Abstract: LTE is a new generation of broadband wireless mobile communication technology in the future. Since LTE provides services for packet domains, it cannot provide circuit domain services like traditional 3G and 2G networks. Therefore, how to provide voice services in LTE networks has become an issue of concern in the industry. This paper mainly introduces the voice service solutions of LTE networks, and analyzes the impact of each scheme on traditional networks and industries.
LTE (Long Term EvoluTIon) is a new generation of broadband wireless mobile communication technology. Based on the system design goal of LTE for packet domain optimization, the network architecture of LTE no longer distinguishes between circuit domain and packet domain, and adopts a unified packet domain architecture. Under the new LTE system architecture, traditional circuit domain voice solutions are no longer supported, and IMS-controlled VoIP services will serve as voice solutions in future LTE networks.
Since the performance indicators of the current VoIP service fail to meet the quality of the existing circuit domain voice service, and the IMS needs to be deployed throughout the network, three different voice solutions are formed on the basis of the existing network: based on the dual standby terminal solution. , CSFB and VoLTE. Both CSFB and VoLTE are LTE voice solutions defined by 3GPP. VoLTE requires comprehensive support and optimization of terminals, wireless and core networks. From the current point of view, the implementation complexity is large. CSFB is a relatively simple voice solution proposed when the industry does not implement VoLTE.
1. Voice solution based on dual standby terminal
The dual standby terminal can stand in the LTE network and the 3G/2G network at the same time, and can receive and transmit signals from both LTE and 3G/2G networks. The dual standby terminal can automatically select voice communication from the 3G/2G mode when making a call. That is to say, the dual standby terminal uses the advantage of still camping on the 3G/2G network to receive and make calls from the 3G/2G network; and the LTE network is only used for data services.
A voice solution based on dual standby terminals is a relatively simple solution. The terminal chip can be implemented with two chips (one 3G/2G chip and one LTE chip) or one multimode chip, and the solution is simple. Since there is no interoperation between the LTE and 3G/2G modes of the dual standby terminal, the terminal does not need to implement different system measurement, and the technology is simple to implement.
Therefore, the essence of the dual standby terminal voice solution is to use the traditional 3G/2G network, which has nothing to do with LTE. There is no requirement for the network, and there is no need to support any interoperability between the LTE network and the traditional 3G/2G network.
Second, CSFB-based voice solution
The main idea of ​​the CSFB solution is to re-access the LTE network back to the 3G/2G circuit domain when the user needs to perform voice service, and initiate or receive voice services according to the service process of the circuit domain.
1, network architecture
To implement CSFB, an SGs interface needs to be established between the MME and the MSC device of the 3G/2G network. SGs association plays a bridge role in CSFB technology, and can connect two different systems to realize the continuous voice service between users.
The CSFB technology affects the existing 3G/2G network. The MSC of the original network needs to add an SGs interface with the MME. The SGSN adds an S3 interface with the MME.
The wireless subsystem (base station, base station controller) of the original 3G/2G network needs to increase the neighbor cell configuration of LTE. In order to make the terminal return to the LTE network as soon as possible after the CSFB to 3G/2G network voice service ends, the wireless subsystem of the original network needs to support the Fast Return function; in order to optimize the delay of the terminal falling back from LTE to 3G/2G, the original Wireless subsystems with networks need to support RIM functions and so on.
2, CSFB key technology
The CSFB idea is to fall back from the LTE network to the 3G/2G circuit domain when the user needs to perform voice service. The way to fall back is to release the LTE wireless link, and carry the redirect field in the release message, indicating the system and frequency point of the terminal re-access, which is called relocation. The relocation method is characterized by simple implementation and small amount of transformation to the original network; the disadvantage is that the delay is relatively large.
In order to reduce the time for the terminal to re-access the 3G/2G network, the 3GPP proposes a relocation function with a system message, and carries a system message of the 3G/2G network in the relocation field. The system message of the 3G/2G network is an eNB transmitted from the BSC/RNC, the SGSN, and the MME to the LTE through the RIM procedure. This method is characterized by a small delay, but the transformation of the original network is large, and the original wireless network needs to be modified.
In order to minimize the amount of modification to the original network, but at the same time to reduce the delay of re-access, the 3GPP specification proposes the DMCR function. The DMCR (Deffered Measurement Control) function is to let the UE fall back to the 3G network to read only part of the system message during the call, without having to read all the system messages before the call is established, thereby reducing the call setup time. However, this function can only be used for 3G networks, and 2G networks do not support DMCR.
Another method for supporting a terminal to fall back from LTE to a 3G/2G network is PS domain handover. This kind of scheme has a small delay, but the support is difficult, and the existing terminal basically does not support this method.
From the perspective of current technical support, industrial realization, performance, etc., the "relocation method with system messages" is widely accepted by the industry.
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