89088468 wireless communications and networks ppt

7/29/2019 89088468 Wireless Communications and Networks Ppt

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Cellular Wireless Networks

ICS 620

Fall 2003Week #9


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Cellular Network Organization

Use multiple low-power transmitters (100 W or

less)

Areas divided into cells

Each served by its own antenna Served by base station consisting of transmitter,

receiver, and control unit

Band of frequencies allocated

Cells set up such that antennas of all neighbors are

equidistant (hexagonal pattern)


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Cellular Spectrum

A band

10 MHz

333 channels

30khz

B band

10 MHz

333 channels

30khz

825 835 845

870 880 890

824 846.5 849

869A band

10 MHz

333 channels

30khz

B band

10 MHz

333 channels

30khz

Phone Transmit

Base Transmit

891.5 894

Aband

Aband

Aband

Aband

Bband

Bband

1 MHz33 chan

1.5 MHz50 chan

2.5 MHz83 chan

1 MHz

33 chan

1.5 MHz

50 chan

2.5 MHz

83 chan

20 MHz Guard


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Frequency Reuse

Adjacent cells assigned different frequencies to

avoid interference or crosstalk

Objective is to reuse frequency in nearby cells

10 to 50 frequencies assigned to each cell Transmission power controlled to limit power at that

frequency escaping to adjacent cells

The issue is to determine how many cells must

intervene between two cells using the same frequency


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Approaches to Cope with

Increasing Capacity

Adding new channels

Frequency borrowingfrequencies are taken fromadjacent cells by congested cells

Cell splittingcells in areas of high usage can besplit into smaller cells

Cell sectoringcells are divided into a number ofwedge-shaped sectors, each with their own set of

channels Microcellsantennas move to buildings, hills,

and lamp posts


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Cellular System Overview


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Cellular Systems Terms

Base Station (BS)includes an antenna, acontroller, and a number of receivers

Mobile telecommunications switching office(MTSO)connects calls between mobile units

Two types of channels available between mobileunit and BS

Control channelsused to exchange informationhaving to do with setting up and maintaining calls

Traffic channelscarry voice or data connectionbetween users


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Steps in an MTSO Controlled Call

between Mobile Users

Mobile unit initialization

Mobile-originated call

Paging Call accepted

Ongoing call

Handoff


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Additional Functions in an MTSO

Controlled Call

Call blocking

Call termination

Call drop Calls to/from fixed and remote mobile

subscriber

bil di i


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Mobile Radio Propagation

Effects

Signal strength

Must be strong enough between base station and mobile

unit to maintain signal quality at the receiver

Must not be so strong as to create too much co-channelinterference with channels in another cell using the

same frequency band

Fading

Signal propagation effects may disrupt the signal andcause errors


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Handoff Performance Metrics

Cell blocking probabilityprobability of a newcall being blocked

Call dropping probabilityprobability that a callis terminated due to a handoff

Call completion probabilityprobability that anadmitted call is not dropped before it terminates

Probability of unsuccessful handoffprobability

that a handoff is executed while the receptionconditions are inadequate


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Handoff Performance Metrics

Handoff blocking probabilityprobability that ahandoff cannot be successfully completed

Handoff probabilityprobability that a handoffoccurs before call termination

Rate of handoffnumber of handoffs per unittime

Interruption durationduration of time during ahandoff in which a mobile is not connected toeither base station

Handoff delaydistance the mobile moves fromthe point at which the handoff should occur to the

point at which it does occur

H d ff S i U d


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Handoff Strategies Used to

Determine Instant of Handoff

Relative signal strength

Relative signal strength with threshold

Relative signal strength with hysteresis Relative signal strength with hysteresis and

threshold

Prediction techniques


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Power Control

Design issues making it desirable to include

dynamic power control in a cellular system

Received power must be sufficiently above the

background noise for effective communication Desirable to minimize power in the transmitted signal

from the mobile

Reduce co-channel interference, alleviate health concerns, save

battery power

In SS systems using CDMA, its desirable to equalize

the received power level from all mobile units at the BS


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Types of Power Control

Open-loop power control

Depends solely on mobile unit

No feedback from BS

Not as accurate as closed-loop, but can react quicker tofluctuations in signal strength

