Multi-network convergence is an important trend in the development of mobile communication network technology. TD-LTE, LTE-Advanced system and WiMAX system are important branches of IMT2000. Based on the analysis of standard protocol stack and network architecture, TD-LTE and LTE-Advanced networks are discussed. WiMAX network convergence solution: Based on the SAE architecture, the TD-LTE and WiMAX loosely coupled convergence schemes and suitable access points are proposed, and the most important network selection and network switching problems in network convergence are discussed and solved. Solution; for LTE-Advanced and WiMAX systems, a tightly coupled fusion scheme is proposed, and a tightly coupled fusion protocol stack is given.
1 IntroductionTD-SCDMA is a TDD mode technology proposed by China for the third generation mobile communication technology standard. It adopts key technologies such as smart antenna, joint detection and relay switching. It is a mobile communication technology with high spectrum utilization and strong anti-interference ability. . With the evolution and development of technology, 3GPP has successively proposed technologies such as TD-LTE and LTE-Advanced. WiMAX is an emerging broadband wireless communication technology that has developed rapidly in recent years. It uses a large number of B3G/4G technologies such as OFDM, MIMO, and HARQ to achieve performance surpass. Although TD-LTE, LTE-Advanced and WiMAX systems belong to different air interface technologies of IMT2000, they have similarities and similarities in many aspects. So whether these two types of high-performance systems can be integrated with each other and how to integrate them is a problem that the industry is very concerned about.
Based on the analysis of standard protocol stack and network architecture, the integration schemes of TD-LTE and LTE-Advanced systems and WiMAX systems are discussed. The evolution and integration schemes of TD-LTE, LTE-Advanced and WiMAX systems are proposed.
2. Research on the integration of TD-LTE and WiMAX2.1 TD-LTE and WiMAX convergence architecture and access point
2.1.1 Fusion Architecture of TD-LTE and WiMAX
At present, the wireless access speed has increased by leaps and bounds, and the traditional mobile PS domain network has been unable to meet the test of high-speed and high-bandwidth future wireless packet services. Therefore, the 3GPP International Organization for Standardization began research on next-generation network architecture, and in the R7 phase, a new mobile PS domain network architecture, SAE (System Architecture EvoluTIon) [4], was introduced. The main idea of ​​SAE is to simplify the existing mobile PS network architecture, reduce the intermediate links of business processing and realize the flattening of the network architecture through network element integration and functional re-division. At the same time, SAE designed an interface for the convergence of LTE networks and other technology networks, and planned a multi-network convergence network architecture. The convergence of the WiMAX network and the TD-LTE can be fused through the network interface between the LTE and the non-3GPP IP network provided by the SAE.
Figure 1 SAE architecture
The architecture of the SAE is as shown in the figure above. The Non-3GPP IP Access is divided into Trusted and Untrusted. This is determined by the operator. For example, the system operator can choose to trust the same carrier or non-3GPP operated by different operators based on the commercial protocol. IP access network. Untrusted Non-3GPP IP Access requires access via ePDG. It is assumed here that the WiMAX system is a Trusted Non-3GPP IP Access network. Then the WiMAX system mainly accesses the 3GPP network through S2a, Gxa, and STa. The specific network architecture of TD-LTE and WiMAX integration is shown in the figure below [5].
Figure 2 LTE and WiMAX convergence architecture
As can be seen from the above figure, in order to guarantee the QoS of WiMAX services, ASN-GW needs to use the Policy and Charging Control (PCC) architecture. In the 3GPP evolution solution, the Policy Decision FuncTIon and the Charging Rule FuncTIon (CRF) are merged into a new functional entity PCRF (Policy and Charging Rules FuncTIon). The WiMAX PCC uses the PCRF to receive QoS parameters.
2.1.2 Access points involved in TD-LTE and WiMAX convergence
In the convergence of TD-LTE and WiMAX, the WiMAX ASN directly accesses TD-LTE through the S2a interface, and the P-GW performs functions such as packet filtering, interception, charging, IP address allocation, and TD-LTE and WiMAX data. It is combined at the P-GW and routed to the external network through the SGi interface.
The user's authentication is mainly that the WiMAX ASN interacts with the AAA server through the STa interface.
