802-11 Modulação

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IEEE 802.11 – MAC/PHY

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802.11

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Nonpersistent / x-persistent CSMA Protocols

• Nonpersistent CSMA Protocol:Step 1: If the medium is idle, transmit immediately

Step 2: If the medium is busy, wait a random amount of time andrepeat Step 1

– Random backoff reduces probability of collisions – Waste idle time if the backoff time is too long

• 1-persistent CSMA Protocol:Step 1: If the medium is idle, transmit immediately

Step 2: If the medium is busy, continue to listen until mediumbecomes idle, and then transmit immediately

– There will always be a collision if two nodes want to retransmit

(usually you stop transmission attempts after few tries)

• p-persistent CSMA Protocol:

Step 1: If the medium is idle, transmit with probability p, and delay for worstcase propagation delay for one packet with probability (1-p)

Step 2: If the medium is busy, continue to listen until mediumbecomes idle, then go to Step 1

Step 3: If transmission is delayed by one time slot, continue with Step 1

– A good tradeoff between nonpersistent and 1-persistent CSMA

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How to Select Probability p ?

• Assume that N nodes have a packet to send and the mediumis busy

• Then, Np is the expected number of nodes that will attempt totransmit once the medium becomes idle

• If Np > 1, then a collision is expected to occur

Therefore, network must make sure that Np < 1 to avoidcollision, where N is the maximum number of nodes that canbe active at a time

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Throughput

0 1 2 3 4 5 6 7 8 9

G

1.0

0.9

0.8

0.7

0.6

0.50.4

0.3

0.2

0.1

0

S

Aloha

Slotted Aloha

1-persistent CSMA

0.5-persistent CSMA

0.1-persistent CSMA

0.01-persistent CSMA

Nonpersistent CSMA

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802.11

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802.11

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IEEE 802.11 MAC Functionality

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802.11

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802.11 Specifications

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IEEE 802.11 features

• ACK protocol

• Medium reservation (RTS/CTS)• Fragmentation

• Multi-channel roaming

• Automatic data-rate fall-back• Cell size / Multi-rate applications

• Power Management

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Accessing the medium CSMA/CD

• Adapters that can detect collisions (e.g. Ethernet adapters)

– Carrier Sensing: listen to the media to determine if it is free

– Initiate transmission as soon as carrier drops

– When collision is detected station defers

– When defer timer expires: repeat carrier sensing and starttransmission

station C

station B

station A

CRS

collision

CRS

CRS

CRSdefer

defer

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Accessing the medium CSMA/CA

• Wireless LAN adapters cannot detect collisions:

– Carrier Sensing - listen to the media to determine if it is free

– Collision Avoidance - minimize chance for collision by starting(random) back-off timer, when medium is sensed free, and prior to

transmission

deferstation C

station B

station A

CRS

CRS

defer

defer

CRS CRS

defer

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CSMA/CA with MAC - level Acknowledgment

Message

ACK

• Collisions still can occur (interference; incapability of sensing other carrier)

– IEEE 802.11 defines “low-level” ACK protocol

– Provides faster error recovery – Makes presence of high level error recovery less critical

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• ACK protocol





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“Hidden stations”: the problem

• Situation that occurs in larger cells (typical outdoor)

– Loss of performance – Error recovery required

BA C

A sends to BC doesn’t detect that, so C might also start sending to B

Collision of messages at B: both messages lost

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“Hidden terminal”: the problem

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“Hidden stations”: the solution

• IEEE 802.11 defines:

– MAC level RTS/CTS protocol (Request to Send / Clear to Send)

– Can be switched off to reduce overhead (when no hidden nodes exist) – More robustness, and increased reliability

– No interruptions when large files are transmitted

A B

RTS: I want to send to B 500 bytes

CTS: OK A, go ahead, so everybody quiet

Data: the 500 bytes of data from A to B

ACK: B received the data OK, so an ACK

C

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• ACK protocol





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Message fragmentation

• IEEE 802.11 defines:

– MAC level function to transmit large messages as smaller frames (user

definable)

