TECHNICAL FIELD

The present invention relates to the field of clock extraction circuits used in digital communication systems, and in particular, to a clock recovery device for usage in a communication system utilizing multiple data streams, and with strong interference.

BACKGROUND In order to successfully detect and decode a modulated data stream in a wireless communication system, it is necessary to accurately synchronize the local sampling clock (also known as clock recovery) in the receiver to that of the transmitter. If the local sampling clock is not properly synchronized then the performance of the communication system may be severely degraded. Several algorithms for clock recovery have been suggested in literature. However, a common drawback of the clock recovery algorithms in literature is that their performance is limited by the presence of strong external interference resulting in that accurate synchronization cannot be achieved. Due to increased user traffic in today's cellular networks, there is an urgent need for increased capacity in already deployed back-hauling networks. Since spectrum is both scarce and strictly regulated by government bodies, the only viable way of increasing the capacity of a network is to improve the spectral efficiency (more bits/s/Hz). Such improved spectral efficiency can for instance be obtained using multi-antenna solutions such as multiple-input multiple-output (MIMO) systems and/or cross-polar interference cancellation (XPIC) systems. In these communication systems, multiple data streams are transmitted over the same frequency and time slots, hence creating strong interference that must be handled by the communication system. However, as mentioned before many existing algorithms for clock recovery cannot function properly in the presence of strong interference. Thus, finding a way to accurately and robustly perform clock recovery in a communication system plagued by strong external interference is therefore highly sought for.

SUMMARY OF THE INVENTION With the above description in mind, then, an aspect of the present invention is to provide a method and a device for accurate and robust clock recovery which seeks to mitigate, alleviate, or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination. A first aspect of the present invention relates to a clock recovery device for establishing synchronization with a received communication signal, comprising means for converting said received communication signal and a reference signal to a digital communication signal and a digital reference signal respectively, wherein said means for converting is adjustable by a sampling frequency, an interference cancellation filter and a first differential phase compensator adapted to produce an interference cancellation signal based on said digital reference signal, a first interference suppressor unit adapted to produce a first interference suppressed signal based on said digital communication signal and a portion of said interference cancellation signal, wherein said portion is determined by a multiplier unit multiplying said interference cancellation signal with a predefined constant WQA, an equalizer filter adapted to produce an equalized signal based on said first interference suppressed signal, a second interference suppressor unit adapted to produce a second interference suppressed signal based on said equalized signal and said interference cancellation signal processed through a second differential phase compensator, a carrier recovery unit adapted to produce a carrier recovered signal from said second interference suppressed signal, a detector unit adapted to detect the presence of a communication signal in said carrier recovered signal and to produce a detected communication signal, a detection error signal, wherein said detection error signal is used for adjusting said interference cancellation filter, said first differential phase compensator, said second differential phase compensator and said equalizer filter, and a locked clock signal, which is set to true if the presence of a communication signal is detected and set to false if no presence of a communication signal is detected in said carrier recovered signal, wherein said clock recovery device is characterized in having a first clock detector unit adapted to produce a first clock detector signal from said first interference suppressed signal, a second clock detector unit adapted to produce second clock detector signal from said received communication signal and said detected communication signal, a switch unit adapted to pass said first clock detector signal from a first switch input to a switch output if said locked clock signal is set to false and to pass said second detector clock from a second switch input to said switch output if said locked clock signal is set to true, and wherein said switch output is connected to a clock loop filter adapted to produce a clock loop filter output signal, based on said switch output, for adjusting said sampling frequency of said converters.

The clock recovery device wherein said first clock detector unit may either a Gardner detector or a Mueller and Muller detector, adapted to produce said first clock detector signal. The clock recovery device wherein said first clock detector unit may further comprise a processing delay unit adapted to delay said delayed received communication signal, a late correlator unit adapted to correlate said detected communication signal with said delayed received communication signal producing a late correlated signal, wherein said delayed received communication signal have been further delayed one symbol in a symbol delay unit, a early correlator unit adapted to correlate said detected communication signal with said received communication signal producing an early correlated signal, two absolute value units are adapted to produce the absolute value of said late correlated signal and said early correlated signal, a processing unit adapted to produce said second clock detector signal as the difference between the absolute value of said late correlated signal and the absolute value of said early correlated signal.

