1. 29-03-2013
A.SANYASI RAO
AMIE, M.Tech, MISTE, MIETE
Assoc. Prof, Dept. of ECE
Balaji Institute of Engineering & Sciences
Allanki Sanyasi Rao
MULTIPLEXING
channels ki
• Multiplexing in 4 dimensions
o space (si) k1 k2 k3 k4 k5 k6
o time (t) c
o frequency (f) t c
o code (c) t
s1
f
• Goal: multiple use s2
f
of a shared medium c
t
• Important: guard spaces needed!
s3
f
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2. 29-03-2013
Allanki Sanyasi Rao
FREQUENCY MULTIPLEX
 Separation of spectrum into smaller frequency bands
 Channel gets band of the spectrum for the whole time
 Advantages:
• no dynamic coordination needed k3 k4 k5 k6
• works also for analog signals
c f
 Disadvantages:
• waste of bandwidth if traffic distributed unevenly
• inflexible
• guard spaces
t
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TIME MULTIPLEX
 Channel gets the whole spectrum for a certain
amount of time
 Advantages:
• only one carrier in the medium at any time
• throughput high even for many users
 Disadvantages:
• Precise synchronization necessary
k1 k2 k3 k4 k5 k6
c
f
t
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3. 29-03-2013
Allanki Sanyasi Rao
TIME & FREQUENCY MULTIPLEX
 A channel gets a certain frequency band for a certain
amount of time (e.g. GSM)
 Advantages:
• better protection against tapping
• protection against frequency selective interference
• higher data rates compared to code multiplex
 Precise coordination required
k1 k2 k3 k4 k5 k6
c
f
t
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CODE MULTIPLEX
k1 k2 k3 k4 k5 k6
 Each channel has unique code
c
 All channels use same spectrum at same time
 Advantages:
• bandwidth efficient
• no coordination and synchronization
• good protection against interference
 Disadvantages: f
• lower user data rates
• more complex signal regeneration
 Implemented using spread spectrum technology
t
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4. 29-03-2013
Allanki Sanyasi Rao
Spread Spectrum
Problem of radio transmission: frequency dependent fading can
wipe out narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal
using a special code
The spread spectrum techniques was developed initially for
military and intelligence requirements.
The essential idea is to spread the information signal over a wider
bandwidth to make jamming and interception more difficult.
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Spread Spectrum (Contd.)
• Spread spectrum is a communication technique that spreads a
narrowband communication signal over a wide range of frequencies for
transmission then de-spreads it into the original data bandwidth at the
receiver.
• Spread spectrum is characterized by:
- wide bandwidth and
- low power
• Jamming and interference have less effect on Spread spectrum
because it is:
- Resembles noise
- Hard to detect
- Hard to intercept
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5. 29-03-2013
Allanki Sanyasi Rao
Narrowband vs Spread Spectrum
Power
Narrowband
(High Peak Power)
Spread Spectrum
(Low Peak Power)
Frequency
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Signal Spreading
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6. 29-03-2013
Allanki Sanyasi Rao
SPREADING AND FREQUENCY SELECTIVE FADING
channel
quality
1 2 5 6
narrowband
3
4 channels
Narrowband guard space frequency
signal
channel
quality
2
2
2
2
2
1 spread spectrum
channels
spread frequency
spectrum
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SPREAD SPECTRUM- MAIN OPERATION
b(t) m(t)
x
c(t)
mt   bt  ct 
M  f   B f * C  f 
B(f)
M(f)
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7. 29-03-2013
Allanki Sanyasi Rao
Why Spread Spectrum..?
Advantages:
•Spread spectrum signals are distributed over a wide range of frequencies
and then collected back at the receiver
•These wideband signals are noise-like and hence difficult to detect or
interfere with
•Initially adopted in military applications, for its resistance to jamming and
difficulty of interception
•More recently, adopted in commercial wireless communications
•Has the ability to eliminate the effect of multipath interference
•Can share the same frequency band with other users
•Privacy due to the pseudo random code sequence (code division
multiplexing)
Disadvantages:
• Bandwidth inefficient
• Implementation is somewhat more complex.
