Preamble signal for wake-up communication in a wireless communication system

Abstract

A method for frequency calibration of a receiver performed in a base station is disclosed. The base station generates a first signal with periodic ASK modulation and transmits the first signal as a preamble or frequency marker signal to facilitate frequency calibration of the receiver. The receiver sweeps its oscillator frequency to detect the presence of the preamble or frequency marker signal. When the presence of the signal is determined, the oscillator frequency of the receiver is calibrated to the transmission frequency of the base station. The base station then transmits a second signal as a wake-up signal to the receiver.

Description

Technical Field

[0001] The embodiments herein relate to a wireless communication device, a base station, and methods therein for wake-up communication in a wireless communication system. Specifically, the embodiments herein relate to a method for frequency calibration of a wake-up receiver in a wireless communication device. Background Art

[0002] In wireless communication systems, there is a strong trend toward supporting devices with ultra-low power consumption. These devices can be small sensor nodes whose batteries are expected to last for many years, or where energy mining is used to achieve battery-free operation. For a wireless communication system to communicate with such a device, a receiver must be operated in that device. To achieve short response times, the receiver must be operated regularly. Consequently, the receiver's power consumption must be limited. Therefore, special ultra-low-power dedicated receivers, so-called wake-up receivers, are often used. However, the capabilities of a wake-up receiver are limited; it can only detect the presence of a wake-up request. When such a request is received, a higher-performance, higher-power main receiver is activated to receive the actual communication.

[0003] To achieve ultra-low power consumption, e.g., less than 100uW, wake-up receivers are typically based on amplitude detection of an on-off keying (OOK) signal. This avoids the need for a power-hungry phase-locked loop (PLL) to generate an accurate local oscillator (LO) signal. However, due to the frequency inaccuracy of the LO signal from a free-running oscillator, only moderate filtering can be implemented before amplitude detection.

[0004] Due to the limited amount of filtering before amplitude detection, wake-up receivers are highly susceptible to interference. Any interference and noise with amplitude modulation in the same frequency range as the wake-up signal that enters the amplitude detector will mask the wake-up signal. It should be understood that the same frequency does not necessarily mean that the interference is transmitted on the same channel. On the contrary, due to the limited ability to filter out signals adjacent to the wake-up signal, it should be understood that signals transmitted in adjacent channels, and potentially even channels further away, will effectively produce the same detrimental effects as co-channel interference. Alternatively, to effectively filter out adjacent interference, frequency generation must be highly accurate, consuming significant power. Furthermore, the amplitude detector is severely nonlinear, producing a very small output for weak input signals. Assuming the amplitude detector has quadratic characteristics for small signals, this means that for every 10dB decrease in the input signal level, the signal-to-noise ratio decreases by 20dB. This quickly becomes a significant disadvantage given the presence of moderate interference at the detector input, as the presence of the interference will limit how much the input signal can be amplified. Therefore, more filtering before amplitude detection is required to achieve a wake-up receiver with high interference immunity. However, in order to employ this filtering, an LO signal of accurate frequency is required, and a method of generating this signal with low power consumption must be provided. Summary of the Invention

[0005] Therefore, an object of embodiments herein is to provide a method for waking up a receiver to generate and calibrate an oscillator signal in a wireless communication system.

[0006] According to a first aspect of the embodiments herein, the object is achieved by a method, performed in a base station, for facilitating frequency calibration of a receiver. The base station generates a first signal having periodic amplitude shift keying (ASK) modulation and transmits the first signal as a calibration signal to the receiver to facilitate frequency calibration of the receiver. The base station then transmits a second signal as a wake-up signal to the receiver.

[0007] According to a second aspect of the embodiments of the present invention, the object is achieved by a method performed in a wake-up receiver. The wake-up receiver has an architecture based on an envelope detector. The wake-up receiver comprises at least: a frequency down-conversion mixer; a digitally controlled oscillator (DCO) configured to generate an oscillator signal; a first filter having a programmable bandwidth; an envelope detector; a second filter having a programmable bandwidth; and a digital processing and control unit configured to determine whether a signal is present and generate a frequency control signal to the DCO and a bandwidth control signal to the first filter and the second filter.

