Why to Apply Digital Transmission?Digital transmission system

• Advantages of Digital Communication Over Analog Modulation: There are many advantages of using Digital Communication over Analog Communication. Some of them are listed as below:The digital communication has mostly common structure of encoding a signal so devices used are mostly similar.

• The Digital Communication's main advantage is that it provides us added security to our information signal.

• The digital Communication system has more immunity to noise and external interference.

• Digital information can be saved and retrieved when necessary while it is not possible in analog.

• Digital Communication is cheaper than Analog Communication.• The configuring process of digital communication system is simple as compared to

analog communication system. Although, they are complex.• In Digital Communication System, the error correction and detection techniques can

be implemented easily.

DigitalTransmission

• ‘Baseband’ means that no carrier wave modulation is used for transmission

Information:- analog:BW & dynamic range- digital:bit rate

Information:- analog:BW & dynamic range- digital:bit rate

Maximization of information transferred

Maximization of information transferred

Transmitted power;bandpass/baseband signal BW

Transmitted power;bandpass/baseband signal BW

Message protection & channel adaptation;convolution, block coding

Message protection & channel adaptation;convolution, block coding

M-PSK/FSK/ASK..., depends on channel BW & characteristics

M-PSK/FSK/ASK..., depends on channel BW & characteristics

wireline/wirelessconstant/variablelinear/nonlinear

wireline/wirelessconstant/variablelinear/nonlinear

NoiseNoise

InterferenceInterference

ChannelChannel

ModulatorModulator

ChannelEncoder

ChannelEncoder

Source encoder

Source encoder

Channel decoder

Channel decoder

Source decoder

Source decoder

DemodulatorDemodulator

Information sink

Information sink

Information source

Information source

Message Message estimate

Received signal(may contain errors)

Transmitted signal

InterleavingInterleaving

Fights against burst errors

Fights against burst errors

DeinterleavingDeinterleaving

In baseband systemsthese blocks are missing

Eeng 360 4

Intersymbol Interference• Intersymbol interference (ISI) occurs when a pulse spreads out in such a way that

it interferes with adjacent pulses at the sample instant.

• Example: assume polar NRZ line code. The channel outputs are shown as spreaded (width Tb becomes 2Tb) pulses shown (Spreading due to bandlimited channel characteristics).

Data 1

bT 0 bT0bT bT

Data 0

bT 0 bT0bT bT

Channel Input

Pulse width Tb

Channel Output

Pulse width Tb

Eeng 360 5

Intersymbol Interference• For the input data stream:

• The channel output is the superposition of each bit’s output:

1 1110 0

bT bT2 bT3 bT40 bT5

A

bT bT2 bT3 bT40 bT5

1 1110 0

bT bT2 bT3 bT40 bT5

Resultant Channel Output Waveform

Band Width

The numerical difference between the upper and lower frequencies of a band of electromagnetic radiation, especially an assigned range of radio frequencies is called Band Width.

Parity Bit:

Parity Checking:

Limitations: