Communication technology began to spread to people's daily lives in the 1980s. Due to the many advantages of this technology, it has been well developed in just a few decades, and the mobile communication business has become more and more efficient. And easy to use technology. In recent years, with the continuous advancement of science and technology, the demand for networks has increased day by day, which has enabled the development of mobile communication services. In order to meet the increasing demands of people, LTE wireless network technology (4G) came into being. New technologies such as aggregate carrier, MIMO, multipoint coordinated transmission, and relay are also generated, which satisfies people's demand for communication-efficient data services. The most critical LTE-Advanced technology has become one of the most important 4G international standards, and has attracted the attention of researchers in the field of communications worldwide. It is believed that in the near future, 4G networks will be universally applied to all users.
1 LTE-Advanced Technology Overview and Analysis 1.1 Development BackgroundThe emergence of mobile communication services has changed the traditional communication methods of human beings, and quickly replaced the communication mode of the original mailing letters, which effectively improved people's lives, greatly improved the efficiency of life, and made it easier for people to understand what is happening around them. Things, the development of mobile communication services is a major step forward in human history.
So far, the communications field has experienced three important stages of development, and each improvement is a major advancement. The first generation is an analog communication system, an advanced mobile phone system introduced by the United States, which implements nationwide voice communication through frequency division multiple access technology. The second generation communication system originated from the CDMAone system in the United States and the global mobile communication system in Europe. It is the beginning of digital communication system. It has achieved unprecedented success in the world. Currently, the global mobile communication system is still the main service in the communication field. the way. With the advancement of technology, users are increasingly demanding the speed and quality of data transmission. Some technologies of the second generation digital communication system have been unable to meet the increasing demands of users, which has promoted the third generation mobile communication system. Development, 3G standards came into being. It includes China-led synchronous code division multiple access technology, global microwave interconnection technology, CDMA2000 in the United States, and wideband code division multiple access technology in Europe and Japan. The 3G mobile communication system is based on CDMA technology and can support multimedia services while increasing data transmission rate. In the past two years, 3G services have been widely used worldwide.
With the development of technology and human progress, the need for better call quality, quality media services, and more efficient and fast data transfer rates requires more advanced network architecture and air interface technology, based on people. The demand for LTE wireless network technology came into being, marking the arrival of the 4G era.
1.2 LTE-Advanced Technical AnalysisLTE wireless network technology is a major change and innovation that has been experienced since the emergence of mobile communication. Many basic technologies in the original 3G era communication system have been improved or replaced, which indicates greater progress in human society. LTE technology has higher requirements for 4G communication systems, larger bandwidth, higher peak rate and user experience in various areas. Statistics show that 80% to 90% of future system throughput will occur in indoor and hotspot scenarios, which will become more important applications in the 4G era. Therefore, to enhance the user experience of hotspot scenarios, the emergence of new technologies is inevitable.
2 LTE-Advanced main technologyIn order to meet the higher requirements of users and adapt to more applications in the future wireless communication market, LTE wireless network technology has generated and introduced many new technologies, such as downlink multi-antenna enhancement technology, aggregate carrier technology, relay and multi-point coordinated transmission. Technology, etc., have greatly improved the performance of wireless mobile communication systems, and are more conducive to user experience.
2.1 Aggregation carrier technologyThe aggregation carrier technology is proposed to make the system bandwidth of the LTE network meet the corresponding requirements. The aggregate carrier technology is to aggregate two or more component carriers to enable receiving higher frequency component carriers, and the communication system using the aggregate carrier technology can still receive one of the component carriers (the carrier of the corresponding frequency before aggregation) ).
The aggregation of carriers is divided into two technologies: continuous and non-continuous, as shown in Figure 1 and Figure 2. The carrier aggregation technology of continuous spectrum can make the configuration of the base station and the terminal more convenient and convenient, and is suitable for the frequency range of the 3.4 GHz to 3.8 GHz band. The aggregate carrier of the discontinuous spectrum has higher flexibility of spectrum aggregation and needs to determine the spectrum. Aggregate the capabilities of supported terminals to design the lowest cost and power loss.
There are two types of carrier aggregation between data flows of multiple component carriers of an LTE network system: physical layer aggregation and MAC layer aggregation. The physical layer aggregation mode means that all component carriers use the same modulation and coding mode and share the same HARQ process and ACK/NACK feedback. In the MAC layer carrier aggregation mode, each carrier has its own transmission block. Each component carrier uses its own independent link adaptation technology, and can select the corresponding modulation and coding mode according to its own link condition. Intermixed and automatically retransmits request HARQ processes and ACK/NACK feedback. In comparison, the MAC layer carrier aggregator is superior to the physical layer carrier aggregation mode.
The LTE downlink can support 4*4 antenna configurations, which are further enhanced in MIMO technology to meet the needs of LTE to improve average spectral efficiency and peak spectral efficiency. Under MIMO technology, downlink single users can support scenarios that are extended to 8*8 configurations, supporting up to 8 layers of transmission; uplinks can support up to 4*4 configurations under MIMO and support up to 4 layers of transmission.
2.2.1 Uplink multi-line enhancement for MIMO
The uplink technology of LTE needs to consider many requirements, such as the low peak-to-peak ratio of the uplink, more antenna configurations, and single carrier transmission per carrier.
