1. What is LTE-A
LTE-A is the abbreviation of LTE-Advanced, which is the follow-up evolution of LTE technology. LTE is commonly known as 3.9G, which shows that the technical indicators of LTE are very close to 4G.
Compared with 4G, LTE has reached the requirements of the 4G standard except for the two indicators of maximum bandwidth and uplink peak rate which are slightly lower than the 4G requirements. The overall technical design of LTE-A, which officially brings LTE into 4G, far exceeds the minimum requirements of 4G.
In June 2008, 3GPP completed the LTE-A technical requirements report and proposed the minimum requirements for LTE-A: downlink peak rate of 1Gbps, uplink peak rate of 500Mbps, and uplink and downlink peak spectrum utilization rates of 15Mbps/Hz and 30Mbps/ Hz. These parameters are far higher than the ITU’s minimum technical requirements indicators and have obvious advantages.
2. Main technical features of LTE-A
In order to meet the various requirements of IMT-Advanced (4G), 3GPP has proposed several key technologies for LTE-Advanced (LTE-A), including carrier aggregation, coordinated multi-point transmission and reception, relay transmission, and multi-antenna enhancement.
The key technologies of the LTE-A system include:
Carrier aggregation
LTE-A supports continuous carrier aggregation and discontinuous carrier aggregation within and between frequency bands, with a maximum aggregate bandwidth of up to 100 MHz. In order to make effective use of carriers in the initial commercial stage of LTE-A, that is, to ensure that LTE terminals can access the LTE-A system, each carrier should be able to be configured as a carrier that is backward compatible with LTE. However, it does not rule out that the design is only used by the LTE-A system. Carrier used.
At present, 3GPP has identified 12 application scenarios for carrier aggregation according to the needs of operators, among which 4 are the continuous and non-continuous carrier aggregation scenarios for FDD and TDD, respectively, as a recent focus.
In the research phase of LTE-A, the relevant research focus of carrier aggregation includes the improvement of spectrum utilization of continuous carrier aggregation and the design of control channels for uplink and downlink asymmetric carrier aggregation scenarios.
Multi-point collaboration
Multi-point cooperation is divided into two categories: multi-point coordinated scheduling and multi-point joint processing, which are suitable for different application scenarios and cannot be completely replaced by each other. The research of multi-point coordinated scheduling mainly focuses on the solution combined with multi-antenna beamforming.
In the recent preliminary assessment of ITU by 3GPP, the multi-point coordination technology is the only technology that can meet the requirements of all scenarios under the condition of the four-antenna configuration of the base station, and at the same time significantly improve the uplink and downlink system performance, so the standardization of multi-point coordination Progress has become the top priority of the 4G candidate project submitted by 3GPP and the ITU-oriented evaluation.
Relay transmission
The future mobile communication system needs to optimize the capacity of urban hotspots based on the traditional cellular network and expand the coverage of blind areas, subways, and rural areas.
The current standardization work in 3GPP focuses on low-power in-band backhaul relay transmission that can be deployed on telegraph poles or external walls. It is small in size and light in weight, and easy to site.
Generally speaking, the relay transmission performance of in-band backhaul is lower than that of traditional microwave backhaul, but in-band backhaul does not require the backhaul frequency band outside the LTE spectrum to further save costs. Therefore, the two have their own market needs and Application scenarios.
Multi-antenna enhancement
In view of increasingly precious frequency resources, multi-antenna technology has been widely adopted by many standards because it doubles the channel capacity by expanding the transmission dimension of the space.
Limited by the influence of the height of the transmitting antenna on the channel, the focus of LTE-A system uplink and downlink multi-antenna enhancement is different.
Based on the multiple downlink multi-antenna modes of the LTE system, the maximum downlink multi-antenna configuration required by LTE-A is 8×8, and the enhancement of multi-user space-division multiplexing is considered the focus of standardization.
The uplink enhancement of LTE-A relative to the LTE system mainly focuses on how to use multiple power amplifiers of the terminal, use uplink transmits diversity to enhance coverage, and uplink spatial multiplexing to increase the uplink peak rate.
OFDM
OFDM is developed from Multi-Carrier Modulation (MCM). OFDM technology is one of the realizations of multi-carrier transmission schemes. Its modulation and demodulation are based on inverse fast Fourier transform (IFFT) and fast Fourier transforms (FFT) respectively.
It is a multi-carrier transmission scheme with the lowest implementation complexity and the most widely used. In the traditional frequency division multiplexing system, the signal spectrum on each carrier does not overlap, so that the receiving end uses traditional filters to separate and extract signals on different carriers.
The OFDM system modulates data symbols on multiple parallel subcarriers with a relatively low transmission rate and orthogonality to each other for transmission. It allows the sub-carrier spectrum to partially overlap, and the receiving end uses the orthogonality between the sub-carriers to recover the transmitted data.
Therefore, the OFDM system has higher spectrum utilization. At the same time, inserting a cyclic prefix between OFDM symbols can eliminate inter-symbol interference caused by multipath effects, and can avoid affecting the orthogonality between subcarriers due to the insertion of guard intervals in a multipath channel environment. This makes the OFDM system very suitable for multipath wireless channel environments.
The advantages of OFDM are its ability to resist multipath fading and high spectrum efficiency. OFDM divides the channel into several sub-channels, and each sub-channel can be considered as flat fading. The fast implementation method of OFDM based on IFFT/FFT can be used. In frequency selective channels, the complexity of an OFDM receiver is simpler than a single carrier system with an equalizer.
Unlike other broadband access technologies, OFDM can run on discontinuous frequency bands, which will facilitate the allocation of multiple users and the application of diversity effects. However, OFDM technology is more sensitive to frequency offset and phase noise, and its peak-to-average power ratio (PAPR) is large.
Wireless relay
LTE system capacity requirements are very high, and such capacity requires a higher frequency band. In order to meet the high-rate transmission requirements of next-generation mobile communication systems, LTE-A technology has introduced wireless relay technology.
User terminals can access the network through intermediate access points to obtain bandwidth services. Reduce the space loss of the wireless link, increase the signal-to-noise ratio, and then increase the channel capacity of edge users. Wireless relay technology includes Repeaters and Relays.
Repeaters are directly forwarded on the radio frequency after receiving the radio frequency signal of the mother base station, which is invisible to the terminal and the base station and does not care whether the target terminal is in its coverage area, so its function is only an amplifier. Its role is limited to increasing coverage, and cannot increase capacity.
Relay technology is based on the original site, by adding some new Relay stations (or relay nodes, relay stations) to increase the distribution density of sites and antennas. These newly added Relay nodes and the original base station (parent base station) are all connected wirelessly, and there is no wired connection with the transmission network.
The downlink data first reaches the parent base station and then is transmitted to the Relay node, which is then transmitted to the end-user. The upside is the opposite. This method shortens the distance between the antenna and the terminal user and can improve the link quality of the terminal, thereby increasing the system’s spectrum efficiency and user data rate.
Self-organizing network
In order to pass the pressure of effective operation and maintenance costs (OPEX) and the complexity of LTE network parameters and structure, 3GPP borrows the concept of a self-organizing network and proposes a new operation and maintenance strategy in R8.
This strategy uses eNodeB as a self-organizing network node and adds self-organizing function modules to it to complete cellular wireless network self-configuration, self-optimization, and self-operation. As a feature of LTE, SON has introduced requirements in R8, and R9 has completed discussions on self-healing and self-optimization capabilities.
The LTE self-organizing network is different from the traditional IP Internet self-organization in that LTE requires that self-organizing nodes can be interconnected, and the network can be self-optimized and self-operated.
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