Gsm Training Gurgaon

GSM INTRODUCTION ADA CELLWORKS PVT LTD

INTRODUCTION •

The global system for mobile communications (GSM) is a set of recommendations and specifications for a digital cellular telephone network (known as a Public Land Mobile Network, or PLMN). These recommendations ensure the compatibility of equipment from different GSM manufacturers, and interconnectivity between different administrations, including operations across international boundaries.

THE GSM NETWORK • • • • •

• •

The GSM network is comprised of the following components: Network Elements The GSM network incorporates a number of network elements to support mobile equipment. They are listed and described in the GSM network elements section of this chapter. GSM subsystems In addition, the network includes subsystems that are not formally recognized as network elements but are necessary for network operation. These are described in the GSM subsystems (non-network elements) section of this chapter. Standardized Interfaces GSM specifies standards for interfaces between network elements, which ensure the connectivity of GSM equipment from different manufacturers. These are listed in the Standardized interfaces section of this chapter.

THE GSM NETWORK CONTINUED •

Network Protocols

•

For most of the network communications on these interfaces, internationally recognized communications protocols have been used • These are identified in the Network protocols section of this chapter. • •

GSM Frequencies The frequency allocations for GSM 900, Extended GSM and Digital Communications Systems are identified in the GSM frequencies section of this chapter.

DIGITAL NETWORKS •

GSM networks are digital and can cater for high system capacities. They are consistent with the world wide digitization of the telephone network, and are an extension of the Integrated Services Digital Network (ISDN), using a digital radio interface between the cellular network and the mobile subscriber equipment.

INCREASED CAPACITY •

The GSM system provides a greater subscriber capacity than analogue systems. GSM allows 25 kHz. Per user, that is, eight conversations per 200kHz. Channel pair (a pair comprising one transmit channel and one receive channel). Digital channel coding and the modulation used makes the signal resistant to interference from the cells where the same frequencies are re-used (co-channel interference); a Carrier to Interference Ratio (C/I) level of 9 dB is achieved, as opposed to the 18 dB typical with analogue cellular. This allows increased geographic reuse by permitting a reduction in the number of cells in the reuse pattern. Since this number is directly controlled by the amount of interference, the radio transmission design can deliver acceptable performance.

CGI : CELL GLOBAL IDENTITY LAI MCC

MNC

LAC

CGI

MCC = Mobile Country Code MNC = Mobile Network Code LAC = Location Area Code CI = Cell Identity

CI

MSISDN CC

98

NDC XXX

SN 12345

CC = Country Code NDC = National Destination Code SN = Subscriber Number

MSISDN •

The Mobile Subscriber ISDN (MSISDN) number is the telephone number of the MS. This is the number a calling party dials to reach the subscriber. It is used by the land network to route calls towards the MSC.

IMSI • IMSI (International Mobile Subscriber Identity) Network IdentityMCC Unique To A Sim. MNC MSIN 404

XX

12345..10

SIM = Subscriber Identity Module MCC = Mobile Country Code MNC = Mobile Network Code MSIN = Mobile Subscriber Identity Number

IMEI • IMEI : Serial number unique to each mobile TAC

FAC

SNR

SP

6

2

6

1

IMEI = International Mobile Equipment Identity TAC = Type Approval Code FAC = Final Assembly Code SNR = Serial Number SP = Spare

SUBSCRIBER IDENTIFICATION • International Mobile Subscriber Identity (IMSI) Just the IMEI identifies the mobile equipment, other numbers • are used to identify the mobile subscriber. Different subscriber identities are used in different phases of call setup. The International Mobile Subscriber Identity (IMSI) is the primary identity of the subscriber within the mobile network and is permanently assigned to that subscriber. • Temporary Mobile Subscriber Identity (TMSI) The GSM system can also assign a Temporary Mobile Subscriber • Identity (TMSI). After the subscriber’s IMSI has been initialized on the system, the TMSI can be used for sending backward and forward across the network to identify the subscriber. The system automatically changes the TMSI at regular intervals, thus protecting the subscriber from being identified by someone attempting to monitor the radio channels. The TMSI is a local number and is always transmitted with the Local numbers and is always transmitted with the Location Area Identification (LAI) to avoid ambiguities.

SUBSCRIBER IDENTIFICATION MODULE (SIM) •

By making a distinction between the subscriber identity and the mobile equipment identity, a GSM PLMN can route calls and perform billing based on the identity of the subscriber rather than the mobile equipment being used. This can be done using a removable Subscriber Information Module (SIM). A ”smart card” is one possible implementation of a SIM module.

•

IMSI. This is transmitted at initialization of the mobile equipment. TMSI This is updated periodically by the PLMN MSISDN This is made up of a country code, a national code and a subscriber number. Location Area Identity (LAI) This identified the current location of the subscriber. Subscriber Authentication Key (KI) This is used to authenticate the SIM.

• • • •

EQUIPMENT IDENTITY NUMBER •

International Mobile station Equipment Identity (IMEI) • Each MS is identified by an International Mobile station Equipment Identity (IMEI) number which is permanently stored in the mobile equipment. On request, the MS sends this number over the signalling channel to the MSC. The IMEI can be used to identify MS,s that are reported stolen or operating incorrectly. • •

Equipment Identity Register ( EIR ) A listing of the allowed IMEI is maintained by the PLMN’s in the Equipment Identity Register (EIR) to validate the mobile equipment.

Frequency Bands Uplink

890 – 915 MHz

Downlink

935 – 960 MHz 25 MHz 1

100 KHz

25 MHz

2

3

4

…………….

200 KHz

124 100 KHz

A 200 KHz carrier spacing has been chosen. Excluding 2x100 KHz edges of the band, this gives 124 possible carriers for the uplink and downlink. The use of carrier 1 and 124 are optional for operators.

GSM Network Architecture BSC HLR BTS

TRAU MSC

BTS BTS BTS BTS

AUC

VLR SMSC

BSC

EIR

PSTN

MS – Mobile Station MS SIM

ME

• Mobile station provides user access to GSM network for voice and data • All GSM mobiles comply to GSM standards • Subscriber data is read from a SIM card that plugs into ME

MS (cont..) • Each MS has a unique number called as IMEI number, which is stored in EIR for authentication purposes • Mobile camps on to the GSM network through the BTS serving the cell • Mobile also scans neighboring cells and reports signal strengths • Mobile transmits and receives voice at 13 kb/s over the air interface

Mobile Station Output Power

20 watts Vehicle and Portable • CLASS 1 8 watts Portable and Vehicle • CLASS 2 5 watts Hand-Held • CLASS 3 2 watts Hand-Held (GSM) • CLASS 4 0.8 watts Hand-Held (DCS 1800) • CLASS 5 • Output power determines: – Accessibility in areas of coverage – Talk Time and Standby time

Mobile Station Identities

MSISDN : Mobile Station ISDN Number It is the human identity used to call a Mobile Station CC

NDC

SN

98

250

00134

MSISDN

• CC – Country Code • NDC – National Destination Code • SN – Serial Number

IMSI (International Mobile Subscriber Identity) 3

2 or 3

MCC

MNC

MSIN

IMSI

Not more than 15 NMSI • MCC – Mobile Country Code • MNC – Mobile Network Code • MSIN – Mobile Subscriber Identity Number

