Patent Description:
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-IoT) 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-IoT 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 <CIT>, <CIT>, <NPL>"; <NPL>"; <NPL>". See <NPL>".

A need exists for advanced techniques of wireless communication.

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. Here, 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 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. In the method, demodulating of the bandpass filtered received sequence of OFDM symbols is based on time-domain processing, and 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. In the method, a duration of the reference signal waveform is shorter than a duration of the received signal waveform, and the correlation is performed across cyclic extensions of the OFDM symbols of the received sequence of OFDM symbols.

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 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. In the method, demodulating of the bandpass filtered received sequence of OFDM symbols is based on time-domain processing, and 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. In the method, a duration of the reference signal waveform is shorter than a duration of the received signal waveform, and the correlation is performed across cyclic extensions of the OFDM symbols of the received sequence of OFDM symbols.

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 MPDCCH (for Machine Type Communication) or PDCCH (for LTE) or NPDCCH (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 PDSCH for a paging message. However, the paging message on PDSCH 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 MPDCCH 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 PDCCH or PDSCH. 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. Hence, 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. Here, 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 preprocessing 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 <NUM>% 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> schematically illustrates a cellular network <NUM>. The example of <FIG> illustrates the network <NUM> according to the 3GPP <NUM> architecture. Details of the fundamental architecture are described in 3GPP TS <NUM>, version <NUM>. <NUM> (<NUM>-<NUM>). While <FIG> and further parts of the following description illustrate techniques in the 3GPP <NUM> framework, similar techniques may be readily applied to different communication protocols. Examples include 3GPP LTE <NUM> and IEEE Wi-Fi technology.

In the scenario of <FIG>, a terminal <NUM> is connectable to the network <NUM>. For example, the terminal <NUM> 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 <NUM> is connectable to the network <NUM> via a radio access network (RAN) <NUM>, typically formed by one or more base stations (BSs) <NUM>. A wireless link <NUM> is established between the RAN <NUM> - specifically between one or more of the BSs <NUM> of the RAN <NUM> - and the terminal <NUM>.

The RAN <NUM> is connected to a core network (CN) <NUM>. The CN <NUM> includes a user plane (UP) <NUM> and a control plane <NUM>. Application data is typically routed via the UP <NUM>. For this, there is provided a UP function (UPF) <NUM>. The UPF <NUM> may implement router functionality. Application data may pass through one or more UPFs <NUM>. In the scenario of <FIG>, the UPF <NUM> acts as a gateway towards a data network <NUM>, e.g., the Internet or a Local Area Network. Application data can be communicated between the terminal <NUM> and one or more servers on the data network <NUM>.

The network <NUM> also includes an Access and Mobility Management Function (AMF) <NUM>; a Session Management Function (SMF) <NUM>; a Policy Control Function (PCF) <NUM>; an Application Function (AF) <NUM>; a Network Slice Selection Function (NSSF) <NUM>; an Authentication Server Function (AUSF) <NUM>; and a Unified Data Management (UDM) <NUM>. <FIG> also illustrates the protocol reference points N1-N22 between these nodes.

The AMF <NUM> 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 <NUM> controls CN-initiated paging of the UEs <NUM>. The AMF <NUM> may keep track of the timing of a DRX cycle of the terminal <NUM>. The AMF <NUM> may trigger transmission of paging signals such as WUSs and/or paging indicators to the terminal <NUM>.

The SMF <NUM> 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 <NUM> and the UPF <NUM>; selection and control of UPFs; configuring of traffic steering; roaming functionality; termination of at least parts of NAS messages; etc..

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

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

<FIG> illustrates aspects with respect to communicating on the wireless link <NUM>. Specifically, <FIG> illustrates aspects with respect to modulation of signals to communicate on the wireless link <NUM>.

Specifically, <FIG>, upper part, illustrates multiple subcarriers <NUM>-<NUM> in frequency domain used for OFDM modulation. Different subcarriers <NUM>-<NUM> 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 <NUM>-<NUM>, e.g., between <NUM> and <NUM> subcarriers.

