Device and system for return link time and frequency tracking

Abstract

The present invention relates to a terminal (100) for communication with a hub over a non-terrestrial network communication channel and arranged to perform time and / or frequency corrections in return link communication towards said hub, said terminal comprising - a modulator (101) arranged to generate a return link signal to be transmitted, - a forward link timing and / or frequency offset estimator (111) arranged to estimate an error signal indicative of a forward link timing and / or frequency offset, said forward link establishing communication towards the terminal, - open loop correction means comprising a local loop filter (114) and a converter (112) and arranged to receive the error signal indicative of the forward link timing and / or frequency offset and to output an error signal indicative of a return link timing and / or frequency offset, - compensation means (103) arranged to receive the error signal indicative of the return link timing and / or frequency offset, to compensate for the return link timing and / or frequency offset and to output an indication of a time and / or frequency correction to the modulator to be applied when generating the return link signal to be transmitted to the hub.

Description

Device and System for Return Link Time and Frequency TrackingField of the invention

[0001] The present invention is generally related to the field of satellite communication systems. More in particular, the invention relates to techniques for satellite terminal time and frequency misalignment correction.Background of the invention

[0002] A typical realization of a satellite communication network is based on a star network in which a transmitter at the gateway transmits in the so-called forward (FWD) link via the satellite to multiple terminals. In case of interactive communication, the terminals transmit back in the so-called return (RTN) link via the satellite towards a receiver at the gateway. In 3GPP, the forward link is also denoted the "downlink" and the return link also denoted the "uplink".

[0003] Satellite networks require accurate time and frequency alignment for proper communication. With multicarrier Orthogonal Frequency Division Multiplex (OFDM) modulations as used in modern 5G New Radio (5GNR) waveforms the required accuracy is even more critical. In recent years, a growing interest is observed in using the terrestrial 5GNR waveform over satellite links. The 5GNR radio waveform is based on multi-carrier OFDM modulation while legacy satellite waveforms like DVB-S2X and DVB-RCS are based on Square Root Raised Cosine (SRRC) pulse shaped single carrier transmissions. Next to some clear advantages of OFDM for multi-channel transmissions, there are some drawbacks. A critical disadvantage is the higher sensitivity to time and frequency misalignment of the terminal. The time and frequency misalignments can be caused by terminal reference clock inaccuracies or by Doppler effects due to moving terminals or satellites. Terminal misalignment in time or frequency results in non-orthogonality of the different subcarriers in the OFDM waveform and causes Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI).

[0004] The 5GNR standardisation body (3GPP) has put forward the use of Global Navigation Satellite Systems (GNSS) and satellite orbit information to pre-compensate the time and frequency errors introduced by mobility effects. This is a critical dependency as the GNSS signal can be missing, jammed or spoofed. Therefore, there is a need for a solution wherein the GNSS information dependency of the communication system is avoided.

[0005] The critical synchronization accuracy needs and the Doppler requirements for 5GNR Non-Terrestrial Networks (NTN) are now reviewed.

[0006] Compared to Terrestrial Networks (TN), the tight synchronization requirement of 5GNR OFDM waveforms becomes increasingly difficult to achieve in Non-Terrestrial Network (NTN) scenariosbecause of two main reasons. First, the radio frequencies are typically roughly 10 times higher (Ku and Ka band instead of L or S-band), making it more difficult to guarantee absolute carrier frequency accuracy and stability. A second reason is that the longer link delays do not allow fast corrections of timing alignment errors with the same mechanism as in terrestrial networks.

[0007] The 3GPP release 17 baseline for NTN links assumes the timing and frequency offsets can be compensated based on the availability of terminal GNSS data and satellite orbit information. The GNSS dependency is new compared to legacy satellite networks. GNSS dependency has some important drawbacks : availability is not always guaranteed (e.g. in hostile environments), the power consumption is not negligible and the logon time increases. So, in some use cases, there is a need for GNSS independent operation.

[0008] The CP-OFDM waveform used in 5GNR is resilient to time mismatches as long as uncertainties are limited and do not exceed the Cyclic Prefix (CP). Misalignment above the CP causes ISI. In 5GNR, the CP length depends on the numerology and ranges from 4.7 ps for numerology 0 to 0.29 ps for numerology 4 (see Table 1 below). So lower numerologies are more resilient to time misalignment.Table 1 : 5GNR Cyclic Prefix length in function of numerology

[0009] The frequency misalignment is also critical, all sub-carrier misalignment causes Inter Carrier interference (ICI). The ICI level is function of the subcarrier spacing (SCS). Table 2 shows simulation results where the ICI level is measured in function of the frequency offset. A frequency misalignment of 1% of the subcarrier spacing, denoted relative SCS offset, causes an acceptable ICI level of -35 dB and should allow operation up to SNR levels of 20 dB without noticeable impact from the frequency misalignment (as 35 dB is much higher than 20 dB, so the noise is roughly 15 dB more powerful than the ICI). Note how in contrast to timing offsets for frequency resilience the higher numerologies are more robust.Table 2 : ICI level in function of frequency misalignment

[0010] Assuming a frequency accuracy need of 1% of SCS, a time accuracy of 0.5 CP (hence, half of the cyclic prefix duration) and taking the worst numerology, the required frequency and time accuracies for limited ICI and ISI are respectively roughly 150 Hz and 0.15 psec. This is a very stringent requirement when compared to legacy single carrier satellite return channel multiple access waveforms like DVB-RCS or the iDirect proprietary MRC (see the paper 'Multi Resolution Coding (MRC) Satellite Waveform', P. Delbeke et al., Ka Conference, 2021). For those legacy satellite waveforms, typical uncertainties of 3 kHz and 10 ps can be accepted, thus orders of magnitudes higher. As some satellite waveforms, e.g. as disclosed in US2022 / 149930 Al, evolve to OFDM modulation (such as in 5G), the time and frequency synchronisation of the waveform becomes more critical, i.e., the uncertainties will require to be much lower than 3 kHz and 10 ps. Application US2022 / 149930 Al only discusses time- multiplexed multi-carrier receiver architecture for the hub, but remains silent time and frequency synchronization for OFDM modulation over satellite.

[0011] First Doppler profiles are determined to model frequency and time misalignments due to the Doppler effect. Typical terminal and satellite mobility use cases for GEO (Geostationary orbit) and LEO (Low Earth Orbit) satellites can be defined. The use cases cover the LEO and GEO scenarios defined in the 3GPP NTN report '3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Solutions for NR to Support Non-Terrestrial Networks (NTN)'.