Several important access points are involved in the convergence scenario:
STa (equivalent to WiMAX R3-AAA interface): used for AAA-based authentication of the UE.
Gxa (equivalent to WiMAX R3-PCC-P): Used to enforce dynamic QoS and charging rules.
S2a (equivalent to WiMAX R2-MIP): Used for the establishment of L3 mobility and access core network links.
S14: Used for different network choices and facilitates optimization of WiMAX-3GPP handover. The interface can also provide an access network discovery and selection function (ANDSF) and a FAF (Forward Attachment Function) functional entity.
Among the above access points, S2a is the most important access point and is the interface for data transmission. Packets of different networks need to be processed at the access point, which is consistent with the characteristics of the external network. Therefore, the design of its protocol stack is very important for data transmission.
The S2a protocol stack is slightly different for PMIPv6 and MIPv4. For PMIPv6, the functional entities of the termination control plane and user plane are non-3GPP IP access (WiMAX) MAG (Mobile Access Gateway) and LMA in the gateway (local mobility anchor point). ), here LMA includes the functionality of a local proxy. The protocol stack of the control plane is PMIPv6 based on IPv6/IPv4, and the protocol stack of the user plane is based on IPv4/v6 forwarding packets. For MIPv4, the functional entities of the termination control and user plane are the MN of the UE and the FA of the non-3GPP IP access (WiMAX), and the HA in the gateway. The control plane protocol stack is MIPv4, and the user plane forwards IPv4 packets at the IPv4 transport layer.
Figure 3 S2a Reference Point Protocol Stack (PMIPv6)
Figure 4 S2a Reference Point Protocol Stack (MIPv4)
From the above analysis, we can see that the S2a protocol stack is relatively simple and meets the needs of data transmission and conversion. It is the most suitable access point for TD-LTE and WiMAX convergence. The network architecture of TD-LTE and WiMAX convergence adopts the way that the core network is shared by all or part of the core network, that is, TD-LTE and WiMAX have independent wireless networks, and the core networks of TD-LTE and WiMAX have their own independent parts, and also have Shared network elements, such as AAA server, PCRF, PGW, etc.
2.2 TD-LTE and WiMAX network selection
In the convergence of TD-LTE and WiMAX networks, network selection and handover are very important issues. Network selection refers to how the newly arrived UE accesses the wireless network. The following figure details the initial network selection process between TD-LTE and WiMAX networks [5]. Here we mainly discuss the solution of single wireless link. That is, the terminal can only receive one link information in TD-LTE or WiMAX.
Figure 5 Network registration process between TD-LTE and WiMAX networks
First, the mobile terminal needs to synchronize with the WiMAX BS and exchange basic capability information, and then triggers the EAP-AKA procedure for terminal authentication. After successful authentication, the terminal and the BS will exchange the handshake key to exchange the security key of the air interface. Registration will then be initiated to trigger the establishment of the BS and ASN-GW data link, thus establishing a service flow based on the protocol downloaded from the AAA server during authentication. At this point, the connection on layer 2 has been established. The network connected by the UE will trigger the DHCP process to obtain the IP address. After the Proxy Binding Update and Proxy Binding Acknowledge information are exchanged, the link will be established between the ASN-GW and the P-GW through the proxy MIP. During this period, the PCRF is from the ASN. The policy parameters are obtained in the -GW and P-GW.
It can be seen from the above process that TD-LTE allows the terminal to access the core network through the PMIP interface (S2a) between the AAA server, the WiMAX ASN-GW and the P-GW. Moreover, the PCC can provide dynamic policies and charging rules for user services.
2.3 TD-LTE and WiMAX network switching
The most important point of switching between TD-LTE and WiMAX systems is to provide a seamless mobile experience for users, ie switching between TD-LTE and WiMAX is transparent to the user. Whether the UE performs handover may be determined by the UE or may be determined by the network side. 3GPP R8 requires the UE to decide to switch, because first, the UE can decide the handover based on its wireless measurement, the user implements the configured parameter selection, and the operator's mobility policy; secondly, the UE does not need to send a policy between the different systems to the network; Third, the impact on the TD-LTE network and the WiMAX network is minimal. For example, the TD-LTE network does not need to receive the measurement report of the WiMAX cell, and does not need to decide whether to perform handover or not, and does not need to track the resource usage of the WiMAX network. The same is true. The following is an example of switching a terminal from a WiMAX network to a TD-LTE network to discuss switching between TD-LTE and WiMAX networks.
