Abstract:
A network station includes a sampling device configured to generate samples of radio frequency activity on a medium. A first correlator is configured to autocorrelate the samples based on a first delay to generate a first autocorrelation value. The first delay matches periodicity of a jam signal. The jam signal indicates a second station is to transmit data. The second station is separate from the network station. A second correlator is configured to autocorrelate the samples to generate a second autocorrelation value. The second autocorrelation value indicates whether the radio frequency activity includes periodic noise. A controller is configured to prevent the network station from transmitting a radio frequency signal on the medium based each of the first autocorrelation value and the second autocorrelation value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation U.S. patent application Ser. No. 11/351,936 filed on Feb. 9, 2006 (now U.S. Pat. No. 8,149,863), which claims the benefit of U.S. Application No. 60/677,220, filed on May 2, 2005. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates in general to wireless communications, and more particularly to discriminating between a periodic interfering signal and a periodic signal of interest. 
     BACKGROUND 
     Carrier Sense Multiple Access (CSMA) is a Media Access Control (MAC) protocol in which a communication node verifies the absence of other traffic before transmitting on a shared physical medium, such as a prescribed radio frequency (RF) band. A wireless communication node, for example, may comprise a device with a radio communication card. Carrier sense signifies that a communication node listens for a carrier wave transmitted by another node when trying to send its own transmission. The presence of a carrier on the medium indicates that the medium is busy, i.e. another node currently is transmitting. If a node that intends to transmit information senses a carrier on the medium, then that node waits for the transmission in progress to finish before initiating its own transmission. 
     CSMA with Collision Avoidance (CSMA/CA) is a protocol in which a communication node (or station) that intends to transmit sends a jam signal. After waiting a sufficient time for all other nodes that may access the medium to receive the jam signal, the node transmits a data frame. Conversely, before a communication node transmits information onto the shared medium, it listens to determine whether a jam signal has been sent by another node. If it detects a jam signal then it delays its own transmission for a random amount of time before again trying to transmit onto the medium. The random delay causes different nodes to wait different periods of time before again trying to transmit and avoids two or more of them sensing the medium at the same time, finding the channel idle, transmitting simultaneously, and having their transmissions collide with each other. 
     A jam signal sent by a wireless communication node comprises a signal pattern that informs other nodes that they should postpone transmitting onto the communication medium. In a CSMA/CA network, a transmitting node typically transmits a jam signal as a preamble to a data packet. The sending node sends the jam signal before transmission of the actual data in order to inform other nodes that the sending node intends to transmit data onto the medium. A jam signal alerts other nodes to back off by different random intervals before transmitting data onto the medium. Backing off by different random amounts reduces the probability of a collision when these other nodes first attempt a transmission retry. 
     Environmental noise is a significant challenge in wireless communications networks. For example, frequencies emitted by a cordless phone, a microwave oven or other appliances can interfere with wireless communications, causing packet fragmentation and data corruption. Another major challenge may arise when multiple radio technologies operate in the same frequency band. Specifically, for example, both IEEE 802.11 (Wi-Fi) networks and 802.15 (Bluetooth) networks operate in the unlicensed 2.4 GHz Industrial Scientific Medical (ISM) frequency band, which can lead to signal interference and result in significant performance degradation when devices are co-located in the same environment. 
     Sinusoidal interferer signals or frequency tones can be especially problematic. For example, there exist wireless networks that employ a CSMA/CA protocol in which a jam signal comprises a prescribed periodic signal of interest. Stations on such networks transmit jam signals before transmitting data to warn other stations that a data transmission is in progress. Stations on such networks also listen for a jam signal transmitted onto the shared medium by other stations before transmitting their own data onto the medium. Stations detect the jam signal using autocorrelation techniques. If a station detects such a jam signal transmitted by another station, then it delays its own transmission by a time interval sufficient for the other data transmission to complete. Unfortunately, autocorrelation of a sinusoidal interferer signal (or tone) can produce a false detection of a jam signal causing a station to unnecessarily delay data transmission, which can result in reduced data throughput. 
     Thus, there has been a need to reduce the impact of periodic noise signals such as sinusoidal signals and tones upon the operation of wireless communications devices. The implementations disclosed herein meet this need. 
     SUMMARY 
     A network station is provided and includes a sampling device configured to generate samples of radio frequency activity on a medium. A first correlator is configured to autocorrelate the samples based on a first delay to generate a first autocorrelation value. The first delay matches periodicity of a jam signal. The jam signal indicates a second station is to transmit data. The second station is separate from the network station. A second correlator is configured to autocorrelate the samples to generate a second autocorrelation value. The second autocorrelation value indicates whether the radio frequency activity includes periodic noise. A controller is configured to prevent the network station from transmitting a radio frequency signal on the medium based each of the first autocorrelation value and the second autocorrelation value. 
