# 关于4GLTE的一些学习笔记
The purpose of modulation is to increase the rate and to raise the signal to the radio frequency so that it can transmit information quickly, but the higher the modulation performance, the higher the signal quality required, which is closely related to the wireless environment. Among them, adaptive modulation and coding (AMC) is based on the information feedback of channel quality to independently select the most suitable modulation method to adjust the size and data rate of the data volume. For a good channel, reduce redundant coding, for a poor channel quality, you need to increase redundant coding to reduce interference;
At present, the most popular antenna technology is MIMO. It has two working modes: multiplexing and diversity. Multiplexing means sending different data on different antennas, which can increase capacity. Diversity mode means that multiple antennas send the same data. Can increase user rate under weak environmental conditions;
Orthogonal frequency division multiplexing technology can only be used when the bandwidth is relatively large. It uses FFT technology. By designing multiple orthogonal subcarriers within a bandwidth range, it can significantly improve the spectrum utilization rate and can counter frequency selection. Sexual decline. But it is more sensitive to frequency deviation, and the peak-to-average ratio is relatively high;
Scheduling means that the base station specifies the sending rate of users. Some commonly used scheduling algorithms include polling algorithm, maximum carrier-to-interference ratio algorithm, proportional fairness algorithm, etc.;
It is worth noting that the interference coordination of the cell uses the secondary frequency at the center and the main frequency at the edge. Inter-cell interference coordination methods include reducing neighboring cell interference, improving cell edge user data throughput, and improving edge user experience. The reduction of interference comes at the expense of capacity. Traditional ICIC and dynamic and static two kinds, adaptive ICIC, is to judge the channel environment to adjust ICIC through MR measurement, it is necessary to turn on ICIC only in high load scenarios.
The self-organizing network is a smarter and more automated network. It includes the base station's self-start, automatic neighbor relationship (ANR), automatic handover optimization (MRO), minimized drive test (MDT) and many other aspects.
Regarding the frame structure of LTE, it is 10ms long and contains 20 time slots. Each time slot is 0.5ms long. Two adjacent time slots form a frame as the scheduling period of LTE. TD-LTE includes three special subframes (downlink pilot time slot (DwPTS), uplink pilot time slot (UpPTS), guard period (GP)) The uplink and downlink subframe configuration is currently 1:3, and the current special subframe The time slot configuration is 10:2:2, and the L-band is 9:3:2. A slot contains 6-7 symbols. The resource unit of LTE is a resource block (RB), its bandwidth is 12KHz, and every 12 subcarriers constitute an RB. There are smaller resource particles (RE), which is mainly proposed to solve the problem of sometimes using too much RB.
The technical principle of TD-LTE has three layers in the protocol stack. Users transmit service data through the user plane and RRC signaling through the control plane.
The physical broadcast channel is a downlink channel, which is mainly used to carry the main information block (MIB). The information block contains three important information: bandwidth, frame number, and PHICH configuration. Each frame has a PBCH, on the first four symbols of time slot 1. But its scheduling period is 40ms, the next three frames are the same as the PBCH on the first frame.
In the random access process, the user sends a random access sequence to the base station (via the PRACH channel), the base station issues a random access response (via the PDSCH channel), and then the user sends high-level information to the base station (via the PUSCH channel). The random access sequence includes the preamble (generated by the cell root sequence), preamble signature, and guard time interval (to ensure that the user sends the preamble sequence to the base station within the corresponding configured base station time window). PRACH has five formats, among which the time window length is a special subframe dedicated to TDD. Generally, the 0 format is sufficient, and the radius of the coverage cell is about 15KM.
The cell search process is PLMN selection request->cell search->system message acceptance->cell selection and camping->PLMN selection response->random access->attach. PSS monitoring (observe whether the time slot is synchronized and obtain the id of the physical layer) and SSS monitoring (monitor frame synchronization, cell IP, TDD/FDD, CP) when the mobile phone is turned on during cell search.
RRC establishment includes control plane connection establishment, public process and user plane connection establishment. After the RRC is established, the base station activates the NAS, and the user can transmit the initial message to the MME.
LTE network architecture and interfaces play an important role in understanding other knowledge. Its schematic diagram is as follows.
There is a tracking area (TA) concept in LTE networks. It is to confirm the location of the mobile station, which is identified by the TA code TAC. A TA can include one or more cells.
Multiple TAs constitute a TA List.
TAI=MCC+MNC+TAC It is a unique identifier, a total of 6 bytes. The user informs the user of the TA List when attaching or during TAU, and the user saves it.
Conditions for TAU triggering:
Enter the new TA List; the periodic tracking area update timer expires; the user reselects to the E-UTRAN cell from other cells.
LTE indoor coverage design is a very important technology, because 70% of the business in the 3G era occurs indoors, and 70% of complaints occur indoors. Including coverage target selection and analysis, preliminary site survey, coverage estimation (capacity estimation), detailed system design, system installation, testing and optimization. Among them, the indoor antenna layout principle is to follow "low power, multiple antennas." In this process, power splitters, combiners, couplers, attenuators, trunk amplifiers, loads, and leaky cables are used.
The optimization of RF is mainly the optimization of radio frequency (design to coverage, handover, interference), and its specific process is to prepare for optimization (determine optimization goals, drive tests, divide clusters, determine test routes, tools and data preparation), and optimize data collection , Problem analysis, adjustment and implementation. When there is weak coverage, cross-area coverage, and uplink and downlink imbalance, optimization is needed.
The problem of pilot pollution is to satisfy that the number of cells is greater than 3 or RSRP(first)-RSRP(4)<=6Db. It is caused by cross-region coverage, environmental impact around the coverage area, unreasonable azimuth, and unreasonable cell layout.
Single-site verification refers to the purpose of monitoring and verification of single-site equipment functions and coverage capabilities. Including the preparation phase (tool setting, test route planning), testing and analysis, if there is a problem, an adjustment suggestion will be given based on the SSV problem record, and if the adjustment suggestion is approved. Just make adjustments and test again. In this way, the verification is completed until the SSV report is given at the end.
The handover algorithm is more complicated and very important. It includes two kinds of same frequency handover and different frequency handover. Among them, the same frequency switching is triggered by the A3 event, and the reporting method of event to cycle is adopted. Co-frequency handover includes three steps: user measurement, base station handover decision, and base station execution of handover commands. Inter-frequency switching directly skips the inter-frequency measurement and switches directly. Therefore, in general, inter-frequency handover is not selected, because the signal quality after the handover cannot be determined without measurement is better than that before the handover. Inter-frequency switching is divided into necessary scene switching and non-essential scene switching. The former is based on coverage, uplink quality, and distance, and the latter is based on business, load, frequency priority, speed, and carrier plasticity. If the inter-frequency trigger is based on coverage, then it is triggered by A3/A4/A5, which is also the reporting method of the cycle, and based on others, it is triggered by the A4 event.