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Physical layer enhancements in 5G‐NR for direct access via satellite systems
Article in International Journal of Satellite Communications and Networking · August 2022
DOI: 10.1002/sat.1461
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Physical Layer Enhancements in 5G-NR for
Direct Access via Satellite Systems
Stefano Cioni1, Xingqin Lin2, Baptiste Chamaillard3, Mohamed El Jaafari3, Gilles
Charbit4, and Leszek Raschkowski5
1European
Space Agency, Noordwijk, NL; 2NVIDIA, Santa Clara, California, USA; 3 Thales Alenia Space, Toulouse, FR; 4MediaTek Wireless Ltd,
Cambourne Business Park, Cambridge, UK; 5Fraunhofer Institute for Telecommunications Heinrich Hertz Institute, Berlin, DE.
Abstract
Among the several features and capabilities introduced by every new 3GPP release on 5G cellular systems, the latest Release
17 will be remembered as the first that specifies a set of enhancements and adaptations to support mobile broadband services
via satellite direct access. Specifically focused on the necessary physical layer mechanism and procedure modifications, this
paper will present in detail the 3GPP work about the inclusion of satellite systems in 5G networks.
I.
Introduction
In the next few years large-scale deployment trend of the fifth generation (5G) wireless networks will
certainly require complementary services in order to offer ubiquitous and reliable coverage across several
geographical areas. Since the beginning, the 5G standard has been called a “network of networks” for the
ambitious goal to expand the typical broadband cellular market. The unique capabilities of non-terrestrial
networks (NTN) can help address the reach of 5G technology in several new use cases [1]. The satellite
communication industry is designing new constellations of satellites with advanced technologies on-board
to serve larger footprints, provide more reliable service, and become more cost-effective [2].
Recently, the third generation partnership project (3GPP) standardization group has witnessed an
increasing interest and participation from the satellite communication industry, aiming at the
convergence of the market needs to deploy an integrated satellite and terrestrial network infrastructure
in the context of 5G services. This synergy has firstly produced a technical report on the NTN channel
model and the challenges to support the 5G New Radio (NR) waveform [3], then a technical report on
possible solutions for 5G NR over NTN systems [4]. In particular, [4] has introduced the main NTN system
assumptions as a function of the satellite orbit (e.g., geosynchronous earth orbit (GEO) or low earth orbit
(LEO)) and the frequency range (e.g., below or above 6 GHz). These inputs, along with the NTN channel
model, have been instrumental in identifying and in prioritizing the necessary modifications to ensure the
first-ever 3GPP release for 5G NR via satellite. The changes have impacts on the entire 5G protocol stack
and network architecture [5]. In particular, the new 3GPP NTN Release 17 work item aims to specify the
enhancements identified for NR according to the following principles [4]: a) frequency division duplexing
(FDD) in satellite systems; b) Earth fixed tracking area assumed with either Earth fixed or moving satellite
cells; c) NTN user equipment (UE) with global navigation satellite system (GNSS) capabilities.
This paper is focused on only the physical layer aspects to be enhanced in NTN systems. Specifically, the
latest 3GPP NTN Release 17 work item has devoted the effort to solving identified issues caused by long
channel propagation delays, large Doppler effects, and moving cells. These NTN inherent challenges will
require revisiting some 5G NR legacy mechanisms and procedures. For completeness, it shall be
underlined that the legacy starting points (i.e., 3GPP 5G NR Release 16) for the NR physical layer
specifications can be found in [6], [7], [8], and [9]. Based on the performance analysis and trade-offs
reported in [4], the first essential enhancements included in Release 17 for NTN systems [10] are timing
relationships, uplink time and frequency synchronization mechanisms, and hybrid automatic repeat
request (HARQ) processes. In addition, interesting discussions have been devoted reviewing the beam
management and bandwidth part operations, including also the necessity of the polarization signaling.
In the following, all these topics have been reviewed and presented as the final agreements included in
the new set of specifications related to the physical layer adaptations for 3GPP NTN Release 17.
II.
Timing Relationship Enhancements
5G NR physical layer involves sophisticated timing relationships [11], including resource allocation timings,
medium access control (MAC) control element (CE) activation timings, UE processing and preparation
procedure timings, among others. These timing relationships are at the core of 5G NR physical layer
specifications and have a direct impact on network and UE implementations. The original timing
relationships in 5G NR were designed mainly targeting terrestrial mobile communications, in which the
propagation delays are usually less than 1 ms. The propagation delays in satellite systems are much longer,
ranging from several milliseconds to hundreds of milliseconds. Addressing such long propagation delays
requires a rethinking of many timing relationships in the 5G NR physical layer [12].
II.A.
Timing relationships enhanced with Koffset
In Release 17, 3GPP enhanced many timing relationships in the 5G NR physical layer with the introduction
of Koffset to handle the long propagation delays in satellite systems. To facilitate describing the design
rationale of Koffset, we use the physical uplink shared channel (PUSCH) timing relationship as a running
example in this section. Besides, for ease of description, we assume that the subcarrier spacing (SCS) value
of the downlink (DL) is the same as that of the uplink (UL).
