Umts Handover
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UMTS Handover There are following categories of handover (also referred to as handoff): Hard Handover Hard handover means that all the old radio links in the UE are removed before the new radio links are established. Hard handover can be seamless or non-seamless. Seamless hard handover means that the handover is not perceptible to the user. In practice a handover that requires a change of the carrier frequency (inter-frequency handover) is always performed as hard handover. Soft Handover Soft handover means that the radio links are added and removed in a way that the UE always keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally soft handover can be used when cells operated on the same frequency are changed. Softer handover Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B (i.e. the site of co-located base stations from which several sector-cells are served. In softer handover, macro diversity with maximum ratio combining can be performed in the Node B, whereas generally in soft handover on the downlink, macro diversity with selection combining is applied. Generally we can distinguish between intra-cell handover and inter-cell handover. For UMTS the following types of handover are specified: Handover 3G -3G (i.e. between UMTS and other 3G systems) FDD soft/softer handover FDD inter-frequency hard handover FDD/TDD handover (change of cell) TDD/FDD handover (change of cell) TDD/TDD handover Handover 3G - 2G (e.g. handover to GSM) Handover 2G - 3G (e.g. handover from GSM) The most obvious cause for performing a handover is that due to its movement a user can be served in another cell more efficiently (like less power emission, less interference). It may however also be performed for other reasons such as system load control. Active Set is defined as the set of Node-Bs the UE is simultaneously connected to (i.e., the UTRA cells currently assigning a downlink DPCH to the UE constitute the active set).
Cells, which are not included in the active set, but are included in the CELL_INFO_LIST belong to the Monitored Set. Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active set belong to the Detected Set. Reporting of measurements of the detected set is only applicable to intra-frequency measurements made by UEs in CELL_DCH state. The different types of air interface measurements are: Intra-frequency measurements: measurements on downlink physical channels at the same frequency as the active set. A measurement object corresponds to one cell. Inter-frequency measurements: measurements on downlink physical channels at frequencies that differ from the frequency of the active set. A measurement object corresponds to one cell. Inter-RAT measurements: measurements on downlink physical channels belonging to another radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to one cell. Traffic volume measurements: measurements on uplink traffic volume. A measurement object corresponds to one cell. Quality measurements: Measurements of downlink quality parameters, e.g. downlink transport block error rate. A measurement object corresponds to one transport channel in case of BLER. A measurement object corresponds to one timeslot in case of SIR (TDD only). UE-internal measurements: Measurements of UE transmission power and UE received signal level. UE positioning measurements: Measurements of UE position. The UE supports a number of measurements running in parallel. The UE also supports that each measurement is controlled and reported independently of every other measurement.
Definition of "femto cell" A femto cell is a device used to improve mobile network coverage in small areas. Femto cells connect locally to mobile phones and similar devices through their normal GSM, CDMA, or UMTS connections, and then route the connections over a broadband internet connection back to the carrier, bypassing the normal cell towers that are arrayed across the countryside. Unlike UMA, femto cells require no special hardware or software support on the mobile devices they connect to. Also known as: "femto" System Architecture Written by David Chambers Tuesday, 04 September 2007 There have been several system architectures proposed and developed by different
femtocell vendors, but the industry is standardising a single, common solution which is being formalised by the 3GPP standards committees. This connects into the operator's core network using the same Iu interface that existing outdoor cellsites use. Femtocells also conform to the standard radio transmission frequencies and protocols used today. The mobile operators telephone switch (MSC) and data switch (SGSN) also communicate to the femtocell controller in the same way as other mobile calls. Therefore, all services including phone numbers, call diversion, voicemail etc. all operate in exactly the same way and appear the same to the end user. The femtocell appears to the standard 3G phone as just another cellsite from the host mobile operator, and can be used by almost any phone including roamers visiting from other countries. The connection between the femtocell and the femtocell controller is termed the Iu-h interface and it uses a secure IP encryption (IPsec) to avoids interception. There is also authentication of the femtocell itself to ensure it is a valid access point. The femtocell connects over broadband IP with a femto-gateway which may handle hundreds of thousands of femtocells. These are consolidated into a single Iu interface which can carry thousands of concurrent calls and data sessions. The standardisation of the Iu-h interface was included in the 3GPP Release 8 standard which will receive formal approval during Q1 2009.