[0008] The wake-up receiver is set to a first bandwidth by selecting digital settings of bandwidth control signals to the first and second filters to receive a first signal transmitted by the base station. The first signal is a periodic ASK modulated signal and is transmitted to facilitate frequency calibration of the receiver.

[0009] The wake-up receiver operates the DCO at different frequencies by scanning the digital settings of the frequency control signal and detecting the presence of a signal for each digital setting of the frequency control signal.

[0010] Then, when a signal is detected, the receiver wakes up to store the digital setting of the frequency control signal.

[0011] The wake-up receiver may be set to a second bandwidth and scan the digital setting of the frequency control signal to detect the presence of a signal, wherein the second bandwidth is narrower than the first bandwidth.

[0012] According to an embodiment of the present invention, the base station generates a first signal with periodic ASK modulation (e.g., OOK modulation) and sends the first signal as a preamble or frequency marker signal to facilitate frequency calibration of the receiver. The first signal can be sent at or close to the frequency of the wake-up signal to be sent. The receiver scans its oscillator frequency to detect whether there is a preamble or frequency marker signal and at what oscillator frequency setting. When it is determined that a signal is present, the oscillator frequency of the receiver is calibrated to the transmit frequency of the base station under the corresponding frequency setting. The base station then sends a second signal to the receiver as a wake-up signal. The receiver uses a frequency-calibrated oscillator to receive the wake-up signal.

[0013] A signal with periodic ASK / OOK modulation allows its frequency to be quickly acquired under varying signal conditions. A method performed in a wake-up receiver provides a scheme where the receiver can perform a fast frequency scan to search for a preamble signal and then perform a more sensitive selective search at the frequency where the preamble signal tone was detected in the first scan. The characteristics of the ASK / OOK signal allow the frequency to be scanned without interrupting reception while the receiver amplitude demodulated output is filtered to find the preamble or marker signal frequency tone. After the frequency scan, the wake-up receiver oscillator is calibrated to the frequency of the preamble signal. Using the first signal transmitted by the base station as the frequency marker signal or preamble signal allows the receiver to use a free running oscillator with lower power consumption while still being narrowband to achieve high selectivity.

[0014] By using this solution, an ultra-low power wake-up receiver with accurate oscillator frequency generation can be achieved, and this allows the use of narrow bandwidth filters, resulting in high selectivity. The combination of ultra-low power and high selectivity is very attractive.

[0015] Therefore, embodiments herein provide an improved method for waking up a receiver to generate a high-precision oscillator signal by calibrating the frequency of a free-running oscillator based on a preamble signal, which results in an ultra-low power and high-selectivity wake-up of the receiver. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Examples of embodiments herein are described in more detail with reference to the accompanying drawings, in which:

[0017] Figure 1 is a schematic block diagram illustrating a wake-up receiver architecture in which the method for frequency calibration according to embodiments of the present invention may be implemented;

[0018] Figure 2 is a flow chart illustrating a method performed in a base station according to an embodiment of the present invention;

[0019] Figure 3is a schematic block diagram illustrating an embodiment of a base station in which the method according to the embodiments herein may be implemented;

[0020] Figure 4 is a flow chart illustrating a method performed in a wake-up receiver according to embodiments herein;

[0021] Figure 5 is a diagram showing frequency spectra with low-frequency and high-frequency modulated tones and different filter options;

[0022] Figure 6 is a diagram showing an OOK waveform with two strong tones;

[0023] Figure 7 is a waveform showing the 9th harmonic and Figure 6 A graph of one cycle of the synthetic waveform;

[0024] Figure 8 It shows Figure 6 A plot of the frequency spectrum of the waveform in ;

[0025] Figure 9 is a graph showing the frequency spectrum of a square wave having a duty cycle of 50%; and

[0026] Figure 10 It also shows Figure 8 and Figure 9 Spectrum diagram. DETAILED DESCRIPTION