– Improves performance in RF polluted environments

– Can be switched off to avoid the overhead in RF clean environments

A hit in a large frame requires re-transmission of a large frame

Fragmenting reduces the frame size and the required time to re-transmit

Hit

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• ACK protocol





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Multi-channel roaming

• IEEE 802.11 systems, support multi-channel roaming

– Access points are set to a fixed frequency – Stations do not need to be configured for a fixed frequency

– Stations switch frequency when roaming between access points

– Stations “associate” dynamically to the access point with best

signal, on power on

• This implies

– Easier configuration

– Faster installation

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Multi-channel roaming

Channel 1

Channel 6

Channel 11

Channel 1

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• ACK protocol





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Automatic rate select

• Wireless PC Card, dynamically switches data-rate

– Fall back to lower data-rate when communications qualitydecreases

• out of range situations

• Interference

– Fall-back scheme:

• 11 Mbps, 5.5 Mbps, 2 Mbps, 1 Mbps

• This implies

– Operating at larger distances

– Robustness in RF polluted areas

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Automatic rate select

• PC Card is capable of supporting different data-rates

“simultaneously”: – e.g. operates at “High” speed in communication to nearby

station and at “Low” speed to station that is further away.

• Data rate capability is maintained in “station associationtable”

• Speed of IEEE Management - and Control frames use

fixed speed determined as “IEEE Basic Rates”, and

controlled by “Multi-cast Rate parameter”.

IEEE 802 11 f t

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• ACK protocol

• Medium reservation (RTS/CTS)

• Fragmentation


• Automatic data-rate fall-back

• Cell size / Multi-rate applications• In-cell relay


• Wired Equivalent Privacy (WEP)• Wireless Distribution System (WDS)

C

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Cell size / Multi Rate applications

• Cell-size can be influenced by “Distance between APs” parameter:

– Distance between APs = Large -> large cell

– Distance between APs = Medium -> medium size cell

– Distance between APs = Small -> small cell

• Cell-size influences capacity per station in the cell

– small cell physically accommodates smaller number of stations than

large cell

– bandwidth per station in small cell greater than in large cell

• Cell size influences data-rate

– larger distance between station and access-point may lead to lower

data-rate


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• Mixture of cell-sizes accommodate mixed applications:

– Office workers:

• High physical station density

• High bandwidth requirement

• Small cell operating at high data rate

• Distance between APs is small – Warehouse operations (such as forklift truck)

• Low physical station density

• Low bandwidth requirement (transaction processing)

• Large cell operating at low data rate• Distance between APs is large

Multi Rate applications

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Multi Rate applications

1 Mbits/sec

2 Mbits/sec

IEEE 802 11 features

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• ACK protocol



• Automatic data-rate fall-back

• Cell size / Multi-rate applications


P M t

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Power Management

• IEEE 802.11, supports power management:

– nothing to send: station in sleep mode – out-bound traffic stored in Access Point (out-bound = from AP to

STA)

– station wake up only for Traffic Information Map (TIM)

– if messages: stay awake to receive them• This implies:

– Prolonged battery life

– Increase usability in hand-held equipment

– Works best in application that have limited bandwidthrequirements (transaction processing)

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IEEE 802.11 – PHY

802 11 Specifications

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802.11 Specifications

802 11

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802.11

802 11 - Technical Comparison

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802.11 Technical Comparison

20 MHz spacing25/30 MHz spacing3 Channels

25/30 MHz spacing3 Channels

Channelization

5.150 – 5.2505.25 0– 5.3505.725 – 5.825 GHz

2.4 – 2.4835 GHz2.4 – 2.4835 GHzFreq. Band

6, 9, 12, 18, 24, 36, 48,54 Mbps

1, 2, 5.5, 11 , 6, 9, 12, 18, 24,36, 48, 54 Mbps

1, 2, 5.5, 11 MbpsData Rate

64-QAM-OFDM16-QAM-OFDMQPSK-OFDMBPSK-OFDM


CCK (8 complex chipspreading)

Modulation

CSMA/CACSMA/CACSMA/CAAccess Method

IEEE 802.11aIEEE 802.11gIEEE 802.11bNetwork Std

802.11b (and g) Physical Layer Channels

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802.11b (and g) Physical Layer Channels

Limites de Potência (EIRP):

· 802.11b/g (2.4 GHz): 30dBm / 1000mW

Obs: Resolução Anatel 365/2004:

Na faixa de radiofreqüências de 2400 - 2483,5 MHz,

potência e.i.r.p. limitada a 400 mW, em localidadescom população superior a 500.000 habitantes.

