Lecture 4 - Spread Spectrum Technologies

Lecture 4 - Spread Spectrum Technologies

Spread Spectrum

Technologies

(1 September, 2006)

 Define spread spectrum technologies and how they are used

range

Objectives

Upon completion of this chapter you will be able to:

Spread Spectrum

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

spectrum because it is:

 Hard to detect

Narrowband vs Spread Spectrum

Frequency

Power

Spread Spectrum (Low Peak Power) Narrowband

(High Peak Power)

Narrow Band vs Spread Spectrum

 Uses only enough frequency spectrum to carry the signal

 High peak power

Spread Spectrum Use

Spectrum available to the public.

 Cordless Telephones

 Global Positioning Systems (GPS)

 Cell Phones

 Personal Communication Systems

 Wireless video cameras

 Wireless Local Area Networks (WLAN)

 Wireless Personal Area Network (WPAN)

 Wireless Metropolitan Area Network (WMAN)

 Wireless Wide Area Network (WWAN)

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)

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"

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

FCC Radio Spectrum

Avalanche transceivers

VHF 30 MHz - 328.6 MHZ Cordless phones, Televisions, RC

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

ISM Frequency Bands

S-Band ISM (802.11b) 2.4 - 2.5 Ghz

 C-Band Satellite downlink 3.7 - 4.2Ghz

C-Band ISM (802.11a) 5.725 - 5.875 Ghz

FHSS

Frequency Hopping Spread Spectrum

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.

Elapsed Time in Milliseconds (ms)

200 400 600 800 1000 1200 1400 1600 2.401

Frequency Hopping Spread Spectrum

An Example of a Co-located Frequency Hopping System

FHSS Contd

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

FHSS Contd

 Dwell Time

( The FCC specifies a dwell time of 400 ms per carrier

frequency in any 30 second time period)

generally around 200-300 us.

FHSS Contd

 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

01 00

fc

FHSS Disadvantages

Standards

 FHSS is subject to interference from other frequencies in the ISM band because it hops across the entire frequency spectrum.

their hopping sequence to increase the number of located systems, however, it is prohibitively

co-expensive.

DSSS

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

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

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).

 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

DSSS Channels

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

±

DSSS Frequency Assignments

Channel 1

2.412 GHz

Channel 6 2.437 GHz

Channel 11 2.462 GHz

25 MHz

25 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

Channel 1 Channel 6 Channel 11

 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

DSSS Encoding and Modulation

DSSS Encoding and Modulation

and two types of modulation schemes depending upon the speed of transmission

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.

 Differential Binary Phase Shift Keying (DBPSK): Two phase

shifts with each phase shift representing one transmitted bit

DSSS Encoding

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.

 A zero bit corresponds to an eleven bit sequence of six 1s.

Direct Sequence Spread Spectrum Contd

 Complementary Code Keying (CCK)

schemes allow 802.11b to transmit data at 1 and 2 Mbps

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

 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:

 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

 Complementary Code Keying (CCK) Contd

CCK chipping bits.

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

 DSSS Modulation

 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

 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

180 Phase Shift

270 Phase Shift

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

FHSS vs DSSS

of transmitter power in Pt-to-Multipoint system

DSSS.

 No such testing alliance exists for FHSS.

FHSS vs DSSS contd

while FHSS is generally between 1-2 Mbps.

its low cost, high speed and interoperability

accelerate.

(WPAN) (Bluetooth), however, it is expected to not

advance into the enterprise

OFDM

802.11a

 Orthogonal Frequency Division Multiplexing (OFDM).

Infrastructure (UNII).

802.11a Network Channel Assignments

Area Frequency Band Channel Center Frequency

 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.

802.11a Channels

 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

OFDM Modulation

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

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.

 The are two major types of modulation schemes: Analog and Digital

Amplitude

Frequency

Phase

Sine Wave

Analog Modulation

height of the carrier wave.

number of oscillation (waves) per

second

starting point of the wave.

Change in Frequency Change in Amplitude

Digital Modulation

1 = 180 0 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

Phase Modulation Extended

the starting point of the wave.

Change in Phase

1 = 180 0 Phase Change

0 = No Phase Change

90 0

180 o 1 0 0 o

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

QUADRATURE AMPLITUDE MODULATION (QAM)

 Quadrature Phase Shift Keying (QPSK)

extends this technique to transmit two bits for

every phase shift.

0000

0001 0011

0010 0110

0111

0101 0100

1100 1101

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.