Digital Communication Systems: Types And Benefits

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INTRODUCTION

The transmission, reception and processing of information with the the utilization of electronic circuits is electronic communication. Information is described as knowledge on intelligence which is communicated or received. It comprises three primary sections: a source, transmission medium and knowledge destination. Information is propagated through a communications system within the sort of symbols such as analog (proportional), such as the human voice, video picture information, or music, or digital(discrete), such as binary-coded number, numeric codes, graphic symbols, microprocessor op-codes, or database information. However, fairly often the source information is unsuitable for transmission in its original form and must be converted to a more suitable form prior to transmission. For instance, with digital communications system, analog information is converted to digital form before to transmission, and with analog communication systems, digital data are converted to analog signals before transmission.

Traditional communications system like analogue modulation techniques, amplitude modulation(AM), frequency modulation (FM) are take over with modern digital communication systems which offers more advantages over traditional analog systems: Ease of processing, simple multiplexing and noise immunity.

Digital communications, however it is a rather ambiguous term that could mean entirely various things to different people. Digital communications systems involve systems where relatively high- frequency analog carriers are modulated by relatively low frequency digital information signals and system involving the transmission of digital pulses.

TYPES OF DIGITAL COMMUNICATION

The term digital communications cover an outsized area of communications techniques, including digital transmission and digital radio. During a communications system, digital transmission is the transmittal of digital pulses between two or more points.. Digital radio is the transmittal of digitally modulated analog carriers between two or more points during a communications system. Digital transmission system require a physical facility between the transmitter and receiver, like a metallic wire pair, a coaxial cable, or an optical fibre cable. In digital radio systems, the transmission medium might be free space, Earth’s atmosphere or a physical facility like a metallic or optical fibre cable.

ADVANTAGES OF DIGITAL COMMUNICATION

Ease with digital signals, as compared to analog signals
Less distortion and interference as compared to analog.
More reliable and cheaper. It more flexible.
Digital techniques lend themselves naturally to signal processing functions that protect against interference and jamming.
Communication is more computer to computer.

DIGITAL RADIO

The property that defines a digital radio system from a standard AM, FM or PM radio system is the nature of the modulating signal. Both digital and analog radio system use analog carriers. Moreover with analog modulation, the modulating signal is analog and it is digital in digital modulation, the source information could have been either analog or digital.

In the above block diagram, in the transmitter, the encoder convert the levels and then encodes or groups the incoming data into a blow that modulates the analog carrier. The modulated carrier is filtered, amplified and then carried through the transmission medium to the receiver. Within the receiver, the incoming signal is filtered, amplified and then applied to the demodulator circuit which again produces the original source information. Clock and carrier-recovery circuits remove carrier and clock timing information from the incoming modulated signal.

DIGITAL DATA WITH ANALOG SIGNALS

This method is implied to send computer information over various transmission channels that need analog signals,a fiber optic networks, computer modems, cellular phone networks, and systems in satellite. In each and every system, an electromagnetic carrier wave is used to transmit the information over large distances and connect digital information users at remote locations. The digital data is utilized to modulate the parameters of carrier wave. The essential process is named as “shift-keying” to differentiate if from the purely analog systems like AM and FM. In analog modulation, there are three parametrs of the carrier wave to change and therefore it has three types of shift keying:

I. Frequency shift keying
II. Phase shift keying
III. Amplitude shift keying

FREQUENCY SHIFT KEYING

Frequency shift keying (FSK) is another relatively simple, low-performance sort of digital modulation. FSK is a form of frequency modulation. The technology is used for communication systems like telemetry, weather balloon radiosondes, caller ID, garage door openers, and low frequency transmission in the VLF and ELF bands. The simplest FSK is Binary FSK. Binary FSK is a form of constant-amplitude angle modulation similar to conventional frequency modulation (FM) except that the modulating signal is a binary signal that varies between two discrete voltage levels rather than a continuously changing analog waveform.

