EBU Technical Review : No. 282 (March 2000)

OFDM for Wireless Multimedia Communications

The broadcast community is well aware that both Digital Audio Broadcasting (i.e. the Eureka-147 DAB system) and terrestrial Digital Video Broadcasting (DVB-T) use orthogonal frequency division multiplexing (OFDM). It is less known, and it came to my rather big surprise, that OFDM has been accepted for the new wireless local area network standards from IEEE 802.11 - High Performance Local Area Network type 2 (HIPERLAN/2) and Mobile Multimedia Access Communication (MMAC) systems. Also, it is expected to be used for the wireless broadband multimedia communication systems (WBMCS) of the future.

Thus, it appears that the multi-carrier multiplexing/modulation system provides a universal basis for all digital terrestrial broadcasting and communications systems that need to combat multipath propagation and narrow-band interference, and yet potentially offer relatively high spectrum efficiency (in terms of useful bits per Hz) and reasonable power efficiency. The fact that the same modulation system could be used for all mobile multimedia systems of the future fits nicely in the conceptual model of technology convergence (which is in many cases a "wishful thinking"). But it also shows that the broadcasting and communications systems are two different worlds that have been developing separately, with little or no co-operation. What a coincidence that both developments resulted in the same technology, namely OFDM!

The critical issue in the design of practical systems is not the choice of the modulation system but the trade-off between various, often conflicting, parameters such as bandwidth, bit-rate and delay spread. The book examines the advantages and disadvantages of OFDM compared with simple single-carrier (SC) systems with equalizers. Furthermore, the book discusses the use of redundancy coding, channel estimation and synchronization. Specific chapters are devoted to the potentially contentious issues of peak-to-average-power and the sensitivity to phase noise and frequency offset. Some solutions are identified.

Communication engineers, more than broadcasters, may be interested in the section devoted to the combination of OFDM and CDMA (code division multiplex access) and frequency-hopping OFDMA (Orthogonal FDM Access).

In summary, the book is a useful guide for broadcast and communications engineers, researchers and students of communication technology. The emphasis is given to two-way communication systems: broadcasting applications are only mentioned in passing, for completeness. One area that is not covered by the book is "on-air multiplexing", the work carried out by the BBC (see EBU Technical Review xxx).

OFDM for Wireless Multimedia Communications
R. van Nee and R. Prasad
Hardbound volume of 260 pages
Ref: ISBN 0-89006-530-6. Price £53.00.
Artech House, London, 2000
http://www.artech-house.com/

Franc Kozamernik

(back to top)

Multirate Digital Signal Processing

The most important characteristic common to all the filters and filter banks discussed in this book is that they use multirate processing of digital signals. A multirate system can, when desired, increase or decrease the sample spacing (and thus the sampling rate) of individual signals before or while processing them. These signals, which then have different sampling rates, can be simultaneously processed in various parts of the multirate system.

This textbook provides an introduction to multirate digital signal processing. It illustrates the properties of various filter banks, enables readers to distinguish between different types of filter banks, and provides design methodologies for multirate filters and filter banks.

The properties of various filter banks are described, enabling readers to distinguish between their diverse types. The book also provides design methodologies for multirate filters and filter banks. Since much of the material is quite advanced, the text includes many figures and examples to aid understanding.

The fundamental theory of multirate signal processing is presented in Chapter 1. Chapters 2 and 3 then summarize the properties of FIR (Finite Impulse Response) filters that are of particular interest in this book. An overview of multirate filters is given in Chapter 5.

Digital filter banks constitute the main subject of this book, and four chapters are devoted to them. Chapter 6 describes two-channel filter banks: quadrature-mirror filter banks (QMF), biortogonal and linear phase filters, used as sub-band coding or transmultiplexing filter banks. Chapter 7 extends this to M-channel filter banks.

