Content for  TR 38.844  Word version:  18.0.0

UE needs to support the asymmetric bandwidth combinations where UL CBW is SmallerCBW and DL CBW is WiderCBW. The asymmetric UL and DL channel bandwidth combination set should be specified in the specification TS 38.101-1.

gNB has to support the irregular channel BW. It is up to the network implementation how it can be implemented, e.g. through a new channel bandwidth, or through RF combining of 2 existing regular channel bandwidths.

• Does not require new channel filters for UE and gNB to be designed and tested
• Requires support of two RF carriers phase aligned on the Tx side to ensure phase continuity on the Rx side
• Requires split of the irregular bandwidth into two sets of PRBs that may be filtered through the next smaller bandwidth after which the signal is combined at the receiver's baseband as a single carrier. For such processing, UE may benefit from supporting intra-band non-contiguous CA.
• Requires UE architecture with capability to split LNA output signal to two separate LO paths for down-conversion. Also requires UE architecture with the ability to combine baseband signals.
• Possible UE architecture alternative to requiring a split LNA, is to use an ADC with a large dynamic range and a subsequent split of the digital signal in two paths with separate NCOs and digital channel filters. Also requires UE architecture with the ability to combine baseband signals.
• For scenarios with less than 10 MHz, second SSB is not excluded but not recommended due to significant additional overhead (duplicated SSB transmission as well as other radio resources such as PDCCH, CSI-RS, PDSCH (for SIB), CSI for Tracking, etc.). However, second SSB could enable or improve the legacy UEs' use of the further PRBs provided by the additional RF carrier in spectrum scenarios with less than 10 MHz.
• "Additional RF carrier" not to be on the channel raster to increase spectrum utilization (up to 2 PRBs), it should be noted that the complete "additional RF carrier" is used only by UEs which support this solution "additional RF carrier" can be used partially (with up to 2 PRBs not available) by legacy UEs which are on the channel raster.
• Proposed BWPs size of the irregular spectrum chunk may have an impact on performance requirements and additional testing
• High spectrum utilization:
  • due to low internal guard band. The spectrum utilization values in the table below use guard band values according to the minimum guard bands defined for the smallest possible channel bandwidths that could be theoretically considered as part of each combined UE CBW. The actual minimum guard band requirements for combined UE CBW configurations would require further discussion in any follow-up WI.
  • as well as no additional CA overhead (duplicated common channels and signals such as SSB, PDCCH and CSI-RS configured both in Pcell and Scell, in addition of the MAC processes associated with CA) due to single baseband carrier usage:

From network perspective: For gNB, no new filter for RF channel is needed. The CA implementation can be reused. The only update is to allow the configuration that the two carriers can be partially overlapped by adjusting the channel spacing. The network can prevent collisions between the two component carriers. For UE, there is no impact and fully backwards compatible. From UE perspective: For gNB, the two RF carriers aggregated by the UE would need to be time-aligned by the gNB to at least the same level required for existing intra-band contiguous CA (TS 38.104 Timing Adjustment Error) to allow UE reception. It is FFS as to whether finer timing alignment would be required. For UE, it is optional support in DL only. For the UE supports DL intra-band non-contiguous CA with corresponding channel bandwidth(s), overlapping CA can be considered by support the configuration with partially overlapping carriers. And in the case no new channel filter for UE is needed. For overlapping CA solution, the legacy channel bandwidth should be supported, hence the minimum guardband should not be less than the minimum guardband of lower UE channel bandwidth than operator licensed bandwidth. To support legacy UEs, channel raster should be applied for each UE channels. And in order to simplify the resource schedule and make the use of single SSB (for irregular bandwidths larger than 10MHz) for CA operation, RB alignment is required. Table 6.3.4-1 shows the channel spacing and spectrum utilization for the example channel bandwidths assuming symmetric CA channel bandwidth.