FAQ    

 

 

How do subframes in 5G differ from those in 4G LTE?

Subframes in 5G New Radio (NR) and 4G LTE differ significantly in terms of flexibility, configuration, and purpose, reflecting the advancements in technology and the broader range of use cases that 5G aims to support.

These differences highlight how 5G NR's frame and subframe structures have been designed to provide the flexibility and efficiency needed to accommodate a much wider range of use cases than 4G LTE, including those requiring high data rates, low latency, and massive connectivity.

Flexibility and Numerology:

  • In 5G NR, subframes offer much greater flexibility due to the concept of numerology, which allows for different subcarrier spacings (e.g., 15, 30, 60, 120, and 240 kHz). This flexibility enables 5G to efficiently support a wide range of applications, from ultra-reliable low-latency communications (URLLC) to massive machine-type communications (mMTC), by adjusting the subcarrier spacing and, consequently, the structure and duration of slots within a subframe.
  • Each numerology in 5G NR results in a different slot duration. For example, with 15 kHz spacing, a slot is 1 ms, but with 30 kHz, a slot is 0.5 ms, and so on. This means the number of slots per subframe can vary depending on the numerology used.
  • 4G LTE uses a fixed subcarrier spacing of 15 kHz, and each subframe is rigidly defined as 1 ms in duration, divided into two slots of 0.5 ms each. There is no concept of multiple numerologies in LTE.

Slot Configuration and Mini-Slots:

  • 5G NR's use of different numerologies allows for a dynamic number of slots within each subframe. For example, with higher subcarrier spacing, more slots can fit into a 1 ms subframe.
  • 5G NR also introduces the concept of mini-slots, which can be as short as 2, 4, or 7 OFDM symbols, allowing transmissions to start at almost any time and not be restricted to slot boundaries. This greatly increases scheduling flexibility and supports low-latency applications.
  • In contrast, 4G LTE's subframe structure is static, with each 1 ms subframe containing exactly two slots, limiting its flexibility in supporting different latency and bandwidth requirements. There are no mini-slots in LTE.

Frame Structure:

  • Both 4G LTE and 5G NR use a 10 ms radio frame, but the internal structure differs. In LTE, each frame consists of 10 subframes (each 1 ms), and each subframe has 2 slots.
  • In 5G NR, the frame is also 10 ms, but the number of slots per subframe (and per frame) depends on the numerology. For example, with 30 kHz subcarrier spacing, there are 20 slots per frame (2 slots per subframe), while with 120 kHz, there are 80 slots per frame (8 slots per subframe).

Dynamic TDD Configuration:

  • 5G NR supports a more dynamic and flexible TDD (Time Division Duplex) configuration, allowing the downlink and uplink patterns within a frame to be adjusted more readily to meet varying traffic demands. This is achieved through flexible slot-based scheduling and dynamic assignment of slots for uplink or downlink.
  • LTE also supports TDD, but with less flexibility in dynamically adjusting the downlink and uplink configuration within the frame structure. The TDD patterns are more static and predefined in LTE.

Summary Table:

Feature

4G LTE

5G NR

Subcarrier Spacing

Fixed (15 kHz)

Flexible (15, 30, 60, 120, 240 kHz)

Subframe Duration

1 ms (fixed)

1 ms (fixed), but slot duration varies

Slots per Subframe

2 (fixed)

Variable (depends on numerology)

Mini-Slots

No

Yes

TDD Flexibility

Limited

Highly dynamic

In summary, 5G NR subframes are designed for maximum flexibility and efficiency, supporting a wide range of services and requirements, while 4G LTE subframes are more rigid and less adaptable to diverse use cases.