WiMAX (Global Interoperable Microwave Access) technology is a broadband wireless access technology based on the IEEE 802.16 series of standards. It can provide high-speed data, voice, and video services in fixed and mobile environments. The characteristics of IPization have developed rapidly in recent years and gradually become one of the development hotspots in the field of broadband wireless access.
As the wireless broadband access technology that solves the best access method of the last mile, WiMAX must use 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. 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 because it can improve transmission efficiency and spectrum utilization without increasing bandwidth.
1.1 Advantages of multi-antenna compared to single antenna Multi-antenna technology has the following advantages over single-antenna technology:
(1) The array gain increases the coherence of the signal after using multiple antennas, thereby obtaining the array gain.
(2) Diversity gain improves diversity gain. Diversity gain is obtained by using multipath, when a certain path performance deteriorates, it will not affect the system performance. In the wireless fading channel, the stability of the received signal strength can be increased to improve the reliability of the transmitted information. Diversity gain can be obtained in three dimensions: space (antenna), time domain (time) and frequency domain (frequency).
(3) Co-channel interference cancellation eliminates co-channel interference. After using multiple antennas, by analyzing different channel responses of interference, the co-channel interference signal is eliminated.
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 and other services, users' demand for data services is increasing. In high-traffic areas such as regions, network deployment will be limited by capacity. In addition, due to the asymmetry of the data service, the size of the capacity is often limited by the level of the downlink rate. Based on these characteristics, taking hybrid networking as an example, the use of 2 × 2 receive diversity multiple input multiple output (MIMO) will increase the capacity of a single site by nearly 20%. Through calculations, MIMO using 2 × 2 receive diversity can save more than 15% of the number of base stations compared to 1 × 2 receive diversity when the area coverage area and capacity requirements remain unchanged, thereby greatly reducing network construction costs in high-traffic areas .
(2) Reduce the expansion cost. In the case of using a receiver based on multi-antenna technology, the use of 2 × 2 receive diversity MIMO antenna can increase the throughput of the sector by 40% to 60%, especially in a multipath environment. In dense urban areas, urban areas, and indoor microcells, the sector throughput rate can be increased by nearly 60%, and these microcells are precisely the areas with high capacity requirements.
Without increasing the carrier frequency and the number of base stations, the use of multi-antenna technology as a capacity expansion solution can not only meet the capacity requirements but also greatly reduce the cost of capacity expansion, and truly achieve low-cost rapid capacity expansion.
(3) Increase the cost of transceivers and antenna feed systems. The introduction of multi-antenna technology makes the processing of transceivers more complicated: the base station must support more than 2 independent transmission channels (two antennas independently encode, modulate, and spread spectrum) And sending) and uplink feedback signaling (such as CQI, ACK / NACK, etc.) signaling of 2 spatial data streams. These will increase the cost of base stations and terminals to a certain extent. In addition, the installation of multiple antennas and antenna feed systems will be more complicated than ordinary antennas. Therefore, the introduction of multiple antennas will also increase the cost of base stations and antenna feed systems to a certain extent. Undoubtedly, on the one hand, the multi-antenna technology improves the transmission capacity. In the case of charging by traffic, the multi-antenna technology can bring operators a multiplied profit margin; on the other hand, the use of 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.
Generally, from the perspective of operating costs, it is necessary to reasonably use multi-antenna technology under the comprehensive consideration of the impact of system and equipment complexity.
The IEEE 802.16 standard supports multi-antenna technologies such as the space-time code of the AlamouTI scheme, adaptive antenna (AAS) and MIMO technology. As a technology that can effectively improve the anti-fading performance of the system, IEEE802.16e takes transmit diversity through space-time coding as an optional option of the standard.
2 Application of multi-antenna technology in WiMAX system
2.1 Adaptive antenna system
AAS can realize automatic adjustment of system parameters, obtain signal-to-noise ratio (SNR) gain, and reduce co-channel interference. The adaptive antenna uses digital signal processing technology to generate a spatial directional beam, align the main beam of the antenna with the expected signal arrival direction, and form a null for interference, suppress interference, and achieve the best reception of the desired signal.
