How LTE Stuff Works?: 5G NR: Time Domain

NR numerologies and frame structure are discussed in the post: 5G NR: Numerologies and FrameStructure. This post extends the discussion about NR time-domain frame/slot structure.

A numerology is defined by Sub Carrier Spacing (SCS) and Cyclic Prefix (CP). Irrespective of the numerology, a radio frame length is 10 ms and subframe length is 1 ms.

In LTE, a subframe is used as a minimum scheduling unit in time-domain while in NR, a slot is used as a dynamic scheduling unit. Additionally, NR supports transmission based on mini-slot (a fraction of a slot) as a minimum scheduling unit. So, it is very important to understand the slot structure and other details about slot-based scheduling etc.

Slots:

Slot Structure:

In NR, the number of OFDM symbols per slot are fixed (= 14 with normal CP and =12 with extended CP)

In LTE, the number of slots per subframe is fixed (= 2). But in NR, the number of slots per subframe varies with numerology (increases with SCS).

OFDM symbol duration reduces with increased SCS -> since the number of OFDM symbols per slot are fixed, slot duration reduces with increased SCS -> since slot duration reduces but subframe duration is fixed, more slots can fit within a subframe.

The following table compares time-domain parameters of LTE and NR for normal CP.

Parameter	LTE	NR
Radio frame length	10 ms	10 ms
Subframe length	1 ms	1 ms
No. of OFDM symbols in a slot	14	14
No. of slots in a subframe	2	Numerology dependent

The following table summarises number of slots in a subframe/frame for each numerology for normal CP.

µ	SCS	No. of slots per subframe = 2^µ	No. of slots per radio frame = 10 * 2^µ	slot duration (ms)
0	15 kHz	1	10	1
1	30 kHz	2	20	0.5
2	60 kHz	4	40	0.25
3	120 kHz	8	80	0.125
4	240 kHz	16	160	0.0625

The following figure illustrates the number of slots in a subframe and slot duration for each numerology for normal CP (click on the image to enlarge).

Slot Formats:

A slot can be classified as downlink (all symbols are dedicated for downlink) or uplink (all symbols are dedicated for uplink) or mixed uplink and downlink transmissions.

In the case of FDD, all symbols within a slot for a downlink carrier are used for downlink transmissions and all symbols within a slot for an uplink carrier are used for uplink transmissions.

TDD:

In TDD, it is possible that slot may not be configured to be fully used for downlink or for uplink. Similar to LTE TDD system, a guard period is necessary for transceiver switching from downlink to uplink and to allow timing advance in the uplink.

NR TDD uses flexible slot configuration. OFDM symbols in a slot can be classified as 'downlink', 'flexible', or 'uplink'. Flexible symbol can be configured either for uplink or for downlink transmissions.

One of intentions of the introduction of the flexible symbols within a slot are to handle the required guard period in NR TDD.

In LTE TDD, UL/DL configuration provided via SIB1 informs UEs that a subframe is used for uplink, downlink or as a special subframe. The configuration of special subframe is also provided via SIB1 which contains the guarding necessary for both timing advance and transceiver switching.

In NR, the configuration of slot format can be done in static, semi-static or fully dynamic fashion. This is discussed in the upcoming sections.

Slot Configuration and slot format determination:

Slot configuration procedure is thoroughly discussed in section 11.1 of 38.213.

NR supports the configuration of slot format in static, semi-static or fully dynamic fashion. Static and semi-static slot configuration is done using RRC while dynamic slot configuration is done using PDCCH DCI.

In TDD, for small/isolated cells, dynamic TDD is suitable to adapt to traffic variations, while for large cells, semi-static TDD may be more suitable for handling interference issues than fully dynamic TDD.

All the mechanisms used in NR to provide the UE with the required DL/UL transmission pattern are discussed in the subsequent sections. It is important to note that the network can combine some or all of the mechanisms discussed below to provide the UE with a desired DL/UL transmission pattern.

Note that if a slot configuration is not provided by the network, all the slots/symbols are considered as flexible by default.

Slot configuration via RRC consists of two parts. First part is configured by the IE tdd-UL-DL-ConfigurationCommon which provides all the UEs in the cell with cell-specific DL/UL pattern.

The second part is configured by the IE tdd-UL-DL-ConfigurationDedicated via dedicated RRC signalling. This UE specific configuration further modifies/allocates the unallocated (flexible) slots and symbols by tdd-UL-DL-ConfigurationCommon.

1a. Slot configuration using RRC (tdd-UL-DL-ConfigurationCommon):

The IE tdd-UL-DL-ConfigurationCommon is either broadcasted within SIB1 or configured to the UE using dedicated RRC signalling.

