2. Wireless local loop (WLL) EE4220 Communications system Dr. Hassan Yousif Electrical Engineering Department College of Engineering Salman Bin Abdulaziz University

3. 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).

5. A general WLL setup

6. 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)

7. 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

8. 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

9. Propagation Considerations for WLL 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

10. SD R Fresnel Zone How much space around direct path between transmitter and receiver should be clear of obstacles? – Objects within a series of concentric circles around the line of sight between transceivers have constructive/destructive effects on communication For point along the direct path, radius of first Fresnel zone: – S = distance from transmitter – D = distance from receiver

11. Atmospheric Absorption Radio waves at frequencies above 10 GHz are subject to molecular absorption – Peak of water vapor absorption at 22 GHz – Peak of oxygen absorption near 60 GHz Favorable windows for communication: – From 28 GHz to 42 GHz – From 75 GHz to 95 GHz

12. Effect of Rain Attenuation due to rain – Presence of raindrops can severely degrade the reliability and performance of communication links – The effect of rain depends on drop shape, drop size, rain rate, and frequency Estimated attenuation due to rain: – A = attenuation (dB/km) – R = rain rate (mm/hr) – a and b depend on drop sizes and frequency

13. Effects of Vegetation Trees near subscriber sites can lead to multipath fading Multipath effects from the tree canopy are diffraction and scattering Measurements in orchards found considerable attenuation values when the foliage is within 60% of the first Fresnel zone Multipath effects highly variable due to wind

14. 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

15. Ideas for U.S. market Supplement Copper Lines – Easier third telephone line – Data service Fixed Mobile Users – Take phone wherever you want / charged on 2 levels – “home” could mean neighborhood – Charged regular mobile rate if you’re on the road

16. Cost Considerations Wireless cost is constant over distance for WLL Wireline depends on distance AND terrain

17. Connection Setup PSTN Switch function WLL Controller AM HLR Transceiver WASU Trunk Air Interface U WLL T WLL 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) WANU Wireless Access Subscriber Unit(WASU) Wireless Access Subscriber Unit(WASU) –located at the subscriber –translates wireless link into a traditional telephone connection

18. Core Network BTS Air Interface BNI (BTS Network Interface) IEE802.16 Refernce Architecture STS Repeater (Optional) Subscriber Network SNI (STS Network Interface) 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

19. 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

20. 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 0o0o 60 o -40dB 10dB 0o0o 120 o 180 o BS antennaSubscriber antenna

21. In-Cell Interference (CDMA) I = (N h – 1)  S  N h  S  voice activity factor  h = total # of houses S = power received at cell site from every house

22. Out-of-Cell Interference Pathloss: 20dB/dec as opposed to 40dB/dec  need to take in account more tiers Only from houses whose antennas are directed at the center cell base station

23. Interference from Another Cell Blue area is region of interferers for C It is Not a perfect pie shape If w = (1/2)*(antenna width) (in radians) W = w+2sin -1 ((R/D)sin(w/2)) If w<<1 and R<<D: W = w (1+(R/D)) is the “pie” arc length

24. Per-Tier Interference Integration over W and all the cells at tier n yields: I n = [  N h Sw/(3sqrt(3))][1/n] for n>4 Interference is proportional to antenna width w and inversely proportional to the tier number. Decreasing the antenna width can greatly reduce interference. As the number of tiers approaches infinity, so does the total interference. Therefore, system capacity is a function of the total number of tiers in the system.

25. Capacity comparison for 5 MHz spectrum allocation Detail IS-95 CDMA IS-136 TDMA ETSI (GSM) MobileWLLMobileWLLMobileWLL Chan. BW (kHz) 125012503030200200 # channels 441671672525 E b /N 0 7 dB 6dB18dB14dB12dB12dB Freq. Reuse 117433 Effective Chan. Per sect. 447.9513.922.782.78 Erlangs per cell Per MHz 38.348.79.8419.69.129.12

26. 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

27. Examples of services provided Marconi WipLL (wireless IP local loop) – Based on Frequency hopping CDMA – Internet Protocol 64kbps to 2.4Mbps rates Committed Information Rate or best effort service Lucent WSS (wireless subscriber system) – 800 to 5000 subscribers per switch – Uses FDMA/FDD 12 Km to 40Km coverage GoodWin WLL – DECT standards – 9.6 kbps rate – Specified conditions -5°С...+55°С, 20...75% humidity

28. Future of WLL / Overview Depends on – economic development – existing infrastructure of a region Offers – market competition – quick deployment – relatively reliable service at low costs

29. 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

30. Questions ? Basie station