Gsm System Tuterialx

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GSM SYS

GSM basics t

- a tutorial, description,

Mobile communications

technology, network and

GSM tutorial includes:

GSM basics tutorial and GSM history

GSM network architectu

GSM interfaces

GSM radio access netwo

GSM frames

GSM frequency bands an

GSM power class, contro

GSM physical & logical GSM codecs / vocoders

GSM handover or hando

The GSM system is the mostbeen a particularly successful

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ability to roam worldwide with the certainty of being able to be able to operate on GSM

networks in exactly the 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 Global

System 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

billion subscriber mark and still steadily increasing.

GSM system overview

The GSM system was designed as a second generation (2G) cellular phone technology. One ofthe basic 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 (time

division 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 for anyone 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. Toachieve this the speech is digitally encoded and later decoded using a vocoder. A variety of

vocoders are available 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 are

nevertheless 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 the

GSM 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

reasonable length. This service has become particularly popular, initially with the young as itprovided a simple, low fixed cost.


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GSM basics

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

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 or

infrastructure technology. By adopting this approach it is possible to define the operation of thewhole network to enable international roaming as well as enabling network elements from

different manufacturers to operate alongside each other, although this last feature is notcompletely 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 stationcontroller (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 be routed to them. Additionally there is the Authentication Centre (AuC), and theEquipment 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.It was 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 manufacturing

industry 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.


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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 technology FDMA / TDMA

Duplex technique FDD

Uplink frequency band890 - 915 MHz

(basic 900 MHz band only)

Downlink frequency band933 -960 MHz(basic 900 MHz band only)

Channel spacing 200 kHzModulation GMSK

Speech codingVarious - original was RPE-

LTP/13

Speech channels per RF

channel8

Channel data rate 270.833 kbps

Frame duration 4.615 ms

GSM History [2]

- a description of the development or history of GSM, Global System for Mobile

communications developed out of the original Groupe Special Mobile

pan_european system.

Today the GSM cell or mobile phone system is the most popular in the world. GSM handsets are

widely 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 and

international cooperation. The GSM history can be seen to be a story of cooperation acrossEurope, 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


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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 themajor systems that were in existence were the AMPS (Advanced Mobile Phone System) that was

used in the USA and many 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.

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

or Extended-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 waspossible 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 the beginnings of the GSM system.

To achieve the basic definition of a new system a meeting was held in 1982 under the auspices of

the Conference of European Posts and Telegraphs (CEPT). They formed a study group called theGroupe 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

and service 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 senseof 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 in cheaper 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.


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GSM launch datesWork 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 by a 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 and

the network infrastructure for GSM would be available.

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

the functional 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

more than 6000 pages. Nevertheless the resultant phone system provided a robust, feature-richsystem. 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 before any 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 that

year. 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 subscribersand there were 25 roaming agreements in place. The growth continued and the next million

subscribers were 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


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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 allowingadditional spectrum 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 for by 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 accordingly

this 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

name which 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 totalnumber 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 Architecture

- a tutorial or overview of the basics of the GSM network architecture design and

technology with details of the base-stations, controllers, MSC, AuC, HLR and

VLR.

The GSM technical specifications define the different elements within the GSM networkarchitecture. 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 systemsnow 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

developments of the newer systems, the basic GSM network architecture has been maintained,


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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)

Simplified GSM Network Architecture

Mobile station

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


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There are a number of elements to the cell phone, although the two main elements are the main

hardware 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 be

transmitted. 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 theuser 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

fundamentally associated with communicating with the mobiles on the network. It consists of

two elements:

Base Transceiver Station (BTS): The BTS used in a GSM network comprises the radiotransmitter receivers, and their associated antennas that transmit and receive to directly

communicate with the mobiles. The BTS is the defining element for each cell. The BTS

communicates with the mobiles and the interface between the two is known as the Uminterface with its associated protocols.

