rf design introduction v3

© Copyright 2005 Wireless Facilities, Inc.

RF Design Introduction

Advanced Technology Group

February 2005


Outline

Radio Frequency Propagation

Link Budget

Digital Modulation

Frequency Planning

Traffic Capacity Analysis

Network Dimensioning

Frequency Hopping


Radio Frequency Propagation


Propagation Modeling

The propagation model will serve as a guideline for determining how a transmitted signal will radiate from a given site, or more specifically, the predicted receive signal strength at a particular point relative to the cell site.

This information will help to determine the effectiveness of a planned cell site, and ultimately, how many sites will be needed to cover a desired area.

Empirical data shows RF propagation to “typically” have three components: Path loss slope, Log-normal fading, and Rayleigh fading.


Cell Size Limitation

Cell size coverage is limited by either horizon distance (“line of sight”) or “path loss”.

Distance to horizon is related to antenna height.

Path loss is related to intrinsic “free space “ path loss due to power spreading in a wave, sometimes modified by additional loss due to reflection and scattering of the wave by buildings and other objects.

Power is lost when the RF wave passes through foliage on trees, etc. This is a seasonally variable effect.

The word “propagation” refers to the direction of power flow in the RF wave, and the power density or power level.


Propagation Path Loss

In a “free space” the “path loss” is 20dB/ decade of distance, or 1/r2.

In urban and suburban areas because of terrain and land covers (buildings, other man made objects, etc.), the “average” loss is in the range of 34 to 40dB/ decade of distance, corresponding to 1/r 3.4 or 1/r 4.

-40-50-60-70-80dBm

0.1 1 10 100 Km


Path Loss Slope

Path loss slope refers to the general trend of receive signal level as the distance from the BTS increases.

This is generally quoted in the logarithmic units of dB/dec.

The path loss slope tells how much the receive signal level drops for every ten-fold increase in distance.

For free space propagation the path loss slope is about 20dB/dec.

However, for the real world mobile communication this is closer to 34 to 40dB/dec.


Mobile Radio Channel Model

Time/SpaceTime/Space

Sign

al P

ower

One commonly used statistical description of the mobile radio channel, models the received signal as a combination ofthree components.

– 1. Propagation Loss – 2. Slow Fading – 3. Fast fading


Slow or Log Normal Fading

Actual received signal levels will deviate around the expected path loss slope.

These fades, or deviations, are mostly the result of terrain changes and the associated vehicle speed.

The nature of this fading has been shown to be log-normal with respect to the path loss slope. Hence, the statistical metric of standard deviation is often used to describe the amount of slow Fading. The standard deviation of this log-normal fading will depend on actual terrain;

however, a 5 to 8dB of standard deviation is generally accepted for “typical” terrain.


Multi-path Fading (cont’d)

Raleigh Scenario:Completely Scattered FieldNo Dominant Direct PathAll Paths have comparable Strengths.

Ricean Scenario:Partially Scattered FieldOne Dominant Strong PathOthers are comparable and weaker

R1

R3R2

Rn

R1R3

R2

Rn

R0


Hata’s Equation (review)

LHaUr(dB) = 69.55 + 26.l6 logfc - 13.82 log (hT) - a(hR) + [44.9 - 6.55 log hT)] log r -CF

The path loss for suburban areas is given byL ha,Su (dB) = L ha,Ur(dB) - 2[log fMHz/28]2 - 5.4

The path loss for open areas is given byL ha,Op(dB) = L ha,Ur(dB) - 4.78 [log fMHz]2 - 18.33log fMHz- 40.94

Range of Validity150 < fMHz < 15001 < rkm < 2030 < (hT)m< 200l < (hR)m < 10


COST231-Hata Model (review)

For PCS bands 1800/1900MHz Okumura-Hata model is not valid.

Instead a modified version called COST231-Hata is usually used.

LHaUr(dB) = 46.3 + 33.9 logfc - 13.82 log (hT) - a(hR) + [44.9 - 6.55 log hT)] log r +CF


Multi-path and Fading Effects

Multi-path and fading effects determine the small distance/ short time power variations.

Multi-path delay leads to two undesirable effects:

Fading, treated mainly by diversity in the receiver.

Inter-symbol interference, which is treated mainly by using an adaptive equalizer.


Typical Mobile Fading (Simulation)

millisecond


Multiple Wave Fading

When many waves pass through a region in different directions, with different amplitudes, all due to reflections from various surfaces (building, trucks, etc.) of the same source wave, the reflected waves will have the opposite E field.

The sum of the traveling waves incident and reflected create a pattern called Standing Wave. This Standing Wave pattern can be the cause of deep fades in some places.

When the Rx antenna is stationary, it may have a very small signal (below “noise” level) at some “bad” spots. Moving only a few centimeters will improve the signal.

Reflected wave has the opposite E field.

Conductive surface Such as metalizedGlass building


Multiple Wave Fading (cont’d)

When the RX antenna is on a moving vehicle, the RF signal strength fades in and out almost periodically.

The theoretical statistical formula for this is known as “Rayleigh fading”, when there is no single strong direct ray.

When there is a major direct ray plus a combination of lower amplitude multiply reflected rays, the statistics describing this case have been analyzed by the mathematician, S. O. Rice, and are called “Rician fading”.

Urban scatteringobjects


Diversity

Multiple antennas are used to receive an overall better signal.

Two Rx antennas separated horizontally by a distance d are commonly used for un-correlated fading at each antenna.

The separation d in general varies with the antenna height h.

Separate receive chains associated with each Rx antenna, are used in the same radio receiver.

Three major methods:Switched DiversityMaximal Ratio Combining DiversityEqual gain (phase shift only) Diversity


Diversity (cont’d)

Time

Sign

al L

evel

in d

B

Signal A Signal B Combined Signal

After reception the two signals can be combined and the fade smoothed out before the message is detected.

Addition of two equal strength RF carriers doubles the voltage, quadruples signal power, while incoherent addition of noise signals only doubles noise power. Signal to noise ratio improves by 4/2=2(3dB).


Switched Diversity

“Front end”Receiver


Antenna B

Diversity Switchselects stronger

PowerDetector

“Front end” receiver for each antenna, amplifiesSignal to useable voltage level.Antenna A

Audio

Switching diversity combining:Switch to the stronger of the two RF signals.Switchover must be done only between the time slots to prevent a phase shift “hit” at the switchover instant.Switching is instantaneous using a 1dB hysteresis.


Equal Gain Diversity


Antenna A


Unity GainPhase Shifter

Antenna B

Detector Audio

Phase measurementand control

++Add together twosignals with in phase carrier waves

Adaptively phase shift or delay one signal to keep the RF carrier in phase with the other signal, then add the two for “equal gain”.


Maximal Ratio Diversity



Antenna A

Phase Shifter

DetectorAmplitude andphase

measurementand control

++Add together twosignals with equal amplitude and inphase carrier waves

Variable gainamplifiers

Amplify the weaker signal to the same level as the stronger for “maximal ratio”.

The term “maximal ratio” refers to maximal signal to noise ratio.

Audio

Antenna B


Effect of Multi-path on Signals

In a mobile radio environment multiple delayed “copies” of the transmitted waveform appear at the receive antenna. Some are “inverted”, some positive.

For a single symbol transmitted from a transmitter, at the receiving end, not only this same symbol is received but also many delayed copies of earlier symbol(s).

These time delay spreads are caused by signal reflection off of high rise buildings, mountains, etc.. The time delay spread intervals, which are measured from the first symbol to the last detectable delayed copy are different in different built environments.

Inter-Symbol Interference (ISI) is the distortion of the signal for one symbol due to the addition of a delayed copy of the earlier symbol(s).

IS-136 specs require an equalizer to handle delay spread up to 41µs (one TDMA symbol duration) or less, with equal (faded) rays.


ISI and Equalizer Action


ISI and Equalizer Action

Control box in the equalizer adjusts the tap coefficients a0, a1, a2, a3 to produce minimum ISI during the SYNCH or CDVCC bit stream input interval of the TDMA frame.

