WO2021032453A1

WO2021032453A1 - Sync location for ntn access

Info

Publication number: WO2021032453A1
Application number: PCT/EP2020/071808
Authority: WO
Inventors: Tzu-Chung Hsieh; Rapeepat Ratasuk
Original assignee: Nokia Technologies Oy
Priority date: 2019-08-16
Filing date: 2020-08-03
Publication date: 2021-02-25
Also published as: EP4014360A1

Abstract

In some example embodiment, there may be provided an apparatus may be configured to at least receive, in a broadcast from a non-terrestrial base station, a common timing advance for a coverage area of the broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; transmit, to the non-terrestrial base station, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by apparatus; and receive, from the non- terrestrial base station, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the non-terrestrial base station.

Description

SYNC LOCATION FOR NTN ACCESS

Field

[0001] The subject matter described herein relates to non-terrestrial networks.

Background

[0002] 5G non-terrestrial networks (NTN) refer to satellites providing a radio access network interface to a user equipment (UE) and backhaul connectivity to a 5G core network including access to a data network. The NTN may thus provide one or more 5G gNB type base stations, each of which serve a coverage area that may include one or more UEs, some of which may be based terrestrially. As used herein, the term “satellite” refers to space borne platforms as well as airborne.

Summary

[0003] In some example embodiment, there may be provided an apparatus may be configured to at least receive, in a broadcast from a non-terrestrial base station, a common timing advance for a coverage area of the broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; transmit, to the non-terrestrial base station, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by apparatus; and receive, from the non- terrestrial base station, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the non-terrestrial base station.

[0004] In some variations, one or more of the features disclosed herein including the following features can optionally be included in any feasible combination. The apparatus may also receive, in the broadcast from the non-terrestrial base station, a plurality of synchronization locations, each of the synchronization locations represented by an additional differential timing advance and an additional frequency compensation. Each of the plurality of synchronization areas may include a corresponding group of synchronization locations providing the additional differential timing advance and the additional frequency compensation. A first synchronization area may include a first group of synchronization locations within an area covered by the first synchronization area. The indication may be received in a random access response decoded by the apparatus or in a timing advance update command. The apparatus may be further caused to at least transmit successively in the random access channel a plurality of preambles, each of the preambles compensated with a corresponding timing advance and a corresponding frequency compensation associated with a different synchronization area included in the plurality of synchronization areas. The successive transmit may cease, when a timer expires or a list including the plurality of synchronization areas is exhausted. The common timing advance and the differential timing advances and the frequency compensations may be received in a system information broadcast. The apparatus may be comprised in or may comprise a user equipment.

[0005] In some example embodiments, an apparatus may be configured to transmit, to a user equipment, a common timing advance for a coverage area of a broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; receive, from the user equipment, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by the user equipment; and send, to the user equipment, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the apparatus.

[0006] In some variations, one or more of the features disclosed herein including the following features can optionally be included in any feasible combination. The apparatus may send, in the broadcast, a plurality of synchronization locations, each of the synchronization locations represented by an additional differential timing advance and an additional frequency compensation. Each of the plurality of synchronization areas may include a corresponding group of synchronization locations providing the additional differential timing advance and the additional frequency compensation. A first synchronization area may include a first group of synchronization locations within an area covered by the first synchronization area. The indication may be sent to the user equipment in a random access response or a timing advance update command. The apparatus may detect one of a plurality of preambles sent by the user equipment, each of the preambles compensated with a corresponding timing advance and a corresponding frequency compensation associated with a different synchronization area included in the plurality of synchronization areas. The apparatus may update, as the coverage area changes, the common timing advance, the plurality of synchronization areas, and the plurality of synchronization locations transmitted in the broadcast.

[0007] The above-noted aspects and features may be implemented in systems, apparatus, methods, and/or articles depending on the desired configuration. The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims.

Description of Drawings

[0008] In the drawings,

[0009] FIG. 1 depicts an example of a portion of non-terrestrial network serving UEs, in accordance with some example embodiments;

[0010] FIG. 2 depicts another example of a portion of non-terrestrial network serving UEs and further depicting synchronization (sync) areas and sync locations, in accordance with some example embodiments; [0011] FIG. 3 depicts an example of a process at the NTN’s gNB for broadcasting sync information, in accordance with some example embodiments;

[0012] FIG. 4 depicts an example of the random access occasion configuration for the sync areas and sync locations, in accordance with some example embodiments;

[0013] FIG. 5 depicts an example of the UEs handling of the sync information provided by the NTN’s gNB, in accordance with some example embodiments;

[0014] FIG. 6 depicts another example of the UEs handling of the sync information provided by the NTN’s gNB;

[0015] FIG. 7 depicts an example of a network node, in accordance with some example embodiments; and

[0016] FIG. 8 depicts an example of an apparatus, in accordance with some example embodiments.

[0017] Like labels are used to refer to same or similar items in the drawings.

Detailed Description

[0018] In non-terrestrial networks (NTN), one or more satellite networks may provide a cellular coverage area, such as a 5G coverage area including a radio access network (RAN). For example, one or more satellites may provide a base station (e.g., a 5G base station, gNB) serving an area including one or more user equipment (UE). The UE may be configured to operate with the NTN’s satellites to access the 5G RAN and core. Alternatively or additionally, the UE may be configured to operate over the terrestrial, public land mobile network and access a terrestrial 5G RAN, such as a 5G gNB type base station. In the case of the NTN base station serving as a 5G gNB base station, it may be coupled to a core network in a manner similar to other gNBs but for the satellite based nature of the NTN’s gNB. In the case of these NTN, there may be different deployment scenarios covering different satellite altitudes, such as at a geostationary orbit (GEO), low earth orbit (LEO), and the like. Table 1 below depicts the GEO and LEO deployment scenarios as described in 3 GPP TR 38.821.

[0019] In non-terrestrial networks (NTN), a satellite may include gNB that provides a cellular coverage area, such as a 5G coverage area including a radio access network (RAN).

For example, the NTN’s satellites may include a base station, such as a 5G base station, gNB. This gNB may have one or more beams each of which may serve an area including one or more user equipment (UE). The UE may be configured to operate with the NTN’s gNB and/or operate over the terrestrial, public land mobile network. In the case of the NTN’s gNB base station, it may be coupled to a core network in a manner similar to other gNBs but for the satellite based nature of the NTN’s gNB. In the case of the NTN, there may be different deployment scenarios covering different satellite altitudes, such as at a geostationary orbit (GEO), low earth orbit (LEO), medium earth orbit (MEO), unmanned airborne system (UAS), high altitude platform sensors (HAPS), and the like. Table 1 below depicts the deployment scenarios as described in 3 GPP TR 38.811.

