NetSim NTN, Verified Against 3GPP TR 38.821

The six reference scenarios in TR 38.821

TR 38.821 defines six reference scenarios by orbit, payload type and beam behaviour (Table 4.2-1 of the report).

The calibration assumptions of Table 6.1.1.1-5 apply to scenarios A, C2 and D2. The four cases on this page are LEO cases. NetSim runs them with a transparent payload, which is Scenario C2.

Four LEO cases: S-band, handheld UE, downlink

The cases cover two altitudes and two frequency re-use factors. All four use satellite parameter Set 1.

Case definitions: TR 38.821 Table 6.1.1.1-9. Re-use options: Table 6.1.1.1-5. Each case has 19 metric beams with 10 UEs per beam, so each CDF contains 190 UEs.

19 metric beams inside an outer interference field

TR 38.821 takes its statistics from the inner 19 beams and requires the surrounding beams to be simulated independently.

Figure 1: SC10 downlink geometry. Axes show distance from the central beam in km.

• Metric beams. The 19 blue cells. Only UEs in these cells enter the CDFs.
• Outer interference beams. The orange cells. They transmit on the same channel and supply the surrounding interference.
• UE positions. The black dots. NetSim places 10 UEs uniformly inside the Voronoi cell of each metric beam.

Re-use factor	Additional tiers	Metric	Outer	Total
1 (SC9, SC14)	2	19	42	61
3 (SC10, SC15)	4	19	108	127

The tier counts follow TR 38.821 Section 6.1.1.1: two additional tiers for FRF = 1, and four additional tiers for FRF > 1.

Figure 2: The same SC10 scenario in NetSim. Stars: beam centres. Red outline: metric beams. Red dots: UEs.

NetSim configuration

One configuration serves all four cases. Only the orbit and the re-use factor change.

Table 1: NetSim parameters for the Set-1 downlink calibration. Table numbers refer to TR 38.821.

The two metrics, and how NetSim computes them

Table 6.1.1.1-5 names the calibration metrics: coupling loss and geometry.

1. Run the case in NetSim
2. Read the downlink records of the radio measurement log
3. Take the time mean for each UE
4. Form the CDF across the 190 UEs

The time mean gives every UE the same weight in the CDF, whatever its number of log records. The beam association log confirms the serving beam of each UE.

Throughput and capacity for the same four cases

The same runs also report application throughput. These are NetSim results with full-buffer traffic. The throughput results in TR 38.821 Section 6.1.1.3 use FTP traffic at set resource utilisation, so the two sets do not compare.

Table 2: Capacity metrics over the 19 metric beams.

Table 3: Percentiles of application throughput. The per-beam value is the sum over the 10 UEs of a metric beam.

Figure 3: CDF of application throughput per UE.

Figure 4: CDF of application throughput per beam.

Throughput follows Geometry SINR. The re-use factor 3 cases deliver about 330 Mbps at both altitudes, because their SINR distributions match. The re-use factor 1 cases deliver less, and their UEs spread over a wider throughput range.

Reproduce these results in NetSim

The example ships with NetSim, with its configuration files and results. The NetSim 5G NTN manual explains it step by step.

References: 3GPP TR 38.821 V16.2.0, “Solutions for NR to support non-terrestrial networks (NTN)”. R1-1913404, “System Level Calibration Results for NTN on DL transmissions”, Thales, RAN1#99.