Non-Slot based scheduling – URLLC Design Considerations

The 5G frame structure provides the foundation for the timing of physical signals. In physical layer aspects, the timing is different for the data block transmission, symbol transmission, and synchronization. However, the radio frame and subframes are fixed in length like LTE in NR.

Independent of data block transmission timing, to support multiple numerologies 5G frame structure offers a flexible substructure for symbol transmission timing. The Slot and symbols are scalable depending on the subcarrier spacing.

The standard slot occupies either 14 or 12 OFDM symbols. Slot length scales with subcarrier spacing.

5g non slot based schedulingThe length of the Slot = 1ms/2µ

As mentioned, the slot length changes based on subcarrier spacing ‘µ’. In other words, the slot length gets shorter as subcarrier spacing gets wider in OFDM.

OFDM symbols in a slot can be classified as ‘downlink’, ‘flexible’, or ‘uplink’. In NR slot format, the DL and UL assignment change at the symbol level whereas in LTE Time Division Duplex the UL/DL assignment is performed at a subframe level.

5G states a subslot structure commonly known as mini-slot. This mini-slot can be used for Ultra-Reliable Low Latency Communications use case and unlicensed band operations. Mini-slots also plays a vital role in LTE-NR coexistence and beamforming.

Based on the specification in 38.912 mini-slot (Non-slot scheduling) is used for DL & UL scheduling of URLL (Ultra-Reliable Low-Latency communication)

Mini-slots consists of 7, 4, or 2 OFDM symbols. Mini-slots can also start immediately without any wait for slot boundaries which helps them in supporting low latency applications.

The Objective of URLLC design is targeted in terms of one-way User Plane latency to be below or equal to 0.5or 1ms. The key enablers are

  • Non-Slot based scheduling
  • Reduced processing time at the User Equipment
  • Front-loaded DMRS makes early channel estimation and coherent demodulation

Mini-slots consists of 7, 4, or 2 OFDM symbols. Mini-slots can also start immediately without any wait for slot boundaries which helps them in supporting low latency applications.

The Objective of URLLC design is targeted in terms of one-way User Plane latency to be below or equal to 0.5or 1ms. The key enablers are

  • Non-Slot based scheduling
  • Reduced processing time at the User Equipment
  • Front-loaded DMRS makes early channel estimation and coherent demodulation

embb transmission

Non-Preemptive scheduling of URLLC doesn’t interrupt on-going eMBB data. The URLLC resources are allocated in the upcoming TTI along with the eMMB data. But there could be a delay in scheduling URLLC service in the upcoming TTI.

 

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