MRN-EN-6-GSM part 1 - home.deib.polimi.ithome.deib.polimi.it/capone/wn/MRN-EN-6-GSM part 1.pdfMRN – 6 – GSM part 1 Mobile Radio Networks ... Unit (TRAU) o GSM voice coding

Politecnico di MilanoFacoltà di Ingegneria dell’Informazione

MRN – 6 – GSM part 1

Mobile Radio NetworksProf. Antonio Capone

A. Capone: Mobile Radio Networks 1

General characteristics of the system


History (1)

o 1982: the CEPT (Conférence Européenne des Administrations des Postes et des Télécommunications) creates special interest group for the definition of a pan-European cellular system: the Groupe Spécial Mobile, GSM

o 1985: Definition of the list of recommendations (standard specifications) that the GSM will work on (they ended up to be about 130: 1500 pages in 12 volumes! ... plus all those of the evolution, phases 2+ and 3 of GSM)


History (2)o 1988: the ETSI (European Telecommunication

Standards Institute) is created by the CEPT and the European Commission, and the GSM standardization activity is moved in the new organization

o 1990: GSM specifications are extended to DCS1800(Digital Cellular System on 1800 MHz),

o 1992: The final version of the standard is released, with the new meaning of the acronym Global System for Mobile Communications

o 1992: Commercial launch of GSM (initially planned for 1991 and then delayed because of the lack of mobile terminals)


History (3)

o 1994-95: Introduction of the SMS

o 1995-97: Commercial introduction of DCS1800 (operating at 1800 MHz)

o 1996: Standardization of enhanced voice codecs

o 1997: Dual-band terminals

o 1999: Standardization of GPRS

o 2000/01: Commercial introduction of GPRS


General characteristics

o Second Generation Digital System (2G)

o Multiple access scheme based on multicarrier TDMA

o Fixed frequency reuseo Services

n Telephony with supplementary servicesn Circuit data services (single channel and

multiple channles)n Packet data (GPRS – General Packet

Radio Service)


Frequencies

o In UK and USA different frequencies around 1900 MHz are used for DCS (1850÷1910 uplink, 1930÷1990 downlink).

890

esteso downlink

esteso uplink

DCS/1800

F[ MHz ]915 935 960

925

1710 1785 188018051805

125+49 carriers 125+49 carriers 374 carriers 374 carroers

uplink downlink

GSM /900

uplink downlink

880


Radio carrierso Radio carriers are spaced of 200 kHz

o On each carrier the transmission rate is 270.833 Kb/s

o Carriers are identified by a ARFCN (Absolute Radio Frequency Channel Number)

o Modulation GMSK (Gaussian Minimum Shift Keying)o Each pair of frequencies for uplink and downlink are

spaced of 45 MHz in GSM 900 and 95 MHz in DCS 1800

f200 kHz


TDMA Frame

o On each carrier the TDMA scheme allows to create up to 8 channels for the transmission of coded voice at 13 Kb/s

3107654310765 2 2

0765431076543 22

TDM Frame - 4.615 ms

BTS Transmits fdown

MS Transmits fup

Time slot = 577 µs


Other characteristics

o Power Controln Power emitted by base station and

mobile terminals is regulated with a closed loop control

o Discontinuous Transmissionn During voice breaks, coding is paused for

reducing interference and energy consumption


System Architecture


Network architecture

BTS

BSC

Fixed Telephone Network

ISDN/PSTN

OMC

EIR

BSC

BSC

Um

Um

Um

Um

Abis

AbisAbis

Abis

BSC

A

A

A

A

E

AuC

MSC

GMSC

VLR

VLR HLR

BSS

NSS

OMSS

RS

Network Switching Subsystem

RadioBase StationSubsystem

BTS

BTS

BTS

Operation and Maintenence Subsystem


Mobile Station (MS)

o Three categories with different nominal power: n vehicular: up to 20 Wn portable: up to 8 Wn personal (hand-terminal): up to 2 W (the

only category currently used)

GSM 900 MHz DCS 1800 MHz GSM 900 MHz DCS 1800 MHz1 . 1 . 1202 8 0,25 960 303 5 4 600 4804 2 . 240 .5 0,8 . 96 .

