Spread Spectrum
Review/Recap
Lecture 22
Overview
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





Relationship between bandwidth of a signal (before
and after and encoding SS)
Benefits of SS
FHSS
Slow and Fast FHSS
DSSS
Relationship between Bit Rate of a Signal (Before and
after DSSS encoding)
CDMA
2
Bandwidth Before and After SS Encoding

Q:- What is the relationship between the
bandwidth of a signal before and after it has
been encoded using spread spectrum?
3
4
Definition of Spread Spectrum

Spread spectrum is a modulation method applied to digitally
modulated signals that increases the transmit signal bandwidth
to a value much larger than is needed to transmit the
underlying information bits.
5
Spread Spectrum Signal Characteristics
1. They are difficult to intercept for unauthorized person.
2. They are easily hidden, it is difficult to even detect their
presence in many cases.
3. They are resistant to jamming.
4. They have an asynchronous multiple-access capability.
5. They provide a measure of immunity to distortion due to
multipath propagation.
6
Spread Spectrum Conditions
•
The signal occupies a bandwidth much larger than is
needed for the information signal.
•
The spread spectrum modulation is done using a
spreading code, which is independent of the data in
the signal.
•
Despreading at the receiver is done by correlating the
received signal with a synchronized copy of the
spreading code.
7
Processing Gain :
The spread spectrum increases the bandwidth
of the message signal by a factor N, called the
processing gain where β is the message signal
bandwidth, βss is the corresponding SS signal
bandwidth.
,N>1
8
9
Spread Spectrum Versus Narrowband Technology
Narrowband wireless communications can be defined as wireless
communications using a single frequency center with no
redundancy to communicate information at high power levels
chosen to overpower interference in that frequency band.
Spread spectrum wireless communications can be defined as
wireless communications using a range of frequencies to
communicate information at low power levels.
Spread spectrum has also been defined as a wireless
communications technology that uses more bandwidth than is
required to deliver information. Spread spectrum also uses low
power and can do so because all interference does not need to be
overcome, due to the redundancy and/or error correction.
10
Spread Spectrum Versus Narrowband Technology
11
12
Spread Spectrum
Spread spectrum
13
Characteristics of
Spread Spectrum


Bandwidth of the transmitted signal W is much greater than the original message
bandwidth (or the signaling rate R)
Transmission bandwidth is independent of the message. Applied code is known both
to the transmitter and receiver
Narrow band
Signal (data)



Wideband signal
(transmitted SS signal)
Interference and noise immunity of SS system is larger, the larger the processing
gain Lc  W / R  Tb / Tc
Multiple SS systems can co-exist in the same band (=CDMA). Increased user
independence (decreased interference) for (1) higher processing gain and higher (2)
code orthogonality
Spreading sequence can be very long -> enables low transmitted PSD-> low
probability of interception (especially in military communications)
14
Characteristics of Spread
Spectrum (cont.)

Processing gain, in general
Lc  W / R  (1/ Tc ) /(1/ Tb )  Tb / Tc , Lc , dB  10log10 ( Lc )
Large Lc improves noise immunity, but requires a larger transmission
bandwidth
 Note that DS-spread spectrum is a repetition FEC-coded systems
Jamming margin M J  Lc  [ Lsys  ( SNR) desp ]
 Tells the magnitude of additional interference and noise that can be
injected to the channel without hazarding system operation. Example:
Lc  30dB,available processing gain


Lsys  2dB, margin for system losses
SNRdesp  10dB, required SNR after despreading (at the RX)
 M j  18dB,additional interference and noise can deteriorate
received SNR by this amount
15
Characteristics of Spread
Spectrum

Spectral efficiency Eeff: Describes how compactly TX signal fits into the
transmission band. For instance for BPSK with some pre-filtering:
Eeff  Rb / BT  Rb / BRF
Lc  Tb / Tc  Lc / Tb  1/ Tc
 BRF , filt : bandwidth for polar mod.
1/ Tc
Lc
BRF 



k
log 2 M Tb log 2 M  M : number of levels
k: number of bits
T
log
M
R
1 b
log 2 M 
2
 Eeff  b 

 M  2k  k  log2 M 
BRF Tb
Lc
Lc
BRF , filt

Energy efficiency (reception sensitivity): The value of  b  Eb / N0
to obtain a specified error rate (often 10-9).
For BPSK the error rate is

1
pe  Q( 2 b ), Q(k ) 
exp( 2 / 2)d 
2 k
QPSK-modulation can fit twice the data rate of BPSK in the same bandwidth.
16
Therefore it is more energy efficient than BPSK.


