GSM Basics and Overview

7/31/2019 GSM Basics and Overview

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GSM basics tutorial and overviewThe GSM system is the most widely used cellular technology in use in the world today. It has been

a particularly successful cellular phone technology for a variety of reasons including the ability to

roam worldwide with the certainty of being able to be able to operate on GSM networks in exactlythe same way - provided billing agreements are in place.

The letters GSM originally stood for the words Groupe Speciale Mobile, but as it became clear this

cellular technology was being used world wide the meaning of GSM was changed to GlobalSystem for Mobile Communications. Since this cellular technology was first deployed in 1991, the

use of GSM has grown steadily, and it is now the most widely cell phone system in the world.

GSM reached the 1 billion subscriber point in February 2004, and is now well over the 3 billionsubscriber mark and still steadily increasing.

GSM system overview

The GSM system was designed as a second generation (2G) cellular phone technology. One of thebasic aims was to provide a system that would enable greater capacity to be achieved than the

previous first generation analogue systems. GSM achieved this by using a digital TDMA (timedivision multiple access approach). By adopting this technique more users could be accommodated

within the available bandwidth. In addition to this, ciphering of the digitally encoded speech was

adopted to retain privacy. Using the earlier analogue cellular technologies it was possible foranyone with a scanner receiver to listen to calls and a number of famous personalities had been

"eavesdropped" with embarrassing consequences.

GSM services

Speech or voice calls are obviously the primary function for the GSM cellular system. To achieve

this the speech is digitally encoded and later decoded using a vocoder. A variety of vocoders areavailable for use, being aimed at different scenarios.

In addition to the voice services, GSM cellular technology supports a variety of other data services.

Although their performance is nowhere near the level of those provided by 3G, they arenevertheless still important and useful. A variety of data services are supported with user data rates

up to 9.6 kbps. Services including Group 3 facsimile, videotext and teletex can be supported.

One service that has grown enormously is the short message service. Developed as part of theGSM specification, it has also been incorporated into other cellular technologies. It can be thought

of as being similar to the paging service but is far more comprehensive allowing bi-directional

messaging, store and forward delivery, and it also allows alphanumeric messages of a reasonablelength. This service has become particularly popular, initially with the young as it provided a

simple, low fixed cost.

GSM basics

The GSM cellular technology had a number of design aims when the development started:


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It should offer good subjective speech quality It should have a low phone or terminal cost Terminals should be able to be handheld The system should support international roaming It should offer good spectral efficiency The system should offer ISDN compatibility

The resulting GSM cellular technology that was developed provided for all of these. The overall

system definition for GSM describes not only the air interface but also the network orinfrastructure technology. By adopting this approach it is possible to define the operation of the

whole network to enable international roaming as well as enabling network elements from different

manufacturers to operate alongside each other, although this last feature is not completely true,especially with older items.

GSM cellular technology uses 200 kHz RF channels. These are time division multiplexed to enable

up to eight users to access each carrier. In this way it is a TDMA / FDMA system.

The base transceiver stations (BTS) are organised into small groups, controlled by a base station

controller (BSC) which is typically co-located with one of the BTSs. The BSC with its associated

BTSs is termed the base station subsystem (BSS).

Further into the core network is the main switching area. This is known as the mobile switching

centre (MSC). Associated with it is the location registers, namely the home location register (HLR)

and the visitor location register (VLR) which track the location of mobiles and enable calls to berouted to them. Additionally there is the Authentication Centre (AuC), and the Equipment Identify

Register (EIR) that are used in authenticating the mobile before it is allowed onto the network and

for billing. The operation of these are explained in the following pages.

Last but not least is the mobile itself. Often termed the ME or mobile equipment, this is the item

that the end user sees. One important feature that was first implemented on GSM was the use of a

Subscriber Identity Module. This card carried with it the users identity and other information to

allow the user to upgrade a phone very easily, while retaining the same identity on the network. Itwas also used to store other information such as "phone book" and other items. This item alone has

allowed people to change phones very easily, and this has fuelled the phone manufacturingindustry and enabled new phones with additional features to be launched. This has allowed mobile

operators to increase their average revenue per user (ARPU) by ensuring that users are able to

access any new features that may be launched on the network requiring more sophisticated phones.

GSM system overview

The table below summarises the main points of the GSM system specification, showing some ofthe highlight features of technical interest.

Specification

Summary for GSM

Cellular System

Multiple access

technologyFDMA / TDMA


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Duplex technique FDD

Uplink frequency band

890 - 915 MHz

(basic 900 MHz band

only)

Downlink frequency band

933 -960 MHz

(basic 900 MHz band

only)

Channel spacing 200 kHz

Modulation GMSK

Speech codingVarious - original was

RPE-LTP/13

Speech channels per RFchannel

8

Channel data rate 270.833 kbps

Frame duration 4.615 ms

GSM HistoryToday the GSM cell or mobile phone system is the most popular in the world. GSM handsets arewidely available at good prices and the networks are robust and reliable. The GSM system is also

feature-rich with applications such as SMS text messaging, international roaming, SIM cards and

the like. It is also being enhanced with technologies including GPRS and EDGE. To achieve this

level of success has taken many years and is the result of both technical development andinternational cooperation. The GSM history can be seen to be a story of cooperation across Europe,

and one that nobody thought would lead to the success that GSM is today.

The first cell phone systems that were developed were analogue systems. Typically they used

frequency-modulated carriers for the voice channels and data was carried on a separate shared

control channel. When compared to the systems employed today these systems were comparatively

straightforward and as a result a vast number of systems appeared. Two of the major systems that

were in existence were the AMPS (Advanced Mobile Phone System) that was used in the USA andmany other countries and TACS (Total Access Communications System) that was used in the UK

as well as many other countries around the world.

Another system that was employed, and was in fact the first system to be commercially deployed

was the Nordic Mobile Telephone system (NMT). This was developed by a consortium of

companies in Scandinavia and proved that international cooperation was possible.


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The success of these systems proved to be their downfall. The use of all the systems installed

around the globe increased dramatically and the effects of the limited frequency allocations were

soon noticed. To overcome these a number of actions were taken. A system known as E-TACS orExtended-TACS was introduced giving the TACS system further channels. In the USA another

system known as Narrowband AMPS (NAMPS) was developed.

New approaches

Neither of these approaches proved to be the long-term solution as cellular technology needed tobe more efficient. With the experience gained from the NMT system, showing that it was possible

to develop a system across national boundaries, and with the political situation in Europe lending

itself to international cooperation it was decided to develop a new Pan-European System.

Furthermore it was realized that economies of scale would bring significant benefits. This was thebeginnings of the GSM system.

To achieve the basic definition of a new system a meeting was held in 1982 under the auspices ofthe Conference of European Posts and Telegraphs (CEPT). They formed a study group called the

Groupe Special Mobile ( GSM ) to study and develop a pan-European public land mobile system.

