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Cordless Systems andWireless Local Loop (WLL)

Chapter 11



Cordless System Operating Environments

Residential ± a single base station can provide in-house voice and data supportOffice

A single base station can support a small officeMultiple base stations in a cellular configuration can support a larger office

Telepoint ± a base station set up in a public place, such as an airport



D esign Considerations for Cordless Standards

Modest range of handset from base station, solow-power designs are used

Inexpensive handset and base station,dictating simple technical approachesFrequency flexibility is limited, so the system

needs to be able to seek a low-interferencechannel when in useUS Standard: PWT (personal wireless telecommunications)Parameters: Table 11.1 page 319



Time D ivision D uplex (T DD )

TDD also known as time-compressionmultiplexing (TCM)D

ata transmitted one direction at a time,with transmissions going both directionsSimple T DD

TD MA T DD



Simple T DD

B it stream is divided into equal segments, compressed intime to a higher transmission rate and transmitted in bursts

Each side must alternate transmissions thus the data rate at

which blocks can be transmitted by EITHER side is1/2( T p+ T b+ T g )

For one side, the effective bits transmitted per second: R = B/2(T p+ T b+ T g )

R = effective data rateB = block size in bitsT p = propagation delayT b = burst transmission timeT g

= guard time (allow channel to settle down)



Transmission Using TDD



Simple T DD

Actual data rate of the medium, A : A = B /T b (block size in bits / time to transmit block)

Combined with previous equation:

Thus the actual data rate on the link is more thandouble the effective data rate seen by either of thetwo sidesB lock Size: as B increased, actual data rate Adecreases but signal delay increases due to

buffering which is bad for voice traffic

¹¹ º ¸

©©ª¨!

b

g p

T T T

R A 12



TD MA T DD

Wireless T DD typically used with T D MAA number of users receive forward channelsignals in turn and then transmit reversechannel signals in turn, all on same carrier frequency

Advantages of T D MA/T DD :Improved ability to cope with fast fading(spatial diversity with antennas)

Improved capacity allocation (can allocate between forward and reverse channels)



Wireless Local LoopWired technologies responding to need for reliable, high-speed access by residential,

business, and government subscribers

ISDN

, xD

SL, cable modemsIncreasing interest shown in competing wirelesstechnologies for subscriber accessWireless local loop (WLL)

N arrowband ± offers a replacement for existingtelephony servicesB roadband ± provides high-speed two-way voice anddata service



WLL Configuration

Can use directional(gain) antennas



Advantages of WLL over Wired

Cost ± wireless systems are less expensive due tocost of cable installation that¶s avoided (labor)Installation time ± WLL systems can be installedin a small fraction of the time required for a newwired systemSelective installation ± radio 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 WLL

Most high-speed WLL schemes use millimeter wave frequencies (10 GHz to about 300 GHz)

There are wide unused frequency bands available above

25 GHzAt these high frequencies, wide channel bandwidthscan be used, providing high data ratesSmall size transceivers and adaptive antenna arrays can

be used



Propagation Considerations for WLL(continued)

Millimeter wave systems have someundesirable propagation characteristics

Free space loss increases with the square of thefrequency; losses are much higher in millimeter wave rangeAbove 10 GHz, attenuation effects due torainfall and atmospheric or gaseous absorption

are largeMultipath losses can be quite high



Fresnel ZoneHow much space around direct path betweentransmitter and receiver should be clear of obstacles including the ground?

Objects within a series of concentric circles around theline of sight between transceivers haveconstructive/destructive effects on communication

For point along the direct path, radius of firstFresnel zone (most serious interference region):

S = distance from transmitter D = distance from receiver

DS

S D R !

