Gsm optimization

5/24/2011

Prepared by Legend Technical Team

Copy Rights © LEGEND Co. 2010


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RF People
RF people work in either
RF Planning RF Optimization
Responsibilities Responsibilities

− Nominal Plan Design. − Maintain the Network„s Accessibility
− Sites Survey. KPIs.
− Validation from field. − Maintain the Network‟s Retainability
− Set RF design (Structure, Azimuth, KPIs.
Height, Tilt, Cables type). − Maintain the Network‟s Service
− Frequency Plan. Integrity KPIs.
− Neighbor Plan. − Study and Apply new features.
− Sites Acceptance. − Try to think of innovative solutions to
maximize the Network capacity.
RF Planning KPIs: To provide
coverage outdoor & indoor and to offer
traffic with acceptable grade of service. They have to maintain the
performance of
the Network as good as possible.



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Part I: Radio Network Planning

Course Outlines:
− Planning Process and Procedures.
− Sites and Hardware Equipment.
− Technical Site Survey & Validation.
− Coverage and Capacity Dimensioning.
− Frequency and Neighbor Planning.



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Part II- Radio Network Optimization

What will be our concern during this part of the course?
 RF Optimization

How the RF Optimization people can maintain the KPIs?

 By studying the different radio network features and studying the controlling
parameters of each feature and how to tune them in a smart way to achieve
the target KPIs.

What are we going to study during this part of the course?
 Most of the Radio Network features and their controlling parameters.
 KPIs monitoring and analysis.
 Trouble shooting and Tuning.


Part II- Radio Network Optimization

Course Outlines:
− Idle Mode Behavior.
− Handover.
− HCS (Hierarchical Cell Structure).
− Concentric & Multi Band Cells.
− CLS ( Cell Load Sharing).
− Frequency Hopping.
− Intra Cell Handover.
− Dynamic HR Allocation.
− Power Control.
− GSM to UMTS Cell Reselection and Handover.
− Trouble Shooting and KPIs monitoring.


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GSM Revision

• GSM stands for “ Global System for Mobile Communication”

• GSM
– Second Generation for Mobile System.
– Digital System.
– Efficient Use of the Spectrum.
– Speech privacy and security.
– Better resistance to interference (Introducing the frequency Hopping)
– Efficient use of the power battery (Introducing the power control)
– GSM Networks are called “PLMN: Public Land Mobile Networks” i.e.
the Radio Sites are located on land, not using satellites.


GSM Revision

• GSM System can work in different bands as follows:

Frequency Band-Down Link Frequency Band-Up Link
GSM 800 869  894 MHz 824  849 MHz
E-GSM (Extended GSM) 925  935 MHz 880  890 MHz
P-GSM 900 (Primary GSM) 935  960 MHz 890  915 MHz
GSM 1800 (DCS) 1805  1880 MHz 1710  1785 MHz
GSM 1900 (PCS) 1930  1990 MHz 1850  1910 MHz

– DCS: Digital Cellular System PCS: Personal Communication Services.

• But what do we mean by frequency Band?
• What is the DL and UL?
• Why DL is higher than UL band?


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GSM Revision
• Frequency Band
– The range of frequencies which the operator is allowed to use for
transmission and reception.

• Down Link and Up link bands
– DL band is the range of frequencies used by the Base station when
transmitting to the MS while the UL band is the range of frequencies used by
the Mobile station when transmitting to the Base Station.


GSM Revision

• Why DL band is higher than the UL band?
– As freq then attenuation with air
– Since Power BaseStation > Power MobileStation then it is wise to configure the
higher frequencies that will be attenuated fast to the side that is using
higher power (BTS).


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Access Techniques

 What do we mean by Multiple Access techniques?
These are the Techniques through which many MSs can access the shared media
which is the air interface.
i. FDMA ( Frequency Division Multiple Access)
− Each MS is assigned a dedicated frequency through which he can
talk.
ii. TDMA (Time Division Multiple Access)
− All MSs are using the same frequency but each of them will be
utilizing it only over a certain period of time called Time Slot (TS)

 In GSM System we’re using TDMA over FDMA where the frequency
band is divided into no. of frequencies each of which is shared
among no. of MSs, where each MS will be assigned a certain TS on
certain frequency.


GSM Revision
• For P-GSM (GSM 900)
– UL Band 890MHz  915MHz, DL Band 935MHz  960MHz
– Each Band is 25 MHz
– Guard Band between DL and UL is 20 MHz
– Duplex Distance = 45 MHz
– Carrier separation = 200 KHz
– No. of frequencies = 124
Downlink 935 – 960 MHz

Uplink 890 – 915 MHz

200 KHz
890.2 890.6 Uplink
1 2 3 4 121 122 123 124
121
890 890.4 915 F (MHz)

935.2 935.6 Downlink
1 2 3 4 121 122 123 124
121
935 935.4 960 F (MHz)
GSM 900 Frequency Allocation


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GSM Revision

• For the all GSM Bands

System P-GSM 900 E-GSM 900 GSM(DCS) 1800 GSM(PCS) 1900

Uplink (MS  BS) 890 – 915 MHz 880 – 915 MHz 1710 – 1785 MHz 1850 – 1910 MHz
Downlink(BS MS) 935 – 960 MHz 925 - 960 MHz 1805 - 1880 MHz 1930 - 1990 MHz

Wavelength  33 cm  33 cm  17 cm  16 cm

Bandwidth 25 MHz 35 MHz 75 MHz 60 MHz

Duplex distance 45 MHz 45 MHz 95 MHz 80 MHz

Carrier separation 200 kHz 200 kHz 200 kHz 200 kHz

No. of carriers 124 174 374 299

Channel rate 270.8 kbps 270.8 kbps 270.8 kbps 270.8 kbps


GSM Network Architecture


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Core Network (NSS: Network Switching System)

 MSC (Mobile Switching Center)
– Routing/Switching of calls between 2 end users within the
GSM Network.
– Charging & Billing.
– Paging of MSs is originated from the MSC
– Access to PSTN (Public Switched Telephone Network)
– Act as a Gateway for other networks.
– Controls no. of BSCs connected to it.


Core Network (NSS: Network Switching System)

 HLR (Home Location Register)
– Centralized Network data base stores and manages all mobile subscriptions.
– Example: IMSI, MSISDN, MSRN, Services subscribed/restricted for that user.
IMSI,MSISDN.ppt
 VLR (Visitor Location Register)
– It is co-located with the MSC.
– Stored in it a copy of the user’s profile on temporary basis.
 AUC (Authentication Center)
– Provides the HLR with the authentication parameters and ciphering Keys used
by the MSC/VLR to authenticate certain user. (Triplets: RAND, SRES, Kc)
Authentication.ppt
 EIR (Equipment Identification Register)
– Used to authenticate the user equipment through the IMEI.
IMEI = International Mobile Equipment Identification


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BSS (Base Station System)
 BSC (Base Station Controller)
– It controls the air interface, it takes the decisions based on the reports came
from the MS and BTS.
– Channel Allocation.
– Controls the Handover Process.
– Dynamic Power Control.
– Frequency Hopping.