The clock recovery device wherein said means for converting said received communication signal and said reference signal may comprise at least one analog to digital converter with adjustable sampling frequency, and at least one filter.

The clock recovery device wherein said constant W _D A may be set to a predetermined constant value when said locked clock signal is set to false, and wherein said constant W _D A is set to zero when said locked clock signal is set to true.

The clock recovery device wherein said constant W _D A may be linearly scaled down to zero when the presence of a communication signal is detected in said detector unit.

A second aspect of the present invention relates to a clock recovery method for establishing synchronization with a received communication signal, comprising the steps, converting a received communication signal and a reference signal to a digital communication signal and a digital reference signal respectively, wherein said converting is adjustable by a sampling frequency, producing an interference cancellation signal by processing said digital reference signal through an interference cancellation filter and a first differential phase compensator, producing, in a first interference suppressor unit, a first interference suppressed signal based on said digital communication signal and a portion of said interference cancellation signal, wherein said portion is determined by multiplying said interference cancellation signal with a predefined constant W _D A, producing an equalized signal by processing said first interference suppressed signal through an equalizer filter, producing, in a second interference suppressor unit, a second interference suppressed signal based on said equalized signal and said interference cancellation signal processed through a second differential phase compensator, producing, in a carrier recovery unit, a carrier recovered signal from said second interference suppressed signal, detecting, in a signal detector unit, the presence of a communication signal in said carrier recovered signal, and producing a detected communication signal, a detection error signal, and a locked clock signal, adjusting said interference cancellation filter, said first differential phase compensator, said second differential phase compensator and said equalizer filter with said detection error signal, setting said locked clock signal, wherein said locked clock signal is set to true if the presence of a communication signal is detected and set to false if no presence of a communication signal is detected in said carrier recovered signal wherein the method is characterized by producing a first clock detector signal from said first interference suppressed signal in a first clock detector unit, producing a second clock detector signal from said received communication signal and said detected communication signal in a second clock detector unit, switching, in a switch unit, to a first switch input, passing said first clock detector signal from said switch input to a switch output, if said locked clock signal is set to false, and switching to a second switch input, passing said second detector clock from said second switch input to said switch output, if said locked clock signal is set to true, producing a clock loop filter output signal based on said switch output and adjusting said sampling frequency of said converters based on said clock loop filter output signal. The clock recovery method wherein said producing of said first clock detector signal from said first interference suppressed signal is performed either by a Gardner detector unit or a Mueller and Muller detector.

The clock recovery method wherein said producing of a second clock detector signal may further comprise the steps, delaying, in a processing delay unit, said delayed received communication signal, correlating, in a late correlator unit, said detected communication signal with said delayed received communication signal and producing a late correlated signal, wherein said delayed received communication signal have been further delayed one symbol in a symbol delay unit, correlating, in a early correlator unit, said detected communication signal with said received communication signal producing an early correlated signal, producing, in two absolute value units, the absolute value of said late correlated signal and said early correlated signal, and processing, in a processing unit, said second clock detector signal as the difference between the absolute value of said late correlated signal and the absolute value of said early correlated signal.

The clock recovery method may further comprise the step of setting said constant W _D A to a predetermined constant value when said locked clock signal is set to false, and to zero when said locked clock signal is set to true.

The clock recovery method may further comprises the step of scaling down said constant W _D A linearly to zero when the presence of a communication signal is detected in said detector unit.