Several basic criteria that the new cellular technology would have to meet were set down for the

new GSM system to meet. These included: good subjective speech quality, low terminal andservice cost, support for international roaming, ability to support handheld terminals, support for

range of new services and facilities, spectral efficiency, and finally ISDN compatibility.

With the levels of under-capacity being projected for the analogue systems, this gave a real sense

of urgency to the GSM development. Although decisions about the exact nature of the cellular

technology were not taken at an early stage, all parties involved had been working toward a digital

system. This decision was finally made in February 1987. This gave a variety of advantages.Greater levels of spectral efficiency could be gained, and in addition to this the use of digital

circuitry would allow for higher levels of integration in the circuitry. This in turn would result incheaper handsets with more features. Nevertheless significant hurdles still needed to be overcome.For example, many of the methods for encoding the speech within a sufficiently narrow bandwidth

needed to be developed, and this posed a significant risk to the project. Nevertheless the GSM

system had been started.

GSM launch dates

Work continued and a launch date for the new GSM system of 1991 was set for an initial launch of

a service using the new cellular technology with limited coverage and capability to be followed bya complete roll out of the service in major European cities by 1993 and linking of the areas by

1995.

Meanwhile technical development was taking place. Initial trials had shown that time division

multiple access techniques offered the best performance with the technology that would be

available. This approach had the support of the major manufacturing companies which would

ensure that with them on board sufficient equipment both in terms of handsets, base stations andthe network infrastructure for GSM would be available.


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Further impetus was given to the GSM project when in 1989 the responsibility was passed to the

newly formed European Telecommunications Standards Institute (ETSI). Under the auspices of

ETSI the specification took place. It provided functional and interface descriptions for each of thefunctional entities defined in the system. The aim was to provide sufficient guidance for

manufacturers that equipment from different manufacturers would be interoperable, while not

stopping innovation. The result of the specification work was a set of documents extending to morethan 6000 pages. Nevertheless the resultant phone system provided a robust, feature-rich system.

The first roaming agreement was signed between Telecom Finland and Vodafone in the UK. Thus

the vision of a pan-European network was fast becoming a reality. However this took place beforeany networks went live.

The aim to launch GSM by 1991 proved to be a target that was too tough to meet. Terminals

started to become available in mid 1992 and the real launch took place in the latter part of thatyear. With such a new service many were sceptical as the analogue systems were still in

widespread use. Nevertheless by the end of 1993 GSM had attracted over a million subscribers and

there were 25 roaming agreements in place. The growth continued and the next million subscriberswere soon attracted.

Global GSM usage

Originally GSM had been planned as a European system. However the first indication that the

success of GSM was spreading further a field occurred when the Australian network provider,

Telstra signed the GSM Memorandum of Understanding.

Frequencies

Originally it had been intended that GSM would operate on frequencies in the 900 MHz cellular

band. In September 1993, the British operator Mercury One-to-One launched a network. TermedDCS 1800 it operated at frequencies in a new 1800 MHz band. By adopting new frequencies new

operators and further competition was introduced into the market apart from allowing additionalspectrum to be used and further increasing the overall capacity. This trend was followed in many

countries, and soon the term DCS 1800 was dropped in favour of calling it GSM as it was purely

the same cellular technology but operating on a different frequency band. In view of the higher

frequency used the distances the signals travelled was slightly shorter but this was compensated forby additional base stations.

In the USA as well a portion of spectrum at 1900 MHz was allocated for cellular usage in 1994.

The licensing body, the FCC, did not legislate which technology should be used, and accordinglythis enabled GSM to gain a foothold in the US market. This system was known as PCS 1900

(Personal Communication System).

GSM success

With GSM being used in many countries outside Europe this reflected the true nature of the namewhich had been changed from Groupe Special Mobile to Global System for Mobile

communications. The number of subscribers grew rapidly and by the beginning of 2004 the total


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number of GSM subscribers reached 1 billion. Attaining this figure was celebrated at the Cannes

3GSM conference held that year. Figures continued to rise, reaching and then well exceeding the 3

billion mark. In this way the history of GSM has shown it to be a great success.

GSM Network ArchitectureThe GSM technical specifications define the different elements within the GSM network

architecture. It defines the different elements and the ways in which they interact to enable the

overall network operation to be maintained.

The GSM network architecture is now well established and with the other later cellular systems

now established and other new ones being deployed, the basic GSM network architecture has been

updated to interface to the network elements required by these systems. Despite the developmentsof the newer systems, the basic GSM network architecture has been maintained, and the elements

described below perform the same functions as they did when the original GSM system was

launched in the early 1990s.

GSM network architecture elements

The GSM network architecture as defined in the GSM specifications can be grouped into four

main areas:

Mobile station (MS) Base-station subsystem (BSS) Network and Switching Subsystem (NSS) Operation and Support Subsystem (OSS)


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Simplified GSM Network Architecture

Mobile station

Mobile stations (MS), mobile equipment (ME) or as they are most widely known, cell or mobile

phones are the section of a GSM cellular network that the user sees and operates. In recent yearstheir size has fallen dramatically while the level of functionality has greatly increased. A further

advantage is that the time between charges has significantly increased.

There are a number of elements to the cell phone, although the two main elements are the mainhardware and the SIM.

The hardware itself contains the main elements of the mobile phone including the display, case,

battery, and the electronics used to generate the signal, and process the data receiver and to betransmitted. It also contains a number known as the International Mobile Equipment Identity

(IMEI). This is installed in the phone at manufacture and "cannot" be changed. It is accessed by the

network during registration to check whether the equipment has been reported as stolen.

The SIM or Subscriber Identity Module contains the information that provides the identity of the

user to the network. It contains are variety of information including a number known as the

International Mobile Subscriber Identity (IMSI).

Base Station Subsystem (BSS)

The Base Station Subsystem (BSS) section of the GSM network architecture that is fundamentallyassociated with communicating with the mobiles on the network. It consists of two elements:

Base Transceiver Station (BTS): The BTS used in a GSM networkcomprises the radio transmitter receivers, and their associated antennas that

transmit and receive to directly communicate with the mobiles. The BTS is thedefining element for each cell. The BTS communicates with the mobiles andthe interface between the two is known as the Um interface with its associatedprotocols.

Base Station Controller (BSC): The BSC forms the next stage back into theGSM network. It controls a group of BTSs, and is often co-located with one ofthe BTSs in its group. It manages the radio resources and controls items suchas handover within the group of BTSs, allocates channels and the like. Itcommunicates with the BTSs over what is termed the Abis interface.