IMEI (International Mobile Equipment Identity) TAC

FAC

SNR

SP

IMEI

6

2

6

1

15

• TAC – Type Approval Code • FAC – Factory Assembly Code • SNR – Serial Number • SP – Spare digit (usually used to specify software version)

SIM ( Subscriber Identity Module) Space to insert SIM photo

• Removable module inserted when the subscriber wants to use the ME • Two sizes: credit card size and stamp size • SIM features and contents are personalized by the Service Activator • ROM – 6kb to 16 kb • RAM – 128 bytes to 256 bytes • EEPROM – 3kb to 8 kb

Contents of SIM • Serial Number • IMSI, Subscriber Key Ki, Ciphering Key Kc • Algorithms for authentication and ciphering • Network Code • PIN, PUK • Charging Information • Abbreviated Dialling • Supplementary Features (e.g. Call barring)

SIM Security • Two level protection • When mobile is turned on, it will ask for user to enter PIN (Personal Id Number) • 3 tries for PIN, after that PIN locked • To unblock PIN, there is PUK (Pin Unblock Key) • 10 attempts of PUK allowed • After that SIM is blocked

BTS (Base Transceiver Station) • BTS has a set of Transceivers (TRXs) to communicate with mobiles in its area • One BTS covers one or more than one cell • The capacity of a cell depends on number of transceivers in the cell • BTS is connected to the BSC through Abis Interface which is 2Mbps • BTS transmits and receives voice at 13kbps over air interface to the mobiles. • BTS commands mobiles to set Tx. Power, timing advance and Handovers

BTS

BSC – Base Station Controller

• Several BTSs are connected to the BSC • BSC Manages channel allocation, handovers and release of channels at connected BTSs • BSC connects to the BTS via the Abis interface and to the MSC on A interface • BSC has the entire database of cell parameters associated with the BTSs. • No mobile data is stored in the BSC • Less connections for MSC as intelligence is made common to all BTSs by the BSC

BSC

TRAU – Transcoder Rate Adaptation Unit MSC and TRAU BSC

PSTN

BTS

13 kbps

16 kbps

16 kbps

64 kbps

TRAU (cont..)

• The MSC is based on ISDN switching. The Fixed Network is also ISDN based. • ISDN has speech rate of 64 kbps. Mobile communicates at 13 kbps. • TRAU converts the data rates between 13kbps GSM rate to 64kbps Standard ISDN rate • TRAU can be collocated with the BTS, BSC or MSC or it can be a separate unit.

Location of Transcoder • Collocated with MSC, BSC, BTS • Separate Unit

MSC

Transco der

BSC

MSC – Mobile Switching Centre HLR

BSC BSC

VLR

BSC BTSs

PSTN

MSC (cont..) • Exchange where calls are established, maintained and released • Database for all subscribers and their associated features. • Communicates with the BSCs on the A interface and with PSTN on fixed line. • MSC is weighted on the number of subscribers it can support. E.g. an MSC of 1 lac subscribers means one MSC is enough till subscriber base increases upto 1 lac, beyond which another MSC is required.

Multiple MSCs

• When there is more capacity, there are more than one MSCs. • All MSCs have to communicate with one another and to the outside world. • Very complicated to connect each MSC to each other and each MSC to PSTN • So there is a concept of GMSC (Gateway MSC)

MSC

BSC

GMSC BSC

MSC

PSTN

HLR – Home Location Register • MSC has all subscriber database stored in HLR • HLR has all permanent subscriber database • HLR has a database which describes the subscriber’s profile i.e. basic features and supplementary services • MSC communicates with the HLR to get data for subscribers on call

VLR – Visiting Location Register

• A subscription when activated is registered in VLR • VLR has all the subscriber numbers which are active. • VLR has a temporary database of all active subscribers (on/off, location information) HLR

MSC

VLR

VLR (cont..) HLR MSC

VLR

VLR

MSC

• MSC communicates with HLR for subscribers coming from different MSCs. If the subscriber is found valid, then it registers the subscriber in the VLR

AUC – Authentication Centre • Authentication is a process by which a SIM is verified • Secret data and the verification process algorithm are stored in AUC • AUC is the element which carries out the verification of the SIM • AUC is associated with the HLR

MS

MSC

HLR

AUC

EIR (Equipment Identity Register)

• EIR is the Mobile Equipment Database which has a series of IMEIs • MSC asks the Mobile to send its IMEI • MSC then checks the validity of IMEI with the EIR • All IMEIs are stored in EIR with relevant classifications

MSC

EIR

Classification of IMEIs White list: This contains the IMEI of type approved mobiles Black List: List of IMEIs which should be barred because either they are stolen or are not functioning properly Grey list: List of IMEIs which are to be evaluated before they are put in black list

Billing Centre (BC) • BC Generates the billing statement for each subscriber • BC may be directly connected to the MSC or through a mediation device • MSC sends CDRs (Call Detail Records) to the BC • According to the template of pulse rates and units set, BC creates a bill according to the destination called and the call duration

Billing Centre (BC) (cont..) Templates for unit costs

CDRs

OMC – Operations and Maintenance Centre • Also called the NOC (Network Operations centre) • It is the central monitoring and remote maintenance centre for all network elements • OMC has links to BSCs and MSCs

OMC – Operations and Maintenance Centre • Also called the NOC (Network Operations centre) • It is the central monitoring and remote maintenance centre for all network elements • OMC has links to BSCs and MSCs

OMC BTSs

BSC

BTSs

BSC

BTSs

BSC

OMC System

OMC Terminals

GSM Channels

GSM Channels • Physical Channel – One time slot on one carrier is called physical channel.

• Logical Channel – Information carried by physical channels is called logical Channels. – Logical channels are mapped on physical channels.

Logical Channels • Traffic channels: Used for speech and data – Full Rate(TCH/F)

– Half Rate(TCH/H)

• Control channels: Used for signaling .i.e.

setting up a radio connection, call or controlling an MS during conversation – BCH(Broadcast channels) – CCCH(common control channels) – DCCH(dedicated control channels)

Traffic Channels(TCH)

Traffic Channels(TCH) TCH/F (full Rate)

TCH/H (half Rate)

Control Channels(CCH) CCH(Control Channel)

BCH

CCH

SCH

CCCH

Synch. Chanels FCCH

RACH

CBCH

PCH/ AGCH

DCCH

SDCCH

FACCH

ACCH

SACCH

BCH(Broadcast Channels) • BCCH(Broadcast Control Channels) – Downlink Only. – Broadcast information of the serving cell (System Information). – Transmitted on timeslot zero of BCCH carrier. – Reads only by idle mobile at least once every 30 secs.

BCH(Broadcast Channels) cont’d • SCH(Synchronisation Channels) – Downlink Only – Carries information for frame synchronisation. – Contains frame number and BSIC(Base Station Identity Code).

BCH(Broadcast Channels) cont’d • FCCH(Frequency Correction Channels) – Downlink Only. – Enable MS to synchronies to the frequency.

CCCH(Common Control Channel) • RACH(Random Access Channel) – Uplink only. – Used by the MS when making its first access to the Network. – The reason for access could be initiation of a call or a page response.

CCCH(Common Control Channel) cont’d • AGCH(Assess Grant Channel) – Downlink only. – Used for acknowledgement of the access attempt sent on RACH. – Used by the network to assign a signaling cannel upon successful decoding of access bursts.

CCCH(Common Control Channel) cont’d • PCH(Paging Channel) – Downlink only. – The network will page the MS ,if there is a incoming call or a short Message. – It contains the MS identity number, the IMSI or TMSI.

