At present, the global information age has arrived, the total amount of data has exploded, and people’s demand for data and information is increasing. The birth of LTE is to continuously optimize wireless communication technology to meet customers’ higher requirements for wireless communication.
What is LTE?
LTE is a long-term evolution of the UMTS technical standard formulated by the 3GPP organization, in 2004 The project was formally established and launched at the 3GPP Toronto meeting in December.
LTE is a wireless data communication technology standard. The current goal of LTE is to use new technologies and modulation methods to improve the data transmission capacity and data transmission speed of wireless networks, such as new digital signal processing (DSP) technologies, which were mostly proposed around 2000.
The long-term goal of LTE is to simplify and redesign the network architecture to make it an IP-based network, which will help reduce potential undesirable factors in the 3G transition.
LTE technology mainly has two mainstream modes, TDD and FDD, and the two modes have their own characteristics. Among them, FDD-LTE is widely used internationally, while TD-LTE is more common in my country.
The LTE (Long Term Evolution) project is an evolution of 3G, a transition between 3G and 4G technologies, and a global standard of 3.9G.
It has improved and enhanced the 3G air access technology, using OFDM and MIMO as the only standard for its wireless network evolution. It provides a peak rate of 100 Mbit/s for downlink and 50 Mbit/s for uplink under a 20MHz spectrum bandwidth, which improves the performance of cell-edge users, increases cell capacity and reduces system delay.
In order to better understand LTE, we have listed 41 basic knowledge of LTE for your reference.
Summary of 41 Basic Knowledge of LTE
Basic Knowledge of LTE 1. What kinds of SIB are there? What is the function?
There are 12 types of SIB. SIB1 contains scheduling information and access-related information of other cells.
SIB2 carries all UE radio resource configuration information;
SIB3 carries cell reselection information of the same frequency, different frequencies, and different systems;
SIB4 carries the reselection information of neighboring cells only on the same frequency;
SIB5 carries inter-frequency E-UTRAN network reselection information;
SIB6 carries the reselection information of the different system UTRAN network;
SIB7 carries reselection information of different system GSM networks;
SIB8 carries the reselection information of the different system CDMA2000 network;
The remaining 4 SIBs contain information on the home base station and some auxiliary notification information.
Basic Knowledge of LTE 2. What should I pay attention to for single-site verification and testing?
Access, upload, download, and coverage; check PCI, TAC, check whether adjacent areas of the same site are added; check site installation problems (reverse antenna connection, reverse transmission, blocking, etc.)
LTE test mainly focuses on PCI (cell identification code), RSRP (average power of reference signal, indicating the quality of cell signal coverage.
Function: main cell selection and reselection, power control)
SINR:
Signal to interference plus noise ratio (reflects signal quality, SINR value has a greater impact on throughput.
Function: used for power control, feedback on signal quality, when signal quality is greater than a large threshold, and signal strength is greater than a small threshold Then reduce 1DB power;
When the signal strength is greater than the large threshold, and the signal quality is greater than the small threshold, the power is reduced by 1dB;
When the signal strength is less than the small threshold, or the signal quality is less than the small threshold, add 1, 3DB power)
RSSI (Received Signal Strength Indicator, refers to the total power received by the mobile phone, including useful signal, interference, and noise floor)
PUSCH Power (UE transmit power)
Transmission mode (TM3 is dual-stream mode)
Throughput DL, Throughput UL uplink, and downlink rate, drop rate, connection success rate, handover success rate.
RSRQ (mainly used for handover, reflecting the load of the cell).
Basic Knowledge of LTE 3. How is the usage of LTE frequency resources? What are the applicable scenarios?