     In one aspect, a method of controlling access to a wireless communications medium is provided. RF activity on the medium is sampled. The samples are autocorrelated to produce a first running autocorrelation value indicative of autocorrelation computed with a first delay substantially matching periodicity of a signal of interest. The running first value is monitored to determine whether it is possibly indicative of the signal of interest, such as a periodic non-sinusoidal preamble of a data packet, for example. Samples also are autocorrelated to produce a second running autocorrelation value indicative of autocorrelation computed with a second delay different from the first delay. The running second value is monitored to determine whether the second value is indicative of an interferer signal, such as a sinusoidal interferer (SSI), for instance. Transmission of an RF transmit signal, such as a data packet, onto the medium is delayed in response to the first value indicating that activity on the medium includes the signal of interest during a time when the second value indicates that activity on the medium does not include an interferer signal. 
     In a further aspect, transmission of an RF transmit signal onto the medium is permitted in response to either, the first value indicating that activity on the medium does not include the signal of interest, or the first value indicating that activity on the medium possibly includes the signal of interest during a time when the second value indicates that activity on the medium includes an interferer signal. 
     In another aspect, a wireless communication apparatus is provide and includes signal processing circuitry operable to produce a first running autocorrelation value indicative of autocorrelation of RF samples computed with a first delay substantially matching periodicity of a signal of interest and to monitor the first value to determine whether the first value is possibly indicative of the signal of interest. The signal processing circuitry is further operable to produce a second running autocorrelation value indicative of autocorrelation computed with a second delay different from the first delay and to monitor the second value to determine whether the second value is indicative of an interferer signal. Moreover, RF transmission control circuitry is operable to prevent transmission of an RF transmit signal on the medium in response to the first value indicating that activity on the medium includes the signal of interest when the second value indicates that activity on the medium does not include an interferer signal. 
     In a further aspect, the RF transmission control circuitry is operable to permit transmission of an RF transmit signal onto the medium in response to either, the first value indicating that activity on the medium does not include the signal of interest, or the first value indicating that activity on the medium possibly includes the signal of interest during a time when the second value indicates that activity on the medium includes an interferer signal. 
     A still a further aspect, a wireless communication apparatus is provided and includes means for producing a first running autocorrelation value indicative of autocorrelation of RF samples computed with a first delay substantially matching periodicity of a signal of interest means for producing a second running autocorrelation value indicative of autocorrelation computed with a second delay different from the first delay. Structure corresponding to the two means for correlating may include the programmable circuitry of a digital signal processor, for example, or alternatively, may include ASIC circuitry custom designed to implement the autocorrelation functions. The apparatus also includes means for monitoring the first value to determine whether the first value is possibly indicative of the signal of interest and means for monitoring the second value to determine whether the second value is indicative of an interferer signal. Structure corresponding to the two means for monitoring may include the programmable circuitry of a digital signal processor, for example, or alternatively, may include ASIC circuitry custom designed to implement the autocorrelation functions. The apparatus also includes means for preventing transmission of an RF transmit signal onto the medium in response to the first value indicating that activity on the medium includes the signal of interest when the second value indicates that activity on the medium does not include an interferer signal. Structure for implementing the means for preventing may include Media Access Controller (MAC) device, for example, which may be programmed in a processor or implemented as an ASIC device, for example. In a further aspect, means for preventing transmission of an RF is provided, which prevents transmission of an RF transmit signal on the medium in response to the first value indicating that activity on the medium includes the signal of interest when both the second value and the third value indicate that activity on the medium does not include an interferer signal. The structure for the means for preventing may be implemented as part of the MAC device. 
     Thus, comparison of two (or more) autocorrelation results, one computed using a delay substantially matched to a periodicity of an RF signal of interest and the another computed using a different delay, can be used to determine whether an RF signal detected on a wireless communication medium is the signal of interest or an interferer. These and other features and advantages of the implementations disclosed will be understood from the following description in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustrative block diagram of a wireless communications station that implements a system and method in accordance with the present disclosure. 
         FIG. 2  is an illustrative timing diagram representing a jam signal transmitted by the station of  FIG. 1  pursuant to a CSMA/CA protocol prior to transmitting a data packet. 
         FIG. 3  is an illustrative schematic diagram of processing circuitry operable to produce autocorrelation values for the station of  FIG. 1  in accordance with the present disclosure. 
         FIG. 4  is an illustrative state transition flow diagram representing a jam signal detection process in accordance with the present disclosure, that uses autocorrelation results from the multiple autocorrelations performed by the processing circuitry of  FIG. 3  to discriminate between receipt of a jam signal and receipt of one or more periodic interference signals in order to determine whether to delay a data transmission by the station of  FIG. 1 . 
         FIG. 5  is an illustrative graph representing simulation results for a wireless station that employs circuitry of  FIG. 3  to discriminate between a jam signal and one or more sinusoidal interferer signals in accordance with the present disclosure. 
         FIG. 6  is an illustrative block diagram representing monitor circuitry operable to implement an autocorrelation metric that prescribes the length of time during which autocorrelators implemented by the processing circuitry of  FIG. 3  should maintain a prescribed value in order to prompt a state transition in accordance with the present disclosure. 
         FIG. 7  is an illustrative state diagram representing state transitions of the circuitry of  FIG. 6  in response to the duration of time during which autocorrelators exceed their respective high thresholds or fall below their respective low thresholds. 
