Abstract:
An apparatus and method for canceling a Direct Current (DC) offset in a Broadband Wireless Access (BWA) communication system are provided. The method includes canceling a DC offset by performing self-calibration in a modulator/demodulator (modem), attempting initial synchronization acquisition in the modem, and if the initial synchronization acquisition succeeds, canceling a DC offset by performing DC offset trace and cancellation.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. § 119 to an application filed in the Korean Intellectual Property Office on Nov. 21, 2007 and assigned Serial No. 10-2007-0119305, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to a Broadband Wireless Access (BWA) communication system. More particularly, the present invention relates to an apparatus and method for canceling a Direct Current (DC) offset in a receiver of an Orthogonal Frequency Division Multiplexing (OFDM) system that is mainly used in a BWA communication system. 
         [0004]    2. Description of the Related Art 
         [0005]    Orthogonal Frequency Division Multiplexing (OFDM) enables a faster transmission using a relatively less frequency band and known to be robust in a multiple path channel environment. As such, OFDM is more widely employed in recent communication systems such as wireless Local Area Network (LAN), Wideband Broadband (Wibro), etc. 
         [0006]    A receiver of an OFDM communication system demodulates a signal received from an antenna, via a Radio Frequency (RF) end, and then restores the signal to an original state. A signal received from an antenna goes under a frequency down-conversion and a signal-magnitude amplification, so that a receiver can acquire a desired signal. To this end, several types of analog devices such as a mixer and an amplifier are used. These devices meet the standards to some degree, but input signals are distorted due to drawbacks in inter-device insulating properties and orthogonality. One of main distortion causes is a DC offset. 
         [0007]    The DC offset is generated because an RF of an OFDM wireless communication system is self-mixed in an analog circuit or because differential baseband devices are mismatched. The DC offset may deteriorate reception performance in instances of mixing an unexpected signal at a baseband end and receiving the baseband signal at a receiver, that is, during the process of inputting a signal to an Analog-to-Digital Converter (ADC). 
         [0008]    In modern receivers, a direct conversion method is mostly used. The direct conversion method is a method in which an RF end directly converts to a baseband frequency. As the receiver using the direct conversion method directly converts a received RF signal to a baseband signal without converting the RF signal to an Intermediate Frequency (IF) signal, a filter, a mixer, an amplifier, etc. used for converting to an IF signal can be omitted. However, because a received RF signal and a local oscillation signal have the same frequency, a DC offset may be generated more, thus there is a need for canceling the DC offset. 
         [0009]    At present, there are several technologies for correcting a DC offset existing within a Radio Frequency Integrated Circuit (RFIC). However, in most technologies, an RFIC measures and corrects its own DC offset, thus a modulator/demodulator (modem) fails know exactly the magnitude of a DC offset of its substantially received signal. As a result, if there is a DC offset in a device within an RFIC, the DC offset may not be canceled or fails to be reduced. There is a technology for estimating and correcting a DC offset in real time during a predetermined period of time irrespective of synchronization. However, this method has a problem that it may cause a rather higher error in an OFDM system. To address this, there is also a method of canceling a DC offset using a preamble only after the synchronization. However, there is a problem that, if DC offset correction is performed only after the synchronization acquisition, it is difficult to acquire an initial synchronization when there is a high DC offset in a non-corrected signal. 
         [0010]    If a receiver attempts to acquire an initial synchronization, gains of amplifiers within the receiver increase at a low signal level, hence even a DC offset increases. If there is a high DC offset in the input signal of a modem, there is a problem of synchronization acquisition being increased only at a strong electric field and being decreased at a weak electric field if it is intended to reduce a gain within an RFIC and reduce a magnitude of a DC offset. Further, there is a problem of causing a deterioration in the reception sensitivity of a receiver that can acquire an initial synchronization and a decrease of a dynamic range of a reception unit of a terminal. 
         [0011]      FIG. 1  is a graph illustrating a conventional process of DC offset variation of an RFIC. As shown, upon power on, most RFICs perform the initialization and its own calibration. Because the self-calibration is implemented by the RFIC, a modem is unaware of the magnitude of a substantially received DC offset. As a result, if there is a DC offset in a device within the RFIC, the DC offset may not be canceled or fails to be reduced to a desired level. 
         [0012]    Also, although calibration is performed (i.e., although synchronization is achieved), an RFIC performs reception/transmission (Rx/Tx) switching. Thus, if temperature changes as time goes by, it cannot be calibrated with an initial calibration value. Further, there is a problem in that, if a Mobile Station (MS) in a sleep state or idle state for a lengthy time powers on and attempts a synchronization acquisition, a proper synchronization acquisition cannot be achieved using an initial value. 
       SUMMARY OF THE INVENTION 
       [0013]    An aspect of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, one aspect of the present invention is to provide an apparatus and method for canceling a Direct Current (DC) offset in a Broadband Wireless Access (BWA) communication system. 
         [0014]    Another aspect of the present invention is to provide an apparatus and method for, upon attempting initial synchronization acquisition and even after synchronization acquisition, effectively canceling a DC offset in a receiver of a Mobile Station (MS) used in a BWA communication system. 
