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GSM Basic Principle
Course Objectives:
·Aware of the Development Background of GSM technology
·Grasp GSM Network structure and Features
·Describe GSM Voice Service Process Procedure
State GSM key technology
Grasp physical and logical channel in air interface
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Contents
1 GSM Basic.....................................................................................................................................................1
1.1 2G Mobile Communication Technology Evolution.................................................................................1
1.2 Mobile Communication Technology Development Trend......................................................................5
1.3 GSM History............................................................................................................................................6
1.4 GSM Features...........................................................................................................................................7
1.5 GSM Specifications.................................................................................................................................8
1.6 GSM Network Structure..........................................................................................................................9
1.7 GSM Protocol Platform.........................................................................................................................12
1.8 Available GSM Services........................................................................................................................15
1.8.1 Telecommunications Services Provided by the GSM............................................................15
1.8.2 Supplementary Services of the GSM System........................................................................16
1.9 Operation Band......................................................................................................................................17
2 GSM Events.................................................................................................................................................21
2.1 Status of Mobile Subscriber...................................................................................................................21
2.1.1 Attach Flag upon MS Power-on.............................................................................................21
2.1.2 Detach upon MS Power-off....................................................................................................22
2.1.3 MS Busy.................................................................................................................................22
2.1.4 Periodical Registration...........................................................................................................22
2.2 Location Update.....................................................................................................................................22
2.2.1 Normal Location Update .......................................................................................................23
2.2.2 Periodical Location Update....................................................................................................23
2.2.3 IMSI Attach............................................................................................................................23
2.3 Handover................................................................................................................................................23
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2.3.1 Purpose of Handover..............................................................................................................23
2.3.2 Classification of Handover.....................................................................................................24
2.4 Cell selection and Reselection...............................................................................................................24
2.4.1 Cell selection..........................................................................................................................24
2.4.2 Cell reselection.......................................................................................................................25
2.5 Authentication ......................................................................................................................................25
2.6 Encryption .............................................................................................................................................26
3 GSM Speech Processing.............................................................................................................................29
3.1 GSM Speech Processing........................................................................................................................29
3.2 Voice encoding.......................................................................................................................................29
3.3 Channel Encoding..................................................................................................................................30
3.4 Interlacing Technology...........................................................................................................................31
3.5 Encryption/Decryption...........................................................................................................................35
3.6 Modulation and Demodulation..............................................................................................................35
4 GSM Key Technologies...............................................................................................................................37
4.1 Diversity Reception................................................................................................................................37
4.2 Discontinuous Transmission..................................................................................................................38
4.3 Power Control........................................................................................................................................39
4.4 Timing Advance.....................................................................................................................................42
4.5 Frequency Hopping................................................................................................................................43
5 Frame Structure and Radio Channels......................................................................................................47
5.1 Radio Frame Structure...........................................................................................................................47
5.2 Physical Channel....................................................................................................................................48
5.3 Logical Channels....................................................................................................................................49
5.3.1 Common Channel...................................................................................................................50
5.3.2 Dedicated Channel..................................................................................................................51
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GSM Basic
5.3.3 Channel Combination.............................................................................................................51
5.4 Mapping between Logical and Physical Channels................................................................................53
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1 GSM Basic
1.1 2G Mobile Communication Technology Evolution
Brief History of Evolution
The outline of GSM history is shown below:
1979 - Europe wide frequency band reserved for cellular
1982 - Groupe Spécial Mobile (GSM) created within CEPT
1986 – Eight proposals put forward by European countries after extensive
research and experiments accepted in Paris
1988 - ETSI took over GSM Committee
1990 - The phase 1 GSM recommendations frozen
1991 - GSM Committee renamed Special Mobile Group and GSM renamed as
Global System for Mobile Communication
1992 - GSM launched for commercial operations
1993 – Major part of GSM phase 2 standard completed
1994 – A new research phase (Phase 2+) added to improve GSM for mobile data
services
Mobile Communication during 1920 ~ 1940
In 1920, mobile communication system was first used by military while in 1940’s; it
was put in use for civil purpose.
Mobile communication started flourishing in recent decade. Its development phases are
as follows:
First generation (1G) mobile communication system
Second generation (2G) mobile communication system
Third generation (3G) mobile communication system
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1G during 1980’s
Since 1980's, 1G analog mobile communication system adopts cellular networkingtechnology. Till 1982 Cellular Systems were exclusively Analog Radio Technology.
At the end of 1980’s Analog System was unable to meet continuing demands due to:
Severely confined spectrum allocations
Interference in multipath fading environment
Incompatibility among various analog systems
Inability to substantially reduce the cost of mobile terminals and infrastructure
required
Easy to eavesdrop and misuse the subscriber’s account
Standards of First Generation
Different standards of first generation are shown in Table 1.1 -1.
Table 1.1-1 Different Standards of First Generation
Standard Origin Frequency Band
Advanced Mobile Phone System
(AMPS) North America 800 MHz
Nordic Mobile Telephone System-
450/900 (NMT-450/900)
North Europe
(Scandinavian)450 & 900 MHz
Total Access Communication System
(TACS)U.K. 900 MHz
2G during 1990’s
During 1990s, Digital mobile communication system characterized by digital
transmission, Time Division Multiple Access (TDMA), and narrowband Code DivisionMultiple Access (CDMA) were developed.
Standards of Second Generation
Different standards of second generation are:
GSM
CDMA IS95
Personal Digital Cellular (PDC)
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Advantages of 2G
Compared with 1G mobile communication system, 2G mobile communication systemhas the following advantages:
Provides high spectrum utilization and large system capacity.
Provides diversified services (voice services and low-rate circuit-switched data
services).
Enables automatic roaming.
Provides better voice quality.
Provides good security.
Can be interconnected with ISDN and PSTN.
Basic structure of GSM network is shown in Fig 1.1 -1.
Fig 1.1-1 Basic Structure of GSM Network
Discrepancies of 2G
2G mobile communication system has the following discrepancies:
Provides low-rate data services only and cannot support multi-media service.
For example, Internet data access speed of GSM MS can reach 9.6 kbps
theoretically.
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GSM Basic Principle
Different 2G mobile communication systems in the world use different
frequencies, therefore it is difficult to implement global roaming.
Internet, E-business, and multi-media communication is developing very
rapidly. Failing to provide strong support to data communication has already
constrained the development of 2G system. Demand for higher data rate and
more diversified services leads to evolution from 2G to 3G.
Fig 1.1 -2 shows the evolution process.
IS-95CDMA
PDC
GSM
IS-136
IS-95-B
HSCSDGPRS
IS-136+IS-136HS
IS-2000MC WCDMA
ARIBWCDMA
UTRAWCDMA
IMT-2000
2G 2.5G 3G
EDGEUWC-136
2.75G
Fig 1.1-2 Evolution from 2G to 3G
GSM 2.5G
GSM system (2.5G) Phase2 and Phase2+ were then developed, adopting high-rate
adaptive coding solution. GPRS provides the data rate up to 171 kbps. Two high-rate
data service options are:
High Speed Circuit Switched Data (HSCSD) based on high-speed data bit rate
and circuit switching
General Packet Radio Service (GPRS) based on packet switched data
GSM 2.75G
Enhanced Data Rates for GSM Evolution (EDGE) developed by the European
Telecommunications Standards Institute (ETSI) adopts 8-PSK (Phase Shift Keying)
modulation. It supports data rate up to 384 kbps theoretically. EDGE is more advanced
than GPRS. However, EDGE cannot provide rate up to 2 Mbps as 3G system does.
Therefore EDGE is often called 2.75G.
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1.2 Mobile Communication Technology Development Trend
3G Research during 1980’s
3G research, development, and establishment started in mid 1980’s.
IMT-2000
International Mobile Telecommunication 2000 (IMT-2000) established by International
Telecommunications Union (ITU) introduces 3G.
IMT-2000 introduces:
Mobile data service and some fixed high-speed data services through one or
more radio channels
Fixed network platform
A global standard
IMT-2000 services, which are compatible with other fixed network services
High quality
Use of common band in the world
Small terminals used in the world
Global roaming
Multi-media services and terminals
Higher frequency utilization
Flexibility for development to the next generation
High-speed hierarchical data rate
Rate up to 2 Mbps while stationary
Rate up to 384 kbps during walking speed
Rate up to 144 kbps while in vehicle
Instead of having pure technology, communication system is currently
developing into a mode featuring the combination of services and technology.
Communication technology is estimated to undergo the largest change in future.
It is strategic transition from voice services to data services from the aspect of
market application and service demand. This change has deeply influenced the
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development trend of communication technology.
4G Services
Some researchers and telecom operators describe fourth-generation (4G) mobile
communication system as a new world better than 3G, which can provide:
Many unimaginable applications
Over 100 Mbps data transmission rate, which is 10,000 times of current MSs
and 50 times of 3G MSs
High-performance multi-media contents
Service as a personal identification device through ID application
Service for high-resolution movies and TV programs, acting as bridge of
combined broadcast and new telecommunication infrastructure
Some services such as 4G wireless instant connections, are cheaper than 3G
services.
1.3 GSM History
Because analog mobile communication system had limited expansion capability,
Global System for Mobile Communication (GSM) was developed on demand for
capacity expansion which achieved global success. It operates at 900 MHz band within
European countries.
GSM Development process is as follows:
1982: Conference of European Posts and Telegraphs (CEPT) formed a study
group called the Group Special Mobile (GSM) to study and develop 2G mobile
communication system.
1986: Eight proposals put forward by European countries after massive research
and experiments were accepted in Paris, and on-site experiments were
performed.
