Patent Publication Number: US-7593692-B2

Title: Radar detection circuit for a WLAN transceiver

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present U.S. Utility Patent Application claims priority pursuant to 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 60/502,934, entitled “Radar Detection Circuit for a WLAN Receiver,” filed Sep. 15, 2003, expired, which is hereby incorporated herein by reference in its entirety and made part of the present U.S. Utility Patent Application for all purposes: 

   BACKGROUND 
   1. Technical Field 
   The present invention relates to wireless communications and, more particularly, wideband wireless communication systems. 
   2. Related Art 
   Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof. 
   Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switched telephone network (PSTN), via the Internet, and/or via some other wide area network. 
   Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. 
   As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. As used herein, the term “low IF” refers to both baseband and intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard. 
   One approach to using a higher intermediate frequency is to convert the RF signal to an intermediate frequency sufficiently low to allow the integration of on-chip channel selection filters. For example, some narrow band or low data rate systems, such as Bluetooth, use this low intermediate frequency design approach. 
   Active mixers used in direct conversion radios, as well as radios that employ an intermediate conversion step, typically comprise input transconductance elements, switches and an output load. These active mixers often have varying output signal characteristics due to environmental conditions, such as temperature, and process and manufacturing variations. These varying output signal characteristics can, for example, result in a mixer producing an errant local oscillation signal that affects the accuracy of an output signal&#39;s frequency. Having inaccurate output frequencies can result in many undesirable outcomes, including unwanted signal filtering by a downstream filter. 
   Other approaches are also being pursued to achieve the design goal of building entire radios on a single chip. With all of the foregoing design goals, however, there is being realized an increasing need for additional frequency bands for use by radio receivers and transmitters of all types. Along these lines, a frequency band that has heretofore been reserved exclusively for radar systems is being opened for use for at least some types of wireless communication systems. Among other systems, wireless local area network (LAN) systems are being developed to take advantage of the frequency band that is being opened up which has been reserved for radar. One design issue, however, that accompanies any wireless LAN device that operates in this frequency band is that of coexistence with radar systems. More specifically, a need exists for a wireless LAN transceiver to give priority to a radar when a radar operation is detected. Accordingly, the wireless LAN, in such a scenario, would be required to detect a radar signal within a specified response time and to communicate over a non-overlapping frequency band thereto. 
   Along these lines, recent changes to government regulations will allow wireless LANs (WLANs) to share frequency spectrum with licensed radar systems. Specifically, the frequency bands 5.25-5.35 GHz and 5.47-5.75 GHz will be open in Europe, and perhaps worldwide at some point in the future. Since these frequency bands are shared, the wireless LANs will be required to take a subordinate role to the licensed radar systems. This includes the incorporation of dynamic frequency selection (DFS) within the WLAN that will avoid spectrum that is occupied by a radar. What is needed, therefore, is a circuit and method for determining when a radar signal is present. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention includes a radio receiver that further includes front-end circuitry for receiving, amplifying, down-converting and filtering an RF signal, and circuitry for converting the received down-converted, amplified and filtered RF signal into a digital signal and further includes a radar detection block (circuit) that monitors the incoming digital signals to detect the presence of radar. Generally, the radar signal includes a plurality of equally spaced pulses having a magnitude that is significantly greater than a magnitude of a signal transmitted according to any one of the various 802.11 protocols (though the invention includes any wireless LAN protocol in use). One design goal, however, is to avoid false triggers that result from spurious tones and omissions and that further detects radar signals even in circumstances in which a radar pulse has been masked or eliminated by interference. Accordingly, the radar detection block includes circuitry for detecting and measuring the radar signals in the presence of such interference. More specifically, the radar detection block includes a moving average filter, a threshold comparison state machine, and radar detection logic within software that is executed by a processor for determining whether a radar signal is present. A threshold comparison state machine measures the rise time and magnitude of an incoming signal. A radar signal typically generates a pattern of threshold crossings that, when interpreted by a processor, would yield data to facilitate the radio receiver being able to reach a conclusion that a radar signal is present. Accordingly, the radio receiver communicates with logic within the radio transceiver to inhibit transmissions in frequency bands that overlap the frequency bands of the radar signal. 
   A typical wireless LAN receiver, such as used to implement the 802.11a standard, is not capable of detecting the short, regular pulses of a radar system. This invention includes, therefore, additional circuitry to the wireless LAN receiver circuitry to enable radar pulse detection. There are three main components: a power detector circuit, a state machine, and a software-driven processor. The power detector circuit taps the received signal off the main path and computes the magnitude squared of the incoming (full bandwidth) signal. The output of the power detector circuit is applied to the state machine, which compares the instantaneous power to two different thresholds. This mechanism allows the measurement of the radar pulse start time, rise time, pulse width, and fall time. The state machine records these parameters into a pulse buffer. The software processor reads the pulse buffer and performs further processing as necessary to determine whether or not a radar is present. 
   Other aspects of the present invention will become apparent with further reference to the drawings and specification, which follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered with the following drawings, in which: 
       FIG. 1  is a functional block diagram illustrating a communication system that includes a plurality of base stations and/or access points, a plurality of wireless communication devices and a network hardware component; 
       FIG. 2  is a schematic block diagram illustrating a wireless communication device as a host device and an associated radio; 
       FIG. 3  is a functional schematic diagram of a direct conversion radio transceiver formed according to one embodiment of the present invention; 
       FIG. 4  is a functional schematic diagram of an automatic frequency control (AFC) circuit formed according to one described embodiment of the invention; 
       FIG. 5  is a diagram that illustrates the relative difference between a radar signal waveform and an 802.11 wireless LAN waveform signal; 
       FIG. 6  is a signal diagram illustrating two groups of pulses of a radar signal; 
       FIG. 7  is a diagram that illustrates the measurement of rise time and fall time of the pulses of the radar signal; 
       FIG. 8  is a functional block diagram of a portion of a radio transceiver according to one embodiment of the present invention; 
       FIG. 9  is a functional block diagram of a radar detection block for use in an 802.11a receiver formed according to one embodiment of the present invention; 
       FIG. 10  is a diagram illustrating signal waveforms for radar pulses and 802.11a signals; 
       FIG. 11  is a functional block diagram of a moving average block as employed in one embodiment of the present invention; 
       FIG. 12  illustrates a threshold comparison state machine; and 
       FIG. 13  is a flowchart illustrating a series of steps that are performed according to one embodiment of the present invention; 
       FIG. 14  is a flowchart illustrating a method for determining whether a radar signal is present according to one embodiment of the invention; 
       FIG. 15  is a flowchart illustrating a method for performing radar detection processing; 
       FIG. 16  is a flowchart illustrating a method for performing radar detection processing for missing pulses; 
       FIG. 17  is a flowchart illustrating a method for performing radar detection processing for extra pulses; and 
       FIGS. 18 ,  19 , and  20  illustrate the generation of data that is utilized in the described embodiments of the invention for determining whether a radar is present. 
