With the deployment of communication technologies and new services. The interaction between the market and technology, some new features in the field of communication in the future are gradually emerging. On the one hand, traditional broadband fixed access users are not satisfied with using broadband services only in fixed environments such as homes and offices, and hope to use broadband access mobile services; on the other hand, traditional mobile users are not satisfied with simple voice, SMS and low-speed data services, hoping to use higher data rate services. Changes in user demand have led to the convergence of fixed broadband access services and mobile services in technology and services. Broadband mobility and mobile broadband have gradually become the trend of technology development in the two fields, complementing each other and promoting each other. In terms of mobile broadband, 3gpp/3gpp2 has developed technical standards such as 1xev-do and hsdpa/hsupa to realize broadband data transmission in a mobile environment. In terms of broadband mobility, the IEEE 802 working group has developed technical specifications such as WLAN and WiMAX, aiming to gradually realize broadband mobility along an evolutionary path of fixed, nomadic/portable and mobile. IEEE802.16WiMAX is an important milestone for broadband mobile. . Promoted the evolution and development of mobile broadband [1, 2].
WiMAX is the name adopted by IEEE802.16 technology in marketing. Its physical layer and MAC layer technology are based on the wireless metropolitan area network (WMAN) technology developed in the IEEE802.16 working group. WiMAX is also another name for IEEE802.16d/e technology. . This paper first introduces the system to the IEEE802.16 series of standards, compares the features, advantages and disadvantages of Wi-Fi, WiMAX and 3G standards. Based on the existing network configurations, the WiMAX networking three-step music is elaborated. Detailed networking strategies at different stages are used to support high-speed mobile and seamless communication of high-speed multimedia services.
2. WiMAX wireless access technologyIEEE802.16 is a working group specially established for the development of wireless metropolitan area network standards. Since its establishment in 1999, the working group has been responsible for the development of air interface standards for fixed wireless access, in order to promote wireless based on IEEE802.16 and ETSIHiperMAN protocols. Broadband access devices, and to ensure compatibility and interoperability between them, in April 2001, the industry's major wireless broadband access manufacturers and chip manufacturers jointly established a non-profit industrial trade alliance organization - WiMAX.
2.1 WiMAX series standard
The original IEEE 802.16 protocol is the wireless metropolitan area network standard adopted by the IEEE in December 2001. The standard supports a working frequency band of 10 to 66 GHz and can only be carried in a line-of-sight environment. These restrictions are not conducive to fixed broadband access. The promotion of technology, so in January 2003, the IEEE released the extension protocol IEEE802.16a, the purpose is to enable fixed broadband access technology can also support non-line-of-sight transmission, operating frequency range of 2 ~ 11GHz requires license and license-free Card frequency band. At present, in order to enable the IEEE 802.16 series of standards to transmit data from several megabits per second to several hundred megabits per second, the ability to provide high-speed multimedia service transmission becomes an effective means to solve the "last mile" bottleneck of the access network. The IEEE 802.16a protocol has been improved, and the fused IEEE802.16 REVd protocol, also known as the IEEE 802.16-2004 protocol, has been proposed. Currently, the protocol has become an industry standard, and major manufacturers have designed and launched various standards based on the standard. Fixed wireless access products. The IEEE 802.16e protocol, which is currently being standardized, is an extension of the fixed access technology, which increases the mobility function of the end user, thereby enabling the mobile terminal to switch and roam between different base stations.
2.2 protocol stack model
The IEEE 802.16 family of standards specifically defines the wireless air interface of WiMAX. Its reference model is shown in Figure 1. IEEE802.16 currently only regulates the U interface between fixed user terminals (SS) and base stations (BS), while the IB interface between BSs, BS and RNC (similar to the radio network controller RNC function of WCDMA systems) The A interface between them does not belong to the IEEE 802.16 standard organization work area. The air interface defined by the IEEE 802.16 series of standards consists of a physical layer and a MAC layer, as shown in FIG. The MAC layer supports point-to-multipoint (PMP) structures and is also applicable to mesh topologies. The structure of the MAC layer is designed to support multiple physical layer specifications, and different physical layer technologies are suitable for different wireless propagation environments.
Figure 1 IEEE802.16 protocol model
Figure 2 Protocol Architecture
The IEEE 802.16 MAC layer is divided into three sublayers from high to low.
