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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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Phase-Shift Keying (PSK)
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Phase-Shift Keying (PSK)
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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What is Spread Spectrum??
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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What is Spread Spectrum??
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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Frequency Hoping Spread
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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Frequency Hoping Spread
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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Code-Division Multiple Access
(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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Code-Division Multiple Access
(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
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