Control box continues to make small adjustments in a0, a1, a2, a3 during data reception, to keep the symbol values as close as possible to design levels.


Doppler Shift

When the mobile station moves towards the base, the RF frequency seen by the receiver increases.

When the mobile station moves away from the base, the RF frequency seen by the receiver decreases.

This is the result of Doppler effect, the same physical phenomenon that makes the pitch of a train whistle appear to go up and then down as the train moves toward and then away from you.

For a 100Km/h speed at 1900Mhz, the frequency shift can be +/-176Hz or more.

f’ = f [1+ (v/c) cos θ]

This is usually corrected at the equalizer to prevent false PM.


Link Budget


Link Budget Overview

The link budget will provide an analysis of the communication link between the base station (BTS) and the mobile (MS), and it is one of the first activities performed within the design process.

The link budget will take the coverage objective, the technology and propagation assumption and provide guidelines for:

the average cell radii, BTS transmit powers, and the signal levels which define the cell edge.


Technology Assumptions

BTS TxPower

BTS TxSystem Losses

IsotropicPathloss

Car/ BuildingPenetration Loss

Human BodyLoss

MS RxSystem Losses

MS RxSensitivity

Downlink

MS TxPowerBTS Rx

System Losses

IsotopicPathloss

Car/ BuildingPenetration Loss

Human BodyLoss

MS TxSystem LossesBTS Rx

Sensitivity

UplinkDiversity

Gain

To construct an accurate link budget, the following data is necessary:BTS and MS maximum transmit power;BTS and MS receive sensitivities;Antenna parameters and diversity considerations;Potential infrastructure configuration issues.


Maximum RF Path Loss (review)

RXBSSensitivity

RXMSSensitivity

Path Loss Down Link

Path Loss Up Link

TXBS

TXMS


LBA Inputs

Base and Mobile Receiver Sensitivity ParametersMinimum Acceptable Signal to Noise Ratio Environmental/Thermal Noise AssumptionReceiver Noise Figure

Antenna Gain at Base & Mobile Stations

Hardware Losses (Cable, Connectors, Combiner,....)

Target Coverage Reliability

Propagation Characteristics of the Channel

Receiving Environment

LBA


LBA Outputs

Coverage Design ThresholdsIn-BuildingIn-CarOn-Street

Base Station ERP

Maximum Allowable Path Loss

Cell Size Estimate

Cell Count Estimate

LBA


List of Gains and Losses

GainsPower Amplifier GainBase Station Antenna GainMobile Antenna GainDiversity Gain

Losses Hardware

CombinerCablesConnectorsDuplexer

Air InterfacePropagation LossesFade MarginPenetration Losses

In-carIn-BuildingBody Loss+

Coverage


Cell Edge Signal Thresholds

Outdoor cell edge mobile minimum received signal equals to:MSRXSENS + (Fade margin + Body loss)Ex: Outdoor Cell Edge = -103dBm + (5dB + 3dB) = -95dBm

In car cell edge mobile minimum received signal equals to:MSRXSENS + (Fade margin + Body loss + Car penetration loss)Ex: In Car Cell Edge = -103dBm + (5dB + 3dB + 6dB) = -89dBm

In building cell edge mobile minimum received signal equals to:MSRXSENS + (Fade margin + Body loss + building penetration loss)Ex: In Building Cell Edge = -103dBm + (5dB + 3dB + 20dB) = -75dBm


Cell Site RF Equipment

DuplexerAnt

TxRx

Radio/ PA

Radio/ PA

Radio/ PA

Radio/ PA

Combiner 1930 – 1945MhzBPF

1850 – 1865MhzBPFSpliter LNA


RMC


BTS Hardware Components

Antenna

Duplexor

Receiver Multicoupler

Power Amplifier

Transmit Combiner

Master Oscillator

Radio Receiver and Transmitter


Antenna Parameter

An antenna is a passive device which acts as to focus energy.

It does not amplify RF energy, but merely redirect it.

The important antenna characteristics are: Gain, Horizontal and Vertical beam width, andDiversity performance.

The gain and beam width parameters are interrelated and usually come at the expense of one another.

With regard to antenna reception these parameters quantify the amount of energy that the antenna is able to collect in a particular direction. With regard to transmission, they indicate the amount of power transmitted in a particular direction.


Antenna Gain

The gain of an antenna specifies the extent that the receive or transmit power density is focused along the main beam. This gain is usually stated in terms of decibel gain relative to an isotropic antenna (dBi), or decibel gain relative to ½ wave dipole antenna (dBd).

An isotropic antenna does not exist in reality, it merely serves as conceptual radiating element where energy is propagated uniformly in all directions from a point.

Figure – Basic Propagation Model for an Isotropic Antenna

Power density,ρ = Ptx/ (4πr2) W/m2

x

y

z

Ptx

4πr2


Antenna Beam width

Sectored antennas are commonly used in a mobile environment.

They focus energy within a particular angle in the horizontal plane.

Horizontal Beam width describes the horizontal angle within which the gain of the antenna does not drop below 3dB from the beam gain.

Vertical Beam width describes the vertical angle within which the gain of the antenna does not drop below 3dB from the main beam gain.


Diversity Performance

Receive diversity at the cell site can be used to improve mobile reception and coverage reliability.

Space diversity is commonly used in the mobile environment which uses receive antenna separation to help minimize the effect of Rayleigh fading on the up link.

Rayleigh fading is a consequence of multiple copies of the same signal combining de-constructively at the receive antenna.

By using two or more antennas separated in space, it increases the probability that both antennas will not experience fades simultaneously.


Duplexor

Allows transmitter and receiver to share one antenna.

Uses broadband resonant filters to keep high transmit RF power from entering the receiver chain.


Band Pass Filter

It Is used to suppress the unwanted signals outside the desired receive or transmit band.


Multi-coupler

It is used to amplify and couple the signal received from the antenna to the receivers.

It includes an LNA and a power splitter.


Power Amplifier (PA)

It is used to amplify the output of a radio transmitter from about 10mW to 30W.


Transmit Combiner

Combines the output from several PAs and directs the power toward the antenna.

There are two different types of combiners:

Cavity tuned combiner:It uses tuned cavity (Cylinder) resonant filters. It requires mechanical re-tuning to change RF channels. The auto-tune combiner technology allows for remote frequency setting.

Hybrid combiner:It is wide band and does not require any tuning.


RBS Hardware Losses and Gains

Typical hardware losses and gains for a TDMA cell site (Ericsson equipment) are:

H/W Parameters BTS MSTX PA Output power 30W/ 44.8dBm 0.6W/ 28dBmCombiner loss 4.2dB 0Tx Band Pass Filter loss 0.6dB 0Duplexor loss 0.5dB 0Feeder loss (Cable & Jumpers) x.xdB (depends on feeder length) 0Antenna Gain 17dBi (depends on the antenna type) 0RX Sensitivity -110dBm -103dBmTMA gain/ noise figure 12dB/ 1.5dB

The RBS Rx sensitivity is at 1% BER measured at the RBS cabinet RX connector (input to MultiCoupler unit).


RBS Hardware

DuplexerAnt

TxRx

Radio/ PA

Radio/ PA

Radio/ PA

Radio/ PA

Combiner 1930 – 1945MhzBPF



RMC


Effective Radiated Power (ERP) (review)

Power Amplifier

HardWareLosses

PA LH

GantennaERP

ERP=PA - LH + GAntenna


Receiver Sensitivity (review)

LNA

RX: Receiver sensitivityIs the minimum acceptable input signal level in dBm, at the input of the receiver’s low noise amplifier, required by the system for reliable communication.

RX is a function of:Carrier to Noise Ratio (CNR)

For a given FER, e.g. of about 1%, each type of modulation and coding requires a minimum signal to noise ratio which at the bit level is stated as Eb/No.