Any of the features in the first aspect and the second aspect of the present invention above may, respectively, be combined, in any way possible, as to form different embodiments of the present invention. All of the benefits described in conjunction with the first aspect of the present invention may in the same way be applied to the second aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features, and advantages of the present invention will appear from the following detailed description of some embodiments and variants of the invention, wherein some embodiments or variants of the invention will be described in more detail with reference to the accompanying drawings, in which:

Fig. 1 shows a block diagram of a clock recovery device according to prior art; and

Fig. 2 shows a diagram of the bit error rate (BER) performance as a function of the signal-to-noise rate (SNR) for a quadrature amplitude modulation, according to an prior art; and Fig. 3 shows a block diagram of a clock recovery device according to the present invention; and

Fig. 4 shows a diagram of the bit error rate (BER) performance as a function of the signal-to-noise rate (SNR) for a quadrature amplitude modulation, according to an embodiment of the present invention; and

Fig. 5a shows a flowchart describing a clock recovery method according to the present invention; and

Fig. 5b shows the continuation of the flowchart describing a clock recovery method according to the present invention from figure 5a.

DETAILED DESCRIPTION

Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference signs refer to like elements throughout the description.

Improving spectral efficiency by utilizing multi-antenna solutions, such as multiple-input multiple-output (MIMO) systems and/or cross-polar interference cancellation (XPIC) systems, will almost always lead to degradation of the performance of the clock recovery in the communication system due to the presence of external interference. One way of coping with the strong interference and still achieve accurate clock synchronization between the transmitter and the receiver, is to apply an interference cancellation technique. The block diagram in figure 1 illustrates a clock recovery device 100 utilizing an interference cancellation technique according to prior art. In the clock recovery device 100 a received communication signal and an input reference signal, which usually is a de-correlated version of the communication signal, are converted from the analog domain to the digital domain 101 and filtered 102, producing a digital communication signal and a digital reference signal. The digital reference signal is processed through an adaptive interference cancellation filter 103, generating an interference cancellation signal, while the digital communication signal is delayed 104 to match the delay of the digital reference signal filtered through the interference cancelation filter 103.

In order to suppress the interference in the digital communication signal, the interference cancellation signal is added to the digital communication signal in an interference suppressor unit 105 (marked 'Canceller AddPoint' in fig. ) to produce an interference suppressed signal. In order to reduce any inter symbol interference and remove any frequency offset in the signal the interference suppressed signal is processed through an adaptive equalizer filter 106 with carrier recovery, producing an equalized signal. The equalized signal is then processed in a detector unit 107 in order to detect information symbols in said equalized signal. As a result of the detection process a detection error signal is generated by the detector unit 107. When the clock recovery device 100 is locked or synchronized with the transmitter, also referred to as being in a 'locked mode', a detection error signal is generated from the detector unit 107, while when the clock recovery device 100 in figure 1 is unlocked or not synchronized with the transmitter, also referred to as being in a 'blind mode', no detection error signal is generated from the detector unit 107. The detection error signal from the detector unit 107 is coupled to both the interference cancelation filter 103 and to the equalizer filter 106 creating a control loop. When the clock recovery device 100 operates in a locked mode the detection error signal is used to update the interference cancelation filter 103 and the equalizer filter 106, while when the clock recovery device 100 operates in a blind mode the interference cancelation filter 103 and the equalizer filter 106 are updated using, for instance, a constant modulus algorithm which is well-known in the art. To be able to obtain symbol synchronization the sample frequency of the analog to digital converters must be a controlled and adjusted. Note that the phase or sampling instant of the converters must also be controlled, and that this phase control can be achieved by tuning the converter frequency. This may be done by utilizing a Gardner detector, which is well-known in the art. In order to adjust the sampling frequency of the converters 101 the Gardner detector tap the interference suppressed signal from the output of the interference suppressor unit 105 and produces a clock detector signal which is used to adjust the sampling frequency of the converters 101 . The Gardner algorithm in the Gardner detector is based on delay differencing between the current sample and another sample delayed by half the symbol period, and the clock detector signal produced by the Gardner detector (also referred to as the Gardner error) can be used to determine if the sampling of the converters are correct, early or late, and thus be used to adjust the sampling frequency of the converters.