         FIGS. 8A-8B  are illustrative graphs showing an example of a determination of threshold levels referred to  FIGS. 6-7 . 
         FIG. 9  is an illustrative block diagram of a cellular phone system that includes circuitry for detection of strong high frequency interference in wireless communications in accordance with the present disclosure. 
         FIG. 10  is an illustrative block diagram of a media player that includes for detection of strong high frequency interference in wireless communications circuitry in accordance with the present disclosure. 
         FIG. 11  is an illustrative block diagram of a high definition television (HDTV) that includes for detection of strong high frequency interference in wireless communications circuitry in accordance with the present disclosure. 
         FIG. 12  is an illustrative block diagram of vehicle control systems that includes circuitry for detection of strong high frequency interference in wireless communications in accordance with the present disclosure. 
         FIG. 13  is an illustrative block diagram of a set top box that includes circuitry for detection of strong high frequency interference in wireless communications in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use a novel apparatus and method to discriminate between a signal of interest, such as a periodic jam signal, and one or more periodic interference signals, such as a strong sinusoidal interferer signal, in accordance with the with the present disclosure, and is provided in the context of particular applications and their requirements. Various modifications to the implementations disclosed will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations and applications without departing from the spirit and scope of the present disclosure. Moreover, in the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the implementations disclosed might be practiced without the use of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order not to obscure the description of the implementations disclosed with unnecessary detail. Thus, the present disclosure is not intended to be limited to the implementations shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
       FIG. 1  is an illustrative block diagram of a wireless communications station  100  that implements an apparatus and method in accordance with the present disclosure. In one implementation, the station  100  is suitable for use as a node in a wireless local area network (WLAN). The station  100  includes WLAN circuitry indicated generally within dashed lines  114  and a host system  112 , such as a personal computer, for example. More particularly, the station  100  includes an antenna  102 , an RF transceiver section  104 , analog-to-digital converter circuitry (ADC)  106 , digital-to-analog converter circuitry (DAC)  108 , a baseband processor and MAC section (BPMAC)  110  and a host system  112 . The antenna  102  radiates RF electromagnetic waves encoded with information during transmission and receives RF electromagnetic waves encoded with information during reception. The RF transceiver section  104  filters and amplifies RF signals and performs conversions between RF and IF (intermediate frequency) to generate I and Q (in-phase and quadrature) data for the ADC  106  and DAC  108 . The BPMAC section  110  modulates and demodulates I and Q data, performs carrier sensing, transmission, and receiving of frames. The BPMAC section  110  implements a CSMA/CA protocol in which the station  100  transmits a jam signal prior to sending a data packet. Conversely, in accordance with the protocol, the station  100  listens on the medium for transmission of a jam signal by another station and delays transmission of its own data packet if it detects a jam signal. In one implementation, the BPMAC section  110  implements Orthogonal Frequency Division Multiplexing (OFDM) modulation compliant with the IEEE Standard 802.11g ( 2003 ). 
       FIG. 2  is an illustrative timing diagram representing a signal of interest in accordance with the present disclosure. This particular signal of interest serves as a jam signal that is transmitted pursuant to a CSMA/CA protocol prior to transmitting packet data. In one implementation, the jam signal complies with the 802.11g standard and includes a periodic sequence of pulses in the time domain. The jam signal also sometimes is referred to as the ‘preamble’ in the 802.11g standard. Specifically, the jam signal includes a 7.2 microsecond signal that includes a sequence of ten periodic pulses with peaks separated by 0.72 microseconds. A station transmits the jam signal onto the 2.4 GHz band specified by the standard in order to inform other stations on the network that they should back off and delay their own transmissions until transmission by the station transmitting the jam signal has completed. 
     The jam signal ordinarily is detected through an autocorrelation technique in which a time-delayed version of the received signal is compared with the received signal itself. The delay used by the autocorrelator typically matches the delay between periodic pulses of the jam signal. Thus, receipt of the jam signal results in a high autocorrelation value. Unfortunately, the presence of a strong inband sinusoidal interference signal also can result in a high autocorrelation value, resulting in false detection of a jam signal. While accurate detection of a jam signal by stations limits data collisions that otherwise might occur due to simultaneous data transmissions by different stations, false detections of a jam signal can unnecessarily reduce network data throughput by causing stations to unnecessarily back off when there is no need to do so. 
     Although the jam signal of  FIG. 2  is periodic, it is not a sinusoid. Rather, in this example, it is a periodic square wave. A sinusoidal signal generally can result in a high autocorrelation value for almost any delay between its periodic pulses. The use of a periodic non-sinusoidal signal as the signal of interest (i.e. the jam signal) is more likely to provide a much higher autocorrelation result from an autocorrelator that is tuned to more closely match its periodicity than it is from autocorrelators that are not closely matched to its periodicity. 