LTE international frequency band: 38 (2570-2620MHz) broadband 40M; Applicable scenarios: new outdoor infrastructure
LTE international frequency band: 39 (1880-1920MHz) broadband 20M; Applicable scenario: common-mode upgrade
LTE international frequency band: 40 (2300-2400MHz) broadband 20M; Applicable scenarios: room division
LTE China frequency band: 38 (2570-2620MHz) broadband 40M; Applicable scenarios: new outdoor infrastructure
LTE China frequency band: 39 (1880-1920MHz) broadband 20M; Applicable scenarios: total MOD upgrade
LTE China frequency band: 40 (2300-2400MHz) broadband 20M; Applicable scenarios: room division
The difference between the F band and D band:
The D frequency band is high and the penetration is poor. The F-band is low and has good coverage, but there is more interference.
The D-band has more space loss than the F-band, and the number of sites using the F-band network in the entire network will be less than that of the D-band.
However, the existing TD-SCDMA and the PHS, which have not completely exited the network, are occupied in the F band, while the D band has a rich and clean 190M bandwidth.
Therefore, from a technical point of view, it is relatively difficult to build a network in the D band, especially since the difficulty of upgrading is greater than that of new construction.
Basic Knowledge of LTE 4. RF optimization operation, what are the reasons that affect the downlink rate?
RF optimization process:
1) Pull the network test, familiar with the network situation.
2) Problem analysis.
3) Propose solutions.
4) Optimization and adjustment.
5) Retest, and produce an optimization summary report.
Optimization adjustment method:
RF adjustment mainly includes antenna feeder adjustment, power adjustment, neighbor cell optimization, and PCI optimization adjustment.
Basic Knowledge of LTE 5. What is the calculation method of the sub-frame ratio and special sub-frame ratio?
There are 9 special sub-frame ratio modes, the commonly used ones are 5 (3:9:2), 6 (9:3:2), 7 (10:2:2), and there are 7 conventional sub-frame ratio modes. Commonly used are 1 (2:2) and 2 (1:3).
The algorithm for the number of uplinks and downlink time domain scheduling: a wireless frame is 10ms, and there are 100 wireless frames in one second.
According to the conversion period of 5ms, the uplink and downlink ratio of regular subframes is 1:3, and special subframes are 3:9:2. Calculate, the number of downlinks full schedules per second=3*100*2=600. The number of uplinks full schedules per second = 1*100*2=200.
According to the 5ms conversion period, the uplink and downlink ratio of regular subframes is 1:3, and the special subframes are 10:2:2. The downlink full scheduling number per second = (3+1)*100*2=800. Uplink full schedule per second = 1*100*2=200
Basic Knowledge of LTE 6. What are the filling operation procedures?
There are currently two filling methods:
1) The server fills the packet, the purpose is to detect whether the transmission is faulty.
2) Pouring packets on the base station side to detect the quality of the air interface.
Basic Knowledge of LTE 7. If the SINR is good and the uplink rate is poor, how to judge that it is caused by interference?
Uplink interference is currently located in the background through interference monitoring in signaling tracking management.
Basic Knowledge of LTE 8. What are the special time slot functions?
DwPTS:
12 symbols at most and 3 symbols at least, which can be used to transmit downlink data and signaling;
UpPTS:
No control signaling or data is sent on UpPTS. The length of the UpPTS is 2 or 1 symbol. When 2 symbols are used for short RACH or Sounding RS when 1 symbol is only used for sounding;
GP:
Ensure that the uplink signals of UEs with different antenna distances are aligned at the antenna air interface of the eNB; provide uplink and downlink conversion time (the eNB’s uplink to downlink conversion actually has a small conversion time Tud, less than 20us);
The GP size determines the size of the supported cell radius. LTE TDD can support up to 100km; avoiding uplink and downlink interference between adjacent base stations.
Basic Knowledge of LTE 9. What are the problems encountered in cluster optimization?
Do cluster optimization after a single test, and do grid optimization after cluster optimization.
Problems encountered in cluster optimization:
Weak coverage, overlapping coverage, no primary serving cell, cross-area coverage, neighboring cell missing allocation, ping-pong handover, PCI mode 3 interference, signal quality is very good, but the download rate is low, and it cannot be fully scheduled.
Basic Knowledge of LTE 10. In the process of single inspection, what are the types of antenna feeder connections? Is the TDS reversed when the engineering side is connected?
