DIGITAL DATA TRANSMISSION,LINE CODING AND PULSE SHAPING

Made By: AAYUSH KUMARRoll No.: 1209731002

What is Digital Data Transmission?• A mode of transmission in which all information to

be transferred, such as voice, image or text data, is converted in the digital (mainly binary numbers) before transmission. At the end point, binary code is converted back into original format. • Digital transmission can deliver a signal, free of

ghosts, interference, and picture noise. It provides sharper, clearer and faster transmission, using less bandwidth to transmit more information than analog.

Transmission modes

Parallel Transmission• In Parallel transmission, by grouping,

we can send n data bits at a time instead of 1.• Its advantage over serial transmission

is in terms of speed.

Block diagram of parallel transmission

Serial transmission • In Serial transmission, one bit follows

another• It advantage over parallel transmission is

that it reduces cost of transmission. But communication with device is parallel hence conversion devices are required at the interface between the sender and the line (p to S) and between the line and receiver (S to P)

Block diagram of serial transmission

Asynchronous, synchronous and isochronous• In telecommunications, asynchronous communication

is transmission of data, generally without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream

• The transmission of data in which both stations are synchronized by a clock is known as synchronous transmission of data. Codes are sent from the transmitting station to the receiving station to establish the synchronization, and the data are then transmitted in continuous streams.

• Isochronous transmission transmits asynchronous data over a synchronous data link so that individual characters are only separated by a whole number of bit-length intervals

Line Coding

• Converting a string of 1’s and 0’s (digital data) into a sequence of signals that denote the 1’s and 0’s.

• For example a high voltage level (+V) could represent a “1” and a low voltage level (0 or -V) could represent a “0”.

Line coding and decoding

Relationship between data rate and signal rate• The data rate defines the number of bits sent per

sec - bps. It is often referred to the bit rate.• The signal rate is the number of signal elements

sent in a second and is measured in bauds. It is also referred to as the modulation rate.• Goal is to increase the data rate whilst reducing

the baud rate.

Signal element versus data element

Line coding schemes

Unipolar NRZ

• All signal levels are on one side of the time axis - either above or below• NRZ - Non Return to Zero scheme is an example of

this code. The signal level does not return to zero during a symbol transmission.• Scheme is prone to baseline wandering and DC

components. It has no synchronization or any error detection. It is simple but costly in power consumption.

Unipolar NRZ scheme

Unipolar RZ

• In unipolar RZ form, the waveform has zero value when symbol ‘0’ is transmitted and waveform has ‘v’ volts when ‘1’ is transmitted.

• The ‘v’ volts is present for half time period and for the remaining time period the waveform returns to zero.

Polar - NRZ

• The voltages are on both sides of the time axis.• Polar NRZ scheme can be implemented with two

voltages. E.g. +V for 1 and -V for 0.• There are two versions: • NZR - Level (NRZ-L) - positive voltage for one symbol and

negative for the other• NRZ - Inversion (NRZ-I) - the change or lack of change in

polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not.

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Polar NRZ

Polar - RZ• The Return to Zero (RZ) scheme uses three

voltage values. +, 0, -. • Each symbol has a transition in the middle. Either

from high to zero or from low to zero.• This scheme has more signal transitions (two per

symbol) and therefore requires a wider bandwidth.• No DC components or baseline wandering.• Self synchronization - transition indicates symbol

value.• More complex as it uses three voltage level. It has

no error detection capability.

Polar RZ scheme

Split-Phase Manchester

• In manchestere format,if symbol 1 is to be transmitted,Then a positive half interval pulse is followed by a negative half interval pulse.

• If symbol zero is to be transmitted,then a negative half interval pulse is followed by a positive half interval pulse.

• Hence for any symbol the pulse takes +ve as weel as –ve values.

Bipolar - AMI and Pseudoternary• Code uses 3 voltage levels: - +, 0, -, to represent

the symbols (note not transitions to zero as in RZ).• Voltage level for one symbol is at “0” and the

other alternates between + & -.• Bipolar Alternate Mark Inversion (AMI) - the “0”

symbol is represented by zero voltage and the “1” symbol alternates between +V and -V.• Pseudoternary is the reverse of AMI.

Bipolar schemes: AMI and pseudoternary

Bipolar Characteristics

• It is a better alternative to NRZ.• Has no DC component or baseline wandering.• Has no self synchronization because long runs of “0”s results in

no signal transitions.• No error detection.

Pulse Shaping

• In electronics and telecommunications, pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to its purpose or the communication channel, typically by limiting the effective bandwidth of the transmission.

• By filtering the transmitted pulses this way, the intersymbol interference caused by the channel can be kept in control.

• Typically pulse shaping occurs after line coding and modulation.

Need for Pulse Shaping

In communications systems, two important requirements of a wireless communications channel demand the use of a pulse shaping filter.

1) generating bandlimited channels

2) reducing inter symbol interference (ISI) from multi-path signal reflections.

Time vs. Frequency Domain for a Sinc Pulse

The sinc pulse,above , meets both of these requirements because it efficiently utilizes the frequency domain to utilize a smaller portion of the frequency domain, and because of the windowing affect that it has on each symbol period of a modulated signal.

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