"Summary
The fifth generation mobile communication (5G) low frequency band (SUB-6GHz) has begun commercial, 5G millimeter wave technology is gradually mature, and it is expected to be commercially available in 2022. The study of the sixth generation mobile communication (6G) has also started, and the 6G vision and the core technology papers have begun. This paper mainly discusses the application and core role of millimeter wave technology in 5G and future 6G.
Introduction
It is well known that the fifth generation mobile communication (5G) is divided into a low frequency band (SUB6GHz) and a high frequency band (mm wave). my country's low-frequency segment 5G has started commercial use in 2019, and the spectrum of millimeter wave 5G has not been officially released, but 24.75 ~ 27.5 GHz and 37 to 42.5 GHz have been approved as experimental frequency bands. 5G millimeter wave technology based on large-scale MIMO tends to mature, and it is expected to be commercially used in 2022.
In recent years, there are more and more papers, reports and reports on 6G vision and core technology at home and abroad, and there are some consensus. On the network architecture, 6G will be a network that fused from 5g (b5g) after a large number of low-ranking satellites and the ground, thereby enabling the first time to achieve full coverage against the entire Earth surface and its near space. 29% of the Earth surface is land, 71% is the ocean, 1G ~ 5G mobile communication network has not fully covered 29% of land. Therefore, 6G will be a revolution in the history of human mobile communications. In the core technology, some proposals are gradually recognized, such as panic, holography, artificial intelligence, and more. Broadband transmission technology is the basis for supporting the communication network. For 6G, the integrated high-speed communication network to realize the empty earth sea, broadband transmission technology will be the core. For the ground 5G network, the spectral resource of the millimeter wave band has begun to implement broadband high-speed transmission. For 6G, the millimeter wave frequency band will be the first choice for the user links, satellite down, and the satellite to the ground station. For example, SpaceX's StarLink mainly uses the KA and Q band, and the O3b medium-track satellite network adopts Ku and KA band. Can affirm that millimeter wave technology will be one of the most important support technologies of 6G networks. It is reported that Taihaz will be 6G core technology, this point of view is worth discussing. In fact, it is limited to the semiconductor process characteristics, in the terahertz band (usually 300 ~ 10,000 GHz, also known as terahertz), transmit power, receiver noise coefficient, difficulty, cost, etc. Application of Taihaz needs breakthrough bottlenecks.
Since the bandwidth of 400 MHz or even wider, high sampling rate ADC / DAC, massive data real-time processing and a large number of radio frequency channels and antenna have become a bottleneck based on 5G millimeter wave based on large-scale MIMO technology. For this reason, there is currently a mixed multi-beam scheme of the phase-controlled sub-array in the active antenna unit (AAU) of the commercialization of 5G millimeters. The solution greatly reduces the number of radiofrequency transceivers, and partially overcomes the above bottleneck problem, but at the expense of array gain and communication capacity.
In theory, large-scale MIMO technology based on all digital multi-beams will be the goal of future mobile communications, but the above bottleneck problem is currently difficult to overcome. To this end, we propose an asymmetric millimeter wave large-scale MIMO system architecture to overcome the bottleneck problems described above while approaching the best performance.
This article will discuss the problems faced by 5G millimeter wave and to discuss the possible technical route in the 6G evolution process, in order to inspire researchers in 5G / 6G millimeter wave technology.
