Abstract:
An embodiment of the present invention provides a method for communicating data in the presence of jamming interference, comprising transmitting a first signal carrying the data from a transmitter to a receiver at a data transmission rate, determining at the receiver that the first signal has been corrupted by the jamming interference, sending a reply from the receiver to the transmitter, indicating that the first signal was corrupted, responsive to the reply, transmitting a second signal carrying the data from the transmitter to the receiver substantially without back-off of the transmission rate, and processing the first and second signals at the receiver to recover the data therefrom.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Applications No. 60/274,499, filed Mar. 9, 2001, and No. 60/297,862, filed Jun. 13, 2001. Both of these provisional applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to wireless data communication networks, and specifically to receivers for wireless local area networks that must operate in the presence of jamming.  
       BACKGROUND OF THE INVENTION  
       [0003]     Wireless local area networks (WLANs) are gaining in popularity, and new applications are being developed. The original WLAN standards, such as “Bluetooth” and IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a and 802.11b extensions to the original standard, in order to enable higher data rates. The 802.11a standard (including Annex G of the standard) envisions data rates over 20 Mbps over short distances in the 2.4 and 5.5 GHz bands, using Coded Orthogonal Frequency Division Modulation (COFDM). The COFDM system uses multiple subcarriers, which are modulated using phase shift keying (PSK) or quadrature amplitude modulation (QAM). The 802.11b standard defines data rates up to 11 Mbps in the 2.4 GHz band using Quadrature PSK (QPSK) or 8-ary PSK (8PSK). These modulation schemes are further described in the IEEE 802.11 standards, which are incorporated herein by reference.  
         [0004]     Wireless modems are sensitive to unintentional jamming by high-power narrowband signals in the communication band of the desired signal. Jamming is particularly problematic in the 2.4 and 5.5 GHz bands, which have been set aside by the Federal Communications Commission (FCC) for unlicensed use. Modems operating in this range under the 802.11 standards must typically deal with strong jamming signals generated by other communication devices such as cordless telephones and Bluetooth transmitters. Receivers known in the art generally use adaptive notch filters to remove such interference. Techniques of adaptive filtering, however, suffer from slow convergence and lack the capability to deal with transient in-band jamming. Bluetooth signals, for example, hop to a new frequency in the 2.4 GHz band once every microsecond, in a hop pattern that appears random to a non-Bluetooth device. Even using very fast adaptation, every such hop in an adaptive filtering system will generally cause a burst of errors in an COFDM or M-PSK receiver.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with one aspect of the present invention, a wireless receiver applies one or more of a number of novel approaches to estimate and remove jamming interference from received Frequency Division Modulation (FDM) signals. These approaches include the following: 
        Detecting the frequencies at which jamming occurs, and erasing these frequencies from the processed signal before demodulating the signal to recover the data. Typically, convolutional coding is applied to the transmitted signal, and the signal is demodulated using a Viterbi decoder in the receiver. In an exemplary embodiment, certain frequencies can be erased or given reduced weights during the decoding process in a manner that permits the signal to be decoded based on the remaining frequencies. Alternatively, the principle of erasing jammed frequencies can also be applied using other demodulation techniques, as are known in the art.     Detecting the phase of the jamming signal, using a phase detector, and then reconstructing the jamming signal so as to cancel it from the processed signal. The phase detector operates most effectively in this manner when the jamming signal is strong, and this method is therefore preferably used when the strength of the jamming signal prevents sufficiently complete removal solely through the preceding approach of frequency “erasure.” Due to factors such as the finite length of the time window used in transforming the received signal to the frequency domain for demodulation, the effects of a strong jamming signal in a narrow band may be felt over a much wider range of frequencies in the receiver. Cancellation of the jamming signal before transformation to the frequency domain can help to overcome this problem.     Demodulating the jamming signal, when the method of modulation used in the jamming signal is known (for example, Frequency Shift Keying—FSK—modulation used for Bluetooth signals). The receiver can then accurately reconstruct the jamming signal in order to remove it from the processed signal. This approach is preferably used under the strongest jamming conditions.     Active digital or analog cancellation of the jamming signal. This approach is possible when the jamming signal is “cooperative,” for example, a signal generated by a Bluetooth transmitter collocated with a FDM receiver, so that the transmitter output is known to the receiver.        