1987: After on-site test, demonstration, and comparison, GSM member countries
have reached an agreement that digital system adopts narrowband Time Division
Multiple Access (TDMA), Regular Pulse Excitation-Long Term Prediction
(RPE-LTP), voice coding, and Gaussian Minimum Shift Keying (GMSK)
modulation.
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1998: Eighteen European countries reached GSM Memorandum of
Understanding (MOU).
1989: GSM took effect.
1991: First GSM network was deployed in Europe.
1992: GSM standard was frozen.
1993: Major part of GSM phase II standard was completed.
1994: A new research phase (Phase 2+) was added to further improvement of
GSM as a platform of mobile data services.
1.4 GSM Features
GSM system has the following features:
High Spectrum efficiency
GSM system features high spectrum efficiency due to the high-efficient modulator,
channel coding, interleaving, balancing, and voice coding technologies adopted.
Large capacity
Volumetric efficiency (number of channels/cell/MHz) of GSM system is three to five
times higher than that of Total Access Communication System (TACS).
High voice quality
Digital transmission technologies and GSM specifications, voice quality is irrelevant
with radio transmission quality.
Open interfaces epic
GSM standard provides open air interface, also open interfaces between networks and
those between network entities, such as A interface and Abis interface.
High security
MS identification code encryption makes eavesdropper unable to determine the MS
number, ensuring subscriber’s location security. Voice encryption, signaling data, and
identification codes make the eavesdropper unable to receive the communication
contents.
Interconnection with Integrated Services Digital Network (ISDN) and PSTN.
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GSM Basic Principle
GSM can interconnect with other networks through current standard interfaces, such as
Integrated Service User Part (ISUP) or Telephone User Part (TUP).
Roaming function
GSM supports roaming by introducing Subscriber Identity Module (SIM) card that
separates subscriber from the terminal equipment.
Diversified services
GSM provides diversified services, tele-services, bearer services, and supplementary
services.
Inter-cell handover
During conversation, MS continues to report the detailed radio environment of local
cell and neighboring cells to serving base station. If inter-cell handover is required, MS
sends a handover request to serving base station.
1.5 GSM Specifications
European Telecommunications Standards Institute (ETSI) initiated and made GSM
standard.
ETSI developed GSM in several phases and set up more Special Mobile Groups
(SMG) to make the related GSM standard.
GSM detailed specifications conform on functions and interfaces only, not on
hardware. Purpose is to reduce the restriction on designers, enabling the operators to
purchase equipment from different manufacturers.
GSM technical specifications consist of 12 fields:
Field 1: General
Field 2: Services
Field 3: Network Functions
Field 4: MS-BS Interfaces and Protocols
Field 5: Physical Layer on Radio Path
Field 6: Speech Coding
Field 7: MS Terminal Adaptor
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Field 8: BS-MSC Interface
Field 9: Network Inter-working
Field 10: Service Inter-working
Field 11: Equipment and Model Acceptance Specification
Field 12: Operation and Maintenance
1.6 GSM Network Structure
Fig 1.6 -3 shows the basic GSM network structure.
BSC TRAU MSC/VLR
SMC
GMSC
AUC
IWF EIR
HLR
PSTNISDNPDN
BTS
MS
BTS
MS
Traffic & Signaling
Signaling
Fig 1.6-3 GSM Network Structure
GSM system consists of:
Network Subsystem (NSS)
Base Station Subsystem (BSS)
Operation and Maintenance Subsystem (OMS)
Mobile Station (MS)
Network Switching Subsystem
NSS is the core element of network switching which interfaces with subscriber services
for voice and data.
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GSM Basic Principle
NSS Main components are:
Mobile Switching Centre (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identification Register (EIR)
Authentication Centre (AUC)
Short Message Centre (SMC)
Home Location Register - HLR is a central database of a system. HLR stores all the
information related to subscribers, including the roaming authority, basic services,
supplementary services, and current location information. It provides routing
information for MSC for call setup. HLR may cover several MSC service areas or even
the whole PLMN.
Visitor Location Register - VLR stores all subscriber information in its coverage area
and provides call setup conditions for the registered mobile subscribers. As a dynamic
database, VLR must exchange large volume of data with HLR to ensure data validity.
When an MS leaves the controlling area of a VLR, it registers in another VLR. The
original VLR deletes the temporary records of that subscriber. VLR integrated within
MSC.
Equipment Identification Register - EIR stores the parameters related to MS. It can
identify, monitor, and block the MS. ERI preventing unauthorized MS from accessing
the network.
Authentication Centre - AUC is a strictly protected database that stores subscriber
authentication information and encryption parameters. AUC integrated with HLR
physically.
Base Station Subsystem BSS serves as a bridge between NSS and MS. It performs
radio channel management and wireless reception and transmission. Base Station
Controller (BSC) and Base Transceiver Station (BTS) are main components of BSS.
Base Station Controller - Located between MSC and BTS, it controls and manages
more than one BTS. It performs radio channel assignments. BTS and MS transmit
power control, and inter-cell handover. BSC is also small a switch that converge and
connects local network with the MSC through A interface. Abis interface connects BTS
to BSC.
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Base Transceiver Station - BTS is wireless transceiving equipment controlled by the
BSC in BSS. BTS carries radio transmission. It performs wired-related wireless
conversion, radio diversity, radio channel encryption, and hopping. Um interface
connects BTS to MS.
Transcoding and Rate Adaptation Unit - TRAU Located between BSC and MSC,
TRAU transcodes between 16 kbps RPE-LTP codes and 64 kbps A law PCM codes.
Operation and Maintenance Subsystem OMS is operation & maintenance part of
GSM. Functional units in GSM are connected to OMS internal networks. OMS
monitors various functional units in GSM network, submits status report, and performs
fault diagnosis.
OMS consists of two parts: OMC – System (OMC-S) and OMC-Radio (OMC-R). The
OMC-S performs operation and NSS maintenance, while OMC-R performs operation
and BSS maintenance.
Mobile Station
MS is subscriber equipment in GSM, it can be vehicle installed or hand portable. MS
consists of mobile equipment and SIM.
Mobile equipment processes voice signals, receives and transmits radio signals.
SIM stores all information required for identifying a subscriber and security
information, preventing unauthorized subscribers. Mobile equipment cannot access
GSM network without a SIM card.
Network Service Area
GSM service area refers to the total area covered by networks of all GSM operators.
Network consists of several MSC service areas, each of which consists of several cells.
Logically, several cells form a location area (LA).
MSC Service Area - A Public Land Mobile Network (PLMN) includes multiple MSC
service areas. MSC service area refers to the MSC coverage area, that is, the total area
covered by BTS under control of BSC connected to MSC. All MSs in the service area
table register in local VLR. Therefore, in actual network, MSC is always integrated
with VLR as a node.
Location Area - Each MSC/VLR service area includes multiple of LAs. MS can move
freely without performing location update in LA. Hence, LA is the paging area of a
broadcast paging message. An LA belongs to one MSC/VLR only, that is, LA cannot
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cross MSC/VLR. The system can identify different LA via LA Identity (LAI).
Cell - LA contains several cells. Each cell has a unique Cell Global Identification(CGI), which indicates a basic radio coverage area in a network.
Fig 1.6 -4shows the relationship among different coverage areas in a GSM network.
GSM service areaThe total network coverage provided by all GSM operators
PLMN service areaThe network coverage provided by a GSM operator
MSC service area
The area controlled by an MSC
Location areaAn area for location update and paging
CellA service area provided by a
specific BTS
Fig 1.6-4 Relationship among Coverage Areas in a GSM Network
1.7 GSM Protocol PlatformGSM technical specifications make clear and normative definition of interfaces and
protocols between subsystems and various functional entities. Interface refers to the
point where two adjacent entities are connected. Protocol defines the rules for
information exchange at the connection point.
GSM Interfaces
Fig 1.7 -5 shows the GSM interfaces.
MS BTS BSC MSC
VLR VLR
HLR
MSCEIR
Sm
Um
Abis A B
D
C
E F
G
Fig 1.7-5 shows the GSM interfaces.
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Sm Interface: Man-machine interface implemented in MS. It is an interface between
subscribers and PLMN. MS consists of keyboard, LCD, and SIM card.
Um Interface: Radio interface between MS and BTS. It is an important interface in
PLMN. Digital mobile communication network has different radio interface as
compared to analogue mobile communication network.
A Interface: It is an interface between BSC and MSC. Base station management
information, call processing interface, mobility management information, and specific
communication information are transferred through A interface.
Abis Interface: It is an interface between BSC and BTS. Supports all services
provided to subscribers. Also supports the control of BTS radio equipment and
management of radio resources assigned.
B Interface: It is an interface between MSC and VLR. VLR is a database locating and
managing MS when MS roams in the related MSC control area. MSC can query the
current location of MS from VLR and update MS location. When subscriber uses a
special supplementary service or changes a relevant service, MSC notifies the VLR.
Sometime VLR also updates information in HLR.
C Interface: It is an interface between MSC and HLR. C interface transfers
management and route selection information. When a call finishes, MSC sends the
billing information to HLR. When PSTN cannot get location information of a mobile
subscriber, the related GMSC queries HLR of the subscriber to obtain the roaming
number of the called MS, and then transfers it to the PSTN.
D Interface: It is an interface between HLR and VLR. Exchanges MS location
information and subscriber management information. To enable a mobile subscriber to
originate or receive calls in the whole service area, data must be exchanged between
HLR and VLR. VLR notifies HLR about the current location of MS belonging to HLR,
and then provides MS roaming number. HLR sends VLR all the data required to
support the services of the MS. When an MS roams to the service area of another VLR,
HLR notifies the previous VLR to delete the relevant MS information. When MS uses
supplementary services, or some parameters are changed, D interface is also used to
exchange the related information.
