Acquisition and cell search:
Before the LTE terminal and the LTE network can communicate, it needs to perform:
find and obtain synchronization with a cell in the network;
need to receive and decode information, also known as cell system information, in order to communicate and operate normally in the cell;
Overview of LTE cell search:
The terminal not only needs to perform cell search when powering on and initially accessing the system, but also needs to continuously search, synchronize and estimate the reception quality of neighboring cells to support mobility;
The steps of LTE cell search:
1. Obtain frequency synchronization and symbol synchronization with a cell;
2. Obtain the frame timing of the cell and determine the start point of the downlink frame;
3. Determine the physical layer cell identity of the cell.
In order to assist cell search, two special signals are transmitted on each downlink component carrier: the primary synchronization signal PSS and the secondary synchronization signal SSS;
In the case of FDD, PSS is sent in the last symbol of the first time slot of subframes 0 and 5, while SSS is sent in the penultimate symbol of the simultaneous slot;
In the case of TDD, PSS is sent in the third symbol of subframes 1 and 6, while SSS is sent in the last symbol of subframes 0 and 5.
Once the terminal detects and recognizes the PSS of the cell, it can be known that
the 5ms timing of the cell and the location of the SSS are known from this, and it has a fixed position offset from the PSS; the
cell identity in the cell identity group, but the terminal cannot do it by itself The cell identification group is detected, and only the possibility of cell identification is reduced from 504 to 168.
Once the terminal detects the PSS, it can know the location of the SSS, which can be seen:
frame timing; cell identification group (168).
Once the terminal has captured the frame timing and physical layer cell identity, it can identify the cell-specific reference signal;
If it is the initial cell search, the terminal is in RRC_IDLE mode, the reference signal is used for channel estimation, and the subsequent BCH transmission channel decoding obtains the most basic set of system information;
In the case of mobility measurement, the terminal is in RRC_CONNECTED mode, and the terminal measures the received power of the reference signal; if the measurement meets a configurable condition, it will trigger to send a measurement report of the received power of the reference signal to the network;
PSS structure:
The 3 PSSs are 3 Zadoff-Chu sequences with a length of 63 bits extended by 5 0s on each side and mapped to 73 subcarriers; the center subcarrier is not actually transmitted because it happens to encounter a DC subcarrier;
Therefore, only 62 elements in the 63-bit long ZC sequence are actually transmitted;
Therefore, PSS occupies 72 resource elements (not including the DC carrier) in subframes 0 and 5 (TDD) and subframes 1 and 6 (FDD), then these resource elements cannot be used for DL-SCH transmission;
SSS structure:
SSS occupies 72 resource elements in the center of subframes 0 and 5 (TDD and FDD) (not including DC carrier);
SSS design helps:
Two SSSs (SSS1 in subframe 0 and SSS2 in subframe 5) can each correspond to 168 different cell identity groups and take 168 different values;
the value set applied to SSS2 should be the same as that applied to SSS1 The value set of is different so that the frame timing can be detected from the reception of a single SSS.
SSS1 is based on the frequency interleaving of two 31-bit m-sequences X and Y. Each of X and Y can take 31 different values;
The set of effective combinations of X and Y for SSS1 (SSS2) is 168, allowing detection of physical layer cell IDs;
Since X and Y between SSS1 and SSS2 are exchanged, frame timing can be obtained;
system message:
Two different mechanisms for sending system information:
1. A limited amount of system information, corresponding to the so-called master information block MIB, using BCH transmission;
2. The main part of the system information, corresponding to the so-called system information block SIB, in the information sharing channel DL -Transmission on SCH.
MIB and BCH transmission:
The MIB includes:
information about the bottom width of the downlink cell; information about
the PHICH configuration of the cell; and the
system frame number SFN. There are 10 unused information bits.