Closed-loop power control

Adjusts signal strength in reverse channel based onmetric of performance

BS makes power adjustment decision andcommunicates to mobile on control channel


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Traffic Engineering

Ideally, available channels would equalnumber of subscribers active at one time

In practice, not feasible to have capacity

handle all possible load ForNsimultaneous user capacity andL

subscribers

LNblocking system


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First-Generation Analog

Advanced Mobile Phone Service (AMPS)

In North America, two 25-MHz bands allocated

to AMPS

One for transmission from base to mobile unit

One for transmission from mobile unit to base

Each band split in two to encourage

competition

Frequency reuse exploited


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Frequency Division MultipleAccess

Definition - FDMA is a multiple accessmethod in which users are assigned

specific frequency bands. The user hassole right of using the frequency bandfor the entire call duration. (Qualcomm, 1997)


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FDMA

Frequency Division Multiple Access

F

requency

Time

Chan A

Chan B

Chan C

Chan D


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AMPS Operation

Subscriber initiates call by keying in phonenumber and presses send key

MTSO verifies number and authorizes user

MTSO issues message to users cell phoneindicating send and receive traffic channels

MTSO sends ringing signal to called party

Party answers; MTSO establishes circuit and

initiates billing information Either party hangs up; MTSO releases circuit,

frees channels, completes billing

Diff B t Fi t d


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Differences Between First and

Second Generation Systems

Digital traffic channelsfirst-generation systemsare almost purely analog; second-generationsystems are digital

Encryptionall second generation systemsprovide encryption to prevent eavesdropping

Error detection and correctionsecond-generationdigital traffic allows for detection and correction,giving clear voice reception

Channel accesssecond-generation systems allowchannels to be dynamically shared by a number ofusers


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Time Division Multiple Access

Definition - TDMA is an assignedfrequency band shared among a fewusers. However, each user is allowed to

transmit in predetermined time slots.Hence, channelization of users in the sameband is achieved through separation in

time.(Qualcomm, 1997)


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TDMA

Time Division Multiple Access

F

requency

Time

Chan A

Chan B

Mobile Wireless TDMA Design


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Mobile Wireless TDMA Design

Considerations

Number of logical channels (number of time slotsin TDMA frame): 8

Maximum cell radius (R): 35 km

Frequency: region around 900 MHz Maximum vehicle speed (V

m):250 km/hr

Maximum coding delay: approx. 20 ms

Maximum delay spread (m): 10 s

Bandwidth: Not to exceed 200 kHz (25 kHz perchannel)


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GSM Network Architecture


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Mobile Station

Mobile station communicates across Um interface(air interface) with base station transceiver insame cell as mobile unit

Mobile equipment (ME)physical terminal, suchas a telephone or PCS

ME includes radio transceiver, digital signal processorsand subscriber identity module (SIM)

GSM subscriber units are generic until SIM isinserted

SIMs roam, not necessarily the subscriber devices


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Base Station Subsystem (BSS)

BSS consists of base station controller andone or more base transceiver stations (BTS)

Each BTS defines a single cell

Includes radio antenna, radio transceiver and alink to a base station controller (BSC)

BSC reserves radio frequencies, manageshandoff of mobile unit from one cell toanother within BSS, and controls paging


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Network Subsystem (NS)

NS provides link between cellular network and

public switched telecommunications networks

Controls handoffs between cells in different BSSs

Authenticates users and validates accounts Enables worldwide roaming of mobile users

Central element of NS is the mobile switching

center (MSC)

Mobile Switching Center (MSC)


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Mobile Switching Center (MSC)

