5G NR Physical Layer

5G New Radio (NR) has a lot of similar but also many different characteristics compared to existing LTE architecture.  These technological improvements in the radio and physical side will make 5G a more flexible and modernized technology.  For example, 5G NR has a more flexible bandwidth operation, with support in sub-6GHz but also in the millimeter-wave frequencies.  These new frequencies will offer larger channel bandwidths to result in higher data rates.  Shorter TTI and shorter symbol times will be able to provide low-latency configurations, while a clean sub-frame structure will lead to a future-proof design and expansion.  In existing systems, reference signals are transmitted in specific periods, while the system information is transmitted in a broadcast fashion.  5G, on the other hand, will have a more lean design, with no always-on reference signals and minimized transmission, which is not always directly related to user data.  NR will also have a beam-centric design with beam management support and, of course, interworking across several spectrum bands, including the LTE spectrum.

NR Characteristics

NR supports different numerologies, adopted to the frequency layer, and each specific use case.  The baseline numerology is 15 kHz, with a scalable subcarrier spacing (SCS) of 15 kHz *2^m. The Symbol length and Cyclic Prefix (CP) scales accordingly with 60 kHz SCS, which also supports ‘normal’ CP of 4.67 us. Depending on SCS, NR supports 7 or 14 symbols per slot, while no DC carrier is present in NR Frequency bands.

The modulation schemes used are BPSK, QPSK, 16QAM, 64QAM, and 256QAM.  NR also supports π/2-BPSK to further reduce the Peak-to-Average Power Ratio (PAPR) for the Uplink.  The basic Frame structure is 12 subcarriers per Physical Resource Block (PRB).  The sub-frame duration is fixed to 1ms.  As mentioned above, the slot length is 7 or 14 symbols for up to SCS 60kHz and 14 symbols for more than SCS 60kHz.  There is also symbol-level alignment across different SCSs with the same CP overhead. NR supports extended CP at least for 60kHz SCS, while it also supports explicitly signaled reserved resources.

In NR, a slot is the equivalent of a sub-frame in LTE TDD.  A slot consists of downlink symbols, uplink symbols, and ‘unknown’ or ‘flexible’ symbols.  No explicit symbols are used for the DL-UL switching gap.  NR supports at most 256 slot formats, which are further described in 3GPP Rel-15 specification.  In LTE, all of the symbols in a subframe are used either for DL or UL transmission, however, in NR, there is a flexible slot formation where specific slot segments are divided for a specific use. The network configures UL/DL allocations for the transmission period.  The configuration is cell-specific and possible to reconfigure the UL/DL allocation for the transmission period per UE.  It is also possible to configure the same UL/DL allocation as TD-LTE with the special subframe configuration.

NR Techniques in the physical layer

Furthermore, NR has the possibility of mini-slots.  The mini-slot is described as the minimum scheduling unit in NR and is suitable for low–latency transmission and the support of Ultra-Reliable, Low-Latency Communications (URLLC). The mini-slot can start at any OFDM symbol and contains DMRS at positions relative to the start of the mini-slot.  Mini-slots may pre-empt ongoing eMBB transmissions.  The mini-slot length is typically 1 slot length, while 2 slot lengths are used for URLLC.  In a mini-slot, the design principles are the same as in typical slots.  The mini-slot can be punctured into other transmissions and provides efficient multiplexing of URLLC services with, for example, enhanced Mobile Broadband (eMBB) traffic.

NR is using high numerologies for shorter slot lengths.  This means mini-slots for low-latency transmissions, and fast scheduling using asynchronous HARQ.  Flexible scheduling and fast HARQ processing mean explicit and dynamic HARQ, Downlink/Uplink scheduling, and un-scheduled transmissions such as grant-free transmissions.  Furthermore, NR supports ‘Self-contained’ sub-frames.  In this case, the UE receives and transmits data within one sub-frame.  Self-contained sub-frames include the HARQ, while there is a significant reduction in latency.

NR allows flexible bandwidth, with up to 400 MHz carrier-channels compared to 20 MHz for LTE.  NR can have up to 16 component carriers, and the overall bandwidth depends on the frequency band.  Obviously, millimeter-wave frequencies can support larger bandwidths with higher speeds.  Of course, not all devices must support the full network carrier bandwidth. Parts of the channel bandwidth can be configured by the network as a Bandwidth-part.  This configuration takes place per UE.  Up to 4 bandwidth parts are used for UL and DL carriers per UE.  Each bandwidth part may have different SCS, location, and bandwidth.  The UE transmits and receives the signal on one bandwidth part, indicated by the control channel.

Initial Access Procedure in NR

5G NR has a new procedure for the initial access of each UE to the gNB.  The initial access in NR is implemented by the NR Synchronization & Common Control Channel.  The SS-PBCH block is used for the initial cell search and RRC measurements.  The SS block carries the symbol and frame timing, CP-length and duplex mode, and the RS sequence for the physical cell Id (PCI).  The Narrow Band Physical Broadcast Channel (NPBCH) carries the MIB and position of SS/PBCH block and CORESETs (Control Resource set).  The SS/PBCH block always spans the same number of tones and the SS/PBCH bandwidth scales with SCS.  The SS/PBCH block can have any location within the channel bandwidth and the sub-frame.  The location of SS/PBCH block is relative to the lowest tone, while PRB is part of the PBCH information.  PBCH also delivers the location and size of the CORESET for the RMSI (Residual Minimum System Information ) and OSI (Other System Information).

For the system information, the UE reads the PBCH providing the basic cell configuration and finds the downlink control channel which schedules the shared channel.  It then reads the minimum system information providing scheduling information for all other system information blocks. The UE reads other required system information and requests on-demand system information, such as the system information that is only relevant to a specific UE.

NR Channel Coding

NR uses HARQ and Scheduling enabling technologies for Channel Coding.  LDPC coding is used for the data channel. This provides implementation and latency advantages over other coding schemes and supports incremental redundancy and chase combining HARQ.  In incremental redundancy, every re-transmission contains different information. In chase combining, every re-transmission contains similar information as far as data an parity bits are concerned, and the receiver has to combine the received bits with the same bits from previous transmissions.

In polar coding, the channels are polarized through the successive combining of binary channels.  Some channels capacity approach 1 and are used for data transmissions, while others approach zero. These are frozen bits. The maximum Code block sizes are 512 control channels for the Downlink and for the Uplink.