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Planning & BSS

Submitted By:-

Saurabh Mehra &

Aniket Varadpande

GET 09

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Air interfaceThe Air Interface carries the Radio Waves.The Um interface is the interface between theMS and the BTS. Voice is modulated on a radio frequency carrier and transmitted on the

Air Interface. The frequency used in GSM is in UHF range i.e. 30 -3000 MHz. Ultra highfrequency radio waves are typically generated by oscillating charges on a transmittingantenna. In the case of a radio station, the antenna is often simply a long wire (a dipole)

fed by an alternating voltage/current source, that is, charge is placed on the antenna by

the alternating voltage source. We can think of the electric field as being disturbances

sent out by the dipole source and the frequency of the oscillating electric field (theelectromagnetic wave) is the same as the frequency of the source.

Each antenna has a unique radiation pattern. This pattern can be represented graphicallyby plotting the received time-averaged power, as a function of angle with respect to thedirection of maximum power in a log-polar diagram. The pattern is representative of the

performance of the antenna in a test environment. However, it only applies to the free-

space environment in which the test measurement takes place. Upon installation, thepattern becomes more complex, due to the extra factors affecting propagation under field

conditions. Thus, the real effectiveness of any antenna is measured in the field.

Antenna Basics

An antenna is a device that is made to efficiently radiate and receive radiated

electromagnetic waves. There are several important antenna characteristics that should be

considered when choosing an antenna for your application as follows: Antenna radiation patterns

Power Gain

Directivity Polarization

Antenna Radiation Patterns

An antenna radiation pattern is a 3-D plot of its radiation far from the source. Antenna

radiation patterns usually take two forms, the elevation pattern and the azimuth pattern.The elevation pattern is a graph of the energy radiated from the antenna looking at it from

the side as can be seen in Figure (a) . The azimuth pattern is a graph of the energy

radiated from the antenna as if you were looking at it from directly above the antenna asshown in fig (b). When you combine the two graphs you have a 3-D representation of

how energy is radiated from the antenna as shown in fig (c)

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Power Gain

The power gain of an antenna is a ratio of the power input to the antenna to the power

output from the antenna. This gain is most often referred to with the units of dBi, which

is logarithmic gain relative to an isotropic antenna. An isotropic antenna has a perfectspherical radiation pattern and a linear gain of one.

Gain (with reference to the isotropic radiator dBi) = Gain (with reference to /2-Dipole

dBd) + 2.15 dB

Directivity

The directive gain of an antenna is a measure of the concentration of the radiated power

in a particular direction. It may be regarded as the ability of the antenna to direct radiated

power in a given direction. It is usually a ratio of radiation intensity in a given directionto the average radiation intensity.

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Polarization

Polarization is the orientation of electromagnetic waves far from the source. There areseveral types of polarization that apply to antennas. They are Linear, which comprises,

Vertical, Horizontal and Oblique, and circular, which comprises, Circular Right Hand

(RHCP); Circular Left Hand (LHCP), Elliptical Right Hand and Elliptical Left Hand.Polarization is most important to get the maximum performance from the antennas. For

best performance the polarization of the transmitting antenna should be matched to that

of the receiving antenna.

Half-Power-Beam-Width

This term defines the aperture of the antenna. The HPBW is defined by the points in thehorizontal and vertical diagram, which show where the radiated power has reached half

the amplitude of the main radiation direction. These points are also called 3 dB points.

VSWR

An impedance of exactly 75 Ohm can only be practically achieved at one frequency. The

power delivered from the transmitter can no longer be radiated without loss because ofthis incorrect compensation. Part of this power is reflected at the antenna and is returned

to the transmitter the forward and return power forms a standing wave with

corresponding voltage minima and maxima (Umin/Umax). This wave ratio (Voltage

Standing Wave Ratio) defines the level of compensation of the antenna.

A VSWR of 1.5 is standard within mobile communications. In this case the realcomponent of the complex impedance may vary between the following values:

Maximum Value: 50 Ohms x 1.5 = 75 Ohms

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Minimum Value: 50 Ohms : 1,5 = 33 Ohms

VSWR= [1+ (Reflection Coefficient)]/[1-(Reflection Coefficient)]

Since Reflection Coefficient is the magnitude of ratio of the V(Reflected)/V(Transmitted)

, its value is always>= 0.The above implies that VSWR is always >= 1

Ideally, VSWR should be = 1, when Reflection Coeffient is equal to 0, i.e. no signal is

being reflected which is practically not possible.

Different antennas and their comparison:

Antenna downtilting

The problem often faced is that the base station antenna provides an overcoverage.

If the overlapping area between two cells is too large, increased switching between the

base station (handover) occurs.There may even be interference of a neighbouring cell

with the same frequency. Downtilting the antenna limits the range by reducing the fieldstrength in the horizon.Antenna downtilting is the downward tilt of the vertical patterntowards the ground by a fixed angle measured w.r.t the horizon.with appropriate downtilt,the received signal strength within the cell improves due to the placement of the main

lobe within the cell radius and falls off in regions approaching the cell boundary and

towards the reuse cell.

There are two methods of downtilting

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Mechanical downtilting

Electrical downtilting.

Mechanical downtilting

It consists of physically rotating an antenna downward about an axis from its vertical

position. In a mechanical downtilt as the front lobe moves downward the back lobe

moves upwards. This is one of the potential drawback as compared to the electricaldowntilt because coverage behind the antenna can be negatively affected as the back

lobe rises above the horizon. Additionally , mechanical downtilt does not change the

gain of the antenna at +/- 90deg from antenna horizon.

Electrical Downtilt

Electrical downtilt uses a phase taper in the antenna array to angle the pattern

downwards. This allows the the antenna to be mounted vertically. Electrical downtiltaffects both front and back lobes. If the front lobe is downtilted the back lobe is also

downtilted by equal amount. Electrical downtilting also reduces the gain equally at all

angles on the horizon. .

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Antenna diversity

In a typical cellular radio environment, the communication between the cell site andmobile is not by a direct radio path but via many paths. Hence the signal that arrives

at the receiver is either by reflection from the flat sides of buildings or by diffraction

around man made or natural obstructions. When various incoming radiowaves arriveat the receiver antenna, they combine constructively or destructively, which leads to a

rapid variation in signal strength. These signal fluctuations are known as multipath

fading. Multipath fading causes rapid changes in signal strength over a short distanceor time,random frequency modulation due to Doppler Shifts on different multipath

signals and time dispersion caused by multipath delays.

Diversity techniques have been recognised as an effective means which enhances the

immunity of the communication system to the multipath fading. GSM therefore

extensively adopts diversity techniques that include

1. interleaving in time domain

2. frequency hopping in frequency domain3. spatial diversity in spatial donmain

4. polarization diversity in polarization domain

Spatial and polarisation diversity techniques are realised through antenna systems.

A diversity antenna system provides a number of receiving branches or ports from whichthe diversified signals are derived and fed to a receiver. The receiver then combines the

incoming signals from the branches to produce a combined signal with improved qualityin terms of signal strength or signal-to-noise ratio (S/N).

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Spatial diversity antenna systems

The spatial diversity antenna system is constructed by physically separating tworeceiving base station antennas.Once they are separated far enough, both antennas

receive independent fading signals. As a result, the signals captured by the antennas

are most likely uncorrelated.The further apart are the antennas, the more likely thatthe signals are uncorrelated.

The types of the configuration used in GSM networks are:

horizontal separation

vertical separation

Two antenna spatial diversity

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Polarization diversity antenna systems

A dual-polarisation antenna consists of two sets of radiating elements which radiate or, inreciprocal, receive two orthogonal polarised fields. The antenna has two input connectors

which separately connect to each set of the elements. The antenna has therefore the

ability to simultaneously transmit and receive two orthogonally polarised fields.

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Channels in GSM

In GSM frequency division duplex is used for duplex transmission. Uplink refers to

signal transmission from MS to BTS and downlink refers to signal transmission from

BTS to MS.

for GSM 900 890-915 MHz for uplink

935-960 MHz for downlink.

for GSM 1800 1710-1785 MHZ for uplink

1805-1880 MHz. for downlink

In GSM the frequency band is divided into channels of 200 KHz each. Hence 125 carriersin GSM 900 and 375 carriers in GSM 1800.the duplex distance in GSM 900 is 45 MHz

and that of GSM 1800 is 95 MHz. FDM is combined with TDMA to increase the no ofusers. In TDMA each radio frequency channel is divided into consecutive periods of time

known as time slots.In GSM each radio channel is divided into 8 time slots.Hence eachtime slot per user is allotted.these TDMA timeslots are called physical channels.Each

time slot lasts for 0.577 sec thus 8 time slot last for 4.615 ms. These time slots are used

for traffic as well as signalling.the TDMA frame cyclicaly repeat time after time.