   

   DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1  is a functional block diagram illustrating a communication system  10  that includes a plurality of base stations or access points (AP)  12 - 16 , a plurality of wireless communication devices  18 - 32  and a network hardware component  34 . The wireless communication devices  18 - 32  may be laptop host computers  18  and  26 , personal digital assistant hosts  20  and  30 , personal computer hosts  24  and  32  and/or cellular telephone hosts  22  and  28 . The details of the wireless communication devices will be described in greater detail with reference to  FIG. 2 . In one embodiment of the present invention, every access point  12 - 16  and every wireless communication device  18 - 32  that is operable to communicate in the radar bands described herein is capable of detecting radar signals and to cease communicating in radar bands so long as a signal is present. In an alternate embodiment of the invention, the various access points  12 - 16  are capable of detecting a radar and further to communicate with any of the wireless communication devices  18 - 32  to prompt the wireless communication devices  18 - 32  to cease transmitting in the radar band in which radar is detected. Accordingly, one or more of the wireless communication devices  18 - 32  is operable to cease communications in the radar band based upon the received prompt from the wireless communication device  12 - 16 . 
   The base stations or AP  12 - 16  are operably coupled to the network hardware component  34  via local area network (LAN) connections  36 ,  38  and  40 . The network hardware component  34 , which may be a router, switch, bridge, modem, system controller, etc., provides a wide area network connection  42  for the communication system  10 . Each of the base stations or access points  12 - 16  has an associated antenna or antenna array to communicate with the wireless communication devices in its area. Typically, the wireless communication devices  18 - 32  register with the particular base station or access points  12 - 16  to receive services from the communication system  10 . For direct connections (i.e., point-to-point communications), wireless communication devices communicate directly via an allocated channel. 
   Typically, base stations are used for cellular telephone systems and like-type systems, while access points are used for in-home or in-building wireless networks. Regardless of the particular type of communication system, each wireless communication device includes a built-in radio and/or is coupled to a radio. 
     FIG. 2  is a schematic block diagram illustrating a wireless communication device  18 - 32  as a host device and an associated radio  60 . For cellular telephone hosts, the radio  60  is a built-in component. For personal digital assistants hosts, laptop hosts, and/or personal computer hosts, the radio  60  may be built-in or an externally coupled component. 
   As illustrated, the host wireless communication device  18 - 32  includes a processing module  50 , a memory  52 , a radio interface  54 , an input interface  58  and an output interface  56 . The processing module  50  and memory  52  execute the corresponding instructions that are typically done by the host device. For example, for a cellular telephone host device, the processing module  50  performs the corresponding communication functions in accordance with a particular cellular telephone standard. 
   The radio interface  54  allows data to be received from and sent to the radio  60 . For data received from the radio  60  (e.g., inbound data), the radio interface  54  provides the data to the processing module  50  for further processing and/or routing to the output interface  56 . The output interface  56  provides connectivity to an output device such as a display, monitor, speakers, etc., such that the received data may be displayed. The radio interface  54  also provides data from the processing module  50  to the radio  60 . The processing module  50  may receive the outbound data from an input device such as a keyboard, keypad, microphone, etc., via the input interface  58  or generate the data itself. For data received via the input interface  58 , the processing module  50  may perform a corresponding host function on the data and/or route it to the radio  60  via the radio interface  54 . 
   Radio  60  includes a host interface  62 , a digital receiver processing module  64 , an analog-to-digital converter  66 , a radar detection block  67 , a filtering/gain module  68 , a down-conversion module  70 , a low noise amplifier  72 , receiver filter module  71 , a transmitter/receiver (Tx/Rx) switch module  73 , a local oscillation module  74 , a memory  75 , a digital transmitter processing module  76 , a digital-to-analog converter  78 , a filtering/gain module  80 , an IF mixing up-conversion module  82 , a power amplifier  84 , a transmitter filter module  85 , and an antenna  86 . The antenna  86  is shared by the transmit and receive paths as regulated by the Tx/Rx switch module  73 . The antenna implementation will depend on the particular standard to which the wireless communication device is compliant. 
   The radar detection block  67  is operable to receive a digital low frequency signal from ADC  66  and to process such digital low frequency signal to determine whether a radar signal is present. Radar detection block  67 , upon detecting a radar signal, prompts radio  60  to inhibit communications in frequency bands that overlap with radar and, in the case of ongoing communications, to switch outgoing communications to a non-overlapping frequency band (non-overlapping with radar). 
   The digital receiver processing module  64  and the digital transmitter processing module  76 , in combination with operational instructions stored in memory  75 , execute digital receiver functions and digital transmitter functions, respectively. The digital receiver functions include, but are not limited to, demodulation, constellation demapping, decoding, and/or descrambling. The digital transmitter functions include, but are not limited to, scrambling, encoding, constellation mapping, modulation. The digital receiver and transmitter processing modules  64  and  76  may be implemented using a shared processing device, individual processing devices, or a plurality of processing devices. Such a processing device may be a microprocessor, micro-controller, digital signal processor, microcomputer, central processing unit, field programmable gate array, programmable logic device, state machine, logic circuitry, analog circuitry, digital circuitry, and/or any device that manipulates signals (analog and/or digital) based on operational instructions. The memory  75  may be a single memory device or a plurality of memory devices. Such a memory device may be a read-only memory, random access memory, volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, and/or any device that stores digital information. Note that when the digital receiver processing module  64  and/or the digital transmitter processing module  76  implements one or more of its functions via a state machine, analog circuitry, digital circuitry, and/or logic circuitry, the memory storing the corresponding operational instructions is embedded with the circuitry comprising the state machine, analog circuitry, digital circuitry, and/or logic circuitry. The memory  75  stores, and the digital receiver processing module  64  and/or the digital transmitter processing module  76  executes, operational instructions corresponding to at least some of the functions illustrated herein. 
