Abstract:
A jamming detector and jamming detecting method are disclosed. In accordance with the present invention, distribution of a received signal is compared with a reference distribution of a specific signal model. Deviation of the signal distribution from the reference distribution can be obtained according to the comparison results. The existence of jamming is determined according to the deviation. If the signal distribution significantly deviates from the reference distribution, it means that jamming exists. An anti-jamming circuit or algorithm can be activated only when jamming is detected, thereby power consumption is reduced.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to anti-jamming for a CDMA-like receiver, more particularly, to a jamming detection for the receiver. 
       BACKGROUND OF THE INVENTION 
       [0002]    RF interference, which is also referred to as “jamming”, sometimes occurs when a CDMA-like system (such as GNSS) receiver operates. Jamming may be caused by various sources such as channel cohabitance, harmonics from mobile phones, satellites, TV, FM radio, radar and hostile sources. Jamming may surreptitiously degrade accuracy of the receiver and damage the integrity of the receiver. An automatic gain control (AGC) circuit in the receiver keeps the receiver in its optimized operating range by detecting the overall strength of the signal and automatically adjusting the gain of the receiver to maintain an approximately constant average output power level. To design the behavior of the AGC circuit, the input signal characteristics must be taken into consideration to minimize the SNR degradation caused by quantization. For example, an AGC unit of a GNSS receiver usually utilizes a specific input signal model such as Gaussian model. When there is jamming incorporated into the input signal, the AGC unit cannot perform well for the predetermined input signal model, which may cause further signal distortion. Similarly, other functions of the receiver, such as signal acquisition ability and navigation accuracy, may also be influenced by the jamming. Therefore, anti-jamming operation is required. However, it is a waste of power if jamming mitigation function is always activated since jamming occurs occasionally. The signaling environment may often be jamming-free. Executing anti-jamming operation during jamming-free period is a significant waste of power. It will be more economic and efficient if the anti-jamming operation is only executed when there is indeed jamming in the input signal. Therefore, an effective jamming detection scheme is necessary. 
       SUMMARY OF THE INVENTION 
       [0003]    In accordance with the present invention, the amplitude or power distribution of a received signal is compared with a reference distribution of a specific signal model, which can be obtained from signal probability distribution function (PDF) or cumulated distribution function (CDF) or statistical experiments. If the signal distribution is very different from the reference distribution, it means that jamming is present. To compare the signal distribution and the reference distribution, the reference distribution is divided into several steps according to signal magnitudes, for example. The actual samples of each step of the received signal are counted and the counted result is compared with an expected value. Then, deviation of the signal distribution from the reference distribution can be obtained according to the comparison results. The existence of jamming is determined according to the deviation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a block diagram schematically showing a receiver with jamming detection function in accordance with the present invention; 
           [0005]      FIGS. 2A and 2B  are diagrams respectively and schematically illustrating different signal models; 
           [0006]      FIG. 3  is a diagram showing a probability distribution of Gaussian random variable; and 
           [0007]      FIG. 4  is a flow chart illustrating a method of an embodiment in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]      FIG. 1  is a block diagram schematically showing a receiver with jamming detection function in accordance with the present invention. The receiver includes an antenna (or antenna group)  10  for receiving RF (radio frequency) signals, an RF processing unit  20  for processing the RF signals such as filtering out noises, amplifying the amplitudes and converting the RF signals into IF (intermediate frequency) signals. The receiver further has an AGC (automatic gain control) unit  30  for adjusting the amplitude of the signal. To achieve optimal performance such as minimizing SNR degradation, the AGC unit  30  is often designed based on a specific signal model. The signal model may be white noise (also referred to as Gaussian or normal distribution) signal model as shown in  FIG. 2A , or single-tone sine wave signal model as shown in  FIG. 2B . The output signal of the AGC unit  30  is passed to an ADC (analog-to-digital converter)  40 . The ADC  40  samples the signal into digital samples for successive processing. 
         [0009]    In accordance with of the present invention, the receiver further has a plurality of counters  45  to count the number of samples of different amplitude steps. That is, the magnitude distribution of the signal is divided into several steps, each step is defined with an amplitude range. The corresponding counters are used to count actual numbers of samples which fall in respective steps. 
         [0010]    In the present embodiment, it is assumed that the signal model utilized by the AGC unit  30  in a GNSS receiver is zero-mean Gaussian model.  FIG. 3  shows a probability distribution of Gaussian random variable. For example, if the amplitude distribution is classified into 8 steps (each step size equals to the signal standard deviation), for 10,000 samples, there should be 135 samples falling into the first step, 214 samples falling into the second step, 1359 samples falling into the third step, 3413 samples falling into the fourth step, 3413 sample falling into the fifth step, 1359 sample falling into the sixth step, 214 sample falling into the seventh step, and 135 sample falling into the eighth step. The above values are called expected step counts H i  (i=1 to 8). Therefore, in the present embodiment, the receiver contains eight counters  45  to count the samples of eight step. However, it is also possible to use other numbers of counters  45 . For example, the receiver may contain four counters  45 , and each counter counts the samples of two adjacent steps. Other arrangement can also be used. 