The transmission timing of PUSCH in the uplink is controlled by the time domain resource assignment
(TDRA) field in the downlink control information (DCI) transmitted in the physical downlink control
channel (PDCCH). The TDRA field is used as an index into a radio resource control (RRC) configured table
providing information on when the PUSCH should be transmitted relative to the reception of the PDCCH.
Specifically, for a scheduling DCI received in downlink slot 𝑛𝑛, the uplink slot where the UE shall transmit
the PUSCH is determined as slot 𝑛𝑛 + 𝐾𝐾2 , where 𝐾𝐾2 is the slot offset relative to the downlink slot 𝑛𝑛, where
the scheduling DCI is received. The value range of 𝐾𝐾2 is {0, 1, …, 32}.
In satellite systems, each UE needs to apply a large timing advance (TA) value to compensate the roundtrip time (RTT) between UE and 5G Node B (gNB). This is illustrated in Figure 1. When TA becomes large,
the cardinality of the set of values of 𝐾𝐾2 that can be used is reduced significantly or even becomes zero.
As illustrated in Figure 1, the consequence is that even by using the maximum value of 𝐾𝐾2 , from the UE’s
perspective, the uplink slot 𝑛𝑛 + 𝐾𝐾2 where PUSCH is supposed to be transmitted may occur before the
downlink slot 𝑛𝑛 where the scheduling DCI is received. To resolve this issue, the scheduling offset Koffset is
introduced to enhance the PUSCH transmission timing. Specifically, for a scheduling DCI received in
downlink slot 𝑛𝑛, the uplink slot where the UE shall transmit the PUSCH is determined as slot 𝑛𝑛 + 𝐾𝐾2 +
𝐾𝐾offset . With appropriate value of 𝐾𝐾offset , the uplink slot 𝑛𝑛 + 𝐾𝐾2 + 𝐾𝐾offset can occur after the downlink
slot 𝑛𝑛 at the UE side.
Figure 1: An illustration of PUSCH transmission timing with and without Koffset enhancement.
Besides the PUSCH transmission timing relationship, Koffset is similarly introduced to enhance several other
timing relationships in the 5G NR physical layer. The list of timing relationships enhanced by Koffset is
summarized as follows.
• The transmission timing of DCI scheduled PUSCH, including channel state information (CSI)
transmission on PUSCH.
• The transmission timing of random access response (RAR) grant scheduled PUSCH.
• The timing of the first PUSCH transmission opportunity in type-2 configured grant.
• The transmission timing of hybrid automatic repeat request acknowledgement (HARQ-ACK) on
the physical uplink control channel (PUCCH), including HARQ-ACK on PUCCH to message B (MsgB)
in 2-step random access.
• The transmission timing of PDCCH ordered physical random access channel (PRACH).
• The timing of the adjustment of uplink transmission timing upon reception of a corresponding
timing advance command.
• The transmission timing of aperiodic sounding reference signal (SRS).
• The CSI reference resource timing.
As Koffset is used in several timing relationships in initial random access, a cell specific Koffset value needs to
be signaled in system information. The value range of the cell specific Koffset is 0 – 1023 ms, which is
sufficient to accommodate different types of satellite systems. To cater for UE specific propagation delay
to further improve scheduling efficiency, a UE-specific Koffset value can be provided and updated by
network with a MAC CE. Specifically, the MAC CE provides a differential UE-specific Koffset value in the
range of 0 – 63 ms. The full UE-specific Koffset value equals the cell specific Koffset value minus the differential
UE-specific Koffset value.
When UE is not provided with a UE-specific Koffset value, the cell specific Koffset value signaled in system
information is used in all the timing relationships that require Koffset enhancement. When UE is provided
with a UE-specific Koffset value (which is equal to the sum of the cell specific Koffset value and a UE-specific
differential value provided in MAC CE), the UE-specific Koffset is used in all the timing relationships that
require Koffset enhancement except for the following timing relationships, where the cell specific Koffset
value signaled in system information is always used:
•
•
•
•
•
The transmission timing of RAR grant scheduled PUSCH.
The transmission timing of message 3 (Msg3) retransmission scheduled by DCI format 0_0 with
cyclic redundancy check (CRC) scrambled by temporary cell radio network temporary identifier
(TC-RNTI).
The transmission timing of HARQ-ACK on PUCCH to contention resolution the physical downlink
shared channel (PDSCH) scheduled by DCI format 1_0 with CRC scrambled by TC-RNTI.
The transmission timing of HARQ-ACK on PUCCH to MsgB scheduled by DCI format 1_0 with CRC
scrambled by MsgB radio network temporary identifier (MsgB-RNTI).
The transmission timing of PDCCH ordered PRACH.
The main reason of always using the cell specific Koffset value in the aforementioned timing relationships is
to make sure that there is no ambiguity between network and UE about the value of Koffset applied.
II.B.
MAC CE timing relationships enhanced with Kmac
MAC CE action timing in 5G NR has an application delay of 3 ms, inherited from the Long-Term Evolution
(LTE) standard. Specifically, when the UE would transmit a PUCCH with HARQ-ACK information in uplink
slot 𝑛𝑛 corresponding to a PDSCH carrying a MAC CE command, the UE action and assumption on downlink
configuration indicated by the MAC CE command starts from the first downlink slot after downlink
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,µ
slot 𝑛𝑛 + 3𝑁𝑁𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,µ
, where 𝜇𝜇 is the SCS configuration for the PUCCH and 𝑁𝑁𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
is the number of
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,µ
slots per subframe for SCS configuration 𝜇𝜇 (therefore, the duration of 3𝑁𝑁𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
is exactly 3 ms).