[0027] Figure 1An example of a wake-up receiver 100 with an envelope detector-based architecture is shown. As can be seen, wake-up receiver 100 includes a band-select filter 102 for filtering RF signals received at an antenna; a frequency down-conversion mixer 104 configured to convert the received RF signal to a baseband or intermediate frequency (IF) signal; and a digitally controlled oscillator (DCO) 106 configured to generate an oscillator signal for mixer 104. Mixer 104 is followed by baseband / IF amplification and filtering. Wake-up receiver 100 may also include a first baseband / IF amplifier 108, a first baseband / IF filter 110 with a programmable bandwidth, a second baseband / IF amplifier 112, and an envelope detector 114. Due to the lower frequencies, sharpening filtering in first baseband / IF filter 110 may be applied to minimize interference reaching envelope detector 114. Wake-up receiver 100 may also include a third amplifier 116 and a second baseband / IF filter 118 with a programmable bandwidth following envelope detector 114. An analog-to-digital converter (ADC) or a simple comparator 120 is then used to convert the baseband / IF signal into a digital signal, such as a digital signal in the form of a bit sequence. The wake-up receiver 100 also includes a digital processing and control unit 122, which is configured to receive the digital signal and determine whether a signal is present, and to generate a frequency control signal 124 to the DCO 106, and bandwidth control signals 126 and 128 to the first filter 110 and the second filter 118, respectively.

[0028] The DCO 106 is the core and main power consumer of the wake-up receiver 100. To achieve ultra-low power consumption, a ring oscillator can be used. When it comes to the mixer 104, a passive structure is attractive for minimizing power consumption and flicker noise.

[0029] Compared to an ADC that can produce a multi-bit representation of a signal, using a comparator produces a single bit. However, comparators are very low power, and the single bit eliminates the need for automatic gain control (AGC), making it well suited for ultra-low power receivers.

[0030] According to embodiments herein, a signal is sent to assist the wake-up receiver 100 in tuning its DCO 106 frequency. The modulation of the auxiliary signal may be ASK with a periodic modulation pattern, such as OOK. Figure 2 A method is described that is performed in a base station to facilitate frequency calibration of a wake-up receiver 100. The method comprises the following actions.

[0031] Action 210

[0032] The base station generates a first signal with periodic ASK (e.g., OOK) modulation. By using an ASK / OOK helper signal with only amplitude information, the DCO frequency of the wake-up receiver 100 can be quickly stepped or continuously scanned without significantly distorting the received signal.

[0033] There are different options for the ASK / OOK modulation. A first option is to use a single tone modulation with a relatively high frequency of about, for example, 1 MHz. A second option can be to use a lower frequency tone modulation, and a third option can be to generate an ASK / OOK modulation containing two strong frequency tones, one at a lower frequency and one at a higher frequency. An ASK / OOK signal containing more than one strong modulation frequency component can still be well suited for a single bit representation in a power efficient ASK / OOK receiver. To achieve this, the high frequency harmonics can be effectively enhanced by replacing each zero crossing in the fundamental square wave with a wavelet of a harmonic frequency square wave.

[0034] Thus, according to some embodiments herein, the first signal with periodic ASK / OOK modulation can contain a first frequency modulation tone and a second frequency modulation tone. By starting with a square wave with a lower frequency, an OOK signal with multi-tone content can be created. Then each zero crossing is replaced by a square wave, i.e., a wavelet with a higher frequency, forming multiple zero crossings to increase the high frequency content of the signal.

[0035] Thus according to some embodiments herein, the first signal can be a periodic OOK signal, and can be generated by replacing each zero crossing in a square wave of a first frequency with a wavelet of a square wave of a second frequency.

[0036] Action 220

[0037] The base station transmits the first signal as a calibration signal to the receiver to facilitate frequency calibration of the wake-up receiver 100.

[0038] The first signal can be used as a preamble of the wake-up signal, and can be transmitted at or close to the frequency at which the wake-up signal is to be transmitted. The first signal is transmitted for a long enough time so that the wake-up receiver 100 is likely to find it, and then the actual wake-up signal can be transmitted at the same frequency as the first signal.

[0039] Action 230

[0040] The base station transmits the second signal as a wake-up signal to the wake-up receiver 100.

[0041] The second signal is transmitted at the same frequency as the first signal after a period of time from when the first signal has been transmitted.

[0042] An alternative is to send the first signal and then after a period of time send a wake-up signal at an adjacent frequency while still sending the first signal. If not acknowledged by the target device (e.g. the wake-up receiver 100), the transmission of the first signal can then continue while more attempts to contact the target device are made by sending the wake-up signal. In this case, the first signal serves as a frequency marker indicating the frequency location of the wake-up signal.

