The structure of a 5G radio frame, defined under the 5G New Radio (NR) standard, is designed to support a wide range of frequencies, bandwidths, and use cases, from enhanced mobile broadband (eMBB) to ultra-reliable low-latency communications (URLLC) and massive machine-type communications (mMTC). Here's an overview of its structure:
Frame Duration: Like 4G LTE, a 5G NR radio frame has a duration of 10 milliseconds (ms) and is divided into 10 subframes, each 1 ms long.
Subframes and Slots: Each subframe can be further divided into slots. The number of slots per subframe in 5G NR is flexible and depends on the numerology (μ) used, which is defined by the subcarrier spacing. The subcarrier spacing can vary from 15 kHz (used in LTE) up to 120 kHz or more, with the spacing doubling with each increase in numerology. This flexibility allows 5G NR to support different deployment scenarios and bandwidth requirements.
Slot Formats in TDD: A slot in TDD can be a mix of downlink, uplink, and flexible symbols within the same slot. This adaptability is crucial for accommodating diverse traffic demands and for TDD (Time Division Duplex) configurations. (NOTE: Flexible symbols are the symbols that can be used for downlink or uplink depending on scheduling.)
Numerology (μ): 5G NR introduces the concept of numerology, which determines the subcarrier spacing and the number of slots per subframe. The most common numerologies are:
- μ = 0: 15 kHz subcarrier spacing (same as LTE)
- μ = 1: 30 kHz subcarrier spacing
- μ = 2: 60 kHz subcarrier spacing
- μ = 3: 120 kHz subcarrier spacing
- μ = 4: 240 kHz subcarrier spacing (mainly for FR2, i.e., mmWave)
As the numerology increases, the slot duration decreases, allowing for lower latency and higher throughput.
Resource Grid: The radio frame is represented as a resource grid, which is a time-frequency matrix. The horizontal axis represents time (organized into frames, subframes, slots, and OFDM symbols), and the vertical axis represents frequency (organized into subcarriers). Each cell in this grid is called a Resource Element (RE).
Bandwidth Parts (BWPs): 5G NR introduces the concept of Bandwidth Parts, which allows a device to operate on a portion of the total carrier bandwidth. This helps reduce power consumption and enables efficient support for devices with different capabilities.
Carrier Aggregation and Spectrum Flexibility: 5G NR supports carrier aggregation and can operate in both sub-6 GHz (FR1) and mmWave (FR2) frequency ranges, providing high flexibility in spectrum usage.
Summary Table:
Parameter |
Description |
Frame Duration |
10 ms |
Subframes per Frame |
10 (each 1 ms) |
Slots per Subframe |
Varies with numerology (2, 4, 8, 16, etc.) |
Subcarrier Spacing |
15, 30, 60, 120, 240 kHz |
Resource Elements |
1 subcarrier × 1 OFDM symbol |
Bandwidth Part |
Subset of total bandwidth for UE operation |