Similarly, the UE action and assumption on uplink configuration indicated by the MAC CE command starts
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,µ
from the first uplink slot after uplink slot 𝑛𝑛 + 3𝑁𝑁𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
.
When downlink and uplink frame timing are aligned at gNB, the MAC CE timing relationships do not
require enhancement. When downlink and uplink frame timing are not aligned at gNB, the UE action and
assumption on downlink configuration indicated by the MAC CE command in PDSCH requires extension.
This is illustrated in Figure 2, which shows that from the gNB’s perspective, the downlink slot 𝑚𝑚 = 𝑛𝑛 +
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,µ
3𝑁𝑁𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
+ 1 may occur before the actual uplink slot 𝑛𝑛 where the PUCCH with HARQ-ACK
information acknowledging the MAC CE command would be received. To resolve this issue, Kmac is
introduced to extend the MAC CE timing relationship in the downlink. Specifically, when the UE would
transmit a PUCCH with HARQ-ACK information in uplink slot 𝑛𝑛 corresponding to a PDSCH carrying a MAC
CE command, the UE action and assumption on downlink configuration indicated by the MAC CE
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,µ
command starts from the first downlink slot after downlink slot 𝑛𝑛 + 3𝑁𝑁𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
+ 𝐾𝐾𝑚𝑚𝑚𝑚𝑚𝑚 . The value of
𝐾𝐾mac is equal to the offset of the gNB’s downlink and uplink frame timing. The downlink slot 𝑚𝑚′ = 𝑛𝑛 +
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,µ
+ 𝐾𝐾𝑚𝑚𝑚𝑚𝑚𝑚 + 1 occurs after the actual uplink slot 𝑛𝑛 where the PUCCH with HARQ-ACK
3𝑁𝑁𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
information acknowledging the MAC CE command would be received by gNB.
Figure 2: An illustration of MAC CE timing relationship with and without Kmac enhancement when downlink and uplink frame
timing are not aligned at gNB.
Note that, as also illustrated in Figure 2, the UE action and assumption on uplink configuration indicated
by the MAC CE command does not require extension: it still starts from the first uplink slot after uplink
𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠,µ
slot 𝑛𝑛 + 3𝑁𝑁𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
.
The UE-gNB RTT is approximately equal to the sum of UE’s TA and Kmac. Therefore, Kmac can find application
in, e.g., determining the starts of ra-ResponseWindow and msgB-ResponseWindow, which are delayed by
an estimate of UE-gNB RTT in satellite systems. Similarly, in beam failure recovery procedure, for PRACH
transmission in uplink slot 𝑛𝑛, the UE monitors the corresponding PDCCH starting from downlink slot 𝑛𝑛 +
4 + 𝐾𝐾𝑚𝑚𝑚𝑚𝑚𝑚 within a corresponding RAR window.
III.
Enhancements on UL time and frequency synchronization
3GPP has specified two waveforms for NR based upon orthogonal frequency division multiplexing
(OFDM): Cyclic prefix OFDM (CP-OFDM) applicable to both the UL and DL, and discrete Fourier transform
spread OFDM (DFT-S-OFDM) applicable to the UL only. For OFDM based NR waveforms, it is required that
UL transmissions from distinct devices are time and frequency aligned before OFDM demodulation is
performed by the base station [13] [14]. When it is not the case, it leads to inter carrier interference (ICI)
and inter symbol interference (ISI), which results in reception performance degradation.
In the following, we first present the legacy NR synchronization mechanisms, and then we discuss the
enhancements on time and frequency synchronization mechanisms introduced in NR Release 17 to
support NTN systems.
III.A.
Legacy timing and frequency synchronization
For timing synchronization, any timing misalignment between received signals should fall within the cyclic
prefix to avoid ISI and ICI. To ensure such alignment, NR specifies a well-known mechanism for the
network to enforce transmit timing advance (TA) from the devices. The TA refers to a negative offset
between the start of a given downlink slot as observed by the device and the start of the same slot in the
uplink [15], as depicted in Figure 3.
Figure 3: Uplink/Downlink radio frame timing at the UE.
The TA value is determined by the UE with the following formula:
(1)
𝑇𝑇TA = �𝑁𝑁TA + 𝑁𝑁TA,offset � × 𝑇𝑇c 1
𝑁𝑁TA,offset is broadcasted in the system information and it was specified to ensure that an UL radio frame
finishes before the start of the subsequent DL radio frame. 𝑁𝑁TA is an offset specific to each device and
under the control of the network via timing advance commands (TAC). It has been established that this
offset should be ideally adjusted to the exact round trip delay between the device and a reference point 2
(RP) considered by the network for UL transmission time alignment.