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DSSS Channels (802 11)

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DSSS Channels (802.11)

5 MHz offset between each channel.

3 Non-Overlapping Channels exist in US: 1, 6, and 11

These are very important when doing Access Point layout and Site Surveying.

Channels- 802.11b

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Spectrum: 83MHz

Channels: Three 22MHz stationary channels. Only 3 non-overlapping.

Speeds: 1, 2, 5.5, and 11 Mbps data rate

1 2 3 4 5 6 7 8 9 10 11

24002483

Notes: With direct sequence the energy is spread out over a wide area of the band. 802.11b channels have a bandwidth of 22Mhz. This

will allow 3 non-overlapping, non-interfering channels to be used in the same area. This is also the 802.11 channel scheme. If there is a

severe signal interference in one area, it is possible to change to another channel and totally avoid the interference. Changing channels

does not happen automatically in DS, and must be done with re-configuration. Note actual throughput is much less due to protocol

overheads – best case 6Mbps.

Wi-FiWi-Fi

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Wi Fi

802.11b/g

Three non-overlapping channels in 2.4 – GHz band (20MHz Wide)

CH 1CH 1 CH 11CH 11CH 6CH 6

DSSS

Channel Setup

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Channel 1 Channel 11

Channel 1Channel 6

Channel 6

Channel 11

Channel 11

Channel 11Channel 6

Channel 1

Site Survey Channel Mapping

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Channel 1

Channel 6

Channel 11

Channel 1

Channel 6

Channel 11

Channel 11

Channel 1

Channel 6

Channel 11

Notes: Deciding on the placement and number of access points is the first critical step in determining proper coverage. Very few gaps in the

coverage should be left since those will be dead air and the client could lack connectivity in those locations. Bandwidth requirements do have an

impact on the coverage areas; that will be discussed in an upcoming slide.

The second critical area is to map out the channel assignments and make sure that there is minimal overlap between channels that cover the same

frequency. Channels 1, 6 and 11 do not have overlapping frequencies and are normally used for roaming applications with Direct SequenceAccess Points.

802.11b Coverage

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1 Mbps DSSS

5.5 Mbps DSSS

11 Mbps DSSS

2 Mbps DSSS

Notes: 802.11b coverage at 1 and 2 Mbps is identical to the other 2 Mbps products with the added benefit of support for 5.5 and 11 Mbps.

When compared to FH, A 2Mb FH product will typically cover what a DS will cover while running 5.5 Mbps. A DS system also has the

ability to datarate shift when moving, allowing the same person operating at 11Mbps, then move to 5.5, 2, and finally still communicate at the

outside ring at 1 Mbps. This rate shifting happens without losing connection, and without any interaction from the user.

30mW Cell Size Comparison

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30 milliWatt client and

Access Point range capabilities

11 Mbps DSSS

25 – 30 meter radius

5.5 Mbps DSSS30 – 60 meter radius

2 Mbps DSSS60 – 90 meter radius

Notes: Default power/coverage. UT environments don’t tend to be typical.

Maximum coverage for every cell isn’t necessarily a goal. Our biggest problem is that APs hear one another, but don’t provide good

signal. So, for decent coverage we need more APs, but then we run into problems of overlapping channels. Even if that wasn’t the

problem, you might want more bandwidth because there are lots of people in an area to cover.

By adjusting transmitter power, you can change the coverage.

Site Survey Bandwidth Layout

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2 Mbps 2 Mbps 2 Mbps 2 Mbps 2 Mbps


5.5 Mbps 5.5 Mbps 5.5 Mbps 5.5 Mbps 5.5 Mbps

5.5 Mbps 5.5 Mbps 5.5 Mbps 5.5 Mbps 5.5 Mbps



Notes: Bandwidth requirements are also factors in coverage mapping, since the distance from an Access Point has a bearing on the available

bandwidth. The above example provides for seamless roaming, but not at a constant speed. Here, we would take advantage of our Multirate

technology and step down in bandwidth, in order to gain greater coverage distances with a single Access Point. If 11 Mbps is required

everywhere, then the Access Points would need to be relocated so that ONLY the “red” 11 Mbps circles are touching each other. This would

require a greater amount of Access Points, but consistent bandwidth would be achieved. Notice that the data rate decreases as the coverage

distance increases.

