TECHNICAL FIELD

Various techniques are related to Orthogonal Frequency Division Multiplex Modulation and non-coherent decoding of respective signals.

BACKGROUND

Wireless communication employs battery-powered devices (hereinafter, terminals) that can connect to an access node to transmit and/or receive data. To reduce energy consumption, low-power modes are sometimes employed. When the terminal is operated in such a low-power mode, an associated access node transmits an appropriate signal to prepare the terminal for subsequent communication of data (a process sometimes referred to as paging).

There are various paging signals known that are employed in connection with paging. A new concept of paging signals, the so-called wake-up signal (WUS), has been introduced in the Third Generation Partnership (3GPP) to Machine Type Communication (MTC) and Narrowband Internet of Things (NB-loT) protocols. The objective of the WUS is to reduce the total energy cost in the UE for listening for paging. The WUS is expected to be sent prior to further paging signals, such as a paging indicator on a physical data control channel. Examples of physical data control channels include Physical Downlink Control Channel (PDDCH) or MTC PDDCH (MPDCCH) or NB-loT PDCCH (NPDCCH). The terminal may selectively decode the physical data control channel and the subsequent data shared channel - such as the Physical Data Shared Channel (PDSCH) - for a further paging signal, the paging message, upon detecting the WUS.

Example implementations of WUSs are described in 3GPP TSG RAN Meeting #74 contribution RP-162286“Motivation for New Wl on Even further enhanced MTC for LTE”; 3GPP TSG RAN Meeting #74 contribution RP-162126“Enhancements for Rel- 15 eMTC/NB-loT”; and 3GPP TSG RAN WG1 #88 R1 -1703139“Wake Up Radio for NR”. See 3GPP TSG RAN WG2#99 R2-1708285.

SUMMARY

A need exists for advanced techniques of wireless communication.

This need is met by the features of the independent claims. The features of the dependent claims define embodiments.

A method includes obtaining a reference signal waveform which is defined in accordance with a non-coherent modulation scheme. The method also includes shaping the reference signal waveform to obtain at least one signal waveform associated with one or more subcarriers of a plurality of subcarriers. The method also includes inputting the at least one signal waveform to at least one corresponding channel of a multi-channel orthogonal frequency division multiplex, OFDM, modulator. The method further includes transmitting an OFDM symbol output by the OFDM modulator. Said shaping approximates a baseband representation of a part of the OFDM symbol associated with the one or more subcarriers to the reference signal waveform.

For example, it would be possible that said obtaining of the reference signal waveform includes cutting and cropping a further reference signal waveform. As such, the reference signal waveform can correspond to a segment of the further reference signal waveform. The further reference signal waveform can also be defined in accordance with the non-coherent modulation scheme. The cutting an cropping can be at sections of the further reference signal waveform which are mapped to cyclic prefixes (CPs) of the OFDM symbol and a further OFDM symbol. The OFDM symbol and the further OFDM symbol can be adjacent to each other in a sequence of OFDM symbols.

It is possible that the sequence of OFDM symbols is transmitted. Flere, it would be possible that the further reference signal waveform is cut and cropped to obtain multiple respective segments, each segment being associated with a respective OFDM symbol of the sequence; and the shaping and OFDM modulation can be performed for each segment.

The further reference signal waveform may hence be longer than the reference signal waveform. By cutting and cropping, it becomes possible to fit a section of the further reference signal waveform - which corresponds to the reference signal waveform - into the OFDM symbol.

A device is configured to obtain a reference signal waveform which is defined in accordance with a non-coherent modulation scheme. The device is further configured to shape the reference signal waveform to obtain at least one signal waveform associated with one or more subcarriers of a plurality of subcarriers. The device is further configured to input the at least one signal waveform to at least one corresponding channel of a multi-channel orthogonal frequency division multiplex, OFDM, modulator. The device is further configured to transmit an OFDM symbol output by the OFDM modulator. The shaping approximates a baseband representation of a part of the OFDM symbol associated with the one or more subcarriers to the reference signal waveform.

For example, the device may be a UE, an access point, a base station, etc..

A method includes receiving a sequence of OFDM symbols. The OFDM symbols each comprise a plurality of subcarriers. The method also includes applying a bandpass filter to the received sequence of OFDM symbols. The bandpass filter is aligned with one or more predefined subcarriers of the plurality of subcarriers. The method also includes demodulating the bandpass filtered received sequence of OFDM symbols based on a, typically, non-coherent modulation scheme.

A device includes a low-power receiver. The low-power receiver is configured to receive a sequence of OFDM symbols. The OFDM symbols each comprise a plurality of subcarriers. The low-power receiver is also configured to apply a bandpass filter to the received sequence of OFDM symbols. The bandpass filter can be aligned with one or more predefined subcarriers of the plurality of subcarriers. The low-power receiver is also configured to demodulate the bandpass filtered received sequence of OFDM symbols based on a non-coherent modulation scheme.

Here, receiving can correspond to obtaining a signal in the RF domain at an analog front-end of the low-power receiver. By demodulating the bandpass filtered received sequence of OFDM symbols, a WUS can be detected.

A method includes cutting and cropping a reference signal waveform at sections of the reference signal waveform mapped to cyclic extensions of OFDM symbols of a sequence of OFDM symbols. By said cutting and cropping, reference signal waveform segments are obtained. The reference signal waveform and the reference waveform segments are defined in accordance with a non-coherent modulation scheme. The method further comprises for each one of the reference signal waveform segments: shaping the respective reference signal waveform segment to obtain a respective at least one signal waveform associated with one or more subcarriers of a plurality of subcarriers; and inputting the respective at least one signal waveform to at least one corresponding channel of a multi-channel OFDM modulator; and transmitting a respective OFDM symbol of the sequence of OFDM symbols output by the OFDM modulator. Said shaping approximates a baseband representation of a part of the OFDM symbol associated with the one or more subcarriers to the reference signal waveform segment.

It is generally possible that all signal waveforms obtained from the shaping are input to the same channel(s) of the multi-channel OFDM modulator.

A device is configured to cut and crop a reference signal waveform at sections of the reference signal waveform mapped to cyclic extensions of OFDM symbols of a sequence of OFDM symbols. By said cutting and cropping, reference signal waveform segments are obtained. The reference signal waveform and the reference waveform segments are defined in accordance with a non-coherent modulation scheme. The device is further configured to perform, for each one of the reference signal waveform segments: shaping the respective reference signal waveform segment to obtain a respective at least one signal waveform associated with one or more subcarriers of a plurality of subcarriers; and inputting the respective at least one signal waveform to at least one corresponding channel of a multi-channel OFDM modulator; and transmitting a respective OFDM symbol of the sequence of OFDM symbols output by the OFDM modulator. Said shaping approximates a baseband representation of a part of the OFDM symbol associated with the one or more subcarriers to the reference signal waveform segment.

A method includes receiving a sequence of OFDM symbols comprising a plurality of subcarriers. The method further includes applying a bandpass filter to the received sequence of OFDM symbols, the bandpass filter being aligned with one or more predefined subcarriers of the plurality of subcarriers. The method also includes demodulating the bandpass filtered received sequence of OFDM symbols based on a non-coherent modulation scheme.

A device is configured to receive a sequence of OFDM symbols comprising a plurality of subcarriers. The device is further configured to apply a bandpass filter to the received sequence of OFDM symbols, the bandpass filter being aligned with one or more predefined subcarriers of the plurality of subcarriers. The device is further configured to demodulate the bandpass filtered received sequence of OFDM symbols based on a non-coherent modulation scheme.

In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a cellular network relying on paging of a terminal according to various examples. FIG. 2 schematically illustrates subcarriers of an orthogonal frequency division multiplex (OFDM) scheme and further schematically illustrates on-off-keying according to various examples.

FIG. 3 schematically illustrates multiple channels and allocated resource elements in an OFDM resource grid according to various examples.

