On November 14, 2017, the Ministry of Industry and Information Technology issued a frequency usage plan for the 5G system in the 3 000 MHz-5 000 MHz frequency band (mid-band). my country became the first country in the world to release a frequency usage plan for the 5G system in the mid-band. The plan clarifies the frequency bands of 3 300 MHz-3 400 MHz (the principle upper limit for indoor use), 3 400 MHz-3 600 MHz, and 4 800 MHz-5 000 MHz as the working frequency bands of the 5G system. The 5G system frequency usage plan first released by the Ministry of Industry and Information Technology this time will play an important leading role in the development of my country's 5G system technology research and development, testing and standard formulation and the maturity of the industrial chain [1]. As a key element of 5G business, 5G terminal is an important value carrier for the future 5G industry [2], and its software and hardware technical requirements (such as baseband chips, radio frequencies, antennas, etc.) are all affected or restricted by the 5G frequency band. Among them, the self-interference problem caused by the coexistence of the original LTE frequency band and the 5G frequency band on the terminal side is a hot topic in the industry.
There are currently two network architecture deployment methods in the industry, namely SA (Standalone, independent) architecture and NSA (Non-standalone, non-independent) architecture [3]. If the 5G network is deployed according to the NSA architecture, the terminal is required to support dual connection [4, 5] technology, using dual radios to connect 4G and 5G networks at the same time and perform dual reception and dual transmission. At this time, the nonlinearity of the radio frequency device and other factors may easily lead to the existence of the terminal The self-interference problem, that is, the uplink may produce harmonic and intermodulation interference on the downlink reception, causing the sensitivity of the receiving end to decrease [2].
Starting from the root of the terminal self-interference problem, the article analyzes the self-interference problem that the terminal may cause in the new 5G frequency band and LTE frequency band in detail, and discusses the current industry’s different solutions to this problem and its requirements for the terminal and network. , And finally put forward relevant strategy suggestions from the operator's point of view.
2 Terminal self-interference problem analysis 2.1 Dual connection technology 5G application backgroundIn essence, the DC (Dual Connectivity, dual connectivity) technology and CA (Carrier Aggregation, carrier aggregation) technology proposed in the R12 standard version of the 3GPP are both LTE multi-connection technologies. CA aggregates at the MAC (Media Access Control) layer, which requires high synchronization. In order to avoid the delay and synchronization requirements in the MAC layer scheduling process, the data is in PDCP (Packet Data Convergence Protocol, packet data). The convergence protocol) layer is divided and combined, and then the user data stream is simultaneously transmitted to the user through multiple base stations. The dual-connection technology standard is relatively mature, and it can help operators deploy 5G networks more quickly on the basis of the original LTE network. It has become a key technology for non-independent networking under 5G to achieve interoperability [6-7]. 5G different networking architectures The following interoperability solutions and specific technical requirements for the terminal are shown in Table 1:
Table 1 Interoperability solutions and technical requirements for terminals
It can be seen from Table 1 that dual connectivity is a key technology for implementing interoperability solutions under non-independent networking, and the terminal needs to support dual-channel radio frequencies in hardware to simultaneously connect to LTE and 5G NR networks.
2.2 Terminal dual-connection self-interferenceAccording to 3GPP’s definition of dual connectivity in literature [4] and related descriptions in literature [8], 5G terminals under non-independent networking need to support dual-receiving and dual-transmitting mechanisms, and connect LTE eNB and 5G gNB at the same time, using two NBs from two NBs. Wireless resources. However, terminals that support dual connections may have self-interference issues. Under the NSA architecture, the terminal is required to maintain dual-receiving and dual-transmitting (that is, to maintain dual-uplink connections in the LTE frequency band and the NR band). Due to the nonlinearity of radio frequency devices and other factors, the uplink dual-transmitting will cause downlink harmonics and intermodulation interference. Cause the sensitivity of the receiving end to decrease.
(1) Harmonic interference
The ideal power amplifier (PA) amplifies the input power with a certain amplification factor a. The actual PA can ensure linear amplification when the input power is low, and when the input power is large, it will enter the non-linear region and the output will be high. Order variable. The details are shown in Figure 1:
Figure 1 Comparison diagram of PA ideal and actual input and output
The terminal transmits signals on the transmitting frequency band f0, and at the same time, if the receiving frequency band is n×f0 (n=2, 3, ...), the receiver will be affected by harmonics, resulting in a decrease in receiver sensitivity, as shown in Figure 2(a) Shown. There are two ways of interference caused by harmonics to the receiving end: that is, PA output PCB (Printed Circuit Board, printed circuit board) interference and transmitting antenna output interference.
(2) Intermodulation interference
When two or more interference signals are added to the receiver at the same time, the combined frequency of the two interferences may be exactly equal to or close to the useful signal frequency and pass the receiver smoothly. This interference is called intermodulation interference, as shown in Figure 2. (B) Shown. Among them, third-order intermodulation is the most serious. For example, the second-order intermodulation is f2-f1, and the third-order intermodulation is 2f2-f1, 2f1-f2,...
(A) Harmonic interference (b) Intermodulation interference
Figure 2 Schematic diagram of harmonic interference and intermodulation interference
2.3 Examples of theoretical analysis of 5G frequency band self-interferenceCombining the frequency range of a domestic operator’s existing LTE network in the B1, B3 and B5 frequency bands and the 5G frequency band currently planned by the Ministry of Industry and Information Technology (including 3 400 MHz—3 600 MHz in n78 and 4 800 MHz—5 000 in n79) MHz), a theoretical example analysis of the terminal self-interference problem, the interference frequency band of the downlink receiving end of the terminal and the harmonic interference and intermodulation interference frequency band of the transmitting end are shown in Table 2~Table 4.