       FIG. 3  is an illustrative schematic diagram of signal processing circuitry  300  embodied in the BPMAC  110  in accordance with the present disclosure. The processing circuitry  300  implements three autocorrelators,  302 ,  304 ,  306 . In one implementation, a first autocorrelator (corr — 1)  302  autocorrelates a received signal with a version of the signal delayed by 0.4 microseconds. A second autocorrelator (corr — 2)  304  autocorrelates a received signal with a version of the signal delayed by 0.8 microseconds. A third autocorrelator (corr — 3)  306  autocorrelates a received signal with a version of the signal delayed by 1.2 microseconds. One implementation uses a 40 MHz clock, and therefore, corr — 1  302  captures 16 samples; corr — 2  304  captures 32 samples; and corr — 3  306  captures 48 samples. 
     The delay associated with the second autocorrelator  304  is configured to produce a maximum autocorrelation value upon receipt of the prescribed signal of interest (jam signal) described above. The delays of the first and third autocorrelators  302  and  306  are configured to produce increased autocorrelation values upon receipt of any sinusoidal interference signal. As explained more fully below, the station  100  advantageously uses the combination of autocorrelation values produced by the three autocorrelators  302 ,  304  and  306  to distinguish between interference signals and a jam signal. 
     More specifically, all three autocorrelators typically will produce higher autocorrelation values for a sinusoidal interferer, but the second autocorrelator  304 , which is tuned to the periodicity of the signal of interest produces a significantly higher autocorrelation value than do the first and third autocorrelators  302  and  306  in response to the signal of interest. The difference in autocorrelation values in response to sinusoidal interferers and in response to a signal of interest is used to advantage in the disclosed implementation to differentiate between sinusoidal interferer signals and a signal of interest. 
     It will be appreciated that the principles of the present disclosure can be achieved with the use of only two autocorrelators: one tuned to the delay of the signal of interest; and the other tuned to a different delay. More specifically, since as mentioned above, virtually any sinusoidal signal, regardless of its periodicity results in a high value of autocorrelation, only one additional autocorrelator tuned to a delay different from that of the signal of interest is required to detect a sinusoidal interferer with a delay different from that of the signal of interest. However, two additional autocorrelators tuned to two different delays, each different from that of the signal of interest, are employed in the disclosed implementation in order to enhance confidence in the result. 
     Referring to  FIG. 3 , the first second and third autocorrelators  302 ,  304  and  306  are identical except for their delays. For that reason, only one of the three autocorrelators is described in detail. Corresponding components of the three autocorrelators are described with identical reference numerals. Components of the first autocorrelator  302  are labeled with unprimed reference numerals. Components of the second autocorrelator  304  are labeled with single primed reference numerals. Components of the third autocorrelator  306  are labeled with double primed reference numerals. 
     Block  308  represents adjacent channel reject (ACR) circuitry and low pass filtering (LPF) circuitry. Pursuant to the 802.11g standard, individual communication channels are 20 MHz wide. The ACR circuitry passes signals in the 20 MHz channel used by the station. Block  310  represents a sample storage buffer. In one implementation, the sample buffer includes FIFO circuitry, which includes D flip-flops. Real and Imaginary sample values are loaded into the FIFO circuitry during each clock cycle. 
     As mentioned above, the delay of the first autocorrelator  302  is 0.4 microseconds, and the number of samples taken using a 40 MHz clock is D1=16 samples. The delay of the second autocorrelator  304  is 0.8 microseconds, and the number of samples taken using that clock is D2=32 samples. The delay of the third autocorrelator  302  is 1.2 microseconds, and the number of samples taken using the same clock is D3=48 samples. 
     During each clock cycle, multiplier circuitry  316  multiplies the current (time=0) sample value  314  by the complex conjugate value  316  of one of the D1-1 delayed samples, producing the contribution of the current sample to the autocorrelation value  326 . During each clock cycle, multiplier circuitry  318  multiplies each of the D1-1 delayed samples  320  by the complex conjugate value  322  of a corresponding 2D1-1 delayed sample, producing the autocorrelation contribution of the (D−1)th delayed sample that needs to be subtracted. Subtraction circuitry  324  subtracts the products produced by multiplier  318  from the product produced by multiplier  316 . 
     Accumulator circuitry  326  sums the values produced by the subtraction circuitry  326  to produce a corr — 1 value. In one implementation, summation circuitry  328  sums the individual corr — 1 values accumulated based on reception by multiple antennas (not shown). The 802.11n standard specifies MIMO (multiple input multiple output), which involves the use of multiple antennas to receive a signal. If there is an only a single antenna, then no summation is required. A complex number output value P n  provided by the summation circuitry  328 , or of the accumulator circuitry  326  alone if there is no summation circuitry, is provided to magnitude computation circuitry, which weighs the real and imaginary parts appropriately to produce the magnitude of the complex valued autocorrelation. 
     In one implementation, the magnitude of P n  is computed as,
 
 mag ( Pn )=| Re|+Mu|Im |, where  Mu  is a scalar constant value.
 
     A smoothing filter  332  having a single pole α s  smoothes the accumulation result. The output is a value corr1_acc, which in the illustrated implementation, has a maximum value for a signal having a period of 0.4 microseconds. 
