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.
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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.
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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
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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
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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.
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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.
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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
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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
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"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.
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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.
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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 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
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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:
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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
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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.
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It is also necessary for the system to add background or comfort noise when the
transmitter is turned off because complete silence can be very disconcerting for the
listener. Accordingly this is added as appropriate. The noise is controlled by the SID
(silence indication descriptor).
GSM Frame 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:
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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.
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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.
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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.
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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.
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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
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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
GSM power level table for GSM 1800
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
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POWER LEVEL
NUMBER
POWER
OUTPUT LEVEL
DBM
10 10
11 8
12 6
13 4
14 2
15 0
GSM power level table for GSM 1900
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
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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.
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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:
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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
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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.
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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-
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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.
15.85 Higher bit rate used to give cleaner speech and is particularly
useful when ambient audio noise levels are high.
18.25 Higher bit rate used to give cleaner speech and is particularly
useful when ambient audio noise levels are high.
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BIT
RATE
(KBPS)
NOTES
19.85 Higher 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.
23.85 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.
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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).
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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
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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.