48929564 GSM Tutorial

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GSM Tutorial

This GSM tutorial is split into several pages:

[1] GSM basics tutorial and overview

[2] GSM history

[3] GSM network architecture

[4] GSM interfaces

[5] GSM radio air interface / access network [6] GSM frames, superframes and hyperframes

[7] GSM frequency bands and allocations

[8] GSM power class, control and amplifiers

[9] GSM physical and logical channels

[10] GSM codecs / vocoders

[11] GSM handover or handoff

GSM basics tutorial and overview [1]

- a tutorial, description, overview about the basics of GSM - Global System for Mobile

communications with details of its radio interface, infrastructure technology, network and operation.

The 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 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 of the basic aims was to provide a system that would enable greater capacity to beachieved than the previous first generation analogue systems. GSM achieved this by

using a digital TDMA (time division multiple access approach). By adopting thistechnique 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. To

achieve 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 it provided a simple, low fixed cost.

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

started:

y It should offer good subjective speech quality

y It should have a low phone or terminal cost

y Terminals should be able to be handheld

y The system should support international roaming

y It should offer good spectral efficiency

y 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 thenetwork or infrastructure 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 abase 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 be routed 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. 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 operatorsto 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 of the highlight features of technical interest.

SPECIFICATION SUMMARY FOR GSM CELLULAR SYSTEM

Multiple access technology FDMA / TDMA

Duplex technique FDD

Uplink frequency band 933 -960 MHz

(basic 900 MHz band only)

Downlink frequency band 890 - 915 MHz

(basic 900 MHz band only)

Channel spacing 200 kHz

Modulation GMSK

Speech coding Various - original was RPE-LTP/13

Speech channels per RF channel 8

Channel data rate 270.833 kbps

Frame duration 4.615 ms

GSM summary

The GSM system is the most successful cellular telecommunications system to date.

With subscriber numbers running into billions and still increasing, it has been proved

to have met its requirements. Further pages of this GSM tutorial or overview detail



many of the GSM basics from the air interface, frame and slot structures to the logical

and physical channels as well as details about the GSM network.

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 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 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 limitedfrequency 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 technologyneeded to be more efficient. With the experience gained from the NMT system, showing

that it was possible to develop a system across national boundaries, and with thepolitical 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 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 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 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 begained, 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.

GSM launch dates

Work continued and a launch date for the new GSM system of 1991 was set for aninitial 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 waspassed 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-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 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 subscribers and 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

Originally it had been intended that GSM would operate on frequencies in the 900 MHzcellular band. In September 1993, the British operator Mercury One-to-One launched a

network. Termed DCS 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 additional 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 in1994. 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 total number of GSM subscribers reached 1 billion. Attaining this figure wascelebrated 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 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 developments of 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:

y Mobile station (MS)

y Base-station subsystem (BSS)

y Network and Switching Subsystem (NSS)

y 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 mobile

phones are the section of a GSM cellular

network that the user sees and operates. In

recent years their size has fallen dramatically

while the level of functionality has greatlyincreased. 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

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 thephone 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

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

y Base Transceiver Station (BTS): The BTS used in a GSM network comprises the

radio transmitter 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 Um interface with its associated protocols.

y Base Station Controller (BSC): The BSC forms the next stage back into the GSM

network. 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 the group 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 wholemobile network. The major elements within the core network include:

y Mobile Switching services Centre (MSC): The main element within the core

network area 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 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.y Home Location Register (HLR): This database contains all the administrative

information 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 for operational reasons.y Visitor Location Register (VLR): This contains selected information from the

HLR that enables the selected services for the individual subscriber to be

provided. The VLR can be 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 more convenient.

y Equipment Identity Register (EIR): The EIR is the entity that decides whether a

given mobile equipment may be allowed onto the network. Each mobile

equipment has a number known as the International Mobile Equipment Identity.

