Gsm (an overview)
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  • 1. AEC,ASANSOL Global System for Mobile communication(GSM) An Overview Chandra Kishor 10/23/2012A description, overview about the basics of GSM - Global System for Mobile communications withdetails of its radio interface, infrastructure technology, network and operation.
  • 2. Global System for Mobile communication(GSM) 2012GSM basics and overview:(a description, overview about the basics of GSM - Global System for Mobilecommunications 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 hasbeen a particularly successful cellular phone technology for a variety of reasons including theability to roam worldwide with the certainty of being able to be able to operate on GSMnetworks 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 becameclear this cellular technology was being used worldwide the meaning of GSM was changed toGlobal System for Mobile Communications. Since this cellular technology was first deployedin 1991, the use of GSM has grown steadily, and it is now the most widely cell phone systemin the world. GSM reached the 1 billion subscriber point in February 2004, and is now wellover 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. Oneof the basic aims was to provide a system that would enable greater capacity to be achievedthan the previous first generation analogue systems. GSM achieved this by using a digitalTDMA (time division multiple access approach). By adopting this technique more userscould be accommodated within the available bandwidth. In addition to this, ciphering of thedigitally encoded speech was adopted to retain privacy. Using the earlier analogue cellulartechnologies it was possible for anyone with a scanner receiver to listen to calls and a numberof famous personalities had been "eavesdropped" with embarrassing consequences.GSM servicesSpeech or voice calls are obviously the primary function for the GSM cellular system. Toachieve this the speech is digitally encoded and later decoded using a vocoder. A variety ofvocoders are available for use, being aimed at different scenarios.In addition to the voice services, GSM cellular technology supports a variety of other dataservices. Although their performance is nowhere near the level of those provided by 3G, theyare nevertheless still important and useful. A variety of data services are supported with userdata rates up to 9.6 kbps. Services including Group 3 facsimile, videotext and teletex can besupported.One service that has grown enormously is the short message service. Developed as part of theGSM specification, it has also been incorporated into other cellular technologies. It can bethought 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 messagesof a reasonable length. This service has become particularly popular, initially with the youngas it provided a simple, low fixed cost.By: Chandra Kishor Page 2
  • 3. Global System for Mobile communication(GSM) 2012GSM basicsThe GSM cellular technology had a number of design aims when the development started: It should offer good subjective speech quality It should have a low phone or terminal cost Terminals should be able to be handheld The system should support international roaming It should offer good spectral efficiency The system should offer ISDN compatibilityThe resulting GSM cellular technology that was developed provided for all of these. Theoverall system definition for GSM describes not only the air interface but also the network orinfrastructure technology. By adopting this approach it is possible to define the operation ofthe whole network to enable international roaming as well as enabling network elements fromdifferent manufacturers to operate alongside each other, although this last feature is notcompletely true, especially with older items.GSM cellular technology uses 200 kHz RF channels. These are time division multiplexed toenable up to eight users to access each carrier. In this way it is a TDMA / FDMA system.The base transceiver stations (BTS) are organised into small groups, controlled by a basestation controller (BSC) which is typically co-located with one of the BTSs. The BSC with itsassociated BTSs is termed the base station subsystem (BSS).Further into the core network is the main switching area. This is known as the mobileswitching centre (MSC). Associated with it is the location registers, namely the homelocation register (HLR) and the visitor location register (VLR) which track the location ofmobiles 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 mobilebefore it is allowed onto the network and for billing. The operation of these are explained inthe following pages.Last but not least is the mobile itself. Often termed the ME or mobile equipment, this is theitem that the end user sees. One important feature that was first implemented on GSM wasthe use of a Subscriber Identity Module. This card carried with it the users identity and otherinformation to allow the user to upgrade a phone very easily, while retaining the sameidentity on the network. It was also used to store other information such as "phone book" andother items. This item alone has allowed people to change phones very easily, and this hasfuelled the phone manufacturing industry and enabled new phones with additional features tobe launched. This has allowed mobile operators to increase their average revenue per user(ARPU) by ensuring that users are able to access any new features that may be launched onthe network requiring more sophisticated phones.By: Chandra Kishor Page 3
  • 4. Global System for Mobile communication(GSM) 2012GSM system overviewThe table below summarises the main points of the GSM system specification, showing someof the highlight features of technical interest. Specification Summary for GSM Cellular System Multiple access technology FDMA / TDMA Duplex technique FDD Uplink frequency band 890 - 915 MHz (basic 900 MHz band only) Downlink frequency band 933 -960 MHz (basic 900 MHz band only) Channel spacing 200 kHz Modulation GMSK Speech coding Various - original was RPE-LTP/13 Speech channels per RF channel 8 Channel data rate 270.833 kbps Frame duration 4.615 ms Today the GSM cell or mobile phone system is the most popular in theworld. GSM handsets are widely available at good prices and the networks are robust andreliable. 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 technologiesincluding GPRS and EDGE. To achieve this level of success has taken many years and is theresult of both technical development and international cooperation. The GSM history can beseen to be a story of cooperation across Europe, and one that nobody thought would lead tothe success that GSM is today.The first cell phone systems that were developed were analogue systems. Typically they usedfrequency-modulated carriers for the voice channels and data was carried on a separateshared control channel. When compared to the systems employed today these systems werecomparatively straightforward and as a result a vast number of systems appeared. Two of themajor systems that were in existence were the AMPS (Advanced Mobile Phone System) thatwas used in the USA and many other countries and TACS (Total Access CommunicationsSystem) 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 commerciallydeployed was the Nordic Mobile Telephone system (NMT). This was developed by aconsortium of companies in Scandinavia and proved that international cooperation waspossible.The success of these systems proved to be their downfall. The use of all the systems installedaround the globe increased dramatically and the effects of the limited frequency allocationswere soon noticed. To overcome these a number of actions were taken. A system known asBy: Chandra Kishor Page 4
