The factors that determine the subframe structure in 5G NR include:
- Subcarrier spacing: The subcarrier spacing is the distance between two adjacent subcarriers. A higher subcarrier spacing means a wider channel bandwidth, which can support higher data rates. However, a higher subcarrier spacing is also more susceptible to interference.
- Number of subcarriers: The number of subcarriers is the total number of subcarriers in a channel. A higher number of subcarriers means a wider channel bandwidth, which can support higher data rates. However, a higher number of subcarriers also requires more complex signal processing, which can increase latency and power consumption.
- Number of symbols: The number of symbols is the total number of OFDM symbols in a subframe. A higher number of symbols means a longer subframe duration, which can support higher data rates. However, a higher number of symbols also increases latency.
In 5G NR, similar to LTE, a radio frame is fixed at 10 milliseconds (ms) and consists of 10 subframes, each of which is 1 ms long. However, unlike LTE, which has a fixed subcarrier spacing (SCS) of 15 kHz, 5G NR supports scalable numerology for more flexible deployments covering a wide range of services and carrier frequencies.
In LTE (4G), a single subcarrier spacing of 15 kHz is used. This fixed subcarrier spacing was chosen as a compromise to support various services, including voice, data, and mobile broadband, while maintaining compatibility with legacy 2G and 3G systems.
- 15 kHz Subcarrier Spacing: In LTE, each subcarrier is spaced 15 kHz apart from its neighbouring subcarriers. This spacing is uniform across the entire LTE spectrum.
- Cyclic Prefix: To ensure orthogonality between subcarriers and mitigate the effects of multipath propagation, LTE uses a fixed cyclic prefix duration for all subcarriers. The cyclic prefix is a guard interval inserted before each OFDM symbol.
5G New Radio (NR) introduces a significant departure from LTE by offering a flexible Orthogonal Frequency Division Multiplexing (OFDM) numerology, which includes variable subcarrier spacing. Here’s how 5G NR’s subcarrier spacing differs from LTE:
- Flexible Subcarrier Spacing: 5G NR supports a range of subcarrier spacings, which can vary from 15 kHz to 240 kHz. This flexibility allows 5G NR to be highly adaptable to different use cases, frequency bands, and service requirements.
- Proportional Cyclic Prefix: As the subcarrier spacing changes, the duration of the cyclic prefix is adjusted proportionally. The cyclic prefix remains an essential component of OFDM to maintain orthogonality between subcarriers. In other words, the guard interval adapts to the subcarrier spacing to ensure efficient communication.
5G subcarrier spacing (SCS) is the distance between two adjacent subcarriers in an OFDM (Orthogonal Frequency Division Multiplexing) signal. It is a key parameter that affects the bandwidth and performance of a 5G network.
5G NR supports a variety of SCS values, including 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz. These SCS values are mapped to numerologies 0, 1, 2, 3, and 4, respectively.
SCS = 15 * 2^μ
μ is the numerology
Consider the following example:
Subcarrier spacing: 30 kHz
Using the formulae above, we can calculate the following parameters:
OFDM symbol duration = 10^3 / (14 * 2^1) = 35 μs
Number of OFDM symbols per slot = 14 * 2^1 = 28
Cyclic prefix duration = 4.7 μs
Subframe duration = 1 ms
Radio frame duration = 10 ms
Below diagram will be showing subcarrier mapping based on numerology
A higher numerology indicates a wider SCS. This means that there are fewer subcarriers, but each subcarrier is wider. Wider subcarriers are more resistant to interference, but they also require more bandwidth.
The choice of numerology depends on a number of factors, including the desired bandwidth, the available spectrum, and the channel conditions. For example, a high numerology with a wide SCS may be used in a macrocell deployment to provide high throughput to users over a large area. A low numerology with a narrow SCS may be used in a small cell deployment to provide high reliability to users in dense urban areas.
- It’s Not like that every numerology can be used for every physical channel.
- Numerologies has been specified as per certain type of physical channel.
This means that not every numerology, which is a set of parameters that define the physical layer structure of a 5G NR system, can be used for every physical channel. There are specific numerologies that are specified for certain types of physical channels.
- Slot Length changes depending on numerology.
- Increase in subcarrier spacing will decrease slot length.
- Symbol per slot will same be based on slot configuration like for configuration 0 number of symbols for a slot is 14 and for configuration 1 it will be 7.
Numerology, Radio Frame Structure and Slot Length
In this setup, each subframe consists of just a single slot, resulting in a radio frame containing a total of 10 slots. Within each slot, there are 14 OFDM symbols.
In this arrangement, each subframe is comprised of 2 slots, which implies that a radio frame consists of a total of 20 slots. Within each slot, there are 14 OFDM symbols.
In this particular configuration, a subframe comprises 4 slots, resulting in a radio frame containing a total of 40 slots. Within each slot, there are 14 OFDM symbols.
Within this setup, each subframe encompasses 8 slots, signifying that a radio frame comprises a total of 80 slots. Each individual slot contains 14 OFDM symbols.
In this configuration, a subframe is composed of 16 slots, resulting in a radio frame containing a total of 160 slots. Each slot consists of 14 OFDM symbols.