Ch 9. Wireless Local Loop

1. Ch 9. Wireless Local Loop Myungchul Kim mckim@icu.ac.kr

2. Wireless Local Loops Wireless Local Loops Free Space Optics

3. 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

4. WLL Configuration

5. 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

6. 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

7. Wireless Local Loops Inter- Exchange Switch Telephone LAN Toll Connecting Trunks Wireless Local Loop Offerings (MMDS, LMDS) PBX, TV Intertoll Trunks Telephone Inter- Exchange Switch Local Control Office Toll Connecting Trunks Wired Local Loop

8. 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

9. 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

10. 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)

11. IEE802.16 Refernce Architecture SNI (STS Network Interface) BNI (BTS Network Interface) Air Interface Core Network Subscriber Network STS BTS Repeater (Optional) 802.16.1: 10GHz-66GHZ 802.16.2: Coexistence 802.16.3: 2-11 GHZ Subscriber Network = (LAN, PBX, IP-based network) Core Network = PSTN. Internet BTS = Base transceiver station STS = Subscriber transceiver station

12. 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

13. 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

14. 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.html • Broadband Wireless Association (http://www.broadband-wireless.org/)