 
         [0010]     A receiver configured in accordance with the present invention may include an anti-jamming controller, which monitors the jamming characteristics, and selects one of the above methods depending on the level and nature of jamming. Several jamming signals may be monitored and dealt with simultaneously in this manner. Alternatively, the receiver may be designed to implement only one these anti-jamming methods, against a single jamming signal or multiple jamming signals. If the jamming is sufficiently weak, the controller turns off the anti-jamming function.  
         [0011]     In certain embodiments of the present invention, the phase detector used in determining the phase of the jamming signal comprises a phase-locked loop (PLL), preferably a digitally-implemented PLL. A buffer typically stores one or more blocks of samples of the incoming signal while the phase detector and jamming cancellation circuitry operates on the samples. This block-oriented mode of operation allows the PLL to be operated in non-causal fashion, running both forward and backward in time, in order to accurately estimate transitions and other parameters of the jamming signal. Alternatively, the phase detector may comprise a filter, such as a finite impulse response (FIR) or infinite impulse response (IIR) filter, with an automatic frequency control (AFC) circuit.  
         [0012]     In further exemplary embodiments of the present invention, other techniques are used to improve the anti-jamming performance of a transmitter/receiver pair. In one such exemplary embodiment, when the receiver determines that a packet has been corrupted by jamming but that the packet header may nonetheless enable identification of the identity of the transmitter that sent the packet, the receiver sends a NACK (non-acknowledge) signal to the transmitter. The transmitter, upon receiving the NACK, retransmits the packet without back-off. Additionally or alternatively, an outer code is added to the transmitted data in each packet. Preferably, for the purpose of outer coding, each packet is divided into several code words, with Reed-Solomon codes.  
         [0013]     Although the inventors have developed these techniques particularly for use in FDM schemes, such as those specified by IEEE standard 802.11a, the principles of these techniques may also be applied, mutatis mutandis, to processing of signals based on other modulation schemes, such as M-PSK (as specified by the 802.11b standard) and Code Division Multiple Access (CDMA) schemes. These principles may also be applied to different types of narrowband jamming signals with different modulation schemes, such as PSK modulation, QAM modulation or CDMA.  
         [0014]     In one aspect, the present invention relates to a receiver capable of receiving a signal carrying data via multiple subcarriers at respective subcarrier frequencies. The receiver includes an anti-jamming (AJ) processor, adapted to assess jamming interference on the subcarrier frequencies and, responsive thereto, to assign respective reliability metrics to the subcarriers. The receiver further includes a demodulator adapted to demodulate the signal using the reliability metrics and thereby recover the data.  
         [0015]     In another aspect, the present invention comprises a receiver capable of receiving a signal carrying data in the presence of jamming interference. The receiver includes an anti-jamming (AJ) processor adapted to process the received signal so as to determine a frequency, phase and amplitude of the jamming interference. A jamming cancellation circuit, coupled to the AJ processor, removes the jamming interference from the received signal responsive to the frequency, phase and amplitude determined by the AJ processor. The receiver further includes a demodulator adapted to demodulate the signal after removal of the jamming interference therefrom so as to recover the data.  
         [0016]     In yet another aspect, the present invention comprises a method for communicating data in the presence of jamming interference. The method includes transmitting a first signal carrying the data from a transmitter to a receiver at a data transmission rate. If it is determined at the receiver that the first signal has been corrupted by the jamming interference, then a reply is sent from the receiver to the transmitter, indicating such corruption has occurred. In response to the reply, a second signal carrying the data from the transmitter to the receiver is transmitted substantially without back-off of the transmission rate. The first and second signals are then processed at the receiver in order to recover the data therefrom.  
         [0017]     The present invention also contemplates a method for processing a received signal carrying data via multiple subcarriers at respective subcarrier frequencies. The method includes assessing jamming interference on the subcarrier frequencies. In response to the assessed interference, respective reliability metrics are assigned to the subcarriers. The signal is then demodulated using the reliability metrics so as to recover the data.  
         [0018]     In a further aspect, the present invention relates to a method for recovering data from a signal received in the presence of jamming interference. The method includes processing the received signal so as to determine a frequency, phase and amplitude of the jamming interference. Jamming interference is removed from the received signal responsive to the determined frequency, phase and amplitude. The signal is then demodulated after removal of the jamming interference therefrom so as to recover the data.  
