Crack the TD-LTE voice problem
Regarding the TD-LTE voice evolution path, all parties in the industry are actively studying. Currently, there are several different solutions in the industry. The International Organization for Standardization 3GPP has proposed an EPS (Evolved Packet System) network evolution architecture, including wireless network (LTE) and core network (SAE). In the EPS network architecture, the wireless network adopts LTE / TD-LTE (Long Term Evolution of UTRAN) to improve spectrum efficiency, reduce delay and optimize packet data, and utilizes a series of advanced technologies such as OFDMA / MIMO to make the uplink throughput up to 100Mbit / s, the downlink throughput can reach 50Mbit / s. The core network uses EPC (evolved packet core network). EPC is a flattened network architecture that uses technologies such as one-channel (one-tunnel) and direct connection to the user plane, which can theoretically greatly shorten the service delay. Although the EPS network is a full packet-switched network, voice services will remain an indispensable and important service for a long time. Therefore, the EPS network must not only support the rapidly growing data services, but also continue to provide high-quality voice. business. However, due to factors such as initial coverage and performance of the EPS network deployment, there may be many problems such as voice continuity in providing voice services. To ensure the smooth development of high-quality voice services on EPS networks, various standards organizations are also actively researching and proposing A variety of voice business solutions, this article focuses on several more mature solutions. The EPS network architecture of the 3GPP access mode is shown in the figure. In the figure, E-UTRAN is the access network architecture adopted by TD-LTE. It optimizes the traditional 3G network architecture and adopts a simplified and flat network structure. The access network contains only the base station (eNodeB), no longer There is RNC. eNodeB has an independent resource management function, and each eNodeB realizes mutual coordination and cooperation through direct interconnection. The simplified structure can effectively improve the overall communication efficiency of the system and provide better coordination for the newly introduced design concept of full packet switching in the EPS system. EPC is the core network part, including MME (mobility management entity), S-GW (service gateway), P-GW (packet data network gateway), HSS (home subscriber server) and PCRF and other functional entities. According to the possible forms of TD-LTE terminals that can provide voice services, voice service solutions are roughly divided into two types, TD-LTE / TD-SCDMA / GSM (GPRS) multimode single standby terminals and TD-LTE / TD-SCDMA / GSM (GPRS) multi-mode dual standby terminal, the former can use CSFB (circuit domain fallback) or SRVCC (single radio frequency voice call continuity) scheme to provide voice services, the latter directly use 2G / 3G network to provide voice services. Provide voice service based on circuit domain network In the early stages of TD-LTE network construction, operators already have a mature 2G / 3G network. For the protection of CS investment, combined with the EPS deployment strategy, the original CS domain voice solution can be used to provide voice services, while TD -The LTE network only handles data services (including IMS data services). In this case, when the UE under the coverage of TD-LTE is processing voice services, the terminal may first fall back to the CS (circuit domain) domain of the 2G / 3G network and process the voice services in the CS domain. In addition, since the wireless modules of TD-LTE and CS dual-mode single standby terminals are in single radio frequency mode, that is, dual-mode or multi-mode single standby terminals with TD-LTE and 2G / 3G access capabilities, TD-LTE connection is used When entering, it cannot receive / transmit circuit domain service signals. In order to enable the terminal to initiate voice services and receive paging of voice services under TD-LTE access, and to correctly handle the PS services being performed by the terminal under TD-LTE access, the CSFB technology was generated. However, the use of the CSFB scheme has a prerequisite, that is, only when the overlapping coverage area of ​​TD-LTE and 2G / 3G, and the terminal has the CSFB function, can the circuit domain fall back. It can be seen that the CSFB solution implements voice services based on the CS domain, so the voice services of the TD-LTE network can be opened without deploying an IMS system. The key to achieving the CSFB function is that the MSC can establish an SGs interface with the MME in order to realize the joint location update and paging operations of LTE and 2G / 3G. In addition, in order to realize the CSFB function, UE, MME, MSC, E-UTRAN, SGSN all need to upgrade to add additional functions. Provide voice services based on IMS network SRVCC refers to the business continuity of voice calls when a single radio frequency terminal is switched from an EPS network to a 2G / 3G network. The SRVCC solution is suitable for operators who have deployed an IMS network. The TD-LTE network can already provide voice services based on the packet domain, but LTE has not reached the scenario of full network coverage. As users move, ongoing voice services will face