However, a problem with the clock recovery device 100 in figure 1 arises if relatively high bandwidth effects must be compensated for by the interference cancelation filter 103 and the equalizer filter 106. An example of such a problematic situation is for instance when the interference cancelation filter 103 must compensate for random differences in the receiver oscillator phase, e.g., caused by phase noise. In this case the early addition 105 of the interference cancellation signal implies an increased delay in the control loop (marked in figure 1 ) for updating the phase of the interference cancellation signal. The effect of the long loop delay may be that the bit error rate (BER) performance as a function of the signal-to-noise rate (SNR) exhibits an unwanted change of the slope 203, as shown in the plot 200 in figure 2, where the BER 201 is plotted as function of the SNR 202 for a quadrature amplitude modulation (QAM) system with phase noise and with prolonged loop delay.

However, according to the present invention the drawbacks mentioned above may be reduced by changing clock recovery algorithm between a blind method and a data-aided method depending on the receiver lock state. In this way it is possible to achieve a significant shortening of the control loops when the receiver is in a locked state, and thus enable an increase in tracking bandwidth. The present invention utilizes the Gardner clock detection method, described above, coupled with an interference suppression technique when the receiver is unlocked, i.e. in a blind mode, and no detection error signal is available from the detector unit 107. The receiver does not need to be locked onto the transmitted signal in order for the symbol clock to be recovered; neither does it require the transmitter to transmit any type of known data such as pilot symbols. Upon entering into a locked mode the clock recovery device according to the present invention changes clock detector strategy to a correlator based clock detector method. The correlator based clock detector method does not require any interference suppression and therefore loop delays in the system may be significantly reduced. A block diagram of a clock recovery device 300 according to an embodiment of the present invention is shown figure 3. In the clock recovery device 300 a received communication signal and a reference signal, which usually is a de-correlated version of the communication signal, are converted 301 from the analog domain to the digital domain (using at least one analog to digital converter with an adjustable sampling frequency) and filtered 302, producing a digital communication signal and a digital reference signal. The digital reference signal is processed through an adaptive interference cancellation filter 303 and subjected to a differential phase compensation, generating an interference cancellation signal, while the digital communication signal is delayed 304 to match the delay the digital reference signal is subjected to when filtered in the interference cancelation filter 303. The interference cancellation signal is coupled to a multiplier 305, which, by the constant WQA, decides which portion of the interference cancellation signal to be added before the equalization process 307 at ^' Canceller AddPoint Γ 306 in figure 3, and after the equalization process 307 at 'Canceller AddPoint ΙΓ 309 in figure 3. In order to suppress the interference in the digital communication signal, the interference cancellation signal, modified by the constant WQA, is added to the digital communication signal in a first interference suppressor unit 306 to produce a first interference suppressed signal. In order to reduce any inter symbol interference and remove any frequency offset in the signal the first interference suppressed signal from the first interference suppressor unit 306 is processed through an adaptive equalizer filter 307 producing an equalized signal. The interference cancellation signal from the interference cancelation filter 303 is delayed 308 to match the delay that the interference suppressed signal is subjected to when filtered in the equalizer filter 307. Before the delayed interference suppressed signal is added to the equalized signal, producing a second interference suppressed signal, in a second interference suppressor unit 309, it is subjected to differential phase compensation in a second differential phase compensator 323. The second interference suppressed signal is processed through a carrier recovery unit 310 producing a carrier recovered signal. The carrier recovered signal is then processed in a detector unit 31 1 in order to detect information symbols in said carrier recovered signal. As a result of the detection process a detection error signal, a detected communication signal and a locked clock signal (not shown in figure 3) is generated by the detector unit 31 1 .

As in the case with the clock recovery device 100 in figure 1 , the clock recovery device 300 according to the present invention may also operate in either a locked mode or in a blind mode. When the clock recovery device 300 is in a locked mode, and thus being synchronized with the transmitter, a detection error signal is generated from the detector unit 31 1 , while when the clock recovery device 300 is in a blind mode, and thus not being synchronized with the transmitter, no detection error signal is generated from the detector unit 31 1 . The detection error signal from the detector unit 31 1 is coupled to both the interference cancelation filter 303 with the first differential phase compensator, to the equalizer filter 307, and to the second differential phase compensator 323 , creating multiple control loops with different loop delays.

