BASIC DIGITAL COMMUNICATION TRANSFORMATIONS
The basic signal processing functions that may be considered
as transformations are divided into nine categories.
1.
Formatting and Source Coding:
Formatting and source coding are both data digitalization procedures.
In addition to digitization, source coding includes data compression.
2. Baseband
Signaling
The creation of PCM waveforms or line codes is a part of the
baseband signaling process.
3. Bandpass
signaling
When the references used during demodulation are a measure of
all the signal properties (including phase), the technique is referred to be
coherent. The procedure is called non-coherent when phase information is not
utilized.
4. Equalization
In systems where channel-induced
ISI (Intersymbol interference) can cause signal distortion, an equalization
filter is required.
5. Channel Coding
The two approaches of channel
coding are waveform coding and structured sequencing. The usage of new
waveforms is a part of waveform coding. Redundant bits are employed in
structured sequences.
6. Multiplexing and multiple access
Resource sharing is at the heart of both multiplexing and multiple
access. Local multiplexing occurs, whereas distant multiple access occurs.
In military applications, spreading is used to defend against
interference and provide privacy. Signals can travel in frequency, time, or
both directions.
8.
Encryption
The main aims of encryption and decryption are communication privacy and
authentication. Maintaining privacy entails preventing unwanted individuals
from collecting information from the channel (eavesdropping). Authentication
prevents unauthorized individuals from inserting false signals (spoofing) into
the channel.
9.
Synchronization
The estimate of both time and frequency is required for synchronization.
In both frequency and phase, coherent systems must synchronize their frequency
reference with the carrier. Phase synchronization is not required for
non-coherent systems.
The digital communication transformations are shown in the figure.
Performance criteria
Signals that represent numbers are sent across a digital
communication system. The recipient knows a priori that these digits make up a
finite set or alphabet. The probability of wrongly detecting a digit or of
mistake is a figure of merit for digital communication systems (Pe).