Abstract:
Disclosed is a method and an apparatus for self-calibrating a Direct Current (DC) offset and an imbalance between orthogonal signals, which may occur in a mobile transceiver. In the apparatus, a transmitter of a mobile terminal functions as a signal generator, and a receiver of the mobile terminal functions as a response characteristic detector. Further, a baseband processor applies test signals to the transmitter, receives the test signals returning from the receiver, and compensates the imbalance and DC offset for the transmitter side and the receiver side by using the test signals. The test signal is applied to only one of the I channel path and the Q channel path, and an RF band signal output from the transmission side by the test signal is used as an input signal to the reception side.

Description:
PRIORITY  
       [0001]     This application claims the benefit under 35 U.S.C. §119(a) of an application filed in the Korean Industrial Property Office on Dec. 8, 2005 and assigned Serial No. 2005-119864, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method and an apparatus for self-calibration in a mobile transceiver, and more particularly to a method and an apparatus for self-calibrating a Direct Current (DC) offset and imbalance between orthogonal signals.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, basic causes of degrading performance of mobile transceivers include undesired or non-ideal characteristics, such as a DC offset and an I/Q imbalance.  
         [0006]     The DC offset is caused by self mixing by a mixer in a wireless receiver. The DC offset occurs when a signal of a Local Oscillator (LO) returns after leaking toward an antenna or when a Radio Frequency (RF) modulation signal input through the antenna is leaked to the LO. The DC offset value generated in this way may saturate a Base band (BB) circuit.  
         [0007]     The I/Q imbalance is caused by self-defects of an oscillator including a phase retarder and a line interconnecting the oscillator and a mixer. The I/Q imbalance is caused when the phase difference between the in-phase channel signal (I channel signal) and the quadrature-phase channel signal (Q channel signal) generated in an oscillator of a wireless transmitter is not 90 degrees. The I/Q imbalance can be reduced by designing mixers of the I channel demodulator and the Q channel demodulator to be symmetric to each other. However, designing the mixers to be symmetric to each other requires an increase in the volume and current consumption of the mixers. Such I/Q imbalances cause decreases in the Signal to Noise Ratio (SNR), which increases the Bit Error Rate (BER), thereby degrading the performance of the wireless transceiver.  
         [0008]     Therefore, in order to improve the performance of a wireless transceiver, it is necessary to arrange a solution for estimating the DC offset and the I/Q imbalance and calibrating the estimated the DC offset and the I/Q imbalance.  
         [0009]      FIG. 1  illustrates a representative example of a process for self-estimating and self-calibrating a DC offset and an I/Q imbalance which occur in a conventional wireless transceiver. The example shown in  FIG. 1  is disclosed in a Patent Cooperation Treaty (PCT) application No. 2004/023667, entitled “Direct Conversion Transceiver Enabling Digital Calibration,” and a paper by James K. Cavers, entitled “New Methods for Adaptation of Quadrature Modulators and Demodulators in Amplifier Linearization Circuits.” 
         [0010]     For convenience of description, the estimation path is not distinguished into an I channel path and a Q channel path. However, the same application is possible even when the estimation path is distinguished into an I channel path and a Q channel path.  
         [0011]     The solution proposed in  FIG. 1  calibrates both the I/Q imbalance and the DC offset generated at a transmission (TX) side and a reception (RX) side. To this end, calibration for the TX side is first performed, and calibration for the RX side is then performed. In other words, for the calibration for the RX side, the calibration for the TX side must precede it. The calibration for the TX side corresponds to imbalance calibration between I channel and Q channel (TX IQ calibration). The calibration for the RX side includes calibration for the DC offset as well as the imbalance calibration between the I channel and the Q channel.  
         [0012]     In the estimator shown in  FIG. 1 , a discrete detector is used. The discrete detector converts an envelope signal output from a drive amplifier of the TX side into a Baseband (BB) signal and takes a discrete Fourier series for a complex envelope waveform of the BB signal. Based on the discrete Fourier series, the discrete detector estimates the gain imbalance, the phase imbalance, and the DC offset of each of the I channel and the Q channel at the TX side.  
         [0013]     However, in the estimation as described above, it is necessary to consider the non-ideal factors, which include a differential gain and a Direct Current (DC) value. In the above-mentioned paper and patent application, the non-ideality factors are estimated.  
         [0014]     The TX and RX gain imbalance, phase imbalance, and DC offsets of the I channel and Q channel obtained through the estimation may be incorrect.  
         [0015]     Further, as noted from  FIG. 1 , many separate diodes, registers, capacitors, and switches are necessary in order to construct the discrete detector. Also, a great amount of time is required for self-calibration of the DC offset and the imbalance between the orthogonal signals.  
       SUMMARY OF THE INVENTION  
       [0016]     Accordingly, the present invention has been made to solve at least the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an apparatus and a method for self-estimating and self-calibrating the DC offset characteristics and the imbalance characteristics.  
         [0017]     It is another object of the present invention to provide an apparatus and a method for estimating and calibrating the DC offset characteristics and the imbalance characteristics in a single path connected between a transmission side and a reception side.  
         [0018]     It is another object of the present invention to provide an apparatus and a method for estimating the DC offset characteristics of a reception side based on a test signal that is received by the reception side by applying a test signal through an uncalibrated transmission side.  
         [0019]     It is another object of the present invention to provide an apparatus and a method for estimating the imbalance characteristics of a reception side based on a test signal that is received by the reception side by applying a test signal through an uncalibrated transmission side.  
         [0020]     It is another object of the present invention to provide an apparatus and a method for estimating the imbalance characteristics of an uncalibrated transmission side based on a test signal that is received by an already calibrated reception side by applying a test signal through the uncalibrated transmission side.  
         [0021]     It is another object of the present invention to provide an apparatus and a method for estimating the DC offset characteristics and the imbalance characteristics of a reception side by applying a test signal to only one of the I channel path and the Q channel path of the transmission side.  
         [0022]     It is another object of the present invention to provide an apparatus and a method for estimating the DC offset characteristics and the imbalance characteristics of a transmission side by applying a test signal to only one of the I channel path and the Q channel path of the transmission side.  
         [0023]     In order to accomplish these and other objects, there is provided a method for self-calibration in a transceiver having a test path for applying a Radio Frequency (RF) band signal from a transmission side to a reception side, the method comprising sequentially generating a first in-phase channel test signal and a second in-phase channel test signal in an analog baseband of a transmission side at a predetermined time interval; converting the first in-phase channel test signal and the second in-phase channel test signal of the analog baseband to a first RF band signal and a second RF band signal according to an order in which the first in-phase channel test signal and the second in-phase channel test signal are generated, and then applying the first RF band signal and the second RF band signal to the reception side through the test path; outputting first and second in-phase channel test signals and first and second quadrature-phase channel test signals by converting the first RF band signal and the second RF band signal to analog baseband signals by means of a first carrier for an in-phase channel and a second carrier for a quadrature-phase channel, respectively; calibrating a DC offset characteristic for in-phase channel reception signals in the analog baseband of the reception side by using an average value of the first and second in-phase channel test signals; and calibrating a DC offset characteristic for quardrature-phase channel reception signals in the analog baseband of the reception side by using an average value of the first and second quardrature-phase channel test signals.  