 BTS (Base Transceiver Station)
– It is the Hardware equipment needed to provide the radio coverage.
– Speech Coding/Channel Coding/Interleaving/Ciphering/Burst
formatting/Modulation all these are done within the BTS (RBS=Radio Base
Station)
– Equipment: Cabinet, jumpers, feeders, combiners, antennas.


MS (Mobile Station)

 Mobile Equipment
– Transmit the radio waves.
– Speech coding and decoding.
– Call control.
– Performance measurement of radio link.

 SIM card (Subscriber Identification Module)
– Stores user addresses (IMSI, MSISDN, TMSI).
– Stores authentication key Ki, authentication algorithm A3 and
ciphering algorithm A8&A5
– Stores the subscribed services.


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• Over the Air Interface
– Frequency Band is divided into no. of frequencies.
– Each frequency is divided into 8 Time slots (TS)
– Each user will be assigned 1 TS.
– One time slot duration = duration of 156.25 bits
– 1 Bit duration=3.7 µsec
– Time slot duration =156.25x3.69 µsec= 0.577 msec
– 1 Frame = 8 TSs
– Frame duration=0.577x8= 4.616 msec
– Bit rate on the air interface is 270 Kbps, but for each user it
is 33.8 Kbps


Physical Channels vs. Logical Channels

 Physical channel: Time slot is called the physical channel.
 Logical channel: It is the content that will be sent over the physical
channel.


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Logical Channels

Logical Channels

Traffic Channels Control Channels

Half Rate Full Rate
Broadcast Dedicated
Common

Frequency Correction Channel Paging Channel Fast Associated Control Channel

Synchronization Channel Access Grant Channel Cell Broadcast Control Channel

Broadcast Control Channel Random Access Channel Slow Associated Control Channel

Standalone Dedicated Control Channel


Traffic Channels

 Full Rate Channels (FR)
– Carries user’s speech traffic or user data DL and UL.
– Each user is assigned 1 TS.
– Transmission rate is 13 Kbit/s.

 Half Rate Channels (HR)
– Carries user’s speech traffic or user data DL and UL.
– 2 users will share 1 TS (physical channel), each of them will be utilizing
it each frame.
– Transmission rate is 6.5 Kbit/s


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Control Channels

 These are used to carry signaling or synchronization data,
they’re divided into three types:
– Broadcast Channels (BCH)
– Common Control Channels (CCCH)
– Dedicated Control Channels (DCCH)


BCH (Broad Cast Control Channels)

i. Frequency Correction Channel (FCCH)
– Pure signal is transmitted to help the MS to lock on the frequency of
the BTS and synchronize to its frequency. (DL channel)

ii. Synchronization Channel (SCH)
– Carries the TDMA frame number.
– BSIC (Base Station Identification Code) of the cell. (DL Channel)

iii. BCCH (Broad Cast Control Channel)
– LAI (Location Area Identity)
– Cell parameters (used power, Idle mode parameters,…..etc)
– List of BCCH carries of the neighbor cells i.e. “BA List” (DL Channel)


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CCCH (Common Control Channels)

i. Paging Channel (PCH)
– Used to inform the MS of an incoming call or sms, where the MS’s
IMSI/TMSI will be sent over it. (DL channel)

ii. Random Access Channel (RACH)
– Used by the MS to ask for an SDCCH to respond to the request send on
the paging channel /initiate a call/location update/IMSI attach-detach.
(UL Channel)

iii. AGCH (Access Grant Channel)
– Used by the network to assign an SDCCH sub-channel for the MS. (DL
channel)


DCCH (Dedicated Control Channels)

i. Standalone Dedicated Control Channel (SDCCH)
– Used for signaling purposes: call setup, location update, IMSI attach-detach
– Used to send/receive SMSs in idle mode. (DL/UL channel)

ii. Slow Associated Control Channel (SACCH)
– Always allocated in conjunction with traffic channel/SDCCH channel to
transmit measurement reports.
– DL measurement reports will include commands from the network to the
MS to adjust its power level.
– Information about the Time Advance.
– UL measurement reports will include information about the MS own power,
received SS & Quality from serving cell and SS from neighbor cells.
– Used to send SMSs in active mode. (DL/UL channel)


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DCCH (Dedicated Control Channels)

iii. Fast Associated Control Channel (FACCH)
– Used to send necessary Handover information.
– Work in stealing Mode such that 1 TCH channel is replaced by FACCH
to send the HO information. (DL/UL channel)

iv. Cell Broad Cast Channel (CBCH)
– It is sent point to multi point i.e. from the cell to the mobiles attached
to it, this channel may carry information about the traffic, weather
reports,…etc. (DL channel)


Mapping of Logical Channels on the Physical channels

Mapping on TS0/BCCH carrier (DL)

51 consecutive control frames = 1 Control multi frame
Where F:FCCH, S:SCH, B:BCCH, C:PCH/AGCH


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 Mapping on TS0/BCCH carrier (UL)

 TS0 in UL is reserved for the RACH, for the MS to access the system.



 Mapping on TS1/BCCH carrier (DL)

Where D:SDCCH, A:SACCH


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Mapping on TS1/BCCH carrier (UL)



 Mapping on TS2/BCCH carrier (DL/UL) if it will be used by certain
MS in active mode

26 consecutive Traffic frames = 1 Traffic multi frame

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TDMA Multi Frames Structure

 Traffic Multi Frames
– Traffic Multi Frame = 26 consecutive traffic frames (4.61msec x 26
=120msec)
 Control Multi Frames
– Control Multi Frame = 51 consecutive Control frames (4.61msec x 51
=235msec)

• Super Frame
 51 consecutive Traffic Multi Frames or 26 consecutive Control
Multi Frames
– Super Frame = 6.12 seconds

• Hyper Frame
 2048 consecutive super Frames
– Hyper Frame = 3 hours and 29 minutes nearly.


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IMSI : International Mobile Subscriber Identity
IMSI = MCC + MNC + MSIN
MCC= Mobile Country Code (3 digits)
MNC= Mobile Network Code (2 digit )
MSIN= Mobile Subscriber Identification Number (10 digits)

MCC MNC MSIN
(3 digits) (2 digits) (10 digits)

Ex: IMSI = MCC-MNC-MSIN = 602-03-1234567890 where,
602  Egypt Country Code
03 Etisalat Network Code
1234567890  Mobile Subscriber Identification Number


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MSISDN : Mobile Station Integrated Services Digital Network
MSISDN = CC + NDC + SN
CC= Country Code (2-3 digits)
NDC= Network Destination Code (2-3 digit )
SN= Subscriber Number ( max 10 digits)

CC NDC SN
(2-3 digits) (2-3 digits) (max. 10 digits)

Ex: MSISDN = CC-NDC-SN =+20-10-1234567 where,
10 Vodafone Network Code
1234567  Subscriber Number


LAI : Location Area Identity
LAI = MCC + MNC + LAC
MCC= Mobile Country Code (2-3 digits)
MNC= Mobile Network Code (2-3 digit )
LAC= Location Area Code ( max 5 digits)

MCC MNC LAC
(2-3 digits) (2-3 digits) (max.5 digits)

Ex: LAI= MCC-MNC-LAC = 602-01-12345 where,
01 Mobinil Network Code
12345  Location Area Code


