5G NR specifies the number of signals and physical channels that are responsible for the communication between the 5G system and the user equipment devices (UEs). These signals and physical channels may be used for the Downlink (DL) and Uplink (UL) communication. It is vital for 5G systems in order to provide all these 5G use cases to have a more flexible and scalable approach. However, some of the LTE concepts are also present in 5G NR regarding the communications channels and reference signals structure.
Physical broadcast channel (PBCH)
Physical Broadcast Channel is used to carry cell/network specific system information. This information is transferred via the Master Information Block (MIB). MIB contains the basic set of parameters that are needed to read the downlink signal and help demodulate the Physical Downlink Control Channel (PDCCH).
According to 3GPP 38.331 Section 6.2.2 6, the bit string carries the 6 most significant bits out of 10 bits. The 4 LSB bits are conveyed in the PBCH transport block as part of channel coding. For <6GHz, the subcarrier spacing values are either 15 or 30kHz, while different values are used for millimeter-wave frequencies.
The PBCH is used to signal the Control Resource Set (CORESET), a common search space, and the necessary PDCCH parameters. CORESET is made up of multiple resource blocks in the frequency domain and 1, 2, or 3 OFDM symbols in the time domain. CORESET, the equivalent of the control region in LTE, has a similar function as PDCCH in LTE. However, in 5G NR, the frequency domain size is not fixed, and there may be several of them.
CORESET frequency spans in multiples of 6 resource blocks. 12 Resource Elements (1RB) make up one Resource Element Group (REG). A Common Control Element (CCE) equals 6 REGs. A CCE can transmit up to 140 bits of signaling.
5G NR supports one more aggregation level than LTE. Aggregation Levels serve to encode signaling information into several CCEs to increase robustness, and therefore coverage. Reference signals located in PDCCH are UE specific, i.e., a Downlink Control Information (DCI) dedicated to a particular UE will have UE’s DM-RS configuration from PDSCH. DCI is placed within a CCE.
Physical Downlink Control Channel (PDCCH)
Physical Downlink Control Channel has a similar concept as in LTE since it carries Downlink Control Information (DCI), namely: DL Allocation, UL Scheduling grants, and UL Power control commands. However, the resource allocation for PDCCH has a couple of new units: REG Bundle and CORESET.
PDCCH has a common search space and a UE specific search space. The search space consists of a set of CCEs upon which the UE tries to blindly detect PDCCH transmission.
L1 signaling is achieved by the DCI (Downlink Control Information). The DCI has various formats for the information that is sent to define resource allocations. Resource Allocation means the allocation of PDSCH resources (RBs) by PDCCH in both frequency and time domain. Regarding the frequency domain, it represents the number of resource blocks, while for the time domain, the number of OFDM symbols. In LTE, the time domain resource allocation was fixed to 4 subframes after (delay of n+4, LTE’s low latency feature allows n+3), and the duration was always 1 subframe = 1 ms. In NR, the time domain resource allocation can start in almost any OFDM symbol. It can last up to 14 OFDM symbols, but it cannot span in multiple slots.
Physical Downlink Shared Channel (PDSCH)
The Physical Downlink Shared Channel (PDSCH) contains the downlink user data. It is responsible for converting user data into PDSCH data and transmitting this data through each transmission element, such as an antenna. In LTE, most of the transport parameters are fixed or automatically determined by transport process algorithm and only small numbers of parameters are configured by DCI. However, in 5G NR there are a lot more configurable parameters, by the DCI but also by the Radio Resource Control (RRC) messaging. This means that the whole process would be configurable and become more flexible.
Reference Signals in NR
Several types of Reference Signal are used in 5G NR compared to the Cell-Specific Reference Signal (CRS) as we know them from LTE. CRS was replaced by PDSCH DM-RS, and they are transmitted only in RBs, which are scheduled to the UE. This concept is known as a lean design and profoundly reduces inter-cell interference. NR Reference signals are tailored for specific roles and can be flexibly adapted for different deployment scenarios and spectrum.
PSS & SSS Synchronization Signal Block
Primary and Secondary Synchronization Signals (PSS/SSS) are used for time/frequency synchronization and cell search procedure. Unlike LTE, where there is always a center frequency position, SSB in 5G NR can have an arbitrary frequency position. In 5G NR, the new concept of Synchronization Signal Block combines PSS, SSS, and PBCH transmitted together. PSS and SSS are mapped to 127 subcarriers around a lower end of the bandwidth (subcarrier 80-206) used for UE downlink synchronization.
