Patent Publication Number: US-8111783-B2

Title: Capturing and using radio events

Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention relate to capturing radio events that can be used to augment or optimize spectrum analysis of, for example, a wireless frequency channel. 
     BACKGROUND OF THE INVENTION 
     The explosive growth in wireless applications and devices over the past few years has produced tremendous public interest benefits. Wireless networks and devices have been deployed in millions of offices, homes, and more recently, in increasing numbers of public areas. These wireless deployments are forecast to continue at an exciting pace and offer the promise of increased convenience and productivity. 
     This growth, which is taking place mostly in the unlicensed bands, is not without its downsides. In the United States, the unlicensed bands established by the Federal Communications Commission (FCC) consist of large portions of spectrum at 2.4 GHz and at 5 GHz, which are free to use. The FCC currently sets requirements for the unlicensed bands such as limits on transmit power spectral density and limits on antenna gain. It is well recognized that as unlicensed band devices become more popular and their density in a given area increases, efficient use of those bands may suffer. 
     Compounding general increased usage is the fact that the types of signaling protocols used by devices in the unlicensed bands are not designed to cooperate with signals of other types also operating in the bands. For example, a frequency hopping signal (e.g., a signal emitted from a device that uses the Bluetooth™ communication protocol or a signal emitted from certain cordless phones) may hop into the frequency channel of an IEEE 802.11 wireless local area network (WLAN), causing interference with operation of the WLAN. Spectrum analysis of a wireless channel can provide helpful insights into potentially interfering signals or energy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system having a traffic monitoring section and spectrum monitoring section; 
         FIG. 2  is a block diagram of a system including a spectrum analysis engine used to plot and classify pulses detected in a selected frequency band in accordance with an embodiment of the present invention; 
         FIG. 3  is a timing diagram showing relationships between fast Fourier transform (FFT) blocks and detected pulses; 
         FIG. 4  shows a timing diagram that includes monitored radio events for a radio event record in accordance with an embodiment of the present invention; 
         FIG. 5  shows an example of a radio event record in accordance with an embodiment of the present invention; 
         FIG. 6  is a block diagram of a system including a spectrum analysis engine and wireless demodulator using a shared radio in accordance with an embodiment of the present invention; 
         FIG. 7  is a timing diagram showing protocol-based radio events in accordance with an embodiment of the present invention; 
         FIG. 8  shows another example of a radio event record including protocol-based radio events in accordance with an embodiment of the present invention; 
         FIG. 9  is a timing diagram that illustrates when raw in-phase/quadrature (I/Q) sample data is preferably collected, and not collected, in accordance with an embodiment of the present invention; 
         FIG. 10  is a state diagram that illustrates when I/Q samples are collected, and not collected, in accordance with an embodiment of the present invention; and 
         FIG. 11  is a flowchart depicting a pulse post processing methodology in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     Embodiments of the present invention provide systems and methods for reducing processing in a spectrum analysis system having a radio, an optional demodulator, and a spectrum analysis module. A method includes collecting in-phase/quadrature (I/Q) data representing energy detected by the radio in a frequency band, performing a fast Fourier transform (FFT) on the I/Q data resulting in a stream of FFT blocks, identifying, using the stream of FFT blocks, a pulse in the frequency band, and recording in a radio events record (RER) a plurality of radio events and corresponding timestamps that are indicative, respectively, of a type and time of individual state changes in the radio during the collecting step. With such an RER, it is then possible to, for example, more accurately identify a beginning time and an end time of the pulse, even where the beginning time or end time might fall within boundaries of a given FFT block. 