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CGI : Cell Global Identity
CGI = LAI + CI = MCC + MNC + LAC + CI
MCC = Mobile Country Code (2-3 digits)
MNC = Mobile Network Code (2-3 digit )
LAC = Location Area Code ( max 5 digits)
CI = Cell Identity ( max 5 digits)

MCC MNC LAC CI
(2-3 digits) (2-3 digits) (max. 5 digits) (max. 5 digits)

Ex: CGI = MCC-MNC-LAC-CI = 602-01-12345-11223 where,
01 Mobinil Network Code
12345  Location Area Code
11223  Cell Identity

IMEI : International Mobile Equipment Identification
IMEI = TAC + FAC + SNR + spare (15 digits)
TAC = Type Approval Code, determined by a central GSM body(6 digits)
FAC = Final Assembly Code, identified the manufacturer (2 digit )
SNR = Serial Number( 6 digits)
spare = A spare bit for future use, when transmitted by MS it is always zero.
( 1 digit)


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AUC : Authentication Center
- In the AUC the below table is stored, such that for each user there is a unique
authentication key (Ki)
User# IMSI Authentication Key
User1 MCC+MNC+MSIN1 Ki1

- On authenticating certain user, the AUC will generate the triplets: RAND,SRES,Kc
- AUC generates a random no. “RAND” and send it to the MS
- Both the AUC and the MS will use RAND + Ki and Algorithm A3 to produce the
SRES(Signed Response)
RAND1 RAND1
A3 (SRES1)_AUC A3 (SRES1)_MS
Ki1 Ki1

AUC side MS Side
- VLR will take the results from AUC and MS and if:
(SRES1)_AUC = (SRES2)_MS  then the MS is Authenticated 

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AUC : Authentication Center
- The AUC is responsible also for generating the ciphering Key (Kc) for each user.
RAND1 RAND1
A8 Kc_AUC A8 Kc_MS
Ki1 Ki1

AUC side MS Side
- Kc_AUC should be equal Kc_MS, so the data encrypted by the network can be
de-ciphered by the MS.
Ciphering Process:
TDMA Frame no. Kc_AUC TDMA Frame no. Kc_MS

A5 A5

Ciphered Speech
Speech + + Speech

Network side MS side



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• MS in Idle Mode

– Doesn’t have a dedicated channel, but able to access the Network and
able to be reached by the Network.

– MS will always try to camp on the best cell based on the signal
strength criterion.

– MS will continuously monitor the serving and neighbor BCCH carriers
to decide which cell to camp on.

– The purpose behind studying the Idle Mode Behavior is to always
ensure that the MS is camped on the cell where it has the highest
probability of successful communication.


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• MS Tasks during Idle Mode

– PLMN Selection.
– Cell Selection.
– Cell Reselection.
– Location Updating.
– Monitor the Incoming Paging.



– PLMN Selection
– Cell Selection.


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• PLMN Selection Criterion
– PLMN identity is defined as “MCC+MNC” which is part of the LAI,
where LAI=MCC+MNC+LAC.
MCC: Mobile Country Code - MNC: Mobile Network Code - LAC:
Location Area Code

– When the MS is powered “ON”, it will check if it needs to perform a
Location Update by comparing the new LAI with the old stored one.

– An MS will need to make a PLMN selection only incase:
1. MS is powered “ON” for the 1st time i.e. No PLMN was registered on
before
(No Information on MCC&MNC is stored on SIM)
2. Old PLMN is not available any more (Out of coverage/Roaming)


– When the MS has to do a PLMN selection due to one of the previous
cases, the selection mode will depend on the MS settings either
Automatic or Manual.
– Automatic PLMN Selection Mode steps:
1. Home PLMN.
2. Each PLMN stored on the SIM card in priority order.
3. Other PLMNs have Signal Strength > -85 dBm.
4. All other PLMNs in order of decreasing Signal Strength.

– Manual PLMN Selection Mode:
1. Home PLMN.
2. All other available PLMNs and give the user the choice to select.


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 National Roaming
– If National Roaming is permitted then a MS can register on a PLMN in
its home country other than its home PLMN.

– National Roaming may be allowed on a certain location areas (LAs) of
the visitor PLMN.

– MS should periodically try to access back his home PLMN, but this
periodic attempts will occur only on automatic selection mode.



– PLMN Selection.
– Cell Selection


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• Cell Selection Criterion

– The Cell Selection algorithm tries to find the most suitable cell in the
selected PLMN and make the MS camp on.

– Cell Selection is done by the MS itself.

– During Idle Mode the Network doesn’t know the cell which the MS is
camping on, it only knows the Location Area where the mobile registed
himself in.


• Cell Selection Criterion Scan RF Frequencies one by
one and calculates the Average
received signal strength over 3
 5 seconds

Tune to the RF Frequency with Tune to the next higher
the highest average received frequency that wasn‟t tried
signal strength before

Check if the chosen frequency is a
BCCH carrier frequency or not No
Yes
MS will synchronize to the BCCH
frequency and read system
information (LAI,BA List,…etc)

Check if PLMN is desired or not
No
Yes
Check if Cell is barred or not
Yes
No
Check if C1 > 0 or not
Yes No

Camp on the Cell


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– Scanning RF Frequencies may occur in 2 ways:
1. Normal Scanning: Scan all Frequencies in the band ex:124 freq. in
GSM900 Band.

2. Stored List Scanning: Scan the Frequencies in the Idle BA list (BCCH
Allocation) stored on the MS SIM before being switched off.
(BA list can have maximum 32 frequencies)

 If MS found cell belongs to the desired PLMN but not suitable, the
MS will start to scan the Idle BA list of this cell.


– Cell is said to be suitable if:
1. Cell belongs to the desired PLMN
 If at least 30 strongest frequencies from GSM900 band were tried
and no suitable
cell was found, then the MS will try another PLMN based on PLMN
criterion.

2. Cell is not Barred ( CB = NO)
 Some cells can be barred for access at selection and reselection or
given lower
priority based on settings of parameters: CB

3. C1 > 0


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– C1 is called “Cell Selection Quantity”
– It is calculated at the MS based on the below equation:
C1 = (Received SS – ACCMIN) – max (CCHPWR-P,0)
ACCMIN  Minimum allowed DL received SS at the MS in order to access the
system
CCHPWR  Maximum allowed transmitting power by the MS in the UL.
P  Maximum out put power of the MS according to its class.
N.B:
1. ACCMIN and CCHPWR are cell parameters sent to the MS at the BCCH
channel.
2. If CCHPWR > P then C1 will decrease and so the Received SS should be large
enough to keep C1 > 0 (May be this cell is not designed for this MS class)
3. ACCMIN, CCHPWR, P are all measured in dBm, where C1&C2 are measured
in dBs



– PLMN Selection.
– Cell Selection.
– Cell Reselection


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• Cell Reselection Criterion
– After a cell has been selected, the MS will start the cell reselection
measurements to know if it is better to stay on the current cell or to
camp on another cell.
– Cell reselection measurements:
1. Monitors the SS (Signal Strength) of the BCCH carrier of the serving
cell.
2. Monitors the SS of the BCCH carrier of all defined neighbors in the
Idle BA list.
3. Continuously read system information sent on the serving BCCH
carrier at least every 30 seconds.
4. Continuously read system information sent on the BCCH carrier for
the six strongest neighbors at least every 5 minutes.
5. Try to decode BSIC of the six strongest neighbors every 30 seconds to
assure that it is still monitoring the same cells.