  • 5. Global System for Mobile communication(GSM) 2012E-TACS or Extended-TACS was introduced giving the TACS system further channels. In theUSA another system known as Narrowband AMPS (NAMPS) was developed.New approachesNeither 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 thatit was possible to develop a system across national boundaries, and with the political situationin 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 bringsignificant 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 theauspices of the Conference of European Posts and Telegraphs (CEPT). They formed a studygroup called the Groupe Special Mobile ( GSM ) to study and develop a pan-European publicland mobile system. Several basic criteria that the new cellular technology would have tomeet were set down for the new GSM system to meet. These included: good subjectivespeech quality, low terminal and service cost, support for international roaming, ability tosupport handheld terminals, support for range of new services and facilities, spectralefficiency, and finally ISDN compatibility.With the levels of under-capacity being projected for the analogue systems, this gave a realsense of urgency to the GSM development. Although decisions about the exact nature of thecellular technology were not taken at an early stage, all parties involved had been workingtoward a digital system. This decision was finally made in February 1987. This gave a varietyof advantages. Greater levels of spectral efficiency could be gained, and in addition to this theuse of digital circuitry would allow for higher levels of integration in the circuitry. This inturn would result in cheaper handsets with more features. Nevertheless significant hurdlesstill needed to be overcome. For example, many of the methods for encoding the speechwithin a sufficiently narrow bandwidth needed to be developed, and this posed a significantrisk to the project. Nevertheless the GSM system had been started.GSM launch datesWork continued and a launch date for the new GSM system of 1991 was set for an initiallaunch of a service using the new cellular technology with limited coverage and capability tobe followed by a complete roll out of the service in major European cities by 1993 andlinking of the areas by 1995.Meanwhile technical development was taking place. Initial trials had shown that timedivision multiple access techniques offered the best performance with the technology thatwould be available. This approach had the support of the major manufacturing companieswhich 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.By: Chandra Kishor Page 5
  • 6. Global System for Mobile communication(GSM) 2012Further impetus was given to the GSM project when in 1989 the responsibility was passed tothe newly formed European Telecommunications Standards Institute (ETSI). Under theauspices of ETSI the specification took place. It provided functional and interfacedescriptions for each of the functional entities defined in the system. The aim was to providesufficient guidance for manufacturers that equipment from different manufacturers would beinteroperable, while not stopping innovation. The result of the specification work was a set ofdocuments extending to more than 6000 pages. Nevertheless the resultant phone systemprovided a robust, feature-rich system. The first roaming agreement was signed betweenTelecom Finland and Vodafone in the UK. Thus the vision of a pan-European network wasfast 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. Terminalsstarted to become available in mid 1992 and the real launch took place in the latter part ofthat year. With such a new service many were sceptical as the analogue systems were still inwidespread use. Nevertheless by the end of 1993 GSM had attracted over a millionsubscribers and there were 25 roaming agreements in place. The growth continued and thenext million subscribers were soon attracted.Global GSM usageOriginally GSM had been planned as a European system. However the first indication thatthe success of GSM was spreading further a field occurred when the Australian networkprovider, Telstra signed the GSM Memorandum of Understanding.FrequenciesOriginally 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 anetwork. Termed DCS 1800 it operated at frequencies in a new 1800 MHz band. By adoptingnew frequencies new operators and further competition was introduced into the market apartfrom allowing additional spectrum to be used and further increasing the overall capacity. Thistrend was followed in many countries, and soon the term DCS 1800 was dropped in favour ofcalling it GSM as it was purely the same cellular technology but operating on a differentfrequency band. In view of the higher frequency used the distances the signals travelled wasslightly 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, andaccordingly this enabled GSM to gain a foothold in the US market. This system was knownas PCS 1900 (Personal Communication System).GSM successWith GSM being used in many countries outside Europe this reflected the true nature of thename which had been changed from Groupe Special Mobile to Global System for MobileBy: Chandra Kishor Page 6
