A method and an apparatus for cyclic-prefix (CP) handling in OFDM-based on / off keying(OOK)generation for low power (LP) devices

Abstract

The invention relates to a method performed by a transmitter for transmitting an OOK signal within a cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) signal, comprising: obtaining an OOK slot s(n) that spans multiple OFDM symbols having FFT size N and cyclic-prefix length NCP, each symbol having duration NSymbol = N + NCP; determining a relative time shift d0 between an OOK component that conveys the OOK and a data component by evaluating, for candidate shifts d, a correlation metric indicative of a match between samples spaced by N across CP boundaries of per-symbol OOK signals, and selecting d0 that maximizes a sum of said correlations over the OOK slot; forming a shifted OOK slot s0(n) = s(n − d0); for each OFDM symbol i in the shifted OOK slot, deriving an OOK symbol xi(n) and splitting xi(n) into a CP portion xi cp(n) of length NCP and a main portion xi main(n) of length N; generating an OFDM-based OOK waveform from the split portions; combining the generated OOK waveform with a data waveform on disjoint subcarrier sets; and transmitting the resulting CP-OFDM signal.

Description

[0001] DESCRIPTION

[0002] A METHOD AND AN APPARATUS FOR CYCLIC-PREFIX (CP) HANDLING IN OFDMBASED ON / OFF KEYING(OOK) GENERATION FOR LOW POWER (LP) DEVICES

[0003] TECHNICAL FIELD

[0004] The invention relates to a method for low power communication based on OOK in a compatible manner with OFDM systems. We propose a method to mitigate the effect of CP insertion which is essential in OFDM system.

[0005] PRIOR ART

[0006] Orthogonal frequency-division multiplexing (OFDM)-based on-off keying (OOK) refers to conveying a simple amplitude envelope (ON / OFF) using, or alongside, an OFDM waveform. Instead of dedicating a separate narrowband wake-up carrier, the transmitter shapes a portion of the multicarrier signal so that, in the time domain, the composite waveform exhibits a detectable pattern of “on” and “off” intervals. A conventional data receiver can continue to demodulate its subcarriers as usual, while a low-power wake-up radio only needs to sense the presence or absence of energy according to a known pattern. Also, this OOK waveform can be used to convey information bits to low power devices such as Ambient loT devices.

[0007] In cyclic prefix (CP)-OFDM, each N-sample symbol is preceded by a CP formed by copying its last NCP samples to the front; when an OOK signal is conveyed as a time-domain envelope, this prefixing step generally disrupts the intended ON / OFF shape at symbol boundaries because the first NCP samples of the transmitted symbol repeat the tail of the previous useful part rather than the start of the desired envelope. As a result, the per-symbol OOK envelope is no longer continuous across the CP / main junction, producing a mismatch that degrades WUS detectability at a low-power wake-up radio and significantly deteriorates the BER performance.

[0008] Numerous studies have investigated the cyclic prefix (CP) problem in OFDM-based OOK generation. One approach involves identifying the CP part, either discarding it [2] or benefiting from it in other aspects such as error correction and improved detection capabilities [2], The low power (LP) device may assume the same CP length to simplify the process. Otherwise, the duration between transition edges is used to determine both the location and length of CP[5]. To enable this, accurate synchronization is necessary, and this is also studied in [3]. This remains quite challenging for practical low power devices as discussed in [5]. Even if we assume the LP device has a local oscillator capability which cannot be done by passive devices, the Sampling frequency offset is still a huge issue which makes this type of approach impractical for the great majority of devices.

[0009] Another method involves leaving guard zeros in the CP location to avoid symbol transition in that interval [4], More generally, another method ensures that inserting CP will have no transition edge between the last OOK chip of the current OFDM symbol and the first chip from the next OFDM symbol [5]. Using Manchester Line Code (MLC), this mitigates the CP effect to some extent. However, this has the drawback of being spectrally inefficient, and the performance still degrades, especially for large numbers of chips per symbol.

[0010] [1] WO2024237836A1 METHODS FOR CYCLIC PREFIX HANDLING FOR LOW-POWER RECEIVERS.

