Research on WiMAX Network Application Scheme Based on IEEE 802.16e Technology
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The IEEE 802.16 working group recently released the wireless metropolitan area network standards IEEE 802.16-2004 and IEEE 802.16e, which support fixed and mobile broadband wireless access. Both IEEE 802.16-2004 and IEEE 802.16e are specifications for the physical layer and the media access layer. The wireless metropolitan area network standard IEEE 802.16-2004 is a fixed broadband wireless access specification developed by IEEE 802, and IEEE 802.16e is a supplementary and revised version of IEEE 802.16-2004, in the hope of providing user stations with functions and services that can move at vehicle speeds based on IEEE 802.16-2004. In order to accelerate the application and industrial chain of IEEE 802.16 technology, the industrial organization WiMAX Forum further provides the interconnection and interoperability capabilities based on IEEE 802.16 technology, as well as the functions and testing specifications of network resource management and control, by defining the frequency bands, application scenarios and interoperability of IEEE 802.16 technology applications.
Due to the potential support of IEEE 802.16e for the mobility of wireless broadband and the active promotion of the WiMAX Forum, the application of WiMAX mobile networks based on IEEE 802.16 is becoming a hot topic in the industry. IEEE 802.16m was selected as one of the candidate solutions for the next generation wireless communication standard (IMT-advanced). Compared with IEEE 802.16e, in order to meet the performance requirements of IMT-2000 and IMT-advanced, IEEE 802.16m emphasizes the addition of some enhanced physical layer functions, such as relay, multicast, power control and multi-antenna technology, etc. However, in terms of supporting mobility and interconnection with IP technology, there is no obvious difference between WiMAX networks based on IEEE 802.16m and IEEE 802.16e. This paper will first study the mobile service capabilities of IEEE 802.16e, and then focus on the network solution of mobile WiMAX based on IP, which can also be used as a future network application solution based on IEEE 802.16m technology.
1. IEEE 802.16e supports mobility* capabilities
Based on 1EEE 802.16-2004, IEEE 802.16e mainly expands the physical layer and media access layer to support multi-user communication and network mobility* capabilities. The following analyzes its enhanced functions based on the physical layer and MAC layer respectively.
1.1 Physical layer enhancements and features of IEEE 802.16e
Orthogonal frequency division multiplexing (OFDM) technology is a sound communication technology for effective information transmission in the channel. This technology uses multiple parallel subcarriers (subcarrier frequencies) that transmit low-rate data to achieve high data rate communication. The advantage of OFDM technology is that it is easy to simplify the channel equalization process and supports multi-user channel allocation and link adaptation in the time domain and frequency domain, thereby further improving the spectrum utilization of the OFDM system. Compared with OFDM, the use of OFDMA can bring more flexibility, that is, according to the characteristics of different channels and the demand for data volume, the channel and power resources can be allocated through sub-channel diversity, thereby more effectively improving the efficiency of resource allocation.
IEEE 802.16e further adopts scalable OFDMA (sealable orthogonal frequency division multiplexing access, SOFDMA), which allows the system to easily adapt to different channel bandwidths by extending the FFT size under a constant subcarrier frequency. For example, if the subcarrier frequency is set to 10.94 kHz, by adjusting the FFT size, it can flexibly support 1.25~20 MHz bandwidth. The
scalable OFDMA system uses diversity and proximity to achieve subcarrier permutation or dispersion in subchannels. Among them, the purpose of diversity is to randomly combine subcarriers into subchannels to provide frequency diversity and average inter-cell interference. Typical diversity permutation methods include downlink FUSC (fully used subcarrier), downlink PUSC (partially used subcarrier) and uplink PUSC. Figure 1 (a) and Figure 1 (b) list the subcarrier distribution methods of downlink PUSC and uplink PUSC respectively. Downlink PUSC adopts a cluster structure, that is, a cluster is composed of appropriate subcarriers in each pair of OFDM symbols in the downlink PUSC, and each OFDM symbol includes 14 continuous subcarriers for data and pilot. The uplink PUSC adopts a slice structure, 12 subcarriers form a slice, and 6 slices are reorganized and permuted to form a time slot. That is, a time slot includes 48 data and 24 pilot subcarriers distributed in 3 OFDM symbols. Among them, the data subcarrier is used for data transmission, and the pilot subcarrier is used for estimation and synchronization.
The research on proximity permutation includes downlink AMC and uplink AMC, which can support multi-user diversity in OFDM (A) systems and facilitate link adaptive processing. Among them, continuous subcarriers from the same OFDM codeword form a bin, and one time slot of AMC is defined as a combination of multiple bins. The combinations are: [6 bins, 1 codeword], [3 bins, 2 codewords], [2 bins, 3 codewords], [1 bin, 6 codewords]. The subcarrier diversity permutation in the AMC permutation mode is more suitable for mobile systems, while the continuous permutation mode is suitable for fixed, nomadic and low-speed mobile environments.
1.2 IEEE 802.16e MAC layer enhancements and features The
mobile * capabilities of IEEE 802.16e are more reflected in the improvement of the MAC layer. The key MAC layer technologies provided include mobile * support, switching and power saving mode.
