Receiver with offset compensation

Receiver for receiving a received signal with a carrier frequency loop (18) which generates a carrier frequency deviation detection signal (TF) for detecting a carrier frequency of the received signal in a first carrier frequency capture range; a carrier phase loop (32) which generates a carrier phase deviation detection signal (TP) for detecting a carrier phase of the received signal, and settles when the carrier frequency deviation detection signal (TF) is within a second carrier frequency capture range; and with an offset control circuit (29) which changes the carrier frequency deviation detection signal (TF) and/or changes the second carrier frequency capture range by means of an offset control signal until a carrier phase lock detection circuit (21) indicates to the offset control circuit that the carrier phase offset of the received signal is less than an adjustable threshold value.

TECHNICAL FIELD

The invention relates to a receiver with offset compensation for receiving a received signal, which contains its [sic] offset control circuit for changing a carrier frequency deviation detection signal and/or a carrier frequency capture range.

BACKGROUND ART

Quadrature amplitude modulation QAM is a digital modulation method which is particularly suitable for transmitting high data rates and combined ASK (Amplitude Shift Keying) and PSK (Phase Shift Keying), i.e. the signal carrier is modulated in amplitude and phase. The amplitude and the frequency or phase of a harmonic oscillation are modulated by two different time functions. In QAM, different variants are possible. In a 64-QAM, 6 bits per data symbol are coded. With increasing level (16-, 64-, 256-, 1 024-QAM) the bandwidth efficiency increases but so does the signal/noise power ratio required for reliable transmission.

FIG. 1shows a first QAM receiver according to the prior art. The data coming from a data source are transmitted by a transmitter via a transmission channel to the QAM receiver and delivered by this QAM receiver to a downstream data sink. During the data transmission via the real data transmission channel, the received signal, as a rule, exhibits linear distortion and an additional noise component. This noise component can be modeled by additive white Gaussian noise (AWGN). The QAM receiver has the task of reconstructing the bit sequence of the data source from the received signal. For this purpose, the received analog signal is first converted into a digital signal by an analog/digital converter ADC and then delivered to a mixing stage. A downstream receiving filter suppresses possible interference signals outside the transmission frequency band. The reliability of detection is increased by suitably dimensioning a matched filter (MF). The matched filter is specially adapted to a basic transmit pulse so that the greatest possible signal/noise power ratio (SNR) is achieved at the detection times. The impulse response of the matched filter MF is usually equal to the basic transmit pulse mirrored in time or displaced by one bit period. The matched filter is a digital receiving filter inside the receiver which is adapted to a transmit filter inside a transmitter, in such a manner that the amplitude of the received signal is maximum at the sampling times. The matched filter MF can be adaptively designed so that it can be adapted to the transmission channel. An adaptive equalizer which compensates for the distortion of the transmission channel can be provided before or after the matched filter.

The output signal of the matched filter MF is supplied to a carrier phase detector which is provided for detecting the carrier phase of a received digital signal. The carrier phase detector TPD delivers a carrier phase detection deviation signal TP to a downstream digital loop filter. The digital loop filter and the downstream NCO (numerically controlled oscillator) supply a digital control value for the mixing stage.

The QAM receiver according to the prior art, shown inFIG. 1, is constructed in one stage. Frequency and phase estimation are done in one stage. This has the disadvantage that a predetermined frequency range must be scanned for the carrier frequency of the received signal in a great number of small search steps in order to ensure that the target value is located within a narrow frequency capture range. The conventional QAM receiver shown inFIG. 1, therefore, needs a relatively large amount of time for such a search process.

For this reason, the QAM receiver according to the prior art shown inFIG. 2was proposed. Such a QAM receiver is described in German patent application 101 33 898.8. The QAM receiver according to the prior art, shown inFIG. 2, is constructed in two stages. The QAM receiver contains a carrier frequency loop for detecting a carrier frequency of the received signal in a first carrier frequency capture range followed by a carrier phase loop for detecting a carrier phase of the received signal in a second carrier frequency capture range. Compared with the single-stage QAM receiver shown inFIG. 1, separating the frequency and phase estimation has the advantage that the frequency estimation by the carrier frequency loop has a large carrier frequency capture range.

The carrier frequency loop of the conventional QAM receiver shown inFIG. 2has a carrier frequency detector which follows the two matched filters MF. The carrier frequency detector generates a carrier frequency deviation detection signal TF which is supplied to a numerically controlled oscillator NCO via a digital loop filter. The NCO delivers a control signal to the mixing stage.

The downstream carrier phase loop is constructed in a similar manner and contains a carrier phase detector TPD which generates a carrier phase deviation detection signal TP. The carrier phase deviation detection signal TP is also supplied to a loop filter which digitally filters the carrier phase deviation detection signal TP and delivers it to a downstream controlled oscillator NCO. The NCO forms a control signal which is supplied to a further mixing stage.

