A General Communication System can be viewed as a Transmitting unit and a Receiving Unit connected by a medium(Channel). Obviously Transmitter and Receiver consists of various sub systems (or) blocks.
Our basic aim is to understand the various modules and sub systems in the system. If we are trying to understand the design and various features of DCS , it is plus imperative that we have to understand how we should design a transmitter and we must understand how to design a very good quality Receiver. Therefore one must know the features of the channel to design a good Transmitter as well as receiver that is the channel and it’s contribution will come repeatedly in digital Communications.
Source:- the primary block (or) the starting point of a DCS is an information source, it may be an analog/digital source , for example the signal considered is analog in nature, then the signal generated by the source is some kind of electrical signal which is random in nature. if the signal is a speech signal (not an electrical signal) that has to be converted into electrical signal by means of a Transducer, which can be considered as a part of source itself.
Sampling & Quantization:- the secondary block involves the conversion of analog to discrete signal
this involves the following steps
Sampling:- it is the process that involves in the conversion of Continuous Amplitude Continuous Time (CACT) signal into Continuous Amplitude Discrete Time (CADT) signal.
Quantization:- it is the process that involves in the conversion of Continuous Amplitude Discrete Time (CADT) signal into Discrete Amplitude Discrete Time (DADT) signal.
Source Encoder:- An important problem in Digital Communications is the efficient representation of data generated by a Discrete Source, this is accomplished by source encoder.
” The process of representation of incoming data from a Discrete source into a more suitable form required for Transmission is known as source encoding”
Note:-The blocks Sampler, Quantizer followed by an Encoder constructs ADC (Analog to Digital Converter).
∴ the output of Source encoder is a Digital Signal, the advantages of Source coding are
- It reduces the Redundancy.
- Minimizes the average bit rate.
Channel encoder:-Channel coding is also known as error control coding. Channel coding is a technique which reduces the probability of error by reducing Signal to Noise Ratio at the expense of Transmission Band Width.The device that performs the channel coding is known as Channel encoder.
Channel encoding increases the redundancy of incoming data , this also involves error detection and error correction along with the channel decoder at the receiver.
Spreading Techniques:- Spread Spectrum techniques are the methods by which a signal generated with a particular Band Width is deliberately spread in the frequency domain, resulting in a signal with a wider Band width.
There are two types of spreading techniques available
- Direct Sequence Spread Spectrum Techniques.
- Frequency Hopping Spread Spectrum Techniques.
The output of a spreaded signal is very much larger than incoming sequence. Spreading increases the BW required for transmission, which is a disadvantage even though spreading is done for high security of data.
SS techniques are used in Military applications.
Modulator:- Spreaded sequence is modulated by using digital modulation schemes like ASK, PSK, FSK etc depending up on the requirement, now the transmitting antenna transmits the modulated data into the channel.
Receiver:- Once you understood the process involved in transmitter Block. One should perform reverse operations in the receiver block.
i.e the input of the demodulator is demodulated after that de-spreaded and then the channel decoder removes the redundancy added by the channel encoder ,the output of channel decoder is then source decoded and is given to Destination.