FIG. 4 schematically illustrates a base station according to various examples.

FIG. 5 schematically illustrates a terminal according to various examples.

FIG. 6 schematically illustrates a main receiver and a low-power receiver according to various examples.

FIG. 7 schematically illustrates a main receiver and a low-power receiver according to various examples.

FIG. 8 schematically illustrates communication between a UE and a base station according to various examples.

FIG. 9 schematically illustrates a wireless interface and an antenna according to various examples.

FIG. 10 schematically illustrates details of the wireless interface of FIG. 9 according to various examples.

FIG. 1 1 schematically illustrates WUS waveforms according to various examples.

FIG. 12 schematically illustrates a wireless interface and an antenna according to various examples.

FIG. 13 schematically illustrates fitting a reference signal waveform into OFDM symbols of a sequence of OFDM symbols according to various examples. FIG. 14 schematically illustrates a low-power receiver according to various examples.

FIG. 15 schematically illustrates a main receiver according to various examples.

FIG. 16 is a flowchart of a method according to various examples.

FIG. 17 is a flowchart of a method according to various examples.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only.

The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof.

Hereinafter, techniques of wirelessly transmitting and/or receiving (communicating) are described. In the various examples described herein, various communication systems may be used. For example, a communication network may be employed. The communication network may be a wireless network. For sake of simplicity, various scenarios are described hereinafter with respect to an implementation of the communication network by a cellular network. The cellular network includes multiple cells. Each cell corresponds to a respective sub-area of the overall coverage area. Other example implementations include a multi-area wireless network such as a cellular WiFi network, etc..

Specifically, various examples are described in which a base station (BS) of a cellular network transmits a signal. However, as a general rule, the techniques described herein can be readily applied to other kinds and types of transmitting devices. For example, a UE may transmit a signal, e.g., in peer-to-peer communication.

In the various examples described herein, various kinds and types of signals may be communicated. An example of a signal that may be communicated is a WUS. While reference is primarily made to communicating WUS hereinafter, in other examples, other kinds and types of signals may be communication. The respective techniques described for communicating WUS can be readily applied for communicating other kinds and types of signals.

Hereinafter, techniques of paging a terminal is described. For this, paging signals can be transmitted to the terminal. Paging signals may include a WUS, a paging indicator, and/or a paging message.

Hereinafter, WUS techniques are described. The WUS techniques enable a terminal to transition a main receiver into a low-power state, e.g., for power-saving purposes. In some examples, the low-power state of the main receiver may be an inactive state. The inactive state can be characterized by a significantly reduced power consumption if compared to an active state of the main receiver. For example, the main receiver may be unfit to receive any data in the inactive state such that some or all components may be shut down. Wakeup of the main receiver from the inactive state is then triggered by a WUS. The inactive state can be associated with various operational modes of the terminal, e.g., a disconnected mode or idle mode. Here, a data connection between the terminal and the cellular network can be released.

For example, the WUS may be detected by a low-power receiver of the terminal. The low-power receiver may be configured to perform time-domain processing to detect the WUS. The time-domain processing can be in the baseband. The low-power receiver may be unfit to perform Orthogonal Frequency Division Multiplex, OFDM demodulation. For example, the low-power receiver may be configured to perform non- coherent decoding. For non-coherent decoding, knowledge of a reference phase is not required for signal detection.

The low-power receiver and main receiver may be implemented within the same hardware component(s) or may be implemented by at least one different hardware component.

The WUS may help to avoid blind decoding of a control channel on which paging signals and/or paging messages are communicated. Since typically such blind decoding is comparably energy inefficient, power consumption can be reduced by using WUSs. This is explained in greater detail hereinafter: For example, in reference implementations without WUS transmission, during paging occasions (POs), the terminal is expected to blind decode the control channels MPDCCFI (for Machine Type Communication) or PDCCFI (for LTE) or NPDCCFI (for NB-IOT) for P-RNTI as paging identity. If presence of a paging indicator including the P-RNTI is detected, the terminal continues to decode a subsequent PDSCFI for a paging message. Flowever, the paging message on PDSCFI may be indicative of paging of other UEs, and not for the given terminal. In this case, the given terminal needs to go back to sleep until the next PO. Moreover, in applications where the paging rate is very low, the cost of terminal idle listening can become relatively high. Under this condition, the terminal needs to monitor the control channel without receiving any paging indication and/or a false paging indication for another terminal. In MTC, it could be even worse as the respective MPDCCFI control channel is transmitted with the highest number of repetitions which reflect the maximum extended coverage used in that cell. Using the WUS helps to avoid blind decoding of the physical control channel.

Communication of the WUS may be time-aligned with a discontinuous reception (DRX) cycle of the terminal. A DRX cycle includes ON-durations and OFF-durations. The low- power receiver can be selectively activated during the ON-durations.

Sometimes, the operational mode of the terminal associated with WUS communication is referred to as WUS mode. As a general rule, there may be multiple WUS modes available, e.g., modes in which the terminal is registered at the network as connected or idle, etc.. A data connection may or may not be established between the terminal and the network when receiving a WUS.

Various concepts have been described to facilitate communication of a WUS. In some reference implementations, the WUS is designed based on OFDM modulation principles to allow its orthogonality to other signals transmitted in the same OFDM symbol. In such reference implementations, this OFDM-modulated WUS, however, needs to be detected by the same power-hungry fast Fourier transform (FFT) based or multi-carrier-based receiver used to decode the PDCCFI or PDSCFI. In detail, to be able to detect and decode the WUS with such a design, the terminal needs to first decode a costly synchronization signal (SS) or it needs to stay synchronized with the network by keeping some certain parts of its circuit on, i.e., its channel estimator. Both, the use of a power-hungry receiver as well as the decoding of costly SS, reduce the resulting energy saving from the WUS. For example, where OFDM demodulation is used to detect the WUS, it has been found that the resulting energy saving is very limited in scenarios where there is a tight requirement on reachability of the terminal and the terminal is configured with DRX cycles. It has been found that using WUS only helps to significantly reduce the energy consumption, in certain scenarios where the terminal is in the extended coverage or is configured with extended DRX cycles that have longer OFF-durations. hereinafter, techniques are described which facilitate reduction the power consumption using a WUS if compared to such reference implementations. Specifically, techniques are described which facilitate reducing the power consumption for various scenarios, e.g., not only for extended coverage and extended DRX cycles.

According to examples, this is achieved by an OFDM-based, single carrier WUS that can be decoded by a low-power receiver. The WUS can be fit into one or more OFDM symbols. The transmitted WUS can be both orthogonal to the rest of the OFDM symbols and can be detected by a non-coherent low-power receiver that does not require tight synchronization. No costly synchronization signal is needed for the WUS detection and therefore the WUS design can lead to an extensive energy saving specially for scenarios where there is a tight requirement on reachability of the terminal, i.e., the terminal is configured with DRX cycles having short OFF-durations. According to examples, such and further effects may be achieved by implementing a particular transmitter-side strategy for generating the WUS.

For example, a reference WUS waveform may be obtained. The reference WUS waveform may be defined in accordance with a non-coherent modulation scheme, e.g., on-off keying (OOK) or frequency-shift keying (FSK). A corresponding mapping of the non-coherent modulation scheme may be applied to obtain the reference WUS waveform. The WUS waveform may be defined in the baseband, and later modulated onto a carrier or a set of OFDM subcarriers of one or more OFDM symbols in sequence.

The reference WUS waveform may be shaped. A corresponding shaping block can output at least one WUS waveform.

The shaping block can prepare the reference WUS waveform for OFDM modulation. For example, each WUS waveform of the at least one WUS waveform output by the shaping block may be associated with a respective subcarrier of a plurality of subcarriers (WUS subcarriers). In other words, the shaping block can output one or more WUS waveforms that correspond to OFDM subcarriers.

