5G NR Physical Layer | 5G PHY Layer
5G NR Physical Layer | 5G PHY Layer Overview
This 5G NR physical layer provides overview of PHY layer modules as per 5G New radio 3GPP standard. It describes processing of PDSCH and PUSCH channels through 5G physical layer.
Introduction:
The 5th generation wireless access tachnology is known as NR (New Radio). It follows 3GPP series of standards similar to GSM, CDMA and LTE. 3GPP organization has been developing specifications for 5G NR since few years. First specifications have been published in Dec. 2017 which suppors NSA (Non Standalone) where in 5G compliant UE relies on existing LTE for initial access and mobility. In June 2018, SA versions of 5G NR spefications have been finalized which works independent of LTE. There are three different use cases of 5G NR technology viz. eMBB (Enhanced Mobile Broadband), mMTC (Massive machine type communications) and URLLC (Ultra Reliable Low Latency Communication). The 5G PHY layer is specified in 3GPP TS 38.200 series of documents.
There are two main components in 5G NR network viz. UE (i.e. mobile subscriber) and gNB (i.e. base station). gNBs are connected with 5G Core in the backend. The connection from gNB to UE is known as downlink which uses PBCH, PDSCH and PDCCH channels for carrying different data/control informations. The connection from UE to gNB is known as uplink which uses PRACH, PUSCH and PUCCH channelsRefer 5G NR Network Architecture >>.
5G NR Numerology
5G NR Supports two frequency ranges FR1 (Sub 6GHz) and FR2 (millimeter wave range, 24.25 to 52.6 GHz). NR uses flexible subcarrier spacing derived from basic 15 KHz subcarrier spacing used in LTE. Accordingly CP length is choosen. This is shown in the table-1.
| Μ | Δf = 2μ.15 | Cyclic Prefix |
|---|---|---|
| 0 | 15 KHz | Normal |
| 1 | 30 KHz | Normal |
| 2 | 60 KHz | Normal, Extended |
| 3 | 120 KHz | Normal |
| 4 | 240 KHz | Normal |
| 5 | 480 KHz | Normal |
Both frequency ranges FR1 and FR2 use different 5G numerology as mentioned in the table-2. Subcarrier Spacing of 15/30 KHz is supported for below 6 GHz 5G NR where as 60/120/240 KHz is supported for mmwave bands. Maximum bandwidth of 100 MHz is supported in sub-6 GHz where as 400 MHz is supported in mmwave frequency ranges. In LTE, maximum BW of 20 MHz was used.
| Parameters | Sub-6 GHz Range | MmWave Range |
|---|---|---|
| Carrier aggregation | upto 16 carriers | |
| BW per carrier | 5/10/15/20/25/40/50/60/80/100 MHz | 50/100/200/400 MHz |
| Subcarrier spacing | 15/30/60 KHz | 60/120/240 KHz |
| Modulation Scheme | DL/UL: 256 QAM | |
| MIMO | DL: upto 8 layers, UL: upto 4 layers | DL: upto 2 layers, UL: upto 2 layers |
| Duplex mode | TDD (focus), FDD | TDD |
| Access scheme | DL: CP-OFDM, UL:CP-OFDM, DFT spread OFDM | |
| Subcarrier spacing (KHz) | 15 | 30 | 60 | 120 | 240 |
| Symbol duration (µs) | 66.7 | 33.3 | 16.7 | 8.33 | 4.17 |
| CP duration (µS) | 4.7 | 2.3 | 1.2 (Normal CP), 4.13 (Extended CP) | 0.59 | 0.29 |
| Max. nominal system BW (MHz) | 50 | 100 | 100 (sub-6 GHz), 200 (mmwave) | 400 | 400 |
| FFT size (max.) | 4096 | 4096 | 4096 | 4096 | 4096 |
| Symbols per slot | 14 | 14 | 14 (normal CP), 12 (extended CP) | 14 | 14 |
| Slots per subframe | 1 | 2 | 4 | 8 | 16 |
| Slots per frame | 10 | 20 | 40 | 80 | 160 |
5G NR Frame Structure
The figure depicts resource grid of 5G NR with symbols in time axis and subcarriers in frequency axis. 12 subcarriers form one PRB (Physical Resource Block). 5G NR supports 24 to 275 PRBs in a single slot. Occupied BW of 34.56 MHz (minimum) and 396 MHz (maximum) can be achieved for 120 KHz subcarrier spacing. One SS/PBCH Block occupies 4 OFDM Symbols in time domain and 24 PRBs in frequency domain. 5G NR SS consists of PSS and SSS as specified for LTE.
Understanding 5G Physical Layer Using PDSCH And PUSCH
Uplink channels: PRACH, PUSCH, PUCCH
There are specific physical signals present in both downlink and uplink for various purposes. Front loaded DMRS (Demodulation Reference signal) is used for both PDSCH and PUSCH channels. We will consider OFDM with CP for both downlink and uplink chain. Uplink also uses DFT Spread OFDM with CP for improved coverage.
PDSCH Channel Processing Through 5G NR Physical Layer
➤As shown in the figure, CRC is added to each of the transport blocks to provide error detection.
➤This is following by LDPC base graph as per transport block size (small or large).
➤Now transport block is segmented into code blocks. CRC is appended to each of these code blocks.
➤Each of the code blocks are individually encoded using LDPC encoder, which are rate matched after encoding process.
➤Code block concatenation is performed to form codewords for transmission over PDSCH channel. About 2 codewords are transmitted simultaneously on single PDSCH channel. Single codeword is used for 1 to 4 layers where as 2 codewords are used for 5 to 8 number of layers.
➤All the codewords are scrambled and modulated to generate complex data symbols before layer mapping. It uses QPSK, 16QAM, 64QAM and 256QAM modulation schemes.
➤The modulated data symbols are mapped to either 4 or 8 layers.
➤The layers are mapped with number of antenna ports reserved for PDSCH use and complex modulated data symbols are mapped to RBs (Resource Blocks) in the resource grid as per subcarrier spacing. Antenna ports range is {1000,...,1011}. DMRS values are inserted during resource element mapping used for channel estimation and equalization at the UE receiver. OFDM signal is generated after RE (Resource Element) mapping.
➤The downlink PDSCH is received by UE which consists of reverse modules of 5G NR physical layer in order to decode the transport block back before passing the information to upper layers.
PUSCH Channel Processing Through 5G Physical Layer
3GPP TS 38.201 : General description
3GPP TS 38.202 : Services provided by physical layer
3GPP TS 38.211 : Physical channels and modulation
3GPP TS 38.212 : Multiplexing and channel coding
3GPP TS 38.213 : Physical layer procedures for control
3GPP TS 38.214 : Physical layer procedures for data
3GPP TS 38.215 : Physical layer measurements
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