Abstract:
A system including a sampling module that generates samples of RF signals on a first channel during first, second, and third periods, which do not overlap. A difference module determines a first difference between i) a first count of polarity reversals during the first period and ii) a second count of polarity reversals during the second period; a second difference between i) the second count and ii) a third count of polarity reversals during the third period; and a third difference between the first and second differences. A third module determines a frequency of the RF signals based on at least one of the first and second counts, determines a frequency variation of the RF signals based on the first and second counts, and identifies a radar type of the RF signals based on at least one of the third difference and the frequency variation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/493,473, filed Jul. 26, 2006, which claims the benefit of U.S. Provisional Application No. 60/749,222, filed Dec. 9, 2005. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to wireless networks, and more particularly to a system and method for detecting and measuring frequency variations in wireless signals. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     The I.E.E.E. has defined several different standards for configuring wireless networks and devices. For example, 802.11, 802.11(a), 802.11(b), 802.11(g), 802.11(h), 802.11(n), 802.16, and 802.20. According to these standards, wireless network devices may be operated in either an infrastructure mode or an ad-hoc mode. 
     In the infrastructure mode, the wireless network devices or client stations communicate with each other through an access point. In the ad-hoc mode, the wireless network devices communicate directly with each other and do not employ an access point. The term client station or mobile station may not necessarily mean that a wireless network device is actually mobile. For example, a desktop computer that is not mobile may incorporate a wireless network device and operate as a mobile station or client station. 
     Referring now to  FIG. 1 , a first wireless network  10  is illustrated in an infrastructure mode. The first wireless network  10  includes one or more client stations  12  and one or more access points (AP)  14 . The client station  12  and the AP  14  transmit and receive wireless signals  16 . The AP  14  is a node in a network  18 . The network  18  may be a local area network (LAN), a wide area network (WAN), or another network configuration. The network  18  may include other nodes such as a server  20  and may be connected to a distributed communications system  22  such as the Internet. 
     Referring now to  FIG. 2 , a second wireless network  24  operates in an ad-hoc mode. The second wireless network  24  includes multiple client stations  26 - 1 ,  26 - 2 , and  26 - 3  that transmit and receive wireless signals  28 . The client stations  26 - 1 ,  26 - 2 , and  26 - 3  collectively form a LAN and communicate directly with each other. The client stations  26 - 1 ,  26 - 2 , and  26 - 3  are not necessarily connected to another network. 
     To minimize radio frequency (RF) interference, some wireless networks may operate in a 5 GHz band. However, regulatory requirements governing the use of the 5 GHz band vary from country to country. Some countries utilize the 5 GHz band for military radar communications. Therefore, wireless networks operating in the 5 GHz band generally employ dynamic frequency selection (DFS) to avoid interference with radar communications. A network device generally employs DFS to a different channel of the 5 GHz band to avoid interfering with radar communications. 
     In infrastructure mode, the AP  14  transmits beacons to inform the client stations  12  that the AP uses DFS. When the client stations  12  detect radar on a channel, the client stations  12  notify the AP  14 . Based on this information, the AP  14  uses DFS to select the best channel for network communications that will not interfere with radar. 
     In ad-hoc mode, one client station may be designated as a DFS owner. The DFS owner is responsible for collecting reports from the other client stations. If any station in the ad-hoc network detects radar, the DFS owner uses DFS to select the best channel for network communications that does not interfere with radar. For example, if station  26 - 1  is the DFS owner, it will be responsible for collecting reports from stations  26 - 2  and  26 - 3 . If any station  26 - 1 ,  26 - 2 , and  26 - 3  detects radar, station  26 - 1  will used DFS to select the best channel and notify stations  26 - 2  and  26 - 3  to switch to that channel. 
     SUMMARY 
     A system comprises a sampling module, a counter module, and a frequency characteristic module. The sampling module samples radio frequency (RF) signals on a first channel for a first predetermined period and a second predetermined period that is subsequent to the first predetermined period. The counter module increments first and second counts when the samples collected during the first and second predetermined periods reverse polarity, respectively. The frequency characteristic module determines a frequency of the RF signal based on at least one of the first and the second counts and determines frequency variation of the RF signal based on the first and second counts. At least one of the first and second counts is equal to the frequency. 
     In another feature, the frequency characteristic module compares the first and second counts to determine the frequency variation. The frequency characteristic module determines that the RF signal is tone radar when the first and second counts are approximately equal. 
