Patent Description:
RF receivers can be split in two main categories: those based on a Zero Intermediate Frequency architecture (ZIF) and those based on the Near Zero Intermediate frequency (NZIF) architecture.

An NZIF receiver converts a received radio signal to an intermediate frequency whose carrier frequency is in the order of magnitude of the baseband signal bandwidth but is significantly lower than the radio carrier frequency to be demodulated.

NZIF receivers are subject to misfunctioning in the presence of blockers or adjacent channels in the vicinity of the received signal frequency and with strong intensities as compared to the signal of interest.

<CIT> discloses swapping IF when determining that the energy of a received signal demodulated by the NZIF receiver using a first IF is larger than a preset value.

There is a need to provide a method for demodulating the RF received signals which allow a correct behavior of the receiver in the presence of blockers or adjacent channels.

One embodiment addresses all or some of the drawbacks of known methods for demodulating RF received signals.

One embodiment provides a method for demodulating a RF signal comprising the steps of:.

An embodiment provides an RF circuit configured to :
detect if an analog to digital converter of a Near Zero Intermediate Frequency receiver is in a clipping state; and
if yes:.

selecting the intermediate frequency corresponding to the lowest value of said first and second values.

According to an embodiment, the selected intermediate frequency is further used by the Near Zero Intermediate Frequency receiver for demodulating a next frame of the received signal.

According to an embodiment, if the analog to digital converter of the Near Zero Intermediate Frequency receiver is not in a clipping state, the current intermediate frequency is kept.

According to an embodiment, the clipping state is detected when a given number of samples of the analog to digital are equal to maximum or minimum values of the analog to digital converter.

According to an embodiment, the clipping state detection is performed for each frame of the received signal.

According to an embodiment, the clipping state detection is performed at the end of each frame.

According to an embodiment, the first and second energy indications are wide-band received signal strength indications.

According to an embodiment, the method or the circuit comprises filtering the received signal with an analog low-pass filter and the bandwidth of the wide-band received signal strength indications is defined by the bandwidth of the analog low-pass filter.

According to an embodiment, the method or the circuit comprises amplifying the RF signal and splitting the amplified signal into a first and a second paths.

According to an embodiment, the method or the circuit comprises:.

According to an embodiment, said analog low-pass filter bandwidth is centered on the local oscillator frequency.

According to an embodiment, the method or the circuit comprises mixing the digitalized signals with a third signal having the selected intermediate frequency.

According to an embodiment, the first and second energy indications correspond to the square root of the sum of square of first path signal and square of quadrature path signal.

According to an embodiment, the first and second energy indications are determined after the analog to digital conversion by said analog to digital converter and before the digital mixing.

For the sake of clarity, only the operations and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail.

In the following disclosure, unless indicated otherwise, when reference is made to absolute positional qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or to relative positional qualifiers, such as the terms "above", "below", "higher", "lower" , etc., or to qualifiers of orientation, such as "horizontal", "vertical", etc., reference is made to the orientation shown in the figures.

Unless specified otherwise, the expressions "around", "approximately", "substantially" and "in the order of" signify within <NUM> %, and preferably within <NUM> %.

In the following, methods and NZIF based circuits are implemented to detect if the presence of normalized blockers or adjacent channels induces an unwanted signal in a RF NZIF based circuit receiver. In the case where the presence of an unwanted signal is detected, the intermediate frequency is adapted as function of a channel energy indication to get rid of the unwanted signal.

<FIG> illustrates an embodiment of a RF circuit <NUM>. More precisely, <FIG> illustrates an RF transceiver architecture.

The RF circuit <NUM> comprises for example a NZIF receiver <NUM>.

In the represented example, the NZIF receiver <NUM> comprises a first module RXFE (Receive Front-End) coupled, or preferably connected, to a second module BB (Base Band). The second module BB is coupled, or preferably connected, to a third module ADC (Analog Digital Converter), which is coupled, or preferably connected, to a fourth module DFE (Digital Front End).

The represented NZIF receiver comprises an analog RF filter <NUM> which is for example a band-pass filter configured to filter the received signal having a frequency band with a central frequency Frx.

