As shown below:
The schematic diagram and flow chart of the entire cell search and synchronization process are as follows:
1) The UE starts up, receives a signal (PSS) at several central frequency points where there may be an LTE cell, and determines whether there is a possible cell around the frequency point by receiving the signal strength. If the UE saves the frequency point of the last shutdown, If the carrier information is turned on, it will try on the cell that was last camped; if not, it will do a full-band scan in the frequency band allocated to the LTE system, and find a signal with a strong signal to try;
2) Then receive PSS (primary synchronization signal) around this center frequency point, which occupies 6RB of the center frequency band, so it can be compatible with all system bandwidths, the signal is repeated in 5ms cycle, transmitted in subframe #0, and is ZC The sequence has strong correlation, so it can be directly detected and received. According to this, the cell ID in the cell group can be obtained, and the slot boundary of 5ms can be determined at the same time. By checking this signal, the length of the cyclic prefix can be known and the used Is FDD or TDD (because TDD's PSS is placed in a special sub-frame, the position is different, based on this to make a judgment) because it is 5ms repetition, because it can not get frame synchronization in this step;
3) After 5ms slot synchronization, the SSS is searched forward on the basis of PSS. The SSS consists of two end random sequences, and the mapping of the front and rear fields is reversed. Therefore, as long as two SSSs are received, the 10ms boundary can be determined. The purpose of frame synchronization. Since the SSS signal carries the cell group ID, the physical layer ID (CELL ID) can be obtained by combining with the PSS, so that the structural information of the downlink reference signal can be further obtained.
4) After obtaining the frame synchronization, the PBCH can be read. The downlink reference signal structure is obtained through the above two steps. The demodulation reference signal can further synchronize the precise time slot with the frequency, and can also perform channel estimation for demodulating the PBCH. . The PBCH is transmitted on slot #1 of subframe #0, which is close to the PSS. By demodulating the PBCH, the system frame number and bandwidth information, as well as the configuration of the PHICH and the antenna configuration can be obtained. The system frame number and the number of antennas are relatively clever: the SFN (system frame number) is 10 bits long, that is, the value is 0-1023. In the MIB (master informaTIon block) broadcast of the PBCH, only the first 8 bits are broadcast, and the remaining two bits are determined according to the position of the frame in the PBCH 40 ms period window, the first 10 ms frame is 00, the second frame is 01, and the third frame is The frame is 10 and the fourth frame is 11. The PBCH 40ms window phone can be determined by blind detection. The number of antennas is implicit in the CRC of the PBCH. After calculating the CRC of the PBCH, it is XORed with the MASK corresponding to the number of antennas.
5) At this point, the UE implements timing synchronization with the ENB;
To complete the cell search, it is not enough to only receive the PBCH, because the PBCH only carries very limited system information, and more detailed system information is carried by the SIB, so it is necessary to receive the SIB (System Information Module), ie UE. Receive BCCH information carried on the PDSCH. To do this, you must do the following:
1) receiving the PCFICH, at which time the time-frequency resource of the channel can be calculated according to the physical cell ID, and the number of symbols of the PDCCH is obtained by receiving and decoding;
2) look up the candidate PDCCH sent to the SI-RNTI (Radio Network Identifier) ​​in the common search space of the PDCCH channel domain. If one is found and the relevant CRC check is passed, it means that there is a corresponding SIB message, so Receiving the PDSCH, and decoding the SIB to the upper layer protocol stack;
The SIB is continuously received, and the upper layer (RRC) judges whether the received system message is sufficient. If it is enough, the reception of the SIB is stopped, and the cell search process is almost finished.
Third, the random access processAfter the synchronization and cell search process ends, followed by the random access procedure, the schematic diagram of the entire random process is as follows:
1. UE sends preamble sequence to ENB on PRACH
Physical non-synchronizaTIon random access procedure
Physical channel: PRACH
Message: preamble sequence
2. ENB responds to the UE with a response message
Address to RA-RNTI on PDCCH
Random access response grant
Physical channel: PDSCH
The information transmitted by the ENB to the UE includes at least the following contents: RA-preamble identifier, Timing Alignment information, initial UL-grant and assignment of Temporary C-RNTI.
Note:
The RA-preamble identifier refers to the identifier of the preamble sent by the UE, and is related to the index.
Timing Alignment information is the timing advance information. Because there is a delay in the wireless transmission of space, the ENB calculates this delay amount and tells the UE to determine the actual time of the next data transmission.
UL-grant: Authorizes the UE to transmit information on the uplink. With this information, the UE can perform the next RRC connection request. It will give the size of the transport block that the UL-SCH can transmit, with a minimum of 80 bits.
3. RRC connection request (UE->> ENB)
Complete some basic configuration before making an RRC connection request:
Apply the default physical channel configuration
Apply the default semi-persistent scheduling configuration
Apply the default MAC main configuration
Apply the CCCH configuration
Apply the time Alignment Timer Common included in System Information Block Type2;
》 Start timer T300;
" initiate transmission of the RRC Connection Request message in accordance with
The RRC layer generates an RRC connection request and transmits it through CCCH: CCCH - "UL-SCH -" PDSCH
Get the UE-identity, either provided by the upper layer (S-TMSI) or by the random value. If the UE registers with the TA (tracking area) of the current cell, the upper layer can provide the S-TMSI and set the establishment clause to be consistent with the upper layer.
4. RRC connection setup (ENB—>>UE)
The UE receives information such as the radio resource configuration sent by the ENB, establishes a related connection, and enters an RRC connection state.
Action about physical layer:
Addressed to the Temporary C-RNTI on PDCCH
If the UE detects the RA success but does not have a C-RNTI, the temporary C-RNTI is promoted to the C-RNTI, otherwise it is discarded. If the UE detects RA success and already has a C-RNTI, it continues to use the original C-RNTI.
5. RRC connection setup complete (UE->> ENB)
After the establishment of the RRC connection is completed, the UE indicates to the ENB that the response information of the connection is received, which should be to ensure the reliability of the connection.
If the UE does not successfully receive the RRC connection setup message, the ENB should resend. Otherwise RRC connection setup complete is not necessary.
After completing the above process, you can enter the normal data transmission process.
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