1.2.1 Mobility support of 802.16e
To support handover and other mobility*, IEEE 802.16 provides functions such as obtaining network topology, scanning and associating the target base station, ranging, and cell reselection at the MAC layer. The mobile station scans neighboring base stations to determine a new diversity set. The scanning steps include: identifying a suitable base station; synchronizing with its downlink transmission and estimating its channel quality; ranging enables the mobile station to complete the synchronization process with a certain base station. Ranging can be conflict-based or non-conflict-based. Non-conflict-based ranging provides a faster and more reliable synchronization method, but at the cost of consuming resources; associating enables the mobile station to record the number of successful scanning and ranging of base stations in the diversity set, accelerating the transfer of the mobile station's services to the target base station; and neighbor list broadcasting enables the base station to generate a neighbor list with the help of network site backhaul to support the handover service of the mobile station. The list information is in the message element "handoffNeighbor preference" of MOB_NBR_ADV. The base station periodically sends a neighbor list, and each base station maintains a MAC address mapping table of neighbor base stations and their indexes.
1.2.2 MAC layer switching capability
Hard switching is a mode that must be supported in IEEE 802.16e. Under hard switching, the high-level connection and the convergence sublayer data of the MAC layer can be buffered and then seamlessly transferred to the target base station. Macro diversity handover (MDHO) and fast base station switching (FBSS) are enhanced optional switching modes. MDHO is supported for both uplink and downlink transmissions, and it allows the mobile station to transmit and receive transmissions with multiple base stations in the diversity set at the same time. The difference between FBSS and MDHO is that in FBSS, although the mobile station is synchronized with all candidate base stations, it only communicates with one central base station. Hard switching, MDHO and FBSS technologies provide mobile support at different application levels, among which MDHO and FBSS can reduce handover delays and support effective resource and network management.
2. WiMAX Network Reference Model
Based on IEEE 802.16 technology, the WiMAX Forum provides a network architecture that supports mobility to support mobile services above the MAC layer and roaming and handover services at different network nodes. The
network reference model (NRM) shown in Figure 2 includes the logical functional entities of the access service network (ASN) and the connection service network (CSN). ASN consists of one or more base stations and one or more ASN-GWs (ASN gateways). It is a complete set of network functions that provides key functions such as radio resource management (RRM), data forwarding, data integrity, key distribution, etc., to provide wireless access services to WiMAX users. Among them, the RRM function can be completed at the base station or ASN-GW. A node that completes this function can request other base stations to obtain the required information, and use this information to help determine candidate base stations to meet the needs of processing such as handover and load balancing. In key distribution, a pairwise master key (PMK) is calculated on the mobile station side and forwarded to the central authority in the ASN-GW. PMK and base station identifier are used together to generate authentication key (AK). A new AK is required when switching to the target base station. Distributed computing is used to support the generation of a new AK for the corresponding target base station in ASN-GW and send it to the target base station as switching information. This processing method can avoid performing user authentication process at each switch, thereby reducing processing delays. CSN needs to provide WiMAX users with core business capabilities such as AAA and DHCP servers, databases, etc. Different logical entities interoperate through various reference points (R1, R4 and R5, etc.).
Study 3, IP-based mobile WiMAX network
From the above analysis, it can be seen that IEEE 802.16e provides the ability to support mobility in the physical layer and MAC layer, while the WiMAX Forum provides the network interface and interconnection model above the MAC layer, and also includes the ability to provide mobility management, resource management and AAA services. The following study is about mobile network services based on mobile IP and WiMAX technology.
3.1 IP-based mobile WiMAX application model
Figure 3 shows the IP-based mobile WiMAX application model. The model includes the following functions: providing the ability to logically divide the above steps and IP-based routing and connection management to support different application scenarios in isolated and interconnected modes; supporting multiple NSPs to share the ASN network of one NAP; supporting one NSP to provide services to multiple ASNs to manage one or more NAPs; supporting mobile stations or SSs to discover and select NSPs to access; supporting NAPs to adopt one or more ASN network topologies; supporting access to services of different operators through interconnection; providing open reference points for network entities in different groups, so that different operators can implement different functional combinations based on different entities. To achieve IP mobile network management, the network should support mobile IP technology, that is, CSN needs to provide WiMAX users with IP connection services, network switching and system roaming capabilities.
3.2 Mobile IP Technology
Mobile IP allows mobile stations to change their access points to the Internet without changing their IP addresses, that is, allowing mobile stations to maintain transmission and high-level connections when performing handovers. Packets directed to mobile stations are first routed to the home network, and the mobile station's home agent intercepts the packet and tunnels it to the current address that the mobile station often reports. The NWG group of the WiMAX Forum allows two types of mobility solutions. Using the user MIP (Client MIP) solution, mobility management can be completed using traditional mobile IP signaling. In the proxy MIP (proxyMIP), the network side can initialize a mobile IP client process, and the virtual client completes the mobile IP signaling process.