After leaving TD-LTE coverage, the problem that the voice can't keep continuous, at this time, with the help of SRVCC technology, the voice can be switched to the circuit domain, so as to ensure that the voice call is not interrupted. SRVCC is actually a handover process. Through the cooperation of IMSSRVCCAS and the bearer network entity MME / MSCServer, the continuity of voice services is achieved. The difference between SRVCC and CSFB is that CSFB falls in the overlapping coverage area of ​​EPS and 2G / 3G, while SRVCC only switches to the 2G / 3G network circuit domain when the LTE network loses coverage. It can be seen that the SRVCC solution is implemented based on IMS, so an IMS system needs to be deployed on the network. In addition, only after the IMS voice service is enabled on the TD-LTE network will SRVCC be used in specific scenarios. In order to achieve uninterrupted voice calls, the three modules of EPC, CS and IMS are required to work together. Among them, the MSC in the circuit domain needs to be upgraded to an enhanced MSC in order to support the handover process from the MME, and support the IMS to CS handover and association CS handover and domain transfer; MME in EPC needs to be able to separate voice and non-voice parts from PS bearer, initiate SRVCC handover to voice bearer part and coordinate PS handover and SRVCC handover; The HSS in the IMS needs to insert the SRVCCVDN (VCC domain transfer number) into the MME during the UE attachment process. The basic workflow of SRVCC is: In order to realize SRVCC, the voice call is anchored in the IMS system. When initiating the TD-LTE to 2G / 3G domain transfer, the MME (with UESRVCC related information) first obtains the handover indication from the TD-LTE network, and then triggers the SRVCC process to the MSCServer, and the MSCServer initiates the session transfer process to the IMS and completes The CS handover procedure to the target cell. After the handover is complete, the MSCServer sends a response (including the necessary handover command information) to the MME and transfers it to the UE for access to the 2G / 3G network. In this way, the voice call is transferred from TD-LTE to the 2G / 3G network, thereby maintaining the continuity of voice services. Based on multimode dual standby terminal When the TD-LTE / TD-SCDMA / GSM (GPRS) multi-mode dual standby terminal uses the TD-LTE mode for data services, the terminal can simultaneously call in or call out voice services in the TD-SCDMA / GSM (GPRS) mode; When using the TD-SCDMA mode for data services, the terminal establishes concurrent services under TD-SCDMA to make incoming or outgoing voice services; when using the GSM mode for data services, the terminal uses the GSM mode for incoming voice services or Exhale. The scheme of providing voice services based on multi-mode dual standby terminals does not place special requirements on the network. As mentioned above, although the evolution to LTE is the future development direction of mobile networks, because LTE technology is more suitable for data services, and for a long time in the future, people cannot do without voice services, so operators need Choose a solution to provide voice services in a network where LTE is deployed. At present, the more commonly used solutions are CSFB, SRVCC and multi-mode dual standby terminal solutions. CSFB technology is suitable for the scenario of overlapping coverage of 2G / 3G circuit domain and TD-LTE wireless network. The network structure is simple and does not require the deployment of IMS. CSFB technology can effectively use the existing CS network investment. However, CS voice will cause TD-LTE Frequent mode switching under LTE affects the user's business experience; in addition, the CSFB technology requires TD-LTE and 2G / 3G to have the same coverage, and puts forward new requirements for both the network and the terminal (need to upgrade network equipment such as MSCServer and need the terminal to support CSFB Functions, etc.) Therefore, the CSFB technology is only a transitional solution to provide voice services in the TD-LTE network. The SRVCC technology is suitable for operators who have deployed IMS networks and have been able to provide IMS-based voice services in TD-LTE networks, but TD-LTE has not reached the scenario of full network coverage. With the large-scale construction of the TD-LTE network and the large number of IMS systems used to provide voice services, the probability of SRVCC voice switching will become smaller and smaller, and you can gradually provide users with a better and better seamless, high-quality voice service experience . However, SRVCC technology needs to deploy IMS network and SRVCCAS (application server); it needs to upgrade part of the existing network MSCServer; it needs e-UTRAN, MME and HSS support; in addition, it also needs the terminal to support IMS client and SRVCC capability. TD-LTE / TD-SCDMA / GSM (GPRS) multi-mode dual standby terminals can choose TD-SCDMA / GSM mode to establish voice services, and select the network attached to the PS domain to establish packet domain services. There are no additional requirements for the network, but the terminal implementation quite complicated. Wind Mill Transformer, Special Designed Transformer, Special Designed Wind Transformer, OEM Wind Transformer Hangzhou Qiantang River Electric Group Co., Ltd.(QRE) , https://www.qretransformer.com