17-08-2012, 01:58 PM
Introduction to Digital Data Transmission
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INTRODUCTION
This book is concerned with the transmission of information by electrical means using
digital communication techniques. Information may be transmitted from one point to another
using either digital or analog communication systems. In a digital communication
system, the information is processed so that it can be represented by a sequence of discrete
messages as shown in Figure 1–1. The digital source in Figure 1–1 may be the result
of sampling and quantizing an analog source such as speech, or it may represent a naturally
digital source such as an electronic mail file. In either case, each message is one of a
finite set containing q messages. If q = 2, the source is referred to as a binary source, and
the two possible digit values are called bits, a contraction for binary digits. Note also that
source outputs, whether discrete or analog, are inherently random. If they were not, there
would be no need for a communication system.
For example, expanding on the case where the digital information results from an
analog source, consider a sensor whose output voltage at any given time instant may assume
a continuum of values. This waveform may be processed by sampling at appropriately
spaced time instants, quantizing these samples, and converting each quantized
sample to a binary number (i.e., an analog-to-digital converter). Each sample value is
therefore represented by a sequence of 1s and 0s, and the communication system associates
the message 1 with a transmitted signal s1(t) and the message 0 with a transmitted
signal s0(t). During each signaling interval either the message 0 or 1 is transmitted with no
other possibilities.
COMPONENTS OF A DIGITAL COMMUNICATIONS SYSTEM
The mechanization and performance considerations for digital communications systems
will now be discussed in more detail. Figure 1–3 shows a system block diagram that is
more detailed than that of Figure 1–1. The functions of all the blocks of Figure 1–3 are
discussed in this section.
General Considerations
In most communication system designs, a general objective is to use the resources of
bandwidth and transmitted power as efficiently as possible. In many applications, one of
these resources is scarcer than the other, which results in the classification of most channels
as either bandwidth limited or power limited. Thus we are interested in both a transmission
scheme’s bandwidth efficiency, defined as the ratio of data rate to signal
bandwidth, and its power efficiency, characterized by the probability of making a reception
error as a function of signal-to-noise ratio. We give a preliminary discussion of this
power-bandwidth efficiency trade-off in Section 1.2.3. Often, secondary restrictions may
be imposed in choosing a transmission method, for example, the waveform at the output
of the data modulator may be required to have certain properties in order to accommodate
nonlinear amplifiers such as a traveling-wave tube amplifier (TWTA).
[attachment=31846]
INTRODUCTION
This book is concerned with the transmission of information by electrical means using
digital communication techniques. Information may be transmitted from one point to another
using either digital or analog communication systems. In a digital communication
system, the information is processed so that it can be represented by a sequence of discrete
messages as shown in Figure 1–1. The digital source in Figure 1–1 may be the result
of sampling and quantizing an analog source such as speech, or it may represent a naturally
digital source such as an electronic mail file. In either case, each message is one of a
finite set containing q messages. If q = 2, the source is referred to as a binary source, and
the two possible digit values are called bits, a contraction for binary digits. Note also that
source outputs, whether discrete or analog, are inherently random. If they were not, there
would be no need for a communication system.
For example, expanding on the case where the digital information results from an
analog source, consider a sensor whose output voltage at any given time instant may assume
a continuum of values. This waveform may be processed by sampling at appropriately
spaced time instants, quantizing these samples, and converting each quantized
sample to a binary number (i.e., an analog-to-digital converter). Each sample value is
therefore represented by a sequence of 1s and 0s, and the communication system associates
the message 1 with a transmitted signal s1(t) and the message 0 with a transmitted
signal s0(t). During each signaling interval either the message 0 or 1 is transmitted with no
other possibilities.
COMPONENTS OF A DIGITAL COMMUNICATIONS SYSTEM
The mechanization and performance considerations for digital communications systems
will now be discussed in more detail. Figure 1–3 shows a system block diagram that is
more detailed than that of Figure 1–1. The functions of all the blocks of Figure 1–3 are
discussed in this section.
General Considerations
In most communication system designs, a general objective is to use the resources of
bandwidth and transmitted power as efficiently as possible. In many applications, one of
these resources is scarcer than the other, which results in the classification of most channels
as either bandwidth limited or power limited. Thus we are interested in both a transmission
scheme’s bandwidth efficiency, defined as the ratio of data rate to signal
bandwidth, and its power efficiency, characterized by the probability of making a reception
error as a function of signal-to-noise ratio. We give a preliminary discussion of this
power-bandwidth efficiency trade-off in Section 1.2.3. Often, secondary restrictions may
be imposed in choosing a transmission method, for example, the waveform at the output
of the data modulator may be required to have certain properties in order to accommodate
nonlinear amplifiers such as a traveling-wave tube amplifier (TWTA).