     In one implementation, a process of distinguishing a signal of interest from a jam signal is implemented using circuitry in the BPMAC  110  that uses the states of the first (corr — 1), second (corr — 2) and third (corr — 3) autocorrelators  302 ,  304  and  306  to control enabling and disabling of data transmission by the station  100 . The autocorrelator state-based transmission control can be summarized as follows. 
     corr — 1=0 &amp;&amp; corr — 3=0 &amp;&amp; corr — 2=1 &amp;&amp; symb_err=0→disable transmit 
     corr — 1=1∥corr — 3=1∥corr — 2=0)∥symb_err=1→enable transmit. 
     Basically, the relative states of the autocorrelators  302 ,  304  and  306  are used to determine whether a received signal is a jam signal or an interferer signal. If a jam signal is received, then data transmission is postponed. If a sinusoidal signal is received, but no jam signal is received, then data transmission is not postponed and may proceed immediately. Each autocorrelator has its own prescribed threshold value. A given autocorrelator has logical 1 (one) state if it produces an autocorrelation value that meets its prescribed threshold for that autocorrelator. That given autocorrelator has a logical 0 (zero) state if it produces an autocorrelation value that does not meet the prescribed threshold for that autocorrelator. In addition, a check is made for an occurrence of a symbol error. If a symbol error has occurred, then data transmission is not postponed notwithstanding the autocorrelator states. 
       FIG. 4  is an illustrative flow diagram representing such a jam signal detection process  400  in accordance with the present disclosure. The process uses autocorrelator results from the multiple autocorrelators of  FIG. 3  to discriminate between receipt of a jam signal and receipt of one or more sinusoidal interferer signals in order to determine whether to delay a data transmission. In decision step  402 , a determination is made as to whether a state of the first autocorrelator (corr — 1)  302  indicates that a periodic interferer signal is being received that has a period that closely matches the first autocorrelator delay, i.e. 1.2 microseconds. If in step  402  a state of corr — 1  302  indicates no receipt of a periodic interference signal, i. e. corr — 1=0, then go to step  404 . 
     In decision step  404 , a determination is made as to whether a state of the second autocorrelator (corr — 2)  304  indicates that a periodic interferer signal is being received that has a period that closely matches the first autocorrelator delay, i.e. 0.8 microseconds. If in step  404  a state of corr — 2  304  indicates receipt of a jam signal, i. e. corr — 2=1, then go to step  406 . 
     In decision step  406 , a determination is made as to whether a state of the third autocorrelator (corr — 3)  306  indicates that a periodic interferer signal is being received that has a period that closely matches the first autocorrelator delay, i.e. 0.4 microseconds. If in step  406  a state of corr — 3  306  indicates no receipt of a periodic interferer signal, i. e. corr — 3=0, then go to step  408 . 
     In decision step  408 , a determination is made as to whether a symbol error has occurred. If in step  408  a determination is made that no symbol error has occurred, i.e. symb_err=0, then data transmission input to RF transmission control circuitry portion  410 , which prevents immediate RF transmission. 
     If in step  402  a state of corr — 1  302  indicates receipt of a periodic interference signal, i. e. corr — 1=1; and in step  404  a state of corr — 2  304  indicates receipt of a jam signal, i. e. corr — 2=1; and in step  406  a state of corr — 3  306  indicates receipt of a periodic interference signal, i. e. corr — 3=1; or in step  408  a determination is made that symbol error has occurred, i.e. symb_err1, then data transmission is permitted by RF transmission control circuitry portion  403 . 
     In one implementation, the state transition process illustrated in  FIG. 4  is implemented in the BPMAC  110  as hardware logic circuits and/or firmware, which may include a combination of programmable logic and embedded code. RF transmission control circuitry portion, which enables RF transmission, is implemented in MAC logic circuitry and/or firmware that proceeds with a data transmission if the conditions tested in decision steps  402 ,  404 ,  406  and  408  all indicate that the received signal is a jam signal. RF transmission control circuitry portion  410 , which temporarily delays RF transmission, is implemented in MAC logic circuitry and/or firmware that delays transmission by a substantially random time interval if one or more of the conditions tested in decision steps  402 ,  404 ,  406  and  408  indicate that the received signal is not a jam signal. 
       FIG. 5  is an illustrative graph representing simulation results for the station  100 , which employs circuitry of  FIG. 3  to discriminate between a jam signal and one or more sinusoidal interferer signals. The bottom-most horizontal line  502  represents a threshold of about 35 for the value for corr1_acc produced by the first autocorrelator  302 . The middle horizontal line  504  represents a threshold of about 40 for the value for corr2_acc produced by the second autocorrelator  304 . The top-most horizontal line  506  represents a threshold of about 75 for the value for corr3_acc produced by the third autocorrelator  306 . For each autocorrelator, its threshold value represents a minimum autocorrelation value indicative of the presence of a signal having a period matching the autocorrelator&#39;s delay. More particularly, each threshold value corresponds to a count. The individual thresholds (e.g. 35, 40, 75) are arrived at by comparing the statistics (distribution) of the autocorrelation values produced by a particular autocorrelator, corresponding to the “interferer” portion and the “interferer+desired signal” portion of the received signal. See  FIGS. 8A-8B , discussed below. The instant values of autocorrelation are compared with these thresholds. 