Then, the at least one WUS waveform is input to an OFDM modulator. For example, the OFDM modulator can operated based on an inverse FFT (IFFT). The OFDM modulator outputs an OFDM symbol.

Then, this OFDM symbol can be transmitted. For example, a transmitter is controlled to transmit the OFDM symbol.

An OFDM symbol can include multiple sub-carriers, each sub-carrier carrying a respective signal. The signals of the sub-carriers are orthogonal to each other.

A receiver may be configured to perform a non-coherent demodulation in accordance with the non-coherence modulation scheme, e.g., OOK or FSK. Flence, even though OFDM symbols are transmitted over-the-air, the receiver may be able to interpret the part of the OFDM symbols corresponding to the WUS waveform. For example, for OOK, a correlation may be performed in time domain. Flere, the expected reference WUS waveform may be compared with the corresponding part of the OFDM symbols, e.g., as part of a time-domain correlation.

The shaping approximates a baseband representation of a part of the OFDM symbol correspond to the WUS subcarriers (WUS part of OFDM symbol) to the reference WUS waveform. Thereby, a low-power receiver can detect the WUS part of the OFDM symbol.

Sometimes, a scenario may occur where the reference WUS waveform does not fit into a single OFDM symbol: Various techniques are based on the finding that using longer reference WUS waveforms, sometimes a higher accuracy in the detection of the WUS at the receiver can be achieved.

As a general rule, it is possible that the duration of the reference WUS waveform is longer than the duration of a single OFDM symbol of a sequence of OFDM symbols. In such a scenario, it is possible to appropriately pre-process the reference WUS waveform to achieve a mapping to multiple OFDM symbols. In one example, the pre- processing includes cutting and cropping multiple segments from the reference WUS waveform, wherein each of these segments is mapped to a corresponding OFDM symbol of the sequence. Then, for each segment, techniques as described above, i.e., shaping and OFDM modulation, can be applied.

For example, the reference WUS waveform can be cut at positions of the reference WUS waveform that are mapped to boundaries between subsequent OFDM symbols of the sequence.

For example, the reference WUS waveform can be cropped at positions that are mapped to CPs of the OFDM symbols of the sequence. Cropping can pertain to discarding corresponding parts of the reference WUS waveform. Such cropping, i.e., removing a fraction of the reference WUS waveform in the CP, can sometimes limit the receiver-side analog front-end bit error rate to a certain noise floor, therefore degrading the WUS detector performance. Various techniques are based on the finding that, as long as the CP is a small fraction of the duration of the respective OFDM symbol, properties of the correlation at the receiver remain essentially unchanged. For instance, in 3GPP LTE, the extended CP is only 7% of the duration of the overall OFDM symbol. In such a scenario, only a minor degradation of the peak of the correlation at the receiver was observed. By choosing the correct type of reference WUS waveform and setting the threshold level of the WUS detector to a certain value, we can achieve a high WUS detection and low WUS false alarm probabilities necessary for WUS design.

FIG. 1 schematically illustrates a cellular network 100. The example of FIG. 1 illustrates the network 100 according to the 3GPP 5G architecture. Details of the fundamental architecture are described in 3GPP TS 23.501 , version 1 .3.0 (2017-09). While FIG. 1 and further parts of the following description illustrate techniques in the 3GPP 5G framework, similar techniques may be readily applied to different communication protocols. Examples include 3GPP LTE 4G and IEEE Wi-Fi technology.

In the scenario of FIG. 1 , a terminal 101 is connectable to the network 100. For example, the terminal 101 may be one of the following: a cellular phone; a smart phone; and IOT device; a MTC device; an NB-IOT device; a sensor; an actuator; etc.

The terminal 101 is connectable to the network 100 via a radio access network (RAN) 1 1 1 , typically formed by one or more base stations (BSs) 1 12. A wireless link 1 14 is established between the RAN 1 1 1 - specifically between one or more of the BSs 1 12 of the RAN 1 1 1 - and the terminal 101 .

The RAN 1 1 1 is connected to a core network (CN) 1 15. The CN 1 15 includes a user plane (UP) 191 and a control plane 192. Application data is typically routed via the UP 191 . For this, there is provided a UP function (UPF) 121 . The UPF 121 may implement router functionality. Application data may pass through one or more UPFs 121 . In the scenario of FIG. 1 , the UPF 121 acts as a gateway towards a data network 180, e.g., the Internet or a Local Area Network. Application data can be communicated between the terminal 101 and one or more servers on the data network 180. The network 100 also includes an Access and Mobility Management Function (AMF) 131 ; a Session Management Function (SMF) 132; a Policy Control Function (PCF) 133; an Application Function (AF) 134; a Network Slice Selection Function (NSSF) 134; an Authentication Server Function (AUSF) 136; and a Unified Data Management (UDM) 137. FIG. 1 also illustrates the protocol reference points N1 -N22 between these nodes.

The AMF 131 provides one or more of the following functionalities: registration management; NAS termination; connection management; reachability management; mobility management; access authentication; and access authorization. For example, the AMF 131 controls CN-initiated paging of the UEs 101 . The AMF 131 may keep track of the timing of a DRX cycle of the terminal 101 . The AMF 131 may trigger transmission of paging signals such as WUSs and/or paging indicators to the terminal 101 .

The SMF 132 provides one or more of the following functionalities: session management including session establishment, modify and release, including bearers set up of UP bearers between the RAN 1 1 1 and the UPF 121 ; selection and control of UPFs; configuring of traffic steering; roaming functionality; termination of at least parts of NAS messages; etc.

As such, the AMF 131 and the SMF 132 both implement control-plane mobility management needed to support a moving terminal.

FIG. 1 also illustrates aspects with respect to a data connection 189. The data connection 189 is established between the terminal 101 via the RAN 1 1 1 and the DP 191 of the CN 1 15 and towards the DN 180. For example, a connection with the Internet or another packet data network can be established. To establish the data connection 189, it is possible that the respective terminal 101 performs a random access (RACFI) procedure, e.g., in response to reception of a paging signal, e.g., a WUS and a paging indicator. A server of the DN 180 may host a service for which payload data is communicated via the data connection 189. The data connection 189 may include one or more bearers such as a dedicated bearer or a default bearer. The data connection 189 may be defined on the Radio Resource Control (RRC) layer, e.g., generally Layer 3 of the OSI model of Layer 2.

FIG. 2 illustrates aspects with respect to communicating on the wireless link 1 14. Specifically, FIG. 2 illustrates aspects with respect to modulation of signals to communicate on the wireless link 1 14.

Specifically, FIG. 2, upper part, illustrates multiple subcarriers 301 -303 in frequency domain used for OFDM modulation. Different subcarriers 301 -303 are orthogonal with respect to each other and thus can each encode specific information with reduced interference. As a general rule, OFDM modulation may employ a variable count of subcarriers 301 -303, e.g., between 20 and 2000 subcarriers.

FIG. 2, lower part, illustrates a signal waveform 305 that is defined in accordance with an OOK modulation. To demodulate data encoded by a carrier or subcarrier 305 using OOK, non-coherent decoding may be employed. The transmitter and receiver may require less precise synchronization in frequency and time.

FIG. 2, lower part also illustrates a bit duration 306. This bit time duration 306 illustrates the time used to encode a single bit.

Various techniques are based on the finding that, in a low-power receiver, simple non- coherent modulation schemes, such as OOK, FSK, are often used for the signal transmission, since it allows low-power low-complex front-end architecture. Hereinafter, techniques are described which facilitate providing orthogonality, while still facilitating a low-power receiver to operated non-coherently, e.g., based on OOK. Frequency duplexing is facilitated between WUS and other signals. Further details with respect to frequency duplexing are illustrated in FIG. 3.