[0020] Table 1: NTN deployment scenarios

[0021] Depending on the deployment scenario, the NTN can have the characteristics of very long propagation delay and high Doppler frequency shift. In addition, the long distance separation between the NTN platform including the gNB and the terrestrially-based UE results in large beam footprints, which may be on the order of 100 kilometers (km) or larger cell sizes as shown in Table 2. The gNB’s beams may be steered to serve a fixed geographic area as the satellite (or HAPS) moves on its trajectory, or the beams may be static with their footprints moving along with the satellite. These characteristics of operation pose a unique challenge for the design of the NTN system. Time and frequency synchronization is the aspect particularly impacted by the condition NTN operates - long propagation, frequency shift and variation, large beam footprints, and the motion of beams. The NTN system may need to support a variety of UEs such as VS AT with a directional antenna to a low complexity, low power class 3 UE for typical IoT devices (see, e.g., Table 1). Table 3 below depicts reference scenarios in Table 3 for potential 5G NR solutions for supporting NTN (see, e.g., 3GPP TR 38.821). [0022] Table 2: Typical beam footprint size

[0023] Table 3: Reference scenario parameters

[0024] FIG. 1 depicts an example of an NTN system 100 including an NTN’s platform 195 (which in this example is a satellite or other airborne platform) of a gNB serving one or more UEs 199A-B, in accordance with some example embodiments. The gNB type base station may provide a Uu interface 166 to the UEs, such as UE 199 A. The NTN’s gNB may have a transmit beam and/or receive beam that serves a cell or coverage area as shown by 177. Moreover, the UEs may also be configured to access a terrestrial gNB via a Uu interface as well in accordance with 3 GPP standards. The gNBs at the NTN 195 may couple via a backhaul 197 to other nodes including 5G core network nodes 198. One or more nodes of the 5G network nodes may be based terrestrially or in the NTN. Although FIG. 1 depicts only a single beam, the NTN may be able to have a plurality of beams as well.

[0025] The time for signal propagation between the NTN 195 and a ground-based UE 199 A may depend on the satellite’s altitude and its elevation angle at the UE. Examples of the maximum round trip delay (at the lowest elevation angle) for GEO and LEO satellite are listed in Table 3. The propagation time varies within a beam footprint where the UEs share the same radio resources. Within the footprint such as cell 177, the maximum propagation delay difference between users, which is d3/c as illustrated in FIG. 1, with c being the speed of light. This delay difference in the worst case is listed in Table 3 as “max. differential delay” for GEO and LEO satellites. These delay differences are in the order of milliseconds (ms), which is about the longest possible configuration of slot time in 5G NR. To align the uplink signals from different users with the gNB’s uplink frame timing, each UE 199A-B may need to apply a timing advance (TA) on downlink timing reference to compensate uplink transmission’s propagation delay. The ideal TA is therefore the round trip time between the NTN’s 195 gNB and UE 199 A, for example.

[0026] If the UE has the knowledge of its location (either through GNSS signals or by configured data) and the UE is capable of decoding the satellite’s ephemeris data broadcast and computing the propagation time, the UE may apply its independently calculated TA for uplink transmission. For other devices, the network may need to provide assistance for TA acquisition. The NTN’s gNB satellite beam may broadcast a common TA using, for example, the shortest round trip time (RTT) in the footprint 177 as 2d₁/c in FIG. 1, but individual UEs may still need to apply an additional TA, which may vary based on the UE’s position in the footprint 177 and the elevation angle with the satellite. The maximum TA may not be enough to cover the beam footprint 177 in NTN. For non-geostationary satellites (e.g., LEO, MEO, HEO, and the like), the motion in the orbit can cause change of the elevation angle and, in turn, the change of RTT over time, requiring the TA to be constantly measured, signaled, and adjusted for every UE. In order to keep TA error within the cyclic prefix of OFDM symbols, the NTN’s gNB may need to send TA commands frequently. Table 4 shows the required TA command period for LEO satellite (see, e.g., 3GPP Tdoc R1-1905994, Huawei, HiSilicon, “Discussion on timing advance and RACH for NTN”). Furthermore, the TA command delay due to the large RTT may result in a TA error exceeding the required TA adjustment accuracy.

[0027] In addition to UL timing synchronization, Doppler frequency shift generated by the motion of satellites, such as the LEO or MEO satellites, may also be a challenge in NTN. The LEO satellite at altitude 600 Km moves at a speed of 7.56 Km/s and may cause a frequency shift as much as 24 parts per million (ppm). This translates to a frequency error of 24 KHz for 2 GHz carrier and 720 KHz for 30 GHz carrier, greater than the OFDM subcarrier spacing likely to be used for those frequencies in NR. The radio unit on the NTN’s 196 satellite may pre- compensate the Doppler shift in transmission and post-compensate in reception. Nonetheless, within the beam footprint 177, UEs may experience varying residual frequency shifts. The worst case is when the satellite is at 90° elevation angle with respect to the beam center on the ground, and Doppler pre-compensation and post-compensation become zero. In that worst case, the largest residual frequency error appears at the edge of beam footprint and is listed in Table 5 for different footprint diameters and carrier frequencies. This residual frequency error may lead to a longer delay for the initial cell search, degrade random access preamble sequence detection and data block decoding, and cause higher uplink (UL) interference among different UEs. The UE may not be able to rely on downlink (DL) synchronization channel to estimate residual Doppler shift because UE may not be able to separate the Doppler shift from the local oscillator induced frequency offset.

[0028] Table 4: Required TA command period for LEO at 600 Km altitude

[0029] Table 5: Worst case residual Doppler shift for LEO at 600 Km altitude

[0030] In some example embodiments, one or more fixed geographic locations may be pre-configured at the NTN. The one or more fixed geographic locations may each serve as a reference for the TA value and the Doppler shift value. The network (e.g., the gNB, core network, and/or other node) may calculate the TA and the Doppler shift (referred collectively herein as “sync information”) for each of the reference locations in a NTN gNB beam’s footprint, such as the footprint of coverage area 177. And, this sync information calculation may be performed from time to time (e.g., periodically). In some example embodiments, the NTN’s gNB may broadcast this calculated sync information in the NTN’s gNB’s synchronization signal (SS) block (SSB) or system information block. The gNB may also broadcast the reference location ID and its geographical location or coordinates.

[0031] In some example embodiments, the UE may discover, from the random access response message from the NTN gNB, a reference location (e.g., a sync area or sync location) on earth in its first random access with the NTN.

[0032] In some example embodiments, the UE may discover a reference location (e.g., a sync area or sync location) on earth using its positioning or localization information (e.g., through Global Navigation Satellite System information). When this is the case, the UE may be able to determine a reference location (e.g., closest reference location), which may provide compensation information for the TA and/or the frequency which is better when compared to other reference locations. [0033] In some example embodiments, the UE may also change the reference location after the connection is established with the assistance of the NTN’s gNB via a TA update message. After acquiring the closest reference location, the UE may access its own sync information from the broadcast data of reference locations.