[0011] [2] Jian, R., Ding, Y., Qu, M., Miao, J., Tang, X., Huang, W., & Zhang, Y. (2024, July). Ambient loT : Insight and Challenge of Enabling Technologies for Future Study. In 20246th International Conference on Electronics and Communication, Network and Computer Technology (ECNCT) (pp. 451-458). IEEE.

[0012] [3] WO2024236511 A1 ENERGY EFFICIENT OOK BASED SYNCHRONIZATION SIGNAL

[0013] [4] R1- 2408537 : ” LP-WUS and LP-SS design”

[0014] [5] R1 -2409311 : ” Ambient loT - General aspects of physical layer design”

[0015] [6] 3GPP TR 38.869, “Study on low-power Wake-up Signal and Receiver for NR,” Dec. 2023.

[0016] [7] 3GPP TR 38.848, “Study on Ambient loT (Internet of Things) in RAN,”.

[0017] All the problems mentioned above have made it necessary to make an innovation in the relevant technical field as a result.BRIEF DESCRIPTION OF THE INVENTION

[0018] The present invention relates to method and system to eliminate the above-mentioned disadvantages and bring new advantages to the relevant technical field.

[0019] An object of the invention is to increase wake up signal detectability in OFDM based OOF.

[0020] Another object of the invention is to enhance reliability of OOK detection at low power devices.

[0021] An object of the invention is decrease the negative effect of cyclic prefix on spectral efficiency when performing OFDM based OOF.

[0022] Another object of the invention is to provide OFDM based OOF to devices having reduced complexity in order to wake up low complex devices.

[0023] Another object of the invention is to provide a method works for both legacy data priority and OOK priority.

[0024] Another object of the invention is to provide a method works for both legacy data priority and OOK priority.

[0025] To achieve all the objects mentioned above and that will emerge from the following detailed description, the present invention relates to a method performed by a transmitter to transmit a OOK signal within a cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) signal, Accordingly,

[0026] - obtaining an OOK slot s(n) that spans multiple OFDM symbols having FFT size N and cyclic-prefix length NCp, each symbol having duration NSymboi = N + NCp;

[0027] - determining a relative time shift do between an OOK component and a data component by evaluating, for candidate shifts d, a correlation metric indicative of a match between samples spaced by N across CP boundaries of per-symbol OOK signals, and selecting d0that maximizes a sum of said correlations over the OOK slot;

[0028] - forming a shifted slot s0(n) = s(n - d0);

[0029] - for each OFDM symbol i in the shifted slot, deriving an OOK symbol Xi(n) and splitting Xi(n) into a CP portion Xicp(n) of length NCp and a main portion Ximain(n) of length N;

[0030] - generating an OFDM-based OOK waveform for the OOK from the split portions;

[0031] - combining the generated OOK waveform with a data waveform on disjoint subcarrier sets; and- transmitting the resulting CP-OFDM signal. Thus, mismatch caused by cyclic prefix is reduced and spectral efficiency is increased.

[0032] A possible embodiment of the invention is characterized in that the correlation metric is a sum over i of expectations E{x*(n - d) ■ Xi(n - d + N)} computed for samples n straddling CP boundaries.

[0033] Another possible embodiment of the invention is characterized in that candidate shifts d are integers in [0, NSymboi - 1] and the search resolution is at sample or chip granularity.

[0034] Another possible embodiment of the invention is characterized in that determining do uses an approximated OOK slot generated via DFT-s-OFDM mapping or a least-squares approximation of a desired OOK envelope.

[0035] Another possible embodiment of the invention is characterized in that further comprising: - storing, in a look-up table, a mapping from candidate OOK sequences to respective values of do; and

[0036] - upon preparing to transmit an slot, retrieving a stored do for the corresponding sequence and using the retrieved do in place of performing the determining, for reducing computation resources during transmission. Thus, reducing resources used for determining the time shift.

[0037] Another possible embodiment of the invention is characterized in that determining do uses an ideal OOK slot model for computing the correlation metric.

[0038] Another possible embodiment of the invention is characterized in that further comprising: - when a stored d0is unavailable for a given OOK sequence, performing the determining to obtain d0; and

[0039] - inserting an entry for the given OOK sequence and the obtained d0into the look-up table for subsequent use.

