omo- strengths, NATO AGARD, Radio systems and the ionosphere, geneous smooth earth, J. Inst. Electr. Eng. Part 3. Conf. Proc. 173, Athens, (May). Picquenard, Arme (1984) Radio wave propagation, Philips Techni- Dick, M.I. (1990) Propagation model of CCIR report 894, Ruther- cal Library, McMillan Technical Press. ford Appleton Laboratories, (May). Radio Society of Great Britain, Radio Communication Handbook. Friis, H.T. (1946) A note on a simple transmission formula, IRE Stanniforth, J.A. (1972) Microwave Transmission, English Univer- Proc. (May), pp. 254-256. sity Press. Hitney, H.V. (1990) IONOPROP, ionospheric propagation assess- Stark, Axel (1986) Propagation of Electromagnetic Waves, Rohde ment software and documentation, Artech House, London. & Schwarz. This page intentionally left blank Digital transmission Edwin V Jones BSc MSc PhD CEng MIEE University of Essex Contents 28.1 Design principles 28/3 28.2 Transmission error performance 28/5 Line codes 28/9 Clock extraction 28/12 Frame alignment 28/16 28.6 References 28/19 This page intentionally left blank Design principles 28/3 Design principles 28.1.1 System requirements A simplified block diagram of a digital communication system is shown in Figure 28.1. Information from a data source is processed so that it can be reliably transmitted via a communication channel to a distant data terminal. The user will measure reliability in terms Decision of the difference between the transmitted data and the received data. threshold The system designer's task is to ensure that this difference is always acceptably low. Decision timing Data source Source Channel coding coding Interference Figure 28.2 Baseband waveforms: (a) transmitted signal; (b) bandlimited received signal Channel (with intermediate repeaters if necessary) tion to partially compensate for the characteristics of the channel) Data terminal Decoding Decoding the signal may look like Figure 28.2(b). The task of the receiver is to establish the transmitted sequence. For a binary sequence, a single decision t