Classe Potenza massima nominale [W] Potenza media nominale [mW]


Mobile Station (MS)

o Characteristicsn MS multi-band: can operate in different

bands (900, 1800, 1900, …)n MS multi-slot: can operate activating

multiple slots simultaneously (GPRS only)o MS includes ME (Mobile Equipment) and

SIM (Subscriber Identity Module) n ME is the main part of the mobile terminal

which includes all hw and sw modules. Identified by an IMEI (International Mobile Equipment Identifier)

n SIM include the functions for identifying the user and executing security procedures


Subscriber Identity Module (SIM)

o It’s a smart card (with micro processor and memory) which is essential to activate any ME

o Multi be inserted in the reader slot of the MEo Serial number

n Identify the SIM cardo International Mobile Subscriber Identity (IMSI)

n Identify the user in the networko Security authentication and cyphering information

n A3 and A8 algorithm (used for authentication and cyphering)

n Ki, Kc (authentication and cyphering keys)


Subscriber Identity Module (SIM)

o Temporary Network informationn LAI (Location Area Identifier), last visited

location arean TMSI (Temporary Mobile Subscriber

Identity), temporary identifier used on behalf of the IMSI

o Subscribed serviceso Personal Identity Number (PIN)o Personal Unblocking Number (PUK)o Access rights o Prohibited networkso Call messageso Phone numbers


Base Station System (BSS)o The BSS includes the functional units that deal

with the radio accessn Radio coverage and communication with the

MSs through the radio interfacen Radio resource management

o The BSS includes:n Base Transceiver Station (BTS)

o Implements the low level protocols of the radio interfaces. It executes control commands sent by the BSC

n Base Station Controller (BSC)o Implements high level protocols controlling a

group of BTSs. Receive information on radio interfance status from BTS and issues configuration and management commands


Network Switching Subsystem (NSS)

o The NSS is the circuit switching network of the mobile system

o It is used for circuit based services only (voice)

o It is basically a telephone network with additional functionalities, nodes and databases for managing mobile users


Network Switching Subsystem (NSS)

o the NSS includes:n Mobile Switching Center (MSC):

o Telephone switching station for mobile systemsn Visitor Location Register (VLR):

o It’s a database (usually integrated into the MSC) including temporary information on users visiting the MSC area

n Home Location Register (HLR):o It’s the main database of the network that includes all

information on mobile users. Includes also the information on the currently visited VLR

n Authentication Center (AuC):o Usually associated with the HLR. It is in charge of

authentication functionsn Equipment Identity Register (EIR):

o Include the IMEI of authorized devicesA. Capone: Mobile Radio Networks 19

Operation and Maintenence Subsystem (OMSS)

o The OMSS includes the units in charge of controlling and monitoring network elements by remote

o The OMSS cann configure the functionalities of all

network devicesn Visualize alarms on failures and

anomalous behaviorn Visualize traffic statistical datan etc.


Areas defined in GSMo PLMN (Public Land Mobile Network) Area:

n Service area of an operatoro MSC/VLR Area:

n Area managed by a MSC. Data of users in this area are stored in the VLR associated with the MSC

o Location Area:n A MSC/VLR area includes one or more Location

Areas (LA). LA is identified by a LAI (Location Area Identifier), transmitted by all BTSs in the area on the broadcast channel

o Cell:n area covered by a BTS. Identified by a BSIC (Base

Station Identity Code), transmitted on the broadcast channel


Base Transceiver Station (BTS)

o The BTS is in charge of implementing the low level protocols of the radio interface

o It transmits and receives signals from the MSs implementing modulation, coding and multiplexing functions on physical channels

o It has the task of performing quality measurements on physical channels and to collect those performed by MSs (all measurements are reported to the BSC)

o It interfaces with the BSC with PCM channels at 64 kbit/s

o Connects PCM channels with those of the radio interface (traffic and signaling)


Transcoder Rate Adaptation Unit (TRAU)

o GSM voice coding is at 13 Kbit/s while PCM is at 64 Kbit/s

o The transcoding is performed by the TRAUo The TRAU can be in the BTS, or (more often) in

the BSCo In this last case 13 Kbit/s flows must be

transmitted over 64 Kbit/s channelso On each 64 Kbit/s 4 flows at 13 Kbit/s are

multiplexed (they are first transformed into 16 Kbit/s flows with some padding)

o For each GSM carrier (8 channels at 13 Kbit/s) 3 PCM channels at 64 Kbit/s are necessaryn One for the signaling transported with the

link protocol LAPDn 2 for the 8 traffic channels


Transcoder Rate Adaptation Unit (TRAU)