The first module RXFE comprises an amplifier <NUM>, for example a low noise amplifier LNA (Low Noise Amplifier), configured to amplify the received RF signal filtered by the analog RF filter <NUM>. The amplified signal is then split into two different paths <NUM>, <NUM>. The first module comprises one mixer <NUM>, <NUM> per path. Each of the mixers <NUM>, <NUM> is configured to mix the received RF signal Frx of the corresponding path with a signal having a local oscillator frequency LO. In the disclosed embodiments, the frequency LO corresponds to the sum of the central frequency Frx of the received signal and of an intermediate frequency, IF1 or IF2, determined as function of a value RSSI1 or RSSI2 representative of the energy of the received signal demodulated by the Near Zero Intermediate Frequency (NZIF) receiver <NUM>. The local oscillator frequency LO of path <NUM> corresponds to an in-phase signal LO-I and the local oscillator frequency LO of path <NUM> corresponds to a quadrature signal LO-Q. The in-phase LO-I and quadrature LO-Q signals are respectively real and imaginary parts of a signal with the frequency LO and supplied by a local oscillator <NUM>.

A frequency down-conversion is implemented by the mixers <NUM> and <NUM> respectively. The mixed signals frequency at the output of the mixers <NUM> and <NUM>, respectively in phase and in quadrature, can take two discrete values IF1 or IF2.

The mixed signals at the output of the mixers <NUM>, <NUM> are respectively coupled, or preferably connected, to a low-pass analog baseband filter <NUM>, <NUM> of the second module BB which is configured to filter out frequencies superior to, for example, two or three times the intermediate frequency, for example, <NUM> for an intermediate frequency of <NUM>. When transposed to RF, the low-pass filter <NUM>, <NUM> is equivalent to a band-pass filter centered on the local oscillator frequency LO. In other words, the signals outside of frequency band LO ± baseband BB bandwidth are rejected by filters <NUM>, <NUM>.

The outputs of the filters <NUM>, <NUM> are respectively, coupled, or preferably connected, to a different series of amplifiers (<NUM>, <NUM>, <NUM> for the first path <NUM>, and <NUM>, <NUM>, <NUM> for the second path <NUM>), which are for example programmable gain amplifiers of the second module BB. The number of amplifiers depends on the application.

The output of the amplifiers <NUM> and <NUM> are respectively coupled, or preferably connected, to different analog to digital converters (ADC) <NUM>, <NUM> of the third module to convert the filtered and amplified signals of the second module into digital signals.

The fourth module DFE comprises optional DC offset removal circuits, which are not represented, coupling the output of the analog to digital converters <NUM> to a mixer <NUM> for the first path <NUM> and the analog to digital converter <NUM> to another mixer <NUM> for the second path <NUM>. DC offset removal circuits are configured to remove unwanted DC bias which may originate from the received signal Frx or from the ADC circuits to improve system performance degradation and bit error rate.

In the represented example, an oscillator <NUM> (NCO) of the fourth module DFE, which is for example a numerically controlled oscillator, supplies a signal NCO_IF having the determined intermediate frequency IF1 or IF2 to the mixers <NUM>, <NUM>.

A derotation is implemented by mixers <NUM>, <NUM> for the signals of first and second paths respectively.

A low pass filter <NUM> (LPF) of the first path couples the output of the mixer <NUM> to a first decimator <NUM> (Decimator φ selection8) and another low pass filter <NUM> (LPF) of the second path couples the output of the mixer <NUM> to a second decimator <NUM> (Decimator φ selection8). The decimator is configured to reduce the data rate by removing samples from the data stream without impacting the signal. In the represented example, the decimators are configured to decimate-by-eight. Other configurations are possible, for example, decimate-by-two. Decimate-by-two function is equivalent to a data converter clocked at half the original rate, with an analog anti-aliasing filter at half the original Nyquist bandwidth. The decimation filter by eliminates unwanted signal images. It also eliminates half of the noise power. Since the desired signal remains unchanged and the noise power reduces by half, there is an overall signal-to-noise ratio (SNR) improvement. For any arbitrary decimation factor D, SNR improves by <NUM>*log(D).

The outputs of the decimators <NUM> and <NUM> are respectively coupled, or preferably connected, to digital channel filters <NUM>, <NUM> which are for example band-pass filters with a bandwidth slightly larger than the frequency band bandwidth of the received signal. In a non-illustrated example, the digital channel filters <NUM>, <NUM> couples the low path filters <NUM>, <NUM> to the respective decimators <NUM>, <NUM>.

The outputs of the digital analog filters <NUM>, <NUM> are coupled, or preferably connected, to digital signal processor (DSP) inputs <NUM> (I path) and <NUM> (Q path) to analyze the demodulated received signal. In a non-illustrated example, the digital signal processor (DSP) inputs <NUM> (I path) and <NUM> (Q path) couple the digital channel filters <NUM>, <NUM> to the respective decimators <NUM>, <NUM>.