In the time domain, the transmission pattern is more complicated than LTE. The period for PBCH is 10ms, while for PSS/SSS it is 5ms. SSB period depends on the subcarrier spacing, the frequency range, and other parameters. All these combinations are there to help the User Equipment to identify a cell during Cell Search procedure and to enable Massive MIMO and Beam Sweeping techniques. SSB set duration is a half frame (5 ms) that repeats every 20 ms.
Beam sweeping is used to broadcast basic system information. Beam sweeping guarantees similar coverage for system information broadcast as for a beamformed PDSCH. There are 8 sweeps for 8 different beams (FR1), and then the sweeps repeat.
Massive MIMO antenna (32/64TRX) can create various beam shapes: wide or narrow for wider or further coverage. Wider horizontal coverage can be used for example in public squares, where Horizontal beamwidth of 110° (0 horizontal sweeps) & Vertical beamwidth 6° (-2° to +9°) can be used. Narrower vertical and horizontal beams for longer cell range can be used for high buildings, while very narrow beams for large distances and low buildings.
PCI and cell definition
Cell and Cell ID concept is still maintained in NR. PSS & SSS is used for DL synchronization and cell ID (PCI). NR PCIs are numbered from 0 to 1007 and divided into 3 groups, with each containing 336 cell IDs.
Cells with equal “PCI mod 3” will transmit the same PSS, which means UE views PSS transmission from different cells as multipath and assumes that the PSS is received from a single cell. While PCI mod 3 planning is still recommended by some vendors, others believe that PCI mod 3 has little impact on user experience.
Demodulation Reference Signals (DMRS)
Demodulation Reference Signals (DMRS) are used for downlink data demodulation (channel adaptation) and time-frequency synchronization.
PDSCH/PUSCH DM-RS signals are used for decoding the Physical Downlink Shared Channel (PDSCH), which contains the downlink user data. These signals can be divided into Front Loaded (FL) DMRS which occupies 1 to 2 symbols and Additional (Add) DRMS, which occupies 1 to 3 symbols, and is used in high-speed scenarios at the latter part of a slot.
Different DMRS types allow a different maximum number of ports: Type 1: Single-symbol: 4, dual-symbol: up to 8 orthogonal ports, used for SU-MIMO. Type 2: Single-symbol: 6, dual-symbol: up to 12 orthogonal ports, used for MU-MIMO.
DMRS time-frequency mapping position is divided into two different types, as well. Type A, starting from the 3rd of 4th symbol in the slot, and Type B, starting from 1st symbol on scheduled PDSCH and is practical only for cases there is no CORESET on the 1st OFDM symbol.
PUSCH DM-RS: NR supports a common DMRS structure for UL and DL (PDSCH and PUSCH). The exact time location, DMRS pattern, and scrambling sequence can be used, or it can also be configured differently. Typically Type B is set, where DM-RS starts in OFDM symbol 0. Only type 1 DM-RS supports DFT-S-OFDM (UL), which is transmission without UL MIMO.
DM-RS for PBCH (SSB): The frequency position of PBCH DM-RS is determined by PCI mod 4. Separation can be achieved by PCI mod 4 planning. However, there is still DM-RS to PBCH or vice versa interference. Therefore no planning is required.
CSI reference signals / TRS
CSI-RSs are designed for downlink measurement and reporting channel status info. Three different types of CSI-RS are supported, all having 32 orthogonal ports. Periodic CSI-RS is activated via RRC signaling, has a periodic transmission once configured, and requires no L1 overhead. Aperiodic CSI-RS uses L1 signaling, has low latency and single transmission when triggered. Finally, semi-persistent CSI-RS is activated via MAC CE and has a periodic transmission until deactivated.
TRS (Tracking) is designed for time/frequency tracking and estimation for delay/Doppler spread. It is configured as a CSI-RS with specific parameter restrictions such as time/freq location, RE pattern, etc.
Phase tracking Reference signals (PT-RS)
Phase tracking Reference signals (PT-RS) are designed for UL/DL phase noise compensation on PDSCH and PUSCH. They are associated with DM-RS so the receiver can compensate for phase noise during demodulation.
Sounding reference signals (SRS)
SRSs are designed for the evaluation of uplink quality and timing. However, they can also be used for downlink channel information when channel reciprocity is applicable, such as in TDD. Three different types are standardized, periodic, aperiodic, and semi-persistent. SRS carrier switching is supported for transmitting SRS over more than one carrier using a single uplink transmitter. Up to 6 OFDM symbols can be used for transmission to increase SRS capacity. LTE supports only the last OFDM symbol. SRs are typically configured with a bandwidth of 16 RBs.