1 5g mm wave
5G mm wave commercial system architecture is usually composed of core network (CN), baseband unit (BBU), and active antenna unit (AAU), as shown in FIG. Its basic architecture is a core network to support multiple baseband units, and each baseband unit will support multiple active antenna units. Specifically, the CN is located in the center of network data exchange, mainly responsible for providing core functions such as data transmission, mobile management, session management; BBU is responsible for the baseband digital signal processing, such as coding, multiplexing, modulation, etc .; AAU is mainly responsible for implementing baseband numbers The conversion between the signal and the radio frequency signal, complete the transmitting and reception process. AAU mainly includes AAU baseband part (beam management, etc.), up and down variable modules, and analog beamformer. The baseband part of the AAU mainly completes some digital signal processing of the physical layer, such as beam management, and completes the control of different beam coverage, and the signal with Digital Analog Converter (DAC) and Analog Digital Converter (ADC) completion signals in analog domain and numbers. Domain conversion. Due to the large bandwidth requirements of the 5G millimeter wave system, this will make new requirements for baseband signal processing, and ADC / DAC's ability. The up and down variable module is responsible for implementing the conversion between baseband I / Q signals (or intermediate frequency signals) and millimeter wave radio signal. The upper frequency conversion module is mainly used for transmitting links, including upper drive, filters, power amplifiers, etc., responsible for moving the transmitted signal from baseband I / Q (or intermediate frequency) to the required millimeter wave transmit frequency. Similarly, the lower variable frequency module is primarily used to receive links, including low noise amplifiers, filters, mixer, etc., move the millimeter wave reception signal to baseband I / Q (or intermediate frequency). The analog beamforming network (phase control sub-array) is mainly responsible for reasonably assigning RF signal energy to an antenna array feed port, which constitutes a specific amplitude and phase distribution, thereby forming a particular beam. Mainstream AAU generally supports 4 or more data streams, each of which supports 1 data stream, as shown in Figure 1. The beamforming circuit of each sub-array is composed of a power distribution / synthetic module, a multi-channel transceiver phase and an amplitude control chip, an antenna array or the like. Take the 4-channel data stream transmitting link as an example, each data stream signal reaches the RF frequency through the upper frequency conversion module, by power distribution network, for example, 1 minute 16, and signals such as a multi-channel chip input port. Taking 4 channel chips as an example, the transmit link of each chip contains power amplifiers, phase shifters, switches, etc., which can complete signal conversion of 1 to 4, and accurately control the amplitude and phase of each signal, and then feed output signal feed The antenna unit is entered to achieve the intended beam.
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Figure 1 Commercialization 5G millimeter wave system architecture
The advantage of 5G millimeter wave commercial mixing multi-beam architecture is to complete multi-beam coverage at lower complexity and cost. As shown in FIG. 1, the system can implement independent control of four beams only by using four ADC / DAC channels and up and down frequency channels. However, due to the shortcomings of the channel, the supported data stream is limited, and the system has significant limitations for the expansion of beam volume, which causes insufficient system capacity. At the same time, since the four beams are independently controlled by the four beams, the architecture does not achieve effective use of the full diameter of the antenna, and thus the 6 dB (4 sub-array) or even more array beam gain is lost. Another more effective mixing multi-beam architecture is simultaneous use of beamforming and analog beam of the alignment of the baseband digital portion, realizing the utilization of the all-diameter of the antenna array, producing a higher array beam gain. However, due to the restricted beam width of the phase-controlled sub-array, there is a problem of narrowing the multi-beam scan range, limited coverage. Such architectures can be enlarged through beam switching, and the coverage is enlarged, but at the expense of time delay and increase beam management complexity, it will eventually lead to decrease in system capacity.
In order to obtain system capacity and array gain, another implementation of AAU is a full digital multi-beam array, as shown in Figure 2. The full digital multi-beam array architecture passes each of the antenna units directly corresponding to a radio frequency transceiver channel, each transceiver channel includes a radio frequency transceiver front end (FEM), an upper and lower frequency conversion channel, and an ADC / DAC, and the beam formation is implemented in the baseband digital domain implementation. The advantage of the full digital multi-beam array architecture is that the baseband digital circuit can accurately implement the desired amplitude phase control, and the number of beams is easy to expand, thereby increasing communication capacity. At the same time, each beam formed by this architecture can obtain the full diameter gain of antenna array. However, its disadvantage is also remarkable. Since each antenna unit needs to pick a radio frequency channel, a high-density integration of a large number of radio frequency antennas greatly increases the complexity of hardware design. At the same time, due to the large bandwidth requirements of the 5G millimeter wave system, the requirements of the RF channel, the sample rate of the ADC / DA, and the requirements of the baseband processing rate will increase, resulting in real-time digital signal processing issues of massive data, significantly increase operating costs and work Consumption.