Coverage areas don’t line up like this in reality. Nor are they flat, but rather three dimensional.

802.11a Physical Layer Channels

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Channel

Identifier

Frequency

(MHz)

34 5170

36 518038 5190

40 5200

42 5210

44 5220

46 523048 5240

52 5260

56 5280

60 5300

64 5320

149 5745

153 5765

157 5785

161 5805

802.11a

Limites de Potência (EIRP):

·

· 802.11a (5.15-5.25GHz): 17dBm / 50mW

· 802.11a (5.25-5.35GHz): 24dBm / 250mW

· 802.11a (5.725-5.825GHz): 30dBm / 1000mW

Nota – ver canais selecionados em slide adiante

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802.11a

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Spectrum (US*):

50mW from 5.150 – 5.250 GHz

250mW from 5.250 - 5.350 GHz

1W from 5.725 – 5.825 GHz

Speeds:

6, 12, and 24Mbps for compliances

54Mbps+ expected

Channels:

20 MHz channels

Vendors? 8 - 15

Notes: Spectrum: More spectrum is available at a higher rate

yielding more throughput than 802.11b. In the US, all three ranges

are available. However in other countries only the first two, or only

the first is available. It is unclear how this will effect demand for

products and how many channels will be provided. At 5GHz, these

signals will not travel as far.

As faixas para WLAN e WPAN

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!"#

$# % & ' !$% ( )# %*+,- %*+.#/" 0*+ ,-%*+.1 0*+

" 2 3 "+ 4" 5678# 0*+# &+ &" /"


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Frequency Channel

Number

Transmit

Frequency

Maximum Transmit

Power

U-NII lower band 36 5.180 GHz 50mW (2.5mW/MHz)40 5.200 GHz

44 5.220 GHz

48 5.240 GHz

U-NII middle band 52 5.260 GHz 250mW (12.5mW/MHz)

56 5.280 GHz

60 5.300 GHz

64 5.320 GHz

U-NII upper band 149 5.745 GHz 1000 mW (50mW/MHz)

153 5.765 GHz

157 5.785 GHz

161 5.805 GHz

802.11a (EUA e Américas)


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OFDM for Mobile Communications

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OFDM represents a different system design approach. It can be thought of as acombination of modulation and multiple access schemes that segments a communications channel insuch a way that many users can share it. Whereas TDMA segments according to time, and CDMAsegments according to spreading codes, OFDM segments according to frequency. It is a technique that

divides the spectrum into a number of equally spaced tones, and carries a portion of a user’sinformation on each tone. A tone can be thought of as a frequency, much in the same way that eachkey on a piano represents a unique frequency. OFDM can be viewed as a form of frequency divisionmultiplexing (FDM). However, OFDM has na important special property that each tone is orthogonalwith every other tone. FDM typically requires there to be frequency guard bands between thefrequencies so that they do not interfere with each other. OFDM allows the spectrum of each tone tooverlap, and since they are orthogonal, they do not interfere with each other. By allowing the tones tooverlap, the overall amount of spectrum required is reduced.

Wi-FiWi-Fi

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802.11a OFDM

Eight non-overlapping channels in lower 5-GHz band

Module contents

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• Technologies overview

• Spread Spectrum – Direct Sequence

– Frequency Hopping

• Modulation

– DBPSK/DQPSK

– CCK

OSI Reference Model: PHY

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• Network Oper. System

– Network Layer – Guarantees delivery data

• Drivers

– LLC Layer

– send/receive data

• LAN Controller

– MAC Layer

– data into/out frame

• MODEM

– Physical Layer

– frame into/out phy frame

Physical LayerPhysical Layer

IEEE: MAC LayerIEEE: MAC Layer

IEEE: LLC LayerIEEE: LLC Layer

Network LayerNetwork Layer

Wireless LAN technologies (overview)

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Wireless LAN TechnologiesWireless LAN Technologies

