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Coding Techniques in High Speed Telephone Modems, a Survey

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Coding Techniques in High Speed Telephone Modems, a Survey Coding Techniques in High Speed Telephone Modems, a survey M.Maspaitella Eindhoven University of Technology Faculty of Electrical Engineering Group Information and Communication Theory march, 1991 Coaches: ir. P.G.M. de Bot (TUE), ir. F. van Terwisga (PTH) Abstract In this literature study report, several aspects of coding techniques will be described. ASK, PSK and QAM as major M-ary constellations will be discussed theoretically in order to get an insight of the fundamental principles of digital communications over a band-limited channel. An important factor is the relation between signal energy Es and the minimum Euclidean distance between adjacent points in a constellation. Subsequently, coding schemes with block coding and, more profound, convolutional coding are discussed. Next topic is the trellis coded modulation achieving coding gain with a redundant hit. Based on the theoretical introductions in the proceeding chapters, relevant CCIlT recommendations for modems up to 14.4 kbps will be discussed. From several scientific and commercial sources, information was obtained about the most recent technical developments in high speed modems. Data rates up to 24 kbps are achieved by application ofhighly advanced coding principles. Multicarrier modulation, multidimensional signal constellation, multilevel coding, constellation shaping and trellis precoding are state of the art design principles discussed only in a introductory way. Finally a number of modem 'mile stones' are surveyable adapted in a table. Preface November 1990 a study was initiated to investigate the performances of present high speed modems. The (literature) study should support a current research project at the group information and communication theory at the faculty of electrical engineering of the TUEl. The literature study was carried out as an alternative study project in 2 the last year on the Pedagogisch Technische Hogeschoo1 • After deliberation hetween the author and the concerning institutions, it was agreed to perform a literature study during approximately 6 weeks. Coaches during this project were Paul de Bot from the TUE and Friso van Terwisga as a supporting teacher from the PTH. Practically, research and editing activities were carried out at the TUE Faculty of Electrical Engineering, group Information and Communication Theory. Main purpose of the study was to obtain recent technical information about recent high speed telephone modems. A survey of recent modem designs was required. From different sources several kinds of information were obtained. Specific interest was fixed on coding techniques and performance as well. To support the literature study sufficiently, it was necessary to provide a theoretical description ofdigital coding techniques. As an important spin-off, a summary of existing coding techniques had to be written. This part of the report seemed quite useful as a first introduction in coding techniques. However, for a more in depth study, the reader is referred to other excellent books ahout digital communications. An optional extent ofthe project wac; to demonstrate an (electrical) ac;pect of a telephone modem. However, the remaining time did not allow to complete this part of the project. A brief study learned that applying modems in an experimental set-up requires a large amount of complex equipment and more research determine practical goals. lEindhoven University of Technology, P.O.B. 513, 5600 MB Eindhoven, The Netherlands. 2The Pedagogisch Technische Hogeschool in Eindhoven is a higher technical educational institute, for teachers in technical disciplines. The author is a student at the elektrotechnical department of the PTH. PTH Eindhoven, P.O.B. 826, 5600 AV Eindhoven, The Netherlands. TABLE OF CONTENTS 1 Introduction 6 1.1 Working approach 6 1.2 Content of the report , 6 2 Useful definitions 7 3 Representation of modulated signals 9 3.1ASK 9 3.2 PSK ............................................. .. 10 3.3 QAM ............................................ .. 13 4 Noise 14 5 Minimal distance related to signal energy 16 5.1 4-ASK 16 5.2 4-QAM = 4-PSK 17 5.3 8-PSK 17 5.4 16 QAM 19 6 Multipoint Constellations 19 6.1 Other Constellations. ................................ .. 21 7 Channel coding ........................................... .. 22 7.1 The communication system. ........................... .. 22 7.2 Block coding 23 7.3 Convolutional encoding 24 7.3.1 Example of a convolutional encoder 24 7.3.2 The TrelIis Diagram 26 8 Trellis-Coded Modulation 27 8.1 The basic idea behind trelIis coding 28 8.2 The Viterbi algorithm ................................ .. 28 9 Echo Cancelation 29 10 Coding techniques in operational telephone modems 30 Table 40 11 Recent technical developments 41 11.1 Multicarrier Modulation 41 11.1.1 Applications 41 11.2 Multilevel coding 42 11.2.1 Applications 42 11.3 Advances in trellis-coded modulation 42 11.3.1 Multidimensional trellis coding 43 11.3.2 Applications 43 4 11.4 Shaping 43 11.5 Adaptive bandwidth 44 11.5.1 Trellis precoding 44 12 Current high speed modems 45 12.1 Summary 45 12.2 Notes ........................................... .. 