  • 7. Global System for Mobile communication(GSM) 2012communications. The number of subscribers grew rapidly and by the beginning of 2004 thetotal number of GSM subscribers reached 1 billion. Attaining this figure was celebrated at theCannes 3GSM conference held that year. Figures continued to rise, reaching and then wellexceeding the 3 billion mark. In this way the history of GSM has shown it to be a greatsuccess.The GSM technical specifications define the different elements within the GSM networkarchitecture. It defines the different elements and the ways in which they interact to enablethe overall network operation to be maintained.The GSM network architecture is now well established and with the other later cellularsystems now established and other new ones being deployed, the basic GSM networkarchitecture 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 hasbeen maintained, and the elements described below perform the same functions as they didwhen the original GSM system was launched in the early 1990s.GSM network architecture elementsThe GSM network architecture as defined in the GSM specifications can be grouped into fourmain areas: Mobile station (MS) Base-station subsystem (BSS) Network and Switching Subsystem (NSS) Operation and Support Subsystem (OSS)Simplified GSM Network ArchitectureBy: Chandra Kishor Page 7
  • 8. Global System for Mobile communication(GSM) 2012Mobile stationMobile stations (MS), mobile equipment (ME) or as they are most widely known, cell ormobile phones are the section of a GSM cellular network that the user sees and operates. Inrecent 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 themain 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 andto be transmitted. It also contains a number known as the International Mobile EquipmentIdentity (IMEI). This is installed in the phone at manufacture and "cannot" be changed. It isaccessed by the network during registration to check whether the equipment has beenreported as stolen.The SIM or Subscriber Identity Module contains the information that provides the identity ofthe user to the network. It contains are variety of information including a number known asthe International Mobile Subscriber Identity (IMSI).Base Station Subsystem (BSS)The Base Station Subsystem (BSS) section of the GSM network architecture that isfundamentally associated with communicating with the mobiles on the network. It consists oftwo elements: 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. 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 thecore network. It provides the main control and interfacing for the whole mobile network. Themajor elements within the core network include: 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 additionalBy: Chandra Kishor Page 8
  • 9. Global System for Mobile communication(GSM) 2012 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. 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. 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. 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 by the network during registration. Dependent upon the information held in the EIR, the mobile may be allocated one of three states - allowed onto the network, barred access, or monitored in case its problems. Authentication Centre (AuC): The AuC is a protected database that contains the secret key also contained in the users SIM card. It is used for authentication and for ciphering on the radio channel. Gateway Mobile Switching Centre (GMSC): The GMSC is the point to which a ME terminating call is initially routed, without any knowledge of the MSs 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. 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 networkarchitecture that is connected to components of the NSS and the BSC. It is used to controland monitor the overall GSM network and it is also used to control the traffic load of theBSS. It must be noted that as the number of BS increases with the scaling of the subscriberpopulation some of the maintenance tasks are transferred to the BTS, allowing savings in thecost of ownership of the system.By: Chandra Kishor Page 9
  • 10. Global System for Mobile communication(GSM) 2012The network structure is defined within the GSM standards. Additionally each interfacebetween the different elements of the GSM network is also defined. This facilitates theinformation interchanges can take place. It also enables to a large degree that networkelements from different manufacturers can be used. However as many of these interfaceswere not fully defined until after many networks had been deployed, the level ofstandardisation 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, timeslots and 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 the MAP/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 location update procedure. 10. H interface The H interface exists between the MSC the SMS-G. It transfers short messages and uses the MAP/H protocol. 11. I interface The I interface can be found between the MSC and the ME. 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 manymight like, they do at least provide a large element of the definition required, enabling thefunctionality of GSM network entities to be defined sufficiently.By: Chandra Kishor Page 10
  • 11. Global System for Mobile communication(GSM) 2012One of the key elements of the development of the GSM, Global System for MobileCommunications was the development of the GSM air interface. There were manyrequirements that were placed on the system, and many of these had a direct impact on the airinterface. Elements including the modulation, GSM slot structure, burst structure and the likewere all devised to provide the optimum performance.During the development of the GSM standard very careful attention was paid to aspectsincluding the modulation format, the way in which the system is time division multiplexed,all had a considerable impact on the performance of the system as a whole. For example, themodulation format for the GSM air interface had a direct impact on battery life and the timedivision format adopted enabled the cellphone handset costs to be considerably reduced asdetailed later.GSM signal and GMSK modulation characteristicsThe core of any radio based system is the format of the radio signal itself. The carrier ismodulated using a form of phase sift keying known as Gaussian Minimum Shift Keying(GMSK). GMSK was used for the GSM system for a variety of reasons: It is resilient to noise when compared to many other forms of modulation. Radiation outside the accepted bandwidth is lower than other forms of phase shift keying. It has a constant power level which allows higher efficiency RF power amplifiers to be used in the handset, thereby reducing current consumption and conserving battery life.Note on GMSK:GMSK, Gaussian Minimum Shift Keying is a form of phase modulation that is used in anumber of portable radio and wireless applications. It has advantages in terms of spectralefficiency as well as having an almost constant amplitude which allows for the use of moreefficient transmitter power amplifiers, thereby saving on current consumption, a critical issuefor battery power equipment.The nominal bandwidth for the GSM signal using GMSK is 200 kHz, i.e. the channelbandwidth and spacing is 200 kHz. As GMSK modulation has been used, the unwanted orspurious emissions outside the nominal bandwidth are sufficiently low to enable adjacentchannels to be used from the same base station. Typically each base station will be allocateda 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 systemby splitting the carrier into eight time slots. The basic carrier is able to support a datathroughput of approximately 270 kbps, but as some of this supports the managementoverhead, the data rate allotted to each time slot is only 24.8 kbps. In addition to this errorcorrection is required to overcome the problems of interference, fading and general dataerrors that may occur. This means that the available data rate for transporting the digitallyencoded speech is 13 kbps for the basic vocoders.By: Chandra Kishor Page 11