InfraredInfrared SpreadSpectrum

SpreadSpectrumNarrow BandNarrow Band

DirectSequence

DirectSequence

FrequencyHopping

FrequencyHopping

Wireless LAN technologies (Spread Spectrum)

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• Unlicensed usage (ISM band)

• No line of sight requirement (indoor)

• High link reliability

• Built-in transmission security

• Two techniques used:

– Direct Sequence


Standard RadioTransmission

Spread SpectrumTransmission

Frequency Spectrum (MHz)

2400 2500

Power Power

Frequency Frequency

88 103 2400FM Band

Module contents

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• Modulation

– DBPSK/DQPSK

– CCK

Multiple Access Methods: Multiple users share the available spectrum

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FREQUENCY

TIME

User 3

User 2

User 1

• Multiple users sharethe same frequencychannel sequentially

• Time slot sequencerepeats over andover

TDMA

TIME

FREQUENCY

CODE

CDMAalso known as “Spread Spectrum”

User 3

User 2

User 1

• Channel is “spread” over widefrequency band

• Many users share the samefrequency band at the same time

• Each user is assigned a unique

“code” to identify and separatethem

FREQUENCY

TIME

FDMA

1 2 3

Each user assigneda different frequency

- like ordinary radio

Spread Spectrum Technologies: DS vs. FH

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• Direct Sequence

– Each symbol is transmitted over

multiple frequencies at the same time – Very efficient (no overhead)

– Higher speed than FH at comparabledistances

– System capacity (multiple channels)

higher than FH

• Frequency Hopping

– Sequential use of multiplefrequencies

– Hop sequence and rate will vary

– “End hop waste time”

COMPLETE WAVEBAND ALLOCATED

Time

Time

Spread Spectrum Technologies: Direct Sequence transmitter

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• Spreading: Information signal (I.e. a “symbol”) is multiplied by aunique, high rate digital code which stretches (spreads) its

bandwidth before transmission.• Code bits are called “Chips”.

• Sequence is called “Barker Code”

Source andChannelCoding

RFModulator

Code

Generator

X

Multiplier

Code Bits (Chips)

Digital Signal (Bits)

FrequencySpectrum

f

“Spread” FrequencySpectrum

f

Spread Spectrum Technologies: What happens during “spreading”

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X

=

“symbol”

“Barker” sequence

Result of multiplication

Symbol time ts

“1” “0”

Chiptime tc

• Due to the multiplication of asymbol with Barker code, the“rate-of-change” increases with

a factor 11• This means that cycle rate

increases from 1 MHz to 11MHz

• In terms of spectrum thismeans that after RF modulationthe signal is spread from 2MHz bandwidth to 22 MHz

bandwidth

2 Mhz 22 Mhz

Spread Spectrum Technologies: Direct Sequence receiver

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• At the receiver, the spread signal is multiplied again by a synchronizedreplica of the same code, and is “de-spread” and recovered

• The outcome of the process is the original “symbol”

RF

Demodulator

Channeland

SourceDecoding

CodeGenerator

X

Multiplied

Code Bits (Chips)

De-SpreadSignal

f

“Spread” FrequencySpectrum

f

Digital Signal (Bits)

Spread Spectrum Technologies: De-spreading

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Data

:11 chip code

Direct Sequence

Spread Spectrum

Signal

+11

-11

+1

-1

Symboltime

• When the incoming signal isde-spread, it results in either a

positive (+) or a negative (-)“spike”

• These “spikes” arrive atintervals equal to the symboltime

• A positive spike represents a“1” symbol, a negative spikerepresents a “0” symbol

Spread Spectrum Technologies: Direct Sequence receiver - effect of echoes

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echo

echo

peak

Symboltime

• Echoes may arrive at thereceiver, fluctuations can be

noticed at positions other thanat the symbol time boundaries

• These fluctuations are ignoredas the receiver will onlyinterpret the spike at thesynchronization points(separated from each other bythe symbol time)

802.11

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Module contents

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• Modulation – DBPSK/DQPSK

– CCK

Modulation: DBPSK (Differential Binary Phase Shift Keying)

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I

Q

Bit Input Phase Change (+jωωωω)

0 0

1 π

Table 1, 1 Mb/s DBPSK Encoding Table.

