Draft: Location-based triggering for NTN

Doppler spreading and time selectivity of the channel are already a challenge for conventional terrestrial networks. However, for LEO systems, the satellites can have speeds up 7.56 km/s, which is much faster than 0.14 km/s of a high-speed train in terrestrial networks, the delays in LEO are more varied and longer, and the path loss is larger, because the communication distances up to 10 times longer than in terrestrial networks. Moreover, downlink in LEO presents a high interference from adjacent satellite beams, and all these features contribute to reduced received signal strength variation in these networks compared to terrestrial networks. Typically, the criteria used in algorithms presented in conventional terrestrial networks to trigger a handover from a gNB to another one is based on signal strength measurements, however, the reduced received signal strength variation in NTNs become these algorithms inefficient for LEO systems. Therefore, algorithms with criteria for handover triggering that address the specifics of LEO systems are crucial for an efficient handover processing that ensures a robust communication with low dropping probability. Some criteria for handovers in NTN are as follows:

• Measurement-based triggering: This method is based on signal strength measurement, and as stated above, it may not be efficient. The triggering thresholds and which measurement events to use as triggers, as reference signal received power (RSRP), reference signal received quality (RSRQ), or received signal strength indicator (RSSI), should be configured. This method relies on UE estimates and established channel estimation techniques, however it would require neighbouring cell lists which can be hard because the fast-moving of satellites leads to a fast cell coverage deviation.
• Location-based triggering: This method is based on UE and satellite location, which can be applied jointly (or not) with another trigger as the measurement-based trigger. For instance, for a deterministic satellite movement, it is possible to predict the configure triggering condition, and the initial association of the UE can be performed based on the distance with the nearest satellite, because we can know the location of the UE and NTN satellite to compute the distance.
• Elevation angles of source and target cells based triggering: This method is similar to the previous one, but it is based on the largest elevation angle.
• Time/timer-based triggering: This method uses triggering conditions based on UTC time or a timer-based solution, which can also be applied jointly (or not) with another trigger as the measurement-based trigger. The timer-based handover trigger considers the deterministic satellite movement to predict the time duration for which the satellite’s footprint covers a certain zone.
• Timing advance value-based triggering: This method uses the timing advance value (independently or jointly with another trigger) to trigger a handover to the target cell. It is appropriate to overcome the Random-Access preamble reception issue, where the UE needs to pre-compensate the instant which sends the preamble. However, UEs with GNSS support are required to perform this method.

Therefore, aligned with 3GPP Rel-17, a location-based handover triggering, taking advantage of deterministic satellite movement was implemented, where it is assumed that the locations of the UE and the NTN satellite are known. From a practical point of view, this means that we know how long it takes to perform the handover, and therefore we only need to implement a timer. This timer can be added to the gNB-CU configuration file like this: location_ho_trigger = 15;

Instructions for running this MR in TDD:

Run OAI-gNB-CU

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb-cu.sa.rfsim.conf --sa

Run OAI-gNB-DU 0

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb-du.sa.tdd.band78.106prb.rfsim.pci0.conf --rfsim --rfsimulator.serveraddr 127.0.0.1 --sa

Run OAI-gNB-DU 1

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb-du.sa.tdd.band78.106prb.rfsim.pci1.conf --rfsim --rfsimulator.serveraddr 127.0.0.1 --sa

Run OAI-UE

(For UE to connect first with DU0)

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-uesoftmodem -r 106 --numerology 1 --band 78 -C 3450720000 --rfsim --sa --uicc0.imsi 001010000000001 --rfsimulator.serveraddr server

(Or for UE to connect first with DU1)

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-uesoftmodem -r 106 --numerology 1 --band 78 -C 3649440000 --rfsim --sa --uicc0.imsi 001010000000001 --rfsimulator.serveraddr server

Instructions for running this MR in FDD:

Run OAI-gNB-CU

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb-cu.sa.rfsim.conf --sa

Run OAI-gNB-DU 0

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb-du.sa.fdd.band66.25prb.rfsim.pci0.conf --rfsim --rfsimulator.serveraddr 127.0.0.1 --sa

Run OAI-gNB-DU 1

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb-du.sa.fdd.band66.25prb.rfsim.pci1.conf --rfsim --rfsimulator.serveraddr 127.0.0.1 --sa

Run OAI-UE

(For UE to connect first with DU0)

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-uesoftmodem -r 25 --numerology 0 --band 66 --C 2152250000 --CO -400000000 --ssb 83 --rfsim --sa --uicc0.imsi 001010000000001 --rfsimulator.serveraddr server

(Or for UE to connect first with DU1)

cd ~/openairinterface5g/cmake_targets/ran_build/build
sudo ./nr-uesoftmodem -r 25 --numerology 0 --band 66 --C 2177250000 --CO -400000000 --ssb 83 --rfsim --sa --uicc0.imsi 001010000000001 --rfsimulator.serveraddr server