         [0019]     The present invention will be more fully understood from the following detailed description of various embodiments thereof, taken together with the drawings in which:  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a block diagram that schematically illustrates a wireless data receiver, in accordance with an exemplary embodiment of the present invention;  
         [0021]      FIG. 2  is a block diagram that schematically illustrates an anti-jamming processor, in accordance with an exemplary embodiment of the present invention;  
         [0022]      FIG. 3  is a block diagram that schematically illustrates a phase detector for determining the phase of a jamming signal, in accordance with an preferred embodiment of the present invention;  
         [0023]      FIG. 4  is a block diagram that schematically illustrates circuitry for demodulating and reconstructing a jamming signal, in accordance with an exemplary embodiment of the present invention;  
         [0024]      FIG. 5  is a flow chart that schematically illustrates a method for removing jamming interference from a signal received by a modem, in accordance with an exemplary embodiment of the present invention;  
         [0025]      FIGS. 6A, 6B  and  6 C are timing diagrams that schematically illustrate processing of signals by a modem to removing jamming interference from the signal, in accordance with an exemplary embodiment of the present invention;  
         [0026]      FIG. 7  is a block diagram that schematically illustrates circuitry for digital active cancellation of a jamming signal, in accordance with an exemplary embodiment of the present invention; and  
         [0027]      FIG. 8  is a flow chart that schematically illustrates a method for transmitting and receiving data packets in the presence of jamming, in accordance with an exemplary embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0028]      FIG. 1  is a block diagram that schematically illustrates a wireless receiver  20 , in accordance with an exemplary embodiment of the present invention. In the description that follows, receiver  20  is assumed to be part of a modem used in a wireless LAN (WLAN), operating in accordance with an COFDM modulation scheme. Exemplary schemes of this sort are those put forth by IEEE standard 802.11a, including Annex G of the standard, as noted in the Background of the Invention. The WLAN environment is assumed to be noisy and, in particular, subject to jamming interference from a variety of possible sources, such as signals generated by Bluetooth transmitters. Although the elements of receiver  20  are shown and described in terms of separate functional blocks, it will be apparent to those skilled in the art that many or even all of these blocks may be implemented in a single integrated circuit chip or in a set of such chips. Additionally or alternatively, the digital processing functions described hereinbelow may be implemented in software running on a suitable microprocessor.  
         [0029]     Receiver  20  comprises an analog front end (AFE)  22 , which performs initial analog processing on signals received over the air, as is known in the art. The AFE filters, amplifies, and splits the signals into in-phase (I) and quadrature (Q) parts, which are digitized by analog/digital (A/D) converters  24 . Preferably, the A/D converters sample the incoming signal at a rate of 44 Msps (million samples/sec). The digitized signals are then held in an A/D buffer  26 , which is preferably configured to hold samples from two successive time segments, corresponding to two successive COFDM symbols. In terms of the 802.11a standard, this means that the buffer should hold two I/Q vectors of samples, each comprising 44*80*2 samples.  
         [0030]     Preferably, an automatic gain control (AGC) circuit  28  reads the digitized samples in buffer  26  and analyzes the signals to control the gain of AFE  22 , as is known in the art. A preamble detector (not shown) also uses the data in the buffer to detect the beginning of a data packet and thereby control other elements of receiver  20 . The functions of the preamble detector are beyond the scope of the present patent application and are therefore omitted from the figures for the sake of simplicity.  
         [0031]     An anti-jamming (AJ) processor  30  reads the data in A/D buffer  26 , and uses these data to reduce or eliminate the effects of jamming signals in receiver  20 . Preferably, processor  30  implements a range of different anti-jamming measures, depending on the strength and nature of the jamming signals, as described in detail hereinbelow. By holding two symbols in succession, buffer  26  allows processor  30  to analyze the incoming samples in both forward and reverse temporal directions, so as to accurately determine the phase, frequency and onset/termination of any jamming signals. When the jamming signal is strong, processor  30  reconstructs the signal and applies an AJ canceling block  32  to subtract the reconstructed jamming signal from the samples in buffer  26 . Additionally or alternatively, processor  30  may indicate that certain frequencies in the input signal should be erased, or at least treated as unreliable, when the signals are decoded.  
         [0032]     Following jamming cancellation, the I/Q samples are frequency-corrected by a phase rotator  34 , under the control of an automatic frequency control (AFC) circuit  36 . Each symbol is then converted to the frequency domain, preferably by a Fast Fourier Transform (FFT) processor  38 . The FFT output is held in a buffer  40 , which provides input to AFC circuit  36 , as is known in the art. The FFT results are also used by AJ processor  30  in determining the frequencies of narrowband jamming signals. In the case of narrowband jammers such as a Bluetooth signal, the jamming signals typically appear as sharp peaks in the spectrum of the COFDM signals.  