         [0015]    The above aspects are achieved by providing an apparatus and method for canceling a DC offset in a BWA communication system. 
         [0016]    According to another aspect of the present invention, a method for canceling a Direct Current (DC) offset in a receiver of a Broadband Wireless Access (BWA) communication system includes canceling a DC offset by performing self-calibration in a modulator/demodulator (modem), attempting initial synchronization acquisition in the modem, and if the initial synchronization acquisition succeeds, canceling a DC offset by performing DC offset trace and cancellation. 
         [0017]    According to another aspect of the present invention, an apparatus for canceling a Direct Current (DC) offset in a receiver of a Broadband Wireless Access (BWA) communication system includes a modulator/demodulator (modem). The modem cancels a DC offset by performing its own calibration, attempts initial synchronization acquisition, and if the initial synchronization acquisition succeeds, cancels a DC offset by performing DC offset trace and cancellation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The above features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
           [0019]      FIG. 1  is a graph illustrating a conventional process of DC offset variation of a Radio Frequency Integrated Circuit (RFIC); 
           [0020]      FIG. 2  is a flow diagram illustrating the process of canceling a DC offset according to an exemplary embodiment of the present invention; 
           [0021]      FIG. 3  is a schematic diagram illustrating the process of DC offset calibration according to an exemplary embodiment of the present invention; 
           [0022]      FIG. 4  is a flow diagram illustrating the process of tracing and canceling a DC offset according to an exemplary embodiment of the present invention; and 
           [0023]      FIG. 5  is a block diagram illustrating a construction of a receiver according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Now, embodiments of the present invention will be described herein below with reference to the accompanying drawings. For the purposes of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail. 
         [0025]    According to the teachings of the invention, a DC offset before synchronization acquisition is effectively cancelled as described herein below. Note that when there is a high DC offset in an input signal of a modulator/demodulator (modem), a possibility of synchronization acquisition at a weak electric field is reduced. This causes a deterioration of reception sensitivity of a receiver that can acquire initial synchronization and a decrease of a dynamic range of a receiving unit of a terminal. 
         [0026]    According to an exemplary embodiment of the present invention, in a coarse frame synchronization step implemented just before synchronization acquisition, a fast Automatic Gain Controller (AGC) of a modem determines a gain index for synchronization acquisition, thus sets a gain for amplifiers of a receive end of a Radio Frequency Integrated Circuit (RFIC). 
         [0027]    After that, the modem measures a DC offset with no signal input to the RFIC, determines if the DC offset value is more or less than a reference value and then, if it is determined to be more than the reference value, performs inverse calibration by the magnitude of difference between the measured DC offset value and the reference value. By doing so, the modem can guarantee an excellent performance of initial synchronization acquisition. 
         [0028]    After synchronization acquisition, in a normal operation mode, the modem measures a magnitude of a DC offset value at each frame of a specific time, determines if the DC offset value is more or less than a reference value and then, if it is determined to be more than the reference value, stores the DC offset value in a DC offset correction end of the RFIC. Therefore, the modem can trace the DC offset by measuring and lets the DC offset correction end of the RFIC remove the DC offset by storing. That is, the modem can enhance reception performance of a receiver by increasing a Carrier to Interference Noise Ratio (CINR) of the receiver. 
         [0029]    An exemplary embodiment of the present invention is comprised of a modem for measuring a DC offset and an RFIC for correcting the DC offset, as later shown in  FIG. 5 . 
         [0030]      FIG. 2  is a flow diagram illustrating the process of canceling a DC offset according to an exemplary embodiment of the present invention. 
         [0031]    Referring to  FIG. 2 , in step  205 , if a receiver powers on, a modem and RFIC perform an initialization process and, particularly, the RFIC performs a self calibration process. 
         [0032]    In step  210 , a fast AGC of the modem determines a gain index for initial synchronization acquisition. According to an exemplary embodiment of the present invention, the fast AGC decides a gain index for synchronization acquisition in a coarse frame synchronization step implemented just before synchronization acquisition and thus, sets gains of amplifiers of a receive end of an RFIC. 
         [0033]    In step  215 , the modem applies a set value for the determined gain index to the RFIC. Then, in step  220 , the modem sets an antenna switch to a transmission (Tx) mode, that is, in the Tx mode, performs self calibration process 
         [0034]    In step  225 , the modem measures a DC offset with no signal input to the RFIC in a self calibration mode. And, in step  230 , determines if the DC offset value is more or less than a reference value (e.g., 5 mV). If it is determined to be more than the reference value in step  230 , the modem performs inverse calibration by the magnitude of difference between the measured DC offset value and the reference value in step  235 , thus enabling an excellent performance of initial synchronization acquisition. Then, in step  240 , the modem sets the antenna switch to a reception (Rx) mode and transits to a normal mode. If it is determined to be less than the reference value in step  230 , the modem jumps to step  240  and transits to the normal mode. 
