Wireless Local Loop, Cellular Satellite (specific and adjunct) Microcellular
- 1. Wireless Local Loop (WLL) UNIT 5
- 2. Definition What is WLL? • WLL is a system that connects subscribers to the local telephone station wirelessly. • Systems WLL is based on: – Cellular – Satellite (specific and adjunct) – Microcellular • Other names – Radio In The Loop (RITL) – Fixed-Radio Access (FRA).
- 3. Wireless Local Loop (WLL) • Providing wireless connections to stationary or near stationary stations within a small service area • Generally targeted at the “last mile” or from a point in the neighborhood to the user Advantages of Wireless local loop: • ease of installation • reducing digging, reduce poles, ducts/conduits, … • quick installation of new links (i.e., rapid provisioning) • largely distance insensitive pricing - at least up to some limit • concentration of resources (especially at the multiplexer to the high bandwidth backbone)
- 4. WLL services • Desirable: – Wireless feature should be transparent – Wireline Custom features • Other: – Business related • Hunt groups, • Call transfers • Conference calling – Calling cards, coin phones – V.29 (9600bps) – ISDN (64kbps)
- 5. WLL should provide… • Toll-quality service • Expand from a central office to about 5 miles • Low license cost • Subscriber costs equivalent or better than copper
- 6. Wireless 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
- 7. Broadband wireless services • Wireless Local Loop (WLL) • Demand for broadband internet and computer access from businesses and homes • Fixed wireless equipment have advantages such as fixed path between T-R • Microwave or millimeter radio frequencies used >28GHz - wave length is very small - small high gain antennas. • Especially useful in developing nations with less telecommunications setup.
- 8. Deployment issues • Spectrum • licensed - limited interference, but requires licensing • unlicensed - more interference, but no licensing - generally limited in (maximum and average) power • Service Quality • Users expect it is going to be the same as wireline service • high reliability • low risk of fraud (due to others “hijacking” the link) • Network planning • should support very high penetration levels (for example >90%) • exploits the fact that users are not moving (or rarely move) • antenna height, etc. is generally derived from user density Very popular in the former “East block” of Europe - since there was no need to install a local loop cable to bring users to the local exchange of the PSTN; enabled very rapid provisioning to very large numbers of subscribers.
- 9. WLL Technologies Satellite • A Great chance for the satellite operators (Hughes Network Systems, Inmarsat International Circular Orbit (ICO), Iridium, Globestar, Odyssey, American Mobile Satellite Corporation (AMSC), Asia Cellular Satellite (ACeS), Thuraya, …) • note that some of these operators (such as Hughes) used terrestrial versions of their system Cellular-based • Used in rural and sparse urban settings Low Tier PCS or Microcellular based systems • PACS,PHS, DECT, … Fixed Wireless Access (FWA) • sometimes proprietary point-to-point links • Increasingly LMDS
- 11. 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
- 12. IS-54 architectural reference model for WLL: WANU = Wireless Access Network Unit WASU = Wireless Access Subscriber Unit PSTN WASU WANU UWLL Switch Function AWLL Trunk transceiver AM HLR WLL controller
- 13. Connection Setup Wireless Access Network Unit(WANU) – Interface between underlying telephone network and wireless link – consists of • Base Station Transceivers (BTS) • Radio Controller(RPCU) • Access Manager(AM) • Home Location Register(HLR) PSTN Switch function WLL Controller AM HLR Transceiver WASU Trunk Air Interface UWLL TWLL WANU Wireless Access Subscriber Unit(WASU) – located at the subscriber – translates wireless link into a traditional telephone connection
- 14. Important Results of Fixed to Fixed Propagation in WLLs • Signal channel is not a Rayleigh fading channel: – Power control algorithms are simpler and can be utilized more effectively • Channel Randomness is lost: – Makes analysis difficult • Pathloss exponent is considerably smaller (Why?): – 20dB/dec compared to 40dB/dec – Decreases cell capacity – Allows for larger coverage area
- 15. Fixed to Fixed Propagation(cont’d) • No handoffs necessary: – Decreases hardware costs and system complexity – Increases quality of service through accurate traffic predictions • Allows usage of directional antennas: – Can greatly reduce interference and increase cell capacity -30dB 30dB 0o 60o -40dB 10dB 0o 120o 180o BS antenna Subscriber antenna
- 16. • 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 WLL
- 17. • Millimeter wave systems have some undesirable propagation characteristics – Free space loss increases with the square of the frequency; losses are much higher in millimeter wave range – Above 10 GHz, attenuation effects due to rainfall and atmospheric or gaseous absorption are large – Multipath losses can be quite high Cont.
- 18. Comparison WLL Mobile Wireless Wireline Good LOS component Mainly diffuse components No diffuse components Rician fading Rayleigh fading No fading Narrowbeam directed antennas Omnidirectional antennas Expensive wires High Channel reuse Less Channel reuse Reuse Limited by wiring Simple design, constant channel Expensive DSPs, power control Expensive to build and maintain Low in-premises mobility only, easy access High mobility allowed, easy access Low in-premises mobility, wiring of distant areas cumbersome Weather conditions effects Not very reliable Very reliable