Showing papers on "Orthogonal frequency-division multiplexing published in 2015"

Filtered OFDM: A new waveform for future wireless systems

Abstract: A spectrally-localized waveform is proposed based on filtered orthogonal frequency division multiplexing (f-OFDM). By allowing the filter length to exceed the cyclic prefix (CP) length of OFDM and designing the filter appropriately, the proposed f-OFDM waveform can achieve a desirable frequency localization for bandwidths as narrow as a few tens of subcarriers, while keeping the inter-symbol interference/inter-carrier interference (ISI/ICI) within an acceptable limit. Enabled by the proposed f-OFDM, an asynchronous filtered orthogonal frequency division multiple access (f-OFDMA)/filtered discrete-Fourier transform-spread OFDMA (f-DFT-S-OFDMA) scheme is introduced, which uses the spectrum shaping filter at each transmitter for side lobe leakage elimination and a bank of filters at the receiver for inter-user interference rejection. Per-user downsampling and short fast Fourier transform (FFT) are used at the receiver to ensure a reasonable complexity of implementation. The proposed scheme removes the inter-user time-synchronization overhead required in the synchronous OFDMA/DFT-S-OFDMA. The performance of the asynchronous f-OFDMA is evaluated and compared with that of the universal-filtered OFDM (UF-OFDM), proposed in [1], [2].

TeraHertz Photonics for Wireless Communications

Abstract: Optical fibre transmission has enabled greatly increased transmission rates with 10 Gb/s common in local area networks. End users find wireless access highly convenient for mobile communication. However, limited spectrum availability at microwave frequencies results in per-user transmission rates limited to much lower values, e.g., 500 Mb/s for 5-GHz band IEEE 802.11ac. Extending the high data-rate capacity of optical fiber transmission to wireless devices requires greatly increased carrier frequencies. This paper will describe how photonic techniques can enable ultrahigh capacity wireless data distribution and transmission using signals at millimeter-wave and TeraHertz (THz) frequencies.

Generalization of Orthogonal Frequency Division Multiplexing With Index Modulation

Abstract: Recently, orthogonal frequency division multiplexing (OFDM) with index modulation (OFDM-IM) was proposed. By selecting a fixed number of subcarriers as active subcarriers to carry constellation symbols, the indices of these active subcarriers may carry additional bits of information. In this paper, we propose two generalization schemes of OFDM-IM, named OFDM with generalized index modulation 1 (OFDM-GIM1) and OFDM-GIM2, respectively. In OFDM-GIM1, the number of active subcarriers in an OFDM subblock is no longer fixed. Dependent on the input binary string, different numbers of active subcarriers are assigned to carry constellation symbols. In OFDM-GIM2, independent index modulation is performed on the in-phase and quadrature component per subcarrier. Through such ways, a higher spectral efficiency than that of OFDM-IM may be achieved. Since both generalization schemes proposed suffer from BER performance loss in low SNR region, an interleaving technique is proposed to tackle this problem. Finally, noting that the two generalization schemes are compatible with each other, the combination of these two schemes, named OFDM-GIM3, has also been investigated. Computer simulation results clearly show our proposed scheme's superiority in both spectral efficiency and BER performance compared to existing works.

Ultra-dense networks in millimeter-wave frequencies

Abstract: Demands for very high system capacity and end-user data rates of the order of 10 Gb/s can be met in localized environments by Ultra-Dense Networks (UDN), characterized as networks with very short inter-site distances capable of ensuring low interference levels during communications. UDNs are expected to operate in the millimeter-wave band, where wide bandwidth signals needed for such high data rates can be designed, and will rely on high-gain beamforming to mitigate path loss and ensure low interference. The dense deployment of infrastructure nodes will make traditional wire-based backhaul provisioning challenging. Wireless self-backhauling over multiple hops is proposed to enhance flexibility in deployment. A description of the architecture and a concept based on separation of mobility, radio resource coordination among multiple nodes, and data plane handling, as well as on integration with wide-area networks, is introduced. A simulation of a multi-node office environment is used to demonstrate the performance of wireless self-backhauling at various loads.