1F
During the initial access procedure, 𝑁𝑁TA shall be considered equal to zero by default for preamble
transmission and shall be updated for the first time based on the TAC embedded in the RAR from the
network. Once the UE has entered the network, the gNB becomes responsible for maintaining the timing
advance [5]. TAC embedded in MAC CE updates can be triggered when needed based on the residual
1 𝑇𝑇𝑇𝑇 = 1/(480000 × 4096) seconds
2 DL and UL are frame aligned at the uplink time synchronization reference point with an offset given by 𝑁𝑁
is located at the base station antenna.
𝑇𝑇𝑇𝑇,𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 .Typically,
this reference point
misalignment observed on the uplink transmissions received by the gNB. The maximal TA updates per TAC
are reported in Table 1 for initial access and Table 2 when in RRC CONNECTED state.
Finally, the device is responsible for its timing advance application based on its knowledge of DL receive
timing which comes from the detection of DL signals such as synchronization signals (SS) and reference
signals (RS).
Table 1: Maximum timing advance update per TAC in RAR
3
Numerology (µ )
0
1
2
3
15
30
60
120
Maximum timing advance update during initial access in NR TN
+2ms
+1ms
+0.5ms
+0.25ms
SCS = 15 ∗ 2µ kHz
Table 2: Maximum timing advance update per TAC in MAC CE
Numerology (µ)
0
1
2
3
SCS = 15 ∗ 2µ kHz
15
30
60
120
+/- 0.017ms
+/- 0.008ms
+/- 0.004ms
+/- 0.002ms
Maximum timing advance compensated via timing advance
command in MAC CE
For frequency synchronization, the UE is embedded with a local oscillator which is tuned to synthesize the
device’s frequency reference. Conventionally, UE oscillators are low cost, which results in poor stability
performance in free-running mode. To counter this behaviour, UE’s frequency reference is locked on the
frequency measured from the received DL signals, such as SS and RS. The same local reference is then reused to synthesize UL transmission frequencies. This procedure is considered sufficient to ensure good
frequency alignment at the gNB. It is worth noting that the network has no control over the UL frequency
misalignment error with respect to the gNB’s own reference.
III.B.
NTN UE capabilities
UE frequency and timing control mechanisms specified in 3GPP Release 16 cannot cope with the large
round trip delays, delay variations, and Doppler shifts experienced in NTN. As a consequence, uplink
timing and frequency self-compensation by the UE is introduced in 3GPP Release 17. To a large extent,
this is one of the major enhancements introduced by 3GPP to support NR NTN, and overall a notable
feature for the UE since the first 3GPP cellular networks.
To enhance the legacy synchronization mechanism, the NTN UE has new capabilities. Firstly, the NTN UE
is able to self-acquire its position via GNSS assisted information. Secondly, it is capable of predicting a
satellite orbit (position and velocity) based on specified ephemeris information shared by the network by
implementing adequate orbit propagation solutions.
3 Multiple SCS values are supported. Specifically: 15 kHz (µ = 0), 30 kHz (µ = 1), 60 kHz (µ = 2), and 120 kHz (µ = 3).
III.C.
Enhancements on UL timing synchronization
To enhance the legacy TA mechanism, a new formula has been specified for TA calculation and shall be
applied by UEs in RRC IDLE, INACTIVE or CONNECTED mode:
Where:
common
UE
𝑇𝑇TA = �𝑁𝑁TA + 𝑁𝑁TA,offset + 𝑁𝑁TA,adj
+ 𝑁𝑁TA,adj
� × 𝑇𝑇c
(2)
- 𝑁𝑁TA and 𝑁𝑁TA,offset were already specified in [6] [8] as part of the legacy TA mechanisms;
common
- 𝑁𝑁TA,adj
is derived from the higher-layer parameters TACommon, TACommonDrift, and
common
TACommonDriftVariation if configured, otherwise 𝑁𝑁TA,adj
= 0;
UE
is computed by the UE based on satellite-ephemeris-related higher-layers parameters if
- 𝑁𝑁TA,adj
UE
configured, otherwise 𝑁𝑁TA,adj
= 0;
- 𝑇𝑇𝑐𝑐 is the NR basic time unit [6].
Figure 4: Pictorial view of the TA calculation in NTN, and UL/DL radio frame timing at the NTN UE.
It can be observed from Eq. (2) and Figure 4 that the legacy close loop mechanism is kept as is and the
network still has direct control over the device’s TA by adjusting the 𝑁𝑁TA parameter value. However, the
round trip delays that may be experienced in NTN (up to hundreds of milliseconds for GEO) are not
compatible with the TA update range presented in Table 1. Moreover, the significant delay which can
occur between the network measurements and the associated TAC reception by the device will impact
the close loop correction performance. Finally, the propagation delay variation may go as far as tens of
microseconds per second due to the satellite velocity. In these conditions and based on the values
presented in Table 2, relying only on the legacy TA mechanism leads to impractical signalling overhead.
common
The newly introduced 𝑁𝑁TA,adj
component is a UE self-estimated timing offset derived from network-
controlled parameters. It is referred to as “common” in the sense that it aims to compensate for the
propagation delay common to all the transmissions between the UEs and the base station. In particular,
this common delay includes the propagation delay observed between the serving satellite and the uplink
time synchronization reference point considered by the network. In the case where this reference point
is set at the NTN gateway (GW) antenna (as shown in Figure 4), the common delay corresponds to the
round trip delay experienced on the feeder link between the satellite and the NTN GW.
