My answer is a summary of the paper: “5G New Radio (NR): Unveiling the Essentials of the Next Generation Wireless Access Technology”, Lin et. al., Ericsson.
NR mm [New Radio Millimeter Wave] Use Cases
The 5th generation [5G] wireless access technology, known as New Radio [NR], will address the three primary 5G use cases:
- Enhanced mobile broadband [eMBB].
- Ultra-reliable low-latency communications [URLLC].
- Massive machine type communications [MMTC]/Internet-of-Things [IoT].
Key technology features
- Support for low latency.
- Ultra lean transmission.
- Advanced antenna technologies.
- Multi-user Massive MIMO.
- Analog beam sweeping and analog beamforming.
- Operation in high frequency bands: [millimeter wave (mm)] and inter-operability between 2G, 3G, 4G frequency bands.
- Dynamic TDD [time division duplexing].
Waveform Scaling
- 3GPP will adopt OFDM [orthogonal frequency division multiplexing] with a CP [cyclic-prefix] for both DL [downlink: RRH to UE] and uplink UL [UE to RRH] transmissions.
- CP-OFDM can enable low implementation complexity and low cost for wide bandwidth operations [expected at mm frequencies] and MIMO [multiple-input multiple-output] technologies.
- NR also supports the use of DFT-S-OFDM [discrete Fourier transform spread OFDM] in the UL to improve network coverage.
- NR supports operation in the spectrum ranging from sub-1GHz to millimeter wave bands. Two frequency ranges (FR) are defined in 3GPP Release-15:
FR1: 450 MHz – 6 GHz, commonly referred to as sub-6GHz; and, FR2: 24.25 GHz – 52.6 GHz, referred to as millimeter wave [mm]. - NR adopts flexible subcarrier spacing scaled from the basic 15 kHz subcarrier spacing in LTE.
- This scalable design allows support for a wide range of deployment scenarios and carrier frequencies.
Frame Structure
- A frame has a duration of 10ms and consists of 10 subframes. This is the same as in LTE, facilitating NR and LTE coexistence.
- Each subframe consists of 2 slots of 14 OFDM symbols each. Although a slot is a typical unit for transmission upon which scheduling operates, NR enables transmission to start at any OFDM symbol and last only as many symbols as needed for the communication.
- This type of “mini-slot” transmission can thus facilitate very low latency for critical data as well as minimize interference to other links per the lean carrier design principle that aims at minimizing transmissions.
- Latency optimization has been an important consideration in NR.
RB [Resource Block]
- An RB consists of 12 consecutive subcarriers [same as LTE] in the frequency domain. A single NR carrier in Release-15 is limited to 3300 active subcarriers and to at most 400MHz bandwidth.
- Howver, in contrast to LTE, the maximum bandwidth in FR1 is 100MHz, and the maximum bandwidth in FR2 is 400MHz. Both are much greater than the maximum LTE bandwidth of 20MHz.
- Despite wide bandwidth, the ultra-lean design in NR minimizes always-on transmissions, leading to higher network EE [energy efficiency] and lower interference.
CA [Carrier Aggregation]
- Similar to LTE-A [3GPP Release-12].
- NR supports the possibility to have an NR carrier and an LTE carrier overlapping with each other in frequency
- This enables dynamic sharing of spectrum between NR and LTE. This facilitates a smooth migration to NR from LTE.
- Solutions specified to allow this type of operation are the ability for NR PDSCH [physical downlink shared channel - explianed below] to map around LTE cell CRS [specific reference signals].
- Flexible placements of DCH [DL control channels].
Co-existence with LTE
- Initial access related reference signals and data channels to minimize collisions with LTE reference signals.
- NR also supports SUL [supplementary uplink] which can be used as a low-band complement to the cell’s UL when operating in high frequency bands and a SDL [supplementary DL].
- To allow good forward compatibility support in NR, it is possible to configure certain sets of resources to be unused in any PDSCH transmission.
- This allows 3GPP to develop new physical layer solutions for currently unknown use cases.
- For a carrier with a given subcarrier spacing, the available radio resources in a subframe of duration 1ms can be thought of as a resource grid composed of subcarriers in frequency and OFDM symbols in time.
- Accordingly, each RE [resource element] in the RG [resource grid] occupies one subcarrier in frequency and one OFDM symbol in time.
Bandwidth Part
- To reduce the device power consumption, a UE [user equipment] may be active on a wide bandwidth in case of bursty traffic for a short time, while being active on a narrow bandwidth for the remaining.
- This is commonly referred to as bandwidth adaptation and is addressed in NR by a new concept known as bandwidth part.
- A bandwidth part is a subset of contiguous RBs on the carrier.
- Up to four bandwidth parts can be configured in the UE for each of the UL and DL, but at a given time, only one bandwidth part is active per transmission direction.
- Thus, the UE can receive on a narrow bandwidth part and, when needed, the network can dynamically inform the UE to switch to a wider bandwidth for reception.
Modulation
- Similar to LTE.
- BPSK and QPSK.
- 16-QAM, 64-QAM and 256-QAM with binary reflected Gray mapping.
Channel Coding
- NR control channels use Reed-Muller block codes and CRC [cyclic redundancy check] assisted polar codes [vs. tail-biting convolutional codes in LTE].
- NR data channels use rate compatible quasi-cyclic LDPC [low-density parity-check] codes [vs. turbo codes in LTE].
Duplexing
- The duplexing options supported in NR include FDD [frequency division duplex], TDD with semi-statically configured UL/DL configuration, and dynamic TDD.
- In the TDD spectrum, for small/isolated cells it is possible to use dynamic TDD to adapt to traffic variations.
- For large over-the-rooftop cells, semi-static TDD may be more suitable for handling interference issues than fully dynamic TDD.