         [0024]     In accordance with another aspect of the present invention, there is provided an apparatus for self-calibration in a transceiver having a test path for applying a Radio Frequency (RF) band signal from a transmission side to a reception side, wherein the apparatus sequentially generates a first in-phase channel test signal and a second in-phase channel test signal in an analog baseband of a transmission side at a predetermined time interval; converts the first in-phase channel test signal and the second in-phase channel test signal of the analog baseband to a first RF band signal and a second RF band signal according to an order in which the first in-phase channel test signal and the second in-phase channel test signal are generated, and then applies the first RF band signal and the second RF band signal to the reception side through the test path; outputs first and second in-phase channel test signals and first and second quadrature-phase channel test signals by converting the first RF band signal and the second RF band signal to analog baseband signals by means of a first carrier for an in-phase channel and a second carrier for a quadrature-phase channel, respectively; calibrates a DC offset characteristic for in-phase channel reception signals in the analog baseband of the reception side by using an average value of the first and second in-phase channel test signals; and calibrates a DC offset characteristic for quardrature-phase channel reception signals in the analog baseband of the reception side by using an average value of the first and second quardrature-phase channel test signals.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0026]      FIG. 1  illustrates a representative example of a process for self-estimating and self-calibrating a DC offset and an I/Q imbalance which occur in a conventional wireless transceiver;  
         [0027]      FIG. 2  is a block diagram illustrating a structure of a mobile terminal according to the present invention;  
         [0028]      FIG. 3  is a flowchart of a process for self-calibration by a DSP according to the present invention; and  
         [0029]      FIG. 4  is a graph for illustrating a comparison between a test signal transmitted to the transmission side and a test signal received by the reception side. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]     Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.  
         [0031]     Before description of preferred embodiments, terms used herein are defined as follows:  
         [0032]     I TX : an in-phase channel test signal that is applied through the I channel path of the TX side in order to calibrate the imbalance characteristics between the I channel path and the Q channel path and the DC offset characteristic occurring in the I channel path of the RX side;  
         [0033]     I Rx : an in-phase channel test signal, which is output as a baseband signal by LO II  from a mixer in the I channel path of the RX side, wherein the baseband signal is obtained from an RF RX signal input to the mixer in the I channel path of the RX side, the RF RX signal is obtained from an RF TX signal output from a mixer in the I channel path of the TX side, and the RF TX signal is obtained from the I RX  input through the I channel to the mixer of the TX side;  
         [0034]     Q RX : a quadrature-phase channel test signal, which is output as a baseband signal by LO QQ  from a mixer in the Q channel path of the RX side, wherein the baseband signal is obtained from an RF RX signal input to the mixer in the Q channel path of the RX side, the RF RX signal is obtained from an RF TX signal output from a mixer in the I channel path of the TX side, and the RF TX signal is obtained from the ITx input through the I channel to the mixer of the TX side;  
         [0035]     LO II : a carrier frequency which is used in order to convert a Radio Frequency (RF) band signal to a baseband signal in the I channel path of the RX side;  
         [0036]     LO QQ : a carrier frequency which is used in order to convert an RF band signal to a baseband signal in the Q channel path of the RX side;  
         [0037]     LO Q : a carrier frequency which is used in order to convert a baseband signal to an RF band signal in the I channel path of the TX side; and  
         [0038]     LO Q : a carrier frequency which is used in order to convert a baseband signal to an RF band signal in the Q channel path of the TX side.  
         [0039]     A method for estimating and calibrating imbalance characteristics and DC offset characteristics according to the present invention by a mobile terminal, in which a test signal generated by a transmitter side is provided to a receiver side and is then used to estimate and calibrate the imbalance characteristics and DC offset characteristics, will be described in detail.  
         [0040]     The test signal has a predetermined shape, which includes a shape of a simple wave, such as a sine wave or a cosine wave.  
         [0041]     Each test signal for estimation of the DC offset of the RX side and the imbalance of the RX side and the TX side is applied to only one channel path of the I channel path and the Q channel path. The following embodiments are based on an assumption that a test signal for estimating the DC offset and the imbalance of the RX side is applied to only the I channel path and a test signal for estimating the imbalance of the TX side is applied to only the Q channel path. Of course, it is also possible to apply a test signal for estimating the DC offset and the imbalance of the RX side to only the Q channel path and apply a test signal for estimating the imbalance of the TX side to only the I channel path.  
         [0042]      FIG. 2  is a block diagram which illustrates a structure of a mobile terminal according to the present invention. Although the discussion in the present embodiment is based on a mobile terminal, the present invention can be applied to all apparatuses and systems which can perform wireless communication.  
         [0000]     A. Calibration of DC Offset of the RX Side  
         [0043]     Referring to  FIG. 2 , the TX side includes Digital-to-Analog Converters (DACs)  220 -I and  220 -Q and Low Pass Filters (LPFs)  230 -I and  230 -Q, and mixers  240 -I and  240 -Q, which are arranged along the I channel path and the Q channel path of the TX side, respectively. Further, the RX side includes mixers  260 -I and  260 -Q, LPFs  270 -I and  270 -Q, and Analog-to-Digital Converters (ADCs)  280 -I and  280 -Q, which are arranged along the I channel path and the Q channel path of the RX side, respectively.  
         [0044]     The Digital Signal Processor (DSP)  210  generates predefined test signals and applies the generated test signals to the I channel path of the TX side, in order to estimate the DC offset characteristics. Further, by using a baseband test signal received through the RX side, the DSP  210  estimates the DC offset characteristics. Based on the estimated DC offset characteristics, the DSP calibrates the DC offset of the RX side.  
         [0045]     First, the DSP  210  applies test signals I TX  to the DAC  220 -I, in order to estimate the DC offset characteristics of the RX side. Specifically, the DSP  210  applies two different baseband test signals I TX#1  and I TX#2  at a predetermined time interval, in order to estimate the DC offset characteristics of the RX side. However, no test signal is applied to the DAC  220 -Q at all. Therefore, the operations of the DAC  220 -Q, the LPF  230 -Q, and the mixer  240 -Q in the Q channel path of the TX side will not be considered herein.  
         [0046]     In the discussion below, the operation by the I TX#1  and the operation by the I TX#2  are discriminated from each other. First, the operation when the I TX#1  is applied as a test signal will be described hereinafter. One example of I TX#1  can be defined by Equation (1) 
 