– Cell reselection measurements summary

BSIC BCCH Data (System Information)
Serving Cell - Every 30 Seconds
Six Strongest Neighbors Every 30 Every 5 Minutes
Seconds


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– When Cell Reselection will occur ?

1. Serving Cell became barred ( CB = YES )

2. C1 serving cell falls below zero for more than 5 seconds.

3. MS tried to access the network through this cell unsuccessfully for
the allowed no. of times defined by the parameter MAXRET

4. C2 neighbor cell ( one of the six strongest neighbors) became greater than
C2 serving cell for more than 5 seconds.

5. MS detects Downlink Signaling Failure.


– What will happen when the MS needs to make cell reselection?
 The MS will camp on the cell that has the highest C2 value.

– C2 is called “Cell Reselection Quantity”
C2 = C1 + CRO – TO * H( PT – T ) where PT ≠ 31
C2 = C1 – CRO where PT = 31
0, X<0
Where H(x)
1, X≥0
CRO  Cell Reselection Offset, unit = 2 dB, value range = 0 to 63
TO Temporary Offset, unit = 10 dB, value range = 0 to 7
PT  Penalty Time during which TO is valid
T  Initiated from zero when the MS places the neighbor in the list of
the Six Strongest

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– CRO : defines a signal strength offset to encourage or discourage MSs
to reselect that cell.
– TO : defines a negative temporary offset for certain time according to
settings of PT (Practically this is useful to prevent fast moving MS from
camping on microcells)
– PT: If PT is set to 31, this means that a (–ve) SS offset “CRO” will be
applied to this cell and it appears less favorite for cell reselection.


 Down Link Signaling Failure Algorithm
– The Algorithm of type “Leaky Bucket” and used a counter “D”, where D
= 90/MFRMS
– MFRMS is a cell parameter defines the no. of multiframes between
the transmission of each paging group i.e. if MFRMS=4 then a MS
attached to a certain paging group will wait in sleeping mode for 4
multiframes (4*235msec) until it is up again to listen to paging.
– When the MS is up to listen to its paging group, if the message is not
decoded successfully then D is decremented by 4 and if the message is
decoded correctly then D is incremented by 1.
– If D reaches zero, then a Down Link Signaling Failure is detected and
cell reselection took place.


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 Down Link Signaling Failure Algorithm
– Ex: Assume that MFRMS = 4
Downlink signaling failure counter is initialized: D = round(90/MFRMS)=22.
If the MS unsuccessfully decodes a paging message, then: D = D - 4 = 18.
If the MS successfully decodes a paging message, then: D = D + 1 = 19.

 If D reaches zero, then a Down Link Signaling Failure is detected and
cell reselection took place.

N.B: D can’t exceed the bucket size given by round(90/MFRMS)


 CRH ( Cell Reselection Hysteresis )
– Cell Reselection between two cells lie in two different Location Areas,
will be accompanied by Location Update.

– At the border between cells the Signal level may be comparable, cell
reselection may occur many times accompanied by many location
updating leading to huge signaling load.

– To avoid this, a parameter CRH is introduced such that a cell in another
location area LA2 should have C2LA2 should greater than C2LA1 of
serving cell lie in LA1 by at least CRH in order to be selected.

– If C2LA1 = 5 dB, CRH = 4 dB, then C2LA2 ≥ 9 dB in order to be selected.


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– PLMN Selection.
– Cell Selection.
– Location Updating


• Location Updating
– To make it possible for the mobile subscriber to receive a call and
initiate a call whenever needed, the network must know where the
MS is located whenever it moves that’s why Location Updating is
needed.

– In the Idle Mode, the Network knows the location of the MS on a
Location area resolution not on a cell resolution.

– There are three different types of location updating defined:
1. Normal Location Updating.
2. Periodic registration.
3. IMSI attach & IMSI detach (when the MS informs the network when
it enters an inactive state)


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1. Normal Location Updating
– Initiated by the MS when it enters a cell belongs to a new Location Area
(LA).
– The MS will compare the LAIold stored on the SIM with the LAInew
broadcasted from the new cell and it will found them different so it’ll
perform Location Update type normal.



2. Periodic Registration
– Regularly the MS should update the Network with its current location
Area.
– The Network will inform the MS how often it should report the location
Area he is registering himself in.
– Based on the value of the Parameter T3212 the MS will know how
frequent it should make periodic registration.
– T3212 take values from 1 (6min) to 255 (25.5 Hours), default = 40 (4 Hours)
– MSC has a supervision time = BTDM+GTDM if it doesn’t hear from the MS
during this period, the MSC will consider the MS implicitly detached.
– BTDM+GTDM should > T3212 , to not consider the MS detach before
periodic location update is performed.


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3. IMSI Attach/Detach
– IMSI attach/detach operation is an action taken by the MS to inform the
Network either it will go to inactive state (Power off) or it returned back to
idle mode.
– ATT is a cell parameter that will inform the MS whether IMSI attach/detach
is operational or not.
– If ATT=Yes, then before the MS will be switched off, it will send an IMSI
detach request to the Network, so no paging messages will be sent to this
MS while it is in this state.
– When the MS is switched on again it will send an IMSI attach request to
the Network so now paging messages can be sent normally to this MS.



– PLMN Selection.
– Cell Selection.
– Monitor the Incoming Paging


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• Monitor the Incoming Paging
 Let us revise the DL logical channels and their mapping:
I) BCH(Broadcast Channels): including
FCCH(Frequency Correction Channel)
SCH(Synchronization Channel) Always Mapped on TS0/C0
BCCH(Broadcast Control Channel)
II) CCCH(Common Control Channels): including
PCH(Paging Channel) Always Mapped on TS0/C0
AGCH(Access Grant Channel)
III) DCCH(Dedicated Control Channels): including
SDCCH(Stand Alone Dedicated Control Channel) May be Mapped on either
SACCH(Slow Associated Control Channel) TS1/C0 or TS0/C0
CBCH(Cell Broadcast Channel)
FACH(Fast Associated Control Channel) “ Work in Stealing mode
by replacing the TCH time slot”


Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

F S B B B B C

F S B C F S C C F S C C F S C C F S C C I

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51

Default Mapping on TS0/C0 (BCH+CCCH) “Non
Combined Mode”
51 TDMA Frames = 1 Control Multi-frame


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5/24/2011

Default Mapping on TS1/C0
(SDCCH+SACCH+CBCH(optional))


Combination of Control channels (Different Mapping Criteria)

− Mapping on TS0/C0 is controlled by Parameter called BCCHTYPE

− BCCHTYPE = NCOMB (Non Combined, BCH&CCCH)TS1/C0 will carry
SDCCH+SACCH

= COMB (Combined, BCH&CCCH&SDCCH/4) TS1/C0 will be free for
TCH

= COMBC (Combined with cell broadcast channel CBCH is in use,
BCH&CCCH&SDCCH/4&CBCH)  TS1/C0 will be free for TCH