The design and application of AAS in WiMAX systems are based on time division multiplexing (TDD) mode. Because in TDD mode, the uplink and downlink share the same frequency band resource, you can use the information of the upstream (downstream) channel to get the information of the downstream (upstream) channel, and it is more convenient to use the reciprocity of the upstream and downstream channels at the base station (terminal) Calculate the weight of beamforming. In frequency division multiplexing (FDD) mode, the upstream and downstream channels are generally different, and it is difficult to obtain the lower (upper) row channel information through the upper (lower) row information. To calculate the weight of beamforming, only through feedback, which will increase the overall system overhead. In the WiMAX system, AAS is an optional technology that can be selected to support the technology in both the uplink and downlink. Using AAS technology can increase system capacity, expand coverage, improve communication reliability, and reduce operating costs. When implementing AAS, both multi-beam selection and adaptive methods can be used.
2.2 Multiple input multiple output technology
MIMO technology was first proposed by Marconi in 1908. It uses multiple antennas in base stations and terminals to suppress channel fading, thereby greatly improving channel capacity, coverage, and spectrum utilization. According to the number of transceiver antennas, MIMO can also include single input multiple output (SIMO) and multiple input single output (MISO).
The core of MIMO technology is space-time signal processing, that is, using multiple antennas distributed in space to combine time domain and space domain for signal processing. Therefore, MIMO technology can be seen as an extension of smart antennas. Generalized MIMO technology includes transmit diversity technology and spatial multiplexing technology. Transmit diversity technology refers to transmitting signals containing the same information on different antennas (the specific form of the signals may not be exactly the same) to achieve the effect of spatial diversity, thereby improving the reliability of the channel and reducing the bit error rate. Spatial multiplexing technology is different from transmit diversity. It transmits different information on different antennas to obtain spatial multiplexing gain, thereby greatly improving the system capacity and spectrum utilization. The use of space-time coding and spatial multiplexing in the WiMAX protocol can significantly increase system capacity and spectrum utilization.
At present, MIMO has become an option for multiple antennas in IEEE 802.16, and is also reflected in IEEE 802.16e. The MIMO modes supported by the 802.16 protocol are divided into three types: space-time transmit diversity mode, spatial multiplexing mode, and combined diversity and multiplexing mode.
2.2.1 Space-time transmit diversity The biggest advantage of transmit diversity is that multiple antennas can be used on the base station side, which can avoid the pressure on the terminal equipment caused by the use of multiple antennas on the receiving side, thereby reducing the obstacles caused by the marketization of 802.16.
In MIMO, the space-time transmit diversity mode is mainly realized by space-time coding. The main idea of ​​space-time coding is to use space and time coding to achieve certain space diversity and time diversity, thereby reducing the channel error rate. When using space-time codes, the system can still achieve maximum diversity gain without the transmitter knowing the channel state information. Common space-time codes include space-time block codes (STBC) and space-time trellis codes (STTC). Among them, STBC has been widely used because of its relatively simple coding and decoding process.
The 802.16d standard uses the transmit diversity of two transmitting antennas to counter the deep fading caused by blocking line-of-sight and non-line-of-sight. The main basis is the space-time code (STC) coding of the AlamouTI scheme. The orthogonality between the two sequences of the root transmit antenna. For a two-transmit antenna system, AlamouTI coding can obtain the maximum diversity gain, and from the history of coding development, the AlamouTI scheme is a space-time block code that provides a complete transmit diversity gain for a system with two transmitting antennas. The code only needs to perform simple processing on the received signal, which greatly reduces the calculation complexity.
2.2.2 Spatial multiplexing Spatial multiplexing technology refers to transmitting independent signals at the transmitting end, and decoding at the receiving end by interference suppression, mainly to improve the transmission rate of the system. At present, the methods of using space division multiplexing to increase channel capacity are mainly various layered space-time codes (such as BLAST). Bell Labs' layered space-time algorithm (BLAST) structure does not introduce orthogonality between symbols through signal transformation (coding, modulation, mapping, etc.), but fully utilizes the multipath characteristics of the channel to remove the correlation between signals Sex.