FIG. 3 illustrates aspects with respect to frequency division duplexing (FDD). FIG. 3 illustrates aspects with respect to physical channels 261 -263 implemented on the wireless link 1 14. The wireless link 1 14 implements a plurality of communication channels 261 -263. Transmission frames - e.g., implemented by radio frames, each including one or more subframes- of the channels 261 -263 occupy a certain time duration.

Each channel 261 - 263 includes a plurality of time-frequency resource elements which are defined in time domain and frequency domain. For example, the resource elements may be defined in time domain with respect to the duration of an OFDM symbol. For example, the resource elements may be defined in frequency domain with respect to OFDM subcarriers. More generally, the resource elements may be defined in a time-frequency resource grid.

For example, a first channel 261 may carry WUSs. The WUSs enable the network 100 - e.g., the AMF 131 - to wake up the terminal 101 . The WUSs may thus be communicated in dedicated resource elements of the channel 261 .

A second channel 262 may carry paging indicators which enable the network 100. The paging indicators may thus be communicated in dedicated resource elements of the channel 262. Typically, the paging indicators are communicated on PDCCFI.

As will be appreciated from the above, the WUSs and the paging signals may be different from each other in that they are transmitted on different channels 261 , 262. Different resource elements may be allocated to the different channels 261 -263. For example, in many scenarios the WUS and the paging indicators are transmitted at two different time instances.

Further, a third channel 263 is associated with user data and may hence carry higher- layer user-plane data packets associated with a given service implemented by the terminal 101 and the BS 1 12. Alternatively, control messages may be transmitted via the channel 263, e.g., a paging message or other RRC control data.

FIG. 4 schematically illustrates the BS 1 12. The BS 1 12 includes an interface 1 121 . For example, the interface 1 121 may include an analog front end and a digital front end. The interface may include an OFDM modulator. The interface 1 121 may be coupled to one or more antennas or antenna arrays. The BS 1 12 further includes control circuitry 1 122, e.g., implemented by means of one or more processors and software. For example, program code to be executed by the control circuitry 1 122 may be stored in a non-volatile memory 1 123. In the various examples disclosed herein, various functionality may be implemented by the BS 1 12, e.g.: transmitting an OFDM symbol including a WUS; generating a WUS waveform; signal shaping a reference WUS waveform; etc..

FIG. 5 schematically illustrates the terminal 101 . The terminal 101 includes an interface 101 1 . For example, the interface 101 1 may include an analog front end and a digital front end. Th interface 101 1 may be coupled to an antenna. In some examples, the interface 101 1 may include a main receiver and a low-power receiver. Each one of the main receiver and the low-power receiver may include an analog front end and a digital front end, respectively. The main receiver may include an OFDM demodulator. The low-power receiver may be configured for non-coherent time-domain decoding, e.g., according to OOK or FSK. The terminal 101 further includes a control circuitry 1012, e.g., implemented by means of one or more processors and software. The control circuitry 1012 may also be at least partly implemented in hardware. For example, program code to be executed by the control circuitry 1012 may be stored in a non- volatile memory 1013. In the various examples disclosed herein, various functionality may be implemented by the terminal 101 , e.g.: receiving a WUS; performing a correlation to detect a WUS; receiving a paging indicator and a paging message; operating the low-power receiver and the main receiver; etc.

FIG. 6 illustrates details with respect to the interface 101 1 of the terminal 101 . In particular, FIG. 6 illustrates aspects with respect to a main receiver 1351 and a low- power receiver 1352. In FIG. 6, the main receiver 1351 and the low-power receiver 1352 are implemented as separate entities. For example, they may be implemented on different chips. For example, they may be implemented in different housings. For example, they may not share a common power supply.

The scenario FIG. 6 may enable switching off some or all components of the main receiver 1351 when operating the main receiver in inactive state. In the various examples described herein, it may then be possible to detect WUSs using the low- power receiver 1352. Also, the low-power receiver 1352 may be switched between an inactive state and an active state, e.g., according to a DRX cycle. For example, if the main receiver 1351 is switched on, the low-power receiver 1352 may be switched off, and vice-versa. As such, the main receiver 1351 and the low- power receiver 1352 may be inter-related in operation (indicated by the arrows in FIG. 6).

FIG. 7 illustrates details with respect to the interface 101 1 of the terminal 101 . In particular, FIG. 7 illustrates aspects with respect to the main receiver 1351 and the low-power receiver 1352. In FIG. 7, the main receiver 1351 and the low-power receiver 1352 are implemented as a common entity. For example, they may be implemented on the common chip, i.e., integrated on a common die. For example, they may be implemented in a common housing. For example, they may share a common power supply.

The scenario FIG. 7 may enable a particular low latency for transitioning between reception - e.g., of a WUS - by the wake-up receiver 1352 and reception by the main receiver 1351 .

While in FIGs. 6 and 7 a scenario is illustrated where the main receiver 1351 and the low-power receiver 1352 share a common antenna, in other examples, it would be also possible that the interface 101 1 includes dedicated antennas for the main receiver 1351 and the low-power receiver 1352.

While in the examples of FIGs. 6 and 7 scenarios are illustrated where there is a dedicated low-power receiver 1352, in other examples there may be no low-power receiver. Instead, the WUS may be detected by the main receiver 1351 in a low-power state. For example, the main receiver 1351 may not be fit to demodulate and decode data mapped to Quadrature Phase Shift Keying (QPSK), Binary Phase Shift Keying (BPSK), or Quadrature Amplitude Modulation (QAM) constellations. Rather, the main receiver 1351 may be fit to perform non-coherent decoding of a WUS, when operating in the low-power state. Then, in response to receiving the WUS, the main receiver 1351 may transition into a high-power state in which it is fit to decode and demodulate the ordinary data, e.g., on channel 263, etc.. FIG. 8 is a signaling diagram. FIG. 8 illustrates aspects with respect to communicating between the terminal 101 and the BS 1 12 (cf. FIG. 1 ).

FIG. 8 illustrates aspects with respect to transmitting and/or receiving (communicating) a WUS 4003. According to the various examples described herein, such techniques as described with respect to FIG. 8 may be employed for communicating WUSs 4003. In particular, FIG. 8 also illustrates aspects with respect to the inter-relationship between communication of the WUS 4003 and communication of a paging indicator 4004 and a paging message 4005.

At 3001 , configuration data 4001 is communicated. This is generally optional. The configuration data 4001 is transmitted by the BS 1 12 and received by the terminal 101 . For example, a respective control message may be communicated on the control channel 262, e.g., PDCCFI. For example, the control message may be a Layer 2 or Layer 3 control message. The control data 4001 may be communicated using RRC / higher-layer signaling.

The configuration data 4001 may be indicative of a sequence design for the WUS transmission. The configuration data 4001 may be indicative of time-frequency resources used for the WUS transmission. The configuration data 4001 may be indicative of a schedule of the time frequency resources in an OFDM resource grid used for WUS transmission; i.e., the configuration data 4001 may allocate resource elements to the channel 261 . The schedule may be indicative of a time pattern of the time-frequency resource elements and/or indicative of a frequency pattern of the time- frequency resource elements.

At 3002, a user data 4002 is communicated. For example, the user data 4002 may be communicated on the payload channel 263. For example, the user data 4002 may be communicated along the data connection 189, e.g., as part of a bearer, etc..

The configuration data 4001 and the user data 4002 are communicated using the main receiver 1351 . Then, there is no more data to be communicated between the terminal 101 and the BS 1 12. Transmit buffers are empty. This may trigger a timer. For example, the timer may be implemented at the terminal 101 . After a certain timeout duration set in accordance with the inactivity schedule 201 , the main receiver 1351 of the terminal 101 is transitioned into an inactive state 384 from an active state, 3003. This is done to reduce the power consumption of the terminal 101 . For example, prior to the transitioning the main receiver 1351 to the inactive state 384, it would be possible to release the data connection 189 by appropriate control signaling (not illustrated in FIG. 8). The timeout duration 201 is an example implementation of a trigger criterion for transitioning into the inactive state 384; other trigger criteria are possible. For example, a connection release message may be communicated.

