Spatial Multidimensional Cooperative Transmission Theories And Key Technologies. Lin Bai

next-generation system based on the first three generations of mobile communication systems to better support high-speed broadband mobile communication services. The World Radio Conference in 2007 allocated spectrum for IMT-Advanced, and IMT-Advanced standards began to be collected in March 2008. By October 2009, a total of six candidate proposals were collected, which can be classified into 3GPP LTE-Advanced³ and IEEE802.16m.⁴ At present, international standards for 4G mobile communication technologies mainly include FDD-LTE, FDD-LTE-Advance, TD-LTE, and TD-LTE-Advanced. Among them, TD-LTE and TD-LTE-Advanced are 4G international standards led by China.

      LTE is an evolution of 3G. It improves 3G air access technology, using orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) technologies as its wireless evolution technology. The LTE mobile communication system can provide a downlink rate of 100Mbit/s (TD-LTE) or 150Mbit/s (FDD-LTE), an uplink rate of 50 Mbit/s (TD-LTE) or 40 Mbit/s (FDD-LTE), and peak rates in a 20-MHz spectrum bandwidth. TD-LTE is the 4G international standard led by China, and it is adopted by China Mobile.

      LTE-Advanced can be divided into FDD-LTE-Advanced and TD-LTE-Advanced. It is optimized for indoor environments and uses technologies such as carrier aggregation. It flexibly allocates spectrum for a wider spectrum bandwidth and effectively supports new frequency band and large-bandwidth applications. It can provide a downlink rate of 1 Gbit/s and an uplink rate of 500 Mbit/s in a 100-MHz spectrum bandwidth.

      WiMax is an IEEE 802.16 standard that provides a maximum access rate of 70 Mbit/s and a working frequency range of 2–66 GHz without authorization. The main advantages of WiMax are as follows: (1) it is beneficial to avoid known interference, (2) it is beneficial to save spectrum resources, (3) flexible bandwidth adjustment capability is beneficial for operators to coordinate spectrum resources, and (4) wireless signal transmission distance can be up to 50 km. However, it cannot meet the seamless connection of wireless networks under high speed in terms of mobile performance. Therefore, WiMax is not a wireless mobile communication technology, but only a wireless broadband LAN technology.

      Wireless MAN-Advanced is an upgraded version of WiMax, namely the IEEE 802.16m standard, which has the ability to seamlessly switch under high speed. It can effectively solve mobile performance problems of WiMax. IEEE 802.16m is compatible with 4G networks. Its advantages are as follows: (1) expand network coverage to achieve seamless network connectivity, (2) improve spectrum efficiency, (3) provide 1 Gib/s wireless transmission rate in roaming mode or high-efficiency/strong signal mode.

1.1.5The fifth generation of mobile communication system

      The fifth-generation mobile communication system (5G) is a next-generation wireless mobile communication system that is being developed in order to meet the rapid spread of intelligent terminals and the rapid development of mobile Internet after 4G. It is a wireless mobile communication system for the needs of the human information society after 2020.

      5G has become an active research in the field of mobile communication systems at home and abroad. In 2013, the 7th Framework Program, which was jointly undertaken by 29 participants including China’s Huawei Corporation, launched the mobile and wireless communications enablers for the 2020 information society (METIS) project⁵ for 5G research. China’s 863 Program also launched the first and second phase of 5G major projects in June 2013 and March 2014, respectively. At present, countries around the world are conducting extensive discussions on the development, application requirements, candidate frequency bands, and key technical indicators of 5G, and striving to reach a consensus at the 2015 World Radio Conference. And standardization process started in 2016.⁶

      For the future vision and application of 5G, there have been relevant descriptions in academia and industry, from which we can summarize the technical needs of the future 5G. Compared with the traditional mobile communication network, 5G should have the following basic characteristics such as: (1) the data traffic is increased by 1000 times, (2) the number of networked devices is increased by 100 times, (3) the peak rate is at least 10Gbit/s, (4) the rate users can obtain is 10Mbit/s, and it can be up to 100 Mbit/s for special needs, (5) short delay and high reliability, and (6) high spectrum utilization and low network energy consumption.

      At present, the key technologies of 5G are still in research. Technologies such as large-scale MIMO technology, beamforming technology, and cooperative wireless communication technology all have the possibility to become the key technologies of 5G.

      MIMO technology can effectively improve the spectral efficiency of wireless communication and obtain receive diversity gain (RDG), which is recognized as the core technology of the next-generation mobile communication system. A typical M × N MIMO system is shown in Fig. 1.1.

       Fig. 1.1. An M × N MIMO system.

      Since each receiving antenna will receive a superimposed signal from all transmitting antennas, the received signal can be expressed as

      where y_n, h_nm, sm, and n_n, respectively represent the received signal of the nth receiving antenna, the channel gain from the mth transmitting antenna to the nth receiving antenna, the transmitted signal of the mth transmitting antenna, and the noise of the nth receiving antenna. It can be seen from Eq. (1.1) that each transmitted signal will have N backups at the receiving end, which is called reception diversity. However, signals from different transmitting antennas form interference at the receiving end. In order to detect the transmitted signal at the receiving end, signals from different transmitting antennas must be extracted. Therefore, the detection algorithm of the MIMO receiver is an indispensable component of the MIMO system.

      Besides, beamforming is also a key technology to achieve space diversity gain. Beamforming technology is widely used in directional antenna array radar, sonar hydroacoustic positioning and classification, ultrasonic optical imaging, geophysical exploration, petroleum exploration, biomedical imaging, and wireless communication. At the transmitting end, beamforming technology is used to appropriately weight the signals transmitted by the corresponding antennas in the antenna array to generate a directional virtual beam. Thereby the purpose of enhancing the desired signal and suppressing interference and improving the communication capacity and quality can be achieved. At the receiving end, signals from different receiving antennas are combined in the receiver to achieve coherent superposition and improve the reception quality of the signals. Beamforming technology can be divided into two categories, namely array beamforming based on antenna array and multi-antenna beamforming based on signal pre-processing, which utilize the spatial correlation and independence of different antenna channels, respectively. Array beamforming technology utilizes the strong correlation of spatial channels and the principle of interference of electromagnetic waves. By weighting the correlation of the output signals of multiple antennas in terms of amplitude and phase, the signals will be superimposed in a certain direction and phase cancellation in other directions will be done to enhance the target signal and suppress interference. And for the multi-antenna beamforming technology, it utilizes the independence between different antenna channels to improve the space diversity gain of the system.

      For a cellular communication system, when a user is at the cell edge, a signal from a neighboring cell base station will be received. The conventional method is to simply consider the signal from the neighboring cell base station as an interference signal. Because this competition-causing strategy will significantly reduce communication performance, coordinated multipoint (CoMP) technology has attracted widespread attention in 5G communication systems. By utilizing the interaction of mobile channel information and data information between adjacent base stations, the CoMP technology performs an interference avoidance strategy for the interfered users or joint transmission for mobile users by multiple base stations. Increasing the throughput of edge users and the coverage area of the high data transmission