Energy-Efficient Resource Assignment and Power Allocation in Heterogeneous Cloud Radio Access Networks

Abstract: Taking full advantage of both heterogeneous networks and cloud access radio access networks, heterogeneous cloud radio access networks (H-CRANs) are presented to enhance both spectral and energy efficiencies, where remote radio heads (RRHs) are mainly used to provide high data rates for users with high quality of service (QoS) requirements, whereas the high-power node (HPN) is deployed to guarantee seamless coverage and serve users with low-QoS requirements. To mitigate the intertier interference and improve energy efficiency (EE) performances in H-CRANs, characterizing user association with RRH/HPN is considered in this paper, and the traditional soft fractional frequency reuse (S-FFR) is enhanced. Based on the RRH/HPN association constraint and the enhanced S-FFR, an energy-efficient optimization problem with the resource assignment and power allocation for the orthogonal-frequency-division-multiple-access-based H-CRANs is formulated as a nonconvex objective function. To deal with the nonconvexity, an equivalent convex feasibility problem is reformulated, and closed-form expressions for the energy-efficient resource allocation solution to jointly allocate the resource block and transmit power are derived by the Lagrange dual decomposition method. Simulation results confirm that the H-CRAN architecture and the corresponding resource allocation solution can enhance the EE significantly.

OFDM With Index Modulation Using Coordinate Interleaving

Abstract: Orthogonal frequency division multiplexing with index modulation (OFDM-IM), which uses the indices of the active subcarriers to transmit data, is a recently proposed multicarrier transmission technique. In this letter, we propose coordinate interleaved OFDM-IM (CI-OFDM-IM) by combining OFDM-IM and space-time block codes with coordinate interleaving. In this scheme, the real and imaginary parts of the data symbols are transmitted over different active subcarriers to achieve an additional diversity gain. The average bit error probability (ABEP) of the proposed scheme is derived and its superiority over the reference systems is shown via computer simulations.

Layered ACO-OFDM for intensity-modulated direct-detection optical wireless transmission

Abstract: Layered asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) with high spectral efficiency is proposed in this paper for optical wireless transmission employing intensity modulation with direct detection In contrast to the conventional ACO-OFDM, which only utilizes odd subcarriers for modulation, leading to an obvious spectral efficiency loss, in layered ACO-OFDM, the subcarriers are divided into different layers and modulated by different kinds of ACO-OFDM, which are combined for simultaneous transmission In this way, more subcarriers are used for data transmission and the spectral efficiency is improved An iterative receiver is also proposed for layered ACO-OFDM, where the negative clipping distortion of each layer is subtracted once it is detected so that the signals from different layers can be recovered Theoretical analysis shows that the proposed scheme can improve the spectral efficiency by up to 2 times compared with conventional ACO-OFDM approaches with the same modulation order Meanwhile, simulation results confirm a considerable signal-to-noise ratio gain over ACO-OFDM at the same spectral efficiency

Wireless Information and Energy Transfer for Two-Hop Non-Regenerative MIMO-OFDM Relay Networks

Abstract: This paper investigates the simultaneous wireless information and energy transfer for the non-regenerative multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) relaying system. By considering two practical receiver architectures, we present two protocols, time switching-based relaying (TSR) and power splitting-based relaying (PSR). To explore the system performance limits, we formulate two optimization problems to maximize the end-to-end achievable information rate with the full channel state information (CSI) assumption. Since both problems are non-convex and have no known solution method, we firstly derive some explicit results by theoretical analysis and then design effective algorithms for them. Numerical results show that the performances of both protocols are greatly affected by the relay position. Specifically, PSR and TSR show very different behaviors to the variation of relay position. The achievable information rate of PSR monotonically decreases when the relay moves from the source towards the destination, but for TSR, the performance is relatively worse when the relay is placed in the middle of the source and the destination. This is the first time such a phenomenon has been observed. In addition, it is also shown that PSR always outperforms TSR in such a MIMO-OFDM relaying system. Moreover, the effects of the number of antennas and the number of subcarriers are also discussed.