Multiple POs 202 for communicating the WUS 4003 are then implemented by reoccurring resource elements on the channel 261 .

At some point in time, the BS 1 12 transmits a WUS 4003, 3004. This may be because there is DL data - e.g., payload data or control data - scheduled for transmission to the terminal 101 in a transmit buffer.

In FIG. 8, also the baseband representation of the WUS 4003, the so-called reference WUS waveform b is illustrated. A corresponding length or duration 501 is also depicted..

In response to receiving the WUS 4003, the main receiver 1351 of the terminal 101 is transitioned to the active state, 3005.

Then, at 3006, a paging indicator 4004 is transmitted by the BS 1 12 to the terminal 101 . The paging indicator 4004 is received by the main receiver 1351 . For example, the paging indicator may be transmitted on channel 262, e.g. PDCCFI. For example, the paging indicator may include a temporary or static identity of the terminal 101 . The paging indicator 4004 may include information on a MCS used for communicating a paging message 4005 at 3007. The paging message 4005 may be communicated on a shared channel 263, e.g., physical downlink shared channel (PDSCFI). Then, at 3008, a data connection 189 is set up between the terminal 101 and the BS 1 12. This may include a random access procedure.

Finally, a UL or DL user-data message 4002 is communicated using the newly set up data connection 189 at 3009.

As will be appreciated from FIG. 8, upon transitioning the main receiver 1351 to the active state at 3005, the data connection 189 needs to be re-established. For this reason, the terminal 101 operates in idle mode - when no data connection 189 is set up or maintained - while the inactive state 384 of the main receiver 1351 is active. Flowever, in the various examples described herein, other implementations of the particular mode in which the terminal 101 operates while the inactive state 384 is active are conceivable. For example, the terminal 101 may operate in connected mode while the inactive state 384 is active.

Next, details with respect to the transmission of the WUS 4003 are explained. Specifically, PFIY Layer 1 properties of the WUS 4003 are explained next.

FIG. 9 illustrates aspects with respect to the wireless interface 1 121 of the BS 1 12. FIG. 9 illustrates aspects with respect to transmitting the WUS 4003.

The interface 1 121 includes a WUS signal-shaping block 1501 ; an IFFT block 1502; a parallel-to-serial block 1503; a CP block 1504; a digital-to-analog converter 1505; an analog frontend 1506; and a power amplifier 1507. The interface 1 121 is coupled to one or more antennas 1508.

A reference WUS waveform b is input to the WUS signal-shaping block 1501 . The reference WUS waveform b has a certain length / duration. In the scenario of FIG. 9, it can be assumed that the length of the reference WUS waveform b is short enough to fit in one OFDM symbol.

As a general rule, the reference WUS waveform b can be defined in accordance with a non-coherent modulation scheme, e.g., OOK, Frequency-shift keying (FSK). Hence, information encoded by the reference WUS waveform b can be mapped to a constellation of a non-coherent modulation scheme.

Non-coherent modulation schemes do generally not require a receiver clock to be in- phase, i.e., synchronized with the transmitter, specifically, the carrier signal of the transmitter. In this case, modulation symbols (rather than bits, characters, or data packets) are asynchronously transferred.

As a general rule, the term“waveform” is used herein to the baseband representation of a signal - i.e., not modulated onto a respective carrier and subcarrier. For example, a waveform may be obtained by encoding a bit stream. Interleaving can be applied. Then, to obtain the waveform mapping onto the constellation of the respective modulation can be applied, e.g., a mapping onto the OOK constellation, etc..

The WUS signal-shaping block 1501 shapes the reference WUS waveform b. This shaping is done to facilitate, both, (i) OFDM modulation, as well as (ii) use of a non- coherent low-power receiver at the receiver (not illustrated in FIG. 9).

The reference WUS waveform b is shaped to obtain multiple WUS waveforms x. The various WUS waveforms x are associated with the WUS subcarriers reserved for the WUS channel 261 . The multiple WUS waveforms x are input into respective channels 1552 of the IFFT block 1502.

As a general rule, the count of WUS subcarriers determines the bit duration 306 of the OOK modulation scheme. For example, a smaller count of subcarriers means that a larger bit duration 306 has to be used in the communication. This somewhat corresponds to the Nyquist-Shannon sampling theorem.

Generally, the IFFT block 1502 provides modulation of signal waveforms onto various subcarriers. The OFDM modulation facilitated by the IFFT block 1502 enables FDD: Further channels 1551 , 1553 of the IFFT block 1502 are used to communicate on other channels 262, 263, e.g., with other terminals. A plurality of data signal waveforms C _O , C _Ί - associated with subcarriers different from the WUS subcarriers - are obtained. The data signal waveforms are defined in accordance with a coherent modulation scheme, e.g., QPSK, BPSK, or QAM. The data signal waveforms are then input to the channels 1551 , 1553 of the IFFT block 1502 (also cf. FIG. 10, where details of the IFFT block 1502 are shown). In accordance with FIGs. 9 and 10, a vector representation of the data input to the IFFT block 1502 is as follows:

(1 )

In Eq. (1 )

denotes the data signal waveforms and

denotes the WUS waveform. denotes the set of sub-carriers associated with the wake-up signal waveform x, i.e., {k _Q , ... , k _Q + K— 1}. The center sub-carrier of X is k _c . The IFFT block 1502 transforms from frequency domain to time domain. An output of the IFFT block 1502 corresponds to a set of complex time-domain samples representing the OFDM subcarrier signals.

The operation of the IFFT block 1502 can be represented in time domain as follows:

The baseband representation of the WUS s is denoted b _n . Flere,“baseband” refers to the signal before modulation onto the sub-carriers. Flere, n is the index of the various output channels of the IFFT block 1502.

The IFFT block can be described by a linear transformation F; Eq. (4) can be re-written in matrix notation:

In block 1503, the samples are clocked out to provide the OFDM symbol s of a certain duration. A cyclic extension - implemented by a cyclic prefix (CP) or a cyclic suffix - is added by the CP block 1504, which increases the length of the OFDM symbol. The CP corresponds to a repetition of the end of the OFDM symbol at the beginning of the OFDM symbol. Flence, blocks 1502, 1503, 1504 implement an OFDM modulator as they output a baseband OFDM symbol of a certain duration.

Then, the blocks 1505-1507 are controlled to transform the OFDM symbol into analog domain, modulate it onto the carrier, amplify it, and transmit it on the spectrum.

FIG. 9 illustrates that the baseband OFDM symbol s includes two contributions, i.e., (i) the contribution from the WUS s ^w (the WUS part of the OFDM symbol) and (ii) the contribution from the data signal s° . s ^w is the WUS part of the OFDM symbol s modulated on the WUS subcarriers associated with the channels 1552; and s° is the part of the OFDM symbol s° modulated on the subcarriers associated with the channels 1551 , 1553:

8 = 8 w + ,

8 o

(6). The WUS part s ^w of the OFDM symbol s corresponds to the WUS 4003.

In FIG. 9, the signal shaping block 1501 is configured to shape the reference WUS waveform b such that the baseband representation of the WUS part s ^w of the OFDM symbol s, i.e., b, is approximately equal to b. Such an approach allows for orthogonality between waveforms x ₀ , x ₁ and x when included in the same OFDM symbol s . This is achieved by communicating the WUS 4003, s^as an OFDM-based modulated signal. The signal shaping block 1501 calculates the necessary input x to the IFFT block 1502 on the subcarriers 301-302 designated for the WUS 4003 which is needed to approximate a desired reference WUS waveform b in the time domain. Thereby, the WUS part s^of the resulting OFDM symbol s can be detected by a low-power receiver without the need for further synchronization, while still being orthogonal to the other parts s° of the OFDM signal s.

