Abstract:
A method of transmitting information in a WLAN (Wireless Local Area Network) network and corresponding WLAN communication devices and integrated circuit chips are provided. A correction signal is used for compensating for a dc offset in a data signal containing at least part of the information to be transmitted. The correction signal is varied by making it taking different values. For each of the different values, a strength of an indicator signal indicative of the dc offset is determined. Based upon the determined strength, an optimum value of the correction signal is identified at which the dc offset is minimized. The value of the correction signal is set to the optimum value. Further, a method of transmitting information in a WLAN network is provided, including compensating for a first and second dc offset in a first and second data signal, respectively, using a first and second feedback loop, respectively.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present application relates to a method of transmitting information in a WLAN (Wireless Local Area Network) network and corresponding WLAN communication devices and integrated circuit chips, and in particular to minimizing DC offsets therein.  
         [0003]     2. Description of the Related Art  
         [0004]     A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.  
         [0005]     Today, most WLAN systems use spread spectrum technology, a wideband radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.  
         [0006]     The standard defining and governing WLAN networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11g and 802.11a which allow data rates of 54 Mbps in the 2.4 GHz and 5 GHz spectrum, respectively. Further extensions exist.  
         [0007]     With the growing demand for WLAN systems in the consumer market, product costs and quality have become key factors in the development of WLAN communication devices, i.e. transmitters, receivers or transceivers. Therefore, the low-IF (low-Intermediate Frequency) topology offering the prospect of integrating the RF (Radio Frequency) or IR (Infrared) front-end on-chip for reducing the production costs while providing a high operational performance has become a frequently used design for such WLAN communication devices. In a low-IF WLAN communication device operating in a reception mode, an incoming transmission signal received over a wireless communication medium, i.e. the air, is down-converted from its RF or IR carrier to an intermediate frequency of typically several hundred kHz by mixing it with an LO (Local Oscillator) signal having an accordingly selected frequency. The low-IF signal thus generated can be demodulated on this intermediate frequency or can be further down-converted to baseband after further processing, e.g., filtering.  
         [0008]     The intermediate frequency created by the mixer is defined as the absolute value of the difference between the carrier frequency and the LO frequency. However, since the mixer does not recognize the polarity of the frequency difference between the carrier and the LO signal, down-conversion of two different received frequencies to the same intermediate frequency occurs. Apart from the wanted signal, an unwanted signal at a frequency, often referred to as the image frequency, is down-converted to the intermediate frequency.  
         [0009]     In order to suppress the signal at the image frequency, i.e. to perform image rejection, the analytic transmission signal is converted to complex low-IF signals which are then filtered using active complex filters. In a complex filter, the filtering of positive frequencies is different from the filtering of negative frequencies. Since every frequency component of a complex signal can be written as a sum of two sequences, the first sequence having only a positive frequency component, the second only a negative, complex filters allow for eliminating the image signal in those cases where the image signal is situated on the opposite frequencies of the wanted signal.  
         [0010]     A typical design for an RF (IR) front-end of a low-IF WLAN transceiver is shown in  FIG. 1 . For clarity reasons, only the signal flow in the transmission mode of the low-IF WLAN transceiver has been depicted. When the low-IF WLAN transceiver is in the reception mode, the signal flow (except for the LO signals) takes the opposite direction.  
         [0011]     In detail, when in the reception mode, a transmission signal is provided to a complex mixer  160  for being down-converted to complex low-IF signals by being complex-mixed with the complex signals of a local oscillator  150 : an LO I (In-phase) signal and an LO Q (Quadrature-phase) signal. The I- and Q-signals resulting from the complex mixer  160  are further processed in an I-path  110  and Q-path  140 , respectively. This may include, e.g., amplification, filtering, or further down-conversion. Part of the I-signal resulting from the complex mixer  160  is separated before the I-path  110  is entered, complex-filtered in an active complex filter  120 , and added to the Q-signal leaving the Q-path  140 . Accordingly, part of the Q-signal resulting from the complex mixer  160  is split, complex-filtered in the active complex filter  130 , and added to the I-signal leaving the I-path  110 .  