When the clock recovery device 300 operates in a locked mode the detection error signal is used to update the interference cancelation filter 303, the equalizer filter 307 and the first and the second differential phase compensators 322,323, while when the clock recovery device 300 operates in a blind mode the interference cancelation filter 303 and the equalizer filter 307 are updated using, for instance, a constant modulus algorithm which is well-known in the art. The updating of the first and the second differential phase compensators 322,323 are as described above updated with the same detection error signal, but the first differential phase compensator 322 needs to be updated with a lower bandwidth than the second differential phase compensator 323 due to the different loop delays.

The constant WDA is in the blind mode set to a predetermined constant value, while in locked mode the constant WDA is set close to zero or to exactly zero.

To be able to obtain symbol synchronization the sample frequency of the analog to digital converters 301 must be a controlled and adjusted. When the clock recovery device 300 operates in the blind mode the sampling frequency is adjusted by a first clock detector unit 315. The first clock detector unit 315 may be implemented using any well-known detector algorithm such as the Gardner algorithm (as in the case of the prior art in figure 1 ) or the Mueller and Muller algorithm. However, in this application the first clock detector unit 315 is exemplified using a Gardner detector. In order to adjust the sampling frequency of the converters 301 the Gardner detector tap the first interference suppressed signal (modified by the constant I I/DA) from the output of the interference suppressor unit 306 and produces a first clock detector signal which is coupled to a first switch input of a switch unit 313. When the clock recovery device 300 operates in the locked mode a second clock detector unit based on two correlators (also known as the early-late method) is used. The second clock detector unit 321 produce a second clock detector signal by tapping the delayed digital communication signal before the Canceller AddPoint I, delaying it in a processing delay unit 315 and splitting the delayed digital communication signal into two parts, wherein one part is delayed one symbol 316 before processed in a late correlator unit 317 adapted to produce a late correlated signal by correlating the detected communication signal with said delayed received communication signal having been further delayed by one symbol in a symbol delay unit 316, and wherein the second part is processed in an early correlator unit 318 producing an early correlated signal by correlating the detected communication signal with said received communication signal. Two absolute value units 319 are adapted to produce the absolute values of the late correlated signal and the early correlated signal. A processing unit 320 adapted to produce a second clock detector signal as the difference between the absolute value of said late correlated signal and the absolute value of said early correlated signal. The difference 320 in absolute value 319 of the late correlated signal and the early correlated signal should be zero; otherwise a synchronization error is present. The second clock detector signal is coupled to a second switch input of a switch unit 313. Because the correlator reference is the delayed digital communication signal, the correlators themselves act as matched filters and therefore suppress any interference that is uncorrelated with the communication signal. Hence, this type of clock detector can be used without first suppressing interference at the input to the detector.

The switch unit 313 is adapted to pass the first clock detector signal from the first switch input to a switch output if the locked clock signal coming from the detector unit 31 1 is set to false, and to pass the second detector clock from the second switch input to the switch output if the locked clock signal is set to true. This means that when the clock recovery device 300 operates in the blind mode, the first clock detector unit based on the Gardner method is used, while when the clock recovery device 300 operates in the locked mode the second clock detector unit based on the early-late method is used. The switch output is connected to a clock loop filter adapted to produce a clock loop filter output signal, based on the signal coming from the switch output, for adjusting the sampling frequency of the converters.

When the clock recovery device 300 operates in the blind mode using the first clock detector unit, the Gardner detector, to adjust the sampling frequency of the converters, and communication symbols from the companion transmitter are detected in the detector unit 31 1 , a transition to locked mode is initiated. During the transition the constant WQA, being set to a predetermined constant value, is linearly scaled down to zero or close to zero with a rate adapted so that the interference cancelation filter 303 and the equalizer filter 307 manage to adapt to the new configuration of the clock recovery device 300. The second clock detector unit 321 is initiated and when the early correlator and the late correlator outputs generates stable outputs the switch 313 may switch from relaying the first clock detector signal in the first switch input to the second clock detector signal in the second switch input to the switch output. Thus effectively changing the operation mode of the clock recovery device 300 from operating in a blind mode to operating in a locked mode. All device parameters are then adjusted according to steady state in the locked mode.