Quantized Massive MU-MIMO-OFDM Uplink

Abstract: Coarse quantization at the base station (BS) of a massive multi-user (MU) multiple-input multiple-output (MIMO) wireless system promises significant power and cost savings. Coarse quantization also enables significant reductions of the raw analog-to-digital converter (ADC) data that must be transferred from a spatially-separated antenna array to the baseband processing unit. The theoretical limits as well as practical transceiver algorithms for such quantized MU-MIMO systems operating over frequency-flat, narrowband channels have been studied extensively. However, the practically relevant scenario where such communication systems operate over frequency-selective, wideband channels is less well understood. This paper investigates the uplink performance of a quantized massive MU-MIMO system that deploys orthogonal frequency-division multiplexing (OFDM) for wideband communication. We propose new algorithms for quantized maximum a-posteriori (MAP) channel estimation and data detection, and we study the associated performance/quantization trade-offs. Our results demonstrate that coarse quantization (e.g., four to six bits, depending on the ratio between the number of BS antennas and the number of users) in massive MU-MIMO-OFDM systems entails virtually no performance loss compared to the infinite-precision case at no additional cost in terms of baseband processing complexity.

Signal identification for emerging intelligent radios: classical problems and new challenges

Abstract: Signal identification, which initially found applications in electronic warfare and spectrum monitoring and surveillance, has been recently considered for commercial communications in the context of software defined and cognitive radios. In this article, I present a snapshot of the status of signal identification algorithms, starting from a general description of maximum likelihood (ML) and feature based (FB) approaches to a more detailed discussion of a practical methodology using cyclostationarity-based features. I discuss the cyclostationarity-based features of various signals and the criteria of decision for their identification, while considering classical problems of identifying single carrier linearly digitally (SCLD) modulated signals, as well as new challenges posed by the identification of orthogonal frequency division multiplexing (OFDM), SC frequency domain equalization (SC-FDE), and multiple-transmit antenna signals. I conclude the article with remarks on practical solutions to signal identification and open research issues.

Artificial Neural Network Nonlinear Equalizer for Coherent Optical OFDM

Abstract: We propose a novel low-complexity artificial neural network (ANN)-based nonlinear equalizer (NLE) for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) and compare it with the recent inverse Volterra-series transfer function (IVSTF)-based NLE over up to 1000 km of uncompensated links. Demonstration of ANN-NLE at 80-Gb/s CO-OFDM using 16-quadrature amplitude modulation reveals a Q-factor improvement after 1000-km transmission of 3 and 1 dB with respect to the linear equalization and IVSTF-NLE, respectively.

Optical wireless transmission of 405 nm, 1.45 Gbit/s optical IM/DD-OFDM signals through a 4.8 m underwater channel.

Abstract: In this paper, we experimentally demonstrate wireless transmission of optical intensity modulation/direct detection-orthogonal frequency division multiplexing (IM/DD-OFDM) signals in an underwater channel using a field programmable gate array based real-time transmitter. The real-time transmission of a 405 nm 1.45 Gbit/s optical OFDM signal through a 4.8 m underwater channel with an error vector magnitude of approximately 10% was successfully achieved.

Low-Complexity ML Detector and Performance Analysis for OFDM With In-Phase/Quadrature Index Modulation

Abstract: A novel variant of orthogonal frequency division multiplexing (OFDM) techniques, which carries additional information bits through the index domain including in-phase and quadrature dimensions, is recently proposed. For its nature, we refer to this technique as OFDM with in-phase/quadrature index modulation (OFDM-I/Q-IM) . In this letter, we propose a novel low-complexity detector based on the maximum-likelihood (ML) criterion for OFDM-I/Q-IM, which does not need to know the variance of the noise and the possible realizations of the active subcarrier indices. With the proposed ML detector, the asymptotic average bit error probability (ABEP) and the exact coding gain achieved by OFDM-I/Q-IM are also derived.