UE
The other newly introduced 𝑁𝑁TA,adj
component is a UE self-estimated timing offset derived from network-
controlled satellite-ephemeris-related parameters and the UE self-acquired GNSS position. It aims to
compensate for the UE specific delay experienced on the service link between the UE and the serving
satellite.
Only the network-controlled parameters required to derive these additional contributions are specified.
The rest is left to the network implementation. Details on the definition and signaling of these parameters
are discussed in subsection III.E.
III.D.
Enhancements on UL frequency synchronization
In NTN, the DL and UL transmissions will be impacted by different types of frequency shifts which can be
sorted in two separate categories.
The first category refers to the frequency shifts caused by Doppler effects observed on the service links
due to the relative velocity between the UEs and the serving satellite. In NTN, these shifts can go up to
2.5×10-5 times the carrier frequency considered in a LEO-based network which can result in frequency
offset several times higher than the SCS. These effects have been considered in the enhanced UL
frequency synchronization procedure. The NTN UE is capable of applying a self-estimated compensation
when generating its UL transmission frequencies based on satellite-ephemeris-related higher-layers
parameters and its self-acquired GNSS position. More specifically, this compensation shall take into
account the impacts of the Doppler shift experienced on the service links on (i) the DL signals’ frequencies
used to adjust the UE local frequency reference, and (ii) the UL signals’ frequencies.
The second category includes the frequency shifts caused by Doppler effects observed on the feeder links
and any other frequency offsets introduced by the satellite transponder. This type of frequency offsets
shall be compensated either by the NTN GW or the satellite transponder when deemed necessary. In any
case, from both the base station and UE perspective, this category of residual frequency shifts is
considered to have no significant impact on the system performance and hence no specification impacts
on Release 17.
III.E.
NTN UL synchronization assistance information
Several higher layer parameters have been introduced to assist NTN UL time and frequency
synchronization procedures and most of them have already been referred to. These parameters, as
summarized in Table 3, are calculated based on orbit prediction information shared by the NTN control
center (NCC) and provided either through system information broadcast (SIB19) or through dedicated RRC
reconfiguration messages, as depicted in Figure 5.
Figure 5: Acquisition steps for UL synchronization assistance information in NTN.
There are two possible formats for the parameters related to ephemeris data. One is based on orbital
elements and the other uses position and velocity state vectors defined in the Earth-centered, Earth-fixed
(ECEF) coordinate system. Ephemeris data may be provided with three other parameters: TACommon,
TACommonDrift and TACommonDriftVariation, which are required to derive the common one way
common
propagation delay at a given time and from there estimate the common contribution 𝑁𝑁TA,adj
required
for TA calculation. The epoch time is a reference time for which assistance information (i.e., serving
satellite ephemeris and common TA parameters) is valid for. When it is not explicitly indicated in SIB19,
this epoch time of assistance information is implicitly known as the end of the system information (SI)
window during which the NTN-specific SIB (i.e. SIB19) message is transmitted. When provided through
dedicated signaling, epoch time of assistance information is the starting time of a DL sub-frame, which is
explicitly indicated by a system frame number (SFN) and a sub-frame number.
Table 3: RRC parameters related to UL synchronization assistance information in NTN
Parameter name
EphemerisStateVectorX,
EphemerisStateVectorY,
EphemerisStateVectorZ
EphemerisStateVectorVx,
EphemerisStateVectorVy,
EphemerisStateVectorVz
Description
Satellite ephemeris parameters based on state vector
Indicate the (x,y,z)-coordinate of serving satellite position state vector in ECEF.
The unit is m.
Value range: -43620761.6 m, …+43620760.3 m
The quantization step is 1.3m for position
Indicate the (x,y,z)-coordinate of serving satellite velocity state vector in ECEF.
The unit is m/s.
Value range: -7864.32 m/s, …+7864.26 m/s
The quantization step is 0.06 m/s for velocity
Satellite ephemeris parameters based on orbital elements
Indicate the semi-major axis α. The unit is m.
Value range: 6500 000 m, …, 43000 000 m
The quantization step is 4.249 × 10−3 m
EphemerisEccentricity
Indicate eccentricity e.
Value range: 0, …, 0.015
The quantization step is 1.431 × 10−8
EphemerisArgumentOfPeriapsis
Indicate the argument of periapsis ω.
The unit is Radian.
Value range: 0, …, 2π
The quantization step is 2.341 × 10−8 rad
EphemerisLongitudeOfAscendingNode
Indicate the longitude of ascending node Ω.
The unit is Radian.
Value range: 0, …, 2π
The quantization step is 2.341 × 10−8 rad
EphemerisInclination
Indicate the inclination i.
The unit is Radian.
Value range: -π/2, ..., +π/2
The quantization step is 2.341 × 10−8 rad
EphemerisMeanAnomaly
Indicate the mean anomaly M at epoch time t0.
The unit is Radian.
Value range: 0, …, 2π
The quantization step is 2.341 × 10−8 rad
TA common related parameters
TACommon is a network-controlled common TA, and may include any timing
offset considered necessary by the network.