Base Station Controller (BSC): The BSC forms the next stage back into the GSMnetwork. It controls a group of BTSs, and is often co-located with one of the BTSs in its

group. It manages the radio resources and controls items such as handover within thegroup of BTSs, allocates channels and the like. It communicates 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 the core networkarea of the overall GSM network architecture is the Mobile switching Services Centre

(MSC). The MSC acts like a normal switching node within a PSTN or ISDN, but also

provides additional functionality to enable the requirements of a mobile user to be

supported. These include registration, authentication, call location, inter-MSC handovers


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and call routing to a mobile subscriber. It also provides an interface to the PSTN so that

calls can be routed from the mobile network to a phone connected to a landline.Interfaces to other MSCs are provided to enable calls to be made to mobiles on different

networks.

Home Location Register (HLR): This database contains all the administrativeinformation about each subscriber along with their last known location. In this way, the

GSM network is able to route calls to the relevant base station for the MS. When a user

switches on their phone, the phone registers with the network and from this it is possible

to determine which BTS it communicates with so that incoming calls can be routed

appropriately. Even when the phone is not active (but switched on) it re-registers

periodically to ensure that the network (HLR) is aware of its latest position. There is one

HLR per network, although it may be distributed across various sub-centres to foroperational reasons.

Visitor Location Register (VLR): This contains selected information from the HLR thatenables the selected services for the individual subscriber to be provided. The VLR canbe implemented as a separate entity, but it is commonly realised as an integral part of the

MSC, rather than a separate entity. In this way access is made faster and moreconvenient.

Equipment Identity Register (EIR): The EIR is the entity that decides whether a givenmobile equipment may be allowed onto the network. Each mobile equipment has anumber known as the International Mobile Equipment Identity. This number, as

mentioned above, is installed in the equipment and is checked by the network during

registration. Dependent upon the information held in the EIR, the mobile may be

allocated one of three states - allowed onto the network, barred access, or monitored in

case its problems.

Authentication Centre (AuC): The AuC is a protected database that contains the secretkey 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 MEterminating call is initially routed, without any knowledge of the MS's location. The

GMSC is thus in charge of obtaining the MSRN (Mobile Station Roaming Number) from

the HLR based on the MSISDN (Mobile Station ISDN number, the "directory number" ofa MS) and routing the call to the correct visited MSC. The "MSC" part of the term

GMSC is misleading, since the gateway operation does not require any linking to an

MSC.

SMS Gateway (SMS-G): The SMS-G or SMS gateway is the term that is used tocollectively describe the two Short Message Services Gateways defined in the GSMstandards. The two gateways handle messages directed in different directions. The SMS-

GMSC (Short Message Service Gateway Mobile Switching Centre) is for short messages

being sent to an ME. The SMS-IWMSC (Short Message Service Inter-Working Mobile

Switching Centre) is used for short messages originated with a mobile on that network.

The SMS-GMSC role is similar to that of the GMSC, whereas the SMS-IWMSC

provides a fixed access point to the Short Message Service Centre.


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Operation and Support Subsystem (OSS)

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

must be 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 Interfaces [4]

- a summary or tutorial of the different interfaces used to provide

communication between various elements in a GSM cell phone network

The network structure is defined within the GSM standards. Additionally each interface betweenthe different elements of the GSM network is also defined. This facilitates the information

interchanges 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 many people might like.

1. Um interface The "air" or radio interface standard that is used for exchanges between amobile (ME) and a base station (BTS / BSC). For signalling, a modified version of the

ISDN LAPD, known as LAPDm is used.

2. Abis interface This is a BSS internal interface linking the BSC and a BTS, and it has notbeen totally standardised. The Abis interface allows control of the radio equipment and

radio frequency allocation in the BTS.

3. A interface The A interface is used to provide communication between the BSS and theMSC. The interface carries information to enable the channels, timeslots and the like to

be allocated to the mobile equipments being serviced by the BSSs. The messagingrequired 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 protocolknown as the MAP/B protocol. As most VLRs are collocated with an MSC, this makesthe interface 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.