Modulation: DQPSK (Differential Quadrature Phase Shift Keying)

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I

Q

Dibit pattern (d0,d1)

d0 is first in time Phase Change (+j )

00 0

01 /211

10 3 /2 (- /2)

Table 1, 2 Mb/s DQPSK Encoding Table

CCK : Turbo 11 Mbps approach

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CCK = Complementary Code Keying

• IEEE 802.11 standard for high speed• 11 and 5.5 Mbps data rates

• Outstanding high multi-path performance

• Outstanding low-SNR performance

• Seamless interoperability with existing DS

• Maintains QPSK chips at 11 MHz chip rate

• Maintains 3 frequency channels

CCK : How it works

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• Data bits are encoded to a symbol which istransmitted in the form of 8 chips

• For Data-Rate = Medium Encoding means: – mapping 2 data bits to I or Q channel (in-

Phase, Quaternary Phase)

– mapping 2 data bits to one of 4 ComplexCodewords

• For Data-Rate = High Encoding means: – mapping 2 data bits to I or Q channel (in-

Phase, Quaternary Phase)

– mapping 6 data bits to one of 64Complex Codewords

• Codewords are complex complementarycodes selected from a code set

5.5 MBps

CCK

8 chips

2 bits encoded to4 complex code

words; 2-QPSK

11 MBpsCCK

8 chips

6 bits encoded to

64 complex code

words; 2-QPSK

CCK : Operating at medium speed

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Pick One of

4 ComplexCodes *

MUX1:8

2

DATAIN

I OUT

Q OUT

1.375 MHz

8 chips clocked with 11 MHz

11 MHz

1

1

Data Rate = 4 bits/symbol * 1.375 MSps = 5.5 MBps

S c r am b l er

1

1

CCK : How it works

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Pick One of

64 ComplexCodes

MUX1:8

6

DATAIN

I OUT

Q OUT

1.375 MHz

11 MHz

1

1

Data Rate = 8 bits/symbol * 1.375 MSps = 11 Mbps

S c r am b l er

1

1

CCK : Data rates and symbol rates

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• Bit-rates:

– The 11 chips Barker sequence in Standard DSSS carries onesymbol clocked at 1MHz, which results in a symbol rate of1Msymbol/sec.

– The 8 chips sequence in CCK clocked at 1 MHz, results in asymbol rate of 1.375 Msymbol/sec (i.e. 11/8)

– At date rate = medium, 4 data bits are mapped on one symbol,which results in 5.5 Mbps (i.e. 1.375 * 4)

– At date rate = high, 8 data bits are mapped on one symbol, whichresults in 11 Mbps (i.e. 1.375 * 8)

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CCK : From DSSS BPSK to 11 Mbps CCK

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11 MBps

CCK5.5 MBps

CCK

802.11 DSSS QPSK

2MBps

Barker

QPSK

802.11 DSSS BPSK

1 MBps

Barker

BPSK

11 chips

1 MSps

1 bit used to

BPSK code word

11 chips

1 MSps

2 bits used to

QPSK code word

8 chips

1.375 MSps

2 bits encoded to

4 complex code

words; 2-QPSK

8 chips

1.375 MSps

I, QI, QI, Q

I, Q

6 bits encoded to

64 complex code

words; 2-QPSK

Technical Comparison

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20 MHz spacing25/30 MHz spacing3 Channels

25/30 MHz spacing3 Channels

Channelization

5.150 – 5.2505.25 0– 5.3505.725 – 5.825 GHz

2.4 – 2.4835 GHz2.4 – 2.4835 GHzFreq. Band

6, 9, 12, 18, 24, 36, 48,54 Mbps

1, 2, 5.5, 11 , 6, 9, 12, 18, 24,36, 48, 54 Mbps

1, 2, 5.5, 11 MbpsData Rate



CCK (8 complex chipspreading)

Modulation

CSMA/CACSMA/CACSMA/CAAccess Method

IEEE 802.11aIEEE 802.11gIEEE 802.11bNetwork Std

802.11

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Problemas de Propagação

2

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1.Attenuation: signal getsweaker with distance

3.Shadow

Zone(Dead Spot)

2.Electromagnetic

Interference(EMI) from

Other stations,Microwave ovens, etc.