0 1 2 15 16 17 30 31 0 130 31 … …

125 µs

Frame delimiter signaling

channel1 2 15 16 29 30

MSC BSC BTS

PCM channels(64 Kbit/s)

TRAU

PCM channels(64 kbit/s)

1 PCM channel per voice circuit

1 PCM channelfor 4 voice circuits

GSM channels(13 kbit/s)


BTS structure

o BTS is usually divided inton TRX (Transceiver)

o Radio elements for transmission and reception on a single radio carrier

n BCF (Base Common Function)o Control element of a set of TRX that also

control connection interface with the BSC


BTS structure

….….

Combiner

BSC

A-bis Interface

PCM line or Radio system 2 Mb/s

TX RX

Signal Processing

TRXController

TRX

one TX antenna Splitter

TX RX

Signal Processing

TRXController

TRX

two RX antennas

……………....


Base Station Controller (BSC)

o A BSC controls a large number of BTSs: from a few tens to hundreds

o Main BSC tasks include:n Cell configuration with assignment of traffic

and control channelsn Setup and release of channels at A and Abis

interfacesn Handover managementn Paging managementn Analysis of measurements received by BTSs

and MSs for the handover activation


Base Station Controller (BSC)

o BSC is in charge of the Radio Resource Management (RRM)

o It is also a switching node n Even if it does not implement routing (this is

done by the MSC)n It connects BTS circuits with MSC circuits

performing (often) the trans-coding (TRAU)n It switches circuits during handover (intra-

BSC)

o The BSCs can be co-located with the MSC, or located in different sites


Mobile Switching Centre (MSC)

o The MSC is the telephone switching station of the mobile radio network and it has additional functionalities for mobility management

o It is associated with a VLR for storing information on visiting users

o The MSC is connected to its BSCs and other MSC in the networkn PCM channels (multiplexed with PDH or

SDH) are used for the connectionn Some of the resources are used for the

signaling network SS7o One or more MSC (called Gateway MSC) per

PLMN are connected to the external telephone networks



o A MS can be reached by fixed telephone lines through the number MSISDN

o The call is routed to the GMSC which identifies the HLR, retrieves from it user information through the MSISDN

o The HLR provides the MSRN (Mobile Station Roaming Number)n MSRN is a temporary telephone number

(same structure of the MSISDN) assigned by the visited VLR

n MSRN allows the GMSC to route call to the MSC currently visited by the user



o The MSC is the main element of the signaling network (we’ll see signaling later on)

o The MSC implements the functionsn CM (Connection Management)

o originating call, terminating call, gatewayn MM (Mobility Management)

o location updating, periodic registration, authentication, …o Signaling protocol for communication with other

network elements:n DTAP (Direct Transfer Application Part) for direct

communication with MSn BSSMAP (BSS Management Application Part) for

communication with BSCn MAP (Mobile Application Part) for communication with other

MSC, VLR, HLR, EIR, AuC


Home Location Register (HLR)

o It is a database permanently associated with a GMSC

o It stores data on all MSs associated with GMSC (through the MSISDN)

o The HLR stores permanent information like IMSI MSISDN, registered services, etc.

o The HLR also stores temporary information like the currently visited VLR address, authentication and encryption temporary keys, etc.

HLR


Visitor Location Register (VLR)

o It’s a database for storing temporary information on users in the area of a MSC

o In the VLR all relevant data of visiting users are duplicated from the HLR

o The IMSI is mapped into a TMSI (Temporary Mobile Subscriber Identity) for avoiding transmitting it in clear text over the air interface

VLR


Security procedures

o Authentication:n It is in charge of verifying user identity

and protect the system from non authorized access

o Encryption:n It is in charge of coding the flow of data

so that it cannot decoded by otherso In GSM authentication and encryption

are strongly related for the key management


Security procedureso Ingredients:

n Kio Authentication key of 128 bits stored in the AuC

and in the SIMn RAND

o Random number of 128 bist generated by the AuC and then sent to the MSC

n A3o Algorithm for the authentication stored in the AuC

and the SIMn A8

o Algorithm for generating the encryption key Kc, stored in the AuC and the SIM■ Procedure results:

■ Kc encryption key■ SRES result of the authentication algorithm

Multiple Triples(RAND, SRES, Kc)Are generated for each IMSI and stored in the HLR