[0034] With respect to the deployed preamble (e.g., Physical Random Access Channel, PRACH) formats and satellite altitude, the NTN may be configured with sync areas (SA), such that the round trip time (RTT) variation within the same sync area is less than a maximum timing error in preamble detection, in accordance with some example embodiments. Within a sync area, the NTN may also be configured with multiple sync locations (SL), such that RTT variation is smaller, and the preamble detection is possible with low complexity, in accordance with some example embodiments.

[0035] In addition to the beam-level TA (and beam-level Doppler shift adjustment if Doppler shift compensation is not performed by the satellite), the NTN’s gNB’s beam may broadcast differential sync area-level and sync location-level TA and Doppler shift information (which may include their change rate or may be in the form of a parametric function of time) for the sync areas and sync locations in the beam’s footprint. While the sync information broadcast of all sync areas within a footprint may be mandatory, the sync information of some sync locations may be omitted if the gNB finds no registered user associated with the sync locations.

[0036] In the uplink (UL) frequency -time resource grid of a beam, the gNB may configure a unique random access time-frequency occasion (RAO) for each sync area and for each sync location in the gNB beam’s footprint. More sophisticated preamble detection can be expected in sync areas random access occasions (RAO) for UEs that have not acquired its sync location and have a larger timing error.

[0037] When the UE accesses the NTN network initially (e.g., for the first time of service), the UE may successively perform random access procedures by, for example, transmitting its preamble and waiting for random access response (RAR) and using all sync areas’ sync information (e.g., TA and Doppler shift) of the beam from system info broadcast (obtained on SIB indicated by the SSB). The UE may attempt successive random access occasions using associated sync area sync information, then check for random access response from each of the RAO.

[0038] When a preamble is detected successfully in a sync area random access occasion, the gNB may send a random access response message to the UE indicating the TA value by a sync area ID and a sync location ID and the remaining TA offset.

[0039] The UE may use the sync area ID and the sync location ID in the random access response to access the sync information from system info broadcast for uplink transmission.

[0040] When the gNB wants to adjust an UE’s TA based on its uplink signal timing, the gNB may include a new sync area ID or a sync location ID in the TA update command.

[0041] The UE may save its sync area ID and sync location ID before going to a sleep mode and use them to access sync information from the SIB broadcast upon wake-up.

[0042] The UE may use the same sync area and sync location when it switches to a different serving beam of the same satellite. And, the UE may use the same sync area and sync location for random access procedure during an inter-satellite handover.

[0043] The network (e.g., the gNB, core network, or other node) may configure a set of geographic location points (referred to herein as sync area reference points (SARP)). These sync area reference points may be distributed in the NTN network’s coverage area based on the RA preamble formats used in PRACH. Adjacent sync area reference points may be configured by no more than a certain distance (e.g., the propagation distance of one PRACH slot duration or a fraction of minimum beam footprint), so that an NTN’s beam footprint may always cover multiple sync area reference points and the differential RTT between an arbitrary point in the coverage area and its nearest sync area reference points is within the timing error tolerance of a high complexity preamble detection, which may use multiple reception windows. The geographic area corresponding to the same nearest sync area reference points is called a sync area (SA). The sync area and its corresponding sync area reference points may be identified by a unique sync area ID.

[0044] The network may also configure a denser set of geographic location points (referred to herein as sync location reference points (SLRP)) that are distributed in its coverage area considering the RA preamble formats. Adjacent sync location reference points may be separated by no more than a certain distance (e.g., the propagation distance supported by or corresponding to the PRACH cyclic prefix duration), so that the differential RTT between an arbitrary point in the coverage area and its nearest sync location reference points is within the timing error tolerance of a low complexity preamble detection, which may just use a single reception window. The geographic area corresponding to the same nearest sync location reference points is called a sync location (SL). Since sync location reference points are densely distributed, there may be multiple sync location reference points with the same nearest sync area reference point. In other words, there may be multiple sync locations within a given sync area. A sync location reference point or a sync location may be identified by a combination of the nearest sync area ID and a unique sync location ID within the sync area.

[0045] FIG. 2 depicts an example configuration of the sync area reference points and sync location reference points configuration, in accordance with some example embodiments.

In the example of FIG. 2, there is shown the sync area 210 and corresponding sync locations 212A-D, sync area 220 and corresponding sync locations 222 A-D, and sync area 230 and corresponding sync locations 232A-D. In the example of FIG. 2, the NTN gNB 195 may broadcast in the SIB the sync information for each of the sync areas and sync locations, so that the UEs in the coverage area 177 of the gNB’s beam can adjust TA and frequency compensation as disclosed herein.

[0046] The configuration of sync area reference points and sync location reference points coordinates may be loaded into one or more of the satellites of the NTN 195. With this coordinate information and the information of NTN’ s satellite trajectory and beam direction, the NTN’s gNB may determine the sync areas and sync locations in each beam’s footprint, such as the coverage area 177 shown at FIGs. 1 and 2. Moreover, the NTN’s gNB may determine the sync information (e.g., TA and Doppler frequency shift) for the sync area reference points and sync location reference points at any instant in time. The sync information for a sync area ( TA_SA , Df_SA) may indicate the TA offset and residual Doppler shift. The residual Doppler shift can be indicated in terms of fractional error in unit of ppm, so that the same error applies to the uplink and the downlink. Suppose the beam’s common TA is TA_B and the Doppler shift is compensated at the satellite 195, the TA and residual Doppler shift at the i-th sync area reference point may be determined as follows:

TA (i) = TA_B + (TA_SA)i (1)

Df(i) = (Df)i _· (2)

[0047] The sync information for a SL ( TA_SL , Df_SL) may indicate the TA offset and residual Doppler shift offset with respect to the sync area. The TA and Doppler shift at the j-th sync location reference point of i-th sync area may be determined as follows:

TA(i,j) = TA_B + (TA_SA)_i + (TA_SL) _j (3)

Df(i,j) = (Df_SA)_i + (Df_SL)_j . (4)

[0048] The NTN’s satellite’s 195 beam specific common TA and satellite’s Doppler shift compensation may provide a crude correction of timing and frequency, while the sync information of sync area and sync location may provide an additional two levels of finer timing and frequency corrections. [0049] The NTN’s gNB may convey the sync information of the sync areas and sync locations in a footprint by including the sync information together with the NTN’s gNB beam’s common TA in the system information broadcast, such as SIB 1 (System Information Block type 1) or other part of SIB. The sync information of all sync areas associated with the sync locations in the footprint may need to be broadcast even if the SARP may not be covered by the beam. Referring to FIG. 2 for example, the SA(1) 210 should be appear in the sync information broadcast even though the sync area reference points is outside the footprint or coverage area 177 because there is a sync location 212C in the footprint or coverage area 177 associated with SA(1) 210. In order to minimize signaling overhead, the sync locations (which are not assigned to any RRC connected UE) may be omitted in the sync information broadcast by the NTN.

