Wireless local loop (WLL)

EE4220 Communications systemDr. Hassan YousifElectrical Engineering DepartmentCollege of EngineeringSalman Bin Abdulaziz University

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).

A general WLL setup

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)

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

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

S D

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

DS

SDR

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

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

baRA

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

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

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

Cost Considerations

• Wireless cost is constant over distance for WLL• Wireline depends on distance AND terrain

Connection Setup

PSTN Switch function

WLLController

AMHLR

Transceiver WASU

TrunkAir Interface

UWLL

TWLL

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 subscriberlocated at the subscriber

– translates wireless link into a translates wireless link into a traditional traditional telephone telephone connectionconnection

Core NetworkBTS

Air Interface

BNI (BTS NetworkInterface)

IEE802.16 Refernce Architecture

STS

Repeater(Optional)

Subscriber Network

SNI (STS NetworkInterface)

Subscriber Network = (LAN, PBX, IP-based network)Core Network = PSTN. InternetBTS = Base transceiver station STS = Subscriber transceiver station

802.16.1: 10GHz-66GHZ802.16.2: Coexistence802.16.3: 2-11 GHZ

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

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

In-Cell Interference (CDMA)

• I = (Nh – 1)S NhSvoice activity factor

h = total # of houses

S = power received at cell site from every house

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

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

Per-Tier Interference

• Integration over W and all the cells at tier n yields: In = [NhSw/(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.

Capacity comparison for 5 MHz spectrum allocation

DetailDetail IS-95 CDMAIS-95 CDMA IS-136 TDMAIS-136 TDMA ETSI (GSM)ETSI (GSM)

MobileMobile WLLWLL MobileMobile WLLWLL MobileMobile WLLWLL

Chan. BW Chan. BW (kHz)(kHz)

12501250 12501250 3030 3030 200200 200200

# channels# channels 44 44 167167 167167 2525 2525

EEbb/N/N00 7 dB7 dB 6dB6dB 18dB18dB 14dB14dB 12dB12dB 12dB12dB

Freq. ReuseFreq. Reuse 11 11 77 44 33 33

Effective Chan. Effective Chan. Per sect.Per sect.

44 44 7.957.95 13.9213.92 2.782.78 2.782.78

Erlangs per cellErlangs per cell

Per MHzPer MHz

38.338.3 48.748.7 9.849.84 19.619.6 9.129.12 9.129.12

ComparisonWLLWLL Mobile Mobile

WirelessWirelessWirelineWireline

Good LOS Good LOS componentcomponent

Mainly diffuse Mainly diffuse componentscomponents

No diffuse No diffuse componentscomponents

Rician fadingRician fading Rayleigh fadingRayleigh fading No fadingNo fading

Narrowbeam Narrowbeam directed antennasdirected antennas

Omnidirectional Omnidirectional antennasantennas

Expensive wiresExpensive wires

High Channel reuseHigh Channel reuse Less Channel reuseLess Channel reuse Reuse Limited by Reuse Limited by wiringwiring

Simple design, Simple design, constant channelconstant channel

Expensive DSPs, Expensive DSPs, power controlpower control

Expensive to build Expensive to build and maintainand maintain

Low in-premises Low in-premises mobility only, easy mobility only, easy accessaccess

High mobility High mobility allowed, easy allowed, easy accessaccess

Low in-premises Low in-premises mobility, wiring of mobility, wiring of distant areas distant areas cumbersomecumbersome

Weather conditions Weather conditions effectseffects

Not very reliableNot very reliable Very reliableVery reliable

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

Future of WLL / Overview

• Depends on– economic development– existing infrastructure of a region

• Offers– market competition– quick deployment– relatively reliable service at low costs

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

Questions?

Basie station