Abstract:
The present disclosure provides an apparatus and a method for canceling inter-modulation (IM) products in a transceiver. The apparatus includes: a pre-distortion circuit configured to estimate a first IM product caused by a transmission signal and pre-distort the transmission signal to cancel the first IM product; an IM product calculator configured to calculate a second IM product caused by the transmission signal in a received signal based on the first IM product; and a subtractor configured to subtract the second IM product from the received signal.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to wireless communication technology, and more particularly, to an apparatus and a method for canceling Inter-Modulation (IM) products. 
       BACKGROUND 
       [0002]    Currently, with rapid development of global mobile communications, telecommunication operators are continuously interested in reducing total cost of ownership (TCO) and looking for cost-effective methods for expanding mobile networks. This is necessary for coping with the rapidly growing demands of users. In order to reduce the TCO, a base station can adopt a multi-carrier technique, e.g., a base station can use a number of carriers, or carrier frequencies, within its transceiver. Such base station can be referred to as a multi-carrier base station. A multi-carrier base station can have multiple carriers in one or more transmission frequency bands for one or more radio access technologies (RATs). A multi-carrier base station needs to support higher output power and broader bandwidth than legacy base stations to provide improved coverage and capacity. 
         [0003]    Typically, a transceiver includes non-linear components, which means that the output of each of these components is a non-linear function of its input. Given a multi-carrier signal to be transmitted (hereinafter referred to as “transmission signal”), a non-linear component will produce, at its output, additional spectral components at frequencies different from the carrier frequencies of the transmission signal. These spectral components are known as inter-modulation (IM) products and have a specific relation in frequency with a transmission signal to be transmitted by the transceiver. The IM products include active IM (AIM) products and passive IM (PIM) products. 
         [0004]    In order to cancel the IM product caused by the transmission signal from the signal actually transmitted by a transceiver, an analog or digital pre-distortion technique has been proposed.  FIG. 1  shows a structure of a transceiver  100  provided with a digital pre-distortion (DPD) circuit  110 . The DPD circuit  110  has a coupler  111  for obtaining a copy of a radio frequency (RF) signal output from a power amplifier (PA)  121  and feed it back to a demodulation and down-conversion circuit  112 . The copy of the RF signal contains the transmission signal and an IM product which is caused by the transmission signal and produced in a transmission signal path  120  (including e.g., a Digital-to-Analog Converter (DAC)  123 , a modulation and up-conversion circuit  122  and the PA  121 ). The demodulation and down-conversion circuit  112  down-converts the copy of the RF signal into an intermediate frequency (IF) signal. The DPD circuit  110  also includes an Analog-to-Digital Converter (ADC)  113  for converting the IF signal into a digital signal which is then processed by a Digital Signal Processor (DSP)  114 . The transmission signal is delayed at a delay circuit  115  and the delayed signal is input to the DSP  114  which then estimates the IM product. Finally, the transmission signal is gain adjusted at a gain adjustment circuit  116  according to the estimated IM product from the DSP  114 . In this way, the transmission signal is pre-distorted to cancel the IM product. The transceiver  100  also has an antenna front end  130  including a transmission filter  131 , a reception filter  132  and an antenna  133 . Here, the band pass transmission filter  131  is provided for filtering the IM product before the transmission signal is fed to the antenna  133 . The respective functions of the PA  121 , the modulation and up-conversion circuit  122 , the DAC  123 , the transmission filter  131 , the reception filter  132  and the antenna  133  are well known and thus the details thereof will be omitted here.  FIG. 2  is a schematic diagram of signal spectra with and without DPD. It can be seen from  FIG. 2  that, with DPD, the IM product is significantly reduced. After being filtered by the transmission filter, the IM product is much lower than the transmission signal (Carrier  1  (C 1 ) and Carrier  2  (C 2 ) as shown in  FIG. 2 ). 
         [0005]    Since the IM products are extended over the frequency spectrum out of the transmission frequency bands, there is a risk that some of the IM products will end up in the reception frequency band of the transceiver, thereby causing distortion in a signal received at the transceiver. Since a multi-carrier base station transceiver supports a high transmit power and has a number of carriers arranged over a broad bandwidth, the IM products may cause a considerable distortion in a received signal. For example, it can also be seen from  FIG. 2  that a residual IM product falls into the reception frequency band of the transceiver. 
         [0006]    Furthermore, there are further IM products that cannot be canceled by the DPD or filtered by the transmission filter. For example, these IM products are produced at an antenna, a connector connected to the antenna, a connector connected to the transmission filter, a feeder cable connecting the connectors, and the transmission filter itself, as shown in  FIG. 3 . Although these IM products are much lower than the IM products caused by e.g. the power amplifier, they may cause distortion to a received signal when the IM products fall into the reception frequency band of the transceiver, especially if the received signal is weak.  FIG. 4  shows an example of signal spectra in which an IM product falling into the reception frequency band is comparable to the received signal. 
