Direct sequence spread spectrum (DSSS) spreads data over a wide frequency band by combining the data with a redundant bit sequence called a chipping code. It can transmit at 1, 2, 5.5, and 11 Mbps using different encoding and modulation schemes. Barker coding maps each data bit to an 11-bit sequence for 1-2 Mbps using DBPSK or DQPSK. Complementary code keying maps groups of data bits to unique 8-bit sequences for 5.5-11 Mbps using DQPSK phase shifts. DSSS occupies a 22 MHz band but can tolerate some interference due to its processing gain from spreading the signal.

1. ECE618: Mobile and
Wireless Communication
Unit 2 Spread Spectrum
Technologies
By Dr. Ghanshyam Singh

2. Objectives
Define spread spectrum technologies and how they
are used
Describe modulation and the different data rates
Explain and compare FHSS, DSSS and OFDM
List the factors that impact signal throughput and
range
Upon completion of this topic you will be able to:

3. Multi-carrier & Direct spread

4. Spread Spectrum
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.
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

5. Narrowband vs Spread Spectrum
Frequency
Power
Spread Spectrum
(Low Peak Power)
Narrowband
(High Peak Power)

6. 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

7. Spread Spectrum Use
In the 1980s FCC (Federal Communications Commission)
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)

8. 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

9. Wireless LAN Networks
Wireless LANs RF spread spectrum management techniques
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.
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
Orthogonal Frequency Division Multiplexing (OFDM)
Operates in both the 5 Ghz and 2.4 GHz range with a data rate of between 6
and 54 Mbps.
802.11a divides each channel into 52 low-speed sub-channels
48 sub-channels are for data while the other 4 are pilot carriers.
The modulation scheme can be either BPSK, QPSK or QAM depending
upon the speed of transmission.

10. 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 MHZ Cordless phones, Televisions, RC
Cars, Aircraft, police and business radios.
UHF 328.6 MHz - 2.9 GHz police 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

11. 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

12. FHSS

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

14. Channel 1 Channel 2 Channel 78
Elapsed Time in Milliseconds (ms)
200 400 600 800 1000 1200 1400 1600
2.401
2.479
TransmissionFrequency(GHz)
Dividedinto79
1MHzChannels
Frequency Hopping Spread Spectrum
An Example of a Co-located Frequency Hopping System

15. FHSS Contd
The original 802.11 FHSS standard supports 1 and
2 Mbps data rate.
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.
FHSS hops between channels at a minimum rate of 2.5 times per
second. Each hop must cover at least 6 MHz
The hopping channels for the US and Europe are shown below.

16. FHSS Contd
Dwell Time
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).
Longer dwell time = greater throughput.
Shorter dwell time = less throughput
Hop Time
Is measured in microseconds (us) and is
generally around 200-300 us.

17. FHSS Contd
Gaussian Frequency Shift Keying
The FHSS Physical sublayer modulates the data stream using
Gaussian Frequency Shift Keying (GFSK).
Each symbol, a zero and a one, is represented by a different
frequency (2 level GFSK)
two symbols can be represented by four frequencies (4 level
GFSK).
A Gaussian filter smoothes the abrupt jumps between
frequencies.
fc + fd2fc + fd1fc - fd1fc – fd2
10110100
fc

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

19. DSSS

20. Direct Sequence Spread Spectrum
Spread spectrum increases the bandwidth of the signal
compared to narrow band by spreading the signal.
There are two major types of spread spectrum techniques:
FHSS and DSSS.
FHSS spreads the signal by hopping from one frequency to
another across a bandwidth of 83 Mhz.
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).

21. Direct Sequence Spread Spectrum
DSSS works by combining information bits (data signal) with
higher data rate bit sequence (pseudorandom number (PN)).
The PN is also called a Chipping Code (eg., the Barker chipping
code)
The 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.

22. Signal Spreading
The Spreader employs an encoding scheme (Barker or
Complementary Code Keying (CCK).
The spread signal is then modulated by a carrier employing either
Differential Binary Phase Shift Keying (DBPSK), or Differential
Quadrature Phase Shift Keying (DQPSK).
The Correlator reverses this process in order to recover the original
data.