When a 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 tocomplete the call may be gained. The protocol used for communication is MAP/C, the

letter "C" indicating that 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.


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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 ofthe subscriber.

7. E interface The E interface provides communication between two MSCs. The Einterface exchanges data related to handover between the anchor and relay MSCs using

the MAP/E protocol.

8. F interface The F interface is used between an MSC and EIR. It uses the MAP/Fprotocol. The communications along this interface are used to confirm the status of the

IMEI of the ME gaining access to the network.

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

Burst [5]

- tutorial, overview of the GSM air interface or GSM signal with details of

carrier, slot structure and transmission burst and duplex scheme and power

class.

One 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 requirements

that 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 alldevised to provide the optimum performance.

During the development of the GSM standard very careful attention was paid to aspectsincluding 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, themodulation format 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.


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GSM signal and GMSK modulation characteristics

The core of any radio based system is the format of the radio signal itself. The carrier ismodulated using a form of phase sift keying known as Gaussian Minimum Shift Keying

(GMSK). GMSK was used 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 as having 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.

Click on the link for a GMSK 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 beused from the same 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 bysplitting 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 to

overcome the problems of interference, fading and general data errors that may occur. Thismeans 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


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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 frequenciesspaced 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 the

particular 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 burstperiods 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.

GSM slots showing offset between transmit and receive


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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 considerablyreduces the need for expensive filters to isolate the transmitter from the receiver. It also provides

a space 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

different types 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 themobile, and typically transfers the digitised voice data.

The structure of the normal GSM burst is exactly defined and follows a common format. It

contains 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 the transmitter toramp up its power

2. 57 data bits: This block of data is used to carry information, and most often contains thedigitised voice data although on occasions it may be replaced with signalling information

in the form of the Fast Associated Control CHannel (FACCH). The type of data is

indicated by the flag that follows the data field3. 1 bit flag: This bit within the GSM burst indicates the type of data in the previous field.4. 26 bits training sequence: This training sequence is used as a timing reference and for

equalisation. There is a total of eight different bit sequences that may be used, each 26

bits long. The same sequence is used in each GSM slot, but nearby base stations using thesame radio frequency channels will use different ones, and this enables the mobile to

differentiate between the various cells 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.


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7. 3 tail bits These final bits within the GSM burst are used to enable the transmitter powerto ramp down. They are often called final tail bits, or just tail bits.

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

GSM Normal Burst

GSM synchronisation burstThe 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 for thetransmitter 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.

1. 3 tail bits: Again, these tail bits at the start of the GSM burst give time for thetransmitter 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.


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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 additional marginwhen 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 becausethere 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 than 40% of the time during normal telephone conversations. The most important

element of DTx is the Voice Activity Detector. It must correctly distinguish between voice andnoise inputs, a task that is not trivial. If a voice signal is misinterpreted as noise, the transmitter is

turned off an effect known as 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 thisis added as appropriate. The noise is controlled by the SID (silence indication descriptor).

GSM Frame Structure

- tutorial, overview of the basics of GSM frame structure including the

multiframe, superframe and hyperframe.

The 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

the various 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 the

tutorial, 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.


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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

is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it formsthe basic unit for the definition of logical channels. One physical channel is one burst period

allocated in each TDMA frame.

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 beaconfrequency.

GSM multiframe

The GSM frames are grouped together to form multiframes and in this way it is possible to

establish 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 into multiframesconsisting 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 of the remaining bursts is then usedto accommodate the SACCH, the remaining frame remaining free. The actual position

used alternates between position 12 and 25.

Control multiframe: the Control Channel multiframe that comprises 51 bursts andoccupies 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 beacon frequency as well. This multiframe is


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subdivided into logical channels which are time-scheduled. These logical channels and

functions include the following:o Frequency correction bursto Synchronisation bursto Broadcast channel (BCH)o Paging and Access Grant Channel (PACCH)o 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

the control multiframes are 51 bursts long, the different number of traffic and control

multiframes within the superframe, brings them back into line again taking exactly the sameinterval.

