Block Diagram of Typical Digital Communication
System
The transmitter transmits a waveform from a finite set of
possible waveforms over a finite period in a digital communication system. The
receiver must discern which waveform was supplied by the transmitter from a
noise affected signal.
The diagram shows a simplified block diagram of a typical
digital communication system. Encryption, multiplexing, spreading, multiple
access, and equalization are all available as options.
Signal transformations from the source to the transmitter are represented by the upper blocks: formatter, source encoder, channel encoder, baseband processor/bandpass modulator. Signal transformations from the receiver to the sink are represented by the bottom blocks, which include the Baseband decoder/Bandpass demodulator, channel decoder, source decoder, and Deformatter. The signal processing operations performed by the higher blocks are effectively reversed in the bottom blocks. We'll go through the fundamental functionalities of each of these components.
Figure: Typical digital communication system
1) Information source
The information to be transmitted originates from the source.
The information or message may be available in digital format (eg: computer
data, teletype data). If the accessible information/message is a non-electrical
signal (e.g., video, voice), an input transducer is used to convert it into a
suitable electrical signal. The analogue electrical signal is then sampled and
processed using an analogue to digital converter, resulting in a digital source
output.
2) Formatter
The source information is converted into binary digits during
formatting (bits). After that, the bits are combined to form digital messages
or message symbols. Each of these symbols (mi, where I = 1,2,3,......M) can be
considered a member of a finite alphabet set with M members. As a result, the
message sign mi is binary for M=2 (it constitutes just a single bit). Such
symbols are made up of a sequence of two or more bits for M>2 (M-ary).
3) Source encoder
The process of efficiently converting the output of an
analogue or digital source into a series of binary digits is known as source
encoding or data compression. For analogue sources, source coding produces
analogue-to-digital (A/D) conversion. It also eliminates superfluous
(non-essential) data. Source codes can reduce a system's data rate by
decreasing data redundancy (ie., reduced bandwidth).
In that, they both entail data digitalization, formatting and
source coding are related procedures. Source coding, on the other hand, entails
both data compression and digitisation. Hence, in a typical digital
communication system, either a formatter (for digitising alone) or a source
encoder (for both digitising and compressing) is used.
4) Channel encoder
In a controlled manner, the channel encoder incorporates
redundancy into the binary information sequence. This type of controlled
redundancy can be employed at the receiver to provide data error-correcting
capability. This reduces the impact of noise and interference during the
signal's transmission via the channel. As a result, channel coding improves the
fidelity of the received signal while also increasing the reliability of the
received data. Digital data is reliably transmitted using channel coding.
5) Baseband processor
Each symbol to be transmitted is converted from a binary
representation (voltage levels indicating binary ones and zeros) to a baseband
waveform for low-speed wired transmission. The term "baseband" refers
to a transmission with a frequency range of DC to a few MHz. A pulse modulation
circuit serves as the baseband processor. The Pulse Code-Modulation (PCM)
waveform is generated when binary symbols are pulse-modulated. PCM waveforms
are commonly referred to as Line codes in telephone applications. Following
pulse modulation, each message symbol is converted into a baseband waveform,
gi(t), with i=1,2...M.
6) Bandpass Modulator
The digital signal must be modulated to transmit high-speed
digital data (e.g., in computer communication systems). The fundamental function
of a digital modulator is to convert binary data into high-frequency analogue
signal waveforms (carrier signals). The phrase "bandpass" refers to
the frequency translation of the baseband waveform gi(t) by a carrier wave to a
frequency substantially higher than the spectral content of gi(t) (t). A
bandpass waveform Si(t) is used to represent the digitally modulated signal,
where i=1,2,.....M. The digital modulator might simply transfer the binary
digit 0 to waveform S1(t) and the binary digit 1 to waveform S2 (t). This is
referred to as binary modulation (M=2).