What are the types of reverse connections between the antenna and feeder?
1) Reverse connection clockwise
2) Reverse connection counterclockwise
3) Reverse connection between two sectors
Reasons for reverse connection:
1) Project construction problems,
2) RU data configuration error. Because TDS and TDL share optical fiber, when the engineer is connected reversely, both TDS and TDL are connected reversely.
Basic Knowledge of LTE 11. What are the planning principles of PCI?
1) Principle of non-conflict: The PCI between adjacent cells of the same frequency cannot be the same; the remainder after PCI modulo 3 of two adjacent cells of the same frequency is different;
2) The principle of no confusion: all neighboring cells of the same cell cannot have the same PCI;
3) Principle of optimization: Ensure sufficient reuse distance. PCI multiplexing is separated by at least 4 layers of cells, which is greater than 5 times the cell coverage radius.
4) To avoid PCI conflicts caused by future network expansion, physical cell identification resources should be properly reserved. PCI range: 0～503, reusable. The 504 PCIs are divided into 168 PCI groups. Each group is assigned to an eNB, and each group contains 3 unique IDs.
Huawei’s neighboring area planning software CAN; PCI planning software SCP.
Basic Knowledge of LTE 12. What are the uplink and downlink channels?
Physical channel:
Corresponding to a collection of a series of REs, which need to carry information from higher layers are called physical channels; such as PDCCH, PDSCH, etc.
Physical signal:
Corresponds to a series of REs used by the physical layer, but these REs do not convey any information from the higher layers, such as reference signals (RS), and synchronization signals.
Downlink physical channel:
PDSCH: Physical Downlink Shared Channel. It is mainly used to transmit service data, and can also transmit signaling. Scheduling between UEs through frequency division,
PDCCH: Physical Downlink Control Channel. Carrying resource allocation information for paging and user data, and HARQ information related to user data.
PBCH: Physical Broadcast Channel. Carrying system information such as cell ID is used in the cell search process.
PHICH: Physical Hybrid ARQ Indicator Channel, used to carry HARP ACK/NACK feedback.
PCFICH: Physical Control Format Indicator Channel, used to carry the position information of the OFDM symbol where the control information is located.
PMCH: Physical Multicast channel, used to carry multicast information and downlink physical signals:
RS (Reference Signal): Reference signal, usually also called pilot signal;
SCH (PSCH, SSCH): synchronization signal, divided into primary synchronization signal and secondary synchronization signal; uplink physical channel:
PRACH: Physical Random Access Channel, carrying random access preamble
PUSCH: Physical Uplink Shared Channel, which carries uplink user data.
PUCCH: Physical Uplink Control Channel, carrying HARQ ACK/NACK, scheduling requests, channel quality indicator, and other information.
Uplink physical signal:
RS: Reference signal.
Basic Knowledge of LTE 13. What are the key technologies of LTE?
1) Adopt OFDM technology; Orthogonal frequency division multiple access technologies. Each sub-carrier is orthogonal to each others without interference, so the spectrum of each sub-carrier can overlap according to a certain rule.
That is, the spectrum efficiency is improved, and a guard interval is added between each symbol, which can better overcome ISI and ICI interference.
2) Adopt MIMO (Multiple-Input Multiple-Output) technology;
Multiple input multiple output antenna technologies. Multiple inputs refer to the input of the base station antenna, and multiple outputs refer to the output of the mobile phone antenna. Increase the channel capacity by increasing the transceiver antenna channel.
MIMO has 2 modes. The first is space-division multiplexing. The two antennas receive different data streams, thereby improving the throughput of the transceiver. The second is to transmit diversity.
The two antennas receive the same data stream and then use the maximum ratio. Consolidation of data improves the reliability of data.
3) Scheduling and link adaptation (AMC);
4) HARQ
HARQ is a fast hybrid retransmission technology. It is mainly implemented in the MAC layer and requires the eNB to quickly schedule data. When the feedback information from the mobile phone is not received, the eNB retransmits quickly to improve transmission efficiency.
