Lecture :An Overview of Digital

Communications

Course Objectives• Understand basic digital modulation schemes, and

trade-offs in using different modulation schemes, primarily bandwidth versus power trade-offs

• Understand the principles behind maximum likelihood receiver design

• Understand equalization theory• Understand advanced techniques, like MCM and

SS• Develop the tools and know-how for performance

analysis

Definition

• Digital communications is the exchange of information using _________________ This is in contrast to the conventional technique of analog communications (as used in the telephone, FM radio, and television), which does not restrict the transmitted signal to a finite set.

Why Digital?

• For sources of information that are _______________, such as ASCII characters from a computer keyboard, digital communications is a natural choice.

• Source encoding or data compression algorithms can ______________________

with a controlled amount of signal distortion.

Why Digital? (Cont’d)• Digital signals are more robust to channel

impairments than analog signals because digital signals can be completely ________-, but with analog signals, accumulated noise is bound to the signal.

• Digital modulation, coding, and signal processing techniques can ____________.

• __________ and ____________ is easier.

Figure 1.1 Pulse degradation and regeneration.

Why Digital? (Cont’d)• ________: digital data can be accurately

and rapidly stored and retrieved. (Recall the superiority of the CD to the LP or the magnetic audio tape; Instant random access, and no rewind delays.).

• In sum, advantages of digital communications are_______, _______ _________, ________, and _______.

A Basic Digital Communication System

Channel

EssentialOptional

Types of Waveform Channels

• Wireline channels, e.g., ______or _______• Fiber optic channels• Wireless electromagnetic channels

– goundwave (MF band, 0.3-3 MHz, AM radio)– skywave (HF band, 3-30 MHz, FM radio)– Line-of-sight (______band, 30-300 MHz, TV;

_____, 0.3-3 GHz, cellular systems)• Uuderwater acoustic channels• Storage channels, e.g., _______and ________

Digital vs Analog Performance Criteria

• Criteria for analog communication system:– __________, such as _______________,

_____________, or ______________ between the transmitted and received waveforms.

• Criteria for digital communication system:– _______________

Figure 9.2 Normalized channel capacity versus channel SRN.

Figure 9.3 Normalized channel bandwidth versus channel SRN.

Figure 9.4 Normalized channel bandwidth versus channel Eb/No.

bpsSNR)(WC 360001log 36dBSNR ; 3000Hz WAssume

Downstream with no AD/DA,SNR,C64kbps

References• C. E. Shannon, A Mathematical Theory of Communication,

The Bell System Technical Journal, Vol. 27, p.p. 79-423, 623-656, July, October, 1948.

• G. D. Forney and G. Ungerboeck, Modulation and Coding for Linear Gaussian Channels, IEEE Trans. On Information Theory, Vol. 44, No. 6, p.p. 2384-2415, 1998.

• Ezio Biglieri, J. Proakis, and S. Shamai, Fading Channels: Information-Theoretic and Communications Aspects, IEEE Trans. On Information Theory, Vol. 44, No. 6, pp. 2619-2692, 1998.

• Sergio Verdu, Fifty Years of Shannon Theory, IEEE Trans. On Information Theory, Vol. 44. No. 6, pp. 2057-2078, 1998.