[0040] Another possible embodiment of the invention is characterized in that storing comprises compressing the look-up table by grouping OOK sequences that are shift-equivalent under circular rotation within Nsymboi and assigning a common representative do.Another possible embodiment of the invention is characterized in that further comprising storing a policy that selects, at run time, either an OOK-prioritizing mode or a legacy-data-prioritizing mode according to device class, link quality, or energy budget.

[0041] Another possible embodiment of the invention is characterized in that generating the OFDMbased OOK waveform comprises an OOK-prioritizing mode that includes:

[0042] - producing a first OOK signal from Xicp(n) by zeroing samples outside CP locations in the main part after mapping to subcarriers for OFDM modulation;

[0043] - producing a second OOK signal from Ximain(n) by zeroing samples at CP locations after mapping to subcarriers for OFDM modulation;

[0044] - combining the first and second OOK signals to form the final OOK signal; and

[0045] - adding the OOK signal to the time-domain data waveform after cyclic-prefix insertion is done for the latter.

[0046] Another possible embodiment of the invention is characterized in that the first and second OOK signals are produced by parallel inverse-FFT chains and combined by sample-wise summation after cyclic-prefix insertion.

[0047] Another possible embodiment of the invention is characterized in that the zeroing operations enforce non-overlap between CP and main contributions and reduce leakage into data subcarriers.

[0048] Another possible embodiment of the invention is characterized in that the data waveform remains unchanged relative to a legacy CP-OFDM transmitter.

[0049] Another possible embodiment of the invention is characterized in that generating the OFDMbased OOK waveform comprises a legacy-data-prioritizing mode that includes:

[0050] - discarding the CP portion Xicp(n) of each OOK symbol;

[0051] - mapping the main portion Ximain(n) to subcarriers using conventional DFT-s-OFDM-based OOK that preserves orthogonality with the data subcarriers;

[0052] - inserting the cyclic prefix; and

[0053] - transmitting the result together with the data waveform.

[0054] Another possible embodiment of the invention is characterized in that discarding the CP portion comprises zeroing CP-indexed samples prior to OFDM modulation.Another possible embodiment of the invention is characterized in that orthogonality between WUS subcarriers and data subcarriers is exact at the transmitter, and residual CP mismatch in the transmitted waveform is minimized by the selected do.

[0055] Another possible embodiment of the invention is characterized in that OOK and data occupy disjoint subcarrier sets such that the data receiver is transparent to the WUS and the WuR is transparent to the data.

[0056] Another possible embodiment of the invention is characterized in that the WUS OOK sequence results from line coding using Manchester, NRZ, RZ, or 4B5B coding.

[0057] Another possible embodiment of the invention is characterized in that the WUS slot employs M chips per OFDM-symbol duration with NChiP= Nsymboi / M, and the shift search is performed at chip boundaries.

[0058] Another possible embodiment of the invention is characterized in that applied to wake-up radio signaling or Ambient-loT, further comprising inserting a guard of G samples between adjacent OOK slots.

[0059] Another possible embodiment of the invention is characterized in that further comprising caching so(n) for reuse when retransmitting an identical OOK sequence.

[0060] Another possible embodiment of the invention is characterized in that the transmitter is a wireless transmit / receive unit selected from a base station, access point, relay, mobile device, or loT gateway.

[0061] The disclosed invention leverages two distinct OFDM prefix schemes to implement On-Off Keying (OOK) modulation inherently. This approach offers several notable advantages:

[0062] Spectral Efficiency: Because the CP is actually used as part of the OOK signal, the spectral efficiency is maximized compared to all existing methods to handle the problem.

[0063] Flexible design: Available alternative for the implementation depending on whether the application requires legacy data priority or OOK priority.

[0064] Power Efficiency and low Complexity: Especially at the device side since it is no longer required to identify and discard CP, which consumes much of its energy.Backward Compatibility: The invention uses basic blocks of conventional OFDM based OOK, without any major changes. Plus, the low power (LP) device also remains the same with minimum required complexity. This facilitates easier integration and deployment without requiring significant modifications to current infrastructure.