     In another feature, the sampling module samples the RF signals for a third predetermined period that is subsequent to the second predetermined period and wherein the counter module increments a third count when the samples collected during the third predetermined period reverse polarity. 
     In another feature, the system further comprises a derivative module that determines a first difference between the first and second counts and a second difference between the second and third counts. The derivative module determines a third difference between the first and second differences. The derivative module determines the first and second differences when the first, second, and third counts are greater than a predetermined threshold. 
     In another feature, the frequency characteristic module determines that the RF signal is chirp radar when the third difference is less than a predetermined threshold. The predetermined threshold is approximately zero. 
     In another feature, the frequency characteristic module determines that the RF signal is chirp radar when the third difference is approximately zero. 
     In another feature, the system further comprises a radar module that determines whether the RF signal is one of chirp radar and tone radar based on the frequency variation. The radar module determines that the RF signal is chirp radar when the frequency variation is linear. The radar module determines that the RF signal is tone radar when the frequency variation is approximately zero. 
     In another feature, the system further comprises a dynamic frequency selection (DFS) module that communicates with the radar module and that selects a second channel having a different frequency than the first channel when the radar module determines that the RF signal is one of chirp radar and tone radar. 
     In another feature, at least one of the samples is disregarded and excluded from the first and second counts when an absolute value of the at least one of the samples is less than a predetermined threshold. 
     In another feature, the predetermined threshold is approximately 0.1. 
     In another feature, the first and second periods are adjacent. 
     In another feature, the first, second, and third periods are adjacent. 
     In another feature, a wireless network device comprises the system. 
     In another feature, a radar detection device comprises the system. 
     In still other features, a method comprises sampling radio frequency (RF) signals on a first channel for a first predetermined period and a second predetermined period that is subsequent to the first predetermined period, incrementing first and second counts when the samples collected during the first and second predetermined periods reverse polarity, respectively, determining a frequency of the RF signal based on at least one of the first and the second counts, and determining frequency variation of the RF signal based on the first and second counts. At least one of the first and second counts is equal to the frequency. 
     In another feature, the method further comprises comparing the first and second counts to determine the frequency variation. The method further comprises determining that the RF signal is tone radar when the first and second counts are approximately equal. 
     In another feature, the method further comprises sampling the RF signals for a third predetermined period that is subsequent to the second predetermined period and incrementing a third count when the samples collected during the third predetermined period reverse polarity. 
     In another feature, the method further comprises determining a first difference between the first and second counts and a second difference between the second and third counts. The method further comprises determining a third difference between the first and second differences. 
     In another feature, the method further comprises determining the first and second differences when the first, second, and third counts are greater than a predetermined threshold. 
     In another feature, the method further comprises determining that the RF signal is chirp radar when the third difference is less than a predetermined threshold. The predetermined threshold is approximately zero. 
     In another feature, the method further comprises determining that the RF signal is chirp radar when the third difference is approximately zero. 
     In another feature, the method further comprises determining whether the RF signal is one of chirp radar and tone radar based on the frequency variation. The method further comprises determining that the RF signal is chirp radar when the frequency variation is linear. The method further comprises determining that the RF signal is tone radar when the frequency variation is approximately zero. 
     In another feature, the method further comprises selecting a second channel having a different frequency than the first channel when the RF signal is one of chirp radar and tone radar. 
     In another feature, the method further comprises disregarding and excluding at least one of the samples from the first and second counts when an absolute value of the at least one of the samples is less than a predetermined threshold. 
     In another feature, the predetermined threshold is approximately 0.1. 
     In another feature, the first and second periods are adjacent. 
     In another feature, the first, second, and third periods are adjacent. 
     In still other features, a system comprises sampling means for sampling radio frequency (RF) signals on a first channel for a first predetermined period and a second predetermined period that is subsequent to the first predetermined period. The system comprises counter means for incrementing first and second counts when the samples collected during the first and second predetermined periods reverse polarity, respectively. The system further comprises frequency characteristic means for determining a frequency of the RF signal based on at least one of the first and the second counts and determining frequency variation of the RF signal based on the first and second counts. At least one of the first and second counts is equal to the frequency. 
     In another feature, the frequency characteristic means compares the first and second counts to determine the frequency variation. The frequency characteristic means determines that the RF signal is tone radar when the first and second counts are approximately equal. 
     In another feature, the sampling means samples the RF signals for a third predetermined period that is subsequent to the second predetermined period and wherein the counter means increments a third count when the samples collected during the third predetermined period reverse polarity. 