Thermal/Environmental Noise: Is a combination of

– Antenna Noise (dBm)– Receiver Noise Figure(NF) in dB– Temperature and System Bandwidth


Receiver Sensitivity Calculation (review)

ReceiverNoise Figure

Nin= k T B

(S/N)out(S/N)in

Absolute Sensitivity

RX Sensitivity =

( ) ( )( )( )

log( ) ( )

SNR SNR NFS N SNR NFS N SNR NF

S k T B NF SNR

in dB out dB

in in out dB

in in out dB

in out dB

= +− = += + +

= ⋅ ⋅ + +10

To Demodulator+


Penetration Losses (review)

In-Car

On StreetIn CarIn BuildingBody Loss


Coverage Objective

The term coverage refers to an area having sufficiently strong signal level on both the uplink and downlink that a user can originate a call with acceptable voice quality in both directions.

Coverage requirements are usually stated as:

The need for in building, in car, or outdoor coverage.

The probability of coverage at the cell edge and over the cell area.


Contour Coverage Reliability

Normal Distribution

Due to various shadowing and terrain effects the signal level measured on a circle around the base station shows some random fluctuations around the estimated value given by the propagation model.

This random signal level along the cell boundary has Lognormal variations.


Coverage Probability

Cell area coverage:Refers to the percentage of average useable area by the cell site. This perspective would provide an estimate of successful origination of a call, if the user were to attempt calls at a random positions within the cell boundary.

Cell edge criteria:Gives the threshold of acceptable performance. This perspective will define the cell edge as the distance where a serving cell site can no longer provide a minimum service reliability. As a result, the cell edge will establish an area within which most of the points should have a probability of coverage greater than threshold.


Fast or Rayleigh Fading

Fast fading occurs at a much quicker rate than its log-normal counterpart.

Fast fading is the result of signals reflecting off of man made objects and taking different paths from the transmitter to the receiver.

Because of path differential, similar signals can arrive at the receiver at different times and cause constructive and deconstructive condition.

Fading due to multi-path reflections can be shown to have Rayleighdistribution.

Although Rayleigh fading is present in most environments, its effect is normally not considered by propagation models.

One reason being that it would be difficult to build a database of all man made structures.


Fade Margin Calculation

The process of engineering a cell coverage involves uncertainty as a propagation model with associated error is used to predict signal levels for a certain area.

To combat the uncertainty involved with propagation prediction, a fade margin is used to pad the link budget and provide a confidence factor that a sufficient signal level will be present a certain percentage of the time.

Many natural processes can be characterized by a normal distribution.

A normal distribution can be fully characterized by its mean and standard deviation.

Standard deviation:Statistically speaking, it is used in conjunction with a normal distribution to show how spread out the population of outcomes are with respect to its mean.


Fade Margin Calculation (cont’d)

When applied to RF propagation modeling, standard deviation provides a statistical metric that helps to quantify the uncertainty involved with the model.

As discussed before, 5 to 8 dB of slow fading standard deviation is typically assumed for normal terrain.

This can also be viewed as prediction error.



-118 -110 -102 -94 -86

Distribution of Received Signal Strength

Received Signal Strength

Prob

abili

t y D

ensi

ty

If a system is engineered to meet the mobile sensitivity requirement of –102dBm at the cell edge, there will be a 50% chance that the observed signal strength will be at or above -102dBm. This will provide 50% coverage reliability.



-110 -102 -94 -86 -78

Distribution of Received Signal Strength

Received Signal Strength

Prob

abili

t y D

ensi

ty

%85%15

The incorporation of a fade margin provides increased cell edge reliability. In the above case, defining the cell edge at –94dBm will provide a 85% probability that the actual received signal strength will be above the –102dBm at the cell edge.

This means that a signal can fade 8dB more than the propagation model had expected without dipping below the mobile receive sensitivity.


Area Coverage Reliability

Coverage design objectives are usually defined in terms of area reliability. Area reliability is the percentage of area where the received signal is above the threshold. It can be thought of as the average of contour reliability for all circles of radii r, 0 < r < R.


Area to Contour ReliabilityA

rea

Rel

iabi

lity

σ/n

Contour ReliabilityArea Reliability

σ /n


Fade Margin vs. Contour Reliability

Contour ReliabilityStandard Deviation of Fade Fade Margin


Fade Margin Calculation

For a given

Standard Deviation, σ (urban: σ =5dB, suburban: σ = 6dB, rural: σ = 7dB).

The propagation loss factor, n (Note: A typical propagation loss factor is n = 3.5);

Compute σ /n.

For the required area reliability and computed σ /n

Estimate coverage contour reliability from plot 1 (Area to Contour Reliability)

Use the contour reliability, the standard deviation, σ, and plot 2 (Fade Margin vs. Contour Reliability) to estimate the fade margin.


Down Link Budget CalculationFor sites with short feeder line

BTS EIRP:BTSTX_EIRP = BTSTXPA_OutputPower – BTSTX _SystemLoss +

BTSTX_AntennaGain

Where:BTSTX _SystemLoss = (Combiner loss + TX Filter loss) + Duplexor loss + Feeder loss

= (4.2dB + 0.6dB) + 0.5dB + 2.5dB = 7.8dB

BTSTX_EIRP = 44.8dBm – 7.8dB + 17dBi = 54dBm

This is the maximum EIRP attainable. For a 50dBm EIRP, the PA output should be set at 41dBm (36.2dBm at the RBS cabinet TX connector).

Down Link Budget MaxPathLoss = BTSTX_EIRP – MSRX_SENS = 54dBm – (-103dBm) = 157dBm


Up Link Budget CalculationFor sites with short feeder line

MS EIRP:MSTX_EIRP = MSTXPA_OutputPower = 28dBm

Up Link Budget:MaxPathLoss = MSTX_EIRP – BTSRXSENS_EIRP

BTSRXSENS_EIRP = BTSRXSENS_RMC + DuplexorLoss + FeederLoss -DiversityGain - AntennaGain

= -110dBm + 0.5dB + 2.5dB –3dB –17dBi= -127dBm

MaxPathLoss = 28dBm – (-127dBm) = 155dBm

•


Path Balancing

The limiting path is the uplink which provides the worst link budget (155dBm).

In order to balance the uplink and downlink paths,

The PA setting at the BTS should be set to 43dBm (38.2dBm at the RBS cabinet TX connector) to make the downlink budget about equal to the uplink budget.


Down Link Budget CalculationFor sites with long feeder line

BTS EIRP:BTSTX_EIRP = BTSTXPA_OutputPower – BTSTX _SystemLoss +

BTSTX_AntennaGain

BTSTX _SystemLoss = (Combiner loss + TX Filter loss) + Duplexor loss + Feeder loss

= (4.2dB + 0.6dB) + 0.5dB + 5.5dB = 10.8dBBTSTX_EIRP = 44.8dBm – 10.8dB + 17dBi = 51dBm

This is the maximum EIRP attainable. For a 50dBm EIRP, the PA output should be set at 44dBm (39.2dBm at the RBS cabinet TX connector).

Down Link Budget:MaxPathLoss = BTSTX_EIRP – MSRX_SENS= 51dBm – (-103dBm) = 154dBm


Up Link Budget CalculationFor sites with long feeder line

Up Link Budget:

MaxPathLoss = MSTX_EIRP – BTSRXSENS_EIRP

BTSRXSENS_EIRP = BTSRXSENS_RMC + DuplexorLoss + FeederLoss -DiversityGain - AntennaGain

= -110dBm + 0.5dB + 5.5dB – 3dB – 17dBi = -124dBm

MaxPathLoss = 28dBm – (-124dBm) = 152dBm

•


Path Balancing

The limiting path is the uplink which provides the worst link budget (152dBm). In order to balance the uplink and downlink paths,

The PA setting at the BTS should be set to 43dBm (38.2dBm at the RBS cabinet TX connector) to make the downlink budget about equal to the uplink budget.

Reducing the BTS output power will affect the cell coverage radius.

Tower Mounted Amplifiers (TMA) can be used to improve the over all sensitivity of BTS.