[0065] ON-OFF Keying (OOK) generation aligns closely with the ongoing 3GPP standardization initiatives, particularly those in Release 18 and subsequent studies emphasizing energy efficiency and network optimization. It plays a crucial role in two key technologies under 3GPP's focus: Ambient loT (outlined in 3GPP TR 38.848 [7]) and Wake-up Radio (WUR) (described in 3GPP TR 38.869 [6]). Both technologies primarily rely on OFDM-based OOK to realize the vision of low-power communication. Consequently, OFDM-based OOK is essential to the 5G ecosystem while also laying the groundwork for future 6G needs. As 3GPP's work on 6G prioritizes ultra-low latency, enhanced energy efficiency, and advanced loT connectivity, the significance of Amb-loT and WUR continues to grow.

[0066] BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 : depicts an example of the desired OOK signal for one-OFDM symbol duration.

[0068] Figure 2: depicts an example of the generated OOK signal after inserting the CP.

[0069] Figure 3: shows an example of the CP+OFDM based OOK generation.

[0070] Figure 4: shows the desired OOK slot in time based on the proposed system.

[0071] Figure 5a: depicts an example of implementation of the proposed method that prioritizes OOK.

[0072] Figure 5b: depicts an example of implementation of the proposed method that prioritizes legacy data.

[0073] DETAILED DESCRIPTION OF THE INVENTION

[0074] In this detailed description, the subject matter is explained with references to examples without forming any restrictive effect only in order to make the subject more understandable.The invention is a method for transmitting an OOK signal to a low power device within a cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) waveform, performed by a transmitter that also carries conventional data on disjoint subcarriers.

[0075] A transmitter (not shown) generates and delivers multicarrier waveforms and related control signaling. The transmitter includes one or more processing units configured to map information onto carrier resources, assemble symbols and slots, and shape, filter, and schedule transmissions according to system timing. The transmitter can allocate resources for different functions, such as conventional data, control indicators, and wake-up signaling. It can apply optional precoding, convert between frequency and time domains, add cyclic prefixes, and combine multiple components prior to transmission. Memory provides configuration parameters and stored patterns or sequences. A controller coordinates resource allocation, timing, and mode selection. Examples of suitable transmitters include any wireless transmitter / receiver unit (WTRU) such as a base station, gNB, access point, relay, loT gateway, or a user equipment device acting as a beacon source. Any of these may implement OFDM or DFT-s-OFDM processing chains with subcarrier mapping, inverse fast Fourier transform, cyclic-prefix handling, and symbol / slot framing. Optionally, the transmitter can perform line coding on a bit stream intended for a wake-up function, apply chip-level shaping, and buffer or retrieve predefined sequences from memory.

[0076] A receiver includes at least two functional radios. A main radio performs conventional reception for data and control. It can acquire timing, remove cyclic prefixes, transform symbols to the frequency domain, estimate and equalize channels, demap subcarriers, and decode higher-layer information. It may support sleep or low-duty-cycle operation under control of local policies or external triggers. A wake-up radio operates as a low-power detector specialized for wake-up signaling. Functionally, it can filter an input band, condition gain automatically, sample or envelope-detect a received waveform, and evaluate simple detection metrics such as energy, correlation with a reference pattern, or matched-filter outputs. The wake-up radio can adapt thresholds, manage false alarms, and, upon a positive decision, generate an event that powers or enables the main radio. The wake-up radio is designed for minimal energy consumption and can operate continuously or with very high duty cycle using reduced-bandwidth processing, coarse timing requirements, and simplified analog and digital blocks. In some implementations the two radios share certain front-end components, clocks, or buffers; in others, the wake-up path is implemented alongside but independently of the main radio to further reduce power draw. In operation, the transmitter may designate a subset of resources for wake-up signaling and a different subset for data, schedule these according to systemtiming, and provide configuration so that the receiver can recognize and act on a wake-up indication. The receiver continuously or near-continuously monitors with the low-power wakeup radio while the main radio is idle or in a reduced-power state. When the wake-up radio declares a valid indication, the device can transition the main radio to an active state and proceed with conventional reception. This arrangement enables very low standby power while maintaining responsiveness to external triggers. The components and capabilities described above establish an enabling environment for the subsequent detailed description of example signals, figures, and processing steps. The receiver may also be any wireless transmitter / receiver unit (WTRU) such as a mobile device or an ambient loT device with similar capabilities to those of the wake up receiver mentioned above.