[0012] Several user equipment (UE) mobility profiles are defined and combined with LEO or GEO satellite link delays. For satellite movement low and high LEO passes are emulated. Some terminal mobility profiles are shown in Fig.l. Each subfigure shows the link distance in function of time (bottom), the terminal speed (middle) in direction of satellite and the acceleration (top) in direction of satellite in function of time. Two terminal mobility models are considered : circular UE movement profiles (CircAerol shown in Fig.la) and 4 constant acceleration UE movement profiles (MaxAerol shown in Fig. lb). UE stands for User Equipment, also denoted the terminal. The maximum speeds and accelerations for the different simulated profiles are summarised in Table 3. Note how CircAero4 and MaxAero4 are very severe profiles that go beyond the scope of commercial connectivity.The LEO profiles are modelled based on circular orbits around earth with a zenithal pass. Two reference cases are simulated a low orbit at 400km altitude, denoted LeoLow, and a high orbit LEO profile at 1500km altitude, denoted LeoHigh (Fig.lc and Fig. Id).Table 3 : Simulated Doppler and clock drift profiles, showing the maximum acceleration and maximum speed as well as the different considered clock drifts

[0013] Next to Doppler offsets originating from terminal or satellite movements, the UE clock reference can be subject to drift. The clock drift also introduces time and frequency errors. To assess the clock tracking performance, clock drift profiles are also included in Table 3. The clock drift profile includes fixed drift profiles (ClockDrift 1 and 2 expressed in ppm / sec drift) and random walk phase excursion profiles (ClockDrift 3 and 4 expressed by their Allan variance at 1 sec). Finally, a stable system with no drift and only noise on the channel is added to evaluate the loop performance with only AWGN noise and no time or frequency drifts impairments.

[0014] The link delay is proportional to the link distance d. The instantaneous link delay can be computed as follows:Delay = d / c*2 where d is the link distance and c is the speed of light.

[0015] The frequency offsets are proportional to the speed and the RF frequency. Excluding relativistic effects, the offsets can be computed as:F_Offset = v / c*F_RF*2 where c is the speed of light and F_RF is the RF carrier centre frequency.

[0016] The factor two in the above equations for Delay and F_Offset_accounts for the doubling of the perceived speed and distance changes due to forward and return paths effects. The effect is doubled because both the forward link and the return link are impacted by link changes. To have the correct link delay, the UE to hub distance must be accounted twice. This is correct for the delay changes, but only an approximation for the frequency offset as the RF frequencies for forward and return link can be different. For transparent (bent pipe) satellites two hops delay (UE to Hub and Hub to UE) isassumed between the transmission and the estimator feedback in the loops. The solution also applies for regenerative satellites in which case the link delay is limited to UE to satellite and back.

[0017] In conclusion the more stringent time and frequency synchronisation requirements of 5GNR waveform combined with higher mobility profiles require novel time and frequency solutions for 5GNR over satellite links. The GNSS data as proposed by 3GPP is a solution, but also comes with a major system dependency that can better be avoided. So, there is a need for improved time and frequency tracking solutions without GNSS dependency.

[0018] Next to corrections based on GNSS information, solutions based on pre-compensation of the satellite movement in the satellite are proposed in the prior art, for example in the paper "Beam centric Doppler pre-compensation for LEO-based 5G-NTN" (A. K. Meshram et al., 40th Int'l Comm. Satellite Systems Conf. ( / CSSC2023), Bradford, UK, 2023). The solution proposed in the prior art reduces the residual error via pre-compensation in the satellite but introduces satellite complexity and does not perfectly compensate the timing and frequency offsets for two reasons. First, the pre-compensation is only correct for centre of the beam and will not be perfect for UEs at the beam edges. Second, the precompensation does not account for the effects of UE mobility. Also these drawbacks need to be addressed.

[0019] Higher order tracking loops for changes in satellite link delay were proposed in some prior art works. For example, US2020 / 084736 Al proposes a second order tracking loop for hub alignment to satellite beam hopping plan, i.e., the hub transmit time must follow variations of the uplink delay and clock drifts between hub and satellite. A hub loop only however does not allow for a fast reaction on sudden link delay and / or frequency variations due to the high link latency. In the beam hopping system in US2020 / 084736 Al, there are no sudden link delay variations between the fixed hub location and the satellite.

[0020] Other closed loop time tracking solutions have been proposed, e.g., in "On synchronisation for SC-FDMA waveform over GEO satellite networks" (F. Rossetto et al., 2012 6th Advanced Satellite Multimedia Systems Conference (ASMS) and 12th Signal Processing for Space Communications Workshop (SPSC), Vigo, Spain, 2012). The paper presents a candidate solution for a new generation satellite RCS waveform based on OFDM for GEO satellites. The paper highlights the criticality of time synchronisation within the CP and proposes higher order loops, similar to the hub loop as explained in this document. The paper uses different CP sizes and subcarrier spacing than in latest 5GNR standard, nevertheless, the paper expresses the challenge of complying with UE mobility effects for UE speed of > 40km / u. The current invention proposes a solution (by alternative loops and / or combining with a hub loop) that can deal with much stronger mobility effects (higher speeds and accelerations) from the UE.

[0021] Summarizing, the above-mentioned prior art solutions do not allow tracking fast variations due to UE movements.Summary of the invention

[0022] It is an object of embodiments of the present invention to provide for a terminal for communication over a non-terrestrial network channel and for a system comprising at least one such a terminal and a hub, wherein timing and / or frequency errors can be tracked without being dependent on GNSS data.

[0023] The above objective is accomplished by the solution according to the present invention.

[0024] In a first aspect the invention relates to a terminal for communication with a hub over a non-terrestrial network communication channel and arranged to perform time and / or frequency corrections in return link communication towards the hub. The terminal comprises a modulator arranged to generate a return link signal to be transmitted, a forward link time and / or frequency offset estimator arranged to estimate an error signal indicative of a forward link timing and / or frequency offset, said forward link establishing a communication towards said terminal, open loop correction means comprising a local loop filter and a converter and arranged to receive said error signal indicative of said forward link timing and / or frequency offset and to output an error signal indicative of a return link timing and / or frequency offset, compensation means arranged to receive said error signal indicative of said return link timing and / or frequency offset, to compensate for said return link timing and / or frequency offset and to output an indication of a timing and / or frequency correction to said modulator to be applied when generating said return link signal to be transmitted to said hub.

