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2. Fixed wireless
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2. Fixed wireless
Fixed radio access
Endpoints are stationary and therefore less susceptible to the bandwidth andquality limitations associated with mobile wireless networks
Different configurations
Point to point
Point to multipoint (WLL = Wireless Local Loop)
Local Multipoint Distribution System (LMDS) > 10 GHz
Multichannel Multipoint Distribution System (MMDS) < 10 GHz
Wireless broadband access useful
In (rural) areas where you can't get high-speed access over the traditional
telecommunications infrastructure
In urban areas where the operator doesnt have his own access network
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P2P vs P2MP
P2Pdedicatedbandwidth
P2MPshared
bandwidth
P2MP Sector
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Cost per subscriber by network
CostP
erSubscriber
CPE Cost
PTP
PMP
# of Subscribers
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Architecture
Base Station (BS) connected to public networks
BS serves Subscriber Stations (SS)
To backbone
MUXDACS
STM-1
HUB IDU
ODU/ANTENNA
Remote
IDU
E1
RemoteODU/Antenna
PayphonePOTS
ACCESSMUX
TM
VAXstation 3100
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Fresnel Zone
Line of Sight (LOS) required
Obstructions might obscure a link
Topographic features, such as mountains
The curvature of the earth Buildings and other man-made objects
Trees, water
Radio LOS is not the same as visual LOS
Fresnel zoneis an elliptical area immediately surrounding the visual path. It varies
depending on the length of the signal path and the frequency of the signal
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Point to point: Wi-Fi @ 2.4 GHz
Performance
Small bandwidth up to 11 Mbps
Long distance up to 20 Km
Different technologies
11Mbit DSSS (Direct Sequence Spread Spectrum)
3Mbit FH (Frequency Hopping)
Applications
Best used for IP
Not good for telecom (E1)
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Point to point: >10 GHz
Performance
High bandwidth up to 622 Mbit
Long distance up to 30 Km Interference
Rain dependable solution (to overcome by design)
Important technologies
Compression (
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2. Fixed wireless
Laser
Performance
High Bandwidth up to 1 GB
Short Distance up to 4 km Interference
Fog
No license required / no radio spectrum to purchase
Rapid deployment
Low maintenance
Applications
IP networks 10, 100 and 1000 Mb/sec
Telecom : E1/E2/E3, STM
2Mb/s 34 Mb/s 155 Mb/s 622Mb/s SDH and SONET support
Video
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Laser
There is no typical application
Temporary deployment
Disaster recovery Broadcast applications
Security
Impossible to tap the signal
Modulated light
No reflections
Narrow beam
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Laser and microwave compared
Bandwidth Distance License Frequency
PAV/Opticalaccess
1 Gbps 4 km No
Witcom 54 Mbps 20 km Yes 15, 18, 23, 26, 38
Ceragon 622 Mbps 30 km Yes 7.5, 10, 15, 18, 23, 26,38
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2. Fixed wireless
Point to multipoint
(Formerly) often referred to as
LMDS: >10 GHz
MMDS:
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WiMAX (1/2)
IEEE 802.16 is a groupof WMAN standards
Latest wireless hype
Published in december 2001, in 10 60 GHz spectrum
Amendment 802.16a, approved in January 2003
Non-line-of-sight extensions in the 2 11 GHz spectrum
Up to 70 Mbps at distances up to ~50 km
Initially WiMAX is to be used as a backhaul technology to feed emerging Wi-Fihotspots (and possibly 3G base stations)
In the near future WiMAX claims to offer metro-area mobility for Internetaccess
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Wi-MAX (2/2)
Technology Freq. Band Distance Speed Features
802.16 10-66 Ghz 31 miles 70 MbsRequires Line-of-Sight
(approved 2002)
802.16a 2-11 Ghz 31 miles 70 MbsNo line-of-sight reqd
(approved Mar03)
Intel promises WiMax versions of Centrino for 2004Nokia will launch a WiMax cell phone in 2005
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Overview
Point to point
2.4 GHz
Other frequencies
Laser
Point to multipoint
WiMAX
Annex: multi-carrier modulation
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Multi carrier modulation
Basic idea is to send signal over multiple low-rate carriers instead of a singlehigh-rate carrier
Longer symbol duration on individual carriers causes less inter symbol interference
Multipath effects can be compensated with a much simpler equaliser
OFDM abandoned the use of steep bandpass filters that completely separatedthe spectrum of individual subcarriers
Common practice in older Frequency Division Multiplex (FDM) systems
Instead OFDM uses orthogonal frequencies
Multicarrier technology used in
802.11a (COFDM, WLAN)
802.16 (OFDM, WMAN)
DAB, DVB-T (COFDM)
BWIF (VOFDM) xDSL (DMT)
M lti i d l ti ff ti d l d
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Multi carrier modulation: effective delay spread
Delay spread depends on difference in path lengths
Single carrier modulation
Symbol interval becomes shorter than delay spread as symbol rate is increased
Time dispersion causes intersymbol interference
Equalisation can mitigate this to some extent (number of taps required is
proportional to the delay spread)
Cell size Max Delay Spread
Pico cell 100m 300nsMicro cell 5km 15us
Macro cell 20km 40us
Ts Channel taps Application
802.11a 50ns 6 WLANDVB-T 160ns 90 Audio
DAB 600ns 60 TV broadcast
M lti i d l ti th lit b t b i
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Multi carrier modulation: orthogonality between subcarriers
Frequency spectra of various subcarriers overlap
At the center frequency of any particular subcarrier all other subcarriers attain
a null point Demodulator does not see modulation of the others
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