A survey: Several technologies of non-orthogonal transmission for 5G

Abstract: One key advantage of 4G OFDM system is the relatively simple receiver implementation due to the orthogonal resource allocation. However, from sum-capacity and spectral efficiency points of view, orthogonal systems are never the achieving schemes. With the rapid development of mobile communication systems, a novel concept of non-orthogonal transmission for 5G mobile communications has attracted researches all around the world. In this trend, many new multiple access schemes and waveform modulation technologies were proposed. In this paper, some promising ones of them were discussed which include Non-orthogonal Multiple Access (NOMA), Sparse Code Multiple Access (SCMA), Multi-user Shared Access (MUSA), Pattern Division Multiple Access (PDMA) and some main new waveforms including Filter-bank based Multicarrier (FBMC), Universal Filtered Multi-Carrier (UFMC), Generalized Frequency Division Multiplexing (GFDM). By analyzing and comparing features of these technologies, a research direction of guiding on future 5G multiple access and waveform are given.

Boosting the Efficiency: Unconventional Waveform Design for Efficient Wireless Power Transfer

Abstract: Traditionally, wireless power is delivered through single-carrier, continuous-wave (CW) signals. Most research efforts to enhance the efficiency of wireless power transfer systems have been confined to the circuit-level design. However, in recent years, attention has been paid to the waveform design for wireless power transmission. It has been found that signals featuring a high peak-to-average power ratio (PAPR) can provide efficiency improvement when compared with CW signals. A number of approaches have been proposed, such as multisines/multicarrier orthogonal frequency division multiplex (OFDM) signals, chaotic signals, harmonicsignals, ultrawideband (UWB) signals, intermittent CW (ICW) signals, or white-noise signals. This article reviews these techniques with a focus on multisines/multicarrier signals, harmonic signals, and chaotic signals. A theoretical explanation for efficiency improvement is provided and accompanied by experimental results. Circuit design considerations are presented for the receiver side, and efficient transmission architectures are also described with an emphasis on spatial power combining.

Adaptive Modulation Schemes for Visible Light Communications

Abstract: A major limitation of existing visible light communication (VLC) systems is the limited modulation bandwidth of light-emitting diodes used in such systems. Using adaptive modulation to improve the spectral efficiency for radio communications has been well studied. For VLC with various physical layer schemes, however, how adaptive modulation works is not well understood yet. The goal of this paper is to provide an in-depth analysis of the achievable spectral efficiency of adaptive modulation for three different schemes for high speed VLC: dc-biased optical orthogonal frequency division multiplexing (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), and single-carrier frequency-domain equalization (SC-FDE). We will show that in the low signal-to-noise ratio region, the ACO-OFDM-based adaptive modulation scheme outperforms the other two schemes. SC-FDE-based adaptive modulation achieves a better performance than the DCO-OFDM-based scheme, and it is much simpler than the other two schemes.

Unlocking Spectral Efficiency in Intensity Modulation and Direct Detection Systems

Abstract: A number of inherently unipolar orthogonal frequency division multiplexing (OFDM) modulation schemes have been introduced recently in an attempt to improve the energy efficiency of OFDM-based intensity modulation and direct detection (IM/DD) systems. All such algorithms, including asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) and unipolar orthogonal frequency division multiplexing (U-OFDM), experience an inherent loss in spectral efficiency caused by the restrictions imposed on the OFDM frame structure required for the generation of a unipolar signal. The current paper presents a modified modulation approach, termed enhanced U-OFDM (eU-OFDM), which compensates the spectral efficiency loss in U-OFDM. At the same time, it still allows for the generation of an inherently unipolar modulation signal that achieves better performance in terms of both electrical power and optical power dissipation compared to the conventional state-of-the-art technique direct current (DC)-biased optical OFDM (DCO-OFDM). To the best of the authors' knowledge, the current work also presents the first experimental proof-of-concept demonstration of both U-OFDM and eU-OFDM, and clearly demonstrates the significant energy advantages that these two schemes can introduce in an optical wireless communications (OWC) system.

