No matter in LTE or NR, the physical downlink control channel (PDCCH) is essential for uplink and downlink service scheduling. In addition to PDCCH-related concepts involved in LTE, such as the resource element group (REG), control channel element (CCE), aggregation level, candidates for blind detection, number of blind detection times, search space, radio network temporary identifier (RNTI), and downlink control information (DCI), concepts such as the control resource set (CORESET) and search space configuration are introduced in NR, making the PDCCH in NR more complicated but also more flexible than in LTE.
Physical Downlink Control Channel (PDCCH) in LTE & 5G NR.
In general, the NR PDCCH and LTE PDCCH are similar, both of which are designed in a way that enables UEs to blindly detect the PDCCH in one or more search spaces where the PDCCH may be transmitted. However, NR and LTE have different design objectives, and the NR PDCCH is optimized based on LTE deployment experience in the following aspects:
- NR PDCCH resources are not spread over an entire carrier bandwidth like in LTE. The bandwidths of NR carriers are relatively large and not all NR UEs can receive signals across an entire carrier bandwidth. Therefore, a more general control channel structure needs to be designed.
- PDCCH beamforming is implemented in NR, which is consistent with the “beam-centric NR” design concept. Beamforming greatly helps resolve path loss issues that high frequencies face.
The technical differences in control channels between LTE and NR are described in below.
| Item | LTE | NR | Difference |
|---|---|---|---|
| PDCCH/ePDCCH | Available | Available | In the frequency domain, these channels occupy an entire bandwidth in LTE and a configurable bandwidth in NR. In the time domain, these channels occupy the first several symbols in a slot in both LTE and NR. |
| Demodulation reference signal (DMRS) for PDCCH | N/A | Available | In LTE, always-on cell-specific reference signals (CRSs) are used for PDCCH estimation and demodulation. In NR, PDCCHs are transmitted on demand, and PDCCH-specific DMRSs are used for demodulation for greater beamforming benefits. |
| Physical HARQ indicator channel (PHICH) | Available | N/A | In LTE, the PHICH is mainly used in the synchronous hybrid automatic repeat request (HARQ) mechanism for uplink retransmissions. This channel is not required in NR. In NR, the asynchronous HARQ mechanism is used for both uplink and downlink retransmissions, and HARQ information is transmitted over the PDCCH. |
| Physical control format indicator channel (PCFICH) | Available | N/A | The PCFICH is used for dynamically indicating how many orthogonal frequency division multiplexing (OFDM) symbols in a subframe are used for PDCCH transmissions. In NR, the size of a CORESET does not dynamically change, and the use of idle control resources for data transmissions is implemented differently from that in LTE. Therefore, NR does not require the PCFICH. |
| CORESET | N/A | Available | In LTE, the PDCCH occupies an entire bandwidth in the frequency domain. In NR, since bandwidths are relatively large, CORESET is introduced to allow for more efficient blind PDCCH detection by UEs. |
| Search space | Available | Available | In both LTE and NR, UEs need to blindly detect DCI from PDCCH candidates in a search space. |
| REG | Available | Available | The numbers of resource blocks (RBs) mapped to one REG are different:In LTE, one REG consists of four REs in a symbol.In NR, one REG consists of 12 resource elements (REs) (one RB) in a symbol. |
| CCE | Available | Available | A CCE corresponds to different numbers of REGs.In LTE, a CCE corresponds to nine REGs (36 REs).In NR, a CCE corresponds to six REGs (72 REs when DMRSs are included, or 54 REs when DMRSs are excluded). |