[0025] The proposed solution indeed allows for timing and / or frequency offset correction in the return link direction without any need for GNSS. With 'return link' is meant the link from the terminal via a satellite to the hub. An estimated error signal indicative of a timing and / or frequency offset in the forward link connection (i.e., from the hub towards the terminal) is fed to open loop correction means comprising a local loop filter and a converter. The way the local loop and the converter are arranged may vary from embodiment to embodiment. At the output of the open loop correction means one obtains an error signal indicative of the timing and / or frequency offset in the return link connection. The fact that from an error signal indicative of a forward link timing and / or frequency offset an error signal is derived indicative of a return link timing and / or frequency offset is a key feature of the present invention. When NTN is considered, some specific challenges need to beaddressed, as faster variations of the timing and / or frequency offsets occur and consequently, faster corrections are needed.

[0026] In a preferred embodiment the error signal indicative of the forward link timing and / or frequency offset is the forward link timing and / or frequency offset itself and the error signal indicative of the return link timing and / or frequency offset is a tracked return link timing and / or frequency offset. The open loop correction means in the terminal comprises a converter to convert the forward link timing and / or frequency offset into the return link timing and / or frequency offset. The terminal further comprises computation means to determine a difference (loop error signal) between the error signal indicative of a return link timing and / or frequency offset and the tracked return link timing and / or frequency offset. The local loop filter then receives this difference. In this embodiment the local loop filter is only used locally in the terminal in an open loop return link correction (resulting in the targeted timing and / or frequency offset correction in the return link direction without any need for GNSS). This open loop is referred to as a local forward tracking loop as it is local in the terminal and tracks changes in the return link based on forward link timing and / or frequency offset estimates. Note that it is not the tracking performance on the forward link, however, that is considered. Only the tracking performance on the return link is observed by using a signal from the forward link channel for return link timing and / or frequency offset correction. The local forward tracking loop is in the following also often named the local loop.

[0027] In a preferred embodiment the error signal indicative of the forward link timing and / or frequency offset is an error on a tracked forward link timing and / or frequency offset and the error signal indicative of the return link timing and / or frequency offset is the return link timing and / or frequency offset itself. The local loop filter in the open loop correction means is arranged to receive the error signal indicative of the tracking error on the timing and / or frequency offset in the forward link connection. The local loop filter in an embodiment like this one can be seen as a local forward link tracking loop filter and the filter output is the forward link tracked timing and / or frequency offset. The forward link tracked timing and / or frequency offset is converted to a return link timing and or frequency offset and used in an open loop return link correction (resulting in the targeted time and / or frequency error correction in the return link direction without any need for GNSS). In the forward link tracking loop, a change in the forward link timing and / or frequency offset with respect to the tracked forward link offset is estimated. The error signal indicative of the timing and / or frequency error in the forward link is an error with respect to the tracked timing and / or frequency in the forward link. This is because feedback from the local loop filter received via the forward link demodulator is taken into account in the estimator.

[0028] In a preferred embodiment a local loop comprising the local loop filter is arranged to compensate for a timing and / or frequency offsets only originating from link delay changes and not from reference clock offsets as the two effects require a correction in the opposite direction.

[0029] Advantageously, the local loop comprising the local loop filter can have a smaller time constant than the round-trip time of the return link and the forward link, without any risk of oscillations. It is an advantage that the local loop can react to fast variations in the error, for example variations due to terminal mobility. Another advantage of the local forward tracking loop solution is that that there is no overhead involved as there is no over the air component as it is only local in the terminal. In combination with other embodiments involving a second loop and a further filter and an over the air component as described below, the overhead of this second loop is limited thanks to this fast local forward tracking loop as this allows making the second loop slower (and thus with less overhead).

[0030] In another aspect the invention relates to a system comprising one or more terminals as previously described and a hub arranged to demodulate the return link signal.

[0031] Preferably the hub is arranged to estimate a return link timing and / or frequency error and the estimated return link timing and / or frequency error is fed back in a so-called hub loop to a further filter. Embodiments like this are also called hub loop solutions. A hub loop solution can be seen as a classical closed control loop. This is a loop that is closed over the air from the hub back to the terminal. The local loop does not go over the air.

[0032] In some embodiments the further filter has a filter order higher than 1. This is for example the case if there is only a hub loop and the hub loop filter is the only filter in the tracking loop.

[0033] In advantageous embodiments the hub loop solution and the local loop solution are combined. In the combined solution there are thus two loops. The local loop filter may then be a filter of higher order and the hub loop filter a first order filter.

[0034] In a preferred embodiment a local loop in the terminal comprising the local loop filter is arranged to compensate for return link timing and / or frequency offsets originating from a link delay changes and the hub loop is arranged to compensate for return link timing and / or frequency offsets originating from clock reference offsets between the hub and the terminal(s).

[0035] In one embodiment the hub comprises a 5GNR Physical Broadcast Channel Synchronisation Signal block for estimating the timing and / or frequency alignment information of the downlink.

[0036] In some embodiments the second filter, i.e., the hub loop filter, is comprised in the hub.

[0037] The hub loop may have a time constant larger than the total round-trip time of the return link and the forward link delay. Such a large time constant does not allow fast corrections but avoids the occurrence of oscillations in the loop. In a preferred embodiment, this is the case when thehub loop is combined with the local loop, where the latter has a smaller time constant and can thus correct faster changes.

[0038] In a preferred embodiment the non-terrestrial network is a satellite network.

[0039] In another embodiment the system comprises a regenerative satellite.

[0040] In a preferred embodiment the modulator is an OFDM modulator.

[0041] In a further aspect the invention relates to a method for performing time and / or frequency correction in return link communication from a terminal as described above to a hub over a non-terrestrial network channel. The method comprises :- receiving in an open loop correction means of the terminal an error signal indicative of a timing and / or frequency offset in a forward link connection from the hub towards the terminal,- converting the error signal indicative of the forward link time and / or frequency error into an error signal indicative of a return link timing and / or frequency offset,- applying the error signal indicative of the return link timing and / or frequency offset to a compensation means to compensate for the return link timing and / or frequency offset and to output an indication of time and / or frequency correction,- generating in a modulator of the terminal a return link signal to be transmitted to a hub using the indication of timing and / or frequency offset correction.

[0042] In a preferred embodiment the timing and / or frequency offset is at least partially due to link delay changes between the terminal and the hub.

[0043] For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

[0044] The above and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.Brief description of the drawings

[0045] The invention will now be described further, by way of example, with reference to the accompanying drawings, wherein like reference numerals refer to like elements in the various figures.

[0046] Fig.l illustrates sample mobility profiles modelling terminal or satellite movements.

[0047] Fig.2 illustrates closed loop tracking of time or frequency errors based on forward link correction estimated in the terminal.