The potentially high computational efficiency of modulated filter banks is then exploited in Chapter 8, which describes polyphase structures for two-channel and M-channel filter banks. To complement the discussion on uniform filter banks, Chapter 9 examines the octave filter banks, then introduces the reader to the wavelet transform, as well as deriving the relationship between dyadic wavelets and filter banks.

The book concludes with an overview of current applications. These include sub-band coding of speech, audio and video signals, multi-carrier data transmissions, etc. DAB and ADSL both use a multi-carrier transmission system which is similar to polyphase transmultiplexer filter banks. It is called OFDM (orthogonal frequency division multiplexing).

The contents of this book are derived from a lecture course at the Hamburg University of Technology, a course that is aimed at students of Electronics and Computer Science in their final year of study. It is thus primarily intended to be a course book that complements such lectures.

Multirate Digital Signal Processing: Multirate Systems, Filter Banks, Wavelets
N.J. Fliege
Bound volume of 340 pages
Ref: ISBN: 0-471-49204-3. Price £45.00
John Wiley & Sons, UK, December 1999
http://www.wiley.com/

Mostafa Bibak

(back to top)

Digital Processing of Signals: Theory and Practice

The term "Digital Signal Processing" is used to describe the complete set of operations, arithmetic calculations and numerical manipulations which are performed on the group of numbers representing the signal to be processed, in order to produce another set of numbers representing the processed signal.

Many different functions can be performed in this way; spectral analysis, linear or non-linear filtering, transcoding, modulation, detection, and estimation and extraction of parameters.

Digital processing began with the development of algorithms for the fast calculation of the Discrete Fourier Transform (DFT). This transform is basic to the study of discrete systems and is the digital equivalent of the Fourier Transform (FT) in the analogue case.

The greater part of this book is devoted to the study of one-dimensional time-invariant discrete linear systems. From the point of view of DSP, these are the most important, the most readily accessible and the most useful. Multi-dimensional systems, particularly those with two dimensions, have undergone a certain degree of development. They are applied for example to image analysis; however, their properties are generally deduced from those of one-dimensional systems, of which they are often only simple extensions.

The conversion of analogue signals to numerical (digital) form involves a double approximation; first, in time space - a process called sampling - and, second, in amplitude space - a process called quantization. The approximation value thus obtained is then associated with a number; this process is called coding. To produce the effect on the signal of these two approximations, two basic tools are used; Fourier analysis, and distribution theory. These are studied in Chapter 1.

Chapters 2 and 3 are devoted to fast calculation algorithms. These chapters present the elementary properties of the fast algorithm and the mechanics of their application. They also outline a unified treatment of the algorithms and discuss possible development.

A group of four chapters is devoted to the study of properties of digital filters. Chapter 4 discusses the properties of discrete time-invariant linear systems, reviews the principal properties of the Z-transform and gives the necessary elements for studying these filters. Chapter 5 discusses finite impulse response (FIR) filters: their properties are considered, together with the techniques for calculating the coefficients, and the structure for their realization.

As infinite impulse response (IIR) filters are generally formed as a cascaded arrangement of fundamental elements of the first and second order, Chapter 6 describe these elements and their properties. These elements considerably simplify the approach to this type of system and provide a set of results which are of use in practical applications.

Chapter 7 describes the methods for calculating the coefficients of IIR filters and discusses the problems of their realization, and their limitations. As infinite impulse response filters have properties comparable to those of continuous analogue filters, it is natural to consider their realization in structures of the same type. This is the subject of Chapter 8, which describes ladder structures.

Some equipment used in spectral analysis or in the telecommunications field, for example, involves signals represented by a set of complex numbers. Their properties are discussed in Chapter 9, along with the design of devices for the production or processing of such signals.

If DSP devices are to operate in real time, then they must operate at a rate which is closely related to the sampling frequency of the signals. Thus, their complexity depends on the number of operations to be performed and on the time interval available for their completion. One simplification that can be of great importance is obtained by changing the sampling frequency to match the bandwidth of the useful signal throughout the processing. This is multirate filtering which is discussed in Chapter 10. The effects on the processing characteristics are described, together with methods for their realization. Rules for their use and evaluation are also provided.