E Interface: It is an Interface between MSCs. It exchanges the handover information
between two MSCs. When MS in a conversation moves from one MSC service area to
another MSC service area, inter-cell handover occurs to maintain the conversation. At
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that time, related MSCs exchange the handover information through E interface.
F Interface: It is an interface between MSC and EIR. It exchanges the MSmanagement information, such as IMEI, between MSC and EIR.
G Interface: It is an interface between VLRs. When MS uses a Temporary Mobile
Subscriber Identity (TMSI) to register with a new VLR, the relevant information is
exchanged between VLRs through G interface. This interface also searches IMSI of the
subscriber from VLR that registers TMSI.
GSM Protocol Structure and OSI
2G cellular mobile network GSM adopts Open System Interconnection (OSI) model to
define its protocol structure. Fig 1.7 -6 shows GSM interface protocol model, which
defines the interfaces and protocols between MS and MSC.
Um interface Abis interface A interface
CM
MM
RRM
LAPDm
Radio
CM
MM
RRM
MTP
64
kbit/s
RRM
LAPDm
Radio
LAPD
64
kbit/s
RRM
LAPD
64
kbit/s
MTP
64
kbit/s
SCCP SCCP
MS BTS BSC MSC
Fig 1.7-6 GSM Interface Protocol Model
OSI reference model is a hierarchical structure. According to the hierarchy concept,
communication process can be divided into several logical layers from lowest to
highest layer. In different systems, the entities in the same layer that exchange
information for the same purpose are called peer entities. Entities in adjacent layers
interact with each other through the common layer. The lower layers provide services
to higher layers. The services provided by layer N is a combination of the services and
functions provided by the layers below it.
First layer of Um interface protocol is physical layer, which is marked as L1 and
it is a lowest layer. L1 provides basic radio channels for the information
transmission of higher layers.
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Second layer L2 is data link layer, which is marked as LAPDm. It covers various data
transmission structures and controls data transmission.
Application layer is the third highest layer L3. It covers various messages and
programs, and controls services. L3 includes Radio Resource Management (RRM),
Mobility Management (MM) and Call connection Management (CM).
Abis interface protocol is slightly different from Um interface protocol. Its
physical layer is 64 kbps land line, and link layer is LAPD.
First layer of A interface protocol is 64 kbps land line, and second layer is the
Message Transfer Part (MTP), which is part of Common Channel Signalling7
(CCS7) network. MTP consists of many network protocols and centralizes all
link layer protocols. Signaling connection control part (SCCP) and MTP
together represent a network layer protocol on A interface.
In BSC both MM and CM are transparently transmitted
1.8 Available GSM Services
1.8.1 Telecommunications Services Provided by the GSM
1. Circuit Services
1) Voice Service
Full-rate voice service
Half-rate voice service
Enhanced full-rate voice service
2) Data service
14.4Kbit/s full-rate data service
9.6Kbit/s full-rate data service
4.8Kbit/s full-rate data service
2.4Kbit/s full-rate data service
2. SMS services(support Chinese short messages)
1) Point-to-point short message service
Point-to-point short message service with the mobile user serving as called
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Point-to-point short message service with the mobile user serving as caller
2) Cell Broadcast Short Message
Cell broadcast service originated from the SMS center or the OMC-R.
3. Packet Services
1) GPRS service
2) EDGE service
At present, the point-to-point interactive telecom services are supported,
including:
Access to the database: Allocate service to users as needed, e.g. Internet, and
provide storing and forwarding, as well as information processing for user-to-
user communications.
Session service: Provide bi-directional user-to-user and port-to-port real time
information communication, e.g. Internet Telnet service.
Tele-action service: Applicable to small-volume data processing services, credit
card confirmations, lottery transactions, electronic monitoring, remote meter
reading (water, electricity and gas), monitoring systems, and so on.
1.8.2 Supplementary Services of the GSM System
GSM supplementary services are diversified, including:
Call Forwarding Unconditional: forward all incoming calls to the number specified by
the subscriber.
Barring: barring of outgoing/coming calls.
Call Waiting: When a call is connected for a subscriber, indication of a new coming
call is given to the subscriber. The subscriber can accept, reject or ignore the waiting
call.
Call Hold: A subscriber can suspend the connected call to do other things.
Multiparty Service: A simultaneous communication with up to six parties is allowed.
Closed User Group: The subscribers of CUG are restricted from outgoing and
incoming calls, but they can normally communicate with each other.
Hot Billing: The network generates an instant call billing message from the billing
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manager. It is applicable to leased phone service, including all kinds of call modes.
Bills are generated and presented to the subscriber immediately after the call is ended.
1.9 Operation Band
1. Working band
Currently, the GSM communication system works at 900 MHz, extended 900
MHz and 1800 MHz, or 1900 MHz band in some countries.
1) 900 MHz band
Uplink (MS transmitting and BS receiving) frequency range: 890 MHz ~ 915MHz
Downlink (BS transmitting and MS receiving) frequency range: 935 MHz ~
960MHz
2) Extended 900 MHz band
Uplink (MS transmitting and BS receiving) frequency range: 880 MHz ~ 915
MHz
Downlink (BS transmitting and MS receiving) frequency range: 925 MHz ~ 960MHz
3) 1,800 MHz band
Uplink (MS transmitting and BS receiving) frequency range: 1,710 MHz ~
1,785 MHz
Downlink (BS transmitting and MS receiving) frequency range: 1,805 MHz ~
1,880 MHz
4) 1,900 MHz band
Uplink (MS transmitting and BS receiving) frequency range: 1,850 MHz ~
1,910 MHz
Downlink (BS transmitting and MS receiving) frequency range: 1,930 MHz ~
1,990 MHz
2. Channel interval
The interval between two adjacent channels in any band is 200 kHz.
3. Channel configuration
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GSM Basic Principle
All channels are configured with the same interval.
1) 900 MHz band
The channel numbers are in the range of 1 ~ 124. There are 124 frequency bands
in all.
The relationship between a channel number and nominal central frequency of a
frequency band is illustrated as follows:
Fu (n) = 890 + 0.2 × n-512 (MHz), uplink
Fd (n) = Fu (n) + 45 (MHz), downlink
Where, 1 ≤ n ≤ 124, n is a channel number, or an Absolute Radio Frequency
Channel Number (ARFCN).
2) Extended 900MHz band
The channel numbers are in the range of 0 ~ 124 and 975 ~ 1023. There are 174
frequency bands in all.
The relationship between a channel number and nominal central frequency of a
frequency band is illustrated as follows:
Fu (n) = 890 + 0.2 × n (MHz), 0 ≤ n ≤ 124
Fu (n) = 890 + 0.2 × (n-1024) (MHz), 975 ≤ n ≤ 1023
Fd (n) = Fu (n) + 45 (MHz)
3) 1,800 MHz band
The channel numbers are in the range of 512 ~ 885. There are 374 frequency
bands in all.
The relationship between a channel number and nominal central frequency of a
frequency band is illustrated as follows:
Fu (n) = 1710.2 + 0.2 × (n-512) (MHz)
Fd (n) = Fu (n) + 95 (MHz)
512 ≤ n ≤ 885
4) 1,900 MHz band
The channel numbers are in the range of 512 ~ 811. There are 300 frequency
bands in all.
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2 GSM Events
The relationship between a channel number and nominal central frequency of a
frequency band is illustrated as follows:
Fu (n) = 1850.2 + 0.2 × (n-512) (MHz)
Fd (n) = Fu (n) + 80 (MHz)
512 ≤ n ≤ 811
4. Duplex transceiving interval
1) 900 MHz band
The duplex transceiving interval is 45 MHz.
2) Extended 900 MHz band
The duplex transceiving interval is 45MHz.
3) 1,800 MHz band
The duplex transceiving interval is 95 MHz.
4) 1,900 MHz band
The duplex transceiving interval is 80 MHz.
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2 GSM Events
2.1 Status of Mobile Subscriber
Mobile subscriber is generally in one of the following three states: MS power-on (idle),
MS power-off, and MS busy. Thus, the network needs to process these states
accordingly.
2.1.1 Attach Flag upon MS Power-on
IMSI attach is divided into three cases:
1. If the MS is powered on for the first time, The SIM card does not store the LAI.
MS sends a Location Update Request to the MSC, notifying the GSM system
that this is a new subscriber in this location area. MSC sends a Location Update
Request to the HLR according to the IMSI number sent by this subscriber. HLR
records the number of the MSC sending the request and the corresponding VLR
number, and returns a Location Update Accepted message to the MSC. By now,
MSC has been activated and it will add an Attach flag to the IMSI of the
subscriber in the VLR. Then it sends a Location Update Acknowledgement
message to the MS. The SIM card of the MS records the LAI.
2. If the MS is not powered on for the first time, instead the MS is powered off and
then powered on again, and if the LAI received by the MS is inconsistent with
that stored in the SIM card, the MS sends a Location Update Request to the
MSC. The VLR must judge whether the original LAI is in its own service area.
If yes, MSC only needs to replace the original LAI in the SIM card of thesubscriber with the new LAI.
If no, MSC sends a Location Update Request to the HLR according to the
information in the IMSI of the subscriber. HLR records the number of MSC
sending the request in the database and returns a Location Update Accepted
message. Then MSC adds an Attach flag to the IMSI of the subscriber and
returns the Location Update Acknowledgement message to the MS. MS replaces
the original LAI on the SIM card with the new LAI.
3. If the MS is powered on again, and the LAI received is consistent with the
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original LAI stored in the SIM card. VLR only adds Attach flag to this
subscriber.
2.1.2 Detach upon MS Power-off
After the MS is powered off, the MS sends a Detach Request to the MSC. After the
MSC receives the request, it informs VLR to add the Detach flag to IMSI of this MS.