TACommon
Value range: 0, …, 270.73 ms
The granularity is 4.072 × 10−3 μs.
Indicate drift rate of the common TA
Value range: -52.387 μs⁄s, …, +52.387 μs⁄s
TACommonDrift
The granularity is 0.2 × 10−3 μs⁄s.
Indicate drift rate variation of the common TA.
Value range: 0, …, 0.5894 μs⁄s2
TACommonDriftVariation
The granularity is 0.2 × 10−4 μs⁄s2.
Epoch time and validity duration parameters
EpochTime
Indicate the epoch time for assistance information by a SFN and a sub-frame
number.
Value range: from 0 to 1023 to indicate SFN, and from 0 to 9 to indicate the subframe number.
ntnUlSyncValidityDuration
A validity duration configured by the network for uplink synchronization
assistance information.
The unit is second.
EphemerisSemiMajorAxis
IV.
Enhancements on Hybrid-ARQ
In 5G NR, reliability of data transmission can be ensured by MAC-layer HARQ re-transmission and RLClayer ARQ re-transmission. The former has typically lower latency thanks to the combination of smaller
MAC packet data units (PDUs) in the MAC layer at the receiver side; the latter has higher latency due to
the re-ordering of larger radio link control (RLC) PDU in the RLC layer at the receiver side. In addition, it
shall be remembered that a single RLC PDU typically consist of several MAC PDUs. The RLC ARQ can be
configured to have lower latency by optimization of the RLC parameters.
In 5G NR specifications, the HARQ process with adaptive retransmissions is asynchronous on DL and UL.
In both directions, the receiver tries to decode the transport block (TB), possibly after soft combining with
previous replicas. Since transmissions and retransmissions are scheduled using the same framework, the
device needs to know whether the transmission is a new TB, in which case the soft buffer should be
flushed, or a retransmission occurred, in which case soft combining should be performed. Therefore, an
explicit new-data indicator (NDI) is associated to the scheduled TB as part of the scheduling information
transmitted in the DL or in the UL.
In satellite systems, larger values of RTT can be typically experienced due to the large channel propagation
delay between the UE and the satellite, which depends on the satellite orbit and the elevation angle. Large
satellite RTT increases scheduling delays. Indeed, HARQ stalling occurs due to the longer propagation
delays between the device and the gNB in the satellite bent pipe model, where the UE needs to wait
longer for the reception of a packet or the UL grant to transmit a new packet with HARQ re-transmissions.
The impact of HARQ stalling on maximum data rates can be mitigated. In the legacy specifications, a typical
UE implementation will not expect a new DL packet on the same HARQ process based on PDCCH
monitoring restrictions before it has transmitted HARQ feedback for the DL packet. In case the HARQ
feedback is disabled, in Rel-17 specifications the UE behaviour will be to monitor the PDCCH for a new DL
packet on the same HARQ process after an internal processing delay to decode the current DL packet
without waiting to transmit the HARQ feedback.
Based on the large NTN RTT, the following enhancements to mitigate the impact of HARQ stalling in
Release 17 have been specified: increasing the number of HARQ processes for re-transmissions at the
MAC layer, and disabling the HARQ feedback in the presence of ARQ re-transmissions at the RLC layer.
IV.A.
Number of HARQ processes
Figure 6 illustrates the HARQ timing diagram for THARQ, Tslot, and processing times in the UE and the gNB
for the DL. Table 4 indicates the minimum number of HARQ processes, 𝑁𝑁HARQ,min, to avoid stop-and-wait
issue with peak data rates. 𝑁𝑁HARQ,min can be approximated as [3]
𝑁𝑁HARQ,min ≥
𝑇𝑇HARQ
,
𝑇𝑇slot
where Tslot is 1 ms (i.e., assuming a reference numerology of SCS = 15 kHz), and THARQ is the time duration
between the initial transmission of one TB and the corresponding ACK/NACK complete decoding. A more
detailed formula of 𝑁𝑁HARQ,min in [17] is given as
𝑇𝑇𝑠𝑠𝑠𝑠 + 𝑇𝑇𝑢𝑢𝑢𝑢 + 𝑇𝑇𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑇𝑇𝑛𝑛𝑛𝑛 + 𝑅𝑅𝑅𝑅𝑅𝑅
�
𝑁𝑁𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻,𝑚𝑚𝑚𝑚𝑚𝑚 = �
𝑇𝑇𝑠𝑠𝑠𝑠
where RTT, Tsf, Tue, Tack, and Tnb refers to the round trip time, the sub-frame duration, the UE processing
time, the ACK/NACK transmission time, and the gNB processing time, respectively.