Network Switching Subsystem (NSS)

The GSM network subsystem contains a variety of different elements, and is often termed the core

network. It provides the main control and interfacing for the whole mobile network. The major

elements within the core network include:

Mobile Switching services Centre (MSC): The main element within thecore network area of the overall GSM network architecture is the Mobileswitching Services Centre (MSC). The MSC acts like a normal switching nodewithin a PSTN or ISDN, but also provides additional functionality to enable the


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requirements of a mobile user to be supported. These include registration,authentication, call location, inter-MSC handovers and call routing to a mobilesubscriber. It also provides an interface to the PSTN so that calls can be routedfrom the mobile network to a phone connected to a landline. Interfaces toother MSCs are provided to enable calls to be made to mobiles on differentnetworks.

Home Location Register (HLR): This database contains all theadministrative information about each subscriber along with their last knownlocation. In this way, the GSM network is able to route calls to the relevantbase station for the MS. When a user switches on their phone, the phoneregisters with the network and from this it is possible to determine which BTS itcommunicates with so that incoming calls can be routed appropriately. Evenwhen the phone is not active (but switched on) it re-registers periodically toensure that the network (HLR) is aware of its latest position. There is one HLRper network, although it may be distributed across various sub-centres to foroperational reasons.

Visitor Location Register (VLR): This contains selected information fromthe HLR that enables the selected services for the individual subscriber to be

provided. The VLR can be implemented as a separate entity, but it iscommonly realised as an integral part of the MSC, rather than a separateentity. In this way access is made faster and more convenient.

Equipment Identity Register (EIR): The EIR is the entity that decideswhether a given mobile equipment may be allowed onto the network. Eachmobile equipment has a number known as the International Mobile EquipmentIdentity. This number, as mentioned above, is installed in the equipment and ischecked by the network during registration. Dependent upon the informationheld in the EIR, the mobile may be allocated one of three states - allowed ontothe network, barred access, or monitored in case its problems.

Authentication Centre (AuC): The AuC is a protected database thatcontains the secret key also contained in the user's SIM card. It is used for

authentication and for ciphering on the radio channel. Gateway Mobile Switching Centre (GMSC): The GMSC is the point to

which a ME terminating call is initially routed, without any knowledge of theMS's location. The GMSC is thus in charge of obtaining the MSRN (MobileStation Roaming Number) from the HLR based on the MSISDN (Mobile StationISDN number, the "directory number" of a MS) and routing the call to thecorrect visited MSC. The "MSC" part of the term GMSC is misleading, since thegateway operation does not require any linking to an MSC.

SMS Gateway (SMS-G): The SMS-G or SMS gateway is the term that is usedto collectively describe the two Short Message Services Gateways defined inthe GSM standards. The two gateways handle messages directed in differentdirections. The SMS-GMSC (Short Message Service Gateway Mobile SwitchingCentre) is for short messages being sent to an ME. The SMS-IWMSC (ShortMessage Service Inter-Working Mobile Switching Centre) is used for shortmessages originated with a mobile on that network. The SMS-GMSC role issimilar to that of the GMSC, whereas the SMS-IWMSC provides a fixed accesspoint to the Short Message Service Centre.

Operation and Support Subsystem (OSS)


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The OSS or operation support subsystem is an element within the overall GSM network

architecture that is connected to components of the NSS and the BSC. It is used to control and

monitor the overall GSM network and it is also used to control the traffic load of the BSS. It mustbe noted that as the number of BS increases with the scaling of the subscriber population some of

the maintenance tasks are transferred to the BTS, allowing savings in the cost of ownership of the

system.

GSM Network InterfacesThe network structure is defined within the GSM standards. Additionally each interface between

the different elements of the GSM network is also defined. This facilitates the informationinterchanges can take place. It also enables to a large degree that network elements from different

manufacturers can be used. However as many of these interfaces were not fully defined until after

many networks had been deployed, the level of standardisation may not be quite as high as manypeople might like.

1. Um interface The "air" or radio interface standard that is used for exchanges between a

mobile (ME) and a base station (BTS / BSC). For signalling, a modified version of theISDN LAPD, known as LAPDm is used.

2. Abis interface This is a BSS internal interface linking the BSC and a BTS, and it has not

been totally standardised. The Abis interface allows control of the radio equipment andradio frequency allocation in the BTS.

3. A interface The A interface is used to provide communication between the BSS and the

MSC. The interface carries information to enable the channels, timeslots and the like to beallocated to the mobile equipments being serviced by the BSSs. The messaging required

within the network to enable handover etc to be undertaken is carried over the interface.

4. B interface The B interface exists between the MSC and the VLR . It uses a protocol

known as the MAP/B protocol. As most VLRs are collocated with an MSC, this makes theinterface purely an "internal" interface. The interface is used whenever the MSC needs

access to data regarding a MS located in its area.

5. C interface The C interface is located between the HLR and a GMSC or a SMS-G. Whena call originates from outside the network, i.e. from the PSTN or another mobile network it

ahs to pass through the gateway so that routing information required to complete the call

may be gained. The protocol used for communication is MAP/C, the letter "C" indicatingthat the protocol is used for the "C" interface. In addition to this, the MSC may optionally

forward billing information to the HLR after the call is completed and cleared down.

6. D interface The D interface is situated between the VLR and HLR. It uses the MAP/Dprotocol to exchange the data related to the location of the ME and to the management of

the subscriber.7. E interface The E interface provides communication between two MSCs. The E interface

exchanges data related to handover between the anchor and relay MSCs using the MAP/Eprotocol.

8. F interface The F interface is used between an MSC and EIR. It uses the MAP/F

protocol. The communications along this interface are used to confirm the status of theIMEI of the ME gaining access to the network.


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9. G interface The G interface interconnects two VLRs of different MSCs and uses the

MAP/G protocol to transfer subscriber information, during e.g. a location update

procedure.

10. H interface The H interface exists between the MSC the SMS-G. It transfers short

messages and uses the MAP/H protocol.

11. I interface The I interface can be found between the MSC and the ME. Messagesexchanged over the I interface are relayed transparently through the BSS.

Although the interfaces for the GSM cellular system may not be as rigorouly defined as many

might like, they do at least provide a large element of the definition required, enabling thefunctionality of GSM network entities to be defined sufficiently.

GSM Radio Air Interface, GSM Slot and

BurstOne of the key elements of the development of the GSM, Global System for MobileCommunications was the development of the GSM air interface. There were many requirementsthat were placed on the system, and many of these had a direct impact on the air interface.

Elements including the modulation, GSM slot structure, burst structure and the like were all

devised to provide the optimum performance.

During the development of the GSM standard very careful attention was paid to aspects including

the modulation format, the way in which the system is time division multiplexed, all had a

considerable impact on the performance of the system as a whole. For example, the modulationformat for the GSM air interface had a direct impact on battery life and the time division format

adopted enabled the cellphone handset costs to be considerably reduced as detailed later.

GSM signal and GMSK modulation characteristics

The core of any radio based system is the format of the radio signal itself. The carrier is modulated

using a form of phase sift keying known as Gaussian Minimum Shift Keying (GMSK). GMSK wasused for the GSM system for a variety of reasons:

It is resilient to noise when compared to many other forms of modulation. Radiation outside the accepted bandwidth is lower than other forms of phase

shift keying. It has a constant power level which allows higher efficiency RF power amplifiers

to be used in the handset, thereby reducing current consumption and

conserving battery life.