FSK TRANSMITTER

FSK transmitter is extremely much liked to a conventional modulator and is very often a voltage-controlled oscillator. The carrier frequency is chosen such that it falls halfway between the mark and space frequencies. A logic 1 input shifts the VCO output to the mark frequency and a logic 0 input shifts the VCO output to the space frequency. Consequently, because the binary signal changes back forth between logic 1 and logic 0 conditions, the VCO output shifts or deviates back and forth between the mark and space frequencies. [image: ]

FSK MODULATOR

The FSK modulator has two oscillators which inclused a clock and binary sequence. The oscillator produces a high and low frequency signal and connected to a switch along which includes an internal clock. To avoid the discontinuities of the output waveform when message transmits, a clock is used in both of the oscillators in the interal part. The binary input sequence is applied on the transmitter to settle the frequences according to the input.

PHASE SHIFT KEYING

Phase shift keying is also a form of angle-modulated, constant-amplitude digital modulation. In PSK, the carrier signal varies with sine and cosine inputs at a specific time. It is commonly usable for wireless, LANs, bio-metric, contactless operations, RFID and Bluetooth communications. It is almost like conventional phase modulation except that with the PSK the input signal is a binary digital signal and a limited number of output phases are possible.

BINARY PHASE SHIFT KEYING

In binary phase shift keying, two outputs phases are feasible for a single carrier frequency. One phase of output represents a logic 1 and therefore the other represents logic 0. Consistently with the input digital state, the phase of the output carrier shifts between two angles are 180that is out of phase. Phase reversal keying and bi-phase modulation is almost similar to BPSK. It is a form suppressed-carrier, square-wave modulation of a non-stop wave (CW) signal.

BPSK TRANSMITTER

In a BPSK transmitter, the balanced modulator acts as a phase reversing switch. Relying on the logic condition of the digital input, the carrier in transferred to the output either in phase with the reference carrier oscillator. The balanced modulator has two types of inputs, a carrier in phase with the oscillator and the binary digital data. To operate the balanced modulator accurately, the voltage of digital input should be higher than the peak carrier voltage. This make sure that the digital input has control on the on/off state of diodes. If the binary input is login 1 , then diodes are forward biased while the other diodes are reverse biased and off. Consistent with the polarities, the carrier voltage is developed throughout transformer1 in phase with the carrier voltage across the second transformer.

BPSK MODULATOR

In the BPSK modulator,it has a balance modulator which has a carrier sine wave as input and binary sequence used as a second input. The modulation used a balance modulator which multiplies the two signals with each other. For a zero binary input, the phase will be zero degree and for high input, the phase will be 180 degree.

AMPLITUDE SHIFT KEYING

It is also a form of amplitude modulation. ASK represents digital information as variations in amplitude of the carrier wave. In an amplitude shift keying device, the binary input 1 is represented due to transmission of fixed amplitude carrier wave and fixed frequency for some time. If the signal value is 1, then carrier signal will make transmission other the signal value of 0 will be transmitted. When ASK modulated, the binary signal deliver a zero value for low input and high input for the carrier output.

ASK modulator

The ASK modulator block diagram features a service signal generator, the binary sequence which is furnished from the message signal and the band-limited filter. The generator sends a infinite high-frequency carrier. The binary sequences from the message signal converts the input to be excessive or either low. If its high, it closes the switch while allowing a carrier wave. If the input is low, the switch opens without any voltage. Therefore, the output could be low.

The filter shapes the pulse which relies upon the amplitude and phase applications of the band-limiting filter or pulse-shaping filter.

References

https://fas.org/man/dod-101/navy/docs/es310/DigiComs/digicoms.htm
https://uotechnology.edu.iq/dep-eee/lectures/4th/Electrical/Communication%20engineering%202/part1.pdf
https://en.wikipedia.org/wiki/Amplitude-shift_keying
https://en.wikipedia.org/wiki/Phase-shift_keying

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