A computing circuit <NUM> of the RF circuit <NUM> is for example coupled, or preferably connected, to the outputs of the analog to digital converters <NUM> and <NUM>.

The computing circuit <NUM> is configured to detect if the analog to digital converter <NUM>, <NUM> of at least one of the paths <NUM>, <NUM> is in a clipping state.

The computing circuit <NUM> is for example also configured to determine and store a value RSSI1, RSSI2 representative of the energy of the received signal after demodulation or partial demodulation. The value RSSI1 is representative of the energy of the received signal when a first intermediate frequency IF1 is used for the demodulation by the NZIF receiver and the value RSSI2 is representative of the energy of the received signal when a second intermediate frequency IF2 is used.

The first and second energy indications RSSI1, RSSI2 are for example received signal strength indications (RSSI), for example wide-band received signal strength indications. In other words, the first and second energy indications RSSI1, RSSI2 correspond for example to the RSSI within the bandwidth of the low-pass analog filter <NUM>, <NUM>.

In another example, the first and second energy indications RSSI1, RSSI2 correspond to the square root of the sum of square of first path signal and square of quadrature path signal.

The computing circuit <NUM> is configured to adapt the intermediate frequency of the NZIF receiver <NUM> as a function of the clipping state of the ADC circuits <NUM> and/or <NUM>, and as a function of the received signal strength indication RSSI1, RSSI2.

<FIG> illustrates an embodiment of a RF signal demodulation method. In an example, the demodulation method can be implemented by the RF circuit of <FIG>.

More particularly, <FIG> illustrates steps of a method for selecting the intermediate frequency IF to be used for the demodulation.

In a step <NUM> (IF Selection Algo entry), the demodulation method begins.

In a following step <NUM> (ADC clipping?), a clipping state of an analog to digital converter ADC of a NZIF receiver is investigated for example by the computing circuit <NUM>. The clipping state is for example detected when a given number of samples of the analog to digital <NUM>, <NUM> are equal to maximum or minimum values of the analog to digital converter. In an example of an <NUM>-bit ADC, the minimum value is -<NUM> and the maximum value is +<NUM>. The clipping state detection is performed for example for each frame of the received signal and/or at the end of each frame.

If the ADC is not in a clipping state (output N of bloc <NUM>), the intermediate frequency is not modified and stays as for the previous frame. The selection method of the intermediate frequency ended (step <NUM> - Exit IF selection).

If the ADC is in a clipping state (output Y of bloc <NUM>), a step <NUM> (RSSI1=RSSI@IF1) is performed. In step <NUM>, a first value RSSI1 representative of the energy of a received signal demodulated by the NZIF receiver using the first intermediate frequency IF1 is determined and stored. The value IF1 is for example the current intermediate frequency of the NZIF receiver. In an example of application to narrow band internet of things (NBIOT), the value IF1 is for example set to <NUM>. Step <NUM> is for example performed by the computing circuit <NUM>.

In a following step <NUM> (Revert RX IF), the intermediate frequency is inverted. In other words, an intermediate frequency IF2 is set to the opposite value of IF1 (for example -<NUM>).

In a following step <NUM> (RSSI2=RSSI@IF2), a second value RSSI2 representative of the energy of a received signal demodulated by the NZIF receiver using the opposite intermediate frequency IF2 is determined and stored. Step <NUM> is for example performed by the computing circuit <NUM>.

In a following step <NUM> (RSSI1<RSSI2?), the first value RSSI1 and the second value RSSI2 are compared. If the first value RSSI1 is lower than the second value (branch Y) then a step <NUM> (IF=IF1) is performed. If the first value RSSI1 is superior to the second value (branch N) then a step <NUM> (IF=IF2) is performed.

In step <NUM>, the intermediate frequency is set to the second intermediate frequency IF2.

In step <NUM>, the intermediate frequency is set to the first intermediate frequency IF1.

After step <NUM> or <NUM>, the selection method ends by step <NUM>.

The method of <FIG> allows the selection of an intermediate frequency value, which easily lowers the wide-band energy of the channel reflecting the fact that an unwanted signal is not overlaid to the signal of interest.

