Analysis and Cancellation of Inter-carrier Interference for OFDM Systems Over Time-variant Multipath Fading Channels
Analysis and Cancellation of Inter-carrier Interference for OFDM Systems Over Time-variant Multipath Fading Channels

Author: Yunjun Zhang

Publisher:

Published: 2002

Total Pages: 0

ISBN-13:

DOWNLOAD EBOOK

The orthogonality among the subcarriers of OFDM systems may be impaired by the time-selectivity of the fading channels. The loss of the orthogonality results in ICI, and if it is not treated appropriately, the system performance may not be improved only by increasing the signal-to-noise ratio. In other words, ICI results in error floors. This research work concentrates on the ICI analysis and cancellation, and also on the effects of channel time-selectivity on the OFDM systems over frequency-selective time-variant mobile fading channels. In the first part of this study, a general time-variant frequency-selective WSSUS fading channel model is further characterized to support the OFDM ICI analysis, thus the obtained results are applicable for many specific channels. We then identify the cause of the ICI, i.e. how the orthogonality among the subcarriers of OFDM systems is impaired. The average ICI power and its distribution are obtained based on the general time-variant frequency-selective WSSUS fading channel model. To mitigate the ICI caused by the channel time-selectivity for OFDM systems, in the second part of this study, an efficient ICI cancellation scheme is designed based on the obtained ICI power distribution. The simulation results indicate that a significant performance improvement can be achieved. For OFDM systems, differential encoding can be performed not only between the information bits of the same subcarrier of the consecutive OFDM symbols (inter-frame differential encoding), but also between the information bits of the adjacent subcarriers of the same OFDM symbol (inter-carrier differential encoding). In the third part of this study, we compare the performance of inter-frame and inter-carrier differential detection for OFDM systems over multipath time-variant mobile fading channels. The objective is to identify which differential encoding scheme (inter-frame or inter-carrier) is more robust to the channel time-selectivity, when OFDM systems are experiencing both frequency-selective and time-selective fading. The conditions under which the inter-carrier differential encoding outperforms the inter-frame differential encoding and vice-versa are provided.

Wireless Communications Over Rapidly Time-Varying Channels
Wireless Communications Over Rapidly Time-Varying Channels

Author: Franz Hlawatsch

Publisher: Academic Press

Published: 2011-05-04

Total Pages: 457

ISBN-13: 0080922724

DOWNLOAD EBOOK

As a result of higher frequencies and increased user mobility, researchers and systems designers are shifting their focus from time-invariant models to channels that vary within a block. Wireless Communications Over Rapidly Time-Varying Channels explains the latest theoretical advances and practical methods to give an understanding of rapidly time varying channels, together with performance trade-offs and potential performance gains, providing the expertise to develop future wireless systems technology. As well as an overview of the issues of developing wireless systems using time-varying channels, the book gives extensive coverage to methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, as well as providing models and transceiver methods for time-varying ultra-wideband channels. - An introduction to time-varying channel models gives in a nutshell the important issues of developing wireless systems technology using time-varying channels - Extensive coverage of methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, enables development of high performance wireless systems - Chapters on transceiver design for OFDM and receiver algorithms for MIMO communication channels over time-varying channels, with an emphasis on modern iterative turbo-style architectures, demonstrates how these important technologies can optimize future wireless systems

Wireless Communications
Wireless Communications

Author: Giorgio A. Vitetta

Publisher: John Wiley & Sons

Published: 2013-03-29

Total Pages: 849

ISBN-13: 1118576608

DOWNLOAD EBOOK

This book introduces the theoretical elements at the basis of various classes of algorithms commonly employed in the physical layer (and, in part, in MAC layer) of wireless communications systems. It focuses on single user systems, so ignoring multiple access techniques. Moreover, emphasis is put on single-input single-output (SISO) systems, although some relevant topics about multiple-input multiple-output (MIMO) systems are also illustrated. Comprehensive wireless specific guide to algorithmic techniques Provides a detailed analysis of channel equalization and channel coding for wireless applications Unique conceptual approach focusing in single user systems Covers algebraic decoding, modulation techniques, channel coding and channel equalisation

A Comparative Analysis to Remove CFO in OFDM System
A Comparative Analysis to Remove CFO in OFDM System

Author: Santosh Gupta

Publisher: LAP Lambert Academic Publishing

Published: 2012

Total Pages: 72

ISBN-13: 9783659235856

DOWNLOAD EBOOK

The most important aspect in an OFDM is frequency and time synchronization as this form the reason for orthogonality between the subcarriers. However OFDM is susceptible to synchronization errors between transmitter and receiver local oscillator frequencies. This error is known as the carrier frequency offset (CFO) and causes ICI and also destroys the orthogonality between the subcarriers. In this literature we study the effects that CFO can cause to the SNR in an OFDM system, estimate the amount of frequency offset. Also the performance of the estimate is studied in a multipath environment. This book discusses and investigates the estimation of carrier offset frequency in orthogonal frequency division multiplexing (OFDM) mobile systems. Although OFDM is resistant to multipath fading, it requires a high degree of synchronization to maintain sub-carrier orthogonality. Therefore the level of performance of the system depends first on the accuracy in estimating the carrier frequency offset and then the estimation of the channel.