At this time, HLR does not receive the notice indicating that this subscriber is detached
from the network. After this subscriber is paged, the HLR requests the MSRN from the
MSC/VLR. At this time, the VLR informs the HLR that this MS is powered off.
2.1.3 MS Busy
In this case, the MS is allocated with a traffic channel to transmit the voice or data and
the IMSI of the subscriber is marked as Busy.
2.1.4 Periodical Registration
When the MS sends the IMSI Detach message to the network, it is possible that the
GSM system cannot decode properly due to the poor radio quality or other reasons and
still believes that MS is in Attach status. Or when the MS is powered on but has
roamed beyond the service coverage, i.e., a blind area, the GSM system does not know
it and still believes that the MS is in Attach status. In both cases, if the subscriber is
paged, the system will keep sending paging messages, wasting radio resources.
To solve the above problems, the measure of forced registration is taken in the GSM
system: The MS must make registration at a regular interval. This is called periodical
location update. If the GSM system does not receive the periodical registration
information of the MS, the VLR where the MS resides records the Implicit Detach
status of the MS. When the correct periodical registration information is received
again, the status is changed into Attach status.
2.2 Location Update
When the MS changes the location area, it finds out that the LAI in its SIM card is
inconsistent with the LAI received. Thus, it registers the location information. This
flow is called location update. Location update is originated by the MS. There are three
type of location update:
Normal Location Update.
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2 GSM Events
Periodical Location Update
IMSI Attach.
2.2.1 Normal Location Update
When the MS roams to a new location area, it finds out that the LAI in its SIM card is
inconsistent with the LAI received. Thus, it originates a Location Update Request to
the current MSC/VLR If the new LAI and old LAI below to same MSC/VLR, Location
Update just renew LAI in VLR. If not, the new MSC/VLR should require MS data
from its HLR, HLR send back MS data to new MSC/VLR and inform old MSC/VLR
to delete MS record at the same time. MS register its LAI in new MSC/VLR, HLR
save the new MSC/VLR number.
2.2.2 Periodical Location Update
When MS make periodical register to MSC, periodical location update happens
2.2.3 IMSI Attach
When MS Power On, it will start a location update process to MSC/VLR, the location
update process is same as that in normal location update.
2.3 Handover
When a mobile subscriber who is engaged in a conversation moves from one BSS to
another, handover function ensures that the link set up for this mobile subscriber is not
interrupted. Whether to perform handover is determined by the BSS. When the BSS
finds out that the communication quality of the current radio link degrades, it performs
different types of handover according to the actual situation. MSS can also request the
handover according to the traffic information.
2.3.1 Purpose of Handover
1. Save the calls in progress( bad quality)
2. Cell-boundary handing over to improve ongoing calls (weak signal)
3. Intra-cell hand-over reducing interference within a cell (severe interference)
4. Directed Retry increase call completion success rate
5. Compelled hand-over to balance traffic distribution of inter-cells.
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GSM Basic Principle
2.3.2 Classification of Handover
According to the scope of handover , it can be divided into the following types
1. Intra-cell hand-over
2. Inter-cell hand-over
3. Inter-BSC hand-over of same MSC
4. Inter-MSCs hand-over
According to the synchronous relationship between MS and BTS when handover
happens, there are three type of handover:
1. Synchronous: MS use the same TA both in destination and target cell. This
usually applies to hand-over of same cell or different sectors within the same
cell. This is the hand-over with highest speed.
2. Asynchronous: MS don’t know the TA to be used in target cell. When either of
the two cells doesn’t synchronize with BSC, this mode should be used. The
hand-over speed is low.
3. Pseudo-synchronous: MS is able to calculate out the TA it should use in the
target cell. When both cells have synchronized with BSC, this mode may be
used. The hand-over speed is fast.
2.4 Cell selection and Reselection
2.4.1 Cell selection
After a MS is turned on, it will attempt to contact a common GSM PLMN, so the MS
will select an appropriate cell, and extract from it the parameters of the control channel
and the prerequisite system information. Such a selection process is referred to as cellselection. The quality of a radio channel is an important factor of cell selection. The
GSM specification defines the path loss criterion C1, and such appropriate cell must
ensure that C1>0. The C1 is calculated according to the following formula:
C1=RXLEV-RXLEV_ACCESS_MIN-MAX((MS_TXPWR_MAX_CCH-P), 0)
Where:
The RXLEV is the average reception level.
The RXLEV_ACCESS_MIN is the minimum level at which the MS is allowed to
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2 GSM Events
access.
The MS_TXPWR_MAX_CCH is the maximum power level of the CCH.
The “P” is the maximum transmitted power of the MS.
MAX (X, Y) = X; If X≥Y.
MAX (X, Y) = Y; If Y≥ X.
After the MS selects a cell, it will stay in the selected cell if no major changes have
occurred to various conditions.
2.4.2 Cell reselection
After a MS selects a cell, the MS will stay in the selected cell as long as no major
changes occur to various conditions. At the same time, the MS starts to measure the
signal level of the BCCH carrier of the adjacent cells, records the six adjacent cells
with the highest signal levels, and extracts from them the various system messages and
control messages of each adjacent cell. When the appropriate conditions are met, the
MS will switch from the current cell to another cell, a process known as cell
reselection. Such appropriate conditions include multiple factors, including cell
priority, and whether the cell is prohibited from access. Among them, an important
factor is the quality of the radio channel. When the signal quality of the adjacent cell
exceeds that of the current cell, cell reselection is triggered. For cell reselection, the
channel quality criterion is determined by the C2 parameter, which is calculated
according to the following formula:
2.5 Authentication
Fig. 2.5 -1 shows the authentication process, where RAND is the question asked by
the network side and only the legal subscriber can give the correct answer SRES.
RAND is generated by the random number generator of the AUC on the network side.
It is 128 bits in length. The value of RAND is obtained in a random manner from the
range of 0~2128 –1.
SRES is called a signed response. It is obtained through the calculation of subscriber’s
unique key parameter Ki. It is 32 bits in length.
Ki is stored in the SIM card and AUC in a very confidential way. Even the subscribers
do not know their own Ki. Ki can be of any format and any length.
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GSM Basic Principle
A3 algorithm is the authentication algorithm determined by the carrier. It is also
confidential. The only restriction of the A3 algorithm is the length of the input
parameter (RAND is 128 bits in length) and the size of the output parameter (SRES
must be 32 bits).
Mobile Terminal Network
A3 algorithm
Random number generator Ki RAND
SRES'
SRES
Ki
A3 algorithm
Fig. 2.5-1 Authentication Process
2.6 Encryption
In the GSM, the position of encryption and decryption over the transmission link
allows the transmitting data in all dedicated modes to use the same protection method.
The transmitting method can be the subscriber information (such as voice and data),
subscriber-specific signaling (such as message carrying the called number), or even the
system-specific signaling (such as the message carrying radio measurement result for
the handover).
Encryption and decryption are the exclusive or operation (this algorithm is called the
A5 algorithm) of 114 radio burst pulse code bits and one 114-bit encryption sequence
generated by a special algorithm. To obtain each burst encryption sequence, A5
calculates on two inputs: One is the frame number and the other is the key (Kc) agreed
upon by the MS and network, as shown in Fig. 2.6 -2. Two different sequences are
used over the uplink and downlink. For each burst, one sequence is used for the
encryption inside the MS and meanwhile used as the decryption sequence in BTS. The
other sequence is used for the encryption of BTS and meanwhile used as the decryption
sequence in MS.
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2 GSM Events
A5
Frame No.
(22-bit)Kc (64-bit)
A5
S1
(114-bit)
S2 S1 S2
MS BTS
Frame No.
(22-bit)Kc (64-bit)
(114-bit) (114-bit) (114-bit)
Fig. 2.6-2 Encryption Algorithm
1. Frame number: Frame number is encoded into a serials of three values, which
are 22 bits in total.
Frame number of each burst varies with the type of radio channel. Each burst
dedicated for communication on the same direction uses different encryption
sequence.
2. A5 algorithm
A5 algorithm must be defined in the global range. This algorithm can be
describes into the two 114-bit sequence black boxes generated by a 22-bit
parameter (frame number) and a 64-bit parameter (Kc).
3. Kc
Before the encryption, Kc must be agreed upon by both the MS and network. In
the GSM, the Kc is calculated during the authentication and then stored in the
SIM card permanently. On the network side, this potential key is also stored in
the visited MSC/VLR and ready for use in the encryption.
The algorithm that uses the RAND (same with the one used for authentication)
and Ki to calculate the Kc is called A8 algorithm. Like the A3 algorithm that
calculate the SRES using RAND and Ki, the A8 algorithm also needs to be
determined by the carrier.
Fig. 2.6 -3 shows how the Kc is calculated.
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GSM Basic Principle
Mobile Terminal Network
A8 algorithm
Random number
generator KiRAND
Kc
Ki
Kc
A8 algorithm
Fig. 2.6-3 Kc Calculation Method
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3 GSM Speech Processing
3.1 GSM Speech Processing
In the GSM system, the MS processes voice signals on wireless interfaces as shown in
Fig 3.1 -7.
A/D
D/A
Voicecoding
Channel
coding
Interle
aving
EncryptionBurst pulse
forming
Modulation
Voice
decoding
Channel
decoding Deinterleaving DecryptionBurst pulse
disassembleDemodulation
260bit/20ms 456bit/20ms 33.8kbit/s 270kbit/s
Fig 3.1-7 Voice Processing in the GSM System
The process of sending voice signals is as follows: for analog voice signals, first make
A/D conversion before doing voice coding to output 13Kbit/s digital voice signals. Tocontrol errors in the process of transmission, channel coding and interlacing processing
shall be conducted on digital voice signals, which are then encrypted according to the
input/output bit stream of 1:1. These bits are grouped into 8 1/2 burst pulse sequences
(corresponding to voice signals/20ms segment) before they are transmitted at about
270Kbit/s in the appropriate timeslots.