         [0033]     Based on the FFT results, a metric quantization block  42  assigns a QAM metric to each QAM symbol of the COFDM signal. The QAM metric provides an estimate of the received QAM symbol that is utilized during the decoding process described below. In addition, a channel estimator  44  generates a channel estimate by approximating the phase shift and gain applied to each subcarrier by the communication channel over which the signals are received. Assuming that the symbols were interleaved by the transmitter, a de-interleaver  46  is used to reverse the interleaving. The resultant stream of QAM metrics is then input to a decoder  48 , typically a Viterbi decoder, along with corresponding channel estimates. Subject to the anti-jamming processing described hereinafter, the decoder  48  then regenerates the transmitted bitstream on the basis of this estimated symbol and channel information.  
         [0034]     In accordance with the invention, when channel estimator  44  has determined that certain frequencies have been corrupted by jamming, it indicates to decoder  48  that the QAM metrics of the corresponding subcarriers should be assigned a low reliability or ignored altogether in the decoding process. The reliability of the subcarriers is typically represented by respective reliability metrics applied by the decoder. The nature of FDM signal transmission with convolutional coding, together with Viterbi decoding, makes it possible to drop certain carriers at the decoding stage without losing data in the bitstream.  
         [0035]     The bitstream output of decoder  48  is used in higher-level functions of receiver  20 , including an ACK (acknowledgment) detector  50  and a MAC (media access control) interface  52 , as are known in the art. The decoder output is also used by channel estimator  44  and by AFC circuit  36 . For the purposes of AFC, the bitstream output of decoder  48  is re-encoded, following a short chain-back and interleaving process, and is compared to the samples stored in buffer  40  in order to determine the frequency correction to be applied by rotator  34 .  
         [0036]      FIG. 2  is a block diagram showing details of AJ processor  30 , in accordance with an exemplary embodiment of the present invention. As pictured in  FIG. 2  and described hereinbelow, processor  30  provides a range of different AJ measures, which are implemented selectively by an AJ controller  60 , depending on the strength and other characteristics of the jamming signal. The measures are typically selected based on the following alternative jamming scenarios: 
        1. Jamming signal absent or too weak to detect. In this case, each subcarrier channel is assigned a reliability metric based on the quality of reception at the corresponding frequency. The metric is applied by decoder  48 .     2. Jamming signal is detectable, but not strong enough to reconstruct its phase and other parameters. The subcarrier channels that are jammed are assigned an erasure metric. The erasure metric is typically a reliability measure that, when applied to jammed channels, is so low as to cause decoder  48  to effectively ignore such channels. Other subcarriers receive reliability metrics based on the quality of reception, as described above.     3. Jamming signal is strong enough to allow reconstruction. In this case, an interference estimation circuit  62  reconstructs the phase and other parameters of the jamming signal, while the input signal is held in buffer  26 . Details of circuit  62  are shown in  FIGS. 3 and 4 . The reconstructed jamming signal is subtracted from the input signal by adders  66  in AJ canceling block  32 . The reliability or erasure metrics of the subcarrier channels are preferably assigned or adjusted following AJ cancellation.     4. Jamming signal is strong enough to allow reconstruction, and its modulation scheme is known. This will be the case, for example, when the jamming is known to be caused by FSK signals, as are generated by Bluetooth transmitters. In this case, circuit  62  can not only estimate the phase of the jamming signal, but can actually demodulate the signal and derive other signal parameters, such as modulation index and timing. The reconstructed jamming signal is subtracted from the signal in buffer  26 , and the subcarrier metrics are assigned, as described above.     5. Jamming signal is generated by a known source, such as a Bluetooth transmitter collocated with receiver  20 . For example, it is expected that some data communication devices will be configured with both a Bluetooth modem for low-rate communications and an 802.11-compliant modem for higher-rate transmissions. In this case, a digital active cancellation circuit  64  can receive an input from the Bluetooth transmitter (or other known jamming source), and can use this input to determine precisely the anti-jamming correction to be applied by AJ canceling block  32 . Details of circuit  64  are shown in  FIG. 7 .          