ICI Cancellation Using Zero-padded Conjugate Transmission
ICI Cancellation Using Zero-padded Conjugate Transmission

Author: Amandeep Kaur

Publisher: LAP Lambert Academic Publishing

Published: 2014-03

Total Pages: 84

ISBN-13: 9783659223839

DOWNLOAD EBOOK

With the rapid growth of digital communication in last few years, the need for high speed data transmission is increased. To support the high speed services like video, data and voice need efficient wireless systems. OFDM is a most efficient multicarrier modulation system used for achieving high data rates in mobile environment because of its multicarrier modulation technique and ability to convert a frequency selective fading channel into several nearly flat fading channels. Also, it becomes very efficient technique for data transmission over multipath fading environment due to its properties like high bandwidth efficiency and resistance to multipath fading. Also, its implementation becomes easier with the help of Fast Fourier Transform and Inverse Fast Fourier Transform for demodulation and modulation respectively.

Intercarrier Interference Reduction and Channel Estimation in OFDM Systems
Intercarrier Interference Reduction and Channel Estimation in OFDM Systems

Author: Yihai Zhang

Publisher:

Published: 2011

Total Pages:

ISBN-13:

DOWNLOAD EBOOK

With the increasing demand for more wireless multimedia applications, it is desired to design a wireless system with higher data rate. Furthermore, the frequency spectrum has becomea limited and valuable resource, making it necessary to utilize the available spectrum efficiently and coexist with other wireless systems. Orthogonal frequency division multiplexing (OFDM)modulation is widely used in communication systems to meet the demand for ever increasing data rates. The major advantage of OFDM over single-carrier transmission is its ability to deal with severe channel conditions without complex equalization. However, OFDM systems suffer from a high peak to average power ratio, and they are sensitive to carrier frequency offset and Doppler spread. This dissertation first focuses on the development of intercarrier interference (ICI) reduction and signal detection algorithms for OFDM systems over time-varying channels. Several ICI reduction algorithms are proposed for OFDM systems over doubly-selective channels. The OFDM ICI reduction problem over time-varying channels is formulated as a combinatorial optimization problem based on the maximum likelihood (ML)criterion. First, two relaxation methods are utilized to convert the ICI reduction problem into convex quadratic programming (QP) problems. Next, a low complexity ICI reduction algorithm applicable to $M$-QAM signal constellations for OFDM systems is proposed. This formulates the ICI reduction problem as a QP problem with non-convex constraints. A successive method is then utilized to deduce a sequence ofreduced-size QP problems. For the proposed algorithms, the QPproblems are solved by limiting the search in the 2-dimensional subspace spanned by its steepest-descent and Newton directions to reduce the computational complexity. Furthermore, a low-bit descent search (LBDS) is employed to improve the system performance. Performance results are given to demonstrate that the proposed ICI reduction algorithms provide excellent performance with reasonable computational complexity. A low complexity joint semiblind detection algorithm based on the channel correlation and noisevariance is proposed which does not require channel state information. The detection problem is relaxed to a continuous non-convex quadratic programming problem. Then an iterative method is utilized to deduce a sequence of reduced-size quadratic programming problems. A LBDS method is also employed to improve the solution of the derived QP problems. Results are given which demonstrate that the proposed algorithm provides similar performance with lower computational complexity compared to that of a sphere decoder. A major challenge to OFDM systems is how to obtain accurate channel state information for coherent detection of the transmitted signals. Thus several channel estimation algorithms are proposed for OFDM systems over time-invariant channels. A channel estimation method is developed to utilize the noncircularity of the input signals to obtain an estimate of the channel coefficients ...

Design of Channel Estimation and Equalization for OFDM Systems
Design of Channel Estimation and Equalization for OFDM Systems

Author: Ali Salah Mahdi

Publisher: LAP Lambert Academic Publishing

Published: 2015-01-20

Total Pages: 84

ISBN-13: 9783659671616

DOWNLOAD EBOOK

Orthogonal Frequency Division Multiplexing (OFDM) system is one of the multicarrier techniques which is robust against Inter-symbol-Interference, multipath fading and very easy to apply in transmitters by using inverse fast Fourier transform IFFT and at the receivers by using fast Fourier transform FFT. In a communication system, channel estimation is very important issue for the data detection. In coherent detection, one of the popular techniques is to use pilot tones as a reference signal in OFDM symbols. In the comb-type pilot tones insertion, pilot tones are inserted into each OFDM symbols, but inserting a large number of pilot tones will lead to channel capacity reduction or bandwidth expansion [1-2]. In this work, to overcome this transmission loss, a modified least square (ModLS) algorithm for fast time varying wireless channel at comb-type pilot arrangement in QAM signals for OFDM system is proposed. The simulation results obtained from the proposed algorithm showed a good performance in noisy wireless channels. In addition, it has been compared with least square (LS) algorithm in different signal to noise ratios and different channel tabs.