     In another feature, the system further comprises derivative means for determining a first difference between the first and second counts and a second difference between the second and third counts. The derivative means determines a third difference between the first and second differences. The derivative means determines the first and second differences when the first, second, and third counts are greater than a predetermined threshold. 
     In another feature, the frequency characteristic means determines that the RF signal is chirp radar when the third difference is less than a predetermined threshold. The predetermined threshold is approximately zero. 
     In another feature, the frequency characteristic means determines that the RF signal is chirp radar when the third difference is approximately zero. 
     In another feature, the system further comprises radar means for determines whether the RF signal is one of chirp radar and tone radar based on the frequency variation. The radar means determines that the RF signal is chirp radar when the frequency variation is linear. The radar means determines that the RF signal is tone radar when the frequency variation is approximately zero. 
     In another feature, the system further comprises dynamic frequency selection (DFS) means for communicates with the radar means and selecting a second channel having a different frequency than the first channel when the radar means determines that the RF signal is one of chirp radar and tone radar. 
     In another feature, at least one of the samples is disregarded and excluded from the first and second counts when an absolute value of the at least one of the samples is less than a predetermined threshold. 
     In another feature, the predetermined threshold is approximately 0.1. 
     In another feature, the first and second periods are adjacent. 
     In another feature, the first, second, and third periods are adjacent. 
     In another feature, a wireless network device comprises the system. 
     In another feature, a radar detection device comprises the system. 
     In still other features, a computer program executed by a processor comprises sampling radio frequency (RF) signals on a first channel for a first predetermined period and a second predetermined period that is subsequent to the first predetermined period, incrementing first and second counts when the samples collected during the first and second predetermined periods reverse polarity, respectively, determining a frequency of the RF signal based on at least one of the first and the second counts, and determining frequency variation of the RF signal based on the first and second counts. At least one of the first and second counts is equal to the frequency. 
     In another feature, the computer program further comprises comparing the first and second counts to determine the frequency variation. The computer program further comprises determining that the RF signal is tone radar when the first and second counts are approximately equal. 
     In another feature, the computer program further comprises sampling the RF signals for a third predetermined period that is subsequent to the second predetermined period and incrementing a third count when the samples collected during the third predetermined period reverse polarity. 
     In another feature, the computer program further comprises determining a first difference between the first and second counts and a second difference between the second and third counts. The computer program further comprises determining a third difference between the first and second differences. 
     In another feature, the computer program further comprises determining the first and second differences when the first, second, and third counts are greater than a predetermined threshold. 
     In another feature, the computer program further comprises determining that the RF signal is chirp radar when the third difference is less than a predetermined threshold. The predetermined threshold is approximately zero. 
     In another feature, the computer program further comprises determining that the RF signal is chirp radar when the third difference is approximately zero. 
     In another feature, the computer program further comprises determining whether the RF signal is one of chirp radar and tone radar based on the frequency variation. The computer program further comprises determining that the RF signal is chirp radar when the frequency variation is linear. The computer program further comprises determining that the RF signal is tone radar when the frequency variation is approximately zero. 
     In another feature, the computer program further comprises selecting a second channel having a different frequency than the first channel when the RF signal is one of chirp radar and tone radar. 
     In another feature, the computer program further comprises disregarding and excluding at least one of the samples from the first and second counts when an absolute value of the at least one of the samples is less than a predetermined threshold. 
     In another feature, the predetermined threshold is approximately 0.1. 
     In another feature, the first and second periods are adjacent. 
     In another feature, the first, second, and third periods are adjacent. 