Slot Configuration
- Cell-specific and UE-specific RRC [radio resource control] configurations determine the UL/DL allocations.
- This framework allows configuration of slot patterns identical to LTE TDD frame structures.
- If a slot configuration is not configured, all the resources are considered flexible by default.
- Whether a symbol is used for DL or UL transmission can be dynamically determined according to Layer 1 [PHY] or Layer 2 [MAC] signaling of DCI [DL control information].
- This leads to a dynamic TDD system.
SS [Sync Signals] & PBCH [Physical Broadcast Channel]
- SS + PBCH = SSB [sync signal broadcast] in NR.
- The subcarrier spacing of SSB can be 15kHz or 30kHz in FR1 and 120kHz or 240kHz in FR2.
- By detecting SS, a UE can obtain the physical cell identity, achieve downlink synchronization in both time and frequency domain, and acquire the timing for PBCH.
- PBCH carries the very basic system information.
PRACH [Physical Random Access Channel]
- PRACH is used to transmit a random-access preamble from a UE to indicate to the gNB [gigabit Node-B, vs. eNB (evolved Node-B) in LTE] a random-access attempt and to assist the gNB to adjust the uplink timing of the UE, among other parameters.
- Like in LTE, Zadoff-Chu sequences are used for generating NR random-access preambles due to their favorable properties, including constant amplitude before and after DFT operation, zero cyclic auto-correlation and low cross-correlation.
- In contrast to LTE, NR random-access preamble supports two different sequence lengths with different format configurations, to handle the wide range of deployments for which NR is designed.
Analog Beam Sweeping Technology
- Short preamble formats in both FR1 and FR2 supports the possibility of analog beam sweeping during PRACH reception such that the same preamble can be received with different beams at the gNB.
PDSCH [Physical Downlink Shared Channel]
- PDSCH is used for the transmission of DL user data, UE-specific higher layer information, system information, and paging.
- For transmission of a DL transport block , a transport block CRC is first appended to provide error detection, followed by a LDPC base graph selection.
- NR supports two LDPC base graphs, one optimized for small transport blocks and one for larger transport blocks.
- Segmentation of the transport block into code blocks and code block CRC attachment are performed.
- Each code block is individually LDPC encoded. The LDPC coded blocks are then individually rate matched.
- Finally, code block concatenation is performed to create a codeword for transmission on the PDSCH. Up to 2 codewords can be transmitted simultaneously on the PDSCH.
PUSCH [Physical Uplink Shared Channel]
- PUSCH is used for the transmission of UL-SCH [UL shared channel] (UL-SCH) and Layer 1-Layer2 control information.
- The UL-SCH is the transport channel used for transmitting an UL transport block.
- The physical layer processing of an UL transport block is similar to the processing of a DL transport block.
PDCCH [Physical Downlink Control Channel]
- PDCCH is used to carry DCI such as downlink scheduling assignments and uplink scheduling grants.
- Legacy LTE control channels are always distributed across the entire system bandwidth, making it difficult to control intercell interference.
- NR PDCCHs are specifically designed to transmit a CORSET [configurable control resource set].
- A CORESET is analogous to the control region in LTE but is generalized: the set of RBs and the set of OFDM symbols in which it is located are configurable with the corresponding PDCCH search spaces.
- Such configuration flexibilities of control regions including time, frequency, numerologies, and operating points enable NR to address a wide range of use cases.
PUCCH [Physical Uplink Control Channel]
- PUCCH is used to carry UCI [uplink control information] such as HARQ [hybrid automatic repeat request] feedback, CSI [channel state information], and SR [scheduling request].
- Unlike LTE PUCCH that is located at the edges of the carrier bandwidth and is designed with fixed duration and timing, NR PUCCH is flexible in its time and frequency allocation.
- That allows supporting UEs with smaller bandwidth capabilities in an NR carrier and efficient usage of available resources with respect to coverage and capacity.
- NR PUCCH design is based on 5 PUCCH formats: PUCCH 0,1,2,3 and 4.
DMRS [Demodulation Reference Signals]
- DMRS is used by the receiver to produce channel estimates for demodulation of the associated physical channel. The design of DMRS is specific for each physical channel – PBCH, PDCCH, PDSCH, PUSCH, and PUCCH.
- In all cases, DMRS is UE specific, transmitted on demand, and normally does not extend outside of the scheduled physical resource of the channel it supports.
PTRS [Phase Tracking Reference Signals]
- PTRS is used for tracking the phase of the local oscillator at the receiver and transmitter. This enables suppression of phase noise and common phase error, particularly important at high carrier frequencies such as millimeter wave.
- Due to the properties of phase noise, PTRS can have low density in the frequency domain but high density in the time domain. PTRS can be present both in the downlink [associated with PDSCH] and in the uplink [associated with PUSCH].
CSI-RS for Analog Beamforming
- Similar to LTE: NR CSI-RS is used for DL CSI acquisition.
- CSI-RS in NR also supports RSRP [reference signal received power] measurements for mobility and beam management [analog beamforming], time/frequency tracking for demodulation, and UL reciprocity-based precoding.
- CSI-RS is UE specifically configured, but multiple user can still share the same resource.
SRS [Sounding Reference Signals]
- SRS is used for UL channel sounding.
- The design supports UL link adaptation and scheduling, but in reciprocity operation also downlink precoder selection, link adaptation and scheduling, e.g., for massive multi-user MIMO.
- Contrary to LTE, NR SRS is UE specifically configured. This enables a high degree of flexibility in the system.
The PHY Layer [Layer-1] NR specs have been described in TS38.201[1], TS38.202[2], TS38.211[3], TS38.212[4], TS38.213[5] and TS38.214[6].
Higher layer [Layer-2, Layer-3] specifications are described in the TS 38.300 series[7].
Footnotes