Databases

Home location register (HLR) databasestoresinformation about each subscriber that belongs toit

Visitor location register (VLR) database

maintains information about subscribers currentlyphysically in the region

Authentication center database (AuC)used forauthentication activities, holds encryption keys

Equipment identity register database (EIR)keeps track of the type of equipment that exists atthe mobile station

TDMA Format Time Slot


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TDMA FormatTime Slot

Fields

Trail bitsallow synchronization of transmissionsfrom mobile units

Encrypted bitsencrypted data

Stealing bit - indicates whether block containsdata or is "stolen"

Training sequenceused to adapt parameters ofreceiver to the current path propagationcharacteristics

Strongest signal selected in case of multipathpropagation

Guard bitsused to avoid overlapping with otherbursts


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GSM Speech Signal Processing

GSM Signaling Protocol


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GSM Signaling Protocol

Architecture


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Functions Provided by Protocols

Protocols above the link layer of the GSMsignaling protocol architecture providespecific functions:

Radio resource management Mobility management

Connection management

Mobile application part (MAP)

BTS management


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Code Division Multiple Access

Definition - CDMA is a method in which usersoccupy the same time and frequencyallocations, and are channelized by unique

assigned codes. The signals are separated atthe receiver by using a correlator that acceptsonly signal energy from the desired channel.

Undesired signals contribute only to noise.(Qualcomm, 1997)

CDMA


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CDMA

Code Division Multiple Access

Code


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Capacity

CDMA has the ability to deliver 10 to 20times the capacity as FDMA for the

same bandwidth. CDMA also has acapacity advantage over TDMA by 5 to7 times.


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Advantages of CDMA Cellular

Frequency diversityfrequency-dependenttransmission impairments have less effect onsignal

Multipath resistancechipping codes used forCDMA exhibit low cross correlation and lowautocorrelation

Privacyprivacy is inherent since spreadspectrum is obtained by use of noise-like signals

Graceful degradationsystem only graduallydegrades as more users access the system


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Drawbacks of CDMA Cellular

Self-jammingarriving transmissions frommultiple users not aligned on chip boundariesunless users are perfectly synchronized

Near-far problemsignals closer to the receiverare received with less attenuation than signalsfarther away

Soft handoffrequires that the mobile acquiresthe new cell before it relinquishes the old; this ismore complex than hard handoff used in FDMAand TDMA schemes

Mobile Wireless CDMA Design


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Mobile Wireless CDMA Design

Considerations

RAKE receiverwhen multiple versions of asignal arrive more than one chip interval apart,RAKE receiver attempts to recover signals frommultiple paths and combine them

This method achieves better performance than simplyrecovering dominant signal and treating remainingsignals as noise

Soft Handoffmobile station temporarily

connected to more than one base stationsimultaneously


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Principle of RAKE Receiver

Types of Channels Supported by


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Types of Channels Supported by

Forward Link

Pilot (channel 0) - allows the mobile unit toacquire timing information, provides phasereference and provides means for signal strengthcomparison

Synchronization (channel 32) - used by mobilestation to obtain identification information aboutcellular system

Paging (channels 1 to 7) - contain messages forone or more mobile stations

Traffic (channels 8 to 31 and 33 to 63)theforward channel supports 55 traffic channels

Forward Traffic Channel


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Processing Steps

Speech is encoded at a rate of 8550 bps

Additional bits added for error detection

Data transmitted in 2-ms blocks with forward

error correction provided by a convolutionalencoder

Data interleaved in blocks to reduce effects of

errors

Data bits are scrambled, serving as a privacy mask



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Processing Steps (cont.)