BLAST structure is mainly divided into vertical-Bell Labs layered space-time algorithm (V-BLAST) and diagonal-Bell Labs layered space-time algorithm (D-BLAST): V-BLAST encodes and maps M bit streams After being interleaved and transmitted through independent antennas, the diversity gain is fully explored, and each information stream can be detected separately. D-BLAST also undergoes the same processing first, but each coding block is assigned to a different antenna for transmission, thereby reducing the overall system performance degradation caused by a poor independent channel transmission effect, but it means more complex transceiver device.
The BLAST structure maximizes the spectral efficiency, but generally requires that the number of receiving antennas is greater than or equal to the number of transmitting antennas, which is difficult to achieve in the downlink; in addition, because different links are used to transmit independent signals, then if a link If you are damaged, you will face irreparable errors.
2.2.3 Combined diversity and multiplexing Space-time transmit diversity can obtain additional diversity gain and coding gain, but it cannot increase the data rate; although spatial multiplexing can maximize the average transmission rate of the MIMO system, it can only obtain limited diversity Gain. The combination of space-time transmit diversity and spatial multiplexing can provide both diversity gain and increase system capacity, resulting in a good compromise between high spectral efficiency and transmission quality, but processing is more complicated than using diversity or multiplexing alone.
2.2.4 Smart MIMO
Mobile WiMAX also supports adaptive MIMO conversion (AMS) between various MIMO modes, which is also called smart MIMO. As shown in Figure 1. Smart MIMO chooses the appropriate MIMO mode according to the channel conditions and improves the spectrum utilization rate without reducing the coverage. The intelligent MIMO method can overcome the uncertainty brought by different scenarios, so that the MIMO technology has a wider range of application scenarios. For different terminals under the same network, the number of antennas may be different. Therefore, if the same MIMO transmission method is used in the same cell, it is difficult to achieve the optimal design goal. In addition, the fading experienced by users is also different. Adaptive selection of different MIMO technologies to adapt to channel changes can optimize system performance. To support adaptive MIMO mode selection, the transmitter needs to get more feedback information including channels or weights.
For future mobile communication systems, how to ensure high quality of service (QoS) under non-line-of-sight and harsh channels is a key issue and a research focus in the field of mobile communication. For a single-input single-output (SISO) system, if you want to meet the above requirements, more spectrum resources and complex coding and modulation technologies are required. The limited spectrum resources and the characteristics of mobile terminals restrict the development of SISO systems, so MIMO The key technology of future mobile communication. There are two main manifestations of MIMO technology, namely spatial multiplexing and space-time coding. Both forms have been applied in WiMAX. WiMAX also provides a form of using spatial multiplexing and space-time coding at the same time, supporting MIMO is an optional solution in the protocol.
2.3 Comparison of AAS and MIMO
Both AAS and MIMO are WiMAX multi-antenna technologies mentioned in the 802.16 protocol, but their working principles and application scenarios are not the same. The two technologies are compared below.
(1) System capacity
Both AAS and MIMO use multiple antennas to enhance the transmission signal and obtain additional system capacity compared to a single antenna. AAS generates a single beam with very concentrated energy. In theory, the capacity of the system increases logarithmically with the beam intensity. In the WiMAX system, for the basic SISO configuration, the throughput of the base station is 25 Mb / s. Compared with SISO, the 4-element AAS can increase the throughput by 50% to 33 Mb / s, and the AAS throughput of the 8-element can reach 38. Mb / s, while the capacity of the MIMO system increases linearly with the number of antennas, the capacity of the 2 × 2 MIMO system is twice that of the SISO system, and the capacity of the 4 × 4 MIMO system is twice that of the 2 × 2 system. Therefore, MIMO can obtain a larger system capacity than AAS.
(2) Mobility
One of the biggest improvements of 802.16e compared to 802.16d is to support the mobile environment. AAS relies on accurate and reliable estimation of the channel to obtain the weight of beamforming. In a mobile environment, the channel changes rapidly, and channel estimation will become very complicated. For example, in a system with a bandwidth of 3.5 GHz and a terminal moving rate of 30 km / s, channel estimation needs to be performed every millisecond to ensure performance, and system overhead is very large. Therefore, in high-speed mobile environments, AAS can hardly work. However, MIMO may not require channel information at the transmitting end and can be applied to a mobile environment.
