IEEE - Aerospace and Electronic Systems - August 2021 - 25

Hosseinian et al.
Table 1.
Three Directions for Satcom Advancement
Integration with terrestrial
networks
Main
components
Applications Emergency response,
Offloading, Path diversity
Advantages Coverage extension,
Throughput increase
Disadvantages Delay increase, Ground
infrastructure
Backhauling,
Multicasting/
broadcasting
High throughput
High cost of launch/
maintenance
be implemented either at the satellite base station or in the
gateway. In both cases, the long propagation delay will
affect the optimal response time.
One of the main functions of the access controller that
is significantly affected by long propagation delay is the
random access (RA) procedure, which serves multiple
purposes such as UE and gNB connection establishment,
uplink and downlink timing synchronization, and beam
management. Because multiple UEs can perform the procedure,
a contention-based solution is specified as an iterative
process in which multiple transmissions occur
between UEs and gNB. The longer propagation delay of
the NTN, which is the major differentiating factor
between the NTN and terrestrial networks, is intensified
further in the RA procedure with multiple iterations.
In Release 16, two types ofRA procedures are specified:
4-step and 2-step [14]. The 2-step procedure is an alternative
to the 4-step to cut down the duration of the initial access by
reducing the number ofsequential downlink and uplink transmissions
between UE and gNB. The 4-step RA takes at least
two round trip delays to be completed, whereas the 2-step
needs one less. This makes the 2-step RA suitable for NTN
scenarios in which the propagation delay between gNB and
UE is much larger than the typical terrestrial cases. However,
the current preamble design of Release 16 does not support
the large round trip delay and frequency offset due to the
Doppler shift ofNTN scenarios. With the ephemeris data that
contain the information on the orbital trajectories ofsatellites,
a Global Navigation Satellite System (GNSS)-based UE can
calculate the distance to the satellite, and consequently estimate
the round trip delay. Considering these assumptions,
during the Release 17 NTN normative phase, the RA procedure
can be redesigned and optimized to support NTN UEs.
CONTROLLOOPSANDTIMINGRELATIONSHIPS
While NR considers the highest speed user terminals to
move at 500 km/h (e.g., high speed trains), NTN user
AUGUST 2021
Delay-sensitive services, Data
collection and IoT in remote areas
Reduced propagation delay, Easy
update/softwarization
Hand-over and routing complexity,
Space pollution
terminals can have speeds up to 1200 km/h for aircraft,
and the gNB in some NTN scenarios, such as LEO, can
move at a colossal speed of 7.56 km/s ($27,216 km/h).
To support such high speeds, it will be critical to reduce
the response time ofthe power control loop. The following
possible solutions have been suggested:
1) to reduce the transmission slot duration, for
instance, by using mini slot transmissions;
2) to increase subcarrier spacing (SCS), which leads to
reduced OFDM symbol duration;
3) to improve the timing of mapping and scheduling of
the power control command, if possible, to warrant
a faster response time.
On the other hand, due to long propagation delays, any
NR timing relationship that involves downlink-uplink
timing interactions must be modified and enhanced for
NTN. The timing between sequential downlink and uplink
processes cannot be smaller than the timing advance
applied by UE. The existing NR specifications use several
configuration RRC parameters for different downlink-
uplink interactions to establish a timing relationship
between gNB and UE. However, the ranges of such RRC
parameters are not sufficient to compensate for typical
NTN timing advance values. The enhancement can be
implemented by applying an extra timing offset for UE
configuration.
HYBRIDAUTOMATICREPEATREQUEST(HARQ)
Even in cellular networks, HARQ retransmissions may
lead to extra latency of up to 8 ms, in the case of the frequency-division
duplexing (FDD) mode. A Transmission
Timing Interval (TTI) bundling mechanism on uplink was
designed in LTE to allow retransmission of the same signal
over up to four consecutive subframes without waiting
for HARQ acknowledgement (ACK) to reduce the jitter.
IEEE A&E SYSTEMS MAGAZINE
25
HTS
Ground interface/relay GEO satellites
Mega-constellation satellite
networks
LEO satellites
IEEE - Aerospace and Electronic Systems - August 2021

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