Cellular Engineering

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Cellular Engineering

http://dinendran.wordpress.com/


Introduction

• Cellular Engineering Objectives

• Costs Elements in Network Design

• Design Constraints

• Quality of Service

• Radio Planning Methodology



Radio Network Planning Area

BTS

TRX

Base Transceiver Station

Mount and Antenna

Air Interface (Um Link)

Mobile Station



Objectives of Cellular Engineering

• Adequate Coverage - 1. Contiguous Coverage with least coverage holes.2. Adequate depth of Coverage( Indoor, Outdoor)

meeting Marketing Plans.

• Capacity - Handling Maximum Possible Traffic in Busy Hour with low Blocking Probability

• Quality of Service (QOS) - Service with least Call Drops,congestion and high Setup Success

Rate,Voice Quality Levels

• Network Growth Accommodation - Scope for Coverage and Capacity Expansion Maintaining the

High Quality Levels

• Cost Effective Design - Lowest Possible Costs for Expansion and Maintenance without affecting

Quality Levels



Costs Elements in Network Design

Cost for Quality Network Design• To Design Optimal N/W :- Extensive Modeling and Numerous revision

of design. • Cost in Acquiring the site locations meeting the Design specifications

( Acquire as close to Designed sites)• Extensive Drive tests before commissioning of site.• Integration of field measurements in design.OTHERWISE ----

• Potential Cost due to Improper Design• Revenue loss due to disconnection• Revenue loss due to lost Airtime• Loss of Competitive Edge• Enhanced Service Revenue loss

• Redesigning Cost• Modifications of Cell Parameters• Equipment modification/Change• Relocation/Addition of sites



Design Constraints

The objective of Radio Network Planning is a Technical Realization of the Marketing Requirement, keeping in mind the following constraints

RADIO NETWORK PLANNING

VENDORSPECIFICATIONS

LICENSECONDITIONS

BUDGETGSM

SYSTEM SPECIFICATIONS

RADIO ASPECTS

MARKETING REQUIREMENTS



Design Constraints

License Requirements (Technical Requirements based on License conditions)

• To cover(class 4) 60% of population within 12 months of Commercial launch• Availability of service in 90% of the area for 90% of the time• To achieve certain Grade of Service( System Reliability is included)• Availability of Limited Bandwidth divide over all licensed operators

GSM Specifications (ETSI recommendations for Radio Transmit and Receive)

• Frequency Bands• Mobile Station Transmit Power(Class) class 4 , 2 W , 1800 - 1 W.• BTS Transmit Power• Receiver Sensitivities of MS and BTS, -102 dBm, -104 Dbm.• Carrier to Interference Ratio(C/I), C/I - better than 9 db.

C/A - better than -9db.



Design Constraints

Vendor Specifications• BTS Transmit Power• Receiver Sensitivity• Cable Loss( Generally it is defined per 100 meters)• Antenna Specifications, beam width etc.• BTS Capacity, number of transceivers.

Radio AspectsRadio wave propagation loss.ShadowingMultipath FadingInterferencePower Link Budgets

Budget• Governed by Business Plan• Identifying and Prioritization the areas based on max. return on

investment



Quality of Service Specifications

The Technical plan for Quality service is based on:

Coverage Quality - Determined by• Outdoor Coverage - Averaged coverage Probability of 95% across the cell area• Indoor Coverage - Extra Coverage for Strategic locations• In Car Coverage - Supplementary level of coverage for highways and remote areas

• Interference Margin - Besides the C/I values recommended by ETSI , extra Interferencemargin should be taken

into account.• Blocking Rate - Probability of an unsuccessful call attempt due to unavailability of radio resource (usually <2%)

•Grade of Service - Probability of a lost call, includes reliability of the system.