Noise and Interference limitation

Receiver minimum detectable signal power is limited by some unavoidable signals:

External - co-channel and adjacent channel interference,

Internal - thermal noise, due to motion of separate discrete electrons in wiring.


Thermal Noise

A conductive element with two terminals may be characterized by its resistance, R (ohm). This resistive element contains free electrons that have some random motion if the resistor has a temperature above absolute zero. This random motion causes a noise voltage to be generated at the terminals of the resistor.

Although the noise is small, when it is amplified by a high gain amplifier, it can become a problem. The amount of thermal noise produced in any electrical resister is proportional to the bandwidth B.

The Noise power Pn is given by:

Pn = KTBWhere:

K = 1.38 x 10-23 J/ K so called Boltzmann’s constantT = 273 + Co Absolute temperature of the resistor in degree Kelvin.

For B = 30 Khz, Pn = 1.2 x 10-16 Watts or –129dBm. This is the so called “noise floor”.


Noise Figure

To characterize the effectiveness of a device, a figure of merit is needed that compares the actual (noisy) device with an ideal device (no internal noise source).

A figure of merit, Noise Factor F, is the measure of the degradation of the signal to noise ratio due to the noise added in the device.

F = (S/N)i / (S/N)o,

Noise Figure NF = 10 Log10 F


Noise Figure (cont’d)

F1 G1 F2 G2 Fn Gn…Device # 1 Device # 2 Device # n

Noise Factor (Figure) in a cascade network

F = F1 + [(F2-1)/ G1] + [(F3-1)/ G1G2] + … + [(Fn–1)/ G1G2…Gn-1]

Where:F and G are the noise factor and gain of each stage.


Noise Figure (cont’d)

The preceding equation for Noise Figure in a cascade network states that if the power gain of the first stage is large, the overall noise figure of the network will be essentially that of the first stage. Thus, in a receiving system design it is important that the first stage to have a low noise figure and a large gain so that the noise figure of the overall system will be as small as possible. That is why in a BTS design the TMA (low noise/ high gain amplifier) is placed next to the antenna on top of the tower.

Receiver Sensitivity:The available input signal level, Si, for a given output signal to noise ratio (S/N)o is referred to as the receiver sensitivity.

Si = F (KTB) (S/N)o


Rx Sensitivity at TMA Input

For a C/N = S/N = 17dB and BTSRXSENS = -110dBm at the MultiCouplerinput, we can calculate receiver noise figure:

-110dBm = NF + (-129dBm) + 17dB ⇒ NFBTS = 2dB (1.585)With feeder line loss of 6dB (Ericsson’s recommendation: to maintain a large signal level performance –that is, third order intercept point– a 6dB feeder loss from TMA to the MultiCoupler input is required when TMA is used);

Feeder line, NFfeed = 6dB (3.98) TMA, NFTMA = 1.5dB (1.4) & G = 12dB (15.85)

The overall NF for the BTS receiver is calculated to be:F = 1.4 + [(3.98-1)/15.85] + [(1.585-1)/15.85(1/3.98)] = 1.744 (2.4dB)

Receiver Sensitivity at the input to TMA is:Si = 2.4dB + (-129dBm) + 17dB = -109.6dBm


Up Link Budget With TMA

Up Link Budget:

MaxPathLoss = MSTX_EIRP – BTSRXSENS_EIRP

BTSRXSENS_EIRP = BTSRXSENS_TMA + TopJumperLoss – DiversityGain- AntennaGain= -109.6dBm + 0.2dB –3dB –17dBi= -129.4dBm

MaxPathLoss = 28dBm – (-129.4dBm) = 157.4dBm

With the introduction of TMA to the system, the uplink budget is improved by about 6dB, and the uplink is no longer the limiting path


Cell Size/Count Estimation

Objective:To determine the size and number of cells required to provide coverage for a given area.

Required Input:Maximum Allowable Path Loss (MAPL)Propagation Loss ModelMarket Boundaries


Cell Size/Count Estimation

Link Budget Analysis

Max Allowable Path Loss

Cell Radius Estimate

Cell Count Estimate

Path Loss Model

Field Tests

Market Boundaries


Cell Size Estimation

PL f hh R a h

c b

b m

= + − +− −

69 55 26 16 13 8244 9 6 55

10 10

10 10

. . log . log( . . log ) log ( )

log . . log . log ( ). . log10

10 10

10

69 55 26 16 13 8244 9 6 55

R MAPL f h a hh

c b m

b=

− − + +−

Using Hata’s Empirical Formula

Cell radius estimate can then be derived based on Hata’s formula:


Cell Count EstimationCell Count Estimation


Frequency Planning


Frequency Planning

The frequency reuse of available frequency bands can achieve higher capacity.

Reuse distances have to be high enough so that the co-channel interference, by the links using the same carrier frequency, is sufficiently low.For adequate speech quality, C/I has to exceed a certain threshold.

A cluster of size K is a group of K cells in which each frequency is used just once.

For a Homogenous hexagonal network, with a cluster size K and total number of frequencies Nt the number of frequencies per cell is: Nc = Nt/K

Capacity can be increased by reducing the cluster size. Cluster size reduction can be achieved by:

Reducing the number of interferers by SectorizationReducing interference from other cells by using features such as DTX, Power Control, Frequency Hopping, Antenna Tilting, Smart Antennas, etc.


Frequency Planning (cont’d)

The GSM specification states that the system should work satisfactorily down to C/I = 9dB.

However, it is recommended to use C/I = 12dB as a design figure to provide a useful margin.

It also states that a C/A = –9 dB should be the limit for acceptable adjacent channel interference.

With a 4/12 re-use pattern, the level of adjacent channel interference will be very difficult to reduce because so many of the channel groups are adjacent to each other. Therefore, it is recommended to use a C/A = –3 dB for a design target.

It is very efficient to use a combination of Frequency Hopping, Dynamic BTS and MS Power Control, and DTX.

The mutual interactions between these features provides a very powerful method to increase system performance. This yields that the system can utilize a tighter re-use pattern and thereby higher system capacity.


Frequency Planning (cont’d)

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

TRX1A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

512 513 514 515 516 517 518 519 520 521 522 523

Assuming a 2.5 MHz of spectrum, it provides 12 carrier frequency channels. The 12 available channels can be used to roll out a standard 4/12 re-use pattern, one channel per sector.


4/12 Re-use Pattern

TRX1(BCCH)

512 513 514 515 516 517 518 519 520 521 522 523

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

524 525 526 527 528 529 530 531 532 533 534 535TRX2(TCH)

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3 A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

Assuming a 5 MHz of spectrum, it provides 25 carrier frequency channels. From the 25 channels available, 12 are used for BCCH frequencies on TRX1, and the remaining 12 channels are used for TCH frequencies on TRX2.


Multiple Re-use Pattern

TRX1(c0 filler)

512 513 514 515 516 517 518 519 520 521 522 523

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

TRX2TRX3(TCH)

524 525 526 527 528 529 530 531 532533 534 535

a1 b1 c1 a2 b2 c2 a3 b3 c3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3 A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

b1

B2b3

c1

c2

c3

a1

a2

a3

b1

B2b3

c1

c2

c3

a1

a2

a3b1

B2b3

c1

c2

c3

a1

a2

a3b1

B2b3

c1

c2

c3

a1

a2

a3

b1

B2b3

c1

c2

c3

a1

a2

a3

b1

B2b3

c1

c2

c3

a1

a2

a3

b1

B2b3

c1

c2

c3

a1

a2

a3

From the 25 channels available, 12 are used for a standard 4/12 re-use pattern for BCCH frequencies. The remaining 12 channels can be assigned to a 3/9 re-use pattern. Employing Synthesizer (RF) hopping with BCCH frequency included, TRX1 operates only as c0 filler, and TRX2 will be hopping on all available frequencies.


1/3 Fractional Re-use Pattern

For a small bandwidth allocation, the fractional 1/3 re-use provides a higher number of frequencies to hop on compared to standard 3/9 re-use pattern.

This provides a better service quality since it takes full advantage of frequency hopping. The gain in quality can be turned into a gain in capacity since the load can be increase by the addition of a TRX.