[0043] Thus, according to some embodiments herein, the first signal can be transmitted continuously and the second signal can be transmitted when the receiver is to be requested to wake up.

[0044] According to some embodiments herein, the first signal can be a first frequency marker signal and the second signal can be transmitted at a frequency that is offset relative to the first frequency marker signal.

[0045] To support calibration over a larger frequency range and to improve the accuracy of the calibration, two or more frequency marker signals with different frequencies can be transmitted. For example, a second frequency marker signal can be transmitted, e.g. on the other side of the wake-up signal frequency. Changes in the frequency tuning sensitivity of the DCO in the wake-up receiver 100 can then be compensated for, which otherwise can cause some uncertainty when the DCO frequency is shifted from the first marker signal frequency to the wake-up signal frequency location.

[0046] Figure 3 A block diagram of a base station 300 is shown. The base station 300 comprises, e.g., a receiver 310, a transmitter 320, a processing unit 330, a memory 340, etc. The base station 300 is configured to perform the above described actions 210-230.

[0047] According to embodiments herein, reference will now be made to Figure 4 A method performed in the wake-up receiver 100 for frequency calibration and receiving a signal is described. The method comprises the following actions, which can be performed in any suitable order.

[0048] Action 410

[0049] The wake-up receiver 100 is set to a first bandwidth to receive a first signal by selecting digital settings of the bandwidth control signals 126, 128 to the first filter 110 and the second filter 118. The first signal is a periodic ASK (e.g. OOK) modulated signal and is transmitted to facilitate frequency calibration of the wake-up receiver 100.

[0050] Action 420

[0051] The wake-up receiver 100 operates the DCO 106 at different frequencies by scanning the digital settings of the frequency control signal 124 to detect if a signal is present.

[0052] The wake-up receiver 100 sweeps its DCO frequency while simultaneously filtering the output signal from the envelope detector 114 to detect the tone of the OOK modulated signal. To facilitate rapid acquisition of the oscillator frequency, a two-frequency sweep process can be performed. The first frequency sweep can be performed using a wider baseband / IF filter bandwidth to allow for a faster frequency sweep with more frequency shift during modulation tone detection, since the bandwidth of the signal becomes wider if a higher-frequency modulation tone is used. The digital processing and control unit 122 can therefore adjust the bandwidth of the first baseband / IF filter 110. Following the first baseband / IF filter 110, the signal undergoes envelope detection and is then filtered again by the second baseband / IF filter 118 and amplified. Furthermore, this second baseband / IF filter 118 can be programmable, depending on which tone is to be detected or whether an actual wake-up signal is to be received.

[0053] Action 430

[0054] The wake-up receiver 100 detects the presence of a signal for each digital setting of the frequency control signal 124. The results of the scan (i.e., the detected tone amplitude and the DCO frequency setting) are stored in memory. A narrowband digital filter can then be used to find the amplitude of the OOK tone to further assist in filtering after the detector, and the filtered results are stored in memory to determine whether a signal is present. If an actual wake-up signal is to be received, a correlator can be used instead to find the corresponding bit sequence.

[0055] Action 440

[0056] When a signal is detected, the wake-up receiver 100 stores the digital setting of the frequency control signal 124. That is, for that digital setting, the amplitude of the detected tone is above the threshold and determines that a signal is present. In this way, the DCO is calibrated to the frequency of the first signal.

[0057] Action 450

[0058] This action is optional. The wake-up receiver 100 may be set to a second bandwidth and scan the digital setting of the frequency control signal to detect whether a signal is present, wherein the second bandwidth is narrower than the first bandwidth.

[0059] The first frequency scan will be relatively fast, which combined with the higher bandwidth will result in reduced sensitivity. In this case, the first scan is performed to try to find a frequency marker or preamble signal, and when the signal is found to be stronger than the average (i.e., the noise level), it is desirable to perform a more careful inspection, i.e., a slower second search. Therefore, for those DCO frequencies (if any) whose detected amplitude is significantly stronger than the average, a second search can be performed using a lower baseband / IF filter bandwidth. The sensitivity will then be higher, and the presence of the marker signal or preamble signal can be detected with greater certainty. However, in good signal conditions, the sensitivity of the first scan may be sufficient to make a decision.