DCCH(Dedicated Control Channel) • SDCCH (Stand-alone Dedicated Control Channel) – Uplink and Downlink. – Used for call setup, authentication, ciphering location update and SMS.

DCCH(Dedicated Control Channel) cont’d • SACCH(Slow Associated Control Channel)

– Downlink and Uplink. – Used to transfer signal while MS have ongoing conversation on traffic or while SDCCH is being used. – On the forward link, the SACCH is used to send slow but regularly changing control information to each mobile on that ARFCN, such as power control instructions and specific timing advance instructions

• SACCH(Slow Associated Control Channel) cont’d – The reverse SACCH carries information about the received signal strength and quality of the TCH, as well as BCH measurement results from neighboring cells.

DCCH(Dedicated Control Channel) cont’d • FACCH(Fast Associated Control Channel) – Downlink and uplink. – Associate with TCH only. – It is used to send fast message like hand over message. – Work by stealing traffic bursts.

Mapping on Physical Channels • The Logical channels are mapped on the physical channels. • The TDMA frames are grouped together into multi-frame. – 26 TDMA multi-frame for Traffic. – 51 TDMA multi-frame for control signal.

Channel Combination • Combined – All the controlling signals are in the time slot 0 of the Multi-frame.

• Non Combined – Dedicated controlling signals are in time slot 1 of the Multi-frame.

Combined • Cell with single carrier. – Timeslot 0 :BCCH+CCCH+SDCCH. – Timeslot 1-7 :TCH/FACCH+SACCH.

Non Combined • Cell with Two carrier – Timeslot 0 (of carrier 1) BCCH+CCCH. – Timeslot 1 (of carrier1) SDCCH+SACCH. – Timeslot 2-7 & 0-7(of both carriers) TCH/FACCH+SACCH.

•SYSTEM INFORMATION

SYSTEM INFORMATION 1 • When frequency hopping is used in cell MS needs to know which frequency band to use and what frequency within the band it should use in hopping algorithm. • Cell channel description Cell Allocation Number(CANO)-Informs the band number of the frequency channels used. 00-Band 0(current GSM band) Cell Allocation ARFCN(CA ARFCN):ARFCN’s used for hopping.It is coded in a bitmap of 124 bits.

SYTEM INFORMATION 1 124

123

122

121

024

023

022

021

020

019

018

017

016

015

014

013

012

011

010

009

008

007

006

005

004

003

002

001

SYSTEM INFORMATION 1 • RACH Control Parameters Access Control

Class(ACC) :-Bitmap with 16 bits. All MS spread out on class 0 –9 . Priority groups use class 11-15. A bit set to 1 barred access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not. 0 – All MS can make emergency call. 1 - MS with class 11-15 only can make emergency calls. Cell barred for access(CB):- 0- Yes 1- No

SYSTEM INFORMATION 1 • RACH Control Parameters Re-establishment

allowed(RE):0- Yes 1- No Max_retransmissions(MAXRET):-Number of times the MS attempts to access the Network [1,2,4 or 7]. Tx-integer(TX):- Number of slots to spread access retransmissions when a MS attempts to access the system. Emergency call allowed:- Yes/No.

SYSTEM INFORMATION 2 • System Information Type 2 message consists of the Double BA list which defines the BCCH frequencies used in the neighboring cells. • The Double BA list provides the MS with different frequencies on which to measure, depending on whether the MS is in idle or active mode. • In active mode, the MS should measure on a reduced number of frequencies in order to improve the accuracy of measurements.

SYSTEM INFORMATION 2 • In Idle mode,the MS should measure on larger number of frequencies, so that the time required for the MS to access the network after power on is reduced. • The MS is also informed which PLMN’s it may use. • As well as System Information Type 2,it is also possible to have System Information Type 2 Bis and System information Type 2 Ater, depending on the size of the BA List. • System Information Type 2 Bis/Ter are optional.

SYSTEM INFORMATION 2 • Neighbor Cell Description:-

BA Indicator(BA IND):- Allows to differentiate measurement results related to different list of BCCH frequencies sent to MS. BCCH Allocation number(BANO):- Band 0 is used. • PLMN Permitted(NCCPERM):-This the PLMN color codes permitted and tells the MS which network color codes(NCC) on the BCCH carriers it is allowed to monitor when it is in this cell. .

SYSTEM INFORMATION 2 • RACH Control Parameters Access Control

Class(ACC) :-Bitmap with 16 bits. All MS spread out on class 0 –9 . Priority groups use class 11-15. A bit set to 1 barred access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not. 0 – All MS can make emergency call. 1 - MS with class 1115 only can make emergency calls. Cell barred for access(CB):- 0- Yes 1- No

SYSTEM INFORMATION 2 Re-establishment allowed(RE):0- Yes 1- No Max_retransmissions(MAXRET):-Number of times the MS attempts to access the Network [1,2,4 or 7]. Tx-integer(TX):- Number of slots to spread access retransmissions when a MS attempts to access the system. Emergency call allowed:- Yes/No.

SYSTEM INFORMATION 2 BCCH ARFCN Number(BAIND):- ARFCN’s used for in a Bitmap of 124 bits 124

123

122

121

024

023

022

021

020

019

018

017

016

015

014

013

012

011

010

009

008

007

006

005

004

003

002

001

SYSTEM INFORMATION 3 • The System Information Type 3 contains information on the identity of the current LA and cell identity, because a change means that the MS must update the network. • System Information 3 also as Control Channel Description parameters used to calculate the Paging group. • When the MS is in idle mode it decides which cells to lock to. Information needed by the MS for cell selection is also broadcast in the Type 3 information.

SYSTEM INFORMATION 3 LOCATION AREA IDENTITTY(LAI) 8

7 6 MCC DIG 2 1 1 1 MNC DIG 2

5 1

4

3 2 MCC DIG 1 MCC DIG 1 MNC DIG 1

LAC LAC CELL IDENTITY CI CI

1

SYSTEM INFORMATION 3 • Control Channel Description Attach / Detach(ATT):0 = Allowed 1 = Not Allowed bs_agblk:-Number of block reserved for AGCH [0-7] Ba_pmfrms:-Number of 51 frame multi-frames between transmission of paging messages to MS of the same group T3212:- Periodic location update timer . [1-255 deci hours].

SYSTEM INFORMATION 3 c c h _ c o nPf h y s i c a l c h a n n e l s 0 1 t im e s lo t (0 ) 1 1 t im e s lo t (0 ) 2 2 t im e s lo t (0 ,2 ) 4 3 t im e s lo t (0 ,2 ,4 ) 6 4 t im e s lo t (0 ,2 ,4 ,6 )

c o m b i n e dN o . o f No Yes No No No

CCH 9 3 18 27 36

SYSTEM INFORMATION 3 • Cell options

DTX:-Whether Discontinuous Transmission used or not. PWRC:-Power control on the downlink. 0 = Not used. 1 = Used. Radio link timeout(RLINKT):-Radio link time-out is the time before an MS disconnects due to failure in decoding SACCH message. Sets the timer T100 in the MS.

SYSTEM INFORMATION 3 • Cell Selection Parameters Rxlev_access_min:-

Minimum received signal level at the MS for which it is permitted to access the system. 0-63 = -100 dBm to –47 dBm. Mx_txpwr_cch:- Maximum power the MS will use when accessing the system. Cell_reselect_hysteresis:- Used for cell reselection.