(1) Service-Specific Convergence Sublayer: Provides an interface with higher layers to better adapt to various upper layer protocols through different convergence modes;
(2) Common Part Sublayer: responsible for performing the core functions of the MAC layer, including system access, bandwidth allocation, connection establishment, connection maintenance, etc.
(3) Privacy Sublayer: Provides security-related functions such as encryption, authentication, and key exchange.
2.3 physical layer specification
At present, the most common IEEE802.16-2004 air interface physical layer specification has been stipulated according to the frequency band in which the system works, and can be divided into two categories.
(1) 10 to 66 GHz: The wavelength of electromagnetic waves in this frequency band is in the millimeter wave band, and the wave energy is easily absorbed by the ground and the building. Therefore, there is no obstacle between the transmitting antenna and the receiving antenna, that is, the line of sight transmission. In addition, the transmission signal is also susceptible to rain attenuation, etc., which makes the deployment of the system very high and the coverage area is small. However, the frequency band is rich in resources, the allocated frequency band is wide, and the system capacity is large. The physical layer specification of IEEE802.16 for this band is WMAN_SC, which adopts single carrier modulation technology;
(2) 2 to 11 GHz: This band contains both spectrum license and license-free spectrum resources, mainly for supporting non-line-of-sight transmission. The electromagnetic wave in the frequency band is long, and the transmitting antenna and the receiving antenna do not have to have a line of sight transmission, so the multipath interference problem is prominent. In addition, many other wireless devices also work in this frequency band, such as Bluetooth systems, wireless LANs, etc. How to coexist with these devices without increasing mutual interference is also a problem to be solved. Considering the physical environment in which the system works, this band supports three physical layer specifications:
*WMAN_SCa: adopts single-carrier adaptive modulation strategy, the downlink uses point-to-multipoint broadcast mode for signal transmission, and the base station transmits TDM signals to all SSs in the BS, and the target SS detects that it is allocated to its own time slot. The reception of the start signal. And adopt TDMA mode on the uplink;
*WMAN_OFDM: Orthogonal Frequency Division Multiplexing (OFDM) modulation with 256-point transform, TDM for downlink, and TDMA+OFDMA for uplink access. This air interface is mandatory for license-free bands. ;
*WMAN_OFDMA: OFDM modulation technique using 2048 point transform. Multiple access transmission is realized by allocating a set of subcarriers for each receiver, and TDMA+0FDMA is used as the multiple access mode for both uplink and downlink. Taking into account the NLOS characteristics, advanced technologies such as advanced adaptive antenna system (AAS), ARQ and dynamic frequency selection (DFS) are adopted.
The IEEE 802.16 system can operate in Frequency Division Duplex (FDD) or Time Division Duplex (TDD) mode. FDD requires pairs of frequencies, TDD does not need, and can flexibly implement dynamic adjustment of uplink and downlink bandwidth.
In IEEE 802.16, it is also stipulated that the terminal can adopt the half-duplex frequency division duplex (H-FDD) mode, which reduces the requirements on the terminal transceiver. Thereby reducing the terminal cost.
2.4 transmission rate
IEEE 802.16 does not specify a specific carrier bandwidth, and the system can use a bandwidth from 1.25 to 20 MHz. Considering the carrier bandwidth division of fixed wireless access systems in various countries, IEEE802.16 specifies several series, multiples of 1.25MHz and multiples of 1.75MHz. The 1.25MHz series includes 1.25/2.5/5/10/20MHz and more. The 1.75MHz series includes 1.75/3.5/7/14MHz and the like. For a fixed wireless access system of 10 to 66 GHz, a 28 MHz carrier bandwidth can also be used to provide a higher access rate.
WMAN_SC specifies single-carrier modulation in this frequency band, specifically QPSK and 16QAM, and optional 64QAM. WMAN_SCa can adopt the most modulation methods, support BPSK, QPSK, 16QAM and 64QAM, and even support 256QAM. The modulation mode of each subcarrier of WMAN_OFDM can support BPSK, QPSK, 16QAM and 64QAM, of which 64QAM is optional for the license-free frequency band. The modulation mode of each subcarrier of WMAN_OFDMA can support QPSK, 16QAM, and optional 64QAM.