Figure 6: Timing diagram with one HARQ process for NTN bent pipe satellite
Table 4: Minimum number of the HARQ processes
Constellation Type
Terrestrial
LEO (600 km)
MEO
GEO/HEO
Max. 𝐓𝐓𝐇𝐇𝐇𝐇𝐇𝐇𝐇𝐇
16 ms
50 ms
180 ms
600 ms
𝑵𝑵𝐇𝐇𝐇𝐇𝐇𝐇𝐇𝐇,𝐦𝐦𝐦𝐦𝐦𝐦 processes for 1 ms slot operation
16
50
180
600
In Release 17, an increase of the number of HARQ processes from 16 for legacy NR to 32 as a UE capability
for NTN has been agreed. A 5-bit size for the indication of the HARQ process number in DCI has been
specified. The HARQ soft buffer is not likely to be increased significantly considering that typically NTN
has lower system bandwidth compared to terrestrial networks (e.g., below 6 GHz), and multi-input multioutput (MIMO) with rank 1 is typically assumed in NTN systems.
IV.B.
Disabling of HARQ feedback
In NTN, HARQ feedback can be semi-statically disabled via RRC signaling (i.e., not dynamically via
indication on DCI) and there is no retransmission at the MAC layer. The enabling / disabling of HARQ
feedback is configurable on a per UE and per HARQ process basis. Enhancements for Type-1 and Type-2
HARQ codebooks have been specified in Release 17 in order to provide the feedback to the gNB for DL
data transmission with feedback-disabled HARQ processes. For Type-3 HARQ codebook, the UE should
skip the codebook feedback for a feedback-disabled HARQ processes and the Type-3 codebook size is
reduced by excluding the bit positions of disabled HARQ processes.
In case HARQ feedback is disabled, re-transmission with automatic repeat request (ARQ) is done at the
radio link control (RLC) layer to ensure reliability. Figure 7 shows a comparison of the following setups:
•
•
NR NTN with 16 HARQ processes specified in Release 15 with all UL HARQ feedback disabled via
RRC (i.e., without HARQ retransmission but with ARQ retransmission). There is no stop-and-wait
due to HARQ protocol. Target block error rate (BLER) is 0.1%.
NR NTN with 32 HARQ processes specified in Release 17 with all UL HARQ feedback enabled via
RRC (i.e., with HARQ re-transmission). The number of HARQ processes is sufficient to
•
accommodate RTT experienced in LEO with 600 km orbital height, and thus avoids stop-and-wait
due to HARQ protocol. Target BLERs are 10% and 1%.
NR NTN with 16 HARQ processes with all UL HARQ feedback enabled via RRC (i.e., with HARQ retransmission). There is stop-and-wait due to HARQ protocol. Target BLERs are 10% and 1%.
The throughput curves for a LEO satellite system at 2 GHz carrier frequency with 10 MHz bandwidth and
SCS of 15 kHz were determined by using low-density parity-check (LDPC) BLER curves with various code
rates. It can be observed that when comparing 32 HARQ processes with MAC HARQ as specified in Release
17 with 16 HARQ processes with RLC ARQ, the difference in required signal-to-noise ratio (SNR) is within
a channel quality indicator (CQI) quantization step of 1 dB. This would suggest that using 32 HARQ
processes with MAC HARQ has similar resource efficiency as that using 16 HARQ processes with RLC ARQ
in NR NTN.
Figure 8 shows the required SNR to achieve BLER targets as obtained with link level simulator for code
rate of 0.2975 with quadrature phase shift keying (QPSK) and code rate of 0.4772 with 16 quadrature
amplitude modulation (16QAM), i.e., CQI = 4 and CQI = 8 respectively based on Table 5.2.2.1-4 in [9]. The
tapped delay line (TDL)-D channel profile with a delay spread of 300 ns and a single antenna configuration
have been simulated with a minimum mean square error (MMSE) channel estimator. An SNR gain in the
order of 1 dB is required to achieve a BLER target of 0.1% compared to a BLER target of 10%. This
represents less than a CQI step for the gNB scheduler. There is no significant impact on spectral efficiency
or the achievable maximum throughput when HARQ feedback is disabled. Potential enhancements to
improve reliability of transmission when HARQ is disabled with multiple transmissions of the same TB and
soft combining were discussed, but not agreed to be part of Release 17.
Figure 7: DL throughput vs. required SNR with 30 MHz bandwidth, NTN TDL-D channel, and RTT=32 ms [16].
Figure 8: BLER performance for NTN TDL-D channel profile [16].
V.
Considerations on Beam Management and Polarization
Communications satellites may employ more advanced antennas to create multiple spot beams on the
ground. Typically, larger frequency reuse factors are then used to reduce the co-channel interference (CCI)
among adjacent beams and increase the system capacity. As 5G NR assumes universal frequency reuse,
other strategies had to be found to mitigate the CCI. Beam management and polarization support have
already been discussed during the initial feasibility study on solutions for NR to support NTN [4]. Diverse
proposals were captured, including the following:
•
•
•
•
•
Release 15 beam management can be used for a frequency reuse factor of 1. For a frequency
reuse factor greater than 1, it was proposed that two Release-15 based schemes are possible:
o one bandwidth part (BWP) is used for each satellite beam;
o one component carrier is used per satellite beam;
additional beam management channel state information reference signal (CSI-RS) configurations
to support different satellite implementation needs;
introduce a mechanism where both UL and DL BWPs are switched simultaneously using a single
DCI to support fast satellite beam switching;
the concept of BWP can be used for frequency resource allocation among NTN beams, and that
the network may configure a specific active BWP for UEs in a beam;
increase the number of BWPs for NTN as 5G NR only specified 4 BWPs in UL and DL each.
