When introducing CORESET, it was mentioned that the DCI information carried by the PDCCH appears at the time-frequency position indicated by CORESET. If the UE wants to know the scheduling information or broadcast message of the base station, it needs to demodulate the PDCCH channel. Before understanding the NR PDCCH channel, let's briefly understand the DCI format and main functions carried by the PDCCH channel.
The DCI format is mainly listed in the following table, among which Format 0_0 and Format 0_1 are used to schedule the time-frequency resources of the PUSCH uplink channel, Format 1_0 and Format 1_1 are used to schedule the time-frequency resources of the PDSCH, and Format 2_0 is used to notify the format of the slot , Format 2_1 is used to inform the UE which frequency domain resources (PRB) and time domain resources (OFDM symbols) may not transmit data, and Format 2_3 is used to inform the UE to transmit the SRS control command.
Let's take a look at Format 0_0 and Format 1_0 respectively, to understand how the base station schedules the two typical uplink and downlink of PUSCH and PDSCH. The following table is the scheduling information of Format 0_0:
from the table, 1 bit indicates the control format, 4 bits indicate the frequency domain resource PRB position in the BWP, and X bits indicate the time position (start symbol and length, in Table of RRC instruction pusch-TimedomainAllocationList), 1 bit indicates whether frequency hopping, 5 bits indicate modulation and coding scheme (MCS, from Table 6.1.4.1-1 in the protocol), 1 bit indicates whether data is transmitted, 2 bits indicate transmission Redundancy version number of the data (the data is divided into 4 parts after channel coding, 0, 1, 2, 3), 4 bits indicate the HARQ process, 2 bits indicate the power scheduling of the PUSCH, and whether 1 bit is configured with SUL (one of 5GNR) The new uplink channel is used for decoupling of the uplink and downlink frequency bands to enhance cell coverage).
For the downlink scheduling Format 1_0, the UE needs to request different service resources in different states. Therefore, different resources will be scheduled according to the different states of the UE. When the UE receives demodulated DCI, it will be based on its own state. RNTI descrambling data. For example, the UE in the RRC connected state adopts the tuning parameter format in the following table.
We see that the scheduling of the downlink PDSCH also includes scheduling instructions for time-frequency resources, modulation and coding, etc. In addition, some channel characteristic indicators are added.
These high-level DCI scheduling information bitstreams are sent and processed at the physical layer, mainly including multiplexing of DCI information bitstreams (that is, multiple DCI information connected together), CRC check, channel coding, rate matching, scrambling, modulation Mapping, RE resource mapping of time-frequency location, as shown in the figure below.
From the figure, we see some differences from LTE PDCCH bitstream processing. The first one, the 24-bit CRC check bit is scattered in the payload using interleaving technology. The other one uses RNTI to scramble part of CRC to enhance anti-interference ability. The CRC check bit uses 24 bits, which is longer than the 16 bits of LTE. This is because the PDCCH channel coding in NR uses Polar coding, while the PDCCH in LTE uses convolutional coding. The idea behind Polar encoding is to use a set of channels, including noise-free channels and all noise-free channels, and then the information bits are sent on the noise-free channels, and the polarization of the channels brings gains in decoding performance. The typical method of Polar decoding is to use continuous deletion and linear decoding. Since linear decoding uses a part of CRC bits, this means the loss of the CRC's error correction ability. For example, in the 24-bit CRC, there are 3 bits for Polar linear decoding, and its actual error correction ability is only 21 bits. It is precisely because of this feature that the CRC in the NR uses a longer check bit to offset the effect.
In addition, from the above figure, we found that the CRC check bit is not directly attached to the information bit like LTE, but a part of it is interleaved in the information bit. This is also due to the characteristics of the Polar code, the interleaved CRC The parity bit helps to find the stop bit in advance when decoding the Polar code. Unlike the LTE downlink channel, each channel shares a cell reference (RS) signal for receiving and demodulating each channel and channel sounding. In NR, the PDCCH has its own dedicated DMRS signal, which means that the PDCCH channel can be detected by DMRS. Establish your own antenna parameters, such as beamforming to specify the direction. After all, Massive MIMO is a technical feature of NR.
After the bit stream is modulated into a QPSK complex constellation symbol, it is then mapped to a specific time and frequency resource position. At this time, the CORESET parameter set mentioned in Random 4 is used to allocate resources, and the CCE to REG mapping -to-REG). One CCE contains 6 REGs, and one REG contains one OFDM symbol RB (12 subcarriers) bandwidth. In this way, after excluding the DMRS signal, one CCE carries 54 resource elements (RE) and 108 bits of data. There are two ways of CCE-to-REG mapping: interleaved and non-interleaved. The interleaving method can bring diversity in frequency selection and overcome the frequency selective fading of the channel. The following figure shows the difference between interleaved and non-interleaved mapping.
According to the above figure, when non-interleaved mapping, if it is 1 OFDM symbol, directly fill one CCE in the frequency domain, then fill the next CCE, and when multiple OFDM symbols, take the time domain REG first , And then fill the frequency domain REG until one CCE is filled, and then perform the mapping of the next CCE. When interleaved mapping is used, it is a bit more complicated. The size is indicated by REG-bundling, the size of the REG is filled in the time domain and then the frequency domain, and then the frequency domain (the starting position of the previous REG-bundling) is separated by 6 REG distances for the next REG-bundling size mapping until one CCE size (6 REGs) is filled. The mapping of the next CCE is based on the offset value (in the RRC message), and then the same rule is mapped.
As mentioned earlier, the NR PDCCH has its own DMRS, and each REG (frequency domain size and other RB) occupies a quarter of the time-frequency resources, which is more than one-sixth of the LTE. This is for Beamformed technology using MIMO can bring great channel gain and more reliable transmission.
When each candidate of the PDCCH (ie UE) performs blind detection in the search space to receive its own DCI information, it can do channel estimation in each REG-bundling to obtain more accurate channel parameters, or it can be done within the entire PDCCH bandwidth Channel estimation, that is, it is not limited to the bandwidth within its own CORESET. It is similar to LTE using cell reference signals to perform channel estimation in the entire frequency band, that is, broadband channel estimation. The limitation of this is that the estimated channel value is used for When beamforming, performance is limited. The two channel estimation uses the DMRS method as shown in the figure below.
For more accurate beam management, DMRS of PDCCH can also be transmitted using quasi-collocated (quasi-collocated), together with CSI-RS (Channel-state information RS) signal to jointly determine the direction of the beam, as shown in the following figure. The UE transmits CSI-RS and DMRS in different CORESET, and the PDCCH is jointly received by the CSI-RS and DMRS in the CORESET domain. By detecting the CSI-RS, the best beam can be determined, so that when monitoring the PDCCH, the DMRS using the best beam can be known.
If CORESET is not configured with quasi-joint positioning, PDCCH candidates consider that their channel parameters are consistent with those obtained by SSB. That is, the default is that the PDCCH channel and PBCH channel have the same delay extension, Doppler extension, and spatial reception parameters.
Only one UE can exist in the search space of a PDCCH, that is, different UEs cannot be located in the same search space at the same time. The UE receives the demodulated data one by one according to the possible search space at the time and frequency positions given by the CORESET and SearchSpace parameter sets. When the demodulated CRC is correct and the RNTI matches, the DCI data is obtained. As many search spaces as there are equivalents, there are as many decoding and CRC processing as possible.
Disclaimer: Some pictures in the article are from http://www.sharetechnote.com/, "Next Generation Wireless Access Technology"
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