• Call Success Rate - Proportion of calls connected and held for 2 minutes within the defined coverage area(desired 98%)

•Dropped call rate - Probability of disconnection due to Handover failure, null areas, interference or congestion(usually<5%)



Quality of Service

• Roll-out Plans - Plans considering availability of coverage within certain time limits based on prioritization

• Traffic Forecasts - Important Considerations• Long term projections and trends developed by Marketing• Existing Traffic distributions and typical densities in the

Existing network

• Spectral efficiency - Based on• Frequency Re-use• Clustering• Traffic trend design• TRX allocation• Business plan feedback



Traffic & Coverage Analysis• Traffic • Coverage• Quality

Nominal Cell Plan• Cell Plan • Site Configurations• Sites Prediction• Frequency Plan

Process Design• Coverage Maps,

actual sites• Documentation of detailsImplementation

System/ ParameterTuning/ OptimizationSignal Propagation

Initial PlanningSystem Growth

Surveys/ Site Sel

RF Planning Process

•Testing of site•Drive test

Mkt req, + other constrainsCW + Model Tunning

Tool, Digital Maps

Is Site meeting Quality Norms

NO

YES Integrate with n/w, Commercial launch



Air Interface

• Frequency Allocation - GSM 900 System with Frequency Band 890 - 915 / 935 - 960 MHz

for Up-link & Down-link.

• Channel Concept

• Carrier Separation - 200 kHz

• 124 Carriers in GSM 900 Band

• Every Carrier can be shared by 8 MS (Physical Channels)

• Types Of Channels

• Physical Channels

• Logical Channels

• Physical Channels

• Each Carrier can be shared by a number of MS called Physical channels

• On every Physical channel a number of Logical channels are mapped



Logical Channels

• Each logical channel used for specific purpose e.g.. Paging, call-setup or

speech

• Eleven Logical Channels

• Two used for Traffic and Nine for Control Signaling

Traffic Channels(TCH)Broadcast Channels(BCH)Common Control Channels(CCCH)Dedicated Control Channels(DCCH)



Magnetic Field Electric Field

Propagation Direction

Propagation properties are different across the frequency

spectrum For GSM the UHF(Ultra High Frequency Band)

is used 300 - 3000MHz

Waves• Radio waves are one type of Electro-magnetic waves• Typically Generated as Disturbances sent out by oscillating charges on a Transmitting Antenna• A simple , travelling, plane wave the Electric field, the Magnetic field and the direction of Propagation, all are

perpendicular to each other• Waves can be described by simple Sinusoidal function• Can be Characterized by the length of one cycle of Oscillation called the Wavelength ‘’or equivalently by

its frequency ‘f’ . f = c

= wavelength in meters/cyclef = Frequency in cycles/second(Hz)c = speed of light(3 * 10 e 8 meters/sec for all Electro-magnetic waves)

Radio Wave Propagation• Waves



Free Space attenuation :- The Principle refers to the decay of the signal, travelling inFree Space as Fn( Distance)

of receiver from transmitter.

Isotropic Radiation :- Isotropic antenna radiates the signal energy in all directions. The power of the signal

diminishes as a function of distance r.r

Pt

Receiving antenna

The Power is expressed in dBm which is P (dBm) = 10 log (Pin/0.001) , where Pin is in Watts.Loss and gain are expressed in db.



AntennaMobile Station

d

Absorption• Refraction• Reflection• Diffraction• ScatteringAssuming • Emitted Power Pt• Received Power Pr• Transmitting Antenna Gain Gt• Receiving Antenna Gain Gr• Distance between both Antennas d

Transmission lossL = 10 log Pt / Pr = 10 log {(4d)²/(Gt Gr ²)}

L = 20 log (4d/) - 10 log Gr - 10 log GtThus theoretically Path loss in Free space Lp = 20 log (4d/)=> the received power decreases when distance between antenna increases and the transmission loss increases when the wavelength decreases. This gives the model of first approximation. However for cell planning it is very import to arrive at a model to predict the actual path loss for particular type of terrain.

Factors affecting Radio wave Propagation



Practical Attenuation

In practical we design n/w with real attenuation in the environment, and path loss is much severe than the free space and it is proportional to 1/r n where n is the slope of the terrain/ propagation path loss slope.

The propagation path loss is determined the actual terrain environment and can vary between 2 & 5 I.e on log scale theslope will be between -20 ( Free space) and -50 dB/decade( highly urban).

-20

-35Signal lev

(dBm)

1 10 100 Log r (km)

The steeper path loss slope is caused by - Obstruction in the propagation path.- Reflections from the ground and from objects, multipath reflections

Signal decay depends on - Distance from transmitter, frequency, Antenna Design, Terrain



Propagation Mechanisms

• Reflection– Propagating wave impinges on an object which is large

compared to wavelength– E.g., the surface of the Earth, buildings, walls, etc.