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5/24/2011


− SDCCH may have on of the following 4 configurations based on parameter
SDCCH

− SDCCH = (i) SDCCH/8 (8 SDCCH Sub-channels i.e. make call setup for 8 users)
= (ii) SDCCH/8 including CBCH (7 SDCCH Sub-channels + 1 CBCH)
 For these two cases, the BCCHTYPE=NCOMB and the mapping of the SDCCH
channel is done on TS1/C0

= (iii) SDCCH/4 (4 SDCCH Sub-channels)
= (iv) SDCCH/4 including CBCH(3 SDCCH Sub-channels + 1 CBCH)
 For these two cases, the BCCHTYPE=COMB or COMBC and the mapping of the
SDCCH channel is done on TS0/C0



Non Default Mapping on TS0/C0 (BCH+CCCH)
2*51 TDMA Frames = 2 Control Multi-frame


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5/24/2011

The Table below summarizes all the previous details

Default Mapping (Non Combined) Non Default Mapping (Combined)
BCH+CCCH on TS0/C0 and
BCH+CCCH+SDCCH+SACCH+CBCH on TS0/C0
SDCCH+SACCH+CBCH on TS1/C0
CBCH doesn't exist CBCH exist CBCH doesn't exist CBCH exist

1 block for BCCH 1 block for BCCH 1 block for BCCH 1 block for BCCH

9 blocks for CCCH 9 blocks for CCCH 3 blocks for CCCH 3 blocks for CCCH

8 blocks for SDDCH 7 blocks for SDDCH 4 blocks for SDDCH 3 blocks for SDDCH

1 block for CBCH 1 block for CBCH


Paging Groups
− The MS will monitor the incoming paging in only specific times, and the rest of
the time it will remain in sleeping mode.
− In this way we save the MS battery and we decrease the UL interference on the
system.
− The MS will monitor the incoming paging when the “Paging Group” assigned for
this MS is transmitted only.
− The CCCH block can be used by either PCH or AGCH.
− When the CCCH block is used for paging it will be called “Paging Block”
− The Paging Block consists of 4 consecutive Time slots lie in 4 consecutive frames.
− The Paging Block can be used to page 4/3/2 users according to IMSI or TMSI is
used when paging the MS ( Length IMSI = 2 TS, Length TMSI = 1 TS)
− The group of users belong to the same paging block will be called “Paging
Group”


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Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

F S B B B B C

F S B C F S C C F S C C F S C C F S C C I

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51

Default Mapping on TS0/C0 (BCH+CCCH) “Non
Combined Mode”
51 TDMA Frames = 1 Control Multi-frame


Paging Groups
− As appeared the MS will listen to paging in only specific times.
− The MS will utilize the time between the 4 TS that lie in 4 consecutive frames to
make the required measurements on the neighbor cells.


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Paging Groups
− How many Paging Groups we have? This will depend on a parameter MFRMS
− MFRMS is a parameter defined per cell and it defines how frequent the paging
group assigned for certain MS will be transmitted.
− MFRMS takes values from 1 to 9,
 if MFRMS=1 then the paging group assigned for certain MS will be transmitted
every 1 control Multiframes=235 msec
 if MFRMS=9 then the paging group assigned for certain MS will be transmitted
every 9 control Multiframes = 9*235msec=2.3 seconds.
− If MFRMS is large:
Positive Side: The MS battery life time will increase coz the MS remains in sleeping
mode for longer time + paging capacity will increase.
Negative Side: Call setup time will increase coz the paging won’t be sent to the MS
except when the time of its paging group came.


Paging Strategies
− Paging Strategies are controlled by parameters in the MSC.

− Setting of parameters will decide whether the paging will be local paging (within
the LA) or global paging (within the MSC service area).

− Setting of parameters will decide also whether paging will be done via IMSI or
TMSI.

− Using the parameters we can decide also how the second paging will be incase
the first paging failed, ex: If 1st paging was local with TMSI then we can set the
2nd paging to be global with IMSI.


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5/24/2011

• Related Feature to the Idle Mode Behavior
Adaptive Configuration of Logical Channels (ACLC)

− As we know the SDCCH channel is used for signaling i.e. call setup, while the TCH
channel is used to carry real user traffic (speech/data).

− As per the GSM standards, the GOS for TCH=2% i.e. within 100 calls if 2 of them
are blocked then this will be acceptable, for the SDCCH/8 the GOS=0.5% and for
the SDCCH/4 the GOS=1%

− As we know in the default settings for frequency C0, TS0 is used to carry
BCH+CCCH and TS1 used to carry SDCCH+SACCH, and TS2TS7 used to carry
speech/data


− Now if the signaling load is high, ex: many users need to make call setup, then
high blocking will occur exceeding the acceptable value = 0.5%

− To solve the blocking we have 2 ways:
i) Static configuration of a TCH TS to be used as SDCCH forever
( Now TS1&TS2 used for SDDCH+SACCH and TS3TS7 used to carry speech/data)
 But in this case we lost 1 TCH channel i.e. 5 users can talk simultaneously instead
of 6
ii) Adaptive configuration of a TCH TS to be used as SDCCH/8 when there is high
SDCCH utilization only
( Now TS1&TS2 used for SDDCH+SACCH and TS3TS7 used to carry speech/data,
but when the utilization is back to its normal trend, TS2 will be configured back
automatically as a TCH and used to carry speech/data)


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 Main Controlling Parameters:
ACSTATE: Activates/Deactivates the feature on cell basis, values: ON/OFF
SLEVEL: No. of Idle SDCCH sub-channels below which the feature will work.

 The conditions that should be fulfilled for the ACLC feature to work:
1) ACSTATE=ON
2) No. of Idle SDCCH sub-channels ≤ SLEVEL (Indication for high utilization)
3) No. of already defined SDCCH channels/8 < Max. allowed configuration of
SDCCHs in the cell.
4) No. of Idle TCHs > 4


• Parameters Summary
SCH Parameters
Parameter Name Value Range Recommended Value Unit
BSIC NCC: 0 to 7 BCC: 0 to 7 ─ ─

RACH Control Parameters
MAXRET 1,2,4,7 4 ─

Control Channel Parameters
BCCHTYPE COMB COMBC NCOMB NCOMB ─
0 to 16 (0: No SDCCH/8
SDCCH 1 ─
configured-combined mode)

IMSI Attach/Detach Parameters
ATT Yes, No Yes ─


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• Parameters Summary
Paging Parameters and Periodic Update
Parameter
Value Range Recommended Value Unit
Name
Control Channel Multi
MFRMS 2 to 9 6
frame
AGBLK 0 or 1 0 ─
0 to 255 (0: infinite-No periodic
T3212 40 6 minutes
registeration)

Cell Selection and Reselection Parameters
Parameter
Value Range Recommended Value Unit
Name
ACCMIN − 47 dBm to −110 dBm −110 dBm dBm
GSM900: 13 to 43 in steps of 2 GSM900: 33 dBm
CCHPWR dBm
GSM1800: 4 to 30 in steps of 2 GSM1800: 30 dBm
CRO 0 to 63 0 2 dB
TO 0 to 7 (7:infinite) 0 10 dB
PT 0 to 31 0
CRH 0 to 14 in steps of 2 dB