The process of receiving voice signals is as follows: for the wireless signals sent by
BTS, first do demodulation before decomposing and decrypting burst pulses. After
every 8 1/2 burst pulse sequences are received, they are subjected to interlacing
processing and re-assembled into 456 bit information. After that, do channel decoding
and detect and correct the errors that occur in the middle of transmission before finally
conducting voice decoding of the bit stream generated by the decoder and converting it
analog voices.
3.2 Voice encoding
Given below is a brief introduction to the voice coding process of the GSM system
using full-rate voice coding as an example.
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Currently, what the GSM system uses is 13kb/s voice coding scheme, known as RPE-
LTP (Rule Pulse Excitation-Long Term Prediction). The aim of this scheme is to produce
near-PSTN voice quality when no error occurs.
It first divides the voice into voice blocks by 20ms and samples it with 8kHz frequency
to get 160 sample values. Then each sample value is quantified to generate 16bit digital
voice signals. The 128Kbit/s data stream is obtained this way. As the rate is too high to
be transmitted on the wireless path, it needs to be compressed by a coder. If a full-rate
coder is used, each voice block will be compressed into 260bits to generate 13Kbit/s
source code rate in the end. The process of processing other signals such as channel
coding comes after that.
On the BTS side, BTS can recover 13Kbit/s source rate, but to generate 16Kbit/s rate so
that it can be transmitted on the Abis interface, it is necessary to add 3Kbit/s signaling so as
to control the operation of the remote TC. On the TC side, to accommodate 64Kbit/s
transmission rate of A interface, it is also necessary to conduct rate conversion between
13Kbit/s and 64Kbit/s.
3.3 Channel Encoding
Channel coding serves to improve transmission quality and overcome the negative
impact of interferences on signals.
Using specialized redundancy technology, channel coding inserts redundancy bits in
certain regularity at the transmitting end for coding while the receiving end in the
process of decoding detects error codes and corrects errors as many as possible using
these redundancy bits to recover the originally transmitted information.
The coding schemes as used in GSM are convolutional code and packet code which are
used in a combinational way in actual applications.
Convolutional code: compiles k information bits into n bits. Both k and n are very
small so that they are suitable for transmission in a serial port manner. Besides they
also show very little delay. The coded n code elements are not only related to k
information code elements of this packet, but also to information code elements in the
preceding (N-1), where N is called constraint length. The convolutional code is
generally represented as (n, k, N). The error tolerance of the convolutional codes
increases as N increases while its error rate decreases as N increases. The convolutional
code is mainly designed for error correction. When the demodulator uses the maximum
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5 Frame Structure and Radio Channels
likelihood estimation method, it can generate very effective error correction results.
Convolution code can be expressed as (n, k, N). The error-correction capability in
convolution encoding grows stronger with the rise of N, while the error probability
decreases exponentially as N rises. The convolution code is used to correct errors, and
it is effective when the decoder works in the maximum likelihood estimate mode.
Packet code: This is a kind of shortened loop code, which gets the redundancy bits by
increasing the exclusive-or algorithm of information bits and maps k input information
bits to no output binary code elements (n>k) through exclusive-or algorithm. The
packet code is designed mainly to detect and correct error codes in groups and it is
used in a mixed way with the convolutional code. The packet code is used for detecting
and correcting errors in groups. It is generally used along with the convolution code.
3.4 Interlacing Technology
The occurrence of burst error codes in wireless communication is usually caused by
fading that lasts a long time. It is not enough to detect and correct errors in the above-
mentioned channel coding scheme. To better address the issue of error codes, the
interlacing technology is introduced to the system. The interleaving technology is
adopted in channels to better solve the error problems.
Interlacing is in fact to send separately the original continuous bits of a message block
in certain regularity. In other words, the original continuous block in the middle of
transmission becomes discontinuous and creates a group of interlaced transmission
message blocks. At the receiving end, this kind of interlacing message blocks is
restored (de-interlaced) to original message blocks. To control the operations and
sessions, the TCAP are classified into two layers, CSL and TSL. The CSL is used to
manage the operations and the TSL is used to manage the transactions (sessions), as
shown in Fig 3.4 -8.
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GSM Basic Principle
Packet
Interleaving
Packet after
interleaving
Error code
11 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
11 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
11 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Fig 3.4-8 Interleaving Technology
After the interlacing technology is applied, if a message is lost in the middle of
transmission, it is in fact part of each message block that is lost, but the whole part of
it. The missing messages can be recovered easily with the coding technology.
In the GSM, different coding and interleaving modes are used in different types of
channels. See Table 3.4 -2 for details.
Table 3.4-2 Coding and Interweaving of Circuit Logical Channels
Channel Type
Input
Rate
(Kbit/s)
Input Code
Block bits
Code Output
Code
Block bits
Interleaving DepthCheck Bit Tail Bit
Convolutional
Code Rate
TCH/F
S
Ia 13 50Parity
check, 3 4 1/2456 On eight 1/2 bursts
Ib 13 132
II 13 78
TCH/
HS
Ia 5.6 22Parity
check, 3 6 1/3228 On four 1/2 bursts
Ib 5.6 73II 5.6 17
TCH/F9.6
TCH/H4.8
12
6240 4
1/2, one bit is
removed
every 15 bits.
456Combine on 22
unequal bursts
TCH/F4.8 6 120 32 1/3 456 Ditto
TCH/F2.4 3.6 72 4 1/6 456 On eight 1/2 bursts
TCH/H2.4 3.6 144 8 1/3 456Combine on 22
unequal bursts
SCH 25
Parity
check, 10 4 1/2 78
Combine on one SB
burst
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5 Frame Structure and Radio Channels
Channel Type
Input
Rate
(Kbit/s)
Input Code
Block bits
Code Output
Code
Block bits
Interleaving DepthCheck Bit Tail Bit
Convolutional
Code Rate
RACH 8Parity
check, 64 1/2 36
Combine on one AB
burst
FACCH 184Packet
coding, 404 1/2 456 On eight 1/2 bursts
SACCH
BCCH
SDCCH
AGCH
PCH
184Packet
coding, 404 1/2 456 On four whole bursts
The voice input rate on TCH/FS is 13 Kbit/s, that is, each speech frame lasts 20 ms and
contains 260 bits. According to the interference of different bits on voice, the 260 bits
are divided into I category (182 bits in total) and II category (78 bits in total). The I
category is further divided into Ia and Ib. The Ia bits are very important bits. If any of
them is incorrect, the subscriber will hear a loud noise in 20 ms voice interval. There
are 50 Ia bits and 132 Ib bits. That is, the 260 bits in a speech frame (20 ms) is { d (0),
d (1),…, d (181), d (182), …, d (259)}. The part with a single line is I category, and
that with a double-line is II category. It is similar to the TCH/HS.
Table 3.4 -2 gives the coding and interleaving adopted in different types of
transmission. The first column lists the channels and the related transmission mode.
The Input Code Block column gives the size of the data block (bits) before channel
coding. The Output Code Block column gives the size of the data block (bits) after
channel coding. In Code, the parameters are listed in the same sequence as the coding
sequence. The tail bit is "0". The decoding is in the reverse order.
Following is description of channel coding and interweaving, taking voice
communication for example.
In the GSM, the voice input rate on TCH/FS is 13kb/s, that is, 260 bits are transmitted
every 20ms. The 260 bits are protected by means of segmented coding.
Among the 260 bits, 182 bits adopts 1/2 convolutional coding, and the remaining 78
bits are not protected. Among the 182 bits, 50 bits are performed with parity check and
then with 1/2 convolutional coding. Three information bits are added. Those 50 bits are
called Ia bits. The other 132 bits, called Ib bits, are performed with 1/2 convolutional
coding directly.
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GSM Basic Principle
Fig 3.4 -9 shows the interleaving algorithm of voice signals on TCH/F. After channel
coding, 456 bits are carried in every 20ms. Those bits are divided into eight groups,
with the 57 bits in each group carried in different burst pulses (eight BPs in total). To
maximize irrelevancy between the bit sequences, the bits should be arranged as
described in Table 3.4 -3.
0 1 2 3 4 5 6 78 9 10 11 12 13 14 15
. . . . . . . .
. . . . . . . .
. . . . . . . .
1 2 3 4 5 6 7 8
456bits
0 1 2 3 4 5 6 78 9 10 11 12 13 14 15
. . . . . . . .
. . . . . . . .
. . . . . . . .
456bits
0 1 2 3 4 5 6 78 9 10 11 12 13 14 15
. . . . . . . .
. . . . . . . .
. . . . . . . .
456bits
0 1 2 3 4 5 6 78 9 10 11 12 13 14 15
. . . . . . . .
. . . . . . . .
. . . . . . . .
456bits
57 1 57 1 57 1 57 1 57 1 57 1 57 1 57 1
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
A block
Odd bits
B block
Even bits116 bit 116 bit 116 bit
Fig 3.4-9 Interleaving of Cells
Table 3.4-3 Full-rate speech interleaving algorithm
No. Item Note
1 0, 8, …, 448 Even bits (B block) in BP (N)
2 1, 9, …, 449 Even bits (B block) of BP (N 1)
3 2, 10, …, 450 Even bits (B block) of BP (N 2)
4 3, 11, …, 451 Even bits (B block) of BP (N + 3)
5 4, 12, …, 52 Odd bits (A block) v BP (N 4)
6 5, 13, …, 453 Odd bits (A block) v BP (N 5)
7 6, 14, …, 454 Odd bits (A block) v BP (N 6)
8 7, 15, …, 455 Odd bits (A block) v BP (N + 7)
456 bits are divided into eight groups (rows). Each group has 57 bits (columns),
occupying Block A or Block B of BP (N) to BP (N+7). After interleaving, a BP carries
114 bits of information plus 2 bits of stolen frame (116 bits in total). The 114 bits
contain 57 bits (odd bits) of information block A and 57 bits (even bits) of information
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5 Frame Structure and Radio Channels
block B. The remaining two bits indicate respectively whether the first half BP (odd
bit) and the last half BP (even bit) are subscriber data or fast channel associated
signaling.