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Spread Spectrum Technique
• To apply an SS technique, simply inject the corresponding SS code somewhere
in the transmitting chain before the antenna. The effect is to diffuse the
information in a larger bandwidth. Conversely, you can remove the SS code, at
a point in the receive chain before data retrieval. The effect of a de spreading
operation is to reconstitute the information in its original bandwidth. Obviously,
the same code must be known in advance at both ends of the transmission
channel.
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8. 29-03-2013
Allanki Sanyasi Rao
Narrow Band vs Spread Spectrum
• Narrow Band
- Uses only enough frequency spectrum to carry the signal
- High peak power
- Easily jammed
• Spread Spectrum
- The bandwidth is much wider than required to send to the signal.
- Low peak power
-Hard to detect
-Hard to intercept
- Difficult to jam
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Spread Spectrum Use
• In the 1980s FCC implemented a set of rules making Spread Spectrum
available to the public.
- Cordless Telephones
- Global Positioning Systems (GPS)
- Cell Phones
- Personal Communication Systems
- Wireless video cameras
• Local Area Networks
- Wireless Local Area Networks (WLAN)
- Wireless Personal Area Network (WPAN)
- Wireless Metropolitan Area Network (WMAN)
- Wireless Wide Area Network (WWAN)
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9. 29-03-2013
Allanki Sanyasi Rao
FCC Specifications
• The Code of Federal Regulations (CFR) Part 15 originally only described two
spread spectrum techniques to be used in the licensed free Industrial, Scientific,
Medical (ISM) band, 2.4 GHz, thus 802.11 and 802.11b.
- Frequency Hopping Spread Spectrum (FHSS) and
- Direct Sequence spread Spectrum (DSSS)
• Orthogonal Frequency Division Multiplexing (OFDM) was not covered by the
CFR and would have required licensing.
- 802.11a, employing OFDM, was created to work in the 5GHz Unlicensed
National Information Infrastructure (UNII)
• In May, 2001 CFR, Part 15 was modified to allow alternative "digital modulation
techniques".
- This resulted in 802.11g which employs OFDM in the 2.4 GHz range
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•Direct Sequence (DS) - A carrier is modulated by a digital code
sequence in which bit rate is much higher than the information
signal bandwidth.
• Frequency Hopping (FH) - A carrier frequency is shifted in
discrete increments in a pattern dictated by a code sequence.
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10. 29-03-2013
Allanki Sanyasi Rao
What is pseudorandom number sequences?
is a sequence of numbers that has been computed by
some defined arithmetic process but is effectively a random
number sequence for the purpose for which it is required.
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• Input is fed into a channel encoder
• Produce an analog signal with a relatively narrow bandwidth around
some center frequency.
• Further modulated using a sequence of digits known as a spreading code
or spreading sequence.
• The spreading code is generated by a pseudo noise, or
pseudorandom number generator.
• The effect of this modulation is to increase significantly the
bandwidth (spread the spectrum) of the signal to be transmitted.
• At the receiver, the same digit sequence is used to demodulate the spread
spectrum signal.
• The signal is fed into a channel decoder to recover the data.
•Spread Spectrum signals use fast codes that run many times the information
bandwidth or data rate.
•These special "Spreading" codes are called "Pseudo Random" or "Pseudo
Noise" codes. They are called "Pseudo" because they are not real Gaussian
noise.
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11. 29-03-2013
Allanki Sanyasi Rao
 What can we gain from this apparent waste of spectrum?
 Gain immunity from various kinds of noise and multipath
distortion
 Can be used for hiding and encrypting signals. Only a
recipient who knows the spreading code can recover the
encoded information.
 Several users can independently use the same higher
bandwidth with very little interference (CDMA).
Allanki Sanyasi Rao
GENERATING THE SPREADING (PSEUDO-NOISE) SEQUENCE
• The pseudo-noise (PN) sequence is a periodic binary sequence with a
noise like waveform that is generated by means of a feedback shift
register.
• The feedback shift register consists of m-stage shift registers and a
logic circuit that perform modulo-2 (X-OR) arithmetic.
• A sequence with period 2m-1 is called Maximal-Length sequence
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12. 29-03-2013
Allanki Sanyasi Rao
PN SEQUENCE: EXAMPLE
s1 s2 s3
1 0 0
1 1 0
1 1 1
0 1 1
1 0 1
0 1 0
0 0 1
1 0 0
Spreading code  0 0 1 1 1 0 1 0 . . .
