ANALOG TRANSMISSION

MODULATION OF DIGITAL DATA It is the process of changing one of the characteristics of an analog signal based on the information in a digital signal. e.g. To transmit digital data from one PC to another using a phone-line. Telephone line carries analog signal, so

digital data should be converted using process of Modulation.

Most of us may be familiar with MODEM.

MODEM stands for: MODULATOR/ DEMODULATOR MODEM uses Modulation process

to convert digital data to a form suitable for transmission on

a telephone line.

WHY MODULATION ?1-For allowing multiple signals to share a single physical channel 2-Necessary for wireless communication where the antenna diameter must be at least equal to the wavelength of the carrier signal. This means, for a 3000 Hz signal through space, the antenna diameter must be at least 60 miles!3-For a medium not suitable for digital transmission.

Modulation process uses two types of signals: (a) Information Signal: i- Analog or ii- Digital

(b) Carrier Signal :A high frequency Sine wave One or more characteristics of Carrier are varied in accordance to the information signal

MODULATION PROCESS PRODUCES MODULATED SIGNAL, READY FOR AMPLIFIATION and TRANSMISSION

Sine Wave as a CARRIER

• Think of a Sine Wave as a Carrier Signal,i.e. the signal onto which the information is loaded for sending to the end user

• A Carrier Signal is used as the basis for sending e.m. signals between a transmitter and a receiver, independently of the frequency

Carrier signals

• A Carrier Signal may be considered to travel at the speed of light, c, whether it is in free space or in a metal wire

• Travels more slowly in most substances• The velocity, frequency, and wavelength

of the carrier signal are uniquely connected by

c = f Wavelength

FrequencyVelocity of light

The receiver is tuned to the frequency of the carrier signal that is expected from the transmitter. See next slide

Basic Analog Modulation Techniques• Parameter to be varied Analog Modulation• Amplitude Amplitude Modulation AM• Frequency Frequency Modulation FM• Phase Phase Modulation PM

• RADIO transmission uses AM and FM• TV broadcast uses FM

Basic Digital Modulation Techniques• Parameter to be varied Digital Modulation• Amplitude Amplitude Shift Keying• Frequency Frequency Shift Keying• Phase Phase Shift Keying

More on Digital Modulation • Amplitude Modulation (AM) also known as

amplitude-shift keying. This method requires changing the amplitude of the carrier phase between 0 and 1 to encode the digital signal.

• Frequency Modulation (FM) also known as frequency-shift keying. Must alter the frequency of the carrier to correspond to 0 or 1.

• Phase Modulation (PM) also known as phase-shift keying. At each phase shift, the bit is flipped from 0 to 1 or vice versa.

Modulation Schematics

Coming slides further highlight the process of ASK FSK PSK

BAUD• Baud refers to the number of

times a change of analog signal occurs in the circuit.

• If one signal change carries per bit, then baud and bps are the same, e.g. baud = bps

Bit Rate vs. Baud Rate• bit: a unit of information• baud: a unit of signaling speed• Bit rate (or data rate): b

– Number of bits transmitted per second• Baud rate (or symbol rate): s

– number of symbols transmitted/sec• General formula:• Bit rate = Baud rate X Number of bits per symbol

Concept of Symbol• Symbol: Each modification of the carrier wave to encode informatione.g. Sending one bit (of information) at a time

– One bit encoded for each symbol (carrier wave – change) 1 bit per symbol

e.g. Sending multiple bits simultaneously – Multiple bits encoded for each symbol (carrier wave change) n bits per symbol, n > 1– Need more complicated information coding

schemes

Sending Multiple Bits per Symbol

• Multiple bits per symbol might be encoded using:

• amplitude, frequency, and phase modulation

– e.g., in PSK: – Phase shifts of 0o, 90o, 180o, and 270o

Example: Two-bit ASK4 symbols

EXAMPLE An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the baud rate and bit rate.

Baud Rate = No. of Signal units/sec = 1000Bit rate = Baud rate X No. of bits per signal Unit = 1000 X 4 = 4000 bps

IMPORTANT NOTE If fewer (less) signal units are required to transmit more bits, less BANDWIDTH will be

required on a medium.

