Showing papers on "Orthogonal frequency-division multiplexing published in 2008"

Spatial Modulation

Abstract: Spatial modulation (SM) is a recently developed transmission technique that uses multiple antennas. The basic idea is to map a block of information bits to two information carrying units: 1) a symbol that was chosen from a constellation diagram and 2) a unique transmit antenna number that was chosen from a set of transmit antennas. The use of the transmit antenna number as an information-bearing unit increases the overall spectral efficiency by the base-two logarithm of the number of transmit antennas. At the receiver, a maximum receive ratio combining algorithm is used to retrieve the transmitted block of information bits. Here, we apply SM to orthogonal frequency division multiplexing (OFDM) transmission. We develop an analytical approach for symbol error ratio (SER) analysis of the SM algorithm in independent identically distributed (i.i.d.) Rayleigh channels. The analytical and simulation results closely match. The performance and the receiver complexity of the SM-OFDM technique are compared to those of the vertical Bell Labs layered space-time (V-BLAST-OFDM) and Alamouti-OFDM algorithms. V-BLAST uses minimum mean square error (MMSE) detection with ordered successive interference cancellation. The combined effect of spatial correlation, mutual antenna coupling, and Rician fading on both coded and uncoded systems are presented. It is shown that, for the same spectral efficiency, SM results in a reduction of around 90% in receiver complexity as compared to V-BLAST and nearly the same receiver complexity as Alamouti. In addition, we show that SM achieves better performance in all studied channel conditions, as compared with other techniques. It is also shown to efficiently work for any configuration of transmit and receive antennas, even for the case of fewer receive antennas than transmit antennas.

An overview of limited feedback in wireless communication systems

Abstract: It is now well known that employing channel adaptive signaling in wireless communication systems can yield large improvements in almost any performance metric. Unfortunately, many kinds of channel adaptive techniques have been deemed impractical in the past because of the problem of obtaining channel knowledge at the transmitter. The transmitter in many systems (such as those using frequency division duplexing) can not leverage techniques such as training to obtain channel state information. Over the last few years, research has repeatedly shown that allowing the receiver to send a small number of information bits about the channel conditions to the transmitter can allow near optimal channel adaptation. These practical systems, which are commonly referred to as limited or finite-rate feedback systems, supply benefits nearly identical to unrealizable perfect transmitter channel knowledge systems when they are judiciously designed. In this tutorial, we provide a broad look at the field of limited feedback wireless communications. We review work in systems using various combinations of single antenna, multiple antenna, narrowband, broadband, single-user, and multiuser technology. We also provide a synopsis of the role of limited feedback in the standardization of next generation wireless systems.

An Overview: Peak-to-Average Power Ratio Reduction Techniques for OFDM Signals

Abstract: One of the challenging issues for Orthogonal Frequency Division Multiplexing (OFDM) system is its high Peak-to-Average Power Ratio (PAPR). In this paper, we review and analysis different OFDM PAPR reduction techniques, based on computational complexity, bandwidth expansion, spectral spillage and performance. We also discuss some methods of PAPR reduction for multiuser OFDM broadband communication systems.

Multicarrier Communication Over Underwater Acoustic Channels With Nonuniform Doppler Shifts

Abstract: Underwater acoustic (UWA) channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth, which introduces frequency-dependent Doppler shifts. In this paper, we treat the channel as having a common Doppler scaling factor on all propagation paths, and propose a two-step approach to mitigating the Doppler effect: 1) nonuniform Doppler compensation via resampling that converts a "wideband" problem into a "narrowband" problem and 2) high-resolution uniform compensation of the residual Doppler. We focus on zero-padded orthogonal frequency-division multiplexing (OFDM) to minimize the transmission power. Null subcarriers are used to facilitate Doppler compensation, and pilot subcarriers are used for channel estimation. The receiver is based on block-by-block processing, and does not rely on channel dependence across OFDM blocks; thus, it is suitable for fast-varying UWA channels. The data from two shallow-water experiments near Woods Hole, MA, are used to demonstrate the receiver performance. Excellent performance results are obtained even when the transmitter and the receiver are moving at a relative speed of up to 10 kn, at which the Doppler shifts are greater than the OFDM subcarrier spacing. These results suggest that OFDM is a viable option for high-rate communications over wideband UWA channels with nonuniform Doppler shifts.