 I   TX#1 ( t )=cos  ω   0   t   (1) 
 
         [0047]     The DAC  220 -I converts the applied I TX#1  to an analog signal and then inputs the converted analog signal to the LPF  230 -I.  
         [0048]     The analog signal I TX#1  is filtered by the LPF  230 -I and is then converted to an RF band signal by the mixer  240 -I. The carrier in the mixer  240 -I corresponds to LO I  and the carrier in the mixer  240 -Q corresponds to LO Q . LO I  and LO Q  can be defined by Equation (2) 
 
 LO   I =cos  ω   t  
 
 LO   Q =α1 sin(  ω   t+φ 1)  (2) 
 
         [0049]     In Equation (2), α 1  denotes the gain imbalance characteristic between the I channel path and the Q channel path of the TX side, and φ 2  denotes the phase imbalance characteristic between the I channel path and the Q channel path of the TX side.  
         [0050]     The RF TX signal TX output# of the RF band converted by the mixer  240 -I can be defined by Equation (3)  
                       TX     output   ⁢   #1       ⁡     (   t   )       =       ⁢           I     TX   ⁢   #1       ⁡     (   t   )       ·   A   ·   cos     ⁢           ⁢     ϖ   c     ⁢   ϖ   ⁢           ⁢   t                 =       ⁢       A   ·     cos   ⁡     (     ϖ   -     ϖ   0       )         +       A   ·     cos   ⁡     (     ϖ   +     ϖ   0       )         ⁢   t                     (   3   )             
 
         [0051]     The RF TX signal TX output#1  is transferred to the RX side through a test path formed by the first switch SW# 1  and the second switch SW# 2 . The RF band signal RX input#1  transferred to the RX side can be defined by Equation (4) 
 
 RX   input#1 ( t )= A ·cos(  ω   c   t−  ω     0   t +θ)+ A ·cos(  ω   c   t+  ω     0   t +θ)  (4) 
 
         [0052]     The RF band signal RX input#1  transferred to the RX side through the second switch SW# 2  is converted to a baseband signal by the mixer  260 -I in the I channel path. The mixer  260 -I uses a carrier LO II  which can be defined by Equation (5) 
 
 LO   II =cos  ω   t   (5) 
 
         [0053]     Further, the RF band signal RX input#1  transferred to the RX side through the second switch SW# 2  is converted to a baseband signal by the mixer  260 -Q in the Q channel path. The mixer  260 -Q uses a carrier LO QQ , which can be defined by Equation (6) 
 
 LO   QQ =α2 sin(  ω   t+ φ2)  (6) 
 
         [0054]     In Equation (6), α 2  denotes the gain imbalance characteristic between the I channel path and the Q channel path of the RX side, and φ 2  denotes the phase imbalance characteristic between the I channel path and the Q channel path of the RX side.  
         [0055]     The baseband signal output from the mixer  260 -I is filtered by the LPF  270 -I in the I channel path and is then transferred to the ADC  280 -I, by which it is converted to a digital signal. The digital signal converted by the ADC  280 -I corresponds to I RX#1 . The baseband signal output from the mixer  260 -Q is filtered by the LPF  270 -Q in the Q channel path and is then transferred to the ADC  280 -Q, by which it is converted to a digital signal. The digital signal converted by the ADC  280 -Q corresponds to Q RX#1 . The I RX#1  and the Q RX#1  are defined by Equation (7)  
                       I     RX   ⁢   #1       ⁡     (   t   )       =       ⁢         A   2     ·     cos   ⁡     (         ϖ   0     ⁢   t     -   θ     )         +       A   2     ·     cos   ⁡     (         ϖ   0     ⁢   t     +   θ     )         +     Δ   ⁢           ⁢   I                       Q     RX   ⁢   #1       ⁡     (   t   )       =       ⁢           α   ⁢           ⁢     2   ·   A       2     ·     sin   ⁡     (         ϖ   0     ⁢   t     -   θ   +   ϕ2     )         -                     ⁢           α2   ·   A     2     ·     sin   ⁡     (         ϖ   0     ⁢   t     +   θ   -   ϕ2     )         +     Δ   ⁢           ⁢   Q                     (   7   )             
 
         [0056]     The I RX#1  and the Q RX#1  are provided to the DSP  210 .  
         [0057]     Next, the operation when the I TX#2  is applied as a test signal will be described hereinafter. One example of I TX#2  can be defined by Equation (8) below. 
 