Visible Light Communications Using OFDM and Multiple LEDs

Abstract: Visible-light communication (VLC) systems leverage solid-state illumination devices to create high-speed communication links. Orthogonal frequency-division multiplexing (OFDM) has been considered for these intensity-modulated/direct-detection (IM/DD) optical channels, however, it suffers from high peak-to-average power ratio (PAPR). Moreover, the implementation of linear, power-efficient, high-current, wideband drivers is challenging. In this paper, the concept of spatial summing is developed where wideband, high PAPR OFDM signals are partitioned into many low-PAPR narrowband signals that are transmitted from multiple LEDs. The signals from different LEDs are allowed to sum in space before being detected by a conventional OFDM receiver. Spatial optical-OFDM (SO-OFDM) is proposed in which filtered subsets of carriers are emitted by each LED. In addition, low-PAPR optical single-carrier FDMA (OSC-FDMA) is developed where different collections of LEDs act as virtual users in a multiple-access scheme. The different variations of SO-OFDM and OSC-FDMA are compared to conventional DC-biased optical (DCO) OFDM and are shown to achieve lower PAPR and more robustness to LED nonlinearities leading to error rate performance gains at high signal-to-noise ratios.

PHY-Layer Resiliency in OFDM Communications: A Tutorial

Abstract: This tutorial paper addresses the physical layer security concerns and resiliency of Orthogonal Frequency Division Multiplexing (OFDM) communications; the de facto air-interface of most modern wireless broadband standards including 3GPP Long Term Evolution (LTE) and WiMAX. The paper starts with a brief introduction to the OFDM waveform and then reviews the robustness of the existing OFDM waveform in the presence of noise, multipath fading, and interference. The paper then moves on to build comprehensive adversarial models against OFDM waveforms. Robustness of OFDM is first investigated under AWGN noise and noise-like jamming attack scenarios, then under uncorrelated yet colored interferences from modulated sources (both intentional and unintentional). Finally, the paper explores some of the more recent developments in the field of energy efficient correlated jamming attacks that can disrupt communication severely by exploiting the knowledge of the target waveform structure. Potential countermeasures against such jamming attacks are presented, in an attempt to make a robust and resilient OFDM waveform.

Adaptive Modulation and Coding for Underwater Acoustic OFDM

Abstract: Underwater acoustic channels are fast varying spatially and temporally according to environmental conditions. Adaptive modulation and coding (AMC) is appealing for underwater acoustic communications to improve the system efficiency by matching transmission parameters to channel variations. In this paper, we construct an AMC system with a finite number of transmission modes in the context of underwater orthogonal frequency-division multiplexing (OFDM). We propose the effective signal-to-noise ratio (SNR) computed after channel estimation and channel decoding as a new performance metric for mode switching, which is shown to predict the system performance more consistently than the input SNR and the pilot SNR. Real-time AMC tests have been conducted in a recent sea experiment to maximize the transmission rate with a given transmission power.

Multiple-Input Multiple-Output OFDM with Index Modulation

Abstract: Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a novel multicarrier transmission technique which has been proposed as an alternative to classical OFDM. The main idea of OFDM-IM is the use of the indices of the active subcarriers in an OFDM system as an additional source of information. In this work, we propose multiple-input multiple-output OFDM-IM (MIMO-OFDM-IM) scheme by combining OFDM-IM and MIMO transmission techniques. The low complexity transceiver structure of the MIMO-OFDM-IM scheme is developed and it is shown via computer simulations that the proposed MIMO-OFDM-IM scheme achieves significantly better error performance than classical MIMO-OFDM for several different system configurations.

Adaptive Beamforming With Resource Allocation for Distance-Aware Multi-User Indoor Terahertz Communications

Abstract: Terahertz (THz) communication is envisioned as a key technology for next-generation ultra-high-speed wireless systems. In this paper, we study an indoor multi-user THz communication system with multiple antenna subarrays. To capture the distance-frequency-dependent characteristics of THz channels, we design a hybrid beamforming scheme with distance-aware multi-carrier transmission, including analog beamforming for user grouping and interference cancellation in radio-frequency (RF) domain and digital beamforming with dynamically selected subarrays at baseband. Specifically, an adaptive power allocation and low-complexity antenna subarray selection policy is developed to serve different users at different distances and reduce the cost of active RF circuits simultaneously, where two greedy subarray selection algorithms are proposed. The effectiveness of the proposed adaptive hybrid beamforming and antenna subarray selection algorithms is verified through simulation results, which achieves significant gains over other nonadaptive and non-distance-aware schemes.