P



Zone Concept Applied to GPS Antenna

Ch oke rings on an ISS GPS Antenna (1 of 4)



Atmospheric Absorption

Radio waves at frequencies above 10 GHzare subject to molecular absorption

Peak of water vapor absorption (humidity) at 22 GHz

Peak of oxygen absorption near 60 GHz

Favorable windows for communication:From 28 GHz to 42 GHzFrom 75 GHz to 95 GHz



Effect of RainAttenuation due to rain

Presence of raindrops can severely degrade thereliability and performance of communication linksThe effect of rain depends on drop shape, drop size,rain rate, and frequencyConsider seasonal variations for the area: climate zones

Estimated attenuation due to rain:

A = attenuation (d B /km) R = rain rate (mm/hr)a and b depend on drop sizes and frequency

b

a R A !



Effects of VegetationTrees near subscriber sites can lead to multipathfadingMultipath effects from the tree canopy arediffraction and scatteringMeasurements in orchards found considerableattenuation values when the foliage is within 60%of the first Fresnel zoneMultipath effects highly variable due to wind



OF DM O rt h ogonal Frequency Division Multiplexing

Uses multiple carrier signals at differentfrequencies transmitting some of the bits oneach channelSubchannels are dedicated to a single datasourceSplit the data stream into N substreams (serial to

parallel converter) with R/ N

bps that istransmitted on a separate subcarrier



OF DM Hardware Block Diagram



OF DM Spectral Density



OF DM AdvantagesPeaks of the power spectral density of each subcarrier are separateThe precise relationship among the subcarriers is

referred to as orthogonalityMinimal interferrence between subcarriersFrequency fading doesn¶t affects the entire channelSubcarriers increase the symbol time by a factor of N which decreases the delay spread of the channel(compact bandwidth)



Multipoint D istribution Service (MD S)

Multichannel multipoint distribution service (MM D S)Also referred to as wireless cableUsed mainly by residential subscribers and small

businessesLocal multipoint distribution service (LM D S)

Appeals to larger companies with greater bandwidthdemands

These two wireless broadband technologies are viewed bymany to have essentially failed because the ³economics (of LM D S and MM D S) became very bad very quickly´



Advantages of MM D SMM D S signals have larger wavelengths andcan travel farther without losing significant

power

Equipment at lower frequencies is lessexpensiveMM D S signals don't get blocked as easily

by objects and are less susceptible to rainabsorption



Advantages of LM D S (local)

Relatively high data ratesCapable of providing video, telephony, and

dataRelatively low cost in comparison withcable alternatives





802.16 Standards D evelopmentProvide efficient transport of heterogeneous trafficsupporting quality of service (QoS)Use wireless links with microwave or millimeter wave radiosStandards that are capable of broadbandtransmissions (>2 Mbps)



IEEE 802.16 ProtocolArchitecture



Protocol ArchitecturePhysical and transmission layer functions:

Encoding/decoding of signalsPreamble generation/removalB

it transmission/receptionMedium access control layer functions:

On transmission, assemble data into a frame withaddress and error detection fieldsOn reception, disassemble frame, and perform addressrecognition and error detectionGovern access to the wireless transmission medium



Protocol ArchitectureConvergence layer functions:

Encapsulate P D U framing of upper layers intonative 802.16 MAC/PHY frames

Map upper layer¶s addresses into 802.16addressesTranslate upper layer QoS parameters intonative 802.16 MAC format

Adapt time dependencies of upper layer trafficinto equivalent MAC service



IEEE 802.16.1 Services (air interface)

D igital audio/video multicastD igital telephony

ATMInternet protocolB ridged LA N

B ack-haulFrame relay



IEEE 802.16.3 Services

16.3 is the Air interface for LicensedFrequencies from 2 to 11 GHzVoice transportD ata transportB ridged LA N



IEEE 802.16.1 Frame Format



IEEE 802.16.1 Frame FormatHeader - protocol control information

D ownlink header ± used by the base stationUplink header ± used by the subscriber to convey

bandwidth management needs to base stationB andwidth request header ± used by subscriber torequest additional bandwidth

Payload ± either higher-level data or a MAC

control messageCRC ± error-detecting code

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