MIB transmission time interval TTI is 40ms;
Unlike other downlink transmission channels that use 24-bit CRC check, BCH uses 16-bit CRC check to reduce CRC-related overhead; BCH coding is based on the same 1/3 rate tail-biting convolutional code as the PDCCH control channel, and does not use Turbo coding It is because the BCH transmission block is small;
After channel coding is rate matching, there are coded bit repetition and bit-level scrambling, and then QPSK modulation is performed on the coded and scrambled BCH transport block;
BCH multi-antenna transmission is limited to transmit diversity. Two antenna ports use SFBC, and four antenna ports use combined SFBC/FSTD;
The encoded BCH transport block is mapped to the first subframe of each radio frame in 4 consecutive frames; the BCH mapping is not based on resource blocks, and the BCH is mapped to the first 4 in the second time slot of subframe 0 Symbols, occupying only 72 central subcarriers;
In the case of FDD, the BCH immediately follows the PSS and SSS of subframe 0;
from the initial cell search, the terminal value obtains the cell frame timing;
System information block:
The MIB on the BCH only contains limited system information, and the main part of the system information is contained in different SIBs transmitted by DL-SCH;
LTE defines a series of SIBs:
SIB1 contains information mainly related to a terminal being allowed to camp on the cell;
SIB2 contains information required for the terminal to access the cell;
SIB3 contains information about cell retransmission;
SIB4-SIB8 contains information about neighboring cells Information;
SIB9 contains the name of the home base station;
SIB10-SIB12 contains public warning information;
SIB13 contains information required for MBMS reception;
SIB14 is used to provide enhanced access restriction information to control the possibility of terminal access to the cell;
SIB15 is contained in neighboring Information required for receiving MBMS on the carrier frequency;
SIB16 contains GPS time and coordinated universal time UTC;
SIB17 contains LTE and WLAN interconnection related information;
SIB18 and SIB19 contain side chain connection related information;
SIB20 contains single point-to-many Point related news;
Similar to MIB, SIB is also repeatedly broadcast; how frequently a specific SIB needs to be transmitted depends on how quickly the terminal needs to obtain relevant system information when accessing the cell;
The SIB with the lower sequence number is more urgent in time. Compared with the SIB with the higher sequence number, it is transmitted more frequently. SIB1 is transmitted every 80ms;
SIB is mapped to different system information message SI, SIB1 is always mapped to the first system information message SI-1;
SIB mapping to SI follows: SIBs mapped to the
same SI have the same transmission period; the
total number of information bits mapped to a single SI cannot exceed the upper limit of the number of bits that can be transmitted in a transport block.
The sending period of a given SIB may have different values in different networks;
the mapping of SIB to SI for other SIBs except SIB1 is flexible, and may be different in different networks or even in the same network;
SI-1 transmission has only limited flexibility, SI-1 is always transmitted in subframe 5;
different SIs have different non-overlapping time windows, and the terminal knows which SI is being received;
in relatively small SI and relatively large system bandwidth In this case, one subframe is enough to provide all SI transmission;
A terminal experiencing good channel conditions can decode the entire SI after receiving only a subset of the coded SI mapped subframes, while a terminal in a poor position needs to receive more subframes to decode the SI correctly. This has the advantage :
1. Terminals experiencing good channel conditions need to receive fewer subframes, which can reduce terminal power loss;
2. Combining with Turbo coding and using larger code blocks will produce enhanced channel coding gain.
Random access:
The basic function of the cellular system is that the terminal can apply to establish a network connection, which is called random access;
The purpose of random access:
for the initial access when the
wireless link is established, to reestablish the wireless link after the wireless link establishment fails,
if the terminal is in the RRC_CONNECTED state and the uplink is out of synchronization, when uplink or downlink data arrives , It is necessary to establish uplink synchronization. When
a positioning method based on uplink measurement is used, it is used for positioning purposes.
If a dedicated scheduling request resource has not been configured on the PUCCH, it is used as a scheduling request.
Uplink synchronization is the main goal; when establishing an initial wireless link, the random access process is also used to assign a unique identifier C-RNTI to the terminal;
contention-based random access can be used for all the above purposes;
Random access based on contention-free is only used to re-establish uplink synchronization when downlink data comes;
The four steps of random access:
1. The terminal transmits a random access preamble so that the eNodeB can estimate the terminal's transmission timing.
2. The network sends a time advance command word to adjust the terminal's transmission timing and allocates a random access response to the terminal.