Note on GMSK:

GMSK, Gaussian Minimum Shift Keying is a form of phase modulation that is used in a number of

portable radio and wireless applications. It has advantages in terms of spectral efficiency as well ashaving an almost constant amplitude which allows for the use of more efficient transmitter power

amplifiers, thereby saving on current consumption, a critical issue for battery power equipment.


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Click on the link for aGMSK tutorial

The nominal bandwidth for the GSM signal using GMSK is 200 kHz, i.e. the channel bandwidth

and spacing is 200 kHz. As GMSK modulation has been used, the unwanted or spurious emissions

outside the nominal bandwidth are sufficiently low to enable adjacent channels to be used from thesame base station. Typically each base station will be allocated a number of carriers to enable it to

achieve the required capacity.

The data transported by the carrier serves up to eight different users under the basic system by

splitting the carrier into eight time slots. The basic carrier is able to support a data throughput of

approximately 270 kbps, but as some of this supports the management overhead, the data rate

allotted to each time slot is only 24.8 kbps. In addition to this error correction is required toovercome the problems of interference, fading and general data errors that may occur. This means

that the available data rate for transporting the digitally encoded speech is 13 kbps for the basic

vocoders.

GSM slot structure and multiple access scheme

GSM uses a combination of both TDMA and FDMA techniques. The FDMA element involves the

division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced

200 kHz apart as already described.

The carriers are then divided in time, using a TDMA scheme. This enables the different users of

the single radio frequency channel to be allocated different times slots. They are then able to use

the same RF channel without mutual interference. The slot is then the time that is allocated to theparticular user, and the GSM burst is the transmission that is made in this time.

Each GSM slot, and hence each GSM burst lasts for 0.577 mS (15/26 mS). Eight of these burst

periods are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms(i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physical

channel is one burst period allocated in each TDMA frame.

There are different types of frame that are transmitted to carry different data, and also the framesare organised into what are termed multiframes and superframes to provide overall

synchronisation.

GSM slot structure

These GSM slot is the smallest individual time period that is available to each mobile. It has a

defined format because a variety of different types of data are required to be transmitted.

Although there are shortened transmission bursts, the slots is normally used for transmitting 148

bits of information. This data can be used for carrying voice data, control and synchronisation data.
http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.phphttp://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.phphttp://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.phphttp://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.php


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GSM slots showing offset between transmit and receive

It can be seen from the GSM slot structure that the timing of the slots in the uplink and the

downlink are not simultaneous, and there is a time offset between the transmit and receive. Thisoffset in the GSM slot timing is deliberate and it means that a mobile that which is allocated the

same slot in both directions does not transmit and receive at the same time. This considerably

reduces the need for expensive filters to isolate the transmitter from the receiver. It also provides aspace saving.

GSM burst

The GSM burst, or transmission can fulfil a variety of functions. Some GSM bursts are used for

carrying data while others are used for control information. As a result of this a number of differenttypes of GSM burst are defined.

Normal burst uplink and downlink Synchronisation burst downlink Frequency correction burst downlink Random Access (Shortened Burst) uplink

GSM normal burst

This GSM burst is used for the standard communications between the basestation and the mobile,and typically transfers the digitised voice data.

The structure of the normal GSM burst is exactly defined and follows a common format. Itcontains data that provides a number of different functions:

1. 3 tail bits: These tail bits at the start of the GSM burst give time for thetransmitter to ramp up its power

2. 57 data bits: This block of data is used to carry information, and most oftencontains the digitised voice data although on occasions it may be replaced withsignalling information in the form of the Fast Associated Control CHannel(FACCH). The type of data is indicated by the flag that follows the data field


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3. 1 bit flag: This bit within the GSM burst indicates the type of data in theprevious field.

4. 26 bits training sequence: This training sequence is used as a timingreference and for equalisation. There is a total of eight different bit sequencesthat may be used, each 26 bits long. The same sequence is used in each GSMslot, but nearby base stations using the same radio frequency channels will use

different ones, and this enables the mobile to differentiate between the variouscells using the same frequency.

5. 1 bit flag Again this flag indicates the type of data in the data field.6. 57 data bits Again, this block of data within the GSM burst is used for

carrying data.7. 3 tail bits These final bits within the GSM burst are used to enable the

transmitter power to ramp down. They are often called final tail bits, or just tailbits.

8. 8.25 bits guard time At the end of the GSM burst there is a guard period.This is introduced to prevent transmitted bursts from different mobilesoverlapping. As a result of their differing distances from the base station.

GSM Normal Burst

GSM synchronisation burst

The purpose of this form of GSM burst is to provide synchronisation for the mobiles on the

network.

1. 3 tail bits: Again, these tail bits at the start of the GSM burst give time forthe transmitter to ramp up its power

2. 39 bits of information:3. 64 bits of a Long Training Sequence:4. 39 bits Information:5. 3 tail bits Again these are to enable the transmitter power to ramp down.6. 8.25 bits guard time: to act as a guard interval.

GSM Synchronisation Burst

GSM frequency correction burst

With the information in the burst all set to zeros, the burst essentially consists of a constant

frequency carrier with no phase alteration.


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1. 3 tail bits: Again, these tail bits at the start of the GSM burst give time forthe transmitter to ramp up its power.

2. 142 bits all set to zero:3. 3 tail bits Again these are to enable the transmitter power to ramp down.4. 8.25 bits guard time: to act as a guard interval.

GSM Frequency Correction Burst

GSM random access burst

This form of GSM burst used when accessing the network and it is shortened in terms of the data

carried, having a much longer guard period. This GSM burst structure is used to ensure that it fits

in the time slot regardless of any severe timing problems that may exist. Once the mobile hasaccessed the network and timing has been aligned, then there is no requirement for the long guard

period.

1. 7 tail bits: The increased number of tail bits is included to provide additionalmargin when accessing the network.

2. 41 training bits:3. 36 data bits:4. 3 tail bits Again these are to enable the transmitter power to ramp down.5. 69.25 bits guard time: The additional guard time, filling the remaining time

of the GSM burst provides for large timing differences.

GSM Random Access Burst

GSM discontinuous transmission (DTx)

A further power saving and interference reducing facility is the discontinuous transmission (DTx)

capability that is incorporated within the specification. It is particularly useful because there are

long pauses in speech, for example when the person using the mobile is listening, and during these

periods there is no need to transmit a signal. In fact it is found that a person speaks for less than40% of the time during normal telephone conversations. The most important element of DTx is the

Voice Activity Detector. It must correctly distinguish between voice and noise inputs, a task that is

not trivial. If a voice signal is misinterpreted as noise, the transmitter is turned off an effect knownas clipping results and this is particularly annoying to the person listening to the speech. However

if noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically

decreased.