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5/24/2011


• Handover (Locating) Algorithm

– The Handover (Locating) Algorithm is the basic feature to provide mobility in
the Radio Network.
– Aims At? i) Keep the continuity of a current call with acceptable quality.
ii) Cell size control in-order to decrease total interference in the
system.
– Implemented where? In the BSC.
– Location process initiated when? After Hand Over (HO), Assignment or
Immediate Assignment.
– Inputs to the Algorithm? Signal Strength, Quality measurements &TA for
serving cell and Signal Strength measurements for neighbor cells.
– Output from the Algorithm? List of candidates which the algorithm judges to be
possible candidates for HO (List of HO candidates are ranked and sorted in
descending order)


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– What types of Handover (locating) algorithm we have?
i) SS & Path Loss based Algorithm: Follows the GSM specifications. HO decision is
taken based on both Signal Strength (SS) and Path Loss.

ii) SS based Algorithm: HO decision is taken based on Signal Strength only and
this leads to better performance.
It is less complex, uses less parameters and easy to be maintained in the Radio
Network.


– The main Flow of the Handover (locating) Algorithm goes as follow:

Initiations Filtering Basic Urgency Conditions
Ranking Handling

Auxiliary Radio Network
Features Evaluation

Organizing the List

Sending the List
& Allocation
Reply


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– Initiation
– Filtering.
– Basic Ranking.
– Urgency Conditions Handling.
– Auxiliary Radio Network Features Evaluation.
– Organizing the List.
– Sending the List & Allocation Reply


• Initiation of the Handover (Locating) Process/Algorithm
 The Locating Process is initiated when one of the following occurs:

1. Handover: Normal, Intra Cell HO (IHO), Sub-cell change (OLUL or
ULOL)

2. Assignment: Allocation of TCH channel after completing call setup on
SDCCH.

3. Immediate assignment: You are assigned SDCCH to make call setup, or a
TCH to make call setup on when no free SDCCH channels exist.


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– Initiation
– Filtering
– Basic Ranking.


• Filtering

− Simply it is the process of collecting the required data on Signal Strength
(SS), Quality and Time Advance (TA) for serving and neighbor cells and
average these consecutive measurements over a specified period to rank
these cells.

− This is accomplished in two steps:
1. Measurements preparation
2. SS, Quality and TA filtering


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5/24/2011

• Filtering

1. Measurements preparation
− Data that is measured:

Cell on which measurements Who makes the
Measured Quantity
are reported measurements?
SS DL MS
Quality DL (rxqual_DL) MS
Serving Cell
Quality UL (rxqual_UL) BTS
TA BTS
6 Strongest neighbor cells SS DL MS

− The MS can measure the SS of up to 32 neighbor frequencies but only the six
strongest neighbors (which it succeeded to decode its BSIC over the last 10
seconds) are reported and considered candidates for HO.


• Filtering

1. Measurements Preparation
− SS measurements are delivered as integer values 0  63 corresponds to real SS
from
-110 dBm  - 47 dBm

− Quality is measured based on the BER and it may be represented in two forms:
i) Integers 0 (Best)  7 (Worst)
ii) Decitransformed Quality units (dtqu) from 0 (Best)  70 (Worst)

− Time Advance (TA) is reported as values between 0  63 bit period.
N.B: If TA=1 then the MS is at nearly 0.5 km from the cell


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• Filtering

2. SS, Quality and TA filtering:
− The consecutive measurements for SS, Quality and TA are averaged in some
way based on the equation of the filter used.

− We’ve 5 Types of Filters that may be used, each one has its own equation or its
way to produce output results from the collected consecutive measurements:
A. General FIR filters (Finite Impulse response)
B. Recursive Straight Average filter
C. Recursive exponential filter
D. Recursive 1st order Butterworth filter
E. Median filter


• Filtering

2. SS, Quality and TA filtering:

− In addition to the way each filter use to produce output results from the
consecutive measurements, each filter has what we call filter length which is
the period over which measurements are considered.

− We have controlling parameters on cell basis to select the type of filter used
and the length of the filter.

− Also the type of the filter used in signaling (call setup) and dedicated phases
may be configured separately as we’ll see.


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5/24/2011


– Initiation
– Filtering
– Basic Ranking


• Basic Ranking

− It is called “Basic” coz in this stage ranking is done before handling the urgency
conditions and evaluation of the auxiliary radio network features.

− As mentioned earlier, two algorithms are available for basic ranking (SS&Path
loss based Algorithm and SS based Algorithm) and they’re selected according
to the parameter EVALTYPE

− EVALTYPE=1, SS & Path loss based Algorithm is used for basic ranking taking
into consideration both Signal Strength measurements and the path loss.

− EVALTYPE=3, SS based Algorithm is used for basic ranking taking into
consideration Signal Strength measurements only.


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• Basic Ranking
Basic Ranking Algorithm following the SS based Algorithm will be done on four steps:
A. Correction of Base Station output power. Common for
B. Evaluation of the minimum signal strength condition for neighbors. Both
C. Subtraction of signal strength penalties. Algorithms
D. Rank the Candidates after applying Offsets and Hysteresis.


• Basic Ranking
Basic Ranking Algorithm following the SS based Algorithm
A. Correction of Base Station output power
 The location algorithm aims at making the Pure traffic frequencies to control
the cell
borders and not the BCCH frequencies, coz most of the time the seized TCH
Time slot will be located on a TCH frequency.

 BSPWR is a parameter to set the output power of the BCCH carrier and
BSTXPWR is a parameter to set the output power of the TCH frequencies.

 Correction for the output power will done for both:
(A-i) Correction for Neighbor Cells.
(A-ii) Correction for Serving Cell.


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5/24/2011

• Basic Ranking

(A-i) Correction for Neighbor Cells

− The MS is informed by the BCCH frequencies of the neighbors cells on which he
has to perform his measurements via the Active BA list.

− SS_corrected_DLneighbor = SS_measured_DLneighbor - ( BSPWR - BSTXPWR )


• Basic Ranking
(A-ii) Correction for Serving Cell
1) TCH Time Slot (TS) is on the BCCH frequency
SS_corrected_DLservingcell = SS_measured_DLservingcell - ( BSPWR - BSTXPWR )

2) TCH TS is hopping between a BCCH frequency and a TCH frequency:
SS_corrected_DLservingcell = SS_measured_DLservingcell - ( BSPWR - BSTXPWR )/N ,
Where N is the no. of the hopping frequencies

3) TCH TS is on the OL (Over Laid sub cell)
SS_corrected_DLUnderLaid = SS_measured_DLOverLaid+ ( BSTXPWR Under Laid –
BSTXPWROverLaid )


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• Basic Ranking
B. Evaluation of the minimum Signal Strength condition for Neighbors
− Not all the neighbors are allowed to be ranked!!
− The neighbor should pass the minimum signal strength condition in order to be
ranked.
− SS_corrected_DLneighbor will be compared with respect to parameter called
MSRXMIN,
If SS_corrected_DLneighbor ≥ MSRXMIN  this neighbor will be included in ranking
If SS_corrected_DLneighbor < MSRXMIN  this neighbor will be excluded from
ranking
− If UL measurements are included then SS_corrected_ULneighbor will be
compared with respect to parameter called BSRXMIN,
If SS_corrected_ULneighbor ≥ BSRXMIN  this neighbor will be included in ranking
If SS_corrected_ULneighbor < BSRXMIN  this neighbor will be excluded from
ranking