   In operation, the radio  60  receives outbound data  94  from the host wireless communication device  18 - 32  via the host interface  62 . The host interface  62  routes the outbound data  94  to the digital transmitter processing module  76 , which processes the outbound data  94  in accordance with a particular wireless communication standard (e.g., IEEE 802.11a, IEEE 802.11b, Bluetooth, etc.) to produce digital transmission formatted data  96 . The digital transmission formatted data  96  will be a digital baseband signal or a digital low IF signal, where the low IF typically will be in the frequency range of one hundred kilohertz to a few megahertz. 
   The digital-to-analog converter  78  converts the digital transmission formatted data  96  from the digital domain to the analog domain. The filtering/gain module  80  filters and/or adjusts the gain of the analog baseband signal prior to providing it to the up-conversion module  82 . The up-conversion module  82  directly converts the analog baseband signal, or low IF signal, into an RF signal based on a transmitter local oscillation  83  provided by local oscillation module  74 . The power amplifier  84  amplifies the RF signal to produce an outbound RF signal  98 , which is filtered by the transmitter filter module  85 . The antenna  86  transmits the outbound RF signal  98  to a targeted device such as a base station, an access point and/or another wireless communication device. 
   The radio  60  also receives an inbound RF signal  88  via the antenna  86 , which was transmitted by a base station, an access point, or another wireless communication device. The antenna  86  provides the inbound RF signal  88  to the receiver filter module  71  via the Tx/Rx switch module  73 , where the Rx filter module  71  bandpass filters the inbound RF signal  88 . The Rx filter module  71  provides the filtered RF signal to low noise amplifier  72 , which amplifies the inbound RF signal  88  to produce an amplified inbound RF signal. The low noise amplifier  72  provides the amplified inbound RF signal to the down-conversion module  70 , which directly converts the amplified inbound RF signal into an inbound low IF signal or baseband signal based on a receiver local oscillation signal  81  provided by local oscillation module  74 . The down-conversion module  70  provides the inbound low IF signal or baseband signal to the filtering/gain module  68 . The filtering/gain module  68  may be implemented in accordance with the teachings of the present invention to filter and/or attenuate the inbound low IF signal or the inbound baseband signal to produce a filtered inbound signal. 
   The analog-to-digital converter  66  converts the filtered inbound signal from the analog domain to the digital domain to produce digital reception formatted data  90 . The digital receiver processing module  64  decodes, descrambles, demaps, and/or demodulates the digital reception formatted data  90  to recapture inbound data  92  in accordance with the particular wireless communication standard being implemented by radio  60 . The host interface  62  provides the recaptured inbound data  92  to the host wireless communication device  18 - 32  via the radio interface  54 . 
   As one of average skill in the art will appreciate, the wireless communication device of  FIG. 2  may be implemented using one or more integrated circuits. For example, the host device may be implemented on a first integrated circuit, while the digital receiver processing module  64 , the digital transmitter processing module  76  and memory  75  are implemented on a second integrated circuit, and the remaining components of the radio  60 , less the antenna  86 , may be implemented on a third integrated circuit. As an alternate example, the radio  60  may be implemented on a single integrated circuit. As yet another example, the processing module  50  of the host device and the digital receiver processing module  64  and the digital transmitter processing module  76  may be a common processing device implemented on a single integrated circuit. Further, memory  52  and memory  75  may be implemented on a single integrated circuit and/or on the same integrated circuit as the common processing modules of processing module  50 , the digital receiver processing module  64 , and the digital transmitter processing module  76 . 
   The wireless communication device of  FIG. 2  is one that may be implemented to include either a direct conversion from RF to baseband and baseband to RF or for a conversion by way of a low intermediate frequency. In either implementation, however, for an up-conversion module  82  and a down-conversion module  70 , it is required to provide accurate frequency conversion. For the down-conversion module  70  and up-conversion module  82  to accurately mix a signal, however, it is important that the local oscillation module  74  provide an accurate local oscillation signal for mixing with the baseband or RF by the up-conversion module  82  and down-conversion module  70 , respectively. Accordingly, the local oscillation module  74  includes circuitry for adjusting an output frequency of a local oscillation signal provided therefrom. As will be explained in greater detail, below, the local oscillation module  74  receives a frequency correction input that it uses to adjust an output local oscillation signal to produce a frequency corrected local oscillation signal output. While one embodiment of the present invention includes local oscillation module  74 , up-conversion module  82  and down-conversion module  70  that are implemented to perform direct conversion between baseband and RF, it is understand that the principles herein may also be applied readily to systems that implement an intermediate frequency conversion step at a low intermediate frequency. 
     FIG. 3  is a functional schematic diagram of a direct conversion radio transceiver formed according to one embodiment of the present invention. Referring now to  FIG. 3 , a transceiver system comprises radio circuitry  304  that is coupled to baseband processing circuitry  308 . The radio circuitry  304  performs filtering, amplification, frequency calibration (in part) and frequency conversion (down from the RF to baseband and up from baseband to the RF). Baseband processing circuitry  308  performs the traditional digital signal processing in addition to partially performing the automatic frequency control. As may be seen, the single chip radio circuitry  304  is coupled to receive radio signals that are initially received by the transceiver and then converted by a Balun signal converter, which performs single end to differential conversion for the receiver (and differential to single end conversion for the transmitter end). The Balun signal converters are shown to be off-chip in  FIG. 3 , but they may be formed on-chip with radio circuitry  304  as well. Similarly, while the baseband processing circuitry  308  is shown off-chip, it also may be formed on-chip with radio circuitry  304 . 
   Radio circuitry  304  and, more particularly, circuitry portion  304 A, includes a low noise amplifier  312  that is coupled to receive RF signals from a transceiver port. The low noise amplifier  312  then produces an amplified signal to mixers  316  that are for adjusting and mixing the RF with a local oscillation signal. The outputs of the mixers  316  (I and Q components of quadrature phase shift keyed signals) are then produced to a first HP-VGA  320 . 
