5G/NR - tdd UL/DL Common Configuration 

  

 

 

 

 

tdd UL/DL Common Configuration (tdd-UL-DL-configurationCommon) in a Nutshell

 

 

  • TDD-UL-DL-configurationCommon parameter defines the uplink and downlink configuration for a TDD system.
  • The parameters used to specify this configuration are the period, the number of slots in a radio frame, the number of symbols in a slot
  • TDD-UL-DL-configurationCommon can be configured in different ways depending on the specific requirements of the network, such as the traffic patterns, the number of users, and the available bandwidth.
  • In most cases, network configures one pattern within the period, but 3GPP allows network to configure two patterns within a period and there are some real network that configure two patterns
  • In terms of 3GPP, it is allowed not to configure TDD-UL-DL-configurationCommon at all, but in reality most of the network configures this pattern.
  • Make it sure that you have enough Guard time for DL-to-UL Switching (DL-to-UL switching gap)
  • Make it sure that the configured PRACH Occasion fall into the slots/symbols where UL is configured.

 

 

 

tdd UL/DL Common Configuration (tdd-UL-DL-configurationCommon) in Detail

 

When operating in TDD mode, we have to clearly define on exactly when to expect the transmission and when to expect the reception. In LTE TDD, we defined 7 predefined pattern for UL and DL allocation in a radio frame. In 5G/NR, we don't have any predefined pattern. Instead, we can define the pattern in much more flexible way using several parameters as shown below.

 

 

 

 

 

Rrc Parameters

 

These parameters are defined in 38.331 v15.3.0 as follows :

 

TDD-UL-DL-ConfigCommon ::= SEQUENCE {

    referenceSubcarrierSpacing                 SubcarrierSpacing,

    pattern1                                   TDD-UL-DL-Pattern,

    pattern2                                   TDD-UL-DL-Pattern OPTIONAL,

    ...

}

 

TDD-UL-DL-Pattern ::= SEQUENCE {

    dl-UL-TransmissionPeriodicity              ENUMERATED {ms0p5, ms0p625, ms1,

                                                           ms1p25, ms2, ms2p5, ms5, ms10},

    nrofDownlinkSlots                          INTEGER (0..maxNrofSlots),

    nrofDownlinkSymbols                        INTEGER (0..maxNrofSymbols-1),

    nrofUplinkSlots                            INTEGER (0..maxNrofSlots),

    nrofUplinkSymbols                          INTEGER (0..maxNrofSymbols-1),

    ...,

    [[

       dl-UL-TransmissionPeriodicity-v1530 ENUMERATED {ms3, ms4} OPTIONAL -- Need R

    ]]

}

 

maxNrofSlots INTEGER ::= 320    // Maximum number of slots in a 10 ms period

maxNrofSymbols-1 INTEGER ::= 13 // Maximum index identifying a symbol within a slot

                                   (14 symbols, indexed from 0..13)

 

dl-UL-TransmissionPeriodicity: Periodicity of the DL-UL pattern. If dl-UL-TransmissionPeriodicity-v1530 is conifgured, dl-UL-TransmissionPeriodicity is ignored.

nrofDownlinkSlots : Number of consecutive full DL slots at the beginning of each DL-UL pattern  

nrofDownlinkSymbols : Number of consecutive DL symbols in the beginning of the slot following the last full DL slot

nrofUplinkSlots : Number of consecutive full UL slots at the end of each DL-UL pattern

nrofUplinkSymbols : Number of consecutive UL symbols in the end of the slot preceding the first full UL slot

 

 

 

Transmission Periodicity

 

The applicable periodicity(P) of the UL/DL configuration varies depending on the reference numerology (n_ref). This can be summarized as a table as shown below. I created this table based on the descriptions in 38.213 v16.5 -11.1

 

P(ms)

u_ref (scs Khz)

Applicable u

P/20

Number of Slots in a P

0

1

2

3

4

0.5

Not described

  

40

 

1

2

4

8

0.625

3(120)

3

32

 

 

 

5

10

1.25

2(60), 3(120)

2,3

16

 

 

5

10

20

2.5

1(30), 2(60), 3(120)

1,2,3

8

 

5

10

20

40

5.0

Not described

  

4

5

10

20

40

80

10.0

Not described

  

2

10

20

40

80

160

 

NOTE : I've calculated P/20 to clarify the statement in the specification saying 'The first symbol every periods is a first symbol in an even frame.'.