• Diffraction– Radio path between transmitter and receiver obstructed

by surface with sharp irregular edges– Waves bend around the obstacle, even when LOS does

not exist• Scattering

– Objects smaller than the wavelength of the propagatingwave

– E.g., foliage, street signs, lamp posts



Radio Propagation• Effect of Mobility

– Channel varies with user location and time– Radio propagation is very complex

» Multipath scattering from nearby objects» Shadowing from dominant objects» Attenuation effects

– Results in rapid fluctuations of received power

Mean Less variation the slower you move

Fading :- Fading takes place due to multipath reflections, when reflectedwaves are in same phase ( constructive) signal addition but when out of phase( Destructive) cancellation of signal resulting in Fade.

Macroscopic( Log Normal) :- Long term fading because of terrain, buildings etc.

Microscopic( Rayleigh) :- It is short term fading caused by Movment of MS, radiowaves from many different reflection path received



It has been found that , statistically , the distance between Rayleigh dips isabout half a wavelength.

In GSM we take example carrier freq. Of f = 900 MHz.&

1/2

= c / f where speed of light ( 3 * 10 m/s)

= 33 cm

And the distance between the two rayleigh dips is half a wave length which is 16.5 cm.

Effect of Rayleigh fading



Fade MarginThe fade margin is normally equal to the maximum expected fade.



Multipath Propagation - Radio wave may reflect from a hill , building, truck, airplane,discontinuity in atmosphere.

MULTIPATH FADING

1

2

consider two signals arriving by two different paths to MS.The first one is directly from BTS & second signal is reflected off a building.

(1) (2)

The Phase of the second signal may besuch that it is 180 degrees out of phase with the direct signal.



Multipath propagation and reflection can cause positive or negative effects.

• Ducting due to tunnels, behind the hills.

• Constructive and destructive Interference.

Rayleigh fadingDelay Spread -

Due to multi path propagation effects the sharp pulse which is transmitted arrivesin the receiver as a delayed, flattened bulge and last longer than original pulse.

Doppler Shift :- Frequency of the signal shifts because of MS movement relative to BTS.

Effects



Countermeasures for propagation losses

Equalization - 26 TSC transmitted with each timeslot burst are used to measure the channel characteristics.The predicted distortions in the received signal are subtracted from the received wave form then the original most likely signal is estimated.

Viterbi Algorithm

Channel Estimator

Input Output

Fade Margin

Diversity - Refers to any of several techniques for sampling the received signal more than once to improve the SNR at the receiver.



Different Diversity Schemes

• Time Diversity - Bit Interleaving

• Frequency Diversity - Frequency Hopping

• Antenna Diversity - Space & Polarization - In space diversity mostly two antennas are used at different positions may be Horizontal or Vertical.

Diversity Methods

After obtaining the necessary samples, these samples have to be processed to obtain a good result.There are various possibilities in combining.There are three ways of combiningthe two different signal samples:

• Selection• Equal gain Combining• Maximal Ratio Combining



SELECTION

At a time, only one Ak is set to unity while others are zero.The signal that has the highest instantaneous SNR value is selected.

Antenna A Antenna BLogic

RX



EQUAL GAIN COMBINING

Both the signals are added together and both Ak are equal to 1.

Antenna A Antenna B

Summing

RX



MAXIMAL RATIO COMBINING

Here Ak is proportional to the signal power S and inversly proportional to the noise power Nat input K. The signals coming from the two antennas are phase shifted in order to allignthem in phase before combining them

Antenna A Antenna B

Co-phasingand

Summing

RX

Ak = Sk / Nk



COMPARISON

The maximal Ratio Combiner is the best performance combiner.The equal gain combiner has 0.5 db degradation as compared with the maximal ratio combiner.The selective combiner has a 2db degradation as compared with the maximal ratio combiner

DIVERSITY GAIN

The achievable diversity gain is also dependent on :

• Clutter Density• Speed Vector of MS w.r.t. BTS• BTS antenna height• Difference between the two Signal Levels

The diversity gain is nominally set to 3 db but can be higher in reality.



Antennas

High Frequency Radio waves are generated by oscillating charges on a transmitting

antenna. We can think of electric field as being disturbances sent out by dipole(long

wire) source and the frequency of the oscillating electric field(Electro-magnetic wave)

is same as the frequency of the source.