This gives the flexibility to design the reference WUS waveform b such that if it was directly detected by the low-power receiver, it would appropriately wake up the terminal 101 . For example, the reference WUS waveform b can be designed to be terminal specific or specific to a group of terminals. For example, the reference WUS waveform b can be designed to be cell specific. For example, the reference WUS waveform b can be designed to have a variable length, e.g., depending on the coverage situation of the terminal 101 . For example, the reference WUS waveform b can be designed to have a specific base sequence, e.g., to facilitate code division multiple access (CDMA) for multiple UEs.

As a general rule, various options are available to implement the signal shaping of the signal shaping block 1501 . In one example option, a look-up table may be provided. The look-up table may translate between the reference WUS waveform b and the WUS waveforms x. Thereby, look-up table may have various entries that relate to different possible reference WUS waveforms b. In a further example option, an optimization may be implemented. For this, a feedback path may be implemented that provides a feedback of b to the signal shaping block 1501 . Then, an iterative optimization algorithm may be employed that - e.g., in a numerical simulation - varies the output of the signal shaping block 1501 , i.e., x, until an optimization criterion is met; the optimization criterion can correspond to a difference between the reference WUS waveform b and b. In a further example, the shaping can be based on an analytic approximation of the OFDM modulator 1502-1504. For example, it would be possible that the shaping is based on an approximation of the IFFT block 1502. The approximation of the IFFT block 1502 can be denoted F. Flere, F can be a sub-matrix of F. The dimension of F can be NxK, see Eqs. (1 )-(4). For example, it would be possible to select the WUS-subcarriers X symmetrically around the center subcarrier kc. Thereby, the output of the IFFT block 1502 can be approximated, but orthogonality to the data signal waveforms is maintained. Specifically, it would be possible that the signal shaping at the signal-shaping block 1501 minimizes a difference between b and b. As a general rule, various metrics can be considered to define the difference. An example metric is the least-squares metric, i.e.:

min \\Fx— b\ \ ²

(7)

Eq. 7 can be reformulated as: where x _LS denotes the WUS waveform x as obtained from the least-squares metrics approximation.

As a general rule, while FIG. 9 illustrates transmission of (i) a WUS by (ii) a BS, generally it would be possible to transmit (i) another type of signal by (ii) another device using the techniques described in FIG. 9.

FIG. 1 1 illustrates aspects with respect to such a signal shaping using Eq. 8. In FIG. 1 1 , the dashed line illustrates the reference WUS waveform b and the full line illustrates the baseband representation b of the WUS part s^ of the OFDM symbol s, i.e., the baseband representation of the WUS 4003. As illustrated in FIG. 1 1 , b * b.

FIG. 1 1 is provided for b being mapped to symbols of OOK, using an N=2048 IFFT OFDM system and carrying the WUS on K=64 consecutive subcarriers (out of 72 designated ones). The signals are shown for one full OFDM symbol (2048 time samples) without the cyclic prefix.

This facilitates employing a low-power receiver 1352 for receiving the WUS part s^ of the OFDM symbol s, cf. FIG. 12. Sometimes, a scenario may occur where the duration 501 ( dwus ) (cf. FIG. 8) of the reference WUS waveform b is too long to fit into a single OFDM symbol of duration ( doFDM ). Then, it is possible to obtain, from the reference WUS waveform b several shorter segments, hereinafter referred to as reference WUS waveform segments. It is then possible to treat each reference WUS waveform segment in the same manner as has been explained in connection with FIGs. 9 - 1 1 , i.e., each reference WUS waveform segment constitutes a respective (shorter) reference WUS waveform on its own.

Longer durations 501 of the reference WUS waveform b, requiring more than one OFDM symbol for its transmission, appear, e.g., when the WUS contains more bits than doFDM/dbit where doFDM has been defined as above and C/M is duration 306 in figure 2. FIG. 12 illustrates how this situation is handled

FIG. 12 illustrates aspects with respect to the wireless interface 1 121 of the BS 1 12. FIG. 12 illustrates aspects with respect to transmitting the WUS 4003. The scenario of FIG. 12 generally corresponds to the scenario of FIG. 9; with additional pre-processing to obtain the reference WUS waveform segments bm. Each of the reference WUS waveform segments can be treated as a respective input to the WUS shaping block 1501 in the same manner as discussed for FIG. 9 for the reference WUS waveform b

In the scenario of FIG. 12, there is an additional block 1701 provided which cuts and crops the reference WUS waveform b, to thereby obtain a number of reference WUS waveform segments bm, where m is an integer in {1 ...M}:

In principle, M may be any positive integer.

For each reference WUS waveform segment bm, it is then possible to perform block 1501 , 1552, 1504-1508. Then, the WUS 4003 is transmitted spread out across a sequence of M OFDM symbols. The receiver can perform a correlation considering the overall duration 501 of the WUS 4003 (cf. FIG. 8) and may not need to synchronize with the timing of the OFDM symbols of the sequence. Next, details with respect to block 1701 are explained. In block 1701 the reference WUS waveform b is cut and cropped, at sections which are mapped to the CPs of the M OFDM symbols of the sequence. This is illustrated in connection with FIG. 13. The input 1781 to the bock 1701 is illustrated in FIG. 13, top row.

FIG. 13 illustrates aspects with respect to a sequence 1750 of OFDM symbols 1751

- 1755. Each OFDM symbol 1751 - 1755 has a respective CP 1761 -1766. The duration 502 (doFDivi) of the OFDM symbols 1751 - 1755 is shorter than the duration 501 of the reference WUS waveform b. The duration 503 of the CPs 1761 -1766 is typically a small fraction of the duration 502 of the OFDM symbols 1751 - 1755, e.g., in the range of 3% to 30 % or in the range of 3 % to 10 %.

Due to the relatively short duration 503 of the CPs 1761 -1766, it is possible to crop - i.e., discard - sections of the reference WUS waveform b which correspond to the CPs 1761 -1766, see row 1782. Since the CPs 1761 -1766 also mark the boundaries between adjacent OFDM symbols 1751 - 1755, thereby a total of M=5 reference WUS waveform segments bi, b2, b3, b4, bs are obtained in this example by appropriately cutting the reference WUS waveform b. As a general rule, M may take other values than 5.

Row 1783 then illustrates the input to the CP block 1504 and 1784 illustrates the output of the CP block 1504. As illustrated by the arrows in FIG. 13, bottom row, the end of the respective OFDM symbol 1751 -1755 is added as the CP at the beginning of each OFDM symbol 1751 -1755.

From a comparison of 1781 with 1784, it is apparent that the baseband representation b of the WUS part s'^ of the OFDM symbols 1751 -1755 - including the CPs 1761 -1766

- deviates from the reference WUS waveform b - specifically at the CPs 1761 -1766. Flowever, due to the relatively short duration 503 of the CPs 1761 -1766, this difference does not result in a significant loss of the ability of the non-coherent receiver to detect the WUS 4003. Details with respect to the receiver are illustrated in connection with FIG. 14. FIG. 14 illustrates aspects with respect to the low-power receiver 1352. The low-power receiver 1352 is coupled to an antenna 1601 . The low-power receiver 1352 may include a bandpass filter that restricts the receive bandwidth to the subcarriers 301 - 303 for WUS transmission (not illustrated in FIG. 14). The low-power receiver 1352 includes an analog frontend that may perform demodulation from the carrier. A non- coherent WUS detector 1604 is provided which is configured to demodulate the respective waveform in accordance with the non-coherent modulation scheme associated with the reference WUS waveform b. For the non-coherent demodulation, a SS needs not to be detected first. Rather, time-domain processing of the respective parts of one or more OFDM symbols in accordance with OOK-demodulation or FSK- demodulation reference implementations is possible. For example, a time-domain correlation between the expected reference WUS waveform b and the baseband signal as received can be performed. The transmitter of the OFDM symbol and the receiver of the OFDM symbol(s) do not need to be synchronized. Specifically, even if the duration of the WUS 4003 is longer than duration of a single OFDM symbol, it is possible to perform the time-domain processing without synchronizing to the beginning/ends of the OFDM symbols.