5) High-order modulation
Only in the downlink is there a 64QAM modulation method, and in the uplink, the highest is only a 16QAM modulation method, because the current mobile phone does not support 64QAM modulation, and only CLASS5 supports 64QAM.
Compared with the modulation method of 16QAM, 64QAM increases the modulation rate by 1.5 times.
6) Multi-antenna technology
Mainly used to improve channel capacity.
Basic Knowledge of LTE 14. What are the network elements of LTE and what are the interfaces between them?
1) E-UTRAN, the access network of LTE, is composed of eNodeB, providing user plane and control plane;
2) LTE core network EPC (Evolved Packet Core) is composed of MME, HSS, PCRF, S-GW, and P-GW;
3) eNodeBs are connected to each other through the X2 interface to support the direct transmission of data and signaling;
4) The S1 interface connects the eNodeB and the core network EPC. Among them, S1-MME is the control plane interface for eNodeB to connect to MME, and S1-U is the user plane interface for eNodeB to connect to S-GW
Basic Knowledge of LTE 15. What is the role of PGW and SGW?
(1) MME is a signaling entity, which is mainly responsible for functions such as mobility management, bearer management, user authentication, SGW and PGW selection;
(2) The interface between S-GW termination and E-UTRAN is mainly responsible for user plane processing, data packet routing, and forwarding, and other functions, supporting the switch of different 3GPP access technologies and acting as the anchor point of the user plane when the switch occurs;
(3) The SGi interface between P-GW termination and external data networks (such as the Internet, IMS, etc.) is the EPS anchor point, that is, the anchor point of the user plane data link between 3GPP and non-3GPP networks, and is responsible for managing 3GPP and Data routing between non-3GPP, manages the movement between 3GPP access and non-3GPP access (such as WLAN, WiMAX, etc.), and is also responsible for functions such as DHCP, policy enforcement, and billing.
Basic Knowledge of LTE 16. What is the most impressive case at work?
When the wireless environment is very good, the scheduling number is also full, and the uplink and downlink rates are very low, which is caused by uplink interference or transmission problems through background interference monitoring or packet filling.
Basic Knowledge of LTE 17. The concept of RB and RE.
1) RB
There are 12 consecutive subcarriers in frequency, and one slot in the time domain is called 1 RB. According to a subcarrier bandwidth of 15kHz, it can be concluded that the bandwidth of 1 RB is 180kHz.
2) RE
A symbol on a subcarrier in frequency and in a domain is called an RE. REG=4RE, CCE=9REG, 1RB=84RE.
Basic Knowledge of LTE 18. What is the relationship between PA and PB?
The larger the value of PB, the higher the reference signal PWR rises on the original basis, which can obtain better channel estimation performance, enhance the demodulation performance of PDSCH, and reduce the transmission power of PDSCH (Type B), which can improve the edge User rate.
When the RS power is constant, increase the PA, increase the power of all users in the cell and increase the MCS of all users in the cell, but it will cause power limitation and affect the throughput rate; conversely, reduce the power and MCS of all users in the cell and reduce the cell throughput rate.
Basic Knowledge of LTE 19. What are the switching types of LTE?
1) According to the reason of switching trigger, the LTE switching types can be divided into:
Coverage-based switching, load-based switching, Service-based switching;
2) According to different cell frequency points and cell system attributes between switches, it can be divided into:
Same frequency switching, different frequency switching, different system switching;
3) Switching in eNb station, switching with X2 port, and switching with S1 port.
Basic Knowledge of LTE 20. How does LTE switching events? How to solve the ping-pong switching?
There are 7 kinds of switching events in LTE, which are divided into A1-A5 and B1-B2. The switching event triggers the measurement report, not the handover.
1) A1, the serving cell is better than the absolute threshold; this event can be used to turn off some inter-cell measurements.
2) A2, the serving cell is worse than the absolute threshold; this event can be used to start some inter-cell measurement, because operations such as handover may occur after this event occurs.