A convergence to one particular solution was not possible at the end of this study phase in Release 16.
However, beam management and BWP operations introduced in 5G NR [18] are considered a very good
baseline for future NTN enhancements. In particular, one beam per cell and multiple beams per cell are
currently supported in existing NR specifications and these configurations are the baseline for NTN.
Another important topic concerns the polarization mode configuration and signaling. In NTN, neighboring
cells may use different polarization modes, i.e., right-hand circular polarization (RHCP) and left-hand
circular polarization (LHCP) to mitigate inter-cell interference, whereas legacy 5G NR solely relies on linear
polarization. This not only affects base station, but also UE capabilities as there may be UEs with different
antenna types. Thus, with regard to the polarization support, the 3GPP work item concluded that the
signaling of the polarization mode is beneficial for NTN. The consequence is that the network will
broadcast the DL and UL transmit polarization configuration.
VI.
Conclusions and outlook
The paper has presented in detail the first set of 5G NR physical layer enhancements specified in Release
17 to support NTN. This joint effort between mobile and satellite industries within the 3GPP
standardization group will enable the full integration of satellite in 5G networks. This will address the
challenges of reachability and service continuity in unserved/underserved areas and improve network
resilience and dependability in case of natural and man-made disasters.
The NTN journey in the 5G ecosystem is not finished, but just started. Looking ahead, NTN Release 18 is
going to start from Q2 2022. In addition to some maintenance adjustments derived from initial NTN
deployments, Release 18 will define the enablers for NR based satellite access in frequency bands above
10 GHz in order to serve fixed and moving platforms (e.g., aircraft, vessels) as well as building-mounted
devices (e.g., corporate business and large premises). Furthermore, NTN Release 18 will evaluate and
identify candidate techniques and solutions for coverage enhancements, mobility and service continuity
improvements, and network verified UE location aspects. On the path to 6G, we anticipate that the
integration of terrestrial networks with NTNs will be essential to provide global coverage and bridge the
digital divide [19].
VII.
References
[1] K. Liolis, A. Franchi, B. Evans, “Editorial for Wiley's IJSCN Special Issue on Satellite networks integration with 5G”,
Int’l. J. Satellite Commun. Network, vol. 39, pp. 319-321, Jun. 2021.
[2] R. De Gaudenzi et al., “Future Technologies for Very High Throughput Satellite Systems,” Int’l. J. Satellite
Commun. Network, vol. 38, no. 2, pp. 141–61, Feb. 2020.
[3] 3GPP TR 38.811, “Study on New Radio (NR) to support non-terrestrial networks (NTN)”, Release 15, Sep. 2020.
[4] 3GPP TR 38.821, “Solutions for NR to support non-terrestrial networks (NTN)”, Release 16, Jun. 2021.
[5] 3GPP TS 38.300, “NR and NG Radio Access Network; Overall description”, Release 16, Mar. 2020.
[6] 3GPP TS 38.211, “NR Physical channels and modulation”, Release 16, Mar. 2020.
[7] 3GPP TS 38.212, “NR Multiplexing and channel coding”, Release 16, Mar. 2020.
[8] 3GPP TS 38.213, “NR Physical layer procedures for control”, Release 16, Mar. 2020.
[9] 3GPP TS 38.214, “NR Physical layer procedures for data”, Release 16, Mar. 2020.
[10] 3GPP RP-213691, “Work-Item Description: Solutions for NR to support non-terrestrial networks (NTN)”,
December 2021.
[11] X. Lin et al., “5G New Radio: Unveiling the Essentials of the Next Generation Wireless Access Technology,” IEEE
Communications Standards Magazine, vol. 3, no. 3, pp. 30-37, September 2019.
[12] X. Lin, S. Rommer, S. Euler, E. A. Yavuz and R. S. Karlsson, “5G from Space: An Overview of 3GPP Non-Terrestrial
Networks,” IEEE Communications Standards Magazine, vol. 5, no. 4, pp. 147-153, December 2021.
[13] 3GPP TS 38.133, “Requirements for support of radio resource management”, Release 16, Mar. 2020.
[14] 3GPP TS 38.101, “User Equipment (UE) radio transmission and reception”, Release 16, Mar. 2020.
[15] E. Dahlman, S. Parkvall, and J. Skold, “5G NR: The Next Generation Wireless Access Technology”, Academic Press,
1st edition, 2018.
[16] 3GPP R1-1912125, “Delay-tolerant re-transmission mechanisms in NR-NTN”, Nov. 2019.
[17] 3GPP R1-1906873, “Discussion on the HARQ procedure for NTN”, May 2019.
[18] X. Lin, D. Yu, and H. Wiemann, “A primer on bandwidth parts in 5G new radio”, in “5G and Beyond: Fundamentals
and Standards”, X. Lin and N. Lee, Springer, 2021.
[19] X. Lin, S. Cioni, G. Charbit, N. Chuberre, S. Hellsten and J.F. Boutillon, "On the Path to 6G: Embracing the Next
Wave of Low Earth Orbit Satellite Access," IEEE Communications Magazine, vol. 59, no. 12, pp. 36-42, December
2021.
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