[0050] In case of LEO or MEO satellites for example, the sync information may change over time due to the motion of the NTN’s 195 satellite. The NTN’s 195 gNB may need to take into account the propagation time of the signaling message (as well as additional time lag before the UE’s UL transmission). A time reference for the received sync may help the UEs to interpret the data and possibly derive more accurate sync information to use. The NTN’s 195 gNB may compute the sync information for the instant At after it is received at the center of beam footprint, taking into account the propagation time. The value At can be fixed in the standard, or can be configured in RRC, or can be transmitted together with the sync info. An example of NTN’s 195 gNB process for providing the sync information is illustrated in FIG. 3.

[0051] FIG. 3 depicts an example of a process at the NTN’s gNB for broadcasting sync information, in accordance with some example embodiments.

[0052] At 310, the network may load into the gNB the configured sync area and sync location reference points. For example, the NTN may load sync areas and sync area locations (e.g., sync area 210 and corresponding sync locations 212A-D, sync area 220 and corresponding sync locations 222A-D, and sync area 230 and corresponding sync locations 232A-D) that are located in all the possible coverage areas 177 of the gNB’s beams. The load may include the positon of the sync area and sync area locations. The configured sync area and sync location reference points in all coverage areas over time may be known to the gNB. The gNB will then figure out which SAs and SLs are in a beam’s footprint at a given instant of time.

[0053] At 320, for each spot beam that can be transmitted by the NTN’s gNB 195, the gNB may also compute a common TA for that beam, Doppler shift compensation for the uplink and downlink, and may also determine the sync areas in the footprint of that beam, such as the sync areas in coverage area or beam footprint 177.

[0054] At 330, for each sync area covered by the NTN’s gNB 195 beam, the gNB may compute an additional differential TA (which is differential in the sense that it is in addition to the common TA determined at 320) and Doppler shift. As noted, the common TA is common to the entire coverage area 177 covered by the beam, while the differential TA represents an additional TA that can be added to the common TA to provide a more accurate TA for the associated sync area.

[0055] At 340, for each sync location within a sync area, the NTN’s 195 gNB may also compute differential TA (which is differential in the sense that it is in addition to the common TA determined at 320) and Doppler shift. The differential TA associated with the sync location represents an additional TA that can be added to the common TA and the sync area’s TA to provide an even more accurate TA for the associated sync location.

[0056] At 350, in each spot beam, the NTN’s 195 gNB may broadcast a common TA (which covers the footprint 177 of the beam) and for each sync area the associated differential TA and the Doppler shift, and for each of the sync location the associated differential TAs and the Doppler shift. The broadcast may, as noted be in a SIB, such as SIB1.

[0057] The SIB1 may allow the NTN’s 195 gNB to update the sync information in a period of 160 ms. The UE may interpolate or extrapolate the sync information received in the past for its uplink transmission. In addition to TA and residual Doppler, the change rate of these variables may also be broadcast for the sync areas and sync locations in the footprint to explicitly provide the UE with sync information between two consecutive updates. Alternatively, the TA and residual Doppler may be characterized by a parametric function of time over the update interval, and the sync information may provide coefficients for the TA and residual Doppler. More frequent TA adjustment may be required for data transmission due to the shorter cyclic prefix (CP) length in the OFDM waveform. When that is the case, additional sync information may be broadcast in SIB with a shorter periodicity.

[0058] The time-frequency resources allocated for PRACH (RA preamble) can be shared or separate for different beams, in part depending on frequency reuse configuration. In any case, further division of the RACH occasion (RAO) resource used by one beam may be required for the sync areas and sync locations in the beam’s footprint. For each sync area in the beam’s coverage, a unique RAO (referred to as SA-RAO) in the time-frequency resource grid may be configured without overlapping the resources for RAO of other sync area or sync location in the beam’s coverage. These sync area-RAOs are used by the UEs that have not acquired their nearest sync location or its nearest sync area. The UE may transmit its RA preamble using only the sync information (e.g., TA and Doppler shift) of the sync area from SIB1 broadcast. Since there is no sync location-level sync information being used, the received preamble signal will have larger timing and frequency error, so the gNB may need to perform more complex processing for preamble detection with multiple hypotheses of timing window and possibly frequency shift. Extra guard time may be needed for sync area-RAO to avoid the time-unaligned preamble signal spilling over to neighbor slots. FIG. 4 depicts an example of the RAO configuration, in accordance with some example embodiments. The information of the RAO allocation for sync area and sync location may be also be broadcast in a SIB, such as

SIB1. [0059] Similarly, a unique RAO may also be configured for each sync location in the beam’s coverage (referred to as SL-RAO). The sync location-RAOs are used by UEs that have already acquired their nearest sync area and sync location. The UE may use both the sync area- level and sync location-level sync information when transmitting the preamble. The arrived preamble signal in a sync location-RAO is therefore more aligned with the time and frequency reference at the gNB. The sync location configuration allows the gNB to detect the preamble with low complexity assuming a timing error less than the CP length and negligible frequency shift.

[0060] In the initial access to the NTN’s gNB, the UE may perform the usual search process on the Synchronization Signal (SS) bursts by acquiring the DL frequency and time synchronization from PSS/SSS in the strongest SS block and decoding the Physical Broadcast Change (PBCH)/Master Information Block (MIB) and SIB1. From SIB1, the UE may be informed of the common TA, the sync information (differential TA and residual Doppler shift), and the RAO of the SAs and SLs in the beam’s footprint. The UE may not know its nearest sync area and sync location if the UE has not accessed the network before or it has moved away from the sync area of its previous connection.

[0061] If the UE has observed that its previously used sync area and sync location IDs appear in SIB1, the UE may use the sync information of the sync area, sync location as noted to transmit RA preamble and initiates the random access procedure. Alternately, the UE may select sync location based on the UE’s own knowledge of its location (e.g., via GNSS positioning). Otherwise, the UE may successively transmit a preamble with different sync area assumption, in terms of sync area-RAO and sync info, for all the sync areas in SIBl. The UE may wait until the RAR timer expires before transmitting the next preamble as shown in FIG. 5 or use separate RAR timers for multiple preamble transmissions in a succession before the successful reception of a RAR as shown in FIG. 6. The latter implementation may lead to a shorter latency for random access.