   The outputs of the first HP-VGA  320  are then produced to a first RSSI  328  as well as to a low pass filter  324 . The outputs of the low pass filter  324  are then produced to a second RSSI  332 , as well as to a second HP-VGA  336  and a third HP-VGA  340  as may be seen in  FIG. 3 . The outputs of HP-VGA  340  are then produced to a pair of analog-to-digital converters. The analog-to-digital converters then produce digital data to both radar detection block  342  and a digital block for processing. 
   Radar detection block  342  is operable to monitor incoming signals and determine when a radar signal is present. In one embodiment, radar detection block  342  produces an indication that a radar is present (shown as RADAR DETECT CONTROL SIG. in  FIG. 3 ) to the digital block which the takes specified actions including suspending transmissions in overlapping radar channels. It is understood that the RADAR DETECT CONTROL SIG. may merely be a binary control signal in one embodiment to prompt the digital block to inhibit transmission or it may be pulse data where logic within the digital block evaluates the pulse data to determine whether a radar has been detected. Moreover, as may be seen, radar detection block  342  receives digital low frequency signals from the ADCs. In an alternate embodiment, however, the radar detection block includes its own set of ADCs and receives low frequency analog signals from the inputs of the ADC. Such alternate embodiment may be implemented as a matter of design choice. 
   First RSSI  328  measures the power level of the signal and interference. The second RSSI  332  measures the power level of the signal only. The baseband processing circuitry  308  then determines the ratio of the RSSI measured power levels to determine the relative gain level adjustments of the front and rear amplification stages. In the described embodiment of the invention, if the power level of the signal and interference is approximately equal to or slightly greater than the power level of the signal alone, then the first amplification stages are set to a high value and the second amplification stages are set to a low value. Conversely, if the power level of the signal and interference is significantly greater than the power of the signal alone, thereby indicating significant interference levels, the first amplification stages are lowered and the second amplification stages are increased proportionately. 
   Circuitry portion  304 B includes low pass filters for filtering I and Q component frequency correction signals and mixer circuitry for actually adjusting LO signal frequency. The operation of mixers and phase locked loop for adjusting frequencies is known. Circuitry portion  304 B further includes JTAG (Joint Test Action Group, IEEE1149.1 boundary-scan standard) serial interface (SIO) circuitry  344  for transmitting control signals and information to circuitry portion  304 A (e.g., to control amplification levels) and to a circuitry portion  304 C (e.g., to control or specify the desired frequency for the automatic frequency control). 
   A portion of the automatic frequency control circuitry that determines the difference in frequency between a specified center channel frequency and an actual center channel frequency for a received RF signal is formed within the baseband circuitry in the described embodiment of the invention. This portion of the circuitry includes circuitry that coarsely measures the frequency difference and then measures the frequency difference in the digital domain to obtain a more precise measurement and to produce frequency correction inputs to circuitry portion  304 B. 
   Finally, radio circuitry portion  304 C includes low pass filtration circuitry for removing any interference that is present after baseband processing as well as amplification, mixer and up-converter circuitry for preparing a baseband signal for transmission at the RF. 
     FIG. 4  is a functional schematic diagram of an automatic frequency control (AFC) circuit formed according to one described embodiment of the invention. The AFC circuit of  FIG. 4  comprises an RF signal processing portion  360  and a baseband signal processing portion  362 . Generally, portion  360  is for adjusting an LO signal frequency. Portion  362  is for determining the difference in center channel frequencies between the received RF and the expected frequency value for the received signal. 
   An analog-to-digital converter (ADC)  364  is used to convert the received signal from analog to digital. ADC  364  is coupled to receive an RF signal that has been down-converted to produce a digitally converted signal to frequency synchronization circuitry  368  that measures the frequency difference in a coarse degree of resolution. Digital frequency control circuitry  366  performs its measurements and calibration in the digital domain and provides its results to frequency synchronization circuitry  368  to adjust the frequency difference of frequency synchronization circuitry  368  with a fine degree of resolution. 
   Frequency synchronization circuitry  368 , as a part of determining the difference in center channel frequency for the received signal and an expected value, receives and interprets a pilot signal that defines the expected center channel frequency. Accordingly, after measuring the actual center channel frequency of the received RF, frequency synchronization circuitry  368  is able to determine the frequency difference. Frequency synchronization circuitry  368  then produces a signal defining the difference in center channel frequency for the received signal and an expected value to a signal generator  370 . It is understood that the pilot channel is transmitted as a part of standard wireless network communication protocols for signal control and synchronization purposes. 
   Signal generator  370 , upon receiving the difference in center channel frequency for the received signal and an expected value, produces quadrature phase shift keyed (I and Q) outputs for the received frequency difference (reflecting a frequency adjustment amount) to a pair of digital-to-analog converters (DACs)  372 . The analog outputs of the pair of DACs  372  are then passed to low pass filters  374  and are then up-converted to the RF. The I and Q RF signal components are then produced to mixer circuitry  376  that also receives a specified input from phase locked loop (PLL) circuitry  378  to produce a received RF having a specified center channel frequency. It is understood that mixer circuitry  376  (including PLL circuitry  378 ) further receives control signals from baseband processing circuitry (not shown in  FIG. 4 ) specifying the expected center channel frequency that is specified in the aforementioned pilot channel. 
     FIG. 5  is a diagram that illustrates the relative difference between a radar signal waveform and an 802.11 wireless LAN waveform signal. Generally, it may be seen that radar signal pulses have a significantly higher magnitude than the 802.11 wireless LAN waveform signal. Additionally, radar pulses have a much shorter pulse width. In the example shown, each radar pulse has a 1 microsecond pulse width and is spaced apart by approximately 1 millisecond (though the figure is not to scale). The 802.11 wireless LAN waveform signal, in contrast, has a period that equals at least 40 microseconds and can equal 3 milliseconds. Thus, the waveform of the radar pulses and the 802.11 wireless LAN signal are notably different, however, which facilitates detection by a radar detection block as disclosed herein. 
   Continuing to examine  FIG. 5 , it may be noted that the radar pulses come in blocks. While 5 pulses per block are shown, it is understood that each block of pulses may have a different number of pulses. Generally, the radar pulses are in the 5.25-5.75 GHz frequency band having a 1 to 10 MHz bandwidth. The 802.11 frequency bands that are approximate to the frequency band of radar range from 5.15-5.35 MHz and from 5.725-5.825 MHz. Accordingly, it may be seen that an overlap exists between these two frequency bands for 802.11 with the frequency band for the radar pulses. 