Antennas are characterized by their electrical specifications

• Gain - Amount of Power radiated in a given direction(dB)

• Main Lobe - The beam containing the maximum radiation intensity

• Side Lobe - other beams containing radiation intensity less than the main lobe

• Back Lobe - beam approximately pointed 180 deg from the main lobe

• Half Power Points - at these points the antenna gain is 3dB lower than the main lobe,

also known as -3dB points

• Beamwidth - Angle(degrees) confined between the Half Power points



Polarization - Quantity describing the orientation of the Electric Field or ‘E-plane’

(GSM antennas can be Dual polarized) e.g. a vertical polarized antenna means its

electric field is perpendicular to surface of earth

Null - regions in the radiation pattern where radiation intensity is minimal compared to

adjacent lobes

F/B ratio - ratio between Power radiated in the main(front) direction to the power radiated in

the reverse direction.

-3 dB

-3 dB

Main Lobe

Side Lobe

Back Lobe

Null Region

Half power Beamwidth0

90

180

270

Horizontal Radiation Pattern Vertical Radiation Pattern

0

270

90

180



Antenna Diversity

In general, two types of antenna diversity are used:

Horizontal Antenna Diversity

Polarization DiversityIt is achieved by using two antennas with their polarization planes perpendicular to each other.One antenna can be used as Rx antenna while the other can be used simultaneously as a Rx/Tx antenna.

Configurations:

Rx/Tx Rx

The 45º / 45º configurationThe 0º / 90º configuration

XXX

Rx/Tx Rx

output

Vertical Antenna Diversity

output



– Isolation between transmitter and receiver is required to avoid receiver desensitization

- Receiver in-band noise caused by the co- site transmitter ( spurious signals).The generation of spurious frequencies could be due to non-linear characteristicsin a transmitter.If the spurious signal falls within the passband of a nearby receiver& the signal level is of sufficient amplitude,it can degrade the performance of receiver.

- Gain reduction of Low-noise amplifier caused by a strong off channel signal.When an undesired signal from a nearby “off-frequency” transmitter is sufficientlyclose to a receiver’s operating frequency,that signal may get amplified & get throughRF selectivity of the receiver.

The practical value for the isolation is 30 db

Antenna Isolation

Antenna TiltingTo minimize interference the transmission range is reduced by tilting the antenna main lobe down, can be Mechanical - physically altering the angle, affects both horizontal and vertical radiation pattern

Electrical - Phasing of the electrical currents in the dipole array, affects only vertical radiation



Antenna Coupling Equipment

Two classes of ACE are distinguished :

• Downlink ACE types• Uplink ACE types

Downlink ACE Types:

Diplexer TRDU The Transmitter Diplex unit makes it possible to join thetransmit & receive signal into one antenna.

Combiners TXFU The transmitter filter unit combines two transmit signals into one output signal.It is a narrow band combiner and is only suitable for GSM –900.

TXHU The transmitter hybrid unit is a wide band combiner used for both GSM 900 ,1800.



Uplink ACE Types

Splitter RXMC The receive multicoupler is used to split the received signal to separate receivers.

Attenuators RXDI The receive distributor besides attenuating the signalfrom the splitter,it also distributes the signal over the

different RTs.

Amplifiers LNA Low noise amplifier boosts the received signal to compensate for splitter & feeder losses.

TMA The tower mounted amplifier also boosts the received signal to reduce the impact of feeder cables noise.It should be placed directly below the antenna.

TMB The tower mounted booster is used to boost both the trans & receive power.



TransceiverTransmitter Power - ?Receiver Sensitivity = -104 dBm

ACE Duplexer Unit Loss = 1.0 dB ( Typical)

7/8 “ coaxial cableLoss per 50 meters = 2 dB ( Typical)

AntennaGain = 11 dBi

Path Loss

Mobile

Antenna Gain - 0 dBicable loss - 0 dB

TX Power - 33 dBmRX Sensitivity = -102

dBm

Link Budget Scenario



Link Budget

UplinkSignal level

(dbm)

MS transmit Power( Class 4) 33dBmCable LossAntenna Gain MSBody LossMax Path Loss

Penetration lossFade Margin ( % cell Edge)

Antenna Gain BTSACE Loss( Duplexer)

Feeder LossDiversity GainBTS Receiver Sensitivity

0 dB

0 dBi2 dB

+ 112 dB20 dB14 dB11 dB

1 dB2 dB

3 dB-104 dBm

33dBm33dBm

33dBm31dBm

-115dBm-101dBm

81dBm

-104dBm

-104dBm-107dBm-105dBm



Link Budget

DownlinkSignal level

(dbm)