     Examples of radio events that may be recorded and used in connection with embodiments of the present invention include a gain change associated with an automatic gain control (AGC) loop, a change in a received signal strength indicator (RSSI), and a transmit on/off signal, as well as protocol-based events, such as detection of a packet preamble, a PHY (physical layer) header and/or a passed media access control cyclic redundancy check (MAC CRC). 
     Description of Example Embodiments 
     To obtain insight into a wireless channel or frequency band, a spectrum analysis engine (also referred to as “SAgE”) may be employed. A sensor that includes a SAgE includes, as shown in  FIG. 1 , a spectrum monitoring section  100  to monitor RF activity in the frequency band and a traffic monitoring section  170  that is capable of sending and receiving traffic according to a communication protocol, such as an IEEE 802.11 WLAN protocol. The spectrum monitoring section  100  comprises a radio  110  (primarily for receive operations) that is capable of tuning to receive energy at each channel (or simultaneously all channels in a wideband mode) of, for example, any of the unlicensed bands (2.4 GHz and 5 GHz) in which IEEE 802.11 WLANs operate. An analog-to-digital converter (ADC)  112  is coupled to the radio  110  and converts the downconverted signals from the radio  110  to digital signals. A radio interface (I/F)  120  is coupled directly to the radio  110  and also to the output of the ADC  112 . A spectrum analysis engine (SAGE)  130  is coupled to the radio I/F  120 . In one embodiment, the SAGE  130  includes a spectrum analyzer  132 , a signal detector  134  having a peak detector  136  and one or more pulse detectors  138 , and a snapshot buffer (SB)  140 . A Fast Fourier Transform (FFT) module  133  is coupled between the radio I/F  120  and the spectrum analyzer  132 , or, as shown in  FIG. 1 , included in the spectrum analyzer  132 . The SAGE  130  generates spectrum activity information that is used to determine the types of signals occurring in the frequency band, and captures signals for location measurement operations. A dual port random access memory (RAM)  150  is coupled to receive the output of the SAGE  130  and a processor I/F  160  interfaces data output by the SAGE  130  to a processor  192 , and couples configuration information from the processor  192  to the SAGE  130 . A memory  194  is accessible to the processor  192  for storage of data and program instructions. 
     The functions of the SAGE  130  will be briefly described in further detail hereinafter. The spectrum analyzer  132  generates data representing a real-time spectrogram of a bandwidth of radio frequency (RF) spectrum, such as, for example, up to 100 MHz. The spectrum analyzer  132  may be used to monitor all activity in a frequency band, for example, the 2.4-2.483 GHz ISM band, or the 5.15-5.35 GHz and 5.725-5.825 GHz bands. The FFT module  133  is, for example, a 256 frequency bin FFT processor that provides (I and Q) FFT data for each of 256 frequency bins that span different dwells that span the bandwidth of a frequency band of interest. A spectrum correction block may be included to correct for I and Q channel imbalance by estimating an I-Q channel imbalance parameter related to phase error and amplitude offset between the I and Q channels, and to suppress a side tone resulting from the RF downconversion process. The spectrum analyzer  132  may further comprise a power computation block to output a power value for each FFT frequency bin. The spectrum analyzer  132  may further include a statistics logic block (not shown) that has logic to accumulate statistics for power, duty cycle, maximum power and a peaks histogram. Statistics are accumulated in the dual-port RAM  150  over successive FFT time intervals or blocks. As mentioned, an FFT block may comprise 256 samples (in this case), and thus a “time interval” is equivalent to 256/40 (MHz)=6.4 μs. After a certain number of FFT intervals, determined by a configurable value stored in control registers (not shown), an interrupt is generated to output the statistics from the dual-port RAM  150 . For example, the statistics are maintained in the dual-port RAM  150  for 10,000 FFT intervals before the processor reads out the values. The power versus frequency data generated by the spectrum analyzer  132  may also be used as input to the signal detector. 
     The signal detector  134  detects signal pulses in the frequency band and outputs pulse event information entries, which include one or more of the start time, duration, power, center frequency and bandwidth of each pulse that satisfies configurable pulse characteristic criteria associated with a corresponding pulse detector. 
     In the signal detector  134 , the peak detector  136  looks for spectral peaks in the (power versus frequency data derived from FFT block output), and reports the bandwidth, center frequency and power for each detected peak. The output of the peak detector  136  is one or more peaks and related information. The pulse detectors  138  detect and characterize signal pulses based on input from the peak detector  136 . 
