Spread spectrum modulation

Spread spectrum modulation

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SPREAD SPECTRUM
MODULATION
UNIT-3
PART-2

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lDefine spread spectrum technologies and how
they are used
lDescribe modulation and the different data rates
lExplain and compare FHSS, DSSS
lList the factors that impact signal throughput and
range
OBJECTIVES
Upon completion of this chapter you will be able to:

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Spread Spectrum
l 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 receive.
l Spread spectrum is characterized by:
4 wide bandwidth and
4 low power
l Jamming and interference have less effect on Spread
spectrum because it is:
4 Resembles noise
4 Hard to detect
4Hard to intercept

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Spread Spectrum Use
l In the 1980s FCC implemented a set of rules making Spread
Spectrum available to the public.
4 Cordless Telephones
4 Global Positioning Systems (GPS)
4 Cell Phones
4 Personal Communication Systems
4 Wireless video cameras
l Local Area Networks
4 Wireless Local Area Networks (WLAN)
4 Wireless Personal Area Network (WPAN)
4 Wireless Metropolitan Area Network (WMAN)
4 Wireless Wide Area Network (WWAN)

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FCC Specifications
l 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.
4 Frequency Hopping Spread Spectrum (FHSS) and
4 Direct Sequence spread Spectrum (DSSS)
l

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Wireless LAN Networks
l Wireless LANs RF spread spectrum management
techniques
4 Frequency Hopping Spread Spectrum (FHSS).
* Operates in the 2.4 Ghz range
* Rapid frequency switching – 2.5 hops per second w/ a dwell
time of 400ms.
* A predetermined pseudorandom pattern
* Fast Setting frequency synthesizers.
4 Direct Sequence Spread Spectrum (DSSS)
* Operates in the 2.4 GHz range
* Digital Data signal is inserted into a higher data rate chipping code.
*A Chipping code is a bit sequence consisting of a redundant bit
pattern.
* Barker, Gold, M-sequence and Kasami codes are employed

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FCC Radio Spectrum
VLF 10 kHz - 30 kHz Cable Locating Equipment
LF 30 kHz - 300 kHz Maritime Mobile Service.
MF 300 kHz - 3 MHz Aircraft navigation, ham radio and
Avalanche transceivers.
HF 3 MHz - 30 MHz CB radios, CAP, Radio telephone,
and Radio Astronomy.
VHF 30 MHz - 328.6 MHZCordless phones, Televisions, RC
Cars, Aircraft, police and business radios.
UHF 328.6 MHz - 2.9 GHzpolice radios, fire radios, business
radios, cellular phones, GPS, paging,
wireless networks and cordless phones.
SHF 2.9 GHz - 30 GHz Doppler weather radar, satellite
communications.
EHF 30 GHz and above Radio astronomy, military systems,
vehicle radar systems, ham radio.
Band Name Range Usage

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ISM Frequency Bands
UHF ISM 902 - 928 Mhz
S-Band 2 - 4 Ghz
S-Band ISM (802.11b) 2.4 - 2.5 Ghz
C-Band 4 - 8 Ghz
C-Band Satellite downlink 3.7 - 4.2Ghz
C-Band Radar (weather) 5.25 - 5.925 Ghz
C-Band ISM (802.11a) 5.725 - 5.875 Ghz
C-Band satellite uplink 5.925-6.425 Ghz
X-Band 8-12 Ghz
X-Band Radar (police/weather)9.5-10.55 Ghz
Ku-band 12-18 Ghz
Ku-band Radar (Police) 13.5-15 Ghz
15.7-17.7 Ghz
ISM - Industrial, Scientific and Medical

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DSSS

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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).

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Direct Sequence Spread Spectrum
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.

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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.

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l Fourteen channels are identified, however, the FCC specifies only 11
channels for non-licensed (ISM band) use in the US.
l Each channels is a contiguous band of frequencies 22 Mhz wide with
each channel separated by 5 MHz.
4 Channel 1 = 2.401 – 2.423 (2.412 plus/minus 11 Mhz).
4 Channel 2 = 2.406 – 2.429 (2.417 plus/minus 11 Mhz).
l Only Channels 1, 6 and 11 do not overlap
DSSS Channels

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Spectrum Mask
l A spectrum Mask represents the maximum power output for the
channel at various frequencies.
l From the center channel frequency, 11 MHz and 22 MHZ the signal
must be attenuated 30 dB.
l From the center channel frequency, outside 22 MHZ, the signal is
attenuated 50 dB.
± ±
±

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DSSS Frequency Assignments
Channel 1
2.412 GHz
Channel 6
2.437 GHz
Channel 11
2.462 GHz
25 MHz25 MHz
l The Center DSSS frequencies of each channel are only 5 Mhz apart but
each channel is 22 Mhz wide therefore adjacent channels will overlap.
l DSSS systems with overlapping channels in the same physical space
would cause interference between systems.
4 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.
4 Channels 1, 6 and 11 are the only theoretically non-overlapping
channels.