         [0012]     When the low-IF WLAN transceiver operates in the transmission mode, an input I-signal and an input Q-signal are provided to the I-path  110  and the Q-path  140 , respectively. The signal processing in the I-path  110  and the Q-path  140  may include, e.g., amplification, filtering or up-conversion from baseband to the intermediate frequency. Part of the input I-signal (input Q-signal) is split before the I-path  110  (Q-path  140 ) is entered, complex-filtered in the active complex filter  130  ( 120 ), and added to the Q-signal leaving the Q-path  140  (I-signal leaving the I-path  110 ) to generate the combined Q-signal (combined I-signal). Subsequently, the combined I-signal and the combined Q-signal are provided to the complex mixer  160  for up-conversion to a desired transmission frequency by being complex-mixed with the LO I-signal and LO Q-signal generated by the local oscillator  150 .  
         [0013]     Complex operators like the complex filters  120 ,  130  and the complex mixer  160  are usually made with pairs of real operators, amplifiers, mixers and filters. The performance of the system in which these complex operators are used degrades when they are not perfectly matched. In analog integrated implementations, hence in low-IF WLAN transceivers, mismatch is unavoidable. In particular, the active complex filters  120 ,  130  cause the combined I-signal and combined Q-signal to suffer from a DC (Direct Current) offset when the low-IF WLAN transceiver is operating in the transmission mode. At the complex mixer  160 , the DC offset causes an LO feedthrough, i.e. the transmission signal having a component at the LO frequency.  
         [0014]     In circumstances where only frequencies within a certain frequency mask are to be used for the transmission signals of a WLAN system, the LO feedthrough can cause the transmission signal to have a component outside the allowed frequency mask. Thus, conventional low-IF WLAN transceivers often have the disadvantage of causing illegal spurious emissions.  
         [0015]     Further, the LO feedthrough causes the transmission signal to have a higher overall signal level. This can imply that the signal level is located beyond the range of linear operation of amplifiers used for amplifying the transmission signal. This leads to degradation of the amplification efficiency. In consequence, many prior art low-IF WLAN transceivers suffer from the problem of achieving only insufficient intensity of the transmission signal at the desired transmission frequency.  
         [0016]     In broadband systems, the carrier and LO bands of the transmission signal often happen to overlap each other since they are spaced only by the low intermediate frequency. Thus, the LO leakage gives raise to interferences between the carrier and LO bands of the transmission signal. Therefore, conventional low-IF WLAN transceivers also have the disadvantage of usually suffering from considerable deterioration of the transmission quality.  
       SUMMARY OF THE INVENTION  
       [0017]     An improved method of transmitting information in a WLAN network and corresponding WLAN communication devices and integrated circuit chips are provided that may overcome the disadvantages of the conventional approaches. In particular, embodiments allow for preventing spurious emissions at the LO frequency. Other embodiments offer the advantage of enhanced amplification efficiency of the transmission signal. In further embodiments, interferences between the carrier and LO bands of the transmission signal are avoided and thus the transmission signal quality is increased.  
         [0018]     In one embodiment, a method of transmitting information in a WLAN network is provided. A correction signal is used for compensating for a DC offset in a data signal containing at least part of the information to be transmitted. The correction signal is varied by making the correction signal taking different values. For each of the different values of the correction signal, a strength of an indicator signal indicative of the DC offset is determined. Based upon the determined strength of the indicator signal, an optimum value of the correction signal is identified at which the DC offset is minimized. The value of the correction signal is set to the optimum value.  
         [0019]     In another embodiment, a method of transmitting information in a WLAN network is provided comprising compensating for a first DC offset in a first data signal containing at least a first part of the information to be transmitted using a first correction signal. The method further comprises compensating for a second DC offset in a second data signal containing at least a second part of the information to be transmitted using a second correction signal. The first correction signal is tuned based upon a first indicator signal indicative of the first DC offset using a first feedback loop. The second correction signal is tuned based upon a second indicator signal indicative of the second DC offset using a second feedback loop.  
