From the Publisher:
Multi-carrier modulation, in particular orthogonal frequency division multiplexing (OFDM), has been successfully applied to a wide variety of digital communications applications for several years. Although OFDM has been chosen as the physical layer standard for a diversity of important systems, the theory, algorithms, and implementation techniques remain subjects of current interest. 

This book is intended to be a concise summary of the present state of the art of the theory and practice of OFDM technology. This book offers a unified presentation of OFDM theory and high speed and wireless applications. In particular, ADSL, wireless LAN, and digital broadcasting technologies are explained. 

It is hoped that this book will prove valuable both to developers of such systems, and to researchers and graduate students involved in analysis of digital communications, and will remain a valuable summary of the technology, providing an understanding of new advances as well as the present core technology.

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Multi-Carrier Digital Communications: Theory and Applications of OFDM

In discrete multitone (DMT) transceivers an intelligent guard time sequence, called a cyclic prefix (CP), is inserted between symbols to ensure that samples from one symbol do not interfere with the samples of another symbol. The length of the CP is determined by the length of the impulse response of the effective physical channel. Using a long CP reduces the throughput of the transceiver, To avoid using a long CP, a short time-domain finite impulse response (FIR) filter is used to shorten the effective channels impulse response. This paper explores various methods of determining the coefficients for this time-domain filter. An optimal shortening and a least-squares (LS) approach are developed for shortening the channel's impulse response. To provide a computationally efficient algorithm a variation of the LS approach is explored. In full-duplex transceivers the length of the effective echo path impacts the computational requirements of the transceiver. A new paradigm of joint shortening is introduced and three methods are developed to jointly shorten the channel and the echo impulse responses in order to reduce the length of the CP and reduce computational requirements for the echo canceller.

Impulse response shortening for discrete multitone transceivers

This paper proposes a new blind channel estimation method for orthogonal frequency division multiplexing (OFDM) systems. The algorithm makes use of the redundancy introduced by the cyclic prefix to identify the channel based on a subspace approach. Thus, the proposed method does not require any modification of the transmitter and applies to most existing OFDM systems. Semi-blind procedures taking advantage of training data are also proposed. These can be training symbols or pilot tones, the latter being used for solving the intrinsic indetermination of blind channel estimation. Identifiability results are provided, showing that in the (theoretical) situation where channel zeros are located on subcarriers, the algorithm does not ensure uniqueness of the channel estimation, unless the full noise subspace is considered. Simulations comparing the proposed method with a decision-directed channel estimator finally illustrates the performance of the proposed algorithm.

/pdf/subspace-based-blind-and-semi-blind-channel-estimation-for-2hrjru3bqk.pdf

Subspace-based blind and semi-blind channel estimation for OFDM systems

A modem operating selectively in the voice frequency and higher frequency bands which supports multiple line codes. A DSP is used to implement different existing ADSL line codes on the same hardware platform. The modem negotiates in real time for a desired line transmission rate to accommodate line condition and service cost requirements which may be implemented at the beginning of each communication session by exchange of tones between modems. A four step MDSL modem initialization process provides line code and rate compatibility. The handshake protocol and receiver algorithm for CAP based MDSL modems allows reliable modem synchronization over severely amplitude distorted channels and makes use of a short length sequence to train a synchronizing equalizer at the receiver. The algorithm and corresponding training sequence to train the transmitter filter are provided. After training to this sequence, a matched filter or correlator detects the inverted sync sequence. Detection of the inverted sequence signals commencement of normal reference training of the demodulation equalizers. An internal state machine in an MDSL modem records and monitors line status and notifies state change to other MDSL and host processor. The protocol for exchanging line connection management messages is a simplified LCP for MDSL. In a DMT system, a transmitter filter reduces the length of effective channel impulse response. Iimplementation of the filter combines time domain convolution and frequency domain multiplication to reduce needed computation power. The filter coefficients update may occur through a feedback channel.

Multimode digital modem

Transmitter-induced cyclostationarity has been explored previously as an alternative to fractional sampling and antenna array methods for blind identification of FIR communication channels. An interesting application of these ideas is in OFDM systems, which induce cyclostationarity due to the cyclic prefix. We develop a novel subspace approach for blind channel identification using cyclic correlations at the OFDM receiver. Even channels with equispaced unit circle zeros are identifiable in the presence of any nonzero length cyclic prefix with adequate block length. Simulations of the proposed channel estimator along with its performance in OFDM systems combined with impulse response shortening and Reed-Solomon coding are presented.