     In one implementation, the first, second and third autocorrelators  302 ,  304  and  306  produce running, i.e. continually updated, autocorrelation values. The state of each autocorrelator is determined by whether its value meets a threshold value prescribed for that autocorrelator for a duration of time prescribed for that autocorrelator. Each autocorrelator has a corresponding prescribed threshold value and a corresponding time duration for which it must meet the threshold value in order to transition to a state indicative of the presence of the signal that it is configured to identify. 
     In this illustrative example, the station  100 , receiving WLAN signal begins to listen on a 20 MHz communication channel at time t=0. Between t=0 and t=20 microseconds, only noise is present on the channel. At t=20 microseconds, a sinusoidal interferer (SSI), weaker in power than the desired signal, is added to the channel. During the time interval from t=20 microseconds to approximately t=50 microseconds, only noise and the SSI are present on the channel. Beginning at about t=50 microseconds, a jam signal is added to the channel. The duration of the jam signal is 10 microseconds. Between t=50 microseconds and t=60 microseconds, noise, SSI and jam signal (preamble) are present on the channel. At approximately t=60 microseconds, the SSI and the jam signal cease, leaving noise and rest of the WLAN signal on the channel thereafter. 
     During the time interval, t=20 microseconds to t=50 microseconds, when only noise and an SSI are present, the autocorrelator values or all three autocorrelators  302 ,  304  and  306  plateau above their respective thresholds. The value produced by the first autocorrelator  302  indicated at  502 - 1  is approximately equal to 50. The value produced by the second autocorrelator  304  indicated at  504 - 1  is approximately equal to 64. The value produced by the third autocorrelator  306  indicated at  506 - 1  is approximately equal to 100. The individual values attained by each of the autocorrelators depend on the delay of the samples. 
     During the time interval t=50 microseconds to t=60 microseconds, when noise, SSI and jam signal are present, only the value produced by the second autocorrelator  304 , indicated at  504 - 2 , surpasses its threshold  504 . The autocorrelation values produced by the first and third autocorrelators  302  and  306  are beneath their respective thresholds  502  and  506 . The jam signal referred to here is a periodic signal, but NOT a sinusoid and is stronger in power than the SSI. A sinusoid will generally give rise to a high value of autocorrelation for almost any delay. However, the jam signal peaks ONLY for the autocorrelator with delay that matches its periodicity, and hence during T=50 to T=60, only the second autocorrelator&#39;s output value is high, since SSI is weaker in power than the jam signal. 
     During the time interval after t=60 microseconds, when only noise is present, all three autocorrelator values drop below their thresholds. 
       FIG. 6  is an illustrative block diagram representing monitor circuitry  600  in the BPMAC section  110  operable to implements an autocorrelation metric that prescribes the length of time during which the first and third autocorrelators of  FIG. 3  should maintain a prescribed value in order to prompt a state transition in accordance with the present disclosure. Block  602  represents receipt of respective corr1_acc and corr3_acc values produced by the first and third autocorrelators  302  and  306 , respectively. Hi threshold counter block  604  represents circuitry that counts the number of consecutive clock cycles during which the autocorrelation value corr1_acc exceeds a threshold value of CORR1_TH_HI. The CORR1_TH_HI threshold value is the same as threshold  502 . Every time this CORR1_TH_HI condition is met, the counter value is incremented.) Hi threshold counter block  604  also represents circuitry that counts the number of consecutive clock cycles during which the autocorrelation value corr3_acc exceeds a threshold value of CORR3_TH_HI. HI. The CORR3_TH_HI threshold counter value is the same as threshold  506 . Every time the CORR3_TH_HI condition is met, the counter value is incremented. 
     Lo counter threshold block  606  represents circuitry that counts the number of consecutive clock cycles during which autocorrelation value corr1_acc is less than a threshold value of CORR1_TH_LO. Lo counter threshold block  606  also represents circuitry that counts the number of consecutive clock cycles during which autocorrelation value corr3_acc is less than a threshold value of CORR3_TH_LO. Usually the LO threshold is set the same as the HI threshold. The LO threshold capability adds flexibility in the hardware to provide hysteresis, by providing a separate register for the LO threshold. There are times when it might be desirable to make the FSM come back to state 0 from state 1, with not as much ease as it went from state 0 to state 1. The additional register for LO threshold can help achieve that result. 
     A cord FSM (finite state machine)  608  receives a corr1hi_ctr value and a corr1lo_ctr value from the respective hi counter threshold block  604  and lo counter threshold block  606 . Similarly, a corr3 FSM  610  receives a corr3hi_ctr value and a corr3lo_ctr value from the respective hi counter threshold block  604  and lo counter threshold block  606 . 