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Figure 2 AAU based on full digital architecture
In summary, the millimeter wave bundle array architecture performance is shown in Table 1. The millimeter wave full digital multi-beam array architecture is the best performance representative, which can get the highest communication capacity and beam gain, but its architecture achieves complexity and cost, and urgent need to develop new technologies.
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Table 1 mm wave beam array technology comparison
Millimeter wave technology in 2 6g
Millimeter wave technology will also play an important role in the sixth generation mobile communication system (6G) in addition to 5G. Although the current 6G vision is not fully clear, its basic goal can be seen, as shown in Figure 3. The global wireless communication network is currently only covered by the main place of human surface of the Earth, and there is still a large area of land, such as desert, lake, mountains and rivers, forests, etc., and have not received effective network access. In addition, since human exploration is continuously extended to the ocean, sky, space, etc., which will have a strong demand for access to the wireless communication network. Therefore, the medium and low-track satellite network, that is, the IOS: Internet Of Space, will be an important part of 6G, integrated with the ground B5G system, and realizes the panning link of the empty world sea integrated communication network.
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Figure 3 6G Airpen Network diagram
Due to the diversification of future applications, the intelligence of connection, and the depth of information processing, 6G systems will generate massive data, and require higher rates of transmission support. It has been reported that 6G is expected to enter the TbPS era, which reaches the transmission rate of 1000X Gbps. To achieve this magnificent goal, it is urgent to find spectrum resources suitable for 6G systems. At present, the frequency of low frequency bands (within SUB-6GHz) has been fully developed, and it is difficult to obtain large spectrum bandwidth to support TBPS transmission rates, so spectrum resources will be sought to higher bands. It is well known that the higher the frequency, the smaller the wavelength, the smaller the size of the RF device, but its performance is usually worse, for example, the output power of the power amplifier, the noise coefficient of the low noise amplifier, and the like. Which frequency band is more suitable for 6G demand? It is a simple discussion on the spectrum resources that may adopt 6G. The terahertz band has a wealth of spectrum resources that are not developed, which can achieve smaller device sizes, and achieve large-scale arrays, there are many related research. However, the current phase is mainly limited to semiconductor process characteristics, and the terahertz device capacity is still insufficient, such as insufficient output power, and the noise coefficient indicator. In addition, due to high cost, complex processing processes, these factors will restrict further application of the terahertz band in the 6G era. Compared with the terahertz band, the millimeter wave frequency band has been significantly improved in the 5G era, and the capacity of the device has been greatly improved, the industrial chain is complete and rich, and the array size of the millimeter wave band is also relatively moderate, and most of the 6G system is mostly used. It can be considered to be a gold frequency band that supports 6G.
Unlike 5G systems, a major feature of 6G IT is a higher requirement for the fast wireless connection of sports objects. The operating speed of low-rail satellites is relatively rapid and large, causing a large number of beam scanning ranges and the number of beam connections, which requires rapid dynamic multi-beam tracking, so millimeters based on all-digital multi-beam array architectures. The MIMO system will be one of the important development directions. However, due to the high gain beam characteristics and space of the millimeter wave array architecture, the rapid and designated satellite is becoming aligned, and the establishment of a wireless communication link has put forward huge challenges, and new technologies are urgently needed to overcome.
3 non-symmetric millimeter wave large scale MIMO system
Through the above analysis, the millimeter wave full digital large-scale MIMO system will be the best choice for B5G or even 6G systems, but it is disadvantage, such as high complexity, high cost, power consumption, etc., it will restrict its in the future system. Applications. In order to effectively reduce the complexity, cost, power consumption of millimeter wave full digital multi-beam arrays, and can support dynamic fast multi-beam tracking, we propose the concept of asymmetric millimeter wave large-scale MIMO system, in order to approximate system optimal performance At the same time, overcome the above bottleneck problem.
The currently employed millimeter wave major MIMO system mixed multi-beam array or full digital multi-beam array is designed to symmetrically design multi-beam emission and reception arrays, that is, the number of transmit channels and reception channels is the same, as shown in FIG. 4 (a). The base station side produces the same emission and reception of multi-beams of the gain based on a millimeter wave mixing / full digital multi-beam receiving and transmitting architecture based on symmetric design. Similarly, the terminal side design is similar to the base station side, and the difference is that the array is small. For example, the base station side and the terminal side are symmetrical 64-free 64 receiphed 4 hair 4 received all digital multi-beam arrays.
