Synchronizing with the Scheduler: Dual-Loop Congestion Control for 5G Uplink on Commodity Devices

Current end-to-end congestion-control feedback is too slow to track rapid wireless dynamics in cellular networks. We identify Grant-to-Buffer Ratio (GBR)—the ratio of base-station uplink grants to mobile-reported demand—as a millisecond-scale RAN signal of uplink resource scarcity. Measurements across AT&T, Verizon, and T-Mobile LTE/5G FDD/TDD deployments show that GBR tracks base-station uplink load and reveals congestion earlier than end-to-end feedback. Because GBR is derived from the mandatory BSR-grant exchange, it requires no base-station changes and captures scheduler decisions at their native timescale. We then design GBR-CC, a dual-loop controller that updates the sender rate on each GBR sample, using GBR for fast adaptation and end-to-end delay trends as a conservative fallback. This design lets the sender react before queues inflate while still handling non-radio bottlenecks through the outer loop. GBR-CC runs on commodity mobile devices without extra hardware or external tools. Experiments on commercial cellular networks show that GBR-CC improves average throughput over GCC by 50%, while reducing median playout latency by 32–53% and freeze rate by 60%; compared with BBR, it improves average throughput by 5% and halves median RTT.