3) A3, the neighbor cell is better than the serving cell; the occurrence of this event can be used to determine whether the UE will switch to the neighbor cell.
4) A4, the neighboring cell is better than the absolute threshold;
5) A5, the serving cell is worse than an absolute threshold and the neighbor cell are better than an absolute threshold; this event can also be used to support handover.
At present, it is mainly based on the handover of A3 and A5. When the signal strength of the neighboring cell is higher than the signal strength of the serving cell by a certain value (default 3DB), and the above requirements are still met for a period of time (timer timeout), the mobile phone will Trigger the measurement report and send it periodically until the base station makes a decision to remember;
A5 When the signal strength of the serving cell is lower than a certain handover threshold, the signal strength of neighboring cells is higher than a certain handover threshold, and the above requirements are still met for a period of time (timer timeout), the measurement report is triggered and sent periodically until the base station makes a handover decision.
The two timers in A3 and A5 are set in order to avoid ping-pong switching. The longer the setting time, the less likely it is to cause ping-pong switching, but it may lead to untimely switching. The shorter the set time, the easier it is to cause ping-pong switching, but the switching is more timely.
Basic Knowledge of LTE 21. How many dB is the difference in received power between LTE and TD-S?
The coverage difference is about 15dB, and the LTE received power is the power of the RS and the power of the RE. TDS is to calculate the power of the code channel, and the algorithm is different.
The high-frequency band has poor diffraction and reflection capabilities. In actual propagation, high-frequency signals are absorbed more by the ground and buildings, and the attenuation is large, so the propagation is not far.
The diffraction ability is inversely proportional to the size of the frequency band. The higher the frequency band, the shorter the wavelength, the weaker the diffraction ability, and the stronger the penetration ability; on the contrary, the lower the frequency band, the longer the wavelength, the stronger the diffraction ability, and the weaker the penetration ability.
Basic Knowledge of LTE 22. What is the search process in the cell?
Firstly, UE demodulates the primary synchronization signal (PSS) to achieve symbol synchronization and obtain the ID of the cell group.
Secondly, UE demodulates the secondary synchronization signal (SSS) to achieve symbol synchronization and obtains the cell group ID; (P-SCH occupies the last Symbol of the first slot of subframes 0 and 5 in the time domain, S-SCH Occupies the penultimate Symbol of the first slot of subframes 0 and 5.)
The initial cell search process is as follows:
1) After the UE is powered on, it starts to initialize the cell search and searches the network. Generally speaking, the UE does not know the bandwidth and frequency of the network when it is turned on for the first time.
2) The UE will repeat the basic cell search process, traversing each frequency point of the entire frequency band to try to demodulate the synchronization signal. (This process is relatively time-consuming, and some methods can be used to shorten the UE initialization time in the future. For example, the UE stores the previously available network information and searches for these networks first after booting).
3) Once the UE has searched for available networks and achieved time-frequency synchronization with the network, and obtained the serving cell ID, the cell search is completed. The UE will demodulate the downlink broadcast channel PBCH to obtain system bandwidth, the number of transmitting antennas, and other information.
After completing the above process, the UE demodulates the downlink control channel PDCCH to obtain the paging cycle assigned to the UE by the network. Then wake up from the IDLE state in a fixed paging cycle to demodulate the PDCCH and monitor the paging. If there is a page belonging to the UE, demodulate the designated downlink shared channel PDSCH resource and receive the page.
Basic Knowledge of LTE 23. What is the access process based on competition and non-competition?
Random access process based on contention: random access preamble, random access preamble response, allocation transmission, contention resolution.
Step 1: Send the preamble of random access on the uplink RACH.
Step 2: Send a random access indication on the DL_SCH channel.
Step 3: Send a random access request on the UL_SCH channel.
Step 4: Send random access response on DL_SCH channel
Random access process based on non-competition:
Allocate preamble, random access preamble, and random access response.
Step 1: Allocate the preamble for random access in the downlink dedicated signaling.
Step 2: Send the preamble for random access on the uplink RACH.
Step 3: Receive a random access response message on the DL_SCH channel.
