planning+&+bss

8/6/2019 Planning+&+BSS

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Planning & BSS


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Air interface

The Air Interface carries the Radio Waves.The Um interface is the interface between the

MS 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 transmitting

antenna. 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 disturbancessent out by the dipole source and the frequency of the oscillating electric field (the

electromagnetic 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 the

direction 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, the

pattern 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 beconsidered 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. Antennaradiation 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 poweroutput from the antenna. This gain is most often referred to with the units of dBi, whichis logarithmic gain relative to an isotropic antenna. An isotropic antenna has a perfect

spherical 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 radiatedpower in a given direction. It is usually a ratio of radiation intensity in a given direction

to the average radiation intensity.

Polarization

Polarization is the orientation of electromagnetic waves far from the source. There are

several types of polarization that apply to antennas. They are Linear, which comprises,

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


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(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 thatof the receiving antenna.

Half-Power-Beam-Width

This term defines the aperture of the antenna. The HPBW is defined by the points in the

horizontal and vertical diagram, which show where the radiated power has reached halfthe 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 of

this 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 withcorresponding 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 real

component of the complex impedance may vary between the following values:

Maximum Value: 50 Ohms x 1.5 = 75 Ohms

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

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


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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 >= 1Ideally, 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 andtowards the reuse cell.

There are two methods of downtilting

Mechanical downtilting

Electrical downtilting.


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Mechanical downtilting

It consists of physically rotating an antenna downward about an axis from its verticalposition. 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 electrical

downtilt because coverage behind the antenna can be negatively affected as the backlobe 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 alsodowntilted 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 carriers

in 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 is

grouped into different logical channels. Each logical channel is used for a specificpurpose 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 theneighboring 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 longguard 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 cell

via 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 interferenceFrequency 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 cell

as well as co- and adjacent channel interference in co-sited cells when using frequencyhopping

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 is

allocated.


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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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