[0060] Assuming that the filtering after the detector (i.e., second filter 118) is narrower, the effective bandwidth of the receiver is equal to the geometric mean of the bandwidths of first filter 110 and second filter 118. To effectively improve sensitivity, the bandwidths of both filters should be reduced simultaneously. Reducing the bandwidth of only one filter would be less effective, as a 4x reduction would be required to achieve a 3dB improvement. In contrast, when both filter bandwidths are reduced together, a 4x reduction in the bandwidth of each filter results in a 6dB improvement in receiver sensitivity.

[0061] Before deciding which DCO digital setting of the frequency control signal to use to receive the wake-up signal, multiple scans, full scans as well as partial scans, can be performed using different filter bandwidths (i.e., with different sensitivities and scan speeds). Because the free-running oscillator in the wake-up receiver is calibrated to the base station's transmit frequency, DCO frequency generation can be both accurate and low-power.

[0062] Action 460

[0063] After calibration, the wake-up receiver 100 may receive a second signal transmitted by the base station as a wake-up signal.

[0064] When the wake-up signal is transmitted at the same frequency as the first signal, the wake-up receiver 100 may operate the DCO using the digital setting of the frequency control signal detected for the first signal to receive the second signal.

[0065] When the wake-up signal is transmitted at a frequency offset from the first signal, the wake-up receiver 100 may operate the DCO using a digital setting of the frequency control signal offset from the digital setting at which the first signal was detected to receive the second signal.

[0066] When two or more first signals are transmitted from a base station at different frequencies, the wake-up receiver 100 can operate the DCO using digital settings calculated from two or more digital settings implemented when the two or more first signals with different frequencies are detected to receive the second signal. The calculation can be an interpolation or extrapolation from the two or more digital settings, which allows the DCO to be operated not only at frequencies between frequency markers (i.e., interpolation) but also at frequencies outside frequency markers (i.e., extrapolation).

[0067] While the embodiments herein describe the wake-up receiver 100 as an example architecture in which a simple comparator is used, the embodiments herein are also applicable to different receiver architectures, such as those in which an ADC may be used. Specifically, the transmission of the preamble signal enables power-efficient and accurate frequency generation in wake-up receivers of any type of architecture.

[0068] The wake-up receiver 100 can be implemented in a wireless communication device. The wireless communication device can be any of an ultra-low power device (e.g., a small sensor node), a wireless communication terminal, a user equipment, a machine type communication (MTC) device, a device-to-device (D2D) terminal, or any type of node in a wireless communication system (e.g., a smartphone, a laptop, a mobile phone, a repeater, a mobile tablet computer, or even a small base station communicating within a cell). According to an embodiment of the present invention, the wake-up receiver 100 can be configured to perform any one or all of the above-mentioned method steps or actions 410 to 460.

[0069] In order to detect signals with different OOK modulations, the wake-up receiver 100 has different bandwidth settings. Figure 5 Simplified spectra are shown for different modulation signal options and filter options.

[0070] The task of the baseband / IF filter is to pass the modulating tone with enough margin to allow for short term oscillator drift and, in the case of a frequency sweep, to allow for frequency shifts during settling of the post-detector filter. Figure 5Some different filter options are shown. Filter 1 selects both the low-frequency tone and the high-frequency tone, plus some margin for the sweep frequency shift. This is suitable for a fast sweep if a low-frequency tone, a high-frequency tone, or both are used in the OOK modulated signal. Filter 2 is similar to filter 1, but with a slightly smaller bandwidth. It is suitable for a second sweep to detect the high-frequency tone, and also for a low-frequency tone if present, or for a fast sweep to detect the low-frequency tone. Filter 3 is tailored for a second sweep to detect the high-frequency tone, and due to its narrower bandwidth and suppression of potential low-frequency interference, it will provide increased receiver sensitivity compared to using filter 2. Filter 4 is suitable for a second sweep to detect the low-frequency tone. Not shown in the figure is a notch for DC, which can be performed using filters 1, 2, and 4 to eliminate DC offset.

[0071] For example, for single-tone OOK modulation, a baseband / IF bandwidth with a relatively high frequency, such as 4 times the modulation tone frequency, of approximately 1 MHz can yield sufficient margin to allow a high sweep speed in the first sweep. In the second sweep, where different frequency tuning settings are individually tried, more stringent filtering can be used. If a homodyne receiver is used, a bandpass filter near the modulation frequency can be used.