Security procedureso Authentication:

MS Network

A3

Ki Random number

generator

A3

KiRAND

RAND

SRES(network)SRES(ms)

equal?Yes No

Accessgranted

Accessdenied


Security procedureso Encryption:

MS Network

A8

Ki

A8

KiRANDRAND

KcKc

A5 A5informazioni cifrate

Random number

generator


Security procedures:Roles of network elements

o Authentication Centre (AuC)n It stores secret keys Ki of all usersn Generated random numbers and calculates SRES and

encryption keys Kc

n Provides triples to the other network elements

MS

MSC/VLR

VLR HLRAuC

AuC

richiesta (TMSI) IMSI IMSI

tripletteRAND, SRES,Kc

triplette

RAND

SRES


BSS

Security procedures:Roles of network elements

o Role of the BSS in the encryption:

MSMSC/VLR

VLR

request

RAND

SRES

Kc

A5Encrypted text Clear textA5

A8Kc

Ki


Security procedures:TMSI allocation

o All communications are initiate by the MS that transmits its ID (IMSI) for activating the authentication procedure

o For avoiding transmitting the IMSI in clear over the air interface the VLR allocates to each MS a TMSI (Temporary Mobile Subscriber Identity)

o The IMSI is used only when a TMSI is not available

o At each location update the VLR can allocate a new TMSI to MS


IMSIo It’s the internal user identification numbero It is composed of three fields:

n MCC: Mobile Country Code (3 digits)n MNC: Mobile Network Code (2 digits)n MSIC: Mobile Subscriber Identification

Number (up to 10 digits)o For example 222 01 4572228769, identifies an

Italian SIM (222) of the TIM mobile operator (01)

o The telephone number (MSISDN) is completely independent from the IMSI


Equipment Identity Register (EIR)

o It is an optional network databaseo It includes the ID (IMEI) and the characteristics of

all mobile equipments (manufacturer, country, etc.)

o It can be used for preventing the use of stolen devises

EIR

MSC/VLRMS

EIR

Conn. request

IMEI request

IMEI replyIMEI check

reply


Operation and Maintenance Subsystem (OMSS)

o It is the system in charge of all management and monitoring functions (both technical and administrative) of the network

o It performs billing, traffic control, alarm messages management, BTS and BSC monitoring, configuration, etc.

o It allows to configure by remote all network elements and to check their correct operation


Operation and Maintenance Subsystem (OMSS)

o It is based on a hierarchical architecturen OMC (Operation & Maintenance Centre) regionaln NMC (Network Management Centre)

NMC

HLROMC OMC

EIRMSC

BSC BSC

HLR

EIRMSC

BSC BSCA. Capone: Mobile Radio Networks 44

Radio Interface


Radio interface

f200 kHz

3107654310765 2 2

0765431076543 22

TDM Frame - 4.615 ms

BTS Transmits fdown

MS Transmits fup

Time slot = 577 µs

890915 935 960

925

uplink downlink

880

FDD (Frequency Division Duplexing)45 MHz di separazione uplink/downlink

3 slot offset uplink/downlink


Radio interfaceo Modulation:

n MGSK (Minimum Gaussian Shift Keying)o Phase continuous modulation with Gaussian pulse (intersymbol

interference)o Channel coding:

n Convolutionalo With several different rates

o Equalization:n Known bit sequences transmitted in each physical burstn Equalization filter synthetized at the receiver

o Voice coding:n 13 Kbit/s (RPE codec - full rate)n 12.2 Kbit/s (CELP codec - enhanced full rate)


Transmission and reception

Source coding

Channelcoding Timing Modulation

GMSK

Voice 32 kb/s

Control

Voice Decoder DemodulatorA/D

ConverterEqualizatorChannel Decoder

13 Kb/s 22.8 Kb/s

o From 13 Kbit/s of the codec we get to 32 Kbit/s on the radio channel

22.8 kb/s


Frequency Hopping

o The impact of multipath fading depends on the signal frequency

o There may be carriers with low attenuation and carriers with high attenuation

o Since transmission are protected by FEC coding, it is more convenient to spread errors

o GSM uses a frequency hopping mechanism that changes frequency slot by slot

f


Power Control

o MS power is controlled by the BTS, and BTS power (traffic carriers only) is controlled by the MS

o The power control mechanism is based on a closed loop scheme with up/down commands send in both directions