     The snapshot buffer  140  collects a set of raw digital signal samples useful for signal classification and other purposes, such as location measurements. The snapshot buffer  140  can be triggered to begin sample collection from either the signal detector  134  or from an external trigger source, such as a signal from the processor to capture received signal data for a period of time sufficient to include a series of signal exchanges used for location processing explained hereinafter. Alternatively, the snapshot buffer  140  can be in a free-running state continuously storing captured data, and then in response to detecting the first signal (e.g., a Probe Request frame), the snapshot buffer  140  is put into a post-store mode that extends long enough to capture an ACK frame signal data. 
     The traffic monitoring section  170  monitors packet activity in a wireless network, e.g., a WLAN, and sends and receives certain packets that are used for location measurement processes. For example, the system may transmit an 802.11 Probe Request frame, data frame or request-to-send frame that may be addressed to the device to be located. It should also be understood that traffic monitoring section  170  could also function as a WiFi access point, and not merely operate as an “outside observer.” Included in the traffic monitoring section  170  are a radio transceiver  172  (comprising a transmitter Tx and a receiver Rx) and a baseband signal processor  178 . The radio transceiver  172  and baseband signal processor  178  may be part of a package chipset available on the market today, such as an 802.11 WLAN chipset for any one or more of the 802.11a/b/g/n or other WLAN communication standards. The baseband signal processor  178  is capable of performing the baseband modulation, demodulation and other physical (PHY) layer functions compliant with the one or more communication standards of interest (e.g., IEEE 802.11a,b,g,h, etc.). An I/F  180  couples the baseband signal processor  178  and radio transceiver  172  to the processor  192 . 
     While the system of  FIG. 1  provides significant capabilities, there are ways in accordance with the present invention to enhance spectrum analysis functionality and efficiency. 
       FIG. 2  shows a block diagram of system  200  that provides spectrum analysis with respect to wireless channels as described substantially above. More specifically,  FIG. 2  depicts a radio frequency (RF) receiver  202  that receives over-the-air transmissions, and feeds the received signals to an analog-to-digital converter (ADC)  204  for digitization. The resulting digitized signals are filtered with filters  206  and then passed to spectrum analysis module  210  for simultaneous processing. Spectrum analysis module  210  may be configured, for example, similarly to spectrum monitoring section  100  of  FIG. 1 . Of course, other configurations may be employed. 
     Further, in a preferred embodiment, a plots module  212  and a classification module  214  are in communication with spectrum analysis module  210  and are provided, respectively, to provide a visual representation of the spectrum analysis on a display for a user to view, and to provide an indication of the types of signals that are present in a given band that is being monitored by the spectrum analysis module  210 . 
     Also shown in  FIG. 2  is an automatic gain control (AGC) block  220  and loop  221  that feeds a gain level circuit or module  222  in RF receiver  202 . As is well-known in the art, AGC is an adaptive function that monitors an output signal and provides feedback to adjust the gain to an appropriate level for a range of input signal levels. In other words, AGC may be used to ensure that, e.g., the analog-to-digital converter (ADC)  204  is not saturated or under driven. 
     As mentioned earlier, the SAgE or spectrum analysis module  210  relies to a large extent on an FFT-based approach to spectrum analysis and signal classification. The FFT engine  133  (shown in  FIG. 1 ) within spectrum analysis module  210  computes a spectrogram which is used for display and/or to identify pulses in both time and frequency domains. Pulse identification can be effective in aiding post-processing of the original input I/Q data. 
     In accordance with one implementation of an embodiment of the present invention, FFTs are computed on a block of samples (N FFT ). An FFT based approach provides frequency resolution at the expense of time resolution (they both depend on the size of the FFT block, where larger FFT block lengths result in finer frequency resolution but poorer time resolution). For example, with a sampling rate of 20 MHz:
 