23. Fourteen channels are identified, however, the FCC specifies only 11
channels for non-licensed (ISM band).
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
DSSS Channels

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

25. DSSS Frequency Assignments
Channel 1
2.412 GHz
Channel 6
2.437 GHz
Channel 11
2.462 GHz
25 MHz25 MHz
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.

26. 2.401 GHz 2.473 GHz
Channel 1 Channel 6 Channel 11
22 MHz
3 MHz
f
P
DSSS Non-overlapping Channels
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).
The transmitter spreads the signal
sequence across the 22 Mhz wide
channel so only a few chips will be
impacted by interference.

27. DSSS
Encoding and Modulation

28. 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.

29. DSSS Encoding

30. 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.

31. 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

32. Direct Sequence Spread Spectrum Contd

33. 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 chipping pattern.

34. Complementary Code Keying (CCK) Contd
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.

35. Complementary Code Keying (CCK) Contd
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.

36. Complementary Code Keying (CCK) Contd
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.

37. DSSS
Modulation

38. Differential Binary Phase Shift Keying (DBPSK)
0 Phase
Shift
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.
180 degree
Phase Shift
180 degree
Phase Shift
Previous
carrier symbol

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

40. DSSS Summary
1 Barker Coding 11 chips encoding 1 bit 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
Data Rate Encoding Modulation

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

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

43. 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.)
54 Mbps OFDM
DateRateinMbps

44. OFDM

45. 802.11a
IEEE 802.11a Standard.
Orthogonal Frequency Division Multiplexing (OFDM).
Operates in the 5.0 GHz band.
It Operates in the Unlicensed National Information
Infrastructure (UNII).
200 channels ( channels 1-199) spaced 5 MHz apart.
Supported data rates are 6, 9, 12, 18, 24, 36, 48, and 54,
MBps.
6, 12, and 24 are mandatory. All others are optional.
75-80 Feet
64 users /Access Point

46. 802.11a Network Channel Assignments
Area Frequency Band Channel Center Frequency
USA U-NII Lower Band 36 5.180 Ghz
(5.150-5.250 Ghz) 40 5.200 Ghz
44 5.220 Ghz
48 5.240 Ghz
USA U-NII Middle Band 52 5.260 Ghz
(5.250 – 5.350 Ghz) 56 5.260 Ghz
60 5.280 Ghz
64 5.320 Ghz
USA U-NII Upper Band 149 5.745 Gh
(5.725 – 5.825) 153 5.765 Ghz
157 5.785 Ghz
161 5.805 Ghz
NOTE: 1. U-NII : Unlicensed National Information Infrastructure.
2. 802.11a is specific to the US.

47. OFDM
A mathematical process that allows 52 channels to overlap without
losing their orthogonality (individuality).
48 sub-channel are used for data
Each sub-channel is used to transmit data
4 sub-channel are used as pilot carriers.
The pilot sub-channels are used to monitor path shift and
shifts in sub-channel frequencies (Inter Carrier Interference
(ICI)).
OFDM
OFDM selects channels that
overlap but do not interfere
with one another.
Channels are separated based
upon orthogonality.

48. 802.11a Channels
Lower UNII Band Middle UNII Band
802.11a use the lower and middle UNII 5 GHz bands to create 8 channels.
Each Channel is 20 MHz each.
Each channel is broken into 52 sub-channels with each sub-channel
300 KHz each.
48 Sub-channels are used to transmit data
4 sub-channels are used as Pilot carriers to monitor the channel
8
Channels
52
Sub-Channels
for each 8
channels
Each channel is
20 MHz wide
Lower and
Middle UNII
frequency band