 I   TX#2 ( t )=−cos  ω   0   t   (8) 
 
         [0058]     The I TX#1  and I TX#2  are signals having a phase difference of 180 degrees. Any pair of signals having simple waveforms with a phase difference of 180 degrees can be used as the I TX#1  and I TX#2 .  
         [0059]     The DAC  220 -I converts the applied I TX#2  to an analog signal and then inputs the converted analog signal to the LPF  230 -I.  
         [0060]     The analog signal I TX#2  is filtered by the LPF  230 -I and is then converted to an RF band signal by the mixer  240 -I. The carrier in the mixer  240 -I corresponds to the LO I  defined by Equation (2).  
         [0061]     The RF TX signal TX output#2  of the RF band converted by the mixer  240 -I can be defined by Equation (9)  
                       TX     output   ⁢   #2       ⁡     (   t   )       =       ⁢           I     TX   ⁢   #2       ⁡     (   t   )       ·   A   ·   cos     ⁢           ⁢     ϖ   c     ⁢   ϖ   ⁢           ⁢   t                 =       ⁢         -   A     ·     cos   ⁡     (     ϖ   -     ϖ   0       )         -       A   ·     cos   ⁡     (     ϖ   +     ϖ   0       )         ⁢   t                     (   9   )             
 
         [0062]     The RF TX signal TX output#2  is transferred to the RX side through a test path formed by the first switch SW# 1  and the second switch SW# 2 . The RF band signal RX input#2  transferred to the RX side can be defined by equation (10) below. 
 
 RX   input#2 ( t )=− A ·cos(  ω   c   t−  ω     0   t +θ)− A ·cos(  ω   c   t+  ω     0   t +θ)  (10) 
 
         [0063]     The RF band signal RX input#2  transferred to the RX side through the second switch SW# 2  is converted to a baseband signal by the mixer  260 -I in the I channel path. The mixer  260 -I uses the carrier LO II  defined by Equation (5).  
         [0064]     Further, the RF band signal RX input#2  transferred to the RX side through the second switch SW# 2  is converted to a baseband signal by the mixer  260 -Q in the Q channel path. The mixer  260 -Q uses the carrier LO QQ  defined by Equation (6).  
         [0065]     The baseband signal output from the mixer  260 -I is filtered by the LPF  270 -I in the I channel path and is then transferred to the ADC  280 -I, by which it is converted to a digital signal. The digital signal converted by the ADC  280 -I corresponds to I RX#2 . The baseband signal output from the mixer  260 -Q is filtered by the LPF  270 -Q in the Q channel path and is then transferred to the ADC  280 -Q, by which it is converted to a digital signal. The digital signal converted by the ADC  280 -Q corresponds to Q RX#2 . The IRX#2 and the Q RX#2  are defined by Equation (11)  
                       I     RX   ⁢   #2       ⁡     (   t   )       =       ⁢         -     A   2       ·     cos   ⁡     (         ϖ   0     ⁢   t     -   θ     )         -       A   2     ·     cos   ⁡     (         ϖ   0     ⁢   t     +   θ     )         +     Δ   ⁢           ⁢   I                       Q     RX   ⁢   #2       ⁡     (   t   )       =       ⁢         -       α   ⁢           ⁢     2   ·   A       2       ·     sin   ⁡     (         ϖ   0     ⁢   t     -   θ   +   ϕ2     )         +                     ⁢           α2   ·   A     2     ·     sin   ⁡     (         ϖ   0     ⁢   t     +   θ   -   ϕ2     )         +     Δ   ⁢           ⁢   Q                     (   11   )             
 
         [0066]     The I RX#2  and the Q RX#2  are provided to the DSP  210 .  
         [0067]     The DSP  210  estimates the DC offset characteristic ΔI of the I channel path of the RX side by using I RX#1 and I   RX#2 , and estimates the DC offset characteristic ΔQ of the Q channel path by using Q RX#1  and Q 2   RX#2 . The ΔI and ΔQ can be estimated by using Equation (12)  
                 Δ   ⁢           ⁢   I     =         I     RX   ⁢   #1       +     I     RX   ⁢   #2         2       ⁢     
     ⁢       Δ   ⁢           ⁢   Q     =         Q     RX   ⁢   #1       +     Q     RX   ⁢   #2         2               (   12   )             
 