         [0035]    After that, in step  245 , the modem attempts initial synchronization acquisition. If the initial synchronization acquisition fails in step  250 , the modem resumes the gain index process of step  210  and its subsequent steps for initial synchronization acquisition. 
         [0036]    If the initial synchronization acquisition succeeds in step  250 , the modem performs a DC offset trace and cancellation process in step  260 . The DC offset trace and cancellation process is described in  FIG. 4 . As shown, 
         [0037]      FIG. 3  is a schematic diagram illustrating the process of DC offset calibration according to an exemplary embodiment of the present invention 
         [0038]    In  FIG. 3 , after a modem and RFIC are initialized, a fast AGC determines a gain index for initial synchronization acquisition. The modem performs a self calibration process and then, performs a DC offset calibration process. 
         [0039]      FIG. 4  is a flow diagram illustrating the process of tracing and canceling a DC offset according to an exemplary embodiment of the present invention. 
         [0040]    Referring to  FIG. 4 , in step  405 , a modem initiates measuring a magnitude of a DC offset value. After that, if a frame interrupt occurs at the time of frame reception in step  410 , the modem determines if the measured DC offset value is more or less than a reference value (e.g., 5 mV) in step  415 . If it is determined to be more than the reference value in step  415 , the modem measures a current gain index value in step  420 . 
         [0041]    Then, in step  425 , the modem traces and cancels a DC offset by storing a calibration value in a DC offset correction end of an RFIC depending on the gain index value during a transmission time. If it is aware of the gain index value, the modem can be aware of how to control a DC offset and the gain index is identical to the gain index in step  210  in  FIG. 2  and the fast AGC of the modem determines the gain index for initial synchronization acquisition. The modem lets the DC offset correction end of an RFIC perform inverse calibration by the magnitude of difference between the measured DC offset value and the reference value. In this step, the modem may adjust the magnitude of difference to achieve the gain index or the modem just lets the DC offset correction end of an RFIC perform inverse calibration by the magnitude of difference between the measured DC offset value and the reference value without adjusting. 
         [0042]    Thus, the modem can enhance the reception performance of a receiver by increasing a Carrier to Interference Noise Ratio (CINR) of the receiver. The loop from step  405  to step  415  may be called as ‘the DC offset trace process’ and the step of  425  may be call as ‘the DC offset cancellation process’. 
         [0043]      FIG. 5  is a block diagram illustrating the construction of a receiver according to an exemplary embodiment of the present invention. 
         [0044]    Referring to  FIG. 5 , if power is ON, an RF unit (i.e., an RFIC)  510  performs an initialization process and its own calibration process. The RF unit  510  applies a DC offset calibration value received from a modem  520 , thus performing a calibration process under the control of the modem  520 . 
         [0045]    A fast AGC within the modem  520  determines a gain index for initial synchronization acquisition, provides and applies a set value for the determined gain index to the RF unit  510 , and performs self calibration process in a transmission mode. The modem  520  measures a DC offset with no signal input to the RF unit  510  and determines if the DC offset value is more or less than a reference value (e.g., 5 mV). After that, if it is determined to be more than the reference value, the modem  520  provides its control signal to the RF unit  510 , thereby performing inverse calibration according to the magnitude. 
         [0046]    The modem  520  attempts the initial synchronization acquisition. If the initial synchronization acquisition succeeds, the modem  520  performs a DC offset trace and cancellation process after the initial synchronization acquisition. 
         [0047]    The modem  520  initiates a process of measuring a magnitude of a DC offset in the DC offset trace and cancellation process after the initial synchronization acquisition. If sensing a frame interrupt that occurs at the time of frame reception, the modem  520  determines if the measured DC offset is more or less than a reference value (e.g., 5 mV). If it is determined to be more than the reference value, the modem  520  measures a current gain index value. The modem  520  traces and cancels the DC offset by storing a calibration value in a DC offset correction end of an RFIC depending on the gain index value during a transmission time (using a control signal). 
         [0048]    An exemplary embodiment of the present invention has an advantage of preventing a initial synchronization failure of the receiver performing the initial synchronization at a signal level of a weak electric field under a condition that the receiver performed initial synchronization acquisition at a strong electric field and moved to the weak electric field and the weak electric field is enough to maintain synchronization. 
         [0049]    The present invention enables a modem to directly measure and calibrate the magnitude of an input DC offset rather than allowing an RFIC to measure and calibrate its own DC offset, thereby capable of calibrating the DC offset value with more accuracy and thus guaranteeing an excellent performance during an initial synchronization acquisition. Also, the invention enables a modem to directly measure a DC offset and instruct calibration to an RFIC even during a normal operation, thereby enabling calibration with more accuracy; thus, increasing a CINR of a receiver and enhancing reception performance of the receiver. Further, upon initial synchronization acquisition and even after synchronization acquisition, the teachings of the invention allows effectively canceling a DC offset and obtaining an excellent performance of initial synchronization acquisition when there is an attempt to perform the initial synchronization acquisition at a weak electric field. 
         [0050]    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.