Filter banks for the decomposition and reconstruction of signals have become a basic tool for compression. Their operation is described in Chapter 11, together with the computation methods and implementation structures.

Filters can be determined from a time specification, i.e. the properties vary with time. Chapter 12 is devoted to simple adaptive filters of the most common type in which the coefficients vary according to the gradient algorithm.

Circuits and technological considerations are dealt with in Chapter 13. Examples are given of large-scale integration techniques, specifically designed for digital signal processing.

Telecom equipment represents a preferred area for the application of digital processing, as transmission networks become progressively digitized and as digital computers are introduced at the terminals. Chapter 14 presents a group of typical telecom applications, illustrating the theory and techniques presented in the book as a whole.

The use of digital techniques in signal processing opens up a wide range of potential advantages - such as precise systems design, equipment standardization and the stability of performance characteristics, as well as the ease of monitoring and control. It is the aim of this book to give access to the required digital techniques, by relating theory and practice.

Digital Processing of Signals: Theory and Practice (3rd Edition)
M. Bellanger
Hardbound volume of 431 pages
Ref: ISBN: 0-471-97673-3. Price £55.00
John Wiley & Sons, UK, February 2000
http://www.wiley.com/

Mostafa Bibak

(back to top)

The DAW Buyers Guide

This guide is a new publication form Sypha who have previously published The Tapeless Audio Directory (reviewed in Issue No. 277). Those who are familiar with the earlier publication will immediately recognize the similarities between the two publications. The main difference is that The DAW Buyers guide does not include playout systems for radio but is dedicated to digital audio workstations and tapeless recorders.

The publishers aim to provide a review of the market, so that a potential purchaser can begin the process of selecting the most suitable product(s) for his/her particular needs. There are over 300 entries on equipment in this overcrowded field. These include existing systems along with others soon to be launched (and for completeness, some products which are no longer in production).

The guide includes all types of digital audio workstation, including turnkey systems, rack units, cards with software and software-only packages. It covers disk-based multitracks, dubbers and portable recorders as well as systems for audio editing and recording applications. The complexity ranges from simple PCMCIA-based portables for location recording and editing, to complex systems for multi-machine operations in surround sound for video, television and film production. The guide also covers the whole range of multitrack tape replacement units, as well as systems for CD/DVD pre-mastering and/or preparing audio content for streaming via the Internet.

The information has been gained from questionnaires sent out to the manufacturers. As before, the guide gives, where possible, full information on target markets, hardware and software specifications, operational features, interfaces, networking and file formats, archive and backup. Plans for future development, training and customer support, typical configurations and costs are also given. Suppliers are listed in Europe and elsewhere. More concise "mini listings" are given where full listings is not available.

The guide aims to shows the hardware required for a working system, and clearly indicates what is provided, and what the user is required to supply, or is optionally available.

For the user, the guide includes information on:

	operational strategies and applications;
	which processes are performed in real-time and which are rendered;
	synchronization to external devices;
	control of and by other equipment;
	file management for library/database purposes, and archiving;
	file transfer (including direct reading of files generated externally, as well as file import/export).

As before in these guides from Sypha, there are tutorial sections on the basics of non-destructive editing along with a comprehensive explanation of the terminology, and tips for potential purchasers.

The DAW Buyers Guide
Y. Hashmi and S. Plumbridge
Bound volume of 104 pages
Ref: ISBN: 1-901950-03-4. Price £22.50
Sypha Publications, UK, December 1999
E-mail: sypha@compuserve.com

Richard Chalmers

(back to top)

European Broadcasting Union Case postale 45 Ancienne Route 17A CH-1218 Grand-Saconnex Geneva Switzerland techreview@ebu.ch