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Orthogonal Frequency Division Multiplexing (orthogonal + frequency_division_multiplexing)

Distribution by Scientific Domains

Engineering	93%

Selected Abstracts

Nonlinear signal processing schemes for OFDM modulations within conventional or LINC transmitter structures

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 3 2008
Rui Dinis
This paper considers two classes of digital signal processing schemes, recently proposed by the authors, both combining a nonlinear operation in the time domain with a linear operation in the frequency domain, so as to reduce the envelope fluctuations and allow good power/bandwidth tradeoffs with Orthogonal Frequency Division Multiplexing (OFDM) transmission. These classes can be adopted within both conventional and two-branch, LInear amplification with Nonlinear Components (LINC) transmitter structures . The paper addresses the statistical characterisation of the transmitted frequency-domain blocks, so as to evaluate performances by analytical means, in a unified manner. A set of numerical results, which have been obtained with the help of this analytical method, is presented and discussed. These results allow a comparison between the two classes of signal processing schemes, namely for nonlinear operations of the ,clipping' type, when a conventional transmitter structure is employed. They also allow an accurate evaluation of the phase and gain imbalance effects in the two-branch power amplification case. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Optimum adaptive OFDM systems

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 3 2003
Lorenzo Piazzo
When Orthogonal Frequency Division Multiplexing (OFDM) is used to transmit information over a frequency selective channel, it is convenient to vary the power and the number of bits allocated to each subcarrier in order to optimize the system performance. In this paper, the three classical problems of transmission power minimization, error rate minimization and throughput maximization are investigated in a unified manner. The relations existing among these three problems are clarified and a precise definition of optimum system is given. A general and rigorous way to extend the solution of any of the three problems in order to obtain the solution of the other two is presented. This result is used to devise an efficient algorithm for the error rate minimization. Copyright © 2003 AEI. [source]

Comparison of coded orthogonal frequency division multiplexing and multicarrier code division multiple access systems for power line communications

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 9 2004
P. L. Katsis
Abstract Orthogonal frequency division multiplexing (OFDM) and multicarrier code division multiple access (MC-CDMA) systems are comparatively evaluated for power line communications (PLC) in a frequency-selective fading environment with additive coloured Gaussian noise which is used to model the actual in-home power line channel. OFDM serves as a benchmark in order to measure the performance of various MC-CDMA systems, since multicarrier modulation systems are considered the best candidate for this kind of channel. Both single-user and multi-user cases are taken into account, making use of the appropriate combiner schemes to take full advantage of each case. System efficiency is enhanced by the application of different coding techniques, a fact which shows that powerful coding can make the difference under such a hostile medium. The impact of block interleaving is investigated, while the simulation examines how different modulation schemes fair under the imposed channel conditions as well. The performance of the system is assessed by the commonly used bit error rate vs signal-to-noise ratio diagrams and there is also a comparison regarding throughput efficiency among all the tested systems. As stated in Section 4, a promising PLC application is attained. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Frequency domain equalisation in CDMA detection

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 2 2008
Ihan Martoyo
The present paper introduces frequency domain equalisation (FDE) as an efficient means for single-user detection (SUD) in universal mobile telecommunication systems (UMTS) handhelds. Such an SUD is, at least partially, able to remove multiple access interference (MAI) like a multi-user detector (MUD), however with several important advantages the operation principle of an FDE-SUD is simple and easy to understand, the FDE-SUD shows low numerical complexity and therefore, it is not power hungry like an MUD. The problem of applying FDE without zero-padding or cyclic-prefix insertion is solved by using the overlap-cut method. In this paper we also prove that the performance of the FDE-SUD is practically identical to that of linear MUD for a fully loaded UMTS cell. Furthermore, armed with the idea of FDE for code division multiple access (CDMA), we can construct a common receiver structure for CDMA, time division multiple access (TDMA), orthogonal frequency division multiplexing (OFDM) and multi-carrier CDMA (MC-CDMA) systems. This is an extremely useful idea in enabling a software-defined radio (SDR) which can operate seamlessly in several environments such as UMTS, the IEEE 802.11a and WiMaX. FDE based receivers could be one important building-block for the next generation mobile communications. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Resource allocation with minimum rates for OFDM broadcast channels,