BAND WIDTH IN ASK

BANDWIDTH IN ASK

As shown in the previous slideIf single bit is transmitted per signal unit, Bit Rate = Baud Rate Bandwidth = Baud Rate

BANDWIDTH IN FSK

BANDWIDTH IN FSK

Fig shows the spectrum of FSK signal. Although one bit is transmitted per signal unit, but because of frequency shift if lowest frequency is fc0 and highest frequency is fc1, Bandwidth = Baud rate + fc1 – fc0

EXAMPLE In FSK transmission, if two carriers fc0 and fc1 are separated by 3000 Hz, find the required

bandwidth at 2000 bps. Solution: fc1-fc0 = 3000 Hz Bit Rate = 2000 bps Baud Rate = Bit Rate = 2000 Bandwidth = fc1 – fc0 + Baud Rate Bandwidth = 3000 + 2000 = 5000 Hz

BANDWIDTH IN PSK

BANDWIDTH IN PSK

As shown in the previous slideIf single bit is transmitted per signal unit, Bit Rate = Baud Rate Bandwidth = Baud Rate

Constellation Diagrams or Phase State Diagram for 2PSK(2PSK because two phases and 1 bit per phase)

Example of 4-PSK: 4 Phases, 2 bits per phase

4-PSK Constellation

Tribit using 8-PSK: 8 Phases 3 bits per phase

• Combined Modulation Techniques

• Combining ASK and PSK on the same circuit. HOW?

• Vary Amplitude A and Phase P of the Carrier simultaneously.

• Let x variations in A• Let y variations in P• will give total variations of x times y• Thus corresponding number of bits per variation

QUADERATURE AMPLITUDE MODULATION

Q A MThe modulation technique to vary A and P simultaneously is called QAM IMPORTANT NOTE In QAM, number of variations in A is kept minimum because A is affected by noise. Number of variations in P is limited only by the interpretation of shifts in it. See next slide

QAM is a combination of ASK and PSK so that a maximum contrast between each signal unit (bit, dibit, tribit, and so on) is achieved.

QAM - Is a widely used family of encoding schemes.A common form: 16-QAMUses: • 8 different phase shifts • 2 different amplitude levels,• 16 possible symbols • 4 bits/symbol

16- QAM Constellation

–TCM – Trellis-Coded Modulation

•An enhancement of QAM •Can transmit different number of bits on each symbol (6,7,8 or 10 bits per symbol)

Transmission Modes• Parallel mode

–Uses several wires, each wire sending one bit at the same time as the others• A parallel printer cable sends 8 bits

together • Computer’s processor and motherboard

also use parallel busses (8 bits, 16 bits, 32 bits) to move data around

Serial ModeSends bit by bit over a single wireSerial mode is slower than parallel mode

Parallel Transmission ExampleUsed for short distances (up to 6 meters) (since bits sent in parallel mode tend to spread out over long distances)

(8 separate copper wires)

Serial Transmission ExampleCan be used over longer distances (since bits stay in the order they were sent)

DTE and DCE• Data terminating equipments (DTEs)

are noncommunciations-oriented components of a data communications environment.

• Data communications devices (DCEs) are communications-oriented components of a networks, such as telephone switching equipment, media, modems, etc.

DTE and DTE

DTE

Terminal

DCE

Modem

DCE

Modem

Communications signals

Terminal

DTE

Modem

• A modem is a DCE device. • Modems use amplitude,

frequency, or phase shift to encode more that one bit per baud.

More on Modems• V-series of modem standards (by ITU-T)

– V.22• An early standard, now obsolete• Used FM, with 2400 symbols/sec 2400 bps bit rate

– V.34• Used 8.4 bits/symbol, with 3,428 symbols/sec

multiple data rates(up to 28.8 kbps)• Includes a handshaking sequence that tests the

circuit and determines the optimum data rate

V.90 and V.92 Modems• Combines analog and digital transmission• Uses a technique based on PCM concept

– Results in a max of 56 Kbps data rate

• V.90 Standard– Based on V.34+ for Upstream transmissions (PC

to Switch)– Max. upstream rate is 33.4 Kbps

• V.92 Standard (most recent)– Uses PCM symbol recognition technique for both

ways– Max. upstream rate is 48 kbps

56k Modems• 56k modems, the fastest possible on voice

grade lines, are based on the V.90 and V.92 standards.

• Downstream transmissions (from phone switch to the user’s computer) use a technique based on recognizing the 8-bit digital symbol.

• With the V.90 standard, upstream lower data rates. The max transmissions are still based on the V.34+ standard.

Digital Modulation may not mean what you think it means…

• The transmitted signal is a continuous time signal (or analog signal) regardless of the modulation ‘analog’ or ‘digital’.