MIMO (MulTIpleInputMulTIpleOutput, multiple input multiple output) technology is an important breakthrough in next-generation communication technology. Based on Huawei's system simulation research results and actual field test results, this article from the perspective of enhanced network coverage, improved system capacity and reduced network construction costs, etc. Explore and analyze the application of MIMO technology in mobile WiMAX system.
In recent years, global broadband users based on traditional cable technology have grown rapidly, and the number of 3G mobile users has also steadily increased. The market's demand for mobile broadband is increasing rapidly, which provides huge market space for wireless broadband access technology.
WiMAX802.16e is increasingly being adopted by operators as the preferred fixed and mobile broadband access strategy to provide end users with rich high-bandwidth multimedia services. These strategies pose great challenges to the operator's wireless network. In order to establish and maintain a profitable business model, major improvements are needed in network capacity, user throughput, and network coverage quality. The application of MIMO multi-antenna technology enables 802.16e to cope with these challenges, thus strongly promoting the development of WiMAX networks.
MIMO refers to the use of multiple transmit and receive antennas at the transmit and receive ends, respectively, and signals are transmitted and received through multiple antennas at the transmit and receive ends, thereby improving the quality of service for each user. For the traditional single antenna system, MIMO technology can greatly improve the spectrum utilization rate, so that the system can transmit higher-speed data services in a limited wireless frequency band. The block diagram of a MIMO system with N transmit antennas and M receive antennas is shown in the figure below.
WiMAX802.16e defines three optional MIMO methods, namely STTD (SpaceTImeTransmit Diversity, Space-Time Transmit Diversity), SM (SpaTIal Multiplexing, Space Division Multiplexing) and adaptive switching methods, and defines Matrix A and Matrix B And Matrix C 3 kinds of coding matrix. Huawei's WiMAX 802.16e system now fully supports Matrix A, and will support MIMO Matrix B and MIMO Matrix C in September 2008.
In the WiMAX802.16e system, the combination of MIMO technology and OFDMA technology can greatly improve the network coverage and double the capacity of the WiMAX system, thereby greatly reducing network construction costs and maintenance costs, and strongly promoted the development of mobile WiMAX.
Perfect combination with OFDMA
MIMO can be applied to all wireless communication technologies. In the WiMAX802.16e system, the perfect combination of MIMO and OFDMA (Orthogonal Frequency Division Multiple Access, Orthogonal Frequency Division Multiple Access) can better reflect the technical advantages of MIMO.
MIMO systems can resist multipath fading, but for frequency selective fading, MIMO is still powerless. Other communication systems generally use equalization technology to solve frequency selective fading in MIMO systems. WiMAX's OFDMA technology can overcome frequency selective fading well. The next generation of mobile communications requires high spectrum utilization technology, but OFDMA's ability to improve spectrum utilization is limited after all. Combined with MIMO technology, the spectrum efficiency can be further improved without increasing the system bandwidth. MIMO + OFDMA technology can not only provide higher data transmission rate, but also achieve strong reliability and enhance system stability through diversity. In addition, OFDMA has a strong anti-multipath interference capability due to the low bit rate and the addition of a time guard interval. The multipath delay is less than the guard interval to protect the system from inter-symbol interference, so that single-frequency networks can use broadband The OFDMA system relies on MIMO technology to eliminate shadow effects and truly achieve seamless coverage of the network.
Significantly improve network coverage
Because WiMAX802.16e uses a higher operating frequency band, the propagation loss is higher than other mobile communication systems. How to expand network coverage is also one of the challenges facing WiMAX. The application of MIMO technology in WiMAX system can greatly improve the network coverage. In diversity mode, MIMO increases the cell coverage radius through diversity gain; in multiplexing mode, it increases the cell coverage radius by increasing the diversity gain obtained at the cell edge rate; in adaptive switching mode, the cell edge works in diversity mode, The coverage gain is the same as the diversity-only mode. The system simulation results of Huawei show that in diversity or adaptive switching mode, 2T2R (2Transmit2 Receive, two-transmit and two-receive) MIMO has 2 ～ 10dB compared with SISO (Single Input Single Output) Coverage gain, coverage radius increases by 50% to 90%; in multiplexing mode only, coverage gain of 3 to 5dB can be obtained indirectly at the cell edge. In addition, MIMO can effectively improve the cell edge coverage probability.
