5G/NR - tdd UL/DL Common Configuration |
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tdd UL/DL Common Configuration (tdd-UL-DL-configurationCommon) in a Nutshell
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.
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
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
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
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.
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])
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.
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
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