     In still other features, the systems and methods described above are implemented by a computer program executed by one or more processors. The computer program can reside on a computer readable medium such as but not limited to memory, non-volatile data storage and/or other suitable tangible storage mediums. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is functional block diagram of a wireless network operating in an infrastructure mode according to the prior art; 
         FIG. 2  is a function block diagram of a wireless network operating in an ad-hoc mode according to the prior art; 
         FIG. 3  a functional block diagram of an exemplary system for detecting radar and performing DFS in a wireless network according to the present disclosure; 
         FIG. 4  lists parameters of various exemplary short-pulse radar signals that may be used to detect radar and perform DFS; 
         FIG. 5A  depicts an exemplary response of AGC gain to a chirp radar pulse followed by a wireless data packet; 
         FIG. 5B  depicts exemplary samples of a chirp radar pulse and a wireless data packet received by an analog-to-digital converter (ADC) when automatic gain control (AGC) is off; 
         FIG. 6A  depicts an exemplary response of AGC gain to a burst of three radar pulses followed by a wireless data packet; 
         FIG. 6B  depicts an exemplary decrease and increase in AGC gain in response to a radar pulse; 
         FIG. 7  depicts an exemplary method for measuring frequency of a radar pulse; 
         FIG. 8  is a graph depicting data samples of a baseband signal versus time; 
         FIG. 9  is an exemplary functional block diagram of a frequency module of the system depicted in  FIG. 3 ; 
         FIG. 10  is an exemplary functional block diagram of a frequency characteristic module of the frequency module of  FIG. 9 ; 
         FIG. 11  is graph of number of zero-crossings in a bin as a function of number of bins when chirp radar pulses are centered at a center frequency of a frequency measuring device; 
         FIG. 12  is graph of number of zero-crossings in a bin as a function of number of bins when chirp radar pulses are not centered at a center frequency of a frequency measuring device; 
         FIG. 13  is a flowchart depicted exemplary steps taken by the frequency module of  FIG. 3 ; 
         FIG. 14  is a flowchart depicting exemplary steps taken by the frequency characteristic module of  FIG. 9 ; 
         FIG. 15A  is a functional block diagram of a high definition television; 
         FIG. 15B  is a functional block diagram of a cellular phone; 
         FIG. 15C  is a functional block diagram of a set top box; and 
         FIG. 15D  is a functional block diagram of a media player. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module, circuit and/or device refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     Dynamic frequency selection (DFS) is typically used to avoid interference between radar signals and wireless network communication systems operating in the 5 GHz band. More specifically, DFS is used to select a radar-free channel for wireless network communication from multiple non-overlapping channels in the 5.25-5.35 GHz and 5.47-5.725 GHz frequency ranges. 
     Referring now to  FIG. 3 , a system  40  for radar detection and DFS may include an automatic gain control (AGC) module  42 , a radar module  44 , a clear channel assessment (CCA) module  46 , an analog-to-digital converter (ADC) module  48 , a filter module  50 , and a dynamic frequency selection (DFS) module  52 . 
     The AGC module  42  provides a radio signal strength indicator (RSSI) measurement to the radar module  44 . Based on RSSI, the radar module  44  determines if a radio frequency (RF) signal is stronger than a predetermined threshold DFS th  such as −64 dBm. The CCA module  46  distinguishes legitimate wireless data packets from other signals and activates the radar module  44  when the RF signal is not a legitimate wireless data packet. The radar module  44  measures parameters of the RF signal such as pulse width, frequency, frequency variation, etc. More specifically, the radar module  44  may include a frequency module  53  that measures frequency and frequency variation of an RF signal. The DFS module  52  may compare the parameters measured by the radar module  44  with a set of parameters of known types of radar signals. The system  40  may switch to a different channel if the RF signal is a radar signal of a known type. 
     The system  40  may be implemented in a wireless network device  54  such as an access point or a client station. The wireless network device  54  typically includes an RF transceiver module  56 , a baseband processor (BBP) module  58 , and a medium access controller (MAC) module (or a control module)  60 . 
     The RF transceiver  56  receives RF signals. The BBP module  58  demodulates, digitizes, and filters the RF signal. The BBP module  58  may include the AGC module  42 , the ADC module  48 , and the filter module  50 . The control module  60  may include the radar module  44 , the CCA module  46 , and the DFS module  52 . 
     In some implementations, the radar module  44 , the CCA module  46 , and the DFS module  52  may be implemented in the BBP module  58  of the wireless network device  54 . In still other implementations, at least one of the modules may be implemented by firmware and/or software. Although shown separately for illustrative purposes, at least one of the modules shown in  FIG. 3  may be implemented using a single module. 
     Additionally, the system  40  may be implemented in any other devices and/or systems that may be used to detect radar. Although the disclosure explains how the system  40  may be used to detect chirp radar, a skilled artisan may use the system  40  to detect and measure frequency variations in signals other than radar. 
     Radar signals may be generally classified into three categories: short-pulse radar signals, long-pulse or chirp radar signals, and frequency-hopping radar signals. A table in  FIG. 4  lists sample parameters for four exemplary short-pulse radar signals. In a chirp radar signal, the frequency of the carrier is linearly varied within radar pulses. For example, a typical chirp radar signal may have a pulse width (PW) of 50-100 μS, a pulse repetition interval (PRI) of 1-2 mS, and a chirp width of 5-20 MHz. A typical frequency-hopping radar signal may have a PW of 1 μS, a PRI of 333 μS, and 9 pulses per hop. Parameter values of radar signals used in actual applications may vary. 
