This is an era of communication systems. Communication systems rather than the communication itself have become the need of mankind. Imagining life today without the internet, mobile phones, satellite TVs, and other gadgets are going to be very hard and rather impossible. Moreover, all these communication systems like the internet, telephone systems, mobile systems, or satellite systems are examples of a digital communication system. In this article, we will discuss the basic principles that govern the design of a digital communication system along with the pros and cons of digital communication in comparison.
Block Diagram of Digital Communication System
The first block of the digital communication system is the information source and input transducer. Sources of information can be either
a) Analog information source
b) Digital information source.
Microphone actuated by a speech or a T.V camera scanning a scene can be the examples for analog information sources whereas teletype or the numerical output of a computer which consists of discrete symbols or letters can be considered as the examples for a digital information source.
Analog information is transformed into discrete information through the process of sampling and quantization.
The symbols produced by the information source are given to the source encoder. As these symbols cannot be transmitted directly to the communication channel, they are first converted in digital form i.e. binary sequences of 0’s and 1’s. Each binary ‘1’ and ‘0’ is known as bit and we will call a group of bits as a codeword.
The ultimate task of source encoding is to reduce redundancy. It is also being used for utilizing bandwidth effectively.
Various digital source encoding techniques include Shannon-Fano encoding and Huffman encoding.
Channel Encoder & Decoder
After converting the message or information signal to binary by source encoder, the signal is transmitted through the channel. The communication channel adds noises and interferences to the signal being transmitted. Therefore, errors get accumulated in the binary sequence received at the receiver end. channel encoding that is used to provide noise immunity to the system or to avoid this kind of errors. So to provide noise immunity to the system, we add some redundancy to the input binary sequence. There are channel encoding techniques like linear block codes, cyclic codes, and convolution codes, etc. to provide noise immunity to the system.
Digital Modulator & Demodulator
The Digital modulator is having two inputs, one is channel encoded output (digital output or modulating signal) that we are multiplying it with a high-frequency carrier signal so that we can send it by the antenna.
To send a signal at a longer distance we need to translate a low-frequency digital signal into a high-frequency analog signal. That’s the reason why in a digital modulator, it converts an electrical digital signal into a high frequency modulated signal.
The basic difference between analog and digital modulation is that the modulating signal in digital communication will be digital (i.e. binary codewords). The carrier signal used by digital modulators is always a continuous sinusoidal wave of high frequency.
There are different types of digital modulation techniques like
ASK (Amplitude Shift Keying)
FSK (Frequency Shift Keying)
PSK (Phase Shift Keying)
QPSK (Quadrature Shift Keying)
MSK (Minimum Shift Keying) etc.
However, since these modulators use a continuous carrier wave, therefore they are also known as Continuous Wave(CW) Modulators.
A digital modulation method must have the following parameters.
- The bandwidth needed to transmit the signal
- Probability of symbol or bit error
- Synchronous or asynchronous method of detection
- Complexity of implementation
For example, the ASK wave form will look like the below diagram. Here the amplitude of the carrier wave is varied in accordance with the modulating wave (digital input sequence).
Advantages and Disadvantages of Digital Communication
- The effect of distortion, noise, and interference is less in a digital communication system. This is because the disturbance must be large enough to change the pulse from one state to the other.
- Regenerative repeaters can be used at a fixed distance along with the link, to identify and regenerate a pulse before it is degraded to an ambiguous state.
- Digital Circuits are more reliable and cheaper when compared to analog circuits.
- The Hardware implementation is more flexible than analog hardware because of the use of microprocessors, Very Large Scale Integration (VLSI) chips, etc.
- Signal Processing functions like encryption and compression can be employed to maintain the secrecy of the information.
- The system performance can be improved by using error detecting and error-correcting codes thereby reducing the probability of error.
- Combining Digital signals using Time Division multiplexing (TDM) is simpler than combining analog signals using Frequency Division Multiplexing (FDM).
- The different types of signals such as data, telephone, TV can be treated as identical signals in transmission and switching in a digital communication system.
- By using spread spectrum techniques, signal jamming can be avoided to a certain extent.
- Since the transmitted signal is digital, a large amount of noise interference may be tolerated.
- Digital communication is adaptive to other advanced branches of data processing such as Digital Signal Processing, Data Compression, etc.
Larger System Bandwidth
Digital transmission requires a larger system bandwidth to communicate the same information in a digital format as compared to analog format.
Digital detection requires system synchronization whereas the analog signals generally have no such requirement.