BTS transmit Power dBmFeeder LossACE Loss(Combiner , Duplexer)Antenna Gain BTSMax Path Loss

Penetration lossFade Margin ( % cell Edge)

Body LossAntenna gain MS

Cable LossDiversity GainMS Receiver Sensitivity (Class 4)

2 dB

1 dB11 dB

112 dB20 dB14 dB

2 dB0 dBi0 dB

0 dB-102 dBm

38dBm36dBm

35dBm46dBm

-100dBm- 86dBm- 66 dBm

-102dBm

-102dBm-102dBm-102dBm

38 dBm



Coverage ExtensionReasons•Customer demand to have coverage in a specific area.•Existence of coverage holes.•High losses when Waves Penetrates building•Building Construction within target area.

Methods

•Lower the threshold level of a received signal.•Decrease the front end noise figure F•Increase transmitted power.•Increase BTS antenna height( Doubling the the height may give + 6 dB Gain)

Equipment

•Mast head Amplifier•Diversity receiver•Micro cells / pico cells ( to fill the holes.)•High gain directional antenna



Micro Cells

Micro cell cover areas that are small compared to macro cells . Micro cells increase capacity and coverage and are located in hot spots and dead spots.

Other Coverage Enhancers

Repeater Passive AntennaActive AntennaLeaky Coax ( radiating Cable).



Traffic Theory and Channel Dimensioning

Capacity of a cellular system(Traffic a single cell can carry) depends on:• No. of Channels available for voice/data• Grade of Service - the acceptable probability of a system to be congestedTraffic Theory attempts to obtain useful estimates on number of channels required in a cell depending on• Selected system• Assumed/Real behavior of subscribers

• Traffic - refers to usage of channels usually expressed as:• Holding time per time unit• number of call hours per hourA ( Er) = number of call per hour * measured in hours( Avg Call maintained ).



for one or several channels, measured in Erlangs(E){Erlang - a Danish traffic theorist}

Based on certain assumptions on Subscriber behavior Erlang developed Erlang’s B-Table.

Assumptions

• No queues

• Number of subscribers > Number of channels available

• No dedicated Channels

• Traffic following Poisson Distribution

• Blocked calls abandon the call attempt immediately

This is referred to as “Loss System”. The B-Table relates

• Number of Traffic Channels

• The GoS

• Traffic offered



n 0.007 0.008 0.009 0.01 0.02 0.03 0.05 0.1 0.2 0.4 n1 0.0071 0.0081 0.0091 0.0101 0.0204 0.0309 0.0526 0.1111 0.25 0.6667 1. . . . . . . . . . . .

14 6.9811 7.1154 7.2382 7.3517 8.2003 8.8035 9.7295 11.473 14.413 21.243 14

Erlang’s B - Table

E.g. Assuming one cell has two carriers

=> NO.of Traffic Channels = 2 * 8 -2 = 4

Acceptable GoS = 2%

Interesting part is :

If we assume a typical call lasts for around 60 sec per hour

Traffic generated by each call = 60/3600 = 17 mE

Thus Number of subscriber one cell can support = 8.2/.017 =

492 subscribers

The Traffic That can be offered is = 8.2003 E



Traffic Dimensioning

Dimensioning the network now implies using demographic data to determine the size of the

cells. Once cell size is decided then need is to estimate the no. of carriers required in each

cell, keeping in mind the traffic is not constant

• Day an d Night Variation

• Different day variation

• Mobility during the course of the day

• Other Factors

Also important is the dimensioning of no.Of signaling channels(SDCCH). To calculate the

need for SDCCHs

• The no. of attempts for every procedure that uses the SDCCH

• The time that each procedure holds the SDCCH

must be taken into account

Procedures are:

• Location update

• Periodic registration

• IMSI attach/detach

• Call setup

• SMS



Channel Utilization

Assuming a subscriber Traffic of 23 E with GoS during Busy Hour not exceeding 2%

NO. of channels required for one cell = 32 (From Erlang’s B=Table)

With 32 Channels, channel utilization = Traffic Served/No.of Channels = 23/32 =72 %

Now Assuming 5 sites(cells) are designed to cover the same area(same Traffic) with

acceptable GoS 2%

Cell Traffic(%) Traffic(E) No.of Channels Channel utilization(%)A 40 9.2 16 58B 25 5.75 11 52C 20 4.6 10 46D 15 3.45 8 43

Total 100 23 45

Thus is is observed Splitting into smaller cells, • No.of Traffic Channels required increases • Channel Utilization reducesCapacity and Interference problems prevent the use of most effective Channel Utilization scheme and a compromise is made between• Cost(Efficiency)• Quality



INTERFERENCE

Interference is the reception of unwanted radio signals that influence the receive/transmit path between a Receiver and a transmitter.