[0048] Fig.3 illustrates closed loop tracking of time or frequency errors based on forward link correction estimated in the terminal where the conversion from Rx to Tx occurs after the loop filter.

[0049] Fig.4 illustrates the problem of uplink frequency correction differences for forward link errors originating from clock reference offsets or Doppler offsets.

[0050] Fig.5 illustrates the closed loop tracking using hub side error estimates for terminal transmission corrections.

[0051] Fig.6 illustrates more details on the higher order loop filtering.

[0052] Fig.7 illustrates the combined hub and local loop tracking of Figs.5 and 2.

[0053] Fig.8 illustrates the combined hub and local loop tracking of Figs.5 and 3.

[0054] Fig.9 illustrates time estimator root mean square (rms) error in function of SNR, number of allocated resource blocks and number of DMRS symbols per slot.

[0055] Fig.10 illustrates the delay tracking performance for several loop parameters for UE mobility profile MaxAreol and CircAerol (as defined in Table 3) in GEO and LEO loop delay conditions.

[0056] Fig.11 illustrates the delay tracking performance for several loop parameters for LEO mobility profile.

[0057] Fig.12 illustrates the frequency tracking performance for several loop parameters for UE mobility profile MaxAreol and CircAerol (as defined in Table 3) in GEO and LEO loop delay conditions.

[0058] Fig.13 illustrates the delay tracking performance for several loop parameters for LEO mobility profile.Detailed description of illustrative embodiments

[0059] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims.

[0060] Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0061] It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps orcomponents as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

[0062] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0063] Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[0064] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0065] It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated.

[0066] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0067] The following terminology is used in this description. A timing and / or frequency offset refers to an offset compared to the nominal expected timing and / or frequency in a UE. The origin of the offset can be changes in the link delay (due to UE or satellite mobility) or UE clock misalignment (due to imperfect reference clocks). Both effects can cause changing timing and / or frequency offsets in forward and return link. A UE can estimate the forward link timing and / or frequency offsets and precompensate for the return link timing and / or frequency offsets.When a tracking loop is involved, the term 'tracked timing and / or frequency offset' refers to the output of the tracking loop filter. The 'error on the tracked offset' refers to the input that is provided to the loop filter and is also referred to as the loop error. The tracked timing and / or frequency offset and the error on the tracked timing and / or frequency offset can be used for both the forward and the return links.After a UE pre-compensation of a return link timing and / or frequency offset, a hub can measure an error on the return link timing and / or frequency, which is a remaining error after UE pre-compensation. Time and timing are synonyms. A time offset is the same as a timing offset.

[0068] As opposed to the navigation data (GNSS) based time and frequency compensation method proposed by 3GPP or, for example, in US6675013 Bl, the solution proposed in this invention is based on closed loop tracking of the terminal carrier frequency offsets and symbol time offsets to correct errors introduced by mobility of terminal or satellite.

[0069] Closed loop return link timing correction (feedback from hub receiver to terminal transmitter) is already applied in 5GNR TN with the Timing Advance (TA) mechanism. However, when moving to NTN, the higher link propagation delay is limiting the speed of the TA corrections, and the faster movements require faster corrections. For the frequency correction, there is no standardised closed loop correction solution in the latest 5GNR standard (Release 17). Instead, a more complicated multi-layer GNSS based Doppler pre-compensation solution (involving two-line element set (TLE) broadcasting using the System Information Broadcast (SIB) message in RAN2) is used in Release 17 as a tracking solution was not found. It was explained above that this solution has disadvantages that are overcome in the present invention.

[0070] Similarly, closed loop time and frequency tracking solutions are also applied in legacy DVB-S2X / RCS systems using first order closed loops, but as set out above, when using OFDM, the required tracking accuracy is much higher, making the first order loop correction used in DVB-RCS inadequate.

[0071] NTN links require faster time corrections than what is possible with first order loops and the TA mechanism because of the high link delay and / or faster variations of time (e.g., due to veryhigh speeds and acceleration of LEO satellites). NTN links have higher frequency errors than terrestrial networks (e.g., due to the higher carrier frequencies) and thus require accurate frequency corrections, however not possible in 3GPP due to the lack of frequency error estimation feedback. The corrections should be also faster than in DVB-S2X / RCS2 to avoid ICI with the small sub-carrier spacing in OFDM. To overcome those limitations of first order correction loops, one or more of new control loops, higher order tracking loops and combined loops are proposed. A second order control loop can correct errors and error rates. A third order loop can correct errors, error rates and error rate variations for both time and frequency errors.

[0072] A first solution for GNSS independent timing and carrier frequency offset tracking is terminal slaving to the forward link carrier as in the embodiment depicted in Fig.2. This embodiment is a local forward tracking loop solution and further referred to as the local loop solution. Fig.2 shows a block scheme comprising a terminal (100), a hub (200) and a satellite channel (300) connecting the terminal and the hub. The channel (300) includes the (transparent bent pipe) satellite link (including obviously satellite movement effects) as well as terminal movement effects. The return link modulator (also shortened to "Mod") block (101) in the UE is responsible for generating the return link signal to be transmitted to the hub side. Block (101) in Fig.2 represents the modulator with all its functionalities. This modulator includes forward error correction (FEC) encoding, symbol mapping, sample generation (e.g., including IFFT in case of OFDM), upsampling and conversion to RF frequency. The invention is not limited to a modulation type, i.e., the modulation can generate single carrier square root raised cosine waveform (like in DVB-RCS2) or multicarrier OFDM waveform (like in 5GNR).

[0073] The terminal or UE further comprises a demodulator block (110). From the demodulated signal an estimation (111) of the symbol time and / or carrier frequency error in the downlink is performed. This information is converted in a converter (112) to estimations taking into account differences between return link and forward link parameters. The conversion is required for frequency correction as the radio frequencies can differ between return link and forward link. An error signal indicative of a return link timing and / or frequency offset can then be computed in computation means (113) (e.g., a processor), which is next applied to the local loop filter (114). The converter and the local loop filter are part of an open loop correction means. The resulting signal coming from the converter of the open loop correction means is input to a compensation block (103) where tracked corrections (compensations) for the timing and / or frequency are computed and an indication of timing and / or frequency correction to the return link modulator is output. The local loop filter output is also fed back in a local loop by feeding it to block (113) for use in the error computation. The local loop filter (114) is a higher order filter allowing correction of not only errors but also error rates and variations of error rates.

[0074] A solution as in the embodiment of Fig.2 performs return link time and / or frequency offset corrections. Frequency correction is the most critical task of the two. In this solution, all terminals slave their clock to the terminal forward link estimation in their demodulator (110) and adapt their return link frequency accordingly. The forward link frequency errors are estimated (111) and converted to return link frequencies (112). The error to the tracked offset is computed (113) and a local loop filter (114) tracks the results.