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It is also necessary for the system to add background or comfort noise when the transmitter is

turned off because complete silence can be very disconcerting for the listener. Accordingly this is

added as appropriate. The noise is controlled by the SID (silence indication descriptor).

GSM Frame StructureThe GSM system has a defined GSM frame structure to enable the orderly passage of information.The GSM frame structure establishes schedules for the predetermined use of timeslots.

By establishing these schedules by the use of a frame structure, both the mobile and the base

station are able to communicate not only the voice data, but also signalling information without thevarious types of data becoming intermixed and both ends of the transmission knowing exactly

what types of information are being transmitted.

The GSM frame structure provides the basis for the various physical channels used within GSM,and accordingly it is at the heart of the overall system.

Basic GSM frame structure

The basic element in the GSM frame structure is the frame itself. This comprises the eight slots,

each used for different users within the TDMA system. As mentioned in another page of thetutorial, the slots for transmission and reception for a given mobile are offset in time so that the

mobile does not transmit and receive at the same time.

GSM frame consisting of eight slots

The basic GSM frame defines the structure upon which all the timing and structure of the GSM

messaging and signalling is based. The fundamental unit of time is called a burst period and it lasts

for approximately 0.577 ms (15/26 ms). Eight of these burst periods are grouped into what isknown as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the

basic unit for the definition of logical channels. One physical channel is one burst period allocated

in each TDMA frame.


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In simplified terms the base station transmits two types of channel, namely traffic and control.

Accordingly the channel structure is organised into two different types of frame, one for the traffic

on the main traffic carrier frequency, and the other for the control on the beacon frequency.

GSM multiframe

The GSM frames are grouped together to form multiframes and in this way it is possible toestablish a time schedule for their operation and the network can be synchronised.

There are several GSM multiframe structures:

Traffic multiframe: The Traffic Channel frames are organised intomultiframes consisting of 26 bursts and taking 120 ms. In a traffic multiframe,24 bursts are used for traffic. These are numbered 0 to 11 and 13 to 24. One ofthe remaining bursts is then used to accommodate the SACCH, the remainingframe remaining free. The actual position used alternates between position 12and 25.

Control multiframe: the Control Channel multiframe that comprises 51bursts and occupies 235.4 ms. This always occurs on the beacon frequency in

time slot zero and it may also occur within slots 2, 4 and 6 of the beaconfrequency as well. This multiframe is subdivided into logical channels which aretime-scheduled. These logical channels and functions include the following:

Frequency correction burst Synchronisation burst Broadcast channel (BCH) Paging and Access Grant Channel (PACCH) Stand Alone Dedicated Control Channel (SDCCH)

GSM Superframe

Multiframes are then constructed into superframes taking 6.12 seconds. These consist of 51 traffic

multiframes or 26 control multiframes. As the traffic multiframes are 26 bursts long and thecontrol multiframes are 51 bursts long, the different number of traffic and control multiframeswithin the superframe, brings them back into line again taking exactly the same interval.

GSM Hyperframe

Above this 2048 superframes (i.e. 2 to the power 11) are grouped to form one hyperframe which

repeats every 3 hours 28 minutes 53.76 seconds. It is the largest time interval within the GSM

frame structure.

Within the GSM hyperframe there is a counter and every time slot has a unique sequential number

comprising the frame number and time slot number. This is used to maintain synchronisation of the

different scheduled operations with the GSM frame structure. These include functions such as:

Frequency hopping: Frequency hopping is a feature that is optional withinthe GSM system. It can help reduce interference and fading issues, but for it towork, the transmitter and receiver must be synchronised so they hop to thesame frequencies at the same time.

Encryption: The encryption process is synchronised over the GSMhyperframe period where a counter is used and the encryption process willrepeat with each hyperframe. However, it is unlikely that the cellphone


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conversation will be over 3 hours and accordingly it is unlikely that security willbe compromised as a result.

GSM Frame Structure Summary

GSM Frequencies and Frequency BandsAlthough it is possible for the GSM cellular system to work on a variety of frequencies, the GSM

standard defines GSM frequency bands and frequencies for the different spectrum allocations that

are in use around the globe. For most applications the GSM frequency allocations fall into three or

four bands, and therefore it is possible for phones to be used for global roaming.

While the majority of GSM activity falls into just a few bands, for some specialist applications, or

in countries where spectrum allocation requirements mean that the standard bands cannot be used,different allocations may be required. Accordingly for most global roaming dual band, tri-band or

quad-band phones will operate in most countries, although in some instances phones using other

frequencies may be required.

GSM band allocations

There is a total of fourteen different recognised GSM frequency bands. These are defined in 3GPPTS 45.005.

Ban Uplink Downlin Comments


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d (MHz)k

(MHz)

380380.2 -

389.8

390.2 -

399.8

410410.2 -

419.8

420.2 -

429.8

450450.4 -

457.6

460.4 -

467.6

480478.8 -

486.0

488.8 -

496.0

710698.0 -

716.0

728.0 -

746.0

750747.0 -

762.0

777.0 -

792.0

810806.0 -

821.0

851.0 -

866.0

850824.0 -

849.0

869.0 -

894.0

900890.0 -

915.0

935.0 -

960.0

P-GSM, i.e. Primary or

standard GSM allocation

900880.0 -

915.0

925.0 -

960.0

E-GSM, i.e. Extended GSM

allocation

900876.0 -

915

921.0 -

960.0

R-GSM, i.e. Railway GSM

allocation

900870.4 -

876.0

915.4 -

921.0T-GSM

180

0

1710.0 -

1785.0

1805.0 -

1880.0

190

0

1850.0 -

1910.0

1930.0 -

1990.0

GSM frequency band usage


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The usage of the different frequency bands varies around the globe although there is a large degree

of standardisation. The GSM frequencies available depend upon the regulatory requirements for

the particular country and the ITU (International Telecommunications Union) region in which thecountry is located.

As a rough guide Europe tends to use the GSM 900 and 1800 bands as standard. These bands are

also generally used in the Middle East, Africa, Asia and Oceania.

For North America the USA uses both 850 and 1900 MHz bands, the actual band used is

determined by the regulatory authorities and is dependent upon the area. For Canada the 1900MHz band is the primary one used, particularly for urban areas with 850 MHz used as a backup in

rural areas.

For Central and South America, the GSM 850 and 1900 MHz frequency bands are the most widelyused although there are some areas where other frequencies are used.

GSM multiband phones

In order that cell phone users are able to take advantage of the roaming facilities offered by GSM,it is necessary that the cellphones are able to cover the bands of the countries which are visited.

Today most phones support operation on multiple bands and are known as multi-band phones.

Typically most standard phones are dual-band phones. For Europe, Middle east, Asia and Oceania

these would operate on GSM 900 and 1800 bands and for North America, etc dual band phoneswould operate on GSM 850 and 1900 frequency bands.

