abstract
The fifth generation mobile communication (5g) low frequency band (sub-6ghz) has begun to be commercially available, 5g millimeter wave technology is also gradually mature, and is expected to start to be commercially available in 2022. The research of the sixth generation mobile communication (6g) has also started, and more papers about 6G's vision and core technology have been published. This paper mainly discusses the application and core role of millimeter wave technology in 5g and 6G in the future.
Introduction
As we all know, the fifth generation mobile communication (5g) is divided into low frequency (sub6ghz) and high frequency (millimeter wave). 5g in low frequency band has been commercially used in 2019 in China, and the spectrum of millimeter wave 5g has not been officially released, but 24.75-27.5 GHz and 37-42.5 GHz have been approved as experimental frequency band. 5g millimeter wave technology based on large-scale MIMO tends to mature and is expected to start commercial use around 2022.
In recent years, there are more and more papers, reports and reports about 6G vision and core technology at home and abroad, with different opinions, but some consensus has gradually formed. In terms of network architecture, 6G will be a network integrated by a large number of low and medium orbit satellites and the back 5g (b5g) on the ground, so as to realize the full coverage of the whole earth surface and its near space for the first time. 29% of the earth's surface is land, 71% is sea, and 29% of the land is not covered by 1g-5g mobile communication network. Therefore, 6G will be a revolution in the history of human mobile communication. In terms of core technology, some expressions are gradually recognized, such as ubiquitous, holographic, artificial intelligence and so on. Broadband transmission technology is the foundation of supporting communication network. For 6G, to realize the integrated high-speed communication network of space, space and sea, broadband transmission technology will be the core. For the ground 5g network, it has begun to use the spectrum resources of millimeter wave band to achieve broadband high-speed transmission. For 6G, millimeter wave frequency band will be the first choice for inter satellite link, satellite down coverage user link and satellite to ground station feed link. For example, SpaceX Starlink mainly adopts Ka and Q bands, while O3b medium orbit satellite network adopts Ku and Ka bands. To be sure, millimeter wave technology will be one of the most important supporting technologies of 6G network. It is reported that terahertz will be the core technology of 6G, which is questionable. In fact, limited by the characteristics of semiconductor technology, in the terahertz frequency band (usually 300-10000 GHz, also known as terahertz in the 100-10000 GHz frequency band), transmission power, receiver noise coefficient, manufacturing difficulty, cost, etc. are all bottlenecks that need to be broken through in the application of terahertz.
As the bandwidth reaches 400 MHz or even wider, high sampling rate ADC / DAC, real-time processing of massive data and high-density integration of a large number of RF channels and antennas become the bottleneck of 5g millimeter wave based on large-scale MIMO technology. For this reason, the current commercial 5g millimeter wave active antenna unit (AAU) adopts the hybrid multi beam scheme of phased array. This scheme greatly reduces the number of RF transceivers, which partially overcomes the above bottleneck problems, but at the expense of array gain and communication capacity.
In theory, large-scale MIMO technology based on all digital multibeam will be the goal of mobile communication in the future, but the above bottleneck problem is a very difficult obstacle to overcome at present. For this reason, we propose an asymmetric millimeter wave large-scale MIMO system architecture, in order to overcome the above bottlenecks while approaching the best performance of the system.
In this paper, we will discuss the problems of 5g millimeter wave and the possible technology route in the process of evolution to 6G, in order to inspire the researchers of 5g / 6G millimeter wave technology.
1 5g millimeter wave
5g millimeter wave commercial system architecture is usually composed of core network (CN), baseband unit (BBU) and active antenna unit (AAU), as shown in Figure 1. Its basic architecture is that a core network supports multiple baseband units, and each baseband unit will support multiple active antenna units. Specifically, CN is located in the center of network data exchange, mainly responsible for providing core functions such as data transmission, mobile management and session management; BBU is mainly responsible for baseband digital signal processing, such as coding, multiplexing, modulation, etc.; AAU is mainly responsible for realizing the conversion between baseband digital signal and radio signal, and completing the transmission and reception process. AAU mainly includes AAU baseband part (beam management, etc.), up and down conversion module, and analog beamformer. The baseband part of AAU mainly completes some digital signal processing in physical layer, such as beam management, control of different beam coverage, and signal conversion in analog domain and digital domain with digital analog converter (DAC) and analog-to-digital converter (ADC). Due to the large bandwidth requirement of 5g millimeter wave system, new requirements will be put forward for baseband signal processing and ADC / DAC capability. Up and down conversion module is responsible for the conversion between baseband I / Q signal (or if signal) and millimeter wave RF signal. Upconverter module is mainly used for transmitting link, including upconverter, filter, power amplifier and other devices, which is responsible for moving the transmitting signal from baseband I / Q (or if) to the required millimeter wave transmitting frequency. Similarly, the down conversion module is mainly used for receiving links, including low-noise amplifiers, filters, mixers and other devices, to move millimeter wave received signals to baseband I / Q (or if). The analog beamforming network (phased array) is mainly responsible for allocating the RF signal energy to the antenna array feed port reasonably, forming a specific amplitude and phase distribution, and then forming a specific beam. The mainstream AAU generally supports 4 or more data streams, and each sub array supports 1 data stream, as shown in Figure 1. The beamforming circuit of each sub array consists of power distribution / synthesis module, multi-channel transceiver phase-shift and amplitude control chip, antenna array, etc. Take the 4-way data stream transmission link as an example, each data stream signal reaches the RF frequency through the up conversion module, and is distributed to the multi-channel chip input port through the power distribution network, for example, 1 in 16 channels. Taking the 4-channel chip as an example, the transmission link of each chip includes power amplifier, phase shifter, switch, etc., which can complete the signal conversion from 1 to 4, accurately control the amplitude and phase of each signal, and then feed the output signal into the antenna unit to achieve the desired beam.