Bandwidth Before and After SS Encoding
Q:- What is the relationship between the
bandwidth of a signal before and after it has
been encoded using spread spectrum?
Ans:- The bandwidth is wider after the signal has
been encoded using spread spectrum
17
18
SS Benefits
Q:- List three benefits of spread spectrum.
19
Benefits of Spread Spectrum





Spread spectrum is being used in more and more
applications in data communications.
Security – need a wide BW receiver and precise
knowledge and timing of the pseudorandom sequence
Resistance to jamming and interference – jamming
signals are usually restricted to one frequency
Band sharing – many signals can use the same
frequency band; but… many spread spectrum signals
raise the overall background noise level
Precise timing – can be used in radar where accurate
20
knowledge of transmission time is needed
SS Benefits
Q:- List three benefits of spread spectrum.
 Ans:(1) We can gain immunity from various kinds of
noise and multipath distortion.
(2) It can also be used for hiding and encrypting
signals. Only a recipient who knows the spreading
code can recover the encoded information.
(3) Several users can independently use the same
higher bandwidth with very little interference, using
21
code division multiple access (CDMA).
22
Frequency Hopping SS
23
FHSS
Q:- What is frequency hopping spread spectrum
24
Frequency hopping



This dilemma was recognized prior
to WWII.
In 1942, Hedy Lamarr and pianist
George Antheil patented a “Secret
Communication System”.
Their scheme was for a
frequency hopping
remote control for
torpedo guidance.
Hedy Lamarr
Actress and co-inventor of frequency hopping spread spectrum
25
First spread-spectrum patent


By changing the transmitter frequencies
in a “random” pattern, the torpedo
control signal could not be jammed.
Lamarr proposed using 88
frequencies sequenced
for control.
Frequency switching pattern
26
FHSS Algorithm
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The initiating party sends a request via a predefined frequency or control
channel.
Once the receiving party have received the request, it sends a pseudo
number called seed to the transmitting party via the same channel.
The initiating party uses this seed as a variable in predefined algorithm and
calculates the sequence of frequencies that must now be used to transmit
the signals. Most often the period of frequency change is predetermined.
The initiating party then sends the first piece of information via the lowest
band of newly generated frequency spectrum. Thus acknowledging the
receiving party that it has correctly calculated the sequence.
The communication begins, and both the receiving party and the sending
party change the frequencies along with the calculated order. Starting at the
same point of the time.
27
FHSS Algorithm
In FH Data is divided into chunks and transmitted at different
frequencies at different times.
28
Frequency hopping spread
spectrum

In a frequency hopping spread spectrum system, the carrier
frequency is switched in a pseudorandom fashion.
The transmitter and receiver know the pattern and are
synchronized.
Time (ms)

Dwell time
225
230
235
240
245
250
255
f (MHz)
29
Dwell Time
FHSS systems include characteristics such as dwell time, hopping
sequences, and hop time. These characteristics come together to
make up how the FHSS system will function and the actual data
throughput that will be available.
The amount of time spent on a specific frequency in an FHSS
hopping sequence is known as the dwell time. These channels, 1
MHz of bandwidth each, provide 79 optional frequencies on which
to dwell for the specified length of the dwell time.
30
Hopping sequence
The hopping sequence is the list of frequencies through which the
FHSS system will hop according to the specified dwell time. The
IEEE 802.11 standard, section 14.6.5, states that 1 MHz channels
should be used. These channels exist between 2.402 and 2.480
GHz in the United States and most of Europe.
Every station in a Basic Service Set must use the same hopping
sequence. Every station must also store a table of all the hopping
sequences that are used within the system.
These hopping sequences must have a minimum hop size of 6
MHz in frequency. If the device is currently communicating on the
2.402 GHz frequency, it must hop to 2.408 GHz at the next hop at
a minimum.
31
Frequency hopping transmitter



The binary data to be transmitted is applied to a conventional
two-tone FSK modulator.
A frequency synthesizer produces a sine wave of a random
frequency determined by a pseudorandom code generator.
These two signals are mixed together, filtered and then
transmitted.
32
Frequency hopping transmitter
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
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Typically the rate of frequency change is much higher than the data
rate.
The illustration below shows that the frequency synthesizer changes
4 times for each data bit.
The time period spent on each frequency is called the dwell time
(typically < 10 ms)
33
Frequency hopping spread
spectrum