o The power control step is 2 dBo The objective is keeping received power

constanto The power control reduces interference and

energy consumed by MSs


Synchronization

o Carrier synchronizationn Frequency synchronization

o Slot synchronizationn Time slot synchronization

o Frame synchronizationn Frame number

o Base station synchronization (optional)n Slots and frame


Carrier synchronization

o The carrier frequency synchronization is achieved by the MS listening on the broadcast channel transmitted by the BTS

o On this channel, at periodic intervals, the BTS transmit a special slot in which a known sequence is transmitted

o This is used by the MS to adjust the local frequency


Slot and Frame synchronization

o GSM channels are based on a multi-frame scheme (for instance, the broadcast channel is transmitted only in some frames of the multi-frame)

o Also the Frequency Hopping sequence depends on the multi-frame

o All MSs must know the frame number in the multi-frame to decode the TDMA channels correctly

o The BTS transmits on the broadcast channel some information that allows MSs to obtain the slot synchronization and the Frame Number


Timing advance

o The TDMA transmissions require guard times due to propagation delays

TimeReference

2t Propagation delayt = d vd - distancev – light speed

)2(max iigT t=


Timing advance

o GSM is designed for cells with a radius of up to Rmax=37.8 Km

o The guard time should then be 2t = 2 x 35 / 3 x 108 = 233 µs

o Which is equivalent to 68,25 bits at carrier rate of 270.8 Kb/s


Timing advanceo For reducing the guard time the BTS estimates

the delay and send it to the MS on a control channel.

o The MS can then compensate the delay and reduce guard time

o Guard time of normal slots in GSM is 8.25 bits (30.46 µsec)

1) FirstTransmission

3) TimingAdvance command

2) Delayestimation

4) OtherTransmissions


Physical block (Burst)o The burst is the physical layer PDU transmitted in

a time sloto Due to TDMA scheme each burst is an autonomous

transmission with its own power profile

-70

-30

-6-1+1+4dB

10 8 10 542.8 10 8 10 µs


Bursts

o Normal Burstn Used for data transmission in traffic

channelso Access Burst

n Used for the first access on the Random Access Channel (RACH)

n It has a long guard time since the timing advance mechanism is still not active


Bursts

o Frequency Correction Burstn Used for frequency synchronization on the

Frequency Correction Channel (FCCH) - 142 bits all set to “1”

o Synchronization Burstn Used for transmitting the slot and frame

synchronization information

o Dummy Burstn Padding burstn Used in downlink only for ensuring a

constant power profile to broadcast carrier


Normal Burst

• T-bits: set to 0, used as frame delimiters• S-bits: (stealing bits) indicate is the burst has user data

or signaling• Coded Data: user data (voice, data, etc.), 114 bits• Training Sequence: control bits for the channel

equalization• GP: Guard time

148 bit = 546.12 µs

577 µs

T3

TrainingSequence

26Coded bits

57T3

GuardPeriod8.25

Coded bits57

S1

S1


Access Burst

o First transmission in uplinko It has 156.25 bits

n 8 tailing bitsn 41 synchronization sequencen 36 coded bitsn 3 tailing bitsn 68.25 bits of guard time

577 µs

T8

Synchronisation41

Coded bits36

T3

Guard Period68.25


Frequency Correction Burst

o It includes 148 + 8.25 bitsn 2 x 3 tail control bits n 142 fixed bit sequences

o All set to 0o They allows frequency synchronization

n 8,25 bits of guard period

577 µs

T3

Fixed bit sequences142

T3

GuardPeriod8.25


Synchronization Burst

o It includes 148 + 8.25 bitsn 2 x 3 tail control bits n 2 x 39 coded bits

o 25 bits of informationo that become are 78 coded bitso Divided into to fields of 39 bits each

n 64 bits of training sequencen 8.25 bits of guard period

577 µs

T3

Training sequence64

Coded bits39

T3

GuardPeriod8.25

Coded bits39


Dummy Burst

o Used in downlink on the carrier where the control channels are allocated

o It ensures that power profile is constanto Power control is not used on this carriero It includes 148 + 8.25 bits

n 2 x 3 tail control bits n 142 pseudo-random sequencen 8.25 bits of guard period