When provided via SIB1, this IE is optionally present for TDD cells. For FDD cells, this IE is not configured by the network.

When provided via dedicated RRC signalling, this IE is mandatory present upon SpCell change and upon serving cell (SCell with SSB or PSCell) addition. Otherwise, this IE is not configured by the network.

The structure of the IE tdd-UL-DL-ConfigurationCommon is given below.

*tdd-UL-DL-ConfigurationCommon*
referenceSubcarrierSpacing	ENUMERATED {kHz15, kHz30, kHz60, kHz120, kHz240, spare3, spare2, spare1}
*pattern1*	TDD-UL-DL-Pattern
*pattern2*	TDD-UL-DL-Pattern
*TDD-UL-DL-Pattern*
dl-UL-TransmissionPeriodicity	ENUMERATED {ms0p5, ms0p625, ms1, ms1p25, ms2, ms2p5, ms5, ms10}
nrofDownlinkSlots	INTEGER (0..320)
nrofDownlinkSymbols	INTEGER (0..13)
nrofUplinkSlots	INTEGER (0..320)
nrofUplinkSymbols	INTEGER (0..13)
dl-UL-TransmissionPeriodicity-v1530	ENUMERATED {ms3, ms4}

This IE configures the UE with at least one DL/UL pattern. pattern1 is mandatory and pattern2 is optional but by including pattern2, the network can have additional scheduling flexibility. Both pattern1 and pattern2 contain same parameters but usually of different values.

The procedure for determining DL/UL pattern depends upon whether or not pattern2 is configured within tdd-UL-DL-ConfigurationCommon.

Case1: only pattern1 is configured:

In this case a single DL/UL pattern is repeated periodically according to dl-UL-TransmissionPeriodicity. Steps to determine DL/UL pattern is given below;

Let P1 = dl-UL-TransmissionPeriodicity, d_slots,1 = nrofDownlinkSlots, u_slots,1 = nrofUplinkSlots, d_sym,1 = nrofDownlinkSymbols, and u_sym,1 = nrofUplinkSymbols;

1. Determine µ from referenceSubcarrierSpacing

2. Determine the total no. of slots (S1) within the slot configuration period (P1); S1 = P1 x 2^µ, for example, let P1 = 2.5 ms and µ = 3, in this case, there are 20 slots within the slot configuration period.

3. From total S1 slots reserve first d_slots,1 slots for downlink symbols and last u_slots,1 slots for uplink symbols.

4. Now, reserve first d_sym,1 symbols within the slot immediately following the last full downlink slot and last u_sym,1symbols in the slot preceding the first full uplink slot.

5. Consider the remaining symbols as flexible symbols. These flexible symbols can further be allocated to either downlink or uplink by making use of dedicated configuration which will be discussed in the next section.

No. of flexible symbols = (S1 - d_slots,1 - u_slots,1).14 - d_sym,1 - u_sym,1

Note: Periods are initialised such that, the first symbol every 20 ÷ P periods is a first symbol in an even numbered radio frame.

The following example illustrates how slots and symbols are distributed when only pattern1 is configured;

Case2: both pattern1 and pattern2 are configured:

In this case two DL/UL patterns (pattern1 and pattern2) are placed next to each other. These two concatenated patterns jointly repeat with periodicity given by dl-UL-TransmissionPeriodicity (from pattern1) + dl-UL-TransmissionPeriodicity (from pattern2).

Steps to determine DL/UL pattern is given below;

1. The UE first determines µ, S1, P1, d_slots,1,u_slots,1, d_sym,1, and u_sym,1 according to procedure defined in Case1.

2. Similarly, UE determines S2, P2, d_slots,2,u_slots,2, d_sym,2, and u_sym,2 from parameters given by pattern2.

3. Determine the total no. of slots (S) within the slot configuration period; S = S1+S2 = (P1+P2).2^µ, for example, let P1 = 2.5 ms, P2 = 2.5 ms and µ = 3, there are 40 slots within the configuration period.

4. During first S1 slots of total S slots, allocate slots/symbols by following steps 3, 4 and 5 from Case1 and using parameters from pattern1.

5. During last S2 slots of total S slots, allocate slots/symbols by following steps 3, 4 and 5 from Case1 and using parameters from pattern2.

Note1: Periods are initialised such that, the first symbol every 20 ÷ (P1+P2) periods is a first symbol in an even numbered radio frame. The network should configure P1 and P2 such that, P1 + P2 divides 20 ms.

The following example illustrates how slots and symbols are distributed when both pattern1 and pattern2 are configured, click on the image to enlarge the image;

1b. Slot configuration using RRC (tdd-UL-DL-ConfigurationDedicated):

As discussed in the previous section, by using cell-specific configuration (tdd-UL-DL-ConfigurationCommon), the network configures some slots/symbols as downlink, some as uplink and the rest as flexible.