For  N   FFT =256, frequency resolution=78.125 kHz (20/256), and time resolution=12.8 us; and
 
For  N   FFT =128, frequency resolution=156.25 kHz (20/128), time resolution=6.4 us.
 
     Increasing the FFT size improves resolution, but pulses shorter than an FFT block duration will not be detected, as shown for pulse  302  in  FIG. 3 . Furthermore, pulses with short gaps (1-3 FFTs wide) may be hard to distinguish. 
     Further still, and as also shown in  FIG. 3 , using FFT-based engines for signal detection may also result in loss of resolution on start-time and end-time of pulses. For instance, pulse  304  of  FIG. 3  starts and ends within or inside the boundaries of successive FFT blocks, and thus the pulse&#39;s start and end points do not coincide with the end points or boundaries of the respective FFT blocks. As such, precise pulse start and end points of given pulses may be hard to identify and catalog based only on the output of an FFT engine. 
     Furthermore, as explained above, to compensate for input signal level variations AGC loop  221  is used. Typically, such a loop  221  operates “fast.” Accordingly, it may be important to know when AGC changes occur so as to be able to better interpret the original I/Q capture data in that such AGC changes can detrimentally impact the usefulness of an FFT block coinciding in time with an AGC change. 
     Thus, to address the foregoing, among other things, and in accordance with embodiments of the present invention, a Radio Events Record (RER) is provided to capture state changes of the AGC, among other things, along with corresponding timestamps and indices into the capture stream(s). The RER information is then used to supplement the FFT data in an effort to reduce the processing time of the I/Q data and provide more precise and efficient analysis. 
     With reference now to  FIG. 4 , there is shown a timing diagram including an input signal  402 , comprising individual pulses that changes over time, and a gain setting signal  404  that represents, e.g., a signal provided by AGC loop  221  of  FIG. 2 . A gain change signal  406  is represented by arrows  406   a  and  406   b , which correspond to transition points of the gain setting signal  404 . Element  412  of  FIG. 4  represents a series of timestamps (or markers) that are used to keep track of a precise time at which, e.g., gain changes  406   a  and  406   b  have occurred. In accordance with the present invention, a Radio Events Record (RER) is generated like that shown in  FIG. 5 . For each gain change that the radio executes, a corresponding entry is made in the RER table. The RER table may be implemented in an RER module  230 , as shown in  FIG. 2  that is in communication with AGC module  220 . Spectrum analysis can thus be augmented with these radio events by, e.g., enabling analysis algorithms to more quickly “zero in” on I/Q data that corresponds in time with pulse events of the monitored spectrum. For instance, it may be assumed that a pulse event is bounded, substantially, in time by the timestamps of two successive gain change events. Accordingly, only a subset of the captured I/Q data need be analyzed to identify/classify a given pulse. 
     Similarly, a radio typically monitors measured RSSI  408 . In accordance with the present invention, an RSSI change signal  410  may thus also be generated. Typically, such a signal would be generated when the absolute value of an RSSI change is more than a predetermined value. Thus, as shown in  FIG. 4 , arrows  410   a ,  410   b  and  410   c  represent moments in time when the RSSI has changed more than a predetermined value or threshold. These RSSI change events are likewise recorded along with a corresponding timestamp in the RER table of  FIG. 5  and used to augment, as desired, the spectral analysis functionality of the system. 
     Again with reference to  FIG. 4 , the incoming I/Q sample data  414  is shown corresponding in time with, e.g., the input signal power  402 . The output of the FFT engine, namely FFT output blocks  416 , however, is delayed somewhat due to processing time. Finally, detected pulses  418  that are characterized with further processing are shifted still further in time from the FFT output blocks  416 . By leveraging the recorded events in the RER, it is possible to even better identify the precise timing, in the time domain, of a given pulse and thereby improve overall classification functionality. 
     Thus, in accordance with a first aspect of the present invention, the following information may be captured in an RER like that shown in  FIG. 5 :
         Gain change events   RSSI change events—RSSI increased or decreased by an amount larger than a programmable threshold from previously recorded RSSI values.   Timestamps   Gain state information (For examples, AGC state, gain etc.).   Index into I/Q data with gaps such as during gain change.       