It can be seen that WMAN_OFDMA has the best spectral efficiency, but the theoretical performance of several physical layer technologies is not much different. It should be noted that the WMAN_SCa anti-multipath and frequency selective fading is very poor, so the physical layer configuration is generally not used in the NLOS environment. The coverage of WiMAX for fixed terminals is generally 5 to 15 km. The standard IEEE802.16e supported by low-speed mobile terminals is still under development, requiring coverage of several kilometers and supporting mobility management such as handover.
2.5 Comparison of different network standards
In future wireless network systems, Wi-Fi, WiMAX, and cellular systems (mainly 3G and 3G systems) can provide carrier-grade applications. Wi-Fi and WiMAX are both poor in terms of supporting user mobility. The cellular system can achieve seamless handover; from the perspective of the supported transmission rate, WiMAX has an incomparable advantage. Wi-Fi has a significantly higher transmission rate than the cellular network; from the perspective of single base station coverage, WiMAX has the largest coverage. Cellular network is the second, WLAN is the smallest; from the perspective of network management, since the IEEE802 series protocol only defines the physical layer and MAC layer protocol, WiMAX and Wi-Fi can learn from the functions of authentication and charging of the cellular network [4] ].
3. WiMAX networking technologyAs a wireless air interface standard, the IEEE 802.16-2004 standard promulgated in 2004 can be used as a supplementary network for packet data by mobile operators, and the mobility of user terminals is not supported at this time. However, the future IEEE 802.16e standard will have better mobility support, and the entire network can be covered by a separate network. Therefore, from the perspective of technology evolution, supported user mobility and network coverage, WiMAX networking is a gradual evolution process, from supplementary network to localized individual network to the final full coverage network. The specific evolution steps are shown in Figure 3. Shown [5].
Figure 3 IEEE802.16 evolution strategy
In the above evolution strategy, there is no fixed broadband wireless access system that has been commercialized, such as the Local Multipoint Distribution Service (LMDS) system, because its physical layer uses single-carrier technology and its frequency is 10 to 66 GHz band. The system physical layer described here is based on OFDM technology and uses the 2 to 10 GHz band to support NLoS transmission.
3.1 first stage
WiMAX is particularly well-suited for delivering high-burst data, and its MAC architecture supports both real-time multimedia and simultaneous applications, which means it is especially suitable for broadband wireless transmission. The biggest feature of WLAN is portability. It mainly solves the user's "last 100m" communication demand, and locates high-speed mobile data access in hotspots, but does not support high-speed mobility. The mainstream application is for business users to use portable in hotels, airports and other hotspots. The computer browses the Internet or accesses the corporate server. WiMAX is an extension of today's Wi-Fi systems that can simultaneously perform distance and high QoS requirements.
At this stage, WiMAX and Wi-Fi do not provide high-speed user mobility support. Therefore, in order to achieve full network coverage, a 3G cellular system needs to be combined. As a 3G system, high-speed broadband IP data coverage and roaming cannot be achieved due to system overhead and complexity limitations. Aspect enhancement.
From the above analysis, this stage will be the coexistence phase of Wi-Fi, WiMAX and 3G networks. Wi-Fi is positioned in high-speed mobile data access in hotspots. WiMAX connects different hotspots in series to achieve a wider range of high-speed data access, mainly to solve the "last mile" communication demand, and 3G network positioning. Voice communication for mobile users and low-speed data communication over the entire network. Since both Wi-Fi and WiMAX define the physical layer and the MAC layer of the wireless air interface, it is necessary to use a loose mode for network convergence with the 3G network, so that the user's authentication, encryption, and charging are performed by the 3G network. WiMAXRNC can also directly use the network management entity of the current wired network to complete the corresponding authentication, charging, and network management. The relationship between Wi-Fi, WiMAX and 3G systems is shown in Figure 4.
Figure 4 Wi-Fi/WiMAX/3G first-phase hybrid networking diagram
The network feature at this stage is that Wi-Fi access points are directly connected to the backbone network through WiMAX, which makes the Wi-Fi base station arrangement particularly convenient and mobile, and also solves the 3G network indoors. And the hot spots cover difficult problems, because users can be authenticated, encrypted and billed in the 3G network, thus breaking the boundary between wireless and wired users, and users can use a unique account to seamlessly Communication. Since the function of the WiMAX base station is to manage the communication between the fixed Wi-Fi access points, and the AP points are mainly LOS transmission, the advanced technology such as OFDM can be not used at this time, and the carrier frequency is 10 to 66 GHz, and WMAN- is adopted. SC agreement.
