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Overview of 5G NR Numerology
In the context of 5G New Radio (NR) wireless communication systems, numerology refers to the set of parameters that define the structure of radio frames and subframes, as well as the spacing of resource elements in the frequency and time domains. This framework is essential for supporting diverse use cases, including enhanced mobile broadband (eMBB), ultra-reliable low latency communication (URLLC), and massive machine-type communication (mMTC).
According to the provided documentation, 5G NR numerology is defined by several key parameters:
- Subcarrier Spacing (Δf or SCS): This represents the frequency difference between consecutive subcarriers in the frequency domain. Unlike LTE, which uses a fixed subcarrier spacing, 5G supports multiple subcarrier spacings to cater to different requirements. Supported values include 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz. In some specifications, these are derived as 2^μ × 15 kHz, where μ is the numerology index ranging from 0 to 4 (or up to 5 in some contexts). For instance, 15 kHz is typically used for eMBB, while larger spacings like 60 kHz support URLLC and mobility requirements up to 500 km/h.
- Numerology Index (μ): This integer uniquely identifies the combination of subcarrier spacing and slot duration. It allows the system to configure different transmission numerologies on the same carrier.
- Slot Duration (Ts): The time domain is divided into slots, with durations varying based on the subcarrier spacing. Common slot durations include 1 ms and 0.5 ms. The number of slots per subframe and per frame increases with the numerology index; for example, with μ=0 (15 kHz SCS), there is 1 slot per subframe and 10 slots per frame, while with μ=2 (60 kHz SCS), there are 4 slots per subframe and 40 slots per frame.
- Frame Structure: A 5G frame has a duration of 10 ms and consists of 10 subframes, each of 1 ms. Each subframe contains one or more slots depending on the numerology.
Resource Grid and Elements
The fundamental unit of the 5G physical layer is the Resource Element (RE), defined by one subcarrier in the frequency domain and one OFDM symbol in the time domain.
- Resource Block (RB): An RB consists of 12 consecutive subcarriers in the frequency domain. The resource grid is formed by multiple RBs in frequency and multiple OFDM symbols in time.
- Resource Grid Alignment: Resource block boundaries are aligned across different numerologies. For example, two resource blocks at a subcarrier spacing of Δf occupy the same frequency range as one resource block at a spacing of 2Δf. This allows for efficient spectrum utilization.
- OFDM Symbols and Cyclic Prefix (CP): Each slot consists of a number of OFDM symbols. Typically, a slot has 14 symbols for normal CP and 12 symbols for extended CP. The CP duration is proportionally reduced for larger subcarrier spacings to maintain a consistent overhead ratio (approximately 7.03% for normal CP). Extended CP is specifically used for 60 kHz subcarrier spacing to mitigate delay spread in certain scenarios.
Supported Transmission Numerologies and Parameters
The documentation details specific parameters associated with each numerology. The following table summarizes the relationship between subcarrier spacing, symbol duration, CP duration, maximum bandwidth, and slot structure:
| Subcarrier Spacing (kHz) | Symbol Duration (μs) | CP Duration (μs) | Max. Nominal System BW (MHz) | Symbols per Slot | Slots per Subframe | Slots per Frame |
|---|---|---|---|---|---|---|
| 15 | 66.7 | 4.7 | 50 | 14 | 1 | 10 |
| 30 | 33.3 | 2.3 | 100 | 14 | 2 | 20 |
| 60 | 16.7 | 1.2 (Normal), 4.13 (Extended) | 100 (sub-6 GHz), 200 (mmwave) | 14 (Normal), 12 (Extended) | 4 | 40 |
| 120 | 8.33 | 0.59 | 400 | 14 | 8 | 80 |
| 240 | 4.17 | 0.29 | 400 | 14 | 16 | 160 |
Note: The FFT size for these configurations is typically 4096 points, supporting up to 3300 data subcarriers for a maximum bandwidth of 400 MHz.
Frequency Ranges and Use Cases
5G NR operates in two primary frequency ranges: * FR1: Frequencies below 6 GHz (450 MHz to 6000 MHz). * FR2: Millimeter wave frequencies above 6 GHz (24250 MHz to 52600 MHz).
The choice of numerology often correlates with the specific use case (eMBB, URLLC, mMTC) and the frequency range. For instance, larger subcarrier spacings (e.g., 120 kHz, 240 kHz) are more common in FR2 to compensate for the reduced symbol duration and support high data rates.