     FIG. 6  is a diagram illustrating two groups (blocks) of pulses of a radar signal. Generally, it is a goal to detect a radar signal before a second group of pulses is received. Accordingly, as may be seen for the first group of pulses, a radar detection block, and more particularly, a state machine of the radar detection block in conjunction with a processor, must be able to detect and determine that a radar signal is present from the five pulses shown of the first group of pulses in  FIG. 6 . As is understood, a common characteristic of radars is that the radar antenna oscillates or rotates thereby radiating any one point in space only for a limited time while the point in space is within a beam angle of the radar antenna. Accordingly, even though the radar continuously produces radar pulses, they are seen by the receiver in groups and are not seen as the radar antenna sweeps away. 
   As may further be seen, a plurality of threshold levels is defined. These threshold levels are used by the state machine and the processor to determine that the radar signal is present. The logic for concluding that a radar signal is present in relation to the pulses will be described in greater detail below. Generally, it may be seen that the first threshold level (TH 0 ), in one embodiment of the invention, is defined to be within the range of −80 decibels per meter (dBm) relative to a milliwatt. A second threshold (TH 1 ) is defined at −63 dBm. 
   The thresholds TH 0  and TH 1  are chosen in order to meet the requirements for radar detection sensitivity and to avoid false alarms and thus may be modified so long as this design goal is considered for a particular radar having known signal characteristics. In a first embodiment, absolute thresholds are used. The choice of TH 0  is made by first estimating the probability of detection and probability of false alarm for the expected environment. In an ideal environment (known radar signals in additive white Gaussian noise (AWGN) the computation is straightforward. However, in practice, the thresholds will be adjusted in the described embodiments of the invention in a dynamic fashion in order to maximize radar detection performance. 
     FIG. 7  is a diagram that illustrates the measurement of rise time and fall time of the pulses of the radar signal. More specifically, a time to is defined as being when a rising pulse crosses the first threshold TH 0 . A second time value t 1  is defined when the pulse crosses the second threshold TH 1 . A third time period is defined, t 2 , as being the time when a falling pulse crosses the TH 1  threshold. Finally, a fourth time is defined as t 3  whenever the falling pulse crosses threshold TH 0 . By measuring the rise and fall times, the state machine and processor executed logic, in the described embodiment, may better determine whether a radar pulse was detected. The logic portion performed by the processor may also be implemented with hardware such as application specific integrated computer logic, field programmable gate array logic, etc. These crossings of TH 0  and TH 1  enable a processor, state machine or other logic performing pulse detection operations to measure a rise time, a fall time, a pulse width and a total signal period. These measured signal characteristics make up what is referred herein also as pulse information that is entered within a table for evaluation to facilitate determination as to whether a radar signal is present. 
     FIG. 8  is a functional block diagram of a portion of a radio transceiver  400  according to one embodiment of the present invention. Initially, an RF signal received at an antenna is coupled to a low noise amplifier  402 . Low noise amplifier  402  produces amplified RF to mixers  404  and  406 . Mixer  404  mixes the amplified RF with a local oscillation to down-convert the RF to a baseband frequency signal to create a down-converted I channel signal. Similarly, mixer  406  mixes the received amplified RF signal with a phase shifted local oscillation, wherein the phase is shifted by 90°, to produce a down-converted Q channel signal. Low pass filters  408  and  410  are coupled to receive the down-converted I and Q channel signals, respectively, to produce filtered I and Q channel signals to a pair of variable gain amplifiers  412  and  414 , respectively. Variable gain amplifiers  412  and  414  further receive gain control signals from an 802.11a physical layer digital signal processor  416 . Responsive to the gain control from processor  416 , variable gain amplifiers  412  and  414  provide a corresponding amount of gain to the filtered I and Q signals and produce amplified I and Q signals to analog-to-digital converters  417  and  418 . Analog-to-digital converters  417  and  418  then convert the amplified I and Q signals to digital to produce incoming I and Q digital signal streams to processor  416 . 
   Radio transceiver  400  of  FIG. 8  further includes a radar detection block  420  that is coupled to receive the incoming I and Q digital signal streams to detect the presence of a radar signal. Upon detecting the presence of a radar signal, radar detection block  420  produces control signals to processor  416  to prompt processor  416  to suspend communications over frequency bands that overlap with the radar frequency bands. The operation and structure of radar detection block  420  is described in greater detail below. 
     FIG. 9  is a block diagram of a radar detector block for use in an 802.11a receiver according to one embodiment of the invention. This corresponding 802.11a receiver for the radar block of  FIG. 9  is a direct conversion type receiver, although the invention is applicable to a super-heterodyne receiver as well. (Both are commonly used for 802.11 type wireless LANs.) The inputs to the radar detection block of  FIG. 9  are the outputs of the analog-to-digital converters (ADCs) that are tapped off and produced to the radar detection circuit (as shown in  FIG. 8 , for example). The input signals produced by the ADCs are digital signals that are sampled at a sufficient rate and with a sufficient number of bits to facilitate use for radar detection. For example, a 40 MHz sampling rate at 8 bits of precision is adequate to detect radar systems in an 802.11a environment. 
   As was shown in relation to  FIG. 8 , radar detection block  420  received the incoming I and Q digital signal streams produced by ADC  417  and ADC  418 . As may be seen in  FIG. 9 , the incoming I and Q digital signal streams are received by mixers  422  and  424 . Mixers  422  and  424  are operatively coupled to receive each of the incoming I and Q digital signal streams twice to square each of the incoming I and Q digital signal streams. Accordingly, mixer  422  produces a squared I digital signal stream, while mixer  424  produces a squared Q digital signal stream. The squared I and Q digital signal streams are then produced to an adder  426  that sums the two squared signals to produce a summed I and Q squared signal stream to a switch  428 . 
   In a first position, the summed I and Q squared signal stream is produced to moving average filter  430  that calculates a moving average of the summed I and Q squared signal stream. Moving average filter  430  then produces a moving average value to decibel conversion block  432 , which converts the moving average value produced by filter  430  to decibel units. 