In the handover scheme, a new functional entity FAF (Forward Attachment Function) [5] needs to be established, and the UE can contact the FAF through the S14 interface. Switching from the WiMAX network to the TD-LTE network, the terminal establishes communication through the WiMAX access network and the FAF to obtain information related to handover, and the FAF is equivalent to the eNB; conversely, from the TD-LTE network to the WiMAX network, the terminal passes the TD-LTE The access network establishes communication with the FAF to obtain information related to the handover, in which case the function of the FAF is equivalent to the WiMAX ASN function. In the handover, it is assumed that the WiMAX network supports PMIPv6, so that a PMIPv6 tunnel can be established between the WiMAX network and the P-GW.
Figure 6 WiMAX to TD-LTE handover process
The switching process of TD-LTE and WiMAX is as shown in the figure above. The UE obtains LTE neighbor information through ANDSF/FAF and a mobility policy between systems. The UE then measures the LTE neighboring cell. If the UE decides to initiate the handover, the UE will initiate a pre-registration procedure, which is a typical 3GPP attach procedure. Although the UE is in the WiMAX network, it can be performed through the IP tunnel between the UE and the FAF. The registration process is activated by the Attach Request message sent by the UE to the FAF. The FAF sends the Attach Request message to the SGSN through the normal Iu-PS interface. It will send out the normal LTE authentication process. If the authentication is successful, the SGSN will receive the attach request and send it. The Attach Accept message updates the location information of the UE in the HSS.
After registration, when the UE decides to switch to TD-LTE, the UE may select an LTE cell autonomously or passively. In the passive selection mode, the UE sends Handover Required information to the source WiMAX ASN, including the candidate LTE cell, the source. The WiMAX ASN sends Handover Request Response information including the handover target LTE cell to the UE. After the target cell is selected, the UE sends Handover Request information including the target cell to the FAF. The FAF prepares a suitable radio resource in the target cell by using a normal Relocation procedure, and sends Handover Command information, including information of the target cell, to the UE. At this time, the UE leaves the WiMAX network and accesses the target cell according to the normal handover procedure of the 3GPP, that is, sends Handover Complete information, and completes the Relocation process. After that, the UE creates a suitable PDP context and continues the data transfer.
It should be noted that after the PDP context is created, the UE maintains the connection with the same P-GW, thus maintaining the original IP address. In addition, TD-LTE and WiMAX handovers also need to maintain the QoS of the service. This requires consideration of QoS mapping, resource allocation, and so on. WiMAX allows multiple traffic flows with different QoS, similar to the 3GPP PDP content activation procedure. In addition, traffic classification is also a part to consider. The QoS maintenance between WiMAX and 3GPP is mainly provided by PCC.
It can be seen from the above analysis that through seamless handover, TD-LTE and WiMAX can smoothly realize network interworking and data transmission.
2.4 Summary
The integration of TD-LTE and WiMAX can only be carried out in a loosely coupled manner, using a dual-mode protocol stack, an independent wireless network, and a unified external service export. This fusion method can achieve seamless between TD-LTE and WiMAX systems. Transmission, and by exchanging QoS parameters, the service can be transmitted between different networks, and the fusion mode is implemented by the 3GPP SAE architecture, and the data transmission of the WiMAX network and the TD-LTE network is performed through the S2a interface, through the Gxa interface and the PCRF. Entity communication, realizing the maintenance of service QoS, and implementing user AAA authentication through STa is a relatively simple fusion method for the transformation of the existing network. 
3 LTE-Advanced and WiMAX fusion analysisLTE-Advanced is an evolved version of LTE that needs to support higher peak rates, higher spectrum efficiency, higher user throughput and number of users, and further improvements in the experience of cell edge users. From a technical point of view, the LTE-Advanced and WiMAX physical layers all use similar advanced technologies such as OFDM, MIMO, adaptive link layer technology, and hierarchical QoS guarantee mechanisms. Both are designed as cellular network structures based on the all-IP core network. The structure of the radio access network (RAN) is weakened by the base station controller device entity, and the concept of public radio resource management and control of the base station is adopted. These are networks. The research and design of the interconnection and fusion mechanisms provide good conditions, such as load balancing, dynamic spectrum allocation, and non-destructive switching between systems.