Power control information inserted into traffic

channel

DS-SS function spreads the 19.2 kbps to a rate of

1.2288 Mbps using one row of 64 x 64 Walshmatrix

Digital bit stream modulated onto the carrier using

QPSK modulation scheme

ITUs View of Third-Generation


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ITU s View of Third-Generation

Capabilities

Voice quality comparable to the public switchedtelephone network

144 kbps data rate available to users in high-speedmotor vehicles over large areas

384 kbps available to pedestrians standing ormoving slowly over small areas

Support for 2.048 Mbps for office use

Symmetrical / asymmetrical data transmissionrates

Support for both packet switched and circuitswitched data services

ITUs View of Third-Generation


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ITU s View of Third Generation

Capabilities

An adaptive interface to the Internet to reflect

efficiently the common asymmetry between

inbound and outbound traffic

More efficient use of the available spectrum ingeneral

Support for a wide variety of mobile equipment

Flexibility to allow the introduction of new

services and technologies


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Alternative Interfaces


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CDMA Design Considerations

Bandwidthlimit channel usage to 5 MHz

Chip ratedepends on desired data rate, need for

error control, and bandwidth limitations; 3 Mcps

or more is reasonable Multirateadvantage is that the system can

flexibly support multiple simultaneous

applications from a given user and can efficiently

use available capacity by only providing thecapacity required for each service


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Paging & SMS

Evolution of Paging

Tone Boy, early 1960s

Tone-Voice, late 1960s

Digital Pagers, 1970s

Numeric Paging Systems

Alpha/Numeric Paging Systems


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Paging

Larger coverage area in each site

Signal, Numeric, Alpha-numeric

Marketed by coverage area.

Features--Web messaging, modemmessaging


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Paging

Current Applications

Fax Forwarding

E-Mail Forwarding

Voice Mail Notification

Automated Problem Notification

Two-way Paging


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Wireless Local Loop

Wired technologies responding to need forreliable, high-speed access by residential,

business, and government subscribers

ISDN, xDSL, cable modems

Increasing interest shown in competing wirelesstechnologies for subscriber access

Wireless local loop (WLL)

Narrowbandoffers a replacement for existingtelephony services

Broadbandprovides high-speed two-way voice anddata service


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WLL Configuration

Advantages of WLL over Wired


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Advantages of WLL over WiredApproach

Costwireless systems are less expensive due to

cost of cable installation thats avoided

Installation timeWLL systems can be installed

in a small fraction of the time required for a newwired system

Selective installationradio units installed for

subscribers who want service at a given time

With a wired system, cable is laid out in anticipation of

serving every subscriber in a given area

Propagation Considerations for


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Propagation Considerations forWLL

Most high-speed WLL schemes use millimeter

wave frequencies (10 GHz to about 300 GHz)

There are wide unused frequency bands available above

25 GHz

At these high frequencies, wide channel bandwidths

can be used, providing high data rates

Small size transceivers and adaptive antenna arrays can

be used

Propagation Considerations for


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Propagation Considerations forWLL

Millimeter wave systems have someundesirable propagation characteristics

Free space loss increases with the square of the

frequency; losses are much higher in millimeterwave range

Above 10 GHz, attenuation effects due torainfall and atmospheric or gaseous absorption

are large Multipath losses can be quite high


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Fresnel Zone

How much space around direct path betweentransmitter and receiver should be clear ofobstacles?

Objects within a series of concentric circles around the

line of sight between transceivers haveconstructive/destructive effects on communication

For point along the direct path, radius of firstFresnel zone:

S= distance from transmitter

D = distance from receiver

DS

SDR


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Atmospheric Absorption

Radio waves at frequencies above 10 GHz

are subject to molecular absorption

Peak of water vapor absorption at 22 GHz

Peak of oxygen absorption near 60 GHz

Favorable windows for communication:

From 28 GHz to 42 GHz

From 75 GHz to 95 GHz


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Effect of Rain

Attenuation due to rain Presence of raindrops can severely degrade the

reliability and performance of communication links

The effect of rain depends on drop shape, drop size,

rain rate, and frequency Estimated attenuation due to rain:

A = attenuation (dB/km)