         [0068]     As noted from Equation (12), ΔI can be estimated as a mean value of test signals I RX#1  and I RX#2  which are consecutively received through the I channel path of the RX side, and ΔQ can be estimated as a mean value of test signals Q RX#1  and Q RX#2  which are consecutively received through the Q channel path of the RX side.  
         [0069]     The DSP  210  determines a calibration value for calibrating ΔI and a calibration value for calibrating ΔQ.  
         [0070]     The calibration value for calibrating ΔI is transferred to the DAC  290 -I and is converted to an analog signal by the DAC  290 -I, and the calibration value for calibrating ΔQ is transferred to the DAC  290 -Q and is converted to an analog signal by the DAC  290 -Q.  
         [0071]     The DC offset characteristic for the received signals in an analog baseband in the I channel of the RX side is counterbalanced by the calibration value for calibrating the converted analog signal ΔI. The analog baseband in the I channel of the RX side corresponds to the section from the output port of the mixer  260 -I to the input port or output port of the LPF  270 -I.  
         [0072]     The DC offset characteristic for the received signals in an analog baseband in the Q channel of the RX side is counterbalanced by the calibration value for calibrating the converted analog signal ΔQ. The analog baseband in the Q channel of the RX side corresponds to the section from the output port of the mixer  260 -Q to the input port of the LPF  270 -Q.  FIG. 1  is based on an assumption that the analog baseband corresponds to the section from the output port of the mixer  260 -Q to the output port of the LPF  270 -Q.  
         [0000]     B. Calibration of Imbalance of the RX Side  
         [0073]     The DSP  210  generates predefined test signals and applies the generated test signals to the I channel path of the TX side, in order to estimate the DC offset characteristics. Further, by using a baseband test signal received through the RX side, the DSP  210  estimates the DC offset characteristics. Based on the estimated DC offset characteristics, the DSP calibrates the DC offset of the RX side.  
         [0074]     The DSP  210  applies test signals I TX  to the DAC  220 -I, in order to estimate the DC offset characteristics of the RX side. Specifically, the DSP  210  applies two different baseband test signals I TX#1  and I TX#3  at a predetermined time interval, in order to estimate the DC offset characteristics of the RX side. However, no test signal is applied to the DAC  220 -Q at all. Therefore, the operations of the DAC  220 -Q, the LPF  230 -Q, and the mixer  240 -Q in the Q channel path of the TX side are not taken into consideration.  
         [0075]     In the discussion below, the operation by the I TX#1  and the operation by the I TX#3  are discriminated.  
         [0076]     First, the operation when the I TX#1  is applied as a test signal will be described. The DAC  220 -I converts the applied I TX#1  to an analog signal and then inputs the converted analog signal to the LPF  230 -I.  
         [0077]     The analog signal I TX#1  is filtered by the LPF  230 -I and is then converted to an RF band signal by the mixer  240 -I. The carrier in the mixer  240 -I corresponds to LO I  and the carrier in the mixer  240 -Q corresponds to LO Q . LO I  and LO Q  can be defined by Equation (2) as described above.  
         [0078]     The RF TX signal TX output#1  of the RF band converted by the mixer  240 -I can be defined by Equation (3) as described above.  
         [0079]     The RF TX signal TX output#1  is transferred to the RX side through a test path formed by the first switch SW# 1  and the second switch SW# 2 . The RF band signal RX input#1  transferred to the RX side can be defined by equation (4) as described above.  
         [0080]     The RF band signal RX input#1  transferred to the RX side through the second switch SW# 2  is converted to a baseband signal by the mixer  260 -I in the I channel path. The mixer  260 -I uses a carrier LO II  which is defined by Equation (5) as described above.  
         [0081]     Further, the RF band signal RX input#1  transferred to the RX side through the second switch SW# 2  is converted to a baseband signal by the mixer  260 -Q in the Q channel path. The mixer  260 -Q uses a carrier LO QQ , which is defined by Equation (6) as described above.  
         [0082]     The baseband signal output from the mixer  260 -I is filtered by the LPF  270 -I in the I channel path and is then transferred to the ADC  280 -I, by which it is converted to a digital signal. The digital signal converted by the ADC  280 -I corresponds to I RX#1 . The baseband signal output from the mixer  260 -Q is filtered by the LPF  270 -Q in the Q channel path and is then transferred to the ADC  280 -Q, by which it is converted to a digital signal. The digital signal converted by the ADC  280 -Q corresponds to Q RX#1 . On the assumption that the DC offset has been already calibrated, the I RX#1  and the Q RX#1  are defined by Equation (13)  
                       I     RX   ⁢   #1       ⁡     (   t   )       =       ⁢         A   2     ·     cos   ⁡     (         ϖ   0     ⁢   t     -   θ     )         +       A   2     ·     cos   ⁡     (         ϖ   0     ⁢   t     +   θ     )                           Q     RX   ⁢   #1       ⁡     (   t   )       =       ⁢           α   ⁢           ⁢     2   ·   A       2     ·     sin   ⁡     (         ϖ   0     ⁢   t     -   θ   +   ϕ2     )         -                     ⁢         α2   ·   A     2     ·     sin   ⁡     (         ϖ   0     ⁢   t     +   θ   -   ϕ2     )                       (   13   )             
 
         [0083]     From comparison between Equation (13) and Equation (5), it is noted that Equation (13) does not include ΔI and ΔQ, which are elements due to the DC offset characteristics.  
         [0084]     The I RX#1  and the Q RX#1  are provided to the DSP  210 .  
         [0085]     Next, the operation when the I TX#3  is applied as a test signal will be described. One example of I TX#3  can be defined by Equation (14) 
 
 I   TX#3 ( t )=sin  ω   0   t   (14) 
 
         [0086]     The I TX#1  and I TX#3  are signals having a phase difference of 90 degrees. Any pair of signals having simple waveforms with a phase difference of 90 degrees can be used as the I TX#1  and I TX#3 .  
         [0087]     The DAC  220 -I converts the applied I TX#3  to an analog signal and then inputs the converted analog signal to the LPF  230 -I.  
         [0088]     The analog signal I TX#3  is filtered by the LPF  230 -I and is then converted to an RF band signal by the mixer  240 -I. The carrier in the mixer  240 -I corresponds to the LO I  defined by Equation (2).  
         [0089]     The RF TX signal TX output#3  of the RF band converted by the mixer  240 -I can be defined by Equation (15)  
                       TX     output   ⁢   #3       ⁡     (   t   )       =       ⁢           I     TX   ⁢   #3       ⁡     (   t   )       ·   A   ·   cos     ⁢           ⁢     ϖ   c     ⁢   t                 =       ⁢         -   A     ·     sin   ⁡     (     ϖ   -     ϖ   0       )         +       A   ·     sin   ⁡     (     ϖ   +     ϖ   0       )         ⁢   t                     (   15   )             
 
         [0090]     The RF TX signal TX output#3  is transferred to the RX side through a test path formed by the first switch SW# 1  and the second switch SW# 2 . The RF band signal RX input#3  transferred to the RX side can be defined by Equation (16) below. 
 