Multi-antenna technology further optimizes uplink control channel performance and coverage, and the way transmit diversity is a major factor in determining this performance. According to the investigation and practice evaluation, in the MIMO technology, the signals are modulated and transmitted using mutually orthogonal code sequences, that is, the original code division uplink control channel is replaced by the transmit diversity mode of the SORTD, which is more advantageous for performance optimization.
To increase capacity, the introduction of multi-antenna technology spatial multiplexing is a major requirement for uplink traffic channels. At the same time, simpler open-loop transmission precoding has more performance advantages than transmit diversity, so it is finally determined that the user of the cell boundary directly uses the open-loop transmission precoding to determine the uplink traffic channel.
2.2.2 MIMO downlink multi-line enhancement
For the downlink signal of LTE, a dedicated reference signal is used for transmission, and all uplink and downlink signals are transmitted in principle based on a codebook or a non-codebook. At the same time, codebook-based PMI feedback is adopted for closed-loop MIMO to reduce feedback overhead. At present, a double precoding matrix codebook structure is being used to design an 8-antenna codebook, that is, a product of two matrices is used to represent a codebook matrix, usually one is a matrix basic code, and the other is based on a basic length of a basic code according to channel characteristics. Correction to it. In addition, long-period (spatial correlation) or short-period feedback (fast decay factor) is also applied according to the speed of the channel change to further reduce the feedback overhead.
The LTE technology uses the user's dedicated reference channel for the transmission of the traffic channel. For the same user, different layers of the traffic channel use CDM+FDM mutually orthogonal signals as reference signals. Currently, in order to reduce interference between multi-user streams, LTE network technology is exploring further enhancements to MU-MIMO technology, developing joint signal processing and multi-user diversity gains, and comparing performance and link complexity. Good compromise treatment.
2.3 Relay technology
The relay technology increases the distribution density between the antenna and the station by adding new sites to the original site. The original base station (the parent base station) is connected to the transmission network, and the newly added station is connected to the parent base station by wireless, so there is no wired connection between the new base station and the transmission network. The downlink data transmission is first passed through the parent base station, then transmitted to the relay node through the parent base station, and finally transmitted to the terminal user, and the uplink data transmission is opposite to the downlink. In this way, the user data rate and spectrum efficiency of the system can be improved, the distance between the end user and the antenna is improved, and the quality of the terminal link is improved.
In order to meet the requirements of users for efficient data transmission, a relay technology is proposed to improve the data transmission rate and average throughput of cell edge users. At present, the simpler relay is a two-hop relay, that is, a base station-relay station and a relay station-base station. In this way, a poor quality link can be converted into two better quality link structures. , thereby increasing the capacity and coverage of the link.
Currently, relays can be classified into two types, Type1 trunk and Type2 trunk. Because the Type1 trunk has its own independent cell ID, it can control a single cell by sending its own synchronization and pilot signals, which is equivalent to having its own master for each cell in its coverage. Base station. However, Type2 trunks do not have the capability of having independent cell IDs of Type1, so they cannot form new cells and transmit their own synchronization and pilot signals. Type2 relays are mainly used to increase spectrum efficiency and cell throughput. Table 1 shows the base station information record of whether the mobile terminal relays.
2.4 Multipoint Collaboration Technology (CoMP)
Multi-point coordinated transmission is a kind of antenna technology. It mainly uses distributed, that is, terminals in the cell boundary area. The signals transmitted by itself can be received by multiple cells at the same time, and can receive different requests from multiple cells simultaneously. The signal thus improves the system performance of the cell edge users. In downlink transmission, by coordinating the transmission signals between multiple cells to avoid interference with each other, the efficiency of downlink transmission can be greatly improved. In the uplink data transmission, the interference between the cells can also be coordinated and suppressed, and multiple cells simultaneously receive and combine signals, thereby improving the signal-to-noise ratio of the received signal and achieving better results.
When the traditional transmission mode is adopted, there is interference between cells, which may cause the performance of edge users to be degraded. The multi-point coordinated transmission technology can effectively eliminate mutual interference between cells by sharing information between base stations of each machine. The multi-point cooperation technology can be classified into two types: joint transmission and cooperative scheduling/beamforming according to whether user information is shared between base stations.
Both joint processing or cooperative scheduling/beamforming can improve the performance of the system, but each has its own advantages and disadvantages. Joint transmission means that signals are transmitted from two or more base stations at the same time, so joint transmission can improve user performance better than coordinated scheduling/beamforming. However, in the joint transmission mode, each cell user occupies too much resources, so for the average rate, the joint transmission mode is lower than the cooperative scheduling/beamforming mode, and the joint transmission has a small performance gain for a single user. The cooperative scheduling/beamforming approach has a higher performance gain for single users. For multi-user mode, both the average rate of users and the performance of edge users are greatly improved.
3 ConclusionNew things symbolize development. The introduction of LTE technology marks the continuous progress of mankind. This satisfies people's higher requirements for the network. The change from 3G to 4G era will be another important progress in human society. The continuous improvement of cell throughput, data transmission rate and space interface technology has led to the rise of wireless flattening technology. This is a great progress in the history of wireless network architecture. The emergence and development of LTE-Advanced technology is unstoppable, and its development standards even exceed the original The standards established by the ITU, and LTE-Advanced technology is fully compatible with LTE wireless network technology. This paper introduces the development of LTE-Advanced technology and related major technologies, and explores its key technologies. It is foreseeable that LTE-Advanced technology will be a hot research topic in the field of mobile communications in the world for a long time, which will be more conducive to the development of the fourth generation of communication technology, and human beings will not be far away from entering the 4G era.
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