Lesson 1 Introduction to Digital Communications
After reading this lesson, you will learn about ¾ ¾ ¾ ¾ ¾ Lesson-wise organization of this course Schematic description of a representative digital communication system Milestones in the history of electronic communications Names and usage of electromagnetic bands Typical transmission loss for several physical media Preamble Usage of the benefits of electrical communications in general and digital communications in particular, is an inseparable part of our daily experience now. Innumerable applications due to developments in digital communications have already started influencing our day-to-day activities directly or indirectly. Popularity of the Internet and television are only two of the most obvious examples to prove the point. In fact, it may not be an overstatement today that ‘information highways’ are considered as essential ingredients of national infrastructure in the march of a modern society. It is, however, pertinent to mention that isolated developments only in the field of electrical communications have not caused this phenomenon. Remarkable progresses and technical achievements in several related fields in electronics engineering and computer engineering have actually made applications of several principles and theories of communication engineering feasible for implementation and usage. The purpose of this web course, however, is narrow and specific to the principles of digital communications. This web course on ‘Digital Communications’ is primarily intended for use by undergraduate students who may be preparing for graduate level studies in the area of electrical communications engineering. A teacher, offering an introductory-level course on digital communications, may also find several topics suitable for classroom coverage. The field of Digital Communications is reach in literature and there is no dearth of excellent text books and research papers on specific topics over and above the bulk of tutorial material, technical standards and product information that are available through the Internet. Hence, the onus is clearly on the present authors to justify the need and relevance of this web course on ‘Digital Communications’. To put it humbly, the present authors believe that any ‘web course’ should primarily cater to the quick requirements of the prime target audience (in our case, an undergraduate student preparing for graduate level studies in the area of electrical communications engineering). The usual requirements are believed to be of the following types: a) exposition to a relevant topic or concept, b) examples and problems to highlight the significance or use of certain principles and c) specific data or information in relation to a topic of study in the area of digital communications. Our teaching experience says that some or all of these requirements are indeed met in several textbooks to a good extent. For ready reference, a consolidated Bibliography is appended at the end of this course material. What stand out, probably, in favour of a ‘web course’ are the flexibility in using the material may be covered and the scope of continuous upgradation of the material to cater to specific needs of the audience in future.
The general structure of ’40-Lesson course’ is an indication to the implicit limits (of ‘time to read’ and ‘storage’); hence a balance among the reader requirements a) – c), mentioned above, should be worked out. The present version of this web course is designed with more emphasis on exposing relevant topics and concepts [requirement a)] which may supplement classroom teaching. The course is split in seven Modules as outlined below. The first module consists of four lessons. The present lesson (Lesson #1) gives an outline of major historical developments in the field of research in telecommunications engineering over a period of hundred years. Materials on radio spectrum should help recapitulate a few basic issues. The lesson ends with a general schematic description on a digital communication system. Lesson #2 gives a brief classification of signals and emphasizes the importance of sampling theory. Lesson #3 presents some basic concepts of information theory, which helps in appreciating other central principles and techniques of digital transmission. The concept of ‘information’ is also outlined here. Needs and benefits of modeling an information source are the topics in Lesson #4. The second module is devoted to Random Processes. The module starts with a simple to follow introduction to random variables (Lesson #5). It is often necessary to acquire the skill of defining appropriate functions of one or more random variables and their manipulation to have greater insight into parameters of interest. The topic is introduced in Lesson #6 wherein only functions of one random variable have been considered. A powerful and appropriate modeling of a digital communication system is often possible by resorting to the rich theories of stochastic processes and this remains an important tool for deeper analysis of any transmission system in general. The topic has been treated at an elementary level in Lesson #7. A few commonly encountered random distributions, such as binomial, Poisson, Gaussian and Rayleigh are presented in Lesson #6. An emerging and powerful branch in electrical communication engineering is now popularly known as statistical signal processing and it encompasses several interesting issues of communication engineering including those of signal detection and parameter estimation. The basic backgrounds, laid in Lessons #5 to #8 should be useful in appreciating some of the generic issues of signal detection and parameter estimation as outlined in Lesson #9. The third module on pulse coding focuses on the specific tasks of quantization and coding as are necessary for transmission and reception of an analog electrical signal. It is however, assumed that the reader is familiar with the basic schemes of analog-todigital conversion. The emphasis in this module is more on the effects of quantization error (Lesson #10) while different pulse coding schemes such as Pulse Code Modulation (Lesson #11), Log-PCM (Lesson #12), Differential Pulse Code Modulation (Lesson #13) and Delta Modulation (Lesson #14) are used for possible reductions in the average number of bits that may have to be transmitted (or stored) for a given analog signal. The example of speech signal has been considered extensively.
Appropriate representation of bits (or information bearing symbol) is a key issue in any digital transmission system if the available bandwidth is not abundant. Most of the physical transmission media (e.g. twisted copper telephone line, good quality coaxial cable, radio frequency bands) are, in general, limited in terms of available frequency band (a simple reason for this general observation: demand for good quality digital communication system, in terms of bits to be transferred per second, has been rising with newer demands and aspirations from users). So, it makes sense to look for time-limited energy pulses to represent logical ‘1’-s and ‘0’-s such that the signal, after representation, can be transmitted reliably over the available limited bandwidth. The issue is pertinent for both carrier less (referred as ‘baseband’ in Module #4) transmission as well as modulated transmission (with carrier, Module #5). Several interesting and relevant issues such as orthogonality amongst time-limited energy pulses (Lesson #15), baseband channel modeling (Lesson #17) and signal reception strategies (Lessons #18 - #21) have, hence, been included in Module #4. Module #5 is fully devoted to the broad topic of Carrier Modulation. Several simple digital modulation schemes including amplitude shift keying, frequency shift keying (Lesson #23) and phase shift keying (Lessons #24 - #26) have been introduced briefly. Performance of these modulation schemes in the background of additive Gaussian noise process is addressed in Lesson #27 and Lesson #28. If appreciated fully, these basic techniques of performance evaluation will also be useful in assessing performance of the digital modulation schemes in presence of other transmission impairments (e.g. interference). The basic issues of carrier synchronization and timing synchronization have been elaborated with reasonable illustrations in Lesson #31 and Lesson #32. Module #6 is on error control coding or ‘Channel Coding’ as it is popularly known today. Basics of block and convolutional codes have been presented in three lessons (Lessons #33 - #35). Two more lessons on turbo coding (Lesson #37) and coded modulation schemes (Lesson #36) have been added in view of the importance of these schemes and procedures in recent years. Spread spectrum communication techniques have gained popularity in last two decades in view of their widespread commercial use in digital satellite communications and cellular communications. A primary reason for this is the inherent feature of multiple access that helps simultaneous use of radio spectrum by multiple users. Effectively, several users can access the same frequency band to communicate information successfully without appreciable interference. Basic spread spectrum techniques have been discussed in Lesson #38 of Module #7 before highlighting the multiple access feature in Lesson #40. It is interesting to note that a spread spectrum communication system offers several other advantages such as anti-jamming and low probability of interception. In such non-conventional applications, the issue of code acquisition and fine tracking is of utmost importance as no pilot signal is usually expected to aid the process of code synchronization. To appraise the reader about this interesting and practical aspect of code synchronization the topic has been introduced in Lesson #39. A short Bibliography is appended at the end of Lesson #40.