     Referring now to  FIGS. 5A-6B , a response of AGC gain to different types of radar signals is depicted.  FIG. 5A  depicts a response of AGC gain to a chirp radar pulse followed by a wireless data packet.  FIG. 5B  depicts exemplary samples of a chirp radar pulse  74  and a wireless data packet  72  received by the ADC module  48  when the AGC is off.  FIG. 5B  also depicts a variation  73  in the signal input to the ADC module  48 , when the AGC is off, corresponding to a varying frequency of chirp radar.  FIG. 6A  depicts a response of AGC gain to a burst of three radar pulses followed by a wireless data packet.  FIG. 6B  depicts in detail a decrease  78  and an increase  80  in AGC gain in response to a radar pulse. 
     When the RF transceiver  56  receives an RF signal, the gain of the AGC module  42  typically decreases to a value that is less than a normal value. The gain of the AGC module  42  may return to the normal value after a period of time. The time taken by the gain of the AGC module  42  to return to the normal value depends on various parameters of the RF signal such as signal strength, pulse width, frequency, etc. The AGC module  42  uses a radio signal strength indicator (RSSI) to communicate the strength of the RF signal to the radar module  44 . If RSSI exceeds a threshold value DFS th  such as −64 dBm, the radar module  44  may perform radar detection. 
     The CCA module  46  may determine whether the RF signal is a legitimate wireless data packet. A preamble in a legitimate wireless data packet includes a standard sequence. The CCA module  46  performs a correlation on the sequence in the preamble to determine whether the RF signal is a legitimate wireless data packet. The CCA module  46  uses a CCA signal to activate the radar module  44  when the RF signal is not a legitimate wireless data packet. Thus, the CCA module  46  may prevent false triggering of the radar module  44 . More specifically, the CCA module  46  may prevent the radar module  44  from performing radar detection and DFS when the RF signal is a legitimate wireless data packet. In addition, the CCA module  46  may prevent the radar module  44  from being falsely triggered by Bluetooth jammers. 
     The ADC module  48  converts the RF signal from an analog to a digital format. When the RF signal is no longer being received, the output of the ADC decreases to a low value. The radar module  44  monitors the output of the ADC module  48 . When the output of the ADC module  48  decreases below a predetermined threshold and remains below the predetermined threshold for a period of time, the radar module  44  detects an ADC under-run condition. The ADC under-run condition indicates an end of a pulse of the RF signal. The radar module  44  determines characteristics of the RF signal such as pulse width (PW), frequency, frequency variation, etc., based on the ADC under-run condition. 
     The filter module  50  typically includes a low-pass filter that filters the output of the ADC module  48 . The radar module  44  determines whether the RF signal is single tone radar or chirp radar based on the output of the filter module  50 . More specifically, the frequency module  53  determines the frequency and frequency variation of the RF signal. The frequency module  53  determines characteristics of the RF signal based on the frequency variation. The characteristics of the RF signal may be used to determine whether the RF signal is single tone radar or chirp radar. 
     The radar module  44  determines parameters of the RF signal such as pulse width, frequency, frequency characteristics (e.g., chirp frequency, single tone frequency, etc.), and pulse repetition interval (PRI). The DFS module  52  compares the parameters determined by the radar module  44  to the exemplary parameters shown in the table in  FIG. 4  to determine whether the RF signal is a radar signal of a known type. 
     When the signal strength of the RF signal exceeds DFS th  and when the CCA module  46  indicates that the RF signal is not a legitimate wireless data packet, the radar module  44  measures pulse width of every pulse of the RF. More specifically, the radar module  44  determines a beginning of a pulse based on the RSSI signal generated by the AGC module  42 . The RSSI signal indicates a beginning of a pulse when the AGC gain crosses the −64 dBm threshold. An end of a pulse is indicated by the ADC under-run condition detected by the radar module  44  at the end of every pulse. The radar module  44  calculates the pulse width of the pulse by counting a difference between the time of the beginning of the pulse and the time of the end of the pulse. 
     Additionally, after receiving the ADC under-run signal at the end of the pulse, the radar module  44  generates a signal to reset the gain of the AGC module  42  to the normal value. Unless reset, the gain of the AGC module  42  may take longer to return to the normal value, and incoming data during that time period may be lost. 