577 µs

T3

Pseudo-random sequence142

T3

GuardPeriod8.25


Logical channels

o “Logical channels” in GSM identify different types of information transmitted over the air interface:n Signaling n Traffic data

o They are divided into: n Traffic channels and control channelsn Common channels and dedicated

channels


Logical channels

FCCH=Frequency Correction CHannelSCH=Synchronisation ChannelBCCH=Broadcast Control CHannelPCH=Paging CHannelRACH=Random Access CHannelAGCH=Access Grant CHannel

SDCCH=Stand-alone Dedicated ControlCHannel

SACCH=Slow Associated Control CHannelFACCH=Fast Associated Control CHannelTCH/F=Traffic CHannel Full rateTCH/H=Traffic CHannel Half rate TCH/E=Traffic CHannel Enhanced Full rate

LOGICALCHANNELS

COMMONCHANNELS

DEDICATEDCHANNELS

CommonCONTROLCHANNELS

FCCH SCH BCCH PCH RACH AGCH SDCCH SACCH FACCH TCH/F TCH/H

DedicatedCONTROLCHANNELS

TRAFFICCHANNELS

TCH/E

BroadcastCONTROLCHANNELS


Traffic Channels (TCH)

o Channels transporting user data (voice or data)

o They can be:n Full Rate channels: 22,8 Kb/sn Half Rate channels: 11,4 Kb/s

Tf

trama 1 trama 2

ThTh

Tf Tf

ThTh

slot

Full Rate

Half Ratet

t


Control Channels (CCH)o Used for transporting signaling of different

types (14 types of control channels)o Three categories of CCHs

n Broadcast Channels (BCH): system information transmitted in the downlink

n Common Control Channels (CCCH): shared channels for connection initialization

n Dedicated Control Channels (DCCH):signaling channels dedicated to specific traffic connections


FCCH SCH BCCH PCH RACH AGCH SDCCH SACCH FACCH




Broadcast Channels (BCH)

o FCCH (Frequency Correction Channel):Channel for frequency synchronization (frequency correction bursts only)

o SCH (Synchronization Channel): it includes BTS id (BSIC) frame number (FN)

o BCCH (Broadcast Control Channel):General system information



BCCH (Broadcast Control Channel)

o Number of common control channels (2 bits)

o 1 bit for the common control TDMA scheme type

o Number of block reserved to AGCH (3 bits)

o Periodicity of paging di messages (3 bits)

o Frequency Hopping parameters



Common Control Channels (CCCH)

o PCH (Paging Channel): downlink used by the BTS per informing MSs of incoming calls, broadcasted over the LA

o RACH (Random Access Channel): uplink used by MSs for accessing the network (new calls, location update, etc.). It’s contention based.

o AGCH (Access Grant Channel): downlink, used for replies to RACH requests



Dedicated Control Channels (DCCH)

o SACCH (Slow Associated Control Channel): exchange of measurements during connection between MS and BTS (signal strengths, quality, ….). Multiplexed with the traffic channel (184 bits every 20 ms of measurements)

o FACCH (Fast Associated Control Channel): used for signaling during handover. It partially replaces traffic channel in the TDMA structure.

o SDCCH (Stand-alone Dedicated Channel): Signaling channel assigned in the first phase of call setup after the RACH/AGCH message exchange (identification, authentication, call set-up, …)



Slow Associated Control Channel (SACCH)

o In downlink:n Power control commandsn The same BCCH info (they cannot be decoded by

the MS because it is listening on the traffic channel)

n BCCH-FREQ-NCELL “N” (# carriers of BCCH of neighboring cells)

n BSIC-NCELL “N” (BSIC of neighboring cells)o In uplink: measurements from the MS:

n RXLEV-SERVING-CELL (signal strength from the serving BTS)

n RXQUAL-SERVING-CELL (BER measured on the downlink)

n RXLEV-NCELL “N” (signal strength from neighboring cells)



Connection setup

MS BS

RACH

BCCH

AGCH

SDCCH

TCH + SACCH

Broadcast info

Access request

SDCCH assignment

Signaling traffic + TCH assignment

traffic + signaling


Random Access (RACH)

o On the RACH a random access scheme is usedo Collisions can occuro The correct reception of the access request is

acknowledged with a reply message on the AGCH

o In the RACH a temporary random identifier is included which is also reported in the AGCH reply

o The RACH access protocol is a simple Slotted-ALOHA


Mapping of logical channel into physical channels

o For using different rates for different logical channels the mapping into physical channels (TDMA structure) is based on a multi-frame

0 1 2 31 32 33 34 63 64 65 66 0 1 2

Multi-Frame

Frame 1 Frame 2


Example: SACCHo A normal burst has 114 bits of data

o Using one slot per frame we have 114 [bits]/4.6 [ms]=24.7 Kb/s

o The rate of coded voice is however 22,8 Kb/so We have additional 1,9 Kb/s equivalent to 1

SLOT every 13 frameso SACCH: 1 SLOT every 26 frames for a speed

of 950 bits/sec.