Comparison of Asymmetrically Clipped Optical OFDM and DC-Biased Optical OFDM in AWGN

Abstract: We present theoretical and simulation results for the performance of asymmetrically-clipped optical OFDM (ACO-OFDM) and DC-biased optical OFDM (DCO-OFDM) in AWGN for intensity-modulated direct-detection systems. Constellations from 4 QAM to 1024 QAM are considered. For DCO-OFDM, the optimum bias depends on the constellation size which limits its performance in adaptive systems. ACO-OFDM requires less optical power for a given data rate than DCO-OFDM for all but the largest constellations and is better suited to adaptive systems as the same structure is optimum for all constellations.

RF Imperfections in High-rate Wireless Systems

Abstract: Wireless communication systems are persistently applying wider bandwidths, larger signal dynamics and higher carrier frequencies to fulfill the demand for higher data rates. This results in an ever increasing demand on the performance of low-cost and power-efficient radio frequency (RF) front-ends. Since the RF technology is, consequently, pushed to its operation boundaries, the intrinsic imperfections of the RF IC technology are more and more governing the system performance of wireless modems. "RF Imperfections in High-rate Wireless Systems" therefore presents a new vision on the design of wireless communication systems. In this approach the imperfections of the RF front-ends are accepted and digital signal processing algorithms are designed to suppress their impact on system performance. To illustrate this approach, this book focuses on multiple-antenna orthogonal frequency division multiplexing (MIMO OFDM), which will be applied as basis for the majority of near-future high-rate wireless systems. The basics of MIMO OFDM are introduced and the typically required signal processing in the implementation of such systems is elucidated. This book treats several of the front-end impairments that seriously affect the performance of MIMO OFDM systems: carrier frequency offset, phase noise, IQ imbalance and nonlinearities. To provide an in-depth understanding of the impact of these RF imperfections, analytical performance results are presented in the book. These results are then used to design different compensation approaches based on digital baseband processing. "RF Imperfections in High-rate Wireless Systems" is of interest to wireless system designers, who want to familiarise with the digital compensation of RF imperfections. For researchers in the field of wireless communications this book provides a valuable overview of this emerging research topic.

Cyclostationary Signatures in Practical Cognitive Radio Applications

Abstract: We define a cyclostationary signature as a feature which may be intentionally embedded in a digital communications signal, detected through cyclostationary analysis and used as a unique identifier. The purpose of this paper is to demonstrate how cyclostationary signatures can be exploited to overcome a number of the challenges associated with network coordination in emerging cognitive radio applications and spectrum sharing regimes. In particular we show their uses for signal detection, network identification and rendezvous and discuss these in the context of dynamic spectrum access. We present a theoretical discussion followed by application-oriented examples of the cyclostationary signatures used in practical cognitive radio and dynamic spectrum usage scenarios. We focus on orthogonal frequency division multiplexing (OFDM) based systems and present an analysis of a transceiver implementation employing these techniques developed on a cognitive radio test platform.

Single-carrier frequency domain equalization

Abstract: This paper present an alternative promising approach to ISI mitigation by the use of single-carrier (SC) modulation combined with frequency-domain equalization (FDE).

Optimal and Suboptimal Power Allocation Schemes for OFDM-based Cognitive Radio Systems

Abstract: In this paper, we investigate an optimal power loading algorithm for an OFDM-based cognitive radio (CR) system. The downlink transmission capacity of the CR user is thereby maximized, while the interference introduced to the primary user (PU) remains within a tolerable range. We also propose two suboptimal loading algorithms that are less complex. We also study the effect of a subcarrier nulling mechanism on the performance of the different algorithms under consideration. The performance of the optimal and suboptimal schemes is compared with the performance of the classical power loading algorithms, e.g., water-filling and uniform power but variable rate loading schemes that are used for conventional OFDM-based systems. Presented numerical results show that for a given interference threshold, the proposed optimal scheme allows CR base station (BS) to transmit more power in order to achieve a higher transmission rate than the classical loading algorithms. These results also show that although the proposed suboptimal schemes have certain degradation in performance compared to the optimal scheme, they outperform the classical loading algorithms. We also present numerical results for nulling mechanism. Finally, we investigate the effect of imperfect channel gain information at the transmitter.

Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF

Abstract: We discuss coherent optical orthogonal frequency division multiplexing (CO-OFDM) as a suitable modulation technique for long-haul transmission systems. Several design and implementation aspects of a CO-OFDM system are reviewed, but we especially focus on phase noise compensation. As conventional CO-OFDM transmission systems are very sensitive to laser phase noise a novel method to compensate for phase noise is introduced. With the help of this phase noise compensation method we show continuously detectable OFDM transmission at 25.8 Gb/s data rate (20 Gb/s after coding) over 4160-km SSMF without dispersion compensation.

Multicarrier communication techniques for spectrum sensing and communication in cognitive radios

Abstract: In this tutorial article we review different multicarrier communication methods for the physical layer of cognitive radio systems. There, secondary users need to dynamically and reliably determine spectral holes, and transmit data in these resources without interfering with other parts of the frequency band. To satisfy the first, each SU has to be equipped with a spectrum analyzer. To satisfy the second, it is widely accepted that a multicarrier modulation technique should be adopted. Moreover, to maximize efficiency, it has been recognized that the side-lobes of each subcarrier band must be minimized. Much of the attention in the present literature emphasizes on the use of conventional OFDM, exploiting the fact that fast Fourier transform (FFT) as part of the OFDM modulator can also be used for channel sensing. Herein, we discuss the performance of OFDM, and also introduce filterbanks for multicarrier communication and spectral analysis in a CR setting. Moreover, the multitaper method has been proposed as an effective method for spectrum analysis. Our article provides an insight into the pros and cons of these technologies.

Constant Envelope OFDM

Abstract: This paper describes a transformation technique aimed at solving the peak-to-average power ratio (PAPR) problem associated with OFDM (orthogonal frequency-division multiplexing). Constant envelope OFDM (CE-OFDM) transforms the OFDM signal, by way of phase modulation, to a signal designed for efficient power amplification. At the receiver, the inverse transformation - phase demodulation - is applied prior to the conventional OFDM demodulator. The performance of CE-OFDM is analyzed in additive white Gaussian noise (AWGN) and fading channels. CE-OFDM is shown to achieve good performance in dense multipath with the use of cyclic prefix transmission in conjunction with a frequency- domain equalizer (FDE). By way of computer simulation and hardware realization, CE-OFDM is shown to compare favorably to conventional OFDM.

A compressed sensing technique for OFDM channel estimation in mobile environments: Exploiting channel sparsity for reducing pilots

Abstract: We consider the estimation of doubly selective wireless channels within pulse-shaping multicarrier systems (which include OFDM systems as a special case). A new channel estimation technique using the recent methodology of compressed sensing (CS) is proposed. CS-based channel estimation exploits a channel's delay-Doppler sparsity to reduce the number of pilots and, hence, increase spectral efficiency. Simulation results demonstrate a significant reduction of the number of pilots relative to least-squares channel estimation.

Performance evaluation of frequency planning schemes in OFDMA-based networks

Abstract: In this paper, we study the performance of frequency allocation schemes in forthcoming OFDMA-based systems. These systems include WiMAX and 3G long term evolution. We first develop an analytical model for the collisions for an arbitrary number of users in the different cells. We then calculate the capacity of the system using a Markov model and taking into account the inter-cell interference and its impact on the adaptive modulation. We apply this model to compare four frequency allocation schemes, namely reuse 1, reuse 3, and static and dynamic mixes of reuse 1 and 3. We also considered a fifth scheme, called partial isolation and proposed for 3G LTE systems, that uses different transmission powers in the different frequency bands, in order to reduce interference at cell edge. Our results show that the partial isolation scheme outperforms all the others, especially in the downlink, as it combines the advantage of the reuse 1 scheme (large overall throughput) with that of the mix of reuses 1 and 3 (good cell-edge performance).

