Hi all,
We’re seeing low uplink throughput on our Ericsson 5G FDD layer and trying to figure out what might be holding it back.
If anyone has experience with this, could you share which features or parameters made a real difference for you? Anything around scheduling, power control, MIMO, or general tuning that helped improve UL would be great to hear.
Low uplink (UL) throughput on FDD layers often stems from UE power limitations or interference rather than a lack of PRBs. On Ericsson infrastructure, moving the needle usually requires a mix of aggressive power control tuning and enabling “multi-antenna” features that allow the network to “hear” the UE better.
The features and parameters that typically yield the most significant gains for UL performance:
1. Power Control & Link Adaptation
Since FDD often operates at lower frequencies (sub-2 GHz), UEs at the cell edge frequently hit their maximum transmit power (P_{max}), leading to high BLER and lower MCS.
* p0NominalPusch & alpha: These are the “bread and butter” of UL tuning. Increasing p0NominalPusch (the base power) can improve SINR for cell-center users, but if set too high, it raises the interference floor (IoT) for the whole cell.
* Fractional Power Control (alpha): Setting alpha closer to 1.0 ensures that path loss is fully compensated, which is vital for maintaining throughput at the cell edge.
* UL BLER Target: If your UL BLER is consistently high, check your target. Sometimes relaxing the target (e.g., from 10% to 15%) allows the scheduler to stay on a higher MCS longer, potentially increasing throughput despite the extra retransmissions.
2. Scheduling & Resource Allocation
The scheduler’s ability to react to Power Headroom Reports (PHR) is critical. If the scheduler assigns too many PRBs to a power-limited UE, the power per subcarrier drops, and the UE fails to decode.
* PHR-Aware Scheduling: Ensure the scheduler is effectively using PHR to downsize the PRB allocation for power-limited UEs. It’s better to have a high-density, narrow-band allocation than a wide, weak one.
* Dynamic Transmit Switching (Rel-17): If you are running an FDD+TDD carrier aggregation (CA) setup, this feature allows the UE to switch its UL power dynamically between bands to maximize the UL pipe.
3. Multi-Antenna & MIMO Enhancements
FDD UL throughput is often limited by the UE having only 1 or 2 transmit antennas. You can compensate for this at the base station.
* UL SU-MIMO (2-layer UL): Ensure maxNoOfUlMimoLayers is set to 2. While many FDD UEs only support 1TX, newer flagship devices can do 2-layer UL on FDD if the signal quality is high enough.
* 4T8R Radios (Radio 4890): Upgrading to 8-branch receivers significantly improves “uplink gain.” Having 8 RX antennas at the site allows for better interference rejection and combining, which can boost UL throughput by 20–30% in FDD layers.
* UL MU-MIMO: This doesn’t help a single user’s peak speed, but it massiveley improves cell capacity by allowing two UEs to share the same frequency resources in the uplink.
4. Interference Management
High Interference over Thermal (IoT) is the silent killer of UL throughput.
1- pucchInterferenceThreshold: Helps the gNB distinguish between actual signals and background noise.
2- Dynamic Spectrum Sharing (DSS): If you are sharing FDD spectrum with LTE, ensure the ltersvd (LTE reserved) resources aren’t unnecessarily choking the 5G UL.
3- Inter-cell Interference Coordination (ICIC): Helps manage the “noise” from neighboring cells, especially in dense urban environments.
Checklist for Troubleshooting:
1. Check PHR Distribution: Are 80% of your users at 0dB headroom? If so, you have a coverage/power issue, not a scheduler issue.
2. Monitor UL IoT: If your IoT is > -110dBm, you likely have external interference or over-aggressive power control settings.
3. Check Retransmission Rates: If retx is high while MCS is low, the link is unstable. Focus on power control and alpha tuning.
If you’re seeing low throughput even with good signal (SINR > 15dB), check if UL 256QAM is enabled and supported by the UE, as this can provide a 33% boost in peak rates under ideal conditions.