<FIG> illustrates steps of the method of <FIG> in the presence of an adjacent channel. We suppose the case of an NBIOT signal. However, the method can be applied to other signal frequencies. More specifically, the steps of <FIG> illustrate the case where an adjacent channel <NUM> (ADJACENT CHANNEL) is present with frequencies that crosses the local oscillator frequency LO and lead to the case where the ADC is in a clipping state and where RSSI1 is superior to RSSI2.

In a step A1), the intermediate frequency IF1 is set and a first analog down-conversion is applied to the received signal by mixing the received signal and a signal having the local oscillator frequency LO. The frequency LO is set to be equal to the addition of the intermediate frequency IF1 with the received signal central frequency Frx. In the example of <FIG>, the adjacent channel <NUM> is on the opposite side of the wanted signal compared to the local oscillator frequency LO. At the end of step A1), the central frequency of the signal is shifted from the frequency LO by the intermediate frequency IF1.

In a step B1), the signal obtained at the end of step A1) is digitally processed for a derotation with the intermediate frequency IF1 to down convert the signal to baseband. During step B1), the filtered part of the adjacent channel <NUM> undergo an image rejection but an image <NUM> of the filtered adjacent channel is still present and form a frequency band which extends across the frequency LO and the channel bandwidth.

The resulting down converted signal is filtered by the digital channel filters <NUM> and <NUM> (DIGITAL CHANNEL FILTER) represented in C1). The digital channel filters <NUM> and <NUM> have a bandwidth centered on baseband and extending slightly on both sides of the demodulated signal. Step <NUM> is for example performed during step C1) to determine RSSI1.

A part of the image <NUM> is still present inside the bandwidth of the digital channel filter and may lead to additional energy in the channel and ADC clipping.

<FIG> illustrates steps of the method of <FIG> in the presence of an adjacent channel <NUM>. The adjacent channel <NUM> is similar to the adjacent channel of <FIG>. More precisely, <FIG> represents steps <NUM> and <NUM> of <FIG>.

In a step A'<NUM>), a first analog down-conversion is applied to the received signal by mixing the received signal and a signal having the local oscillator frequency LO. The frequency LO is set to be equal to the addition of the inverted intermediate frequency IF2 with the received signal central frequency Frx. Since IF2 is the opposite of IF1, compared to the example of <FIG>, the adjacent channel is now on the same side of the LO as the wanted signal. At the end of step A1'), the central frequency of the signal is shifted from the frequency LO by the intermediate frequency IF2. The received signal is then filtered by the analog low-pass filters <NUM>, <NUM> (Baseband filter). In the represented example, the bandwidth of the analog filters is two or three times greater than the intermediate frequency and defines the wideband RSSI measurement bandwidth (WIDEBAND RSSI). The adjacent channel part that remains unfiltered by the analog filters <NUM>, <NUM> is now outside the received signal.

In a step B'<NUM>), the signal obtained at the end of step A1') is digitally processed for another derotation with the inverted intermediate frequency IF2 to down convert the signal to baseband. During step B1'), the filtered part of the adjacent channel <NUM> undergo an image rejection but an image <NUM> of the filtered adjacent channel is still present and form a frequency band which extends outside the channel bandwidth.

The resulting down converted signal is filtered by the digital channel filters <NUM> and <NUM> (DIGITAL CHANNEL FILTER) represented in C1'). The digital channel filters <NUM> and <NUM> have a bandwidth centered on baseband and extending slightly on both sides of the demodulated signal. Step <NUM> is for example performed during step C1') to determine RSSI2.

The image <NUM> and the unfiltered part of the adjacent channel are out of the bandwidth of the digital channel filter. They do not add additional energy in the channel, as it can be measured by wideband RSSI, hence preventing ADC clipping.

<FIG> illustrates steps of the method of <FIG> in the presence of a blocker. The example of <FIG> is similar to the example of <FIG> except that the adjacent channel is replaced by a blocker <NUM> (Blocker) having a frequency located inside the analog filter <NUM> bandwidth on the low frequency side.

In a step A2), the intermediate frequency IF1 is set and a first analog derotation is applied to the received signal by mixing the received signal and the signal having the local oscillator frequency LO. The frequency LO is set to be equal to the addition of the intermediate frequency IF1 with the received signal central frequency Frx. In the example of <FIG>, the blocker <NUM> is on the opposite side of the wanted signal compared to the local oscillator frequency LO. At the end of step A2), the central frequency of the signal is shifted from the frequency LO by the intermediate frequency IF1. The received signal is then filtered by the analog low-pass filters <NUM>, <NUM> (Baseband filter). In the represented example, the bandwidth of the analog filters is two or three times greater than the intermediate frequency and defines the wideband RSSI measurement bandwidth (WIDEBAND RSSI).