   Whenever switch  428  couples moving average filter  430  to adder  426 , a switch  438  couples the output of decibel conversion block  432  to a threshold comparison state machine  440 . Accordingly, the moving average, in decibels, is produced to threshold comparison state machine  440  for analysis as will be described below. Whenever switch  428  couples adder  426  to a decibel conversion block  434 , however, the summed I and Q squared signal stream is produced to decibel conversion block  434  which produces the summed I and Q squared signal stream in decibels to a subtractor  436 . Subtractor  436  is further coupled to receive and subtract a receiver gain setting from the summed I and Q squared signal stream in decibels. The output of subtractor  436  is then coupled, by switch  438 , to threshold comparison state machine  440 . Threshold comparison state machine  440  operates as described below to provide preliminary analysis of the detected power levels produced either by decibel conversion block  432  or subtractor  436  to a processor  442 . Radar detection logic  444  within processor  442 , then analyzes the preliminary analysis received from threshold comparison state machine  440  to determine whether a radar signal has been received. 
   This circuit of  FIG. 9  computes the received power and applies either no filtering (option  1  in which both switches are in the low position) or a moving average filter (option  2  in which both switches are in the upper position). Option  1  is most effective for short pulse width radars when no interference is present. Option  2  is most effective with longer radar pulses in interference that looks random as is shown in  FIG. 10 . 
     FIG. 10  is an illustration of signal waveforms for radar pulses and 802.11a signals. In general, the radar detector will not have prior information concerning either the radar pulse width or interference. Consequently, in the described embodiment, logic drives switches  428  and  438  to the lower position (option  1 ) unless an 802.11a frame is being received (as determined during call setup signaling between an access point and a wireless host or communication device in one embodiment of the invention). In the case of 802.11a communications, option  2  is employed (the switches are toggled to the upward position as shown) during the duration of the frame. 
   When option  2  is employed during a received 802.11a frame, the thresholds are set to be relative to the average received power. Since the radar signal is typically a constant envelope signal, while the interference is more like Gaussian noise, the moving average filter will have the effect of improving the radar signal to interference power ratio by a factor of the square root of the filter length. The improvement is limited by the length of the radar pulse (i.e., maximum improvement is when the radar pulse fills the moving average filter). Thus, the threshold level and filter length are jointly selected based on the expected radar pulses length and the detection and false alarm probabilities. 
     FIG. 11  is a functional block diagram of a moving average block as employed in one embodiment of the present invention. The moving average filter effectively is an integrator and can improve the radar signal to interference level for many types of radar signals. In the described embodiment, four delay elements  446  are included coupled in series and having outputs that are further produced to an adder. A summed output from the adder is then produced to a divide by “N” block  448 . The value of “N” in the divide-by-N block is equal to four in the described embodiment since there are four delay elements. The number of delay elements and the divisor “N” may each be modified according to particular requirements. 
   If it is assumed that the radar signal has a constant envelope, the I and Q components then take the form:
 
 I ( n )= A   r  cos(  ω   r   nT +φ)
 
 Q ( n )= A   r  sin(  ω   r   nT +φ)
 
where A r , the radar signal amplitude, and ω r , the down-converted radar signal frequency, are constant (or approximately constant) during the radar pulse. The radar signal is zero outside of the received pulse. In constrast, the received 802.11a signal can be modeled as two Gaussian signals:
 
 I ( n )= G   i ( n )
 
 Q ( n )= G   q ( n )
 
The moving average filter of length n computes the following:
 
           y   =         ∑     i   =   1     k     ⁢     I   i   2       +     Q   i   2             
With an input signal that is composed of a radar signal embedded within an 802.11a frame, the output of the moving average filter is:
 
             I   ⁡     (   n   )       =         A   r     ⁢     cos   ⁡     (         ϖ   r     ⁢   n   ⁢           ⁢   T     +   φ     )         +       G   i     ⁡     (   n   )                       Q   ⁡     (   n   )       =         A   r     ⁢     sin   ⁡     (         ϖ   r     ⁢   n   ⁢           ⁢   T     +   φ     )         +       G   q     ⁡     (   n   )                     y   =         ∑     i   =   1     k     ⁢       A   r   2     ⁡     (         cos   2     ⁡     (         ϖ   r     ⁢   i   ⁢           ⁢   T     +   φ     )       +       sin   2     ⁡     (         ϖ   r     ⁢   i   ⁢           ⁢   T     +   φ     )         )         +       A   r     ⁢     cos   ⁡     (         ϖ   r     ⁢   i   ⁢           ⁢   T     +   φ     )       ⁢       G   i     ⁡     (   i   )         +       G   i   2     ⁡     (   i   )       +       A   r     ⁢     sin   ⁡     (       ϖ   r     ,       i   ⁢           ⁢   T     +   φ       )       ⁢       G   q     ⁡     (   i   )         +       G   q   2     ⁡     (   i   )                     y   =       k   ⁢           ⁢     A   r   2       +       ∑     i   =   1     k     ⁢       A   r     ⁢     cos   ⁡     (         ϖ   r     ⁢   i   ⁢           ⁢   T     +   φ     )       ⁢       G   i     ⁡     (   i   )           +       G   i   2     ⁡     (   i   )       +       A   r     ⁢     sin   ⁡     (         ϖ   r     ⁢   i   ⁢           ⁢   T     +   φ     )       ⁢       G   q     ⁡     (   i   )         +       G   q   2     ⁡     (   i   )               
With the radar and signal model given above, y has a non-central chi-square distribution with 2 k degrees of freedom. Thus, the relative error in the measurement of y is given by:
 
                         Relative   ⁢           ⁢   Error     =       (     standard   ⁢           ⁢   deviation   ⁢           ⁢   of   ⁢           ⁢   y     )     /     (     mean   ⁢           ⁢   of   ⁢           ⁢   y     )                   =       σ   y       m   y                   =           4   ⁢   k   ⁢           ⁢     σ   wlan   4       +     4   ⁢     kA   r   2     ⁢     σ   wlan   2               2   ⁢   k   ⁢           ⁢     σ   wlan   2       +     k   ⁢           ⁢     A   r   2                         
which decreases by a factor of the square root of k with increasing k.