         [0020]     In a further embodiment, a WLAN communication device for transmitting information in a WLAN network comprising a compensator unit, a control unit, an analyzer unit and an identifier unit is provided. The compensator unit is arranged to compensate for a DC offset in a data signal containing at least part of the information to be transmitted using a correction signal. The control unit is arranged to vary the correction signal by making the correction signal taking different values, and to set the value of the correction signal to an optimum value at which the DC offset is minimized. The analyzer unit is arranged to determine for each of the different values of the correction signal a strength of an indicator signal indicative of the DC offset. The identifier unit is arranged to identify, based upon the determined strength of the indicator signal, the optimum value of the correction signal.  
         [0021]     In yet another embodiment, a WLAN communication device for transmitting information in a WLAN network comprising a first compensator unit, a second compensator unit, a first feedback loop and a second feedback loop is provided. The first compensator unit is arranged to compensate for a first DC offset in a first data signal containing at least a first part of the information to be transmitted using a first correction signal. The second compensator unit is arranged to compensate for a second DC offset in a second data signal containing at least a second part of the information to be transmitted using a second correction signal. The first feedback loop is arranged to tune the first correction signal based upon a first indicator signal indicative of the first DC offset. The second feedback loop is arranged to tune the second correction signal based upon a second indicator signal indicative of the second DC offset.  
         [0022]     In still another embodiment, an integrated circuit chip for transmitting information in a WLAN network comprising a compensator circuit, a control circuit, an analyzer circuit and an identifier circuit is provided. The compensator circuit is for compensating for a DC offset in a data signal containing at least part of the information to be transmitted using a correction signal. The control circuit is for varying the correction signal by making the correction signal taking different values, and for setting the value of the correction signal to an optimum value at which the DC offset is minimized. The analyzer circuit is for determining for each of the different values of the correction signal a strength of an indicator signal indicative of the DC offset. The identifier circuit is for identifying, based upon the determined strength of the indicator signal, the optimum value of the correction signal.  
         [0023]     In a further embodiment, an integrated circuit chip for transmitting information in a WLAN network comprising a first compensator circuit, a second compensator circuit, a first feedback loop circuit and a second feedback loop circuit is provided. The first compensator circuit is for compensating for a first DC offset in a first data signal containing at least a first part of the information to be transmitted using a first correction signal. The second compensator circuit is for compensating for a second DC offset in a second data signal containing at least a second part of the information to be transmitted using a second correction signal. The first feedback loop circuit is for tuning the first correction signal based upon a first indicator signal indicative of the first DC offset. The second feedback loop circuit is for tuning the second correction signal based upon a second indicator signal indicative of the second DC offset. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The accompanying drawings are incorporated into and form a part of the specification for the purpose of explaining the principles of the invention. The drawings are not to be construed as limiting the invention to only the illustrated and described examples of how the invention can be made and used. Further features and advantages will become apparent from the following and more particular description of the invention, as illustrated in the accompanying drawings, wherein:  
         [0025]      FIG. 1  is a block diagram illustrating components of a low-IF WLAN transceiver according to prior art;  
         [0026]      FIG. 2  is a flow diagram illustrating a DC offset cancellation process according to an embodiment;  
         [0027]      FIG. 3  is a flow diagram illustrating a complex DC offset cancellation process according to an embodiment;  
         [0028]      FIG. 4  is a block diagram illustrating components of a low-IF WLAN transceiver according to an embodiment;  
         [0029]      FIG. 5  is a block diagram illustrating components of a low-IF WLAN transceiver according to another embodiment; and  
         [0030]      FIG. 6  illustrates the behavior of the LO feedthrough versus the DC correction signal according to an embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     The illustrative embodiments of the present invention will be described with reference to the figure drawings, wherein like elements and structures are indicated by like reference numbers. While the embodiments that will be set forth in the following refer to low-IF WLAN transceivers, other embodiments may relate to other transceivers, transmitters or any type of signal processing devices suffering from DC offsets.  