The resulting signal, whose frequency rapidly jumps
around, effectively scatters pieces of the signal all
over the band.
Someone else monitoring the spectrum would not
recognize that a transmission is being made.
225
230
235
240
245
250
255
f (MHz)
34
Frequency hopping receiver


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The received signal is mixed using a local oscillator driven by the
same pseudorandom sequence.
The output produces the original two-tone FSK signal from which the
binary data can be extracted.
Timing is extremely critical in frequency hopping systems in order to
maintain synchronization.
35
Practical Example: Bluetooth
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2.4 GHz – 2.4835 GHz Operating Range
79 Different Radio Channels
Hops 1600 times per second for data/voice links
Hops 3200 times per second for page and inquiry
scanning
1 Mbps = Rb for Bluetooth Ver 1.1/1.2
3 Mbps = Rb for Bluetooth Ver 2.1
Gaussian Frequency Shift Keying (GFSK)
36
Frequency Hopping SS


Signal is broadcast over seemingly random series of radio
frequencies
 A number of channels allocated for the FH signal
 Width of each channel corresponds to bandwidth of
input signal
Signal hops from frequency to frequency at fixed intervals
 Transmitter operates in one channel at a time
 Bits are transmitted using some encoding scheme
 At each successive interval, a new carrier frequency is
selected
37
Frequency Hopping SS
38
Frequency Hopping SS



Hopping Sequence
 Channel sequence dictated by spreading code
 Pseudorandom number serves as an index into a
table of frequencies
Chip Period
 Time spent on each channel
 FCC regulation  maximum dwell time of 400 ms
 IEEE 802.11 standard  300 ms
Chipping rate
 Hopping rate
39
Frequency Hopping SS


Receiver, hopping between frequencies in
synchronization with transmitter, picks up
message
Advantages

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Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed
only at knocking out a few bits
40
FHSS Performance Considerations

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Large number of frequencies used
Results in a system that is quite resistant to
jamming

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Jamming signal must jam all frequencies
With fixed power, this reduces the jamming power
in any one frequency band
41
FHSS
Q:- What is frequency hopping spread spectrum
Ans:- With frequency hopping spread spectrum
(FHSS), the signal is broadcast over a seemingly
random series of radio frequencies, hopping from
frequency to frequency at fixed intervals. A
receiver, hopping between frequencies in
synchronization with the transmitter, picks up the
message.
42
43
Slow and Fast FHSS
Q:- Explain the difference between slow FHSS
and fast FHSS.
44
(Frequency Hopping Spread Spectrum)

Discrete changes of carrier frequency


sequence of frequency changes determined via pseudo
random number sequence
Two versions
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Fast Hopping: several frequencies per user bit
Slow Hopping: several user bits per frequency
Advantages



frequency selective fading and interference limited to
short period
simple implementation
45
uses only small portion of spectrum at any time
FHSS: Example
tb
user data
0
1
f
1
1
t
td
f3
f2
f1
f
0
t
td
f3
f2
f1
t
tb: bit period
slow
hopping
(3 bits/hop)
fast
hopping
(3 hops/bit)
td: dwell time
46
Comparison between slow
hopping and fast hopping

Slow hopping
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Pros: cheaper
Cons: less immune to narrowband interference
Fast hopping
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Pros: more immune to narrowband interference
Cons: tight synchronization  increased complexity
47
Slow and Fast FHSS
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commonly use multiple FSK (MFSK)
have frequency shifted every Tc seconds
duration of signal element is Ts seconds
Slow FHSS has Tc  Ts
Fast FHSS has Tc < Ts
FHSS quite resistant to noise or jamming

with fast FHSS giving better performance
48
Slow MFSK FHSS
49
Fast MFSK FHSS
50
Slow and Fast FHSS
Q:- Explain the difference between slow FHSS
and fast FHSS.