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

<FIG>, lower part also illustrates a bit duration <NUM>. This bit time duration <NUM> 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>.

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

Each channel <NUM> - <NUM> 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 <NUM> may carry WUSs. The WUSs enable the network <NUM> - e.g., the AMF <NUM> - to wake up the terminal <NUM>. The WUSs may thus be communicated in dedicated resource elements of the channel <NUM>.

A second channel <NUM> may carry paging indicators which enable the network <NUM>. The paging indicators may thus be communicated in dedicated resource elements of the channel <NUM>. Typically, the paging indicators are communicated on PDCCH.

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 <NUM>, <NUM>. Different resource elements may be allocated to the different channels <NUM>-<NUM>. For example, in many scenarios the WUS and the paging indicators are transmitted at two different time instances.

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

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

<FIG> schematically illustrates the terminal <NUM>. The terminal <NUM> includes an interface <NUM>. For example, the interface <NUM> may include an analog front end and a digital front end. Th interface <NUM> may be coupled to an antenna. In some examples, the interface <NUM> 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 <NUM> further includes a control circuitry <NUM>, e.g., implemented by means of one or more processors and software. The control circuitry <NUM> may also be at least partly implemented in hardware. For example, program code to be executed by the control circuitry <NUM> may be stored in a non-volatile memory <NUM>. In the various examples disclosed herein, various functionality may be implemented by the terminal <NUM>, 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> illustrates details with respect to the interface <NUM> of the terminal <NUM>. In particular, <FIG> illustrates aspects with respect to a main receiver <NUM> and a low-power receiver <NUM>. In <FIG>, the main receiver <NUM> and the low-power receiver <NUM> 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> may enable switching off some or all components of the main receiver <NUM> 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 <NUM>. Also, the low-power receiver <NUM> may be switched between an inactive state and an active state, e.g., according to a DRX cycle.

For example, if the main receiver <NUM> is switched on, the low-power receiver <NUM> may be switched off, and vice-versa. As such, the main receiver <NUM> and the low-power receiver <NUM> may be inter-related in operation (indicated by the arrows in <FIG>).

<FIG> illustrates details with respect to the interface <NUM> of the terminal <NUM>. In particular, <FIG> illustrates aspects with respect to the main receiver <NUM> and the low-power receiver <NUM>. In <FIG>, the main receiver <NUM> and the low-power receiver <NUM> 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> may enable a particular low latency for transitioning between reception - e.g., of a WUS - by the wake-up receiver <NUM> and reception by the main receiver <NUM>.

While in <FIG> a scenario is illustrated where the main receiver <NUM> and the low-power receiver <NUM> share a common antenna, in other examples, it would be also possible that the interface <NUM> includes dedicated antennas for the main receiver <NUM> and the low-power receiver <NUM>.

While in the examples of <FIG> scenarios are illustrated where there is a dedicated low-power receiver <NUM>, in other examples there may be no low-power receiver. Instead, the WUS may be detected by the main receiver <NUM> in a low-power state. For example, the main receiver <NUM> 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 <NUM> 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 <NUM> may transition into a high-power state in which it is fit to decode and demodulate the ordinary data, e.g., on channel <NUM>, etc..

<FIG> is a signaling diagram. <FIG> illustrates aspects with respect to communicating between the terminal <NUM> and the BS <NUM> (cf.

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

At <NUM>, configuration data <NUM> is communicated. This is generally optional. The configuration data <NUM> is transmitted by the BS <NUM> and received by the terminal <NUM>. For example, a respective control message may be communicated on the control channel <NUM>, e.g., PDCCH. For example, the control message may be a Layer <NUM> or Layer <NUM> control message. The control data <NUM> may be communicated using RRC / higher-layer signaling.