In a fractional re-use, the available spectrum is divided into two sets:A first set corresponding to the BCCH carrier frequencies re-used according to a 4/12 re-use pattern.A second set for the other frequencies re-used according to the fractional 1/3 re-use pattern.


1/3 Fractional Re-use Pattern (cont.)

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3 A1A2

A3

B1B2

B3

D1D2

D3

C1C2

C3

b1

B2b3

c1

c2

c3

a1

a2

a3

b1

B2b3

c1

c2

c3

a1

a2

a3b1

B2b3

c1

c2

c3

a1

a2

a3b1

B2b3

c1

c2

c3

a1

a2

a3

b1

B2b3

c1

c2

c3

a1

a2

a3

b1

B2b3

c1

c2

c3

a1

a2

a3

b1

B2b3

c1

c2

c3

a1

a2

a3

TRX1(c0 filler)

512 513 514 515 516 517 518 519 520 521 522 523A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

TRX2TRX3(TCH)

a1 b1 c1

524 525 526527 528 529530 531 532533 534 535

Assuming a 5 MHz of spectrum, it provides 25 frequency channels.From the 25 channels available, 12 are used for a standard 4/12 re-use pattern for BCCH frequencies. The other 12 are assigned to a 1/3 fractional re-use pattern for TCH frequencies. The BCCH frequency should not be part of the hopping frequencies set for the fractional re-use.


Traffic Capacity Analysis


Traffic Capacity Requirement

Depending on the subscriber growth forecast, traffic analysis may show congestion soon after initial cell build out.

If Traffic analysis predicts high usage areas within the early days of system deployment, it may be necessary to reduce cell size and increase cell density in these areas to effectively handle offered traffic.


Traffic Usage

The usage of traffic path is defined by two parameters:Calling Rate, or the number of times a route or traffic path is used per unit time, or the call intensity per traffic path during busy hour.Holding time, or the average duration of occupancy of a traffic path by a call.

A traffic path is a channel, time slot, frequency band, line, trunk, switch or circuit over which individual communications pass in sequence.

The Carried Traffic is the volume of traffic actually carried by a switch.

The Offered Traffic is the volume of traffic offered to switch.

Offered Load = Carried Load + Overflow


Traffic Usage (cont’d)

A typical hour-by-hour voice traffic variation for a serving switch in Unite States is shown below.It is seen that the busiest period, the Busy Hour (BH), is between 10:00am-11:00am.

0 6 129 15 18 21 24

110

10

Time of day (Hour)

No. ofCalls (K)


Traffic Usage (cont’d)

The Busy Hour (BH) is defined as the time-consistent hour span of time (not necessarily a clock hour) that has the highest average traffic load for the business day throughout the business season.

The Peak Hour is defined as the clock hour with highest traffic load for a single day.

Since the traffic also varies from month to month, the Average Busy Season (ABS) is defined as the three months (not necessarily consecutive) with the highest average BH traffic load per access line.

Phone systems are not engineered for maximum peak loads, but for typical BH loads.

The blocking probability is defined as the average ratio of blocked calls to total calls and is referred to as the Grade of Service (GoS).


Traffic Measurement Units

Traffic is measured in either Erlangs, 100 Call Seconds (CCS), percentage of occupancy, or peg count.

Erlangs: Traffic intensity is the average number of calls simultaneously in progress during a particular period of time. It’s measured either in Erlangs or 100 call Seconds (CCS).

An average of one call in progress during an hour represents a traffic intensity of 1 Erlangs

1 Erlang=1x3600 call seconds=36 CCS.

Percentage of occupancy is the percentage of time that a server is busy.

Peg count is the number of attempts to use a piece of equipment.


Traffic Intensity and Distribution

A typical average traffic intensity and traffic distribution in U.S. for mobile application are given in tables below.

Average traffic intensity for O/I

Typical average traffic distribution in U.S. metro environment

Environment O/I Call/Line

Metro 3.5-4.0Suburban 2.0-2.5

Rural 1.2-1.5

Traffic type Distribution

Mobile to land 65%Mobile to mobile 5%Land to mobile 30%


Blocking Concept

The number of active calls is a Poisson random variable of mean l/m.

0.2 0.099501 20.4 0.196040.6 0.2867990.8 0.369247

1 0.4412481.2 0.5011621.4 0.5478931.6 0.5809191.8 0.600279

2 0.6065312.2 0.6006822.4 0.5841032.6 0.5584252.8 0.525436

3 0.4869790.44486

0.4007680.3562180.3125010.2706710.231526

4.4 0.195628

Poisson Distribution

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.6 1.2 1.8 2.4 3 3.6 4.2 4.8 5.4 6 6.6 7.2 7.8 8.4 9 9.6

Number of Users

Prob

abili

ty D

ensi

ty

Func

tion Blocking

Probability

λ/µ


Offered vs. Carried Traffic

Offered Traffic

CarriedTraffic

OverflowTraffic

The offered load, A, is split intoCarried calls C(A,N), and Blocked calls B(A,N), or overflow trafficN is number of channels/trunks

Utilization, U, can be defined as theratio between carried load and the number of channels or circuits.U(A,N)=C(A,N)/N


Blocking Formulas

Erlang B Formula provides the probability of blockage at the switch due to congestion or to “all trunks busy”. This is expressed as GoS or the probability of finding N channels busy.

The assumptions in Erlang B formula are:Traffic originates from an infinite number of sources.Lost calls are cleared assuming a zero holding time.Number of serving channels is limitedInter-arrival times of call requests are independent of each other.The probability of a user occupying a channel (service time) is based on exponential distribution.Traffic requests are represented by a Poisson distribution implying exponentially distributed call inter-arrival times.

∑=

= N

i

i

N

iA

AA

ANB

0 !

!),(B(N,A)= blocking probabilityN = number of serving channelsA = offered load


Blocking Formulas (cont’d)

Poisson’s Formula is used to design trunks for a given GoS.

A comparison between Erlang B and Poisson formulas shows that Poisson formula results in higher blocking than that obtained by the Erlang B formula for a given traffic load.Erlang B and Poisson’s formulas are commonly used to calculate the blocking probabilities (or GoS) of the wireline or wireless systems. For Erlang loss system, the carried traffic A’ will be:

The lost traffic is:

∑∞

=

−=Ni

iA

b iAeANP!

),(Pb(N,A)= blocking probabilityN = number of trunksA = offered load

[ ]),(1 ANBAA −=′ A’= Carried traffic load

),( ANBA ⋅



Erlang C Formula assumes that a queue is formed to hold all requested calls that can not be served immediately. This means that blocked customers are delayed.

The assumptions in Erlang C formula are:Infinite sourcesPoisson inputLost calls delayedExponential holding timecalls served in order of arrival

∑−

= −+

−= 1

0 )1(!!

)1(!),( N

i

Ni

N

NANA

iA

NANA

ANCC(N,A)= blocking probabilityN = number of serving channelsA = offered load



Binomial Formula:

The assumptions for the binomial formula are:Finite sourceEqual traffic density per sourceLost calls held

is

Ni

s

b DsD

Ns

sDsP ⎟

⎠⎞

⎜⎝⎛

−⎟⎟⎠

⎞⎜⎜⎝

⎛ −⎟⎠⎞

⎜⎝⎛ −

= ∑−

=

− 11 1

Pb= blocking probabilityD = Expected traffic densityN = number of channels in a group of channelsS = number of sources in a group of sources


Grade of Service

The Grade of Service (GOS) is a measure of the ability of the user accessibility to a trunked system.

Given a specific number of channels available in a system, the GOS is used to define the desired performance of a wireless system by specifying the desired probability of a user obtaining a traffic channel.

GOS is typically given as the likelihood that a call is blocked.

A commonly used value for GOS (blocking probability) is 2%.


Traffic Forecasting

PCS carriers will have to determine the characteristics of their penetration rate for potential customers.