R = rain rate (mm/hr)

a and b depend on drop sizes and frequency

baRA


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Effects of Vegetation

Trees near subscriber sites can lead to multipathfading

Multipath effects from the tree canopy are

diffraction and scattering Measurements in orchards found considerable

attenuation values when the foliage is within 60%

of the first Fresnel zone

Multipath effects highly variable due to wind

Multipoint Distribution Service


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u t po t st but o Se v ce(MDS)

Multichannel multipoint distribution service(MMDS)

Also referred to as wireless cable

Used mainly by residential subscribers and small

businesses

Local multipoint distribution service (LMDS)

Appeals to larger companies with greater bandwidth

demands


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Advantages of MMDS

MMDS signals have larger wavelengths andcan travel farther without losing significantpower

Equipment at lower frequencies is lessexpensive

MMDS signals don't get blocked as easilyby objects and are less susceptible to rainabsorption


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Advantages of LMDS

Relatively high data rates

Capable of providing video, telephony, and

data

Relatively low cost in comparison with

cable alternatives


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802.16 Standards Development

Use wireless links with microwave or millimeterwave radios

Use licensed spectrum

Are metropolitan in scale Provide public network service to fee-paying

customers

Use point-to-multipoint architecture with

stationary rooftop or tower-mounted antennas


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802.16 Standards Development

Provide efficient transport of heterogeneous trafficsupporting quality of service (QoS)

Use wireless links with microwave or millimeter

wave radios Are capable of broadband transmissions (>2

Mbps)


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Protocol Architecture

Physical and transmission layer functions: Encoding/decoding of signals

Preamble generation/removal

Bit transmission/reception

Medium access control layer functions: On transmission, assemble data into a frame with

address and error detection fields

On reception, disassemble frame, and perform address

recognition and error detection Govern access to the wireless transmission medium


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Protocol Architecture

Convergence layer functions: Encapsulate PDU framing of upper layers into

native 802.16 MAC/PHY frames

Map upper layers addresses into 802.16addresses

Translate upper layer QoS parameters intonative 802.16 MAC format

Adapt time dependencies of upper layer trafficinto equivalent MAC service


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IEEE 802.16.1 Services

Digital audio/video multicast

Digital telephony

ATM

Internet protocol

Bridged LAN

Back-haul Frame relay


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IEEE 802.16.3 Services

Voice transport

Data transport

Bridged LAN


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IEEE 802.16.1 Frame Format


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IEEE 802.16.1 Frame Format

Header - protocol control information Downlink headerused by the base station

Uplink headerused by the subscriber to convey

bandwidth management needs to base station

Bandwidth request headerused by subscriber to

request additional bandwidth

Payloadeither higher-level data or a MAC

control message CRCerror-detecting code


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MAC Management Messages

Uplink and downlink channel descriptor

Uplink and downlink access definition

Ranging request and response

Registration request, response and acknowledge Privacy key management request and response

Dynamic service addition request, response and

acknowledge


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MAC Management Messages

Dynamic service change request, response,and acknowledge

Dynamic service deletion request and

response Multicast polling assignment request and

response

Downlink data grant type request

ARQ acknowledgment

Physical LayerUpstream


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y y pTransmission

Uses a DAMA-TDMA technique

Error correction uses Reed-Solomon code

Modulation scheme based on QPSK

Physical LayerDownstream


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y yTransmission

Continuous downstream mode For continuous transmission stream (audio, video)

Simple TDM scheme is used for channel access

Duplexing technique is frequency division duplex

(FDD) Burst downstream mode

Targets burst transmission stream (IP-based traffic)

DAMA-TDMA scheme is used for channel access

Duplexing techniques are FDD with adaptivemodulation, frequency shift division duplexing(FSDD), time division duplexing (TDD)