Table 2 Interference frequency band at the receiving end (downlink)
Table 3 Transmitter harmonic interference frequency band (uplink) MHz
Table 4 Transmitter intermodulation interference frequency band (uplink) MHz
Note: (1) In order to more truly reflect the impact of this issue on specific operators, the B1, B3 and B5 frequency bands in Table 2, Table 3 and Table 4 are the LTE frequency ranges of a certain operator, and n78 and n79 are The Ministry of Industry and Information Technology plans the 5G frequency range instead of the original frequency range defined by the original 3GPP. (2) The combination of B5 and n79 has not yet been defined in 3GPP.
It can be seen from the interference analysis in Table 4 that the main interferences involved include: second harmonic interference (B3 uplink to B42 downlink); third-order intermodulation interference (B3 and n78 uplink to B3 downlink, B5 and n78 uplink to B5 downlink) , B3 and n79 uplink to B3 downlink, B5 and n79 uplink to B5 downlink), fourth-order intermodulation interference (B3 and n78 uplink to B3 downlink).
3 5G terminal dual connection self-interference solutionThe current industry solutions to the problem of harmonic interference include improving the performance indicators of radio frequency front-end devices, adding interference cancellation circuits, uplink and downlink frequency division scheduling, and uplink and downlink time division scheduling. For the problem of intermodulation interference, it is discussed to use uplink and downlink frequency division scheduling or uplink and downlink time division scheduling schemes to solve. However, the seriousness of the problem and the feasibility and effectiveness of existing solutions need to be further studied and verified.
3.1 Improving the performance of RF devices(1) Reduce PA nonlinearity
The fundamental cause of terminal harmonic interference is the non-linearity of the device (as shown in Figure 3). Therefore, improving the performance of the device is the most fundamental solution to reduce terminal harmonic interference. By studying the relationship between device nonlinearity and related performance indicators, optimize related performance indicators to reduce device nonlinearity. At present, 3GPP RAN4 is discussing whether the performance indicators of 5G terminals can be further optimized compared with LTE when the above-mentioned harmonic interference problems exist.
(2) Add filter after PA
Add a harmonic filter at the PA output to suppress harmonics. This method is simple to implement and has a low cost, but by adding a filter, only part of the harmonic interference signal output by the transmitting antenna can be eliminated, and the harmonic signal of the PA output PCB cannot be completely suppressed. Therefore, consider this method and other methods Collaborative and comprehensive use.
3.2 Add interference cancellation circuitRefer to the full-duplex self-interference cancellation method [9-10], such as the analog domain cancellation method and the digital domain cancellation method. Analog circuit domain self-interference cancellation rebuilds the self-interference signal through analog circuit design and directly subtracts the reconstructed self-interference signal from the received signal. The digital domain self-interference cancellation method mainly relies on parameter estimation and reconstruction of the self-interference, from the received signal Subtract the reconstructed self-interference to eliminate the residual self-interference.
3.3 Frequency division schedulingDetermine the frequency resource allocated in the downlink according to the uplink allocation result. For example, do not use the frequency spectrum corresponding to the harmonic main lobe or intermodulation signal, reduce the use frequency of the frequency spectrum corresponding to the harmonic side lobe, and use the frequency spectrum corresponding to the non-harmonic or intermodulation signal normally. This method has requirements for network transformation, and may reduce the peak rate of the network due to avoiding interference spectrum.
3.4 Time division schedulingTime division scheduling is performed according to the ratio of uplink and downlink time slots, the network gives the ratio of uplink and downlink time slots, and the terminal reasonably controls the transmission and reception according to the ratio. For example: for harmonic interference, when LTE transmits, the NR side suspends reception; for intermodulation interference, when LTE and NR are dual transmitted at the same time, the LTE side suspends reception, or does not perform simultaneous dual transmission, and only performs single frequency single transmission in the uplink. Send, that is, only LTE transmission or NR transmission is selected at the same time.
4 Operators' strategic recommendationsAlthough the Ministry of Industry and Information Technology has divided the frequency range of 5G in the mid-band, it has not clarified the frequency range of commercial use approved by various operators. According to theoretical calculations, the second harmonics corresponding to the frequency bands used by some operators' original LTE networks (such as the B3 frequency band of LTE FDD of China Telecom and China Unicom) fall within the currently approved 3.4 GHz-3.6 GHz frequency band. Based on the current frequency band status, operators can choose to adopt the following countermeasures for 5G network deployment: (1) The network directly adopts the SA architecture for deployment, and the terminal adopts a single radio frequency to connect to the LTE network or 5G network at the same time, thereby avoiding dual connection bands. (2) Two-handed preparation, that is, to avoid being allocated to the 5G frequency band that is likely to cause harmonic or intermodulation interference, and on the other hand to fully test and verify the existing interference cancellation schemes, and balance different schemes at the cost Relations with performance, and standardize feasible and effective solutions as soon as possible.
5 concluding remarksThe article discusses the possible self-interference problems of 5G terminals in the new 5G frequency band and LTE frequency band due to the introduction of dual-connection technology, and analyzes the cause of the problem and the mainstream solutions in the industry.
If the operator’s 5G network is deployed in a non-independent (NSA) architecture, the terminal is required to support dual-connection technology for simultaneous transmission and reception of LTE and 5G. Depending on the combination of LTE and 5G frequency bands, the uplink dual transmission may cause intermodulation interference on the downlink reception. The non-linearity of the front-end device may cause harmonic interference to the downlink reception, and ultimately cause the sensitivity of the terminal receiving end to decrease. At present, the industry’s ideas for solving this problem include device performance optimization, radio frequency index improvement, frequency division scheduling, uplink and downlink time division avoidance, etc. However, the feasibility and effectiveness of these solutions have not yet reached a consensus in the industry, and further research is needed. And verification.
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