Exploiting input cyclostationarity for blind channel identification in OFDM systems

A multi-point communications system having an allocated total bandwidth is set forth. The system comprises a head end unit disposed at a primary site. The head end unit includes a receiver for receiving OFDM/DMT modulated data from a plurality of remote transceivers over a predetermined number of bins from a transmission medium. The receiver of the head end unit receives OFDM/DMT modulated data corresponding to a first data stream from a first one of the plurality of remote transceivers over one or more bins of a first subset of the predetermined number of bins and OFDM/DMT data corresponding to a second data stream from a second one of the plurality of remote transceivers over one or more bins of a second subset of the predetermined number of bins. The head end unit and each of the first and second transceivers can receive and/or transmit in at least a given X QAM mode or Y QAM mode. The head end unit monitors actual bandwidth usage of the total allocated bandwidth and uses the measured bandwidth usage to alter the QAM modes used for communications within the system.

Apparatus and method for bandwidth management in a multi-point OFDM/DMT digital communications system

A multi-point communications system is set forth. The communications system comprises a primary transmitter disposed at a primary site and a plurality of receivers respectively disposed at a plurality of secondary sites. The primary transmitter generates OFDM/DMT transmissions over a first number of transmission bins to transmit communication data in those bins. A transmission medium is used for transmitting the OFDM/DMT transmissions from the primary transmitter at the primary site to the plurality of receivers disposed at a plurality of secondary sites. Each of the plurality of receivers employs a unique receiver architecture that digitally processes exclusively a reduced subset of the first number of bins transmitted by the primary transmitter to recover communication data that is transmitted in the reduced subset of the first number of bins. Such a receiver architecture eliminates much of the complexity typically associated with receiving and processing a wide-band OFDM/DMT signal, thereby making the communications system subject to economical and practical implementation in applications having receivers respectively located at a large number of secondary sites.

Multi-point OFDM/DMT digital communications system including remote service unit with improved transmitter architecture

A data communication system is provided including a multicarrier transmitter which encodes and modulates input data into a plurality of carriers, a channel which distorts signals input to it by the transmitter, an echo path which also distorts signals, and a multicarrier receiver which receives signals from the channel and minimizes the distortions through the use of a SIRF. The method of determining the set of time domain SIRF parameters generally includes the steps of approximating the original channel and echo impulse responses, optionally modeling the channel impulse response, optionally modeling the echo impulse response, computing SIRF coefficients based upon the combined approximated or modeled channel impulse response and the approximated or modeled echo impulse response, calculating a SSNR for the calculated SIRF coefficients, and repeating the steps a predetermined number of times to determine those coefficients with the best

Method and apparatus for providing reduced complexity echo cancellation in a multicarrier communication system

A multi-point communications system that comprises a head end unit disposed at a primary site and a plurality of receivers disposed at remote sites. The head end unit includes a transmitter for transmitting OFDM/DMT symbols over a predetermined number of bins across a transmission medium. The OFDM/DMT symbols are transmitted in periodically occurring formatted symbol frames. The cyclic prefix includes a predetermined periodic signal superimposed thereon. The receivers receive the OFDM/DMT symbols over a subset of the predetermined number of bins from the transmission medium and use the superimposed signals to attain symbol alignment. In accordance with a further aspect of the present invention, the receivers apply a predetermined incremental phase shift to received samples corresponding to the received OFDM/DMT symbols to thereby compensate for phase shifts. The multi-point communications system may include a similar system for aligning symbols transmissions from a remote service unit having a transmitter. Such a system includes a plurality of remote service units each including a transmitter for transmitting OFDM/DMT symbols over a predetermined number of bins across a transmission medium. Each of the plurality of remote service units is operable in a symbol alignment mode in which the transmitter transmits a broad band periodic signal. The head end unit includes a receiver for receiving the OFDM/DMT symbols, including the broad band periodic signal, from the transmission medium. The head end unit uses the time position of the broad band periodic signal to align the symbol transmissions of the remote service unit.

Peter J. W. Melsa

Papers

Impulse response shortening for discrete multitone transceivers

Apparatus and method for bandwidth management in a multi-point OFDM/DMT digital communications system

Multi-point OFDM/DMT digital communications system including remote service unit with improved transmitter architecture

Method and apparatus for providing reduced complexity echo cancellation in a multicarrier communication system

Apparatus and method for symbol alignment in a multi-point OFDM or DMT digital communications system