Ans:- Slow FHSS = multiple signal elements per
hop; fast FHSS = multiple hops per signal
element.
51
52
DSSS
Q:- What is direct sequence spread spectrum?
53
Direct Sequence Spread Spectrum
l Spread spectrum increases the bandwidth of the signal compared to
narrow band by spreading the signal.
l There are two major types of spread spectrum techniques: FHSS and
DSSS.
4 FHSS spreads the signal by hopping from one frequency to
another across a bandwidth of 83 Mhz.
4 DSSS spreads the signal by adding redundant bits to the signal
prior to transmission which spreads the signal across 22 Mhz.
* The process of adding redundant information to the signal is
called Processing Gain .
* The redundant information bits are called Pseudorandom
Numbers (PN).
54
DSSS
l DSSS works by combining information bits (data signal) with higher data rate
bit sequence (pseudorandom number (PN)).
l The PN is also called a Chipping Code (eg., the Barker chipping code)
lThe bits resulting from combining the information bits with the chipping code
are called chips - the result- which is then transmitted.
* The higher processing gain (more chips) increases the signal's
resistance to interference by spreading it across a greater number of
frequencies.
* IEEE has set their minimum processing gain to 11. The number of
chips in the chipping code equates to the signal spreading ratio.
* Doubling the chipping speed doubles the signal spread and the required
bandwidth.
55
Signal Spreading
4The Spreader employs an encoding scheme (Barker or Complementary Code
Keying (CCK).
4 The spread signal is then modulated by a carrier employing either Differential
Binary Phase Shift Keying (DBPSK), or Differential Quadrature Phase Shift
Keying (DQPSK).
4 The Correlator reverses this process in order to recover the original data.
56
DSSS Channels
Fourteen channels are identified, however, the FCC specifies only 11 channels for non-licensed
(ISM band) use in the US.
Each channels is a contiguous band of frequencies 22 Mhz wide with each channel separated by
5 MHz.
Channel 1 = 2.401 – 2.423 (2.412 plus/minus 11 Mhz).
Channel 2 = 2.406 – 2.429 (2.417 plus/minus 11 Mhz).
Only Channels 1, 6 and 11 do not overlap
57
Spectrum Mask
A spectrum Mask represents the maximum power output for the channel at various frequencies.
From the center channel frequency, ± 11 MHz and
dB.
From the center channel frequency, outside
± 22 MHZ the signal must be attenuated 30
± 22 MHZ, the signal is attenuated 50 dB.
58
DSSS Frequency Assignments
The Center DSSS frequencies of each channel are only 5 Mhz apart but each channel is 22
Mhz wide therefore adjacent channels will overlap.
DSSS systems with overlapping channels in the same physical space would cause interference
between systems.
Co-located DSSS systems should have frequencies which are at least 5 channels apart,
e.g., Channels 1 and 6, Channels 2 and 7, etc.
Channels 1, 6 and 11 are the only theoretically non-overlapping channels.
25 MHz
Channel 1
2.412 GHz
25 MHz
Channel 6
2.437 GHz
Channel 11
2.462 GHz
59
DSSS Non-overlapping Channels
P
3 MHz
Each channel is 22 MHz wide. In order for two
bands not to overlap (interfere), there must be five
channels between them.
A maximum of three channels may be co-located
(as shown) without overlap (interference).
22 MHz
Channel 1
The transmitter spreads the signal sequence
across the 22 Mhz wide channel so only a few chips
will be impacted by interference.
Channel 6
Channel 11
f
2.401 GHz
2.473 GHz
60


DSSS
Encoding and Modulation
61
DSSS Encoding and Modulation
DSSS (802.11b) employs two types of encoding schemes and two types of modulation
schemes depending upon the speed of transmission.
Encoding Schemes
Barker Chipping Code: Spreads 1 data bit across 11 redundant bits at both 1
Mbps and 2 Mbps
Complementary Code Keying (CCK):
Maps 4 data bits into a unique redundant 8 bits for 5.5 Mbps
Maps 8 data bits into a unique redundant 8 bits for 11 Mbps.
Modulation Schemes
Differential Binary Phase Shift Keying (DBPSK): Two phase shifts with
each phase shift representing one transmitted bit.
Differential Quadrature Phase Shift Keying (DQPSK): Four phase shifts
with each phase shift representing two bits.
62

