Feature: Test & Measurement
Te good news is that free space loss can be managed with improved antenna gain, with beamforming, for example, making it possible for mobile devices on the ground to communicate with satellites.
Figure 2: Illustration of free space loss: If waves are propagated spherically, the transmission power of the geostationary satellite is distributed evenly over a spherical surface. Only a tiny fraction (on the order of 1/120) of the output power reaches the receiver on Earth, corresponding to a whopping 200 dB of attenuation.
technology, a new duplexing method was introduced in FR2. Up to now, 5G has only operated in this region with time division duplexing (TDD). 5G NTN however, will use frequency division duplexing (FDD), which is more spectrum efficient because it avoids the long guard intervals necessary to switch between transmission and reception due to a long delay. As a result, some extra work is required using optimisation methods such as channel estimation, equalisation, and beamforming. With TDD, these methods are implemented directly at the transmitter (channel reciprocity) while FDD requires a bilateral communication loop. Known reference signals from the transmitter
enable the receiver to estimate channel characteristics and feed them back to the transmitter (channel status reporting).
Free space path loss One of the biggest challenges for 5G NTN is free space path loss (FSPL). Tis occurs primarily due to the long distance between the transmitter and receiver. In the most extreme case, an end-user device on the ground is connected to a geostationary satellite 35 786 kilometres (22 236 miles) above the Earth (see Fig. 2). Aside from transmission power, gain
in both the receiving and transmitting antennas is vital for stable communications.
Propagation delay Compared to communications via ground infrastructure, propagation delay, oſten referred to as round trip time (RTT), is much longer with a satellite connection. Suppose an end-user device on the ground requests a data packet from another mobile device on the ground via satellite in geostationary orbit. Te propagation delay from Earth to the satellite and back (two round trips) will be around 544 milliseconds. Compare that to the 20 to 30 millisecond delay measured for terrestrial 5G networks. Existing 5G soſtware protocols, therefore, need to be adapted to these longer delays.
System architecture for 5G NTN Te point of integrating NTN into 5G is to facilitate direct communication between satellites and end-user devices on the ground. Additions and improvements are expected in future 3GPP releases. In the initial implementation of 5G NTN, most of the mobile network will be terrestrial, consisting of base stations (gNB) and the core network. Satellites will act as simple ‘bent pipe’ or ‘transparent payload’ repeaters. Te disadvantage of this approach is that propagation delay (measured from client to server with acknowledgment) includes the distance between the satellite and the Earth four times. However, the advantage of this approach is that most modifications required for 5G NTN can be carried out in the more easily accessible terrestrial network. Future architectures will shiſt more of the signal processing load to the satellites. Some server functions can be integrated into the non- terrestrial components of the network, so that the satellite will handle channel assignment (scheduling) and packet retries. Tis will reduce latency and increase the level of autonomy.
Figure 3: Propagation delay and the Doppler shift in various 5G NTN scenarios. 34 Dec 2024/Jan 2025
www.electronicsworld.co.uk
Influence on propagation delay From the point of view of Earth based transmitters and receivers, a sunup-sundown effect occurs with satellite communications,
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