A list of different categories and the demographic population break down can be as following example:

1) Vehicle Traffic along the roads

Demographic2) Income of people from the age of 15-34 making $15 – 35k (Demo. 1)3) Income of people from the age of 35-44 making $35 – 50k (Demo. 2)4) Income of people from the age of 35-54 making $50k+ (Demo. 3)


Traffic Forecasting (cont’d)

The break down for years 1 – 5 of the penetration rates per category can be as following example:

Year1 Year2 Year3 Year4 Year51) Vehicle Traffic 0.2 0.6 0.9 1.3 1.652) Demo. 1 0.05 0.24 0.55 0.75 0.913) Demo. 2 0.05 0.25 0.55 0.75 0.924) Demo. 3 0.15 0.4 0.65 0.92 1.2Comb. pen. rate 0.45% 1.49% 2.65% 3.72% 4.68%

These demographic penetration rates can be applied to each census block group (using MapInfo) to determine the potential customer base for that census block.

The vehicle traffic count can be determined for each of the roads and then be spread over the appropriate census block group with the associated penetration rate.


Traffic Forecasting (cont’d)

The number from the vehicle traffic count is then combined with the potential demographics user for each census block group.

From this number and the Erlang per subscriber number calculated before, an Erlang value is determined for each census block group based upon the above assumptions and is spread over the associated area.

The study then can be produced based on the coverage area of each potential sector with an Erlang captured and the associated RF channels and RF carriers required.


Network Planning


Two Types of Traffic

The traffic capacity of the wireline/wireless network can be categorized as: Voice/Data traffic (Erlang traffic).Control/Signaling traffic (events traffic).

The signaling traffic capacity calculation is based on occurrence of an event (i.e.: Call Attempt (CA)) and does not involve the duration of the call.The calculation of the voice traffic considers the call duration

The measurement of the voice traffic is based on Erlang B (blocked calls are not retried).

Calls begin Calls end


Two Types of Traffic (cont’d)

The two types of traffic would impact logically different sections of the network:

The signaling traffic will impact:The signaling links.The databases (HLR/VLR).Data storage.Computer hardware (processors).

The voice traffic will impact:The transcoder.The voice trunk/switch.Voice mail.


Signaling Traffic Impact

The following events have major impact on the traffic calculations and processor utilization:

Call OriginationsCall TerminationsAuthenticationsHandoversLocation UpdatesIMSI Attach/Detach proceduresSMS ServicesData Services


Traffic Model

Traffic model includes some parameters:Grade of Service or blocking probability (i.e.: 2%)Busy Hour Call Attempts (BHCA)/sub. Erlang/sub.No. of subscribers and the growth over the planning period.

Example:

P aram eter V alueG oS, A ir Interface 2%G oS, B SC -M SC 0.1%G oS, M SC -P ST N 0.01%B H C A /sub 1 .5 (assum e all active m obiles)D uration of a call 120 secE rlang/sub .05G rowth of the subscribers 20% /yr


Call Mix Model

Call Mix Consists of: Mobile Origination Call (MOC) %Mobile Termination Call (MTC) %Mobile to Mobile (MTM) Attempts %Mobile Call Completion %

Example:

Parameter Value CompletionMOC(M-L) 60% %70MTM(M-M) 5% %40MTC(L-M) 35% %40


Service Mix Model

Service Mix Model includes the probability of using various services per user per call.

Example:

P aram eter V alueR atio o f S M S per call 0 .1Fax/D ata C alls 0 .05R atio o f V o ice M ail per call 0 .1


Capacity Limits

The maximum network capacity (voice/signaling) is given for each network element.

Each element’s system limit is provided for future expansions (max number of processors)

For a voice sensitive element/link ( i.e., MSC, MC) maximum number of: ErlangsSubscribersTrunks

For a signaling sensitive element (HLR, VLR, SM_SC) maximum number of:Transactions/secData linksSubscribers


NSS Elements’ Limits

The BSC limits are:Maximum no. of BTS that can be supported.Maximum no. of Call Attempt (CA).Maximum no. of voice ports it can support (I/O).Maximum no. of Signaling links that can be supported.

The MSC limits are:Maximum no. of BSC that can be supported .Maximum no. of Call Attempt (CA).Maximum no. of voice ports it can support (I/O).Maximum no. of Signaling links that can be supported.


NSS Elements’ Limits (cont’d)

The VLR limits are:Maximum no. of subscribers (Size of the Memory).Maximum no. of transaction/sec processing on the VLR database.

The HLR limits are:Maximum no. of subscribers (Size of Memory).Maximum no. of Signaling links that can be supported.Maximum no. of transaction/sec processing on the HLR database.


Joint Radio & Traffic Design

In principle radio coverage and traffic distribution are to be considered jointly.

However, due to the inherent task complexity, the procedure calculates:First, a suitable radio coverage for the service area, Then, it verifies if that coverage can fulfill the cell capacity requirements deriving from the traffic forecasting.

These two very strictly dependent steps are iterated until a satisfactory solution is derived.

The factors conditioning the resulting cell layout come from either propagation or traffic constraints, depending on the most critical conditions.


Traffic Analysis

As for the traffic modeling, the service area must be characterized based on subscribers' density and distribution.

Geographical maps or territorial databases are utilized to identify the main roads, inhabitant densities, and business areas.

Urban and geographical analysis can be integrated, when necessary, with data relevant to the fixed telecommunication users distribution.

Since the mobility attributes affect signaling network and distributed data base dimensioning significantly, they are also modeled in this step.


Subscriber Forecast

Service Types and percentagesVoice.Short Messages.Fax.Later on: Data/Internet Transactions.....

Service StatisticsAverage Call Duration.Erlangs/Sub.Outgoing vs. Incoming Call Ratios.....

DemographicsService Penetration.Total Number of Subscribers.Distribution of Subscribers.

Mobility of subscribersHandoff Rates.Location Update Rate.


Demographics Analysis

Demographics Analysis means predicting the subscribers density in different areas based on demographic data such as:

Population Density ( Layered by Age Classes).Income Distribution.Household Distribution.Highways and Vehicular Traffic Distribution.Business Area Maps.

The estimate is usually obtained by a weighted combination of these distributions.

$$$$$

$$$$$$$


Demographics Analysis

Vehicular Traffic Dist. Population Dist. Income Dist.

%50 %0

%50%0

%25 %25

%25%25

%30 %20

%40%10

%? %?

%?%?Subscribers Dist.

W1 W3W2


Subs/Cell

Composite Coverage Design(Cell Footprints)

Subscriber Distribution Map


Alternative Subscriber Forecast

Total Population,Service Penetration Factor

Total No. of Subscribers

LBA

MAPLPropagationModel

Market Area

Subscribers’ Density

# Subs/Cell

Cell Area


Traffic Analysis for BTS

# Subs/Cell

Erlangs/Cell

Voice Channels/Cell

RF Channels/Cell

Erlang/Subs

Erlangs Model GoS

Channelization


BTS Dimensioning

BTS

Step-1: RF channelsFor each sector estimate the required number of

Traffic channels (TCH’s).Control channels (BCCH, CCCH and SDCCH) to support TCH’s.

RF channels or TRX’s / BTS.Perform Feasibility Analysis Against Limitation.

Step-2: BackhaulFor the entire BTS:

Estimate the total number of E0 channels needed.Estimate #E1’s/BTS or #BTS’s/E1 !!!


Step-1: Voice Channels

BTS# Subs/Cell

Erlangs/Cell

Erlang/Subs

Erlangs Model GoS

Voice Channels/Sector


Step-1: Control Channels

Use of Time Slots#TRX’s #TCH’s #Erlangs #SDDCH’s TS0 Other TS’s

1 7 2.94 4 1 BCCH+3CCCH+4SDCCH

2 14 6.2 8 1BCCH+9CCCH 8 SDDCH3 22 14.9 8 1BCCH+9CCCH 8 SDCCH4 30 21.9 12 1BCCH+

3CCCH+4SDCCH8 SDCCH

5 38 29.2 12 1BCCH+3CCCH+4SDCCH

8 SDCCH

6 45 35.6 16 1BCCH+9CCCH 2 x 8 SDCCH7 53 43.1 16 1BCCH+9CCCH 2 x 8 SDCCH8 61 50.6 20 1BCCH+

3CCCH+4SDCCH2 x 8 SDCCH


2 x 8 SDCCH


2 x 8 SDCCH

Note: CBCH uses one SDCCH

Number of Control channel required


Step-1: Number of TRX’s

The maximum number of RF Channels per BTS is limited by:Manufacturers Hardware Limitations.Available Spectrum and Target Reuse Factor.