       FIG. 6  is a block diagram depicting an embodiment of the present invention in which a single RF receiver  202  is shared between the spectrum analysis module  210  and a wireless demodulator  208 . More specifically, and as explained with respect to  FIG. 6 , the RF receiver  202  receives over-the-air transmissions and feeds the received signals to an analog-to-digital converter (ADC)  204  for digitization. The resulting digitized signals are filtered with filters  206  and then passed, on the one hand, to the demodulator  208 , such an 801.11x demodulator and, on the other hand, to the spectrum analysis module  210  for simultaneous processing. Demodulator  208  is used as a traffic monitoring section (or as part of a WiFi access point), like that shown in  FIG. 1 . Those skilled in the art will appreciate that demodulator  208  may be configured to monitor other types of traffic including Bluetooth™ communications, among others. RER module  230 , in this embodiment, is in communication not only with AGC  220 , but also with demodulator  208 . 
     There are several benefits to having an RER in connection with the architecture of  FIG. 6  as well. For example, the state of the demodulator  208 , or modem, may provide helpful information for spectrum analysis, particularly if the SAgE is designed to identify, e.g., non-802.11 signals. 
     More specifically, during a radio transmit operation, the receive stream may be useless or unavailable, based on the radio design. As such, data (such as I/Q samples) collected during this time, may be of no practical use. Similarly, if a complete frame of, e.g., an 802.11 protocol is successfully received, that frame or pulse can usually be excluded from further spectrum analysis (to reduce processing overhead), as the classification of such a pulse is readily identifiable. 
     Further, depending on the desired “resolution” or degree of granularity of the radio event data, and as shown in  FIG. 7 , in addition to recording in an RER gain change and RSSI changes, several additional protocol-based events may be recorded, in accordance with a second aspect of the present invention. For example, protocol detection might be further decomposed into various receive states such as sync correlation (not shown), preamble detection, PHY header detection, address matching (not shown), or valid MAC CRC detection. As before, a timestamp is preferably associated with each of the foregoing protocol-based events and, together, stored in an RER table.  FIG. 8  depicts an RER table that includes not only the gain and RSSI changes, but also the protocol-based events. All of this information can then be used to augment or improve spectral analysis post processing. 
     One of the more efficient uses of the protocol-based events is to employ them to suppress “unnecessary” I/Q data capture. For example, with, e.g., a successful receipt of a PHY header, it may be assumed that the entirety of the remainder of the pulse associated with that PHY header contains I/Q data associated with an 802.11 frame. Accordingly, there may be no need to further burden the data bus and associated memory of the system with data that need not be further analyzed. 
     Alternatively, where the RER data indicates that a CRC has not passed, then such data may be used as a trigger to initiate post processing of the related I/Q data in an effort to identify a root cause of the failure. 
     When raw I/Q samples are captured for spectrum analysis purposes, they can also be used to augment the primary protocol. For example, as is explained more fully later herein, Dynamic Frequency Selection (DFS) radar detection may be improved, Root Cause analysis on bad packets may be more easily implemented, and improved timing sync between access points may be realized, among other augmentation benefits. 
       FIG. 9  shows a timing diagram depicting how when a good packet header (of, e.g., an 802.11 frame) is detected, the capture of raw I/Q data may optionally be eliminated. This contrasts with the case wherein a bad packet is detected or no packet is detected at all, as also shown in  FIG. 9 . In the two latter cases, it is preferable to buffer the raw I/Q data samples and perform spectrum analysis of the relevant time periods to determine what energy may have been present, generally, or what energy may have interfered with an expected protocol frame. Of course, the system and methodology of the present invention can also be configured to capture or collect I/Q samples whatever the state of the modem. 
       FIG. 10  depicts a state machine that may be implemented to suppress buffering of “unnecessary” I/Q data and thereby optimize bus and memory utilization. As shown in the figure, a default state might be to buffer I/Q samples. Once, e.g., an 802.11 header is detected, the buffering of the sample can be discontinued until such time as, e.g., the 802.11 packet has ended. Since the several components of an 802.11 packet are well defined (e.g., PHY header through CRC), the length of time of a pulse associated with a given 802.11 packet can be determined. Thus, sample buffering can be discontinued for an amount of time that corresponds to the length of time to receive an entire packet. If the current dwell time (a predefined duration over which data is monitored by spectrum analysis module  210 ) has ended, then sample buffering may also be stopped. 
     The original 802.11 specification defines 20 MHz wide channels, but the new 802.11n specification also defines a new 40 MHz wide channel. In 40 MHz mode, it is possible that the RF receiver  202  will receive a 20 MHz wide signal (either on the lower-end or upper-end of the spectrum). In this case it is possible to buffer samples for only the portion of the spectrum where 802.11 signal is not present (again to save memory and bus bandwidth). This may be accomplished by implementing two decimation filters (not shown), wherein one filter selects the lower-half of the spectrum while the other filter selects the upper-half of the spectrum. 
       FIG. 11  is a flowchart depicting a post process methodology in accordance with an embodiment of the present invention. At step  1102 , a listing of all pulses is prepared. At step  1104 , it is determined, for each pulse, whether an 802.11 header has been detected. If yes, it is then determined, at step  1106 , whether a valid 802.11 CRC has been detected. If yes, then the given pulse is discarded, as indicated by step  1108 . In other words, if the pulse is determined to be associated with a valid 802.11 packet, then there is no need to conduct further spectrum analysis processing to identify the pulse. Consequently, the pulse may be discarded. The process proceeds to step  1114 , explained below. 
     However, if at step  1104  a header was not detected, the nature of the pulse may be unknown and interference analysis for the pulse is performed as indicated by step  1112 . Step  1114  then provides a loop back to perform analysis of a next pulse in the list. If, at step  1104 , a header was detected, but at step  1106  a valid CRC was not detected, then the methodology preferably undertakes a root cause analysis of the failure, as indicated by step  1110 . A root cause may be, for example, a collision, or a false header detect, among others. The process then moves to step  1114 . 
     Thus, as will be understood by those skilled in the art, by leveraging radio events including AGC and RSSI changes as well as protocol-based radio events, it is possible to more quickly process FFT-derived pulse data and “zero in” on other pulses for which further analysis may be beneficial. 
     From the foregoing, those skilled in the art will appreciate the utility of leveraging a Radio Events Record in connection with spectrum analysis. The following table provides a more comprehensive, but non-limiting, list of radio events and the types of records that may be recorded in an RER table in accordance with embodiments of the present invention. A more detailed discussion of each of the radio events and their respective utility in an overall spectrum analysis system follows. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Radio Event 
                 Records written 
               