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Wireless LAN Technology


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Wireless LAN Applications

LAN Extension

Cross-building interconnect

Nomadic Access

Ad hoc networking


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LAN Extension

Wireless LAN linked into a wired LAN onsame premises

Wired LAN

Backbone

Support servers and stationary workstations

Wireless LAN

Stations in large open areas

Manufacturing plants, stock exchange trading floors,and warehouses


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Multiple-cell Wireless LAN


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Cross-Building Interconnect

Connect LANs in nearby buildings

Wired or wireless LANs

Point-to-point wireless link is used

Devices connected are typically bridges or

routers


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Nomadic Access

Wireless link between LAN hub and mobiledata terminal equipped with antenna

Laptop computer or notepad computer

Uses: Transfer data from portable computer to office

server

Extended environment such as campus


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Ad Hoc Networking

Temporary peer-to-peer network set up tomeet immediate need

Example:

Group of employees with laptops convene for ameeting; employees link computers in a

temporary network for duration of meeting


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Wireless LAN Requirements

Throughput Number of nodes

Connection to backbone LAN

Service area Battery power consumption

Transmission robustness and security

Collocated network operation

License-free operation

Handoff/roaming

Dynamic configuration


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Wireless LAN Categories

Infrared (IR) LANs

Spread spectrum LANs

Narrowband microwave

Strengths of Infrared Over


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Microwave Radio

Spectrum for infrared virtually unlimited Possibility of high data rates

Infrared spectrum unregulated

Equipment inexpensive and simple

Reflected by light-colored objects

Ceiling reflection for entire room coverage

Doesnt penetrate walls

More easily secured against eavesdropping Less interference between different rooms


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Drawbacks of Infrared Medium

Indoor environments experience infraredbackground radiation

Sunlight and indoor lighting

Ambient radiation appears as noise in aninfrared receiver

Transmitters of higher power required

Limited by concerns of eye safety and excessive

power consumption Limits range

IR Data Transmissionh i


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Techniques

Directed Beam Infrared

Ominidirectional

Diffused

i d f d


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Directed Beam Infrared

Used to create point-to-point links

Range depends on emitted power and

degree of focusing

Focused IR data link can have range of

kilometers

Cross-building interconnect between bridges or

routers

O i idi i l


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Ominidirectional

Single base station within line of sight of allother stations on LAN

Station typically mounted on ceiling

Base station acts as a multiport repeater

Ceiling transmitter broadcasts signal received

by IR transceivers

IR transceivers transmit with directional beamaimed at ceiling base unit

Diff d


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Diffused

All IR transmitters focused and aimed at apoint on diffusely reflecting ceiling

IR radiation strikes ceiling

Reradiated omnidirectionally

Picked up by all receivers

Spread Spectrum LANC fi i


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Configuration

Multiple-cell arrangement (Figure 13.2)

Within a cell, either peer-to-peer or hub

Peer-to-peer topology

No hub

Access controlled with MAC algorithm

CSMA

Appropriate for ad hoc LANs

Spread Spectrum LANC fi i


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Configuration

Hub topology Mounted on the ceiling and connected to

backbone

May control access

May act as multiport repeater

Automatic handoff of mobile stations

Stations in cell either:

Transmit to / receive from hub only Broadcast using omnidirectional antenna

N b d Mi LAN


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Narrowband Microwave LANs

Use of a microwave radio frequency bandfor signal transmission

Relatively narrow bandwidth

Licensed

Unlicensed

Li d N b d RF


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Licensed Narrowband RF

Licensed within specific geographic areas toavoid potential interference

Motorola - 600 licenses in 18-GHz range

Covers all metropolitan areas

Can assure that independent LANs in nearby

locations dont interfere

Encrypted transmissions prevent eavesdropping

U li d N b d RF


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Unlicensed Narrowband RF

RadioLAN introduced narrowband wirelessLAN in 1995

Uses unlicensed ISM spectrum

Used at low power (0.5 watts or less) Operates at 10 Mbps in the 5.8-GHz band

Range = 50 m to 100 m

89088468 wireless communications and networks ppt

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