[0062] Upon detecting a preamble in a sync area-RAO, the NTN’s gNB may determine the sync location for the UE based on the measured TA. In the RAR message, the gNB sends back, the TA is indicated by sync area ID, SL ID, and the remaining TA, which represents the small TA offset after applying the sync area and sync location’s sync info. Alternately, UE may implicitly determine the sync area ID and sync location ID based on the unique time-frequency location or window of the RAR or based on the RAR response window. Since the large TA value is not directly signaled, less bits are needed in the message, while TA precision can be maintained by the remaining TA field. Suppose an UE receives a TA command: SA ID = i, SL ID = j, remaining TA = TA_R, the TA adjustment for that UE should be TA = TA_B + (TA_SA)_i + (TA_SL) J + TA_{R .} (5)

[0063] The gNB may need to save the UE’s sync area ID, sync location ID for TA update later. The gNB may use the random access (RA)-RNTI to identify the UE, when the preamble is detected and change to cell ©-RNTI when message 3 is received in random access procedure. The UE may also save the sync area ID, sync location ID, and the remaining TA it receives in the RAR message and applies the broadcast sync information of the sync area and sync location for uplink transmission and downlink frequency correction.

[0064] After the random access is complete, the gNB may estimate the UE’s TA based on the arrival time of the UE’s uplink signal and compare with the TA used by the UE (according to the SA ID, SL ID, remaining TA, and the latest sync info). The gNB may keep the TA in sync with its uplink timing by sending a TA update command message to the UE. To minimize the number of bits in the message, the TA adjustment may be indicated by the change of remaining TA if there is no change of sync area and sync location. However, a new sync location ID and a new remaining TA are needed if sync location changes, but sync area remains the same. The sync area ID, sync location ID and a new remaining TA are needed if sync area changes. The sync area ID and sync location ID for a UE may need several updates after the completion of random access since multiple locations may have similar TA at a time instant.

The UE’s movement can also cause sync area and sync location change.

[0065] FIG. 5 depicts an example of a UE performing a random access with the NTN’s gNB, in accordance with some example embodiments.

[0066] At 510, the UE may acquire downlink synchronization via the synchronization signal block (SSB) and decodes the SIB, in accordance with some example embodiments. As noted, the SIB may include the common TA for the footprint 177 of the gNB’s beam, so the UE may receive and update, at 520, the common TA to be used while in the footprint (or cell) 177. The UE may also select, at 520, a candidate sync area, such as SA1 210, SA2220, or SA3 230, which are included in the SIB. For example, UE 199 A may select SA3 230 and then use the TA_SAfor the selected, candidate sync area and its Doppler compensation (and Df_SA) associated with SA3. This T ASA is, however, an additional amount over the common TA.

[0067] At 530, the RACH preamble is transmitted to the NTN’s gNB 195 with a TA that includes the common TA and the TA_SA associated with the selected, candidate sync area and with the Doppler frequency compensation (and Df_SA) associated with the selected, candidate sync area. At 540, if the RAR is received before the timer expiration, the RAR is decoded, at 550, and the RAR may include a sync location for selected, candidate sync area. In the example of FIG. 5, the UE may determine the sync location’s TA_SL from the decoded SIB but adds an additional TA_R determined by the network. As noted, the network may determine the sync location when it determines that the candidate sync area is suitable for the UE. At 540, if the RAR is not received before the timer expiration, the UE may select another candidate sync area in the SIB broadcast at 520. [0068] At 560, the UE’s uplink transmission includes a TA and frequency compensation, in accordance with some example embodiments. The TA includes the common TA (TA_B), the differential TA associated with the selected sync area (TASA), and the TA associated with the sync location (TAs) and any additional TA (e.g., TA_R) which may be provided by the network in the RAR at 550. And, the frequency compensation takes into account the frequency compensation associated with the sync area ( Df_SA) and frequency compensation associated with the sync location (Df_SL).

[0069] FIG. 6 depicts another example of a UE performing a random access with the NTN’s gNB, in accordance with some example embodiments. FIG. 6 is similar to FIG. 5 in some respects but includes the UE having a separate RAR timer for each candidate SA and sending preamble using each candidate SA’s sync information without waiting for any RAR timer to expire.

[0070] At 530, the RACH preamble is transmitted to the NTN’s gNB 195 with a TA that includes the common TA and the TASA associated with the selected, candidate sync area and with the Doppler frequency compensation (and D f_SA) associated with the selected, candidate sync area. The UE sends the preamble to the gNB at 530 in the RAO using the TA of some (if not all) of the sync areas in the SIB, until the UE has exhausted the list of sync areas in the SIB (YES at 640) or all the timers for the RAR expire (YES at 540).

[0071] At 540, if the RAR is received before the timer expiration, the RAR is decoded, at 550, and the RAR may include a sync location for selected, candidate sync area. In the example of FIG. 6, the UE may determine the sync location’s TASL from the decoded SIB but adds an additional TAR determined by the network. As noted, the network may determine the sync location when it determines that the candidate sync area is suitable for the UE.

[0072] At 655, if none of the sync areas is suitable, the UE may wait for an updated SIB broadcast including additional sync areas. [0073] When the UE measures a stronger RSRP from a new SSB and decides to switch to the new beam (e.g. using cell reselection or handover), the UE may use the same sync area ID and sync location ID stored in the UE’s memory to look up the sync information from the new beam’s SIBl. If the new beam is transmitted from the same NTN’s satellite, the UE’s remaining TA may still be valid for the new beam. If the new beam comes from a different satellite of the NTN, the UE may need to initiate a random access in the RAO of the stored sync area ID and sync location ID, applying the sync information for TA and Doppler shift correction. Since the UE’s nearest sync area and sync location are already known in this case, the timing and frequency error of the preamble will be relatively small and easier to detect. That opens up the possibility of using two-step RACH where the UE can transmit the preamble and message 3 simultaneously.

[0074] The UE may also go into sleep mode for a period of time for power saving. Upon wake-up, the UE may tune in to SSB and access the sync information of its sync area ID and sync location ID. The UE may then compute the TA and Doppler shift correction for uplink transmission and wait for TA update command from the NTN’s gNB to finely adjust its TA. Alternatively, the UE may initiate a random access in the RAO of its sync area ID and sync location ID to obtain remaining TA adjustment.

[0075] The number of sync areas and sync locations may be estimated with the preamble format used and the desired received TA variation in SA-RAO and SL-RAO. Table 6 show two examples for S band (2 GHz), assuming the desired TA variation within a sync location is less than the CP length of the preamble.

[0076] Satellite motion may result in change of RTT. For LEO at 600 Km altitude with transparent payload, the maximum RTT variation rate is 40 ms/second. In order to keep the TA error within the CP length of OFDM waveform, the TA update needs be done at a sufficient update frequency. The 160 ms period of SIBl may be good enough at least for some preambles in random access, but in data transmission the cyclic prefix (CP) length is shorter and the SIB1 periodicity falls short of the required update period listed in Table 4. This problem may be mitigated by including TA change rate in the sync information broadcast or parameterizing TA as a function of time for the UE to predict its TA before the next occurrence of SIB1. Alternatively, the gNB can also broadcast the sync information more frequently in other part of SIB for the connected UEs.

