Research on interoperability between LTE and 2G/3G systems

Preface

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. Currently, 2G and 3G networks are running on the Internet, and the number of users 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 standards to commercial use. LTE commercial trial networks have been widely launched around the world.

In order to ensure a good service experience for existing mobile users, the LTE standard has conducted extensive research and made provisions for the interoperability characteristics with existing systems during the formulation process. This article introduces the implementation methods of other operators and technical standards, and proposes the requirements for interoperability between LTE and 2G/3G systems in the early stage of construction.

1 Multiple systems should have good interoperability

To ensure the continuity of services among multiple systems, the LTE specification has made relatively comprehensive provisions for the interoperability between different systems. The main contents can be divided into several aspects, such as cell selection and reselection, data switching, voice switching and wireless connection redirection.

1.1 Cell Selection and Reselection

Similar to the interoperability between 2G and 3G, LTE supports cell reselection between LTE and 2G/3G networks for multi-mode terminals, including:

a) A terminal in idle state reselects a cell between LTE and UMTS/GSM.

b) Cell reselection from UMTS to LTE by a terminal in CELL-PCH and CELL-FACH states.

c) A terminal in GPRS-Packet-IDLE and Transfer modes reselects a cell from GPRS to E-UTRAN.

d) Cell reselection from LTE to GSM or network-assisted (NACC) cell reselection by a terminal in 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 cell with this frequency.

b) E-UTRAN indicates the frequency of UTRAN in the RRC release message, and the terminal starts the reselection process for the cell with this frequency.

c) The BSC indicates the frequency of the E-UTRAN in the RR release message, and the terminal starts the reselection process for the cell with this frequency.

d) E-UTRAN indicates the GSM frequency in the RRC release message, and the terminal starts the reselection process for the cell with this frequency.

1.3 Data Service Switching

When establishing a data service connection, LTE supports bidirectional handover with UMTS/GSM systems, including:

a) Only data service connection is established in LTE, and the terminal in Active state switches from E-UTRAN to UTRAN/GPRS.

b) Only a data service connection is established in UMTS, and the terminal in the Cell-DCH state is switched from UTRAN to E-UTRAN.

c) A data service connection is established in GPRS, and the terminal in the GPRS-Packet-Transfer state switches from GPRS to E-UTRAN.

1.4 Voice Service Switching

For the handover of voice services, LTE is implemented in two stages. 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 packet domain voice services, it is implemented through the single radio voice continuity control (SR-VCC) function, which includes:

a) When the LTE network cannot provide voice services, a terminal with CSFB capability can: redirect from LTE-IDLE state to UTRAN/GSM to establish voice services; from LTE-Active state (i.e., a data service connection is established), initiate the PS Handover process so that the terminal can access UTRAN/GSM and initiate the voice service establishment process.

b) When the LTE network is able to provide IMS voice services, the voice services 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 when it was first formulated was to increase the access rate of wireless mobile broadband. Technically, it only supports packet data services. However, considering that voice is the gold medal service on the current mobile network, LTE has also formulated a large number of interoperability specifications for carrying voice services. According to the different implementation time and methods, it can be mainly divided into three solutions: CSFB, SR-VCC and VoLGA (LTE network carries voice through universal access).

2.1 CSFB

LTE and GSM/WCDMA dual-mode terminals are single-radio mode. When using LTE access, they cannot simultaneously receive/send GSM/WCDMA circuit domain service signals. In order to enable the terminal to receive/send CS services such as voice under LTE access, and to correctly handle ongoing LTE PS services, CSFB technology was created. When the operator has not yet deployed the IMS network, only voice, SMS and other services are provided by the CS domain, and LTE provides data services, 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, the SGs interface needs to be introduced between the MME and the MSC server. The terminal is attached to LTE and attached to the CS domain through SGs, so that other users can call the UE. In this way, the terminal can preferentially reside in 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 network architecture, equipment functions, main processes and other contents of CSFB in the standard specifications have been frozen; in order to reduce the delay problem during the fallback process, 3GPP has proposed to enhance the CSFB function. The main solutions include embedded LAU, security enhancement, SR-VCC-based CSFB and hermit location update.