• RACH Control Parameters.

SYSTEM INFORMATION 4 • Location Area Identification. • Cell Selection Parameters

Rxlev_access_min:- Minimum received signal level at the MS for which it is permitted to access the system. 0-63 = -100 dBm to – 47 dBm. Mx_txpwr_cch:- Maximum power the MS will use when accessing the system. Cell_reselect_hysteresis:- Used for cell reselection.

SYSTEM INFORMATION 4 • RACH Control Parameters max_retransmissions(MAXRET) tx_integer(TX) Cell barred for access(CB). Re-establishment allowed(RE) Emergency Call Allowed Access Control Class (ACC)

SYSTEM INFORMATION 4 • CBCH Description(Optional) : CHN:- This is the channel number for CBCH. It is controlled internally in BSC. TSC:- Training Sequence Code. Base Station Color Code(BCC) part of BSIC is used. CBCHNO:- Absolute RF channel number of CBCH. MAC:- Mobile Allocation in the cell, describes the frequencies to be used in the hopping sequence if frequency hopping is used.

SYSTEM INFORMATION 4 Hopping Channel(H):-Informs if CBCH Channel is hopping or single. ARFCN:- If H=0; MAIO:- If H=1, informs the MS where to start hopping. Values [0-63]. HSN:- If H=1, informs the MS in what order the hopping should take place. Values[0 –63]. HSN=0 Cyclic Hopping. MA:-Indicates which RF Channels are used for hopping. ARFCN numbers coded in bitmap.

SYSTEM INFORMATION 5 • Sent on the SACCH on the downlink to the MS in dedicated mode. • On SAACH, the MS also receives information about the BCCH carrier in each neighboring cell. This may differ from those sent in System information type 2. • It is also possible to have system Information Type 5 Bis and System Information Type 5Ter, depending on the size of the BA list.

SYSTEM INFORMATION 5 • Neighbor Cell Description:BA-IND:-Used by the Network to discriminate measurements results related to different lists of BCCH carriers sent by the MS(Type 2 or 5). Values 0 or 1(different from type 2). BCCH Allocation number:-00-Band 0(current GSM band).

SYSTEM INFORMATION 5 BCCH ARFCN:-Neighboring cells ARFCN’s. Sent as a bitmap. 0-Not used 1-Used. 124

123

122

121

024

023

022

021

020

019

018

017

016

015

014

013

012

011

010

009

008

007

006

005

004

003

002

001

SYSTEM INFORMATION 6 • Ms in dedicated mode needs to know if the LA has changed.If so, it must perform location updating when the call is released. • MS may change between cells with different Radio link timeout and DTX. • Cell Identity. • Location Area Identification. • PLMN permitted.

SYSTEM INFORMATION 6 • Cell options: DTX PWRC Radio Link timeout.

SYSTEM INFORMATION 7/8 • System Information Types 7 and 8 contain Cell Reselect parameters. Their function is to supplement System Information Type 4.

GSM Interfaces • • • • •

(Um) Air interface A bis interface A Interface B Interface C interface

-

MS to BTS BTS to BSC BSC to MSC MSC to VLR MSC to HLR

AUC To other Networks GMSC

D Interface

H Interface HLR C Interface MSC A Interface

VLR

EIR

B Interface F Interface

BSC A bis Interface Air Interface MS

GSM Interfaces • The interfaces between MSC and MS is called A, Abis and Um interfaces. • On these interfaces only three layers are defined.They are not corresponding to the OSI (Open System Interconnection) model.

A Interface • A interface between the BSC and the MSC • The A interface provides two distinct types of information, signalling and traffic, between the MSC and the BSC. • The speech is transcoded in the TRC and the SS7 (Signalling system) signalling is transparently connected through the TRC or on a separate link to the BSC.

Abis Interface • The A-bis interface responsible for transmitting traffic and signalling information between the BSC and the BTS. • The transmission protocol used for sending signalling information on the A-bis interface is Link Access Protocol on the D Channel (LAPD)

(Um) Air Interface • This is the interface between the mobile station and the Base station. • The Air interface uses the Time Division Multiple Access (TDMA) technique to transmit and receive traffic and signalling information between the BTS and MS. • The TDMA technique is used to divide each carrier into eight time slots.These time slots are then assigned to specific users,allowing up to eight conversations to be handled Simultaneously by the same carrier.

7

6

5

Up Link

4

3

2

1

0

1

2 3

0 Down Link 4

5

Time Slot • This interface is the radio interface between the mobile station and the network and uses layer Three messages. • On Layer three messages we have the division of message types into CM (communication Management), MM (Mobility Management), and RR (Radio Resource Management).

6

7

Connection Management (CM) • • •

There are three entities within CM: Call Control(CC) – Which handles the procedures concerning call control. e.g. setup,Change of bearer service. Supplementary Service (SS) – Which handles such as call bearing, call waiting , call forwarding etc. Short Message Service (SMS) – Enables the MS to handle short message transfer to and from the network.

Mobility Management (MM) • Mobility management handles functions for authentication, location updating, identification and others concerning the mobility of the mobile station.

Radio Resource Management (RR) • It contains the functions concerning the radio link. Here we find the capability to establish,maintain and release the radio connection between the network and the mobile station, which includes the handover procedure.

B Interface • The B interface between the MSC and the VLR uses the MAP/TCAP protocol. • Most MSCs are associated with a VLR, making the B interface "internal". • Whenever the MSC needs access to data regarding a MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.

C Interface • The C interface is between the HLR and a MSC. • Each call originating outside of GSM (i.e., a MS terminating call from the PSTN) has to go through a Gateway to obtain the routing information required to complete the call, and the MAP/TCAP protocol over the C interface is used for this purpose. • Also, the MSC may optionally forward billing information to the HLR after call clearing.

D Interface • The D interface is between the VLR and HLR. • It uses the MAP/TCAP protocol to exchange the data related to the location of the MS and to the management of the subscriber.

E Interface • The E interface interconnects two MSCs. • The E interface exchanges data related to handover between the anchor and relay MSCs using the -MAP/TCAP+ISUP/TUP protocol.

F Interface • The F interface connects the MSC to the EIR. • It uses the MAP/TCAP protocol to verify the status of the IMEI that the MSC has retrieved from the MS.

G Interface • The G interface interconnects two VLRs of different MSCs. • It uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure.

Encoding and Interleaving of Information Signal in GSM

Topics for discussion • Speech Encoding • Data Encoding • Interleaving for Voice,Control and Data signals

Speech Encoding • We shall start with a raw voice signal fed into the microphone, travel through the various stages involving vocoding, channel coding etc till it reaches the final burst format on the Air Interface.

Speech Encoding ckt Raw Voice signal

Voice Encoding

Channel coding

interleaving

RF Modulation

Speech Encoding ckt • The voice is sampled at the rate of 50 samples per second. • This results in 20 msec blocks of speech • Each of this 20 msec block is passed on to the 13Kbps vocoder. • There are 260 information bits from the output of the vocoder for every 20 msec input i.e.; 13Kbps *20msec = 260 bits.

Voice Encoding ckt

Vocoder I/p 20 msec speech blocks

13Kbps Vocoder

Vocoder O/p 260 bits

Channel coding • Channel Coding is done to protect the logical channels from transmission errors introduced by the radio path. • The coding schemes depend on the type of the logical channels, hence the coding can differ from speech, control and data .