The UE specific tweak to the slot configuration is necessary to help the network adjust DL/UL pattern based on the UE needs.

The network sends the UE-specific slot configuration using IE tdd-UL-DL-ConfigurationDedicated which further modifies/allocates the unallocated (flexible) slots and symbols.

The IE tdd-UL-DL- ConfigurationDedicated is optional and if the network doesn’t configure this IE, the UE uses tdd-UL-DL-ConfigurationCommon alone to derive the slot configuration.

The configuration in tdd-UL-DL- ConfigurationDedicated overrides only flexible symbols per slot over the number of slots as provided by tdd-UL-DL- ConfigurationCommon i.e., this dedicated configuration cannot change slots/symbols which are already allocated for downlink/uplink via tdd-UL-DL-ConfigurationCommon.

The contents of tdd-UL-DL- ConfigurationDedicated are presented below;

*tdd-UL-DL-ConfigurationDedicated*
slotSpecificConfigurationsToAddModList	List of 1 to 320 TDD-UL-DL-SlotConfig
slotSpecificConfigurationsToreleaseList	List of 1 to 320 TDD-UL-DL-SlotIndex
*TDD-UL-DL-SlotConfig*
slotIndex	INTEGER (0..319)
symbols
allDownlink	NULL
allUplink	NULL
explicit
nrofDownlinkSymbols	INTEGER (0..13)
nrofUplinkSymbols	INTEGER (0..13)

The tdd-UL-DL-ConfigurationDedicated provides individual slot configuration(s) using slotSpecificConfigurationsToAddModList. Each slot configuration contains the following;

slotIndex identifies a slot within a slot configuration period given in tdd-UL-DL-configurationCommon.

symbols structure provides the direction (downlink or uplink) for the symbols within the slot that is being configured.

- allDownlink indicates that all symbols in this slot are used for downlink;

- allUplink indicates that all symbols in this slot are used for uplink;

- explicit indicates explicitly how many symbols in the beginning and end of this slot are allocated to downlink and uplink, respectively.

- nrofDownlinkSymbols is the number of consecutive downlink symbols in the beginning of the slot identified by slotIndex.

- nrofUplinkSymbols is the number of consecutive uplink symbols in the end of the slot identified by slotIndex.

If nrofDownlinkSymbols is not provided, there are no downlink symbols in the slot and if nrofUplinkSymbols is not provided, there are no uplink symbols in the slot. The remaining symbols in the slot are flexible.

The following illustration shows how UE-specific slot configuration overrides flexible slots/symbols provided by the cell-specific configuration, click on the image to view it in full;

It is important to note that a symbol can only be flexible if and only if both UE- and cell-specific slot configurations indicate that the respective symbol to be flexible.

2. Dynamic slot configuration using group scheduling:

This mechanism is mainly intended for the devices that are not being currently scheduled; the devices that are being scheduled would receive the transmit/receive pattern via scheduling grants/assignments (discussed in next section).

As discussed already, the slot configuration using RRC is done in cell-specific and UE-specific manner. The resulting slot configuration may have some more flexible slots/symbols left unallocated.

By making use of layer1 signalling, the remaining (if any) flexible symbols can dynamically be reconfigured. PDCCH DCI format 2_0 whose CRC is scrambled with SFI-RNTI is used for this purpose. For more information on DCI formats, refer to DCI Formats in 5G NR.

DCI format 2_0 is designed to provide slot format indicator (SFI)(s) for a group of UEs. Multiple UEs in the group are allocated with same SFI-RNTI and hence all those UEs decode same PDCCH (DCI). Each UE extracts its own SFI based on the position of SFI within DCI (configured by RRC).

The SFI informs the UE(s) which are downlink, uplink, and flexible symbols.

The IE SlotFormatIndicator within PDCCH-ServingCellConfig provides the configuration of SFIs to a configured group of one or more UEs. The IE structure for SlotFormatIndicator is given below;

*SlotFormatIndicator*
sfi-RNTI	INTEGER (0..65535)
dci-PayloadSize	INTEGER (1..128)
slotFormatCombToAddModList	List of 1 to 16 SlotFormatCombinationsPerCell
slotFormatCombToReleaseList	List of 1 to 16 ServCellIndex
*SlotFormatCombinationsPerCell*
servingCellId	INTEGER (0..31)
subcarrierSpacing	ENUMERATED {kHz15, kHz30, kHz60, kHz120, kHz240, spare3, spare2, spare1}
subcarrierSpacing2
slotFormatCombinations	List of 1 to 512 SlotFormatCombination
positionInDCI	INTEGER (0..127)
*SlotFormatCombination*
slotFormatCombinationId	INTEGER (0..511)
slotFormats	List of 1 to 256 slot formats, each format can take INTEGER (0..255)

dci-PayloadSize informs the UE about the total length of the DCI payload scrambled with SFI-RNTI.

