Estimating Signal Strengths Prior to Field Trials in Wireless Local

Loop NetworksBy

Eng. Indrani HissalleMSc,BSc,CEng,MIESL, MIEEE

Content

1. Abstract2. Related Work3. Implementation

i. Procedureii. Data Collectioniii. Drive Test

4. Propagation models5. Results6. Conclusion

1.Abstract

• Main issue in giving wireless Local Loop connections in a rural area is

Difficulty in conducting FIELD TRIALS .

• Extremely poor conditions of access roads reduces the efficiency of doing field trials.

Cont…

• Fading is another issue occurs when the height of the mobile antennas are well below the height of surrounding structures and ever growing trees.

• Technicians of Sri Lanka Telecom use to climb on these trees and rooftops to locate signals.

Field Trials in Remote areas

Tiring Climb…..

Exhausted….

More to do….

The Technique used…..

• Friis free space equation and Hata/Okumura model to estimate signal strength.

• Then a comprehensive equation was derived using MS EXCEL to use by the customer care officers in SRI LANKA TELECOM.

Purpose….•• To reduce the time consumption for a field trial To reduce the time consumption for a field trial •• To reduce the cost involved in sending a team to a To reduce the cost involved in sending a team to a

rural place rural place •• To avoid unnecessary complaintTo avoid unnecessary complaint•• Finally to have a user friendly service provisioning Finally to have a user friendly service provisioning

method.method.

•• ……. For all of these , It is essential to do the signal . For all of these , It is essential to do the signal strength estimation according to the customerstrength estimation according to the customer’’s s information at the customer care center.information at the customer care center.

Related Work

• Any wire line operator armed with some spectrum could choose to extend the network by deploying a FIXED WIRELESS arm into regions other than carry or burry wires.

• About 50 years ago SLT started giving fixed analog radio solutions to rural customers- multi access system.

• In 1996 WLL system was introduced.

WLL Layout Plan

AT & T XC

Belungala cell 5Link 19

TorwoodCell 6Ink 20

JaffnaCell 9Link 2 & 10

EmbilipitiyaCell 8Link 28

Single treeCell 7Link 27

HantanaCell 4Link 12

CH30

CH79

CH108

CH80

CH30

CH89

EMX 2500

Cont..

• For a new installation of a WLL subscriber line, one must take care to ensure that adequate radio coverage will be provided throughout the lifetime of the service.

• By using path loss models to estimate the received signal level as a function of distance, it becomes possible to predict the connectivity.

Cont…

• A research conducted in Carnagie Mellon University by Allex hills, Jon Schlegel and Ben Jenkins on estimating Signal Strength in the design of an Indoor Wireless Network describes the method of using “Path loss Propagation Models “in Signal Strength measurements in indoor prefix.

• The same procedure can be used to estimate outdoor signal strengths using relevant propagation models for out door

Implementation• Free space propagation serves as a

reference point for just about all path loss models.

• An important objective of propagation model is to predict the actual path loss experienced by the communication links.

• But there are no theoretical models that capture all of the variations experienced in the field.

Cont..

• Therefore we need general answers without knowing specific paths.

• Therefore one must take measurements and use both physics and statistics to reach general conclusions.

• Statistics can help effectively extend what we know from physics, and what we see in measurements

Why Okumura-Hata model?

• It makes corrections to free space propagation models.

• Using antenna height and the frequency of the signal in Okumura-Hata model we can avoid complications like Fresnel zone calculations .

Procedure

1). Using 1: 100000 area map identifies the customer’s location at the Sri Lanka Telecom Regional office when the customer requests a new telephone connection.

2). Measure the closest distance, (‘d’) from the base station to the customer’s location using the map. In future SLT is going to use GIS (Geographical Information System) to locate the customers and service availability.

Cont..3). The customer Care Officer has to enter only

elevation and the measured distance (‘d’) in the computer program given to him.

4). If the final received power is greater than -100dBm, the officer can register the customer for a new WLL connection and the application will be forwarded for further action.

5). If it is not within the range defined, the customer will be kept in the waiting list and field inspection should be done.

ii Data collection

• In order to verify the results, field test called “Drive test” is carried out in fifty customer premises with the help of relevant staff.

• During the field test Handheld Terminal was used to get data and it gives RSSI value, CRC values and the aerial distance.

iii Drive test• An area covered by the radio Local Loop base

station of Nawgala (Kegalle) was selected as the testing ground.

• The equipments in the mobile – Hand Held Terminal (HHT)– Fixed Access Unit (FAU) with a pole – Power Control Unit (PCU) which is fully charged– power extension unit (in case of emergency power

requirement) and a compass.