In a step B2), the signal obtained at the end of step A2) is digitally processed for a derotation with the intermediate frequency IF1 to down convert the signal to baseband. During step B2), the filtered part of the blocker <NUM> undergo an image rejection but an image <NUM> of the filtered blocker is still present and has an image frequency which falls inside the channel bandwidth.

The resulting down converted signal is filtered by the digital channel filters <NUM> and <NUM> (DIGITAL CHANNEL FILTER) represented in C2) and even if the blocker frequency is filtered by the digital filter, a part of the image <NUM> is still present inside the bandwidth of the digital channel filter and may lead to additional energy in the channel and cause ADC clipping.

Step <NUM> is for example performed during step C2) to determine RSSI1.

<FIG> illustrates steps of the method of <FIG> in the presence of a blocker <NUM>.

In a step A2'), a first analog down-conversion is applied to the received signal by mixing the received signal and a signal having the local oscillator frequency LO. The frequency LO is set to be equal to the addition of the inverted intermediate frequency IF2 with the received signal central frequency Frx. Since IF2 is the opposite of IF1, compared to the example of <FIG>, the blocker signal is now on the same side of the LO as the wanted signal. At the end of step A2'), the central frequency of the signal is shifted from the frequency LO by the intermediate frequency IF2. The received signal is then filtered by the analog low-pass filters <NUM>,<NUM> (Baseband filter). In the represented example, the bandwidth of the analog filters is <NUM> or <NUM> times greater than the intermediate frequency and defines the wideband RSSI measurement bandwidth (WIDEBAND RSSI). The adjacent channel part that remains unfiltered by the analog filters <NUM>, <NUM> is now outside the received signal.

In a step B'<NUM>), the signal obtained at the end of step A2') is digitally processed for a derotation with the inverted intermediate frequency IF2 to down convert the signal to baseband. During step B2'), the filtered part of the blocker <NUM> undergo an image rejection but the image <NUM> of the filtered adjacent channel is present at a frequency which extends outside the channel bandwidth on the high frequency side.

The resulting down converted signal is filtered by the digital channel filters <NUM> and <NUM> (DIGITAL CHANNEL FILTER) represented in C2'). The digital channel filter <NUM>, <NUM> has a bandwidth centered on baseband and extending slightly on both sides of the demodulated signal.

The image <NUM> and the unfiltered part of the blocker <NUM> are out of the bandwidth of the digital channel filter. They do not add additional energy in the channel, as it can be measured by wideband RSSI, hence preventing ADC clipping.

Step <NUM> is for example performed during step C2') to determine RSSI2.

<FIG> shows that, by inverting the intermediate frequency, it is possible to prevent the effects of adjacent channels and/or blockers which can otherwise create ADC clipping (cases of <FIG> and <FIG>). In relation with the method of <FIG>, when the ADC is clipping, by inverting the intermediate frequency and comparing indications of the energy in the wideband channel RSSI1 and RRSI2 allows to choose a configuration, which correspond to the case where the energy is the lowest in the wideband, which will in turn lead to prevent ADC clipping.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art. In particular, even if the case of adjacent channels and blockers are present on the low frequency side have been illustrated in <FIG>, the inversed reasoning could be achieved for cases where adjacent channel and blocker are present initially on high frequency side. In case of ADC clipping, inverting the intermediate frequency and selecting the lowest wideband channel energy configuration will provide a solution for preventing the ADC clipping and other adjacent channel/blocker effects.

Moreover, even if the solution inverting the intermediate frequency has been described, an intermediate frequency value close to the opposite value of the initial intermediate frequency value may also be used.

Claim 1:
A method for demodulating a RF signal comprising the steps of:
detecting if an analog to digital converter (ADC) of a Near Zero Intermediate Frequency (NZIF) receiver is in a clipping state; and
if yes:
determining and storing a first value (RSSI1) representative of the energy of a received signal demodulated by the Near Zero Intermediate Frequency (NZIF) receiver using a first intermediate frequency (IF1);
determining and storing a second value (RSSI2) representative of the energy of the received signal demodulated by the Near Zero Intermediate Frequency (NZIF) receiver using a second intermediate frequency (IF2) corresponding to the opposite value of the first intermediate frequency (IF1),
selecting the intermediate frequency corresponding to the lowest value of said first and second values.