Thursday, April 1, 2021

Supplementary Uplink(SUL) in 5G NR

Introduction 

An additional UL carrier located at a lower frequency to allow better coverage in case UE is at the cell edge. For example, a downlink/uplink carrier pair operating in the 3.5 GHz band could be complemented with a supplementary uplink carrier in the 800 MHz band. The UE selects SUL carrier to perform random access based on the configured RSRP threshold rsrp-ThresholdSSB-SUL present in RACH-ConfigCommon. In SUL scenario, the non-SUL uplink carrier is typically significantly more wideband compared to the SUL carrier. Thus, under good channel conditions such as the device located relatively close to the cell site, the non-SUL carrier typically allows for substantially higher data rates compared to the SUL carrier. At the same time, under bad channel conditions, for example, at the cell edge, a lower-frequency SUL carrier typically allows for significantly higher data rates compared to the non-SUL carrier, due to the assumed lower path loss at lower frequencies. 

 Deployment example of supplementary UL

 

When the uplink coverage of the NR carrier is good, UE uses the NR carrier to send and receive data. However, when the UE is moving beyond the uplink coverage of the NR carrier, UE uses the SUL carrier for transmitting data. UE can dynamically select the UL NR or SUL for data transmission, but it cannot use the two carriers at the same time. Relevant band combinations for SUL in SA and EN-DC scenarios can be found in TS 37.872 Version 15.

 A UE can be configured with up to four bandwidth parts in the uplink with a single uplink bandwidth part being active at a given time. If a UE is configured with a supplementary uplink, the UE can in addition be configured with up to four bandwidth parts in the supplementary uplink with a single supplementary uplink bandwidth part being active at a given time. The UE shall not transmit PUSCH or PUCCH outside an active bandwidth part. 

For an active cell, the UE shall not transmit SRS outside an active bandwidth part. The following picture provides more detail about the supplementary uplink operation. In area A where uplink coverage is good, UE uses normal NR UL for uplink transmission. When UE moves to area B where UL coverage is limited, NW can still provide better DL coverage through increased DL TX power, however, that is not possible for the UE due to power restrictions in the UL. Therefore, to achieve better UL coverage UE moves to supplementary UL which is located at a lower frequency band than the NR UL.

Additional uplink cannot be thought of as another carrier as in CA. SUL specifically serves a completely different purpose i.e. better uplink coverage. While the main aim of carrier aggregation is to enable higher peak data rates by increasing the bandwidth available for transmission to/from a device, the typical aim of SUL is to extend uplink coverage, that is, to provide higher uplink data rates in power-limited situations, by utilizing the lower path loss at lower frequencies. One more main difference compared to CA is that UE can transmit on only one UL, not on both uplinks simultaneously. 

 A UE can be configured with up to four bandwidth parts in the uplink with a single uplink bandwidth part being active at a given time. If a UE is configured with a supplementary uplink, the UE can in addition be configured with up to four bandwidth parts in the supplementary uplink with a single supplementary uplink bandwidth part being active at a given time. The UE shall not transmit PUSCH or PUCCH outside an active bandwidth part. For an active cell, the UE shall not transmit SRS outside an active bandwidth part. 

 Initial access in SUL scenarios:

 In SUL scenarios, initial access is supported in each of the uplinks however UE can RACH over any one of the configured UL. When SUL is configured, a configured uplink grant(in other words UL DCIs) can only be signaled for one of the 2 ULs of the cell. In the case of supplementary Uplink, the UE is configured with 2 UL carriers for one DL carrier of the same cell(normal NR UL and supplementary uplink) and uplink transmissions on those two UL carriers are controlled by the network to avoid overlapping PUSCH/PUCCH transmissions in time. Overlapping transmissions on PUSCH are avoided through scheduling while overlapping transmissions on PUCCH are avoided through configuration. Therefore, PUCCH can only be configured for only one of the 2 ULs of the cell by means of RRC signaling to transmit PUCCH on either the SUL carrier or on the conventional (non-SUL) carrier. The process of selecting the carrier(either NR UL or SUL) works as follows, the information about the configured SUL is transmitted as part ServingCellConfigCommonSIB in SIB1. The UE selects either a normal uplink or supplementary UL carrier to perform random access based on the configured RSRP threshold rsrp-ThresholdSSB-SUL present in RACH-ConfigCommon. A cell is an SUL cell. In other words, before initially accessing a cell, a UE will thus know if the cell to be accessed is an SUL cell or not. If the cell is an SUL cell and the device supports SUL operation for the given band combination, initial random access may be carried out using either the SUL carrier or the non-SUL uplink carrier. The cell system information provides separate RACH configurations for the SUL carrier and the non-SUL carrier and a device capable of SUL determines what carrier to use for the random access by comparing the measured RSRP of the selected SS block with a carrier-selection threshold(rsrp-ThresholdSSB-SUL) also provided as part of the cell system information. 