[0077] The invention is a method for transmitting a wake-up signal (WUS) intended for a wake-up radio (WuR) within a multicarrier waveform that also carries conventional data. The WUS is conveyed as an on-off keyed (OOK) envelope synthesized using a subset of resources that are disjoint from those used by the data. The transmitter and receiver capabilities described above provide the enabling environment for the following signal definitions, figures, processing steps, and embodiments.

[0078] Let N denote the FFT size and NCP the cyclic-prefix length; one OFDM symbol has duration Nsymboi = N + NCp samples. An OOK is represented as an OOK-keyed slot s(n) that spans multiple OFDM symbols in time. For the i-th symbol interval in the slot, the corresponding OOK segment is Xi(n). When indexed over Nsymboi samples, Xi(n) can be viewed as the sum of a main portion Ximain(n) of length N and a cyclic-prefix portion Xicp(n) of length NCP. In some implementations, a line-coded bit stream (for example Manchester) defines M chips per symbol, with chip duration NChiP= Nsymboi / M.

[0079] Figure 1 depicts an example of the desired OOK signal for one OFDM-symbol duration. The ideal envelope over Nsymboi samples is shown together with an approximate envelope that would be synthesized from the reserved resources. Figure 2 depicts an example of the OOK signal after cyclic-prefix insertion; having CP per symbol may disturb the intended envelope at symbol boundaries spaced by N samples, illustrating acyclic-prefix-induced mismatch. Figure 3 shows an example of CP+OFDM-based OOK generation in which OOK samples are mapped to the reserved resources, transformed to the time domain, and appended with a cyclic prefix, optionally in parallel with a data chain on disjoint resources. Figure 4 shows a slot-length view of the desired OOK signal, where s(n) spans multiple OFDM symbols and defines per-symbol segments Xi(n). Figure 5a depicts an implementation that prioritizes OOK signal fidelity by handling the CP and main portions in complementary branches; Figure 5b depicts anotherimplementation that prioritizes legacy data processing by discarding CP-indexed OOK samples and preserving strict orthogonality to data resources.

[0080] The general concept is to treat the OOK as a slot-length object and align it relative to the CP-OFDM structure so that the cyclic-prefix-induced envelope mismatch is jointly minimized across all symbols of the slot. The transmitter introduces a relative time shift d between the OOK component and the data component prior to cyclic-prefix insertion and selects an optimal shift d0according to a slot-wide metric. One suitable metric is S(d) =

[0081]

[0082] E[ x‘(n - d) ■ Xi(n - d + N) ], evaluated over samples n at the the CP / main boundary of each Xi(-). The selected shift d0= arg max d S(d) yields a shifted WUS slot s0(n) = s(n - d0) with improved continuity across CP boundaries. The search can be performed at sample resolution or chip resolution, with d drawn from {0, 1, ..., Nsymboi - 1}. The shift may be computed on an ideal model of s(n) or on an approximated OOK constructed by DFT-s-OFDM or least-squares fitting to the reserved resources so that practical synthesis constraints are reflected.

[0083] With s0(n) available, the transmitter derives per-symbol segments Xi(n) and, for implementation convenience, splits each into Xicp(n) and Ximain(n). The data component is generated on a disjoint resource set using a conventional CP-OFDM or DFT-s-OFDM chain. The OOK component is generated in one of two selectable implementations.

[0084] In the implementation of Figure 5a (OOK prioritization), two complementary OOK branches are formed. A first branch retains only the CP-indexed samples derived from Xicp(n) by zeroing samples within the N-sample main region; a second branch retains only the main-indexed samples derived from Ximain(n) by zeroing samples at CP locations. Each branch maps its retained samples to the reserved resources, performs the inverse transform, and inserts a cyclic prefix. The resulting time-domain OOK signals are combined, for example by samplewise summation, to form the final OOK signal, which is then combined with the data waveform after cyclic-prefix insertion is done for the latter. Because the branches are complementary in time, non-overlap is enforced between CP and main contributions, reducing leakage of OOK energy into data resources while preserving the intended envelope across Nsymboi.