               
                   
                   
               
             
            
               
                   
                 Gain change on antenna n 
                 Timestamp and 
               
               
                   
                 (n = 1, 2, . . . ). 
                 index into SB data 
               
               
                   
                   
                 Gain value 
               
               
                   
                   
                 Noise floor level 
               
               
                   
                   
                 RSSI value 
               
               
                   
                 Rx signal on/off indicator 
                 Timestamp and 
               
               
                   
                 for each antenna 
                 index into SB data 
               
               
                   
                   
                 Gain value 
               
               
                   
                   
                 Noise floor level 
               
               
                   
                   
                 RSSI value 
               
               
                   
                 RSSI change 
                 Timestamp and 
               
               
                   
                   
                 index into SB data 
               
               
                   
                   
                 RSSI value 
               
               
                   
                 Transmit events 
                 Timestamp and 
               
               
                   
                   
                 index into SB data 
               
               
                   
                   
                 Transmit on/off 
               
               
                   
                   
                 signal 
               
               
                   
                 802.11 Rx events 
                 Timestamp and 
               
               
                   
                   
                 index into SB data 
               
               
                   
                   
                 802.11 header detect 
               
               
                   
                   
                 (on partial spectrum 
               
               
                   
                   
                 or full-spectrum) 
               
               
                   
                   
                 802.11 CRC valid 
               
               
                   
                   
                 MAC trigger 
               
               
                   