 The initial access steps are as follows 

  • If the RSRP is above the threshold, random access is carried out on the non-SUL carrier. 
  • If the RSRP is below the threshold, random access is carried out on the SUL carrier. 

 The device carrying out a random-access transmission will transmit the random-access message 3 on the same carrier as used for the preamble transmission. For other scenarios when a device may do random access, that is, for devices in connected mode, the device can be explicitly configured to use either the SUL carrier or the non-SUL carrier for the uplink random-access transmissions(Eg: PdcchOrder) 

UCI on SUL:

 In case of Supplementary Uplink (SUL, see TS 38.101-1 [18]), the UE is configured with 2 ULs for one DL of the same cell, and uplink transmissions on those two ULs are controlled by the network to avoid overlapping PUSCH/PUCCH transmissions in time. Overlapping transmissions on PUSCH are avoided through scheduling while overlapping transmissions on PUCCH are avoided through configuration (PUCCH can only be configured for only one of the 2 ULs of the cell) ==> HigherLayer has to configure in such way that pucch/pusch do not overlap when they are in different carriers. In the case of supplementary-uplink operation, a device is explicitly configured(by means of RRC signaling) to transmit PUCCH on either the SUL carrier or on the conventional (non-SUL) carrier and the UL on which PUCCH is configured cannot be changed dynamically, it can be only done through RRC reconfiguration. If a device is to transmit UCI on PUCCH during a time interval that overlaps with a scheduled PUSCH transmission on the same carrier, the device instead multiplexes the UCI onto PUSCH. The same rule is true for the SUL scenario, that is, there is no simultaneous PUSCH and PUCCH transmission even on different carriers. Rather, if a device is to transmit UCI on PUCCH one carrier (SUL or non-SUL) during a time interval that overlaps with a scheduled PUSCH transmission on either carrier (SUL or non SUL), the device instead multiplexes the UCI onto the PUSCH.

 PUSCH on SUL 

Dynamic UL carrier switching is release 16 feature i.e only when the UE has capability(uplinkTxSwitchRequested-r16) Reference: TS 38.214, 6.1.6.3 Uplink switching for supplementary uplink. Therefore, we won't be supporting this release 16 feature. In terms of PUSCH transmission, the device can be configured to transmit PUSCH on the same carrier as PUCCH. Alternatively, a device configured for SUL operation can be configured for dynamic selection between the SUL carrier or the non-SUL carrier using UL/SUL indicator field in the DCI. 

SRS on SUL

When UE has a capability simultaneousTxSUL-NonSUL the UE supports simultaneous transmission of SRS on an SUL/nonSUL carrier and PUSCH/PUCCH/SRS/PRACH on the other UL carrier in the same cell (from 38.306) that means SRS can be separately configured for SUL and NUL.

SUL Configuration

The IE ServingCellConfigCommon is used to configure cell specific parameters of a UE's serving cell

ServingCellConfigCommon ::= SEQUENCE {

    ...

    uplinkConfigCommon                     UplinkConfigCommon                                 OPTIONAL,   -- Need M

    supplementaryUplinkConfig           UplinkConfigCommon                                   OPTIONAL,   -- Need S

   ...

}


ServingCellConfigCommonSIB ::= SEQUENCE {

   ...

    uplinkConfigCommon                  UplinkConfigCommonSIB                               OPTIONAL, -- Need R

    supplementaryUplink                   UplinkConfigCommonSIB                               OPTIONAL, -- Need R

   ...    

}


BWP-UplinkCommon ::=                SEQUENCE {

  ...

    rach-ConfigCommon                   SetupRelease { RACH-ConfigCommon }           OPTIONAL,   -- Need M

    pusch-ConfigCommon                  SetupRelease { PUSCH-ConfigCommon }        OPTIONAL,   -- Need M

  ...

}


Each of these for NUL and SUL

UplinkConfigCommonSIB ::= SEQUENCE {

    frequencyInfoUL              FrequencyInfoUL-SIB, ==> Separate frequency info for NUL and SUL.

    initialUplinkBWP             BWP-UplinkCommon,

  ...

}


Additional optional uplinkConfig in servingCellConfig is defined

ServingCellConfig ::=  SEQUENCE {

    ...

    uplinkConfig                              UplinkConfig                            OPTIONAL,   -- Need M

    supplementaryUplink                 UplinkConfig                             OPTIONAL,   -- Need M                                    

   ...

}

Dedicated RACH configurations for SUL

rach-ConfigDedicated                CHOICE {

        uplink                              RACH-ConfigDedicated,

        supplementaryUplink      RACH-ConfigDedicated

}


on demand SI configuration SUL.