 
     FIG. 12  illustrates a threshold comparison state machine. This device measures the time instants when the radar pulse crosses two thresholds. The output takes the form of 4 time measurements: start time (T 0 ), rise time (T 1 ), pulse width (T 2 ), and fall time (T 3 ). A set of the 4 time measurements is recorded for every complete cycle back to start. 
   The state machine operates as follows. It originates in the start state and observes the incoming power estimate P. When P exceeds threshold P 0 , the start time (T 0 ) is recorded and the state is advanced to Rising 0 . Once in Rising 0 , a counter is initiated to record the total time in that state (T 1 ). If the incoming power estimate drops below P 0 , then the state machine is reset and it returns to Start. As described herein, P 0  and P 1  represent two threshold values used to identify a typical radar pulse. For example, the values of P 0  and P 1  may be set to TH 0  and TH 1  of  FIGS. 6 and 7 . 
   When P exceeds the second threshold, P 1 , the state machine advances to state Rising 1 . The time it spends in this state is recorded in T 2 . When P drops below P 1 , the state machine advances to Falling 0 . The time in this state is recorded in T 3 . If, while in the Falling 0  state, P rises back up to P 1 , then the state machine returns to the Rising 1  state. T 2  is then incremented by the contents of T 3  and T 3  is reset. This process of moving back and forth between the Rising 1  and Falling 0  states can happen multiple times. After P drops below P 0 , the state machine returns to Start and the complete set of 4 time measurements are forwarded to the processor. 
   The final radar detection decision is made by a programmable processor as shown in the embodiment of  FIG. 9 . In the embodiment of  FIG. 8 , however, the final radar detection decision may be made either in the processor or in radar detection block  420 . Referring again to  FIG. 9 , processor  442 , and more particularly, radar detection logic  444 , periodically reviews the pulse data collected by the state machine  440 , and compares it with the characteristics of known radar signals. One key characteristic for determining radar presence, however, is the pulse repetition frequency. Thus, the processor is operable to match multiple received pulses with the same relative spacing. When a sequence of this type is observed, detection is declared. Otherwise, the pulse data is discarded, and the processor waits for new data. This multi-layer approach helps minimize false detections, while maximizing the chances that actual radars are detected. 
   In the described embodiment of the invention, a processor receives the output of the state machine and logic defined therein (in radar detection block  444  of  FIG. 9 , for example) analyzes the output of the state machine to determine whether a radar signal has been received. It should be understood that the following logic, as defined in block  444  and executed by processor  442 , may readily be formed in hardware as described before. 
   The embodiments of the invention include a sequence of steps that are typically executed on a programmable processor.  FIG. 13  is a flowchart that illustrates a series of steps that are performed according to one embodiment of the invention. Generally, the invention includes measuring signal characteristics to determine if a received signal has a characteristic of a radar pulse and to further determine whether a pattern of pulses is consistent with a radar pattern. More specifically, the invention includes determining whether a received signal has exceeded a first threshold (step  450 ) and, when the first threshold is exceeded, a timer is initiated to track or measure a rise time (step  452 ). The time is turned off and the rise time is determined when the rising signal crosses a second threshold. Thus, the invention includes determining that the received signal has crossed the second threshold (step  454 ). 
   The invention further includes determining a pulse width. Thus, once the second threshold has been reached, the invention includes initiating a second timer to measure an amount of time above the second threshold (and therefore the pulse width) (step  456 ). The invention further includes determining a received signal has fallen below the second threshold (step  458 ). The difference in time between the two crossings of the second threshold define the pulse width of a received signal. If the second timer is initialized to start counting from zero, the value of the second timer represents the pulse width. 
   Once the second threshold is crossed in a downward position, meaning the received signal levels has crossed from above to below the second threshold, a third timer is initialized to track a fall time (step  460 ). Once the signal crosses the first threshold in the downward direction, the third timer is stopped and the fall time is determined (step  462 ), which fall is the time required for the signal level to fall from the second to the first threshold. Accordingly, the invention further includes producing first, second and third timer values to logic for determining whether a radar pulse has been received (step  464 ). Finally, if a radar pulse has been received, the method of the embodiment of the invention includes stopping all transmissions in frequency bands that overlap with radar frequency bands (step  466 ). 
     FIG. 14  is a flowchart illustrating a method for determining whether a radar signal is present according to one embodiment of the invention. For the described embodiment, it is assumed hardware of a radio receiver is continually filling a first in/first out (FIFO) with the pulse information. Moreover, in the described embodiment of the invention, the inventive method is repeated at periodic intervals of less than one second. 
   Initially, the invention includes receiving and detecting incoming pulses and placing the pulse information in a FIFO register (step  470 ). Detecting the incoming pulses includes measuring pulse characteristics such as rise time, pulse width and fall time. Thereafter, the invention includes producing pulse information to a processor and clearing the FIFO (step  472 ). Generally, this step includes loading pulse data (pulse information) into the programmable processor (in the described embodiment) or other logic. In one embodiment, the pulse data is loaded by direct transfer such as by direct memory access (DMA). 
   After the pulse data is loaded, a table of pulse data is generated for a series of pulses (step  474 ). If the total number of pulses is less than a specified number, processing is suspended (stopped) until the specified number of pulses is listed within the table (step  476 ). In addition to adding pulse data to the table, the invention includes removing pulse information from the generated table for any pulse having a pulse width less than a specified minimum width amount and greater than a specified maximum width amount (step  478 ). In an alternate embodiment, pulse data is only placed within the table for further analysis if the pulse width is within a specified range for a given pulse. Accordingly, for this embodiment, the step of removing pulse data for such a pulse is unnecessary. In either embodiment, however, pulses that are either too long or too short to be a radar pulse are removed from the table of pulse data entries. Typical radar systems have pulses with a pulse width in the range of one to three microseconds. 
   The invention further includes determining whether a total number of pulses is less than a specified number and, if so, stops further processing until the table has pulse data for a specified number of pulses (step  480 ). In one embodiment of the invention, the specified number is equal to six. Thereafter, the invention includes grouping a plurality of pulse data entries to enable detection of a specified radar pulse (step  482 ). In the described embodiment, the group of pulses are grouped by time. More specifically, a nominal value of 210 milliseconds is used to group pulses. In the very specific embodiment, such a grouping is referred to as an epoch. The epoch or group length is set to be long enough to perform radar detection processing (step  484 ) and to cover the burst lengths sufficiently long to detect desired radar systems. This step takes advantage of a radar characteristic of radar systems of pulses being transmitted and arriving in bursts. Although it is not known exactly how long the bursts will be for a radar, the nominal value of 210 milliseconds should be adequately long to facilitate identifying a received radar signal. Finally, if a radar pulse has been received, the invention includes inhibiting or stopping transmission in frequency bands that overlap with radar frequency bands (step  486 ). 