     Referring now to  FIG. 7 , the frequency module  53  measures the frequency of the RF signal to determine characteristics of the signal. As previously mention, the characteristics may be used to determine whether the RF signal is a single tone radar signal or a chirp radar signal. The frequency module  53  divides a baseband signal into bins of equal periods. The period of each bin is proportional to the resolution of the frequency measurement. In some implementations, the frequency module  53  may determine the frequency of the RF signal when the gain of the AGC module  42  decreases below a predetermined threshold DFS th  (typically −64 dBm). 
     The frequency module  53  determines the frequency of the RF signal for each bin. More specifically, the frequency module  53  counts zero-crossings for each bin to determine the frequency. By counting the number of zero-crossings, the frequency module  53  may utilize less resources than more complex methods that use Fourier transforms (e.g., DFT, FFT). 
     Referring now to  FIG. 8 , a graph depicting data samples of the baseband signal versus time is shown. Due to noise in the RF signal, the sampled data may have multiple zero-crossings like those generally depicted at  86  and  88 . However, the baseband signal may have only two true zero-crossings as shown in  FIG. 8 . Therefore, the frequency module  53  only counts zero-crossings of data samples that have an absolute value (ABS) greater than a predetermined ABS threshold. By counting data samples that have an absolute value greater than the ABS threshold, the frequency module  53  may count two zero-crossings rather than ten zero-crossings for the baseband signal shown in  FIG. 8 . In some implementations, the ABS threshold may be 0.1, although other thresholds may be used. 
     Referring now to  FIG. 9 , an exemplary implementation of the frequency module  53  is depicted. The frequency module  53  may include a sampling module  100 , an ABS module  102 , a polarity comparator  104 , a counter  106 , a timer  108 , a clock  110 , memory  112 , and a frequency characteristic module  114 . 
     The sampling module  100  may communicate with the ADC module  48 , the ABS module  102 , and the timer  108 . The polarity comparator  104  may communicate with the ABS module  102 , the counter  106 , and memory  112 . The counter  106  may communicate with the timer  108  and memory  112 . The clock  110  may communicate with the timer  108 . 
     The clock  110  may periodically set the timer  108  to count down from a predetermined time. The predetermined time may correspond with the period of the bin. While the timer  108  is counting down, the sampling module  100  may collect data samples from the ADC module  48 . The ABS module  102  may receive the samples from the sampling module  100  and discard samples that have an absolute value less than the ABS threshold. The polarity comparator  104  compares the remaining samples to a previous sample stored in memory  112 . If the polarity of the previous sample is opposite of the sample, a zero-crossing has occurred. When a zero-crossing occurs, the polarity comparator  104  directs the counter  106  to increment a count total and stores the sample in memory  112 . The counter  106  stores the count total in memory  112  when the timer  108  expires. The count total sampled during the predetermined time generally corresponds to the frequency of the RF signal for each bin. 
     The frequency characteristic module  114  may communicate with memory  112  and determine frequency characteristics of the count totals (or frequencies) stored in memory  112 . More specifically, the frequency characteristic module  114  compares the frequencies of each bin and determines whether the frequencies vary according to a predetermined pattern. For example, if the frequencies vary according to a linear pattern, the RF signal may be chirp radar. If the frequencies are substantially the same from bin to bin, the RF signal may be tone radar. 
     Referring now to  FIG. 10 , an exemplary implementation of the frequency characteristic module  114  is depicted. The frequency characteristic module  114  may include a tone module  150  and a chirp module  152 . The tone module  114  may determine whether the RF signal is tone radar, and the chirp module  152  may determine whether the RF signal is chirp radar. Although this example only includes the tone and chirp modules  150 ,  152 , other modules may be included to determine other frequency variation characteristics of the RF signal. 
     The tone module  150  may include a tone comparator  154 . The tone comparator  154  may compare the frequencies of each bin stored in memory  112 . If the frequencies are substantially the same, the tone module  150  may determine the RF signal to be tone. To determine whether the frequencies are substantially the same, the tone comparator  154  may also compare the variation of each bin to a tone threshold. 
     The chirp module  152  determines whether the frequency of the RF signal varies linearly, in which case the RF signal may be chirp radar.  FIG. 11  depicts evenly distributed zero-crossings where pulses of chirp radar are centered at a center frequency of a DFS-enabled device receiving the chirp radar signal. Chirp radar may include a down-chirp as generally depicted at  180  and an up-chirp as generally depicted at  182 . Since the down-chirp and up-chirp are linear in nature, the chirp module  152  may determine that the RF signal is chirp radar when the pattern is linear. 
