Supplementary Uplink

Introduction

In 5G NR, a downlink carrier may be associated with two uplink carriers (the non-SUL carrier and the SUL carrier), where the Supplementary Uplink (SUL) carrier is typically located in lower frequency bands, thereby providing enhanced uplink coverage.

Example of Supplementary Uplink: With SUL, the UE is configured with 2 ULs for one DL of the same cell as depicted in figure below:

Example of Supplementary Uplink
Example of Supplementary Uplink

Below are the Operating Bands defined by 3GPP for NR in Frequency Range 1 where the duplex mode is SUL:

Operating Bands supporting SUL as duplex mode
Operating Bands supporting SUL as duplex mode

Supplementary Uplink (in detail)

In conjunction with a UL/DL carrier pair (FDD band) or a bidirectional carrier (TDD band), a UE may be configured with additional, Supplementary Uplink (SUL) which can improve UL coverage for high frequency scenarios.

Since the lower frequency bands are already occupied by LTE primarily, so for enabling early NR deployment in lower-frequency spectra, LTE/NR spectrum co-existence is thought of as the way for an operator to deploy NR in the same spectrum as an already existing LTE deployment. Two co-existence scenarios were identified in 3GPP and guided the NR design:

  1. LTE/NR co-existence in both DL and UL directions
  2. There is co-existence only in the UL direction, typically within the UL part of a lower-frequency paired spectrum, with NR downlink transmission taking place in the spectrum dedicated to NR, typically at higher frequencies. NR supports a supplementary uplink (SUL) to specifically handle this scenario.

SUL implies that a conventional downlink/ uplink (DL/UL) carrier pair has an associated or supplementary uplink carrier with the SUL carrier typically operating in lower-frequency bands. As an 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.

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.

In case of Supplementary Uplink, the UE is configured with 2 UL carriers for one DL carrier of the same cell, 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 (PUCCH can only be configured for only one of the 2 ULs of the cell). In addition, initial access is supported in each of the uplink

Note:

  1. In paired spectrum, DL and UL can switch BWP independently. In unpaired spectrum, DL and UL switch BWP simultaneously. Switching between configured BWPs happens by means of RRC signaling, DCI, inactivity timer or upon initiation of random access. When an inactivity timer is configured for a serving cell, the expiry of the inactivity timer associated to that cell switches the active BWP to a default BWP configured by the network. There can be at most one active BWP per cell, except when the serving cell is configured with SUL, in which case there can be at most one on each UL carrier.
  2. When SUL is configured, a configured uplink grant can only be signaled for one of the 2 ULs of the cell
  3. SUL differs from the aggregated uplink in that the UE may be scheduled to transmit either on the supplementary uplink or on the uplink of the carrier being supplemented, but not on both at the same time.

Random Access in case of Supplementary Uplink (SUL)

For random access in a cell configured with SUL, the network can explicitly signal which carrier to use (UL or SUL). Otherwise, the UE selects the SUL carrier if and only if the measured quality of the DL is lower than a broadcast threshold. Once started, all uplink transmissions of the random-access procedure remain on the selected carrier.

SIB1 ::= SEQUENCE {

    servingCellConfigCommon  ServingCellConfigCommonSIB    OPTIONAL,   — Need R

}

ServingCellConfigCommonSIB ::= SEQUENCE {

    supplementaryUplink  UplinkConfigCommonSIB   OPTIONAL,   — Need R

}

Supplementary Uplink related configuration is present as a part of SIB1. Before initially accessing a cell, a device will thus know if the cell to be accessed is SUL cell or not. If the cell is 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 also provided as part of the cell system information.

  • 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.

In practice, the SUL carrier is thus selected by devices with a (downlink) pathloss to the cell that is larger than a certain value. 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 a 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.

Control Signaling in case of Supplementary Uplink (SUL)

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.

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. In the latter case, the uplink scheduling grant will include SUL/non-SUL indicator that indicates on what carrier the scheduled PUSCH transmission should be carried. Thus, in the case of supplementary uplink, a device will never transmit PUSCH simultaneously on both the SUL carrier and on the non-SUL carrier.

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 not 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.

SUL Carrier Co-Existence with LTE UL Carrier leading to enhanced User Experience

As mentioned above, One SUL scenario is when the SUL carrier is located in the uplink part of paired spectrum already used by LTE. In other words, the SUL carrier exists in an LTE/NR uplink coexistence scenario. In many LTE deployments, the uplink traffic is significantly less than the corresponding downlink traffic. Consequently, in many deployments, the uplink part of paired spectra is not fully utilized. Deploying an NR supplementary uplink carrier on top of the LTE uplink carrier in such a spectrum is a way to enhance the NR user experience with limited impact on the LTE network.

SUL carrier coexisting with LTE Uplink Carrier
SUL carrier coexisting with LTE Uplink Carrier

Reduction in Latency

In the case of TDD, the separation of uplink and downlink in the time domain may impose restrictions on when uplink data can be transmitted. By combining the TDD carrier with a supplementary carrier in paired spectra, latency-critical data can be transmitted on the supplementary uplink immediately without being restricted by the uplink-downlink partitioning on the normal carrier.

Difference between Carrier Aggregation (CA) and supplementary uplink (SUL)

In a typical carrier aggregation scenario:

  • Main aim of carrier aggregation is to enable higher peak data rates by increasing the bandwidth available for transmission to/from a device.
  • The two (or more) carriers are often of similar bandwidth and operating at similar carrier frequencies, making aggregation of the throughput of the two carriers more beneficial. Each uplink carrier is operating with its own associated downlink carrier, simplifying the support for simultaneous scheduling of multiple uplink transmissions in parallel. Formally, each such downlink carrier corresponds to a cell of its own and thus different uplink carriers in a carrier-aggregation scenario correspond to different cells.

While in case of SUL scenario:

  • Main 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
  • The supplementary uplink carrier does not have an associated downlink carrier of its own. Rather, the supplementary carrier and the conventional uplink carrier share the same downlink carrier. Consequently, the supplementary uplink carrier does not correspond to a cell of its own. Instead, in the SUL scenario there is a single cell with one downlink carrier and two uplink carriers.
Carrier Aggregation vs Supplementary Uplink
Carrier Aggregation vs Supplementary Uplink

Is there something possible like Supplementary Downlink also?

Yes, since the carrier aggregation framework allows for the number of downlink carriers to be larger than the number of uplink carriers, some of the downlink carriers can be thought of as supplementary downlinks. One common scenario is to deploy an additional downlink carrier in unpaired spectra and aggregate it with a carrier in paired spectra to increase capacity and data rates. No additional mechanisms beyond carrier aggregation are needed and hence the term supplementary downlink is mainly used from a spectrum point of view.

Is it possible to have combination of SUL Carrier and Carrier Aggregation?

In principle, it is possible to have a combination of SUL carrier and carrier aggregation, for example, a situation with carrier aggregation between two cells (two DL/UL carrier pairs) where one of the cells is SUL cell with an additional supplementary uplink carrier. However, currently there are no band combinations defined for such carrier-aggregation/SUL combinations

References:

  1. 3GPP TS 38.300 version 15.9.0 Release 15
  2. www.sharetechnote.com
  3. “5G NR – The next generation wireless access technology” – By Erik Dahlman, Stefan Parkvall, Johan Sköld

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