Robust Power Control and Beamforming in Cognitive Radio Networks: A Survey

Abstract: Traditional spectrum allocation policies may result in temporarily unused radio spectrum. Cognitive radio (CR) has emerged as a promising technology to exploit the radio spectrum in a more efficient manner by allowing spectrum sharing between secondary users (SUs) and primary users (PUs). Power control and beamforming are two key techniques in CR design used to maximize the benefits of SUs, yet to maintain the quality of service of PUs. In practice, the available system parameters (e.g., channel state information and interference power) to enable power control and beamforming could be uncertain due to various factors such as estimation error and/or measurement error, thus the robustness of the designed algorithms should be considered in order to overcome the effects of parametric uncertainties. The objective of this paper is to give an overview on robust design for power control and beamforming in cognitive radio networks (CRNs). We will analyze modeling methods for parametric uncertainties, introduce various design methodologies, and present robust algorithms that have appeared in the literatures. Finally, some potential issues and future research directions in this field will be presented.

Multiuser MIMO-OFDM for Visible Light Communications

Abstract: Visible light communication (VLC) is emerging as a promising technique to provide ubiquitous wireless connection. In this paper, a multiuser VLC system utilizing multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) is investigated. Since the distances of the multiple transmitter–receiver links are different, their temporal delays are also different, resulting in complex channel gain and phase differences when transformed to the frequency domain. For each subcarrier in OFDM, the corresponding precoding matrix is calculated in the frequency domain to eliminate multiuser interference. Phase information in the frequency domain is first considered, where complex, instead of real, channel matrices are used for precoding, which reduces the channel correlation and achieves better performance. Moreover, minimum dc bias, unified dc bias, and asymmetrically clipped optical OFDM-based schemes are proposed to generate real-valued nonnegative signals for intensity modulation, and their performances are validated via simulations with zero forcing and minimum mean-squared error (MMSE) precoding techniques.

Joint Subcarrier Assignment and Power Allocation in Full-Duplex OFDMA Networks

Abstract: Recent advances in the physical layer have demonstrated the feasibility of in-band wireless full-duplex which enables a node to transmit and receive simultaneously on the same frequency band. While the full-duplex operation can ideally double the spectral efficiency, the network-level gain of full-duplex in large-scale networks remains unclear due to the complicated resource allocation in multi-carrier and multi-user environments. In this paper, we consider a single-cell full-duplex OFDMA network which consists of one full-duplex base station (BS) and multiple full-duplex mobile nodes. Our goal is to maximize the sum-rate performance by jointly optimizing subcarrier assignment and power allocation considering the characteristics of full-duplex transmissions. We develop an iterative solution that achieves local Pareto optimality in typical scenarios. Through extensive simulations, we demonstrate that our solution empirically achieves near-optimal performance and outperforms other resource allocation schemes designed for half-duplex networks. Also, we reveal the impact of various factors such as the channel correlation, the residual self-interference, and the distance between the BS and nodes on the full-duplex gain.

10 Gbps mobile visible light communication system employing angle diversity, imaging receivers, and relay nodes

Abstract: Over the past decade, visible light communication (VLC) systems have typically operated between 50 Mbps and 3.4 Gbps. In this paper, we propose and evaluate mobile VLC systems that operate at 10 Gbps. The enhancements in channel bandwidth and data rate are achieved by the introduction of laser diodes (LDs), angle diversity receivers (ADR), imaging receivers, relay nodes, and delay adaptation techniques. We propose three mobile VLC systems: an ADR relay assisted LD-VLC, an imaging relay assisted LD-VLC (IMGR-LD), and select-the-best imaging relay assisted LD-VLC. The ADR and imaging receiver are proposed for the VLC system to mitigate the intersymbol interference, maximize the signal-to-noise ratio (SNR), and reduce the impact of multipath dispersion due to mobility. The combination of IMGR-LD with a delay adaptation technique adds a degree of freedom to the link design, which results in a VLC system that has the ability to provide high data rates under mobility. The proposed IMGR-LD system achieves significant improvements in the SNR over other systems in the worst case scenario in the considered real indoor environment.