         [0032]     Referring now to  FIG. 2 , a DC offset cancellation process according to an embodiment is shown. This process may allow for minimizing or even completely cancelling a DC offset imposed on a data signal. In step  210 , a DC correction signal may be applied to the input signal path of a low-IF WLAN transceiver for compensating for a DC offset imposed on the input signal. In this context, the meaning of “compensating ” may also include partially compensating. Further, the term “DC correction signal ” may indicate that the respective signal is for compensating for, i.e. correcting, the DC offset. The DC correction signal may be a DC signal, e.g., a DC current and/or voltage, but other types of signals may also be used for compensating for the DC offset.  
         [0033]     In step  220 , the strength of an indicator signal indicative of the DC offset may be determined versus the value of the DC correction signal. The determination of the indicator signal strength may include measuring the amplitude of the indicator signal. The amplitude can have both positive and negative values. Further, the determination may comprise squaring the measured amplitude of the indicator signal and/or calculating the absolute value thereof. If more than one indicator signal is used, the amplitude of each of the indicator signals may be measured. The measured amplitudes may be squared and/or added.  
         [0034]     In step  230 , it is identified whether the signal strength determined in step  220  comprises a local minimum. According to the present embodiment, the presence of a local minimum is identified by comparing the determined strengths of the indicator signal and detecting whether there is a determined strength inferior to both its left-hand and right-hand neighboring strength. The left-hand (right-hand) neighboring strength may be defined as the strength of the indicator signal determined for the next lower (next higher) value of the DC correction signal. In the present embodiment, the indicator signal is selected such that the local minimum of the determined strengths corresponds to a value of the DC correction signal at which the DC offset is minimized. This value will be referred to in the following as the optimum value of the DC correction signal.  
         [0035]     In another embodiment, the identifying step  230  comprises, for each of the determined strengths having a left-hand neighboring strength and a right-hand neighboring strength, calculating a first difference between the determined strength and its left-hand neighboring strength and a second difference between the right-hand neighboring strength and the determined strength. Subsequently, it may be determined whether the determined strengths of the indicator signal comprise a local minimum for which the first difference and the second difference have different signs.  
         [0036]     In a further embodiment, step  230  of identifying whether the determined strengths comprise a local minimum may include interpolating between the determined strengths for generating a smooth strength function. For instance, polynomial spline functions may be used for obtaining the smooth strength function. The first derivative of the smooth strength function may be calculated, and it may be determined whether the first derivative comprises a null. The determined strength or interpolated strength corresponding to the null of the first derivative may be identified as the local minimum.  
         [0037]     In step  240 , it may be queried whether the local minimum corresponding to a minimized DC offset has been identified in step  230 . If this is not the case, the value of the DC correction signal may be varied in step  250 .  
         [0038]     According to the present embodiment, the value of the DC correction signal is set to a start value when step  210  of applying the DC correction signal to the input signal path is performed. Each time step  250  is executed, the value of the DC correction signal may be increased or decreased by a certain step value until a target value is reached. If no local minimum has been identified for the values of the DC correction signal between the start value and the target value, steps  220  to  250  may be repeated for other start values, target values and/or step values.  
         [0039]     In another embodiment, step  250  of varying the value of the DC correction signal comprises continuously increasing or decreasing the value of the DC correction signal, and step  220  of determining the strength of the indicator signal is performed continuously while the value of the DC correction signal is varied. In this embodiment, step  220  results in a continuous function of the strength of the indicator signal against the value of the DC correction signal. Accordingly, step  230  may comprise calculating the first derivative of the strength function, determining whether the first derivative comprises a null and identifying the determined strength corresponding to the null of the first derivative as the local minimum.  