(3) Others Because AAS requires channel information, to a certain extent, its working mode must be TDD, and MIMO can be used normally in TDD or FDD mode. MIMO can be combined with orthogonal frequency division multiplexing (OFDM) technology to achieve better performance, which is also one of the future trends of mobile communication technology.
MIMO technology is very suitable for wireless signal processing in a multi-path environment within the city, including providing spatial diversity and multi-channel parallel transmission. It is a suitable technology to improve the coverage and throughput of WiMAX systems. The adaptive antenna is beneficial to improve the directional spatial gain of the base station and moving objects and suppress the directional interference of the interference signal. However, in an urban multi-path dispersion environment, MIMO may be more popular with device developers. Therefore, MIMO technology is more suitable for the needs of the development of WIMAX. In WiMAX system, it is necessary to support the basic MIMO working mechanism to be compatible with other technologies, so as to make WiMAX more competitive.
3 Conclusion With the continuous innovation of multimedia services, wireless access technology is showing a trend of high bandwidth and IP. Nomadic and mobile broadband access will become an important demand for the future marketization of communications. WiMAX is a broadband wireless access technology specially developed for this demand. It has excellent performance, high efficiency, and low cost. It can meet the requirements of various application scenarios through a flexible system configuration.
Faced with the current fiercely competitive wireless communications market, WiMAX must adopt advanced technology to gain a foothold, while maintaining its advantages of large capacity, wide coverage, non-line-of-sight transmission, and unified standards, etc., and quickly adopt existing technologies Advantages are transformed into product advantages and the scale of commercial use is expanded. Among the key technologies of WiMAX, multi-antenna technologies such as AAS and MIMO have become one of the hotspots of many WiMAX manufacturers because they can greatly improve the capabilities of wireless communication systems. It is necessary to select the appropriate multi-antenna technology according to different scenarios to optimize the performance of the system. At the same time, we must pay more attention to the cost of the actual application of multi-antenna technology and the combination with other technologies (such as OFDM, OFDM, etc.), so that WiMAX can face the competition of long-term evolution (LTE) and wireless interface evolution (AIE), and become a real solution The best wireless broadband access technology for the last mile access method.
As the wireless broadband access technology that solves the best access method of the last mile, WiMAX must use 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. 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 because it can improve transmission efficiency and spectrum utilization without increasing bandwidth.
1.1 Advantages of multi-antenna compared to single antenna Multi-antenna technology has the following advantages over single-antenna technology:
(1) The array gain increases the coherence of the signal after using multiple antennas, thereby obtaining the array gain.
(2) Diversity gain improves diversity gain. Diversity gain is obtained by using multipath, when a certain path performance deteriorates, it will not affect the system performance. In the wireless fading channel, the stability of the received signal strength can be increased to improve the reliability of the transmitted information. Diversity gain can be obtained in three dimensions: space (antenna), time domain (time) and frequency domain (frequency).
(3) Co-channel interference cancellation eliminates co-channel interference. After using multiple antennas, by analyzing different channel responses of interference, the co-channel interference signal is eliminated.
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 and other services, users' demand for data services is increasing. In high-traffic areas such as regions, network deployment will be limited by capacity. In addition, due to the asymmetry of the data service, the size of the capacity is often limited by the level of the downlink rate. Based on these characteristics, taking hybrid networking as an example, the use of 2 × 2 receive diversity multiple input multiple output (MIMO) will increase the capacity of a single site by nearly 20%. Through calculations, MIMO using 2 × 2 receive diversity can save more than 15% of the number of base stations compared to 1 × 2 receive diversity when the area coverage area and capacity requirements remain unchanged, thereby greatly reducing network construction costs in high-traffic areas .
(2) Reduce the expansion cost. In the case of using a receiver based on multi-antenna technology, the use of 2 × 2 receive diversity MIMO antenna can increase the throughput of the sector by 40% to 60%, especially in a multipath environment. In dense urban areas, urban areas, and indoor microcells, the sector throughput rate can be increased by nearly 60%, and these microcells are precisely the areas with high capacity requirements.