 

NOTE : The 'Applicable u' is specified based on the statement in the specification saying 'A UE expects that the reference subcarrier spacing configuration ref μ is smaller than or equal to a subcarrier spacing configuration μ for any configured DL BWP or UL BWP.'

 

NOTE : I calculated the section 'Number of Slots in a P' just to show how many slots for each numerology in a P(Period). It doesn't necessarily mean that all of the numerology is applicable to the specified period.

 

 

 

What if TDD-UL-DL-ConfigCommon is not configured ?

 

UE determines if each of the slot is uplink or downlink and the symbol allocation within each of the slot purely by DCIs as stated in 38.213-11.1 Slot configuration.

    If a UE is not configured to monitor PDCCH for DCI format 2-0, for a set of symbols of a slot that are indicated as flexible by higher layer parameters TDD-UL-DL-ConfigurationCommon or TDD-UL-DL-ConfigDedicated, when provided to a UE, or when TDD-UL-DL-ConfigurationCommon and TDD-UL-DL-ConfigDedicated are not provided to the UE

    • 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

 

 

 

Guard Timing (Symbol gap for DL to UL switching)

 

In terms of 3GPP RRC specification, you can set any values within the range of the values specified in RRC Parameter. However, in terms of lower layer perpective there is some restrictions (or something to take into account) as listed below.

  • You need a certain number of Guard time (Guard symbols) when switches from DL to UL
  • You don't need any guard time when switches from UL to DL

Followings are some of examples that are valid or invalid based on this rule.

 

 

NOTE 1 : Why we need Guard time only for DL to UL switching ? 

 

I don't think 3GPP TS document explicitely states about this requirement, but you may find some other documents (e.g, white papers) explaining about this requirement.  Simply put, (You may refer to Recommendation 10 of Ref [1])

  • we need some time for DL signal reach (propagate to) UE. So UE need some additional time from the end of DL signal. Otherwise, UE may transmit UL signal before it complete the reception of DL signal and as a result there would be interference between DL and UL signal.
  • we don't need any guard time for UL to DL because DL and UL is always propery aligned thanks to timing advance. that is, gNB sends timing advance to make UL signal perfectly aligned in time domain.

Even though it is not clearly/explicitely stated, you may refer to 38.211 - 4.3.2 and following table :

 

< 38.211 - Table 4.3.2-3: Transition time NRx-Tx and NTx-Rx >

 

 

NOTE 2 : Why we need Guard time only for DL to UL switching ? 

 

What would be the best Guard time (Symbol Gaps) ? The answer to this question would vary depending on various factors especially the distance between gNB and UE. Some guide line stated in Recommendation 10 of Ref [1]) is as follows.

  • A GP(Guard Period) of 2 symbols would cater for cell sizes of up to 10.7 km;
  • A GP(Guard Period) of 4 symbols would cater for cell sizes of up to 21.4 km;
  • A GP(Guard Period) of 6 symbols would cater for cell sizes of up to 32.1 km.

 

 

NOTE 3 : Any other consideration ? 

 

This is not directly related to Guard time, but you need to make it sure that the PRACH Occasion you set in RACH config should fall into the UL slot/symbols you configured in the RRC.

 

 

 

Reference

 

[1] 5G TDD Synchronisation Guidelines and Recommendations for the Coexistence of TDD Networks in the 3.5 GHz Range - GSMA (2020)    

[2] 5G TDD Uplink White Paper - NGMN (2022)

[3] TDD : Why a Guard Period only in DL-to-UL Switching ? - TECHTRAINED