SI-SchedulingInfo ::=               SEQUENCE {

   ...

    si-RequestConfig                       SI-RequestConfig                                    OPTIONAL,  -- Cond MSG-1

    si-RequestConfigSUL                SI-RequestConfig                                    OPTIONAL,  -- Cond SUL-MSG-1

    ...

}

RRC will receive RACH configuration for supplementary Uplink separately.

Only one PUCCH Config (either SUL or NUL)

Only one BFR config(either SUL or NUL)

 

Ways to switch SUL to NUL or vice versa

Radom Access Procedure

 During initial access UE checks if the

1> if the RSRP of the downlink pathloss reference is less than rsrp-ThresholdSSB-SUL:

     2> select the SUL carrier for performing Random Access procedure;

1> else:

    2> select the NUL carrier for performing Random Access procedure;

 

Pdcch Order initiated with DCI1_0

NW can initiate SUL to NUL switching or vice versa through pdcch order procedure

1> if the UL/SUL indicator indicates SUL.

     2> select the SUL carrier for performing Random Access procedure;

1> else:

    2> select the NUL carrier for performing Random Access procedure;

Once Ul carrier switch is initiated via random access procedure, the procedure is completed on the selected carrier and the active UL carrier becomes the carrier selected during the random access procedure.

Prach on SUL, msg3 on SUL and msg5(if there is one) also SUL

 

Dynamic switching via DCI0_0, DCI0_1 and DCI0_2

NW can request UE to send PUSCH on normal UL/SUL through DCI indication.

1> if the UL/SUL indicator indicates SUL.

     2> select SUL for sending PUSCH;

1> else:

    2> send the PUSCH over Normal UL;


Allowed Band Combinations

TRP with SUL is always running at 15kHz together with NUL running at 15kHz/30kHz/60kHz.

SA NR SUL band combinations:

Ref TS 38.101 Section 5.2C

NR Band combination for SUL

NR Band (Table 5.2-1)

SUL Band/SCSNR Band SCS

SUL_n41-n80

n41, n80

15kHz

SUL_n41-n81

n41, n81

n81,15kHz

SUL_n41-n95

n41, n95



SUL_n77-n802

n77, n80

n80, 15kHz

SUL_n77-n842

n77, n84



SUL_n78-n802

n78, n80

n80, 15kHz

SUL_n78-n812

n78, n81

n81,15kHz

SUL_n78-n822

n78, n82

n82, 15kHz

SUL_n78-n832

n78, n83

n83,15kHz

SUL_n78-n842

n78, n84

n84, 15kHz

SUL_n78-n862

n78, n86

n86, 15kHz

SUL_n79-n802

n79, n80

n80, 15kHz

SUL_n79-n812

n79, n81

n81, 15kHz

SUL_n79-n84

n79, n84



SUL_n79-n95

n79, n95



NOTE 1:   If a UE is configured with both NR UL and NR SUL carriers in a cell, the switching time between NR UL carrier and NR SUL carrier is 0 us.

NOTE 2:   For UE supporting SUL band combination simultaneous Rx/Tx capability is mandatory.

NOTE 3:   For UE supporting SUL band combination, UL MIMO is not configured on SUL carrier

Operating SUL band combination with downlink CA in FR1

NR Band combination for SUL

NR Band (Table 5.2-1)

CA_n78_SUL_n78-n86

n78, n86


NSA NR SUL band combinations:

Ref TS 37.716 and TS 37.872

Band combination for SUL in NSA

E-UTRA  Band

NR Band

DC_7_SUL_n78-n80

7

n78-n80

DC_20_SUL_n78-n80

20

n78-n80

DC_8_SUL_n78-n80

8n78-n80

DC_1_SUL_n77-n80

1

n77-n80

DC_3_SUL_n77-n84

3

n77-n84

DC_3_SUL_n78-n84

3

n78-n84

DC_3-SUL_n78_n80

3

n78_n80

DC_3-SUL_n79_n80

3

n79_n80

DC_1-SUL_n78_n84

1

n78_n84

DC_8-SUL_n78_n81

8

n78_n81

DC_28-SUL_n78_n83

28

n78_n83

DC_66-SUL_n78_n86

66

n78_n86

DC_8-SUL_n79_n81

8

n79_n81

DC_3-SUL_n78_n82

3

n78_n82

DC_20-SUL_n78_n83

20

n78_n83

References

  • 3gpp Tdocs R1-1720558
  • 3GPP TS 38.331
  • 3GPP TS 38.101
  • 3GPP TS 37.872
  • 3GPP TS 38.716
  • 3GPP TS 38.300 version 15.9.0 Release 15​
  • www.sharetechnote.com​
  • “5G NR – The next generation wireless access technology” – By Erik Dahlman, Stefan Parkvall, Johan Sköld​
  • 5G Uplink Enhancement Technology White Paper ZTE​
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