This number, as mentioned above, is installed in the equipment and is checked bythe 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.

y Authentication Centre (AuC): The AuC is a protected database that contains the

secret key also contained in the user's SIM card. It is used for authentication and

for ciphering on the radio channel.

y Gateway Mobile Switching Centre (GMSC): The GMSC is the point to which a ME

terminating 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" of a 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.

y SMS Gateway (SMS-G): The SMS-G or SMS gateway is the term that is used to

collectively describe the two Short Message Services Gateways defined in the

GSM standards. 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.

Operation and Support Subsystem (OSS)

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 trafficload 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

between the 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 theseinterfaces 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 a mobile (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 not been 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 the MSC. The interface carries information to enable the channels, timeslotsand the like to be allocated 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 the 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 to complete 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.

6. D interface The D interface is situated between the VLR and HLR. It uses theMAP/D protocol 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/E protocol.

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 the IMEI of the ME gaining access to the network.

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 locationupdate 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.

Messages exchanged 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 the functionality 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 Mobile

Communications 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 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 divisionmultiplexed, all had a considerable impact on the performance of the system as a whole.

For example, the modulation 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.



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 was used for the GSM system for a variety of reasons:

y It is resilient to noise when compared to many other forms of modulation.

y Radiation outside the accepted bandwidth is lower than other forms of phase

shift keying.

y It has a constant power level which allows higher efficiency RF power amplifiersto 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 be used 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 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 themanagement 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. 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 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

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 frames are 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

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. This offset 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 a spacesaving.

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.

y Normal burst uplink and downlink y Synchronisation burst downlink

y Frequency correction burst downlink y 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.

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 to ramp up its power

2. 57 data bits: This block of data is used to carry information, and most often

contains the digitised 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 field

3. 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 the same 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.

7. 3 tail bits These final bits within the GSM burst are used to enable thetransmitter power to 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 is

introduced to prevent transmitted bursts from different mobiles overlapping. 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 for the

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.

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

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 has accessed 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

margin 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 isparticularly 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 totransmit 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 and noise 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.



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 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.

GSM frame consisting of eight slots

The basic GSM frame defines thestructure 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 forms the 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 beacon frequency.

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:



y Traffic multiframe: The Traffic Channel frames are organised into multiframes

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 of the remaining

bursts is then used to accommodate the SACCH, the remaining frame remaining

free. The actual position used alternates between position 12 and 25.

y Control multiframe: the Control Channel multiframe that comprises 51 bursts

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 beacon frequency as well.This multiframe is subdivided into logical channels which are time-scheduled.

These logical channels and functions include the following:

o Frequency correction burst

o Synchronisation burst

o 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 26bursts 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 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 uniquesequential 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:

y Frequency hopping: Frequency hopping is a feature that is optional within the

GSM system. 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.

y Encryption: The encryption process is synchronised over the GSM hyperframeperiod where a counter is used and the encryption process will repeat with each

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



GSM Frame Structure

Summary

GSM frame structure

summary

By structuring the GSMsignalling into frames,

multiframes, superframes

and hyperframes, the timing

and organisation is set into

an orderly format that

enables both the GSM mobile

and base station to

communicate in a reliable

and efficient manner. The

GSM frame structure formsthe basis onto which the other forms of frame and hence the various GSM channels are

built.

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 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 3GPP TS 45.005.

BAND UPLINK

(MHZ)

DOWNLINK

(MHZ)

COMMENTS

380 380.2 - 389.8 390.2 - 399.8

410 410.2 - 419.8 420.2 - 429.8

450 450.4 - 457.6 460.4 - 467.6

480 478.8 - 486.0 488.8 - 496.0

710 698.0 - 716.0 728.0 - 746.0

750 747.0 - 762.0 777.0 - 792.0

810 806.0 - 821.0 851.0 - 866.0

850 824.0 - 849.0 869.0 - 894.0

900 890.0 - 915.0 935.0 - 960.0 P-GSM, i.e. Primary or standard GSM

allocation

900 880.0 - 915.0 925.0 - 960.0 E-GSM, i.e. Extended GSM allocation

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

900 870.4 - 876.0 915.4 - 921.0 T-GSM

1800 1710.0 -

1785.0

1805.0 -

1880.0

1900 1850.0 -

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 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 band phones 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 givinggood 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.

