EE302 Lesson 21:
Transmission of Binary
Data in Communication
Systems
Transmission schemes


Before launching into digital modulation we need to
provide an overview of transmission methods.
6 combinations are discussed.
1. Analog channel (no modulation)




Simplest type communications system.
The analog intelligence signal not modulated, but is
transmitted in the baseband channel.
Baseband means the signal is transmitted at its original
base frequencies (voice 300 Hz – 3 kHz)
Example: intercom system.
Analog
input
Analog baseband
channel
Analog
output
2. Standard analog modulation


AM and FM modulation systems previously studied
previously
Example: AM/FM radio stations
Analog
input
Modulator
Analog
channel
Demodulator
Analog
output
3. Digital transmission on digital
channel.


Direct computer/computer or computer to peripheral
communication.
Example: serial, USB, parallel connections
Digital
input
Coder
Digital
channel
Decoder
Digital
output
4. Digital transmission on analog
channel.


Digital signal is converted to analog for transmission on
analog channel.
Example: internet connection via phone line
Digital
input
Modem
Analog
channel
Modem
Digital
output
5. Analog transmission on digital
channel


Analog signal (voice, music) is converted to digital (PCM
encoder).
Example: optical connection between CD player and
amplifier
Analog
input
A/D and coder
Digital
channel
Decoder and D/A
Analog
output
6. Digitized analog signal transmission
on analog channel


Analog signal (voice, music) is convert to digital (PCM
encoder).
Example: Digital cell phone (GSM, CDMA)
Analog
input
A/D and coder
Modem
Analog
Modem
channel
Decoder and D/A
Analog
output
Basic Modem Concepts


Digital data are transmitted over the telephone
and cable television networks by using
broadband communication techniques involving
modulation, which are implemented by a
modem, a device containing both a modulator
and a demodulator.
Modems convert binary signals to analog signals
capable of being transmitted over telephone and
cable TV lines and by radio, and then
demodulate such analog signals, reconstructing
the equivalent binary output.
Basic Modem Concepts

There are four widely used modem types:
1.
2.
3.
4.
Conventional analog dial-up modems.
Digital subscriber line (DSL) modems.
Cable TV modems.
Wireless modems.
Basic Modem Concepts
Figure 11-12: How modems permit digital data transmission on the telephone network.
Modem Modulation Types

The four main types of modulation used in
modern modems are:
Frequency-shift keying (FSK)
1.

Primarily used at lower speeds (<500 kbps) and in a noisy
environment.
Phase-shift keying (PSK)
2.

Operates in narrower bandwidths over a wide range of speeds.
Quadrature amplitude modulation (QAM)
3.

Very high data rates in narrow bandwidths.
Orthogonal frequency division multiplexing (OFDM)
4.

Covered in Section 11-5.
Frequency-shift keying (FSK)





Frequency-shift keying (FSK) is the oldest and
simplest form of modulation used in modems.
In FSK, two sine-wave frequencies are used to
represent binary 0s and 1s.
A binary 0 is usually called a space.
A binary 1 is referred to as a mark.
For example, a space has a frequency of 1070
Hz and a mark has a frequency of 1270 Hz.
These two frequencies are alternately
transmitted to create the serial binary data.
Frequency-shift keying (FSK)
Figure 11-13: Frequency-shift keying. (a) Binary signal. (b) FSK signal.
Phase-shift keying (PSK)



In phase-shift keying (PSK), the binary signal
to be transmitted changes the phase of a sinewave character, depending upon whether a
binary 0 or binary 1 is to be transmitted.
Binary Phase-shift keying (BPSK) uses a
phase shift of 180°.
During the time that a binary 0 occurs, the
carrier is transmitted with one phase; when a
binary 1 occurs, the carrier is transmitted with a
180° phase shift.
Binary phase-shift keying (BPSK)
Figure 11-18: Binary phase-shift keying.
Quadrature phase-shift keying (QPSK)
 One
way to increase the binary data rate while
not increasing the bandwidth required for the
signal transmission is to encode more than 1 bit
per phase change.
 In the system known as quadrature PSK (QPSK
or 4-PSK), more bits per baud are encoded. The
bit rate of data transfer can be higher than the
baud rate, yet the signal will not take up
additional bandwidth.
 In QPSK, each pair of successive digital bits in
the transmitted word is assigned a particular
phase.
 Each pair of serial bits, called a dibit, is
represented by a specific phase.
Quadrature phase-shift keying (QPSK)
Figure 11-24: Quadrature PSK modulation. (a) Phase angle of carrier for different
pairs of bits. (b) Phasor representation of carrier sine wave. (c) Constellation diagram
of QPSK.
Quadrature amplitude modulation (QAM).






One of the most popular modulation techniques used in
modems for increasing the number of bits per baud is
quadrature amplitude modulation (QAM).
QAM uses both amplitude and phase modulation of a
carrier.
In 8-QAM, there are four possible phase shifts and two
different carrier amplitudes.
Eight different states can be transmitted.
With eight states, 3 bits can be encoded for each baud
or symbol transmitted.
Each 3-bit binary word transmitted uses a different
phase-amplitude combination.
Quadrature amplitude modulation (QAM).
Figure 11-29: A constellation diagram of a 8-QAM signal.
Quadrature amplitude modulation (QAM).
0000
0100
1100
90
0001
0101
1101
180
0011
1000
1001
0
0111
1100
1011
270
0010
0110
1110
1010
A constellation diagram of a 16-QAM signal. 4 bits can be encoded.
Quadrature amplitude modulation (QAM).


As each symbol gets closer together, the signal
becomes more susceptible to noise.
64-QAM and 256-QAM are used in digital cable
television and cable modems.
Spectral Efficiency and Noise


Spectral efficiency is a measure of how fast data
can be transmitted in a given bandwidth (bps/Hz).
Different modulation methods give different
efficiencies.
Modulation
Spectral efficiency, bps/Hz
FSK
<1
BPSK
1
QPSK
2
8-PSK
3
16-QAM
4
Spectral Efficiency and Noise



The signal-to-noise (S/N) ratio clearly influences
the spectral efficiency. The greater the noise, the
greater the number of bit errors.
The number of errors that occur in a given time
is called the bit error rate (BER).
The BER is the ratio of the number of errors that
occur to the number of bits that are transmitted
in a one second interval.