Types of Interference:

• Co-channel Interference (occurs when the interfering channel is on the same frequency channel)• Adjacent channel Interference(occurs when the interfering signal is on an adjacent frequency channel)

Carrier to Interference Ratio – How much a signal (C) is interfered by an other signal (I) is given by the carrier to interference ratio (C/I) db.

Interference Protection: To reduce the interference between two frequencies there should be a minimum margin between those frequencies.ETSI recommendations:

Relation Frequency Spacing Minimum C/I

Co-channel 0khz 9db1st Adjacent Channel 200khz -9db2nd Adjacent Channel 400khz -41db3rd Adjacent Channel 600khz -49db



Minimum Spacing in khz

In the cell 600Between 2 co-sites 400Between 2 neighboring cells 200

Fighting the Interference

The C/I ratio can be increased in a number of different ways:• Intelligent frequency management • Frequency Hopping• Antenna pattern design• Accurate tilting• Reduction of Antenna Height• Power Reduction



FREQUENCY REUSE

As the spectrum allocated for a cellular network is limited, there is a limit to the no offrequencies or channels that can be used.Channel reuse is implemented by using the same channels within cells Located at different positions in a cellular network service area.

Cluster Size/ Reuse Factor (K) – The no of cells that are using the same frequencies is called Cluster size, or reuse factor K.

Valid values of K can be found using equation (where i & j are integers):

K = i² + j² + ij

Calculating the Frequency Reuse Distance

The frequency reuse distance (D) can be derived from the K-value :

D / R = 3 K



FREQUENCY HOPPING

In frequency hopping systems, each call hops between a defined set of frequencies.

GSM networks use “slow” frequency hopping.A hop occurs before each time slot is transmitted (every 4.615 millisecond, or 217 hops per second).

Frequency hopping mitigates two problems with transmission quality over the air interface:

• Multipath fading

• Interference

Different types of FH

• Base Band FH

• Synthesizer hopping



Base Band FH

In Base band FH a call hops between different TRX of the same sector

In the above fig , a customer will be TRX 1 (f1) for 1 TDMA frame and in the next frame he will be in TRX2 (f2) and next in TRX3 (f3)

f1f2 f3



Synthesiser FH

In this, the output freq of the TRX changes and the calls will continue on the same Timeslot

The advt of Synthesiser over Base band is that we need only as many TRX as the Capacity , but in the case of Baseband Hopping we need 4 TRX in 1 sector eventhoughThe

capacity of that sector is very less

f1,f2,f3,f4



FH is described by ….

1) HSN ( Hopping sequence number )2) MAIO ( Mobile allocation index offset )

*HSN is an algorithm on which the frequencies should be selected with in the predefined group for hopping frequencies

There are 64 algorithm ( HSN ) ie 0-63

*MAIO is the starting frequency of this algorithm.

The value of MAIO can be 0 to N-1 , where N= number of allocated frequencies



Coverage and Frequency Planning

Since the GSM Radio frequency spectrum is limited, the most challenging task for an

RF engineer is to use the Radio Frequency as efficiently as possible or how the

allocated set of frequencies can be distributed to serve the required area with least

interference.

Interference - is the reception of unwanted signal that affect the air interface between

the receiver and the transmitter.

Types of interference

• Co-channel Interference - occurs when same frequencies from different areas appear

in the same area.

• Adjacent channel interference - occurs when there is inappropriate gap between the

adjacent frequencies serving the same area

The minimum Frequency spacing according to the theoretical GSM standards are:

• Between two sectors of same site(cell) 400 kHz

• Between two Neighboring sectors 200 kHz

• Within a sector of a site(cell) 600 kHz



The frequency plan is living plan or in other words a constantly changing activity

based on

• Network Growth

• Traffic Growth

• Interference Detection

Frequency Planning is carried out following different approaches involving

Clustering, Frequency Hopping depending on the coverage and capacity

requirement.