                   
               
            
           
         
       
     
     Gain Change 
     As is well known in the art, independent AGC loops are typically implemented on each of a plurality of antennas. Thus, radio events may be recorded with respect to each antenna. However, in many cases, it is possible to monitor and record events that are associated with only a single one of the antennas for purposes of the instant invention. Of course, specific circumstances will dictate the appropriate implementation. 
     For a given gain change there is an associated settling time. In accordance with an embodiment of the present invention, I/Q samples captured during the settling time can be discarded or never even buffered. In one implementation, the last valid I/Q sample based on the older gain value is referenced in one RER record and the first valid I/Q sample based on the new gain value is similarly referenced by another RER record. I/Q samples between the markings (i.e., during the gain settling time) may be discarded and marked as invalid. 
     The performance of some detection algorithms requires knowledge of noise floor. For example, a pulse detection circuit uses the noise floor information to detect pulses that are a certain threshold above the noise floor. The noise floor of the receiver depends on the gain value. Accordingly, when the gain changes, I/Q samples must be correctly associated with the corresponding noise floor. The information collected in an RER table can facilitate this association. 
     For FFT processing in, e.g., spectrum analysis module  210 , an FFT block should be marked valid only if the I/Q samples in the FFT block are valid and correspond to a unique gain value. That is, samples taken during a gain change may be of limited use in connection with spectrum analysis. Thus, keeping track of the timing of gain change events in an RER table can improve overall spectrum analysis by eliminating potentially problematic FFT blocks. From a practical standpoint, such FFT blocks are preferably dropped by hardware since, in one current implementation, the statistics logic block (not shown) of the spectrum analysis module  210  is configured to generate data aggregated over a stream of multiple FFT&#39;s, and thus if suspect FFT&#39;s or FFT&#39;s from different gain settings part of data set, the results may be tainted. 
     Further, to save bus bandwidth and memory (e.g., snapshot buffer  140 ), hardware may be designed/configured to suppress writing I/Q samples during a gain transition. More specifically, a gain change event may trigger the hardware to cease I/Q sample buffering, since data collected during the transition may be suspect. 
     It is further possible, in accordance with the present invention, using differences in timestamps between gain change events to determine the duration of a given pulse. Since FFT based spectrum analysis may not provide accurate pulse duration (see, e.g.,  FIG. 3  and related text), the pulse durations derived from gain changes may be used for signal classification. One practical example of a use for this feature of the present invention is for Dynamic Frequency Selection (DFS) radar detection. 
     Specific to 802.11, there are requirements to avoid operating in the same spectrum as DFS radar. On some channels in some countries, Radar is designated as the primary user of the spectrum and 802.11 is relegated to being a secondary user that must not interfere. DFS radar signals have varying pulse widths and pulse repetition intervals, and are usually received in conjunction with other signals (e.g., 802.11 or other sources). A histogram of pulse widths developed with the data in the RER can be used to group pulses with similar duration for subsequent analysis by the classification module  214 , thereby facilitating DFS radar detection. 
     Thus, the RER of the present invention, in combination with the spectrum analysis engine (SAgE)  210 , provides a general purpose solution to DFS detection, rather than a narrow point solution. With spectrum analysis alone, it is difficult to precisely identify a pulse width of a given pulse or a pulse repetition rate. However, by leveraging I/Q samples (using the RER), it is possible in accordance with the present invention to achieve better DFS (or other non-802.11 pulse) detection. For example, software DSP algorithms can be used to analyze data to improve detection and reduce false alarms. Typically, such algorithms may be difficult/expensive to implement as hardware logic. 
     Another use for the record of gain changes and timestamps is to provide accurate start times and samples offsets for various signals for post-processing of I/Q data. For example, the data may be employed to better find a synchronization field of a known protocol (e.g., 802.11). Deriving an accurate estimate of the start time using data in an RER table reduces the search window (and hence the processing overhead) to find a synchronization word. 
     Receiver Signal On/Off Indicator 