To provide better roaming coverage, tri-band and quad-band phones are also available. European

triband phones typically cover the GSM 900, 1800 and 1900 bands giving good coverage inEurope as well as moderate coverage in North America. Similarly North America tri-band phones

use the 900, 1800 and 1900 GSM frequencies. Quad band phones are also available covering the

850, 900, 1800 and 1900 MHz GSM frequency bands, i.e. the four major bands and therebyallowing global use.

GSM Power Control and Power ClassThe power levels and power control of GSM mobiles is of great importance because of the effect

of power on the battery life. Also to group mobiles into groups, GSM power class designations

have been allocated to indicate the power capability of various mobiles.

In addition to this the power of the GSM mobiles is closely controlled so that the battery of the

mobile is conserved, and also the levels of interference are reduced and performance of the

basestation is not compromised by high power local mobiles.

GSM power levels

The base station controls the power output of the mobile, keeping the GSM power level sufficientto maintain a good signal to noise ratio, while not too high to reduce interference, overloading, and

also to preserve the battery life.


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A table of GSM power levels is defined, and the base station controls the power of the mobile by

sending a GSM "power level" number. The mobile then adjusts its power accordingly. In virtually

all cases the increment between the different power level numbers is 2dB.

The accuracies required for GSM power control are relatively stringent. At the maximum power

levels they are typically required to be controlled to within +/- 2 dB, whereas this relaxes to +/- 5

dB at the lower levels.

The power level numbers vary according to the GSM band in use. Figures for the three main bands

in use are given below:

Power level

number

Power output

level dBm

2 39

3 37

4 35

5 33

6 31

7 29

8 27

9 25

10 23

11 21

12 19

13 17

14 15

15 13

16 11

17 9

18 7

19 5

GSM power level table for GSM 900


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Power level

number

Power output

level dBm

29 36

30 34

31 32

0 30

1 28

2 26

3 24

4 22

5 20

6 18

7 16

8 14

9 12

10 10

11 8

12 6

13 4

14 2

15 0


Power level

number

Power output

level dBm


22/32

30 33

31 32

0 30

1 28

2 26

3 24

4 22

5 20

6 18

7 16

8 14

9 12

10 10

11 8

12 6

13 4

14 2

15 0


GSM Power class

Not all mobiles have the same maximum power output level. In order that the base station knows

the maximum power level number that it can send to the mobile, it is necessary for the base stationto know the maximum power it can transmit. This is achieved by allocating a GSM power class

number to a mobile. This GSM power class number indicates to the base station the maximumpower it can transmit and hence the maximum power level number the base station can instruct it

to use.

Again the GSM power classes vary according to the band in use.

GSM GSM GSM GSM


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Power

Class

Number

900 1800 1900

Powe

rlevel

numb

er

Maximu

m

power

output

Powe

rlevel

numb

er

Maximu

m

power

output

Powe

rlevel

numb

er

Maximu

m

power

output

1 PL030 dBm /

1WPL0

30 dBm /

1W

2 PL239dBm /

8WPL3

24 dBm/

250 mWPL3

24 dBm /

250 mW

3 PL337dBm /

5W PL2936 dBm /

4W PL3033 dBm /

2W

4 PL433dBm /

2W

5 PL529 dBm /

800 mW

GSM power amplifier design considerations

One of the main considerations for the RF power amplifier design in any mobile phone is itsefficiency. The RF power amplifier is one of the major current consumption areas. Accordingly, to

ensure long battery life it should be as efficient as possible.

It is also worth remembering that as mobiles may only transmit for one eighth of the time, i.e. for

their allocated slot which is one of eight, the average power is an eighth of the maximum.

GSM logical and physical channelsGSM uses a variety ofchannels in which the data is carried. In GSM, these channels are separatedintophysical channels and logical channels. The Physical channels are determined by the timeslot,

whereas the logical channels are determined by the information carried within the physical

channel. It can be further summarised by saying that several recurring timeslots on a carrierconstitute a physical channel. These are then used by different logical channels to transfer

information. These channels may either be used for user data (payload) or signalling to enable the

system to operate correctly.

Common and dedicated channels


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The channels may also be divided into common and dedicated channels. The forward common

channels are used for paging to inform a mobile of an incoming call, responding to channel

requests, and broadcasting bulletin board information. The return common channel is a randomaccess channel used by the mobile to request channel resources before timing information is

conveyed by the BSS.

The dedicated channels are of two main types: those used for signalling, and those used for traffic.The signalling channels are used for maintenance of the call and for enabling call set up, providing

facilities such as handover when the call is in progress, and finally terminating the call. The traffic

channels handle the actual payload.

The following logical channels are defined in GSM:

TCHf- Full rate traffic channel.

TCH h - Half rate traffic channel.

BCCH - Broadcast Network information, e.g. for describing the current control channel structure.The BCCH is a point-to-multipoint channel (BSS-to-MS).

SCH - Synchronisation of the MSs.FCHMS - frequency correction.

AGCH - Acknowledge channel requests from MS and allocate a SDCCH.

PCHMS - terminating call announcement.

RACHMS - access requests, response to call announcement, location update, etc.

FACCHt - For time critical signalling over the TCH (e.g. for handover signalling). Traffic burst isstolen for a full signalling burst.

SACCHt - TCH in-band signalling, e.g. for link monitoring.

SDCCH - For signalling exchanges, e.g. during call setup, registration / location updates.

FACCHs - FACCH for the SDCCH. The SDCCH burst is stolen for a full signalling burst.

Function not clear in the present version of GSM (could be used for e.g. handover of an eight-ratechannel, i.e. using a "SDCCH-like" channel for other purposes than signalling).

SACCHs - SDCCH in-band signalling, e.g. for link monitoring.

GSM Audio Codec / VocoderAudio codecs or vocoders are universally used within the GSM system. They reduce the bit rate of

speech that has been converted from its analogue for into a digital format to enable it to be carried

within the available bandwidth for the channel. Without the use of a speech codec, the digitisedspeech would occupy a much wider bandwidth then would be available. Accordingly GSM codecs

are a particularly important element in the overall system.

A variety of different forms of audio codec or vocoder are available for general use, and the GSM

system supports a number of specific audio codecs. These include the RPE-LPC, half rate, and

AMR codecs. The performance of each voice codec is different and they may be used under


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different conditions, although the AMR codec is now the most widely used. Also the newer AMR

wideband (AMR-WB) codec is being introduced into many areas, including GSM

Voice codec technology has advanced by considerable degrees in recent years as a result of the

increasing processing power available. This has meant that the voice codecs used in the GSM

system have large improvements since the first GSM phones were introduced.

Vocoder / codec basics

Vocoders or speech codecs are used within many areas of voice communications. Obviously thefocus here is on GSM audio codecs or vocoders, but the same principles apply to any form of

codec.