         [0042]     AJ processor  30  may be configured to apply all of the above measures, or only a subset of them, depending on the expected jamming environment in which receiver  20  must operate and other factors, such as cost of the modem or other device in which the receiver is used. When multiple jamming signals are present simultaneously at different frequencies, AJ controller  60  may decide to apply different measures against different signals, depending on the jamming signals strengths and modulation characteristics. In order to reconstruct multiple jamming signals in real time, AJ processor  30  will typically comprise multiple interference estimation and cancellation circuits or modules. Although the possibility of erasing or reducing the reliability measure of jammed subcarriers (scenario 2 above) is applicable primarily to FDM receivers, the remaining alternatives for reconstructing and canceling the jamming signal at the receiver can be applied in receivers using other modulation schemes, as well, such as M-PSK modulation.  
         [0043]      FIG. 3  is a block diagram that schematically shows details of interference estimation circuit  62 , in accordance with an exemplary embodiment of the present invention. In this embodiment, circuit  62  is based on a second-order digital phase-locked loop (PLL)  70 , which determines the phase of the jamming signal. Alternatively, other types of PLLs may be used, as described, for example, by Lindsey and Chie, in “A Survey of Digital Phase-Locked Loops,”  Proceedings of the IEEE  69 (1981), pp. 410-431, which is incorporated herein by reference. Further alternatively, other means known in the art may be used to determine the phase of the jamming signal, such as a suitable finite impulse response (FIR) filter or an infinite impulse response (IIR) filter.  
         [0044]     PLL  70  generates a waveform s(n), given by s(n)=K exp {j({circumflex over (Φ)}(n)+Φ 0 }, wherein n is the sample index, given by t=nT sample , and {circumflex over (Φ)}(n) is the estimated phase of the jamming signal at sample n. A complex multiplier  72  multiplies samples r(n) from buffer  26 , having phase Φ(n), by s*(n), to generate an error signal: 
 
 e ( n )= r ( n )• s *( n )= K ′ exp { j (Φ( n )−{circumflex over (Φ)}( n )}  (1) 
 
 An arctangent converter  74  converts the I and Q components of e(n) to a phase error: 
 
θ e ( n )=arctan ( r ( n )• s *( n ))   (2) 
 
         [0045]     The phase error is fed to a first amplifier  76 , with gain G 1 , and to a second amplifier  82 , with gain G 2 , via an adder  78  and an accumulator  80 . Computation of the appropriate gain values is described in an Appendix to this specification. The outputs of the amplifiers are summed by an adder  84 , and the result is summed by another adder  86  with the contents of an accumulator  88  and with a constant increment 2πf BT T sample . Here f BT  is the frequency of the jamming signal, determined based on the output of FFT processor  38 . The label “BT” is used to denote that the jamming signal is generated by a Bluetooth transmitter, by way of example, but other narrowband jamming signals can be treated in like manner. The phase error stored in accumulator  88  is converted to I/Q form, by a converter  90 , to generate the new value of the signal s(n). This signal is conjugated using an inverter  92 , and is then input to complex multiplier  72 .  
         [0046]     Assuming that PLL  70  has sufficient bandwidth to track changes in the jamming signal, the waveform s(n) provides a consistent reconstruction of the jamming signal. To determine the correct amplitude K to apply to the waveform, the phase error determined by arctangent converter  74  and the amplitude of the jamming frequency components from FFT processor  38  are fed to a gain estimator  94 . The estimator preferably uses a lookup table to determine the optimal gain value, which is applied by a gain set-up block  96  to the I and Q components of s(n) that are output by converter  90 . The result is a reconstruction of the phase and amplitude of the jamming signal, which is then subtracted by AJ canceling block  32  from the input signal in buffer  26 .  
         [0047]      FIG. 4  is a block diagram that schematically illustrates details of interference estimation circuit  62 , in accordance with another preferred embodiment of the present invention. Here it is assumed that the modulation scheme of the jamming signal is known, allowing the jamming parameters to be accurately estimated by actually demodulating the jamming signal. In this embodiment, too, circuit  62  comprises a phase lock demodulator  100 , which is preferably similar in design and operation to PLL  70 , as described above. The phase estimate of the jamming signal is smoothed by low-pass filtering with an adder  102  and a delay stage  104 . Based on the phase estimate, a parameter estimation block  106  determines other parameters needed to reconstruct the jamming signal, including its modulation index k and its start and stop times. An interference reconstruction block  108  uses the signal phase and the other parameters provided by block  106  to generate the reconstructed jamming signal output s(t).  