3. Like normal scheduling data, the terminal uses the UL-SCH to send the mobile terminal identification to the terminal.
4. The network sends a contention resolution message to the terminal on the DL-SCH.
Step 1: Random access preamble transmission:
The preamble transmission is mainly for the base station to indicate the occurrence of a random access attempt and enable the base station to estimate the time delay between the base station and the terminal; this time delay will be used in the second step to adjust the uplink timing;
The time-frequency resource used for the transmission of the random access preamble is called the physical random access channel PRACH; the network broadcasts to all terminals to inform which PRACH the random access preamble can be transmitted on SIB2;
Each cell has 64 available preamble sequences;
PRACH time-frequency resources:
The PRACH resource has a cell bandwidth corresponding to 6 resource blocks;
the length of the preamble depends on the configured preamble, the basic random access resource period is 1ms, and it can be configured longer;
The eNodeb will avoid scheduling any uplink transmission on the time-frequency resources that have been used for random access. The preamble and user data for random access are orthogonal; it can avoid UL-SCH and random access attempts from different terminals Interference between
For FDD mode, there is at most one random access area in each subframe, that is, multiple random access attempts cannot be multiplexed in the frequency domain;
For TDD mode, multiple random access areas can be configured in a single subframe, because the number of uplink subframes in each radio frame of the TDD system is less;
Preamble structure and sequence selection:
Two parts of the
preamble : a preamble sequence; a cyclic prefix;
The preamble adopts a guard interval to control the uncertainty of timing, so the actual preamble is less than 1ms;
As a part of the preamble, the cyclic prefix allows the base station to perform frequency domain processing; the length of the cyclic prefix approximates the length of the guard interval;
PRACH power setting:
The basis for setting the transmit power of the random access preamble is to measure the cell-specific reference signal on the downlink main component carrier to obtain a downlink path loss estimate; from the path loss estimate, a configurable probability offset can be added Get the initial transmission power of PRACH;
The random access mechanism of LTE allows power to climb, such as increasing the actual PRACH transmit power for each failed random access attempt;
Preamble sequence generation:
The preamble sequence is generated by cyclically offsetting the Zadoff-Chu root sequence.
Cyclic shifted Zadoff-Chu sequence characteristics: the amplitude of the sequence is constant, and the sequence has ideal cyclic autocorrelation;
In order to deal with cells of different sizes, the cyclic shift Ncs is sent as part of the system information;
a disadvantage of using the Zadoff-Chu sequence is that it is difficult to distinguish the delay determined by the frequency offset and the distance;
Preamble detection:
The samples generated in the time domain window are collected and converted to frequency domain representation by FFT, with a window length of 0.8ms;
FFT output represents the received signal of the frequency code, multiplied by the complex conjugate frequency domain representation of the Zadoff-Chu root sequence, The result is input to IFFT; by observing the output of IFFT, it is possible to detect which shift of the Zadoff-Chu root sequence and its delay are transmitted;
Step 2: Random access response:
In response to the detected random access attempt, the network sends a message on the DL-SCH, including: the
sequence number of the random access preamble sequence detected by the network, and which sequence is valid for the response;
calculated by the random access preamble receiver Obtained timing correction value;
scheduling request, indicating what resources the terminal will use to transmit the message of the third step;
temporary identification TC-RNTI, used for further communication between the terminal and the network.
If the network detects multiple random access attempts (from different terminals), the response messages from multiple terminals can be combined and transmitted;
All terminals that send the preamble will monitor the L1/L2 control channel within a configurable time window to obtain a random access response;
As long as terminals performing random access in the same resource use different preambles, random access collisions will not occur, and it can be clearly known from the downlink signaling which terminal the information is for;
When multiple terminals use the same random access preamble at the same time, there is a certain possibility of competition;
Step 3: Terminal identification:
After the second step, the terminal uplink has been time synchronized; in the
third step, the terminal sends the required message to the eNodeB through the UL-SCH resource allocated in the random access response in the second step; the same as the uplink The benefits of scheduling uplink message transmission:
1. First, the amount of information sent when the uplink is not synchronized should be as small as possible.
2. Second, the message transmission adopts a common uplink transmission mechanism to adjust the license size and modulation method. To adapt,
3. Finally, hybrid ARQ using soft combining for uplink messages can be implemented.
Step 4: Contention resolution:
The last step of the random access process includes downlink messages for contention resolution;
It is a contention resolution mechanism to ensure that one terminal will not mistakenly use the identity of another terminal;
If the terminal has allocated C-RNTI, the contention resolution is solved by introducing the terminal using C-RNTI on the PDCCH;
If the terminal does not have a valid C-RNTI, use TC-RNTI and the related DL-SCH containing the contention resolution message to process the contention resolution message;
When the C-RNTI is unique to a terminal, non-target users will ignore the PDCCH transmission;
only the terminal whose reception ID in the fourth step matches the transmission ID in the third step can declare a successful random access;
Paging:
Paging is used for terminals in RRC_IDLE state to establish initial network connection;
Using the same mechanism as normal downlink data transmission on DL-SCH, the mobile terminal monitors L1/L2 control signaling to obtain paging-related downlink scheduling allocation;
The terminal should be put to sleep when it does not need receiver processing, and wake up quickly within a predetermined time interval to monitor the paging information coming on the network; the paging cycle can be customized;
The network configures in which subframe the terminal should wake up and monitor paging; using the identifier IMSI, which subframe within a subframe monitors paging is also deduced from the IMSI;
Paging messages can only be transmitted on some subframes, from one subframe every 32 frames to 4 subframes per frame, to support very high paging capacity;