FIG. 3is a diagram for explaining the operation of the conventional QAM receiver shown inFIG. 2. If the carrier frequency of the received signal is at a nominal frequency fENOM, the carrier frequency loop generates a carrier frequency deviation detection signal TFIDEALhaving the value zero. However, due to interference, the carrier frequency loop generates a real carrier frequency deviation detection signal TFREALwhich has an offset as shown inFIG. 3c.The frequency error after the frequency estimation can have various causes, namely intrinsic noise of the frequency detector, amplitude phase errors in the transmission channel or, for example, a wrongly dimensioned anti-aliasing filter. In all these cases, the characteristic of the frequency detector, which forms an S curve, is distorted in such a manner that the detector output signal does not provide a frequency offset even though there is a frequency offset present. A resultant controlled variable thus has the value zero for a frequency value which deviates from the ideal frequency estimate (FENOM). The real carrier frequency deviation detection signal TFREALthus has a frequency offset.

If the deviation or the error of the frequency estimation by the carrier phase loop is too great, the subsequent phase estimation of the carrier phase of the received signal by the carrier phase loop can no longer compensate for the remaining frequency error. If, as shown in the example ofFIG. 3, the zero value of the real carrier frequency deviation detection signal is located outside the carrier frequency capture range, shown inFIG. 3d,of the carrier phase loop, the carrier phase loop can no longer compensate for this considerable frequency error and the conventional QAM receiver shown inFIG. 2cannot receive a received signal. The carrier frequency capture range of the carrier phase loop depends on the signal/noise ratio SNR of the complete transmission system and can only be extended within narrow limits.

SUMMARY OF THE INVENTION

It is, therefore, the object of the present invention to create a receiver for receiving a received signal which rapidly determines the carrier frequency and operates reliably even with large frequency errors.

According to the invention, this object is achieved by a receiver having the features specified in claim1.

Preferred embodiments of the receiver according to the invention are specified in the subclaims.

The invention creates a receiver for receiving a received signal with a carrier frequency loop which generates a carrier frequency deviation detection signal TF for detecting a carrier frequency of the received signal in a first carrier frequency capture range, a carrier phase loop which generates a carrier phase deviation detection signal TP for detecting a carrier phase of the received signal, and settles when the carrier frequency deviation detection signal TF is within a second carrier frequency capture range, and with an offset control circuit which changes the carrier frequency deviation detection signal TF and/or the second carrier frequency capture range by means of an offset control signal until a carrier phase lock detection circuit of the offset control circuit indicates that the carrier phase offset of the received signal is less than an adjustable threshold value.

In a preferred embodiment of the QAM receiver according to the invention, the second carrier frequency capture range is narrower than the first carrier frequency capture range.

The receiver according to the invention is preferably a QAM receiver.

The carrier frequency loop of the QAM receiver according to the invention preferably has a carrier frequency detector, a first digital loop filter, a first controlled oscillator and a first mixing stage.

The carrier phase loop of the QAM receiver according to the invention preferably has a carrier phase detector, a second digital loop filter, a second controlled oscillator and a second mixing stage.

In a preferred embodiment of the receiver according to the invention, a first matched filter for the in-phase signal component of the received QAM signal and a second matched filter for the quadrature signal component of the received QAM signal are provided between the first mixing stage of the carrier frequency loop and the second mixing stage of the carrier phase loop.

The second mixing stage of the QAM receiver according to the invention is preferably followed by a carrier phase lock detection circuit.

In a particularly preferred embodiment of the receiver according to the invention, the offset control circuit generates a carrier phase offset signal which is added to the carrier phase deviation detection signal TP, filtered by the second digital loop filter of the carrier phase loop, by an adder for shifting the frequency of the carrier frequency capture range of the carrier phase loop.

In an alternative embodiment of the receiver according to the invention, the offset control circuit generates a carrier frequency offset signal which is added to the carrier frequency deviation detection signal TF, generated by the carrier frequency detector, by an adder for compensating for a frequency offset of the received signal.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4shows a first embodiment of the receiver1according to the invention for receiving a received signal. The receiver1receives via a signal input2a received signal which is transmitted by a transmitter4by a transmission channel3. The received analog signal is converted into a digital signal by an analog/digital converter5and supplied as in-phase signal component to a first mixing stage7via a line6aand as quadrature signal component via a line6b.The mixing stage7multiplies the in-phase signal component and the quadrature signal component in the time domain with a control signal present on a line8. The mixed in-phase signal component is supplied to an in-phase matched filter10avia a line9a.The mixed quadrature signal component is delivered to a quadrature phase matched filter10bvia a line9b.The in-phase matched filter is connected via a line11a,and the quadrature phase matched filter10bis connected via a line11b,to a downstream further mixing stage12. In addition, the output signal of the in-phase matched filter10ais applied via a line13a,and the output signal of the quadrature phase matched filter10bis applied via a line12b,to a carrier frequency detector13. The carrier frequency detector13generates a carrier frequency deviation detection signal TF and delivers it via a feedback line14to a digital loop filter15. The digital loop filter15filters the carrier frequency deviation detection signal TF present and applies the filtered signal to a downstream first controlled oscillator17via a line16. The numerically controlled oscillator17generates the control signal for the first mixing stage7in dependence on the filtered carrier frequency deviation detection signal TF. The first mixing stage7, together with the matched filters10a,10b,the carrier frequency detector13, the digital loop filter15and the controlled oscillator17, forms a carrier frequency loop18of the receiver1for detecting the carrier frequency of the received signal in a first carrier frequency capture range.