If speech were digitised in a linear fashion it would require a high data rate that would occupy avery wide bandwidth. As bandwidth is normally limited in any communications system, it is

necessary to compress the data to send it through the available channel. Once through the channel

it can then be expanded to regenerate the audio in a fashion that is as close to the original as

possible.To meet the requirements of the codec system, the speech must be captured at a high enough

sample rate and resolution to allow clear reproduction of the original sound. It must then becompressed in such a way as to maintain the fidelity of the audio over a limited bit rate, error-

prone wireless transmission channel.

Audio codecs or vocoders can use a variety of techniques, but many modern audio codecs use a

technique known as linear prediction. In many ways this can be likened to a mathematical

modelling of the human vocal tract. To achieve this the spectral envelope of the signal is estimated

using a filter technique. Even where signals with many non-harmonically related signals are used itis possible for voice codecs to give very large levels of compression.

A variety of different codec methodologies are used for GSM codecs: CELP: The CELP or Code Excited Linear Prediction codec is a vocoder

algorithm that was originally proposed in 1985 and gave a significantimprovement over other voice codecs of the day. The basic principle of theCELP codec has been developed and used as the basis of other voice codecsincluding ACELP, RCELP, VSELP, etc. As such the CELP codec methodology isnow the most widely used speech coding algorithm. Accordingly CELP is nowused as a generic term for a particular class of vocoders or speech codecs andnot a particular codec.

The main principle behind the CELP codec is that is uses a principle known as"Analysis by Synthesis". In this process, the encoding is performed byperceptually optimising the decoded signal in a closed loop system. One way inwhich this could be achieved is to compare a variety of generated bit streamsand choose the one that produces the best sounding signal.

ACELP codec: The ACELP or Algebraic Code Excited Linear Prediction codec.The ACELP codec or vocoder algorithm is a development of the CELP model.However the ACELP codec codebooks have a specific algebraic structure asindicated by the name.


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VSELP codec: The VSELP or Vector Sum Excitation Linear Prediction codec.One of the major drawbacks of the VSELP codec is its limited ability to codenon-speech sounds. This means that it performs poorly in the presence ofnoise. As a result this voice codec is not now as widely used, other newerspeech codecs being preferred and offering far superior performance.

GSM audio codecs / vocoders

A variety of GSM audio codecs / vocoders are supported. These have been introduced at differenttimes, and have different levels of performance.. Although some of the early audio codecs are not

as widely used these days, they are still described here as they form part of the GSM system.

Codec

name

Bit rate

(kbps)

Compression

technology

Full rate 13 RTE-LPC

EFR 12.2 ACELP

Half rate 5.6 VSELP

AMR12.2 -

4.75ACELP

AMR-WB23.85 -

6.60ACELP

GSM Full Rate / RPE-LPC codec

The RPE-LPC or Regular Pulse Excited - Linear Predictive Coder. This form of voice codec was

the first speech codec used with GSM and it chosen after tests were undertaken to compare it with

other codec schemes of the day. The speech codec is based upon the regular pulse excitation LPCwith long term prediction. The basic scheme is related to two previous speech codecs, namely:

RELP, Residual Excited Linear Prediction and to the MPE-LPC, Multi Pulse Excited LPC. The

advantages of RELP are the relatively low complexity resulting from the use of baseband coding,but its performance is limited by the tonal noise produced by the system. The MPE-LPC is more

complex but provides a better level of performance. The RPE-LPC codec provided a compromise

between the two, balancing performance and complexity for the technology of the time.Despite the work that was undertaken to provide the optimum performance, as technology

developed further, the RPE-LPC codec was viewed as offering a poor level of voice quality. As

other full rate audio codecs became available, these were incorporated into the system.

GSM EFR - Enhanced Full Rate codec


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Later another vocoder called the Enhanced Full Rate (EFR) vocoder was added in response to the

poor quality perceived by the users of the original RPE-LPC codec. This new codec gave much

better sound quality and was adopted by GSM. Using the ACELP compression technology it gavea significant improvement in quality over the original LPC-RPE encoder. It became possible as the

processing power that was available increased in mobile phones as a result of higher levels of

processing power combined with their lower current consumption.

GSM Half Rate codec

The GSM standard allows the splitting of a single full rate voice channel into two sub-channels

that can maintain separate calls. By doing this, network operators can double the number of voice

calls that can be handled by the network with very little additional investment.

To enable this facility to be used a half rate codec must be used. The half rate codec was

introduced in the early years of GSM but gave a much inferior voice quality when compared to

other speech codecs. However it gave advantages when demand was high and network capacitywas at a premium.

The GSM Half Rate codec uses a VSELP codec algorithm. It codes the data around 20 ms frames

each carrying 112 bits to give a data rate of 5.6 kbps. This includes a 100 bps data rate for a modeindicator which details whether the system believes the frames contain voice data or not. This

allows the speech codec to operate in a manner that provides the optimum quality.

The Half Rate codec system was introduced in the 1990s, but in view of the perceived poorquality, it was not widely used.

GSM AMR Codec

The AMR, Adaptive Multi-rate codec is now the most widely used GSM codec. The AMR codecwas adopted by 3GPP in October 1988 and it is used for both GSM and circuit switched UMTS /

WCDMA voice calls.

The AMR codec provides a variety of options for one of eight different bit rates as described in the

table below. The bit rates are based on frames that are 20 millisceonds long and contain 160

samples. The AMR codec uses a variety of different techniques to provide the data compression.The ACELP codec is used as the basis of the overall speech codec, but other techniques are used in

addition to this. Discontinuous transmission is employed so that when there is no speech activity

the transmission is cut. Additionally Voice Activity Detection (VAD) is used to indicate when

there is only background noise and no speech. Additionally to provide the feedback for the userthat the connection is still present, a Comfort Noise Generator (CNG) is used to provide some

background noise, even when no speech data is being transmitted. This is added locally at thereceiver.

The use of the AMR codec also requires that optimized link adaptation is used so that the optimum

data rate is selected to meet the requirements of the current radio channel conditions including its

signal to noise ratio and capacity. This is achieved by reducing the source coding and increasingthe channel coding. Although there is a reduction in voice clarity, the network connection is more

robust and the link is maintained without dropout. Improvement levels of between 4 and 6 dB may


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be experienced. However network operators are able to prioritise each station for either quality or

capacity.

The AMR codec has a total of eight rates: eight are available at full rate (FR), while six are

available at half rate (HR). This gives a total of fourteen different modes.

