In this lecture, we are going to learn about the basic digital communication system, a block diagram of digital communication system, and its functional blocks in a very detailed manner. So let’s start from the knowledge of digital communication systems.
In digital communication, the message signal to be transmitted is digital in nature. This means that digital communication involves the transmission of information in digital form.
Block Diagram of Digital Communication system
The below figure shows the model of a digital communication system.
The overall purpose of the system is to transmit the message or sequences of symbols coming out of a source to a destination point at as high a rate and accuracy as possible. The source and the destination point are physically separated in space and a communication channel connects the course to the destination point.
The communication channel accepts electrical (i.e., electromagnetic) signals and the output of the channel is usually a smeared or distorted version of the input due to the nonideal nature of the communication channel.
In addition to this, the information-bearing signal is also corrupted by unpredictable electrical signals |(i.e. noise) from both man-made and natural causes. Thus, the smearing and the noise introduce errors in the conformation being transmitted and limits the rate at which information can be communicated from the source to the destination.
The probability of incorrectly decoding a message symbol at the receiver is often used as a measure of the performance of a digital communication system.
Now let us have a detailed look at each of the functional blocks in the digital communication system.
1. Discrete Information Source
The information source may be classified into two categories based upon the nature of their output i.e.,
- Analog Information Source
- Discrete Information Source
In the case of analog communication, the information source is analog. Analog information sources, such as microphones actuated by speech emit one or more continuous amplitude signals.
In the case of digital communication, the information source produces a message signal which does not continuously vary with time. rather the message signal is intermittent with respect to time. The output of discrete information sources such as a teletype or the numerical output of a computer consists of a sequence of discrete symbols or letters.
An analog information source may be transformed into a discrete information source through the process of sampling and quantizing. Discrete information sources are characterized by the following parameters:
- Source alphabet
- Symbol rate
- Source alphabet probabilities
- Probabilistic dependence of symbols in sequence
2. Source Encoder and Decoder
The symbol produced by the information source is given to the source encoder. These symbols cannot be transmitted directly. They are first converted into digital form (i.e., binary sequence of 1’s and 0’s) by the source encoder.
Each binary ‘1’ and ‘0’ is known as a bit. The group of bits is called a codeword. The source encoder assigns codewords to the symbols. For each distinct symbol, there is a unique codeword.
The codeword can be 4, 8, 16, or 32 bits long. As the number of bits is increased in each codeword, the symbols that may be represented are also increased.
As an example, 8 bits would have 28, i.e. 256 distinct codewords. This means that 8-bits may be used to represent 256 symbols and similarly, 16 bits may represent 216=65536 symbols and so on. Some typically source encoders are pulse code modulators, Delta modulators, vector quantizers, etc.
Source encoders must have the following important parameters:
- Block size
- Codeword Length
- Average data rate
- Efficiency of the encoder
In last, it may be noted that at the receiver end, some sort of decoder is used to perform the reverse operation to that of the source encoder. It converts the binary output of the channel encoder into a symbol sequence. Some decoders also use memory to store codewords. The decoders and the encoders can be synchronous or asynchronous.
3. Channel Encoder and Decoder
After converting the message or information signal in the form of the binary sequence by the source encoder, the signal is transmitted through the channel. The communication channel adds noise and interference to the signal being transmitted. Hence errors are introduced in the binary sequence received at the receiver end.
Therefore, the errors are also introduced in the symbols generated from these binary codewords, Thus channel coding is done to avoid these types of errors. In fact, the channel encoder adds some redundant binary bits to the input sequence. Also, these redundant bits are always added with some properly defined logic.
As an example, let us consider the codeword from the source encoder to make it 4-bits long. This fourth bit was added in such a manner that the number of 1’s in the encoded word remains even.
The coding and decoding operational the encoder and decoder need the memory and processing of binary data. However, in modern times, due to the use of microcontrollers and computers, the complexity of the encoders and decoders is much reduced.
A channel encoder must have the following important parameters:
- The coding rate depends upon the redundant bits added by the channel encoder.
- The coding method was used.
- Coding efficiency is the raion of data rate at the input to the data rate at the output of the encoder.
- Error control capabilities.
- Feasibility of the encoder and decoder.
4. Digital Modulators and Demodulators
If the modulating signal is digital (i.e. binary codewords), then digital modulation techniques are used.
The carrier signal used by digital modulators is always a continuous sinusoidal wave of high frequency.
In fact, the digital modulators map the input binary sequence of 1’s and 0’s to the analog signal waveforms.
FOr example, if one bit at a time is transmitted, then the digital modulator signal is s1(t) to transmit binary ‘0’ and s2(t) to transmit binary ‘1’ as shown in the below figure.
Here, the signal s1(t) has a low frequency compared to signal s2(t). hence, here even though the modulated signal seems to be continuous, the modulation is discrete. this means that a signal carrier is converted into two waveforms s1(t) and s2(t) because of digital modulation.
At the receiver end, the digital demodulator converts the input modulated signal into a sequence of binary bits.
A digital modulation method must have the following important parameters:
- The bandwidth needed to transmit the signal
- Probability of symbol or bit error.
- Synchronous or asynchronous method of detection
- Complexity of implementation
5. Communication Channel
The connection between the transmitter and receiver is established through a communication channel. The communication channel can take place through wirelines, wireless, or optical fiber channels.
The other media such as optical disks, magnetic tapes, disks, etc. may also be called communication channels since they can also carry data through them,
However, it may be noted that each and every communication channel has some problems. These are:
Signal Attenuation: The signal attenuation in the channel occurs due to the internal resistance of the channel and fading of the signal.
Amplitude and Phase distortion: The transmitted signal is distorted in amplitude and phase due to the non-linear characteristics of the communication channel.
Additive noise interference: Additive noise interference is produced due to internal solid-state devices and resistors etc., used to implement a communication system.
Multipath distortion: The multipath distortion occurs mostly in wireless communication channels. In fact, the signals coming from different paths tend to interfere with each other.
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