• Basic Ranking
B. Evaluation of the minimum Signal Strength condition for Neighbors
− Example: Assume that a MS is connected to cell A that has five neighbors
B,C,D,E&F, the MSRXMIN for all the cells is -104 dBm and the
SS_corrected_DLneighbor for each cell after correcting the BTS o/p power is given
in the below Table

Neighbors SS_corrected_DLneighbor
B -85 dBm
C -110 dBm  Cell C will be excluded
from ranking and won‟t be
D -87 dBm
considered in the next stage
E -70 dBm and the MS will never HO to it
F -100 dBm


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• Basic Ranking
C. Subtraction of signal strength penalties
− Penalties or Punishments will be applied on cells that are for some reasons
temporarily undesirable.
− A Penalty value will decrease the rank of some cells for certain penalty time.
− SS_punished_DL = SS_corrected_DL – Locating Penalties – HCS Penalties
− In the coming slides we’ll talk about the two types of penalties:
(C-i) Locating Penalties
(C-ii) HCS Penalties


• Basic Ranking
1) Due to HO failure: If HO to a neighbor cell failed then we’ve to apply a penalty value
for some time on this neighbor so when basic ranking is done again we don’t go
back to this cell.
Penalty value will be configured using parameter PSSHF (default 63 dB)
Penalty time will be configured using parameter PTIMHF (default 5 sec)


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• Basic Ranking
2) Due to Bad Quality (BQ) Urgency HO:
If a cell was abandon due to BQ, then it should have been the best cell from SS
point of view so without penalties using the basic ranking we’ll be back to this cell.
Penalty value will be configured using parameter PSSBQ (default 7 dB)
Penalty time will be configured using parameter PTIMBQ (default 5 seconds)
3) Due to Excessive TA Urgency HO:
Handled in the same manner like the BQ case.
Penalty value will be configured using parameter PSSTA (default 63 dB)
Penalty time will be configured using parameter PTIMTA (default 30
seconds)


• Basic Ranking
(C-ii) HCS Penalties
− It is related to the HCS (Hierarchical Cell Structure) feature when a MS is detected
as a fast moving mobile (If fast moving mobile feature is activated)
− A penalty will be applied on lower layer cells so in ranking we will prioritize cells in
the same layer of the serving cell and cells in higher layers and in this way
unnecessary HO’s are prevented ( ex: layer2 cells will be prioritized than layer1
cells)


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• Basic Ranking
D. Rank the Candidates after applying Offsets and Hysteresis
− Ranking for neighbor cells will be done after
applying Offsets and Hysteresis.
– Offset: Displace the cell border as compared to
The border strictly given by SS.
Controlling parameter: OFFSET (default: zero dB)
– Hysteresis: To reduce the risk of ping pong HO
a region for Hysteresis is applied
around the cell border.


• Basic Ranking
− If the Hysteresis value is too high there will be a risk that the MS will be connected
to the cell of low SS for long time and if the Hysteresis is too low then there will be
a risk that ping pong HO’s occur.
− So the applied value of Hysteresis will be variable based on the received SS of the
serving cell.
− SS_corrected_DLservingcell will be compared to value HYSTSEP (default -90 dBm),
 If SS_corrected_DLservingcell > HYSTSEP, then the serving cell is strong enough and
high value of Hysteresis will be applied such that Hysteresis value=HIHYST (default
5 dB)
 If SS_corrected_DLservingcell < HYSTSEP, then the serving cell is not strong enough
and low value of Hysteresis will be applied such that Hysteresis value=LOHYST
(default 3 dB)


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5/24/2011

• Basic Ranking

N
SS_corrected_DLservingcell > HYSTSEPo HYST=LOHY
ST
Yes
HYST=HIHYST
Output from
Basic Ranking

Now,
Rankservingcell = SS_corrected_DLservingcell
Rankneighbor= SS_punished_DLneighbor – OFFSETneighbor – HYSTneighbor



– Initiation
– Filtering
– Basic Ranking
– Urgency Conditions Handling


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• Urgency Conditions Handling

− After the Basic Ranking stage a check is made on the serving cell to know if
Urgency conditions are detected or not.
− We have two types of Urgency HO:
1. Bad Quality (BQ) Urgency HO
2. Excessive Time Advance (TA) Urgency HO
− If Urgency conditions are detected then the serving cell should be abandon as
fast as possible, but some of the neighbors will be removed from the candidate
list and the MS will not be able to HO to them as we will see later.
− As seen before, cells that were abandon due to Urgency HO will be subjected
to punishment/penalty.




− The Quality measured at the DL and UL for the serving cell will be compared with
two parameters QLIMDL & QLIMUL (default 50 dtqu) and if:
Or  Urgency HO due to BQ should be
rxqual_DL > QLIMDLperformed
rxqual_UL > QLIMUL
− The Quality may drop like that as a result of Co-Channel Interference or when the
SS became very low.
− When Urgency condition is detected the MS has to leave the cell and make HO to
other cell, but in this case the serving cell is the one that has the highest SS so the
MS has to HO to a cell of worse SS, but is the MS allowed to HO to any worse cell?


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− Is the MS allowed to HO to any worse cell? No, this will be based on a parameter
called BQOFFSET which will ensure that far neighbors won’t be selected.
− If Rankservingcell – Rankneighbor ≤ BQOFFSET+HYST, then this neighbor is near to the
serving cell and it is not much worse than the serving cell and it can be candidate
for HO.
− If Rankservingcell – Rankneighbor > BQOFFSET+HYST, then this neighbor is far from the
serving cell and it will be removed from the candidate list.
− Ex: If Urgency condition is detected where Rankservingcell = -75 dBm and the
neighbors: RankB = -79 dBm ,RankC = -90 dBm ,RankD = -87 dBm and
BQOFFSET=5dB,HYST=0 dB
Rankservingcell – RankB =4dB<BQOFFSET= 5dB Cell B is kept in the candidate list
Rankservingcell –RankC=15dB>BQOFFSET= 5dB Cell C is removed from the candidate list
Rankservingcell – RankD = 8dB > BQOFFSET=5dB  Cell D is removed from the
candidate list



2. Excessive Time Advance (TA) Urgency HO
− TA can be used as a measure for the distance between the BTS and the MS.
− If TA > TALIM (63 bit period)  Urgency HO due to TA is initiated.


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• After Basic Ranking and Evaluation of the Urgency Conditions, the
Serving cell and Neighbor cells will be divided into 3 Groups
Better Cell

Categorization #1 Serving Cell

Worse Cell



– Initiation
– Filtering
– Basic Ranking
– Auxiliary Radio Network Features Evaluation


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• Auxiliary Radio Network Features Evaluation

1. Assignment to Another Cell Evaluation
2. Cell Load Sharing Evaluation
3. Over Laid/Under Laid sub-cell Evaluation
4. IHO Evaluation
5. HCS Evaluation

 After these Evaluations, some candidates will be removed from the HO candidate
list and
Categorization#2 will be performed.