A diagram 400 comparing the BER 401 performance vs. the SNR of a QAM modulated system with phase noise according to prior art 403 and according to the present invention 404 is shown in figure 4. It can clearly be seen that the unwanted change of the slope 403 that can be clearly seen in the prior art 403 (the dotted lines) is gone in the same simulation using the present invention 404 (the line-dot-line) as presented above. A clock recovery method 500 for establishing synchronization with a received communication signal, according to the present invention, may also be defined based on the clock recovery device 300 described in conjunction with figure 3. The clock recovery method 500 may be described in a series of steps as follows.

• Converting 501 a received communication signal and a reference signal to a digital communication signal and a digital reference signal respectively, wherein said converting is adjustable by a sampling frequency.

• Producing 502 an interference cancellation signal by processing said digital reference signal through an interference cancellation filter and a first differential phase compensator.

• Producing 503, in a first interference suppressor unit, a first interference suppressed signal based on said digital communication signal and a portion of said interference cancellation signal, wherein said portion is determined by multiplying said interference cancellation signal with a predefined constant WDA-

• Producing 504 an equalized signal by processing said first interference suppressed signal through an equalizer filter.

• Producing 505, in a second interference suppressor unit, a second interference suppressed signal based on said equalized signal and said interference cancellation signal processed through a second differential phase compensator.

• Producing 506, in a carrier recovery unit, a carrier recovered signal from said second interference suppressed signal.

• Detecting 507, in a signal detector unit, the presence of a communication signal in said carrier recovered signal, and producing a detected communication signal, a detection error signal, and a locked clock signal.

• Adjusting 508 said interference cancellation filter, said first differential phase compensator, said second differential phase compensator and said equalizer filter with said detection error signal.

• Setting 509 said locked clock signal, wherein said locked clock signal is set to true if the presence of a communication signal is detected and set to false if no presence of a communication signal is detected in said carrier recovered signal.

• Producing 510 a first clock detector signal from said first interference suppressed signal in a first clock detector unit.

• Producing 51 1 a second clock detector signal from said received communication signal and said detected communication signal in a second clock detector unit.

• Switching 512, in a switch unit, to a first switch input, passing said first clock detector signal from said switch input to a switch output, if said locked clock signal is set to false, and switching to a second switch input, passing said second detector clock from said second switch input to said switch output, if said locked clock signal is set to true.

· Producing 513 a clock loop filter output signal based on said switch output.

• Adjusting 514 said sampling frequency of said converters based on said clock loop filter output signal. The step of producing said a second clock detector signal may further comprise the following steps below.

- Delaying, in a processing delay unit, said delayed received communication signal.

- Correlating, in a late correlator unit, said detected communication signal with said delayed received communication signal and producing a late correlated signal, wherein said delayed received communication signal have been further delayed one symbol in a symbol delay unit.

- Correlating, in a early correlator unit, said detected communication signal with said received communication signal producing an early correlated signal.

- Producing, in two absolute value units, the absolute value of said late correlated signal and said early correlated signal.

- Processing, in a processing unit, said second clock detector signal as the difference between the absolute value of said late correlated signal and the absolute value of said early correlated signal. The clock recovery method may also comprise the steps below.

- Setting said constant W _D A to a predetermined constant value when said locked clock signal is set to false, and to zero when said locked clock signal is set to true.

- Scaling down said constant W _D A linearly to zero when the presence of a communication signal is detected in said detector unit.

The present invention, according to the embodiments described above, permits clock recovery in high interference conditions without limiting the steady-state tracking bandwidth of control loops such as differential phase tracking loops. This is achieved without the transmitter transmitting any known data such as pilot symbols.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" "comprising," "includes" and/or "including" when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should be regarded as illustrative rather than restrictive, and not as being limited to the particular embodiments discussed above. The different features of the various embodiments of the invention can be combined in other combinations than those explicitly described. It should therefore be appreciated that variations may be made in those embodiments by those skilled in the art without departing from the scope of the present invention as defined by the following claims.