Experimental Study of the Effects of RLC Modes for 5G-NTN applications using OpenAirInterface5G

Provisioning of 5G services via Non-Terrestrial Networks
(5G-NTN) has become a reality. Currently, 5G-NTN is
being developed by adapting the 5G Terrestrial Network (5GTN)
protocol which presents several challenges as the satellite
channel is significantly different from terrestrial channels. The
retransmission mechanism is one of them. Recent works on 5GNTN
have followed the approach of disabling retransmissions
from the 5G protocol stack such as Hybrid Automatic Repeat
Request (HARQ) at the MAC layer and Automatic Repeat Request
(ARQ) at the RLC layer while delegating it to the application
layer. This approach degrades the end-user-throughput and latency.
In this work, we experimentally study the effect of enabling
ARQ retransmission, by using RLC Acknowledged Mode (RLCAM)
in a GEO-based 5G-NTN which is susceptible to packet
loss caused by low SNR. We conduct real-time experiments
to compare the effects of RLC-AM and RLC Unacknowledged
Mode(RLC-UM) on applications(VoIP, video stream, file transfer)
using TCP and UDP for different SNR regimes. We have used
OpenAirInterface5G-NTN, which is developed to perform realtime
5G-NTN experiments, and a satellite channel emulator. We
observe that for low SNR, RLC-AM performs better than RLCUM
in achieving the required bitrate and packet error rate.
The reason being RLC-AM recovers the lost packets earlier in
the protocol stack without delegating the retransmissions to the
application layer. This becomes especially useful when HARQ is
disabled. We believe that our experimental study will complement
the ongoing theoretical research and help improve the procedures
of the Radio Resource Control(RRC) layer in 5G-NTN-specific
applications.