     FIG. 15  is a flowchart of a method for performing radar detection processing. In some cases, it is expected that a valid radar may not be detected due to interference with one or more pulses. Accordingly, the group of pulses for which no radar was detected is evaluated for a missing pulse. Thus, for the grouped plurality of pulse information entries (epoch), the invention includes generating a first list of pulse repetition intervals by subtracting a start time for a given pulse from a start time for an immediately preceding pulse for each pulse in the group (step  490 ). It is understood, of course, that this step cannot be performed for the first pulse. Thereafter, the invention includes quantizing pulse repetition intervals with a specified granularity (step  492 ). Generally, received pulse data has a degree of resolution that is not necessary and may result in false determinations regarding radar detection conclusions. In one embodiment of the invention, the data is quantized to a resolution of 25 milliseconds and a smallest incremental value. Thereafter, the invention includes removing all pulses not having a pulse repetition interval value within a specified range (step  494 ). If the total number of pulses is less than a specified number (six in the described embodiment) the process is stopped and is repeated for a subsequent grouped plurality of pulse information entries (step  496 ). 
   Once a group of pulses (epoch in the described embodiment) contains a group of pulses that is equal to or exceeds the specified number of required pulses (six in the described embodiment), the invention includes determining (by counting) which pulses have the most common and second most common pulse interval values in the group of pulse information entries (step  498 ). The method further includes determining if a total number of most common pulse interval values is greater than or equal to the specified number and therefore determining that a radar pulse has been detected (step  500 ). If the pulse train (group of pulses) does not suggest radar presence, the invention includes examining the pulse train to determine if the pulse train is missing a radar pulse (step  502 ). The specific steps for determining that a radar is present notwithstanding a missing pulse is illustrated in relation to  FIG. 16 . 
   If analysis of the pulse train for missing radar pulses does not suggest radar presence, the invention includes examining the pulse train to determine if the pulse train includes an extra radar pulse (step  504 ). If a radar pulse is detected in any one of the prior steps, the invention further includes suspending transmission in overlapping frequency bands and classify radar by comparing frequency of pulses from first list of pulse repetition intervals to known radar signals (step  506 ). Finally, the invention includes continuing monitoring for radar and, once a radar signal is determined to not be present, resuming transmission of communication signals in overlapping frequency bands (radar bands) (step  508 ). 
     FIG. 15  illustrated a method for determining whether a radar signal is present and whether transmissions in overlapping frequency bands should be suspended. Within the steps of  FIG. 15 , there are two steps for determining whether a radar is present even if a pulse is missing (for example, due to interference) or if there is an extra pulse (for example, due to spurious noise or other noise source) in steps  504  and  506 , respectively. Each of these two steps, however, further includes a series of steps for determining the same. 
     FIG. 16  is a flowchart illustrating a method for performing radar detection processing for missing pulses.  FIG. 17  is a flowchart illustrating a method for performing radar detection processing for extra pulses. Referring now to  FIG. 16 , the method includes evaluating whether a 2*pulse interval of a most common pulse interval value is equal to a pulse interval of a second most common pulse interval value (step  510 ). Additionally, the invention includes evaluating whether the 2*pulse interval of the second most common pulse interval value is equal to the pulse interval of the most common pulse interval value (step  512 ). Also, the invention includes determining if the total number of most common and second most common pulses is greater than the specified number (step  514 ). Finally, the invention includes determining that a radar pulse is present if any of the above three steps are true (step  516 ). Generally, if any of these steps yields a true result, then a radar is present and detectable even if interference prevents receipt of a radar pulse. 
   Another type of interference is the introduction of a signal that appears to be a pulse. Thus, referring now to  FIG. 17 , the inventive method includes analysis for determining a radar signal is present even in the presence of an extra signal appearing as a pulse. More specifically, the invention includes for a grouped plurality of pulse information entries, generating a second list of pulse repetition intervals by subtracting a start time for a given pulse from a start time for a pulse preceding an immediately preceding pulse (step  518 ). Thereafter, the pulse repetition intervals are quantized with a specified granularity (step  520 ). The invention further includes removing all pulses not having a pulse repetition interval value within a specified range (step  522 ). The remaining pulse intervals of a first list of pulse repetition intervals are then compared to the second list of pulse repetition intervals (step  524 ). Finally, the invention includes evaluating whether if pulse periods match from the comparison, and if the total number of pulses in the second list of pulse repetition intervals is greater than a specified number, determining whether a radar is present (step  526 ). 
     FIGS. 18 ,  19  and  20  illustrate the generation of data that is utilized in the described embodiments of the invention for determining whether a radar is present. More specifically,  FIG. 18  illustrates the specific pulse data that is stored or evaluated. In the described embodiment, the pulse data is stored in tabular form and includes identification of the start time, the rise time, the pulse width and the fall time for each of a plurality of pulses. Based on the data of  FIG. 18 ,  FIG. 19  illustrates some of the aforementioned processing for determining whether a radar is present in view of a missing pulse. More specifically, a set of pulse intervals is defined between pulses and is mapped according to quantity. For example, the most common pulse interval is the pulse interval having a value of (start time) E—(start time) A. The frequency of this pulse interval is represented herein as “n1”. Such determinations are then used as described herein for determining radar is present even if a pulse is missing (was not received by the receiver). Finally,  FIG. 20  illustrates some of the processing relating to the presence of additional pulses and the method for detecting the presence of a radar in such cases. One important aspect of the table of  FIG. 20  is that  FIG. 20  illustrates that start times and corresponding pulse widths are based upon every other pulse. Accordingly, the pulse widths are twice as long as expected ordinarily. For example, the first row illustrates subtraction of a start time for pulse “A” from the start time for pulse “I”. 
   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and detailed description. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but, on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the claims. As may be seen, the described embodiments may be modified in many different ways without departing from the scope or teachings of the invention.