slotFormatCombToAddModList configures the UE with a list of SlotFormatCombinations, each of which for a different serving cell.

positionInDCI informs the UE about the (starting) position (bit) of the slotFormatCombinationId (SFI-Index) for the serving cell identified by servingCellId within the DCI payload.

servingCellId is the ID of the serving cell for which the slotFormatCombinations are applicable.

slotFormatCombinations configures the UE with a list with slot format combinations. Each SlotFormatCombination comprises of one or more SlotFormats.

- slotFormatCombinationId is used in the DCI payload to dynamically select the slot format combination.

- slotFormats provides slot formats that occur in consecutive slots in time domain order as listed in this field. Slot formats are explained below;

Slot Formats:

Release 15 version of 38.213 has defined 56 slot formats (Table 11.1.1-1) each of which is a predefined pattern of downlink/flexible/uplink symbols during one slot. The table is presented below for a quick reference.

Format

Symbol number in a slot

56 – 254

Reserved

255

UE determines the slot format for the slot based on tdd-UL-DL-ConfigurationCommon, or tdd-UL- DL-ConfigurationDedicated and, if any, on detected DCI formats

When slot format is set to 255, the UE should not use the above table to determine the slot format. Instead, the slot format is determined based on configuration given by tdd-UL-DL-ConfigurationCommon, or tdd-UL- DL-ConfigurationDedicated or from the received scheduling grants/assignments (discussed in the next section).

Additionally, this mechanism may also be used to override already configured periodic transmissions in uplink (SRS) or downlink (CSI-RS). For example, if the UE is configured to transmit SRS periodically, but the network wants to utilise one of the SRS symbols for downlink; in this case, the network simply sends SFI overriding uplink (allocated for SRS) with downlink.

- It is only possible to override a symbol which was configured to be flexible by RRC (cell- and UE-specific) as SFI can’t change any slots/symbols that are already configured to be uplink or downlink.

3. Slot configuration via dynamic scheduling:

In addition to the above discussed mechanisms, when the network is already scheduling a UE with scheduling grants/assignments, the network could dynamically inform the UE about transmit/receive pattern.

If the UE is not configured with SlotFormatIndicator and during the flexible symbols configured by DL-ConfigurationCommon and tdd-UL-DL-ConfigurationDedicated (if configured);

- the UE receives PDSCH or CSI-RS in the set of symbols of the slot if the UE receives a corresponding indication by a DCI format 1_0, DCI format 1_1, or DCI format 0_1.

- the UE transmits PUSCH, PUCCH, PRACH, or SRS in the set of symbols of the slot if the UE receives a corresponding indication by a DCI format 0_0, DCI format 0_1, DCI format 1_0, DCI format 1_1, or DCI format 2_3.

For more details on DCI formats, refer to DCIFormats in 5G NR.

Mini-slots:

In supporting low-latency applications such as URLLC, time-domain scheduling plays a significant role. There are two ways to support low-latency transmissions w.r.t time-domain scheduling. One way is to dynamically change the SCS/slot duration which itself involves some delay. Another way is only to use only part of a slot i.e., by using as may number of OFDM symbols as necessary to send/receive the data.

NR supports transmissions over a fraction of a slot which is known as mini-slot. For a very low latency transmissions, mini-slots avoid the need to wait until the slot boundary.

With low SCS configurations, the slot duration is high and hence using a fraction of a slot provides significant advantage compared to full slot in low latency scenarios. For example, when using SCS of 15 kHz, the slot duration is 1 ms whereas a mini-slot occupying 2 symbols would last for ≈143 µs.

Mini-slot concept is also advantageous when operating in FR2; As the available bandwidth can be too large in FR2, even few OFDM symbols over a large bandwidth can deliver high amount of data. In other words, allocating full slot over a large bandwidth is unnecessary if the data to be transmitted is not that large.

Another use case of mini-slot based transmission is when operating in unlicensed spectrum wherein, after the successful LBT (listen-before-talk) procedure, the transmission can start as soon as possible without needing to wait until the slot boundary.

A mini-slot length can be 2, 4, or 7 OFDM symbols. A mini-slot contains DM-RS at position(s) relative to the start of the mini-slot.

Note that some UEs targeting certain use cases may not support all mini-slot lengths and all starting positions within a slot.

The figure below illustrates an example time-domain structure for normal CP.

Reference: 3GPP TS 38.211, 38.212, 38.213, 38.214, 38.331, and TR 38.912

5G NR: Time Domain – Slots and Slot Formats