The longest recurrent time period of the structure is called hyperframe and has the

duration of 3 h 28 min 53 sec 760 ms. The TDMA Frame Numbers (FN) are numberedfrom 0 to 2 715 647. One hyperframe is divided into 2048 superframes, which have

duration of 6.12 seconds. The superframe is itself subdivided into multiframes.

There are two types of multiframes in the system:

26 frame multiframe (51 per superframe) with a duration of 120 ms, comprising

26 TDMA frames. This multiframe is used to carry the logical channels TCH,SACCH and FACCH,

51 frame multiframe (26 per superframe) with a duration of 235.4 ms,

comprising 51 TDMA frames. This multiframe is used to carry the logical

channels FCCH, SCH, BCCH,CCCH, SDCCH, SACCH, and CBCH .

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A variety of information is transmitted between the BTS and the MS. The information isgrouped into different logical channels. Each logical channel is used for a specific

purpose such as paging, call set-up and speech. For example, speech is sent on

the logical channel Traffic Channel (TCH). The logical channels are mapped onto the

physical channels.

Logical Channels

The logical channels can be separated into two categories. They are traffic channels and

signaling channels.

There are two forms of TCHs:

full rate TCH (TCH/F) - this channel carries information at a gross rate of 13 kbit/s.

half rate TCH (TCH/H) - this channel carries information at a gross rate of 6.5 kbit/s.

Signaling channels are subdivided into three categories:

Broadcast CHannels (BCH) Common Control CHannels (CCCH) Dedicated Control CHannels (DCCH)

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The following sections describe specific channels within these

categories.

BROADCAST CHANNELS (BCH)

Frequency Correction CHannel (FCCH)

On FCCH, bursts only containing zeroes are transmitted. This serves two purposes. First

to make sure that this is the BCCH carrier, and second to allow the MS to synchronize tothe frequency. FCCH is transmitted downlink only.

Synchronization CHannel (SCH)

The MS needs to synchronize to the time-structure within this particular cell, and alsoensure that the chosen BTS is a GSM base station. By listening to the SCH, the MS

receives information about the frame number in this cell and about BSIC of the chosen

BTS. BSIC can only be decoded if the base station belongs to the GSM network. SCH is

transmitted downlink only.

Broadcast Control CHannel (BCCH)

The MS must receive some general information concerning the cell in order to start

roaming, waiting for calls to arrive or making calls. The needed information is broadcast

on the Broadcast Control CHannel (BCCH) and includes the Location Area Identity(LAI), maximum output power allowed in the cell and the BCCH carriers for the

neighboring cells on which the MS performs measurements. BCCH is transmitted on the

downlink only. Using FCCH, SCH, and BCCH the MS tunes to a BTS and synchronized

with the frame structure in that cell. The BTSs are not synchronized to each other.Therefore, every time the MS camps on another cell, it must listen to FCCH, SCH and

BCCH in the new cell.

COMMON CONTROL CHANNELS (CCCH)

Paging Channel (PCH)

At certain time intervals the MS listens to the PCH to check if the network wants to make

contact with the MS. The reason why the network may want to contact the MS could be

an incoming call or an incoming short message. The information on PCH is a paging

message, including the MSs identity number (IMSI) or a temporary number (TMSI).PCH is transmitted downlink only.

Random Access Channel (RACH)

The MS listens to the PCH to determine when it is being paged.When the MS is paged, it

replies on the RACH requesting a signaling channel. RACH can also be used if the MS

wants to contact the network. For example, when setting up a mobile originating call.RACH is transmitted uplink only.

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Access Grant Channel (AGCH)

The networks assigns a signaling channel (Stand-alone Dedicated Control CHannel(SDCCH)) to the MS. This assignment is performed on the AGCH. AGCH is transmitted

downlink only.

DEDICATED CONTROL CHANNELS (DCCH)

Stand alone Dedicated Control Channel (SDCCH)

The MS as well as the BTS switches over to the assigned SDCCH. The call set-up

procedure is performed on the SDCCH, as well as the textual message transmission (short

message and cell broadcast) in idle mode. SDCCH is transmitted both uplink

and downlink.When call set-up is performed, the MS is told to switch to aTCH.

Slow Associated Control Channel (SACCH)

The SACCH is associated with SDCCH or TCH (i.e. sent on the same physical channel).On the uplink, the MS sends averaged measurements on its own BTS (signal strength and

quality) and neighboring BTSs (signal strength). On the downlink, the MSreceives information concerning the transmitting power to use and instructions on the

timing advance. SACCH is transmitted both uplink and downlink.

Fast Associated Control Channel (FACCH)

If a handover is required the FACCH is used. FACCH works in stealing mode meaning

that one 20 ms segment of speech is exchanged for signaling information necessary for

the handover. Under normal conditions the subscriber does not notice thespeech interruption because the speech coder repeats the previous speech block.

Cell Broadcast Channel (CBCH)

CBCH is only used downlink to carry Short Message Service

Cell Broadcast (SMSCB) and uses the same physical channel as

the SDCCH.

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BURST FORMATS

The bit rate over the air interface is 270.8 kbps. This gives a bit time of 3.692 ms (48/13ms). The time interval of a TS thus corresponds to 156.25 bits. The physical content of a

TS is called a burst.

There are five different types of bursts.

1. Normal Burst (NB):

This burst is used to carry information on traffic and control channels. For TCH itcontains 114 encrypted bits, and includes a guard time of 8.25 bit duration

(30.46 ms). The stealing flag is relevant only for TCH

2. Frequency correction Burst (FB):

This burst is used for frequency synchronization of the MS. It consists of zeroes

only.

3. Synchronization Burst (SB):This burst is used for time synchronization of the MS. It contains a long training

sequence and carries the information of the TDMA Frame Number (FN) and Base StationIdentity Code (BSIC).

4. Access Burst (AB):

This burst is used for random access and handover access. It is characterized by a long

guard period (68.25 bit duration or 252 ms), to cater for burst transmission

from an MS that does not know the timing advance at the first access (or at handover).

This allows for a cell radius of 35 km. The access burst is used on the Random AccessCHannel (RACH) and on the Fast Associated Control CHannel (FACCH) at handover.

5. Dummy Burst:

This burst is transmitted when no other type of burst is to be sent. This means that the

base station always transmits on the frequency carrying the system information, thus

making it possible for the MSs to perform power measurements on the BTS in order todetermine which BTS to use for initial access or which to use for handover. In order to

achieve this, a dummy page and a dummy burst is defined in the GSM recommendations.

CCCH is replaced by the dummy page, when there is no paging message to transmit. This

dummy page is a page to a non-existing MS. In the other TSs not being used, a dummyburst with a pre-defined set of fixed bits is transmitted.

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Call flow

1. Mobile originated call

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2. Mobile terminated call

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Handover

The GSM handover process uses a mobile assisted technique for accurate and fast

handovers, in order to maintain the user connection link quality and manage traffic

distribution. Prior to handover following process takes place:

1. Measurement of radio subsystem downlink performance and signal strengths

received from surrounding cells is made in the MS.

2. These measurements are sent to the BSS for assessment.3. The BSS measures the uplink performance for the MS being served and also

assesses the signal strength of interference on its idle traffic channels.

4. During its idle time (the remaining seven timeslots), the MS switches to theBCCH of the surrounding cells and measures its signal strength.

5. The signal strength measurements of the surrounding cells, and the signal strength

and quality measurements of the serving cell are reported back to the serving cellvia the SACCH once in every SACCH multiframe.

6. This information is evaluated by the BSS for use in deciding when the MS shouldbe handed over to another traffic channel.

The following measurements is be continuously processed in the BSS:

i) Measurements reported by MS on SACCH

- Down link RXLEV

- Down link RXQUAL- Down link neighbor cell RXLEV

ii) Measurements performed in BSS

- Uplink RXLEV- Uplink RXQUAL- MS-BS distance

- Interference level in unallocated time slots

Handover is done on five conditions

Interference

RXQUAL RXLEV

Distance or Timing Advance

Power Budget

The following are the types of handovers

1. Intracell

When handover takes place between two sectors of same cell .it is within same BSC

2. Intercell intra BSCWhen handover takes place between two cells but within same BSC

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3. Intercell inter BSC

When handover take place between two cells which are located in diffent BSCs

4. Inter MSCWhen handover takes place between two cells which located in differnt MSCs

Frequency Hopping

The Frequency Hopping function permits the dynamic switching of radio links from one

carrier frequency to another. Frequency Hopping changes the frequency used by a radio

link every new TDMA frame in a regular pattern.

