chapter 03 - spread spectrum technology

7/31/2019 Chapter 03 - Spread Spectrum Technology

1/33

ITNW 1351

Fundamentals of Wireless

LANsChapter 3

Spread Spectrum Technology


2/33

2

www.ntia.doc.gov/osmhome/allochrt.pdf

http://www.fact-index.com/t/ty/types_of_radio_emissions.html#Bandwidth

http://www.kmj.com/proxim/pxhist.html
http://www.ntia.doc.gov/osmhome/allochrt.pdfhttp://www.fact-index.com/t/ty/types_of_radio_emissions.htmlhttp://www.kmj.com/proxim/pxhist.htmlhttp://www.kmj.com/proxim/pxhist.htmlhttp://www.fact-index.com/t/ty/types_of_radio_emissions.htmlhttp://www.ntia.doc.gov/osmhome/allochrt.pdf


3/33

3

Narrowband Transmission

Frequency

P

o

w

e

r

1. Uses only enoughfrequency to carry

data (& NO MORE)

2. High peak power

(more power required

to use a smaller

frequency range)

3. Signal must stand

out above inherent

RF noise level (noise

floor) in order to be

received

4. Can be jammed

easily

5. Can experience

interference easily


4/33

4

Narrowband Transmission

AMBroadcast

Example:

10 kHz (5 kHz above &

5 kHz below fC)

fC

1340 kHz

1345 kHz1335 kHz


5/33

5

Spread Spectrum Technology

Frequency

P

o

w

e

r

1. Information can besent using

narrowband carrier

signal spread out

over a much larger

frequency range

2. Probability of signal

corruption or

jamming is greatly

reduced

3. Peak power is very

low

4. Signal looks like

noise to most

receivers

three radio bands for

transmission under 1 Watt of

power:902-928 MHz

2400-2483.5 MHz

& 5752.5-5850 MHz


6/33

6

Narrowband vs. Spread Spectrum

Technology

Narrow Band:

1. High peak

power

2. Interference

and jamming

a problem

Spread

Spectrum

Technology

1. Wide band

2. Low peak

power

3. Looks likenoise


7/33

7

Uses of Spread Spectrum

1950s & 1960s - military

Since 1980s

Cordless phones Global positioning systems (GPS)

Digital cellular telephony (code division

multiple access CDMA)

Personal communications system (PCS)


8/33

8


Wireless LANS (WLANs) Wireless personal area network (WPAN)

Bluetooth technology

Wireless metropolitan area network(WMAN) Highly directional, high-gain antennas for long

distance

High-speed RF links with relatively low power Wireless wide area network (WWAN)

See WMAN above


9/33

9


The most common uses of spread

spectrum technology today lie in a

combination of wireless 802.11-

compliant LANs and 802.15-compliant

Bluetooth devices

The two function differently, play within

the same FCC rules, but can interfere witheach other


10/33

10

Bluetooth

The most popular WPAN technologies

Specified by the IEEE 802.15 standard

Hops approximately 1600 times persecond

Other technologies frequency-hopping

HomeRF 2.0 approximately 50 times persecond

802.11 WLAN 5-10 times per second


11/33

11

WMANs

Wireless links that span an entire cityusing high-power point-to-point links tocreate a network

Use licensed frequencies instead of theunlicensed frequencies used withWLANs

For frequency control i.e. organization willnot have to worry about an interferingnetwork

Same applies to WWANs


12/33

12

FCC Specifications

Has specified two types of spread

spectrum technology

1. Direct Sequencing

2. Frequency Hopping

Title 47 Telegraphs, Telephones,

and Radiotelegraphs Part 15

Wireless LAN devices are called part 15

devices


13/33

13

Frequency-hopping Spread

Spectrum (FHSS)

Uses frequency agility to spread data over a

minimum of 75 and a maximum of 79 carrier

frequencies

Frequency agility = a radios ability tochange transmission frequency abruptly

within the usable RF band

2.4 GHz Industrial Scientific Medical (ISM)band

The band is 83.5 MHz wide (802.11)


14/33

14

How FHSS Works

A carrier changes frequency (hops)according to a pseudo-randomsequence

A list of several frequencies to which thecarrier will hop at specified intervalsbefore repeating pattern