 RX   input#3 ( t )=− A ·sin(  ω   c   t−  ω     0   t +θ)+ A sin(  ω   c   t+  ω     0   t +θ)  (16) 
 
         [0091]     The RF band signal RX input#3  transferred to the RX side through the second switch SW# 2  is converted to a baseband signal by the mixer  260 -I in the I channel path. The mixer  260 -I uses the carrier LO II  defined by Equation (5).  
         [0092]     Further, the RF band signal RX input#3  transferred to the RX side through the second switch SW# 2  is converted to a baseband signal by the mixer  260 -Q in the Q channel path. The mixer  260 -Q uses the carrier LO QQ  defined by Equation (6).  
         [0093]     The baseband signal output from the mixer  260 -I is filtered by the LPF  270 -I in the I channel path and is then transferred to the ADC  280 -I, by which it is converted to a digital signal. The digital signal converted by the ADC  280 -I corresponds to I RX#3 . The baseband signal output from the mixer  260 -Q is filtered by the LPF  270 -Q in the Q channel path and is then transferred to the ADC  280 -Q, by which it is converted to a digital signal. The digital signal converted by the ADC  280 -Q corresponds to Q RX#3 .  
         [0094]     The I RX#3  and the Q RX#3  are defined by Equation (17)  
                       I     RX   ⁢   #3       ⁡     (   t   )       =       ⁢         A   2     ·     sin   ⁡     (         ϖ   0     ⁢   t     -   θ     )         -       A   2     ·     sin   ⁡     (         ϖ   0     ⁢   t     +   θ     )                           Q     RX   ⁢   #3       ⁡     (   t   )       =       ⁢         -       α   ⁢           ⁢     2   ·   A       2       ·     cos   ⁡     (         ϖ   0     ⁢   t     -   θ   +   ϕ2     )         +                     ⁢         α2   ·   A     2     ·     cos   ⁡     (         ϖ   0     ⁢   t     +   θ   -   ϕ2     )                       (   17   )             
 
         [0095]     The I RX#3  and the Q RX#3  are provided to the DSP  210 .  
         [0096]     The DSP  210  estimates the imbalance characteristics α 2  and φ 2  between the I channel path and the Q channel path of the RX side by using the I RX#1  and Q RX#1 , and the I RX#3  and Q RX#3 . The α 2  and φ 2  can be estimated by using Equation (18)  
               α2   =           u   ⁢           ⁢     2   2       +     u   ⁢           ⁢     4   2             u   ⁢           ⁢     1   2       +     u   ⁢           ⁢     3   2               ⁢     
     ⁢     ϕ2   =       tan     -   1       (         2   ·   u     ⁢           ⁢     1   ·   u     ⁢           ⁢   3         u   ⁢           ⁢     1   2       -     u   ⁢           ⁢     3   2           )               (   18   )             
 
         [0097]     In Equation (18), α 2  denotes the gain imbalance characteristics between the I channel path and the Q channel path of the RX side, and φ 2  denotes the phase imbalance characteristics between the I channel path and the Q channel path of the RX side.  
         [0098]     Further, u 1 , u 2 , u 3 , and u 4  used in Equation (18) can be defined by Equation (19)  
                       u   ⁢           ⁢   1     =       ⁢     Re   (         S     RX   ⁢   #1       ⁡     (   t   )       ·     ⅇ     j   ⁢           ⁢     w   0     ⁢   t         )                 =       ⁢               I     RX   ⁢   #1       ⁡     (   t   )       ·   cos     ⁢           ⁢     ϖ   0     ⁢   t     -           Q     RX   ⁢   #1       ⁡     (   t   )       ·   sin     ⁢           ⁢     ϖ   0     ⁢   t       =       A   2     ⁢   cos   ⁢           ⁢   θ               ⁢     
     ⁢             u   ⁢           ⁢   2     =       ⁢     Im   ⁡     (         S     RX   ⁢   #1       ⁡     (   t   )       ·     ⅇ       jϖ   0     ⁢   t         )                   =       ⁢             I     RX   ⁢   #1       ⁡     (   t   )       ·   sin     ⁢           ⁢     ϖ   0     ⁢   t     +           Q     RX   ⁢   #1       ⁡     (   t   )       ·   cos     ⁢           ⁢     ϖ   0     ⁢   t                   =       ⁢       -       α2   ·   A     2       ⁢     sin   ⁡     (     θ   -   ϕ2     )                 ⁢     
     ⁢             u   ⁢           ⁢   3     =       ⁢     Re   (         S     RX   ⁢   #3       ⁡     (   t   )       ·     ⅇ     j   ⁢           ⁢     w   0     ⁢   t         )                 =       ⁢             I     RX   ⁢   #3       ⁡     (   t   )       ·   cos     ⁢           ⁢     ϖ   0     ⁢   t     -           Q     RX   ⁢   #3       ⁡     (   t   )       ·   sin     ⁢           ⁢     ϖ   0     ⁢   t                   =       ⁢       A   2     ⁢   sin   ⁢           ⁢   θ             ⁢     
     ⁢             u   ⁢           ⁢   4     =       ⁢     Im   ⁡     (         S     RX   ⁢   #3       ⁡     (   t   )       ·     ⅇ       jϖ   0     ⁢   t         )                   =       ⁢             I     RX   ⁢   #3       ⁡     (   t   )       ·   sin     ⁢           ⁢     ϖ   0     ⁢   t     -           Q     RX   ⁢   #3       ⁡     (   t   )       ·   cos     ⁢           ⁢     ϖ   0     ⁢   t                   =       ⁢         α2   ·   A     2     ⁢     cos   ⁡     (     θ   -   ϕ2     )                         (   19   )             
 