         [0048]     For the specific example of a Bluetooth jamming signal, with Gaussian FSK (GFSK) modulation, the modulated jamming signal can be represented as:  
               v   ⁡     (   t   )       =           2   ⁢           ⁢   E     T       ⁢   exp   ⁢     {     j   ⁡     (       Φ   ⁡     (   t   )       +     Φ   0       )       }               (   3   )             
 
 Here E is the signal energy, and T is the symbol period (1 μs for Bluetooth signals). The continuous phase of v(t) is given by:  
               Φ   ⁡     (   t   )       =       ω   ⁢           ⁢   t     +     2   ⁢           ⁢   π   ⁢           ⁢   k   ⁢       ∫     -   ∞     t     ⁢       m   ⁡     (   η   )       ⁢           ⁢     ⅆ   η           +   θ             (   4   )             
 
 wherein m(t) is the modulating signal, k is the modulation index, ω is the center frequency of the Bluetooth signal, and θ is the random phase of the Bluetooth signal. 
 
         [0049]     In Bluetooth GFSK, the modulating signal is the result of filtering a NRZ (non-return to zero) sequence of the input data bits with a Gaussian filter whose time impulse response is:  
                   h   G     ⁡     (   t   )       =         π     α     ⁢     exp   ⁡     (         -     π   2         α   2       ⁢     t   2       )           ,     α   =         2   ⁢           ⁢   ln   ⁢           ⁢   2       B               (   5   )             
 
 where B is the 3 dB bandwidth of Gaussian filter. The modulating signal can then be expressed as:  
               m   ⁡     (     kT   Sample     )       =       ∑     n   =     -   ∞       ∞     ⁢       x   ⁡     (       (     k   -   n     )     ⁢     T   Sample       )       ⁢       h   G     ⁡     (     nT   Sample     )                   (   6   )             
 
 wherein x(nT Sample ) is the sampled NRZ input signal. In an exemplary implementation of the receiver  20 , the A/D converters  24  operate to sample the NRZ input signal at the rate of 44 Msps. 
 
         [0050]     Given the phase information and signal parameters determined by blocks  100  and  106 , the estimated jamming signal reconstructed by block  108  will have a complex envelope of the form:  
               s   ⁡     (   t   )       =       K   ⁢           ⁢   exp   ⁢     {     j   ⁡     (         Φ   ^     ⁡     (   t   )       +     Φ   0       )       }       =     K   ⁢           ⁢   exp   ⁢     {     j   ⁢       ∑     -     n   0         NT   s       ⁢     (         h   G     ⁡     (   n   )       *       θ   e     ⁡     (   n   )         )         }                 (   7   )             
 
 where T s  is used for brevity to denote T sample . It will be seen that s(t) is, essentially, a delayed version of the original jamming signal given by equation (3). Preferably, equation (7) is used to construct a lookup table, which is used by block  108  in reconstructing s(t). 
 
         [0051]     Reference is now made to  FIGS. 5 and 6 A-C, which schematically illustrate methods for determining the phase of a jamming signal, in accordance with an exemplary embodiment of the present invention.  FIG. 5  is a flow chart, showing the steps carried out by AJ processor  30  and other elements of receiver  20  in estimating parameters of the jamming signal. FIGS.  6 A-C are timing diagrams, showing the timing of processing stages involved in the phase estimation methods of  FIG. 5 .  FIG. 6A  shows the processing stages involved when a jamming signal was present during the preceding COFDM symbol received by receiver  20  and continues through the present symbol;  FIG. 6B  shows the stages when a jamming signal present during the preceding COFDM symbol terminates at the present symbol; and  FIG. 6C  shows the stages when a jamming signal is detected initially during the present symbol, without its having been detected at the preceding symbol.  
         [0052]     The methods illustrated by these figures take advantage of the block-oriented processing structure of receiver  20 . In accordance with this structure, during the time a block of samples is stored in buffer  26 , AJ processor  30  can run a phase detector on the samples at least twice—once in a forward time direction, and once in reverse. This forward/backward operation is advantageous in improving the phase detection performance of interference estimation circuit  62 , including removal possible bias and phase distortion that can accumulate when conventional unidirectional phase estimation is used. It is particularly useful in finding start and stop times of the jamming. In the description that follows, reference is made to PLL  70  as an example of a phase detector than can be run in a bi-directional manner, but the methods of  FIGS. 5 and 6 A-C can similarly be applied using phase detectors of other types.  