The reasons of using Frequency Hopping are:

1. Decreasing the probability of interference

Frequency Hopping will spread the annoyance of interference

over different mobile stations in a particular cell

2. Suppressing the effect of Rayleigh fading

Rayleigh fading (or multipath fading) is caused by different paths followed by the radiosignal. Rayleigh fading can cause coverage holes.Rayleigh fading is location and

frequency dependent. When the mobile station is stationary or moves at a slow speed,Frequency Hopping will significantly improve the level of the air-interface performance.However, when the mobile station moves at a high speed, Frequency Hopping does not

harm, but does not help much either. The more frequencies are used in a particular cell,

the more Frequency Hopping can gain in suppressing the effect of Rayleigh fading.

Process :

The regular pattern by which a radio link changes carrier frequency, is described by thehopping sequence. The hopping sequence can have a cyclic pattern or a pseudo-random

pattern. In order to calculate the hopping sequence, a function is used which maps a

particular TDMA frame to a radio frequency within the set of frequencies, usingparameters such as TDMA frame number and number of frequencies in the set of

frequencies. Both the uplink and the downlink use the same hopping sequence. For this

purpose the parameters used to calculate the hopping sequence are also transferredfrom the BTS to the mobile station. To reduce complexity of the GSM system, the

common channels(BCCH, FCCH, SCH, PAGCH and RACH) do not hop.

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There are two types of frequency hopping schemes

1. Basebang frequency hopping2. Synthesized frequency hopping

Some terms:

HSN(Hopping Sequence Number):

The HSN specifies the order in which the frequencies within the set of frequencies aregoing to hop.if HSN is 0 then hopping takes place is cyclic fashion .fi it between 0 to 63

then hopping takes place in random manner.

MAIO(mobile allocation index offset) :

Mobile Allocation Index Offset (MAIO) is a frequency offset set for all Basic Physical

Channels . Manual MAIO planning prevents adjacent channel interference within a cellas well as co- and adjacent channel interference in co-sited cells when using frequency

hopping

BA(BCCH allocation):

These are the frequencies allocated for BCCH.these are the fixed frequencies and do not

hop .

MA(mobile allocation):

These frequencies are allocated for traffic channels to hop.

Base band frequency hopping

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As shown in fig the BCCH time slot does not hop.timeslot 0 of TRX 2-4 hop over

MA(f2,f3,f4).this hopping group uses HSN-1.all timeslots 1-7 hop over

MA(f1,f2,f3,f4).this hopping group uses HSN-2.

Frequency synthesized hopping:

In this the BCCH TRX does not hop.MAIOs are different for different TRXswithin the same hopping group hence no collisions.in this scheme only one HSN isallocated.

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Transmission

The inability of a BTS to cater more than a threshold of subscribers, and Radio

Waves (900 MHz /1800MHzGSM frequency) to reach beyond a certain distance without

interference, forces us to put a huge number of sites to cover our entire subscriber base.

Now, to control these sites, they need to be connected to BSC. These BSCs have to befurther connected to MSCs. This connection can be made by any type of transmission

medium i.e. it can be Optical Fiber, Microwave link, Satellite Communications, or

Coaxial Cable. The following parameters are considered in transmission planning.

Free space loss

The microwave antennas used for point to point links fall into the category of aperture

antennas, the parabolic dish antenna being the most common example. A propagating

electromagnetic wave has a power density Pd (in watts per square metre) associated withit. The aperture (known for these purposes as the effective aperture Ae) of the antenna

is measured in square metres and the antenna serves to convert the power density into anactual power Pr (the suffix r standing for received) in accordance with the formula

Pr = Pd Ae

Given that the surface area of a sphere of radius r is equal to 4r2, it is possible to say that the power

density Pd is related to the power transmitted Pt by the equation

the power density at same distance is given by

The received power is given by

The effective aperture of isotropic antenna is given by

Our formula for received power now becomes

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Expressing in decibals

Pr (dBm) = Pt (dBm) + Gt (dBi) + Gr (dBi) 20 log10 (4 ) 20 log10 (r ) + 20 log10 ( )

Now (Pt-Pr = Path loss). So

Path Loss = 20 log10 (4 ) + 20 log10 (r ) 20 log10 ( ) Gt Gr

Solving we get

Path Loss = 92.4 + 20 log10 d + 20 log10 f Gt Gr

When gain of antenna is 0 dBi then loss is called as free space loss

FSL = 92.4 + 20 log10 d + 20 log10 f Gt Gr

Fade Margin

The Radio Link is usually designed in such a way that the Received Power PR

(Normal propagation conditions) is much greater than the Receiver ThresholdPTH.

The Fade Margin FM is defined as :

FM (dB) = PR (dBm) - PTH (dBm)

A Fade Margin is required to compensate for the reduction in Rx power caused

by Propagation Anomalies.

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Link budget

A more complete Link Budgetexample (7GHz, 50 km link) is :

Net Path Loss 64 dB

Received Power PR - 34 dBm

Assuming the RX Threshold PTH = -77 dBm, then the Fade Margin is :

FM = PR - PTH = 43 dB

Equivalent earth curvature

An "Equivalent Earth Curvature" can be defined by altering the real Earth

Curvature in order that the radio ray path be straight.In the Equivalent Earthrepresentation the Earth Radius R is multiplied by a factor k. The value of the k-factordepends on the curvature of the radio ray.

The k-factor is a measure of the ray curvature effect, produced by the variation in

the Atmosphere Refraction Index with height. So, the k-factor is related to the

power Gain losses

Transmitted power 30 dBm

Tx feeder and branching losses 1.4 dB

Tx antenna gain 42.5 dB

Free space loss 143.3 dB

Additional propagation loss 3.0 dB

Rx antenna gain 42.5 dB

Rx feeder and branching loss 1.4 dB

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Vertical Refractivity Gradient G. The k-factor indicates the atmosphere state at

a given time and its effect to the radio ray curvature.In a well-mixed atmosphere

(Standard Atmosphere), the Refractivity decreases with height at a constant rate. Thiscorresponds to the so-called Standard Condition, with a stable k-factor, equal to about

4/3.

Other k-factor conditions are :

k < 4/3 Sub-refractive Atm. (Ray Path closer to the earth. )The lowest k value corresponds to the highest probability that the radio ray be obstructed

by the ground.

k > 4/3 Super-refractive Atm.( Ray Path more distant from the earth.)The range of the radio transmission can be significantly expanded. Unexpected

interference can be observed.

Freznel zone

A Fresnel zone is a three-dimensional body, bounded by ellipsoids that have their focal

points at the transmitter and the receiver antennas. The sum of the distances from a point

(P) on the ellipsoid to the transmitter (T) and to the receiver (R) is n/2 wavelengths longerthan the LOS path (S):

distance (P - T) + distance (P - R) = S + n/2,

where n is the number of the Fresnel zone.

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First Fresnel zone

For the first Fresnel zone, n = 1. The radius of the first Fresnel zone is rF1. To keep out ofthis zone, the distance r from the optical LOS should be:

where d1 is the distance between the obstacle and the receiver.

The obstacles may be hills, buildings, or vegetation. The following figure represents the1st Fresnel zone:

Diffraction effect

Shadowing does not always mean that no signal is received behind an obstacle. Radio

waves may bend around obstructions to a certain extent. This effect is called diffraction.The diffraction effect depends on the wavelength in relation to the size of the obstacle.

The diffraction effect is greater if the wavelength increases.

Knife-edge diffraction

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The following figure shows a simple model for a single knife-edge diffraction:

Because of diffraction loss, only a fraction of the transmitted power in A will arrive at the

receiver in B.The parameter v is calculated using the distances d1[m] and d2[m] from the knife edge to

the cell site (BTS) and the MS, respectively, the height of the knife-edge h [m], and the

wavelength [m]:

After calculating v, the diffraction loss Ldiffrcan be found from the curve in the graphshown in the figure below

.

Multipath propagation

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The radio wave may be reflected, from a hill, a building, a truck, an airplane, or adiscontinuity in the atmosphere. In some cases, the reflected signal is significantly

attenuated, while in others almost all the radio energy is reflected and very little

absorbed. The result is that not one but many different paths are followed betweentransmitter and receiver. This is known as multipath propagation.

Reflection and multipath propagation can cause positive and negative

effects:

DuctingMultipath propagation may imply coverage extension by allowing radio signals to reach

behind hills and buildings and into tunnels.The latter effect is known as ducting.