  • 12. Global System for Mobile communication(GSM) 2012GSM slot structure and multiple access schemeGSM uses a combination of both TDMA and FDMA techniques. The FDMA elementinvolves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrierfrequencies spaced 200 kHz apart as already described.The carriers are then divided in time, using a TDMA scheme. This enables the different usersof the single radio frequency channel to be allocated different times slots. They are then ableto use the same RF channel without mutual interference. The slot is then the time that isallocated to the particular user, and the GSM burst is the transmission that is made in thistime.Each GSM slot, and hence each GSM burst lasts for 0.577 mS (15/26 mS). Eight of theseburst periods are grouped into what is known as a TDMA frame. This lasts for approximately4.615 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 theframes are organised into what are termed multiframes and superframes to provide overallsynchronisation.GSM slot structureThese GSM slot is the smallest individual time period that is available to each mobile. It has adefined 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 transmitting148 bits of information. This data can be used for carrying voice data, control andsynchronisation data.GSM slots showing offset between transmit and receiveIt can be seen from the GSM slot structure that the timing of the slots in the uplink and thedownlink are not simultaneous, and there is a time offset between the transmit and receive.By: Chandra Kishor Page 12
  • 13. Global System for Mobile communication(GSM) 2012This offset in the GSM slot timing is deliberate and it means that a mobile that which isallocated the same slot in both directions does not transmit and receive at the same time. Thisconsiderably reduces the need for expensive filters to isolate the transmitter from the receiver.It also provides a space saving.GSM burstThe GSM burst, or transmission can fulfil a variety of functions. Some GSM bursts are usedfor carrying data while others are used for control information. As a result of this a number ofdifferent types of GSM burst are defined. Normal burst uplink and downlink Synchronisation burst downlink Frequency correction burst downlink Random Access (Shortened Burst) uplinkGSM normal burstThis GSM burst is used for the standard communications between the basestation and themobile, and typically transfers the digitised voice data.The structure of the normal GSM burst is exactly defined and follows a common format. Itcontains data that provides a number of different functions: 1. 3 tail bits: These tail bits at the start of the GSM burst give time for 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 the transmitter 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.By: Chandra Kishor Page 13
  • 14. Global System for Mobile communication(GSM) 2012GSM Normal BurstGSM synchronisation burstThe purpose of this form of GSM burst is to provide synchronisation for the mobiles on thenetwork. 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 BurstGSM frequency correction burstWith the information in the burst all set to zeros, the burst essentially consists of a constantfrequency 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 BurstGSM random access burstThis form of GSM burst used when accessing the network and it is shortened in terms of thedata carried, having a much longer guard period. This GSM burst structure is used to ensureBy: Chandra Kishor Page 14
  • 15. Global System for Mobile communication(GSM) 2012that it fits in the time slot regardless of any severe timing problems that may exist. Once themobile has accessed the network and timing has been aligned, then there is no requirementfor 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 BurstGSM discontinuous transmission (DTx)A further power saving and interference reducing facility is the discontinuous transmission(DTx) capability that is incorporated within the specification. It is particularly useful becausethere are long pauses in speech, for example when the person using the mobile is listening,and during these periods there is no need to transmit a signal. In fact it is found that a personspeaks for less than 40% of the time during normal telephone conversations. The mostimportant element of DTx is the Voice Activity Detector. It must correctly distinguishbetween voice and noise inputs, a task that is not trivial. If a voice signal is misinterpreted asnoise, the transmitter is turned off an effect known as clipping results and this is particularlyannoying to the person listening to the speech. However if noise is misinterpreted as a voicesignal 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 isturned off because complete silence can be very disconcerting for the listener. Accordinglythis is added as appropriate. The noise is controlled by the SID (silence indication descriptor).GSM Frame Structure:The GSM system has a defined GSM frame structure to enable the orderly passage ofinformation. The GSM frame structure establishes schedules for the predetermined use oftimeslots.By establishing these schedules by the use of a frame structure, both the mobile and the basestation are able to communicate not only the voice data, but also signalling informationwithout the various types of data becoming intermixed and both ends of the transmissionknowing exactly what types of information are being transmitted.By: Chandra Kishor Page 15