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 designations have been allocated to indicate the power capability of variousmobiles.

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 levelsufficient 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 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

POWER

LEVEL

NUMBER

POWER

OUTPUT

LEVEL

DBM

29 36

30 34

31 32

0 30

1 28



POWER

LEVEL

NUMBER

POWER

OUTPUT

LEVEL

DBM

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 LEVELNUMBER

POWEROUTPUT LEVEL

DBM

30 33

31 32

0 30

1 28

2 26

3 24

4 22

5 20

6 18

7 16

8 14

9 12



POWER LEVEL

NUMBER

POWER

OUTPUT LEVEL

DBM

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 isnecessary for the base station 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

CLASSNUMBER

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 PL0 30 dBm /

1W

PL0 30 dBm /

1W

2 PL2 39dBm /

8W

PL3 24 dBm/

250 mW

PL3 24 dBm /

250 mW

3 PL3 37dBm /

5W

PL29 36 dBm /

4W

PL30 33 dBm /

2W

4 PL4 33dBm /

2W

5 PL5 29 dBm /800 mW



GSM power amplifier design considerations

One of the main considerations for the RF power amplifier design in any mobile phone

is its efficiency. 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 channels [9]- a tutorial, description, overview of GSM channels including transport and logical

channels, SACCH, SDCCH, FACCH, etc.

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

separated into physical 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 enable the 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.

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 enablingcall 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.udio 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 digitised speech 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 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 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 highenough 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 alimited 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 it is possible for voice codecs to give

very large levels of compression.

A variety of different codec methodologies are used for GSM codecs:



y CELP: The CELP or Code Excited Linear Prediction codec is a vocoder algorithm

that was 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. 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

"Analysis by 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.

y 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 as indicated by

the name.

y VSELP codec: The VSELP or Vector Sum Excitation Linear Prediction codec. Oneof the 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 offering far 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

audio codecs are not as widely used these days, they are still described here as theyform 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

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 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 thetechnology 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 to the poor quality perceived by the users of the original RPE-LPC codec. Thisnew codec gave much 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-

channels that can maintain separate calls. By doing this, network operators can doublethe 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 bpsdata 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

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 overallspeech 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 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

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 byreducing the source coding and 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.

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 designationof G.722.2, is based on the earlier popular Adaptive Multi-Rate, AMR codec. AMR-WB

also uses 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 has a 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 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 Activity Detector (VAD); Discontinuous Transmission (DTX)functionality for GSM; and Source Controlled Rate (SCR) functionality for UMTS

applications. Further functionality includes 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 are coded separately to reduce the codec complexity. This split also serves to

focus the bit allocation into 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 adaptiveexcitation 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 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 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 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 orwhen 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.

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

useful when ambient audio noise levels are high.







BIT

RATE

(KBPS)

NOTES



23.05 Not suggested for full rate GSM channels.

23.85 Not 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 aresult, 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 configurations that are available:

y Configuration A: 6.6, 8.85, and 12.65 kbit/s

y Configuration B: 6.6, 8.85, 12.65, and 15.85 kbit/s

y Configuration C: 6.6, 8.85, 12.65, and 23.85 kbit/sIt 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 including multimedia messaging, streaming audio, etc.

GSM codecs summary

There has been a considerable improvement in the GSM audio codecs that have been in

use. Starting with the original RTE-LPC speech codec and then moving through the

Enhanced Full Rate, EFR codec and the GSM half rate codec to the AMR codec which isnow the most widely used and provides a variable rate that can be tailored to the

individual conditions. Also the newer AMR-WB codec wills ee increasing use. Although

with newer technologies such as LTE, Long Term Evolution which uses an all IP based

system, codecs are still used to provide data compression and improved spectral

efficiency, the idea of a codec will still be used, although some of the GSM codecs that

are in use today will be superseded.