Mode

Bit

rate

(kbps)

Full Rate (FR) /

Half rate (HR)

AMR 12.2 12.2 FR

AMR 10.2 10.2 FR

AMR 7.95 7.95 FR / HR

AMR 7.40 7.40 FR / HR

AMR 6.70 6.70 FR / HR

AMR 5.90 5.90 FR / HR

AMR 5.15 5.15 FR / HR

AMR 4.75 4.75 FR / HR

AMR codec data rates

AMR-WB codec

Adaptive Multi-Rate Wideband, AMR-WB codec, also known under its ITU designation of

G.722.2, is based on the earlier popular Adaptive Multi-Rate, AMR codec. AMR-WB also uses anACELP basis for its operation, but it has been further developed and AMR-WB provides improved

speech quality as a result of the wider speech bandwidth that it encodes. AMR-WB has a

bandwidth extending from 50 - 7000 Hz which is significantly wider than the 300 - 3400 Hzbandwidths used by standard telephones. However this comes at the cost of additional processing,

but with advances in IC technology in recent years, this is perfectly acceptable.

The AMR-WB codec contains a number of functional areas: it primarily includes a set of fixed rate

speech and channel codec modes. It also includes other codec functions including: a Voice ActivityDetector (VAD); Discontinuous Transmission (DTX) functionality for GSM; and Source

Controlled Rate (SCR) functionality for UMTS applications. Further functionality includes in-bandsignaling for codec mode transmission, and link adaptation for control of the mode selection.

The AMR-WB codec has a 16 kHz sampling rate and the coding is performed in blocks of 20 ms.

There are two frequency bands that are used: 50-6400 Hz and 6400-7000 Hz. These are codedseparately to reduce the codec complexity. This split also serves to focus the bit allocation into the

subjectively most important frequency range.


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The lower frequency band uses an ACELP codec algorithm, although a number of additional

features have been included to improve the subjective quality of the audio. Linear prediction

analysis is performed once per 20 ms frame. Also, fixed and adaptive excitation codebooks aresearched every 5 ms for optimal codec parameter values.

The higher frequency band adds some of the naturalness and personality features to the voice. The

audio is reconstructed using the parameters from the lower band as well as using randomexcitation. As the level of power in this band is less than that of the lower band, the gain is

adjusted relative to the lower band, but based on voicing information. The signal content of the

higher band is reconstructed by using an linear predictive filter which generates information fromthe lower band filter.

Bit

rate

(kbp

s)

Notes

6.60

This is the lowest rate for AMR-WB. It is used for circuit switched

connections for GSM and UMTS and is intended to be used only

temporarily during severe radio channel conditions or during

network congestion.

8.85

This gives improved quality over the 6.6 kbps rate, but again, its

use is only recommended for use in periods of congestion or

when during severe radio channel conditions.

12.65

This is the main bit rate used for circuit switched GSM and UMTS,

offering superior performance to the original AMR codec.

14.25Higher bit rate used to give cleaner speech and is particularly

useful when ambient audio noise levels are high.





19.85 Higher bit rate used to give cleaner speech and is particularlyuseful when ambient audio noise levels are high.

23.05 Not suggested for full rate GSM channels.

23.85Not suggested for full rate GSM channels, and provides speech

quality similar to that of G.722 at 64 kbps.


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Not all phones equipped with AMR-WB will be able to access all the data rates - the different

functions on the phone may not require all to be active for example. As a result, it is necessary to

inform the network about which rates are available and thereby simplify the negotiation betweenthe handset and the network. To achieve this there are three difference AMR-WB configurations

that are available:

Configuration A: 6.6, 8.85, and 12.65 kbit/s Configuration B: 6.6, 8.85, 12.65, and 15.85 kbit/s Configuration C: 6.6, 8.85, 12.65, and 23.85 kbit/s

It can be seen that only the 23.85, 15.85, 12.65, 8.85 and 6.60 kbit/s modes are used. Based on

listening tests, it was considered that these five modes were sufficient for a high quality speech

telephony service. The other data rates were retained and can be used for other purposes includingmultimedia messaging, streaming audio, etc.

GSM handover or handoffOne of the key elements of a mobile phone or cellular telecommunications system, is that the

system is split into many small cells to provide good frequency re-use and coverage. However asthe mobile moves out of one cell to another it must be possible to retain the connection. Theprocess by which this occurs is known as handover or handoff. The term handover is more widely

used within Europe, whereas handoff tends to be use more in North America. Either way, handover

and handoff are the same process.

Requirements for GSM handover

The process of handover or handoff within any cellular system is of great importance. It is a

critical process and if performed incorrectly handover can result in the loss of the call. Dropped

calls are particularly annoying to users and if the number of dropped calls rises, customer

dissatisfaction increases and they are likely to change to another network. Accordingly GSMhandover was an area to which particular attention was paid when developing the standard.

Types of GSM handover

Within the GSM system there are four types of handover that can be performed for GSM only

systems:

Intra-BTS handover: This form of GSM handover occurs if it is required tochange the frequency or slot being used by a mobile because of interference,or other reasons. In this form of GSM handover, the mobile remains attached to

the same base station transceiver, but changes the channel or slot. Inter-BTS Intra BSC handover: This for of GSM handover or GSM handoff

occurs when the mobile moves out of the coverage area of one BTS but intoanother controlled by the same BSC. In this instance the BSC is able to performthe handover and it assigns a new channel and slot to the mobile, beforereleasing the old BTS from communicating with the mobile.

Inter-BSC handover: When the mobile moves out of the range of cellscontrolled by one BSC, a more involved form of handover has to be performed,


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Inter-system handover

With the evolution of standards and the migration of GSM to other 2G technologies including to

3G UMTS / WCDMA as well as HSPA and then LTE, there is the need to handover from onetechnology to another. Often the 2G GSM coverage will be better then the others and GSM is often

used as the fallback. When handovers of this nature are required, it is considerably more

complicated than a straightforward only GSM handover because they require two technically verydifferent systems to handle the handover.

These handovers may be called intersystem handovers or inter-RAT handovers as the handover

occurs between different radio access technologies.

The most common form of intersystem handover is between GSM and UMTS / WCDMA. Here

there are two different types:

UMTS / WCDMA to GSM handover: There are two further divisions of thiscategory of handover:

Blind handover: This form of handover occurs when the base stationhands off the mobile by passing it the details of the new cell to the

mobile without linking to it and setting the timing, etc of the mobile forthe new cell. In this mode, the network selects what it believes to be theoptimum GSM based station. The mobile first locates the broadcastchannel of the new cell, gains timing synchronisation and then carriesout non-synchronised intercell handover.

Compressed mode handover: using this form of handover the mobileuses the gaps I transmission that occur to analyse the reception of localGSM base stations using the neighbour list to select suitable candidatebase stations. Having selected a suitable base station the handovertakes place, again without any time synchronisation having occurred.

Handover from GSM to UMTS / WCDMA: This form of handover issupported within GSM and a "neighbour list" was established to enable this

occur easily. As the GSM / 2G network is normally more extensive than the 3Gnetwork, this type of handover does not normally occur when the mobileleaves a coverage area and must quickly find a new base station to maintaincontact. The handover from GSM to UMTS occurs to provide an improvement inperformance and can normally take place only when the conditions are right.

The neighbour list will inform the mobile when this may happen.

GSM Basics and Overview

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Transcript of GSM Basics and Overview