3.3 System switching and roaming services of WiMAX network application model The WiMAX application model can support intra-system switching within the same gateway (ASN-GW), switching between different gateways and roaming services. The following is further analyzed in conjunction with the IEEE 802.16e protocol.
3.3.1 Intra-ASN intra-system switching
According to the physical layer and MAC layer capabilities provided by IEEE 802.16e and the WiMAX network reference model, the intra-ASN system switching is completed. The R8 interface (not shown in Figure 3) can be used to achieve communication between base stations. In the OFDMA system, the switching can be determined based on the carrier-to-interference ratio (CINR) parameter. In various types of switching, continuous CINR measurement is required. In accordance with the relevant provisions of the IEEE 802.16e protocol, the system can use scan request and response messages to enable the mobile station to periodically scan neighboring base stations, trigger neighboring scanning and initiate switching. The method includes the following steps:
Research on WiMAX network application scheme based on IEEE 802.16e technology (1) When the mobile station detects that the CINR signal of the serving base station is lower than the deletion threshold (H_Delete_Threshold), it can initiate a neighboring cell scan to measure the CINR value of the neighboring base station. When the mobile station detects that the CINR value of a neighboring base station is higher than the increase threshold (H_Add Threshold) of the serving base station signal, it initiates a handover request.
(2) Both the base station and the mobile station can decide the process of selecting the target base station. The mobile station can complete the handover process by scanning and selecting the target base station; the mobile station can also feedback the measurement results to the serving base station through the MOB_Mobile Station SHO_REQ message, and the serving base station decides the final selected target base station.
3.3.2 Inter-ASN system handover
The inter-ASN-GW handover is completed by using the mobile IP technology, MIP-based and proxy MIP-based mobility management methods. In the MIP-based handover mode, the MIP client resides in the mobile station, the MIP function is implemented by the mobile station, and the inter-ASN-GW handover is initiated by the network side. The method includes the following steps:
(1) The triggering condition of the handover occurs;
(2) The ASN-GW sends a proxy announcement message to the mobile station;
(3) After receiving the proxy announcement with the new care-of address (CoA), the mobile station sends a MIP registration message to the home network agent (HA) to ensure the continuity of the mobile station session; (4) The MIP registration message is forwarded to the HA, and the HA returns a MIP registration response, and the currently serving visited network agent (FA) finally returns a MIP registration response to the mobile station.
In the handover mode based on proxy MIP, the ASN-GW assumes the proxy MIP function. The MIP client resides on the ASN-GW, and the ASN-GW implements the MIP function for the mobile station agent. In the proxy MIP mode, the ASN-GW obtains the relevant information required for MIP registration from the AAA server during the authentication phase, including the DHCP service address, and generates authentication extension security information. The cross-ASN-GW handover process is transparent to the mobile station.
The following steps are included:
(1) The serving ASN-GW notifies the target ASN-GW to establish a new MIP session; (2) The target ASN-GW sends a MIP registration to the HA;
(3) A new MIP session is established. After the MIP is successfully registered, the HA will send the subsequent forward messages of the mobile station to the target ASN-GW; (4) Trigger the target ASN-GW to establish a tunnel with the base station;
(5) A new Intra-ASN tunnel relationship is established between the target ASN-GW and the base station.
3.3.3 Roaming
The roaming function enables WiMAX users to use the network services provided by the system, including authentication and billing, within the coverage area of the visited network, thereby providing users with a wide range of coverage and service access. As shown in Figure 4, WiMAX supports roaming. ASN provides wireless access, the V-CSN of the visited NSP provides users with Internet access, AAA proxy and other functions, and the H-CSN of the home NSP provides users with Internet access, user authentication, authorization and billing and other functions. Roaming users can access the Internet network through V-CSN or H-CSN. Main process steps:
Research on WiMAX network application solutions based on IEEE 802.16e technology (1) Mobile station accesses the network;
(2) The H-CSN of the home NSP performs user/device authentication and IP address allocation for the mobile station; (3) Optionally, if the user supports standard MIP or proxy MIP services, the system will also use standard MIP or proxy MIP to establish a MIP session for the user; (4) The user can access the Internet network through the V-CSN; (5) If there is a mandatory tunnel requirement, the user will access the Internet network through the H-CSN.
When the user signs up for a WiMAX user that supports roaming, and roaming is supported between his home network and the visited network, then the user can access the WiMAX system in the visited network regardless of whether he uses nomadic, portable or fully mobile services, and thus use the services provided by the system.
4. Conclusion
This paper comprehensively studies the service capabilities that support mobility in the IEEE 802.16e technical specification and WiMAX network structure, and further provides an application model for mobile WiMAX networks based on IP, providing a reference for the formation of applications and industrial chains based on IEEE 802.16 technology. The study further found that although IEEE 802.16e provides a series of mobile service capabilities at the physical layer and MAC layer of WiMAX networks, and WiMAX provides mobility management, authentication, and network interfaces, the WiMAx Forum has not yet improved and optimized to support interconnected mobile management and resource management, and network applications based on mobile IP applications are still under study. Further research will be conducted to optimize the mobile service management of WiMAX networks under network resources.
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