4. Propagation models• Friis free space Model

– The free space propagation model is used to predict received signal strength when the transmitter and receiver have a clear, unobstructed line of straight path between them.

• The free space power received by a receiver antenna, which is separated from a radiating transmitter antenna by a distance of d, is given by the Friis free space equation.

• Pr = Pt [ Gt Gr λ2/ (4π)2d2L]……………..(4.1)

Large scale path loss model

• The average large scale path loss for an arbitrary T-R separation is expressed as a function of distance by using a path loss exponent ‘n’

• LP (d)= LP (d0)+10nlog(d/ d0)…….(4.3)

Path Loss Exponents

2~3Obstructed in Factories

4~6Obstructed in building1.6~1.8In building LOS3~5Shadowed urban area 2.7~3.5Urban area 2Free space

Path Loss Exponent(n)

Environment

OKUMURA model• Okumura’s model is one of the most widely used

models for signal prediction in urban areas.• Not really “a model” as much as a set of curves

which are corrections to free-space.• Describes the attenuation and variations of field

strength for varied terrain. • Applicable for frequencies in the range 150MHz

to 1920MHz and distance of 1km to100km. It can be used for base station antenna heights ranging from 30m to 1000m.

Chart of Okumura model

Use Okumura model to calculate path loss1. Determine free space path loss2. Look up table for median attenuation A3. Add correction factors due to antennas and environments

Okumura model properties1. Based completely on measurement, no analytical explanation2. Simple, but accurate for predicting path loss of cellular & land mobiles3. Not good in rural areas, slow response to rapid changes in terrain

L50= Lp + Amu(f,d)-G(hte)-G(hre) –GAREA…(4.4)

Hata model

– Empirical formulation to match Okumura model

– Valid in 150—1500 MHz– He presented the urban area propagation loss

as a standard formula and supplied correction Equations for application to other situations.

– Okumura-Hata model is very popular and terrain profile is taken into account.

Okumura-Hata model

• L50 =69.55+26.16logfc – 13.82 log(hte) - a(hre) +(44.9 - 6.55loghte )log d

• For a small to medium sized city the mobile antenna correction factor is given by• a(hre) = (1.1 logfc –0.7) (hre)-(1.56logfc-0.8) dB

• For a large city it is given by• a(hre) = 8.29(log1.54hre)2-1.1 d B for fc<=300MHz• a(hre) = 3.2(log11.75hre)2-4.97 d B for fc.>=300MHz

To obtain path loss in open rural areas

• (L50)’ the formula is modified as

– (L50)’ = L50 - 4.78 (log fc)2 – 18.336logfc -40.94 ……………….….(4.12)

• Therefore the median received power = Effective Radiated Power - all losses + Receiving antenna gain.

Kegalle area map

C o mparisio n o f measured and C alculated P o wero n WLL

-120.00

-100.00

-80.00

-60.00

-40.00

-20.00

0.00

20.00

D ist ance/ ( m)

MeasuredPowerCalculatedPowerError

Comparision of Measured and Calculated Power on CDMA

-120.00

-100.00

-80.00

-60.00

-40.00

-20.00

0.00

20.00

40.00

1.2861.4261.7141.8002.0502.3142.7503.1713.1713.4533.9424.2854.3714.6005.3137.4287.6277.8138.0568.3528.7859.80011.40011.62317.997

Distance/(m)

Pow

er/(d

Bm

)

Calculated Pow er

Measured Pow er

Error

Results

• The median received power estimates with actual RSSI measurements have been compared in this project.

• These equations can be used for estimating signal strength to satisfy a customer request at their registration with newly proposed path loss exponents.

Proposed New Path Loss Exponents

5

6

Hilly area

Mountainous area

Path Loss Exponent (n)

Environment

Conclusion• With this proposal we can satisfied the

customers and same time we may save a reasonable amount of money for the company.

• For any modulation method the same path loss models with new “Path Loss Exponent” can be used and signal strength measurements for planning and optimizing the CDMA system can be improved with those results.

Thanks to…• Dr. Sanath Alahakoon, Head of the electrical

and electronic department of university of peradeniya,

• Dr. Mathini Sellathurai of Cardiff university of wales, uk.

• Dr Atputharajah of university of Peradeniyawho were tremendously cooperative on all accounts.

• Mr. Chrsity Alwis , Chief Network Officer, Sri Lanka Telecom

• SLT/SAB Staff and staff of Faculty of Engineering.