107 Gb/s coherent optical OFDM transmission over 1000-km SSMF fiber using orthogonal band multiplexing

Abstract: Coherent optical OFDM (CO-OFDM) has emerged as an attractive modulation format for the forthcoming 100 Gb/s Ethernet. However, even the spectral-efficient implementation of CO-OFDM requires digital-to-analog converters (DAC) and analog-to-digital converters (ADC) to operate at the bandwidth which may not be available today or may not be cost-effective. In order to resolve the electronic bandwidth bottleneck associated with DAC/ADC devices, we propose and elucidate the principle of orthogonal-band-multiplexed OFDM (OBM-OFDM) to subdivide the entire OFDM spectrum into multiple orthogonal bands. With this scheme, the DAC/ADCs do not need to operate at extremely high sampling rate. The corresponding mapping to the mixed-signal integrated circuit (IC) design is also revealed. Additionally, we show the proof-of-concept transmission experiment through optical realization of OBM-OFDM. To the best of our knowledge, we present the first experimental demonstration of 107 Gb/s QPSK-encoded CO-OFDM signal transmission over 1000 km standard-single- mode-fiber (SSMF) without optical dispersion compensation and without Raman amplification. The demonstrated system employs 2x2 MIMO-OFDM signal processing and achieves high electrical spectral efficiency with direct-conversion at both transmitter and receiver.

Resource Allocation for Delay Differentiated Traffic in Multiuser OFDM Systems

Abstract: Most existing work on adaptive allocation of sub- carriers and power in multiuser orthogonal frequency division multiplexing (OFDM) systems has focused on homogeneous traffic consisting solely of either delay-constrained data (guaranteed service) or non-delay-constrained data (best-effort service). In this paper, we investigate the resource allocation problem in a heterogeneous multiuser OFDM system with both delay-constrained (DC) and non-delay-constrained (NDC) traffic. The objective is to maximize the sum-rate of all the users with NDC traffic while maintaining guaranteed rates for the users with DC traffic under a total transmit power constraint. Through our analysis we show that the optimal power allocation over subcarriers follows a multi-level water-filling principle; moreover, the valid candidates competing for each subcarrier include only one NDC user but all DC users. By converting this combinatorial problem with exponential complexity into a convex problem or showing that it can be solved in the dual domain, efficient iterative algorithms are proposed to find the optimal solutions. To further reduce the computational cost, a low-complexity suboptimal algorithm is also developed. Numerical studies are conducted to evaluate the performance of the proposed algorithms in terms of service outage probability, achievable transmission rate pairs for DC and NDC traffic, and multiuser diversity.

Channel estimation methods for preamble‐based OFDM/OQAM modulations

Abstract: In this paper, OFDM/OQAM is proposed as an alternative to conventional OFDM with cyclic prefix (CP) for transmission over multi-path fading channels. Two typical features of the OFDM/OQAM modulation are the absence of a guard interval (GI) and the fact that the orthogonality property only holds in the real field and for a distortion-free channel. Thus, the classical channel estimation (CE) methods used for OFDM cannot be directly applied to OFDM/OQAM. Therefore, we propose an analysis of the transmission of an OFDM/OQAM signal through a time-varying multi-path channel and we derive two new CE methods. The first proposed method only requires the use of pair of real pilots (POP). In a second method, named interference approximation method (IAM), we show how the imaginary interference can be used to improve the CE quality. Several preamble variants of the IAM are compared with respect to the resulting instantaneous power. Finally, the performance results obtained for the transmission of an OFDM/OQAM signal through an IEEE 802.22 channel using the POP method and three variants of IAM are compared to those obtained with CP-OFDM. Copyright © 2008 John Wiley & Sons, Ltd.

Sidelobe suppression in OFDM-based spectrum sharing systems using adaptive symbol transition

Abstract: In this letter, we introduce a new method for orthogonal frequency division multiplexing (OFDM) sidelobe suppression. An extension is added to OFDM symbols that is calculated using optimization methods to minimize adjacent channel interference (ACI) while keeping the extension power at an acceptable level. Using this technique, interference to adjacent signals is reduced significantly at the cost of a small decrease in the useful symbol energy. The proposed method can be used by cognitive radio (CR) systems to shape the spectrum of OFDM signals and to minimize interference to licensed users (LU), or to reduce the size of guard bands used in conventional OFDM systems.

Self-Organizing Dynamic Fractional Frequency Reuse in OFDMA Systems

Abstract: We describe an algorithm for sub-carrier and power allocation that achieves out-of-cell interference avoidance through dynamic fractional frequency reuse (FFR) in downlink of cellular systems based on orthogonal frequency division multiple access (OFDMA). The focus in on the constant-bit-rate (CBR) traffic type flows (e.g., VoIP). Our approach is based on the continuous "selfish" optimization of resource allocation by each sector. No a priori frequency planning and/or inter-cell coordination is required. We show, both analytically (on a simple illustrative example) and by simulations (of a more realistic system), that the algorithm leads the system to "self-organize" into efficient frequency reuse patterns.