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 6 2007
Carolin Huppert
Downlink transmissions with minimum rate requirements over orthogonal frequency division multiplexing (OFDM) channels are commonly done by means of scheduling algorithms. However, regarding it from an information theoretical point of view, this is not optimal since broadcast techniques can achieve higher rates. The drawbacks of the optimum broadcast algorithm are that the signalling overhead is larger than for scheduling and also the computational complexity is much higher. In this paper we propose an algorithm which overcomes these points. This algorithm is a hybrid algorithm combining scheduling and broadcast approaches. Thus, it combines advantages of both methods. Furthermore, we present modifications to this algorithm to avoid irresolvable decoding dependencies. We show by means of simulation results that the proposed algorithm operates close to the optimum performance and that it outperforms a pure scheduling approach. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Multi-user detection in OFDM systems using CDMA and multiple antennas

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 6 2003
Christoph Degen
This paper deals with linear multi-user detection in orthogonal frequency division multiplexing (OFDM) systems. The separation of concurrently transmitted signals is based on space division and code division multiple access (SD/CDMA). The examined OFDM schemes are compared against frequency domain equalization for single-carrier transmission schemes (SC/FDE). Both show strong similarities. Therefore in this paper, we focus on a unified treatment of multi-user detection systems based on OFDM or SC/FDE. The trade-off between exploitation of diversity and user separation is analyzed for different spreading techniques and a varying number of receive antennas. The performance is evaluated in terms of both uncoded and coded bit error ratio (BER). Furthermore, a novel time domain spreading technique for OFDM systems is proposed for which the peak-to-average transmitter power ratio (PAPR) is independent of the applied spreading code. Copyright © 2004 AEI [source]

Array antenna assisted doppler spread compensator for OFDM

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 5 2002
Minoru Okada
This paper proposes a novel array-antenna-assisted Doppler spread compensator for orthogonal frequency division multiplexing (OFDM), which is sensitive to fast time-variation of the radio propagation channel. In the proposed compensator, a linear array antenna is installed on top of the vehicle. The compensator estimates the received signal at a certain point on the linear array antenna by using space domain interpolation. Because the relative position of the estimated receiving point with respect to the ground does not change during the effective symbol duration of an OFDM signal, the time variation due to the movement of the vehicle can be compensated for. Computer simulation shows that the compensator can compensate for the bit error rate performance degradation due to time-variation of the channel when the velocity of the vehicle is up to 180km/h and a two-element array antenna is used at the carrier frequency of 600 MHz. The bit error rate performance can be further improved by using a four-element array antenna. [source]

Performance of robust symbol-timing and carrier-frequency estimation for OFDM systems

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 5 2009
Nan-Yang YenArticle first published online: 7 NOV 200
Abstract In recent years, many maximum likelihood (ML) blind estimators have been proposed to estimate timing and frequency offsets for orthogonal frequency division multiplexing (OFDM) systems. However, the previously proposed ML blind estimators utilizing cyclic prefix do not fully characterize the random observation vector over the entire range of the timing offset and will significantly degrade the estimation performance. In this paper, we present a global ML blind estimator to compensate the estimation error. Moreover, we extend the global ML blind estimator by accumulating the ML function of the estimation parameters to achieve a better accuracy without increasing the hardware or computational complexity. The simulation results show that the proposed algorithm can significantly improve the estimation performance in both additional white Gaussian noise and ITU-R M.1225 multipath channels. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Low complexity bit allocation algorithm for OFDM systems

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 11 2008
Changwook Lee
Abstract A bit allocation algorithm is presented for orthogonal frequency division multiplexing (OFDM) systems. The proposed algorithm is derived from the geometric progression of the additional transmission power required by the subcarriers and the arithmetic,geometric means inequality. Consequently, this algorithm has a simple procedure and low computational complexity. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Kalman filter-based channel estimation and ICI suppression for high-mobility OFDM systems

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 10 2008
Prerana Gupta
Abstract The use of orthogonal frequency division multiplexing (OFDM) in frequency-selective fading environments has been well explored. However, OFDM is more prone to time-selective fading compared with single-carrier systems. Rapid time variations destroy the subcarrier orthogonality and introduce inter-carrier interference (ICI). Besides this, obtaining reliable channel estimates for receiver equalization is a non-trivial task in rapidly fading systems. Our work addresses the problem of channel estimation and ICI suppression by viewing the system as a state-space model. The Kalman filter is employed to estimate the channel; this is followed by a time-domain ICI mitigation filter that maximizes the signal-to-interference plus noise ratio (SINR) at the receiver. This method is seen to provide good estimation performance apart from significant SINR gain with low training overhead. Suitable bounds on the performance of the system are described; bit error rate (BER) performance over a time-invariant Rayleigh fading channel serves as the lower bound, whereas BER performance over a doubly selective system with ICI as the dominant impairment provides the upper bound. Copyright © 2008 John Wiley & Sons, Ltd. [source]