2.2 SR-VCC

SR-VCC mainly solves the seamless switching between VoIP voice and CS voice controlled by IMS for single radio terminals. The premise of SR-VCC technology is to build an IMS network to implement VoIP services. At the same time, SR-VCC technology requires the MSC server to support the Sv interface. In order to facilitate switching, VoIP needs to be anchored in IMS. Currently, SR-VCC only supports one-way switching from E-UTRAN to UTRAN/GERAN. MME first receives a switching request and an indication message indicating that this is SR-VCC processing from E-UTRAN, and then triggers the switching 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 specifications have been basically determined. In order to reduce the problem of excessive delay on the IMS side during the switching process, 3GPP has proposed enhanced SR-VCC specifications such as anchoring solutions at the SIP level and at the EPC gateway level.

2.3 VoLGA

The main idea of ​​VoLGA is to use LTE as an IP access network, and to access the CS core network to complete the processing of voice services through a newly added network entity VANC (VoLGA Access Network Controller) to simulate RNC or BSC. VANC supports two working modes: A mode and Iu mode, which are respectively for GERAN and UTRAN networks.

Unlike the previous two, VoLGA was standardized by an industrial alliance. The original intention was to use it as an IP access network in the early stages of LTE deployment. CS domain services still use the original 2G/3G network. Therefore, this solution did not solve the problem of ultimately providing voice in the LTE system and did not receive widespread support from mainstream operators and equipment manufacturers.

3. Complete interoperability solutions bring complex upgrades to existing network equipment

To ensure the continuity of user services across multiple systems, comprehensive interoperability specifications need to be developed at the network level. For new equipment, these factors can be taken into account during R&D (only increasing R&D costs); for existing equipment, it is necessary to consider how to upgrade and transform it to meet new technical requirements.

Just as the introduction of 3G networks required the upgrading and transformation of 2G equipment, LTE interoperability also poses a large number of transformation tasks for existing equipment.

3.1 SGSN

LTE network only supports PS domain services, so SGSN is one of the core network elements that interoperate with 2G/3G networks after the introduction of LTE. There are many requirements for upgrading SGSN, which requires adding supported interfaces, processes, protocols, fields and other aspects.

a)Support S3, S4, S6d and other interfaces and related functions.

b) To implement the CSFB function, the Suspend and Resume processes must be supported.

c) Support the RIM process between the 2G network and the LTE network, including the signaling process of the Gb interface and the Gn interface.

d) Support IPv6 single stack, IPv4, IPv6 dual stack PDP types.

e) Support EPC DNS address resolution, SRV lookup, and select P-GW (GGSN) based on the weight information fed back by DNS.

3.2 HLR

To ensure the success rate of called calls in the border area of ​​two adjacent MSCs within the LTE coverage area, the HLR to which the called user belongs needs to support the Roaming Retry process. At the same time, it also needs to be upgraded to support HLR/HSS integration.

3.3 MSC

If the existing MSC is upgraded to support CSFB, all MSCs within the LTE coverage area must support the following functions.

a) Mobility management based on SGs interface, including: IMSI location update request, establishing the mapping relationship between IMSI and SGs interface in VLR; IMSI detachment, deleting the mapping relationship between IMSI and SGs interface in VLR.

b) Voice service processes, including paging on SGs interfaces.

At the same time, GMSC also needs to be upgraded to support the Sv interface and interactively handle the SR-VCC switching process through the Sv interface.

3.4 2G/3G RAN Equipment

RAN equipment is the basic unit of the mobile network, completing the information interaction process of the user air interface. For the interoperability content, the two-way reselection of the idle cell and the reselection or switching from LTE to 2G/3G cell in the data connection state can mostly be completed through software upgrades; but for the fallback function of voice services, the existing RAN equipment can be upgraded to support the R8 version through software, but for enhanced CSFB, most of them do not support it, and a large number of software and hardware equipment need to be replaced.

