Understanding Self-Contained Slots in 5G NR

One of the most significant architectural shifts in 5G New Radio (NR) compared to LTE is the focus on minimizing latency. A key enabler for Ultra-Reliable Low-Latency Communication (URLLC) is the concept of the Self-Contained Slot.

While "Self-Contained Slot" is not strictly a formal 3GPP term, it is widely used in the industry to describe a slot configuration that contains both Downlink (DL) and Uplink (UL) transmission opportunities, separated by a Guard Period (GP), within a single slot duration. This design allows for immediate feedback and faster turn-around times.

Low Latency

In legacy LTE TDD systems, the HARQ (Hybrid Automatic Repeat Request) process often spans multiple subframes. A device might receive data in subframe n, process it, and send the acknowledgement (ACK/NACK) in subframe n+k (where k >= 4). This introduces significant latency.

5G NR addresses this by enabling a much tighter loop: receive data and send acknowledgement in the same slot.

Anatomy of a Self-Contained Slot

A typical Self-Contained Slot is structured to handle a complete transmission cycle. It generally consists of three parts:

1. Downlink (DL) Control: The beginning of the slot is almost always reserved for DL control information (PDCCH). This informs the UE about the scheduling (e.g., "Receive data now" or "Transmit data soon").
2. Guard Period (GP): A necessary silent period that allows the device hardware to switch from reception mode to transmission mode (Rx-to-Tx switching).
3. Uplink (UL) Control: The end of the slot is used for UL control information (PUCCH), where the UE sends its ACK/NACK feedback.

Types of Self-Contained Slots

Depending on the traffic flow, the slot can be configured as DL-dominant or UL-dominant.

1. DL-Dominant Self-Contained Slot

In this configuration, the majority of the slot is used for Downlink data (PDSCH).

• Structure: DL Control + DL Data + GP + UL Control
• Use Case: Primarily for high-throughput downlink data. The UE receives a large chunk of data and immediately sends back the ACK/NACK at the end of the same slot.

2. UL-Dominant Self-Contained Slot

In this configuration, the majority of the slot is used for Uplink data (PUSCH).

• Structure: DL Control + GP + UL Data + UL Control
• Use Case: Primarily for uplink transmission. The UE receives a scheduling grant at the start of the slot and immediately transmits its data.

Benefits

• Significantly Reduced Latency: By closing the HARQ loop within a single slot, the Round Trip Time (RTT) is drastically lower than in LTE.
• Flexible TDD Operation: The integrated Guard Period allows for fast switching between DL and UL, essential for TDD deployments in higher frequency bands (mmWave) where beam reciprocity is used.
• Efficient Spectrum Usage: Dynamic adaptation of the DL/UL ratio within slots allows the network to better match instantaneous traffic demands.