3.2 second stage
At this stage, WiMAX increases the portable and slow mobility characteristics of mobile devices, but end users do not have the ability to switch between different WiMAX base stations. In addition to high-speed data transmission, WiMAX terminals can also perform VoIP voice communication. At this time, WiMAX user terminals with portability and mobility are called mobile user terminals (MSS). In order to achieve network-wide communication, the terminal has Wi-Fi/WiMAX/3G multi-mode function. At this time, the convergence problem between different Wi-Fi/WiMAX/3G networks is more complicated, in order not to change the original configuration between different networks. It is recommended to use loose coupling mode.
In order to support user mobility, IEEE802.16e protocol is required. For WiMAX MSS, the WiMAX BS is similar to an access point (AP), and its function is similar to that of the base station in the 3G system. The specific reference model As shown in Figure 5. Both the MSS and the AP entity need to add an upper layer protocol for radio resource management, mobility management, and communication event task management. Compared with the U and A interfaces of the fixed node, the MU and MA are respectively improved.
Figure 5 WiMAX/3G reference protocol model
Figure 6 Wi-Fi/WiMAX/3G second-phase hybrid networking diagram 1
The relationship between Wi-Fi, WiMAX and 3G systems is shown in Figure 6. Compared with the network architecture of the first phase, the communication function of the MSS is mainly added.
As in the first phase, this phase mainly borrows 3G IP core network by loosely coupling, and performs network management such as addressing, authentication, service authorization, encryption and billing for IEEE802.16SS/MSS to reduce the investment of the core network. cost. At this time, an interface needs to be specifically defined in the 3G packet core network for communicating with the WiMAX RNC. In order to support the portable and low mobility characteristics of the MSS, the physical layers of WMAN_OFDM and WMAN_OFDMA are needed to overcome the impact of NLOS. In this case, the working frequency band needs to be 2 to 11 GHz, and the SS uses MAN_SC in the frequency range of 10 to 66 GHz, or in the frequency range of 2 to 11 GHz. WMAN_SCa communicates.
3.3 third stage
With the gradual expansion of WiMAX networks, the IEEE 802.16e standard is constantly improving and supporting mobile mobility, and WiMAX can be used to cover the entire network. Compared to the second phase of networking. Additional core network devices supporting WiMAX are required, as shown in Figure 7. In the future, the whole network coverage of WiMAX mobile terminals will be supported mainly in the 2-6 GHz frequency band, and the RNC in WiMAX is responsible for centralized network control and as a retransmission entity for communication between the wireless network and the core network. The information exchange between the base station of the WiMAX and the RNC may adopt an MPLS or IP-in-IP tunneling protocol that differentiates the service QoS, and the communication between different RNCs or the information exchange between the RNC and the core network adopts a fast label switching path ( LSP) exchange protocol.
Figure 7 Wi-Fi/WiMAX/3G second-phase hybrid networking diagram 2
4 ConclusionDue to the repeated commercialization of 3G cellular systems, various wireless access technologies have been rapidly developed. In the future wireless communication field, it is difficult to have any technology or standard that can dominate the world, but will be a combination of various wireless access technologies, and various mobile communication systems are compatible and cooperative. WiMAX is an emerging broadband metro access standard. It adopts many advanced and mature technologies, can provide high transmission rate and strong QoS guarantee, and can support user mobility with the improvement of standards, thus achieving seamless coverage of the entire network.
In view of the existing cellular systems and the development of strong Wi-Fi, WiMAX networking and use should first be positioned to complement the high-speed data support of existing networks. With the continuous improvement of technology and network configuration, give full play to its technological advantages. And the flexibility of networking, and finally reach the entire network coverage through independent networking, which can provide high-speed data transmission and guarantee QoS, and can seamlessly roam throughout the network. In addition to the need to continue to improve the IEEE802.16e standard protocol and provide interconnection and cooperation with Wi-Fi and 3G networks, WiMAX also needs to launch corresponding terminal and base station devices as soon as possible to ensure that the working frequency band does not affect existing electronic devices. Interference with the system, and as the number of users increases, the price of the equipment drops to a level acceptable to the public as soon as possible.
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