[0075] A terminal slaving to the forward link as presented in Fig.2 is sometimes considered as a solution for terminal local reference clock offset corrections in fixed terminals (hence, without mobility). In this first solution also called local loop solution, on the contrary, the approach is used to correct Doppler effects on the radio link, which was not considered in the prior art for satellite links. In this embodiment with only the local loop, the local reference clock in the terminal is assumed to be good enough (i.e., accurate enough with negligible offset) or to be corrected by other means. As explained below, the local loop solution cannot be used to correct both clock effects and mobility effects together.

[0076] In an alternative preferred embodiment, the local loop presented in Fig.2 can be rearranged as in Fig.3, based on the observation that the downlink receiver (120) comprises elements of the local loop correction of the return link transmission based on forward link offset estimates. Indeed, in the UE, the forward link demodulation typically also requires a loop for the forward link synchronisation. In that case, the local loop filter (114) comprised in the open loop correction means and already presented above can become part of the forward link receiver to track the forward link time and / or frequency error. The conversion is now applied after the local loop filter (114) and the converter (112) converts tracked forward link timing and / or frequency offsets to return link timing and / or frequency offsets. For the frequency this means conversion of downlink radio frequency errors to uplink radio frequency errors which are different for Frequency Division Duplexing (as used in NTN). For the time tracking loop, the conversion function doubles and inverses the error to apply a correction to compensate for both the changes in forward and return link delay.

[0077] A notable difference between embodiments in Fig.2 and Fig.3 is the error estimation function (111). In both cases a time and / or frequency error is estimated. In Fig.2, however, the error estimation (111) range comprises the full offset (e.g. up to 800 kHz, so not the incremental offset compared to a tracked offset) in the forward link as the forward link demodulator (110) receives no feedback from the estimator (111). To compare the error to the tracked return link offset an extra error computation function (113), calculating the difference between the tracked offset and the measured offset before feeding the loop filter (114) is required. In Fig.3, the forward link demodulator (110) does receive feedback (e.g. the tracked frequency and / or time) from the loop filter (114) and therefore theestimator (111) only needs to estimate the tracking error on the forward link timing and / or frequency offset. Therefore, a block like (113) is not needed in the embodiment according to Fig.3.

[0078] In both embodiments of the local loop ( Fig.2 and Fig.3), the forward link frequency offset measurement (111) in the UE does not allow differentiating frequency offsets originating from Doppler effects and significant offsets originating from local reference clock offsets. These two effects require an opposite correction as explained in the paper 'Doppler Shift Estimation in 5G New Radio Non-Terrestrial Networks' (X.Lin et al., IEEE Global Communications Conference 2021) and illustrated in Fig.4. A wrong allocation of the frequency offset source results in a doubling of the error instead of compensating the offset. Due to the open loop correction of the return link based on forward link tracking, the local loop solution can only correct either the clock reference offsets when no mobility (no Doppler) is present, or, as a novel idea proposed here, the Doppler effects in cases where the terminal reference clock is assumed very good. The good reference clock can be available at the terminal side or corrected via another loop mechanism, as proposed further in this description.

[0079] As an example, a required frequency accuracy of 150 Hz, an RF frequency of 30 GHz and a local loop tracking of the Doppler frequency offsets (but not the reference clock offsets) are assumed. The local clock accuracy should then be better than 150 Hz / 30 GHz = 5ppb, which may be very challenging for low cost terminals.

[0080] The local loop cannot correct both the reference clock offsets and the Doppler drift offsets but offers some notable advantages over the hub loop. First, it does not require periodic time or frequency error control messages from the hub to the User Equipment (i.e., the terminal). This limits the control messages overhead cost. This also means it can be implemented without the need for new MAC layer messages in 3GPP standard. Second, the loop can be fast, as it is local and thus there are no long latencies between different blocks in the loop, thus there is no impact from the long satellite roundtrip time.

[0081] The local loop can track both frequency and delay offsets caused by changes on the link delay due to mobility of satellite or terminal. However, in that case, for the frequency offsets the local loop cannot correct local reference clock offsets. For the delay error the local loop can only estimate and track changes due to Doppler effect and not correct the absolute link delay error. The terminal has no absolute time reference to measure the absolute time misalignment. The absolute delay correction must come from hub side estimates, for example, via the timing advance (TA) mechanism described in 5GNR.

[0082] A hub loop solution for time and / or frequency tracking is detailed in Fig.5. This hub loop solution is similar to some solutions in prior art, e.g. to synchronize a hub with a beam hopping satellite.However, those prior art solutions cannot deal with the mobility and clock offsets and required accuracies considered here. The communication system comprises at least one terminal (100) and a hub (200) that communicate over a non-terrestrial network communication channel (300). In advantageous embodiments the non-terrestrial network channel is a satellite channel.

[0083] Fig.5 shows the return link composed of a modulator (101) at the terminal (UE) side and a demodulator (201) at the hub side. The channel (300) represents the (transparent bent pipe) satellite link (including satellite movements) as well as terminal movements. Hub side estimates (202) of timing and frequency offset on the uplink are used in a hub loop to compensate at the terminal side the transmission time and transmission carrier centre frequency (103). The name "hub loop" indicates that at least part of the loop is performed at the hub side. The hub loop filter (102) forms part of the hub loop and is a higher order filter which can perform proportional or integral corrections. It is not needed that the hub loop filter is located at the hub side. The name only indicates that it is part of the hub loop.

[0084] In some embodiments, like the one depicted in Fig.5, the hub loop filter (102) is located at the UE side. This allows a more granular per sample update of the frequency and time corrections while keeping the feedback error message (link between blocks 203 and 102 in Fig.5) rate low. The only parameter fed back from hub to terminal is the estimated error; all integration operations are performed at the UE side.

[0085] In other embodiments the hub loop filter (102) can be implemented at the hub side, so further limiting the complexity in the terminal. Keeping the complexity at the hub side is advantageous, but it results in certain use cases (depending on the corrections that need to be made) in larger absolute correction steps depending on the loop update period. In certain use cases with higher variations, this solution also requires feeding back more parameters to the UE like, e.g., the correction, the correction drift and the drift variation.