FIG. 15 illustrates aspects with respect to the main receiver 1351 . The main receiver

1351 is coupled to an antenna 161 1 . The main receiver 1351 includes a low noise amplifier 1612, an analog-to-digital converter 1613, a cyclic prefix removal block 1614, a serial-to-parallel conversion 1615, and an FFT block 1616. The FFT block 1616 outputs multiple channels 1551 -1552. The channels 1552 include the WUS waveform x which, however, can be discarded, because the main receiver 1351 is already in active state. The blocks 1614-1616 hence form an OFDM de-modulator.

As a general rule, the low-power receiver 1352 of FIG. 14 and the main receiver 1351 of FIG. 15 may be integrated into the same device, e.g., into the UE 101 . In such an implementation, it is possible that the main receiver 1351 and the low-power receiver

1352 re-use the same antenna. The low-power receiver 1352 and the main receiver 1351 can be individually activated and deactivated, e.g., depending on the operational mode of the UE 101 . FIG. 16 is a flowchart of a method according to various examples. In FIG. 16, blocks 7001 -7004 may be performed by a transmitter, e.g., implemented by the wireless interface 1 121 of the BS 1 12 (cf. FIGs. 9 and 12) or implemented by a wireless interface of another device, e.g., the UE 101 .

At block 7001 , a reference waveform is obtained, e.g., a reference WUS waveform. The reference waveform can be in accordance with a non-coherent modulation scheme, i.e., mapped to the constellation of, e.g., OOK, FSK etc.. Flence, if the reference waveform was received by a low-power receiver, it could be decoded by the low-power receiver and could trigger activation of the main receiver.

In some scenarios, the reference waveform may be obtained by cutting and cropping a further reference waveform - as such the reference waveform may implement a segment of the further reference waveform. This may be helpful where the further reference waveform is longer than the duration of OFDM symbols of a sequence of OFDM symbols.

Next, at block 7002. the reference waveform is signal shaped. For example, Eq. (8) could be used. The signal shaping changes the reference waveform and one or more waveforms associated with one or more selected subcarriers of an OFDM modulation are obtained by the signal shaping. The signal shaping can be based on an approximation of a transformation function of the OFDM modulation. The signal shaping can be configured to minimize a difference between (i) a baseband representation of a part of the OFDM symbol on the selected subcarriers and (ii) the reference waveform.

Block 7003 corresponds to OFDM modulation. It is possible to consider further data signal waveforms on other subcarriers.

At block 7004, the OFDM symbol obtained from the OFDM modulation of block 7003 is transmitted.

In scenarios in which block 7001 includes cropping and cutting a further reference waveform, blocks 7001 -7004 can be re-executed for all reference waveform segments that fit into the further reference waveform (cf. FIG. 13). In such a scenario, a sequence of OFDM symbols is transmitted wherein the further reference waveform is spread out across the OFDM symbols of the sequence of OFDM symbols.

In some scenarios, it would be possible that the cropping and cutting is selectively executed. For example, it would be possible that the duration of the further reference signal waveform is determined. Then, based on the duration, it would be possible to either execute or not execute the cutting and cropping. For instance, the duration of the further reference signal waveform could be compared with the duration of the OFDM symbols. If the duration of the further reference signal waveform is larger than the duration of the OFDM symbols, then the cutting and cropping may be executed; otherwise the cutting and cropping may not be executed.

FIG. 17 is a flowchart of a method according to various examples. In FIG. 17, blocks 7005 and 7006 may be implemented by a receiver, e.g., the main receiver 1351 of the wireless interface 101 1 of the terminal 101 . Blocks 7007 and 7008 may be implemented by a receiver, e.g., the low-power receiver 1352 of the wireless interface 101 1 of the terminal 101 .

The method of FIG. 17 is inter-related with the method of FIG. 16. The method of FIG. 17 may be used to receive one or more OFDM symbols transmitted at one or more iterations of block 7004 (obtained from one or more iterations of blocks 7002 and 7003). For example, a sequence of multiple OFDM symbols may be received.

Block 7005 corresponds to operation of the main receiver, if activated. Flere, the one or more OFDM symbols are received across the entire carrier bandwidth, i.e., including all subcarriers, in block 7005. In this regard, receiving can correspond to obtaining at the analog frontend a signal in the radio-frequency regime. Then, OFDM demodulation is applied, in block 7006. This may include receiving a SS and synchronization with the transmitter. Further, the CPs of the one or more OFDM symbols are removed, e.g., prior to applying an FFT. This operation requires synchronization.

Block 7007 corresponds to operation of the low-power receiver, e.g., if the main receiver is in inactive state. Flere, reception of the one or more OFDM symbols, at block 7007, is restricted to the bandwidth of the selected subcarriers encoding the waveform, e.g., by employing a bandpass filter. Then, at block 7008, OOK decoding - or, generally, another non-coherent decoding - is applied.

Block 7008 may include time-domain processing such as a correlation. Here, the correlation can be implemented between the reference signal waveform and the received signal waveform of the bandpass filtered one or more OFDM symbols. As a general rule, it is possible that the duration of the reference signal waveform is shorter than the duration of the received signal waveform: e.g., the reference WUS waveform may be detected within a longer time duration due to limited synchronization between transmitter and receiver. In still other words, a number of samples of the reference signal waveform may be smaller than a number of samples of the received signal waveform. For a WUS, the short WUS may be searched within a long signal.

In some examples, the CPs of the one or more OFDM symbols can be subject to the correlation. In other words, the search for the reference signal waveform can be implemented irrespective of the position of the CPs - the correlation can be implemented across the CPs. Thus, the CPs do not need to be removed from the one or more received OFDM symbols. This simplifies the setup of the low-power receiver. The reduced accuracy due to differences between the reference signal waveform and the received signal waveform at the CPs of the one or more OFDM symbols does not significantly affect the ability to detect the reference signal waveform, due to the limited length of the CPs.

Summarizing, techniques have been described in which a low-power receiver can be used. The low-power receiver operates in accordance with one or more non-coherent modulation schemes, such as OOK and FSK. Respective techniques may be applied for WUS transmission, since it allows low-power low-complexity front-end architectures to be used in the receiver.

Above, techniques have been described which facilitate tailoring inherently non-multi- carrier WUS techniques to multi-carrier based systems. Specifically, the WUS can be an OFDM-based modulated signal on one or more WUS subcarriers, to thereby avoid interfering with the simultaneous OFDM transmission on other subcarriers. As has been described, the WUS may span M OFDM symbols, where M can be 1 or larger. For cases of M> 1 , M segments of the a reference WUS waveform may be obtained by cutting and cropping the reference WUS waveform, where each segment is mapped to one OFDM symbol including its CP. For each segment, it is possible to remove the respective part of the reference WUS waveform that is mapped to the CP of the respective OFDM symbol.

The reference WUS waveform or multiple segments of the reference WUS waveform are shaped by using a signal shaping block and an OFDM modulator, respectively. The signal shaping calculates the necessary input to the OFDM modulator on the WUS subcarriers needed to approximate the reference WUS waveform or the respective reference WUS waveform segments in the time domain. The CP is added to make the resulting signal orthogonal to the other OFDM subcarriers.

With a reference WUS waveform spanning M OFDM symbols, the signal shaping and the OFDM modulator are applied M times, once per reference WUS waveform segment.

By creating the WUS in above manner and using the designated WUS subcarriers on the OFDM modulator, the resulting WUS becomes entirely orthogonal to the other signals transmitted in that OFDM symbol and the WUS still can be detected by a low- power receiver (i) without knowing relative timing between WUS and OFDM frames, (ii) without the need for further removing of the CP, and (iii) without the need to perform frame synchronization. All this allows for a simple implementation of the low-power receiver and for a reduced power consumption.

The low-power receiver therefore only needs to wake-up early enough to hear WUS when it arrives, i.e. low time synchronization requirements (saving energy). Moreover, with correct type of WUS, there is a robustness against frequency errors and it is possible to use low-power frequency synthesizers, again resulting in energy saving.

Removing a fraction of the reference WUS waveform in the CP and shaping, however, limit the AFE bit error rate to a certain error floor, therefore degrading the WUS detector performance. As long as the CP is a small fraction of the OFDM symbol, the correlation properties of the WUS remains essentially unchanged. For instance, in LTE, the extended cyclic prefix is only 7% of the OFDM symbol. By choosing the correct type of WUSs and setting the threshold level of the WUS detector to a certain value, it is possible to achieve a high WUS detection and low WUS false alarm probabilities necessary for WUS design.

Further summarizing, the following examples have been described:

Example 1 . A method, comprising:

- obtaining a reference signal waveform ( b , bi-bs) which is defined in accordance with a non-coherent modulation scheme,

- shaping the reference signal waveform ( b, bi-bs) to obtain at least one signal waveform ( x ) associated with one or more subcarriers ( K) of a plurality of subcarriers (301 -303),

- inputting the at least one signal waveform to at least one corresponding channel (1552) of a multi-channel orthogonal frequency division multiplex, OFDM, modulator (F, 1502, 1503, 1504), and

- transmitting an OFDM symbol (s) output by the OFDM modulator (F, 1502, 1503, 1504),

wherein said shaping approximates a baseband representation (b) of a part (s ^w ) of the OFDM symbol (s, 1751 -1755) associated with the one or more subcarriers (301 - 303) to the reference signal waveform ( b, bi-bs ).

Example 2. The method of example 1 ,

wherein said obtaining of the reference signal waveform segments ( bi-bs ) comprises:

- cutting and cropping a further reference signal waveform (b) at sections of the further reference signal waveform (b) mapped to cyclic extensions (1759) of the OFDM symbol (s, 1751 -1755) and a further OFDM symbol (s, 1751 -1755), the OFDM symbol and the further OFDM symbol being adjacent to each other in a sequence (1750) of OFDM symbols. Thus, the reference signal waveform may correspond to a segment of the further reference signal waveform.

Example 3. The method of example 2,

wherein the OFDM modulator adds the respective cyclic extensions to the OFDM symbols.

Example 3A. The method of example 2 or example 3, further comprising:

- determining a duration (501 ) of the further reference signal waveform ( b ), wherein said cutting and cropping is selectively executed depending on the duration (501 ).

Example 3B. The method of example 3A, further comprising:

- comparing the duration (501 ) of the further reference signal waveform with a duration (502) of the OFDM symbol,

wherein said cutting and cropping is executed if the duration of the further reference signal waveform is longer than the duration of the OFDM symbol.

Example 4. The method of any one of the preceding examples, further comprising:

- selecting a count of the one or more subcarriers based on a bit duration (306) of the non-coherent modulation scheme.

Example 5. The method of any one of the preceding examples,

wherein the reference signal waveform (b, b1 -b5) is at least a segment of a wake-up signal waveform (b, b1 -b5).

Example 6. The method of any one of the preceding examples,

wherein said shaping is based on an approximation of the OFDM modulator (F, 1502, 1503, 1504).

Example 7. The method of any one of the preceding examples,

wherein said shaping minimizes a difference between the baseband representation (b) of the OFDM symbol (s ^w ) on the one or more subcarriers (301 -303) and the reference signal waveform (b, b1 -b5), wherein the difference is defined using a least-squares metric.

Example 8. The method of any one of the preceding examples, further comprising:

- obtaining a plurality of data signal waveforms {co, C _ί ) associated with further subcarriers (301 -303) of the plurality of subcarriers (301 -303), the further subcarriers (301 -303) being different from the one or more subcarriers (301 -303), the plurality of data signal waveforms {co, C _ί ) being defined in accordance with a coherent modulation scheme, and

- inputting the plurality of data signal waveforms {co, C _ί ) to corresponding channels (1551 , 1552) of the OFDM modulator.

Example 9. The method of example 8,

wherein the coherent modulation scheme is selected from the group comprising: BPSK; QPSK; and QAM.

Example 10. The method of any one of the preceding examples,

wherein the non-coherent modulation scheme comprises e.g. On-Off-Keying or Frequency Shift Keying.

Example 1 1 . A method, comprising:

- receiving a sequence of OFDM symbols comprising a plurality of subcarriers,

- applying a bandpass filter to the received sequence of OFDM symbols, the bandpass filter being aligned with one or more predefined subcarriers (301 -303) of the plurality of subcarriers, and

- demodulating the bandpass filtered received sequence of OFDM symbols based on a non-coherent modulation scheme.

Example 12. The method of example 1 1 ,

wherein said demodulating of the bandpass filtered received sequence of OFDM symbols is based on time-domain processing.

Example 13. The method of example 12 wherein the time-domain processing comprises a correlation between a reference signal waveform and a received signal waveform of the bandpass filtered received sequence of OFDM symbols.

Example 14. The method of example 15,

wherein a duration of the reference signal waveform is shorter than a duration of the received signal waveform,

wherein the correlation is performed across cyclic extensions of the OFDM symbols of the received sequence of OFDM symbols.

Example 15. The method of any one of examples 13 to 16, further comprising:

- controlling a main receiver (1352) to receive the sequence of OFDM symbol, and

- demodulating the received sequence of OFDM symbols by inputting the OFDM symbols of the received sequence of OFDM symbols into an OFDM demodulator (1614-1616).

Example 16. A device (101 , 1 12) configured to:

- obtain a reference signal waveform (b, b1 -b5) which is defined in accordance with a non-coherent modulation scheme,

- shape the reference signal waveform (b, b1 -b5) to obtain at least one signal waveform ( x ) associated with one or more subcarriers ( K) of a plurality of subcarriers (301 -303),

- input the at least one signal waveform to at least one corresponding channel (1552) of a multi-channel orthogonal frequency division multiplex, OFDM, modulator (F, 1502, 1503, 1504), and

- transmit an OFDM symbol (s ) output by the OFDM modulator (F, 1502, 1503,

1504),

wherein said shaping approximates a baseband representation (b) of a part of the OFDM symbol (s, 1751 -1755) associated with the one or more subcarriers (301 - 303) to the reference signal waveform (b, b1 -b5).

Example 17. The device (101 , 1 12) of example 16 wherein the device (101 , 1 12) is configured to perform the method of any one of examples 1 - 10.

Example 18. A device (101 ) comprising a low-power receiver (1352) configured to:

- receive a sequence of OFDM symbols comprising a plurality of subcarriers,

- apply a bandpass filter to the received sequence of OFDM symbol, the bandpass filter being aligned with one or more predefined subcarriers (301 -303) of the plurality of subcarriers, and

- demodulate the bandpass filtered received sequence of OFDM symbols based on a non-coherent modulation scheme.

Example 19. The device of example 18, wherein the device is configured to perform the method of any one of examples 1 1 to 15.

Although the invention has been shown and described with respect to certain preferred embodiments, equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications and is limited only by the scope of the appended claims.

For illustration, above, various examples have been described with respect to communicating WUS. Flowever, the particular type of the signal - i.e., the action it triggers at the receiver - is not germane for the functioning of the techniques described herein. In other examples, other kinds and types of signals may be communicated.

For further illustration, above, various examples have been described in which a WUS is transmitted by a BS. In other examples, a WUS may be transmitted by devices other than a BS. For example, it would be possible to transmit a WUS - or another signal, as explained above - by a UE, e.g., on a sidelink channel or, generally, using peer-to- peer communication.