5G/NR - Self Contained Slot

 

 

 

Self Contained Slot

Simply put, self contained slot is the slot that contains everything within the single slot. What do you mean by 'everything' in this context ?

The meaning of 'everything' in this context is 'DL scheduling (DCI), DL data transmission(PDSCH) and Ack/Nack for the transmission (PUCCH)'. Therefore, self contained slot mean the slot where the scheduling, data transmission and HARQ feedback happens within the same slot as examplified below.

NOTE : The example shown above is self contained slot for PDSCH. There can be self contained slot for PUSCH as well. In case of PUSCH self contained slot, you would see a slot where UL scheduling (DCI) and PUSCH transmission would happen in the same slot. In this case, k2 should be 0.

  • The same picture twice : the drawing shows two consecutive slots with identical contents, because a self-contained slot is a shape that repeats rather than a one-off arrangement.
  • Blue, red, grey, green, in that order : the blue block at the front is the DCI, and the red run is the PDSCH it grants. The grey symbols are used by neither. The green block at the end is the PUCCH carrying the Ack.
  • K0 = 0 puts the data in the slot that scheduled it : the first green arrow marks the offset from the DCI to the PDSCH. Setting it to zero keeps the grant and the data together.
  • K1 = 0 puts the Ack in the same slot again : the second green arrow marks the offset from the PDSCH to the PUCCH. Both offsets are zero, and that pair is the whole definition.
  • The grey symbols are not wasted by accident : they sit between the PDSCH and the PUCCH, and they give the UE time to stop receiving and start transmitting.

Followings are the additional topics to be covered in this note

Why self-contained slot ?

Main purpose of the purpose of a self-contained slot is to support ultra-reliable and low-latency communications (URLLC) and key benefits of self-contained slot can be listed as below ::

  • Low Latency: By allowing downlink control, downlink data, uplink control, and uplink data to occur within the same slot, the time between sending information and receiving an acknowledgment (round-trip time) is greatly reduced. This is critical for applications that require immediate or near-immediate responses, such as autonomous driving, remote surgery, or real-time gaming. I think this is the biggest benefit.
  • Improved Efficiency: Self-contained slots can improve the efficiency of the network by reducing the amount of time it takes to perform a communication cycle, thus freeing up resources for other users and services.
  • Simplified Scheduling: The base station can schedule both downlink and uplink transmissions more easily within a single slot, simplifying the scheduling algorithm and reducing the communication overhead.
  • Flexibility: This structure allows for more dynamic and flexible use of the spectrum, as different slots can be configured with different proportions of downlink and uplink according to the current demand.

The latency argument is worth putting in numbers, because it is the reason the other three benefits exist. In an ordinary slot the Ack for a PDSCH arrives some slots later. The gNB waits that long before it knows whether to retransmit. With K1 set to zero the Ack arrives in the slot that carried the data, and the retransmission decision can be made immediately.

That shortens the loop the network actually cares about. A HARQ round trip stops being a multi-slot affair and becomes a within-slot one. Every layer above it inherits the saving. Scheduling gets simpler for the same reason, since the gNB is not tracking outstanding acknowledgements across a queue of slots.

  • The HARQ loop is what shortens : the benefit is not faster transmission, it is a faster answer about whether the transmission worked.
  • Flexibility and latency come from the same property : a slot that carries both directions can be re-proportioned per slot, which is also what lets the Ack fit inside it.

What makes a slot self-contained ?

Nothing in the specification is called self-contained, and there is no field to switch it on. The arrangement is an outcome, and three separate configurations have to agree before a slot takes the shape drawn above.

The first two are the timing offsets. K0 is the slot offset from the DCI to the PDSCH it schedules. It comes from a resource allocation table, in the row that the Time domain resource assignment field of the DCI selects. K1 is the offset from the PDSCH to the Ack, and the network configures a list of candidate values that the DCI then indexes into. Both are explained on the Dynamic Scheduling page.

The third is the TDD pattern. A slot can only carry a PUCCH if it actually has uplink symbols in it, and that is set by tdd-UL-DL-ConfigurationCommon. The pattern gives each slot a number of downlink symbols and a number of uplink symbols, with flexible symbols in between. A slot configured as downlink throughout has nowhere to put the Ack, whatever K1 says.

So the recipe is short but strict. Give the slot both downlink and uplink symbols, choose K0 = 0 so the data lands in the slot that granted it, and choose K1 = 0 so the Ack lands there too. Miss any one of the three and the slot is an ordinary one.

  • Self-contained is a description, not a setting : no IE carries the name, and the shape emerges from the scheduling offsets and the TDD pattern together.
  • K0 and K1 both have to be zero : zero on one alone gives a slot that holds the grant and the data, or the data and the Ack, but not all three.
  • The TDD pattern is the precondition : uplink symbols have to exist in that slot before any choice of K1 can place a PUCCH in it.
  • The cost is the turnaround gap : symbols spent switching from receive to transmit carry nothing, which is why a self-contained slot fits less data than a downlink-only one.

What are the challenges ?

Until now (as of Dec 2023), I don't see many of the commercial device supporting this type of slots(NOTE : I have heard of some UE modem(only a few) claim to support self contained slot, but haven't heard that any network operator support this feature in the field). Does it mean that there would be too much challenges for implementing and deploying this feature ?

Yes, there are many challenges for this and some of those challenges is listed below :

  • Hardware Requirements: Existing cellular infrastructure may not support the rapid switching and processing required for self-contained slots. This necessitates upgrades to base stations and the deployment of new hardware that can handle the more complex and time-sensitive nature of self-contained slot processing.
  • Signal Processing Complexity: The base station must rapidly process downlink and uplink information within the same slot. This increases the complexity of signal processing and requires more advanced algorithms, which can increase the cost and power consumption of base stations.
  • Network Synchronization: Self-contained slots require precise synchronization between the base station and user equipment. Any timing misalignment can disrupt the tight transmission schedule, leading to decreased efficiency and increased latency.
  • Interference Management: With both uplink and downlink transmissions happening in close temporal proximity, there is an increased risk of interference, which requires advanced interference management and cancellation techniques.
  • Backward Compatibility: As 5G networks are being deployed alongside existing 4G LTE infrastructure, ensuring backward compatibility can be challenging. Devices and network elements need to be designed to support both old and new protocols and slot structures.
  • UE Capabilities: Not all user devices may support the new slot structures. Users with older devices may not benefit from the improvements offered by self-contained slots, leading to a fragmented experience.
  • Energy Consumption: More complex processing could lead to increased energy consumption in both the network infrastructure and user devices, potentially impacting battery life and operational costs.
  • Resource Allocation: Determining the optimal allocation of resources for downlink and uplink in a self-contained slot requires sophisticated algorithms, especially in dynamic network environments with varying traffic patterns and load.

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