[0086] Fig.6 shows a more detailed scheme of the (hub or local) loop filter, excluding modulation and demodulation functionality. A same loop filter structure is used for time and frequency error corrections. The (hub or local) loop filter receives its input from forward link time and / or frequency errors in case of the local loop (Fig.2 or 3) or from hub side return link waveform time and / or frequency estimations in case of the hub loop (Fig.5). The loop in Fig.6 also shows an important difference between both loops, namely the position of the channel or the Doppler effects insertion. For the local loop the reference clock used in the forward link is impaired by Doppler effects (300b) whereas for the hub loop, the impairment is in the loop (300a). In both cases the loop will zero the difference between the two received inputs (113 or 203 on Fig.6) resulting in a waveform aligned to the hub timeand / or frequency reference in the return link demodulator. When the return link is well aligned to the hub time and frequency reference, demodulation is possible without ISI and ICI.

[0087] The loop filter is characterised by three parameters ki, k2and k3shown in Fig.6. The parameter ki is the proportional correction factor, k2is the integral correction factor and k3is the integral of integral correction factor making it a third order loop and allowing tracking second order variations. The loop can also be characterised in function of time constant Tc, damping factor (^) and k3 factor (k3f). Tc defines the loop speed. The damping factor (^) expresses the compromise between fast settling time and allowed overshoot and oscillations on step response. k3 is defined by a k3 factor which defines how much slower the third order correction is. In order to avoid influence of k3 on the second order loop parameters Tc and damping factor, as a rule of thumb k3 factor is set to 10. The relation between the knparameters and loop parameters like time constant (Tc) and damping factor (^) is given in Table 4.Table 4: Loop filter types and parameters

[0088] The loop performance for step and ramp responses is also given in Table 4. The carrier frequency offset is proportional to speed, so a constant speed results in a fixed frequency offset, while the delay is proportional to distance, so a constant speed results in a ramp variation of the delay. For a constant speed, a zero-delay steady state error cannot be achieved via a first order loop, it can be achieved via a second order loop or higher.

[0089] In 5GNR, the implementation of higher loop order requires some changes in the (terminal side) loop algorithms but no changes in waveform. Only limited signalling improvements are required. In particular, new Medium Access Control (MAC) messages are required. MAC Control Element (CE) messages exist for timing correction and a new MAC CE message can be defined for the frequency corrections. The new MAC CE messages allow sending both the time and frequency errors back to the terminal. The MAC CE message for Timing Advance (TA) correction already exists in standard and can be reused.

[0090] The hub loop offers with respect to the local loop the advantage of correcting frequency and time errors originating from both Doppler offsets and local reference offsets. This makes it an attractive solution. Compared to the local loop solution, the loop speed is limited by the satellite link delay and requires a relatively high overhead due to the need for frequent control corrections (for example, every 10 ms) fed back from the hub to the UE. The local loops as presented in Fig.2 and Fig.3 are faster and do not require any extra control overhead over the link but this comes at the cost of not allowing for correction of the local reference offsets or drifts.

[0091] In the embodiment presented in Fig.7 both loops are combined, i.e., a fast, local loop and a slower hub loop, thereby combining the advantages of local and hub loops. The local loop corrects all effects introduced by terminal or satellite mobility. This can be done in a fast local loop without causing any extra control overhead. As explained above, the local loop introduces an offset caused by the reference clock offset or drifts as those cannot be corrected via the local loop. The second, slower hub loop corrects the errors introduced by the clock offset. This second loop can be much slower (as that offset does not change quickly, as there is no need to correct fast changes due to mobility).

[0092] Similarly to the solution with only the local loop, in an alternative preferred embodiment, the local loop part of the combined loop solution can leverage the forward link receiver as presented in Fig 8. This can be advantageous when the forward link receiver also needs a tracking loop. In that case the functionality is reordered and the local loop filter (114) is part of the forward link receiver (120).

[0093] An additional advantage of the embodiment in Fig.3 and Fig.8 over the embodiment proposed in Fig.2 and Fig.7 is the limited error detector range required. In the embodiment of Fig.2, the offset detector estimator (111) range should cover the whole loop tracking range. This can be quite inefficient in computational power. In the embodiment of Fig.3, the forward link estimator measures the loop offset and the detection range can be limited to the maximum loop error. The loop tracking range can be more than 800 kHz for frequency and more than 10ms for time on a LEO pass, while the loop error should be a smaller than the subcarrier spacing for frequency and smaller than the CP length for time.

[0094] In 5GNR, the slower hub loop can be implemented via the existing advance mechanism for delay tracking. For carrier frequency corrections, a new control message is required for the slower hub loop.

[0095] An extra advantage of the local loop and combined loop solution is that the local time and frequency offsets estimates required for fast Doppler corrections can be based on the forward link waveform that is always present. Tracking can remain active even when there is no active return transmission. In particular, the 5GNR Physical Broadcast channel (PBCH) is always present in theforward link waveform and has the necessary Synchronisation signal block (SSB) allowing accurate time and frequency offset estimation.

[0096] In the tracking solutions presented above, the communication system silently assumes that the terminal gets synchronized to the network at startup or during terminal logon. A solution for the logon challenge over satellite links was proposed in 'Robust Logon Waveform Deployed in Large Mobile Satellite Access Networks' , (D. Delaruelle, Ka Conference, 2019) and US1145680 Bl.

[0097] The standardised 5GNR Physical Random Access Channel (PRACH) waveforms for terminal logon are designed for terrestrial networks (TN) and only allow limited time and frequency uncertainties. NTN logon without GNSS information and resulting high time and frequency uncertainties also requires some substantial changes to the 5GNR logon channel waveform. The solutions proposed in this invention allow reducing the GNSS dependency, limiting the GNSS need to only the logon phase.

[0098] First the error estimator (111, 202) quality is considered as this may have an impact on the overall tracking loop performance. Realistic estimators inaccuracies are considered in loop simulations presented below. The parameters and channels considered for the simulations are chosen for the sake of the illustrating the invention and are obviously not limiting the invention. The simulation considers the 5GNR physical uplink shared channel (PUSCH). The time error estimation is performed on known symbols, more specifically the demodulation reference symbols (DMRS), in the received waveform in the hub. As the DMRS overhead is more or less constant with the allocation bandwidth (or the number of allocated resource blocks (RB)), there are more DMRS symbols for higher bandwidth allocations and therefore the estimator quality is improving with higher bandwidth allocations. To avoid poor time error estimators in low bandwidth allocations, the DMRS density can be increased for lower bandwidths. Fig.9 shows time error estimator performance in function of SNR, number of RBs (denoted nrb) and DMRS overhead (denoted EDmrs and expressed in number of DMRS symbols per slot) for PUSCH transmissions. The estimator noise must be taken into account in the overall loop performance. The estimator does not have to strictly meet the accuracies specified above (0.15 ps), but a poor estimator requires long averages and limits the loop speed (slow loops) to limit the self-noise in the loop. If the loop is too fast, the estimator noise is dominant in loop tracking errors.