         [0053]     The method of  FIG. 5  is initiated each time a block of samples corresponding to a new COFDM symbol is received in buffer  26 , at a symbol reception step  110 . This new symbol is referred to in  FIGS. 6A-6C  as COFDM Symbol N-1. The subsequent steps taken by AJ processor  30  depend on whether or not a jamming signal was detected at the previous symbol, as determined at a previous symbol status step  112 . If a jamming signal was detected at the previous symbol, PLL  70  runs over the samples in both forward and reverse directions, at a PLL running step  114 . For efficient convergence of the PLL, the jamming signal frequency is held at the same value as it had at the preceding symbol, and the initial phase values in accumulators  80  and  88  are also set to the values determined at the conclusion of processing of the preceding symbol. After running PLL  70 , the power and phase of the jamming signal are estimated, as described above, at an estimation step  116 . If the jamming signal terminates during Symbol N-1 ( FIG. 6B ), the transit time (i.e., the identification of the sample during the symbol interval at which the jamming terminated) is also estimated.  
         [0054]     Based on the power and phase estimates determined at step  116 , the jamming signal is reconstructed and subtracted out of the current block of samples by AJ cancellation block  32 , at a jamming erasure step  118 . After subtraction of the jamming signal and frequency correction by rotator  34 , FFT processor  38  operates to transform the symbol to the frequency domain, at a FFT step  120 . AJ controller  60  checks the frequency spectrum of the symbol to confirm the existence and removal of the jamming signal, as well as to determine the residual level of interference at the jamming frequency and in other FDM frequency bins. The AJ controller accordingly issues reliability or erasure metrics for these bins, to be applied by Viterbi decoder  48 , at a bin update step  122 . To the extent that the jamming terminated during the current symbol, the next symbol is processed assuming, at step  112 , that no jamming signal was detected during the preceding symbol. The absence of a jamming signal during the next symbol is preferably verified by the FFT performed on the samples of the next symbol.  
         [0055]     When a new jamming signal is detected in the current symbol at step  112 , without the jamming signal having been present in the previous symbol, the frequency and amplitude of the new jamming signal must be estimated before further processing can take place. These estimates are made by running a FFT on the raw samples in buffer  26 , at a preliminary FFT step  124 . Based on the FFT spectrum, the center frequency and gain of the jamming signal are found, at a frequency determination step  126 . Using this information, PLL  70  is run on the samples of the current symbol in a forward direction, then in reverse, and then forward again, at a PLL rerunning step  128 . The results of step  128  are used to estimate the power, phase and start time of the jamming signal during the current symbol, at a start estimation step  130 .  
         [0056]     The estimated parameters of the jamming signal are used to correct the COFDM samples at step  118 . The FFT performed on the corrected samples at step  120  is, in this case, the second FFT performed on the samples of the current symbol. As in the previous case, the FFT enables AJ controller to confirm the jamming frequency (to be used in processing the next symbol, as well) and to determine the metrics to be passed to decoder  48 . Because of the delay in carrying out the second FFT at step  120 , as exemplified by  FIG. 6C , PLL  70  is preferably run on the next block of samples first in the reverse direction, and only afterwards in the forward direction.  
         [0057]      FIG. 7  is a block diagram that schematically shows details of active cancellation block  64  ( FIG. 2 ), in accordance with an exemplary embodiment of the present invention. As noted above, this block is used when there is an actual link or similar cooperative relationship established between receiver  20  and a source of jamming interference, such as a Bluetooth transmitter  152  collocated with the receiver.  
         [0058]     In order to cancel the Bluetooth jamming signal out of the samples of the COFDM symbol, the samples from buffer  26  are input to a complex negative rotator  140 . In operation, the negative rotator  140  functions to frequency align the Bluetooth jamming signal with the baseband Bluetooth transmit signal in order to facilitate establishment of time alignment therebetween. Delay blocks  142  and  144  apply successive delays of T/2 to the samples, wherein T is the Bluetooth symbol period. The samples and their delayed counterparts are then input to a bank of correlators  146 ,  148  and  150  for correlation with delayed versions of the actual signals generated by Bluetooth transmitter  152  provided by a T/2 delay block  156 . For purposes of clarity, the output of the T/2 delay block  156  is not explicitly depicted as being separately connected to each correlator  146 ,  148  and  150 . The results of early correlator  146 , which operates on the undelayed samples, and of late correlator  150 , which operates on the results delayed by T, are input to absolute value blocks  160  and  162 , which provide the real square amplitudes of the complex correlation values. The amplitudes are summed together by an adder  164  and provided to early late filter  158 .  