Although these functions can be realized through software upgrades, most of the support for interfaces and protocols requires the replacement of hardware or even platforms to meet the requirements. Therefore, the complete interoperability requirements will inevitably lead to large-scale adjustments to existing network equipment.

4. Learn from others’ experience to polish jade

Interoperability is not an isolated issue. Different telecom operators and different technical standards are faced with this choice. Therefore, paying attention to and understanding their practices will provide a good reference and reference for Chinese operators and E-UTRAN, UTRAN and GERAN technologies.

4.1 The selection of foreign operators can be used as a reference for Chinese operators

As of January 2011, there are 17 commercial LTE networks in 15 countries and regions around the world. As the LTE network has just started to be built, its coverage and number of users are still at an extremely early stage; at the same time, most of the LTE terminals on the market are data cards or routers. In this case, the risk of upgrading existing network element equipment for interoperability is far from proportional to the benefits. Therefore, most operators choose an independent LTE networking method and adopt a full dual-SIM dual-standby mode.

Subsequently, with the increase in the number of LTE network users, the rise in user service requirements and management complexity, the integration of core network equipment and the maturity of terminal equipment, foreign LTE operators will gradually introduce interoperability content based on the standard functions defined by 3GPP. First, realize two-way cell reselection between systems in the idle state; at the same time, for the initiated data call, enable PS switching from LTE to UTRAN/GERAN. In the early stage of LTE network construction, data card services were the main focus, so the above functions can basically achieve the purpose of using 2G/3G coverage to make up for the lack of LTE coverage.

With the diversification of LTE terminal types, when they initiate or receive voice calls, they can use CSFB to fall back to the CS domain of the 2G/3G network to complete the call. In the future, due to market competition, operators may also open LTE-based VoIMS and require LTE and 2G/3G systems to support SR-VCC. When the terminal leaves the LTE coverage area, it can smoothly switch to the 2G/3G network. As the volume of business increases, it is also possible to enable interoperability based on business volume balancing between multiple systems.

4.2 Implementation methods of other technical standards can be used as reference for E-UTRAN

As one of the main 3G technologies, cdma2000 has also formulated a large number of specifications on the interoperability between 1x and EV-DO systems. There are four main cdma 2000 interoperability solutions: hybrid terminal, dual receiver, cross paging and VCC. Among them, the hybrid terminal and dual receiver solutions use periodic time slot monitoring, so paging information is easily lost in non-monitoring time slots, and the terminal consumes more power.

At present, the interoperability between 1x and EV-DO mostly adopts the cross-paging solution, which realizes the registration of two networks and the reception of two-network paging by a single network through the access network. The EV-DO system sets the cdma 2000 CS domain message notification application protocol at the air interface application layer, so that the terminal can receive cdma 2000 1x paging or short messages issued by the EV-DO system while monitoring EV-DO, without the need to frequently switch between the two networks, thereby reducing the overhead required for network switching, extending the standby time of the terminal, and facilitating the rapid response to 1x voice calls. Cross-paging requires the setting of the A1/A1P interface between the DO AN and the 1x MSC, so only the 1x MSC needs to be modified, and other network elements of the mobile network do not need to be modified.

The goal of interoperability between 1x and EV-DO is also to achieve VCC, which requires adding a VCC AS functional entity in IMS to manage terminal registration and routing in the CS/PS domain and assist in completing the switching function from VT/VoIP to CS voice.

5 Conclusion

The construction of LTE network will not be completed overnight. The frequency bands it can use and the characteristics of the services it carries determine that the main purpose of LTE construction is to absorb data services in hot spots. Therefore, the interoperability between LTE and 2G/3G must first solve the two-way switching of data services. For voice services, they will not be the focus of LTE networks for a long time in the future. Therefore, we can consider learning from the experience of cdma2000 and adopting the cross-paging method to implement voice services back to 2G/3G networks. This not only solves the interoperability needs of users, but also avoids the transformation of a large number of network elements and improves the investment benefits of operators.