Without increasing the carrier frequency and the number of base stations, the use of multi-antenna technology as a capacity expansion solution can not only meet the capacity requirements but also greatly reduce the cost of capacity expansion, and truly achieve low-cost rapid capacity expansion.
(3) Increase the cost of transceivers and antenna feed systems. The introduction of multi-antenna technology makes the processing of transceivers more complicated: the base station must support more than 2 independent transmission channels (two antennas independently encode, modulate, and spread spectrum) And sending) and uplink feedback signaling (such as CQI, ACK / NACK, etc.) signaling of 2 spatial data streams. These will increase the cost of base stations and terminals to a certain extent. In addition, the installation of multiple antennas and antenna feed systems will be more complicated than ordinary antennas. Therefore, the introduction of multiple antennas will also increase the cost of base stations and antenna feed systems to a certain extent. Undoubtedly, on the one hand, the multi-antenna technology improves the transmission capacity. In the case of charging by traffic, the multi-antenna technology can bring operators a multiplied profit margin; on the other hand, the use of 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.
Generally, from the perspective of operating costs, it is necessary to reasonably use multi-antenna technology under the comprehensive consideration of the impact of system and equipment complexity.
The IEEE 802.16 standard supports multi-antenna technologies such as the space-time code of the AlamouTI scheme, adaptive antenna (AAS) and MIMO technology. As a technology that can effectively improve the anti-fading performance of the system, IEEE802.16e takes transmit diversity through space-time coding as an optional option of the standard.
2 Application of multi-antenna technology in WiMAX system
2.1 Adaptive antenna system
AAS can realize automatic adjustment of system parameters, obtain signal-to-noise ratio (SNR) gain, and reduce co-channel interference. The adaptive antenna uses digital signal processing technology to generate a spatial directional beam, align the main beam of the antenna with the expected signal arrival direction, and form a null for interference, suppress interference, and achieve the best reception of the desired signal.
The design and application of AAS in WiMAX systems are based on time division multiplexing (TDD) mode. Because in TDD mode, the uplink and downlink share the same frequency band resource, you can use the information of the upstream (downstream) channel to get the information of the downstream (upstream) channel, and it is more convenient to use the reciprocity of the upstream and downstream channels at the base station (terminal) Calculate the weight of beamforming. In frequency division multiplexing (FDD) mode, the upstream and downstream channels are generally different, and it is difficult to obtain the lower (upper) row channel information through the upper (lower) row information. To calculate the weight of beamforming, only through feedback, which will increase the overall system overhead. In the WiMAX system, AAS is an optional technology that can be selected to support the technology in both the uplink and downlink. Using AAS technology can increase system capacity, expand coverage, improve communication reliability, and reduce operating costs. When implementing AAS, both multi-beam selection and adaptive methods can be used.
2.2 Multiple input multiple output technology
MIMO technology was first proposed by Marconi in 1908. It uses multiple antennas in base stations and terminals to suppress channel fading, thereby greatly improving channel capacity, coverage, and spectrum utilization. According to the number of transceiver antennas, MIMO can also include single input multiple output (SIMO) and multiple input single output (MISO).
The core of MIMO technology is space-time signal processing, that is, using multiple antennas distributed in space to combine time domain and space domain for signal processing. Therefore, MIMO technology can be seen as an extension of smart antennas. Generalized MIMO technology includes transmit diversity technology and spatial multiplexing technology. Transmit diversity technology refers to transmitting signals containing the same information on different antennas (the specific form of the signals may not be exactly the same) to achieve the effect of spatial diversity, thereby improving the reliability of the channel and reducing the bit error rate. Spatial multiplexing technology is different from transmit diversity. It transmits different information on different antennas to obtain spatial multiplexing gain, thereby greatly improving the system capacity and spectrum utilization. The use of space-time coding and spatial multiplexing in the WiMAX protocol can significantly increase system capacity and spectrum utilization.
At present, MIMO has become an option for multiple antennas in IEEE 802.16, and is also reflected in IEEE 802.16e. The MIMO modes supported by the 802.16 protocol are divided into three types: space-time transmit diversity mode, spatial multiplexing mode, and combined diversity and multiplexing mode.