  • 16. Global System for Mobile communication(GSM) 2012The GSM frame structure provides the basis for the various physical channels used withinGSM, and accordingly it is at the heart of the overall system.Basic GSM frame structureThe basic element in the GSM frame structure is the frame itself. This comprises the eightslots, each used for different users within the TDMA system. As mentioned in another pageof the tutorial, the slots for transmission and reception for a given mobile are offset in time sothat the mobile does not transmit and receive at the same time.GSM frame consisting of eight slotsThe basic GSM frame defines the structure upon which all the timing and structure of theGSM messaging and signalling is based. The fundamental unit of time is called a burst periodand it lasts for approximately 0.577 ms (15/26 ms). Eight of these burst periods are groupedinto 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 oneburst period allocated in each TDMA frame.In simplified terms the base station transmits two types of channel, namely traffic andcontrol. Accordingly the channel structure is organised into two different types of frame, onefor the traffic on the main traffic carrier frequency, and the other for the control on the beaconfrequency.GSM multiframeThe GSM frames are grouped together to form multiframes and in this way it is possible toestablish a time schedule for their operation and the network can be synchronised.There are several GSM multiframe structures: Traffic multiframe: The Traffic Channel frames are organised into multiframes consisting of 26 bursts and taking 120 ms. In a traffic multiframe, 24 bursts are used for traffic. These areBy: Chandra Kishor Page 16
  • 17. Global System for Mobile communication(GSM) 2012 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. 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 SuperframeMulti frames are then constructed into super rames taking 6.12 seconds. These consist of 51traffic multiframes or 26 control multiframes. As the traffic multiframes are 26 bursts longand the control multiframes are 51 bursts long, the different number of traffic and controlmultiframes within the superframe, brings them back into line again taking exactly the sameinterval.GSM HyperframeAbove this 2048 superframes (i.e. 2 to the power 11) are grouped to form one hyperframewhich repeats every 3 hours 28 minutes 53.76 seconds. It is the largest time interval withinthe GSM frame structure.Within the GSM hyperframe there is a counter and every time slot has a unique sequentialnumber comprising the frame number and time slot number. This is used to maintainsynchronisation of the different scheduled operations with the GSM frame structure. Theseinclude functions such as: Frequency hopping: Frequency hopping is a feature that is optional within the 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. Encryption: The encryption process is synchronised over the GSM hyperframe period 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 3 hours and accordingly it is unlikely that security will be compromised as a result.By: Chandra Kishor Page 17
  • 18. Global System for Mobile communication(GSM) 2012GSM Frame Structure SummaryGSM Frequencies and Frequency Bands:Although it is possible for the GSM cellular system to work on a variety of frequencies, theGSM standard defines GSM frequency bands and frequencies for the different spectrumallocations that are in use around the globe. For most applications the GSM frequencyallocations fall into three or four bands, and therefore it is possible for phones to be used forglobal roaming.While the majority of GSM activity falls into just a few bands, for some specialistapplications, or in countries where spectrum allocation requirements mean that the standardbands cannot be used, different allocations may be required. Accordingly for most globalroaming dual band, tri-band or quad-band phones will operate in most countries, although insome instances phones using other frequencies may be required.GSM band allocationsThere is a total of fourteen different recognised GSM frequency bands. These are defined in3GPP TS 45.005.By: Chandra Kishor Page 18
  • 19. Global System for Mobile communication(GSM) 2012 Band Uplink Downlink Comments (MHz) (MHz) 380 380.2 - 390.2 - 389.8 399.8 410 410.2 - 420.2 - 419.8 429.8 450 450.4 - 460.4 - 457.6 467.6 480 478.8 - 488.8 - 486.0 496.0 710 698.0 - 728.0 - 716.0 746.0 750 747.0 - 777.0 - 762.0 792.0 810 806.0 - 851.0 - 821.0 866.0 850 824.0 - 869.0 - 849.0 894.0 900 890.0 - 935.0 - P-GSM, i.e. Primary or standard 915.0 960.0 GSM allocation 900 880.0 - 925.0 - E-GSM, i.e. Extended GSM 915.0 960.0 allocation 900 876.0 - 915 921.0 - R-GSM, i.e. Railway GSM 960.0 allocation 900 870.4 - 915.4 - T-GSM 876.0 921.0 1800 1710.0 - 1805.0 - 1785.0 1880.0 1900 1850.0 - 1930.0 - 1910.0 1990.0GSM frequency band usageThe usage of the different frequency bands varies around the globe although there is a largedegree of standardisation. The GSM frequencies available depend upon the regulatoryrequirements for the particular country and the ITU (International TelecommunicationsUnion) region in which the country is located.As a rough guide Europe tends to use the GSM 900 and 1800 bands as standard. These bandsare 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 isdetermined by the regulatory authorities and is dependent upon the area. For Canada the 1900MHz band is the primary one used, particularly for urban areas with 850 MHz used as abackup in rural areas.For Central and South America, the GSM 850 and 1900 MHz frequency bands are the mostwidely used although there are some areas where other frequencies are used.By: Chandra Kishor Page 19
  • 20. Global System for Mobile communication(GSM) 2012GSM multiband phonesIn order that cell phone users are able to take advantage of the roaming facilities offered byGSM, it is necessary that the cellphones are able to cover the bands of the countries which arevisited.Today most phones support operation on multiple bands and are known as multi-bandphones. Typically most standard phones are dual-band phones. For Europe, Middle east, Asiaand Oceania these would operate on GSM 900 and 1800 bands and for North America, etcdual 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 giving goodcoverage in Europe as well as moderate coverage in North America. Similarly North Americatri-band phones use the 900, 1800 and 1900 GSM frequencies. Quad band phones are alsoavailable covering the 850, 900, 1800 and 1900 MHz GSM frequency bands, i.e. the fourmajor bands and thereby allowing global use.GSM Power Control and Power Class:The power levels and power control of GSM mobiles is of great importance because of theeffect of power on the battery life. Also to group mobiles into groups, GSM power classdesignations have been allocated to indicate the power capability of various mobiles.In addition to this the power of the GSM mobiles is closely controlled so that the battery ofthe mobile is conserved, and also the levels of interference are reduced and performance ofthe basestation is not compromised by high power local mobiles.GSM power levelsThe 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 mobileby sending a GSM "power level" number. The mobile then adjusts its power accordingly. Invirtually all cases the increment between the different power level numbers is 2dB.The accuracies required for GSM power control are relatively stringent. At the maximumpower levels they are typically required to be controlled to within +/- 2 dB, whereas thisrelaxes to +/- 5 dB at the lower levels.The power level numbers vary according to the GSM band in use. Figures for the three mainbands in use are given below:By: Chandra Kishor Page 20
