Difference between RTT vs differential time delay

Both terms i.e. Round Trip Time (RTT) and differential time delay relate to latency, but measure different things.

Round Trip Time (RTT)

• It is the total time taken for a signal to travel from the transmitter (e.g., UE or gNB) to the receiver (e.g., satellite or gateway) and back again.
• In NTN, it represents the end to end propagation delay over the uplink and downlink paths combined.

$RTT = Tup + Tdown$

Where, Tup : Uplink Propagation Delay Tdown : Downlink Propagation Delay

• Measures how long a signal takes to make a complete round trip between ground and space nodes.
• Depends on satellite altitude and path geometry.
• Typical values : ~240 ms (GEO), ~10-20 ms (LEO)

Differential Time Delay

• It is the difference in signal arrival times at different receivers (or different points in a beam) caused by variations in propagation distance from the satellite.
• It is the delay variation across the coverage area of a satellite beam.

$ΔT=∣T1​−T2​∣$

Where, T1 : Propagation delay to UE1 (at one point of the beam) T2 : Propagation delay to UE2 (at another point of the beam)

• Represents how much longer (or shorter) the signal takes to reach one user compared to another within the same satellite footprint.
• Caused by the curvature of the Earth and the slanted path to users at the beam edge.
• Typical values : Up to few milliseconds for GEO, Tens to hundreds of microseconds for LEO

Summary : RTT is the absolute propagation delay, while differential time delay is the variation in that delay across a single cell’s coverage area.