Patent Publication Number: US-2016248459-A1

Title: Systems and methods for automatic gain control using a carrier estimation path

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to wireless communications. More specifically, the present disclosure relates to automatic gain control (AGC) using a carrier estimation path by a near field communication (NFC) device. 
     BACKGROUND 
     Advances in technology have resulted in smaller and more powerful personal computing devices. For example, there currently exist a variety of portable personal computing devices, including wireless computing devices, such as portable wireless telephones, personal digital assistants (PDAs) and paging devices that are each small, lightweight, and can be easily carried by users. More specifically, the portable wireless telephones, for example, further include cellular telephones that communicate voice and data packets over wireless networks. Many such cellular telephones are being manufactured with relatively large increases in computing capabilities, and as such, are becoming tantamount to small personal computers and hand-held PDAs. Further, such devices are being manufactured to enable communications using a variety of frequencies and applicable coverage areas, such as cellular communications, wireless local area network (WLAN) communications, near field communication (NFC), etc. 
     When NFC is implemented, an NFC enabled device may receive signals from another NFC device. The carrier level of the signals may affect the quality of the communication. Therefore, benefits may be realized performing automatic gain control using a carrier estimation path. 
     SUMMARY 
     A method is described. The method includes determining a carrier level estimation of a carrier signal received by a communication device using a carrier estimation path. The method also includes adjusting an attenuation based on the carrier level estimation. 
     The method may also include adjusting at least one of a band-pass filter gain and a low-pass filter gain based on the carrier level estimation. 
     The communication device may be an initiator near field communication (NFC) device. The carrier estimation path may be included in a receiver. The carrier estimation path may preserve the carrier signal by bypassing a band-pass filter. The carrier level estimation may be determined during a continuous wave period. 
     Determining the carrier level estimation may include enabling the carrier estimation path. The carrier signal may be received at the carrier estimation path. The carrier signal may be converted to a DC level. The DC level may include an in-phase component and a quadrature-phase component. The DC level may be measured. 
     The method may also include determining an automatic gain control (AGC) gain table based on the carrier level estimation. Determining the AGC gain table may include applying a range of attenuation values. An attenuator gain may be determined for each attenuation value based on the carrier level estimation. Attenuation values may be selected to produce nominal steps within an attenuation gain range. The selected attenuation values may be assigned to index numbers of the AGC gain table. 
     Adjusting the attenuation may include determining a baseband signal amplitude based on the carrier level estimation. Whether to reduce the attenuation may be determined based on whether the baseband signal amplitude is less than a target value. 
     Adjusting the attenuation may also include applying a maximum attenuation during a continuous wave period. The baseband signal amplitude may be determined based on the carrier level estimation and the maximum attenuation. The baseband signal amplitude may be determined to be less than a target value. The attenuation may be reduced to bring the baseband signal amplitude within a threshold of the target value. 
     An electronic device is also described. The electronic device includes a processor, memory in electronic communication with the processor and instructions stored in the memory. The instructions are executable by the processor to determine a carrier level estimation of a carrier signal received by the electronic device using a carrier estimation path. The instructions are also executable to adjust at least one of an attenuation, a band-pass filter gain and a low-pass filter gain based on the carrier level estimation. 
     A computer-program product is also described. The computer-program product includes a non-transitory computer-readable medium having instructions thereon. The instructions include code for causing an electronic device to determine a carrier level estimation of a carrier signal received by the electronic device using a carrier estimation path. The instructions also include code for causing the electronic device to adjust at least one of an attenuation, a band-pass filter gain and a low-pass filter gain based on the carrier level estimation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating one configuration of a wireless communication system; 
         FIG. 2  is a flow diagram illustrating a method for automatic gain control (AGC) using a carrier estimation path; 
         FIG. 3  is a block diagram illustrating one configuration of an initiator receiver; 
         FIG. 4  is a flow diagram illustrating a method for determining an AGC gain table using a carrier estimation path; 
         FIG. 5  is a flow diagram illustrating a detailed method for determining an AGC gain table using a carrier estimation path; 
         FIG. 6  is a flow diagram illustrating a method for dynamically adjusting attenuation using a carrier estimation path; 
         FIG. 7  is a flow diagram illustrating a detailed method for dynamically adjusting attenuation using a carrier estimation path; and 
         FIG. 8  illustrates certain components that may be included within an electronic device. 
     
    
    
     DETAILED DESCRIPTION 
     It should be noted that some communication devices may communicate wirelessly and/or may communicate using a wired connection or link. For example, some communication devices may communicate with other devices using an Ethernet protocol. The systems and methods disclosed herein may be applied to communication devices that communicate wirelessly and/or that communicate using a wired connection or link. In one configuration, the systems and methods disclosed herein may be applied to a communication device that communicates with another device using near field communication (NFC). 