         [0059]     Meanwhile, the actual signals generated by Bluetooth transmitter  152  are delayed by a variable delay block  154 . The length of the delay is determined by an early/late filter  158 . The delayed signals are subjected to an additional T/2 delay, by a fixed delay block  156 . The delayed signal output from block  154  is combined with the output of on-time correlator  148  by a phase shift determination block  166 , to find the phase shift of the modulation of the Bluetooth signal relative to the COFDM symbols. This phase shift is applied to a complex positive rotator  168  in order to generate the reconstructed Bluetooth signal for subtraction from the COFDM samples by AJ cancellation block  32 .  
         [0060]     Although the preferred embodiments described above make particular reference to FDM schemes, such as those specified by IEEE standard 802.11a, the principles of these techniques may also be applied, mutatis mutandis, to processing of signals based on other modulation schemes, such as M-PSK (as specified by the 802.11b standard) and Code Division Multiple Access (CDMA) schemes. Similarly, although these preferred embodiments deal by way of example with interference caused by Bluetooth transmitters, the methods of the present invention support coexistence of WLANs with multiple narrowband jamming sources with frequency modulation signals. The principles of the present invention may also be applied to other narrowband jammers with different modulation schemes, such as PSK modulation, QAM modulation or CDMA. In such cases, when the modulation scheme of the jamming source is known, interference estimation block  62  and active cancellation block  64  make use of the particular modulation characteristics of the jamming signal, instead of the GFSK characteristics of Bluetooth. Adaptation of the designs shown in  FIGS. 4 and 7  to operate with other modulation schemes will be straightforward for those skilled in the art.  
         [0061]      FIG. 8  is a flow chart that schematically illustrates a method for transmitting and receiving packets over a WLAN in the presence of jamming, in accordance with another preferred embodiment of the present invention. This method relies on a novel protocol, which is implemented by a transmitter and receiver in the WLAN independent of any other AJ measures that may be used in the receiver, such as those described with reference to the preceding figures. Present WLAN protocols, such as those specified by IEEE standards 802.11a and 802.11b, provide for the transmitter to back off (i.e., to reduce) its transmission rate when it determines that packets are being lost due to interference. The reduced transmission rate makes it easier for the receiver to decode the packets, but of course, it reduces the throughput of the data link. While this step may be necessary in the presence of broadband interference, it is unnecessarily severe when only narrowband jamming is concerned.  
         [0062]     Thus, when the receiver determines that a portion of the data in a packet have been corrupted, at a packet reception step  170 , it does not immediately discard the packet, but rather tries to determine the source of the packet and the reason for the data corruption. The receiver attempts to identify the source of the packet by deciphering the source address, typically a MAC address, in the packet header, at an address reading step  172 . If the address is indecipherable, the packet is simply discarded, in accordance with existing protocols, at a discard step  174 .  
         [0063]     On the other hand, if the receiver is able to read the packet source address, and determines that the corruption of the data in the packet was due to jamming, the receiver sends a NACK (non-acknowledge) signal to the transmitter, at a NACK step  176 . The NACK signal tells the transmitter to retransmit the packet without back-off, at a retransmission step  178 . When the retransmitted packet is received, it may be uncorrupted, particularly if the jamming has abated. On the other hand, if the jamming signal continues, the retransmitted packet may also contain corrupted data, but it is probably a different portion of the data from that which was corrupted in the initial packet. Thus, at a decoding step  180 , the receiver decodes the entire contents of the retransmitted packet, with the assistance of the data from the initial packet. In this manner, the jamming interference is overcome, with a less drastic reduction of data throughput than is caused by protocols known in the art.  
         [0064]     Other techniques may also be used to improve the throughput of COFDM transmissions in the presence of jamming. For example, the number of subcarrier channels used may be increased from the 64 frequencies provided by the 802.11a standard to 128 frequencies, provided that sufficiently accurate frequency estimation is used to maintain orthogonality between the channels. Alternatively or additionally, an outer code may be added to the transmitted data in each packet, to be used in reconstructing COFDM symbols that are erased due to jamming. Preferably, for the purpose of outer coding, each packet is divided into several code words with Reed-Solomon codes. For this same purpose, repeat transmission of the symbols may be used, at the cost of reducing the maximum data rate. For example, the transmitter may use a 16 QAM, rate 2/3 convolutional code with repetition, in place of the 16 QAM, rate 1/2 code specified by the standard.  
         [0065]     It will be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.  
       APPENDIX—COMPUTATION OF PLL PARAMETERS  
       [0066]     Attention is drawn to the above-referenced Provisional Patent Application No. 60/297,862, which describes computation of various parameters and initial conditions pertinent to operation of the DLL  70  of  FIG. 3 .