     The receiver  202  may be temporarily turned off for various reasons such as tuning a synthesizer, resetting AGC loops, power save, etc. These on/off events may also be recorded in an RER table for at least the reasons indicated with respect to gain change events. As well, the receiver signal off event may be used by hardware as a trigger to write records from a previous receive burst and to clear hardware state machines to process a subsequent burst. 
     RSSI Changes 
     Receive signal levels could change for a short burst without causing gain changes (due, e.g., to hysteresis in the AGC loops  221 ). RSSI changes may thus also be recorded since RSSI changes can provide accurate timestamps for the start and end of a pulse. RSSI changes may also be recorded when the gain changes as well. The following processing is associated with instances of RSSI changes. 
     A sudden jump in signal levels can cause spectral splatter in FFT outputs. Using an RER table to “mark” FFTs with signal transitions can prevent spectral splatter from corrupting spectrum plots. 
     RSSI change events can also be used to identify start and end of pulses (in the same fashion described above for gain change events). Additionally, it is not uncommon for short pulses to overlap with longer pulses in time, and RSSI change event records can be used to identify such short pulses. 
     Further, signals using Time-Domain Duplex (TDD) can cause the AGC loop  221  to attack each burst. Depending on the duration of the pulses and the gap between pulses, this may cause too many gain changes (which is detrimental to spectrum analysis for the reasons noted elsewhere herein). One approach to address this phenomenon is to put hysteresis in the AGC to avoid gain changes. Where such hysteresis is implemented, RSSI records can be used as explained above to record, more accurately, the start and end of the pulses. Accurate start and end time of pulses also helps to reduce processing overhead when I/Q samples are post-processed to find the synchronization word, as mentioned earlier in connection with gain change events. 
     Similar to the discussion for using gain changes to detect DFS radar, RSSI records can be used in conjunction with gain records to identify pulse widths for radar detection. DFS radar requirements list signal levels above which 802.11 radios must vacate the channel on detection of radar. Since the RSSI values are captured for these pulses they provide an easy filter for radar identification. A big challenge for radar detection is avoidance of false detect on off-channel signals. Detectors based purely on pulse widths and RSSI levels can easily provide false triggers on strong off-channel radar or other signals with similar characteristics. For example, an off-channel signal at −20 dBm may be suppressed by 40 dB by a receive filter but it might still be above the DFS threshold. However, the measured RSSI value (−60 dBm) will not be consistent with the gain value—front-end gains (e.g., LNA) before receive filtering would be turned off because they see a very strong signal. By checking for consistency between gain and RSSI records the falsing on off-channel can be reduced. Other mechanisms to reduce false detections include post processing I/Q data for pulses matching statistic tests. Frequency tests can be used to detect off channel radar, and Chirp detection can rule out falsing on other signals such as 802.11 collisions. 
     Transmit Events 
     For radios that can transmit and receive (e.g., 802.11 radios), spectrum analysis is preferably turned off during the duration when the radio is in transmit mode. If not, the receiver will only be able to see the transmitted signal which will drown any other signal present in the network. By monitoring transmit on/off events, unnecessary processing by the spectrum analysis module  210  can be avoided. 
     802.11 Receive Events 
     As mentioned, preamble, header, and valid MAC CRC can be detected and used to optimize the processing of I/Q data. Furthermore, a trigger could be based on an Address match. With 802.11 it could be matching an implied address, as the source Address is not included in some control frames. Frame matching, as part of overall 802.11 receive events, could also be aimed at specific types of frames, such as ones matching a detected type of attack, or special packets sent to calibrate for Beam forming or locking timing between access points. 
     From the foregoing, those skilled in the art will appreciate how the implementation of a radio events record can help to further improve spectrum analysis processing in connection with a wireless channel being used for, e.g., 802.11 type traffic. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the systems and methods described herein consistent with the principles of the present invention without departing from the scope or spirit of the invention. Although several embodiments have been described above, other variations are possible consistent with the principles of the present invention. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed embodiments. The specification and examples are exemplary only, and the true scope and spirit of the invention is defined by the following claims and their equivalents.