DSSS Encoding
63
Barker Chipping Code
802.11 adopted an 11 bit Barker chipping code.
Transmission.
The Barker sequence, 10110111000, was chosen to spread each
1 and 0 signal.
The Barker sequence has six 1s and five 0s.
Each data bit, 1 and 0, is modulo-2 (XOR) added to the eleven bit
Barker sequence.
If a one is encoded all the bits change.
If a zero is encoded all bits stay the same.
Reception.
A zero bit corresponds to an eleven bit sequence of six 1s.
A one bit corresponds to an eleven bit sequence of six 0s.
64
Barker Sequence
65
Direct Sequence Spread Spectrum ….
66
Complementary Code Keying(CCK)
Barker encoding along with DBPSK and DQPSK modulation
schemes allow 802.11b to transmit data at 1 and 2 Mbps
Complementary Code Keying (CCK) allows 802.11b to
transmit data at 5.5 and 11 Mbps.
CCK employs an 8 bit chipping code.
The 8 chipping bit pattern is generated based upon the data
to be transmitted.
At 5.5 Mbps, 4 bits of incoming data is mapped into a
unique 8 bit chipping pattern.
At 11 Mbps, 8 bits of data is mapped into a unique 8 bit
67
chipping pattern.
Complementary Code Keying (CCK)
To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..
The unique 8 chipping bits is determined by the bit pattern of the 4 data bits to be transmitted. The
data bit pattern is:
b0, b1, b2, b3
b2 and b3 determine the unique pattern of the 8 bit CCK chipping code.
Note: j represents the imaginary number, sqrt(-1), and appears on the imaginary or quadrature axis of the
complex plane.
68
Complementary Code Keying (CCK)
To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..
The unique 8 chipping bits is determined by the bit pattern of the 4 data bits to
be transmitted. The data bit pattern is:
b0, b1, b2, b3
b0 and b1 determine the DQPSK phase rotation that is to be applied to
the chip sequence.
Each phase change is relative to the last chip transmitted.
69
Complementary Code Keying (CCK)
To transmit 11 Mbps 8 data bits is mapped into 8 CCK chipping
bits.
The unique 8 chipping bits is determined by the bit pattern of
the 8 data bits to be transmitted. The data bit pattern is:
b0, b1, b2, b3, b4, b5, b6 ,b7
b2, b3, b4 ,b5, b6 and b7 selects one unique pattern of
the 8 bit CCK chipping code out of 64 possible sequences.
b0 and b1 are used to select the phase rotation sequence.
70
DSSS
Modulation
71
Differential Binary Phase Shift Keying (DBPSK)
A Zero phase
shift from the
previous symbol is
interpreted as a 0.
A 180 degree
phase shift from the
previous symbol is
interpreted as a 1.
72
Differential Quadrature Phase
Shift Keying (DQPSK)
A Zero phase shift from the
previous symbol is interpreted
as a 00.
A 90 degree phase shift
from the previous symbol is
interpreted as a 01.
A 180 degree phase shift
from the previous symbol is
interpreted as a 11.
A 270 degree phase shift
from the previous symbol is
interpreted as a 10.
73
DSSS Summary
Data Rate
Encoding
1
Barker Coding 11 chips encoding 1 bit
Modulation
DBPSK
2
Barker Coding
11 chips encoding 1 bit
DQPSK
5.5
CCK Coding
8 chips encode 8 bits
DQPSK
11
CCK Coding
8 chips encode 4 bits
DQPSK
74
75
Direct Sequence Spread Spectrum


Another method of realizing spread spectrum is called
direct sequence spread spectrum (DSSS).
In a DSSS system the message bit stream is modified
by a higher rate pseudonoise (PN) sequence (called a
chip sequence).
76
Direct Sequence Spread
Spectrum –DSSS

In direct-sequence spread spectrum (DSSS), the serial binary data is XORed
with a pseudo-random binary code which has a bit rate faster than the
binary data rate, and the result is used to phase-modulate a carrier.
 chipping rate – bit rate of the pseudorandom code
 the faster you change the phase of a carrier, the more BW the signal
takes up – looks like noise
UNMODULATED
CARRIER
SLOW SPEED PSK
HIGH SPEED PSK
many clock (chipping rate)
pulses in one data bit time
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DSSS
1
data
1
0
1
time of one data bit
carrier modulated by the data
Pseudo Random
Sequence
“chip”
data  PRS
XOR 
carrier modulated by the data  PRS
power
UNMODULATED
CARRIER
SLOW SPEED PSK
HIGH SPEED PSK
78
frequency
Direct Sequence Spread
Spectrum (cont’d)

Observations



A signal that would normally occupy a few kHz BW
is spread out 10 to 10,000 times its BW.
The fast phase modulation spreads the energy of
the signal over a wide BW – appears as noise in a
conventional receiver.
Also called CDMA – Code Division Multiple Access


used in satellites – many signals can use the same
transponder
used in cell phones – many users in same BW
79
Direct Sequence Spread
Spectrum