3.5 Encryption/Decryption
There are encryption measures available in the GSM system. They are applicable to
voice, data and signaling. They are independent of the data type and work for normal
bursts only. Encryption is accomplished by exclusive or operation of an encryption
sequence (computed by A5 encryption algorithm via key Kc and frame number) and
114 information bits on a normal burst.
The original transmission data can be obtained by using the same sequence at the
receiving end to conduct exclusive-or operation with the encryption sequence.
3.6 Modulation and Demodulation
Modulation and demodulation are the last step in signal processing. Using GMSK
modulation mode at a rate of 270.833 k Baud, GSM usually conducts demodulation
with Viterbi algorithm (with a balanced demodulation method). Demodulation is the
reverse process of modulation.
GMSK is a special digital FM modulation mode. The modulation rate is 270.833
kilobauds. The Frequency Shift Keying (FSK) modulation with bit rate four times of
frequency offset is called MSK (Minimum Shift-frequency Keying). In GSM, the
Gaussian demodulation filter is used to further reduce the modulation spectrum. It can
cut the frequency conversion speed.
The GMSK can be expressed by a I/Q diagram. If there is no Gaussian filter, when a
series of constant 1s are sent, the MSK signal will be kept in the state that is higher than the center frequency 67.708 kHz of the carrier. If the center frequency of the
carrier serves as the fixed phase reference, the signal 67.708 kHz will cause steady
increment of phase. The phase rotates 360° at 67,708 times per second. In a bit period
(1/270.833 kHz), the phase moves 1/4 a circle in the I/G diagram, that is, 90°. The data
1 can be looked as 90° plus the phase. Two 1s makes a phase increment by 180°, three
1s makes a increment by 270°, and so on. The data 0 indicates the same phase change
in the reverse direction.
The actual phase track is strictly controlled. In the GSM, digital filter and 1/Q or digital
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GSM Basic Principle
FM modulator are used to generate correct phase track accurately. The Root Mean
Square (RMS) between the actual track and the ideal track allowed by GSM
specifications cannot exceed 5°, and the peak deviation cannot exceed 20°.
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4 GSM Key Technologies
4.1 Diversity Reception
The diversity reception technology is usually introduced to the GSM system to receive
on several tributaries the signals with little relativity but carrying the same information
and then output the signals after they are combined. In this way, the impact of fading
on the stability of receiving signals can be played down.
There are ways of diversity as follows: space diversity, frequency diversity, time
diversity and polarization diversity.
1. Space Diversity
Two receiving antennas are set in the space to receive independently the same
signals before combining and outputting them. In this way, the degree of fading
can be dramatically lowered. This is the so-called space diversity. The space
diversity is based on the fact that the field strength varies randomly with the
space. The longer the distance, the more variant the multi-path transmission will be, and the less relevant the receiving filed strength will be. The relevancy refers
to the similarity between the signals. Therefore, the necessary distance must be
determined. According to the test and statistics, CCIR suggests the spacing
between two antennas should be larger than 0.6 wavelength, namely d>0.6λ, to
achieve a satisfactory diversity result and that it should be better to near the odd
number multiplication of λ/4. Even if the distance between antennas is shortened
to be λ/4, good diversity effect can be achieved.
2. Time Diversity
Time diversity is to send the same message with some delays or part of the
message in different time within the delay range tolerable by the system. In the
GSM system, time diversity is achieved by the interlacing technology. In the
GSM, interleaving technology is adopted to implement the time diversity.
3. Frequency Diversity
Frequency diversity means more than two frequencies send a signal
concurrently. The receiving end combines the signals of different frequencies
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and reduces or eliminates the impact of fading with different paths of the
wireless carrier waves of varied frequencies. The frequency diversity is effective
and requires one set of antenna only. Frequency diversity in GSM is
implemented by frequency hopping technology.
4. Polarization Diversity
Polarization diversity is to receive signals by making two pairs of receiving
antennas with polarization direction into some angles against each other, which
can generate a good diversity result. The two sets of polarized antennae in
polarity diversity can be integrated in one set of antenna. Thus, only one
receiving antenna and one transmitter antenna are required in a cell. If duplexer is adopted, only one transceiving antenna is required. It saves antennas greatly.
4.2 Discontinuous Transmission
There are two voice transmission modes. One is continuous voice coding (one speech
frame every 20ms) no matter whether the subscriber is talking or not. Another is
discontinuous transmission (DTX) with 13kb/s coding in voice activation period and
500b/s coding in non voice activation period. In addition, a comfort noise frame (20ms
per frame) is transmitted every 480ms, as shown in Fig 4.2 -10.
There are two purposes of employing the DTX mode: one is to lower the general
interference level in the air; the second is to save the power of transmitters. However,
the DTX may slightly lower the transmission quality. Therefore, the DTX mode and
common mode are optional.
TRAU BTS
BTS MS
Comfort
noise frame
Speech frame
480 ms
Fig 4.2-10 Speech Frame Transmission in DTX Mode
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5 Frame Structure and Radio Channels
4.3 Power Control
Power control means to control the actual transmitting power (keep it as low as
possible) of MS or BS in radio propagation, so as to reduce the power consumption of
MS/BS and the interference of the entire GSM network. Needless to say, the
prerequisite of power control is to ensure the good communication quality of the
ongoing calls. The power control process is simply illustrated in Fig 4.3 -11.
A B
Fig 4.3-11 Power Control
As shown in Fig. 1.5-16, the MS at point A is far from the BS antenna. Because the
propagation loss of electric wave in air is in direct proportion to n power of the
distance, the MS at A needs higher transmit power to ensure good communication
quality. Comparatively, point B is closer to the BS transmission antenna, hence smaller
transmission loss; therefore, to obtain similar communication quality, a mobile phone
at point B can use lower transmission power during communication. When a mobile
phone in communication is moving from point A towards point B, the power control
can reduce its transmitting power gradually. On the contrary, if it is moving from point
B towards point A, the power control can increase its transmitting power gradually.
The power control is classified as uplink power control and downlink power control,
they function separately. By uplink power control, it means to control the MS
transmitting power, while downlink power control means to control the BS transmitting
power. No matter uplink power control or downlink power control, the uplink or
downlink interference is suppressed as the transmit power is reduced. Meanwhile the
power consumption of the MS or base station is reduced. The most obvious benefits are
the average conversation quality of the whole GSM network is greatly increased, and
the MS standby time is prolonged.
1. Power control process
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GSM Basic Principle
The original information used for decision making during a power control
process is obtained from the measurement data of the MS and BS and
corresponding control decision can be made after processing and analyzing of
the original data. Similar to the handover control process, the whole power
control process is shown in Fig 4.3 -12.
Measurement data saving
Average measurement data
processing
Power control decision
making
Power control command
sending
Measurement data correction
Fig 4.3-12 Power Control Process
1) Measurement data saving
The measurement data related to power control includes uplink signal level,
uplink signal quality, downlink signal level, and downlink signal quality.
2) Average measurement data processing
To reduce the influence of complex radio transmission on the measurement
values, the smooth processing of the measurement data usually adopts the
forward averaging method. That is, the average value of multiple measurement
values is used to make a power control decision. The parameter setting in
averaging calculation may vary with the types of the measurement data, i.e.,
quantity of the measurement data to be used may be different.
3) Power control decision making
In the decision making of power control, there are three parameters: a threshold,
an N value, and a P value. Among the latest N average values, if there are P
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5 Frame Structure and Radio Channels
parameters exceed the threshold, the signal level is too high or the signal quality
is good; if there are P parameters are lower than the threshold, the signal level is
too low or the signal quality is poor.
According to the condition of the signal level or quality, the mobile phone or BS
can judge how to control the transmitting power, and the increase or decrease
amplitudes are determined by the pre-configured values.
4) Power control command sending
According to the power control decision, the corresponding control command is
sent to the BS, which will then execute the command or transfer it to MS.
5) Measurement data correction
After power control, the original measurement data and average values are
useless. If the useless information is still kept, it may cause incorrect power
control decision. Therefore, it is necessary to discard the outdated data or update
it for later use.
The fastest power control can be performed once every 480 ms, which is the
highest speed that the measurement data is reported. In other words, an entire
power control process is executed once in at least 480ms.
2. High-speed power control
The control extent of the power control process recommended by ETSI is fixed
as 2dB or 4dB normally. However, in most practical cases the fixed power
control extent is unable to achieve optimal effects, for a simple example:
When an MS initiates a call at a location very near to the BS antenna, its start
transmitting power is the max. transmitting power of the MS in the system
message broadcast in the cell BCCH (MS_TXPWR_MAX_CCH). It’s obvious
that at this time as the MS is quite close to the MS antenna, the power control
process is supposed to reduce its transmitting power as fast as possible.
However, it can hardly be achieved by the power control process recommended
by the ETSI specifications, because only 2dB or 4dB is decreased each time. In
addition, there is an interval between every two power control processes
(because enough new measurement data need be collected). Therefore, it takes a
long time to reduce the transmit power of the MS to a proper value. It is the
same in the downlink direction. Obviously this is disadvantageous in terms of
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GSM Basic Principle
reducing interference to the whole GSM network. To improve this, the power
control extent each time should be increased, which is the core idea of the high-
speed power control.
