wireless networks topic 08

8/2/2019 Wireless Networks Topic 08

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Signal Encoding

TechniquesTopic 8


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Phase-Shift Keying (PSK)

Two-level PSK (BPSK)

Uses two phases to represent binary digits

( )

=ts ( )tfA c2cos( ) +tfA c2cos1binary0binary

=

( )tfA c2cos

( )tfA c2cos1binary

0binary


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Differential PSK (DPSK)

Phase shift with reference to previous bit

Binary 0 signal burst of same phase as previous signal

burst

Binary 1 signal burst of opposite phase to previous

signal burst


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Four-level PSK (QPSK)

More efficient use of bandwidth if each signal element represents

more than one bit e.g. shifts of/2 (90o)

each signal element represents two bits

split input data stream in two & modulate onto the phase of the carrier

( )

=ts

+ 42cos

tfA c 11

+

4

32cos

tfA c

432cos tfA c

4

2cos

tfA c

01

00

10


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Four-level phase Shift Keying

(QPSK)


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Quadrature Amplitude Modulation

QAM is a combination of ASK and PSK

Two different signals sent simultaneously on the

same carrier frequency

( ) ( ) ( ) tftdtftdts cc 2sin2cos 21 +=


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Quadrature Amplitude Modulation


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Reasons for Analog Modulation

Modulation of digital signals

When only analog transmission facilities are

available, digital to analog conversion required

Modulation of analog signals

A higher frequency may be needed for effective

transmission

Modulation permits frequency divisionmultiplexing


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Basic Encoding Techniques

Analog data to analog signal

Amplitude modulation (AM)

Angle modulation

Frequency modulation (FM)

Phase modulation (PM)


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Spread Spectrum


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What is Spread Spectrum??

Definition: Spread spectrum is a form of wireless

communications in which the frequency of thetransmitted signal is deliberately varied

This results in a much greater bandwidth than the

signal would have if its frequency were not varied


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Conventional wireless communication:signal has a frequency, usually specified in MHz

and GHz, that does not change with time (except

for small, rapid fluctuations that occur as a resultof modulation)When you listen to a signal at 103.1 MHz on an FM

stereo receiver, the signal stays at 103.1 MHz. It

does not go up to 105.1 MHz or down to 99.1MHz


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The digits on the radio's frequency dial stay thesame at all times

The frequency of a conventional wireless signal is

kept as constant as the state of the art will permitThe bandwidth can be kept within certain limits,

and so the signal can be easily located by

someone who wants to retrieve the information.


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Problem with the conventional

Communication systemThere are at least two problems with conventional wirelesscommunications that can occur under certain

circumstances:

First, a signal whose frequency is constant issubject to catastrophic interference. This occurs

when another signal is transmitted on, or very

near, the frequency of the desired signal

Catastrophic interference can be accidental (as inamateur-radio communications) or it can be

deliberate (as in wartime)


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Problem with the conventional

Communication systemSecond, a constant-frequency signal is easy tointercept, and is therefore not well suited to

applications in which information must be kept

confidential between the source and destination


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Purpose of Spread Spectrum

Spread spectrum was originally developed to improve the

reliability and security of radio transmissions (primarily for

military communications systems). Prior to World War II

Spread Spectrum approach to wireless communications isemployed today in Wi-Fi and some cellular networks to

obtain the following benefits:

Enhanced reliability:

Mitigates the impact of wireless interference on acommunication channel


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Purpose of Spread Spectrum

Increased Bandwidth:Exploits additional wireless spectrum to better utilize and

share bandwidth among multiple channels

Improved security:

limits the ability of attackers to intercept transmissions Themain idea behind spread spectrum is to separate a wireless

communication into a set of related transmissions, send the

messages over a wide range of radio frequencies, then

collect and re-combine signals on the receiving side.


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Spread Spectrum

Input is fed into a channel encoder

Produces analog signal with narrow bandwidth

Signal is further modulated using sequence of digits

Spreading code or spreading sequence Generated by

pseudonoise, or pseudo-random number generator

Effect of modulation is to increase bandwidth of signal

to be transmitted


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Spread Spectrum

On receiving end, digit sequence is used to demodulate

the spread spectrum signal

Signal is fed into a channel decoder to recover data


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Spread Spectrum


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Frequency Hoping Spread

Spectrum (FHSS) Signal is broadcast over apparently random series of

radio frequencies (transmitter) hopping from frequency to frequency at fixed interval

Transmitter operates in one channel at a time

Bits are transmitted using some encoding scheme

At each successive interval, a new carrier frequency isselected

The sequence of channels used is dictated by a spreading

code

A receiver, hopping between frequencies insynchronization with the transmitter

Pick up the message


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Spectrum Advantages

Eavesdroppers hear only unintelligible blips (few bits)

Attempts to jam signal on one frequency succeed only at

knocking out a few bits


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Spectrum

requency opp ng prea


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requency opp ng preaSpectrum System

(Transmitter)


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Frequency Hopping SpreadSpectrum System (Receiver)


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Direct Sequence Spread Spectrum

(DSSS) Each bit in original signal is represented by multiple

bits in the transmitted signal

Spreading code spreads signal across a wider

frequency band Spread is in direct proportion to number of bits used

10 bits spreading code spread the signal across a frequency

band that is 10 times greater than a 1 bit spreading code

One technique combines digital information stream

with the spreading code bit stream using exclusive-OR

XOR ??


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Direct Sequence Spread Spectrum

(DSSS)


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DSSS Using BPSK


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Code-Division Multiple Access

(CDMA) In CDMA the users are spread across both

frequency and time in the same channel

Unique digital codes, rather than separate RF

frequencies or channels are used to differentiate

subscribers

The codes are shared by both the mobile stations

(cellular phone) and the base station, and are

called pseudo random code sequences or

pseudo-noise code sequences.


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(CDMA) It can provide secure communication in hostile

environment such that the transmitted signal is

not easily detected or recognized by unwanted

listeners

It can reject interference whether it is the

unintentional interference by another user

simultaneously attempting to transmit through thechannel, or the intentional interference by a

hostile transmitter attempting to jam the

transmission


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(CDMA)

Another application is in multiple access

communication in which a number of independent

users can share a common channel without anexternal synchronizing mechanism


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CDMA Example

Ifk=6 and code is a sequence of 1s and -1s For a 1 bit, A sends code as chip pattern

For a 0 bit, A sends complement of code

Receiver knows senders code and performs electronicdecode function

= received chip pattern

= senders code

( )665544332211 cdcdcdcdcdcddS

u+++++=


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CDMA Example

User A code = To send a 1 bit =

To send a 0 bit =

User B code = To send a 1 bit =

Receiver receiving with As code

(As code) x (received chip pattern) User A 1 bit: 6 -> 1

User A 0 bit: -6 -> 0

User B 1 bit: 0 -> unwanted signal ignored

wireless networks topic 08

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