiuser interference (MUI) and mitigation of time- and frequency-selective effects constitute major challenges in the design of third-generation wireless mobile systems. Relying on block spreading and judiciously chosen time-frequency guard intervals, we propose a multiuser transceiver that eliminates MUI deterministically and guarantees symbol detectability in the presence of unknown time- and frequency-selective fading. Blind channel estimation is also investigated. Simulation results demonstrate the validity of the theoretical results and show improved performance of the proposed transceiver over a multi-user time-frequency RAKE receiver.

We study space-time block coding for single carrier block transmissions through frequency selective multipath channels of order L. We prove that maximum diversity of order 2(L + 1) can be achieved with two transmit and one receive antennas. We show that simple linear receiver processing can achieve full antenna diversity gains, and investigate the performance versus complexity tradeoffs that emerge when the embedded multipath diversity is also exploited. Simulation results demonstrate that joint exploitation of multi-antenna and multipath diversities yields significantly enhanced performance in frequency selective multipath channels.

We compare single user digital multi-carrier spread spectrum modulation with direct sequence spread spectrum in the presence of frequency-selective multipath fading. We derive closed-form expressions for the bit error probability and show that MC-SS is more robust to multipath fading than is DS-SS.

We study space-time (ST) block coding for single carrier zero-padded (ZP) block transmissions through finite impulse response (FIR) multipath fading channels of order L. We prove that the maximum diversity of order 2(L+1) is achieved with two transmit and one receive antenna. Simulations demonstrate that joint exploitation of multi-antenna diversity and multipath diversity leads to significantly enhanced performance.

Novel blind channel estimators based on the finite alphabet property of information symbols are derived in this paper for OFDM and related multicarrier code-division multiple access (MC-CDMA) systems. The resulting algorithms are applicable not only to standard OFDM transmitters with cyclic prefix, but also to the zero padded OFDM transmissions that improve symbol recovery at the expense of altering the transmitter and complicating the equalizer. Based on FFT-processed received data, channel identifiability is guaranteed regardless of channel zero locations and various channel estimation algorithms become available by trading on the complexity for performance. Unlike existing blind channel estimators, the proposed alternatives require short data records especially for PSK transmissions. The inherent scalar ambiguity is easily resolved because it has unit amplitude and phase values drawn from a finite set. Decoupling channel from symbol estimation enables a phase-directed operation that improves upon decision-directed schemes that are known to suffer from error propagation. Practical issues are also addressed including the presence of frequency guard intervals, constellation and power loading, various frame designs, coded transmissions as well as semi-blind and online implementations for systems with training sequences. The algorithms are tested with simulations and also compared with existing alternatives in a realistic HIPERLAN/2 setting.

A generalized frequency-hopping (GFH) orthogonal frequency-division multiple-access (OFDMA) system is developed in this paper as a structured long code direct-sequence code-division multiple-access (DS-CDMA) system in order to bridge frequency-hopped multicarrier transmissions with long code DS-CDMA. Through judicious code design, multiuser interference is eliminated deterministically in the presence of unknown frequency-selective multipath channels. Thanks to frequency-hopping, no single user suffers from consistent fading effects and constellation-irrespective channel identifiability is guaranteed regardless of channel nulls. A host of blind channel estimation algorithms are developed trading off complexity with performance. Two important variants, corresponding to slow- and fast-hopping, are also addressed with the latter offering symbol recovery guarantees. Performance analysis and simulation results illustrate the merits of GFH-OFDMA relative to conventional OFDMA and long code DS-CDMA with pseudorandom noise codes and RAKE reception.

Optimal transmitter-receiver designs obeying the water-filling principle are well-known and widely applied when the propagation channel is deterministically known and frequently updated at the transmitter. Because the latter may be costly or impossible to acquire in rapidly varying wireless environments, we develop in this paper statistical water-filling-like criteria for stationary random fading channels. The resulting optimal designs require only knowledge of the channel's correlation matrix that does not require frequent updates and can be easily acquired. Applied to a multiple transmit-antenna paradigm, the optimal precoder and loading algorithm outperform not only the conventional equal-power allocation across all antennas but also their deterministic water-filling counterparts in fast fading scenarios.

The generalized frequency hopping (GFH) OFDMA system developed in this paper permeates FH benefits to multicarrier CDMA transmissions through unknown frequency-selective multipath channels. No single user suffers from consistent fading effects and multiuser interference is eliminated by design. Constellation-irrespective channel identifiability is guaranteed, and a host of blind channel estimation algorithms are developed, trading off complexity with performance. An important variant corresponding to fast-hopping is also addressed. Performance analysis and simulation results illustrate the merits of GFH-OFDMA relative to conventional OFDMA and long code CDMA with RAKE reception.