The high-speed power control can, according to the actual signal strength and
quality, work out the power control extent to be realized, without the limitation
of the fixed extent, thus solving the power control problem without much effort
when the MS makes the initial access. Of course its functions are not limited to
this situation. It can work in many cases e.g. fast moving mobile phones,
sudden interference or obstacles. Whenever large extent power control is
required, the high-speed power control process is the ideal solution.
4.4 Timing Advance
In the GSM, because TDMA is adopted in the air interface, the MS must employ the
TSs allocated to it only, and remain inactive in other time. Otherwise, it may affect the
MSs using other TSs on the same carrier.
In the GSM, the MS requires three intervals between timeslots when receiving or
transmitting signals. See Fig 4.4 -13.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1Offset
Downlink:
Uplink:
Sent by the BTS Sent by the MS
TDMA frame number
TDMA frame number
Fig 4.4-13 Uplink and Downlink Offset of TCH
Suppose an MS occupies TS2 and moves away from the base station, the messages sent
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GSM Basic Principle
sequence. Finally, the signals are sent via the RF filter to the antenna for transmission.
The receiver determines the receiving frequency according to FH synchronization
signals and FH sequence, receives corresponding signals after FH for demodulation.
The basic structure of FH is illustrated in Fig 4.5 -14.
Synchronization
circuit
Frequency
modulation
sequence
generator
Variable
frequency
synthesizer
Message
modulation
Up
converter
Send
Message
demodulationDown converter
Receive
Fig 4.5-14 Basic Structure of FH
Features of frequency hopping technology: The frequency hopping technology can be
employed to increase the working band of the system so as to enhance the anti-
jamming and anti-jamming capability of the communication system. Frequency
hopping can help improve and protect the pulse of the effective information part from
the impact of Rayleigh fading in the communication environment. After frequency
hopping is done, the original data are recovered by means of channel decoding. The
times of frequency hopping are increased to boost frequency hopping gains so as to
enhance the anti-jamming and anti-fading capability of the system.
The frequency hopping technology is actually to avoid external interferences so that
they cannot follow the changes of frequencies, thus avoiding or markedly lowering
same-channel interference and frequency selective fading. The reason to increase the
number of hoppings is that the gain of frequency hopping system is equal to the ratio of
frequency hopping system bandwidth to N minimum frequency hopping intervals.
Usually, the FH number should be greater than three. If frequency diversity is also
available for the FH system, and a message is transmitted by several groups of
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5 Frame Structure and Radio Channels
frequency hopping simultaneously and then judged by the law of large numbers, more
subscribers can use services at the same time with least mutual interference.
The frequency hopping comprises baseband hopping and RF hopping.
The baseband hopping enables the transmit and receive frequencies of each
carrier unit to remain unchanged. At different frame number (FN) moment, the
frame unit sends data to different carrier units.
RF FH is to control the frequency synthesizer of each transceiver, making it hop
according to different schemes in different timeslots.
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5 Frame Structure and Radio Channels
GSM air interface uses TDMA based frame structure. Communication services are
obtained by transmission of information using logical channels on physical channels.
Mapping between the logical channel and physical channel is the process that arranges
the information to be sent to the suitable TDMA frames and timeslots.
5.1 Radio Frame Structure
Five levels of GSM radio frame structure are timeslot, TDMA frame, multiframe,
superframe and hyperframe.
Timeslot is the basic unit of a physical channel.
TDMA frame consists of eight timeslots. It is a basic unit occupying carrier
bandwidth. Each carrier has eight timeslots.
There are two types of multiframes:
One type of multiframe consists of 26 TDMA frames. This type of multiframe is
used in TCH, SACCH, and FACCH.
The other type of multiframe consists of 51 TDMA frames. This type of
multiframe is used in BCCH, CCCH, and SDCCH.
The superframe is a consecutive 51 x 26 TDMA frame. It consists of 51 26-
multiframes or 26 51-multiframes.
The hyperframe consists of 2,048 superframes.
Fig 5.1 -15 shows GSM frame structure.
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0
TDMA frame
00 01
1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
02
0 1 2 3 4 5 6 7
0 1 2 3 4 22232425
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1 2 3 4 4748 4950
0 1 2 3 47 48 49 50
0 1 2524
2042 2043 204420452046 20476543210
1 26-multiframe = 26 TDMA frames (120 ms) 1 51-multiframe = 51 TDMA frames (3036/13 ms)
1 superframe = 1326 TDMA frames (6.12s)= 51 26-multiframe or 26 51-multiframes
1 hyperframe = 2048 superframes = 2715648 TDMA frames
Fig 5.1-15 GSM Frame Structure
5.2 Physical Channel
GSM adopts mixed technology of Frequency Division Multiple Access (FDMA) and
Time Division Multiple Access (TDMA). GSM features high frequency utilization.
FDMA - enables 124 carrier frequencies (carriers for short) to be assigned to the
uplink (from the MS to the BTS) 890 MHz – 915 MHz or downlink (from the BTS to
the MS) 935 MHz – 960 MHz in GSM900 band. Interval between carriers is 200 kHz.
Carriers in the uplink and downlink are in pairs called duplex communication mode.
Interval between duplex receiving and transmitting carrier pair is 45 MHz.
TDMA - enables each carrier of GSM900 band to be divided into eight time segments.
Each time segment is called a timeslot. See Fig 5.2 -16.
This type of timeslot is called a channel or a physical channel. Eight consecutive
timeslots on a carrier constitute a TDMA frame, that is, a carrier of GSM provideseight physical channels.
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5 Frame Structure and Radio Channels
16/25 ms
200 kHz
Timeslot
Time
Frequency
Fig 5.2-16 Time-Frequency Structure of Physical Channel
Eight timeslots in TDMA frame are called physical channels.
5.3 Logical Channels
Each physical channel is time multiplexed with different logical channels. Logical
channels carry various signaling or traffic information based on user and network
requirements. To provide signaling traffic control, logical channels map on physical
channels.
Logical channels are classified into Common Channel and Dedicated Channel.
EnhancedFull-ratechannel
LogicalChannels
Fast AssociatedControl Channel
(FACCH)
Slow AssociatedControl Channel
(SACCH)
Stand-aloneDedicated ControlChannel (SDCCH)
FrequencyCorrection
Channel (FCCH)
Common ControlChannel (CCCH)
DedicatedChannel
BroadcastChannel (BCH)
DedicatedControl Channel
(DCCH)
Traffic Channel(TCH)
Broadcast ControlChannel (BCCH)
Paging Channel(PCH)
Random AccessChannel (RACH)
Access GrantChannel (AGCH)
CommonChannel
Half-ratechannel(TCH/H)
Full-rateChannel(TCH/F)
Synchroniza-tion Channel
(SCH)
Fig 5.3-17 GSM Logical Channels
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5.3.1 Common Channel
Common Channel is classified in two main types:
Broadcast Channel (BCH): BCH transmits broadcast messages from base station to
MS. Broadcast Channel is unidirectional channel from base station to MS. It is of three
types:
Frequency Correction Channel (FCCH): It carries the information used to correct
the MS frequency. MS receives frequency correction information through FCCH and
corrects its time base frequency.
Synchronization Channel (SCH): It carries frame synchronization (TDMA frame
number) information and Base Station Identity Code (BSIC) to MS.
Broadcast Control Channel (BCCH): It broadcasts general information of BTS. For
example, broadcasts the local cell and neighboring cell information, and
synchronization (time and frequency) information. MS listens to BCCH periodically to
obtain the information transmitted on it, such as the Local Area Identity, List of
Neighboring Cell, frequency table used in local cell, cell identity, power control
indication, intermittent transmission permission, access control, and CBCH
description. BCCH carrier is transmitted by base station at a fixed power, and its signal
strength is measured by all MSs.
Common Control Channel (CCCH): CCCH is point-to-multipoint bi-directional
channel. It carries signals required to set up a connection between base station and MS.
It is of three types:
Paging Channel (PCH): It broadcasts paging messages from base station to MS. It is a
downlink channel.
Random Access Channel (RACH): MS sends information to base station through this
channel when accessing the network at random. The information sent includes response
to the paging message of base station and access of mobile-originated call. MS also
applies for a Stand-alone Dedicated Control Channel (SDCCH) from base station
through this channel. RACH is an uplink channel.
Access Grant Channel: The base station sends the assigned SDCCH to the MS that
accesses the network successfully through this channel. The AGCH is a downlink
channel.
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5 Frame Structure and Radio Channels
5.3.2 Dedicated Channel
Dedicated channel is a traffic channel which carries voice and data. Some types of
dedicated channel are used for the control purpose.
Dedicated Channel is classified in two main types:
Dedicated Control Channel (DCCH): DCCH is a point-to-point bi-directional
channel between base station and MS. It is of three types:
Stand-alone Dedicated Control Channel (SDCCH): It carries signaling and
channel information between base station and MS, such as the authentication
and registration signaling messages. During the establishment of a call, SDCCH
supports bi-directional data transmission and short messages transfer.
Slow Associated Control Channel (SACCH): Through this channel, base
station sends power control message and frame adjustment message to MS, and
receives signal strength report and link quality report from MS.
Fast Associated Control Channel (FACCH): It carries inter-cell handover
signaling messages between base station and MS.
Traffic Channel (TCH): TCH carries voice and data. According to switching mode,
TCH can be divided into circuit-switched channel and data-switched channel.
According to transmission rate, TCH can be divided into full-rate channels and half-
rate channels.
