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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
A. Capone: Mobile Radio Networks 2
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)
A. Capone: Mobile Radio Networks 3
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)
A. Capone: Mobile Radio Networks 4
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
A. Capone: Mobile Radio Networks 5
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)
A. Capone: Mobile Radio Networks 6
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
A. Capone: Mobile Radio Networks 7
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
A. Capone: Mobile Radio Networks 8
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
A. Capone: Mobile Radio Networks 9
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
A. Capone: Mobile Radio Networks 10
System Architecture
A. Capone: Mobile Radio Networks 11
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
A. Capone: Mobile Radio Networks 12
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]
A. Capone: Mobile Radio Networks 13
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
A. Capone: Mobile Radio Networks 14
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)
A. Capone: Mobile Radio Networks 15
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
A. Capone: Mobile Radio Networks 16
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
A. Capone: Mobile Radio Networks 17
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
A. Capone: Mobile Radio Networks 18
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.
A. Capone: Mobile Radio Networks 20
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
A. Capone: Mobile Radio Networks 21
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)
A. Capone: Mobile Radio Networks 22
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
A. Capone: Mobile Radio Networks 23
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)
A. Capone: Mobile Radio Networks 24
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
A. Capone: Mobile Radio Networks 25
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
……………....
A. Capone: Mobile Radio Networks 26
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
A. Capone: Mobile Radio Networks 27
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
A. Capone: Mobile Radio Networks 28
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
A. Capone: Mobile Radio Networks 29
Mobile Switching Centre (MSC)
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
A. Capone: Mobile Radio Networks 30
Mobile Switching Centre (MSC)
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
A. Capone: Mobile Radio Networks 31
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
A. Capone: Mobile Radio Networks 32
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
A. Capone: Mobile Radio Networks 33
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
A. Capone: Mobile Radio Networks 34
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
A. Capone: Mobile Radio Networks 35
Security procedureso Authentication:
MS Network
A3
Ki Random number
generator
A3
KiRAND
RAND
SRES(network)SRES(ms)
equal?Yes No
Accessgranted
Accessdenied
A. Capone: Mobile Radio Networks 36
Security procedureso Encryption:
MS Network
A8
Ki
A8
KiRANDRAND
KcKc
A5 A5informazioni cifrate
Random number
generator
A. Capone: Mobile Radio Networks 37
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
A. Capone: Mobile Radio Networks 38
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
A. Capone: Mobile Radio Networks 39
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
A. Capone: Mobile Radio Networks 40
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
A. Capone: Mobile Radio Networks 41
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
A. Capone: Mobile Radio Networks 42
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
A. Capone: Mobile Radio Networks 43
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
A. Capone: Mobile Radio Networks 45
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
A. Capone: Mobile Radio Networks 46
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)
A. Capone: Mobile Radio Networks 47
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
A. Capone: Mobile Radio Networks 48
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
A. Capone: Mobile Radio Networks 49
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
A. Capone: Mobile Radio Networks 50
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
A. Capone: Mobile Radio Networks 51
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
A. Capone: Mobile Radio Networks 52
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
A. Capone: Mobile Radio Networks 53
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=
A. Capone: Mobile Radio Networks 54
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
A. Capone: Mobile Radio Networks 55
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
A. Capone: Mobile Radio Networks 56
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
A. Capone: Mobile Radio Networks 57
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
A. Capone: Mobile Radio Networks 58
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
A. Capone: Mobile Radio Networks 59
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
A. Capone: Mobile Radio Networks 60
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
A. Capone: Mobile Radio Networks 61
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
A. Capone: Mobile Radio Networks 62
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
A. Capone: Mobile Radio Networks 63
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
A. Capone: Mobile Radio Networks 64
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
A. Capone: Mobile Radio Networks 65
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
A. Capone: Mobile Radio Networks 66
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
A. Capone: Mobile Radio Networks 67
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
CommonCONTROLCHANNELS
FCCH SCH BCCH PCH RACH AGCH SDCCH SACCH FACCH
DedicatedCONTROLCHANNELS
BroadcastCONTROLCHANNELS
A. Capone: Mobile Radio Networks 68
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
BroadcastCONTROLCHANNELS
A. Capone: Mobile Radio Networks 69
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
BroadcastCONTROLCHANNELS
A. Capone: Mobile Radio Networks 70
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
CommonCONTROLCHANNELS
A. Capone: Mobile Radio Networks 71
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, …)
DedicatedCONTROLCHANNELS
A. Capone: Mobile Radio Networks 72
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)
DedicatedCONTROLCHANNELS
A. Capone: Mobile Radio Networks 73
Connection setup
MS BS
RACH
BCCH
AGCH
SDCCH
TCH + SACCH
Broadcast info
Access request
SDCCH assignment
Signaling traffic + TCH assignment
traffic + signaling
A. Capone: Mobile Radio Networks 74
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
A. Capone: Mobile Radio Networks 75
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
A. Capone: Mobile Radio Networks 76
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
A. Capone: Mobile Radio Networks 77
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
A. Capone: Mobile Radio Networks 78
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
A. Capone: Mobile Radio Networks 79
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
A. Capone: Mobile Radio Networks 80
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.
A. Capone: Mobile Radio Networks 81
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
A. Capone: Mobile Radio Networks 82
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
A. Capone: Mobile Radio Networks 83
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
A. Capone: Mobile Radio Networks 84
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)...
A. Capone: Mobile Radio Networks 85
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
A. Capone: Mobile Radio Networks 86
Signaling coding:SDCCH, BCCH, PCH, AGCH
184 bit
Tail4 bit184 bit
convolutional code 1/2
456 bit
FIRE code (224, 184)
40 bit
A. Capone: Mobile Radio Networks 87
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