[0085] In the implementation of Figure 5b (legacy-data prioritization), the CP portion Xicp(n) is discarded, or equivalently CP-indexed samples are zeroed before modulation. Only Ximain(n) is mapped to the reserved resources using conventional DFT-s-OFDM-based OOK; a cyclic prefix is then inserted. This preserves strict resource orthogonality between the OOK component and the data component. Although residual CP mismatch may remain in the timedomain OOK signal, the prior slot-level alignment using d0reduces the discontinuity and improves low power receiver detectability while leaving the legacy data path unchanged.To reduce computation during transmission, a look-up table stored in memory can map candidate OOK sequences to respective shifts d0. When preparing to transmit, if a sequence matches an entry, the stored d0is retrieved and used in place of running the shift search. If no entry exists, the transmitter computes d0for that sequence, uses it for the current slot, and inserts a new entry thereafter. The table can be compressed by grouping sequences that are shift-equivalent under circular rotation within Nsymboi and assigning a common representative d0. A run-time policy may select between the Figure 5a and Figure 5b implementations according to device class, link quality, or energy constraints.

[0086] The OOK sequence that defines the WUS may result from line coding, such as Manchester, NRZ (Non-Return-to-Zero), RZ (Return-to-Zero), or 4B5B (Four-Bit / Five-Bit (block) line coding), and can be chip-shaped if desired. The reserved resources for OOK generation may be contiguous or non-contiguous, and the arrangement applies to CP-OFDM as well as DFT-s-OFDM. The guard between adjacent WUS slots may be set to G samples if system timing calls for an inter-slot separation. Alternative continuity metrics equivalent to S(d) can be employed, for example minimizing the squared difference between boundary samples of Xi(n - d) and Xi(n - d + N) or maximizing a normalized correlation. The search resolution for d can be adapted to the chip structure or to implementation limits.

[0087] For Wake up receivers, the device uses its low-power wake-up radio to monitor for the WUS while the main radio is idle or in a reduced-power state. Because the WUS and data occupy disjoint resources and the transmitter has aligned the WUS across CP boundaries, the WuR can operate with simplified timing requirements, for example by using energy detection, noncoherent correlation, or a simple matched filter to a known chip shape. Upon declaring a valid WUS, the device can enable the main radio to proceed with conventional reception and higher-layer processing.

[0088] Across the embodiments described, the method preserves spectral efficiency by avoiding additional pilots or guard symbols for alignment, avoids reliance on precise timing at the WuR, and remains compatible with legacy data processing. The selectable implementations provide a choice between emphasizing the fidelity of the OOK envelope or preserving strict data-path orthogonality, and the optional look-up table reduces processing cost when WUS sequences recur.

[0089] The scope of protection of the invention is specified in the attached claims and cannot be limited to those explained for sampling purposes in this detailed description. It is evident that a person skilled in the art may exhibit similar embodiments in light of the above-mentioned facts without drifting apart from the main theme of the invention.

Claims

CLAIMS1. A method performed by a transmitter for transmitting an OOK signal within a cyclic- prefix orthogonal frequency-division multiplexing (CP-OFDM) signal, comprising: - obtaining an OOK slot s(n) that spans multiple OFDM symbols having FFT size N and cyclic-prefix length NCp, each symbol having duration NSymboi = N + NCp;- determining a relative time shift do between an OOK component that conveys the OOK signal and a data component by evaluating, for candidate shifts d, a correlation metric indicative of a match between samples spaced by N across CP boundaries of per-symbol OOK signals, and selecting d0that maximizes a sum of said correlations over the OOK slot;- forming a shifted OOK slot s0(n) = s(n - d0);- for each OFDM symbol i in the shifted OOK slot, deriving an OOK symbol Xj(n) and splitting Xj(n) into a CP portion Xicp(n) of length NCp and a main portion Ximain(n) of length N;- generating an OFDM-based OOK waveform from the split portions;- combining the generated OOK waveform with a data waveform on disjoint subcarrier sets; and- transmitting the resulting CP-OFDM signal.