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 GSMframe structure.

Within the GSM hyperframe there is a counter and every time slot has a unique sequentialnumber comprising the frame number and time slot number. This is used to maintain

synchronisation of the different scheduled operations with the GSM frame structure. Theseinclude functions such as:

Frequency hopping: Frequency hopping is a feature that is optional within the GSMsystem. It can help reduce interference and fading issues, but for it to work, the

transmitter and receiver must be synchronised so they hop to the same frequencies at the

same time.

Encryption: The encryption process is synchronised over the GSM hyperframe periodwhere a counter is used and the encryption process will repeat with each hyperframe.

However, it is unlikely that the cellphone conversation will be over 3 hours andaccordingly it is unlikely that security will be compromised as a result.


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GSM Frame Structure Summary

GSM Frequencies and Frequency Bands [7]

- a tabular summary of the frequencies and frequency bands allocations and

spectrum used by the GSM cellular telecommunications system.

Although 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 phonesusing other frequencies may be required.


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GSM band allocations

There is a total of fourteen different recognised GSM frequency bands. These are defined in3GPP TS 45.005.

BandUplink

(MHz)

Downlink

(MHz)Comments

380380.2 -

389.8390.2 - 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 -

716.0728.0 - 746.0

750747.0 -

762.0777.0 - 792.0

810806.0 -

821.0851.0 - 866.0

850824.0 -

849.0

869.0 - 894.0

900890.0 -

915.0935.0 - 960.0

P-GSM, i.e. Primary or standard

GSM allocation

900880.0 -

915.0925.0 - 960.0

E-GSM, i.e. Extended GSM

allocation

900 876.0 - 915 921.0 - 960.0R-GSM, i.e. Railway GSMallocation

900870.4 -

876.0915.4 - 921.0 T-GSM

18001710.0 -

1785.0

1805.0 -

1880.0

19001850.0 -

1910.0

1930.0 -

1990.0

GSM frequency band usage

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


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requirements for the particular country and the ITU (International Telecommunications Union)

region in which the country 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 1900

MHz 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

widely used 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 bandphones would 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 in

Europe 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

thereby allowing global use.

GSM Power Control and Power Class [8]

- tutorial, overview of the GSM power control, GSM power levels, power class

and power amplifier design.


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The 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 designationshave 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

sufficient to maintain a good signal to noise ratio, while not too high to reduce interference,overloading, and also to preserve the battery life.

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 powerlevels 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 mainbands 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


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Power level

number

Power output level

dBm

16 1117 9

18 7

19 5

GSM power level table for GSM 900

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

8 149 12

10 10

11 8

12 6

13 4

14 2

15 0


Power level

number

Power output level

dBm

30 33

31 32

0 30

1 28

2 26


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Power level

number

Power output level

dBm

3 244 22

5 20

6 18

7 16

8 14

9 12

10 10

11 8

12 613 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 basestation to 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

maximum power 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

Power

Class

Number

GSM 900 GSM 1800 GSM 1900

Power

level

number

Maximum

power

output

Power

level

number

Maximum

power

output

Power

level

number

Maximum

power

output

1 PL030 dBm /

1WPL0

30 dBm /

1W

2 PL239dBm /8W

PL324 dBm/250 mW

PL324 dBm /250 mW

3 PL337dBm /

5WPL29

36 dBm /

4WPL30

33 dBm /

2W


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GSM

Power

Class

Number

GSM 900 GSM 1800 GSM 1900

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

GSM logical and physical channels [9]

- a tutorial, description, overview of GSM channels including transport and

logical channels, SACCH, SDCCH, FACCH, etc.

GSM uses a variety ofchannels in which the data is carried. In GSM, these channels are

separated intophysical 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

carrier constitute 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 enablethe system to operate correctly.

Common and dedicated channels

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 random

access channel used by the mobile to request channel resources before timing information is

conveyed by the BSS.