Constructive and destructive interference

The interference due to multipath propagation manifests itself in the following ways:

1. Rayleigh fading: random phase shift creates rapid fluctuations in the signal strength

2. Delay spread in the received signal3. Intersymbol interference: the delay spread in the received signal causes each symbol to

overlap with adjacent symbols

4. Doppler shift: the shift in frequency on different paths causes random frequencymodulation.

Network Optimization

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The optimization of the network, is done to check the performance of the network, just

after it is made operational and to get best possible quality of service. The objective ofoptimization procedure is:

To check whether the network meets the customers given requirements, on thebasis of which network was designed.

To check whether the parameters and configurations are defined correctly or not.

To find out and suggest changes in the defined parameters and configurations to

achieve best possible quality of service.

Quality of service can be characterized by factors such as contiguity of coverage,

accessibility to the network, speech quality and number of dropped and blocked

calls. A number of parameters are checked as a measure of quality of service by

using a drive test system.

Drive Test system comprises of a test mobile phone, software to control and log data

from the phone and a Global positioning system receiver for position information. A

drive test system can only indicate the type of problem in the network that exists, itdoesnt indicate cause of the problem but with the help of knowledge of possible reasons

of a problem, one can trace the cause. Following steps are taken to fulfill the objective of

network optimization using a drive test tool.

1. Collection of Data and extraction of relevant information from it.

2. Analysis of the extracted data.

3. Suggesting changes in the network configurations based on the analysis.

Collection of Data and extraction of relevant information:

Drive test involves setting up a call to best carrier and driving along the roads. While

driving the radio parameters and air interference signal data are collected as a log file. In

general following parameters are checked during the drive test for different categories of

terrains like dense urban, sub-urban, rural, highways and for different clutters like inbuilding, residential areas, commercial areas, industrial areas etc.

1. Rx Level.2. Rx Quality

3. Timing Advance4. Handover parameters5. Data of six best neighbor cells.

6. Layer 2 and layer 3 messages.

From the data collected various information can be extracted which depict the

performance of BTS sites and the network as a whole. Following information can be

extracted from this drive test data.

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1. Coverage edge probability

2. Coverage area probability3. Speech quality

4. Frequency and BSIC reuse

5. Neighbor cell definition details.6. Handover details.

Edge Probability: To get an idea of coverage area, coverage boundary of all the cellsbased on received signal level (RXLEV), is obtained and is plotted over the geographical

map of the area. The coverage boundary of a cell is considered to be made up of equal

received level points on the field.

With the help of this coverage plot the edge probability or the probability of getting asignal level better than a specific value over the boundary of all cells is obtained which

helps in determining the performance of the network with respect to coverage boundary

requirements given by the customer.

Area Probability: The obtained signal levels from the cells at all the points of the

network, are then used to make, a best server plot. This best server plot is drawn bycategorising it on the basis of in building coverage, in car coverage and on street

coverage. These categories are defined on the basis of the coverage area where a good

quality conversation is required. The details about threshold defined for these categoriesare discussed in coverage planning report.

Speech Quality: Speech quality is a very important aspect for determining the quality of

service for whole of the network. Speech quality is inferred by the RXQUALmeasurements during the drive test. RXQUAL, is the Bit error rate (BER) derived from

the 26 bits midamble on TDMA burst. Its level characterizes speech quality where 0

indicates the highest quality and 7 the worst. Thus during drive test, poor quality areascan be found and marked by looking over the quality on the scale of 0 to 7. RXQUAL

can be poor due to poor RXLEV, Co-channel interference, adjacent channel interference

or multipath. RXQUAL is measured and tested for all the categories of clutter and terrain.

Frequency and BSIC reuse: From the collected data the frequency reuse pattern with

the BSIC (Base Station Identity Codes) planning of all the cells of the network can be

obtained. The reuse distance for all the reused frequencies can be determined

Neighbour cells definition details: With the help of collected data 6 best serving

neighbours of all the cells can be determined. The drive test window of the antennasystem gives details of 6 best neighbours at an instance.

Handover details: There are certain other very important parameters which has to bechecked during drive test as these parameters directly reflects on the performance of the

network, like handover margin, handover threshold, values of handover timers, offset and

penalty for the handovers.

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With a call established, and measuring on the cell edge, we can display the phone

measurements of serving and neighbour cells. The difference between the RXLEV of the

server and that of neighbours can be monitored on the amplitude and time scale. At somepoint on the drive-test route, one of the neighbours RXLEV will become stronger than

the servers signal level and when this difference of the two exceeds the handover

margin, for atleast a timing set in the handover required counter in BSS, a handover willoccur. Thus by simultaneously monitoring RXQUAL during the handover, the value of

the handover margin can be determined and a decision can be made whether that value is

appropriate for the quality of service desired.

Analysis of extracted data:

The information extracted from the collected data is then analysed to compare it with theagreed benchmarks related to coverage, quality, handover success rate etc and is used to

infer the cause of the deviation from given requirements and set benchmarks. It is also

used to infer cause of detected problem in the network if there is any.

There are special coverage requirement which are discussed in coverage planning report

under special coverage category these specific coverage requirements are matched tofind out whether the requirement of customer is taken care of or not.

RXQUAL is also matched with the given requirement. If RXQUAL is poor and RXLEVis sufficiently good it can reasonably be deducted that the cause is interference. Generally

a test frequency which has no adjacent or Co channel present in that area is used to find

out if interference is because of multipath. If it is not because of multipath then spectrum

analyser can be used to find out whether it is an adjacent channel interference or it can bededucted that it is Co channel interference.

A handover margin on the high side will result in a handover occurring after the user hasexperienced some deterioration in quality. High handover margins can result in poor

reception and dropped calls, while very low values of handover margin can produce

Ping-Pong effects as a mobile switches too often between cells.

With the help of collected data it can be found out weather uplink and downlink are

balanced or not. If even after having good RXLEV and RXQUAL, calls are dropping or

even when RXLEV and RXQUAL of serving cell is better than that of neighbour cell,handover is taking place, it indicates that the link needs to be balanced.

BSIC for all the cells are also checked and verified with what is defined in the BSS. Ifsame BSIC is defined for cells having same BCCH frequency and these cells coexist in

the neighbour list then understandably lot of handovers will be unsuccessful.

Layer 2 and 3 messages can be used for analysing cause of a particular handover failure,

call drop, very poor speech quality or any other abnormality in the performance of the

network.

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Suggesting changes in the network configurations based on the analysis:

After detection of the causes of the deviation from the requirement or network relatedproblems, measures are taken to improve the performance of the network and to match

customers requirement.

Network performance can be influenced by the network parameters. The configuration

parameters can be divided into two groups hard configuration and soft configuration,

depending on the type of control and action required to modify them.

Hard Configuration: The hard configuration parameters are aspects of base station

configuration and include antenna type, antenna gain, antenna orientation, effective

height of antenna radiation centre, use of space diversity, antenna feeder loss andeffective isotropic radiated power (EIRP).

Changes in this configuration are made to meet the requirements and to deal with the

analysed problems. For an example if certain area is affected by interference resulting inpoor quality then one of the way to reduce interference level is by shrinking the coverage

area. Shrinking of coverage area can be achieved by reducing EIRP that is by replacingthe existing antenna with a lower gain or narrower horizontal beam width antenna system

and by reducing transmitted power under limitation of not loosing the link balance. Most

effective solution used to shrink coverage area is by increasing antenna downtilt and/orreducing antenna height. Similarly to improve coverage in certain areas the transmitted

power of BTS can be changed, antennas with different gain or beamwidth can be used

and the height of antenna system can be changed.

For further specific coverage and quality requirements pico or micro cells can be installed

inside the residential places, commercial buildings, stadiums and car parks etc. A pico

cell is nothing but a cell with very low EIRP in comparison to a Macro Cell. Note that theneighbour list for these pico cells is defined differently than that for normal Macro cells.

Micro cell has also got lesser coverage area than that of Macro cells.

Repeaters can also be used for providing coverage to specific areas. There can be

Channel selective or Band selective Repeaters where band selective repeaters amplifies

the whole GSM band and transmit it towards the area required to be covered while

channel selective repeaters receive power from selected channels of one or more than oneparent cells, amplify it and direct it towards the area required to be covered. In the similar

way if capacity requirement of certain area is more, then the coverage of a cell is needed

to be compressed by any of the means discussed above so that it may cater to lessernumber of customers.

If the mentioned measures dont work for matching coverage and capacity requirement

then relocation or addition of site can also be suggested. If interference is observedduring drive test then apart from reducing coverage area, frequency plan for the network

can be redefined and reuse distances can be increased.