In August 2007, in Tokyo, Japan, Huawei tested the actual use of MIMO together with local operator J: COM. The test results show that the use of MIMOMatrixA technology can effectively increase the coverage radius of the WiMAX802.16e system, and can greatly reduce the number of base stations required for network construction.
Double the system capacity
WiMAX802.16e system can provide extremely high data throughput and mobility, allowing users to be online at any time, even if they are in a mobile state can also get a true broadband experience. MIMO technology can double the system throughput and spectrum efficiency in multiplexing mode, and can double the single user peak rate; in diversity mode, increase the system throughput and spectrum efficiency by increasing the ratio of high-order modulation ; In the adaptive switching mode, the cell center works in the multiplexing mode, and the cell edge works in the diversity mode, so the system throughput and spectrum efficiency enhancement effects are somewhere in between. The application of OFDMA and MIMO technology enables the WiMAX802.16e system to maximize spectrum efficiency and provide the high speed and high bandwidth required for high-quality mobile video and TV services.
Huawei's system simulation results show that the use of MIMO2 × 2MatrixB technology can increase the WiMAX 16e system throughput by about 30% to 60%, and at the same time can double the peak rate of users to ensure that WiMAX users get a better business experience.
Significantly reduce network construction and maintenance costs
In dense urban areas and CBD areas, there are many high-end users, which require high system throughput and peak rate, and are prone to capacity constraints. After adopting MIMOMatrixB technology, the WiMAX system's downlink capacity is increased by 55%, and the uplink capacity is increased by 33%. In capacity-constrained scenarios, the number of base stations will be reduced by about 25%. Compared with other multi-antenna technologies such as AAS (Adaptive Antenna System also Advanced Antenna System), the capacity gain advantage of MIMO in dense urban areas is more obvious, which can effectively reduce the high traffic area. Network construction cost or expansion cost.
For scenarios with limited coverage, using MIMO technology can increase the coverage radius by more than 50%, increase the coverage area of ​​a single site by nearly 100%, and can save 40% to 60% of the number of base stations under a certain coverage area. The introduction of MIMO in suburban and rural areas can achieve the maximum coverage of the fewest base stations and greatly reduce the initial network construction cost.
In addition, in terms of installation and maintenance, the AAS antenna is at least a 4-beam antenna, which requires a higher antenna installation space. Compared with the traditional antenna, the AAS antenna requires significantly more feeders and increases the amount of engineering. At the same time, the width of the tower cable tray also has to be Widened; the antenna used by AAS has a large weight, which requires higher installation load capacity. In addition, the AAS antenna has a larger volume, poor wind resistance, and high wind resistance requirements. It is not suitable for use in areas with high winds or typhoons. Compared with AAS, MIMO requires fewer antennas. An ordinary ± 45 degree dual-polarized antenna can support 2 × 2 MIMO. It requires low antenna installation space and load bearing, is easy to deploy and maintain, and can effectively save antenna installation costs. And maintenance costs.
Conclusion
With the rapid development of broadband data users and mobile communication users, and the trend from narrowband voice communication to broadband high-speed data communication, how to increase network capacity on a certain spectrum resource and how to improve network coverage performance under a certain number of sites, Become a key issue to be considered in the future communication network construction. The WiMAX802.16e system introduces MIMO multi-antenna technology, and is perfectly combined with OFDMA. By enhancing the spatial resolution of the system, improving spectrum utilization and improving network capacity and coverage performance, thereby reducing network construction costs and maintenance costs. WiMAX802.16e has gradually become the most competitive wireless broadband access technology.
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