• Clustering - The limited spectrum allocated limits the number of frequencies to be

used e.g. 4.2 MHz bandwidth provides only 21 carriers for serving the large number

of subscribers thus forcing to reuse of same frequencies in different positions of the

network area. A unique kind of distribution id designed with the available set of

frequencies to be replicated over the whole area this is called clustering.

The number of frequencies that can be used within a cluster depends on

• Available Frequencies

• Interference relations between frequencies

• Amount of Traffic in the area



3/9 Cluster 4/12 Cluster



Area of Regular Hexagon

r = 2R

R

R

Area of Equilateral Triangle(Each Side = R) = (1/4) R² 3

Thus for a Regular Hexagon( Six equilateral triangles)

Area = (3/2) R² 3 = 2.6 R²



Types of Sites

Omni Directional : The omni site radiates in all the directions. The ideal coverage shape of omni is circular.

R

Sectored : Site is divided in to sectors. Each sector has directive antenna, which has high levelof directivity. They also handles much more traffic than Omni cell and coverage wise they cover double the area. The typical examples of sectored site are 3 sector and 6 sector sites.As number of frequencies are radiated in a particular direction the interference is much low as compared to omni.



Omni Directional Sectored



DRIVE TEST

Purpose- Initial network coverage verification and benchmarking

- Coverage Verification before and after changes- Locating and measuring interference- Locate coverage holes- Logging excessive handovers due to poor network design- Preventive maintenance- Simultaneous measurements of the other networks

Tools - Test Mobile Phone- Lap Top having Drive Test Tool ( e.g. Neptune, Tems etc)- UPS- GPS- Vehicle



- CELL ID including BSIC, LAC, and time slot- RXLEVEL for the serving and the neighbour cells- RXQUALITY for the serving cell- BCCH, BSIC for the serving and the neighbour cells- TIMING ADVANCE- TRANSMIT POWER- LAYER 3 MESSAGES- GPS POSITION DATA

Data Collection

Drive Test Route Planning

- Primary route(street level) Includes all major roads,highways etc

- Secondary route(street level)Includes all other small streets, subdivisions and compounds.

- Miscellaneous routes (in-building and special locations)Includes golf courses, beach resorts, shopping malls, department stores, convention centers, hotels and resorts

Drive Test can be divided in two categories , With long calls and short calls.



Common Problems

Cell DraggingCell Dragging -- Calls may drag a cell beyond the desired handover boundary. This might result dropped calls or bad Rxquality.

Corrective Measures

• Create an appropriate neighbour cell list• Change HO parameters such as thresholds, margin, cell baring, etc• Check serving cell’s cell identifier in the neighbour cell’s neighbour list• Check neighbour cell’s BCCH, BSIC, LAC, Cell ID, etc

Dropped CallsDropped Calls -- Caused by either RF environments or incorrect system parameters

Corrective Measures

• Check if an appropriate neighbour cell list is defined• Check HO parameters e.g margins.• Existing or new coverage holes• Interference, Co-channels, Adjacent channels or External interference• Serving cells might go down • Abnormalities such as call setup failure



Frequent Handovers( Toggling)Frequent Handovers( Toggling) -- Serving keep changing and as a result of bad audio quality

Corrective Measures• Check if an appropriate neighbor cell list is defined• Check HO parameters • Interference, Co-channels, Adjacent channels or External interference• Lack of dominant server• Poor coverage• Not optimal antenna configuration• Hardware Problem e.g more feeder loss, more diplexer loss, less RT power etc.

System BusySystem Busy -- System busy on several call attempts and site appears consistently on the traffic report

Corrective Measures

Short Term•Add additional RTs•Upgrade the BTS Configuration.•Increase SDCCH if there is SDCCH congestion.