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After carefully studying the statistical data about the network performance if it is found

that congestion for some particular sites are more and call successful rate is less, then

more resources (TRX) can be added to improve availability of the traffic channels oradditional BTS sites can also be added but this addition has a limit because of limited

available frequency spectrum hence with higher number of sites or frequency used, reuse

distance of the sites will reduce which will increase interference and hence the qualitywill go poorer.

There are lots of other ways by which capacity can be increased without much affectingthe speech quality.

1. Addition of Micro and pico cells.

2. Using Underlay and overlay cells.3. Deploying frequency hopping

Everytime TRXs are added in the network, frequency plan of the network or a portion of

the network has to be changed which will further require to analyse the network usingdrive test system, to monitor the networks performance. It is possible that after addition

of certain TRXs frequency reuse distance will decrease to such a level that it willintroduce unacceptable amount of interference and deployed frequency plan will require

to be redefined.

Soft Configurations: Other parts of the system can be controlled with soft parameters.

These affect operation of algorithms within the system, and include categories such as

common BTS parameters, cell access parameters etc. GSM defines around 150 soft

parameters. For an example if it is found from the BSS statistics details that excessive

handovers hence more utilisation of resources is taking place then reduction of overlap of

the cell coverage areas can avoid them.

Defined BSICs for the cells specially for cells transmitting same frequencies are set to be

different otherwise lot of unsuccessful handovers will take place. Even then, if it is foundthat number of unsuccessful handover is high then redefining the neighbour list in BSS

can control it. Several neighbours for a serving cell can be defined in GSM. Usually, we

want a handover to be made to the strongest neighbour, but in some cases frequent

handovers to this best neighbour can result in congestion in this cell, affecting the usersinitiating calls from that cell. The situation can also occur in reverse, when a handover

required to the best neighbour can result in a rejection due to unavailability of resources,

causing the handover to be attempted to the next best neighbour, which can delay theprocess and deteriorate the quality further. Under certain circumstances, we may need to

remove a potential neighbour from the neighbour list and provide alternatives.

In the idle mode, the mobile always prefers to remain with or move to the best serving

cell. The best cell is decided on the basis of uplink and downlink path balance in the

cells. This balance is calculated by GSM defined C1 calculations. C1 calculations force

the mobile to move to the strongest cell. In certain cases, such as macro-micro cell

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architecture, optimisation may require that in certain areas the mobile not remain in the

best cell, but instead remain in a cell depending on traffic loading. C2 parameters provide

the option of adding fixed positive or negative offsets to the C1 calculation in each cell.So, although C1 might be better for a neighbour cell, the application of C2 parameters

could delay reselection. C2 parameters also allow the mobile to apply temporary offsets

for a period known as penalty time, which helps reduce Ping-Pong effects. With the helpof carefully done drive test these parameters like offset or penalty time for handovers can

also be checked and verified.

Optimisation of the network using drive test system is an iterative process thus after

deploying discussed changes in the network drive test is done again and mentioned steps

are repeated until required performance objective is fulfilled.

Drive test is very effective part of the optimisation of the network but drive test data is

not very effective to find out some of the very specific problems their cause and solutions

to rectify them. For finding requirement of capacity, exact cause of handover failures and

reduced call success rate one has got to be dependent on statistics obtained from the BSS.On the other hand drive test is the only medium with the help of which users perspective

of quality of service can be visualised hence simultaneous monitoring of the BSSstatistics and drive test data gives most practical and optimum cause and the solution of a

problem.

Drive test

Area: Lucknow cantonment board

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Components and instruments:

1. GPS antenna2. Sony Ericsson handset connected to laptop

3. TEMS 6.1 a software for data collection for Ericsson

Purpose:

There were many customer complaints in that area.

Observations:

Using handset a call is made to a no 7777 where all parameters are tested. the parameterssuch as RxQual,RxLev are determined. for RxQual 0-7 levels are considered where 0 is

considered as best while 0 as worst. for RxLev the power level is measured in dBm

where -50 to -60 dBm is considered sas good power level. the handoffs were also studied

the handoffs such as intra cell and intercell were studied. At some places handoffs werenot successful and there the call was dropped.

The GUI of the software contained following

1. Map

2. Radio Parameters

3. C/I4. Serving Neighbour

5. C/A

6. GPS7. Current Channel

Introduction to Mapinfo

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1. .xls file with the required data and containing the latitude and longitude columns.

Make sure that no column is blank and the values of latitude and longitude are not

mixed

2. Save the text.xls file as .txt file say temp.txt

3. Open Mapinfo software

4. Click on open button and select Text Files (Tab-delimited) i.e. .txt files. Locate

the test.txt file created and click on open. Select delimiter as tab and check titles

on the 1st row.

5. A table will be created in Mapinfo.

6. Now from the menu select the option Create Points.

7. A box will open and it will ask for X and Y co-ordinates.

8. In the X co-ordinate column select the longitude column & in the Y co-

ordinate select the latitude column. Also select the desired symbol

9. Close Mapinfo and open the .tab file created by it.

10. The Points given will be plotted on the map having the desired symbol which has

been already selected.

11. We can add layers also using the Mapinfo tool.

12. Click on open and you can open Railways, Roadways etc.

13. Once opened you can change the layer properties by right clicking on it.

14. Hence, clutter can also be added in the similar way. We can manage different sites

shown according to the .xls file which are clearly plotted on the map and

frequency planning and other parameters can be managed.

15. LABELS can also be added by checking the labels box and clicking on the

label. Then the desired label can be selected from the dropdown box.

16. Click on TableMaintenance Table Structure. A dialog box appears showing

a list of fields with their name and type. Check the required boxes which you want

to make searchable.

Comparison of SDH and PDH

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PDH SDH

The reference clock is not synchronized

throughout the network

The reference clock is synchronized

throughout the network

Multiplexing / Demultiplexing

operations have to be performed from

one level to the next level step by step

The synchronous multiplexing results in

simple access to SDH system has

consistent frame structures throughout

the hierarchy.

The payload is not transparent The payload is transparent

PDH system has different frame

structures at different hierarchy levels

SDH system has consistent frame

structures throughout thehierarchy.

Physical cross-connections on the same

level on DDF are forced if any

Digital cross- connections are provided

at different signal levels and in different

ways on NMS

G.702 specifies maximum 45Mpbs &

140Mpbs & no higher order (faster)

signal structure is not specified

G.707 specified the first level of

SDH.That is, STM-1, Synchronous

Transport Module 1st Order & higher.

(STM-1,STM-4,STM-16, STM-64)

PDH system does not bear capacity totransport B-ISDN signals.

SDH network is designed to be atransport medium for B-ISDN, namelyATM structured signal.

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Few services are available It will transport variety of services.

Limited amount of extra capacity for

user / management

It will transport service bandwidths

Sufficient number of OHBs is available

Bit - by - bit stuff multiplexing Byte interleaved synchronous

multiplexing.

BTS & BSC Training

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Introduction to Base Transceiver Station (BTS)

A base transceiver station or cell site (BTS) is a piece of equipment that facilitates

wireless communication between user equipment (UE) and a network.

Though the term BTS can be applicable to any of the wireless communication standards,

it is generally and commonly associated with mobile communication technologies like

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GSM and CDMA. In this regard, a BTS forms part of the base station subsystem (BSS)

developments for system management. It may also have equipment for encrypting and

decrypting communications, spectrum filtering tools (band pass filters) etc. antennas mayalso be considered as components of BTS in general sense as they facilitate the

functioning of BTS. Typically a BTS will have several transceivers (TRXs) which allow

it to serve several different frequencies and different sectors of the cell (in the case ofsectorised base stations). A BTS is controlled by a parent base station controller via the

base station control function (BCF). The BCF is implemented as a discrete unit or even

incorporated in a TRX in compact base stations. The BCF provides an operations andmaintenance (O&M) connection to the network management system (NMS), and

manages operational states of each TRX, as well as software handling and alarm

collection. The basic structure and functions of the BTS remains the same regardless of

the wireless technologies.

Nokia Ultrasite EDGE BTS: -

This section describes the units of Nokia Ultrasite EDGE BTS. Following are the units

and their explanation.

Base Operations and Interfaces Unit (BOI):-

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The BOIx unit handles the control functions common among other units in the

BTS. These functions include:

1. BTS initialization and self testing2. Configuration

3. O & M signaling

4. Software download5. Main clock functions

6. Timing functions

7. Collection and management of the external and internal alarms8. Message delivery to the BSC

9. Cabinet Control

Dual Band Diplex Filter Unit (DU2A): -

The DU2A unit combines output form DVxx or RTxx units into one antenna

feeder. It is mounted on the top of the BTS cabinet.

Transceiver Baseband Unit (BB2x) : -The BB2x unit is a digital signal processing board, consisting of two independent

baseband modules. Each module functions independently for its own TRX unit.The BB2x unit also controls frequency hopping.