If the number of RF’s needed is not feasible, cell splitting or more sectorization may be needed.

At the end of this step all BTS’s should have acceptable number of RF channels.

Voice Channels/Sector

Total RF channels

Control Channels/Sector

BTS


Step-2: Backhaul Consideration

Add the number of TCH’s needed on all sectors and calculate the numbers of E0’s needed.

If TRAU is at the BTS # E0 Channels = # TCH’s.

If TRAU is at BSC or MSC# E0 Channels = # TCH’s/4, rounded up????

Add One or two E0’s for Signaling/Control Information.

Estimate the number of E1’s neededTotal # E0 channels/30 = # E1 links


Step 2: Backhaul Consideration

If #E0/30 > 1more than one E1 is neededOne may limit the #E1/BTS to one. In such a case the number of TCH’s per BTS may be limited by E1 capacity, i.e. roughly 28*4=112 TCH’s per BTS.

If #E0/30 < 1Multiple BTS’s may be connected in a Daisy Chain Configuration.

BTS

BTS

BTS

BSC


Example, GSM planning

Problem Statement: Using the following data for a GSM system, calculate:1. Average busy hour traffic per subscriber2. Traffic capacity per cell3. Required number of BSs per zone and the hexagonal cell radius for the zone.System’s Data:

Subscriber usage per month=120 minutesDays per month=24Busy hours per day=5Allocated spectrum=5 MHzFrequency reuse plan=4/12RF channel width=200 KHz, full rateCapacity of a BTS=32 ErlangsSubscribers in the zone=60,000Area of the zone=500 km2


Solution

Erlang per subscriber=

Number of RF carriers=

RF carrier per sector=

TCHs per sector= 2x8 = 16Traffic capacity of a sector at 2% (GoS) = 9.82 ErlangsTraffic per BTS = 9.82 X 3 ≈ 29.5 < 32 Erlangs

Maximum subscribers per BTS=

Number of BTS in a zone=

Average hexagonal cell radius=

0167.060524

120=

××

25200

5000=

234

25=

×

17660167.0

5.29≅

341766

000,60≅

km38.26.234

500=

×


BSC interfaces Review

BSC <-> BTSVoice Ports (E1 trunk)Abis Ports (64kpbs LAPD link)

BSC <-> MSC/VLRVoice Ports (E1 trunk)A link (64kbps SS7 F link)

BSC <-> OMC (R)Data link (X.25 data link)

BSC

MSCMSC

BTS2BTS2

OMCOMC

BTSnBTSnBTS1BTS1


BSC <=> BTS Link

The number of the voice ports (E0) required between the BTS(s) and BSC is determined by the BTS and the traffic channels allocated for the offered traffic.

The number of signaling links required can be derived form the number of traffic channels allocated.

Normally an E0 link will be sufficient to carry the maximum voice/signaling data to/from a BTS.

BTS

BSC


BSC <=> BTS Voice Ports

If TRAU is at the BTSTotal voice ports = total TCH used by the BTS (all of the sectors).

If TRAU is at BSC or MSCTotal voice ports = total TCH used by the BTS (all of the sectors)/ 4, rounded up!

BTS

BSC

It is possible that a full E1 link may not be required by a BTS.

In this case several BTSs can be connected to a single E1 in Daisy Chain Configuration.


TRAU Locations

BTS TRAU BSC To Fixed Networks

MSCTo MS

BTS MSCBSC TRAU To Fixed NetworksTo MS

BTS MSC To Fixed NetworksTo MS BSC TRAU

AInterface

A-bisInterface

RF AirInterface

13 kbps encoded voice / 12 kbps data

16 kbps transmission64 kbps transmission

Physical site


BSC <=> BTS Signaling Ports

The number of Abis signaling links can be determined from: BHCA or call arrival rate obtained from

Total Erlangs from all BTS sectors connected to a BSC.Average Call Duration.

Number of SMS and Location Updates/Call.Abis Message Sizes.

BTS

BSC


BSC<=>MSC/VLR: Voice Ports

Aggregate the Erlang from all of the BTS’s, call it eBTS-BSCPerform an Erlang B look up with a GoS of BSC (usually smaller than BTS GOS) and eBTS-BSC to determine the number of voice channels required.From number of Voice Channels find the number of E0 channels needed

If TRAU is at the BSC # E0’s = # Voice CH’sIf TRAU is at the MSC # E0’s = # Voice CH’s/4, rounded up

BTS1BTS1

BTS2BTS2 BBSSCC

TRAUTRAU

e1

e2

BTS2BTS2en

eBTS-MSC

MSCMSC


BSC <-> OMC

The data interface between the BSC and OMC is based on the X.25 data protocol.A single X.25 data link can be planned for this OMC interface. The capacity of this link depends on the BSC sizing and number of BTSs connected.

19.9kbps or higher is recommended.Usually a 64kbps E0 link is sufficient.

The connection from BSC to OMC may be indirect through MSC.

BSCOMC


BSC Dimensioning (review)

The BSC capacity determines its ability to connect to, and process information received by, all the signaling links from the BTS, the MSC and the OMC. This capacity is usually expressed in terms of

Max_BTS: Total No of BTSs that can be supported/controlled,Max_TRX: Maximum number of TRXs in the connected BTSs,Max_CA: Maximum number of CA,Max_PORT: Total Number of Ports (input and output together).

BTS BSC

MSC/VLR

BTS

BTS

OMC


BSC Dimensioning

For a given system once all of the trunk traffic to the BSC has been identified the capacity requirement can be determined.

The Total Erlang (or BHCA) from all of the BTS < Max_CA The total number of ports required by the BSC< Max_PORTNumber of Connected BTS’s < Max_BTSNumber of TRX’s on Connected BTS’s < Max_TRXThe total number of Signaling links < Maximum No. of signaling links supported

Once the capacity and performance requirement has been identified the equipment (No. of boards etc.) can be determined.


MSC/VLR Interfaces

MSC

PSTNPSTN

OtherOtherMSCsMSCs

OMCOMC

MCMCBSCsBSCs

HLR/ACHLR/AC SMSM--GWGW

SS7 Network

EIREIR

MSC/VLR voice interfaces:BSCsOther MSCsPSTNMC (VMS)

MSC/VLR signaling link interfaces:BSCsSS7 Network (Redundant SS7 A-link)

HLR/ACSM-gateway

PSTN (SS7 ISUP Signaling)Other MSC (SS7 F-link)

EIR (SS7 F-link)OMC (X.25 link)


MSC/VLR <-> BSC Voice Ports

The Number of MSC ports, needed for MSC to BSC voice transmissions is the sum of all E0 channels from all of the BSCs

MSC

BSC3BSC3BSC2BSC2BSCnBSCnBSC1BSC1

Nports = NBSC1 + NBSC2 +...+ NBSCn


MSC/VLR <-> MSC Voice Ports

MSC/VLR <->MSCVoice trunks are required between MSCs to support:

MTM calls without routing the call to the PSTN Inter-MSC HOMC traffic across MSC’s

Initially an E1 link will be planned between each MSC pair which are subject to inter-MSC handover.

MSC1

MSC2


MSC/VLR<->PSTN

The Number of Voice Ports can be determined from:Total Erlangs from all of the BSCs (already calculated)GoS from the traffic modelErlang B table

MSC PSTN

eBSC1

eBSC2

eBSCn GoSMSC


MSC/VLR Signaling links

The signaling links are based on a designed SS7 backbone: It is assumed that an existing network is used. And that the SS7 network is designed to handle the traffic from the PLMN.