       FIG. 12  depicts unevenly distributed zero crossings where pulses of chirp radar are not centered at the center frequency of the DFS-enabled device receiving the radar signal. As with  FIG. 11 , chirp radar that is not centered at the center frequency is also linear in nature. Down-chirps of the RF signal are generally depicted at  184  and up-chirps are generally depicted at  186 . 
     Referring back to  FIG. 10 , the chirp module  152  may include a high pass module  156 , a chirp comparator  158 , and a derivative module  160 . Bins that have a low frequency may be sensitive to random noise and are excluded when determining the frequency characteristics. Therefore, the high pass module  156  may be used to remove bins with a frequency less than a frequency threshold from consideration when determining the frequency variation. 
     The derivative module  160  may approximate a first and second derivative of the frequencies of each bin stored in memory  112 . The second derivative may be used to determine whether the frequency variation is linear. More specifically, if the second derivative is approximately zero, the frequency variation is linear. If the frequency variation is linear, the chirp module  152  may determine that the RF signal is chirp radar due to the linear characteristics of chirp radar. 
     To determine the first and second derivatives of the frequencies, the derivative module  160  may use difference equations. The first derivative may be determined with the following equation:
 
 d   i   =|z   i   −z   i+1 |
 
where d i  is the first derivative, z i  is the frequency in a current bin, and z i+1  is the frequency in the next bin. The second derivative may be determined with the following equation:
 
 s=|d   i   −d   i+1 |
 
where s is the second derivative, d i  is the first derivative of the frequency in a current bin, and d i+1  is the first derivative of the frequency in the next bin.
 
     The chirp comparator  158  determines whether the second derivative is approximately zero. More specifically, the chirp comparator  158  may compare the second derivative of the frequencies to a chirp threshold that is slightly greater than zero. If the second derivative is less than the chirp threshold, then the second derivative is approximately zero. If the second derivative is approximately zero, the chirp module  150  may determine RF signal to be chirp radar. 
     Referring now to  FIG. 13 , exemplary steps taken by the frequency module  53  to determine the frequency of the RF signal are generally depicted at  200 . The process starts in step  202  when the RF transceiver  56  receives the RF signal. In step  203 , the clock  110  sets the timer  108  to countdown from a predetermined time. In step  204 , the counter  106  is initialized to zero. The sampling module  100  collects data samples from the ADC module  48  in step  206 . 
     In step  208 , the ABS module  102  determines an absolute value of the samples. The ABS module  102  determines whether the absolute value of the sample is greater than the ABS threshold in step  210 . If the absolute value of the sample is greater than the ABS threshold, the polarity comparator  104  determines whether the polarity of the sample has transitioned in step  212 . If the polarity of the sample has transitioned, the counter  106  increments in step  214 . In step  216 , the frequency module  53  determines whether the timer  108  has expired. If the timer  108  has expired, the value that the counter  106  has incremented to is stored in memory  112  in step  218  and the process ends in step  220 . If the timer  108  has not expired, the process returns to step  206 . 
     If the ABS module  102  determines that the absolute value of the sample is not greater than the ABS threshold in step  210 , the ABS module  102  discards the sample in step  222 . The frequency module  53  determines whether the timer  108  has expired in step  216 . If the timer  108  has expired, the value that the counter  106  has incremented to is stored in memory  112  in step  218  and the process ends in step  220 . If the timer  108  has not expired, the process returns to step  206 . 
     If the polarity comparator  104  determines that the polarity of the sample has not transitioned in step  212 , the frequency module  53  determines whether the timer  108  has expired in step  216 . If the timer  108  has expired, the value that the counter  106  has incremented to is stored in memory  112  in step  218  and the process ends in step  220 . If the timer  108  has not expired, the process returns to step  206 . 
     Referring now to  FIG. 14 , exemplary steps taken by the frequency characteristic module  114  to determine whether the frequencies stored in memory vary according to a particular pattern are generally depicted at  250 . The process begins in step  252 . In step  253 , the frequency characteristic module  114  reads the frequency of a bin from memory  122 . In step  254 , the tone comparator  154  determines whether the frequencies are substantially the same from bin to bin. More specifically, the tone comparator  154  compares the frequency read from memory  122  to a frequency that was previously read from memory. If the frequencies from bin to bin are substantially the same, the tone module  150  may determine that the RF signal is tone radar in step  256  and the process ends in step  258 . 