Here N=2m – 1=7, length of the sequence
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PROPERTIES OF MAXIMAL-LENGTH SEQUENCES
• In each period of a maximal-length sequence, the number
of 1’s and the number of 0’s in the sequence always differ
by 1.
• The autocorrelation function of a maximal-length sequence
is periodic and binary valued.
Tb
Rc    ct ct   dt
1
Tb  2
T
 b
2
 N 1
 1    Tc

Rc     NT c
 1 for the rest of the p eriod
 N

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13. 29-03-2013
Allanki Sanyasi Rao
Chip rate, Rc is the rate at which the
no. of bits of the PN sequence
occur.
The duration of each bit is
TC= 1/Rc
Therefore, the period of the
waveform is Tb = NTc
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W1 and W2N
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14. 29-03-2013
Allanki Sanyasi Rao
Sequence Generation
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Chip Sequences
Encoding Rules
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15. 29-03-2013
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CDMA Multiplexer
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CDMA Demultiplexer
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16. 29-03-2013
Allanki Sanyasi Rao
DIRECT SEQUENCE SPREAD SPECTRUM (DSSS)
Direct Sequence (DS) - A carrier is modulated by a digital code
sequence in which bit rate is much higher than the information signal
bandwidth.
• Most widely recognized technology for spread spectrum.
• This method generates a redundant bit pattern for each bit to be
transmitted. This bit pattern is called a chip.
• The longer the chip, the greater the probability that the Original data
can be recovered, and the more bandwidth required.
In a spread spectrum system, the process gain (or ‘processing gain') is
the ratio of the spread bandwidth to the unspread bandwidth. It is
usually expressed in decibels (dB).
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• The amount of spreading is dependent upon the ratio of chips per bit
of information (which is the processing gain Gp for DSSS)
• A direct sequence modulator is then used to carrier modulate the
carrier using binary phase shift keying (BPSK)
• At the receiver, the information is recovered by multiplying the signal
with a locally generated replica of the code sequence.
• Each bit represented by multiple bits using spreading code
• Spreading code spreads signal across wider frequency band
• In proportion to number of bits used
• 10 bit spreading code spreads signal across 10 times bandwidth
of 1 bit code
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17. 29-03-2013
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Processing Gain
The process gain (or ‘processing gain') is the ratio of the spread
bandwidth to the unspread bandwidth. It is usually expressed in
decibels (dB).
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DSSS USING BPSK
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18. 29-03-2013
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19. 29-03-2013
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RANGING USING DSSS
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Frequency Hopping Spread Spectrum (FHSS)
• When using FHSS, the frequency spectrum is divided into channels.
Data packets are split up and transmitted on these channels in a
random pattern known only to the transmitter and receiver.
At the transmitter, the original signal is broadcasted
over a series of radio frequencies, hopping from
different frequencies in a fixed pattern.
The receiver should use the same hopping pattern
simultaneously with the transmitter in order to
receive the data correctly.
The spreading code specifies the sequence of
channels and the receiver should use the same
code to tune into the sequence of channels that are
used by the sender.
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20. 29-03-2013
Allanki Sanyasi Rao
• The information signal is transmitted on different frequencies
• Time is divided in slots
• In each slot the frequency is changed
• The change of the frequency is referred to as slow if more than one
bit is transmitted on one frequency, and as fast if one bit is
transmitted over multiple frequencies
• The frequencies are chosen based on the spreading sequences
 Each channel used for fixed interval
• Eg: 300 ms in IEEE 802.11
• Sequence dictated by spreading cod
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FREQUENCY HOPPING SPREAD SPECTRUM (FHSS)
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21. 29-03-2013
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FREQUENCY SELECTION IN FHSS (EXAMPLE 1)
Normally ‘K’ successive bits of input data sequence represents 2k = M
symbols. Those distinct M symbols are transmitted with the help of M-
ary FSK modulation system. When spread spectrum modulation is to
be used, then the M-ary FSK signal is further modulated to generate
wideband signal.