The configuration data <NUM> may be indicative of a sequence design for the WUS transmission. The configuration data <NUM> may be indicative of time-frequency resources used for the WUS transmission. The configuration data <NUM> 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 <NUM> may allocate resource elements to the channel <NUM>. 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 <NUM>, a user data <NUM> is communicated. For example, the user data <NUM> may be communicated on the payload channel <NUM>. For example, the user data <NUM> may be communicated along the data connection <NUM>, e.g., as part of a bearer, etc..

The configuration data <NUM> and the user data <NUM> are communicated using the main receiver <NUM>.

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

Multiple POs <NUM> for communicating the WUS <NUM> are then implemented by reoccurring resource elements on the channel <NUM>.

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

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

In response to receiving the WUS <NUM>, the main receiver <NUM> of the terminal <NUM> is transitioned to the active state, <NUM>.

Then, at <NUM>, a paging indicator <NUM> is transmitted by the BS <NUM> to the terminal <NUM>. The paging indicator <NUM> is received by the main receiver <NUM>. For example, the paging indicator may be transmitted on channel <NUM>, e.g. PDCCH. For example, the paging indicator may include a temporary or static identity of the terminal <NUM>. The paging indicator <NUM> may include information on a MCS used for communicating a paging message <NUM> at <NUM>. The paging message <NUM> may be communicated on a shared channel <NUM>, e.g., physical downlink shared channel (PDSCH).

Then, at <NUM>, a data connection <NUM> is set up between the terminal <NUM> and the BS <NUM>. This may include a random access procedure.

Finally, a UL or DL user-data message <NUM> is communicated using the newly set up data connection <NUM> at <NUM>.

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

Next, details with respect to the transmission of the WUS <NUM> are explained. Specifically, PHY Layer <NUM> properties of the WUS <NUM> are explained next.

<FIG> illustrates aspects with respect to the wireless interface <NUM> of the BS <NUM>. <FIG> illustrates aspects with respect to transmitting the WUS <NUM>.

The interface <NUM> includes a WUS signal-shaping block <NUM>; an IFFT block <NUM>; a parallel-to-serial block <NUM>; a CP block <NUM>; a digital-to-analog converter <NUM>; an analog frontend <NUM>; and a power amplifier <NUM>. The interface <NUM> is coupled to one or more antennas <NUM>.

A reference WUS waveform b is input to the WUS signal-shaping block <NUM>. The reference WUS waveform b has a certain length / duration. In the scenario of <FIG>, 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 <NUM> 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>).

The reference WUS waveform b is shaped to obtain multiple WUS waveforms x̃. The various WUS waveforms x̃ are associated with the WUS subcarriers reserved for the WUS channel <NUM>. The multiple WUS waveforms x̃ are input into respective channels <NUM> of the IFFT block <NUM>.

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

Generally, the IFFT block <NUM> provides modulation of signal waveforms onto various subcarriers. The OFDM modulation facilitated by the IFFT block <NUM> enables FDD: Further channels <NUM>, <NUM> of the IFFT block <NUM> are used to communicate on other channels <NUM>, <NUM>, e.g., with other terminals. A plurality of data signal waveforms x<NUM>, x<NUM> - associated with subcarriers different from the WUS subcarriers - are obtained. The data signal waveforms x<NUM>, x<NUM> are defined in accordance with a coherent modulation scheme, e.g., QPSK, BPSK, or QAM. The data signal waveforms x<NUM>, x<NUM> are then input to the channels <NUM>, <NUM> of the IFFT block <NUM> (also cf. <FIG>, where details of the IFFT block <NUM> are shown).

In accordance with <FIG> and <FIG>, a vector representation of the data input to the IFFT block <NUM> is as follows: <MAT>.

In Eq. (<NUM>), <MAT> denotes the data signal waveforms and <MAT> denotes the WUS waveform. <IMG> denotes the set of sub-carriers associated with the wake-up signal waveform x̃, i.e., {k<NUM>,. , k<NUM> + K - <NUM>}. The center sub-carrier of <IMG> is kc.