Basic Knowledge of LTE 24. How to calculate the rate?
100 (number of RBs under 20M bandwidth) × 12 (each RB has 12 sub-carriers) × 14 (OFDM symbol) × 6 (each sub-carrier carries 6BIT information) × 1000 (converted into seconds) ÷ 1000 (converted into K) ÷ 1000 (converted to M) × 2 (MIMO2) × 75% (excluding 25% overhead) = 151.2 (downlink peak value, premise TDD, conventional CP, 64QAM)
Basic Knowledge of LTE 25. What are the main factors affecting LTE single-user downlink and uplink throughput rates?
1) Antenna transceiver mode, MIMO antenna number, and mode, beamformed antenna array gain (including antenna number);
2) The quality of the spatial channel, including signal strength, interference conditions, spatial channel correlation, UE moving speed, and UE receiver performance;
3) TDD is also related to the ratio of uplink and downlink subframes, and the channel configuration in FDDTDD is related (for example, how much CFI is, whether there is MBMS support);
4) It is also related to the number of users;
5) PS data transmission performance influencing factors; terminal: mobile phone capabilities (class1-5), terminal software configuration;
6) Air interface: RSRP/SINR is relatively low, coding method, rank value, air interface resources (number of RBs), air interface delay, scheduling frequency; ENB: base station hardware failure, base station processing capacity.
Basic Knowledge of LTE 26. Reasons and solutions that affect the downlink rate.
1) Weak coverage can be solved by adjusting the antenna feeder system, power adjustment, and new stations.
2) The signal quality is poor and the SINR is low. It can be adjusted through the antenna feeder system, power adjustment, neighbor cell optimization, and parameter optimization.
3) The signal quality is very good but the number of dispatches is not satisfied. It may be caused by multiple users, equipment failures, transmission failures, and air interface quality. Background coordination is required for positioning. Currently, positioning is mainly done through packet filling.
4) Hardware alarm, submit the project for resolution.
5) Transmission failure, submit the project for solution.
6) Test equipment and software problems, and solve them by restarting the equipment and software, or replacing the equipment.
7) The uplink and downlink are unbalanced. I haven’t encountered it for the time being. You can mention the traffic tracking.
Basic Knowledge of LTE 27. What is the interference of MOD6? What is the interference of MOD3?
As long as the MOD6 interference is the interference of the downlink reference signal because the reference signal is fixed on the 0th and 4th symbols in the time domain in one RB, it is not fixed in the frequency domain and is sent on every 6 subcarriers. The transmission on which sub-carrier depends on the value of PCI mod6.
If the value of PCI mod 6 is 0, it is sent on the 0th and 6th subcarriers on the 0th symbol and on the 3rd and 9th subcarriers on the 4th symbol.
If the value of PCI mod 6 is 1, then it is sent on the 1st and 7th subcarriers on the 0th symbol and on the 4th and 10th subcarriers on the 4th symbol, and so on.
In this way, it can be known that the reference signals sent by different subcarriers are for different PCIs. If the adjacent cells have the same PCImod6, the reference signal will be sent on the same subcarrier so that the reference signal will have interference.
MOD3 is divisible by 3 and the remainder. The PCI of the neighboring cell requires a different mod3 value because the base station sends a downlink synchronization signal to the mobile phone. There are three main synchronization signals (0, 1, 2) specified in the 3GPP specification, in which synchronization is used. The signal is determined by the value of PCI mod 3. When the value of PCI mod 3 is 0, the 0th synchronization signal is used, and so on.
Different synchronization signals are orthogonal to each other, and there is no interference between each other.
The mobile phone can distinguish the cells based on the synchronization signal. If the adjacent cells have the same PCI mod 3, they will use the same synchronization signal, and the synchronization signals will interfere with each other, resulting in a decrease in the SINR value.
Basic Knowledge of LTE 28. What are the shortcomings of OFDM and MIMO?