Rate of the GSM full-rate channel is 13 kbps, and that of the GSM half-rate channel is
6.5 kbps. In addition, the enhanced full-rate channel has same rate as the full-rate
channel, which is 13 kbps. However, it has better compressed coding scheme than full-
rate channel. That is why enhanced full-rate channel provides better voice quality.
5.3.3 Channel Combination
In actual application, different types of logical channels are mapped on the same
physical channel. This is called channel combination.
Following are nine GSM channel combinations:
Full-rate traffic channel (TCHFull): TCH/F + FACCH/F + SACCH/TF
Half-rate traffic channel (TCHHalf): TCH/H (0, 1) + FACCH/H(0, 1) +
SACCH/TH (0, 1)
Half-rate1 traffic channel (TCHHalf2): TCH/H (0, 0) + FACCH/H (0, 1)
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GSM Basic Principle
+SACCH/TH (0, 1) + TCH/H (1, 1)
SDCCH: SDCCH/8 (0,…
, 7) + SACCH/C8 (0,…
, 7)
Main broadcast control channel (MainBCCH): FCCH + SCH + BCCH + CCCH
Combined broadcast control channel (BCCHCombined): FCCH + SCH +
BCCH + CCCH + SDCCH/4 (0,…,3) + SACCH/C4 (0,…, 3)
Broadcast channel (BCH): FCCH + SCH + BCCH
Cell broadcast channel (BCCHwithCBCH): FCCH + SCH + BCCH + CCCH +
SDCCH/4 (0,…, 3) + SACCH/C4 (0,…, 3) + CBCH
Slow dedicated control channel (SDCCHwithCBCH): SDCCH + SACCH +
CBCH
Among the above channel combinations, CCCH = PCH + RACH + AGCH. As a
downlink channel, only CBCH carries cell broadcast information and shares the
physical channel with SDCCH.
Each cell broadcasts FCCH and SCH. The basic combination in the downlink direction
includes FCCH, SCH, BCCH and CCCH (PCH + AGCH). It is allocated to TN0 of
BCCH carrier configured for a cell, as shown in Fig 5.3 -18.
SF B C
R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R
51 frames
SF C C SF C C SF C C I
R R R R R R R R R R
D0 D1 D2D
3D4 D5 D6 D7 A0 A1 A2 A3
SF C C
R R R R R R R R R R
III
D0 D1 D2D
3D4 D5 D6 D7 A4 A5 A6 A7 III
A1 A2 A3 III
A5 A6 A7 III
D0 D1 D2 D3D
4D5 D6 D7 A0
D0 D1 D2 D3 D4 D5 D6 D7 A4
SF B C SF C C SF D0
D1
SF D2
D3
ISF A0
A1
SF B C SF C C SFD
0
D
1SF
D
2
D
3ISF
A
2
A
3
D3
D3
R R
R R
A2 A3
A0 A1
D
2D
2
SF
SF
D
0
D
1D
0
D
1
R R R R R R R R R R R R R R R R R R R R R R R
R R R R R R R R R R R R R R R R R R R R R R R
F: FCCH S: SCH
B: BCCH C: CCCH (CCCH=PCH+AGCH+RACH)
R: RACH D: SDCCH
A: SACCH/C I: Idle
BCCH+CCC
HDownlink
BCCH+CCC
HUplink
8 SDCCH/8
Downlink
8 SDCCH/8Uplink
BCCH+CCC
H+4SDCCH/4Downlink
BCCH+CCC
H+4SDCCH/4
Uplink
(a) FCCH+SCH+BCCH+CCCH
(b) SDCCH/8(0,...,7)+SACCH/C8(0,...,7)
(c) FCCH+SCH+CCCH+SDCCH/4(0,...,3)+SACCH/C4(0,...,3)
Fig 5.3-18 Frame Channel Structure
For half-rate voice channel combination, each timeslot has two half-rate sub-channels
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5 Frame Structure and Radio Channels
and corresponding SACCH, with 26 TDMA frames as a multiframe.
Fig 5.3 -19 shows the frame structure.
H
0
H
0
S
1
S
0
H
1
H
0
H
0
H
0
H
0
H
0
H
0
H
0
H
0
H
0
H
0
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
26 frames
Fig 5.3-19 Half-Rate Voice Channel Frame Structure
5.4 Mapping between Logical and Physical Channels
Logical channels in GSM are much more than the eight physical channels that a GSM
carrier can provide. If each logical channel is configured with a physical channel, the
eight physical channels provided by a carrier are not enough.
In such case, extra carriers must be added. However, the communication in this way is
not highly effective. The way to solve this problem is to multiplex the CCCH, that is,
multiplex the CCCH on one or two physical channels.
Mapping between physical channels and logical channels in GSM is as follows:
Base station has N carriers, and each carrier has eight timeslots. Define the carriers asf 0, f 1, f 2 … Downlink starts from timeslot 0 (TS0) of f 0. TS0 is used to map with control
channel only. f 0 is also called broadcast control channel (BCCH).
Fig 5.4 -20 shows BCCH and CCCH on TS0 multiplexing.
012 7012 701
FS B C FS C C FS C C FS C C FS C C I
TDMA
frame
BCCH+CCCH
Downlink
F (FCCH): MS synchronizes its frequency through it.S (SYCH): MS reads TDMA frame number and Base Station Identity Code (BSIC)
through it.
B (BCCH): MS reads the general inforamtion of the cell through it.I (IDLE): Idle frame, containing no information. It serves as the end flag of the
multi-frame.
Fig 5.4-20 Multiplexing of BCCH and CCCH on TSO
BCCH and CCCH occupy total 51 TS0s. Although only the TS0 of each frame is
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GSM Basic Principle
occupied, the total length is 51 TDMA frames in terms of time. Each time when an idle
frame appears, the multiframe ends. After that, a new multiframe starts from F and S.
Repeat like this, and TDMA multiframe is constructed.
When there is no paging or call connected, the base station always transmits on f 0. This
enables MS to detect the signal strength of the base station to determine the cell to be
used.
For the uplink, the TS0 on f 0 does not include the above channels. It is used for the MS
access only, that is, it is used as the RACH.
Fig 5.4 -21 shows the TS0 of 51 consecutive TDMA frames.
012 7012 701
RR
TDMA
frame
RACH
UplinkRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR
Fig 5.4-21 Multiplexing of RACH on TSO
BCCH, FCCH, SCH, PCH, AGCH, and RACH are all mapped on TS0. RACH is
mapped on uplink, and the rest are mapped on downlink.
TS1 on downlink f 0 is used to map DCCH to physical channel.
Fig 5.4 -22 shows the mapping relationship.
012 7012 701
D0 I
TDMAframe
SDCCH+SA CCCH
Downlink
D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3 I I
D0 ID1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7 I I
Fig 5.4-22 Multiplexing of SDCCH and SACCH on TS1 (Downlink)
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5 Frame Structure and Radio Channels
Since the bit rate in call setup and registration is quite low, eight dedicated control
channels can be placed on one timeslot to improve the multiplexing ratio of the
timeslot.
SDCCH and SACCH have 102 timeslots in total, that is, 102 time division
multiplexing (TDM) frames.
DX (D0, D1 …) of SDCCH is used in the early time when a call is set up. When the
MS transfers to the TCH, and the subscriber starts the conversation or the release is
triggered after registration, the DX is used by other MSs.
AX (A0, A1 …) of the SACCH transfers unimportant control information, such as
radio measurement data, that is TS0 of 51 consecutive TDMA frames.
TS1 on the uplink f 0 has the same structure with the TS1 on the downlink f 0. They have
an offset in time, which means bi-directional connection can be performed at the same
time for an MS.
Fig 5.4 -23 shows the multiplexing of the SDCCH and SACCH on TS1 of the uplink
f 0.
012 7012 701
A5
TDMAframe
SDCCH+ SACCCH
Uplink
A6 A7 D0 D1 D2 D3 D4 D5 D6 D7
A1 A2 A3 D0 D1 D2 D3 D4 D5 D6 D7
I I I
I I I
A0
A4
DX: same as uplink AX: Same as downlink
Fig 5.4-23 Multiplexing of SDCCH and SACCH on TS1 (Uplink)
Uplink and downlink TS0 and TS1 on f 0 are used by the logical control channel, while
other six physical channels (TS2 to TS7) are used by TCH.
Fig 5.4 -24 shows the mapping from TCH to physical channel.
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GSM Basic Principle
T=TCH A=SACCH I=Idle
Fig 5.4-24 TCH Multiplexing
Fig 5.4 -24 shows TS2 time division multiplexing.
TCH carries voice or data. SACCH carries control commands such as the command to
change the output power.
Idle I does not contain any information but is used in measurement.
TDM is implemented on TS2 with 26 timeslots as a cycle.
The idle timeslot I serves as the beginning or end of the repeated sequence.
Uplink TCH is of the same structure with the downlink TCH. They only have a time
offset, which is three timeslots. That is, the TS2 of the uplink and that of the downlink
do not appear simultaneously, which means that the MS does not send or receive data
at the same time.
Fig 5.4 -25 shows the offset between the uplink and downlink of the TCH.
0
TDMA frame number
Uplink C0
00 01
From BTS to MS
From MS to BTS
Downlink C0
45MHz (GSM900)
95MHz (DCS1800)
Offset
1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0
TDMA frame number
00 01
1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Fig 5.4-25 Offset between Uplink and Downlink of the TCH
The conclusion is that on carrier f 0:
TS0: a logical control channel, with repeat cycle of 51 timeslots.
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5 Frame Structure and Radio Channels
TS1: a logical control channel, with repeat cycle of 102 timeslots.
TS2: a logical traffic channel, with repeat cycle of 26 timeslots.
TS3 to TS7: logical traffic channels, with repeat cycle of 26 timeslots.
The TS0 to TS7 of other f 0 – f N are all traffic channels.