     If the tone comparator  154  determines that the frequencies are not substantially the same from bin to bin in step  254 , the frequency comparator  156  determines whether the frequency is greater than a frequency threshold in step  260 . If the frequency is not greater than the frequency threshold, the frequency characteristic module  114  determines whether there are more bins in memory  112  in step  262 . If there are more bins in memory  112 , the frequency module  114  reads the frequency of the next bin stored in memory  112  in step  253 . 
     If the frequency comparator  156  determines that the frequency is greater than the frequency threshold, the derivative module  160  determines the first derivative in step  264  and the second derivative in step  266 . In step  268 , the chirp comparator  158  determines whether the second derivative is less than the chirp threshold. If the second derivative is less than the chirp threshold, the chirp module  152  determines that the RF signal is chirp radar in step  270  and the process ends in step  258 . If the second derivative is not less than the chirp threshold, the process ends in step  258 . 
     Referring now to  FIGS. 15A-15D , various exemplary implementations of the system  40  are shown. Referring now to  FIG. 15A , the system  40  can be implemented in signal processing and/or control circuits  422  of a high definition television (HDTV)  420 . The HDTV  420  receives HDTV input signals in either a wired or wireless format and generates HDTV output signals for a display  426 . In some implementations, signal processing circuit and/or control circuit  422  and/or other circuits (not shown) of the HDTV  420  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other type of HDTV processing that may be required. 
     The HDTV  420  may communicate with mass data storage  427  that stores data in a nonvolatile manner in devices such as optical and/or magnetic storage devices. The devices may include, for example, hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The HDTV  420  may be connected to memory  428  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The HDTV  420  also may support connections with a WLAN via a WLAN network interface  429 . 
     Referring now to  FIG. 15B , the system  40  can be implemented in signal processing and/or control circuits  452  of a cellular phone  450  that may include a cellular antenna  451 . In some implementations, the cellular phone  450  includes a microphone  456 , an audio output  458  such as a speaker and/or audio output jack, a display  460  and/or an input device  462  such as a keypad, pointing device, voice actuation and/or other input device. The signal processing and/or control circuits  452  and/or other circuits (not shown) in the cellular phone  450  may process data, perform coding and/or encryption, perform calculations, format data and/or perform other cellular phone functions. 
     The cellular phone  450  may communicate with mass data storage  464  that stores data in a nonvolatile manner in devices such as optical and/or magnetic storage devices. The devices may include, for example, hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The cellular phone  450  may be connected to memory  466  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The cellular phone  450  also may support connections with a WLAN via a WLAN network interface  468 . 
     Referring now to  FIG. 15C , the system  40  can be implemented in signal processing and/or control circuits  484  of a set top box  480 . The set top box  480  receives signals from a source such as a broadband source and outputs standard and/or high definition audio/video signals suitable for a display  488  such as a television and/or monitor and/or other video and/or audio output devices. The signal processing and/or control circuits  484  and/or other circuits (not shown) of the set top box  480  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other set top box function. 
     The set top box  480  may communicate with mass data storage  490  that stores data in a nonvolatile manner. The mass data storage  490  may include optical and/or magnetic storage devices such as hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The set top box  480  may be connected to memory  494  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The set top box  480  also may support connections with a WLAN via a WLAN network interface  496 . 
     Referring now to  FIG. 15D , the system  40  can be implemented in signal processing and/or control circuits  504  of a media player  500 . In some implementations, the media player  500  includes a display  507  and/or a user input  508  such as a keypad, touchpad and the like. In some implementations, the media player  500  may employ a graphical user interface (GUI) that typically employs menus, drop down menus, icons and/or a point-and-click interface via the display  507  and/or user input  508 . The media player  500  further includes an audio output  509  such as a speaker and/or audio output jack. The signal processing and/or control circuits  504  and/or other circuits (not shown) of the media player  500  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other media player function. 
     The media player  500  may communicate with mass data storage  510  that stores data such as compressed audio and/or video content in a nonvolatile manner. In some implementations, the compressed audio files include files that are compliant with MP3 format or other suitable compressed audio and/or video formats. The mass data storage may include optical and/or magnetic storage devices such as hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The media player  500  may be connected to memory  514  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The media player  500  also may support connections with a WLAN via a WLAN network interface  516 . Still other implementations in addition to those described above are contemplated. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.