     As described herein, an initiator NFC device may recognize a target NFC device and/or tag when within range of the coverage area of the NFC device and/or tag. The term NFC tag refers to an integrated circuit that provides NFC functionality. After a target NFC device and/or tag have been located, the initiator NFC device may obtain sufficient information to allow for communications to be established. Communications between the devices may be enabled over a variety of NFC RF technologies, such as, but not limited to, NFC-A, NFC-B, NFC-F, etc. 
     Various configurations are now described with reference to the Figures, where like reference numbers may indicate functionally similar elements. The systems and methods as generally described and illustrated in the Figures herein could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of several configurations, as represented in the Figures, is not intended to limit scope, as claimed, but is merely representative of the systems and methods. 
       FIG. 1  is a block diagram illustrating one configuration of a wireless communication system  100 . The wireless communication system  100  may include an initiator NFC device  102 . The initiator NFC device  102  may communicate with a target NFC device  139 . The initiator NFC device  102  may also be referred to as a poller, polling device or initiator. The target NFC device  139  may also be referred to as a listener, listening device or target. 
     NFC is an inductively coupled communication. Therefore, the initiator NFC device  102  may also be referred to as an inductively coupled communication device. The antenna  106   a  of the initiator NFC device  102  produces a radiated field (also referred to as a magnetic field or an electromagnetic field) that is received by the antenna  106   b  of the target NFC device  139 . 
     The initiator NFC device  102  and the target NFC device  139  may use one or more NFC signaling technologies to communicate with each other. The NFC signaling technologies may include NFC-A, NFC-B and NFC-F. NFC-A may be referred to as type-A, NFC-B may be referred to as type-B and NFC-F may be referred to as type-F. The NFC signaling technologies differ in the modulation schemes employed. 
     NFC has four different tag types, which support a subset of the NFC signaling technologies. Type 1 tags (T1T) use NFC-A communication without data collision protection. Type 2 tags (T2T) use NFC-B communication with anti-collision. Type 3 tags (T3T) use NFC-F with anti-collision. Type 4 tags (T4T) can use either NFC-A (T4AT) or NFC-B (T4BT) with anti-collision. 
     In one configuration, the initiator NFC device  102  and the target NFC device  139  may be operable to communicate using NFC through various interfaces, such as a frame radio frequency (RF) interface, ISO-data exchange protocol (DEP) RF interface and NFC-DEP RF interface. In another configuration, the initiator NFC device  102  and the target NFC device  139  may establish an NFC-DEP RF protocol based communication link with link layer connections defined through a logical link control protocol (LLCP). In still another configuration, the initiator NFC device  102  and the target NFC device  139  may be operable to be connected to an access network and/or core network (e.g., a CDMA network, a GPRS network, a UMTS network, and other types of wireline and wireless communication networks). 
     The initiator NFC device  102  may poll for nearby NFC devices. The target NFC device  139  may begin to listen when it comes within a few centimeters of the initiator NFC device  102 . The initiator NFC device  102  will then communicate with the target NFC device  139  in order to determine which signaling technologies can be used. In one case, the initiator NFC device  102  may be acting as a reader. In other words, the initiator NFC device  102  may be in a reader mode. In this case, a user may place a target NFC device  139  in the vicinity of the initiator NFC device  102  to initiate a payment transaction. 
     The initiator NFC device  102  may generate an RF field to communicate with the target NFC device  139 . The initiator NFC device  102  may modulate the RF field to send a signal (e.g., data) to the target NFC device  139 . Once the target NFC device  139  receives that signal, the initiator NFC device  102  may transmit a continuous wave to maintain the RF field. The continuous wave may have a carrier frequency. In the case of NFC, the carrier frequency may be 13.56 megahertz (MHz). 
     The target NFC device  139  may receive the RF field. The target NFC device  139  may respond by performing modulation on top of the continuous wave. The initiator NFC device  102  may receive the modulated signal and may try to decode it. 
     In summary, during a first communication period, the initiator NFC device  102  may transmit data to the target NFC device  139 . In the first communication period, the initiator NFC device  102  is in an initiator transmit (TX) mode. During a second communication period, the target NFC device  139  may respond back. In the second communication period, the initiator NFC device  102  is in an initiator receive (RX) mode. 
     A continuous wave period may be the period of time when the initiator NFC device  102  starts generating the continuous wave and before the target NFC device  139  modulates the continuous wave. During the continuous wave period, the initiator NFC device  102  may receive a carrier signal  108  of the continuous wave at an initiator receiver  104 . In other words, the initiator NFC device  102  may receive the carrier signal  108  that it transmits in the continuous wave. The strength of the carrier signal  108  is referred to as the carrier level of the carrier signal  108 . The carrier level may be measured at a particular point in a radio path. 
     With different antenna  106  configurations, the carrier level of the carrier signal  108  may vary. In one case, the initiator NFC device  102  and the target NFC device  139  may have different antennas  106 . The antennas  106  may have different dimensions or sizes. This may occur when the initiator NFC device  102  and the target NFC device  139  are made by different manufacturers or are different models made by the same manufacturer. Differences in antenna  106  dimensions may result in large variations of the carrier level. 