[0077] Table 6: Examples of SA and SL density estimation

[0078] FIG. 7 depicts a block diagram of a network node 700, in accordance with some example embodiments. The network node 700 may be configured to provide one or more network side functions, such as a base station (e.g., gNB) and/or other network nodes. For example, the gNB may be included in a satellite or other airborne platform as part of an NTN. The gNB may be configured to transmit, to a user equipment, a common timing advance for a coverage area of a broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; receive, from the user equipment, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by the user equipment; and send, to the user equipment, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the gNB. The gNB may send, in the broadcast, a plurality of synchronization locations, each of the synchronization locations represented by an additional differential timing advance and an additional frequency compensation. Each of the plurality of synchronization areas may include a corresponding group of synchronization locations providing the additional differential timing advance and the additional frequency compensation. A first synchronization area may include a first group of synchronization locations within an area covered by the first synchronization area. The indication may be sent to the user equipment in a random access response. The gNB may detect one of a plurality of preambles sent by the user equipment, each of the preambles compensated with a corresponding timing advance and a corresponding frequency compensation associated with a different synchronization area included in the plurality of synchronization areas. The gNB may update, as the coverage area changes, the common timing advance, the plurality of synchronization areas, and the plurality of synchronization locations transmitted in the broadcast.

[0079] The network node 700 may include a network interface 702, a processor 720, and a memory 704, in accordance with some example embodiments. The network interface 702 may include wired and/or wireless transceivers to enable access other nodes including base stations, the Internet, and/or other nodes. The memory 704 may comprise volatile and/or non volatile memory including program code, which when executed by at least one processor 720 provides, among other things, the processes disclosed herein with respect to the network node.

[0080] FIG. 8 illustrates a block diagram of an apparatus 10, in accordance with some example embodiments. The apparatus 10 may comprise or be comprised in a user equipment. The apparatus may be configured to at least receive, in a broadcast from a non-terrestrial base station, a common timing advance for a coverage area of the broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; transmit, to the non-terrestrial base station, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by apparatus; and receive, from the non-terrestrial base station, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the non-terrestrial base station. The apparatus may also receive, in the broadcast from the non terrestrial base station, a plurality of synchronization locations, each of the synchronization locations represented by an additional differential timing advance and an additional frequency compensation. Each of the plurality of synchronization areas may include a corresponding group of synchronization locations providing the additional differential timing advance and the additional frequency compensation. A first synchronization area may include a first group of synchronization locations within an area covered by the first synchronization area. The indication may be received in a random access response decoded by the apparatus. The apparatus may be further caused to at least transmit successively in the random access channel a plurality of preambles, each of the preambles compensated with a corresponding timing advance and a corresponding frequency compensation associated with a different synchronization area included in the plurality of synchronization areas. The successive transmit may cease, when a timer expires or a list including the plurality of synchronization areas is exhausted. The common timing advance and the differential timing advances and the frequency compensations may be received in a system information broadcast. The apparatus may be comprised in or may comprise a user equipment.

[0081] The apparatus 10 may include at least one antenna 12 in communication with a transmitter 14 and a receiver 16. Alternatively transmit and receive antennas may be separate. The apparatus 10 may also include a processor 20 configured to provide signals to and receive signals from the transmitter and receiver, respectively, and to control the functioning of the apparatus. Processor 20 may be configured to control the functioning of the transmitter and receiver by effecting control signaling via electrical leads to the transmitter and receiver. Likewise, processor 20 may be configured to control other elements of apparatus 10 by effecting control signaling via electrical leads connecting processor 20 to the other elements, such as a display or a memory. The processor 20 may, for example, be embodied in a variety of ways including circuitry, at least one processing core, one or more microprocessors with accompanying digital signal processor(s), one or more processor(s) without an accompanying digital signal processor, one or more coprocessors, one or more multi-core processors, one or more controllers, processing circuitry, one or more computers, various other processing elements including integrated circuits (for example, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), and/or the like), or some combination thereof. Accordingly, although illustrated in FIG. 8 as a single processor, in some example embodiments the processor 20 may comprise a plurality of processors or processing cores.

[0082] The apparatus 10 may be capable of operating with one or more air interface standards, communication protocols, modulation types, access types, and/or the like. Signals sent and received by the processor 20 may include signaling information in accordance with an air interface standard of an applicable cellular system, and/or any number of different wireline or wireless networking techniques, comprising but not limited to Wi-Fi, wireless local access network (WLAN) techniques, such as Institute of Electrical and Electronics Engineers (IEEE) 802.11, 802.16, 802.3, ADSL, DOCSIS, and/or the like. In addition, these signals may include speech data, user generated data, user requested data, and/or the like.

[0083] For example, the apparatus 10 and/or a cellular modem therein may be capable of operating in accordance with various first generation (1G) communication protocols, second generation (2G or 2.5G) communication protocols, third-generation (3G) communication protocols, fourth-generation (4G) communication protocols, fifth-generation (5G) communication protocols, Internet Protocol Multimedia Subsystem (IMS) communication protocols (for example, session initiation protocol (SIP) and/or the like. For example, the apparatus 10 may be capable of operating in accordance with 2G wireless communication protocols IS-136, Time Division Multiple Access TDMA, Global System for Mobile communications, GSM, IS-95, Code Division Multiple Access, CDMA, and/or the like. In addition, for example, the apparatus 10 may be capable of operating in accordance with 2.5G wireless communication protocols General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), and/or the like. Further, for example, the apparatus 10 may be capable of operating in accordance with 3G wireless communication protocols, such as Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access 2000 (CDMA2000), Wideband Code Division Multiple Access (WCDMA), Time Division- Synchronous Code Division Multiple Access (TD-SCDMA), and/or the like. The apparatus 10 may be additionally capable of operating in accordance with 3.9G wireless communication protocols, such as Long Term Evolution (LTE), Evolved Universal Terrestrial Radio Access Network (E-UTRAN), and/or the like. Additionally, for example, the apparatus 10 may be capable of operating in accordance with 4G wireless communication protocols, such as LTE Advanced, 5G, and/or the like as well as similar wireless communication protocols that may be subsequently developed.

[0084] It is understood that the processor 20 may include circuitry for implementing audio/video and logic functions of apparatus 10. For example, the processor 20 may comprise a digital signal processor device, a microprocessor device, an analog-to-digital converter, a digital-to-analog converter, and/or the like. Control and signal processing functions of the apparatus 10 may be allocated between these devices according to their respective capabilities. The processor 20 may additionally comprise an internal voice coder (VC) 20a, an internal data modem (DM) 20b, and/or the like. Further, the processor 20 may include functionality to operate one or more software programs, which may be stored in memory. In general, processor 20 and stored software instructions may be configured to cause apparatus 10 to perform actions. For example, processor 20 may be capable of operating a connectivity program, such as a web browser. The connectivity program may allow the apparatus 10 to transmit and receive web content, such as location-based content, according to a protocol, such as wireless application protocol, WAP, hypertext transfer protocol, HTTP, and/or the like.