  • 21. Global System for Mobile communication(GSM) 2012 Power level Power output level number 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 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 1800By: Chandra Kishor Page 21
  • 22. Global System for Mobile communication(GSM) 2012 Power level Power output level number 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 10 10 11 8 12 6 13 4 14 2 15 0 GSM power level table for GSM 1900GSM Power classNot all mobiles have the same maximum power output level. In order that the base stationknows the maximum power level number that it can send to the mobile, it is necessary for thebase station to know the maximum power it can transmit. This is achieved by allocating aGSM power class number to a mobile. This GSM power class number indicates to the basestation the maximum power it can transmit and hence the maximum power level number thebase station can instruct it to use.Again the GSM power classes vary according to the band in use. GSM GSM 900 GSM 1800 GSM 1900 Power Class Number Power Maximum Power Maximum Power Maximum level power level power level power number output number output number output 1 PL0 30 dBm / PL0 30 dBm / 1W 1W 2 PL2 39dBm / PL3 24 dBm/ PL3 24 dBm / 8W 250 mW 250 mW 3 PL3 37dBm / PL29 36 dBm / PL30 33 dBm / 5W 4W 2W 4 PL4 33dBm / 2W 5 PL5 29 dBm / 800 mWBy: Chandra Kishor Page 22
  • 23. Global System for Mobile communication(GSM) 2012GSM power amplifier design considerationsOne of the main considerations for the RF power amplifier design in any mobile phone is itsefficiency. The RF power amplifier is one of the major current consumption areas.Accordingly, to ensure long battery life it should be as efficient as possible.It is also worth remembering that as mobiles may only transmit for one eighth of the time, i.e.for their allocated slot which is one of eight, the average power is an eighth of the maximum.GSM logical and physical channels:a tutorial, description, overview of GSM channels including transport andlogical channels, SACCH, SDCCH, FACCH, etc.GSM uses a variety of channels in which the data is carried. In GSM, these channels areseparated into physical channels and logical channels. The Physical channels are determinedby the timeslot, whereas the logical channels are determined by the information carriedwithin the physical channel. It can be further summarised by saying that several recurringtimeslots on a carrier constitute a physical channel. These are then used by different logicalchannels 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 channelsThe channels may also be divided into common and dedicated channels. The forwardcommon channels are used for paging to inform a mobile of an incoming call, responding tochannel requests, and broadcasting bulletin board information. The return common channel isa random access channel used by the mobile to request channel resources before timinginformation is conveyed by the BSS.The dedicated channels are of two main types: those used for signalling, and those used fortraffic. The signalling channels are used for maintenance of the call and for enabling call setup, providing facilities such as handover when the call is in progress, and finally terminatingthe 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 channelstructure. The BCCH is a point-to-multipoint channel (BSS-to-MS).SCH - Synchronisation of the MSs.By: Chandra Kishor Page 23
  • 24. Global System for Mobile communication(GSM) 2012FCHMS - 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). Trafficburst 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 aneight-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:- an overview, description or tutorial detailing the basics of GSM audio codecsor vocoders including LPC-RPE, EFR, Full Rate, Half Rate, AMR codec andAMR-WB codec as well as CELP, ACELP, VSELP, speech codectechnologies.Audio codecs or vocoders are universally used within the GSM system. They reduce the bitrate of speech that has been converted from its analogue for into a digital format to enable itto be carried within the available bandwidth for the channel. Without the use of a speechcodec, 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 theGSM system supports a number of specific audio codecs. These include the RPE-LPC, halfrate, and AMR codecs. The performance of each voice codec is different and they may beused under different conditions, although the AMR codec is now the most widely used. Alsothe newer AMR wideband (AMR-WB) codec is being introduced into many areas, includingGSMVoice codec technology has advanced by considerable degrees in recent years as a result ofthe increasing processing power available. This has meant that the voice codecs used in theGSM system have large improvements since the first GSM phones were introduced.By: Chandra Kishor Page 24
  • 25. Global System for Mobile communication(GSM) 2012Vocoder / codec basicsVocoders or speech codecs are used within many areas of voice communications. Obviouslythe focus here is on GSM audio codecs or vocoders, but the same principles apply to anyform of codec.If speech were digitised in a linear fashion it would require a high data rate that wouldoccupy a very wide bandwidth. As bandwidth is normally limited in any communicationssystem, it is necessary to compress the data to send it through the available channel. Oncethrough the channel it can then be expanded to regenerate the audio in a fashion that is asclose to the original as possible.To meet the requirements of the codec system, the speech must be captured at a high enoughsample rate and resolution to allow clear reproduction of the original sound. It must then becompressed in such a way as to maintain the fidelity of the audio over a limited bit rate, error-prone wireless transmission channel.Audio codecs or vocoders can use a variety of techniques, but many modern audio codecs usea technique known as linear prediction. In many ways this can be likened to a mathematicalmodelling of the human vocal tract. To achieve this the spectral envelope of the signal isestimated using a filter technique. Even where signals with many non-harmonically relatedsignals are used it is possible for voice codecs to give very large levels of compression.A variety of different codec methodologies are used for GSM codecs: CELP: The CELP or Code Excited Linear Prediction codec is a vocoder algorithm 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. 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. VSELP codec: The VSELP or Vector Sum Excitation Linear Prediction codec. One of the major drawbacks of the VSELP codec is its limited ability to 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.By: Chandra Kishor Page 25
