Key Technology Analysis of WiMAX system

The huge demand for wireless access to the Internet and wireless multimedia data services has promoted the rapid development of wireless communication technology. Broadband, IP, and mobile communication technology will become the future development trend. WiMAX is a broadband wireless access technology based on the series of standards. It has developed rapidly over the past two years and has gradually become a hot topic in the Development of Metro broadband wireless access technology.

WiMAX systems have two major technical standards: one is to meet the WiMAX802.16d standard for fixed broadband wireless access, and the other is to meet the WiMAX802.16e standard for fixed and mobile broadband wireless access technology. At present, the WiMAX Forum is performing 16d product certification and is also stepping up the development of 16e standards. Key WiMAX technologies include the following:

-- OFDM/OFDMA.Orthogonal Frequency Division Multiplexing (OFDM) is a high-speed transmission technology. It is one of the key technologies in the future wireless broadband access system/next generation cellular mobile system, 3GPP has used OFDM technology as its main candidate Technology for LTE research. In WiMAX systems, OFDM is a physical layer technology. There are two main application methods: OFDM physical layer and OFDMA physical layer. The OFDM physical layer of a wireless MAN uses OFDM modulation. The OFDM orthogonal carrier set is generated by a single user and transmits data streams concurrently for a single user. TDD and FDD duplex modes are supported. The uplink adopts the TDMA multiple access mode. The downlink adopts the TDM multiplexing mode. The STC transmitter diversity and AAS adaptive antenna systems can be used. The OFDMA physical layer of a wireless MAN uses OFDMA multi-access mode. It supports TDD and FDD duplex modes, and can adopt STC transmission diversity and AAS. The OFDMA system supports 2048, 1024, 512, and 128 FFT points. Generally, downward data streams are divided into logical data streams. These data streams can be modulated and encoded in different ways and connected to users with different channel features at different signal power. Upstream data streams are connected through multi-address access through sub-channels, and the uplink data streams are transmitted through the downstream Media Access Protocol MAP. Although the OFDM technology is very sensitive to phase noise, the standard-defined ScalableFFT allows you to select different modulation methods based on different wireless environments to ensure that the system can work in a high-performance manner.

-- HARQ.Because HARQ technology improves the spectrum efficiency, it can significantly increase the system throughput. At the same time, because retransmission can bring about a merge gain, it indirectly expands the coverage of the system. Although the 16e Protocol specifies that the channel encoding methods include convolution code CC), convolution Turbo code CTC), and low-density checksum (low-density checksum) encoding, for the HARQ method, according to the current protocol, in 16e, only the CC and ctc harq methods are supported. Specific provisions: In the 16e protocol, the hybrid automatic retransmission requires HARQ) method is optional in the MAC part. The HARQ function and related parameters are determined and negotiated using message SBC during network access or re-access. HARQ is based on each connection. It can use message DSA/DSC to determine whether each service stream has the HARQ function.

-- AMC.AMC has unique technical requirements in WiMAX Applications. As AMC technology needs to determine the encoding scheme and modulation scheme to be used based on channel conditions, therefore, the AMC technology must implement the AMC function according to the technical characteristics of WiMAX. Unlike CDMA, WiMAX physical layer uses OFDM technology, therefore, channel factors that have an important impact on OFDM demodulation performance, such as latency expansion, Doppler frequency shift, PAPR value, and cell interference, must be considered in the AMC algorithm to adjust the system encoding and modulation method, to achieve the Instantaneous Optimal performance of the system. The WiMAX standard defines a variety of encoding and modulation modes, including convolution encoding, grouped Turbo encoding (optional), convolution Turbo code (optional), zero-bit-tail convolution code ZeroTailbaitingCC (optional), and low-income-C (optional ), corresponding to different bit rates, such as 1/2, 3/5, 5/8, 2/3, 3/4, 4/5, and 5/6.

-- MIMO.For future mobile communication systems, how to ensure high QoS in non-line-of-sight (NLOS) and harsh channels is also a key issue in mobile communication. For SISO systems, more spectrum resources and complex coding and modulation technologies are required to meet the above requirements, the limited spectrum resources and the features of mobile terminals all restrict the development of the SISO system. Therefore, MIMO is a key technology for mobile communication in the future. MIMO technology has two forms: Space multiplexing and empty-time encoding. Both forms are applied in WiMAX protocol. The Protocol also provides the format of Space reuse and empty-time encoding. At present, MIMO technology is being developed and applied to various high-speed wireless communication systems, but few mature products are currently available. It is estimated that the MIMO technology is developed and implemented, it still takes some time to make a breakthrough. Supporting MIMO is an optional solution in the Protocol. The Protocol has a complete definition of MIMO. MIMO technology can significantly improve the system capacity and spectrum utilization, and greatly improve the system performance, it will be supported by most device manufacturers in the future.