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(A) symmetrical millimeter wave major scale MIMO system architecture
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(b) Asymmetric millimeter wave large size MIMO system architecture
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Symmetrical and asymmetric millimeter wave large size MIMO system architecture
The basic principle of the non-symmetric millimeter wave major MIMO system is to disconnect the full digital multi-beam emission and receive arrays, that is, the transmit array and the reception array size are different, and the specific form is shown in FIG. 4 (b). The base station side employs a large-scale full-digital multi-beam-transmit array and a smaller scale all-digital multi-beam receiving array, thereby generating a narrower emission multi-beam and wider receiving multi-beam; the terminal side can still maintain a conventional symmetric form It is also possible to use asymmetrical form. For example, the base station side is collected by symmetrical 64-shot 64 (64T64R) into an asymmetric 64T16R, while the terminal side remains unchanged or turned into an asymmetrical 4T2R or 4T1R array, with a traditional 64-cell element 4 sub-array (4 × 4 sub-array) 4 Data flow mixing multi-beam system compared to 64T16R full digital multi-beamless asymmetric array due to full diameter operation, the beam gain is 6 dB, if the end side remains unchanged, downlink The gain is 6 dB. If the number of terminal receiving array units is reduced from 4 to 1, the downlink gain is unchanged. For uplink, if the terminal side remains unchanged, the uplink gain is not changed, but the number of non-symmetrial reception array and the number of radio frequency channels are 16 rather than 64. Therefore, the non-symmetric millimeter wave full digital large-scale MIMO array has greatly reduced the complexity, cost and power consumption of the system while almost maintaining the advantages of a large-scale MIMO array of symmetrical allograms.
Through the above analysis, the non-symmetrical system and the traditional symmetry mixing multi-beam system are advantageous on the link gain, but the non-symmetrical system has the following characteristics:
(1) Transmit and receive array beam asymmetry.The non-symmetrical system makes full use of the full caliber, realizing the high gain of the emission array, the receiving array low gain width beam, holding the link gain, or higher.
(2) Beam scan range. Since the non-symmetric system still uses a full digital multi-beam array architecture, its beam scanning range is consistent with the symmetric full digital multi-beam system, with a large beam scan range.
(3) Beam alignment and management are easier. Since the size of the non-symmetric system receives the scale, the receiving beam is wider, which will greatly reduce the DOA calculation and beam-alignment difficulties and the complexity of beam management, especially for the scene of the 6G Implicant.
(4) System capacity is high. The number of beams of the asymmetric large-scale MIMO array system is much more than the number of beams of the current commercial mixing multi-beam array, so that more data streams can be supported to increase system capacity.
(5) Hardware design complexity is reduced. On the base station side, the receiving channel is largely reduced, for example, the number of channels is reduced from 64 to 16. This will greatly reduce hardware costs, especially for broadband signals, high-precision ADC chips and RF channels, while significantly reducing the processing amount of baseband signal and processing algorithm.
However, while the millimeter wavefesi-symmetrical large-scale MIMO system brings advantages, it will also usher in the challenge of the corresponding key technologies, for example, due to the use of asymmetrical launch and reception arrays, the upper and downlink channels are non-mutually easy, this needs Study non-interi channel characteristics and channel models.
Currently, research on non-symmetric millimeter wave major MIMO arrays is still in its starting stage, but this is a useful attempt and exploration, expect to promote the establishment of B5G and 6G new system architectures.
4 summary
This article briefly summarizes the role of millimeter wave technology in 5G and its evolution. First, the basic architecture and main problems of the current 5G commercial millimeter wave major MIMO system are analyzed. At the same time, high-performance full-digital multi-beam architecture; secondly, the potential application of millimeter wave technology in 6G system is discussed; in the end, introduction Our preliminary idea of the non-symmetric millimeter wave major scale MIMO system and brief analysis of its advantages and disadvantages. In summary, millimeter wave technology will play an increasingly important role in future mobile communication systems, and continue to promote millimeter wave technology research and serve future society.
This article is from RF Baihuantan
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