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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

is stolen 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-

rate channel, 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 / Vocoder [10]

- an overview, description or tutorial detailing the basics of GSM audio codecs or

vocoders including LPC-RPE, EFR, Full Rate, Half Rate, AMR codec and AMR-

WB codec as well as CELP, ACELP, VSELP, speech codec technologies.

Audio 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


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carried within the available bandwidth for the channel. Without the use of a speech codec, the

digitised speech would occupy a much wider bandwidth then would be available. AccordinglyGSM 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

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 the

focus 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 a

very 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 thechannel 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 be

compressed 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 atechnique 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 isestimated using a filter technique. Even where signals with many non-harmonically related

signals are used it is 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 thatwas originally proposed in 1985 and gave a significant improvement over other voice

codecs of the day. The basic principle of the CELP codec has been developed and used as

the basis of other voice codecs including ACELP, RCELP, VSELP, etc. As such the

CELP codec methodology is now the most widely used speech coding algorithm.


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Accordingly CELP is now used as a generic term for a particular class of vocoders or

speech codecs and not a particular codec.

The main principle behind the CELP codec is that is uses a principle known as "Analysisby Synthesis". In this process, the encoding is performed by perceptually optimising the

decoded signal in a closed loop system. One way in which this could be achieved is to

compare a variety of generated bit streams and choose the one that produces the best

sounding signal.

ACELP codec: The ACELP or Algebraic Code Excited Linear Prediction codec. TheACELP codec or vocoder algorithm is a development of the CELP model. However the

ACELP codec codebooks have a specific algebraic structure as indicated by the name.

VSELP codec: The VSELP or Vector Sum Excitation Linear Prediction codec. One ofthe major drawbacks of the VSELP codec is its limited ability to code non-speech

sounds. This means that it performs poorly in the presence of noise. As a result this voice

codec is not now as widely used, other newer speech codecs being preferred and offeringfar superior performance.

GSM audio codecs / vocoders

A variety of GSM audio codecs / vocoders are supported. These have been introduced at

different times, and have different levels of performance.. Although some of the early audiocodecs are not as widely used these days, they are still described here as they form part of the

GSM system.

Codec nameBit rate

(kbps)Compression technology

Full rate 13 RTE-LPC

EFR 12.2 ACELP

Half rate 5.6 VSELP

AMR 12.2 - 4.75 ACELP

AMR-WB 23.85 - 6.60 ACELP

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

LPC with 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 ExcitedLPC. The advantages of RELP are the relatively low complexity resulting from the use of


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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 codecprovided 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

Later another vocoder called the Enhanced Full Rate (EFR) vocoder was added in response tothe poor quality perceived by the users of the original RPE-LPC codec. This new codec gavemuch better sound quality and was adopted by GSM. Using the ACELP compression technology

it gave a 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-channelsthat 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 capacity

was 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 mode indicator 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 poor

quality, it was not widely used.

GSM AMR Codec


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The AMR, Adaptive Multi-rate codec is now the most widely used GSM codec. The AMR codec

was 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 indicatewhen there is only background noise and no speech. Additionally to provide the feedback for the

user that 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

the receiver.

The use of the AMR codec also requires that optimized link adaptation is used so that theoptimum 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 codingand increasing the 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 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.

ModeBit 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 / HRAMR 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


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an ACELP 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 hasa bandwidth extending from 50 - 7000 Hz which is significantly wider than the 300 - 3400 Hz

bandwidths used by standard telephones. However this comes at the cost of additionalprocessing, 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

Activity Detector (VAD); Discontinuous Transmission (DTX) functionality for GSM; and

Source Controlled Rate (SCR) functionality for UMTS applications. Further functionalityincludes in-band signaling 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 arecoded separately to reduce the codec complexity. This split also serves to focus the bit allocationinto the subjectively most important frequency range.

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 are

searched 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 random

excitation. As the level of power in this band is less than that of the lower band, the gain isadjusted 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

from the lower band filter.

Bit

rate

(kbps)

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 onlytemporarily during severe radio channel conditions or during network

congestion.

