Foreword The development of mobile broadband technology has enriched the types of data services carried by mobile networks and promoted 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 increase 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 standards to commercial use. The LTE commercial trial network has been widely deployed worldwide. In order to ensure a good business experience perception for existing mobile users, in the process of formulating LTE standards, a lot of research has been conducted on the interoperability characteristics with existing systems, and regulations have been made. This paper introduces the requirements for interoperability between the early stage of LTE construction and the 2G / 3G system through the introduction of other operators and technical standard implementation methods. 1 There should be good interoperability between multiple systems In order to ensure the continuity of business use among multiple systems, the LTE specification provides more complete provisions for interoperability between different systems. The main content 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 2G / 3G interoperability, LTE supports multi-mode terminal cell reselection between LTE and 2G / 3G networks, including: a) Cell reselection of the terminal in idle state between LTE and UMTS / GSM. b) Cell reselection of terminals in CELL-PCH and CELL-FACH state from UMTS to LTE. c) Cell reselection of GPRS-Packet-IDLE and Transfer mode terminals from GPRS to E-UTRAN. d) Cell reselection or network assisted (NACC) cell reselection of the terminal in the RRC connected state from LTE to GSM. 1.2 Wireless connection redirection Wireless connection redirection can be performed between LTE and GSM / UMTS, including: a) The RNC indicates the frequency of E-UTRAN in the RRC reject and RRC release messages, and the terminal starts the reselection process for the cell at that frequency. b) E-UTRAN indicates the frequency point of UTRAN in the RRC release message, and the terminal starts the reselection process for the cell at this frequency point. c) The BSC indicates the frequency of E-UTRAN in the RR release message, and the terminal starts the reselection process for the cell at this frequency. d) E-UTRAN indicates the frequency of GSM in the RRC release message, and the terminal starts the reselection process for the cell of this frequency. 1.3 Data service switching When establishing a data service connection, LTE supports two-way switching between UMTS / GSM systems, including: a) Only data service connection is established in LTE, and the terminal in the AcTIve state switches from E-UTRAN to UTRAN / GPRS. b) Only data service connection is established in UMTS, and the terminal in Cell-DCH state switches from UTRAN to E-UTRAN. c) Establish a data service connection at GRPS, and the terminal in GPRS-Packet-Transfer state switches from GPRS to E-UTRAN. 1.4 Voice service switching For the switching of voice services, LTE is divided into two stages to achieve. When the LTE network cannot provide voice services, it is implemented through the circuit domain voice fallback (CSFB) function; when the LTE network can provide voice services in the packet domain, through a single radio frequency Voice continuous control (SR-VCC) function, including: a) When the LTE network cannot provide voice services, a terminal with CSFB capability can achieve: from the LTE-IDLE state, redirect to UTRAN / GSM to establish voice service; from the LTE-AcTIve state (that is, the establishment of a data service connection), Initiating the PS Handover process enables the terminal to access in UTRAN / GSM and initiates the 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 on voice services The main purpose of the LTE standard at the beginning of the formulation was to increase the wireless mobile broadband access rate. Technically, it only supports packet data services. However, considering that the current gold service on the mobile network is voice, LTE has also developed a large number of interactive voice-bearing services. Operating specifications. According to the different implementation time and method, it can be divided into three schemes: CSFB, SR-VCC and VoLGA (LTE network carries voice through universal access). 2.1 CSFB LTE and GSM / WCDMA dual-mode terminals are in single-radio mode. When using LTE access, they cannot receive / transmit GSM / WCDMA circuit domain service signals simultaneously. In order to enable the terminal to receive / transmit CS services such as voice under LTE access, and to be able to correctly handle the ongoing LTE PS service, the CSFB technology has been generated. When the operator has not deployed an IMS network, only the CS domain provides voice and SMS services, and 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. The realization of CSFB function needs to introduce SGs interface between MME and MSC server. The terminal attaches to LTE, and at the same time attaches to the CS domain through 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 only return to the 2G / 3G network to initiate a CS voice call when voice services are needed. At present, the CSFB network architecture, device functions, main processes and other content in the standard specification have been frozen; in order to reduce the delay in the fallback process, 3GPP has also proposed to enhance the CSFB function. The main solutions are embedded LAU, security Enhanced, CSFB based on SR-VCC and hermit location update etc. 2.2 SR-VCC SR-VCC mainly solves the seamless switching between IMS-controlled VoIP voice and CS voice for a single radio frequency terminal. It is the premise of the SR-VCC technology to build an IMS network to implement VoIP services. At the same time, the SR-VCC technology requires the MSC server to support the Sv interface. To facilitate handover, VoIP needs to be anchored in IMS. Currently, SR-VCC only supports unidirectional handover from E-UTRAN to UTRAN / GERAN. The MME first receives the handover request and the instruction message describing this processing for SR-VCC from E-UTRAN, and then triggers the handover process between it and the MSC server enhanced for SR-VCC through the Sv reference point. At present, the application scenarios, functional architecture and main processes of SR-VCC in the standard specification have been basically determined. In order to reduce the problem of too long delay on the IMS side during the handover process, 3GPP has also proposed: at the SIP level and at the EPC Enhanced SR-VCC specifications such as the anchoring scheme at the gateway level. 2.3 VoLGA The main idea of ​​VoLGA is to use LTE as an IP access network, simulate a 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 2 working modes: A mode and Iu mode, which are aimed at GERAN and UTRAN networks respectively. The difference from the previous two is that VoLGA is formulated by the Industry Alliance. The original intention is to use it as an IP access network in the early stage of LTE deployment. The CS domain business still uses the original 2G / 3G network, so this solution has not been resolved. In the end, the problem of providing voice in the LTE system has not been widely supported by mainstream operators and equipment manufacturers. 3 Complete interoperability solution brings complex upgrade of existing network equipment The user's experience of business continuity between multiple systems requires the establishment of a complete interoperability specification at the network level. For new equipment, these factors can be taken into account during research and development (only increase the cost of research and development); for existing equipment, you need to consider how to upgrade to meet new technical requirements. Just as the introduction of a 3G network to upgrade 2G equipment, the interoperability of LTE also presents a large number of transformation tasks for existing equipment. Traffic Facilities,Waterproof Traffic Facilities,Outdoor Traffic Facilities,Traffic Control Devices Yangzhou Heli Photoelectric Co., Ltd. , https://www.heli-eee.com