We'll explain the details for a 3G UMTS mobile phone system here, because this is the most common technology being used for femtocells today. Other mobile phone systems would operate in a very similar way. The femtocell appears to the standard 3G phone as just another cellsite from the host mobile operator, and can be used by almost any phone including roamers visiting from other countries. The mobile operators telephone switch (MSC) and data switch (SGSN) also communicate to the femtocell gateway in the same way as for other mobile calls. Therefore, all services including phone numbers, call diversion, voicemail etc. all operate in exactly the same way and appear the same to the end user. The connection between the femtocell and the femtocell controller uses a secure IP encryption (IPsec), which avoids interception and there is also authentication of the femtocell itself to ensure it is a valid access point.
The figure below illustrates the system architecture and context for femtocell operation
Inside the femtocell are the complete workings of a mobile phone basestation. Additional functions are also included such as some of the RNC (Radio Network Controller) processing, which would normally reside at the mobile switching centre. Some femtocells also include core network element so that data sessions can be managed locally without needing to flow back through the operators switching centres. The key functions are integrated onto a single chip, such as the PC302 from picoChip. These and other chip manufacturers document the different components in more detail in their reference designs. In addition to these highly integrated chips, a radio frontend (such as from Bitwave) and a highly accurate frequency reference crystal oscillator are also required. The extra capabilities of a femtocell demand it to be self-installing and self-configuring. This requires considerable extra software which scans the environment to determine the available frequencies, power level and/or scrambling codes to be used. This is a continuous process to adapt to changing radio conditions, for example if the french windows are opened in a room containing the femtocell. Within the operators network, femtocell gateways aggregate large numbers of femtocell connections (typically 100,000 to 300,000) which are first securely through high capacity IP security firewalls. Read further to learn about which radio technologies are used for femtocells.
A femto cell router is a small device, the size of any Wi-Fi router, which is in effect a miniature base station. The radio is a standard based radio such as UMTS/HSPA which the operator will likely require a license to operate on. The router connects to a DSL line. The idea is to enable the subscriber to make and receive mobile calls indoors, with low signal levels which has a number of benefits to the operator: - Hopefully accelerate fixed line substitution. - Reduce the cost of building a full macro layer network. - Lock subscriber in to the operator and reduce the likelihood of churn. - Provide a viable medium for content distribution (the ugly walled garden paradigm) Basically, there are a number of ways, the first of which is the conventional hierarchy using an RNC . Just like any node-b is connected back to an RNC which is then in turn is connected to the core network, the femtocell routers can be treated like individual nodebs and connect to RNCs. This may appeal to big manufacturers who already have substantial deployments and many RNC on the ground. The downside to this approach is the limited chances of inter-operable devices if the operators chooses to diversify their suppliers. Although the Iub interface (base station to RNC) is standardised, the reality is most implementations are proprietary. Typically operators don't like to put all their eggs in one basket and would prefer to get solutions from various suppliers. The other downside of this approach is that most available RNC solutions are geared towards Macro/Micro type of deployments. In other words they are built in order to support relatively small number of cells with a huge number of subscribers in each cell. They don't scale very well to support Femto cell deployments with almost as many cells as there are subscribers, and a handful of subscribers per cell. An alternative approach is to use UMA (or now called GAN). UMA was originally conceived to support dual mode cellular/WiFi-over-Internet type of connectivity. When the mobile is detected indoors within the range of pre-determined WiFi coverage, a UMA concentrator does the core network negotiation on behalf of the mobile, e.g. registering, and updating location ... etc. It is thought that a similar procedure can be used for femto cells. When the mobile is detected within the coverage of a femto cell, UMA kind of hand shaking takes place to ensure that: 1. the mobile is allowed to access the network at this particular femto cell, 2. the traffic between the mobile and the core network is forwarded accordingly. Using this method a UMA concentrator is required, so in a way, the solution is still hierarchical and it is certainly one of the criticisms of this method is that it is not a fully flat architecture. Kineto is one of the companies promoting this approach. Yet another alternative approach is enabling femto cell connectivity through an IMS service. In this approach, the femto cell router talks SIP over the internet back to an IMS service which acts as a bridge between the femto layer and the rest of the network. Although this is considered the "flattest" approach, it is yet to understand how it will work in practice, the delay in working fully featured IMS platforms is one of the concerns of femto cell development companies.
Frequency-Division Duplexing Frequency-division duplexing (FDD) means that the transmitter and receiver operates at different carrier frequencies. The term is frequently used in ham radio operation, where an operator is attempting to contact a repeater station. The station must be able to send and receive a transmission at the same time, and does so by slightly altering the frequency at which it sends and receives. This mode of operation is referred to as duplex mode or offset mode. Uplink and downlink sub-bands are said to be separated by the frequency offset. Frequency-division duplexing can be efficient in the case of symmetric traffic. In this case time-division duplexing tends to waste bandwidth during the switch-over from transmitting to receiving, has greater inherent latency, and may require more complex circuitry. Another advantage of frequency-division duplexing is that it makes radio planning easier and more efficient, since base stations do not "hear" each other (as they transmit and receive in different sub-bands) and therefore will normally not interfere each other. On the converse, with time-division duplexing systems, care must be taken to keep guard times between neighboring base stations (which decreases spectral efficiency) or to synchronize base stations, so that they will transmit and receive at the same time (which increases network complexity and therefore cost, and reduces bandwidth allocation flexibility as all base stations and sectors will be forced to use the same uplink/downlink ratio)
Cells, which are not included in the active set, but are included in the CELL_INFO_LIST belong to the Monitored Set. Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active set belong to the Detected Set. Reporting of measurements of the detected set is only applicable to intra-frequency measurements made by UEs in CELL_DCH state. The different types of air interface measurements are: Intra-frequency measurements: measurements on downlink physical channels at the same frequency as the active set. A measurement object corresponds to one cell. Inter-frequency measurements: measurements on downlink physical channels at frequencies that differ from the frequency of the active set. A measurement object corresponds to one cell. Inter-RAT measurements: measurements on downlink physical channels belonging to another radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to one cell. Traffic volume measurements: measurements on uplink traffic volume. A measurement object corresponds to one cell. Quality measurements: Measurements of downlink quality parameters, e.g. downlink transport block error rate. A measurement object corresponds to one transport channel in case of BLER. A measurement object corresponds to one timeslot in case of SIR (TDD only). UE-internal measurements: Measurements of UE transmission power and UE received signal level. UE positioning measurements: Measurements of UE position. The UE supports a number of measurements running in parallel. The UE also supports that each measurement is controlled and reported independently of every other measurement.
Definition of "femto cell" A femto cell is a device used to improve mobile network coverage in small areas. Femto cells connect locally to mobile phones and similar devices through their normal GSM, CDMA, or UMTS connections, and then route the connections over a broadband internet connection back to the carrier, bypassing the normal cell towers that are arrayed across the countryside. Unlike UMA, femto cells require no special hardware or software support on the mobile devices they connect to. Also known as: "femto" System Architecture Written by David Chambers Tuesday, 04 September 2007 There have been several system architectures proposed and developed by different
femtocell vendors, but the industry is standardising a single, common solution which is being formalised by the 3GPP standards committees. This connects into the operator's core network using the same Iu interface that existing outdoor cellsites use. Femtocells also conform to the standard radio transmission frequencies and protocols used today. The mobile operators telephone switch (MSC) and data switch (SGSN) also communicate to the femtocell controller in the same way as other mobile calls. Therefore, all services including phone numbers, call diversion, voicemail etc. all operate in exactly the same way and appear the same to the end user. The femtocell appears to the standard 3G phone as just another cellsite from the host mobile operator, and can be used by almost any phone including roamers visiting from other countries. The connection between the femtocell and the femtocell controller is termed the Iu-h interface and it uses a secure IP encryption (IPsec) to avoids interception. There is also authentication of the femtocell itself to ensure it is a valid access point. The femtocell connects over broadband IP with a femto-gateway which may handle hundreds of thousands of femtocells. These are consolidated into a single Iu interface which can carry thousands of concurrent calls and data sessions. The standardisation of the Iu-h interface was included in the 3GPP Release 8 standard which will receive formal approval during Q1 2009.
We'll explain the details for a 3G UMTS mobile phone system here, because this is the most common technology being used for femtocells today. Other mobile phone systems would operate in a very similar way. The femtocell appears to the standard 3G phone as just another cellsite from the host mobile operator, and can be used by almost any phone including roamers visiting from other countries. The mobile operators telephone switch (MSC) and data switch (SGSN) also communicate to the femtocell gateway in the same way as for other mobile calls. Therefore, all services including phone numbers, call diversion, voicemail etc. all operate in exactly the same way and appear the same to the end user. The connection between the femtocell and the femtocell controller uses a secure IP encryption (IPsec), which avoids interception and there is also authentication of the femtocell itself to ensure it is a valid access point.
The figure below illustrates the system architecture and context for femtocell operation
Inside the femtocell are the complete workings of a mobile phone basestation. Additional functions are also included such as some of the RNC (Radio Network Controller) processing, which would normally reside at the mobile switching centre. Some femtocells also include core network element so that data sessions can be managed locally without needing to flow back through the operators switching centres. The key functions are integrated onto a single chip, such as the PC302 from picoChip. These and other chip manufacturers document the different components in more detail in their reference designs. In addition to these highly integrated chips, a radio frontend (such as from Bitwave) and a highly accurate frequency reference crystal oscillator are also required. The extra capabilities of a femtocell demand it to be self-installing and self-configuring. This requires considerable extra software which scans the environment to determine the available frequencies, power level and/or scrambling codes to be used. This is a continuous process to adapt to changing radio conditions, for example if the french windows are opened in a room containing the femtocell. Within the operators network, femtocell gateways aggregate large numbers of femtocell connections (typically 100,000 to 300,000) which are first securely through high capacity IP security firewalls. Read further to learn about which radio technologies are used for femtocells.
A femto cell router is a small device, the size of any Wi-Fi router, which is in effect a miniature base station. The radio is a standard based radio such as UMTS/HSPA which the operator will likely require a license to operate on. The router connects to a DSL line. The idea is to enable the subscriber to make and receive mobile calls indoors, with low signal levels which has a number of benefits to the operator: - Hopefully accelerate fixed line substitution. - Reduce the cost of building a full macro layer network. - Lock subscriber in to the operator and reduce the likelihood of churn. - Provide a viable medium for content distribution (the ugly walled garden paradigm) Basically, there are a number of ways, the first of which is the conventional hierarchy using an RNC . Just like any node-b is connected back to an RNC which is then in turn is connected to the core network, the femtocell routers can be treated like individual nodebs and connect to RNCs. This may appeal to big manufacturers who already have substantial deployments and many RNC on the ground. The downside to this approach is the limited chances of inter-operable devices if the operators chooses to diversify their suppliers. Although the Iub interface (base station to RNC) is standardised, the reality is most implementations are proprietary. Typically operators don't like to put all their eggs in one basket and would prefer to get solutions from various suppliers. The other downside of this approach is that most available RNC solutions are geared towards Macro/Micro type of deployments. In other words they are built in order to support relatively small number of cells with a huge number of subscribers in each cell. They don't scale very well to support Femto cell deployments with almost as many cells as there are subscribers, and a handful of subscribers per cell. An alternative approach is to use UMA (or now called GAN). UMA was originally conceived to support dual mode cellular/WiFi-over-Internet type of connectivity. When the mobile is detected indoors within the range of pre-determined WiFi coverage, a UMA concentrator does the core network negotiation on behalf of the mobile, e.g. registering, and updating location ... etc. It is thought that a similar procedure can be used for femto cells. When the mobile is detected within the coverage of a femto cell, UMA kind of hand shaking takes place to ensure that: 1. the mobile is allowed to access the network at this particular femto cell, 2. the traffic between the mobile and the core network is forwarded accordingly. Using this method a UMA concentrator is required, so in a way, the solution is still hierarchical and it is certainly one of the criticisms of this method is that it is not a fully flat architecture. Kineto is one of the companies promoting this approach. Yet another alternative approach is enabling femto cell connectivity through an IMS service. In this approach, the femto cell router talks SIP over the internet back to an IMS service which acts as a bridge between the femto layer and the rest of the network. Although this is considered the "flattest" approach, it is yet to understand how it will work in practice, the delay in working fully featured IMS platforms is one of the concerns of femto cell development companies.
Frequency-Division Duplexing Frequency-division duplexing (FDD) means that the transmitter and receiver operates at different carrier frequencies. The term is frequently used in ham radio operation, where an operator is attempting to contact a repeater station. The station must be able to send and receive a transmission at the same time, and does so by slightly altering the frequency at which it sends and receives. This mode of operation is referred to as duplex mode or offset mode. Uplink and downlink sub-bands are said to be separated by the frequency offset. Frequency-division duplexing can be efficient in the case of symmetric traffic. In this case time-division duplexing tends to waste bandwidth during the switch-over from transmitting to receiving, has greater inherent latency, and may require more complex circuitry. Another advantage of frequency-division duplexing is that it makes radio planning easier and more efficient, since base stations do not "hear" each other (as they transmit and receive in different sub-bands) and therefore will normally not interfere each other. On the converse, with time-division duplexing systems, care must be taken to keep guard times between neighboring base stations (which decreases spectral efficiency) or to synchronize base stations, so that they will transmit and receive at the same time (which increases network complexity and therefore cost, and reduces bandwidth allocation flexibility as all base stations and sectors will be forced to use the same uplink/downlink ratio)