In general, when studying LTE-Advanced and WiMAX interconnect structures, you need to consider the following issues:
â— While providing mutual cooperation between networks, it is necessary to compromise the fairness between networks.
â— Reasonably define structural entities to enable communication between LTE-Advanced and WiMAX in a more cost-effective manner.
• Define the total capacity, metrics, and capabilities of each inter-networking entity.
• The interconnect architecture should be flexible and support the collaboration of other new networks without introducing too many new nodes and interfaces.
Since the network structure of LTE-Advanced and WiMAX is relatively simple, and the network architecture of LTE-Advanced has not been finalized, LTE-Advanced and WiMAX may be closely integrated in the air interface. If LTE-Advanced follows the network architecture of LTE, then the architecture of heterogeneous interconnection between LTE-Advanced and WiMAX systems requires the addition of necessary nodes and interfaces. Therefore, a very important concept and function is introduced to complete the inter-network cooperation: Generic Link Layer (GLL) [6]. In this converged network architecture, LTE-Advanced and WiMAX access networks complement each other for network coverage, and GLLs are introduced into data bearer nodes, such as multimode terminals and bearer gateways, to increase collaboration on the original link layer mechanism. Features such as packet queuing, high-level header compression, segmentation, and retransmission. At the same time, the coordinated radio resource management function is introduced into the original radio resource control layer (LTE-Advanced) and MAC (WiMAX) to ensure that the overall radio resources can be more effectively utilized after network convergence.
3.1 General Link Layer Technology GLL
GLL can be seen as a new communication layer added to the original protocol layer to provide uniform link layer data processing for different wireless access mechanisms. The design of the GLL can be coupled to the MAC layer to different degrees. Generally speaking, the higher the degree of coupling, the higher the complexity of the system interconnection, but the higher the multi-access gain. The functions of the GLL mainly include:
â— As an aggregation layer of different access technologies, it provides a unified interface for various high-level protocols (such as the network layer) to achieve the purpose of shielding different wireless access technologies.
â— Control and supplement the RLC (Radio Link Control)/MAC functions of different access technologies to achieve efficient use of resources and maximize application layer performance.
• Maintain a modular structure of the network protocol layer to support the convergence of different access technologies.
• Provides scheduling of user data packets between different networks to facilitate network diversity gain.
â— Provide link layer status information to the upper layer to support efficient mobility management between access networks.
The following figure shows the WiMAX and LTE-Advanced network convergence reference protocol architecture after GLL. Where: PDCP represents a packet convergence protocol; BMC represents a broadcast, multicast control protocol; CS represents a convergence sublayer; CPS represents a general partial sublayer; SS represents an encryption sublayer. The protocol architecture is based on a tightly coupled approach where GLL is placed on layer 2 of the original protocol but below layer 3. Since the control and data planes have been separated in LTE, the GLL defines the control plane (GLL-C) and the user plane (GLL-U), respectively. In the user plane, MAC data of different formats based on different networks are processed by the GLL-U layer to provide a data stream defined by a unified format in the upper layer. At the control level, GLL-C collects the underlying feedback information of each network and passes it to the collaborative resource management unit for dynamic resource management.
Figure 7 GLL-based internet protocol architecture
It can be seen that the convergence of LTE-Advanced and WiMAX can meet the requirement of tight convergence, that is, two networks can share the core network and air interface, but only on the wireless transmission side. In this degree, the convergence is WiMAX and The true convergence of the 3GPP series of networks.
4. to sum upBased on the analysis of standard protocol stack and network architecture, this paper analyzes and discusses the TD-LTE, LTE-Advanced network and WiMAX network convergence scheme: based on the SAE architecture, proposes a TD-LTE and WiMAX loosely coupled fusion scheme and suitable access points. The most important network selection and network switching problems in network convergence are discussed, and the solution is given. For LTE-Advanced and WiMAX solutions, a tightly coupled fusion scheme and a tightly coupled protocol stack are given. It can be seen from the analysis that with the development of technology, the integration of 3GPP series networks and WiMAX networks is getting closer and closer. It is believed that in the near future, 3GPP series networks can achieve full integration with WiMAX and develop together.
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