[0072] For single-tone OOK modulation with relatively low-frequency modulation, a simpler, narrower low-pass filter than a bandpass filter can be used in the second sweep. However, the risk of noise and interference increases at lower frequencies.

[0073] For a signal with OOK modulation containing two strong tones (one at a lower frequency and one at a higher frequency), the wake-up receiver 100 can then select the tone frequencies to search for and the baseband / IF filters to implement. A disadvantage of a two-tone modulated signal is that, being transmitted at the same power as a single-tone signal, each tone will have slightly worse signal strength. However, a benefit of a two-tone signal is that it reduces the risk of errors in waking up the receiver by ensuring the presence of both tones (e.g., by detecting the higher frequency tone in the first scan and the lower frequency tone in the second scan).

[0074] To maximize the performance of detecting a preamble / marker signal having two tones when processed by a single-bit wake-up receiver 100, the OOK modulation should have an effective average level of 50% both in the high frequency region and over a longer time interval corresponding to the period of the low frequency tone. This will maximize the noise margin of the two detected amplitude levels corresponding to the digital zero and digital one of the two tones. Figure 6Such a composite OOK waveform is shown with two strong tones, the fundamental tone at 100kHz and the 9th harmonic tone at 900kHz. It can be seen that filtering out 0.5 of DC in the waveform will result in a clear decision between 0 and 1 for both the high and low frequency tones.

[0075] Figure 7 The amplitude waveform of the 9th harmonic (ie, the square wave shown at the top) and Figure 6 One cycle of the composite waveform (i.e., the waveform shown at the bottom) is plotted, with equally spaced time markers also plotted. It can be seen that all edges align with the equally spaced time markers, so no additional oversampling beyond the 9th harmonic is required. It can also be observed that all wavelets in the composite waveform, such as those indicated by reference numerals 701 and 702, are in phase with the 9th harmonic square wave, resulting in the maximum amplitude of that frequency component in the composite waveform.

[0076] Figure 8 Shown Figure 6 The spectrum of the waveform in [ ] shows strong power at the fundamental and the 9th harmonic. As can be seen, the waveform contains only DC, the fundamental tone, and odd harmonics. The 9th harmonic is the strongest of the odd harmonics and has a similar amplitude to the fundamental. Therefore, an OOK signal can contain more than one strongly modulated frequency tone while still fitting well into a single-bit representation in a power-efficient OOK receiver.

[0077] As a comparison, Figure 9 The figure shows a 50% duty cycle and Figure 6 Figure 1 shows the spectrum of an ordinary square wave with the same waveform as in Figure 1. It can be seen that in this case the 9th harmonic has a much smaller amplitude of approximately -23dB compared to the fundamental tone of approximately -3.9dB.

[0078] Figure 10 It also shows Figure 8 and Figure 9 The spectra of two signals. Figure 6 The spectrum of the waveform is marked with a circle, and the spectrum of the regular square wave is marked with a cross. This helps compare the amplitudes of the tones in the two waveforms. When the two-tone signal is used instead of the regular square wave, the power of the fundamental tone drops from -3.9dB to -7.6dB, a decrease of 3.7dB. At the same time, the ninth harmonic increases from -23.0dB to -9.0dB, a 14dB increase. Thus, a 3.7dB decrease in fundamental tone power is traded for a 14dB increase in the ninth harmonic, an attractive compromise.

[0079] In summary, the embodiments herein provide a method performed in a base station to facilitate receiver frequency calibration by sending a signal with periodic OOK as a preamble or frequency marker signal. The wake-up receiver performs a frequency scan to search for the frequency marker signal or preamble signal and calibrates its oscillator frequency to the frequency of the preamble or frequency marker signal. The wake-up receiver can then receive the wake-up signal using the calibrated oscillator. The characteristics of OOK allow the frequency to be scanned without interrupting reception while filtering the receiver AM demodulation output to find the preamble / marker signal tone. Using the signal sent by the base station as a frequency marker or preamble allows the receiver to use a free-running oscillator while still being narrowband to achieve high selectivity. High selectivity can therefore be achieved with ultra-low receiver power consumption.

