“Throughput is low” is not a diagnosis. In 5G as in 4G, the downlink throughput of a device results from a chain of factors: mobility, received signal level, signal quality, error rate, the share of resources the scheduler allocates to it, the number of MIMO layers and the modulation and coding scheme. To move from symptom to cause, each degraded second is assigned the first verified cause, in an order that runs from the most fundamental factor to the finest.
The eight rules, in order
- Mobility: a handover, a PSCell change, a radio link failure or a re-establishment within the second explains the drop on its own.
- Coverage: the serving cell RSRP is below the coverage-hole threshold.
- Radio quality: SINR is insufficient, due to interference or noise.
- BLER: the error rate remains above target despite link adaptation.
- Slots: the device is scheduled in only a small share of slots, because of cell load or low application demand.
- RB: few resource blocks per allocation; the bandwidth is shared with other users.
- Rank: fewer than 1.5 layers on average despite good SINR.
- MCS: a low modulation and coding scheme despite good SINR, a sign of conservative link adaptation.
Why the order matters
A second containing a handover also shows degraded SINR and high BLER: attributing it to interference would skew the breakdown. Likewise, rank 1 is a cause only if SINR would have allowed more — at 3 dB, a single layer is normal. The “rank” and “MCS” rules therefore apply only when SINR is rated good, and a second that matches no rule is classified as “undetermined” rather than forced into a category.
Setting the thresholds
- Low throughput: on a 100 MHz n78 carrier, a threshold of 300 to 400 Mbit/s isolates the truly abnormal seconds; in LTE, it depends on the bandwidth and on carrier aggregation.
- Insufficient SINR: 5 to 8 dB, depending on the requirement.
- SINR rated good, for the rank and MCS rules: 10 to 13 dB.
- Data transfer: only the seconds with a transfer in progress are analyzed, excluding the first and last second of each period.
Reading the breakdown
- A breakdown dominated by radio quality points to interference: cell overshooting, pilot pollution, poorly managed neighbor relations.
- A large share of rank 1 at good SINR often indicates a hardware fault: a faulty RF branch, crossed polarizations, an incorrectly connected antenna.
- A low share of slots points to cell load or to the application itself.
- The “undetermined” share points beyond the radio: transport, server, device, or transitions between cells.
Proving the cause
Every low-throughput period must be replayable: its position on the map, its curves, its signaling messages. Correlating two parameters then turns a hypothesis into a demonstration — for example, a layer count that stays at 1 whatever the SINR, or a transmit power that follows the downlink load.