All non-call-associated signaling in GSM is grouped under MAP (Mobile application part).

Non-call-associated signaling implies all signaling dealing with mobility management, security, activation/deactivation of supplementary services and so on.

Planning a fix SS7 packet network is a major task. Many large operators design their own SS7 network (STPs).


MSC/VLR Capacity

The MSC/VLR capacity measures:The MSC/VLR ability to connect to and to process information received by all the signaling links from BSC(s), HLR and OMC. The MSC capacity is usually expressed in terms of

Maximum no of BSC that can be supported/controlled (a hard value),Maximum no of Call Attempt (CA),Maximum no of voice ports it can support (I/O),Maximum no of Signaling link can be supported.

The VLR capacity limits are based onNumber of subscribers (less of a limiting factor),Transaction/sec processing on the VLR database.


MSC/VLR Dimensioning

For a given system, once all of the voice ports and signaling links to the MSC have been identified, the size of MSC can be determined.

The total Erlang from all of the BSCs < Maximum Erlang supported by the MSC.The total number of voice ports required < Maximum ports supported by the MSC.The total CA from all of the BSCs < Maximum CA supported by the MSC.The total number of signaling links required < Maximum signaling links supported by the MSC.


MSC/VLR Dimensioning (cont’d)

The VLR limitations must also be metTotal no. of subscribers < Maximum no. of subscribersTotal no. of transactions/sec < Maximum no. of transaction/sec

If the required traffic is greater than the MSC/VLR limits, then provide different alternatives

Increase the number of MSCs or plan for a larger MSC/VLRIf other MSCs already exist, then determine the possibility of sharing with other MSCs.

The best alternatives are selected based on system constraints.


Distributed v.s. Centralized MSC Designs

Comparison of distributed vs. centralized designs:

Distributed design Centralized DesignAllows for easy expansion Not as easyReliability/availability Any minor change may effect the systemEasier to adapt to IN standard Harder to adoptFaster introductions of services SlowerLess complex and easier to maintain Harder to maintainCost More (facilities to interconnect) Less costly

MSCSTP

HLR/ACHLR/AC

VLRVLR

EIREIR MSC/VLR/HLR/AC/EIR


Planning/Configuration Steps

Review Inputs: Average Size and Capacity of Links and Network ElementsBTS Locations

BSC PlanningPreferred LocationsBTS-BSC ConfigurationsBTS-BSC Assignment

GMSC/MSC PlanningMSC Preferred LocationsBSC-MSC assignment

HLR Location, Redundant HLROMC Location

From Dimensioning

From RF Design


Frequency Hopping


Frequency Hopping

FH is used in GSM to improve the system’s performance and quality in the multipath fading environment and to reduce the required S/N ratio.GSM uses Slow FH in which the hopping rate is less than the message bit rate.

In GSM the operating frequency is changed only with every TDMA frame.The hopping rate is 216.7 hops per second which corresponds to a frame duration of 4.615 sec.

The mobile transmits at different frequencies for different time slots. A frequency synthesizer is used to change and settle on a new frequency within a fraction of one time slot (577 µs).

F1 F2 F3 F4 F5 F6 F7 F8

T=1 T=2 T=3


Frequency Hopping (cont’d)

FH provides frequency diversity to overcome Rayleigh fading which may cause fades of 40 to 50 dB deep on the received signal.

FH also provides interference diversity (interference averaged over multiple users).

FH reduces the S/N ratio required for an acceptable QoS, from 12 dB for a non-hopping radio link to 9 dB (approx.), improving the overall network’s capacity.

Different hopping algorithms can be assigned to the MSCyclic Hopping Random Hopping


Frequency Hopping (cont’d)

In the Mobile Station, in FH mode, only three time slots are available to transmit, receive and monitor while in the BTS all eight time slots are capable of transmitting and receiving to support eight MSs in one frame.

The Broadcast Channels (BCH) comprising of FCCH, SCH and BCCH are not allowed to hop.

All dedicated channel types can hop (TCH/SDCCH/FACCH/SACCH).

Two different implementation schemes of SFH are used in BSs which are base-band hopping and RF hopping.

Hybrid hopping is a combination and compromise of the two implementation schemes.


Base-band Hopping

ANT

TS handler transmitter f0




TRX 1

TRX 2

TRX 3

TRX 4

filtercombiner

Bus for routing of bursts

Each transmitter is assigned with a fixed frequency. At transmission, all bursts, irrespective of which connection, are routed to the appropriate transmitter of the proper frequency. The mobile is hopped around the transmitters and receivers.The advantage with this mode is that narrow-band low loss filter combiners can be used.


Base-band Hopping (cont’d)

BCCH TSBCCH TS--0 does not hop0 does not hop

BCCH 0 00 000

TS-0 TS-1 TS-2 TS-3 TS-4 TS-5 TS-6

0

TS-7

TRX1

1 1 1 1 11 1

2 2 2 22 2

3333333

2

TRX2

TRX3

TRX4

f1

f2

f3

f4

0

1

2

TS-0 of TRX2-4 hoppingover MA(f2,f3,f4)

TS1-7 of TRX1-4 hoppingover MA(f1,f2,f3,f4)

Different MAIOsDifferent MAIOsfor same TS of for same TS of different TRXsdifferent TRXswithin the same within the same HSN to avoid HSN to avoid collisioncollision.


RF Hopping

TS handler transmitter f0 . . . fn

TS handler

TS handler

TS handler

TRX 1

TRX 2

TRX 3

TRX 4

transmitter f0 . . . fn



ANThybridcombiner

ANThybridcombiner

One transmitter handles all bursts that belong to a specific connection. In contrast to base-band hopping, the transmitter tunes to the correct frequency at the transmission of each burst.The advantage of this mode is that the number of frequencies that can be used for hopping is not dependent on the number of transmitters. It is possible to hop over a lot of frequencies.The disadvantage with synthesizer (RF) hopping is that wide-band hybrid combiners have to be used. This type of combiner has approximately 3dB loss making more than two combiners in cascade impractical.


RF Hopping (cont’d)

BCCH TRX does not hopBCCH TRX does not hop

BCCH 0 00 000

TS-0 TS-1 TS-2 TS-3 TS-4 TS-5 TS-6

0

TS-7

TRX1

0 0 0 0 0

1 1 1 11 1

2222222

1

TRX2

TRX3

TRX4

MA={f1}

0

1

2

Different MAIOs for same TS of Different MAIOs for same TS of different TRXs within the same different TRXs within the same hopping group to avoid collisionhopping group to avoid collision..

0 0

MA={f2, f3, f4…..}


Hopping Algorithms

Cyclic frequency hopping: the frequencies are changed, once every TDMA frame, in a consecutive order (e.g. …,f1,f2,f3,f4,f1,f2,f3,f4,…).

Random frequency hopping: a random hopping sequence is implemented as a pseudo-random sequence. 63 independent sequences are defined.

Hopping Sequence Number (HSN) will specify which of the 63 sequences to be used (e.g. …,f1,f4,f4,f3,f1,f2,f4,f1,…).

The random hopping mode is superior for averaging the co-channel interference. Random hopping is the hopping mode of choice for high capacity networks.


Hopping Algorithms (cont’d)

Orthogonal hopping sequence: for each transceiver, in the same channel group, in the same cell, they will be assigned with the same HSN, i.e. they hop in the same way.

In order not to interfere with each other, they must not use the same frequency at the same time. The problem is solved by using an offset in the hopping sequence, referred to as Mobile Allocation Index Offset (MAIO). Two transceivers bearing the same HSN but different MAIO will never use the same frequency in the same TDMA frame.

(e.g., …,f1,f4,f4,f3,f1,f2,f4,f1,……,f2,f1,f1,f4,f2,f3,f1,f2,……,f3,f2,f2,f1,f3,f4,f2,f3,……,f4,f3,f3,f2,f4,f1,f3,f4,…).

rf design introduction v3

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