         [0040]     Once step  240  yields that the determined signal strengths comprise a local minimum, the optimum value of the DC correction signal may be identified in step  260  as the value of the DC correction signal corresponding to the local minimum of the strength of the indicator signal. Finally, in step  270 , the value of the DC correction signal may be set to the optimum value.  
         [0041]     It is noted that the sequence of steps shown in  FIG. 2  has been chosen for illustration purposes only and is not to be understood as limiting the invention. For instance, steps  250  and  220  may be performed alternately until the target value of the DC correction signal is reached, and afterwards step  230  of identifying whether the determined signal strengths comprise a local minimum may be performed.  
         [0042]     In the present embodiment, the determined strength of the indicator signal reaches a local extremum, particularly a local minimum, only if the DC offset is minimized. In other embodiments, the determined strength of the indicator signal may have a local maximum when the DC correction signal is minimized or may include a plurality of local extrema. In such circumstances, step  230  may comprise verifying whether further criteria are fulfilled in order to identify whether the determined strength comprises a local extremum that corresponds to a minimized DC offset. For example, when a continuous strength function is used, a second derivative of the strength function may be calculated and it may be determined whether the second derivative corresponding to the null of the first derivative is positive or negative in order to determine whether the local extremum identified by the null of the first derivative is a local minimum or a local maximum, respectively.  
         [0043]     Turning now to  FIG. 3 , a flow diagram illustrating a complex DC offset cancellation process is shown. The complex DC offset cancellation may be employed in systems where DC offsets imposed on a plurality of signals are to be minimized. The complex DC offset cancellation may even be used for minimizing DC offsets residing on cross-coupled signals.  
         [0044]     In step  310 , the DC offset cancellation described above with reference to  FIG. 2  may be applied to a first signal suffering from a first DC offset. In the depicted embodiment, the DC offset cancellation is applied to an I-signal in a system where complex I- and Q-signals are used. Once the optimum value of the first DC correction signal applied to the I-signal has been identified in step  260 , the value of the first DC correction signal is set to this optimum value in step  270  and is kept at this value while step  320  is performed. In step  320 , the DC offset cancellation according to the process illustrated in  FIG. 2  is performed anew for identifying an optimum value of a second DC correction signal applied to the Q-signal and setting the value of the second DC correction signal to this optimum value.  
         [0045]     According to the present embodiment, it is not necessary to switch off the remaining signals carrying DC offsets while the DC offset on one signal is minimized, even when the respective signals are cross-coupled. In particular, it may be unnecessary to switch off the Q-signal (I-signal) while step  310  ( 320 ) of performing the DC offset cancellation on the I-signal (Q-signal) is executed.  
         [0046]     It is noted that steps  310  and  320  may also be performed in the inverse order. The sequence of steps  310  and  320  may be supplemented with corresponding further steps of performing the DC offset cancellation when DC offsets on more than two signals are to be minimized. The same or different indicator signals may be used for minimizing DC offsets on a plurality of signals.  
         [0047]     In  FIG. 4 , components of a low-IF WLAN transceiver according to an embodiment are shown. The low-IF WLAN transceiver may be arranged for minimizing or even completely cancelling DC offsets residing on the combined I-signal and/or the combined Q-signal which may be caused by the active complex filters  420 ,  430  based upon the complex DC offset cancellation process illustrated in  FIG. 3 . The components  410  to  460  may correspond to the components  110  to  160  described above with reference to  FIG. 1 . DC correction controllers  490 ,  495  may be used for applying DC correction signals to the Q-path  440  and the I-path  410 , respectively, in step  210 , varying the value of each of the DC correction signals in step  250 , and setting the value of each of the DC correction signals to an optimum value in step  270 . In other embodiments, at least one of the DC correction signals may be applied at any other point between the I-path  410  (or the Q-path  440 , respectively) and the complex mixer  460 .  