Long Term• Build a new cell site to off-load traffic



Handover BoundaryHandover Boundary -- Handovers do not occur at the desired HO boundary, the result is an imbalance in traffic distribution across the system

Corrective Measures• Check if an appropriate neighbour cell list is defined• Check HO parameters • Inappropriate antenna configurations of the serving and neighbour cells• Interference, Co-channels, Adjacent channels or External interference• No TCH available (neighbour cells congestion)



SignalQuality

Range (in BER)

01234567

BER < 0.20.2 < BER < 0.40.4 < BER < 0.80.8 < BER < 1.61.6 < BER < 3.23.2 < BER < 6.4

6.4 < BER < 12.8BER > 12.8

SIGNAL QUALITY LEVELSSIGNAL QUALITY LEVELS



Short call Parameters

A sequence is fed in to the tool e.g 100 calls with a holding time of 60 secs which repeats Automatically. The parameters which are monitored are• Call set up success rate.• Call setup time.• Access Delay( Time between channel request and call alert)• Call success rate.• Call drop rate• Handover success• Handover failure



Narrow Band or Carrier Wave (CW) Measurements

The figure below shows a geographical representation of CW measurement equipment

GPS

BTS

MS

Radio Path

TXCW

RX

Signal

Level

COMPUTER

Time Data Storage

Average VarianceLocation time speedLocation



Survey Transmitter Parameters

PARAMETER VALUE

Transmitter Output Power 43 dbmTransmitter Feeder Cable loss 2 dbmTransmitter feeder cable length 10 mTransmitter omni-directional antenna gain 7 dbi

Effective Isotropic Radiated Power 48 dbm

CW MEASUREMENT

For analysis the out-of-band frequency is chosen



While doing CW the following things to be kept in mind

1 - Choose test sites such that each site coverage area has nearly all the clutters.2 - The route of each site should have all the clutters( numbers of bins for all the clutters

must be same)3 - The height of site = average clutter height + 3 meters.4 - Use differential GPS.5- Take panoramic photos of CW survey sites and near by area.6 - It is recommended that route length should be at least 80 kms.7 – Take around 15 files , 13 for tuning the model and 2 for validating it.

The CW ( based on Signia Tool ) have got two ASCII text files data file has decimal lat long and received power strengths Header ( .hd) has got all the information e. g site id,site name, lat long of test site, frequency , height of antenna etc.



Okumura-Hata Model

L = a + a log f - a log h - a hm + { a - a log h} log d - Le

a, a, a, a, a are adjustable parameters

f frequency(MHz)

h Base Antenna Height

hm Mobile Station height

d Distance between both antennas

Le Correction factor

a Mobile antenna height correction factor



Standard Macro Cell Model ( Used By Asset.)

Standard Macro Cell model is based on Okumara – Hata Model. The addition is inclusion ofclutter And height.Model is valid for freqencies between 150 MHz and 2 GHz.

Prx = Ptx – Ploss

Ploss = k1 + k2log(d) + k3 ( Hms) + k4 log( Hms) + k5 log( Heff)+ k6 log( Heff) log(d) +k7 diff+ C_ Loss.

d = distance betn base station and mobile stations ( kms)Hms = Height of mobile station from ground.Heff = efefctive base station antenna height.diff = Diffraction loss.k1 & k2 = intercept and slope.k3 = mobile antenna height factor.k4 = okumara hata multiplying factor for Hmsk5 = effective antenna height gain.k6 = okumara hata type mutiplying factor for log(Heff) log(d).k7 = Diffraction.C_ loss = Clutter loss.( height and separation is also included).



Model Tuning

While calibrating the model we need to compare it with propagation data, so CW measurement help you produce an accurate prediction model that functions correctly.

• Filter the data• Analysis of data , S.D, Mean Error.• Tune the values of K1 – K7 to get S.D < 8 and mean error tending to ZERO.• Fine tune the values of clutter offset to get the final value of S.D and mean error to zero.• The S.D < 8 – very good. S.D < 10 acceptable.





TOOLs Used

• Planning Tool - Asset from Aircom International UK.• Frequency Planning – ILSA from Aircom• Drive test Tool – Tems from Erricssion and Neptune from Aircom.• Analysis – FICS for Tems and Probe for Neptune.• PMS – Matrica.



SURVEYSTypes of surveys• Site Selection Surveys - is conducted before installing a site to consider following aspects

• Location meeting the search-ring requirements• Space for antennas• Antenna Separations• Obstacles Nearby• Space for Radio Equipment• Power supply / Back-up• Transmission link• Coverage area study• Contract with the owner

• Drive Test Surveys - conducted regularly • To ensure the health and proper functioning of the Network and its

elements • To detect Interference areas• To meet the Subscribers need• To provide better coverage• To solve Quality problems


Documents

Cellular Engineering