The output signals of the two matched filters10a,10b,delivered via the lines11a,11b,are supplied to the second mixing stage12. The second mixing stage12multiplies the output signals present from the two matched filters10a,10bin the time domain with a control signal present on a line19. The mixed signals are supplied to a carrier phase lock detection circuit21via lines20a,20b.In addition, the output signals of the second mixing stage12are applied to a carrier phase detector23via lines22a,22b.The carrier phase detector23generates a carrier phase deviation detection signal TP which is applied to a digital loop filter25via a feedback line24. The digital loop filter25filters the carrier phase deviation detection signal TP applied and delivers it to an adder27via a line26. The adder27adds the filtered carrier phase deviation detection signal TP to a carrier phase offset signal TP offset which is delivered by an offset control circuit29of the receiver10[sic] according to the invention via an offset control line28. The aggregate signal produced by the addition is delivered by the adder27via a line30to a downstream controlled oscillator31which generates the control signal for the second mixing stage12. The second mixing stage12, together with the carrier phase detector23, the second digital loop filter25and the second controlled oscillator31, forms a carrier phase loop32of the receiver1according to the invention.

The carrier phase lock detection circuit following the carrier phase loop32indicates to the offset control circuit29via a line33whether the carrier phase offset of the received signal is less than an adjustable threshold value. This tells the offset control29that the carrier frequency of the received signal has been determined with sufficient accuracy.

FIG. 5shows the operation of the first embodiment of the QAM receiver1according to the invention as shown inFIG. 4. The offset control circuit29generates a carrier phase offset signal TP Offset which is added to the carrier frequency deviation detection signal TP, filtered by the digital loop filter25, by the adder27for shifting the frequency of the carrier frequency capture range of the carrier phase loop32. As a result, the carrier frequency capture range of the carrier phase loop is shifted from the position shown inFIG. 5dinto the position shown inFIG. 5e.The center frequency of the carrier frequency capture range is shifted. The offset control29shifts the carrier frequency capture range of the carrier phase loop32until the carrier phase lock detection circuit21indicates that the zero value of the carrier frequency deviation detection signal TFREALlies within the shifted carrier frequency capture range of the carrier frequency loop32.

The carrier frequency capture range of the carrier phase loop32can be changed by the offset control circuit29in accordance with various search strategies. The carrier frequency capture ranges can be scanned either linearly in succession beginning at a lower center frequency up to an upper center frequency by the offset control circuit29or the offset control circuit begins in a centrally positioned carrier frequency capture range and varies the carrier frequency capture ranges around this centrally positioned carrier frequency capture range. In a preferred embodiment of the receiver, the shifted carrier frequency capture ranges overlap to some extent.

FIG. 6shows a further embodiment of the receiver1according to the invention. In this embodiment components contained identically in the first embodiment are provided with the same reference symbols.

The QAM receiver1shown inFIG. 6also contains a carrier frequency loop18and a carrier phase loop32. In the second embodiment shown inFIG. 6, the offset control circuit29generates a carrier frequency offset control signal TFOFFSETwhich is applied to an adder35within the carrier frequency loop18via a line34. At its other input, the adder35receives the carrier frequency deviation detection signal TF generated by the carrier frequency detector13and adds it to the carrier frequency offset control signal TFOFFSETdelivered by the offset control circuit29via the line24to form an offset-compensated carrier frequency deviation detection signal TF′ which is supplied to the digital loop filter15via a line36.

FIG. 7shows the operation of the second embodiment of the receiver1according to the invention, shown inFIG. 6. The zero value of the real carrier frequency deviation detection signal as shown inFIG. 7cis shifted by the offset signal TFOFFSETgenerated by the offset control circuit29. The offset control circuit29changes the carrier frequency deviation detection signal by means of the offset signal until the carrier frequency deviation detection signal TF′ is located within the carrier frequency capture range of the carrier phase loop32as shown inFIG. 7d.This is indicated to the offset control circuit29by the carrier phase lock detection circuit29. The carrier frequency deviation detection signal TFREALcan be changed by the offset control circuit29in accordance with various search strategies. The zero value is changed, for example, linearly from a lower limit frequency to an upper limit frequency until it falls within the carrier frequency capture range of the carrier phase loop. Other search strategies with alternate searching around a predetermined starting value are also possible.