       FIG. 7  is an illustrative state diagram  700  representing state transitions of FSMs  608  and  610  of the monitor circuitry  600  in response to the duration of time during which autocorrelation values produced by the first and/or third autocorrelators  302  and  306  exceed their respective high thresholds or fall below their respective low thresholds. The FSM  608  transitions to state 704 in which corr1_peak=1 if corr1hi_ctr&gt;=CORR1_CTR_TH_HI. The FSM  608  transitions to state 702 in which corr1_peak=0 if corr1lo_ctr&gt;=CORR1_CTR_TH_LO. The FSM  610  transitions to state 704 in which corr3_peak=1 if corr3hi_ctr&gt;=CORR3_CTR_TH_HI. The FSM  610  transitions to state 702 in which corr3_peak=0 if corr3lo_ctr&gt;=CORR3 CTR_TH_LO. 
     It will be understood that the state transitions computed as explained with reference to  FIGS. 6-7  result in the states referenced in decisions steps  402  and  404  in  FIG. 4 . Thus, for example, the first and third autocorrelators  302  and  306  have to maintain a value above a prescribed threshold for at least a prescribed amount of time in order to prompt a state change to corrj_peak=1 (j=1,3) in its respective FSM  608  or  610 . Conversely, once a FSM  608  or  610  has transitioned to the corrj_peak=1 state, it has to fall below a prescribed threshold for at least a prescribed amount of time in order to prompt a state change back to corrj_peak=0. The time interval requirements guards against state transitions occurring in response to environmental noise, for example. Thus, the FSMs  608  and  610  are designed robustly enough to ascertain whether autocorrelation values continue to substantially meet their respective thresholds for at least substantially their respective counts despite minor fluctuations due to environmental factors unrelated to whether a detected signal is a signal of interest or an interferer, for example. 
       FIGS. 8A-8B  are illustrative graphs showing an example of how hi threshold levels described with reference to  FIGS. 6-7  can be produced. Results such as those shown by way of example in  FIGS. 8A-8B  are arrived at empirically.  FIG. 8A  represents a measure of probability density function (PDF) versus autocorrelation value for one of the autocorrelators attuned to identify an SSI. The curve labeled  802  shows results in presence of a sinusoidal interferer only. The curve labeled  804  shows results in presence of a noise and a jam signal only. The curve  802  shows that autocorrelator value has a highest probability of a value of about 29 in the presence of only an interferer. The curve  804  shows that autocorrelator value has a highest probability of a value of about 11 in the presence of only a packet and noise. The separation of the two curves is significant because it is the reason that the autocorrelator values for the first and third autocorrelators  302  and  306  drop off during the 50-60 microsecond time interval shown in  FIG. 5 . Basically, the first and the third autocorrelator do not give rise to a high value of autocorrelation in the presence of a jam signal. 
       FIG. 8B  shows a measure of cumulative density function (CDF) versus autocorrelation value for one of the autocorrelators. The curve  802 - 1  represents inverse cumulative distribution function (1-area) of the autocorrelation value produced by samples corresponding to the “interferer” portion of the packet and curve  804 - 1  represents cumulative distribution function of the autocorrelation value produced by samples corresponding to the “nosie+desired signal” portion of the packet. The value at the crossing point of curves  802 - 1  and  804 - 1  is selected as the threshold value for the autocorrelator represented by these curves. In this example, that value is 19.47. 
     In very noisy environments, the values cor1_acc and corr3_acc can fluctuate significantly in the presence of a SSI signal. A problem that can result is that the fluctuation may prevent a state transition of FSM  608  and/or FSM  610  to state  704  shown in  FIG. 7 , because the fluctuations may preclude the autocorrelation values from staying above the hi threshold long enough to trigger such a state transition. As a result, it is possible that only the second autocorrelator will have a value that exceeds its threshold, and as a result, a false detection of a jam signal could be declared, causing unnecessary transmission delay. 
     In order to mitigate the problem of noise causing the first or third autocorrelators to be unable to exceed their hi thresholds despite the presence of an SSI signal, the BPMAC  110  includes circuitry that monitors the values of corr1_acc and corr3_acc after corr2_acc after the state transition process of  FIG. 4  results in a transition to temporarily disable RF transmission. If both corr1_acc and corr3_acc exceed their respective thresholds before the passage of the right index of the supposed jam signal, i.e. 7.2 microseconds elapsed, then a symbol error (symb_err=1) is declared, and the state transition process of  FIG. 4  results in permitting RF transmission. 
       FIG. 9  is an illustrative block diagram of a cellular phone system  950  that includes circuitry for detection of strong high frequency interference in wireless communications in accordance with the present disclosure. The cellular phone  950  includes a cellular antenna  951 . Signal processing and/or control circuits  952  communicate with a WLAN  968  interface and/or memory  966  or mass data storage  964  of the cellular phone  950 . In some implementations, the cellular phone  950  includes a microphone  956 , an audio output  958  such as a speaker and/or audio output jack, a display  960  and/or an input device  962  such as a keypad, pointing device, voice actuation and/or other input device. WLAN interface  968  includes the detection circuitry (not shown). The signal processing and/or control circuits  952  and/or other circuits (not shown) in the cellular phone  950  may process data, perform coding and/or encryption, perform calculations, format data and/or perform other cellular phone functions. The cellular phone  950  may be connected to memory  966  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The cellular phone  950  also may support connections with a WLAN via the WLAN network interface  968 . 