     In another case, the carrier level may vary due to the motion of the initiator NFC device  102  or the target NFC device  139 . In this case, even if the antennas  106  are the same, different distances between the initiator NFC device  102  antenna  106   a  and the target NFC device  139  antenna  106   b  may result in large variations of the carrier level. In one scenario, the initiator NFC device  102  and the target NFC device  139  may be at a far distance, which results in a large carrier level and small modulation index. In another scenario, as the initiator NFC device  102  and the target NFC device  139  are brought closer, the carrier level goes down and modulation index goes up. In either scenario, the carrier level may be higher than the initiator NFC device  102  can handle without proper attenuation. 
     If the carrier level is too high, the initiator receiver  104  may become saturated. The initiator NFC device  102  may perform automatic gain control (AGC) on the received signal. In one approach, the initiator NFC device  102  may set the attenuation  114  of the carrier signal  108  to handle the highest carrier level. However, this approach may result in unnecessary attenuation of the carrier signal  108  and sideband signal, which may degrade sideband signal to noise ratio. Therefore, benefits may be realized by estimating the carrier level and performing AGC based on the carrier level estimation  122 . The AGC may include adjusting one or more of an attenuation  114 , band-pass filter (BPF) gain  116  and low-pass filter (LPF) gain  118  for the initiator receiver  104 . 
     The initiator receiver  104  may include a functional path  110 . Components that may be included in the functional path  110  may include an attenuator, mixer, band-pass filter (BPF), low-pass filter (LPF), analog-to-digital converter (ADC) and modem. One configuration of an initiator receiver  104  is illustrated in  FIG. 3 . During normal operation (e.g., non-carrier level estimation operation), the carrier signal  108  is processed using the functional path  110 . As part of the signal processing on the functional path  110 , the carrier signal  108  may be filtered by the BPF. 
     To determine the carrier level estimation  122 , the initiator receiver  104  may include a carrier estimation path  112 . The carrier estimation path  112  may preserve the carrier signal  108  by bypassing the BPF. The initiator NFC device  102  may activate (e.g., enable) the carrier estimation path  112  during a continuous wave period. During the continuous wave period, the carrier signal  108  may be received at the carrier estimation path  112 . A carrier level estimation block  120  may then obtain a carrier level estimation  122 . 
     In one implementation, the initiator NFC device  102  may convert the analog carrier signal  108  to a digital signal that is measured by the carrier level estimation block  120 . For example, the initiator NFC device  102  may down-convert the carrier signal  108  to a DC level. After down conversion, the DC level may include an in-phase component and a quadrature-phase component. The DC level may include two channels (e.g., paths). One channel may be an in-phase channel (I dc ) and the other channel may be a quadrature-phase channel (Q dc ). The DC level of the whole system corresponds to the carrier level. The carrier level estimation block  120  may include a DC estimation block that estimates the carrier level by measuring the DC level. 
     The initiator NFC device  102  may determine the gain of the functional path  110  using the carrier level estimation  122 . The path gain (G p )  128  may be determined according to Equation (1). 
         G   p   =−Ac+ 10·log 10( I   dc   2   +Q   dc   2 )  (1)
 
     In Equation (1), Ac is the carrier level in decibels (dB). I dc  (e.g., the I-channel) and Q dc  (e.g., the Q-channel) may be determined by the carrier estimation block  120 . 
     The initiator NFC device  102  may adjust the attenuation  114  of the initiator receiver  104  based on the carrier level estimation  122 . The initiator NFC device  102  may perform different operations using the carrier level estimation  122 . 
     In one case, adjusting the attenuation  114  may include determining an AGC gain table  126  based on the carrier level estimation  122 . The AGC gain table  126  may be calibrated during a bench calibration. The AGC gain table  126  may be a lookup table (LUT) that maps different values of attenuation  114 , BPF gains  116  and LPF gains  118 . 
     An AGC gain table determination module  124  may determine the AGC gain table  126 . The AGC gain table determination module  124  may apply a carrier signal  108  with a known carrier level (Ac). The AGC gain table determination module  124  may then apply a range of attenuation  114  values while keeping the gain of other initiator receiver  104  components fixed. 
     The AGC gain table determination module  124  may determine the path gain (Gp)  128  for each attenuation  114  value based on the carrier level estimation  122 . This may be accomplished according to Equation (1) above. From the path gains  128 , the AGC gain table determination module  124  may determine an attenuator gain  130  for each attenuation value. There may be a relatively fixed difference (Δ) between the path gain (Gp)  128  and the attenuator gain (Ga)  130 . Therefore, the attenuator gain (Ga)  130  may be determined as Ga=Gp−Δ. 
     The AGC gain table determination module  124  may select attenuation  114  values to produce nominal steps within an attenuator gain range. The AGC gain table determination module  124  may assign the selected attenuation  114  values to index numbers of the AGC gain table  126 . An example of an AGC gain table  126  is described in connection with  FIG. 4  below. 