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2.401 GHz 2.473 GHz
Channel 1 Channel 6 Channel 11
22 MHz
3 MHz
f
P
DSSS Non-overlapping Channels
4 Each channel is 22 MHz wide. In
order for two bands not to overlap
(interfere), there must be five
channels between them.
4 A maximum of three channels may
be co-located (as shown) without
overlap (interference).
4 The transmitter spreads the signal
sequence across the 22 Mhz wide
channel so only a few chips will be
impacted by interference.

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DSSS Encoding and Modulation
l DSSS (802.11b) employs two types of encoding schemes
and two types of modulation schemes depending upon the
speed of transmission.
l Encoding Schemes
4Barker Chipping Code: Spreads 1 data bit across 11 redundant
bits at both 1 Mbps and 2 Mbps
4Complementary 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.
l Modulation Schemes
4 Differential Binary Phase Shift Keying (DBPSK): Two phase
shifts with each phase shift representing one transmitted bit.
4 Differential Quadrature Phase Shift Keying (DQPSK): Four
phase shifts with each phase shift representing two bits.

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DSSS Encoding

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Barker Chipping Code
l 802.11 adopted an 11 bit Barker chipping code.
l Transmission.
4 The Barker sequence, 10110111000, was chosen to spread
each 1 and 0 signal.
* The Barker sequence has six 1s and five 0s.
4 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.
l Reception.
4A zero bit corresponds to an eleven bit sequence of six 1s.
4A one bit corresponds to an eleven bit sequence of six 0s.

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Barker Sequence
One Bit
1
0
1 0 1 1 0 1 1 1 0 0 0 1 0 1 1 0 1 1 1 0 0 0
Chipping Code
(Barker Sequence)
Original Data
Spread Data
0 1 0 0 1 0 0 0 1 1 1 1 0 1 1 0 1 1 1 0 0 0
Six 0s = 1 Six 1s = 0
One Bit
10110111000

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Direct Sequence Spread Spectrum Contd

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Complementary Code Keying (CCK)
l Barker encoding along with DBPSK and DQPSK modulation
schemes allow 802.11b to transmit data at 1 and 2 Mbps
l Complementary Code Keying (CCK) allows 802.11b to
transmit data at 5.5 and 11 Mbps.
l CCK employs an 8 bit chipping code.
4 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 chipping pattern.

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Complementary Code Keying (CCK) Contd
l To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..
l The unique 8 chipping bits is determined by the bit pattern of the 4
data bits to be transmitted. The data bit pattern is:
4 b0, b1, b2, b3
4 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.

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Complementary Code Keying (CCK) Contd
lTo transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..
lThe unique 8 chipping bits is determined by the bit pattern of the 4 data
bits to be transmitted. The data bit pattern is:
4 b0, b1, b2, b3
4 b0 and b1 determine the DQPSK phase rotation that is to be
applied to the chip sequence.
4 Each phase change is relative to the last chip transmitted.

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Complementary Code Keying (CCK) Contd
l To transmit 11 Mbps 8 data bits is mapped into 8
CCK chipping bits.
l The unique 8 chipping bits is determined by the
bit pattern of the 8 data bits to be transmitted. The
data bit pattern is:
4 b0, b1, b2, b3, b4, b5, b6 ,b7
4 b2, b3, b4 ,b5, b6 and b7 selects one unique
pattern of the 8 bit CCK chipping code out of 64
possible sequences.
4 b0 and b1 are used to select the phase rotation
sequence.

