5G SA Uplink Coverage at the Cell Edge
An operator deploys 5G SA at 3.5 GHz TDD with 100 MHz bandwidth, 30 kHz SCS, and a DDDSU slot pattern on 64T64R radios. Sites are 500–700 m apart. Downlink at the cell edge still delivers 180–250 Mbps, but uplink drops to 1–3 Mbps, and PRACH detection failures reach 12% at the same locations. RRC setup success rate is 99.2% overall but 94% for users beyond 600 m. The operator has 1.8 GHz and 700 MHz spectrum available, but only 20 MHz at 1.8 GHz is free. The marketing team wants a 20 Mbps uplink guarantee for industrial users across the coverage area. A proposal is to add 12 new sites; another is to change the TDD pattern to DSUUU. You must decide how to meet the uplink SLA without doubling site count.
1. Identify the dominant coverage limitation and explain why downlink KPIs look healthy while uplink fails.
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The cell is uplink-limited, not downlink-limited. At 3.5 GHz the path loss is high, and the UE transmit power is far lower than the gNB transmit power. The gNB can use 64T64R beamforming and high EIRP on the downlink, so edge users still see 180–250 Mbps. On the uplink, the UE is the power-limited transmitter; massive MIMO receive gain helps but cannot fully close the link for a 20 Mbps guarantee at 600–700 m.
The DDDSU pattern is also downlink-heavy: only one full uplink slot in five, plus special-slot uplink symbols. PRACH failures at 12% and RRC failures beyond 600 m confirm that random access and uplink control are failing before downlink throughput degrades. Adding sites would fix it, but 12 new sites is a cost and deployment-time decision. The right first step is to improve uplink link budget selectively, not to raise downlink power or simply add capacity.
2. Evaluate DSUUU versus SUL at 1.8 GHz for the 20 Mbps uplink guarantee.
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DSUUU increases uplink scheduling opportunities, but it does not change the fundamental UE power and path-loss problem. It may raise average uplink throughput for mid-cell users, while reducing downlink capacity and possibly hurting latency for downlink-heavy traffic. At the edge, the UE still cannot deliver 20 Mbps reliably because the link budget is short; DSUUU alone is therefore not sufficient.
SUL at 1.8 GHz is the stronger option. Lower frequency gives roughly 8–10 dB lower path loss than 3.5 GHz, which directly improves random access and uplink coverage. With 20 MHz at 1.8 GHz, the cell can support the industrial SLA if scheduling is reserved for edge users rather than all users. Combine SUL for cell-edge uplink with 3.5 GHz for downlink and mid-cell capacity; configure uplink switching based on RSRP and service type. This meets the SLA without 12 new sites, but it requires careful capacity planning on the 20 MHz carrier.
3. What KPIs and thresholds would you use to verify the fix and avoid degrading downlink?
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Verify both coverage and service quality. Track PRACH detection success (target >99%), RRC setup success (>99.5%), uplink throughput at the 5th percentile for industrial users (≥20 Mbps), and uplink delay. Check SUL usage ratio and 1.8 GHz PRB utilization to ensure the 20 MHz carrier is not oversubscribed. On the downlink, compare edge throughput, latency, and PRB utilization before and after; a DSUUU change would show a downlink loss, while SUL should leave 3.5 GHz downlink largely unchanged.
Also monitor inter-frequency handover and SUL-to-3.5 GHz switching success, because poor switching can create new drops. Use a per-cell edge-bucket report, not only network averages. Acceptance should require the industrial SLA at the edge while the overall downlink throughput and latency remain within agreed bounds.