[0099] The frequency error estimator is based on two phase measurements consecutive in time and performed on known symbols, here DMRS symbols, spaced by one 5G-NR frame of 10 ms. The quality of the frequency estimate also changes with SNR. The hub loop update period is assumed to be one update per 5GNR frame of 10 ms. This requires and overhead message for each UE every 10 ms.

[0100] Fig.10 shows hub loop delay tracking results only accounting for terminal mobility (2 aero terminal mobility profiles) but including respective loop delays for GEO (Figs.10a and 10b) and LEO (Figs. 10c and 10d)) satellites. The Doppler effects from the satellite are not yet included for the sake of the analysis. The figures show transient delay changes due to mobility profile for circular and maximum acceleration profiles for different loop orders (denoted LpOrd). In each subfigure, the top figure shows the changing link delay and the tracked link delay (which appear to coincide as the error is much smaller than the absolute delay), the bottom figures show the differences between both corresponding to the tracking error. Showing both results on top of each other is interesting as it can be verified for which absolute delays the relative time error is highest. The dotted horizonal black line is the maximum allowed time misalignment for optimal performance for 15 kHz SCS, thus ± 0.15 ps. Figs.10a and 10b show the terminal mobility profiles with GEO satellite delay with loop time constants (denoted TC) of 0.5 second and 1 second. The damping factor is denoted d. Faster loops are not possible in GEO conditions due to link delay. Figs.10c and lOd show similar results for terminal mobility profiles with LEO satellite delay. The time constants are 0.2 and 0.5 seconds. TC can be smaller for LEO because of the smaller loop delay in LEO with respect to GEO, resulting in better tracking performance. Comparing Fig.lOa and 10c (or 10b and lOd), one can see that LEO conditions allows faster loops because of the shorter link delay, and consequently, better tracking of the mobility profiles delay changes. It is clear that a first order filter (LpOrderl) never achieves the requirements, even when only considering terminal mobility. A second order loop meets the requirement but has a non-zero steady state error for constant acceleration (Fig.lOb). The third order component will zero the steady state error for constant acceleration leaving only a transient at the acceleration change points.

[0101] Next the hub loop delay tracking performance is considered, only accounting for satellite delay variations, more specifically for LEO satellite movements, in Fig.ll. The low LEO orbit (denoted LEOOrbitL) (Fig.11a) is much more critical than the high orbit (denoted LEOOrbitH) (Fig. lib) because of the higher associated accelerations. The lower loop delay (compared to GEO), combined with the third order tracking loop gives good performances in both LEO cases, although a second order loop with TC = 0.2 also achieves requirements, but may be too risky when accounting for both satellite delay variations as well as terminal mobility. Furthermore, as mentioned below, second order loops have a steady state error in the presence of accelerations.

[0102] As shown on Fig.10 and 11, for none of the use cases, a first order loop meets the target requirements. The second and third order loops are better. The third order loop can zero the steady state error for constant acceleration segment, while the second order loop has a steady state error in the presence of accelerations (hence, when the slope of the delay changes), so the third order loop is preferred. This can best be seen in Fig.lOb.

[0103] UE mobility tracking in GEO conditions is more critical than LEO satellite mobility. For LEO satellites, the relative speeds and accelerations from the satellite are higher, but the loop delay is smaller, so allowing faster tracking and thus allowing to follow those variations better.Table 5: Delay tracking performance for GEO and LEO delay in all tested mobility profiles, showing RMS and peak errors for several SNR levels

[0104] Table 5 shows the RMS and peak errors for all the tested profiles for third order loops with selected optimal delay tracking loop parameters. For hub loop GEO profiles, the loop filter parameters are : Tc = 1 sec, = 1.4 and k3factor=10. For hub loop LEO profiles, the same filter is used with Tc=0.2 sec and for the local loop, the same filter parameters are used with Tc-0.05 sec. In LEO conditions the performance is mostly limited by the error detector performance confirmed by the same RMS and peak errors for most mobility profiles in the table. While for GEO, the performance is mostly limited by the loop speed confirmed by the similar performance for all SNR levels. One can see that the 0.15 ps peak error target is achieved for all profiles except the CircAero4 and the MaxAero4 with Geo delay loop. The fast timing and / or frequency offset variations can be tracked successfully with the local loop as shown in Table 5. In a preferred embodiment the advantages of both hub loop and local loopare combined. The local loop allowing fast tracking of UE movements is combined with the hub loop for UE clock offset corrections.

[0105] Similarly, to the time tracking results, Figs.l2a-b-c-d show frequency tracking results. In this case the black dotted lines show the frequency tracking success criteria of 150 Hz (valid for numerology 0 or 15 kHz SCS). Again, tracking terminal movement in the presence of LEO link delays (Fig.10c and Fig.lOd) is less critical than in the presence of GEO delays (Fig.lOa and Fig.10b) because faster loops are allowed. Similarly, a first order loop does not meet the requirement and the lower time constants give the best results. The frequency transients for the abrupt changes in acceleration create high error peaks.

[0106] The delay is proportional to the distance while the frequency is proportional to the speed. The constant acceleration thus is equivalent to a ramp profile for the frequency offset that can be zeroed with a second order loop. For the delay (Fig.10b) on the contrary a third order loop is required to zero the constant acceleration step. Consequently, in a preferred embodiment where the UE mobility effects are corrected by the local loop, a second order local loop is selected for the frequency tracking and a third order local loop is selected for the time tracking.

[0107] In a combined loop, where the hub loop corrects mainly the clock offsets, the preferred hub loop settings are a first order frequency tracking loop to zero the clock offset (between UE and gNB) and a second order loop delay tracking loop to zero the delay offset changes caused by misaligned clocks.

[0108] LEO satellite movements tracking performance is shown in Fig.13 for low (Fig.13a) and high (Fig.13b) pass profiles. A second order loop with Tc=0.2 or a third order loop with Tc=0.5 do comply with the required 150 Hz accuracy.

