The term digital communication covers a broad area of communications techniques, including digital transmission and digital radio.
Digital transmission, is the transmission of digital pulses between two or more points in a communication system.
Digital radio, is the transmission of digital modulated analog carriers between two or more points in a communication system.
Why Digital?
There are many reasons
- The primary advantage is the ease with which digital signals, compared to analog signal, are regenerative.
The shape of the waveform is affected by two mechanisms:
(1) As all the transmission lines and circuits have some non-ideal transfer function, there is a distorting effect on the ideal pulse.
(2) Unwanted electrical noise or other interference further distorts the pulse waveform.
Both of these mechanisms cause the pulse shape to degrade as a function of distance. During the time that the transmitted pulse can still be reliably identified, the pulse is thus regenerated. The circuit that perform this function at regular intervals along a transmission system are called
regenerative repeaters. - Digital circuits are less subject to distortion and interference than analog circuits.
- Digital circuits are more reliable and can be produced at lower cost than analog circuits. Also, digital hardware lends itself to more flexible implementation than analog hardware.
- Digital techniques lend themselves naturally to signal processing functions that protect against interference and jamming.
- Much data communication is computer to computer, or digital instrument or terminal to computer. Such digital terminations are naturally best served by digital link.
Elements of Digital Communication System
Input Transducer:
The input messages can be categorized as analog (continuous form) or digital (discrete form). The message produced by a source must be converted by a transducer to a form suitable for the particular type of communication system employed.
Source Encoder/ Decoder
The purpose of source coding is to reduce the number of bits required to convey the information provided by the
information source. The task of source coding is to represent the source
information with the minimum of symbols. High compression rates (Good compression rates) make be
achieved with source encoding with loss-less or little loss of information.
Channel Encoder/ Decoder
A way of encoding data in a communications channel that adds patterns of redundancy into the transmission path in order to lower the error rate. The task of channel coding is to represent the source information in a manner that minimizes the error probability in decoding.
Digital Modulator:
It is used to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network, or over a limited radio frequency band. It is used for following purposes:
- For ease of radiation
- To reduce noise and interference
- For channel assignment
- For multiplexing or transmission of several message over a single channel
- To overcome equipment limitation
Channel:
There are different forms of channel. The signal undergoes degradation from transmitter to receiver due to noise, fading, interference etc.
Equalization
Equalization is, the technique, used to keep the balance between frequency components within an electronic signal, i.e. to minimizes inter symbol interference (ISI). Read about Equalization in detail here.
Different Types of Equalizers:
There are different types of equalizers which are used on the basis of their functionality. Some of them are listed below:
- Linear Equalizers
- Adaptive Equalizers
- Blind Equalizers
- Decision Feedback Equalizers
- Turbo Equalizers
- Viterbi Equalizers
Synchronization
- Symbol/ Timing synchronization
- Frequency synchronization
a) Carrier frequency synchronization
b) Sampling frequency synchronization
Channel Estimation
A channel estimate is only a mathematical estimation of what is truly happening in nature. It Allows the receiver to approximate the effect of the channel on the signal. The channel estimate is essential for removing inter symbol interference, noise rejection techniques etc.
Two basic types of techniques are used:
- Data-aid algorithm
- Non-data-aid algorithm
Output Transducer
The output transducer completes the communication system. The device converts the electric signal at its input into the form desired for the system user.
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