Reflected Signal

Laptop Direct Signal

4. MultipathInterference

Direct and reflected signals may cancel out

Blocking

Object

Wi-Fi Multipath

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Propagação com Percurso Múltiplo

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A interferência entre os múltiplos percursos pode ser

destrutiva, dando origem a desvanecimento da portadora

modulada recebida, e consequente degradação da qualidadede transmissão (BER).

Access Point 802.11)

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Sistemas de Antenas com Diversidade

• São usados para combater a distorção e o

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• São usados para combater a distorção e odesvanecimento resultantes de propagação compercurso múltiplo.

• Utilizam duas antenas idênticas, a pequena distânciauma da outra, cobrindo a mesma área de serviço.

Exemplo: Cisco AIR-ANT3351, usada para aumentar

o alcance dos adaptadores de clientes.Ganho: 2,2 dBi

Características das Antenas

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• Ganho- medida do acréscimo de potência na direcção de máxima radiação

• Diagrama de Radiação- gráfico da distribuição espacial da potência radiada (geralmenterepresentado em dois planos, horizontal e vertical)

• Polarização- gráfico da variação temporal da intensidade do campo, num planonormal à direcção de propagação

Antenas

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Exemplos de Cobertura de Antenas

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Omni Directional Directional Patch Yagi/Parabolic

Diagrama de Radiação – Exemplo

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13.5 dBi Yagi

Cisco AIR-ANT1949 Horizontal Vertical

Line of Sight/Earth Curvature

Many obstructions may block line of sight

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Many obstructions may block line of sight.

•Topographic features such as hills or mountains

•Vegetation such as trees or vines

•Man-made objects like buildings or towers

•The curvature of the Earth.

Line of Sight disappears at 6 miles (9.65 km) due to the curvature of the earth.

Fresnel Zone

The area around the visual line-of-sight that radio waves spread out into after they

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The area around the visual line-of-sight that radio waves spread out into after they

leave the antenna. This area must be clear or else signal strength will weaken.Fresnel Zone is an area of concern for 2.4 GHz wireless systems. Although 2.4GHz signals pass rather well through walls, they have a tough time passing

through trees. The main difference is the water content in each. Walls are very dry:trees contain high levels of moisture. Radio waves in the 2.4 GHz band absorb into

water quite well.

C (h // )

Cantenna

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• Cantenna (htttp://www.cantenna.com)

–Extends the range of a client and/or access point

–Legitimate uses as well

Belkin: Wireless Pre-802.11n Router

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Linksys Wireless-G Router with SRX

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The Linksys Wireless-G Router with SRX combines

smart antenna technology with standards-basedWireless-G (802.11g) networking. By overlaying the

signals of two Wireless-G compatible radios, the

"Multiple In, Multiple Out" (MIMO) technology

effectively doubles the data rate.

Similar to Belkin's "pre N" wireless router, Linksys

doesn't claim to be "pre" anything. Like Belkin's

MIMO technology, it claims better range and speed,

reducing "dead spots" in the wireless coverage area.

The new Linksys SRX combines an 802.11g, access

point with a built-in 4-port full-duplex 10/100 Switch

and Router to share a high-speed cable or DSL

Internet connection.

." *"

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$%%%&'()

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+,,("-(

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& %& %&

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1 2 2

3$ '2 .

• “Wireless LAN war drivers routinely cruise their immediate areas in

War Driving

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ycars equipped with laptops loaded with a wireless LAN card, anexternal high-gain antenna and a GPS receiver. The wireless LAN

card and GPS receiver feed signals into freeware, such asNetStumbler, which detects APs and their identifiers along with theirGPS-derived locations. NetStumbler also automatically detectswhether or not built-in Wi-Fi Wired Equivalent Privacy (WEP) isturned on or off.

• More malevolent war-drivers may use Air-Snort or Kismet, toolsdesigned to crack WEP (Wired Equivalent Privacy).

War Chalking

•War Chalking is a language devoted to publicly labeling Wireless Networks

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yacross the world (this is more of a myth…not been proven this is done at all).

•http://www.warchalking.org

802-11 Modulação

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