       FIG. 10  is an illustrative block diagram of a media player  1000  that includes circuitry for detection of strong high frequency interference in wireless communications in accordance with the present disclosure. In one implementation, the media player may include an MP3 player, for example. The media player  1000  may communicate with mass data storage  1010  that stores data such as compressed audio and/or video content in a nonvolatile manner. In some implementations, the compressed audio files include files that are compliant with MP3 format or other suitable compressed audio and/or video formats. The mass data storage may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. The media player  1000  may be connected to memory  1014  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The media player  1000  also may support connections with a WLAN via the WLAN network interface  1016 . Still other implementations in addition to those described above are contemplated. 
     Signal processing and/or control circuits  1004  communicate with a WLAN interface  616  and/or mass data storage  1010  and/or memory  1014  of the media player  1000 . The WLAND interface  616  includes the detection circuitry (not shown). In some implementations, the media player  1000  includes a display  1007  and/or a user input  1008  such as a keypad, touchpad and the like. In some implementations, the media player  1000  may employ a graphical user interface (GUI) that typically employs menus, drop down menus, icons and/or a point-and-click interface via the display  1007  and/or user input  1008 . The media player  1000  further includes an audio output  1009  such as a speaker and/or audio output jack. The signal processing and/or control circuits  1004  and/or other circuits (not shown) of the media player  1000  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other media player function. 
       FIG. 11  is an illustrative block diagram of a high definition television (HDTB)  1100  that includes circuitry for detection of strong high frequency interference in wireless communications in accordance with the present disclosure. Performance monitor circuitry may be implemented in a WLAN interface  1129 . The HDTV  1100  receives HDTV input signals in either a wired or wireless format and generates HDTV output signals for a display  1126 . In some implementations, signal processing circuit and/or control circuit  1100  and/or other circuits (not shown) of the HDTV  1100  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other type of HDTV processing that may be required. 
     The HDTV  1100  may communicate with mass data storage  1127  that stores data in a nonvolatile manner such as optical and/or magnetic storage devices. The HDTV  1100  may include mass storage  1127  such as HDD. The HDTV  1100  also may include memory  1128  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The HDTV  1100  also supports connections with a WLAN via the WLAN network interface  1129 , which includes the detection circuitry. 
       FIG. 12  is an illustrative block diagram of a vehicle  1200  a WLAN interface  848  that includes circuitry for detection of strong high frequency interference in wireless communications in accordance with the present disclosure. In some implementations, a powertrain control system  1232  receives inputs from one or more sensors  1236  such as temperature sensors, pressure sensors, rotational sensors, airflow sensors and/or any other suitable sensors and/or that generates one or more output control signals such as engine operating parameters, transmission operating parameters, and/or other control signals. 
     Other control systems  1240  of the vehicle  1200  may likewise receive signals from input sensors  1242  and/or output control signals to one or more output devices  1244 . In some implementations, the control system  1240  may be part of an anti-lock braking system (ABS), a navigation system, a telematics system, a vehicle telematics system, a lane departure system, an adaptive cruise control system, a vehicle entertainment system such as a stereo, DVD, compact disc and the like. Still other implementations are contemplated. 
     The powertrain control system  1232  may communicate with mass data storage  1246  that stores data in a nonvolatile manner. The mass data storage  1246  may include optical and/or magnetic storage devices such as, for example, hard disk drives HDD and/or DVDs. The powertrain control system  1232  may be connected to memory  1247  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The powertrain control system  1232  also may support connections with a WLAN via a WLAN network interface  1248 . The control system  1240  may also include mass data storage, memory and/or a WLAN interface (all not shown). 
       FIG. 13  is an illustrative block diagram of a set top box  1100  that includes WLAN interface  996  with circuitry for detection of strong high frequency interference in wireless communications in accordance with the present disclosure. The set top box  1300  receives signals from a source  1381  such as a broadband source and outputs standard and/or high definition audio/video signals suitable for a display  1388  such as a television and/or monitor and/or other video and/or audio output devices. The signal processing and/or control circuits  1384  and/or other circuits (not shown) of the set top box  1300  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other set top box function. 
     The set top box  1300  may communicate with mass data storage  1390  that stores data in a nonvolatile manner. The mass data storage  1390  may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. The set top box  1300  may be connected to memory  1394  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The set top box  1300  also may support connections with a WLAN via the WLAN network interface  1396 . 
     While certain implementations disclosed herein are described with reference to various illustrative features and aspects, the implementations may be modified to include other features and aspects. For example, in an implementation described above, two autocorrelators are provided with delays that straddle the delay of the autocorrelator having a delay attuned to the period of the jam signal. However, fewer or greater than a total of three autocorrelators may be employed. The implementations disclosed herein are therefore to be broadly interpreted and construed as including other alternative variations and modifications.