The IFFT block <NUM> transforms from frequency domain to time domain. An output of the IFFT block <NUM> corresponds to a set of complex time-domain samples representing the OFDM subcarrier signals.

The operation of the IFFT block <NUM> can be represented in time domain as follows: <MAT>.

The baseband representation of the WUS <MAT> is denoted b̃n. Here, "baseband" refers to the signal before modulation onto the sub-carriers. Here, n is the index of the various output channels of the IFFT block <NUM>.

The IFFT block can be described by a linear transformation F; Eq. (<NUM>) can be rewritten in matrix notation: <MAT>.

In block <NUM>, 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 <NUM>, 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. Hence, blocks <NUM>, <NUM>, <NUM> implement an OFDM modulator as they output a baseband OFDM symbol of a certain duration.

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

<FIG> illustrates that the baseband OFDM symbol s includes two contributions, i.e., (i) the contribution from the WUS sW (the WUS part of the OFDM symbol) and (ii) the contribution from the data signal sO. sW is the WUS part of the OFDM symbol s modulated on the WUS subcarriers associated with the channels <NUM>; and sO is the part of the OFDM symbol sO modulated on the subcarriers associated with the channels <NUM>, <NUM>: <MAT>.

The WUS part sW of the OFDM symbol s corresponds to the WUS <NUM>.

In <FIG>, the signal shaping block <NUM> is configured to shape the reference WUS waveform b such that the baseband representation of the WUS part sW of the OFDM symbol s, i.e., b̃, is approximately equal to b. Such an approach allows for orthogonality between waveforms x<NUM>, x<NUM> and x̃ when included in the same OFDM symbol s. This is achieved by communicating the WUS <NUM>, sW as an OFDM-based modulated signal. The signal shaping block <NUM> calculates the necessary input x̃ to the IFFT block <NUM> on the subcarriers <NUM>-<NUM> designated for the WUS <NUM> which is needed to approximate a desired reference WUS waveform b in the time domain. Thereby, the WUS part sW 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 sO 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 <NUM>. 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 <NUM>. 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 <NUM>. 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 x̃. 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 b̃ to the signal shaping block <NUM>. Then, an iterative optimization algorithm may be employed that - e.g., in a numerical simulation - varies the output of the signal shaping block <NUM>, i.e., x̃, until an optimization criterion is met; the optimization criterion can correspond to a difference between the reference WUS waveform b and b̃. In a further example, the shaping can be based on an analytic approximation of the OFDM modulator <NUM>-<NUM>. For example, it would be possible that the shaping is based on an approximation of the IFFT block <NUM>. The approximation of the IFFT block <NUM> can be denoted F̃. Here, F̃ can be a sub-matrix of F. The dimension of F̃ can be NxK, see Eqs. (<NUM>)-(<NUM>). For example, it would be possible to select the WUS-subcarriers <IMG> symmetrically around the center subcarrier kc. Thereby, the output of the IFFT block <NUM> 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 <NUM> minimizes a difference between b̃ 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.: <MAT>.

Eq. <NUM> can be reformulated as: <MAT> where x̃LS denotes the WUS waveform x̃ as obtained from the least-squares metrics approximation.

As a general rule, while <FIG> 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>.

<FIG> illustrates aspects with respect to such a signal shaping using Eq. <NUM>. In <FIG>, the dashed line illustrates the reference WUS waveform b and the full line illustrates the baseband representation b̃ of the WUS part sW of the OFDM symbol s, i.e., the baseband representation of the WUS <NUM>. As illustrated in <FIG>, b̃ ≈ b.

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

This facilitates employing a low-power receiver <NUM> for receiving the WUS part sW of the OFDM symbol s, cf.

Sometimes, a scenario may occur where the duration <NUM> (dWUS) (cf. <FIG>) 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 <FIG>, i.e., each reference WUS waveform segment constitutes a respective (shorter) reference WUS waveform on its own.

Longer durations <NUM> 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 dbit is duration <NUM> in <FIG>. <FIG> illustrates how this situation is handled.

<FIG> illustrates aspects with respect to the wireless interface <NUM> of the BS <NUM>. <FIG> illustrates aspects with respect to transmitting the WUS <NUM>. The scenario of <FIG> generally corresponds to the scenario of <FIG>; with additional preprocessing 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 <NUM> in the same manner as discussed for <FIG> for the reference WUS waveform b.

In the scenario of <FIG>, there is an additional block <NUM> 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 {<NUM>.

In principle, M may be any positive integer.

For each reference WUS waveform segment bm, it is then possible to perform block <NUM>, <NUM>, <NUM>-<NUM>. Then, the WUS <NUM> is transmitted spread out across a sequence of M OFDM symbols. The receiver can perform a correlation considering the overall duration <NUM> of the WUS <NUM> (cf. <FIG>) and may not need to synchronize with the timing of the OFDM symbols of the sequence.

Next, details with respect to block <NUM> are explained. In block <NUM> 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>. The input <NUM> to the bock <NUM> is illustrated in <FIG>, top row.

<FIG> illustrates aspects with respect to a sequence <NUM> of OFDM symbols <NUM> - <NUM>. Each OFDM symbol <NUM> - <NUM> has a respective CP <NUM>-<NUM>. The duration <NUM> (dOFDM) of the OFDM symbols <NUM> - <NUM> is shorter than the duration <NUM> of the reference WUS waveform b. The duration <NUM> of the CPs <NUM>-<NUM> is typically a small fraction of the duration <NUM> of the OFDM symbols <NUM> - <NUM>, e.g., in the range of <NUM>% to <NUM> % or in the range of <NUM> % to <NUM> %.

Due to the relatively short duration <NUM> of the CPs <NUM>-<NUM>, it is possible to crop - i.e., discard - sections of the reference WUS waveform b which correspond to the CPs <NUM>-<NUM>, see row <NUM>. Since the CPs <NUM>-<NUM> also mark the boundaries between adjacent OFDM symbols <NUM> - <NUM>, thereby a total of M=<NUM> reference WUS waveform segments b<NUM>, b<NUM>, b<NUM>, b<NUM>, b<NUM> are obtained in this example by appropriately cutting the reference WUS waveform b. As a general rule, M may take other values than <NUM>.

Row <NUM> then illustrates the input to the CP block <NUM> and <NUM> illustrates the output of the CP block <NUM>. As illustrated by the arrows in <FIG>, bottom row, the end of the respective OFDM symbol <NUM>-<NUM> is added as the CP at the beginning of each OFDM symbol <NUM>-<NUM>.

From a comparison of <NUM> with <NUM>, it is apparent that the baseband representation b̃ of the WUS part sW of the OFDM symbols <NUM>-<NUM> - including the CPs <NUM>-<NUM> - deviates from the reference WUS waveform b - specifically at the CPs <NUM>-<NUM>. However, due to the relatively short duration <NUM> of the CPs <NUM>-<NUM>, this difference does not result in a significant loss of the ability of the non-coherent receiver to detect the WUS <NUM>. Details with respect to the receiver are illustrated in connection with <FIG>.

<FIG> illustrates aspects with respect to the low-power receiver <NUM>. The low-power receiver <NUM> is coupled to an antenna <NUM>. The low-power receiver <NUM> may include a bandpass filter that restricts the receive bandwidth to the subcarriers <NUM>-<NUM> for WUS transmission (not illustrated in <FIG>). The low-power receiver <NUM> includes an analog frontend that may perform demodulation from the carrier. A non-coherent WUS detector <NUM> 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 <NUM> 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> illustrates aspects with respect to the main receiver <NUM>. The main receiver <NUM> is coupled to an antenna <NUM>. The main receiver <NUM> includes a low noise amplifier <NUM>, an analog-to-digital converter <NUM>, a cyclic prefix removal block <NUM>, a serial-to-parallel conversion <NUM>, and an FFT block <NUM>. The FFT block <NUM> outputs multiple channels <NUM>-<NUM>. The channels <NUM> include the WUS waveform x̃ which, however, can be discarded, because the main receiver <NUM> is already in active state. The blocks <NUM>-<NUM> hence form an OFDM de-modulator.

As a general rule, the low-power receiver <NUM> of <FIG> and the main receiver <NUM> of <FIG> may be integrated into the same device, e.g., into the UE <NUM>. In such an implementation, it is possible that the main receiver <NUM> and the low-power receiver <NUM> re-use the same antenna. The low-power receiver <NUM> and the main receiver <NUM> can be individually activated and deactivated, e.g., depending on the operational mode of the UE <NUM>.

<FIG> is a flowchart of a method according to various examples. In <FIG>, blocks <NUM>-<NUM> may be performed by a transmitter, e.g., implemented by the wireless interface <NUM> of the BS <NUM> (cf. <FIG> and <FIG>) or implemented by a wireless interface of another device, e.g., the UE <NUM>.

At block <NUM>, 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.. Hence, 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 <NUM>. the reference waveform is signal shaped. For example, Eq. (<NUM>) 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 <NUM> corresponds to OFDM modulation. It is possible to consider further data signal waveforms on other subcarriers.

At block <NUM>, the OFDM symbol obtained from the OFDM modulation of block <NUM> is transmitted.

In scenarios in which block <NUM> includes cropping and cutting a further reference waveform, blocks <NUM>-<NUM> can be re-executed for all reference waveform segments that fit into the further reference waveform (cf. 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> is a flowchart of a method according to various examples. In <FIG>, blocks <NUM> and <NUM> may be implemented by a receiver, e.g., the main receiver <NUM> of the wireless interface <NUM> of the terminal <NUM>. Blocks <NUM> and <NUM> may be implemented by a receiver, e.g., the low-power receiver <NUM> of the wireless interface <NUM> of the terminal <NUM>.

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

Block <NUM> corresponds to operation of the main receiver, if activated. Here, the one or more OFDM symbols are received across the entire carrier bandwidth, i.e., including all subcarriers, in block <NUM>. 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 <NUM>. 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 <NUM> corresponds to operation of the low-power receiver, e.g., if the main receiver is in inactive state. Here, reception of the one or more OFDM symbols, at block <NUM>, is restricted to the bandwidth of the selected subcarriers encoding the waveform, e.g., by employing a bandpass filter. Then, at block <NUM>, OOK decoding - or, generally, another non-coherent decoding - is applied.

Block <NUM> 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 <NUM> or larger. For cases of M><NUM>, 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 <NUM>% 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.

For illustration, above, various examples have been described with respect to communicating WUS. However, 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.

Claim 1:
A method, comprising:
- cutting and cropping a reference signal waveform (b) at sections of the reference signal waveform (b) mapped to cyclic extensions (<NUM>-<NUM>) of OFDM symbols (s, <NUM>-<NUM>) of a sequence (<NUM>) of OFDM symbols, to obtain reference signal waveform segments (b<NUM>-b<NUM>), the reference signal waveform and the reference waveform segments being defined in accordance with a non-coherent modulation scheme,
wherein 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 (x̃) associated with one or more subcarriers (K) of a plurality of subcarriers (<NUM>-<NUM>),
- inputting the respective at least one signal waveform to at least one corresponding channel (<NUM>) of a multi-channel orthogonal frequency division multiplex, OFDM, modulator (F, <NUM>, <NUM>, <NUM>), and
- transmitting a respective OFDM symbol (s) of the sequence (<NUM>) of OFDM symbols output by the OFDM modulator (F, <NUM>, <NUM>, <NUM>),
wherein said shaping approximates a baseband representation (b̃) of a part (sw) of the OFDM symbol (s, <NUM>-<NUM>) associated with the one or more subcarriers (<NUM>-<NUM>) to the reference signal waveform segment (b<NUM>-b<NUM>).