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Uplink Scheduling

In terrestrial 5G NR, uplink transmissions follow a dynamic grant procedure: the UE sends a Scheduling Request, the gNB responds with a UL Grant, the UE sends a Buffer Status Report, and the gNB allocates resources for data transmission on PUSCH. This multi-step procedure assumes sub-millisecond round-trip times. In NTN, the one-way propagation delay ranges from approximately 4 ms (LEO 600 km) to 240 ms (GEO), resulting in round-trip delays of 8 ms to over 500 ms. A four-step dynamic grant procedure would therefore add hundreds of milliseconds of scheduling delay before data transmission can begin.

To address this, 3GPP TS 38.214 Section 6.1.2.3 and TS 38.321 Section 5.8.2 define Configured Grant (CG) as a mechanism where the gNB pre-configures periodic uplink transmission resources for each UE via RRC signaling. Once configured, the UE transmits autonomously on PUSCH without waiting for individual UL grants.

NetSim Implementation

NetSim implements Configured Grant Type 1 as a cyclic round-robin scheduler operating in time-division mode (TDM):

• Each associated UE receives one dedicated UL slot per cycle.

• The cycle length equals the number of associated UEs (\(N_{UE}\)). UE \(i\) transmits in slot \((n \cdot N_{UE} + i)\) where \(n = 0, 1, 2, \ldots\)

• In each assigned slot, the scheduled UE receives all available PRBs (after overhead deduction).

• The available PRBs per UE per slot = \(N_{PRB} - \lceil OH_{UL} \cdot N_{PRB} \rceil\).

• Each beam runs its own independent cyclic round-robin schedule.

• MCS is selected per-UE based on the UL link budget: UL SNR \(\rightarrow\) CQI \(\rightarrow\) MCS.

• HARQ is disabled in NTN by default and operates as a passthrough. Packet delivery flows through the HARQ transmit path, but no retransmissions occur.