2.2.1 Space-time transmit diversity The biggest advantage of transmit diversity is that multiple antennas can be used on the base station side, which can avoid the pressure on the terminal equipment caused by the use of multiple antennas on the receiving side, thereby reducing the obstacles caused by the marketization of 802.16.
In MIMO, the space-time transmit diversity mode is mainly realized by space-time coding. The main idea of ​​space-time coding is to use space and time coding to achieve certain space diversity and time diversity, thereby reducing the channel error rate. When using space-time codes, the system can still achieve maximum diversity gain without the transmitter knowing the channel state information. Common space-time codes include space-time block codes (STBC) and space-time trellis codes (STTC). Among them, STBC has been widely used because of its relatively simple coding and decoding process.
The 802.16d standard uses the transmit diversity of two transmitting antennas to counter the deep fading caused by blocking line-of-sight and non-line-of-sight. The main basis is the space-time code (STC) coding of the AlamouTI scheme. The orthogonality between the two sequences of the root transmit antenna. For a two-transmit antenna system, AlamouTI coding can obtain the maximum diversity gain, and from the history of coding development, the AlamouTI scheme is a space-time block code that provides a complete transmit diversity gain for a system with two transmitting antennas. The code only needs to perform simple processing on the received signal, which greatly reduces the calculation complexity.
2.2.2 Spatial multiplexing Spatial multiplexing technology refers to transmitting independent signals at the transmitting end, and decoding at the receiving end by interference suppression, mainly to improve the transmission rate of the system. At present, the methods of using space division multiplexing to increase channel capacity are mainly various layered space-time codes (such as BLAST). Bell Labs' layered space-time algorithm (BLAST) structure does not introduce orthogonality between symbols through signal transformation (coding, modulation, mapping, etc.), but fully utilizes the multipath characteristics of the channel to remove the correlation between signals Sex.
BLAST structure is mainly divided into vertical-Bell Labs layered space-time algorithm (V-BLAST) and diagonal-Bell Labs layered space-time algorithm (D-BLAST): V-BLAST encodes and maps M bit streams After being interleaved and transmitted through independent antennas, the diversity gain is fully explored, and each information stream can be detected separately. D-BLAST also undergoes the same processing first, but each coding block is assigned to a different antenna for transmission, thereby reducing the overall system performance degradation caused by a poor independent channel transmission effect, but it means more complex transceiver device.
The BLAST structure maximizes the spectral efficiency, but generally requires that the number of receiving antennas is greater than or equal to the number of transmitting antennas, which is difficult to achieve in the downlink; in addition, because different links are used to transmit independent signals, then if a link If you are damaged, you will face irreparable errors.
2.2.3 Combined diversity and multiplexing Space-time transmit diversity can obtain additional diversity gain and coding gain, but it cannot increase the data rate; although spatial multiplexing can maximize the average transmission rate of the MIMO system, it can only obtain limited diversity Gain. The combination of space-time transmit diversity and spatial multiplexing can provide both diversity gain and increase system capacity, resulting in a good compromise between high spectral efficiency and transmission quality, but processing is more complicated than using diversity or multiplexing alone.
2.2.4 Smart MIMO
Mobile WiMAX also supports adaptive MIMO conversion (AMS) between various MIMO modes, which is also called smart MIMO. As shown in Figure 1. Smart MIMO chooses the appropriate MIMO mode according to the channel conditions and improves the spectrum utilization rate without reducing the coverage. The intelligent MIMO method can overcome the uncertainty brought by different scenarios, so that the MIMO technology has a wider range of application scenarios. For different terminals under the same network, the number of antennas may be different. Therefore, if the same MIMO transmission method is used in the same cell, it is difficult to achieve the optimal design goal. In addition, the fading experienced by users is also different. Adaptive selection of different MIMO technologies to adapt to channel changes can optimize system performance. To support adaptive MIMO mode selection, the transmitter needs to get more feedback information including channels or weights.