Compressed channel sensing

Abstract: Reliable wireless communications often requires accurate knowledge of the underlying multipath channel. This typically involves probing of the channel with a known training waveform and linear processing of the input probe and channel output to estimate the impulse response. Many real-world channels of practical interest tend to exhibit impulse responses characterized by a relatively small number of nonzero channel coefficients. Conventional linear channel estimation strategies, such as the least squares, are ill-suited to fully exploiting the inherent low-dimensionality of these sparse channels. In contrast, this paper proposes sparse channel estimation methods based on convex/linear programming. Quantitative error bounds for the proposed schemes are derived by adapting recent advances from the theory of compressed sensing. The bounds come within a logarithmic factor of the performance of an ideal channel estimator and reveal significant advantages of the proposed methods over the conventional channel estimation schemes.

Call Admission Control Optimization in WiMAX Networks

Abstract: Worldwide interoperability for microwave access (WiMAX) is a promising technology for last-mile Internet access, particularly in the areas where wired infrastructures are not available. In a WiMAX network, call admission control (CAC) is deployed to effectively control different traffic loads and prevent the network from being overloaded. In this paper, we propose a framework of a 2-D CAC to accommodate various features of WiMAX networks. Specifically, we decompose the 2-D uplink and downlink WiMAX CAC problem into two independent 1-D CAC problems and formulate the 1-D CAC optimization, in which the demands of service providers and subscribers are jointly taken into account. To solve the optimization problem, we develop a utility- and fairness-constrained optimal revenue policy, as well as its corresponding approximation algorithm. Simulation results are presented to demonstrate the effectiveness of the proposed WiMAX CAC approach.

Wireless LAN Comes of Age: Understanding the IEEE 802.11n Amendment

Abstract: During the initial development of the IEEE 802.11n (11n) amendment for improving the throughput of wireless LANs, a lot of excitement existed surrounding the potential higher throughput (i.e., faster downloads), and increased range (distance) achievable. However, delays in the development of this standard (which began in 2003, and is still in the final draft stages) as well as vendor, customer reluctance to adopt the pre-11n offerings in the marketplace, have generally slowed interest in this next-generation technology. However, there is still much to be excited about. The latest draft of IEEE 802.11n (Draft 3.0) offers the potential of throughputs beyond 200 Mbps, based on physical layer (PHY) data rates up to 600 Mbps. This is achieved through the use of multiple transmit and receive antennas, referred to as MIMO (multiple input, multiple output). Using techniques such as spatial division multiplexing (SDM), transmitter beamforming, and space-time block coding (STBC), MIMO is used to increase dramatically throughput over single antenna systems (by two to four times) or to improve range of reception, depending on the environment. This article offers an exposition on the techniques used in IEEE 802.11n to achieve the above improvements to throughput and range. First, the current generation WLAN devices (11a/b/g) are described in terms of the benefits offered to end users. Next, the evolution of the Tin amendment is discussed, describing the main proposals given, and illustrating reasons for the delay in standardization. Then, the changes to the PHY for 11n are presented. A description of channel modeling with MIMO is shown, followed by the signal processing techniques employed, including MIMO channel estimation and detection, space-time block coding (STBC), and transmitter beamforming. Simulation results are presented which illustrate the benefits of these techniques, versus the existing a/g structures, for both throughput and range. Finally, a brief section outlining considerations for the rapid prototyping of a baseband design based on the 802.11n PHY is presented. We conclude with a discussion of the future for 11n, describing the issues addressed with Drafts 2.0 and 3.0, as well as its place in a wireless market with WiMAX and Bluetooth.

Long-haul transmission of16×52.5 Gbits/s polarization-division- multiplexed OFDM enabled by MIMO processing (Invited)

Abstract: Focus Issue on Orthogonal-Frequency-Division Multiplexed Communications Systems and Networks We discuss the realization and performance of polarization-division-multiplexed orthogonal frequency division multiplexing (PDM-OFDM) for long-haul transmission systems. Polarization demultiplexing of the PDM signal at the receiver is realized by employing a multiple-input multiple-output (MIMO) detector. Using a recirculating loop a long-haul transmission experiment is reported of 52.5 Gbits/s PDM-OFDM (40 Gbits/s after coding) over 4160 km of standard single-mode fiber (SSMF). In this transmission experiment, 16 wavelength-division-multiplexed (WDM) channels are transmitted at 50 GHz channel spacing, and we show that MIMO processing in the receiver enables both polarization demultiplexing and a large PMD tolerance.