         [0099]     In Equation (19), S RX#1 (t) is equal to I RX#1 (t)+jQ RX#1 (t), and S RX#3 (t) is equal to I RX#3 (t)+jQ RX#3 (t).  
         [0100]     The DSP  210  calculates calibration values K and L for calibrating the imbalance characteristics of the RX side by using the estimated α 2  and φ 2 . K and L can be defined by Equation (20)  
               K   =       -   tan     ⁢           ⁢   ϕ2       ⁢     
     ⁢     L   =     1     α2   ⁢           ⁢   cos   ⁢           ⁢   ϕ2                 (   20   )             
 
         [0101]     Based on the calculated K and L, a first calibrator  212  within the DSP  210  calibrates the imbalance characteristics between the I channel reception signal and the Q channel reception signal. The calibration of the imbalance characteristics is to make the I channel reception signal and the Q channel reception signal have a desired phase difference (90 degrees) between them. Therefore, it will do if the calibration of the imbalance characteristic is performed for only one of the I channel reception signal and the Q channel reception signal.  FIG. 2  is based on an assumption that calibration is performed on the Q channel reception signal.  
         [0102]     The first calibrator  212  adds the Q channel reception signal having been multiplied by the calibration value L and the I channel reception signal having been multiplied by the calibration value K, thereby outputting a new Q channel reception signal for which the imbalance characteristic has been calibrated. The calibration of the imbalance characteristic by the first calibrator  212  can be defined by Equation (21) 
 
 Q   TX     —     calibration   =K×I   RX   +L×Q   RX   (21) 
 
         [0103]     In Equation (21), Q TX     —     calibration  denotes the Q channel reception signal for which the imbalance characteristic has been calibrated, I RX  denotes the I channel reception signal, and Q RX  denotes the Q channel reception signal.  
         [0000]     C. Calibration of Imbalance of the TX Side  
         [0104]     The DSP  210  applies test signals to the I channel path and the Q channel path of the TX side in order to estimate the imbalance characteristic between the I channel path and the Q channel path of the TX side. The test signals include I TX  and Q TX , which can be defined by Equation (22) 
 
I TX =0 
 
Q TX =1  (22) 
 
         [0105]     The DSP  210  applies I TX  and Q TX  to the TX side, and then receives I RX  and Q RX  through the I channel path and the Q channel path of the RX side. A process of applying I TX  and Q TX  to the TX side and then receiving I RX  and Q RX  is the same as the process described above, so detailed description thereof will be omitted here.  
         [0106]     The DSP  210  estimates the imbalance characteristics α 1  and φ 1  between the I channel path and the Q channel path of the TX side based on I RX  and Q RX . α 1  and φ 1  can be estimated by using Equation (23)  
               α1   =         I   RX   2     +     Q   RX   2           ⁢     
     ⁢     ϕ1   =       tan     -   1       ⁢       I   RX       Q   RX                   (   23   )             
 
         [0107]     In Equation (23), α 1  denotes the gain imbalance characteristic between the I channel path and the Q channel path of the TX side, and φ 1  denotes the phase imbalance characteristic between the I channel path and the Q channel path of the TX side.  
         [0108]     The DSP  210  calculates calibration values M and N for calibrating the imbalance characteristics of the RX side by using the estimated α 1  and φ 1 . The values M and N can be calculated by using Equation (24)  
               M   =       -   tan     ⁢           ⁢   ϕ1       ⁢     
     ⁢     N   =     1     α1   ⁢           ⁢   cos   ⁢           ⁢   ϕ1                 (   24   )             
 