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, isthat the system is split into many small cells to provide good frequency re-use and

coverage. However as the 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.



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 GSM handover 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:

y Intra-BTS handover: This form of GSM handover occurs if it is required to

change 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.

y 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 into anothercontrolled 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 communicating with the mobile.

y Inter-BSC handover: When the mobile moves out of the range of cells controlled

by one 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 by the MSC.

y 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 RFsection 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 theother radio channels looking for beacon frequencies that may be stronger or more

suitable. In addition to this, when the mobile communicates with a particular BTS, one

of the responses it makes is to send out a list of the radio channels of the beacon

frequencies of neighbouring BTSs via the Broadcast Channel (BCCH).

The mobile scans these and reports back the quality of the link to the BTS. In this way

the mobile assists in the handover decision and as a result this form of GSM handover is

known as Mobile Assisted Hand Over (MAHO).



The network knows the quality of the link between the mobile and the BTS as well as

the strength of local BTSs as reported back by the mobile. It also knows the availability

of channels in the nearby cells. As a result it has all the information it needs to be able

to make a decision about whether it needs to hand the mobile over from one BTS to

another.

If the network decides that it is necessary for the mobile to hand over, it assigns a new

channel and time slot to the mobile. It informs the BTS and the mobile of the change.

The mobile then retunes during the period it is not transmitting or receiving, i.e. in anidle period.

A key element of the GSM handover is timing and synchronisation. There are a number

of possible scenarios that may occur dependent upon the level of synchronisation.

y Old and new BTSs synchronised: In this case the mobile is given details of the

new physical channel in the neighbouring cell and handed directly over. The

mobile may optionally transmit four access bursts. These are shorter than the

standard bursts and thereby any effects of poor synchronisation do not cause

overlap with other bursts. However in this instance where synchronisation is

already good, these bursts are only used to provide a fine adjustment.

y Time offset between synchronised old and new BTS: In some instances theremay be a time offset between the old and new BTS. In this case, the time offset is

provided so that the mobile can make the adjustment. The GSM handover then

takes place as a standard synchronised handover.

y Non-synchronised handover: When a non-synchronised cell handover takes

place, the mobile transmits 64 access bursts on the new channel. This enables the

base station to determine and adjust the timing for the mobile so that it can

suitably access the new BTS. This enables the mobile to re-establish the

connection through the new BTS with the correct timing.

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 one technology 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 very different 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:

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

category of handover:

o Blind handover: This form of handover occurs when the base station

hands 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 for the new

cell. In this mode, the network selects what it believes to be the optimum



GSM based station. The mobile first locates the broadcast channel of the

new cell, gains timing synchronisation and then carries out non-

synchronised intercell handover.

o Compressed mode handover: using this form of handover the mobile uses

the gaps I transmission that occur to analyse the reception of local GSM

base stations using the neighbour list to select suitable candidate base

stations. Having selected a suitable base station the handover takes place,

again without any time synchronisation having occurred.y Handover from GSM to UMTS / WCDMA: This form of handover is supported

within GSM and a "neighbour list" was established to enable this occur easily. As

the GSM / 2G network is normally more extensive than the 3G network, this type

of handover does not normally occur when the mobile leaves a coverage area and

must quickly find a new base station to maintain contact. The handover from GSM

to UMTS occurs to provide an improvement in performance and can normally

take place only when the conditions are right. The neighbour list will inform the

mobile when this may happen.

Summary

GSM handover is one of the major elements in performance that users will notice. As a

result it is normally one of the Key Performance Indicators (KPIs) used by operators to

monitor performance. Poor handover or handoff performance will normally result in

dropped calls, and users find this particularly annoying. Accordingly network operators

develop and maintain their networks to ensure that an acceptable performance is

achieved. In this way they can reduce what is called "churn" where users change from

one network to another.

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