8.85This gives improved quality over the 6.6 kbps rate, but again, its use isonly recommended for use in periods of congestion or when during severe

radio channel conditions.

12.65This 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 whenambient audio noise levels are high.


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Bit

rate

(kbps)

Notes

15.85Higher bit rate used to give cleaner speech and is particularly useful 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.

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 between

the handset and the network. To achieve this there are three difference AMR-WB configurationsthat 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 onlistening 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 purposesincluding multimedia messaging, streaming audio, etc.

GSM handover or handoff [11]- tutorial or overview of the essentials of GSM handover or handoff from one cell

to another and detailing types of handover and methodologies used.

One 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. The

process 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.


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Requirements for GSM handoverThe 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 to change thefrequency 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 occurswhen the mobile moves out of the coverage area of one BTS but into another controlled

by the same BSC. In this instance the BSC is able to perform the handover and it assigns

a new channel and slot to the mobile, before releasing the old BTS from communicatingwith the mobile.

Inter-BSC handover: When the mobile moves out of the range of cells controlled byone BSC, a more involved form of handover has to be performed, handing over not only

from one BTS to another but one BSC to another. For this the handover is controlled bythe MSC.

Inter-MSC handover: This form of handover occurs when changing between networks.The two MSCs involved negotiate to control the handover.

GSM handover process

Although there are several forms of GSM handover as detailed above, as far as the mobile is

concerned, they are effectively seen as very similar. There are a number of stages involved in

undertaking a GSM handover from one cell or base station to another.

In GSM which uses TDMA techniques the transmitter only transmits for one slot in eight, and

similarly the receiver only receives for one slot in eight. As a result the RF section of the mobile

could be idle for 6 slots out of the total eight. This is not the case because during the slots in

which it is not communicating with the BTS, it scans the other radio channels looking for beacon


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- summary, overview or tutorial about the basics of GSM EDGE cellular

evolution technology providing enhanced data rates for GSM.

GSM EDGE tutorial includes:

GSM EDGE technology

EDGE network architecture Modulation, slot, & burst EDGE MCS coding schemes and classes

Evolved EDGE

EDGE is an evolution to the GSM mobile cellular phone system. The name EDGE stands forEnhanced Data for GSM Evolution and it enables data to be sent over a GSM TDMA system at

speeds up to 384 kbps. In some instances GSM EDGE evolution systems may also be known as

EGPRS, or Enhanced General Packet Radio Service systems. Although strictly speaking a"2.5G" system, the GSM EDGE cellular technology is capable of providing data rates that are a

distinct increase on those that could be supported by GPRS.

EDGE evolution is intended to build on the enhancements provided by the addition of GPRS

(General Packet Radio Service) where packet switching is applied to a network. It then enables a

three-fold increase in the speed at which data can be transferred by adopting a new form ofmodulation. GSM uses a form of modulation known as Gaussian Minimum Shift Keying(GMSK), but EDGE evolution changes the modulation to 8PSK and thereby enabling a

significant increase in data rate to be achieved.

GSM EDGE basics


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Parameter Details

Multiple Access Technology FDMA / TDMA

Duplex Technique FDDChannel Spacing 200 kHz

Modulation GMSK, 8PSK

Slots per channel 8

Frame duration 4.615 ms

Latency Below 100 ms

Overall symbol rate 270 k symbols / s

Overall modulation bit rate 810 kbps

Radio data rate per time slot 69.2 kbps

Max user data rate per time slot 59.2 kbps (MCS-9)Max user data rate when using 8 time slots 473.6 kbps **

GSM EDGE specification highlights

Note:

** A maximum user data rate of 384 kbps is often seen quoted as the data rate for GSM

EDGE. This data rate corresponds to the International Telecommunications Union (ITU)

definition of the data rate limit required for a service to fulfill the International Mobile

Telecommunications-2000 (IMT-2000) standard(i.e. 3G) in a pedestrian environment.