     In another case, the adjusting the attenuation  114  may include dynamically adjusting the attenuation  114  during normal operation of the initiator receiver  104 . For example, a dynamic attenuation adjustment module  132  may adjust the attenuation  114  at various times during operation, including during communication with a target NFC device  139 . 
     In one implementation, the dynamic attenuation adjustment module  132  may determine a baseband signal amplitude (A bb )  134  based on the carrier level estimation  122 . This may be accomplished according to Equation (2). 
         A   bb =√{square root over ( I   dc   2   +Q   dc   2 )}  (2)
 
     The dynamic attenuation adjustment module  132  may determine the baseband signal amplitude  134  during a continuous wave period. In one case, the continuous wave period may include the initial initiator transmission of the continuous wave to the target NFC device  139  before the target NFC device  139  modulates the continuous wave. Another continuous wave period may include the transition from initiator transmission to initiator reception. 
     The dynamic attenuation adjustment module  132  may determine whether to reduce the attenuation  114  based on whether the baseband signal amplitude  134  is less than a target value  136 . If the baseband signal amplitude  134  is not less than the target value  136 , then the dynamic attenuation adjustment module  132  may not change the attenuation  114 . In this case, the carrier signal  108  is neither saturating the initiator receiver  104  nor being excessively attenuated. 
     If the baseband signal amplitude  134  is much less than the target value  136 , then the carrier signal  108  may be attenuated more than necessary. In this case, the dynamic attenuation adjustment module  132  may reduce the attenuation  114  to bring the amplitude of the baseband signal  134  within a threshold of the target value  136 . 
     Upon determining the attenuation  114 , the initiator NFC device  102  may adjust at least one of a band-pass filter gain  116  and a low-pass filter gain  118  accordingly. For example, the selected attenuation  114  may be mapped to a band-pass filter gain  116  and a low-pass filter gain  118 . In one implementation, this mapping may be provided by the AGC gain table  126 . By selecting an attenuation  114 , the band-pass filter gain  116  and the low-pass filter gain  118  may be set accordingly. 
     The described systems and methods may provide design flexibility and improve efficiency. The initiator NFC device  102  may adapt to different tags and its own antenna  106   a . For example, the initiator NFC device  102  can move from large to small antenna and calibrate the initiator receiver  104 . This makes the initiator NFC device  102  more robust. Furthermore, the described systems and methods will greatly reduce the amount of manual tuning of the initiator NFC device  102 . 
       FIG. 2  is a flow diagram illustrating a method  200  for AGC using a carrier estimation path  112 . The method  200  may be performed by an initiator NFC device  102 . The initiator NFC device  102  may receive  202  a carrier signal  108 . During a continuous wave period, the initiator NFC device  102  may receive a carrier signal  108  of the continuous wave. In other words, the initiator NFC device  102  may receive the carrier signal  108  that it transmits in the continuous wave. 
     The initiator NFC device  102  may determine  204  a carrier level estimation  122  of the carrier signal  108 . The carrier level represents the strength of the carrier signal  108  as measured at a particular point in a radio path. The initiator NFC device  102  may activate a carrier estimation path  112  that bypasses a band-pass filter in the initiator receiver  104 . A carrier level estimation block  120  may then obtain a carrier level estimation  122 . 
     In one implementation, the initiator NFC device  102  may convert the analog carrier signal  108  to a digital signal to estimate the carrier level. The initiator NFC device  102  may down-convert the carrier signal  108  to a DC level. After down conversion, the DC level may include an in-phase component and a quadrature-phase component. The initiator NFC device  102  may estimate the carrier level by measuring the DC level. 
     The initiator NFC device  102  may adjust  206  the attenuation  114  of the initiator receiver  104  based on the carrier level estimation  122 . The initiator NFC device  102  may perform different operations using the carrier level estimation  122 . 
     In one case, the initiator NFC device  102  may adjust  206  the attenuation  114  to determine an AGC gain table  126  based on the carrier level estimation  122 . The initiator NFC device  102  may apply a carrier signal  108  with a known carrier level. The initiator NFC device  102  may then apply a range of attenuation values while keeping the gain of other initiator receiver  104  components fixed. 
     The initiator NFC device  102  may determine the path gain  128  for each attenuation value based on the carrier level estimation  122 . This may be accomplished according to Equation (1) above. From the path gains  128 , the initiator NFC device  102  may determine an attenuator gain  130  for each attenuation value. The initiator NFC device  102  may select attenuation  114  values to produce nominal steps within an attenuation gain range and assign the selected attenuation  114  values to index numbers of the AGC gain table  126 . 