Channel Coding for speech 260 bits class 1b

Class 1a 50 Bits

3 parity

132

class 2

4 tail bits

Convolutional coder ½ coder, k=5 456 bits=378 bits from Convolution coder + 78 class 2 bits

Channel coding for Speech • The 260 bits of speech info from the vocoder is broken down into three parts. • Class 1a- 50 bits , these represent the filter coefficients of the speech and are the most important for proper detection of the speech at the receiver and hence are given maximum protection. 3 additional parity bits are derived from the class 1a bits for cyclic redundancy check (CRC).

Channel coding for Speech cont’d • Class 1b - 132 bits are not parity checked but are fed into the convolutional coder along with 4 tail bits which are used to set the registers in the receiver to a known state for decoding purpose. • Class 2- 78 bits, these are not so important and are not protected but are combined with the output of the convolution coder.

Control Channel Coding 184 bits Control data 184 Fire coded

40 4 tail parity bits ½ Convolutional Coder 456 bits output

Control Channel Coding • The control information is received in blocks of 184 bits. • These bits are first protected with a cyclic code called as Fire code, which is useful in correction and detection of burst errors. • 40 Parity bits are added, along with 4 tail bits. • These 228 bits are given to the CC whose output is again 456 bits at a bitrate of 22.8Kbps. • The control channels include the RACH, PCH, AGCH etc.

Data Channel Coding 240 bits Data

4 tail bits

½ Convolutional Coder Output= 488 bits After Puncturing Output=456 bits

Data Channel Coding • The data bits are received in blocks of 240 bits. These are directly convolution coded after adding 4 tail bits. • The output of the CC is now 488 bits, which actually increases the bitrate to 24.4 Kbps. • To keep the bitrate constant on the air interface we need to puncture the output of the CC. Hence, we have a final bitrate of 22.8 Kbps again .

Channel Coding cont’d • The above explanation was given keeping in view a full rate Traffic, Control, or Data channel. • For Half rate or Lesser rates the same principle of channel coding holds good, with slight differences in the encoding process.

Interleaving • Having encoded the logical channel information, the next step is to build its bit stream into bursts that can be transmitted within the TDMA frame structure. This is the stage where the interleaving process is carried out. • Interleaving spreads the content of one information block across several TDMA timeslots or bursts.

Interleaving cont’d • • • • •

The following interleaving depths are used : Speech – 8 blocks Control – 4 blocks Data – 22 blocks The interleaving process for a speech block is shown wherein which a 456 bit speech block is divided into 8 blocks of 57 bits each and each of these odd and even 57 bit blocks are interleaved diagonally on to alternate bursts on the TDMA frame.

Speech Interleaving Speech block N-1

456 bit speech data 8* 57 bits each = 456 bits Of Speech block N

T+F

57 57 odd T+F odd Of N-1 Of N

57 57 Even T+F Even Of N-1 Of N

The speech is spread over 8 such normal bursts Each normal burst consists of two blocks of 57 bit speech from different 20msec blocks (say N, N-1) along with 26 bit training sequence T and 2 flag F plus 6 start stop bits .

Control Data Interleaving 456 bits control data 114

114

114

114

TDMA Burst blocks The control data is spread over 4 blocks using rectangular interleaving instead of diagonal interleaving as in speech the receiver will have to wait for at least 2 multiframes before being able to decode the control message

Data Interleaving 456 bit data block 114 First 6 bits

114

114

114

First 6 bits Last 6 bits

Burst 1

Burst 2

Burst 3

Burst 4

Burst 19

Last 6 bits

Burst 22

Data Interleaving cont’d • Here the data block of 456 bits is divided into 4 blocks of 114 bits each. • The first 6 bits from each of the 114 bit blocks is inserted in to each frame, the second 6 bits from each of the 114 bits into the next frame and so on spreading each 114 block over 19 TDMA bursts while the entire 456 bits is spread over 22 TDMA bursts. • Thus the data interleaving is said to have a depth of 22 bursts.

Data Interleaving cont’d • The reason why data is spread over such along period of time is that if data burst is corrupted or lost, only a small part of it is lost which can be reproduced at the receiver. • This wide interleaving depth does produce a time delay during transmission but that is acceptable since it does not affect the data signal quality at the receiver, unlike speech where delay could result in bad quality of signal to the subscriber. • *Note – The interleaving used in data is diagonal interleaving.

Interleaving Advantage Before Deinterleaving 3 successive bursts corrupted

After Deinterleaving The corrupted bursts are spread over a length equal to the interleaving depth so that the effect of the errors is minimized.

Air Interface Bitrate • The information which is now coded and interleaved at 22.8 Kbps now has to be transmitted over the Air interface to the BTS. • The information burst is not sent directly , but is sent in ciphered form within a burst envelope. This ciphering is done using ciphering keys and algorithms known both by the mobile and the BSS.

Air Interface Bitrate cont’d • The Kc is the ciphering key and A5 algorithm are applied to the information(speech or data) which increases the bitrate to a final rate of 33.8 Kbps from/to each mobile. • If we assume all 8 timeslots of the cell to be occupied then the bitrate of the Air interface comes to 33.8 * 8= 270.4 Kbps/channel.

Air Interface Bitrate cont’d Information Block 22.8 Kbps

Kc

A5 Algorithm Sent on Air interface Ciphered information burst 33.8 Kbps

Air Interface Bitrate cont’d TDMA Fn

1 2 3 4 5 6 7 8

Cell coverage area

TDMA Fn+1

Cell rx’s 8*33.8 KBps = 270.4 Kbps Per TDMA frame

Mobile Tx’s at 33.8 Kbps

Decoding and Deinterleaving at the Receiver • At the receiver the reverse process of Deinterleaving and decoding have to take place respectively, so as to recover the information from the signal. • After Deinterleaving the signal will be decoded which is the reverse process of the Convolutional coding, using Viterbi decoders. • The decoder can recover lost or corrupted data up to 4 successive bits, because the memory depth of the CC is 4(for k=5).

Channelization • Frequency band has several application segments • Certain blocks of the Band are reserved for certain applications by regulating authorities • Technologies have decided their frequency bands • E.g. AMPS/DAMPS: 824-894 MHz

Channelization methods Channelization can be done primarily by three methods: – FDMA (Frequency Division Multiple Access) – TDMA (Time Division Multiple Access) – CDMA (Code Division Multiple Access)

Power

FDMA

Time

Frequency • E.g. AMPS band is divided into 30 KHz channels (1666 Freq. channels) • Television Channels (Star, Zee, Sony,..)