         [0048]     A switch between the DC correction controllers  490 ,  495  and the local minimum identifier  480  may be used for connecting either the DC correction controller  490  operating on the Q-path  440  or the DC correction controller  495  operating on the I-path  410  to the local minimum identifier  480 , thereby allowing for completing the DC offset cancellation on one of the signals before the DC offset cancellation on the other signal is started according to  FIG. 3 . The switch may be controlled, e.g., by the local minimum identifier  480  or a separate switch control unit.  
         [0049]     In the depicted embodiment, the transmission signal is used for the indicator signal. The strength of the indicator signal may be determined by a signal strength analyzer  470 . In particular, the signal strength analyzer may determine the strength of the LO feedthrough, i.e. the strength of a component of the transmission signal at the LO frequency. For this purpose, the signal strength analyzer  470  may comprise a means for measuring the amplitude of the LO feedthrough. Further, the signal strength analyzer  470  may comprise means for squaring and/or calculating the absolute value of the measured amplitude.  
         [0050]     The determined strength of the indicator signal may be provided to a local minimum identifier  480 . The local minimum identifier  480  may be arranged for associating each of the determined strengths with the corresponding value of the DC correction signal applied to the I-path  410  or the Q-path  440  in order to determine the strengths of the indicator signal versus the values of the DC correction signal according to step  220 . For this purpose, the local minimum identifier  480  may not only receive the determined strengths from the signal strength analyzer  470  but also the corresponding value of the DC correction signal from the DC correction controllers  490 ,  495 . Alternatively, the local minimum identifier  480  may send control signals to the DC correction controllers  490 ,  495  to make the DC correction controllers  490 ,  495  apply DC correction signals having values predefined by the local minimum identifier  480 . The sub-step of associating the determined strengths with the values of the DC correction signal may alternatively be performed by the signal strength analyzer  470  or a separate associating unit. Other methods for associating the determined strengths with the corresponding values of the DC correction signal may be applied.  
         [0051]     The local minimum identifier  480  may further be employed for identifying in step  230  whether the determined strengths of the indicator signal comprise a local minimum. Therefore, the local minimum identifier  480  may comprise means for comparing the determined strengths, means for calculating differences between neighboring determined strengths, means for interpolating between the determined strengths of the indicator signal and/or means for calculating derivatives of an indicator signal strength function. Upon having found in step  240  that the determined strengths comprise a local minimum, the local minimum identifier  480  may identify in step  260  an optimum value of the DC correction signal corresponding to the local minimum of the determined strengths. Thereupon, the local minimum identifier  480  may communicate the optimum value to the DC correction controller  490  or the DC correction controller  495  so that the respective DC correction controller can set the value of the DC correction signal to the optimum value according to step  270 .  
         [0052]     Referring now to  FIG. 5 , components of a low-IF WLAN transceiver according to another embodiment are shown. The low-IF WLAN transceiver may be adapted to perform the complex DC offset cancellation process shown in  FIG. 3  in order to minimize or completely cancel DC offsets imposed on the combined I-signal and/or the combined Q-signal. The components  510  to  560  may correspond to the components  110  to  160  described with reference to  FIG. 1 . The components  580  to  595  of the low-IF WLAN transceiver may correspond to the components  480  to  495  discussed with respect to  FIG. 4 .  
         [0053]     The determination of the strength of the indicator signal in step  220  may be performed based on the combined I-signal and the combined Q-signal. The signal strength analyzer  570  may comprise means for measuring the amplitudes of the combined I-signal and the combined Q-signal. Further, the signal strength analyzer  570  may comprise means for squaring the measured amplitudes, for adding the squared measured amplitudes, and for using the result of the addition as the indicator signal. Further, the signal strength analyzer  570  and/or the local minimum identifier  580  may be arranged for associating the strengths of the indicator signal thus determined with the corresponding values of the DC correction signal according to the method described above with reference to  FIG. 4 .  
         [0054]     In further embodiments, WLAN communication devices or integrated circuit chips may be provided, that are arranged for performing the above described methods and processes.  