The main disadvantages of OFDM are high-frequency synchronization requirements and a high peak-to-average ratio. The main shortcomings of MIMO are as follows: it has high requirements for SINR, which is suitable for the vicinity of the base station, but not for the edge of the cell.
Basic Knowledge of LTE 29. What is ICIC? What is the ICIC principle? What is the effect?
ICIC (Inter-Cell Interference Coordination) inter-cell interference coordination, TD-LTE adopts co-frequency networking, which is easy to introduce co-frequency interference, especially for edge users.
Adjacent cells are divided by frequency bands to stagger the resources of their respective edge users to achieve the goal of reducing co-frequency interference. Traditional ICIC method: Generally, it is a static ICIC solution. The edge frequency points are manually divided, but the allocation is fixed and the spectrum utilization rate is low.
Huawei adopts an adaptive ICIC solution: Adaptive ICIC is automatically controlled by OSS, which can increase cell edge throughput by 40%.
- ) Self-adaptive ICIC centrally manages and formulates the entire network cell edge mode through M2000, with high reliability and little human intervention;
- ) Effectively improve the effect of static ICIC on frequency utilization in scenarios where network traffic is unevenly distributed;
- ) It is possible to correct the slow convergence of dynamic ICIC interference to the entire network.
Basic Knowledge of LTE 30. How much is 64QAM better than 16QAM?
A symbol of 16QAM can carry 4 bits of information, and a symbol of 64QAM can carry 6 bits of information. Its efficiency is increased by 1.5 times.
Basic Knowledge of LTE 31. What is the plan of PRACH?
PRACH main planning parameters include prachconfindex, PRACH configuration index number, which defines the PRACH type, transmission period, version number, Rootsegindex, prachCS, and the planning is mainly based on the cell radius.
Basic Knowledge of LTE 32. What is the transmission mode of PRACH?
TM1:
Single antenna port, information is sent through a single antenna;
TM2:
In transmit diversity, two antennas transmit the same amount of data, and the receiving end combines information through the maximum ratio, which reduces the bit error rate and improves the reliability of transmission;
TM3:
Open-loop space-division multiplexing, the terminal does not feedback information, and the transmitter sends information through predefined channel information;
TM4:
Closed-loop space-division multiplexing, the terminal feeds back information, and the transmitter uses the feedback information to calculate the modulation method to send;
TM5:
The multi-user MIMO base station uses the same frequency domain resources to send multiple data streams to different users, and the receiving end cancels and nulls the data streams according to multiple antennas;
TM6:
Single-layer closed-loop space-division multiplexing.
When the terminal feeds back RI=1, the transmitter uses single-layer precoding to adapt to the current channel;
TM7:
Single-stream beamforming has 8 antenna elements.
The transmitter uses the uplink signal to estimate the characteristics of the downlink channel. When the signal is sent in the downlink, each antenna is multiplied by the corresponding characteristic weight to make the transmitted signal have beamforming characteristics;
TM8:
Double-stream beamforming.
Basic Knowledge of LTE 33. What does CQI mean? What are its functions?
CQI is a channel quality indicator, which reflects the wireless link quality.
The receiving end adjusts the coding mode through the received CQI indication information.
Basic Knowledge of LTE 34. What is TA? What are the planning principles?
TA is the tracking area, which is the same as the LA in the 2G network.
First, the TA cannot be too large or too small, because the TA is the area for paging and location update. If the TA is too large, the larger the paging information sent by the eNB, the larger the resource occupied by the downlink channel.
TA is too small, the more frequent the location update, the more signaling of control messages, occupying system overhead.
Second, the TA boundary cannot cross the MME.
Third, try not to be in places with high business volume.
Fourth, make reasonable plans based on rivers, traffic arteries, and mountain topography.
Basic Knowledge of LTE 35. Why track RSSI when doing uplink services?
RSSI tracking is the detection of the received power of the base station. When there is no service or downlink service, the uplink transmission power is low, and the difference may not be observed, which will cover some problems. It is used as an uplink service. The terminal transmission power can be increased, and then observe the RSSI change accurately.
Observe whether the received power on different antenna ports is too different. If the difference is more than 4-5dB in many cases, it can be basically confirmed that there is an imbalance in the room subsystem. (Dual-flow compartment, single-flow compartment does not have this problem.)
Basic Knowledge of LTE 36. What is the implementation process of MCS scheduling?
The UE measures the SINR and reports the RI and CQI index to the eNodeB. The eNodeB performs TM and MCS scheduling according to the RI and CQI index fed back by the UE; MCS is generally determined by CQI, IBLER, PC+ICIC, etc.
The downlink UE maps to the CQI according to the measured CRS SINR and reports it to the eNB. The uplink eNB obtains the uplink CQI through DMRS or SRS measurement.
For the CQI (full-band or sub-band) or uplink CQI reported by the UE, the eNB first adjusts the CQI according to PC constraints, ICIC constraints, and IBLER conditions, and then maps the 4-bits CQI to 5-bits MCS.
The 5bits MCS is delivered to the UE through the PDCCH. The UE can look up the table to obtain the modulation mode and TBS according to the MCS, perform downlink demodulation or uplink modulation, and the eNB performs downlink modulation and uplink demodulation according to the MCS.
Basic Knowledge of LTE 37. How to calculate power?
In the case of 20Mhz bandwidth and 2*20w antenna configuration, the default downlink power configuration is: PA=-3, PB=1, RS=15dbm; the calculation method is as follows: PDSCH without RS EPRE=10lg[20*1000/(12 *100)]=12dBm; PDSCH with RS EPRE=10lg[20*1000/*(12*100)]=12dBm, then RS EPRE=(total power-PDSCH power)/2=15dBm.
For example, for a single carrier 20M bandwidth configuration, there are a total of 1200 sub-carriers, and RSRP power = RU total output power -10log1200.
If it is a single-port 20W RU, the RSRP power can be calculated to be 43-10log1200=12.2dBm;
ρA represents the ratio of the power of the data subcarrier to the power of the pilot subcarrier of the OFDM symbol (type A symbol) without the pilot.
ρB represents the ratio of the data sub-carrier power of the pilot OFDM symbol (type B symbol) to the pilot sub-carrier power.
Basic Knowledge of LTE 38. What is the role of TAU?
Enter a new TA, its TAI is not in the TAI LIST stored by the UE; assign a new GUTI to the user;
Change the status of UE and MME from EMM-DEREGISTERED to EMM-REGISTERED;
IDLE users can request the establishment of user plane resources through the TAU process.
Basic Knowledge of LTE 39. How is RRC established?
RRC connection request:
The UE sends it on SRB0 through UL_CCCH, carrying the initial (NAS) identification and establishment reason of the UE, etc. This message corresponds to Msg3 of the random access procedure;
RRC connection establishment:
The eNB sends on SRB0 through DL_CCCH, carrying the complete configuration information of SRB1, and this message corresponds to Msg4 of the random access process;
The RRC connection is established:
The UE sends on SRB1 through UL-DCCH and carries uplink NAS messages, such as Attach Request, TAU Request, Service Request, Detach Request, etc., and the eNB performs S1 port establishment according to these messages.
Basic Knowledge of LTE 40. How is ERAB established?
The P-GW initiates a bearer modification request, and the S-GW sends it to the MME; the MME sends an E-RAB modification request message to the eNB to modify one or more bearers. The E-RAB modification list information includes the QoS of each bearer;
After receiving the E-RAB modification request message, the eNB modifies the data radio bearer;
The eNB returns an E-RAB modification response message, the E-RAB modification list information contains the successfully modified bearer information, and the E-RAB modification failure list message contains the unsuccessful modification bearer message.
Basic Knowledge of LTE 41. How does the LTE page?
The MME sends a paging message to the eNB, and each paging message carries information about a paged UE;
The eNB reads the TA list in the Paging message, and performs air interface paging on the cells belonging to the list under it;
If the UE has previously informed the MME of the DRX message through the NAS, the MME will inform the eNB of the information through the paging message.
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