     In another case, the initiator NFC device  102  may adjust  206  the attenuation  114  by dynamically adjusting the attenuation  114  during normal operation of the initiator receiver  104 . For example, the initiator NFC device  102  may determine a baseband signal amplitude (A bb )  134  based on the carrier level estimation  122 . This may be accomplished according to Equation (2) above. The initiator NFC device  102  may determine the baseband signal amplitude  134  during a continuous wave period. 
     The initiator NFC device  102  may determine whether to reduce the attenuation  114  based on whether the baseband signal amplitude  134  is less than a target value  136 . If the baseband signal amplitude  134  is not less than the target value  136 , then the initiator NFC device  102  may not change the attenuation  114 . If the baseband signal amplitude  134  is less than the target value  136 , then initiator NFC device  102  may reduce the attenuation  114  to bring the amplitude of the baseband signal  134  within a threshold of the target value  136 . 
       FIG. 3  is a block diagram illustrating one configuration of an initiator receiver  304 . The initiator receiver  304  may be implemented in accordance with the initiator receiver  104  described in connection with  FIG. 1 . 
     The initiator receiver  304  may include a functional path  310  that may receive and process a carrier modulated signal  308  during normal operation. An attenuator  374  may attenuate the carrier modulated signal  308  so that the internal circuit can operate on it. The attenuator  374  may be implemented as a capacitive or resistive divider that may change its attenuation  114  based on a control signal. In one configuration, a code may be a register value that controls the attenuator  374  for different attenuation  114 . A higher code may produce more attenuation  114 . 
     The mixer  376  may down-convert the attenuated signal  375  to baseband using a local oscillator signal  354 . The down-converted signal  377  may be a DC and baseband signal that includes an I-channel and a Q-channel. In the functional path  310 , the down-converted signal  377  may then pass through a band-pass filter  378  and a low-pass filter  380  to prepare the baseband signal so the ADC  382  can sample it and convert it to a digital signal  383 . The down-converted carrier may be filtered out of the band-pass filtered signal  379 , leaving the baseband signal  379 . The low-pass filtered signal  381  may be provided to the ADC  382 . A modem  384  may receive and process the digital signal  383 . 
     The carrier estimation path  312  may bypass the band-pass filter  378  to preserve the down-converted carrier level in order to tune the gain of the whole path. A low-pass filter  380  may be maintained to provide anti-aliasing filtering for the ADC  382 . 
     For the carrier estimation path  312 , the carrier signal  308  is received and down-converted to DC. Therefore, a DC level that reflects the carrier signal  308  strength is received at the ADC  382 . 
     The modem  384  may include a DC estimation block  386  that can process the digital signal  383  to determine a carrier level estimation  322 . This may include measuring the in-phase component (e.g., I dc ) and the quadrature-phase component (e.g., Q dc ) of the DC level. 
     The modem  384  may also include an AGC module  390  to perform attenuation and gain control based on the carrier level estimation  322 . The AGC module  390  may determine an attenuation  114  for the attenuator  374 . The AGC module  390  may also determine a BPF gain  116  for the BPF  378  and a LPF gain  118  for the LPF  380 . This may be accomplished as described above in connection with  FIG. 1 . 
     In one configuration, the BPF gain  116  and the LPF gain  118  may be mapped to attenuation  114  values. Therefore, by determining the attenuation  114 , the BPF gain  116  and the LPF gain  118  may be set based on the selected attenuation  114 . The AGC module  390  may send one or more attenuation/gain control signals  388  to the attenuator  374 , BPF  378  and LPF  380  to adjust their respective attenuation  114  and gain levels. 
       FIG. 4  is a flow diagram illustrating a method  400  for determining an AGC gain table  126  using a carrier estimation path  112 . The method  400  may be performed by an initiator NFC device  102  to calibrate an attenuator  374 . The initiator NFC device  102  may receive a carrier signal  108  during a continuous wave period. The carrier signal  108  may be a known carrier level. The initiator NFC device  102  may enable a carrier estimation path  112 . 
     The initiator NFC device  102  may apply  402  a range of attenuation  114  values. Each attenuation  114  value may produce a different amount of attenuation  114  by the attenuator  374 . For example, the attenuation values  114  may be a series of attenuation codes that may adjust the capacitance or resistance of the attenuator  374 . 
     The initiator NFC device  102  may determine  404  an attenuator gain  130  for each attenuation value based on the carrier level estimation  122 . This may be accomplished by first determining the path gain (Gp)  128  according to Equation (1) above. From the path gain  128 , the initiator NFC device  102  may determine an attenuator gain  130  for each attenuation value. The attenuator gain  130  may be a fixed difference (Δ) from the path gain  128 . Therefore, the attenuator gain (Ga)  130  may be determined as Ga=Gp−Δ. The initiator NFC device  102  may store the attenuator gains  130  in memory. 
     The initiator NFC device  102  may select  406  attenuation  114  values to produce nominal steps within an attenuation gain range. The initiator NFC device  102  may assign  408  the selected attenuation  114  values to index numbers of the AGC gain table  126 . 