         [0007]    Patent document U.S. Pat. No. 8,055,235B1 describes a method for interference cancellation in which a cancellation signal and an interference signal is summed in analog domain. However, this method is only applicable to cancel large interference signals, but is not applicable to cancel IM products at very low level, especially PIM products produced at the passive components shown in  FIG. 3 . Patent documents US2011/0149714 and US20090068974 describe similar interference cancellation techniques that cancel interferences in digital domain. But again, they are not applicable to cancel IM products at very low level, especially PIM products. Patent document CA2658070 describes a method for blindly detecting PIM products. However, it fails to provide any solution for canceling them. Patent document US2011/0075754A1 describes a method for removing PIM products generated in a wave guide and an antenna that fall into a reception frequency band. However, it cannot detect or cancel IM products produced at the components shown in  FIG. 3  and cannot detect or cancel IM products at very low level, such as −100 dBm. 
         [0008]    There is thus a need for an improved solution for canceling IM products caused by a transmission signal from a received signal. 
       SUMMARY 
       [0009]    It is an object of the present disclosure to overcome at least some of the above problems in the prior art. 
         [0010]    According to a first aspect of the present disclosure, an apparatus for canceling inter-modulation (IM) products in a transceiver is provided. The apparatus includes: a pre-distortion circuit configured to estimate a first IM product caused by a transmission signal and pre-distort the transmission signal to cancel the first IM product; an IM product calculator configured to calculate a second IM product caused by the transmission signal in a received signal based on the first IM product; and a subtractor configured to subtract the second IM product from the received signal. 
         [0011]    In an embodiment, the transceiver includes a transmission signal path, a reception signal path and an antenna front end. The first IM product is produced in the transmission signal path and the second IM product is produced in the antenna front end and the reception signal path. 
         [0012]    In an embodiment, the pre-distortion circuit is configured to estimate, during a training period, a reference IM product caused by a reference transmission signal. The IM product calculator includes: a coefficient estimation unit configured to estimate, during the training period, a coefficient characterizing an equivalent transfer function of the antenna front end and the reception signal path based on the reference IM product and a received signal containing an IM product caused by the reference transmission signal; and an IM product calculation unit configured to calculate the second IM product based on the first IM product and the coefficient. 
         [0013]    In an embodiment, the coefficient estimation unit is configured to estimate the coefficient by using a pilot-based estimation algorithm in which the reference IM product is used as a pilot. 
         [0014]    According to another embodiment of the present disclosure, a method for canceling inter-modulation (IM) products in a transceiver is provided. The method includes: estimating a first IM product caused by a transmission signal and pre-distorting the transmission signal to cancel the first IM product; calculating a second IM product caused by the transmission signal in a received signal based on the first IM product; and subtracting the second IM product from the received signal. 
         [0015]    In an embodiment, the transceiver includes a transmission signal path, a reception signal path and an antenna front end. The first IM product is produced in the transmission signal path and the second IM product is produced in the antenna front end and the reception signal path. 
         [0016]    In an embodiment, the method further includes: estimating, during a training period, a reference IM product caused by a reference transmission signal, and estimating, during the training period, a coefficient characterizing an equivalent transfer function of the antenna front end and the reception signal path based on the reference IM product and a received signal containing an IM product caused by the reference transmission signal. The second IM product is calculated based on the first IM product and the coefficient. 
         [0017]    In an embodiment, the coefficient is estimated by using a pilot-based estimation algorithm in which the reference IM product is used as a pilot. 
         [0018]    With the embodiments of the present disclosure, the IM product estimated by the pre-distortion circuit is reused to calculate the IM product caused by the transmission signal in the received signal and finally cancel the calculated IM product from the received signal. In this way, the IM product caused by the transmission signal, including AIM components and PIM components, can be removed from the received signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above and other objects, features and advantages will be more apparent from the following description of embodiments with reference to the figures, in which: 
           [0020]      FIG. 1  shows a structure of a transceiver provided with a DPD circuit according to the prior art; 
           [0021]      FIG. 2  is a schematic diagram of signal spectra with and without DPD; 
           [0022]      FIG. 3  shows a structure of an antenna front end; 
           [0023]      FIG. 4  is a schematic diagram of signal spectra illustrating a PIM product falling into reception frequency band; 
           [0024]      FIG. 5  shows a structure of a transceiver including an apparatus for canceling IM products according to an embodiment of the present disclosure; 
           [0025]      FIG. 6  is a schematic diagram of signal spectra illustrating the process of canceling IM products from a received signal; and 
           [0026]      FIG. 7  is a flowchart illustrating a method for canceling IM products according to an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The embodiments of the present disclosure will be detailed below with reference to the drawings. It should be noted that the following embodiments are illustrative only, rather than limiting the scope of the present disclosure. 
         [0028]      FIG. 5  shows a structure of a transceiver  500  including an apparatus  510  for canceling IM products according to an embodiment of the present disclosure. The transceiver  500  can be used in a base station. As shown in  FIG. 5 , the transceiver  500  further includes a transmission signal path  520 , an antenna front end  530  and a reception signal path  540 . The transmission signal path  520  includes e.g., a Digital-to-Analog Converter (DAC), a modulation and up-conversion circuit and a Power Amplifier (PA). The antenna front end  530  includes e.g., an antenna, a connector connected to the antenna, a connector connected to the transmission filter, a feeder cable connecting the connectors, a transmission filter and a reception filter. The reception signal path  540  includes e.g., a Low Noise Amplifier (LNA), a demodulation and down-conversion circuit and an Analog-to-Digital Converter (ADC). All of the components included in the transmission signal path  520 , the antenna front end  530  and the reception signal path  540  are well known in the art and the details thereof will be omitted here. 