-- QoS mechanism.In the WiMAX standard, the MAC layer defines a complete QoS mechanism. The MAC layer can set different QoS parameters for each connection, including the rate, latency, and other indicators. The four scheduling types defined by the WiMAX system are only for upstream business flows. For downstream business flows, only QoS parameter restrictions are imposed based on the Application Type of the business flow. That is, different application types have different QoS parameter restrictions, because the downstream bandwidth allocation is triggered by the data in the Buffer in BS. The QoS parameters defined here are all for air interfaces and are required for these four services.

-- Sleep mode.To adapt to the characteristics of mobile communication systems, the 16e Protocol adds the terminal Sleep mode and Idle mode. The Sleep mode aims to reduce the energy consumption of MS and reduce the use of ServingBS air resources. Sleep mode is a state in which MS temporarily suspends the ServingBS service within a specified period of pre-negotiation. From the perspective of ServingBS, MS in this state is in the unavailability state. The Idle mode provides MS with a more power-saving working mode than the Sleep mode. after entering the Idle mode, MS is only at discrete intervals, periodically receives downstream broadcast data, including paging messages and MBS services. When multiple BS are moved, no switchover or network access is required. The difference between Idle mode and Sleep mode is that in Idle mode, MS does not have any connections, including management connections. in Sleep mode, MS has management connections and may also have business connections; in Idle mode, MS does not need to be switched over BS. in Sleep mode, MS switches over BS. Therefore, in Idle mode, the overhead of MS and base stations is lower than Sleep; in Idle mode, MS regularly registers the location with the system. In Sleep mode, MS always keeps in touch with the base station and does not need to register.

-- Switch technology.The 16e Standard specifies a required switching mode, which is abbreviated as HOhandover in the Protocol). In fact, it is what we usually call hard switching. In addition, two optional switching modes are provided: MDHO macro diversity switching) and FBSS quick BS switching ). Hard switch must be supported in WiMAX16e, which is called HO in the Protocol. The mobile station can obtain information of adjacent cells through messages broadcast by the current BS service, you can also request to allocate scan intervals or sleep intervals to scan and locate neighboring base stations to obtain information about adjacent cells, evaluate the information, and find potential target cells. Switching can be initiated by MS decision-making or BS decision-making. During Fast Base Station switching to FBSS, MS only communicates with AnchorBS. Fast refers to switching from one AnchorBS to another AnchorBS without performing steps in the HO process. Support for FBSS is optional for MS and BS. For macro diversity switching MDHO), MS can send and receive data between multiple BS at the same time, so that the diversity merging gain can be obtained to improve the signal quality. MDHO support is optional for MS and BS.

The WiMAX technology Standard Edition officially released by IEEE is WiMAX802.16-2004. This version is the official release of 16d. It is mainly used for Fixed Broadband Wireless Access and does not have the mobile function, therefore, the application model can be used for the integrated access of small and medium-sized enterprises, the bearer of telecommunication services, and some industry users and things. Compared with the LMDS system, WiMAX has many technical advantages. The series of standards are applicable to man applications. The throughput of the system varies with the bandwidth, modulation, and encoding modes of the channels, A single carrier can provide a maximum data throughput of Mbps, depending on the bandwidth, modulation, and encoding modes used. The coverage radius varies depending on the power, antenna, frequency, and environment. Based on different environmental characteristics, the coverage radius of the IEEE 802.16 system can be 3km to 15km or even greater, providing users with high-speed data access services within a large range; at the same time, WiMAX supports non-line-of-sight transmission and non-outdoor antenna user terminal mode, allowing the network to directly access the user's "desktop", expanding the application scenario. The operating frequency of the system can be from 2 GHz to 38 GHz. If the WiMAX band is eventually divided into lower frequencies, the system performance will be greatly reduced due to rain attenuation. However, when the spectrum resources become increasingly tight, the identification of WiMAX frequency bands is still a problem that WiMAX needs to face. 802.16e can provide low-speed mobile access services, which is not characteristic of traditional fixed wireless access technology. Based on the characteristics of business development, market expansion needs, and application scenarios, how to give full play to the technical characteristics and advantages of WiMAX, and combined with the current situation of wireless networks, it is of special significance for operators to provide users with more convenient and high-quality services and create new business growth points.