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FHSS CYCLES
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22. 29-03-2013
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Two versions
Fast Hopping: several frequencies per user bit (FFH) --- symbol rate
is lesser than the hop rate i.e., hop rate is faster.
Slow Hopping: several user bits per frequency (SFH) --- symbol rate
is higher than hop rate i.e., hop rate is slower.
Tb
user data
0 1 0 1 1 t
f
Td
f3 slow
f2 hopping
f1 (3 bits/hop)
Td t
f
f3 fast
f2 hopping
f1 (3 hops/bit)
t
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(EXAMPLE 2)
• FHSS uses the 2.402 – 2.480 GHz frequency range in the ISM
band.
• It splits the band into 79 non-overlapping channels with each
channel 1 MHz wide.
Transmission Frequency (GHz)
2.479
1 MHz Channels
Divided into 79
2.401
200 400 600 800 1000 1200 1400 1600
Elapsed Time in Milliseconds (ms)
Channel 1 Channel 2 Channel 78
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23. 29-03-2013
Allanki Sanyasi Rao
BANDWIDTH SHARING (EXAMPLE 3)
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(EXAMPLE 4)
• A number of channels are
allocated for the FH signal.
• Typically, there are 2k
carrier frequencies forming
2k channels.
• The width of each channel
corresponds to the
bandwidth of the input
signal.
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24. 29-03-2013
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Allanki Sanyasi Rao
Transmitter of FH/MFSK
FSK FH/MFSK
signal signal
M-ary FSK
Mixer
Modulator
Binary data
Sequence
Frequency hops
Frequency
Synthesizer
‘t’ successive bits of
PN sequence
1 2 t generator
PN
Sequence
Generator
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25. 29-03-2013
Allanki Sanyasi Rao
Receiver of FH/MFSK
Received FSK
FH/MFSK signal signal Non coherent
Mixer M-ary FSK
detector Binary
Sequence output
Frequency hops
Frequency
Synthesizer
PN
Sequence
Generator
Allanki Sanyasi Rao
Application of FHSS – Multipath Suppression
Multipath Interference Problem
In mobile communication, the signal reach to the receiver from different
paths.
There is one direct path and many indirect paths due to reflections from
nearby objects.
The signal due to indirect paths interfere with the required signal in
amplitude as well as phase. It is called multipath fading.
How FHSS overcomes multipath effect?
The carrier frequency of the transmitted signal hops faster than the
differential time delay between the direct signal and reflected signals.
Therefore the reflected signal energy will fall in different frequency slots.
This signal energy will be treated as interference by the matched filter of the
receiver. It is then filtered out and only signal from direct path is available at
the output.
The hopping rate must be fast enough to eliminate interference due to small
time delays between direct and reflected paths.
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26. 29-03-2013
Allanki Sanyasi Rao
Synchronization in Spread Spectrum Systems
Spread spectrum systems are essentially synchronous. The PN sequences
generated at the receiver and the transmitter must be same and locked to
each other so that the transmitted signal can be extracted.
The synchronization of the spread spectrum systems can be considered in
two parts:
•Acquisition
•Tracking
The acquisition means initial synchronization of the spread spectrum signal.
The tracking starts after acquisition is complete. The tracking maintains the
PN generator at the receiver in synchronism with the transmitter.
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Acquisition of DS signal using Serial search
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27. 29-03-2013
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The received signal is correlated with the generated PN sequence. This
cross correlation is performed over the time interval of NTC.
The output of the Correlator is compared with a threshold. If it exceeds
the threshold, then the required signal is obtained. If the threshold is
not exceeded, then the PN generator output is advanced by half chip
duration ( 1 T ) and the correlation is performed.
C
2
The output of the Correlator is again compared with the threshold and
the procedure is repeated.
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Acquisition of FH signal using Serial search
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28. 29-03-2013
Allanki Sanyasi Rao
The VCO consists of frequency synthesizer, PN generator and clock
generator. The received signal is correlated with the output of VCO.
The tuned filter passes only the intermediate frequency f0.
The envelop detector generates the output which is compared with the
threshold voltage.
When the input frequency and frequency of VCO are same, then output
of threshold detector is high and the clock generator starts running
continuously. Then the signal is said to have acquired and tracking
starts.
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