         [0055]     In  FIG. 6 , the behavior of the indicator signal versus the value of the DC correction signal according to an embodiment, e.g., in the low-IF WLAN transceiver described with respect to  FIG. 4 , is shown. In this embodiment, the LO feedthrough is used for the indicator signal. The strength function  610  may represent the behavior of the determined strength of the indicator signal when the value of a first DC correction signal, e.g., the DC correction signal applied to the I-path  410 , is varied. The first local minimum  620  may be reached when the DC offset on a first signal, e.g., the combined I-signal, is minimized. The value  630  of the DC correction signal corresponding to the first local minimum  620  may be selected as the optimum value for the first DC correction signal.  
         [0056]     Once a first local minimum  620  for the first path, e.g., the I-path, has been found, the process for the second path, e.g., the Q-path, may be started. The value of the first DC correction signal may be kept at the optimum value  630 , and the strength of the indicator signal may pass along the curve  640  while the value of the second DC correction signal, e.g., the DC correction signal applied to the Q-path  440 , may be varied. When the DC offset residing on a second signal, e.g., the combined Q-signal, is minimized, the strength of the indicator signal may reach the second local minimum  650 . The optimum value of the second DC correction signal may correspond to the value  660  at which the second local minimum  650  is reached. Thus, when setting the values of the first and second DC correction signals to the optimum values  630 ,  660 , both the DC offsets on the first and on the second signal may be minimized.  
         [0057]     As apparent from the above description of embodiments, methods and corresponding devices for performing DC offset cancellation are provided. In a low-IF WLAN transceiver, I/Q-signals may be generated out of an analytic signal to perform image rejection. Active complex cross-coupled filters  120 ,  130 ,  420 ,  430 ,  520 ,  530  may be used to generate these complex signals. Active parts may suffer from DC offset which may cause LO feedthrough in a transmitter. The method according to the presented embodiments may reduce this DC offset for a complex cross-coupled structure.  
         [0058]     The presented DC offset cancellation may be applied in combination with AMD&#39;s Am1780 WLAN transceiver.  
         [0059]     As discussed above, a DC offset at either the combined I-signal or the combined Q-signal depicted in  FIGS. 4 and 5  or at both the combined I-signal and the combined Q-signal may generate an LO feedthrough at the transmission signal. In one embodiment, the amplitude of the LO may be measured and fed into the local minimum identifier  480 . As a first step to reduce the DC offset causing the LO feedthrough, a DC voltage may be injected with the DC correction controller  495  to reduce the overall LO feedthrough. This may be accomplished such that a current is injected starting at a certain level and increased by a certain step. The local minimum identifier  480  may be used to find a first minimum  620 .  
         [0060]     When the minimum  620  is found for the I-path  410 , the same process may be started for the Q-path  440  using the local minimum identifier  480  and the DC correction controller  490 . A second local minimum  650  may then be found. Both I and Q correction values may be selected now that a local minimum  620 ,  650  or the maximum LO rejection, respectively, has been reached. It may not be required to switch either the Q- or the I-path off during calibration of the other path.  
         [0061]     Additionally, another type of implementation has been presented. Instead of using the transmission signal to detect the LO leakage, the combined I-signal and the combined Q-signal may be used to determine the DC offset. The correction mechanism may be the same as in the embodiment where the transmission signal is used for the indicator signal. An advantage of this method may be the simplicity. It may not be required to switch paths off. Local minima  620 ,  650  may be used to find the maximum value for image rejection. Thus, a fast algorithm may be possible to find the total minimum.  
         [0062]     While the invention has been described with respect to the physical embodiments constructed in accordance therewith, it will be apparent to those skilled in the art that various modifications, variations and improvements of the present invention may be made in the light of the above teachings and within the purview of the appended claims without departing from the scope of the invention. In addition, those areas in which it is believed that those of ordinary skill in the art are familiar, have not been described herein in order to not unnecessarily obscure the invention described herein. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims.