Foreword
The development of mobile broadband technology enriches the types of data services carried by mobile networks and promotes the growth of mobile data services. The rapid growth of mobile data has put forward new bandwidth requirements for mobile access. At present, 2G and 3G networks are running on the network, and the number of users carried has reached 920 million. In order to further improve the access rate of mobile networks and alleviate the problem of limited spectrum resources, the International Standards Organization has accelerated the formulation of LTE technical standards and shortened the time from standard to commercial. The LTE commercial trial network has been widely deployed worldwide.
In order to ensure the good service experience perception of existing mobile users, the LTE standard has conducted a lot of research and regulations on the interoperability characteristics with existing systems in the development process. This paper introduces the requirements for interoperability between the initial stage of LTE construction and 2G/3G systems by introducing the implementation methods of other operators and technology standards.
1 Multiple inter-systems should have good interoperability
In order to ensure the continuity of service usage between multiple systems, the LTE specification provides relatively complete provisions for interoperability between different systems. The main contents can be divided into cell selection and reselection, data switching, voice switching, and wireless connection redirection. several aspects.
1.1 Cell selection and reselection
Like interworking between 2G/3G, LTE supports multi-mode terminal cell reselection between LTE and 2G/3G networks, including:
a) Cell reselection between LTE and UMTS/GSM in the idle state.
b) Cell reselection from UMTS to LTE for terminals in CELL-PCH and CELL-FACH states.
c) Cell reselection from GPRS to E-UTRAN for GPRS-Packet-IDLE and Transfer mode terminals.
d) Cell reselection from LTE to GSM or network assisted (NACC) cell reselection for terminals in the RRC connected state.
1.2 Wireless Connection Redirection
Wireless connection redirection can be performed between LTE and GSM/UMTS, including:
a) The RNC indicates the frequency of the E-UTRAN in the RRC reject and RRC release messages, and the terminal starts the reselection process for the frequency point cell.
b) The E-UTRAN indicates the frequency point of the UTRAN in the RRC release message, and the terminal starts the reselection process for the frequency point cell.
c) The BSC indicates the frequency of the E-UTRAN in the RR release message, and the terminal starts the reselection process for the frequency point cell.
d) The E-UTRAN indicates the frequency of the GSM in the RRC release message, and the terminal starts the reselection process for the frequency point cell.
1.3 Data Service Switching
When establishing a data service connection, LTE supports bidirectional handover with the UMTS/GSM system, including:
a) Only the data service connection is established in LTE, and the terminal in the AcTIve state switches from E-UTRAN to UTRAN/GPRS.
b) Only the data service connection is established in the UMTS, and the terminal in the Cell-DCH state is switched from the UTRAN to the E-UTRAN.
c) Establish a data service connection in the GRPS, and the terminal in the GPRS-Packet-Transfer state switches from GPRS to E-UTRAN.
1.4 Voice Service Switching
For the switching of voice services, LTE is implemented in two phases. When the LTE network cannot provide voice services, it is implemented by the circuit domain voice backoff (CSFB) function; when the LTE network can provide packet domain voice services, the single radio is used. Voice continuous control (SR-VCC) function is implemented, including:
a) When the LTE network cannot provide voice services, the CSFB-capable terminal can implement: redirecting to UTRAN/GSM to establish voice services from the LTE-IDLE state; and from the LTE-AcTIve state (that is, establishing a data service connection), The PS Handover process is initiated to enable the terminal to access the UTRAN/GSM and initiate a voice service establishment process.
b) When the LTE network can provide IMS voice service, the voice service on the LTE side can be switched to the UMTS/GSM network through the SR-VCC function.
2 The focus of LTE interoperability is still voice service
The main purpose of the LTE standard at the beginning of the development is to increase the wireless mobile broadband access rate. Technically, only the packet data service is supported. However, considering that the current gold service on the mobile network is voice, LTE has also developed a large number of mutual Operating specifications. According to the implementation time and mode, it can be divided into three schemes: CSFB, SR-VCC and VoLGA (LTE network carries voice through universal access).
2.1 CSFB
The LTE and GSM/WCDMA dual-mode terminals are in the Single-radio mode. When using LTE access, the GSM/WCDMA circuit domain service signals cannot be simultaneously received/transmitted. In order to enable the terminal to receive/send CS services such as voice under LTE access, and to correctly process the ongoing LTE PS service, CSFB technology is generated. When the IMS network is not deployed, the CS domain provides services such as voice and SMS. When LTE provides data services, the CSFB technology can trigger the terminal to fall back from LTE access to GSM/WCDMA network access and perform CS services. To implement the CSFB function, an SGs interface needs to be introduced between the MME and the MSC server. The terminal is attached to the LTE and is attached to the CS domain through the SGs so that other users can call the UE. In this way, the terminal can preferentially reside on the LTE network to enjoy high-speed data services, and return to the 2G/3G network to initiate CS voice calls when voice services are required.
At present, the CSFB network architecture, device functions, main processes, etc. in the standard specification have been frozen; in order to reduce the delay in the rollback process, 3GPP has proposed enhanced CSFB function. The main solutions are embedded LAU and security. Enhanced, SR-VCC-based CSFB and hermit location updates.
2.2 SR-VCC
SR-VCC mainly solves the seamless handover between IMS voice and CS voice controlled by IMS. The IMS network has been set up to implement VoIP services, which is the premise of SR-VCC technology. At the same time, SR-VCC technology requires the MSC server to support the Sv interface. In order to facilitate handover, VoIP needs to be anchored in IMS. Currently SR-VCC only supports one-way handover from E-UTRAN to UTRAN/GERAN. The MME first receives a handover request from the E-UTRAN and an indication message for describing this as SR-VCC processing, and then triggers a handover procedure between it and the MSC server enhanced for SR-VCC through the Sv reference point.
The application scenarios, functional architecture, and main processes of the SR-VCC in the current standard have been basically determined. In order to reduce the problem of excessive IMS delay in the handover process, 3GPP has proposed: at the SIP level and at the EPC. Enhanced SR-VCC specification such as anchoring scheme at the gateway level.
2.3 VoLGA
The main idea of ​​VoLGA is to use LTE as an IP access network, simulate the RNC or BSC through a newly added network entity VANC (VoLGA access network controller), and access the CS core network to complete the voice service processing. VANC supports two modes of operation: A mode and Iu mode, for GERAN and UTRAN networks, respectively.
Different from the former two, VoLGA is standardized by the Industrial Alliance. The original intention is to use it as an IP access network in the early stage of LTE deployment. The CS domain service still uses the original 2G/3G network, so the solution is not solved. The problem of providing voice in the LTE system is not widely supported by mainstream operators and equipment manufacturers.
3 Complete interoperability solution brings complex online equipment upgrades
The continuous experience of users in multiple systems requires a comprehensive interoperability specification at the network level. For new equipment, these factors can be taken into account during development (only increase in research and development costs); for existing equipment, it is necessary to consider how to upgrade to meet new technical requirements.
Just as the introduction of 3G networks to upgrade 2G equipment, LTE interoperability has also proposed a lot of transformation tasks for existing equipment.
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