[0085] Apparatus 10 may also comprise a user interface including, for example, an earphone or speaker 24, a ringer 22, a microphone 26, a display 28, a user input interface, and/or the like, which may be operationally coupled to the processor 20. The display 28 may, as noted above, include a touch sensitive display, where a user may touch and/or gesture to make selections, enter values, and/or the like. The processor 20 may also include user interface circuitry configured to control at least some functions of one or more elements of the user interface, such as the speaker 24, the ringer 22, the microphone 26, the display 28, and/or the like. The processor 20 and/or user interface circuitry comprising the processor 20 may be configured to control one or more functions of one or more elements of the user interface through computer program instructions, for example, software and/or firmware, stored on a memory accessible to the processor 20, for example, volatile memory 40, non-volatile memory 42, and/or the like. The apparatus 10 may include a battery for powering various circuits related to the mobile terminal, for example, a circuit to provide mechanical vibration as a detectable output. The user input interface may comprise devices allowing the apparatus 20 to receive data, such as a keypad 30 (which can be a virtual keyboard presented on display 28 or an externally coupled keyboard) and/or other input devices.

[0086] As shown in FIG. 8, apparatus 10 may also include one or more mechanisms for sharing and/or obtaining data. For example, the apparatus 10 may include a short-range radio frequency (RF) transceiver and/or interrogator 64, so data may be shared with and/or obtained from electronic devices in accordance with RF techniques. The apparatus 10 may include other short-range transceivers, such as an infrared (IR) transceiver 66, a Bluetooth™ (BT) transceiver 68 operating using Bluetooth™ wireless technology, a wireless universal serial bus (USB) transceiver 70, a Bluetooth™ Low Energy transceiver, a ZigBee transceiver, an ANT transceiver, a cellular device-to-device transceiver, a wireless local area link transceiver, and/or any other short-range radio technology. Apparatus 10 and, in particular, the short-range transceiver may be capable of transmitting data to and/or receiving data from electronic devices within the proximity of the apparatus, such as within 10 meters, for example. The apparatus 10 including the Wi-Fi or wireless local area networking modem may also be capable of transmitting and/or receiving data from electronic devices according to various wireless networking techniques, including 6LoWpan, Wi-Fi, Wi-Fi low power, WLAN techniques such as IEEE 802.11 techniques, IEEE 802.15 techniques, IEEE 802.16 techniques, and/or the like.

[0087] The apparatus 10 may comprise memory, such as a subscriber identity module (SIM) 38, a removable user identity module (R-UIM), an eUICC, an UICC, and/or the like, which may store information elements related to a mobile subscriber. In addition to the SIM, the apparatus 10 may include other removable and/or fixed memory. The apparatus 10 may include volatile memory 40 and/or non-volatile memory 42. For example, volatile memory 40 may include Random Access Memory (RAM) including dynamic and/or static RAM, on-chip or off-chip cache memory, and/or the like. Non-volatile memory 42, which may be embedded and/or removable, may include, for example, read-only memory, flash memory, magnetic storage devices, for example, hard disks, floppy disk drives, magnetic tape, optical disc drives and/or media, non-volatile random access memory (NVRAM), and/or the like. Like volatile memory 40, non-volatile memory 42 may include a cache area for temporary storage of data.

At least part of the volatile and/or non-volatile memory may be embedded in processor 20. The memories may store one or more software programs, instructions, pieces of information, data, and/or the like which may be used by the apparatus for performing operations disclosed herein. Alternatively or additionally, the apparatus may be configured to cause the operations disclosed herein with respect to the base stations/WLAN access points and network nodes including the UEs.

[0088] The memories may comprise an identifier, such as an international mobile equipment identification (IMEI) code, capable of uniquely identifying apparatus 10. The memories may comprise an identifier, such as an international mobile equipment identification (IMEI) code, capable of uniquely identifying apparatus 10. In the example embodiment, the processor 20 may be configured using computer code stored at memory 40 and/or 42 to the provide operations disclosed herein with respect to the UE.

[0089] Some of the embodiments disclosed herein may be implemented in software, hardware, application logic, or a combination of software, hardware, and application logic. The software, application logic, and/or hardware may reside on memory 40, the control apparatus 20, or electronic components, for example. In some example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer- readable media. In the context of this document, a “computer-readable medium” may be any non-transitory media that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer or data processor circuitry, with examples depicted at FIG. 8, computer-readable medium may comprise a non-transitory computer-readable storage medium that may be any media that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer.

[0090] Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein may be enhanced operations of NTN. Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein may be TA and Doppler shift correction without frequent user specific signaling, and no need for additional overhead for large TA adjustment in RAR and TA update command.

[0091] The subject matter described herein may be embodied in systems, apparatus, methods, and/or articles depending on the desired configuration. For example, the base stations and user equipment (or one or more components therein) and/or the processes described herein can be implemented using one or more of the following: a processor executing program code, an application-specific integrated circuit (ASIC), a digital signal processor (DSP), an embedded processor, a field programmable gate array (FPGA), and/or combinations thereof. These various implementations may include implementation in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device. These computer programs (also known as programs, software, software applications, applications, components, program code, or code) include machine instructions for a programmable processor, and may be implemented in a high-level procedural and/or object-oriented programming language, and/or in assembly/machine language. As used herein, the term “computer-readable medium” refers to any computer program product, machine-readable medium, computer-readable storage medium, apparatus and/or device (for example, magnetic discs, optical disks, memory, Programmable Logic Devices (PLDs)) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions. Similarly, systems are also described herein that may include a processor and a memory coupled to the processor. The memory may include one or more programs that cause the processor to perform one or more of the operations described herein. [0092] Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations may be provided in addition to those set forth herein. Moreover, the implementations described above may be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. Other embodiments may be within the scope of the following claims.

[0093] If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined. Although various aspects of some of the embodiments are set out in the independent claims, other aspects of some of the embodiments comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims. It is also noted herein that while the above describes example embodiments, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications that may be made without departing from the scope of some of the embodiments as defined in the appended claims. Other embodiments may be within the scope of the following claims. The term “based on” includes “based on at least.” The use of the phase “such as” means “such as for example” unless otherwise indicated.

Claims

WHAT IS CLAIMED

1. An apparatus comprising: at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to at least: receive, in a broadcast from a non-terrestrial base station, a common timing advance for a coverage area of the broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; transmit, to the non-terrestrial base station, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by apparatus; and receive, from the non-terrestrial base station, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the non-terrestrial base station.