     In one configuration, the AGC gain table  126  may be a look up table (LUT). In this configuration, the LUT may have index numbers (also referred to as a gain table numbers) with corresponding attenuation codes. An attenuation code may be a register value that controls the attenuator  374  for the different attenuation  114 . The attenuation code may also be referred to as attenuator code. 
     Each index number of the AGC gain table  126  may also have an associated band-pass filter (BPF) code that indicates a BPF gain  116 . Similarly, each index number of the AGC gain table  126  may have an associated low pass filter (LPF) code that indicates a LPF gain  118 . 
       FIG. 5  is a flow diagram illustrating a detailed method  500  for determining an AGC gain table  126  using a carrier estimation path  112 . The method  500  may be performed by an initiator NFC device  102 . 
     The initiator NFC device  102  may enable  502  a carrier estimation path  112 . The carrier estimation path  112  may preserve a carrier signal  108  by bypassing a band-pass filter  378 . 
     The initiator NFC device  102  may input  504  a known carrier level (Ac) during the calibration process. For example, the carrier level may be 1 Vsp. The input carrier level should not saturate the ADC  382  even with the smallest attenuation  114  (e.g., when an attenuator code=0). 
     The initiator NFC device  102  may set  506  an attenuator code. The attenuator code may be the amount of attenuation  114  produced by the attenuator  374 . The initiator NFC device  102  may apply a range of attenuator codes (e.g., attenuation  114  values). Each attenuator code may produce a different amount of attenuation  114  by the attenuator  374 . In one configuration, the attenuator code may be a cap code. 
     The initiator NFC device  102  may measure  508  the DC levels I dc  and Q dc  at the I-channel and Q-channel. For example, a DC estimation block  386  may measure the I component and the Q component of the DC level provided by the ADC  382 . In one configuration, the initiator NFC device  102  may check the DC offset with no input signal and subtract DC offset from the I dc  and Q dc . 
     The initiator NFC device  102  may estimate  510  the path gain (Gp)  128 . This may be accomplished according to Equation (1) using the known carrier level (Ac) and the measured I dc  and Q dc . 
     The initiator NFC device  102  may then calculate  512  the attenuator gain (Ga)  130  from the path gain (Gp)  128 . The attenuator gain (Ga)  130  may be determined as Ga=Gp−Δ, where Δ is the fixed difference from the path gain  128 . 
     The initiator NFC device  102  may determine  514  whether to set another attenuator code. Therefore, the initiator NFC device  102  may loop through the range of attenuator codes (from 0 to 255, for example) and determine the associated attenuator gain (Ga)  130  of each attenuator code. 
     When the initiator NFC device  102  determines  514  to not set another code (e.g., upon exhausting the range of attenuator codes), the initiator NFC device  102  may form  516  an attenuation table with nominal steps within an attenuation gain range. The initiator NFC device  102  may select attenuator codes from the range of attenuator codes to produce the nominal steps within the attenuation gain range. The attenuation table may be included in an AGC gain table  126  (e.g., LUT). 
       FIG. 6  is a flow diagram illustrating a method  600  for dynamically adjusting attenuation  114  using a carrier estimation path  112 . The method  600  may be performed by an initiator NFC device  102  during normal operation of the initiator receiver  104 . For example, the initiator NFC device  102  may be communicating with a target NFC device  139 . 
     The initiator NFC device  102  may receive a carrier signal  108 . The initiator NFC device  102  may enable the carrier estimation path  112  during a continuous wave period. The carrier estimation path  112  may bypass the BPF  378  of the initiator receiver  104 . In one case, the continuous wave period may include the initial initiator transmission of the continuous wave to the target NFC device  139  (before the target NFC device  139  modulates the continuous wave). Another continuous wave period may include the transition from initiator transmission to initiator reception. 
     The initiator NFC device  102  may apply  602  a maximum attenuation  114  during the continuous wave period. The maximum attenuation  114  may be the maximum attenuation  114  in an AGC gain table  126 . For example, the maximum attenuation  114  may correspond to a maximum attenuator code in the AGC gain table  126 . The maximum attenuation  114  may ensure that the initiator receiver  104  is not saturated by the carrier signal  108 . 
     The initiator NFC device  102  may determine  604  a baseband signal amplitude  134  based on the carrier level estimation  122  and the maximum attenuation  114 . While the maximum attenuation  114  is applied, the initiator NFC device  102  may convert the analog carrier signal  108  to a DC level. The DC level may include an in-phase component and a quadrature-phase component. The DC level may be converted to a digital signal. The initiator NFC device  102  may perform the carrier level estimation  122  by measuring the DC level of the in-phase channel (I dc ) and the quadrature-phase channel (Q dc ). The initiator NFC device  102  may determine the baseband signal amplitude (A bb )  134  according to Equation (2). 