Power

TDMA Time

Frequency • E.g. AMPS has 3 timeslots on each 30 KHz channel

CDMA • Frequency channel is divided into code channels • E.g. in IS-95 CDMA, 1.228 MHz channel is divided into 64 Code Channels • Each user has a particular code • Codes are orthogonal to each other, do not interfere with each other

Duplex Access Methods

Duplex Amplitude Time

Tx

F1

Rx

F2

Frequency

• Frequency Division Duplex (FDD) • Transmit on one frequency and receive on another frequency

Amplitude

Time Division Duplex Time

Rx Tx F1

Frequency

• Time division duplex • Tx and Rx is on the same frequency but on different times

GSM Air Interface

• Separate Bands for Uplink and Downlink

– Downlink: 935-960Mhz (EGSM: 925-960MHz) – Uplink: 890-915 MHz (EGSM: 880-915 MHz)

•

TDMA and TDMA Multiplex 124 Frequency Channels (ARFCN) for GSM900 – 1 to 124 fro current band – 975 to 1023 for E-GSM – 200kHz Channels – 8 Mobiles share ARFCN by TDMA –

GSM Air interface (1800) – 1800: Downlink: 1805-1880 MHz – 1800: Uplink: 1710-1785 MHx

• 374 ARFCNs • Separation of 95 MHz • ARFCNs are numbered from 512 to 885 inclusive

The GSM Burst Control Bit

Tail Bits

3

57 Data

1

Control Bit

26 Midamble

1

Tail Bits

57 Data

3 8.25 Guard Period

Speech Coder 20 ms blocks

• RPE/LTP coder (Regular Pulse Speech Coder excitation/Long term Prediction) • Converts 64 kbps Bits Ordered speech to 13 kbps • At the end we get 13kbps speech i.e. 260 bits in 20 ms

50 very important bits

132 important bits

78 other bits

Error Correction Type 1a 50 3(CRC) Type 1b 132 Type II 78 Reordering

Type 1b Tail Type 1b 25 66 3 66 25 4 Type II 78 Type 1a Type 1a Half rate convolutional code 378 456 bits from 20 ms of speech

Type II 78

Diagonal Interleaving 456 bits from 20ms of speech

456 bits from 20ms of speech

57 57 57 57 57 57 57 57

57 57 57 57 57 57 57 57

57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 • Traffic channel (TCH) bursts carry two 57 bit blocks (114) • Each 120 ms of speech = 456*6 = 2736 bits 2736/114 = 24 bursts i.3. 24 frames Multiframe has 26 frames in 120ms. There are 2 spare frames .. 1 SACCH, 1 Idle

Convolutional Coding and Interleaving HELLO • Bits to be Tx ed: • Convolutionally encoded: HHEELLLLOO EE HH LL LL OO • Interleaved: • Bits Rx ed: EE HH LL LL OO • De-Interleaved: HHEELLLLOO • Viterbi Decoded: HELLO

Speech Coding Process 456 bits 20 ms Speech Coder 260 bits 13 kbps

Transceiver (BTS) 13 kbps

260 bits

Transcoder Handler

50 1a

132 1b

78 II

Channel Coder 456 bits

22.8 kbps

16 kbps 260 + 60 = 320 bits TRAU frame Abis

TRAU frame • 260 bits info + 60 TRAU bits = 320 bits/20ms = TRAU frame • 60 bits contain frame Information data which indicates speech, data, O&M, full rate/half rate • 60 bits = 35 synchronization + 21 control + 4 timing

Midamble or Training Bits Control Bit Control Bit Tail Bits Tail Bits 3

57 Data

1

26 Midamble

1

57 Data

3

8.25 Guard Period

• 8 midamble patterns (Colour codes) of 26 bits (BSIC) • RACH and SCH have longer 41 and 64 bit Midambles • Equalizer estimates channel impulse response from midamble • Mathematically construct inverse filter • Uses inverse to decode bits

Downlink and Uplink • Uplink lags downlink by 3 timeslots • Uplink and downlink use same timeslot number • Uplink and downlink use same channel number (ARFCN) • Uplink and downlink use different bands (45 MHz apart for GSM 900)

Measurements made by MS and BTS Uplink RXLEV (-48 to -110 dbm) Uplink RXQUAL (0-7) Uplink RXLEV (-48 to -110 dbm) Uplink RXQUAL (0-7)

• 0 3 5 7

RxQual < 0.2% 1 0.4 – 0.8 % 1.6 – 3.2 % 6.4 – 12.8 %

0.2 – 0.4 % 4 0.8 – 0.16 % 6 3.2 – 6.4 %

Mobile Power Control Path Loss

Power Command

• Mobile is commanded to change its Transmit Power • Change in Power is proportionate to the Path Loss • Change in Power is done in steps of 2 dbs

Timing Advance • TDMA approach requires signals to arrive at BTS at the correct time • A mobile at 30 km will be late by 100micro seconds • Timing advance is in the range of 0-62 • One unit is 550m • So maximum cell size is 63*0.55 = ~35 kms

Concepts of Channels in GSM • A company vehicle is used for several purposes in a day • Similarly in GSM, the timeslots are used for different purposes at different times

Frames and Multiframes Control Channel Multiframe

0

Traffic Channel Multiframe

50

0

4.615 ms

Frame Time Slot

0 1 2 3 4 5 6 7 3 Data 1 Midamble 1 Data 3 8.25 bits 156.25 bits

576.92 micro sec

25

GSM Operations • Location Update • Mobile Originated Call • Mobile Terminated Call • Handover • Security Procedures • Cell Barring

• DTX • Cell Broadcast • Short Message Service • Emergency calls • Supplementary Services • Roaming

Mobile Turn On • Mobile Searches for Broadcast Channels (BCH) • Synchronizes Frequency and Timing • Decodes BCH sub-channels (BCCH) • Checks if Network Allowed by SIM • Location Update • Authentication

Location Area BTS BTS BTS

Location LocationArea Area11

BSC

BTS BTS

Location Location Area Area22

BSC

BTS BTS BTS

BSC

MSC

Location Area Identity MCC

MNC

LAC

• Location area is the area covered by one or more BTSs where a mobile can move freely without updating the system • One Location area can be covered by one or more BSCs, but ony one MSC.

Importance of Location Area • Reduce Paging load • Resource Planning Smaller Location Areas – Location update increases Larger Location Areas – Paging load increases

What is Location Update? • MSC should know the location of the Mobile for paging • Mobile is continuously changing location area • Mobile when changes Location Area informs the MSC about its new LA • Process of informing MSC about new Location area is Location Update

Types of Location Updates 1. Normal Location Update 2. IMSI Attach 3. Periodic Location Update Hi, I am in Location area xxx

IMSI Attach • Mobile turns off and sends an IMSI Detach to MSC • Mobile turns on again and compares LAI • If same, sends an IMSI attach to MSC Is the received LAI same as before

If same, Sends IMSI attach

Normal Location Update • Mobile Turns on Power • Reads the new LAI • If different, does a Location Update

Is the received LAI same as before

If differe n t, does Location Update

Periodic Location Update • The periodic location Update time is set from OMC/MSC • After the periodic location update timer expires, the mobile has to do a location update

What happens at Location Update? • Mobile changes location area • Reads the new Location Area from BCCH • Sends a RACH (request for channel) • Gets a SDCCH after AGCH • Sends its IMSI and new and old LAI in a Location Update request to MSC on SDCCH

What happens at location update cont.. ….. . . • MSC starts Authentication • If successful, Updates the new Location area for the Mobile in the VLR • Sends a confirmation to the Mobile • Mobile leaves SDCCH, and comes to idle mode

Mobile Originated Call Channel Request Immediate Assign Service Request Authentication Ciphering Set Up Call Proceeding Assignment Alerting Connection

Mobile Terminated Call Paging Channel Request Immediate Assign Paging Response Authentication Ciphering Set Up Call Confirmed Assignment Alerting Connection

Security Features • Authentication –Process to verify Authenticity of SIM –Mobile is asked to perform an operation using identity unique to • Ciphering SIM – Process of coding speech for secrecy – The speech bits are EXORed with bit stream unique to MS

Security Features (TMSI Reallocation) Location Update TMSI Allocation Call Setup TMSI Reallocation GSM Infrastructure

Mobile

TMSI- Temporary Mobile Subscriber Identity

Security Features (Identity Check) Identity Check Sends IMEI EIR White listed /Grey Listed/ Black Listed mobiles

Handover

Cell 1

Cell 2

Handover is a GSM feature by which the control/communication of a Mobile is transferred from one cell to another if certain criteria’s are met. It is a network initiated process.