46 Table 47 References ................................................ .. 48 A Glossary Of Notations 50 5 1 Introduction 1.1 Working approach In an early stage, documentation was requested from different suppliers. A study had to be made of coding techniques to get a sufficient understanding of the basic coding techniques. This was necessary to translate the scientific literature into an adaptable and readable introduction in PSK, ASK, QAM and trellis coding. The study of relevant literature, and the writing of this report were the main part of this investigation. 1.2 Content of the report Like the investigation, the report can be divided into two main parts. The first part deals with a theoretical explanation of coding techniques, applied in modems. A brief summary of common telephone circuit techniques is provided as well. In the second part a survey will be presented of the high speed telephone modem market. At first some CCIlT standards and recommendations will be explained. Next, a summary will be given from recent developments on the high speed modem market. For this is a literature study, there are many references to relevant scientific and commercial articles. Features and performances of different models will be com­ pared. For reasons stated before in the preface, no further attention is paid to the attempt to visualize some effects with measuring equipment. 6 2 Useful definitions To get some more insight in the working principles of modems, several basic aspects will be described in this chapter. First we will discuss some essential definitions Transmission line The telephone line as a data transmission line includes some typical properties. The Public Telephone Network, which is the backbone of most data communications, was originally developed for voice and not for data transmission. It is optimized for a satisfactory voice communication between telephone subscribers. As most voice energy is concentrated between 300 Hz and 2500 Hz, the bandwidth of a telephone circuit is approximately between 300 Hz and 3200 Hz in order to satisfy the average telephone voice subscriber. Therefore it is also called a band-limited channel. To protect other cables from being disturbed, The telephone company also limits the signal level to protect other cables from being disturbed wit Inter Symbol Inter­ ference (lSI). Modem A modem transmits binary data over a telephone line. This is achieved by modulating data bits onto a band-limited carrier signal. At the other end of the line the signal must be demodulated in order to obtain the digital data. Keying Digital information is characterized by the property that it always is represented in a discrete value. (e.g. +5V (one), -5V (zero» When we modu late an analogue carrier with discrete values, the signal is said to be coded or keyed. Modem specifications Baudrate The baudrate expresses the number of different signal points the modem transmits per second. Hence this is also defined as the symbol rate, as every point can be considered of a symbol-word defined by a bit, dibit, tribit etc. Data rate An important criterion to review a modem is to measure its data rate. The data rate indicates the speed of the incoming information bits that will be transmitted. The data rate is notated in bits per second (bps). At this moment high-speed modems offer performances up to a 19200 bps. 7 SiN ratio The signal to noise ratio provides information of the signal level related to a given noise component on the transmission channel. This information is useful to have an impression of how 'loud ' the signal level must be, in order to make a distinction between noise on the channel and a transmitted signal. SiN is notated in dB. Bit E"or Rate The Bit Error Rate (BER) indicates the probability an error occurs. Usually, the performance of modems is expressed as a required SiN at a certain BER. For example, a BER of 10-6 can be achieved at a SiN of 30 Db. Telephone line To send (digital) information over a telephone line data must be modulated on an analogue carrier signal. A suitable carrier wave form is a sine wave. For digital applications we want the analogue signal to have a limited amount of discrete values. These: different values, or positions, can be achieved by several forms of modulation or coding. We can make a distinction between FSK, ASK, PSK and QAM. The data is protected for being disturbed (by ego noise) by coding techniques. In the next paragraph the different modulation techniques will be discussed briefly. For a more in depth study of typical telephone techniques the reader is referred to [Bingham]. 8 3 Representation of modulated signals Representations from a modulated waveform, known from analogue modulation are: Sj(t)-Aj(t)cos(wt+4>,) A j: Amplitude 4>,: Phase Otherwise: Sj(t)-Aj(t)cos(wt+4»­ x(t) coswt+y(t) sinwt quadrature components x(t)-Aj(t)cos4>, and y(t)-A j(t)sin4>, unit vectors coswt and sinwt x(t) and y(t) can be considered as two low-pass signals, modulated on two orthogonal unit vectors in a two dimensional signal space.
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