For future mobile communication systems, how to ensure high quality of service (QoS) under non-line-of-sight and harsh channels is a key issue and a research focus in the field of mobile communication. For a single-input single-output (SISO) system, if you want to meet the above requirements, more spectrum resources and complex coding and modulation technologies are required. The limited spectrum resources and the characteristics of mobile terminals restrict the development of SISO systems, so MIMO The key technology of future mobile communication. There are two main manifestations of MIMO technology, namely spatial multiplexing and space-time coding. Both forms have been applied in WiMAX. WiMAX also provides a form of using spatial multiplexing and space-time coding at the same time, supporting MIMO is an optional solution in the protocol.
2.3 Comparison of AAS and MIMO
Both AAS and MIMO are WiMAX multi-antenna technologies mentioned in the 802.16 protocol, but their working principles and application scenarios are not the same. The two technologies are compared below.
(1) System capacity
Both AAS and MIMO use multiple antennas to enhance the transmission signal and obtain additional system capacity compared to a single antenna. AAS generates a single beam with very concentrated energy. In theory, the capacity of the system increases logarithmically with the beam intensity. In the WiMAX system, for the basic SISO configuration, the throughput of the base station is 25 Mb / s. Compared with SISO, the 4-element AAS can increase the throughput by 50% to 33 Mb / s, and the AAS throughput of the 8-element can reach 38. Mb / s, while the capacity of the MIMO system increases linearly with the number of antennas, the capacity of the 2 × 2 MIMO system is twice that of the SISO system, and the capacity of the 4 × 4 MIMO system is twice that of the 2 × 2 system. Therefore, MIMO can obtain a larger system capacity than AAS.
(2) Mobility
One of the biggest improvements of 802.16e compared to 802.16d is to support the mobile environment. AAS relies on accurate and reliable estimation of the channel to obtain the weight of beamforming. In a mobile environment, the channel changes rapidly, and channel estimation will become very complicated. For example, in a system with a bandwidth of 3.5 GHz and a terminal moving rate of 30 km / s, channel estimation needs to be performed every millisecond to ensure performance, and system overhead is very large. Therefore, in high-speed mobile environments, AAS can hardly work. However, MIMO may not require channel information at the transmitting end and can be applied to a mobile environment.
(3) Others Because AAS requires channel information, to a certain extent, its working mode must be TDD, and MIMO can be used normally in TDD or FDD mode. MIMO can be combined with orthogonal frequency division multiplexing (OFDM) technology to achieve better performance, which is also one of the future trends of mobile communication technology.
MIMO technology is very suitable for wireless signal processing in a multi-path environment within the city, including providing spatial diversity and multi-channel parallel transmission. It is a suitable technology to improve the coverage and throughput of WiMAX systems. The adaptive antenna is beneficial to improve the directional spatial gain of the base station and moving objects and suppress the directional interference of the interference signal. However, in an urban multi-path dispersion environment, MIMO may be more popular with device developers. Therefore, MIMO technology is more suitable for the needs of the development of WIMAX. In WiMAX system, it is necessary to support the basic MIMO working mechanism to be compatible with other technologies, so as to make WiMAX more competitive.
3 Conclusion With the continuous innovation of multimedia services, wireless access technology is showing a trend of high bandwidth and IP. Nomadic and mobile broadband access will become an important demand for the future marketization of communications. WiMAX is a broadband wireless access technology specially developed for this demand. It has excellent performance, high efficiency, and low cost. It can meet the requirements of various application scenarios through a flexible system configuration.
Faced with the current fiercely competitive wireless communications market, WiMAX must adopt advanced technology to gain a foothold, while maintaining its advantages of large capacity, wide coverage, non-line-of-sight transmission, and unified standards, etc., and quickly adopt existing technologies Advantages are transformed into product advantages and the scale of commercial use is expanded. Among the key technologies of WiMAX, multi-antenna technologies such as AAS and MIMO have become one of the hotspots of many WiMAX manufacturers because they can greatly improve the capabilities of wireless communication systems. It is necessary to select the appropriate multi-antenna technology according to different scenarios to optimize the performance of the system. At the same time, we must pay more attention to the cost of the actual application of multi-antenna technology and the combination with other technologies (such as OFDM, OFDM, etc.), so that WiMAX can face the competition of long-term evolution (LTE) and wireless interface evolution (AIE), and become a real solution The best wireless broadband access technology for the last mile access method.
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