Criteria for Handover • Receive Quality (RXQUAL) on uplink and downlink • Receive Signal Strength (RXLEV) on uplink and downlink • Distance (Timing Advance) • Interference Level • Power Budget

Handover Decision • BSC process the measurements reported by Mobile and the BTS

BTS

BTS BTS

BTS

BTS BTS

Mobile has measurements of six neighbors

Handover Decision (cont..) • BSS performs averaging function on these measurements every SACCH frame (480ms) • Handover Decision algorithm is activated after a set number of SACCH frame periods by comparison against thresholds

Types of Handovers • INTRA-CELL HANDOVERS • INTER-CELL HANDOVERS • INTRA-BSC HANDOVERS • INTER-BSC HANDOVERS • INTER-MSC HANDOVERS

INTRA-CELL HANDOVER C1 C0

Handover between timeslots of same frequency Handover between different frequencies of the same cell (to reduce interference) MSC is not aware about this

Inter-cell Handover

Cell 1

BTS

Cell 2

Handover between cells of the same BTS

Inter-cell Handover (cont..) • MSC is told about HO • BTS -> BSC -> MSC • Why MSC is informed? – In case of change of LA, MSC may need LAC for paging. As MS is busy, a link already exists. So, MSC can send a tone in case of call waiting, and does not need to page again. – This is needed also for billing and call tracing

INTRA-BSC Handover BTS MSC

BSC BTS

This HO takes place if the cell to which handover is to be done belongs to the same BSC

Inter BSC Handover BSC

BTS

BSC

BTS

MSC

The MSC is completely involved in this Handover

Inter MSC Handover MSC

BSC

BTS

MSC

BSC

BTS

GMSC/ PSTN/ Backbone

In this case the handover takes place through the interconnecting element which can be GMSC or PSTN or private Backbone between the MSCs

Cell Barring BTS Cell Barring is a GSM feature by which certain mobiles could be barred access to certain cells Cell barring is activated/deactivated at BTS level Cell barring is done for mobile categories and priorities

Cell Barring • Every mobile has an access class • The access class is stored in the SIM – Classes 0-9 are termed normal calsses – Classes 11-15 are emergency classes

• Every cell has a set parameter which defines which access classes are barred for the particular cell. This parameter is broadcasted on the BCCH

What is DTX?

• DTX (Discontinous Transmission) • Each direction of Transmission is only 50% • Transmitter is switched ON for useful information frames Need for DTX •To increase battery life •To reduce the average interference level DTX is done by DTX handlers which have the following functions.

VAD (Voice Activity Detector) • Senses for speech in 20ms blocks • Removes stationary noise • VAD is an energy detector • Compares Energy of filtered speech threshold • It determines which 20ms blocks contain speech and it only forwards those frames

Evaluation of Background Noise • Background noise is always present with speech • DTX cuts off this noise with speech • Gives an uncomfortable feeling to the listener • VAD takes care of this by inserting comfort noise at the receiving end when speech discontinues.

Emergency Calls

• GSM specs define 112 as an emergency number • ‘112’ is accessible with or without SIM • Without SIM it is sent on the best channel • Mobile on sensing ‘112’ sets the establishment cause to emergency call in the RACH • Routing of this call be done to a desired location defined in the switch

Cell (Re)selection • Cell reselection is done using C1 path loss criterion. • The purpose is to ensure that the MS is camped on to the cell with the best transmission quality. • The MS will camp on to the cell with the highest C1 value if C1 > 0.

The following parameters are used to calculate the C1 criterion • The received signal at the MS side. • Rxlev_access_min - broadcast on the BCCH - The minimum received level at the MS required for access to the network. • Ms_txpwr_max_cch - the maximum power that an MS may use when initially accessing the network. • The maximum power of the MS

C1 = A - Max(B,0) • A = Received level Average Rxlev_access_min. • B = MS_txpwr_max_cch - maximum output power of the MS

Cell Reselect Hysteresis • Cell reselection on the border of two location areas result in a location update. When an MS moves on the border of two location areas lots of location updates take place. To avoid these location updates, the reselect hysteresis is introduced. • A location update is performed only if: – The C1 value of the new location area is higher than the C1 value in the current location area and – The received signal strengths have at least a difference of the reselect hysteresis.

Cellular concept

Why to use the cellular concept ?  Solves the problem of Spectral congestion and user capacity by means of frequency reuse.  Offers high capacity in a limited spectrum allocation.  Offers system level approach, using low power transmitters instead of a single, high power transmitter (large cell) to cover larger area.

 A portion of the total channels available is allocated to each base station.  Neighboring base stations are assigned different groups channels, in order to minimize interference.

Cell shape

1-Omni-directional cell-site (Omnidirectional antenna). 2-Rhombus-shaped sectors (Directive antenna). 3-Hexagonal shaped sectors (Directive antenna).

Cell size Large cell : (up to 70km in diameter) It exists where : 1-Radio waves are unobstructed. 2-Transmission power can cover the area. 3-low subscriber density.

Small cell : (up to 2km in diameter) It exists where : 1-Radio waves are obstructed. 2-Low transmission power to decrease interference. 3-High subscriber density.

Types of cells 1-Macro-cells 3-Pico-cells.

2-Micro-cells. 4-Umbrella-cells.

What is a cluster ?  A cluster is a group of cells.  No channels are reused within a cluster.  It is the unit of design.

Cluster size  Definition : It is The number of cells per cluster

Where :

N = i^2 + ij + j^2

i = 0, 1, 2….& j = 0,1,2…. etc. N = 1 , 3 , 4 ,7, 9 , 12 ,……

Types of clusters 1-N=7 omni frequency plan (2directional). 2-N=7 trapezoidal frequency plan (1-directional). 3-N=9 omni frequency plan. 4-Tricellular plans a) N=3 tricellular plan (3/9). b) N=4 tricellular plan (4/12).

Channel assignment strategies  Considerations : 1) Max. capacity. 2) Min interference. 3) Perfect handover.

 Types of assignment strategies :    

1) Fixed : Each cell has permanent predetermined set of voice channels. New calls served by unused channels of this cell. Borrowing strategy if all channels are occupied. High probabiltity that call is Blocked if channels are occupied.( disadv.)

2) Dynamic :  Channels are not allocated to different cells permanently.  Each new call BTS requests new channel from MSC.  MSC allocate a channel, by using an algorithm that takes into account: 1- Frequency is not already in use. 2- Min. reuse distance to avoid cochannel interference.

 Adv. of dynamic assignment strategy : 1) Increase channel utilization ( Increase trunking efficiency ). 2) Decrease probability of a blocked call.

Frequency reuse Concept

Reuse cluster

Co-channel Reuse ratio (Q) :  R : cell radius.  D : reuse distance.

 Q = D/R. =

sqrt(3N). Where : N : cluster size

Handover

Definition : procedure that allows MS to change the cell or time-slot to keep as good link as possible during all the call.

