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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 that support fixed and mobile broadband wireless access. Both IEEE 802.16-2004 and IEEE 802.16e are specifications for the physical layer and media access layer. Among them, the wireless metropolitan area network standard IEEE 802.16-2004 is a fixed broadband wireless access specification formulated by IEEE 802, and IEEE 802.16e is a standard for IEEE 802.16- Supplementary and revised version of 2004, with the expectation that on the basis of IEEE 802.16-2004, it is expected to provide user stations with functions and services capable of moving at vehicle speeds. In order to accelerate the application of IEEE 802.16 technology and the formation of the industrial chain, by defining the frequency band, application scenarios and interoperability of IEEE 802.16 technology application, the industrial organization WiMAX Forum further provides the capability of interconnection and interoperability based on IEEE 802.16 technology, and Network resource management and control functions and testing specifications.
As IEEE 802.16e potentially supports the mobile capability 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 of discussion in the industry. IEEE 802.16m was selected as one of the candidate solutions for the next-generation wireless communication standard (IMT-ad-vanced). Compared with IEEE 802.16e, in order to meet the performance requirements of IMT-2000 and IMT-advanced, IEEE 802.16m emphasized the addition of Some enhanced physical layer functions, such as Relay, multicast, power control and multi-antenna technology, but in terms of supporting mobility and interconnection with IP technology, WiMAX networks based on IEEE 802.16m and IEEE 802.16e are not obvious The difference. This article will first study the mobile service capabilities of IEEE 802.16e, and then focus on the IP-based mobile WiMAX network solution, which can also be used as a future network application solution based on IEEE 802.16m technology.
1 IEEE 802.16e supports mobility service capabilities
On the basis of 1EEE 802.16-2004, IEEE 802.16e is mainly extended at the physical layer and media access layer to support multi-user communication and network mobility service capabilities. The following is an analysis of its enhanced functions based on the physical layer and the MAC layer.
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 sub-carriers (sub-carrier 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 different channel characteristics and data volume requirements, channel and power resources are allocated through sub-channel diversity, thereby more effectively improving the efficiency of resource allocation.
IEEE 802.16e further adopts expandable OFDMA (sealable orthogonal freqtaency division multiplexingaccess, SOFDMA). Under a constant sub-carrier frequency, by extending the FFT size, the system can easily adapt to different channel bandwidths. For example, if the sub-carrier frequency is set to 10.94 kHz, by adjusting the FFT size, the bandwidth of 1.25 to 20 MHz can be flexibly supported.
The scalable OFDMA system adopts diversity and proximity methods to realize sub-carrier replacement or dispersion in sub-channels. Among them, the purpose of diversity is to randomly combine sub-carriers into sub-channels to provide frequency diversity and average inter-cell interference. Typical diversity replacement methods include downlink FUSC (fully used subcarrier), downlink PUSC (partially used subcarrier) and uplink PUSC. Fig. 1(a) and Fig. 1(b) respectively enumerate the subcarrier distribution modes of downlink PUSC and uplink PUSC. The downlink PUSC adopts a cluster structure, that is, a string is composed of appropriate subcarriers in each pair of OFDM symbols in the downlink PUSC, and each OFDM symbol includes 14 consecutive subcarriers for data and pilot. The uplink PUSC adopts a tile structure, 12 sub-carriers form a tile, and 6 tiles are reorganized and replaced to form a time slot. That is, one 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 (pilot) subcarrier is used for estimation and synchronization.
Proximity permutation includes downlink AMC and uplink AMC, which can support multi-user diversity in an OFDM(A) system, which is more convenient for link adaptation processing. Among them, consecutive sub-carriers from the same OFDM symbol form a bin. A slot of AMC is defined as a combination of multiple bins. The combination methods are:[6个bin，1个码元],[3个bin，2个码元],[2个bin，3个码元], [1个bin，6个码元]. AMC replacement mode subcarrier diversity replacement is more suitable for mobile systems, while continuous replacement mode is suitable for fixed, nomadic and low-speed mobile environments.
1.2 MAC layer enhancements and features of IEEE 802.16e
The mobility service capability of IEEE 802.16e is more reflected in the improvement of the MAC layer. The key MAC layer technologies provided include mobility service support, handover, and power-saving modes.
1.2.1 Mobility support for 802.16e
In order to support handover and other mobility services, IEEE 802.16 provides functions at the MAC layer such as network topology acquisition, scanning of target base stations, correlation, ranging, and cell reselection. The mobile station scans neighboring base stations to determine a new diversity set. The scanning step includes; identifying a suitable base station; synchronizing with its downlink transmission and estimating its channel quality; ranging to enable the mobile station to complete the synchronization process with a certain base station. Ranging can be based on conflict and non-conflict. The non-conflict-based ranging provides a faster and reliable synchronization method, but at the cost of resources; correlation enables the mobile station to record the number of successful scanning and ranging of the base station of the diversity set, accelerating the transfer of the mobile station’s business To the target base station; and the neighbor list broadcast enables the base station to generate a neighbor list by means of the network site back-haul (back-haul) 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 the neighbor list, and each base station maintains the MAC address mapping table and its index of the neighbor base station.
1.2.2 MAC layer switching capability
Hard handover is a mode that must be supported in IEEE 802.16e. Under hard handover, the high-level connection and the convergence sub-layer data of the MAC layer can be buffered and then seamlessly transferred to the target base station. Macrodiversity handover (MDHO) and fast base station handover (FBSS) are enhanced optional handover modes. MDHO supports both uplink and downlink transmissions. It allows the mobile station to simultaneously transmit and receive transmissions with multiple base stations in the diversity set. The difference between FBSS and MDHO is that although the mobile station synchronizes with all candidate base stations in FBSS, it only communicates with a central base station. Hard handover, MDHO and FBSS technologies provide mobile support at different application levels. MDHO and FBSS can reduce handover delay and support effective resource and network management.
2 WiMAX dull 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, as well as roaming and handover services at different network nodes.
The network reference model (NRM) shown in Fig. 2 includes the logical functional entities of the access service network (ASN) and the connection service network (CSN). ASN is composed of one or more base stations and one or more ASN-GW (ASN gateway). It is a complete set of network functions, providing such as radio resource management (RRM), data forwarding, data integrity, key distribution, etc. The key function is to provide wireless access services to WiMAX users. Among them, the RRM function can be completed in 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 the base station identifier are used together to generate an authentication password (authenTIcation key, AK). When switching to the target base station, a new AK is required. Using distributed computing to support the generation of a new AK corresponding to the target base station in the ASN-GW, and send it to the target base station as handover information. This processing method can avoid performing the user authentication process at each handover, thereby reducing processing delay. CSN needs to provide WiMAX users with core business capabilities such as AAA and DHCP servers and databases. Different logical entities complete interoperability through various reference points (R1, R4, R5, etc.).
3 IP-based mobile WiMAX network
It can be seen from the above analysis that IEEE 802.16e provides the ability to support mobility in the physical layer and the MAC layer, and the WiMAX Forum provides network interfaces and interconnection models above the MAC layer, including the provision of mobility management and resources. Management and AAA service capabilities. The following research is based on mobile network services based on mobile IP and WiMAX technology.
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