  • 26. Global System for Mobile communication(GSM) 2012GSM audio codecs / vocodersA variety of GSM audio codecs / vocoders are supported. These have been introduced atdifferent times, and have different levels of performance.. Although some of the early audiocodecs are not as widely used these days, they are still described here as they form part of theGSM system. Codec name Bit rate Compression technology (kbps) 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 ACELPGSM Full Rate / RPE-LPC codecThe RPE-LPC or Regular Pulse Excited - Linear Predictive Coder. This form of voice codecwas the first speech codec used with GSM and it chosen after tests were undertaken tocompare it with other codec schemes of the day. The speech codec is based upon the regularpulse excitation LPC with long term prediction. The basic scheme is related to two previousspeech codecs, namely: RELP, Residual Excited Linear Prediction and to the MPE-LPC,Multi Pulse Excited LPC. The advantages of RELP are the relatively low complexityresulting from the use of baseband coding, but its performance is limited by the tonal noiseproduced by the system. The MPE-LPC is more complex but provides a better level ofperformance. The RPE-LPC codec provided a compromise between the two, balancingperformance and complexity for the technology of the time.Despite the work that was undertaken to provide the optimum performance, as technologydeveloped 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 codecLater another vocoder called the Enhanced Full Rate (EFR) vocoder was added in response tothe poor quality perceived by the users of the original RPE-LPC codec. This new codec gavemuch better sound quality and was adopted by GSM. Using the ACELP compressiontechnology 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 asa result of higher levels of processing power combined with their lower current consumption.By: Chandra Kishor Page 26
  • 27. Global System for Mobile communication(GSM) 2012GSM Half Rate codecThe GSM standard allows the splitting of a single full rate voice channel into two sub-channels that can maintain separate calls. By doing this, network operators can double thenumber of voice calls that can be handled by the network with very little additionalinvestment.To enable this facility to be used a half rate codec must be used. The half rate codec wasintroduced in the early years of GSM but gave a much inferior voice quality when comparedto other speech codecs. However it gave advantages when demand was high and networkcapacity was at a premium.The GSM Half Rate codec uses a VSELP codec algorithm. It codes the data around 20 msframes each carrying 112 bits to give a data rate of 5.6 kbps. This includes a 100 bps data ratefor a mode indicator which details whether the system believes the frames contain voice dataor not. This allows the speech codec to operate in a manner that provides the optimumquality.The Half Rate codec system was introduced in the 1990s, but in view of the perceived poorquality, it was not widely used.GSM AMR CodecThe AMR, Adaptive Multi-rate codec is now the most widely used GSM codec. The AMRcodec was adopted by 3GPP in October 1988 and it is used for both GSM and circuitswitched UMTS / WCDMA voice calls.The AMR codec provides a variety of options for one of eight different bit rates as describedin the table below. The bit rates are based on frames that are 20 millisceonds long and contain160 samples. The AMR codec uses a variety of different techniques to provide the datacompression. The ACELP codec is used as the basis of the overall speech codec, but othertechniques are used in addition to this. Discontinuous transmission is employed so that whenthere 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. Additionallyto provide the feedback for the user that the connection is still present, a Comfort NoiseGenerator (CNG) is used to provide some background noise, even when no speech data isbeing transmitted. This is added locally at the receiver.The use of the AMR codec also requires that optimized link adaptation is used so that theoptimum data rate is selected to meet the requirements of the current radio channel conditionsincluding its signal to noise ratio and capacity. This is achieved by reducing the sourcecoding and increasing the channel coding. Although there is a reduction in voice clarity, thenetwork connection is more robust and the link is maintained without dropout. Improvementlevels of between 4 and 6 dB may be experienced. However network operators are able toprioritise 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 areavailable at half rate (HR). This gives a total of fourteen different modes.By: Chandra Kishor Page 27
  • 28. Global System for Mobile communication(GSM) 2012 Mode Bit rate Full Rate (FR) / (kbps) 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 ratesAMR-WB codecAdaptive Multi-Rate Wideband, AMR-WB codec, also known under its ITU designation ofG.722.2, is based on the earlier popular Adaptive Multi-Rate, AMR codec. AMR-WB alsouses an ACELP basis for its operation, but it has been further developed and AMR-WBprovides 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 thanthe 300 - 3400 Hz bandwidths used by standard telephones. However this comes at the costof additional processing, but with advances in IC technology in recent years, this is perfectlyacceptable.The AMR-WB codec contains a number of functional areas: it primarily includes a set offixed rate speech and channel codec modes. It also includes other codec functions including:a Voice Activity Detector (VAD); Discontinuous Transmission (DTX) functionality forGSM; and Source Controlled Rate (SCR) functionality for UMTS applications. Furtherfunctionality includes in-band signaling for codec mode transmission, and link adaptation forcontrol of the mode selection.The AMR-WB codec has a 16 kHz sampling rate and the coding is performed in blocks of 20ms. There are two frequency bands that are used: 50-6400 Hz and 6400-7000 Hz. These arecoded separately to reduce the codec complexity. This split also serves to focus the bitallocation into the subjectively most important frequency range.The lower frequency band uses an ACELP codec algorithm, although a number of additionalfeatures have been included to improve the subjective quality of the audio. Linear predictionanalysis is performed once per 20 ms frame. Also, fixed and adaptive excitation codebooksare 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 randomexcitation. As the level of power in this band is less than that of the lower band, the gain isadjusted relative to the lower band, but based on voicing information. The signal content ofthe higher band is reconstructed by using an linear predictive filter which generatesinformation from the lower band filter.By: Chandra Kishor Page 28