Dual Variable Gain Duplex Filter (DVxx): -

The DVxx unit performs duplex operation of TX and RX signals through acommon antenna and filters and amplifies main and diversity receiver signals

before they pass through the M2xA unit to the TRx unit.

The DVxx unit contains a variable-gain LNA for optimal amplification of the

receive signal with or without the optional Masthead Amplifier (MNxx) unit. The

high gain LNA is fixed and used without the optional MNxx. The low-gain LNA

is variable and used only with the MNxx unit; the low-gain LNA is set accordingto antenna cable attenuation values. Feeder loss values are entered during HW

configuration, which sets the correct DVxx unit gain accordingly at the RX band

for optimal performance.

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Masthead Amplifier and Bias Tee units: -

Masthead Amplifier unit:Following are the functions of Masthead Amplifier

1. 33db RX gain in GSM/EDGE.2. Low RX noise figure (improved RX sensitivity and SNR)

3. Low TX loss in a compact, low volume, lightweight, sealed enclosure.

Bias Tee Unit (BPxx):

The BPxx unit provides DC power to the MNxx using the RF cable.

Receiver Multicoupler Unit (M2xA): -The M2xA unit distributes RX signals to the TRx units. The 2-way unti is used

in most Wideband Combiner or combining bypass configurations. One unit

performs signal splitting for both main and diversity branches.

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Power Supply Unit: -

The PWSx unit converts AC or DC input voltage to DC voltage required for theNokia Ultraite EDGE BTS. The PWSx unit distributes the appropriate voltage

through the backpane to the units. The PWSx unit also supplies power for MNxx

unit.

Transceiver Unit (TRx) : -

The transceiver unit contains one transmitter, one main receiver, one diversity

receiver.

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The TSxx unit performs RF modulation/demodulation and amplification for one

RF carrier to handle the following signal i.e. uplink signals from the mobile

stations to the BTS or the downlink signals from the BTS to the MS.

These functional sections communicate with the Transceiver Baseband (BB2x)

and Base Operations and Interfaces (BOIx) units through the backplane. Thefunctional sections process the following signals i.e. data signals between the

TSxx and BB2x units initialization and control signals from the BB2x unit to the

TSxx unit status and alarm signals from the TSxx unit to the BB2x unit.

The RF section of the receiver converts the carrier frequency signal to the IF frequency.The IF sections of the receiver perform channel filtering to prevent interfering

frequencies from distorting the signal. The IF sections also provide automatic gain

control.

Transmission Unit (VSxx) : -

The transmission unit connects the BTS to the rest of the network using wireline

or radio interface.

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The FXC RRI unit is the radio link transmission unit for the Nokia Ultrasite

EDGE BTS. The FXC RRI support two Flexbus connections (coaxial cable),16x2 Mbit/s each. The Flexbus connects the RRi transmission units for the Nokia

FlexiHopper Microwave, multiple BTS cabinets located at the same site can also

be connected together using a Flexibus.

The FXC RRI transmission unit operates as a repeater and interconnects Nokia

Ultrasite EDGE BTS and the BSC using loop, chain, star and point-to-pointnetwork configuration.

The main features of the FC E1/T1 transmission unit are: one Abis line interface to the

Mbit/s (E1) or 1.5 Mbit/s (T1) transmission line operation as the termination point in a

chain or star configuration balanced interface that can be configured to E1 or T1 mode

interface statistics gathered in compliance with ITU-T G.826 and ANSI T1.403

Recommendations handling of timeslot 0 at 2 Mbit/s interfaces. The 2 Mbit/s E1 frame/

multiframe structure complies with ITU-T G.704/706 Recommendations transmitting and

receiving functions at the 2 Mbit/s interfaces.

Wideband Combiner Unit (WCxA) : -

The WCxA unit combines two transmitter outputs into one. When sing the

WCxA, the DVxx unit is required.

With 2-way Wideband Combining, the WCxA unit combines the transmit (TX) signals

from two Transceiver (TSxx) units and feeds the combined signal to the antenna through

the TX port of the Dual Variable Gain Duplex Filter (DVxx) unit.

Signal Flow in the BTS: -

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Description:-

In the uplink path, the BTS receives signals from the MS; in the downlink path,

the BTS sends the signals to the BTS. Uplink and downlink signals travel

through the Air interface on different frequencies with the higher frequencycarrying downlink signal.

Uplink Signal Path:The uplink signal path involves the following actions:

o The antenna picks up a signal from the air interface.

o The antenna passes the signal to the Masthead Amplifier and the Bias Tee

and then to the optional Dual Band Diplex filter.

o The signal passes through Dual Variable Gain Duplex Filter to the

Receiver Multicoupler and then to the Transceiver module.

o The Transceiver Module converts the received signal to the Intermediate

Frequncy levels and filters the signal.

o The transceiver module then sends the signal to the Transceiver Baseband

unit for digital processing.

o The transceiver baseband unit then sends the processed signal to thetransmission unit which passes the signal to the BSC through the Abis

interface.

Downlink Signal Path:

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The downlink signal path involves the following actions:

o The BSC receives the signal from the network and sends the signal to the

transmission unit of the BTS through the Abis interface.

o The transmission unit passes the signal to the transceiver baseband unit

for the digital signal processing.

o The transceiver baseband unit sends the processed signal to thetransceiver unit.

o The transceiver unit filters the signal and raises the carrier frequency and

amplifies it.

o Then the transceiver baseband sends the signal to the wideband combiner

or directly to the dual variable gain duplex filter.

o Then the signal passes through the options dual band diplex filter to the

bias tee and masthead amplifier and through the antenna to the mobilestation.

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GSM Speech and Channel Encoding: -

Speech Encoding:RPE-LPC (Regular Pulse Excited - Linear Predictive Coder):

In modern land-line telephone systems, digital coding is used. The electrical variations

induced into the microphone are sampled and each sample is then converted into a digitalcode. The voice waveform is then sampled at a rate of 8 kHz. Each sample is thenconverted into an 8 bit binary number representing 256 distinct values. Since we sample

8000 times per second and each sample is 8 binary bits, we have a bit rate of 8kHz X 8

bits = 64kbps. This bit rate is unrealistic to transmit across a radio network sinceinterference will likely ruin the transmitted waveform. GSM speech encoding works to

compress the speech waveform into a sample that results in a lower bit rate using RPE-

LPC. A LPC encoder fits a given speech signal against a set of vocal characteristics.

The best-fit parameters are transmitted and used by the decoder to generate syntheticspeech that is similar to the original. Information from previous samples is used to predict

the current sample. The coefficients of the linear combination of the previous samples,

plus an encoded form of the residual, the difference between the predicted and actualsample, represent the signal. Speech is divided into 20 millisecond samples, each of

which is encoded as 260 bits, giving a total bit rate of 13 kbps. This way GSM can

transmit 4 times (floor [64kbps/13kbps]) as many phone calls as a regular land-linetelephone.

Channel Encoding:

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Once we have a compressed digital signal, we must add a number of bits for error control

to protect the signal from interference. These bits are called redundancy bits. The GSM

system uses convolutional encoding to achieve this protection. The exact algorithms useddiffer for speech and for different data rates. The method used for speech blocks is

described below.

Bit Composition of the Speech Signal:

The RPE-LPC Encoder produces a block of 260 bits every 20 ms. It was found (though

testing) that some of the 260 bits were more important when compared to others. Belowis the composition of these 260 bits.

Class Ia - 50 bits (most sensitive to bit errors)

Class Ib - 132 bits (moderately sensitive to bit errors)

Class II - 78 bits (least sensitive to error)As a result of some bits being more important than others, GSM adds redundancy bits to

each of the three Classes differently. The Class IA bits are encoded in a cyclic encoder.

The Class Ib bits (together with the encoded Class IA bits) are encoded using

convolutional encoding. Finally, the Class II bits are merely added to the result of theconvolutional encoder. Below is the operation of each encoder as related to each Class of

bits.

Cyclic Encoding:

The Class IA bits are encoded using a cyclic encoder to add three bits of redundancy. Theresulting Class IA bits are of the form:

where b0,b1,b2 are the three redundancy bits added by the cyclic encoder and m0,...,m49are the original Class IA bits. The cyclic encoder produces 50+3=53 bits.

Cyclic codes are linear codes (the sum of any two codes is also a codeword), as we have

seen in class. In addition to being linear, a cyclic shift, or rotate, of a codeword producesanother codeword Since the code used in GSM is a (53,50) code the generatorpolynomial used in the encoding is of degree 53-50 = 3. The specific polynomial used in

GSM is x^3 + x + 1. The following block diagram can produce the codeword. Once the

data has been completely shifted through the system, the contents of Reg0 through Reg2will contain the three additional bits.