Figure 1 commercial 5g millimeter wave system architecture
The advantage of 5g MMW commercial hybrid multibeam architecture is that it can complete multibeam coverage with low complexity and cost. As shown in Figure 1, the system only uses four ADC / DAC channels and up and down frequency conversion channels to realize independent control of four beams. However, due to the insufficient number of channels and limited data flow, the system has significant limitations for the expansion of the number of beams, resulting in insufficient system capacity. At the same time, because four beams are controlled by subarray independently, this architecture does not realize the effective utilization of the full aperture of the antenna, so it will lose 6 dB (four subarrays) or even more array beam gains. Another effective hybrid multibeam architecture is to use both baseband digital beamforming and phased array analog beamforming to achieve full aperture utilization of the antenna array and generate higher array beam gain. However, due to the limited beam width of phased array, there will be the problem of narrow scanning range of multi beam and limited coverage. This architecture can expand the coverage through beam switching, but at the expense of delay and beam management complexity, it will eventually reduce the system capacity.
In order to obtain the system capacity and array gain at the same time, another implementation form of AAU is all digital multi beam array, as shown in Figure 2. All digital multi beam array architecture directly corresponds each antenna unit to a RF transceiver channel, each transceiver channel includes RF transceiver front-end (FEM), up and down conversion channel, ADC / DAC, etc., and the beamforming is all realized in the baseband digital domain. The advantage of all digital multi beam array architecture is that it can precisely achieve the required amplitude and phase control by using baseband digital circuit, and the number of beams is easy to expand, thus increasing the communication capacity. At the same time, each beam formed by this architecture can obtain the full aperture gain of the antenna array. However, its disadvantages are also obvious. Because each antenna unit needs to be connected to a RF channel, the high-density integration of a large number of RF antennas greatly increases the complexity of hardware design. At the same time, due to the large bandwidth demand of 5g millimeter wave system, the requirements for the bandwidth of RF channel, the sampling rate of ADC / DAC, and the baseband processing rate will increase, resulting in the real-time digital signal processing problem of massive data, greatly increasing the operation cost and power consumption.
Figure 2 AAU based on all digital architecture
In conclusion, the performance comparison of MMW multibeam array architecture is shown in Table 1. Millimeter wave all digital multi beam array architecture is the representative of the best performance, which can get the highest communication capacity and beam gain, but its architecture has high complexity and cost, so it is urgent to develop new technology.
Table 1 MMW multibeam array technology comparison
Millimeter wave technology in 26G
In addition to being fully utilized in 5g, millimeter wave technology will also play an important role in the 6th generation mobile communication system (6g). Although the current 6G vision is not fully clear, its basic objectives can be seen, as shown in Figure 3. At present, the global wireless communication network only covers the main human settlements on the earth's surface, and there are still large areas of land, such as deserts, lakes, mountains and forests, which are not effectively connected to the network. In addition, as the tentacles of human exploration continue to extend to the ocean, sky, space and other areas, these areas will have a strong demand for access to wireless communication networks. Therefore, as an important part of 6G, the low and medium orbit satellite network, namely the Internet of space, will be integrated with the b5g system on the ground to realize the ubiquitous link of the integrated communication network of space, space and sea.
Figure 3 Schematic diagram of 6G air network
Due to the diversification of future applications, the intelligent connection, and the depth of information processing, 6G system will generate massive data, which needs a higher rate of transmission support. It is reported that 6G is expected to enter the era of terabits (TBPs), that is, to achieve a transmission rate of 1000x Gbps. In order to achieve this goal, it is urgent to find suitable spectrum resources for 6G system. At present, the frequency of low frequency band (within sub-6ghz) has been fully developed, and at the same time, it is difficult to obtain larger spectrum bandwidth to support the transmission rate of TBPs. Therefore, we will seek for spectrum resources in higher frequency band. As we all know, the higher the frequency, the shorter 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 so on. So which frequency band is more suitable for 6G? Here is a brief discussion on the possible spectrum resources of 6G. Terahertz band has abundant undeveloped spectrum resources, which can realize small device size and large-scale array. There are many related researches. However, the current stage is mainly limited by the semiconductor process characteristics, terahertz device capacity is still insufficient, such as insufficient output power, poor noise coefficient index and so on. In addition, because of its high cost and complex processing technology, these factors will restrict the further application of THz band in 6G era. Compared with terahertz band, millimeter wave band has been fully developed in 5g era, and the device capacity has been greatly improved. The industrial chain is complete and rich. At the same time, the array size of millimeter wave band is relatively large