A novel search method to reduce PAPR of an OFDM signal using partial transmit sequences

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 2 2007
Hao-Chung Tu
Abstract One of the main drawbacks of orthogonal frequency division multiplexing (OFDM) is the high peak-to-average power ratio (PAPR) of the transmitted OFDM signal. Partial transmit sequence (PTS) technique can improve the PAPR statistics of an OFDM signal. As ordinary PTS technique requires an exhaustive search over all combinations of allowed phase factors, the search complexity increases exponentially with the number of sub-blocks. In this paper, we propose a novel PTS technique with reduced complexity that each level inverts twice of phase factor bits from previous level. Then we also use initial random phase sequence to find the better search way of PAPR reduction. Numerical results show that the proposed method can achieve significant reduction in search complexity with little performance degradation. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Comparison of coded orthogonal frequency division multiplexing and multicarrier code division multiple access systems for power line communications

Adaptive predistortion of COFDM signals for a mobile satellite channel

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 2 2003
Nibaldo Rodriguez
Abstract In this paper, we consider the optimization of the performance of QPSK and 16-QAM coded orthogonal frequency division multiplexing (COFDM) signals over the non-linear and mobile satellite channel. A high power amplifier and Rician flat fading channel produces non-linear and linear distortions; an adaptive predistortion technique combined with turbo codes will reduce both types of distortion. The predistorter is based on a feedforward neural network, with the coefficients being derived using an extended Kalman filter (EKF). The conventional turbo code is used to mitigate Rician flat fading distortion and Gaussian noise. The performance over a non-linear satellite channel indicates that QPSK COFDM followed by a predistorter provides a gain of about 1.7 dB at a BER of 3×10,3 when compared to QPSK COFDM without the predistortion scheme and 16-QAM COFDM provides a gain of 0.5 dB output back-off and 1.2 dB signal to noise ratio at a BER of 3×10,5 when compared with an adaptive predistorter based on the Harmmerstein model. We also investigate the influence of the guard time interval and Doppler frequency effect on the BER performance. When the guard interval increases from 0 to 0.125T samples and the normalized Doppler frequency is 0.001, there is a gain of 0.7 and 1 dB signal to noise ratio at a BER of 6×10,4 for QPSK and 16-QAM COFDM, respectively. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Performance evaluations of microwave power amplifiers with single- and multi-carrier quadrature amplitude modulations

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 12 2006
Chien-Chang Huang
Abstract This article presents the performance evaluations of microwave power amplifiers (PAs) for different kinds of quadrature amplitude modulation (QAM) schemes including ,/4-DQPSK and 16-QAM for single-carrier formats and orthogonal frequency division multiplexing (OFDM) for multi-carrier configuration, by using the time-domain signal processing technique and the hybrid Volterra analysis. The influences due to various peak-to-average power ratios (PAPRs) of the modulation signals for the PA performances such as output powers, adjacent channel power ratios (ACPRs), and error vector magnitudes (EVMs) are shown with experimental verifications. A figure-of-merit of PAs based on linearity and efficiency is presented to establish a criterion for choosing the best working condition. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2375,2378, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22020 [source]

Performance assessment of next-generation wireless mobile systems

BELL LABS TECHNICAL JOURNAL, Issue 4 2009
Krishna Balachandran
The deployment of third generation (3G) systems such as Universal Mobile Telecommunications System (UMTS) and code division multiple access (CDMA) 1X is now ubiquitous. Furthermore, several operators have already deployed evolved 3G systems such as high speed packet access (HSPA) and 1× evolution data optimized (1× EV-DO). These 3G and evolved 3G systems are based on spread spectrum technologies. Now the industry has begun to focus on next-generation wireless systems based on orthogonal frequency division multiplexing (OFDM) technology, flat-Internet Protocol (IP) architectures, and advanced multiple input-multiple output (MIMO) capabilities. These next-generation technologies, primarily are: 3rd Generation Partnership Project (3GPP)-based evolved UMTS terrestrial radio access network (EUTRAN), also called Long Term Evolution (LTE); 3rd Generation Partnership Project 2 (3GPP2)-based Ultra Mobile Broadband (UMB); and Institute of Electrical and Electronics Engineers (IEEE) 802. 16e-based Worldwide Interoperability for Microwave Access (WiMAX). These technologies are in the final specifications to initial deployment phases in selected markets. This paper provides an overview and performance analysis of these next-generation technologies based on their initial releases. Additionally, a comparative performance analysis of these technologies is also presented. © 2009 Alcatel-Lucent. [source]