GSM chose to use cyclic encoding due to the ability to quickly determine if errors are

present. The three redundancy bits produced by the cyclic encoder enable the receiver toquickly determine if an error was produced. If an error was produced the current 53 bit

frame is discarded and replaced by the last known "good" frame.

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Convolutional Encoding:

The resulting 53 bits of the cyclic encoder are added to the 132 Class Ib bits (plus a tail of

4 extra bits so that the encoder may be flushed) and encoded using the convolutional

encoder. The convolutional encoder adds one redundancy bit for every bit that it seesbased on the last four bits in the sequence. Below is a block diagram detailing the

convolutional encoder.

The convolutional encoder retains a memory of the last four bits in the sequence (a single

bit is retained in each flip-flop). These four bits are added together using a modulo-2adder. The resulting bit is sent to the output via path 1. The encoder sends a second bit to

the output via path 2. As a result, the convolutional encoder encodes one input bit intotwo output bits. In GSM there are 4 flip-flops, and the convolution performed is of D^4

+D^3 + 1 and D^4 +D^3 + D + 1.

Once the convolutional encoder has encoded the bits, a new bit sequence of 378

(2*(53+132+4=189)=378) bits is produced. These 378 bits are directly added to the 78

Class II bits (directly added since these bits are least sensitive to error). As a result, the

channel encoded bit sequence is now 378+78=456 bits long. Therefore, each 20 ms burstproduces 456 bits at a bit rate of 22.8 kbps.

Interleaving:

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Interleaving is the processes of rearranging the bits. Interleaving allows the error

correction algorithms to correct more of the errors that could have occurred during

transmission. By interleaving the code, there is less possibility that a whole chuck of codecan be lost. Consider this example to see how interleaving works. We need to transmit

20 bits. Furthermore, 10 bits can be transmitted in one transmission burst, and the error

correcting mechanism can correct 3 errors per 10 bits. Take a look at the following twoscenarios:

With interleaving the receiver is able to get all 20 bits correctly but without interleavingwe lose 1 complete burst. In GSM the interleaving is much more complicated than the

simple example above. The 456 bits outputted by the convolutional encoder are divided

into 57 bit blocks by selecting the 0th, 8th, 16th through 448th bits in the first block, the

1st, 9th 17th through 448th bits in the 2nd block and so on to have 8 blocks. Then the bits

in the first 4 blocks are placed in the even bit positions for the total block of 456 bits, andthe bits in the second set of 4 blocks are placed in the odd positions.

Ciphering:Ciphering is used to encrypt the data so that no one can overhear the conversation of

another user. In GSM the two parties involved in encrypting and decrypting the data are

the Authentication Center (AuC) and the SIM card in the mobile phone. Each SIM cardholds a unique secret key, which is known by the AuC. The SIM card and AuC then,

follow a couple algorithms to first authenticate the user, and then encrypt and decrypt the

data. For authentication, the AuC sends a 128-bit random number to the mobile phone.The SIM card uses it's secret key and the A3 algorithm to perform a function on therandom number and sends back the 32-bit result. Since the AuC knows the SIM card's

secret key, it performs the same function, and checks that the result obtained from the

mobile phone matches the result it obtained. If it does, the mobile user is authenticated.Once authentication has been performed, the random number and the secret key are used

in the A8 algorithm to obtain a 64-bit ciphering key. This ciphering key is used with the

TDMA frame number in the A5 algorithm to generate a 114 bit sequence. Note: the

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ciphering key is constant throughout a conversation, but the 114 bit sequence is different

for every TDMA frame. The 114 bit sequence is XORed with the two 57 bit blocks in a

TDMA burst. The only user that can decrypt the data is the mobile phone or the AuCsince they are the only ones that have access to the secret key, which is needed to

generate the ciphering key, and the 114 bit sequence. Note that the A3, A5, and A8

algorithms are not known to the public domain; however some information about A5 hasbeen leaked. It is known that A5 has a 40-bit key length, which allows for the encryption

to be broken in a matter of days, but since cellular calls have a short lifetime, the

weakness of the algorithm is not an issue.

Description of BSC3i

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The Nokia GSM/EDGE BSC3i is a modern fault-tolerant system for GSM networks. The

BSC3i is based on modular software and hardware architecture. The distributed

architecture of BSC3i is implemented with a high capacity and redundant multiprocessorsystem DX200 computing platform. The system enables the distributed processing

capacity of several computer units.

The main function of the BSC3i is to control and manage the BSS and the radio channels.

Functionality of BSC3i: -The BSC3i manages a variety of tasks ranging from channel administration to short

message service. The main functionalities are below

Management of terrestrial channels

Indication of blocking on A interface

Allocation of traffic channels between BSC and BTS

Management of Radio Channels Management of channel configuration i.e. how many traffic channels and

signaling channels can be used in the BSS. This is in connection with the radio

network configuration.

Management of Traffic Channels (TCH) and SDCCH. i.e. resource management,

channel allocation, link supervision, channel release, power control.

Management of BCCH, CCCH i.e. channel management, random access, access

grant, paging, management of PCCCH.

Management of Frequency Hopping

The BSC is in charge of the frequency hopping management which enables use of radio

resource and enhanced voice quality.

Handovers

The frequency of the mobile is changed in connection with the handovers which areexecuted and controlled by the BSC.

Maintenance

The BSC offers the possibility for the following maintenance:

Fault localization for the BSC

Reconfiguration of the BSC

Reconfiguration support of the BSC

Updating software

User Interface

The BSC has a user friendly interface with plain text messages and commands, which are

easy to learn and use. The user interface compiles with the recommendations of the ITU.

These are known as MML commands.

Measurements and Observations

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The BSC measures the traffic, observes signaling events, and traces a specific call. It then

forwards these results to OMC

SMS

The BSC forwards mobile originating and mobile terminating short messages

transparently.

Cell Broadcast Messages

Cell broadcast provides the BSC with the short message service cell broadcastcapabilities defined by GSM recommendations. It is used for broadcasting short messages

to mobile stations in a specified area.

BSC3i supports Full Rate traffic channels, Half Rate traffic channels, and Enhanced FullRate traffic channels.

Adaptive Multi Rate Codec

AMR introduces new set of codecs and adaptive algorithm form codec changes thus canprovide significantly better speech quality and more capacity on air interface. We can

achieve very good speech quality in full rate mode even in low C/I conditions

GPRS and EDGE

BSC3i is capable of providing and is compatible with data services handling i.e. GPRSand EDGE.

Dynamic Abis Allocation

The Dynamic Abis allocation functionality allocates Abis transmission capacity to cellswhen needed instead of reserving a full fixed transmission link per TRX. It is

implemented as a software feature. Quality of Service is improved by it.

Architecture of BSC3i:-

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two Clock System Units (CLS) installed in a CLOC-B cartridge

up to seven Base Station Controller Signalling Units (BCSU) with packet controlunits installed in CC3C-A cartridges

two Marker and Cellular Management Unit (MCMU) installed in CC4C-A

cartridges

one Operation and Maintenance Unit (OMU) installed in CM2C-A cartridge

up to 64 Exchange Terminal Plug-in Units (64 ET4/ET2 units withGSW1KB/S11.5 or 62 ET2 units with earlier configurations)

four LAN switches (ESB26/ESB20-A) installed in the CC4C-A cartridges with

the MCMU

four Fan Trays (FTRB) for forced ventilation.

Base Station Controller and Signalling Unit (BCSU): -

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The BCSU performs those functions of the BSC that are highly traffic dependent i.e. on

the volume of the traffic. One BCSU can handle traffic from around 200 TRXs. The

BCSU is housed in the cartridge of its own. It consists of two parts, which correspond toA and Abis interface. Packet control Units (PCUs) are also connected to BCSU.

The A interface part of the BCSU does the following functions: performing the distributed functions of the message Transfer Part (MTP) and

Signalling Connection and Control Part (SCCP)

controlling the mobile and base station signaling (BSSAP)

performing all message handling and processing functions of the signaling

channels connected to it

The Abis interface part of the BCSU controls the air interface channels associated with

the TRXs and the Abis signaling channels. Every speech circuit on the Abis interface is

mapped one-to-one to a GSM specific speech/data channel on the air interface. Thehandover and the power control algorithms reside in this particular unit.

Bit Group Switch (GSWB): -

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The Bit Group Switch conveys the traffic passing through the BSC and switches the tones

to the subscribers of the exchange and to the trunk circuits. The GSWB also establishes

the necessary conditions to the signaling units and the internal data transmissionchannels, and is responsible for the sub-multiplexing functions of the BSC. It switches on

8, 16, 32, 64 kbit/s level.