GSM EDGE network architecture

- a summary, overview or tutorial about the basics of the enhancements required

for the GSM EDGE network architecture including the GGSN and SGSN.

In order that the GSM EDGE upgrade can be implemented, additions are required within the

EDGE network architecture to be able to cater for the packet data that is carried by the system.

The additional network entities required are the same as those used for GPRS and also for

UMTS.

With the introduction of the new entities within the network, it was still necessary for the new

EDGE network elements and those from the existing GSM elements to work along side one

another. Accordingly the introduction of GPRS and EDGE technology saw the addition of some

new entities within the over network architecture.


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The two main elements that are required by the GSM EDGE network architecture are the GGSN

and SGSN. These enable the network to be able to cater for the packet data that is passed overthe network.

GSM EDGE network architecture upgrades

Although in practice a variety of elements are required within the network architecture, the mainnew network architecture entities that are needed for the EDGE upgrade are:

SGSN: GPRS Support Node - this forms a gateway to the services within the network. GGSN: Gateway GPRS Support Node which forms the gateway to the outside world. PCU: Packet Control Unit which differentiates whether data is to be routed to the packetswitched or circuit switched networks.

A simplified view of the GSM EDGE network architecture can be seen in the diagram below.

From this it can be seen that it is very similar to the more basic GSM network architecture, but

with additional elements.

GSM EDGE network architecture


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SGSNThe SGSN or Serving GPRS Support Node element of the GPRS network provides a number of

takes focussed on the IP elements of the overall system. It provides a variety of services to the

mobiles:

Packet routing and transfer Mobility management Authentication Attach/detach Logical link management Charging data

There is a location register within the SGSN and this stores location information (e.g., current

cell, current VLR). It also stores the user profiles (e.g., IMSI, packet addresses used) for all the

GPRS users registered with the particular SGSN.

GGSN

The GGSN, Gateway GPRS Support Node is one of the most important entities within the GSM

EDGE network architecture.

The GGSN organises the inter-working between the GPRS / EDGE network and external packetswitched networks to which the mobiles may be connected. These may include both Internet and

X.25 networks.

The GGSN can be considered to be a combination of a gateway, router and firewall as it hides

the internal network to the outside. In operation, when the GGSN receives data addressed to a

specific user, it checks if the user is active, then forwarding the data. In the opposite direction,packet data from the mobile is routed to the right destination network by the GGSN.

PCU

The PCU or Packet Control Unit is a hardware router that is added to the BSC. It differentiates

data destined for the standard GSM network (circuit switched data) and data destined for the

EDGE network (Packet Switched Data). The PCU itself may be a separate physical entity, or

more often these days it is incorporated into the base station controller, BSC, thereby saving

additional hardware costs.


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GSM EDGE network upgradingOne of the key elements for any network operator is the cost of capital expenditure (capex) to

buy and establish a network. Capex costs are normally very high for a new network, and

operators endeavour to avoid this and use any existing networks they may have to make the

optimum use of any capital. In addition to the capex, there are the operational costs, (opex).These costs are for general maintenance and other operational costs that may be incurred.Increasing efficiency and reliability will reduce the opex costs.

Any upgrade such as that from GSM to EDGE will require new investment and operators are

keen to keep this to the minimum. The upgrades for the EDGE network are not as large as

starting from scratch and rolling out a new network.

The EDGE network adds to the existing GSM network. The main new entities required within

the network are the SGSN and GGSN, and these are required as the starting point.

The base station subsystems require some updates. The main one is the addition of the PCU

described above. Some modifications may be required to the BTS, but often only a software

upgrade is required, and this may often be achieved remotely. In this way costs are kept to aminimum.

GSM EDGE Modulation, Slot, Burst and Air

Interface

- summary, overview or tutorial about the basics of GSM EDGE air interface

including the modulation scheme, slot, burst configurations.

The air interface for GSM EDGE, including the modulation, as well as the slot and burststructures, have been developed to be compatible with the overall GSM concept. In this wayEDGE cellular technology is able to operate alongside the existing GSM systems by adding an

EDGE upgrade.

