Spread spectrum modulation

February 2005 Copyright 2005 All Rights Reserved 1

SPREAD SPECTRUM MODULATION

UNIT-3

PART-2

Copyright 2005 All Rights Reserved 2February 2005

Define spread spectrum technologies and how they are used

Describe modulation and the different data rates Explain and compare FHSS, DSSS List the factors that impact signal throughput and

range

OBJECTIVES

Upon completion of this chapter you will be able to:


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


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)


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)


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


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


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


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 .

The redundant information bits are called Pseudorandom Numbers (PN).


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).

The Correlator reverses this process in order to recover the original data.


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 12.412 GHz

Channel 62.437 GHz

Channel 112.462 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.


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.


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.


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.

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.


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 Data0 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


Direct Sequence Spread Spectrum Contd


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.


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.



To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits..


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.



To transmit 11 Mbps 8 data bits is mapped into 8 CCK chipping bits.


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.


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




90 Phase Shift

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

Data Rate Encoding Modulation


FHSS


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.


Channel 1 Channel 2 Channel 78

Elapsed Time in Milliseconds (ms)

200 400 600 800 1000 1200 1400 1600

2.401

2.479

Tra

ns

mis

sio

n F

req

ue

nc

y (

GH

z)

Div

ided

into

79

1 M

Hz

Ch

ann

els

Frequency Hopping Spread SpectrumAn Example of a Co-located Frequency Hopping System


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.


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.


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


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.


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.


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.


Co-location Comparison

1 5 10 15 20

10

20

30

40

Number of Co-located Systems

11 Mbps DSSS3 Mbps FHSS (sync.)

3 Mbps FHSS (no sync.)

Da

te R

ate

in M

bps


End of Lecture

Education

Spread spectrum modulation