         [0109]     A second calibrator  214  within the DSP  210  calibrates the imbalance characteristics between the I channel transmission signal and the Q channel transmission signal by using the calculated M and N. The calibration of the imbalance characteristics is to make the I channel transmission signal and the Q channel transmission signal have a desired phase difference (90 degrees) between them.  
         [0110]     The second calibrator  214  adds the Q channel transmission signal having been multiplied by the calibration value M and the I channel transmission signal, thereby outputting a new I channel transmission signal for which the imbalance characteristics have been calibrated. Further, the second calibrator  214  multiplies a calibration value N by the Q channel transmission signal, thereby outputting a new Q channel transmission signal for which the imbalance characteristics have been calibrated.  
         [0000]     Process  
         [0111]      FIG. 3  is a flowchart of a process for self-calibration by a DSP according to the present invention. In  FIG. 3 , steps  310  and  318  correspond to steps for calibrating the DC offset characteristics of the RX side, steps  320  and  328  correspond to steps for calibrating the imbalance characteristics of the RX side, and steps  330  and  332  correspond to steps for calibrating the imbalance characteristics of the TX side.  
         [0112]     Referring to  FIG. 3 , in step  310  the DSP  210  applies a baseband test signal I TX#1  to the I channel path of the TX side in order to calibrate the DC offset of the RX side. However, no separate test signal is applied to the Q channel path.  
         [0113]     In step  312 , the DSP  210  receives the test signals I RX#1  and Q RX#1  through the I channel path and the Q channel path of the RX side, respectively. The test signals I RX#1  and Q RX#1  received from the RX side originate from the test signal I TX#1  applied to the TX side.  
         [0114]     In step  314  the DSP  210  applies a baseband test signal I TX#2  to the I channel path of the TX side in order to calibrate the DC offset of the RX side. In this step also, no separate test signal is applied to the Q channel path.  
         [0115]     In step  316 , the DSP  210  receives the test signals I RX#2  and Q RX#2  through the I channel path and the Q channel path of the RX side, respectively. The test signals I RX#2  and Q RX#2  received from the RX side originate from the test signal I TX#2  applied to the TX side.  
         [0116]     In step  318 , the DSP  210  estimates and calibrates the DC offset characteristics of the RX side. Specifically, the DSP  210  estimates the DC offset characteristics of the I channel path and the Q channel path of the RX side by using the received test signals I RX#1 , Q RX#1 , I RX#2 , and Q RX#2 . The DC offset characteristics of the I channel path and the Q channel path of the RX side can be estimated by using Equation (12) described above. Then, the DSP  210  determines DC offset calibration values for calibrating the estimated DC offset characteristics of the I channel path and the Q channel path of the RX side.  
         [0117]     The DSP converts the determined DC offset calibration values to analog signals and provides the analog signals to the I channel path and the Q channel path of the RX side, thereby calibrating the DC offset characteristics for the I channel reception signal and the Q channel reception signal.  
         [0118]     The above discussion is based on an assumption that the DSP applies the second test signal I TX#2  after receiving a signal corresponding to the first test signal I TX#1 . However, it is also possible to sequentially apply the first and second test signals and then sequentially receive signals corresponding to the test signals.  
         [0119]     In step  320 , the DSP  210  applies a baseband test signal I TX#1  to the I channel path of the TX side in order to calibrate the imbalance characteristics of the RX side. No separate test signal is applied to the Q channel path.  
         [0120]     In step  322 , the DSP  210  receives the test signals I RX#1  and Q RX#1  through the I channel path and the Q channel path of the RX side, respectively. The test signals I RX#1  and Q RX#1  received from the RX side originate from the test signal I TX#1  applied to the TX side.  
         [0121]     In step  324  the DSP  210  applies a baseband test signal I TX#3  to the I channel path of the TX side in order to calibrate the imbalance characteristics of the RX side (step  324 ). In this step also, no separate test signal is applied to the Q channel path.  
         [0122]     In step  326 , the DSP  210  receives the test signals I RX#3  and Q RX#3  through the I channel path and the Q channel path of the RX side, respectively. The test signals I RX#3  and Q RX#3  received from the RX side originate from the test signal I TX#3  applied to the TX side.  
         [0123]     In step  328 , the DSP  210  estimates the gain imbalance characteristics α 2  and the phase imbalance characteristics φ 2  by using the received test signals I RX#1 , Q RX#1 , I RX#3 , and Q RX#3 . The gain imbalance characteristics α 2  and the phase imbalance characteristics φ 2  can be estimated by using Equation (18) defined above.  
         [0124]     The DSP  210  determines calibration values K and L for calibrating the estimated imbalance characteristics between the I channel path and the Q channel path of the RX side by using the gain imbalance characteristics α 2  and the phase imbalance characteristics φ 2 . The calibration values K and L can be estimated by using Equation (20) defined above.  
         [0125]     The DSP calibrates the imbalance characteristics between the I channel reception signal and the Q channel reception signal by using the calibration values K and L. The calibration of the imbalance characteristics can be achieved by outputting a new Q channel reception signal, which is obtained by adding the I channel reception signal multiplied by K and the Q channel reception signal multiplied by L.  
         [0126]     The above discussion is based on an assumption that the DSP applies the second test signal I TX#3  after receiving a signal corresponding to the first test signal I TX#1 . However, it is also possible to sequentially apply the first and second test signals and then sequentially receive signals corresponding the test signals.  
         [0127]     In step  330 , the DSP  210  applies test signals I TX  and Q TX  for calibrating the imbalance characteristics of the TX side to the TX side. The test signals are applied to the I channel path or the Q channel path, respectively. It is assumed that the test signal I TX  has a value of 0 and the test signal Q TX  has a value of 1. No signal is applied to the I channel path of the TX side at all.  
         [0128]     In step  332 , the DSP  210  applies test signals I RX  and Q RX  from the RX side. The test signals I RX  and Q RX  received through the I channel path and the Q channel path of the RX side originate from the test signals I TX  and Q TX  applied to the TX side.  
         [0129]     In step  334 , the DSP  210  estimates and calibrates the gain imbalance characteristics of the TX side. Specifically, the DSP  210  estimates the gain imbalance characteristic axl and the phase imbalance characteristic φ 1  by using the received test signals I RX  and Q RX  The gain imbalance characteristic cal and the phase imbalance characteristic φ 1  can be estimated by using Equation (23) defined above.  
         [0130]     The DSP  210  calculates the calibration values M and N for calibrating the imbalance characteristics between the I channel path and the Q channel path of the TX side by using the gain imbalance characteristic α 1  and the phase imbalance characteristic φ 1 . The calibration values M and N can be calculated by using Equation (11) defined above.  
         [0131]     The DSP  210  calibrates the imbalance characteristics between the I channel transmission signal and the Q channel transmission signal by using the calibration values M and N. The calibration of the imbalance characteristics can be achieved by outputting a new I channel transmission signal obtained by adding the I channel transmission signal and the Q channel transmission signal multiplied by M, and by outputting a new Q channel transmission signal obtained by multiplying the Q channel transmission signal by N.  
         [0132]      FIG. 4  is a graph for illustrating a comparison between a test signal (TX signal) transmitted to the TX side and a test signal (RX signal) received from the RX side.  FIG. 4  is based on an assumption that the DC offset characteristic and the imbalance characteristic of the RX side have been already calibrated.  
         [0133]     As noted from  FIG. 4 , the TX signal and the RX signal coincide with each other due to α 1  and φ 1  caused by the imbalance characteristics of the TX side. Therefore, the present invention has proposed a solution for estimating and then compensating α 1  and φ 1 . By calibrating the imbalance characteristics of the TX side as described above, it is possible to make the TX signal and the RX signal coincide with each other.  
         [0134]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.