In addition to this EDGE technology re-uses many of the features of the existing systems

allowing both technologies to utilise the same base stations, etc. This provides a lower cost

option to upgrade the network rather than having to deploy a completely new system.

With EDGE operating alongside GSM and GPRS, it has been necessary for the air interface to

accommodate all signals, often catering for all three simultaneously. This approach, while


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proving some technical challenges has been very successful, as demonstrated by the number of

operators whose networks are able to accommodate all three signals.

GSM EDGE modulation characteristics

One of the ways in which EDGE is able to provide higher data rates is to use a differentmodulation scheme for higher data rates. However the GMSK modulation scheme used for thebasic GSM system is still used for the lower data rates.

GMSK was chosen for the original 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.

The GMSK modulation format is is used for the lower data rate transfers. The advantages mean

that it is well suited for situations where lower data rates can be tolereated.

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 as having an almost constant amplitude which allows for the use of more efficienttransmitter power amplifiers, thereby saving on current consumption, a critical issue for battery

power equipment.

Click on the link for a GMSK tutorial

In order to enable data to be transmitted a form of phase modulation known as Octonary PhaseShift Keying, 8PSK was used. This form of modulation has a number of advantages that meant it

was chosen for carrying high speed EDGE data:

Able to operate within the existing GSM / GPRS channel structure. Able to operate within the existing GSM / GPRS channel bandwidth. Able to operate within the existing GSM / GPRS channel coding structure. Provides a higher data capability than the existing GSM GMSK modulation scheme.

The 8-PSK modulation scheme fulfils these requirements. It has the equivalent bandwidth and

adjacent channel interference levels to GMSK. This makes it possible to integrate EDGE


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channels into the existing GSM / GPRS network and frequency plan as well as keeping the same

channel coding structure.

Note on PSK:

Phase shift Keying, PSK is a form of modulation used particularly for data transmissions. If

offers an effective way of transmitting data. By altering the number of different phase states

which can be adopted, the data speeds that can be achieved within a given channel can beincreased, but at the cost of lower resilience to noise an interference.

Click on the link for a PSK tutorial

The 8PSK modulation method is a linear method in which three consecutive bits are mappedonto one symbol in the I/Q plane as shown below

8PSK Modulation Constellation

Using 8-PSK, the rate at which symbols are sent remains the same. However each symbol now

represents three bits instead of one. This means that the actual data rate is increased by a factorof three.

The "distance" between the different positions on the constellation diagram is shorter using

8PSK modulation than when using GMSK. This means that there is an increased risk of any of

the symbols being misinterpreted, especially in the presence of interference or noise. This occurs

because it is more difficult for the radio receiver to detect which symbol it has received. Toovercome this, additional error coding may be required to protect against the possibility of errors.

However increased levels of error protection require additional data to be sent and this reduces

the data throughput of the required data.


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In view of this, it is found that when the signal is poor GMSK can be more effective than 8PSK,

and as a result, the overall EDGE modulation scheme is a mixture of GMSK and 8PSK.

GSM EDGE time slots

EDGE, GPRS and GSM have to all operate along side each other in a network. It is a primaryrequirement that the evolutionary technologies are able to all operate on the same network. Thisensures the service offered to existing customers using older phones along with those paying

additional rates for the premium EDGE services. This means that the network has to support both

services operating simultaneously.

Accordingly different slots within the traffic frames will need to be able to support differentstructures and different types of modulation dependent upon the phones being used, the callsbeing made and the prevailing conditions. It is quite possible that one slot may be supporting a

GSM call, the next a GPRS data connection, and the third an EDGE connection using GMSK or

8PSK.

GPRS network architecture

- a summary, overview or tutorial about the basics of the GSM EDGE datacoding and error correction and MCS classes.

Most of the data being sent over an EDGE link will consist of TCP/IP packets. These packets are

longer than a single EDGE packet payload and therefore it is necessary to split the TCP/IP

packets into smaller section and these are known as "chunks

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