49. OFDM
Modulation

50. Modulation Background
In order to properly understand OFDM modulation we need to do
a quick review of various modulation techniques.
James Clark Maxwell, 1864, first developed the idea that
electromagnetic magnetic waves arose as a combination electric
current and magnetic field – an electromagnetic wave.
Heinrich Hertz , in 1880s, developed the first Radio
Frequency device that sent and received electromagnetic waves
over the air
The name Hertz (Hz) was given to the unit of frequency
measurement representing one complete oscillation of an
electromagnetic wave. This is also called cycle per second.
Kilohertz = thousands of cycles per second
Megahertz = millions of cycles per second
Gigahertz = billions cycles per second

51. Modulation Background Contd
The oscillating electromagnetic wave, also called a sine wave, is shown below.
This wave can be used as a carrier signal to carry information.
The information can be imposed upon the carrier through a process called
modulation which is accomplished by modifying one of three physical wave
characteristic. These physical characteristics are:
Amplitude – The height of the wave
Frequency – the number of oscillation (cycles) per second.
Phase – the starting point of the wave (when compared to the starting point of
the previous wave.
There are two major types of modulation schemes: Analog and Digital
Amplitude
Frequency
Phase
Sine Wave

52. Analog Modulation
Amplitude Modulation
varies the height of the
carrier wave.
Frequency
Modulation varies the
number of oscillation
(waves) per second
Phase Modulation
changes the starting point
of the wave.
Change in
Phase
Change in
Frequency
Change in
Amplitude
1 = 1800
Phase Change
0 = No Phase Change

53. Digital Modulation
1 = 1800
Phase Change
0 = No Phase Change
Amplitude Shift
Keying (ASK) changes
the amplitude of the
carrier wave to represent
a 0 or 1.
Frequency Shift
Keying (FSK) changes
the frequency of the
carrier wave to
represent a 0 or 1.
Phase Shift Keying
(PSK) changes the
phase of the carrier wave
to represent a 0 or 1.
180 degree
phase change

54. Phase Modulation Extended
Phase Modulation
changes the starting point
of the wave.
Change in
Phase
1 = 1800
Phase Change
0 = No Phase Change
900
2700
180o
0o
1 0
Phase shift can also be represented on an x/y axis
constellation such that:
In this instance we can transmit 1 bit for every phase
shift.
This is called Binary Phase Shift Keying (BPSK) in
802.11a
π radians)1 = 1800
Phase Change (
0 = No Phase Change
π radians)1 = 1800
Phase Change (
0 = No Phase Change
BPSK

55. QUADRATURE AMPLITUDE MODULATION (QAM)
900
270
0
00
135
o
01
11 10
35
o
315
o
225
o
180
o
0
o
2 bits/phase
Quadrature Phase Shift Keying (QPSK)
extends this technique to transmit two bits for
every phase shift.
0000
0001
0011
00100110
0111
01010100
1100
1101
1111
1110 1010 1011
1001 1000
90
0
270
0
180
o
0
o
4 bits/phase
Quadrature Amplitude Modulation
(QAM) generalizes these techniques to
encode information in both phase (by
employing PSK techniques such as BPSK
and QPSK) with amplitude.
For example, in the diagram a right, each
quadrature contains 4 amplitudes (16 levels)
and can therefore transmit 4 bits per phase.
00 = 350
Phase Change
01 = 1350
Phase Change
11 = 2250
Phase Change
10 = 3150
Phase Change
QPSK
QAM

56. QAM Extended
In the diagram at right,
each quadrature
contains 8 amplitudes
(64 levels) and can
therefore transmit 6 bits
per phase.
90
0
270
0
180
o
0
o

57. Summary of OFDM Encoding/Modulation
64 Phase shifts can encode 6 bits /phase shift resulting is a transmission rate of
either 48 or 54 Mbps depending upon the number of sub-channels (R) used for error
correction.
Coding Rate (R) is the ratio of sub-channels carrying data to sub-channels
carrying error correction code. E.G., 1/2 would indicate that 24 sub-channels (1/2 X
48 = 24) are being used for error correction while the remaining 24 sub-channels are
used for data transmission.
The Length of the each Symbol is equal to number of sub-carriers times the
bits /transition. e.g., 48 X 6 = 288.

58. Summary of OFDM Encoding/Modulation