Types of handover  IntraCell : bet. 2 channels of same cell.  InterCell : bet. 2 channels of 2 different cell & same BTS.  InterBTS (intra BSC) : 2 cells of different BTS Same BSC.  InterBSC : bet. 2 cells of different BSC’s & same MSC.

Measurements before handover 1- Measurements from MS to BSC : a) Strength of BTS signal. b) Quality of BTS signal. c) Signal strength of 6 neighbor BTS’s.

2-Measurements from BTS to BSC : a) Strength of MS signal. b) Quality of MS signal. c) Distance between serving BTS & MS.

Different causes of Different causes of handover Handover Emergency HO

Level

Quality

Distance

Better cell HO

PBGT Interference

Traffic causes

Basic handover algorithms a)“Min. acceptable performance” algorithm: MS power is increased when quality deceases till handover is the only way. b) “Power budget “ algorithm: Prefer direct handover when quality deceases without increasing MS power first .

Handover priority 1) UL quality cause (or interference). 2) DL quality cause (or interference). 3) UL level cause. 4) DL level cause. 5) Distance cause. 6) Better cell cause.

Inter ference

Sources of interference include: 1) Another mobile in the same cell. 2) A call in progress in the neighboring cell. 3) Other BTS’s operating in the same frequency band.

Interference effects : • In voice channel causes crosstalk • In control channels it leads missed and blocked calls due to errors in the digital signaling.

Main types of interference : 1) Co-channel interference. 2) Adjacent channel interference.

1) Co-channel interference • •

Source : Near cell using same frequency. It is a function of reuse distance(D/R). General rule :

io = No. of co-channel interfering cells. S = Signal power from a desired BS. Ii = interference power caused by the ith interfering co-channel cell BS.

• Another form : C/I = 10 log {(1/n)(D/R)*m} Where : m = propagation constant (dep’s on nature of environment) n = number of co-channel interferers. Can be minimized by : Choosing minimum reuse distance = (2.5….3)(2R).

2) Adjacent channel interference • Source : A cell using a frequency adjacent to the one in another cell due to imperfect reciever’s filter.

• General rule : ACI= -10 Log[(d1/d2)*m] – Adj ch isolation. Where : d1: distance between MS & proper BTs d2: dist. Bet MS & adj BTS causing interference. Adj ch isolation = Filter isolation = - 26db. Can be minimized by :

1-careful filtering 2-careful channel assignments 3-Directional antenna.

Traffic engineering theor y

Why do we need to know traffic?  The amount of traffic during peak hours allows us to dimension our wireless system for a certain GOS.

 GOS : probability of having a call blocked during busy hour (block rate).

Traffic intensity (E)  Erlang : A unit of traffic intensity measure.  1 Erlang = 1 circuit in use for 1 hour.  T ( in Erlangs) = [No. of calls per hour*average call holding time(sec.)] / [3600]

Typical traffic profile

Traffic tables Erlang B Table

Blocked calls are not held

Erlang C Table

Blocked calls are held in the queue indefinitely

Poisson Table

Blocked calls are held in the queue for a time = the mean holding time of

Erlang – B table  P(N;T) = [ (T^N)*exp(-T) ] / N!

N 2 4 10 20 40

GOS 1% GOS 2% 0.153 0.869 4.46 12.0 29.0

0.223 1.093 5.084 13.182 30.997

Trunking  Sharing channel among several users.  Trunking efficiency (nT) : Measures the number of subscribers that each channel in every cell can accommodate. nT = (traffic in Erlangs / no. of channels)*100.

 Trunking efficiency in presence of one operator :

 Trunking efficiency in presence of two operators :

N = 7 , 312 one N = 7 , 312 / 2 = 156 one direction voice direction voice channel for each channels operator. No. of channels / cell = No. of channels / cell 312 / 7 = 44 ch./cell. = 156 / 7 = 22 From Erlang-B table ch./cell. @GOS 2%,this’s From Erlang-B table equivalent to 35 @GOS 2%,this’s Erlangs equivalent to 15 Erlangs. nT = 35 / 44 = 79.55. nT = 15 / 22 = 68.18.

System capacit y

 S : total duplex channels available for use = k*N Where:

N : cluster size. k : No. of channels / cell.

 C : total No. of duplex channels in system; C = M*k*N. Where :

M : No. of times the cluster is repeated.

Improving system capacity  Cell splitting.  Sectoring.

Cell splitting

Sectoring  We use directional antennas instead of being omnidirectional

What does sectoring mean?  We can now assign frequency sets to sectors and decrease the re-use distance to fulfill : 1) More freq reuse. 2) Higher system capacity. 3) Improve S/I ratio ( better signal quality ).

 How S/I ratio is improved? -e.g. In 120 degree sectoring there’s only 2 interferers instead of 6 incase of omnidirectional N=7 cluster.

Directional frequency reuse  Here we use 7/21 pattern for frequency allocation.

Comparison between various t ypes of clusters

N = 7 omni frequency plan :  n = 6 , m = 4.  D / R = 4.583.  1) Co-channel interference ratio : C / I = 18.6 dB.  2) Adjacent channel interference : ACI = -26 dB @ d1= d2.

N = 7 trapezoidal frequency plan  n = 2 , m = 4.  D / R = 6.245.  1) Co-channel interference ratio : C / I = 28.8.  2) Adjacent channel interference : disappears because the channels are assigned alternatively to the cells.

 Trunking efficiency : • 312 one direction voice channels N=7  312 / 7 = 44.57 ~ 44 ch./cell. • From Erlang-B table @ GOS = 2% T = 35 E.  nT = 35 / 44 = 79.55 %.

N = 9 omni frequency plan  n = 4 , m = 4.  D / R = sqrt ( 3 * 9 ) = 5.2.  1) Co-channel interference : C / I = 22.6 dB.  2) Adjacent channel interference : ACI = -38 dB @ d2 = 2 (d1).

 Trunking efficiency : • 312 one direction voice channels N=9  312 / 9 = 34.67 ~ 34 ch./cell. • From Erlang-B table @ GOS = 2% T = 25.529 E.  nT = 25.529 / 34 = 75.085 %. Conclusion : nT 7 > nT 9 But C/I 7 > C/I 9 ACI 7 > ACI 9

4 / 12 cell pattern  n = 1 , m = 4.  D / R = sqrt (3* 4) = 3.732.  C / I = 22.87 dB.  Trunking efficiency : • No. of channels/cell = 312 / 12 = 26 ch./cell. • From Erlang-B table @ GOS = 2 %.  T = 18.4 E/cell.  nT = 18.4 / 26= 70.77%.

3 / 9 cell pattern  n = 1 , m = 4.  D / R = sqrt (3* 3) = 3.  C / I = 19.1 dB.  Trunking efficiency : • No. of channels/cell =312 / 9 = 34 ch./cell. • From Erlang-B table @ GOS = 2 %.  T = 25.5 E/cell.  nT = 25.5 / 24 = 75 %.

120 degree cell sectoring  n = 2 , m = 4.  D / R = sqrt(3 * 7) = 4.583.  Co-channel interference : C / I = 23.436 + 6dB(due to isolation) = 29.436 dB.

 Trunking efficiency : • No. of channels/cell = 312 / 21 = 14.857. • From Erlang-B @ GOS=2%  T= 8.2003.  nT = 8.2003 / 14.857 =56.216%.

 References : • Motorola CP02 • NOKIA SYSTRA

If any Query Contact 9903867731 [email protected]