2. The method of claim 1 , wherein the correlation metric is a sum over i of expectations E{Xi*(n - d) ■ Xi(n - d + N)} computed for samples n straddling CP boundaries.

3. The method of claim 1, wherein candidate shifts d are integers in [0, NSymboi - 1] and the search resolution is at sample or chip granularity.

4. The method of claim 1, wherein determining do uses an approximated OOK slot generated via DFT-s-OFDM mapping or a least-squares approximation of a desired OOK envelope.

5. The method of claim 1 , wherein determining do uses an ideal WUS slot model.

6. The method of claim 1 , further comprising:- storing, in a look-up table, a mapping from candidate WUS OOK sequences to respective values of d0; and- upon preparing to transmit an OOK slot, retrieving a stored d0for the corresponding sequence and using the retrieved d0in place of performing the determining, for reducing computation resources during transmission.

7. The method of claim 6, further comprising:- when a stored d0is unavailable for a given OOK sequence, performing the determining to obtain do; and- inserting an entry for the given OOK sequence and the obtained do into the look-up table for subsequent use.

8. The method of claim 6, wherein storing comprises compressing the look-up table by grouping OOK sequences that are shift-equivalent under circular rotation within NSymboi and assigning a common representative do.

9. The method of claim 6, further comprising storing a policy that selects, at run time, either an OOK-prioritizing mode or a legacy-data-prioritizing mode according to device class, link quality, or energy budget.

10. The method of claim 1, wherein generating the OFDM-based OOK waveform comprises an OOK-prioritizing mode that includes:- producing a first OOK signal from Xicp(n) by zeroing samples outside CP locations in the main part and mapping to subcarriers for OFDM modulation;- producing a second OOK signal from Ximain(n) by zeroing samples at CP locations and mapping to subcarriers for OFDM modulation;- inserting a cyclic prefix into each of the first and second OOK signals;- combining the first and second OOK signals to form the OOK component; and - adding the OOK component to the time-domain data waveform after cyclic-prefix insertion.11 . The method of claim 10, wherein the first and second OOK signals are produced by parallel inverse-FFT chains and combined by sample-wise summation after cyclic- prefix insertion.

12. The method of claim 10, wherein the zeroing operations enforce non-overlap between CP and main contributions and reduce leakage into data subcarriers.

13. The method of claim 10, wherein the data waveform remains unchanged relative to a legacy CP-OFDM transmitter.

14. The method of claim 1, wherein generating the OFDM-based OOK waveform comprises a legacy-data-prioritizing mode that includes:- discarding the CP portion Xicp(n) of each OOK symbol;- mapping the main portion Ximain(n) to subcarriers using conventional DFT-s-OFDM- based OOK that preserves orthogonality with the data subcarriers;- inserting the cyclic prefix; and- transmitting the result together with the data waveform.

15. The method of claim 14, wherein discarding the CP portion comprises zeroing CP- indexed samples prior to OFDM modulation.

16. The method of claim 14, wherein orthogonality between OOK subcarriers and data subcarriers is exact at the transmitter, and residual CP mismatch in the transmitted waveform is minimized by the selected do.

17. The method of claim 1 , wherein OOK and data occupy disjoint subcarrier sets such that the data receiver is transparent to the OOK signal and the low power receiver is transparent to the data.

18. The method of claim 1 , wherein the OOK sequence results from line coding using Manchester, NRZ, RZ, or 4B5B coding.

19. The method of claim 1, wherein the OOK slot employs M chips per OFDM-symbol duration with NchiP= NSymbOi / M, and the shift search is performed at chip boundaries.

20. The method of claim 1, applied to wake-up radio signaling or Ambient-loT, further comprising inserting a guard of G samples between adjacent WUS slots.21 . The method of claim 1 , further comprising caching So(n) for reuse when retransmitting an identical OOK slot sequence.

22. The method of claim 1, wherein the transmitter is a wireless transmit / receive unit selected from a base station, access point, relay, mobile device, or loT gateway.

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