[0109] Table 6 shows all frequency tracking results for several Doppler profiles with Geo delay, Leo delay and local loop tracking. In local loop tracking no clock drift error is assumed as it cannot be tracked. (In local loop config, the clock drift must be corrected by another slower hub loop.) All tests are configured with the optimal third order loop with =1.4 and k3factor=10. For the GEO delay profiles the optimal Tc is 1 sec, for the LEO delay profiles the loop Tc is set to 0.2 sec and for the local tracking there is no delay in the loop and the loop time constant is put to 0.05 sec. The optimal values were derived by means of simulations but can also easily be explained. The faster the loop the better the tracking. However, the Tc cannot be shorter than the loop delay as this would result in an unstable oscillating loop.Table 6: Frequency tracking performance for GEO and LEO delay in all tested mobility profiles.Showing RMS and peak errors for several SNR levels

[0110] One can observe that the performance is mostly independent of the SNR for both the GEO and LEO loop delay cases. This is an indication that the performance is mostly limited by the loop delay and not by the estimator quality (as the latter does degrade for lower SNRs as shown in Fig.7). Only for the local loop the estimator accuracy can be limiting.

[0111] Assuming a maximum peak error of 150 Hz one can see that in GEO loop delay cases only the CircAerol and 3 profiles comply and that only the more stringent clock drift profile comply. For LEO profiles, the situation is better where most tested profiles comply. Clearly the local loop gives best results for Doppler offsets tracking. No local reference clock offset was assumed in the local loop case and an extra hub loop is required for tracking of the local reference clock offsets.

[0112] The better performance of the local loop for Doppler offset tracking motivates the use of a combination of the hub loop and the local loop. The local loop best track Doppler effects causedby terminal or satellite movement (but misses the required reference clock offset corrections) while the hub loop can be slower and only track the clock offsets. Terminal mobility with abrupt acceleration changes combined with GEO delay is more challenging than the LEO satellite movements tracking, despite the higher speeds and accelerations of the satellite.

[0113] In the embodiments above, a transparent or bent pipe satellite is assumed. The solution applies equally well for regenerative satellites where the hub is in the satellite. The only difference is that the loop delay is halved, and the hub loops can be made a bit faster. The presented results are thus the worst case as the bent pipe satellite loop delay was accounted for.

[0114] In the described embodiments, a satellite link was assumed. The solution also applies to other channel with high mobility or high link delay condition where the first order loops cannot track the time or frequency error changes fast enough. These could be other NTN channels or even TN channel with larger cells or fast moving UEs.

[0115] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention may be practiced in many ways. The invention is not limited to the disclosed embodiments.

[0116] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored / distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

Claims1. Terminal (100) for communication with a hub over a non-terrestrial network communication channel and arranged to perform time and / or frequency corrections in return link communication towards said hub, said terminal comprising a modulator (101) arranged to generate a return link signal to be transmitted, a forward link timing and / or frequency offset estimator (111) arranged to estimate an error signal indicative of a forward link timing and / or frequency offset, said forward link establishing a connection communication towards said terminal, open loop correction means comprising a local loop filter (114) and a converter (112) and arranged to receive said error signal indicative of said forward link timing and / or frequency offset and to output an error signal indicative of a return link timing and / or frequency offset, compensation means (103) arranged to receive said error signal indicative of said return link timing and / or frequency offset, to compensate for said return link timing and / or frequency offset and to output an indication of a time and / or frequency correction to said modulator to be applied when generating said return link signal to be transmitted to said hub.

2. Terminal (100) as in claim 1, wherein said error signal indicative of said forward link timing and / or frequency offset is said forward link timing and / or frequency offset itself and said error signal indicative of said return link timing and / or frequency offset is a tracked return link timing and / or frequency offset, and wherein said converter (112) is arranged to convert said forward link timing and / or frequency offset into said return link timing and / or frequency offset, the terminal comprising computation means (113) to determine a difference between said return link timing and / or frequency offset and said tracked return link timing and / or frequency offset and said local loop filter is arranged to receive said difference.

3. Terminal (100) as in claim 1, wherein said error signal indicative of said forward link timing and / or frequency offset is an error on a tracked forward link timing and / or frequency offset and said error signal indicative of said return link timing and / or frequency offset is said return link timing and / or frequency offset itself and wherein said local loop filter (114) is arranged to receive said error signal on said tracked forward link timing and / or frequency offset and said converter (112) is arranged to convert a filtered output signal of said local loop filter into said return link timing and / or frequency offset.

4. Terminal as in any of the previous claims, wherein a local loop comprising said local loop filter is arranged to compensate for return link timing and / or frequency offset originating from link delay changes.

5. Terminal as in any of the previous claims, wherein said local loop comprising said local loop filter has a time constant smaller than a total round trip time of said return link and said forward link delay.

6. System comprising one or more terminals (100) as in any of the previous claims and a hub arranged to demodulate said return link signal.

7. System as in claim 6, wherein said hub is arranged to estimate an error on said return link time and / or frequency and wherein said estimated error on the return link time and / or frequency is fed back in a hub loop to a further filter (102).

8. System as in claim 7 , wherein a local loop comprising said local loop filter is arranged to compensate for a return link time and / or frequency offset originating from link delay changes and wherein said hub loop is arranged to compensate for a return link timing and / or frequency offset originating from clock reference offsets between said hub and said one or more terminals.

9. System as in claim 7 or 8, wherein said further filter is a first order filter.

10. System as in any of claims 6 to 9, wherein said hub is arranged to transmit a 5GNR Physical Broadcast Channel Synchronisation Signal block for estimating said forward link time and / or frequency offset.

11. System as in any of claims 6 to 10, wherein said further filter is comprised in said hub.

12. System as in any of claims 6 to 11, wherein said hub loop has a time constant larger than said local loop time constant.

13. System as in any of claims 7 to 12, wherein said local loop filter is a second order filter and said further filter is a first order filter.

14. System as in any of claims 7 to 12, wherein said local loop filter is a third order filter and said further filter is a second order filter.

15. System as in in any of claims 6 to 11, comprising a regenerative satellite.

16. System as in any of the previous claims, wherein said modulator is an OFDM modulator.

17. Method for performing time and / or frequency correction in return link communication from a terminal (100) as in any of claims 1 to 5 to a hub (200) over a non-terrestrial network channel, the method comprising :- receiving in an open loop correction means of said terminal an error signal indicative of a timing and / or frequency offset in a forward link connection from said hub towards the terminal,- converting said error signal indicative of said forward link time and / or frequency error into an error signal indicative of a return link timing and / or frequency offset,- applying said error signal indicative of said return link timing and / or frequency offset to a compensation means (103) to compensate for said return link timing and / or frequency offset and to output an indication of time and / or frequency correction,1- generating in a modulator of said terminal a return link signal to be transmitted to a hub using said indication of time and / or frequency offset correction.

18. Method for performing time and / or frequency correction as in claim 17, wherein said timing and / or frequency offset is at least partially due to link delay changes between said terminal and said hub.

Images