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DSSS
Modulation

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Differential Binary Phase Shift Keying (DBPSK)
0 Phase
Shift
4A Zero phase shift from the
previous symbol is interpreted as
a 0.
4A 180 degree phase shift from
the previous symbol is interpreted
as a 1.
180 degree
Phase Shift
180 degree
Phase Shift
Previous
carrier symbol

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Differential Quadrature Phase Shift Keying (DQPSK)
4A Zero phase shift from the previous
symbol is interpreted as a 00.
Previous
carrier symbol
0 Phase
Shift
4A 90 degree phase shift from the previous
symbol is interpreted as a 01.
4A 180 degree phase shift from the previous
symbol is interpreted as a 11.
4A 270 degree phase shift from the previous
symbol is interpreted as a 10.
90 Phase
Shift
180 Phase
Shift
270 Phase
Shift

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DSSS Summary
1 Barker Coding 11 chips encoding 1 bitDBPSK
2 Barker Coding 11 chips encoding 1 bit DQPSK
5.5CCK Coding 8 chips encode 8 bitsDQPSK
11 CCK Coding 8 chips encode 4 bitsDQPSK
Data Rate Encoding Modulation

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FHSS

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Frequency Hopping Spread Spectrum
l Carrier changes frequency (HOPS)
according to a pseudorandom Sequence.
4 Pseudorandom sequence is a list of frequencies. The
carrier hops through this lists of frequencies.
4 The carrier then repeats this pattern.
4 During Dwell Time the carrier remains at a certain
frequency.
4 During Hop Time the carrier hops to the next frequency.
4 The data is spread over 83 MHz in the 2.4 GHz ISM
band.
4 This signal is resistant but not immune to narrow band
interference.

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Channel 1 Channel 2 Channel 78
Elapsed Time in Milliseconds (ms)
200400 600 8001000120014001600
2.401
2.479
Transmission Frequency (GHz)
Divided into 79
1 MHz Channels
Frequency Hopping Spread Spectrum
An Example of a Co-located Frequency Hopping System

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FHSS Contd
l The original 802.11 FHSS standard supports 1 and
2 Mbps data rate.
4 FHSS uses the 2.402 – 2.480 GHz frequency range in the ISM band.
4 It splits the band into 79 non-overlapping channels with each channel
1 MHz wide.
4 FHSS hops between channels at a minimum rate of 2.5 times per
second. Each hop must cover at least 6 MHz
4The hopping channels for the US and Europe are shown below.

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FHSS Contd
l Dwell Time
4 The Dwell time per frequency is around 100 ms
(The FCC specifies a dwell time of 400 ms per carrier
frequency in any 30 second time period).
4 Longer dwell time = greater throughput.
4 Shorter dwell time = less throughput
l Hop Time
4 Is measured in microseconds (us) and is
generally around 200-300 us.

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FHSS Contd
l Gaussian Frequency Shift Keying
4 The FHSS Physical sublayer modulates the data stream using
Gaussian Frequency Shift Keying (GFSK).
4 Each symbol, a zero and a one, is represented by a different
frequency (2 level GFSK)
4 two symbols can be represented by four frequencies (4 level
GFSK).
4 A Gaussian filter smoothes the abrupt jumps between
frequencies.
f
c
+ f
d2
f
c
+ f
d1f
c
- f
d1
f
c
– f
d2
10110100
f
c

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FHSS Disadvantages
l Not as fast as a wired Lan or the newer WLAN
Standards
l Lower throughput due to interference.
4FHSS is subject to interference from other frequencies in
the ISM band because it hops across the entire frequency
spectrum.
l Adjacent FHSS access points can synchronize
their hopping sequence to increase the number of co-
located systems, however, it is prohibitively
expensive.

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FHSS vs DSSS
l DSSS is more susceptible to narrow band noise.
4 DSSS channel is 22 Mhz wide whereas
4 FHSS is 79 Mhz wide.
l The FCC regulated that DSSS use a maximum of 1 watt
of transmitter power in Pt-to-Multipoint system.
l DSSS costs less then FHSS
l FHSS can have more systems co-located than
DSSS.
4 DSSS systems have the advantage in throughput
l The Wi-Fi alliance tests for DSSS compatibility
4 No such testing alliance exists for FHSS.

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FHSS vs DSSS contd
l DSSS generally has a throughput of 5-6 Mbps
while FHSS is generally between 1-2 Mbps.
l Both FHSS and DHSS are equally insecure.
l DSSS has gained much wider acceptance due to
its low cost, high speed and interoperability.
4This market acceptance is expected to
accelerate.
4 FHSS advancement includes HomeRF and 802.15
(WPAN) (Bluetooth), however, it is expected to not
advance into the enterprise.

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Co-location Comparison
1 5 10 15 20
10
20
30
40
Number of Co-located Systems
11 Mbps DSSS
3 Mbps FHSS (sync.)
3 Mbps FHSS (no sync.)
Date Rate in Mbps

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End of Lecture