2. The apparatus of claim 1, wherein the apparatus is further caused to at least receive, in the broadcast from the non-terrestrial base station, a plurality of synchronization locations, each of the synchronization locations represented by an additional differential timing advance and an additional frequency compensation.

3. The apparatus of claim 1, wherein each of the plurality of synchronization areas includes a corresponding group of synchronization locations providing the additional differential timing advance and the additional frequency compensation.

4. The apparatus of any of claims 1-3, wherein a first synchronization area includes a first group of synchronization locations within an area covered by the first synchronization area.

5. The apparatus of any of claims 1-4, wherein the indication is received in a random access response decoded by the apparatus or a timing advance update command.

6. The apparatus of any of claims 1-5, wherein the apparatus is further caused to at least transmit successively in the random access channel a plurality of preambles, each of the preambles compensated with a corresponding timing advance and a corresponding frequency compensation associated with a different synchronization area included in the plurality of synchronization areas.

7. The apparatus of claim 6, wherein the successive transmit ceases, when a timer expires or a list including the plurality of synchronization areas is exhausted.

8. The apparatus of any of claims 1-7, wherein the common timing advance and the differential timing advances and the frequency compensations are received in a system information broadcast.

9. The apparatus of any of claims 1-8, wherein the apparatus is comprised in or comprises a user equipment.

10. An apparatus comprising: at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to at least: transmit, to a user equipment, a common timing advance for a coverage area of a broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; receive, from the user equipment, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by the user equipment; and send, to the user equipment, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the apparatus.

11. The apparatus of claim 10, wherein the apparatus is further caused to at least send, in the broadcast, a plurality of synchronization locations, each of the synchronization locations represented by an additional differential timing advance and an additional frequency compensation.

12. The apparatus of claim 11, wherein each of the plurality of synchronization areas includes a corresponding group of synchronization locations providing the additional differential timing advance and the additional frequency compensation.

13. The apparatus of any of claims 10-12, wherein a first synchronization area includes a first group of synchronization locations within an area covered by the first synchronization area.

14. The apparatus of any of claims 10-13, wherein the indication is sent to the user equipment in a random access response or a timing advance update command.

15. The apparatus of any of claims 10-14, wherein the apparatus is further caused to at least detect one of a plurality of preambles sent by the user equipment, each of the preambles compensated with a corresponding timing advance and a corresponding frequency compensation associated with a different synchronization area included in the plurality of synchronization areas.

16. The apparatus of any of claims 10-15, wherein the apparatus comprises or is comprised in a base station.

17. The apparatus of any of claims 10-16, wherein the apparatus updates, as the coverage area changes, the common timing advance, the plurality of synchronization areas, and the plurality of synchronization locations transmitted in the broadcast.

18. A method comprising: receiving, in a broadcast from a non-terrestrial base station, a common timing advance for a coverage area of the broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; transmitting, to the non-terrestrial base station, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by a user equipment; and receiving, from the non-terrestrial base station, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the non terrestrial base station.

19. The method of claim 18, further comprising receiving, in the broadcast from the non-terrestrial base station, a plurality of synchronization locations, each of the synchronization locations represented by an additional differential timing advance and an additional frequency compensation.

20. The method of claim 18, wherein each of the plurality of synchronization areas includes a corresponding group of synchronization locations providing the additional differential timing advance and the additional frequency compensation.

21. The method of any of claims 18-20, wherein a first synchronization area includes a first group of synchronization locations within an area covered by the first synchronization area.

22. The method of any of claims 18-21, wherein the indication is received in a random access response decoded by the apparatus or a timing advance update command.

23. The method of any of claims 18-22, further comprising transmitting successively in the random access channel a plurality of preambles, each of the preambles compensated with a corresponding timing advance and a corresponding frequency compensation associated with a different synchronization area included in the plurality of synchronization areas.

24. The method of claim 23, wherein the transmitting successively ceases, when a timer expires or a list including the plurality of synchronization areas is exhausted.

25. The method of any of claims 18-24, wherein the common timing advance and the differential timing advances and the frequency compensations are received in a system information broadcast.

26. A method comprising: transmitting, to a user equipment, a common timing advance for a coverage area of a broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; receiving, from the user equipment, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by the user equipment; and sending, to the user equipment, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to a base station.

27. The method of claim 26, further comprising sending, in the broadcast, a plurality of synchronization locations, each of the synchronization locations represented by an additional differential timing advance and an additional frequency compensation.

28. The method of claim 26, wherein each of the plurality of synchronization areas includes a corresponding group of synchronization locations providing the additional differential timing advance and the additional frequency compensation.

29. The method of any of claims 26-28, wherein a first synchronization area includes a first group of synchronization locations within an area covered by the first synchronization area.

30. The method of any of claims 26-29, wherein the indication is sent to the user equipment in a random access response or a timing advance update command.

31. The method of any of claims 26-30, wherein the apparatus is further caused to at least detect one of a plurality of preambles sent by the user equipment, each of the preambles compensated with a corresponding timing advance and a corresponding frequency compensation associated with a different synchronization area included in the plurality of synchronization areas.

32. The method of any of claims 26-31 further comprising updating, as the coverage area changes, the common timing advance, the plurality of synchronization areas, and the plurality of synchronization locations transmitted in the broadcast.

33. An apparatus comprising: means for receiving, in a broadcast from a non-terrestrial base station, a common timing advance for a coverage area of the broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; means for transmitting, to the non-terrestrial base station, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by a user equipment; and means for receiving, from the non-terrestrial base station, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the non-terrestrial base station.

34. The apparatus of claim 33 further comprising means for performing a function recited in any of claims 19-25.

35. An apparatus comprising: means for transmitting, to a user equipment, a common timing advance for a coverage area of a broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; means for receiving, from the user equipment, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by the user equipment; and means for sending, to the user equipment, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to a base station.

36. The apparatus of claim 35 further comprising means for performing a function recited in any of claims 27-32.

37. A non-transitory computer-readable storage medium including program code which when executed by at least one processor causes operations comprising: receiving, in a broadcast from a non-terrestrial base station, a common timing advance for a coverage area of the broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; transmitting, to the non-terrestrial base station, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by a user equipment; and receiving, from the non-terrestrial base station, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to the non terrestrial base station.

38. A non-transitory computer-readable storage medium including program code which when executed by at least one processor causes operations comprising: transmitting, to a user equipment, a common timing advance for a coverage area of a broadcast and a plurality of synchronization areas, each of the synchronization areas represented, in the broadcast, by a differential timing advance and a frequency compensation; receiving, from the user equipment, in a random access channel a preamble compensated based on the common timing advance, and further compensated by the differential time advance and the frequency compensation associated with one of the plurality of synchronization areas selected by the user equipment; and sending, to the user equipment, an indication to use the selected one of the plurality of synchronization areas for compensation of an uplink transmission to a base station.