     The initiator NFC device  102  may determine  606  that the baseband signal amplitude (A bb )  134  is less than a target value  136 . If the baseband signal amplitude  134  is much less than the target value  136 , then the initiator NFC device  102  may reduce  608  the attenuation  114  to bring the baseband signal amplitude  134  within a threshold of the target value  136 . For example, the initiator NFC device  102  may determine whether the baseband signal amplitude  134  is less than the target value  136  by a first threshold of the target value  136 . If the baseband signal amplitude  134  is less than this first threshold, then the initiator NFC device  102  may reduce  608  the attenuation  114  to bring the baseband signal amplitude  134  within a second threshold of the target value  136 . 
     It should be noted that the method  600  involves a one-point dynamic AGC procedure. In other words, the attenuation  114  is adjusted based on a single carrier level estimation. This reduces the amount of time for adjusting the initiator receiver  104  gain. Other search algorithms (e.g., a binary search) may require more measurements, which may slow down calibration and reduce efficiency. 
       FIG. 7  is a flow diagram illustrating a detailed method  700  for dynamically adjusting attenuation  114  using a carrier estimation path  112 . The method  700  may be performed by an initiator NFC device  102  during normal operation of the initiator receiver  104 . For example, the initiator NFC device  102  may be communicating with a target NFC device  139 . The initiator NFC device  102  may receive a carrier signal  108 . 
     The initiator NFC device  102  may enable  702  a carrier estimation path  112  during a continuous wave period. The carrier estimation path  112  may preserve a carrier signal  108  by bypassing a band-pass filter  378 . 
     The initiator NFC device  102  may set  704  the maximum attenuation  114  in an AGC gain table  126 . For example, the initiator NFC device  102  may set  704  the attenuation  114  of the attenuator  374  to the first entry in the AGC gain table  126 . In one configuration, the AGC gain table  126  may be a LUT. 
     The initiator NFC device  102  may measure  706  the I dc  and the Q dc . The initiator NFC device  102  may convert the analog carrier signal  108  to a DC level that includes an in-phase channel (I dc ) and a quadrature-phase channel (Q dc ). In one configuration, the initiator NFC device  102  may convert the DC level to a digital signal and then measure  706  the I dc  and the Q dc . 
     The initiator NFC device  102  may determine  708  whether a baseband signal amplitude (A bb )  134  is less than a target value (Vref)  136 . A bb    134  may be determined according to Equation (2). If A bb    134  is less than the target value  136 , the initiator NFC device  102  may calculate  710  a gain difference (G d ). This may be accomplished according to Equation (3). 
         G   d =20·log 10(0.5/ A   bb )  (3)
 
     The initiator NFC device  102  may select  712  an attenuation  114 , BPF gain  116 , LPF gain  118  from the AGC gain table  126  based on the gain difference (G d ). In one implementation, the initiator NFC device  102  may jump to entry [1+floor(G d /2)] in the AGC gain table  126 . This equation assumes a 2 dB step in attenuation gain  130  and may be modified for other step sizes. This AGC gain table  126  entry may bring A bb    134  within a threshold of the target value  136 . 
     The initiator NFC device  102  may wait  714  for a first settling time. The initiator NFC device  102  may then disable  716  the carrier estimation path  112 , enable the functional path  110  and may wait  718  for a second settling time before continuing normal operations. 
     If the initiator NFC device  102  determines  708  that A bb    134  is not less than the target value  136 , the initiator NFC device  102  may disable  716  the carrier estimation path  112  and enable the functional path  110 . In this case, the initiator NFC device  102  does not adjust the attenuation  114 . The initiator NFC device  102  may then wait  718  for the second settling time before continuing normal operations. 
       FIG. 8  illustrates certain components that may be included within an electronic device  802 . The electronic device  802  may be an access terminal, a mobile station, a user equipment (UE), etc. For example, the electronic device  802  may be the initiator NFC device  102  of  FIG. 1 . 
     The electronic device  802  includes a processor  803 . The processor  803  may be a general purpose single- or multi-chip microprocessor (e.g., an Advanced RISC (Reduced Instruction Set Computer) Machine (ARM)), a special purpose microprocessor (e.g., a digital signal processor (DSP)), a microcontroller, a programmable gate array, etc. The processor  803  may be referred to as a central processing unit (CPU). Although just a single processor  803  is shown in the electronic device  802  of  FIG. 8 , in an alternative configuration, a combination of processors (e.g., an ARM and DSP) could be used. 
     The electronic device  802  also includes memory  805  in electronic communication with the processor (i.e., the processor can read information from and/or write information to the memory). The memory  805  may be any electronic component capable of storing electronic information. The memory  805  may be configured as random access memory (RAM), read-only memory (ROM), magnetic disk storage media, optical storage media, flash memory devices in RAM, on-board memory included with the processor, EPROM memory, EEPROM memory, registers and so forth, including combinations thereof. 