[0080] When the word "comprise" or "comprising" is used, it should be interpreted as non-limiting, ie meaning "consisting at least of.

[0081] The embodiments herein are not limited to the preferred embodiments described above. Various alternatives, modifications, and equivalents may be used. Therefore, the above embodiments should not be considered to limit the scope of the present invention, which is defined by the appended claims.

Claims

1. A method for facilitating frequency calibration of a receiver (100), performed in a base station (300), the method comprising: generating (210) a first signal having a periodic ASK modulation, the ASK modulation comprising two frequency tones, one frequency tone at a first frequency and one frequency tone at a second frequency; sending (220) the first signal as a calibration signal to the receiver to facilitate frequency calibration of the receiver; as well as sending (230) a second signal as a wake-up signal to the receiver, wherein the first frequency is lower than the second frequency, and The ASK modulation has an effective average level of 50% in a frequency region corresponding to the second frequency and in a time interval corresponding to a frequency tone period at the first frequency.

2. The method according to claim 1, wherein The second signal is transmitted at the same frequency as the first signal after a period of time from when the first signal has been transmitted.

3. The method according to claim 1, wherein The first signal is a first frequency marker signal, and the second signal is transmitted at a frequency that is shifted relative to the first frequency marker signal.

4. The method according to claim 1, wherein The first signal is continuously transmitted, and the second signal is transmitted when a receiver wake-up is to be requested.

5. The method according to any one of claims 3 to 4, further comprising: A second frequency marker signal is sent at a second marker frequency to support receiver calibration over a larger frequency range.

6. The method according to claim 1, wherein The periodic ASK signal is an OOK signal and is generated by replacing each zero crossing in the square wave of the first frequency with a wavelet of the square wave of the second frequency.

7. A method performed in a wake-up receiver (100), wherein: The wake-up receiver has an envelope detector-based architecture and comprises at least: frequency down-conversion mixer (104); A digitally controlled oscillator DCO (106) configured to generate an oscillator signal; A first filter (110) having a programmable bandwidth; Envelope detector (114); A second filter (118) having a programmable bandwidth; and a digital processing and control unit (122) configured to: determine whether a signal is present and generate a frequency control signal (124) to the DCO (106) and bandwidth control signals (126, 128) to the first filter and the second filter; The method comprises: Setting (410) the wake-up receiver to a first bandwidth to receive a first signal by selecting a digital setting of the bandwidth control signal to the first filter and the second filter, wherein the first signal is a periodic ASK modulated signal comprising two frequency tones and is transmitted by a base station for facilitating frequency calibration of the receiver, wherein one frequency tone is at a first frequency and one frequency tone is at a second frequency; operating (420) the DCO at different frequencies by sweeping a digital setting of the frequency control signal; For each digital setting of the frequency control signal, detecting (430) whether a signal is present; and storing (440) said digital setting of said frequency control signal when a signal is detected, wherein the first frequency is lower than the second frequency, and The ASK modulation has an effective average level of 50% in a frequency region corresponding to the second frequency and in a time interval corresponding to a frequency tone period at the first frequency.

8. The method according to claim 7, further comprising: The wake-up receiver is set (450) to a second bandwidth and scans the digital setting of the frequency control signal to detect the presence of a signal, wherein the second bandwidth is narrower than the first bandwidth.

9. The method according to any one of claims 7 to 8, further comprising: The DCO is operated (460) using the digital setting of the frequency control signal detected for the first signal to receive a second signal sent by a base station as a wake-up signal.

10. The method according to any one of claims 7 to 8, further comprising: The DCO is operated (460) using a digital setting of the frequency control signal offset from the digital setting of the detected first signal to receive a second signal sent by a base station as a wake-up signal.

11. The method according to any one of claims 7 to 8, further comprising: The DCO is operated (460) using a digital setting calculated from two or more digital settings implemented when two or more first signals having different frequencies are detected to receive a second signal transmitted by a base station as a wake-up signal.

12. A base station (300) configured to perform the method steps according to any one of claims 1 to 6.

13. A wake-up receiver (100) configured to perform the method steps according to any one of claims 7 to 11.

14. A wireless communication device comprising the wake-up receiver (100) according to claim 13.

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