  • 29. Global System for Mobile communication(GSM) 2012 Bit Notes rate (kbps) 6.60 This is the lowest rate for AMR-WB. It is used for circuit switched connections for GSM and UMTS and is intended to be used only temporarily during severe radio channel conditions or during network congestion. 8.85 This gives improved quality over the 6.6 kbps rate, but again, its use is only recommended for use in periods of congestion or when during severe radio channel conditions. 12.65 This is the main bit rate used for circuit switched GSM and UMTS, offering superior performance to the original AMR codec. 14.25 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. 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 differentfunctions on the phone may not require all to be active for example. As a result, it isnecessary to inform the network about which rates are available and thereby simplify thenegotiation between the handset and the network. To achieve this there are three differenceAMR-WB configurations that are available: Configuration A: 6.6, 8.85, and 12.65 kbit/s Configuration B: 6.6, 8.85, 12.65, and 15.85 kbit/s Configuration C: 6.6, 8.85, 12.65, and 23.85 kbit/sIt can be seen that only the 23.85, 15.85, 12.65, 8.85 and 6.60 kbit/s modes are used. Basedon listening tests, it was considered that these five modes were sufficient for a high qualityspeech telephony service. The other data rates were retained and can be used for otherpurposes including multimedia messaging, streaming audio, etc.GSM handover or handoff- tutorial or overview of the essentials of GSM handover or handoff from onecell to another and detailing types of handover and methodologies used.One of the key elements of a mobile phone or cellular telecommunications system, is that thesystem 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 theconnection. The process by which this occurs is known as handover or handoff. The termhandover is more widely used within Europe, whereas handoff tends to be use more in NorthAmerica. Either way, handover and handoff are the same process.By: Chandra Kishor Page 29
  • 30. Global System for Mobile communication(GSM) 2012Requirements for GSM handoverThe process of handover or handoff within any cellular system is of great importance. It is acritical 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 whendeveloping the standard.Types of GSM handoverWithin the GSM system there are four types of handover that can be performed for GSM onlysystems: 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. 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 another controlled by the same BSC. In this instance the BSC is able to perform the handover and it assigns a new channel and slot to the mobile, before releasing the old BTS from communicating with the mobile. 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. Inter-MSC handover: This form of handover occurs when changing between networks. The two MSCs involved negotiate to control the handover.GSM handover processAlthough there are several forms of GSM handover as detailed above, as far as the mobile isconcerned, they are effectively seen as very similar. There are a number of stages involved inundertaking a GSM handover from one cell or base station to another.In GSM which uses TDMA techniques the transmitter only transmits for one slot in eight,and similarly the receiver only receives for one slot in eight. As a result the RF section of themobile could be idle for 6 slots out of the total eight. This is not the case because during theslots in which it is not communicating with the BTS, it scans the other radio channels lookingfor beacon frequencies that may be stronger or more suitable. In addition to this, when themobile communicates with a particular BTS, one of the responses it makes is to send out alist of the radio channels of the beacon frequencies of neighbouring BTSs via the BroadcastChannel (BCCH).By: Chandra Kishor Page 30
  • 31. Global System for Mobile communication(GSM) 2012The mobile scans these and reports back the quality of the link to the BTS. In this way themobile assists in the handover decision and as a result this form of GSM handover is knownas Mobile Assisted Hand Over (MAHO).The network knows the quality of the link between the mobile and the BTS as well as thestrength of local BTSs as reported back by the mobile. It also knows the availability ofchannels in the nearby cells. As a result it has all the information it needs to be able to make adecision 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 newchannel and time slot to the mobile. It informs the BTS and the mobile of the change. Themobile then retunes during the period it is not transmitting or receiving, i.e. in an idle period.A key element of the GSM handover is timing and synchronisation. There are a number ofpossible scenarios that may occur dependent upon the level of synchronisation. 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. Time offset between synchronised old and new BTS: In some instances there may 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. 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 handoverWith the evolution of standards and the migration of GSM to other 2G technologies includingto 3G UMTS / WCDMA as well as HSPA and then LTE, there is the need to handover fromone technology to another. Often the 2G GSM coverage will be better then the others andGSM is often used as the fallback. When handovers of this nature are required, it isconsiderably more complicated than a straightforward only GSM handover because theyrequire two technically very different systems to handle the handover.These handovers may be called intersystem handovers or inter-RAT handovers as thehandover occurs between different radio access technologies.The most common form of intersystem handover is between GSM and UMTS / WCDMA.Here there are two different types: UMTS / WCDMA to GSM handover: There are two further divisions of this category of handover:By: Chandra Kishor Page 31
  • 32. Global System for Mobile communication(GSM) 2012 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. 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.By: Chandra Kishor Page 32