Transcript Satellite Communications

Ch 9. Wireless Local Loop
Myungchul Kim
[email protected]
Wireless Local Loops
•Wireless Local Loops
•Free Space Optics
Wireless MAN (Local Loop)
• Wired technologies responding to need for
reliable, high-speed access by residential,
business, and government subscribers
– ISDN, xDSL, cable modems
• Increasing interest shown in competing wireless
technologies for subscriber access
• Wireless local loop (WLL)
– Narrowband – offers a replacement for existing
telephony services
– Broadband – provides high-speed two-way voice and
data service
WLL Configuration
Advantages of WLL over Wired
Approach
• Cost – wireless systems are less expensive due to
cost of cable installation that’s avoided
• Installation time – WLL systems can be installed
in a small fraction of the time required for a new
wired system
• Selective 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 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
• Undesirable characteristics of millimeter WF
– Free space loss increases with the square of the
frequency; losses are much higher in millimeter WF
– Above 10 GHz, attenuation effects due to rainfall and
atmospheric or gaseous absorption are large
– Multipath losses can be quite high due to vegetation
Wireless Local Loops
Telephone
InterExchange
Switch
LAN
PBX, TV
Toll
Connecting
Trunks
Wireless
Local Loop
Offerings
(MMDS, LMDS)
Intertoll
Trunks
Telephone
Wired
Local
Loop
Local
Control
Office
Toll
Connecting
Trunks
InterExchange
Switch
MMDS and LMDS
• Multichannel multipoint distribution service (MMDS)
–
–
–
–
–
Older standard for 2.15 GHZ to 2.68 GHZ
Also referred to as wireless cable (competes with cable TV)
Used mainly by residential subscribers and small businesses
Single MMDS channel can offer 27 Mbps over 50 km
Individual subscribers at 300 kbps to 3 Mbps
• Local multipoint distribution service (LMDS)
– Newer standard for 30 GHZ (US), 40 GHZ (Europe)
– Appeals to larger companies with greater bandwidth demands
– Can deliver upto 37 Mbps within 2 to 4 km
– point-to-multipoint communication system for
digital two-way voice, data, Internet, and video
Tradeoffs between MMDS and LMDS
• MMDS
– MMDS signals have larger wavelengths and can travel
farther without losing significant power
– Equipment at lower frequencies is less expensive
– MMDS signals don't get blocked as easily by objects
and are less susceptible to rain absorption
– It has been proposed to assign a new frequency band
dedicated to digital MMDS services, but this is
impractical
• LMDS
– Relatively high data rates
– Capable of providing video, telephony, and data
– Relatively low cost in comparison with cable
alternatives
802.16 Standards Development
• Standards for LMDS air interface and functions
– Use wireless links with microwave or millimeter wave
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
– Provide efficient transport of heterogeneous traffic
supporting quality of service (QoS)
– Are capable of broadband transmissions (>2 Mbps)
IEE802.16 Refernce Architecture
SNI
(STS Network
Interface)
Subscriber
Network
BNI
(BTS Network
Interface)
Air
Interface
STS
BTS
Core
Network
Repeater
(Optional)
Subscriber Network = (LAN, PBX, IP-based network)
Core Network = PSTN. Internet
BTS = Base transceiver station
STS = Subscriber transceiver station
802.16.1: 10GHz-66GHZ
802.16.2: Coexistence
802.16.3: 2-11 GHZ
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
• Convergence layer functions:
– Encapsulate PDU framing of upper layers into native 802.16
MAC/PHY frames
– Map upper layer’s addresses into 802.16 addresses
– Translate upper layer QoS parameters into native 802.16 MAC
format
– Adapt time dependencies of upper layer traffic into equivalent
MAC service
Free-Space Optics (FSO)
• FSO uses lasers to transmit data, but instead of
enclosing the data stream in a fiber optic cable, the
data is transmitted through the air.
• FSO systems can support data rates between 1.25G
bit/sec to 150G bit/sec (theoretically) with link lengths
that can vary from more than 600 feet up to about a
mile.
• Common FSO networks support around 2.5 Gbps of
data, voice and video communications between 1000
to 2000 feet.
• FSO transceivers can be located on a rooftop, on a
corner of a building or indoors behind a window to
support the last mile.
• Highly secure line of sight communications in the last
mile
Useful Web Sites for WLLs
– Broadband wireless exchange magazine
(http://www.bbwexchange.com/)
– IEEE 802.16 Working Group on Fixed
Broadband Wireless Standards
(http://grouper.ieee.org/groups/802/16/index.ht
ml
– Broadband Wireless Association (http://www.broadbandwireless.org/)