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Understanding 5G NR Radio Frames and Slots

Understanding 5G NR Radio Frames and Slots
Understanding 5G NR Radio Frames and Slots

The 5G New Radio (NR) framework, as specified by the 3rd Generation Partnership Project (3GPP), introduces a sophisticated structure for radio frames and slots to support diverse deployment scenarios and end-user applications. This article provides an in-depth analysis of these structures, their configurations, and their significance in 5G NR.


5G NR Radio Frames

Fixed Duration and System Frame Number (SFN)

A 5G radio frame has a fixed duration of 10 milliseconds (ms) and is indexed using the System Frame Number (SFN), which ranges from 0 to 1023, cycling every 10.24 seconds. The SFN is acquired from the Physical Broadcast Channel (PBCH) when accessing a cell. The SFN is represented using 10 bits, with the 6 Most Significant Bits (MSB) included in the layer 3 payload of the PBCH via RRC signaling, and the 4 Least Significant Bits (LSB) added by the Physical layer.


Subframes and Slot Duration

Each 5G subframe has a fixed duration of 1 ms, resulting in 10 subframes per radio frame. The slot and symbol durations vary depending on the numerology, as outlined in 3GPP TS 38.211. The number of slots per subframe and the number of symbols per subframe depend on the numerology, with the normal cyclic prefix containing 14 symbols per slot and the extended cyclic prefix containing 12 symbols per slot.


Numerology and Slot Configurations

Numerology Impact on Slots

Numerology in 5G NR determines subcarrier spacing and, consequently, the slot duration. The numerologies specified in 3GPP TS 38.211 include subcarrier spacings of 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz. Figure 1 illustrates the symbols per 1 ms subframe for each numerology, showing that the slot durations vary accordingly.


Frequency Division Duplex (FDD) and Time Division Duplex (TDD)

In FDD, symbols in the downlink carrier are used for Base Station transmissions, while symbols in the uplink carrier are used for UE transmissions. In TDD, the symbols are divided into subsets for Base Station transmissions, UE transmissions, and a guard period to accommodate propagation delays and transceiver switching.


Uplink-Downlink Configuration

Uplink-Downlink Switching in TDD

The uplink-downlink configuration for TDD is flexible and can be dynamically adjusted based on short-term requirements. The Base Station can broadcast an uplink-downlink configuration in SIB1, which is not constrained to a pattern specified by 3GPP, allowing for dynamic reconfiguration using RRC signaling or layer 1 signaling on the PDCCH.


Guard Periods

Guard periods are crucial for accommodating propagation delays and transceiver switching. Figure 2 illustrates a general uplink-downlink configuration with a guard period when switching from downlink to uplink.


Dynamic TDD and Cross-Link Interference

Dynamic TDD allows rapid reconfiguration of symbols between uplink and downlink. However, this can lead to cross-link interference, where uplink transmissions in one cell interfere with downlink transmissions in a neighboring cell, and vice versa. Figure 3 illustrates the concept of cross-link interference.


Slot Format Indicator (SFI)

Configuring Slot Formats

Slot formats define the uplink and downlink transmission patterns within a slot. These can be pre-configured using RRC signaling and dynamically adjusted using DCI Format 2_0. Each Slot Format Combination is linked to a specific Serving Cell and has a specific position within the payload of DCI Format 2_0.


Slot Format Combinations

3GPP TS 38.213 specifies 56 standard slot formats, primarily based on cycles of 14 symbols (one slot), with some based on 7 symbols (half a slot) for low latency applications. Slot Format 0 is for downlink-only symbols, and Slot Format 1 is for uplink-only symbols. Figure 4 illustrates the interleaving of uplink and downlink Slot Formats in an FDD cell configuration.


Conclusion

The 5G NR framework defined by 3GPP introduces a versatile and robust structure for radio frames and slots, enabling diverse deployment scenarios and dynamic resource allocation. Understanding these configurations is crucial for optimizing 5G network performance and ensuring reliable communication.


References

  • 3GPP. (n.d.). TS 38.211 V15.2.0: NR; Physical channels and modulation. Retrieved from 3GPP

  • 3GPP. (n.d.). TS 38.213 V15.2.0: NR; Physical layer procedures for control. Retrieved from 3GPP

  • 3GPP. (n.d.). TS 38.331 V15.2.0: NR; Radio Resource Control (RRC) protocol specification. Retrieved from 3GPP

For further reading and access to the complete list of specifications, visit the 3GPP website at www.3gpp.org.

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