Carrier remains at a certain frequencyfor a specified time (dwell time), then Uses small amount of time to hop to the

next frequency (hop time)


15/33

15

How FHSS Works

When the list of frequencies has been

exhausted, the transmitter repeats the

sequence

The process of repeating the sequence

continues until the information is

completely received

The receiver radio is synchronized to

the transmitting radios hop sequence


16/33

16

How FHSS Works

F

r

e

q

u

e

n

c

y

Elapsed Time


17/33

17

Frequency-hopping is not totally immune

to narrowband interference

If a particular frequency is interfered with, only

that portion of the spread spectrum signal

would be lost Lost data would be retransmitted

In order for a frequency-hopping system tobe 802.11, it must operate in in the 2.4

GHz ISM band


18/33

18

Frequency-hopping systems operate using

a specified hopping pattern called achannel

26 hop patterns or a subset thereof

Synchronized radios

12 co-located systems as a max


19/33

19

Direct Sequence Spread Spectrum

(DSSS)

Vast majority of wireless LAN

equipment uses this technology

A method of sending data in which the

tx and rx systems are both on a 22

MHz-wide set of frequencies

Wide channel = more data at higher rate


20/33

20

DSSS

2.401

GHz

2.473

GHz

P

ow

e

r

Frequency

1. 11 Channels

2. Each channel is 22 MHz wide

3. Center frequencies are 5 MHz apart


21/33

21

DSSS Channel 1 Example

fC

= 2.412 GHz

11 MHz 11 MHz

22 MHz

2.401 GHz 2.423 GHz


22/33

22

fC = 2.412 GHz

2.401 GHz 2.423 GHz

fC = 2.417 GHz

2.406 GHz 2.428 GHz

5 MHz

DSSS Ch 1 & Ch 2 Spacing


23/33

23

DSSSCh #

FCCfC (GHz)

ETSI

fC (GHz)

1 2.412 N/A

2 2.417 N/A

3 2.422 2.422

4 2.427 2.427

5 2.432 2.432

6 2.437 2.437

7 2.442 2.442

8 2.447 2.447

9 2.452 2.452

10 2.457 2.457

11 2.462 2.462


24/33

24

DSSS Nonoverlapping

channels

P

o

w

e

r

Frequency

22 MHz 3 MHz

Ch 1 Ch 6 Ch 11


25/33

25

Comparing FHSS & DSSS

Factors to consider:

Narrowband interference present in environment

Co-location for failover, load balancing, or added

capacity Cost of FHSS & DSSS systems

Equipment compatibility and availability

Data rate and throughput

Security

Standards support


26/33

26


Narrowband interference

FHSS = greater resistance

DSSS systems affected more because of the use of

22 MHz contiguous bands instead of 79 MHz usedby FHSS


27/33

27


Cost

DSSS equipment cost much less than FHSS

Because of rapid adoption in the

marketplace


28/33

28


Co-location for failover, load balancing, and

added capacity

FHSS has advantage because of its

ability for many more frequency-hoppingsystems to be co-located than DSSS

79 discrete carrier frequencies

DSSS has maximum co-location of threeaccess points


29/33

29


Equipment compatibility & availability

Wireless Ethernet Compatibility Alliance

(WECA) tests 802.11b compliant DSSS

LAN equipment to insure compatibility

and interoperation (Wi-Fi standard)

No such tests for FHSS


30/33

30


Data rate and throughput

Frequency-hopping slower than DSSS

Both systems have a throughput (actual

data sent) of about of their data rate

Due to interframe spacing between data

frames for control signals etc.

No data sent when FHSS is changingfrequencies

Expect 5 to 6 Mbps on the 11 Mbps setting


31/33

31


Security???

Standard sets of hop sequences and

compliance with IEEE or WLIF make

breaking code of hop sequences fairlyeasy

The radio broadcasts the channel

number in clear text with each beacon

MAC address of AP can be see with

each beacon


32/33

32


Standards support

DSSS has gained wide acceptance

because of low cost, high speed, and

Wi-Fis interoperability standards


33/33

33

THE

END

chapter 03 - spread spectrum technology

Documents