NR - Downlink Control

In NR system, the downlink control information (DCI) is carried by physical downlink control channel (PDCCH), which physically occupied a region in the 2-dimension resource grid, which is essentially comprised of a certain number of resource elements (RE's), a.k.a., control resource set (CORESET). Different from the counterpart in LTE, i.e., PDCCH region which always covers the whole frequency range, CORESET does not span the whole bandwidth, since the bandwidth in NR system is remarkably large while not all the NR UE's are able to perform reception over the whole bandwidth.

Due to the static feature of CORESET, the indication of its size is not necessary. Therefore, there is no PCFICH in NR system. Additionally, irrespective of downlink or uplink, asynchronous HARQ is utilized in NR. Hence, PHICH is not needed either.

In NR, the channel coding scheme for PDCCH is based on polar codes. It can support 512 coded bits (prior to rate matching¹) and up to 140 information bits. Targetting an enhanced coverage, a beamformed single layer transmission is adopted for PDCCH. In this sense, it is similar to ePDCCH in LTE system.

Just as its name indicates, the basic element for PDCCH is control channel element (CCE), which is comprised of 6 resource element groups (REG's), and a REG is defined as one resource block (RB) in one OFDM symbol². An aggregation level number of CCE's forms one PDCCH. The value of aggregation level can be 1, 2, 4, 8, 16³.

The frequency-domain resource allocation fields indicate the resource blocks in the active BWP. Regarding the indication, there are a couple of methods, termed type-0 and type-1 respectively. The former is based on a bitmap, and will bring about large signaling overhead. While the latter covers the a starting position in the frequency domain and an amount of resources.

The resource allocation scheme can be configured as type-0, type-1, and dynamic selection between the both²⁷. Note that the allocation is indicated in the term of VRB. For type-0, each VRB is directly mapped to a PRB without interleaving. For type-1, there are two alternatives for the VRB-to-PRB mapping, interleaved and non-interleaved.

Generally, many transmission parameters are per-BWP configured. The information obtained are based on the current BWP, which should be transformed and translated before applied to the target BWP by padding/truncating for each DCI field.

The time domain allocation field in DCI is essentially an entry index of the corresponding table. Totally two tables are configured by RRC signalling, for downlink scheduling assignment and uplink scheduling grant respectively. A table has up to 16 entries, with each entry consisting of

• A slot offset relative to the DCI belonging slot
• The starting OFDM symbol for data transmission
• The number of OFDM symbols in the slot
• For downlink, the PDSCH mapping type.

Slot aggregation is a tool for coverage enhancement, in which a TB is repeated across up to 8 slots. The related configuration is separately signalled by RRC signalling.

Regarding TB size determination, NR adopts a hybrid method. For smaller TB, NR defines a table to lookup. While for larger TB, the TB size can be computed based on a formula.

Considering the 5-bit MCS field, there are 32 combinations. Depending on whether 256QAM is supported or not, there are 28 or 29 MCS levels and the other 4²⁸ or 3²⁹ are reserved for retransmission.

According to the indication of the scheduled resources, the number of RE's available can be obtained. By subtracting the RE's for DMRS, and CSI-RS/SRS, the number of RE's available for data transmission can be further acquired. According to the spectral efficiency indicated by MCS, an intermediate number of information bits can be got whereby table lookup or formula-based computation can be carried out. After that, the TB size can be determined.

For retransmission, the TB size is equal to that for initial transmission. Based on the modulation order indicated by the reserved MCS level, the code rate can be implicitly derived.