Alternatively, by using M=2K independent waveforms Si(t),
i=1,2,........M, one waveform for each of the 2K potential bit sequences, the
modulator may broadcast K coded information bits at a time. This is referred to
as M-ary modulation (M>2). The bandpass modulator is used to transmit
digital data efficiently. The baseband processor block is not necessary if the
bandpass modulator is present. As a result, these two blocks seem to be
mutually exclusive.
CHANNEL
The physical medium through which the signal is sent from the
transmitter to the receiver is referred to as the communication channel. The
channel in wireless transmission could be the atmosphere (free space).
Telephone channels, on the other hand, typically use a range of physical media,
including wirelines, optical fibre cables, and wireless (microwave radio).
A variety of conceivable methods can randomly corrupt the
broadcast signal, including additive thermal noise caused by electronic
devices, man-made noise, such as automotive ignition noise, and atmospheric
noise, such as electrical lightning discharges during thunderstorms.
The channel characteristics, which can be characterized in
terms of the channel's impulse response hc(t), affect the
transmitted signal Si(t) as it propagates across the channel.
Moreover, additive random noise n(t) alters the signal at numerous places along
its path. As a result, the received signal x(t) must be referred to as a
corrupted version of the signal Si (t). The signal x(t) received can
be written as follows:
x(t)= Si(t)
* hc(t) + n(t) i=1,2…..M
where * represents the convolution operation, and
n(t) represents the
noise process.
RECEIVER SECTION
1. Baseband decoder
The line
coded pulse waveform is converted back to transmitted data sequence by the
baseband decoder block.
2. Band pass demodulator
Each received bandpass waveform x(t) is frequency down-converted
by the receiver front end and/or the demodulator. The recovery of a waveform is
characterised as digital demodulation (baseband pulse). In preparation for
detection, the demodulator returns x(t) to an appropriately shaped baseband
pulse z(t). Detection is defined as concluding the digital interpretation of a
waveform.
Typically, the receiver and demodulator are connected to several
filters.
(i) Filtering to eliminate high-frequency phrases
that aren't wanted (in the Frequency down-conversion of bandpass waveforms).
(ii) Filtering for pulse shaping
(iii) The poor impulse response of the channel causes
filtering option by equalisation to reverse any degrading effects on the signal
Finally, the detector converts the shaped pulse into an
approximation of the data symbols that were communicated (binary or M-ary).
In most cases, reference waveforms are used to demodulate.
The process is described as coherent when the reference is a measure of all
signal properties (especially phase). Non-coherent processes are those in which
phase information is not utilised.
3. Channel decoder
The channel decoder receives estimates of the transmitted
data symbols. The channel decoder uses knowledge of the channel encoder's code
and the redundancy in the received data to try to recreate the original
information sequence. The frequency with which mistakes occur in the decoded
sequence is a measure of how well the demodulator and decoder work. The
probability of bit error is an essential metric of system performance (Pe).
4. Source decoder
The input sequence from the channel decoder is accepted by
the source decoder. It attempts to recreate the original signal from the source
using knowledge about the source encoding technique utilised. The signal at the
source decoder's output is an approximation of the actual source output because
of channel decoding problems and probable distortion induced by the source
decoder. The digital communication system's distortion causes the mismatch
between this estimate and the actual digital signal.
5. Deformatter
If the original information source was not in digital data
format and the receiver's output must be in the same format as the original, a
deformatter block is necessary. It converts digital data into discrete
(keyboard characters) or analogue (numbers) formats (speech signal).
6. Information sink
If a non-electrical analogue output is required, the output
transducer translates the estimated digital signal to the appropriate analogue
output. A computer, data terminal equipment, or a user can all be used as an
information sink.
7.
Synchronization
All signal processing inside a digital communication system is controlled by synchronisation and its key element, a clock signal. It is involved in practically every block's functionality. Synchronization entails calculating both time and frequency. Both in frequency and phase, coherent systems must synchronise their frequency reference with the carrier. Phase synchronisation is unnecessary for non-coherent systems.