148 bit = 546.12 µs

577 µs

T3

TrainingSequence

26Coded bits

57T3

GuardPeriod8.25

Coded bits57

S1

S1


T T T T T T T T T T T T T T T T T T T T T T T TA -

0 2512

o Traffic CHannels (T) and Slow Associated Control Channel (SACCH) (A) are multiplexed together

o In a multi-frame of 26 frames (120 ms)

TCH+SACCH


TCH+SACCH

Downlink, Uplink

1 2 3 4 5 6 7 8 9 10 1112131415161718192021222324250

TCH/FR

SACCH

577 µs4,615 ms

0 21 43 75 6Normalburst

Normalburst

Normalburst

Normalburst

Normalburst

Normalburst

Normalburst

Normalburst

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

TCH/FR

IDLE


Common control channels

o One slot (slot 0) an a carrier (carrier 0) is assigned to common control channels with a multi-frame of 51 frames (235.38 ms)

7 0 1 2

supertrama

trama 0 trama 2

7 0 1 2 7 0 1 2 7 0 1 2

trama 50


Common control channels

o In the downlink• FCH• SCH• BCCH• CCH

o In the uplink Random Access Channel (RACH)

R R R R R R R R R R R R R R R R R R R R RR R RR R R R R R

F S B C F S B C F S B C -0 50 frame.


SDCCH

o Another slot is used for creating 8 Stand-Alone Dedicated Control Channel (SDCCH) (S) channels

o Used for call setup and other signaling (LA, SMS)

o The 8 channels are assigned 3 slots each in a 26 frames multi-frame

S S S S S S S S S S S S S S S S S S S S S S S SA -0 2512


Voice coding: 13 Kb/so Every 20 ms codec generates 260 bits that are divided

into 3 groups (class 1a – 50 bits, class 1b - 132 bits, class 2 – 78 bits)

Class 1a50 bit

Class 1b132 bit

Class 278 bit

Class 1a53 bit

Class 1b132 bit

Class 278 bit

Class 1a + Class 1b378 bit

Class 278 bit

Cyclic code (53, 50)

Tail4 bit

Convolutional code 1/2

456 bit

■ 456 bits in 20 ms equivalent to a rate of 22.8 Kb/s


Voice coding: 13 Kb/s

o 4 Normal Burst transmitted into 4 frames 4.6*4 = 18.4 ms o But one every 13 is for signaling therefore on average

18.4*13/12=20 ms

20 ms coded voice

456 bit

Fragment 1

114 bit

Fragment 2

114 bit

Fragment 3

114 bit

Fragment 4

114 bit

Normal Burst

T3

TrainingSequence

26Coded bits

57T3

Coded bits57

S1

S1


Voice coding: 13 Kb/s

o Interleaving: 4 blocks of 114 bits

B(i,1) B(i,2) B(i,3) B(i,4) B(i+1,1) B(i+1,2) B(i+1,3) B(i+1,4) ... B(i+3,3) B(i+3,4)

Interleaving

B(i,1) B(i+1,1) B(i+2,1) B(i+3,1) B(i,2) B(i+1,2) B(i+2,2) B(i+3,2) B(i,4) B(i+1,4) B(i+2,4) B(i+3,4)...


Data channel coding: 9.6 Kb/s

60 bit 60 bit 60 bit 60 bit

240 bit Tail4 bit

488 bit

convolutional code 1/2

456 bit

puncturing di 32 bit

48 bit12


Signaling coding:SDCCH, BCCH, PCH, AGCH

184 bit

Tail4 bit184 bit

convolutional code 1/2

456 bit

FIRE code (224, 184)

40 bit


Documents

MRN-EN-6-GSM part 1 - home.deib.polimi.ithome.deib.polimi.it/capone/wn/MRN-EN-6-GSM part 1.pdfMRN – 6 – GSM part 1 Mobile Radio Networks ... Unit (TRAU) o GSM voice coding