5G/NR - Scheduling

Scheduling Overview

Scheduling is the process of allocating resources for transmitting data. As in LTE (actually in all cellular communication), NR scheduling is dictated by Network and UE is just following what network tells. Overall scheduling mechanism in NR is pretty much similar to LTE scheduling, but NR has finer granularity than LTE especially in terms of time domain scheduling at physicallayer. In this page, I will just go through very high level view of NR scheduling process. For the detailed procedure/operation, I will create separate pages and point to those pages.

High Level Factors for Scheduling

At high level view, NR Scheduling is not much different from LTE scheduling in terms of overall concept. In this page, I will describe on the overview of NR Scheduling and the further details will be linked to several separate pages. The overall skelleton of this page is based on Based on "38.300 - 10 Scheduling" even though most part is written in my own words and illustrations.

Scheduling Structure at PHY Layer

In terms of Physical layer, there are roughly two categories of scheduling that are time domain scheduling and frequency domain scheduling. Frequency Domain Scheduling in NR is almost same as LTE. Time Domain Scheduling in NR is similar to LTE TDD schduling, but there are several time domain scheduling factors that are not supported in LTE.

Actually Scheduling is a huge part in NR protocol and there are so many factors and terminologies that you need to understand. Since it is difficult to put all those details in one page, I have written several different pages with the details for each individual aspect/factors listed below. Just try to get the big picture about the scheduling in this page and follow through the link mentioned in comments column in the following table.

Domain	Factors/Parameters	Comments
Time	TDD DL/UL CommonConfiguration	In terms of concept, this is very similar to LTE TDD DL/UL configuration, but NR configuration provides more flexibility. For the details, refer to this page.
	TDD DL/UL DedicatedConfiguration	In LTE, once a subframe is scheduled all the symbols in the subframe are used for data transmission. But in NR, it is possible to set only a few symbols in the subframe for data transmission and reception. It means in NR, symbol level scheduling is possible. TDD DL/UL Dedicated Configuration is one of mechanisms for symbol level scheduling. For the details, refer to this page.
	SlotFormat	SlotFormat is also a kind of mechanism for symbol level scheduling. 3GPP defines 61 different type of symbol level scheduling type (i.e, SlotFormat). For the details, refer to this page.
	SLIV	This is another kind of mechanism for symbol level scheduling for a slot. For the details, refer to this page.
	K0, K1, K2	In LTE TDD, there are simiar parameters like K1 and K2(K1 is similar to K in TDD LTE and K2 is similar to k in TDD LTE). But K1, K2 in NR has more flexibility. However, there is no parameters in LTE that is equivalent to K0. In other words, in LTE K0 is always 0, so we don't need to define this as a parameter. For the details of K0, K1, K2, refer to this page.
Frequency	RA Type	This is similar to LTE RA Type. NR supports two RA Types - Type 0 (Bitmap) and Type 1(RIV). For the details of RA Types, refer to this page.
Frequency	RBG Size	This is same concept as RBG size in LTE Resource Allocation Type 0. In LTE, RBG size can be 1,2,3,4 depending on channel bandwidth. Similarily in in NR the RBG size can only be 2,4,8 or 16 depending on the bandwidth of the BWP. But in NR, for the same BWP width, the RBG size varies depending on configuration type. For the details of RBG Size, refer to this page.

NOTE : In reality, the scheduling is applied dynamically in the context of HARQ process. I would suggest you to look into this page to get the overall understandins on how HARQ process works.

Type of Downlink Scheduling

There are two types of scheduling for downlink. One is called 'Dynamic Scheduling' and the other one SPS(Semi Persistent Scheduling'). Dynamic scheduling is the mechanism in which each and every PDSCH is scheduled by DCI(DCI 1_0 or DCI 1_1). SPS is the mechanism in which the PDSCH transmission is scheduled by RRC message. SPS is explained in detail here.

NOTE : For simplicity, HARQ Ack/Nack process and (possible) retransmission is not shown in this illustration.

NOTE : CS stands for Configured Scheduling

Types for Uplink Scheduling

As in downlink scheduling, there are two types of scheduling for uplink. One is called 'Dynamic Scheduling' and the other one CS(Configured Scheduling'). Dynamic scheduling is the mechanism in which each and every PUSCH is scheduled by DCI(DCI 0_0 or DCI 0_1). CS is the mechanism in which the PUSCH transmission is scheduled by RRC message. The further details about CS is explained here.

Trigger for PUSCH scheduling can be summarized as in the following table (I wrote this table based on the description on 38.214-6.1).

ConfiguredGrantConfig	rrc-ConfiguredUplinkGrant	Scheduling Trigger
Not Configured	Not Configured	Dynamic Scheduling by DCI < Without CS > shown below
Configured	Not Configured	Semi-static scheduling with DCI Trigger < With Type 2 CS > shown below
Configured	Configured	Semi-static scheduling without DCI Trigger < With Type 1 CS > shown below

NOTE :: For simplicity, (possible) retransmission is not shown in this illustration.

NOTE :: CS stands for Configured Scheduling