The operation of GSWB is controlled and supervised by the MCMU (Marker and

Cellular Management unit). It performs all the necessary connecting and releasing

functions.

It consists of a power supply and four plugin units each having 32 4Mbit/s interfaces.

Capacity of GSWB is 256 2 Mbit/s PCMs.

It is fully digital, one-staged, and non-blocking time switch with full availability and it is

very simple.

Clock and Synchronization Unit (CLS): -

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The clock and synchronization unit (CLS) distributes the timing reference signals to the

functionality units in the BSC3i. It can operate plesiochronously synchronously with the

timing references it receives from the digital PCM trunks. To oscillator of the CLS isnormally synchronized to external source, usually MSC, through PCM line.

Upto three additional PCM inputs are provided for redundancy. The processor chooses

the highest priority interface which is in order, for the phase lock form the six

synchronization inputs.

Marker and Cellular Management Unit (MCMU): -The MCMU controls and supervises the GSWB and performs the hunting, connecting

and releasing of the switching network circuits. The range of tasks it handles makes up acombination of general marker function and radio resource management.

The MCMU is connected to the other computer units of the exchange, OMU and BCSU,

through the message bus. It performs the control of a switching matrix and the BSC-

specific management functions of the radio resources.

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Marker functions of the MCSU include the connection and release of the circuits of the

switching matrix. When the MCMU performs these functions, it exchanges messages

with other Call Control Computers via the Message Bus (MB).

The Switch Control Interface writes the required connections into the switch control

memory and reads it contents.

The Cellular Management functions of the MCMU are responsible for the cells and radio

channels that are controlled by the BSC. This responsibility is centralized in the MCMU.It keeps track of the radio resources requested by the MSC and the handover procedures

of the BSC. It also manages the configuration of the network.

One BSC3i always includes two MCMUs that are permanently connected to the

duplicated pair of GSWB, the active MCMU to the active GSWB and passive MCMU topassive GSWB.

The integrated LAN switch provides access to the operators IP network.

Call Control Computers: -In BSC3i the call control functions are executed by the microcomputers, called the Call

Control Computers. The Call Control Computers have and identical Central ProcessingUnits (CPU) which is based on the most suitable commercially available Intel

microprocessors. The CPU board contains a microprocessor and a local Random Access

Memory (RAM). All different plug-in units of the CPU are connected using PCI bus.

Operation and Maintenance Unit (OMU): -The Operation and Maintenance Unit (OMU) is an interface between the MSC and ahigher level network management system and the user. The OMU can also be used for

local operations and maintenance. The OMU receives fault indications from the BSC. It

can produce local alarm printouts to the user or send the fault indications. In the event offault, the OMU automatically activates appropriate recovery and diagnostic procedures

within the BSC. Recovery can also be activated by the MCMU if OMU is lost.

Traffic control functions

Maintenance functions

System configuration administration functions

System management functions

OMU has its own Call Control Computers as well as local I/O interfaces. It has

microcomputer of its own, alarm interface, message bus interface, peripheral deviceinterface, analog X.25 interface, digital X.25 interface and Ethernet interface.

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The alarm interface connects internal wired alarms to the OMU from the BSC cartridges,power supply, air conditioning equipment etc.

Exchange Terminal (ET): -The ET performs the electrical synchronization and adaptation of external PCM lines. It

performs the HDB3 (ET2E), or B8ZS or AMI (ET2A) coding and decoding, inserts thealarm bits in the outgoing direction and produces PCM frame structure. All 2.048 Mbit/s

interfaces for the MSC, the SGSN and the BTSs are connected to the ET. The ETs adapt

the external PCM circuits to the GSWB an synchronize to the system clock.

In the incoming direction, the ET decodes 2.048 Mbit/s signal of the circuit to data

signals. The decoder decodes the line code into binary format.In the outgoing direction, the Et receives a binary PCM signal from the switching

network and generates PCM frame structure. The resulting signal is converted into a line

code and transmitted further onto 2.048 Mbit/s.

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Visit to BSC (Sitapur Road)We had an introductory visit to the BSC and were shown different equipment and their

configuration present on the site. Following are the notes regarding the visit.

There are in all 3-earthing pits on the site. They are all connected together. Remember,

the earthing wire is green in colour.

A diesel generator (DG) is used for backup in case of mains supply failure. Its power

rating is 40kVA and the internal DG battery is 12V.

A permanent mains is provided to the site. There is a Switch Fuse Unit (SFU) connected

to the servo which is nothing but a transformer for a regulated power supply. The supplythen reaches the Insulated Transformer which stabilizes the voltage. Auto Mains Failure

(AMF) is attached, it turns into action whenever there is a mains failure, at that time it

automatically switches on the DG. MCBs are present for each and every unit i.e. DG,

Insulated transformer etc.

The IDU or the Indoor Unit is provided by NEC, SRAL. IF cable connects the IDU to the

ODU. A cable known as Tributary cable is used for alarms.

The mains power reaching the shelter is fed to the Power Plant. The Power Plant contains

a rectifier module and a Low Voltage Disconnector (LVD).The rectifier module converts the input AC into DC which is generally around 54V. The

Low Voltage Disconnector (LVD) disconnects the circuit in case the voltage falls below

44V.

Next in the shelter is Nokia Ultrasite BTS. It contains Power Cards (2 in number) each

-48V, BB2F cards (depending on the number of TRX), BOI card, E1/T1 or RRI cards(max.4), duplexer (DVDx), multicoupler (M2HA), wideband combiner and TRXs.

Next is the Fiber Rack: - XDH 300 is used for making fiber link to the MSC. Just

besides it is Krone which is used for E1 patching. Because there will be losses whileconnecting by twisting the cable and these losses can be couple of db which is

unacceptable, hence, Krone is used for easier lossless patching.

Surpass 7070 MUX is used.

There is a centralized temperature control system. I contains cooling fans which are

included in the cabinet core mechanics, HETA which is the BTS cabinet heater. Thesoftware in the BOI unit controls the unit temperature, hence, controls the fan speed and

the temperature. There is a cold sensor in HETA, if the temperature goes below -10 oC,

heater will start automatically.

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Hands-on experience: Nokia BTS Manager & MML sessions: -

Login to Nokia BTS Manager:

Launch the Nokia BTS manager software

It asks to key in the BSC id and the BCF (i.e. the BTS)

Authorized Username and password has to be keyed in along-with the above

mentioned parameters

Once successfully authenticated, the software shows a GUI of the BTS. It shows the

number of TRX, BB2F etc i.e. the hardware present on the BTS. It also shows the status

of the equipment, if it is not functioning properly then it is indicated by red alarm. If anyTRX is idle, i.e. no call is going on it then it is shown yellow. During TRX addition the

TRX is remotely configured using BTS manager. The alarm window is also shown just

next to the virtual BTS which shows the history of alarms incurred.

MML Sessions:-

The software being used for the MML sessions is Reflection. It is actually software from

where we can login to the desired BSC and check its components and for that fact we canmonitor all the BTSs under the BSC.

Short code of the desired BSC for e.g. BLKO1 is entered once reflection is started, then itrequires authentication. Once authenticated, the command line interface is visible where

the MML commands can be issued.

Activity: New TRX configuration using Nokia BTS manager, hub manager and MML

commands

Three steps are to be followed while performing the TRX addition

1. Signalling should be unlocked from the BSC end

2. Mapping of the E1 i.e. Abis interface using Nokia Hub Manager3. Unlock traffic from the BSC end i.e. make TRX fully functional

First of all we should know the BCF number of the site and the number of TRX to beadded to the BTS and the sector.

Open the Reflection software and enter the BSC short-code i.e. BLKO1. enter the

authentication details then key in the following commands

ZEEI : BCF = 45;

The above command will give the number of TRXs working or in the configuring state.

Working TRXs are represented by WO. The PCM number column will also be visible.

This PCM number will be added to the next command.

ZDTI:::PCM=512;

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The corresponding PCM number is added to the above command and we get the

signaling timeslots. We note the timeslot number corresponding to the TRX we are going

to add.

ZERO:BCF=45, TRX=10;

The above command gets you the traffic timeslot number which are going to map on the

Abis. Note it down.

To unlock the signaling from the BSC end we need to enter the following command

ZDTC:T045A:WO:;

Here 045 is the BSCF number of the site and A (in hex) means TRX number i.e. 10 th

TRX.

Once th signaling has been unlocked from the BSC end, open the Nokia BTS manager. Inthe BSC parameters enter the BSC ID and the BCF.

Once logged in open the BTS Hub Manager