Understanding 5G – Part 2
5G NR Physical Layer Overview
This is the second article in a multi part series on 5G technology and how it will help transform the wireless networks of tomorrow. We provided a brief introduction to 5G in Part 1 https://www.ttswireless.com/5g-overview-part1-introduction/ . Now we will forward the 5G-NR (New Radio) physical layer.
5G NR Resource Grid
Let’s begin with an overview of the 5G-NR resource grid displayed in figure below. The physical layer of 5G-NR carries over many aspects of LTE including:
- Can be described in terms of a resource grid in frequency and time domain
- Built on OFDMA based physical resource blocks consisting of 12 x subcarriers
- Frame structure of 10ms in length which is further divided into 10 x 1ms subframes
- FDD and TDD operations supported
- Modulation ranges from π/2-BPSK to 256-QAM as with advanced LTE
Important differences to current LTE include:
- DFT spreading for 5G-NR is optional for uplink transmission. It may be enabled or disabled, unlike LTE where DFTS-OFDM (also known as SC-FDMA) was mandated.
- 5G NR specifications allow for significant flexibility in configuration of fundamental parameters of the physical layer which include the subcarrier spacing and the number of slots per subframe.
A supported subcarrier spacing is termed a “numerology” in NR and is summarized in the below table. Each numerology has a set number of slots per subframe. Since each frame always consists of 10 x subframes, the slots per frame is simply a given by a multiple of 10.
Note that only 60kHz suppors extended cyclic prefix.
Frequency Range Designations and Operating Bands
5G-NR is broken out into two distinct frequency range designations: FR1 & FR2 (commonly known as “mmWave”). FR1 does not support subcarrier spacing above 60kHz, while this is the minimum for FR2.
Each frequency range designation is then further divided into a number of NR Operating bands that are given by an “n” prefix followed by an integer. FR1 has 84 operating bands whilst FR2 only has 4 operating bands. These operating bands vary significantly in bandwidth and are detailed in 3GPP 38.101-1 & 38.101-2. The operating bands for FR2 are larger and in the order of GHz wide.
*The maximum transmission bandwidth will also depend on subcarrier spacing and number of resource blocks
5G-NR channel raster numbering is given by NR Absolute Radio Frequency Channel Number (NR-ARFCN) and goes up to 3,279,165 for FR2.
Spatially Planned Parameters
Spatially parameter planning for NR requires planning of PCI and PRACH like with LTE. However, it is important to note that there are double the number of PCIs supported in 5G. There are 336 PCI groups of 3 identifies which to give a total of 1008 unique PCI numbers. For the PRACH, the root sequence numbers range from 0 to 837 inclusive as with LTE.
Keep following for more bits on details behind 5G!