Detection, Synchronization, and Doppler Scale Estimation with Multicarrier Waveforms in Underwater Acoustic Communication

Abstract: 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.

A Novel Approach to MIMO Transmission Using a Single RF Front End

Abstract: In this paper we introduce a new perspective to the implementation of wireless MIMO transmission systems with increased bandwidth efficiency. Unlike traditional spatial multiplexing techniques in MIMO systems, where additional information can be sent through the wireless channel by feeding uncorrelated antenna elements with diverse bitstreams, we use the idea of mapping diverse bitstreams onto orthogonal bases defined in the beamspace domain of the transmitting array far-field region. Using this approach we show that we can increase the capacity of wireless communication systems using compact parasitic antenna architectures and a single RF front end at the transmitter, thus paving the way for integrating MIMO systems in cost and size sensitive wireless devices such as mobile terminals and mobile personal digital assistants.

Phase Noise Effects on High Spectral Efficiency Coherent Optical OFDM Transmission

Abstract: There are three major advantages for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission using digital signal processing. First, coherent detection is realized by digital phase estimation without the need for optical phase-locked loop. Second, OFDM modulation and demodulation are realized by the well-established computation-efficient fast Fourier transform (FFT) and inverse FFT. Third, adaptive data rates can be supported as different quadrature amplitude modulation (QAM) constellations are software-defined, without any hardware change in transmitter and receiver. However, it is well-known that coherent detection, OFDM, and QAM are all susceptible to phase noise. In this paper, theoretical, numerical, and experimental investigations are carried out for phase noise effects on high spectral efficiency CO-OFDM transmission. A transmission model in the presence of phase noise is presented. By using simulation, the bit error rate floors from finite laser linewidth are presented for CO-OFDM systems with high-order QAM constellations. In the experiments, the phase noise effects from both laser linewidth and nonlinear fiber transmission are investigated. The fiber nonlinearity mitigation based on receiver digital signal processing is also discussed.

Channel-aware scheduling algorithms for SC-FDMA in LTE uplink

Abstract: Single-carrier frequency division multiple access (SC-FDMA) has been selected as the uplink access scheme in the UTRA Long Term Evolution (LTE) due to its low peak-to-average power ratio properties compared to orthogonal frequency division multiple access. Nevertheless, in order to achieve such a benefit, it requires a localized allocation of the resource blocks, which naturally imposes a severe constraint on the scheduler design. In this paper, three new channel-aware scheduling algorithms for SC-FDMA are proposed and evaluated in both local and wide area scenarios. Whereas the first maximum expansion (FME) and the recursive maximum expansion (RME) are relative simple solutions to the above-mentioned problem, the minimum area-difference to the envelope (MADE) is a more computational expensive approach, which, on the other hand, performs closer to the optimal combinatorial solution. Simulation results show that adopting a proportional fair metric all the proposed algorithms quickly reach a high level of data-rate fairness. At the same time, they definitely outperform the round-robin scheduling in terms of cell spectral efficiency with gains up to 68.8% in wide area environments.

Nonbinary LDPC Coding for Multicarrier Underwater Acoustic Communications

Abstract: 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.

Electronic Dispersion Compensation

Abstract: The performance of different electronic equalization and processing schemes for 40- and 10-Gb/s optical transmission over single-mode fiber (SMF) are discussed, from the point of their ability to compensate chromatic dispersion (CD) and polarization mode dispersion (PMD). In addition, the impact of fiber nonlinearity and modulation format on equalization is also investigated. The main objective of this paper is to present an overview and a comparison of the performances rather than a detailed explanation of the principles of the different equalization schemes. The equalizers which will be covered are analog equalizer (feedforward and decision feedback type), maximum likelihood sequence estimator (MLSE), electronic precompensation, coherent/intradyne detection with digital signal processing (DSP) equalization, DSP-based optical orthogonal frequency division multiplexing (OFDM), and turbo equalization.