     Data  807   a  and instructions  809   a  may be stored in the memory  805 . The instructions may include one or more programs, routines, sub-routines, functions, procedures, code, etc. The instructions may include a single computer-readable statement or many computer-readable statements. The instructions  809   a  may be executable by the processor  803  to implement the methods disclosed herein. Executing the instructions  809   a  may involve the use of the data  807   a  that is stored in the memory  805 . When the processor  803  executes the instructions  809 , various portions of the instructions  809   b  may be loaded onto the processor  803 , and various pieces of data  807   b  may be loaded onto the processor  803 . 
     The electronic device  802  may also include a transmitter  811  and a receiver  813  to allow transmission and reception of signals to and from the electronic device  802  via an antenna  817 . The transmitter  811  and receiver  813  may be collectively referred to as a transceiver  815 . The electronic device  802  may also include (not shown) multiple transmitters, multiple antennas, multiple receivers and/or multiple transceivers. 
     The electronic device  802  may include a digital signal processor (DSP)  821 . The electronic device  802  may also include a communications interface  823 . The communications interface  823  may allow a user to interact with the electronic device  802 . 
     The various components of the electronic device  802  may be coupled together by one or more buses, which may include a power bus, a control signal bus, a status signal bus, a data bus, etc. For the sake of clarity, the various buses are illustrated in  FIG. 8  as a bus system  819 . 
     In the above description, reference numbers have sometimes been used in connection with various terms. Where a term is used in connection with a reference number, this may be meant to refer to a specific element that is shown in one or more of the Figures. Where a term is used without a reference number, this may be meant to refer generally to the term without limitation to any particular Figure. 
     The term “determining” encompasses a wide variety of actions and, therefore, “determining” can include calculating, computing, processing, deriving, investigating, looking up (e.g., looking up in a table, a database or another data structure), ascertaining and the like. Also, “determining” can include receiving (e.g., receiving information), accessing (e.g., accessing data in a memory) and the like. Also, “determining” can include resolving, selecting, choosing, establishing and the like. 
     The phrase “based on” does not mean “based only on,” unless expressly specified otherwise. In other words, the phrase “based on” describes both “based only on” and “based at least on.” 
     The term “processor” should be interpreted broadly to encompass a general purpose processor, a central processing unit (CPU), a microprocessor, a digital signal processor (DSP), a controller, a microcontroller, a state machine, and so forth. Under some circumstances, a “processor” may refer to an application specific integrated circuit (ASIC), a programmable logic device (PLD), a field programmable gate array (FPGA), etc. The term “processor” may refer to a combination of processing devices, e.g., a combination of a digital signal processor (DSP) and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a digital signal processor (DSP) core, or any other such configuration. 
     The term “memory” should be interpreted broadly to encompass any electronic component capable of storing electronic information. The term memory may refer to various types of processor-readable media such as random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable PROM (EEPROM), flash memory, magnetic or optical data storage, registers, etc. Memory is said to be in electronic communication with a processor if the processor can read information from and/or write information to the memory. Memory that is integral to a processor is in electronic communication with the processor. 
     The terms “instructions” and “code” should be interpreted broadly to include any type of computer-readable statement(s). For example, the terms “instructions” and “code” may refer to one or more programs, routines, sub-routines, functions, procedures, etc. “Instructions” and “code” may comprise a single computer-readable statement or many computer-readable statements. 
     The functions described herein may be implemented in software or firmware being executed by hardware. The functions may be stored as one or more instructions on a computer-readable medium. The terms “computer-readable medium” or “computer-program product” refers to any tangible storage medium that can be accessed by a computer or a processor. By way of example, and not limitation, a computer-readable medium may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray® disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. It should be noted that a computer-readable medium may be tangible and non-transitory. The term “computer-program product” refers to a computing device or processor in combination with code or instructions (e.g., a “program”) that may be executed, processed or computed by the computing device or processor. As used herein, the term “code” may refer to software, instructions, code or data that is/are executable by a computing device or processor. 
     Software or instructions may also be transmitted over a transmission medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of transmission medium. 
     The methods disclosed herein comprise one or more steps or actions for achieving the described method. The method steps and/or actions may be interchanged with one another without departing from the scope of the claims. In other words, unless a specific order of steps or actions is required for proper operation of the method that is being described, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the claims. 
     Further, it should be appreciated that modules and/or other appropriate means for performing the methods and techniques described herein, such as those illustrated by  FIG. 2 , and  FIGS. 4-7 , can be downloaded and/or otherwise obtained by a device. For example, a device may be coupled to a server to facilitate the transfer of means for performing the methods described herein. Alternatively, various methods described herein can be provided via a storage means (e.g., random access memory (RAM), read only memory (ROM), a physical storage medium such as a compact disc (CD) or floppy disk, etc.), such that a device may obtain the various methods upon coupling or providing the storage means to the device. Moreover, any other suitable technique for providing the methods and techniques described herein to a device can be utilized. 
     It is to be understood that the claims are not limited to the precise configuration and components illustrated above. Various modifications, changes and variations may be made in the arrangement, operation and details of the systems, methods, and apparatus described herein without departing from the scope of the claims.