0195136047.pdf

730 COMMUNICATION SYSTEMS

∼=^1

2

e−^18

4. 243

√

π

∼= 1. 01 × 10 −^9

The PSK system is clearly superior to the ASK system, based on equal values ofEb/N 0.

Digital Transmission on Fading Multipath Channels

We have discussed thus far digital modulation and demodulation methods for the transmission

of information over two types of channels, namely, an additive noise channel and a linear filter

channel. While these channel models are appropriate for a large variety of physical channels, they

become inadequate in characterizing signal transmission over radio channels whose transmission

characteristics vary with time. Time-varying behavior of the channel is exhibited by the following:

• Signal transmission via ionospheric propagation in the HF band(see Figure 15.0.3), in
  which signal fading is a result of multipath signal propagation that makes the signal arrive
  at the receiver via different propagation paths with different delays.


• Mobile cellular transmission, between a base station and a telephone-equipped automobile,
  in which the signal transmitted by the base station to the automobile is reflected from
  surrounding buildings, hills, and other obstructions.


• Line-of-sight microwave radio transmission, in which signals may be reflected from the
  ground to the receiving antenna due to tall obstructions or hilly terrain in the path of
  propagation.


• Airplane-to-airplane radio communications, in which secondary signal components may
  be received from ground reflections.
  Such channels may be treated as linear systems with time-varying impulse response by
  adopting a statistical characterization. Models for time-variant multipath channels will not be
  considered here.

15.4 Learning Objectives

Thelearning objectivesof this chapter are summarized here so that the student can check whether

he or she has accomplished each of the following.

• Basic ideas about waveguides.


• Transmission line with matched impedances.


• Calculating the signal power at the output of a radio transmission system.


• Fundamentals of antennas.


• Signal-to-noise power ratio at the system output, as a measure of performance of the
  communication system.


• Amplitude modulation and message demodulation in analog communication systems.


• Frequency modulation and demodulation in analog communication systems.


• Block diagrams of superheterodyne radio receivers.


• Basic notions of television broadcasting.


• Cellular-telephone-system concepts.


• Sampling, quantization, and coding in digital communication systems.