MIMO SAR OFDM Chirp Waveform Diversity Design With Random Matrix Modulation

Abstract: Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) has received much attention due to its interesting application potentials, but effective waveform diversity design is still a technical challenge. In a MIMO SAR, each antenna should transmit a unique waveform, orthogonal to the waveforms transmitted by other antennas. The waveforms should have a large time-bandwidth product, low cross-correlation interferences, and a low peak-average ratio. To reach these aims, this paper proposes an orthogonal frequency division multiplexing (OFDM) chirp waveform with random matrix modulation. The designed waveforms are time-delay and frequency-shift decorrelated. Referring to MIMO SAR high-resolution imaging, the proposed OFDM chirp waveform parameters are optimally designed, and their performances are analyzed through the ambiguity function and range-Doppler-based MIMO SAR imaging algorithm. Extensive and comparative simulation results show that the waveforms have the superiorities of high range resolution, constant time domain and almost constant frequency-domain modulus, large time-bandwidth product, low peak-average ratio, and low time-delay and frequency-shift correlation peaks. More importantly, this scheme can easily generate over three orthogonal waveforms with a large time-bandwidth product.

MIMO Full-Duplex Precoding: A Joint Beamforming and Self-Interference Cancellation Structure

Abstract: This paper presents a single- or multi-user Multiple-Input-Multiple-Output (MIMO) Full-Duplex (FD) precoding transceiver structure applicable for single-carrier and Orthogonal Frequency Division Multiplexing (OFDM) systems. The structure increases the dimensionality at the transmitter, which allow for the cancellation of self-interference and forward beamforming to be jointly processed using precoding at the transmitter. The FD Precoding (FDP) structure allows for various joint precoding algorithms and different optimization objectives. We present separate and joint precoding designs for sum-rate maximization and a theoretical analysis of when the separate design is optimal. The joint designs make use of Sequential Convex Programming (SCP). Extensive simulation results using both channel models and measured data show that the FDP structure can provide very significant performance gains over existing techniques for both SU- and MU-MIMO systems. In particular, the FDP structure provides between 1.6 and 1.8 times the spectral efficiency of optimized half-duplex for many of the tested SU-MIMO scenarios.

Behavioral Modeling and Predistortion of Power Amplifiers Under Sparsity Hypothesis

Abstract: A simple and flexible technique for improving the modeling and predistortion of power amplifiers is presented. The technique, which relies on the sparsity assumption for the kernel coefficients of the full Volterra (FV) behavioral model, combines a greedy algorithm for the selection of the active coefficients, a maximum likelihood method for their estimation and an information criterion for determining the best model. The approach has been applied to the design of reduced-parameters FV-based digital predistorters for three power amplifiers driven with orthogonal frequency division multiplexing signals, following the LTE and DVB-T2 standards, and a multichannel wideband code-division multiple access signal. Results show that the proposed linearizers meet the spectral masks and error vector magnitude constraints of the referred standards and provide a reduction better than 45% in the number of parameters, compared to the FV predistorters.

Underwater Acoustic Communication

Abstract: Underwater wireless communications over distances in excess of about 100 m are established using acoustic signals. Acoustic signals propagate as pressure waves, whose energy absorption limits the available bandwidth. As a result, existing technology provides bit rates on the order of several kilobits per second for transmission over distances on the order of several kilometers. Additional challenges are presented by multipath propagation that causes frequency selectivity, random time variation, and Doppler effects that occur due to low speed of sound (1500 m/s). This article overviews the development of acoustic modem technology, which evolved over the past several decades from noncoherent modulation/detection techniques to bandwidth-efficient phase-coherent modulation/detection. A survey of techniques for channel equalization in single-carrier systems as well as recent advances in multicarrier acoustic communications is also presented. Keywords: Acoustic; communications; coherent; equalization; channel estimation; phase synchronizations; multipath; Doppler; sparse channels; diversity combining; beamforming; multiuser detection; interference suppression; time reversal; multi-input–multi-output (MIMO) processing; multicarrier modulation; OFDM; adaptive modulation; underwater networks