Receiver



Receiver must know the pseudorandom sequence
of the transmitter and have a synchronizing circuit
to get in step with this pseudorandom digital signal.
The receiver using an identically programmed PN
sequence compares incoming signals and picks out
the one with the highest correlation.
Other signals using different PN sequences appear
as noise to the receiver and it doesn’t recognize
them.
80
Processing gain

The measure of the spreading is called the
processing gain, G, which is the ratio of the DSSS
bandwidth, BW, divided by the data rate, fb .
BW
G
fb

The higher the processing gain, the greater the DSSS
signal’s ability to fight interference.
81
DSSS Signal
The spread signal has the same power as the narrowband signal, but far more
sidebands
Amplitudes are very low and just above the random noise level
Transmitter and receiver are using the same PN sequence, so signal will be
82
recognized
Benefits of Spread Spectrum





Spread spectrum is being used in more and more
applications in data communications.
Security – need a wide BW receiver and precise knowledge
and timing of the pseudorandom sequence
Resistance to jamming and interference – jamming signals
are usually restricted to one frequency
Band sharing – many signals can use the same frequency
band; but… many spread spectrum signals raise the
overall background noise level
Precise timing – can be used in radar where accurate
knowledge of transmission time is needed
83
DSSS
Q:- What is direct sequence spread spectrum?

Ans:- With direct sequence spread spectrum
(DSSS), each bit in the original signal is
represented by multiple bits in the transmitted
signal, using a spreading code.
84
85
Bit Rate in DSSS Before and After

Q:- What is the relationship between the bit
rate of a signal before and after it has been
encoded using DSSS
86
DSSS

Direct Sequence

each bit in the original signal is represented by multiple bits in the transmitted signal,
known as a chipping code

the chipping code spreads the signal over a wider frequency band in direct proportion to
the number of bits used.
87
Bit Rate in DSSS Before and After

Q:- What is the relationship between the bit
rate of a signal before and after it has been
encoded using DSSS
Ans:- For an N-bit spreading code, the bit rate
after spreading (usually called the chip rate) is N
times the original bit rate.
88
89
CDMA
Q:- What is CDMA?
90
CDMA Transceiver Block
Diagram
91
Lets walk through an example?
92
Multiplication
1
x
1
=
1
1
x
-1
=
-1
-1
x
1
=
-1
-1
x
-1
=
1
93
CDMA example
Low-Bandwidth Signal:
High-Bandwidth Spreading Code:
...repeated...
94
CDMA example
Low-Bandwidth Signal:
High-Bandwidth Spreading Code:
Mix is a simple multiply
… and transmit.
95
CDMA example
To Decode / Receive, take the signal:
96
CDMA example
To Decode / Receive, take the signal:
Multiply by the same Spreading Code:
… to get ...
… which you should recognise as...
97
CDMA example
To Decode / Receive, take the signal:
Multiply by the same Spreading Code:
… to get ...
98
(Discuss noise)
To Decode / Receive, take the signal:
Multiply by the same Spreading Code:
… to get ...
99
What if we use the wrong code?
Take the same signal:
Multiply by the wrong Spreading Code:
100
What if we use the wrong code?
Take the same signal:
Multiply by the wrong Spreading Code:
… for example, let's just shift the same code left a bit:
101
What if we use the wrong code?
Take the same signal:
Multiply by the wrong Spreading Code:
… for example, let's just shift the same code left a bit:
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What if we use the wrong code?
Take the same signal:
Multiply by the wrong Spreading Code:
… you get ...
… which clearly hasn't recovered the original signal.
Using wrong code is like being off-frequency. 103
This obviously shows that
timing is critical. To receive a signal, you not only
need to be generating the RIGHT code, but your
TIMING needs to be locked very tightly to the
received signal too.
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CDMA
Q:- What is CDMA?

Ans:- CDMA allows multiple users to transmit
over the same wireless channel using spread
spectrum. Each user uses a different spreading
code. The receiver picks out one signal by
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matching the spreading code.
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Summary







Relationship between bandwidth of a signal (before
and after and encoding SS)
Benefits of SS
FHSS
Slow and Fast FHSS
DSSS
Relationship between Bit Rate of a Signal (Before and
after DSSS encoding)
CDMA
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