         [0029]    The apparatus  510  includes a pre-distortion (DPD) circuit  511 , an IM product calculator  512  and a subtractor  513 . 
         [0030]    The pre-distortion circuit  511  estimates an IM product (hereinafter denoted as IM 1 ) caused by a transmission signal and pre-distorts the transmission signal to cancel IM 1 . Here, IM 1  includes IM components produced in the transmission signal path  520 , e.g., the 3 rd  order and/or the 5 th  order IM products. The pre-distortion circuit  511  can estimate IM 1  using any known DPD technique. 
         [0031]    The IM product calculator  512  calculates an IM product (hereinafter denoted as IM 2 ) caused by the transmission signal in a received signal based on IM 1 . Here, IM 2  includes IM components produced in the antenna front end  530  and the reception signal path  540 . The IM product calculator  512  includes a coefficient estimation unit  514  and an IM product calculation unit  515 , which will be detailed below. 
         [0032]    During a training period, the pre-distortion circuit  511  estimates a reference IM product (hereinafter denoted as x) caused by a reference transmission signal. The coefficient estimation unit  514  estimates a coefficient characterizing an equivalent transfer function (hereinafter denoted as h) of the antenna front end  530  and the reception signal path  540  based on x and a received signal (hereinafter denoted as y) containing an IM product (hereinafter denoted as im) caused by the reference transmission signal. 
         [0033]    In particular, the IM product im contains IM components produced in the antenna front end  530  and the reception signal path  540  and possibly a residual IM component leaked from the transmission filter. All these components are caused by the same reference transmission signal. That is, the IM product im and the reference IM product x are caused by the same reference transmission signal and thus have the same characteristics in frequency domain. Therefore, it is possible to calculate im from x: 
         [0000]    
       
      
       im=h·x.  
      
     
         [0000]    where h is an equivalent transfer function of the antenna front end  530  and the reception signal path  540 , as described above. 
         [0034]    Therefore, the received signal y can be expressed as: 
         [0000]    
       
      
       y=im+r=h·x+r.  
      
     
         [0000]    where r denotes the combination of a wanted signal and a noise but does not include any IM products. 
         [0035]    Since both the wanted signal and the noise can be considered as white noises, the coefficient estimation unit  514  can use x as a pilot to estimate h by using any pilot-based channel estimation algorithm (such as Minimum Mean Square Error (MMSE), Least Square (LS), etc.) during the training period. 
         [0036]    Since the estimated h can be considered to be quite stable. After the training period, the IM product calculation unit  515  can calculate IM 2  based on IM 1  and the estimated h: 
         [0000]      IM2= h·IM 1. 
         [0037]    Then, the subtractor  513  subtracts IM 2  from the received signal. In this way, the IM product caused by the transmission signal in the received signal can be canceled. 
         [0038]      FIG. 6  is a schematic diagram of signal spectra illustrating the process of canceling IM products from a received signal. It can be seen from  FIG. 6  that the IM product, including possibly any residual IM components after DPD and filtering at the transmission filter, can be canceled from the received signal. 
         [0039]      FIG. 7  is a flowchart illustrating a method  700  for canceling IM products according to an embodiment of the present disclosure. The method  700  can be performed at the transceiver  500 . The method  700  includes the following steps. 
         [0040]    At step S 710 , an IM product (IM 1 ) caused by a transmission signal is estimated and the transmission signal is pre-distorted to cancel IM 1 . 
         [0041]    At step S 720 , an IM product (IM 2 ) caused by the transmission signal in a received signal is calculated based on IM 1 . 
         [0042]    At step S 730 , IM 2  is subtracted from the received signal. 
         [0043]    In an embodiment, the method  700  further includes a step of estimating, during a training period, a reference IM product (x) caused by a reference transmission signal, and estimating, during the training period, a coefficient characterizing an equivalent transfer function (h) of the antenna front end and the reception signal path based on x and a received signal (y) containing an IM product (im) caused by the reference transmission signal. IM 2  is calculated based on IM 1  and h. 
         [0044]    It is to be noted that the contents described above in relation to the transceiver  500  of  FIG. 5  also apply to the method  700  according to the present disclosure. 
         [0045]    With the embodiments of the present disclosure, the IM product estimated by the pre-distortion circuit is reused to calculate the IM product caused by the transmission signal in the received signal and finally cancel the calculated IM product from the received signal. In this way, the IM product caused by the transmission signal, including AIM components and PIM components, can be removed from the received signal. 
         [0046]    The present disclosure has been described above with reference to the preferred embodiments thereof. It should be understood that various modifications, alternations and additions can be made by those skilled in the art without departing from the spirits and scope of the present disclosure. Therefore, the scope of the present disclosure is not limited to the above particular embodiments but only defined by the claims as attached.