5G EN-DC: What is Single Uplink?

The abbreviation EN-DC (eUTRAN New Radio – Dual Connectivity) indicates that in this type of 5G deployment, 4G LTE and 5G NR will be used simultaneously. However, it looks like that the combination of some frequency bands might prove to be a bit problematic due to self interference as laid out in this interesting whitepaper recently published by Keysight (see page 4).

In essence, the paper lays out that uplink transmission (from the mobile’s point of view) in the 3.5 GHz band might significantly interfere with reception in the 1.8 GHz band in downlink direction. This is rather bad news as especially in Europe, these two frequency bands are one of the combinations that make sense for an early 5G deployment in cities.

3GPP To The Rescue

The standard use of 5G EN-DC described in 3GPP requires simultaneous transmission and reception of LTE and NR signals by the UE. To mitigate the self interference issue, 3GPP has specified which band combinations are allowed to stray from the stringent requirement for simultaneous operation. Band 3 + n78 (that’s 1.8 GHz for LTE and 3.x GHz for NR) is such an exception.

To fix the issue, the uplink needs to alternate between 1.8 and 3.5 GHz transmission. While 3.5 GHz uplink transmission is ongoing, no data must be scheduled by the network on the 1.8 GHz downlink LTE carrier. I guess this is possible as the LTE eNB is aware when 3.x GHz uplink transmissions occur, as it is responsible for originally configuring the connection and 3GPP has specified a number of mechanisms to control the use of the uplink on different bands. The paper doesn’t go into much detail on how things are specified for this so I had a closer look into the 3GPP specs myself and here is what I found:

3GPP TS 36.331 (LTE RRC Specification)

"tdm-PatternConfig - UL/DL reference configuration indicating the time during which a UE configured with EN-DC is allowed to transmit. This field is used when power control or IMD issues require single UL transmission."

RRCConnectionReconfiguration-v1510-IEs ::= SEQUENCE {
nr-Config-r15 CHOICE {
tdm-PatternConfig-r15 CHOICE {
    release NULL,
    setup SEQUENCE {
         subframeAssignment-r15 SubframeAssignment-r15,
         harq-Offset-r15 INTEGER (0.. 9)

3GPP TS 38.331 (NR RRC Specification, IE BandCombinationList)

MRDC-Parameters ::= SEQUENCE {
  singleUL-Transmission ENUMERATED {supported} OPTIONAL,

  -- R4 1-10: Support of EN-DC with LTE-NR coexistence in UL
  sharing from UE perspective
  ul-SharingEUTRA-NR ENUMERATED {supported}

  -- R4 1-11: Switching time between LTE UL and NR UL for
  EN-DC with LTE-NR coexistence in UL sharing from UE
  ul-SwitchingTimeEUTRA-NR ENUMERATED {type1, type2}

  -- R4 2-4: Simultaneous reception and transmission for
  inter-band EN-DC (TDD-TDD or TDD-FDD)
  simultaneousRxTxInterBandENDC ENUMERATED {supported} 

38.306 (UE Radio Access Capabilities)

singleUL-Transmission --> IE in RF, Physical layer parameters, points to TS 38.101-3, and there, the
relevant table contains a "Single UL allowed" column.

Some more details from 3GPP on the terms used above:

tdm-Pattern: Indicates whether the UE supports the tdm-Pattern for single UL transmission associated functionality. Support is conditionally mandatory for UEs that do not support dynamic power sharing and for UEs that indicate single UL for any BC [...].

singleUL-Transmission: Indicates that the UE does not support simultaneous UL transmissions as defined in TS 38.101-3 [4]. The UE may only set this bit for certain band combinations defined in TS 38.101-3 [4]. [...]

ul-SharingEUTRA-NR: Indicates whether the UE supports EN-DC with EUTRA-NR coexistence in UL sharing from UE perspective.

ul-SwitchingTimeEUTRA-NR: Indicates support of switching type between LTE UL and NR UL for EN-DC with LTE-NR coexistence in UL sharing from UE perspective. Type1 indicates UE supports switching within less than 0 us and type2 indicates UE supports switching within less than 20 us.

simultaneousRxTxInterBandENDC: Indicates whether the UE supports simultaneous transmission and reception in TDD-TDD and TDD-FDD inter-band EN-DC. It is mandatory for certain TDD-FDD and TDD-TDD band combinations defined in TS 38.101-3 [4].

P.S.: Fun fact… Can anyone tell me how it is possible to have a switching time of “less than 0 us” (type 1)???