WiMAX (Worldwide Interoperability for Microwave Access) technology is a broadband wireless access technology based on the IEEE 802.16 series of standards. It provides high-speed data, voice and video services in both fixed and mobile environments, combining mobile and broadband. The characteristics of IP have developed rapidly in recent years and have gradually become one of the development hotspots in the field of broadband wireless access. As a wireless broadband access technology that solves the last mile of the best access method, WiMAX must adopt multi-antenna technology to improve its competitiveness.
1 Advantages of multi-antenna technology With the rapid development of wireless communication technology, the serious shortage of spectrum resources has increasingly become a "bottleneck" in the development of wireless communication industry. How to fully exploit and utilize limited spectrum resources and improve spectrum utilization is one of the hot topics in the current communication industry.
Multi-antenna technology is widely favored for its ability to increase transmission efficiency and spectrum utilization without increasing bandwidth.
1.1 Advantages of multiple antennas compared to single antennas
Multi-antenna technology has the following advantages over single-antenna technology:
(1) Array gain
The use of multiple antennas increases the coherence of the signal, resulting in an array gain.
(2) Diversity gain
Increased diversity gain. The diversity gain is obtained by using multipath, and does not affect the performance of the system when the performance of one path deteriorates. In the wireless fading channel, the stability of the received signal strength can be increased to improve the reliability of the transmitted information. The diversity gain can be obtained in three dimensions: space (antenna), time domain (time), and frequency domain (frequency).
(3) Co-channel interference cancellation
Co-channel interference is eliminated. After using multiple antennas, the co-channel interference signals are eliminated by analyzing the different channel responses of the interference.
1.2 Economics of multi-antenna technology
(1) Increase transmission capacity and reduce network construction costs in high traffic areas
With the continuous promotion of data services, especially the application of mobile TV, high-speed wireless Internet access, etc., the demand for data services is increasing. In high-traffic areas such as dense urban areas and hotspots, network deployment will be limited by capacity. . In addition, due to the asymmetry of data services, the capacity is often limited by the downlink rate. Based on these characteristics, in the case of hybrid networking, multi-input multiple-output (MIMO) using 2&TImes;2 receive diversity will increase the single-site capacity by nearly 20%. By calculation, in the case of the same area coverage and capacity requirements, MIMO using 2&TImes;2 receive diversity can save more than 15% of the number of base stations compared to 1&TImes;2 receive diversity, thereby greatly reducing the cost of networking in high traffic areas. .
(2) Reduce the cost of expansion
In the case of using a multi-antenna based receiver, the MIMO antenna using 2&TImes;2 receive diversity can increase the throughput of the sector by 40% to 60%, especially in dense urban areas and urban areas with complex multipath environments. In the case of microcells such as indoors, the sector throughput rate can be increased by nearly 60%, and these microcells are precisely areas with high capacity requirements.
By increasing the carrier frequency and the number of base stations, the multi-antenna technology can be used as a capacity expansion solution to meet the capacity requirements and greatly reduce the expansion cost, realizing low-cost and fast capacity expansion.
(3) Increase the cost of transceivers and antenna feeder systems
The introduction of multi-antenna technology makes the processing of the transceiver more complicated: the base station must support more than 2 independent transmission channels (two antennas are independently coded, modulated, spread and transmitted) and the uplink feedback of two spatial data streams Signaling (such as CQI, ACK/NACK, etc. signaling) processing. These will increase the cost of base stations and terminals to a certain extent, and the installation of multiple antennas and antenna feeder systems will be more complicated than ordinary antennas. Therefore, the introduction of multiple antennas will also increase the cost of base stations and antenna feeder systems to some extent. Undoubtedly, on the one hand, multi-antenna technology is used to increase transmission capacity. In the case of traffic-based charging, multi-antenna technology can bring operators multiple profit margins; on the other hand, multi-antenna technology will bring base stations and terminals. Greater implementation complexity. Relatively speaking, in addition to the necessary signaling and measurement information, the use of multi-antenna technology in the upgrade has less impact on the network.
From the perspective of operating costs, multi-antenna technology needs to be used rationally under the influence of system and equipment complexity.
The IEEE 802.16 standard supports multi-antenna techniques such as space time code for Alamouti schemes, adaptive antenna (AAS) and MIMO technology. As a technology that can effectively improve the anti-fading performance of the system, IEEE802.16e uses transmit diversity achieved by space-time coding as an option for the standard.
2.1 Adaptive Antenna System
AAS can automatically adjust system parameters, obtain signal-to-noise ratio (SNR) gain, and reduce co-channel interference. The adaptive antenna utilizes digital signal processing technology to generate a spatially directional beam, so that the main beam of the antenna is aligned with the direction of arrival of the desired signal, and at the same time, the interference is formed into a null, the interference is suppressed, and the optimal reception of the desired signal is achieved.
The design and application of AAS in WiMAX systems is based on Time Division Multiplexing (TDD) mode. Because in the TDD mode, the uplink and downlink share the same frequency band resource, the information of the upper (lower) channel can be used to obtain the information of the lower (upper) channel, and the base station (terminal) can utilize the reciprocity of the uplink and downlink channels. The weight of the beamforming is calculated. In the frequency division multiplexing (FDD) mode, the uplink and downlink channels are generally different, and it is difficult to obtain the lower (upper) channel information by the information of the upper (lower) lines. To calculate the weight of beamforming, only through feedback, this will increase the overhead of the entire system. In the WiMAX system, AAS is an optional technology that can be optionally supported in both the uplink and downlink. AAS technology can increase system capacity, expand coverage, improve communication reliability, and reduce operating costs. AAS can be implemented in either multi-beam or adaptive mode.
2.2 Multiple Input Multiple Output Technology
MIMO technology was first proposed by Marconi in 1908. It uses multiple antennas at base stations and terminals to suppress channel fading, thereby greatly improving channel capacity, coverage and spectrum utilization. MIMO may also include single input multiple output (SIMO) and multiple input single output (MISO) depending on the number of transmit and receive antennas.
The core of MIMO technology is space-time signal processing, which combines the time domain and the spatial domain for signal processing by using multiple antennas distributed in space. Therefore, MIMO technology can be seen as an extension of smart antennas. Generalized MIMO techniques include transmit diversity techniques and spatial multiplexing techniques. Transmit diversity technology refers to transmitting signals containing the same information on different antennas (the specific forms of the signals are not necessarily identical), achieving the effect of spatial diversity, thereby improving channel reliability and reducing bit error rate. Spatial multiplexing technology, unlike transmit diversity, transmits different information on different antennas to obtain spatial multiplexing gain, which greatly increases system capacity and spectrum utilization. The WiMAX protocol uses both space-time coding and spatial multiplexing to significantly increase system capacity and spectrum utilization.
At present, MIMO has become an option for multiple antennas in IEEE 802.16, and is also embodied in IEEE802.16e. The MIMO modes supported by the 802.16 protocol are divided into three types: space-time transmit diversity mode, spatial multiplexing mode, and a combination of diversity and multiplexing.
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