− The Locating Algorithm may be initiated after immediate assignment to know
whether it is better for the MS to take a TCH time slot on the current cell or not.
− If during the signaling phase a better cell was found after ranking, then
“Assignment to Better Cell” will be initiated.
− If during the signaling phase no better cell was found, then the MS will normally
be assigned a TCH time slot on the current cell.
− If the Better/Serving cells were congested then “Assignment to Worse Cell” will
be initiated if possible.


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− Is the MS allowed to take TCH time slot on any worse cell? No, this will be based
on a parameter called AWOFFSET which will ensure that far neighbors won’t be
selected.
− Only if Rankservingcell – Rankneighbor ≤ AWOFFSET+HYST, then this neighbor is near to
the serving cell and it is not much worse than the serving cell and assignment to it
can be done.
− If Rankservingcell – Rankneighbor > AWOFFSET+HYST, then this neighbor is far from the
serving cell and it will be removed from the candidate list.



2. Cell Load Sharing (CLS) Evaluation
− This feature is used to reduce congestion on the serving cell.
− When CLS is activated and the load on the serving cell becomes higher than
certain threshold then:
i) Valid CLS HO candidates are defined
ii) Re-calculation of their ranking values will be performed.


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2. Cell Load Sharing (CLS) Evaluation
i) Valid CLS HO candidates are defined as follow:
− Load on neighbor cells < CLS load threshold
− Internal cells: lies in the same BSC
− Same Layer

ii. Re-calculation of their ranking values will be performed
− We’re going to recalculate the Ranking values of the valid CLS neighbors with
reduced Hysteresis so these worse neighbors will appear with higher SS than they
really are and the MS can make HO to them and relief the congestion on the
current cell.
 This feature will be discussed in details afterwards.



3. OL/UL Sub-Cell Evaluation
− The OL/UL feature provides a way of increasing the traffic capacity in a cellular
network without building new sites.

− Since OL subcell serves smaller area than the corresponding UL subcell a smaller
reuse distance can be used in in the OL subcell than in the under laid.

− The OL/UL evaluation may result in a recommendation to change the subcell from
the one currently in use, this evaluation is based on:
DL SS, TA serving Cell, Distance to cell border, Traffic Load in the cell


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4. Intra Cell HO (IHO) Evaluation
− The IHO feature provides a way to improve the speech quality during the
conservation when bad quality is detected while the SS is high.

− This is can be accomplished by changing the channel the connection is currently
using within the same cell.



5. Hierarchical Cell Structure (HCS) Evaluation
− The HCS feature provides the possibility to give priority to cells that are not
strongest but provide sufficient SS.

− The priority of a cell is given by associating a layer to the cell.

− We have 8 layers from layer 1 (Highly prioritized) to layer 8 (least prioritized).

− Micro cells are prioritized than Macro cells for capacity purposes.

− Cells of lower layers will be ranked higher than cells of higher layers in the HO
candidate list.

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• After the Auxiliary Radio Network features evaluation some
candidates may be prioritized and the order of the candidate list will
be modified.
The Serving cell and Neighbor cells will be divided into 3 Groups
Above S
Serving Cell
Categorization #2 (SC)
Below S



– Initiation
– Filtering
– Basic Ranking
– Organizing the List


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• Organizing the List

− The final list will contain maximum up to six neighbors + the serving cell and
categorized as follows: Serving Cell (SC), Above S, Below S

− To reach the final form before sending the list the following steps will be done:
A. Removal of Candidates
B. Ordering the Candidate list based on the Current Conditions.



A. Removal of Candidates
 Some Candidates may be removed coz:
− Some Controlling timers are active and preventing HO to certain cell:
TALLOC: This timer prevents HO on a target cell for some time after assignment/HO
failure due to congestion on target cell. (N.B: No penalties are applied on this
cell)
TURGEN: This timer prevents HO on a target cell for some time after urgency HO
failure due to congestion on target cell. (N.B: No penalties are applied on this
cell)

N.B: TALLOC and TURGEN are BSC parameters (Default Values= 2 SACCH periods


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B. Ordering the Candidate list based on the Current Conditions
− Means what? Means in what order the 3 categories (Above S, S, Below S) will be
arranged before sending the candidate list. This will be based on some condition
flags.
− Condition flags: 1  Assignment Request Arrived
2  Assignment to Worst Cell is in use
3  Excessive TA detected
4  BQ Urgency HO
5  OL/UL Subcell load change or IHO


B. Ordering the Candidate list based on the Current Conditions
Condition flags: 1  Assignment Request Arrived 2  Assignment to Worse Cell is in
use 3  Excessive TA detected 4  BQ Urgency HO 5  OL/UL
Subcell load change or IHO
Condition Flags
Case Ordering Comment
1 2 3 4 5
1 0 x 0 0 0 Above S Normal Case
Serving Cell has BQ so it should be abandon
2 0 x 0 1 0 Above S Below S
either to the Above S or Below S cell
An Assignment request came and the AW flag is
3 1 0 0 0 0 Above S S
not raised
An Assignment request came and the AW flag is
4 1 1 0 0 0 Above S S Below S
raised

Serving Cell has BQ so it should be abandon but
coz the OL/UL subcell change flag is raised, then
5 0 x 0 1 1 Above S S Below S the serving cell is included coz this subcell
change may solve the issue with no need to go
for a below worse cell


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– Initiation
– Filtering
– Basic Ranking
– Organizing the List


• Sending the List & Allocation Reply

− The resulting candidate list will form the basis on which HO will be performed.

− Empty list means that no options are better than remaining on the current cell
and no HO will occur.

− The channel allocation reply may be success or failure.

− Failure may be due to congestion or signaling failure on the target cell.

− Based on the result of allocation either success/failure, some actions will be taken
like applying some penalties or enabling of certain timers as we saw previously.


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• Example1:
− Assume that the o/p from the Filtering stage for the SS measurements is as below
and we want to prepare the Basic Ranking Candidate list for HO:
Cell SS(dBm)
A -70
B (Serving Cell) -74
C -78
D -68
E -80
F -92
G -95
Where,
BSPWR = BSTXPWR, MSRXMIN = -90 dBm,
Cell A was abandon due to BQ urgency HO (PSSBQ=7dB)
SS based Algorithm is in use where OFFSET=0, HYSTSEP= -90 dBm,
HIHYST= 5 dB, LOHYST= 3 dB


• Solution:

A) Correction of Base Station output power:
− Since BSPWR = BSTXPWR then the current measurements will be kept as it is.
− SS_corrected_DLneighbor = SS_measured_DLneighbor
− SS_corrected_DLserving = SS_measured_DLserving

B) Evaluation of the minimum Signal Strength condition for Neighbors
− The SS for neighbors will be compared against MSRXMIN = -90 dBm
Cell SS(dBm)
A -70 Cell F and Cell G have SS < MSRXMIN then they will be
B (Serving Cell) -74
removed from the list and can’t be candidates for HO.
C -78
D -68
E -80
F -92
G -95


73

Gsm optimization

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