         [0011]    The receiver further has a gain control &amp; jamming detection unit  50 . The gain control &amp; jamming detection unit  50  has a jamming detection part  52  and a gain control part  54 . In practice, these two parts can be implemented by different blocks or implemented by the same block executing both functions. The counters  45  respectively count the actual samples of the signal falling in the respective steps and notify the gain control &amp; jamming detection unit  50  with the results. The jamming detection unit  50  compares the actual results with the ideal values or measures the discrepancy between two distributions to determine whether there is jamming incorporated with the signal. If the actual results indicate that input sample distribution is very different from the reference one, the gain control &amp; jamming detection unit  50  determines that jamming is detected. Then, the gain control &amp; jamming detection unit  50  generates a jamming indicator and passes the same to an anti-jamming unit  60 , so that the anti-jamming unit  60  can be activated to eliminate the jamming. When receiving the jamming indicator indicating that jamming is detected, the anti-jamming unit  60  may eliminate the jamming by any proper schemes. For example, the anti-jamming unit  60  can utilize a frequency domain approach such as FFT (Fast Fourier Transform) to remove jamming in frequency domain. Alternatively, the anti-jamming unit  60  can utilize a time domain approach such as an adaptive filter to filter out the jamming and remove the jamming from the input signal. In the field of anti-jamming, there are various schemes available, and any of them can be applied in the anti-jamming unit  60 . 
         [0012]    After the jamming is removed from the signal, the clean signal is passed to a correlator  70  executing correlation. The output of correlator  70  is passed to a processor to execute operations such as signal acquisition and signal tracking. The gain control &amp; jamming detection unit  50  also generates a gain control signal according to the counter results. The gain control &amp; jamming detection unit  50  sends the gain control signal to the AGC  30  to control the AGC unit  30 . For example, if the signal is un-jammed, the gain control part  54  of the gain control &amp; jamming detection unit  50  can adjust the gain of the AGC unit  30  so that the sample distribution in each ADC step approximates the reference distribution. Furthermore, when jamming is detected, the AGC unit  30  can switch from a current mode to another mode, which is more appropriate for the current signal. However, the gain control &amp; jamming detection unit  50  may also generate a control signal for controlling other components of the receiver. 
         [0013]    The counters  45  and the jamming detection part  52  of the gain control &amp; jamming detection unit  50  can be considered together and referred to as a jamming detector  100 , as indicated in  FIG. 1 . The counters are used as a distribution measurement part of the jamming detector  100  to measure the amplitude distribution of the received signal by counting the sample number of each step. The jamming detector  100  checks the output of the ADC  40  to determine whether there is jamming or not. If so, the jamming detector  100  notifies the anti-jamming unit  60 . Then the anti-jamming unit  60  properly operates to remove or reduce the jamming accordingly. By the provision of the jamming detector, the anti-jamming unit  60  does not need to always be in the operating mode. Instead, the anti-jamming unit  60  only operates when the jamming indicator indicates that there is jamming. Therefore, unnecessary power consumption can be avoided. 
         [0014]    The signal distribution is represented by amplitude histogram, that is, the signal distribution is classified into several groups by sample amplitude, and there are counters to count the sample numbers falling into each group. There are various ways to classify the groups. The amplitude groups can be divided into multiple ADC steps. It is possible that just a few MSB (most significant bits) of the ADC steps are used. Moreover, the amplitude group division can be done in software. In the above embodiment, the sizes of the respective steps are uniform. However, even non-uniform group size design is workable. 
         [0015]    The deviation X of the actual signal distribution from the ideal signal model can be calculated by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where k is the total step number (k=8 in this case), h i  is the actual sample number of the samples falling in the i-th step, and H i  is the ideal sample number for the i-th step. Taking the Gaussian distribution case mentioned above as an example, H 4 =3413. If the deviation X exceeds a threshold X n,a , that is: 
         [0000]      X≧X n,a ,  (2) 
         [0000]    then it is determined that there is jamming. The threshold X n,a  is determined by the gain control &amp; jamming detection unit  50  depending on the total sample number n (n=10000 in this case) for the signal and the desired accuracy a. 
         [0016]    The checking scheme above is only an example. Any other suitable statistic determination scheme can also be used. For example, besides directly comparing the counted sample number of each step with the expected value, it is also possible to calculate the differences or ratios between the counted sample numbers of the respective steps and judge if the magnitude distribution of the input signal is within an acceptable range with respect to a predetermined signal model according to the calculated differences or ratios. For example, square of differences or chi-square test can be used by the jamming detection part  52  of the gain control &amp; jamming detection unit  50  to check the deviation degree of the signal distribution of the received signal from the ideal distribution. 
         [0017]      FIG. 4  is a flow chart illustrating the respective steps of the method of the above embodiment in accordance with the present invention. Firstly, a signal model is selected in step S 102  of the method. Generally, the AGC unit  30  is set with a predetermined signal model in advance. The gain control &amp; jamming detection unit  50  will judge whether there is jamming by using the predetermined signal model as a reference to check the ADC output. The amplitude distribution of the signal model is divided into k steps (S 104 ). The number k is given in advance. Although the described embodiment deals with the magnitude distribution, other types of distribution such as frequency response are also applicable. Then, ideal sample number H i  (i=1 to k) of each step of the distribution is determined (S 106 ). Generally, the ideal sample numbers H i  can be easily obtained from statistic theories. As described, the ADC  40  samples the signal with a predetermined sampling rate, and the samples of the respective groups are counted by the counters  45  as h i  (S  108 ). One counter  45  is in charge of counting samples of one ADC step or several conjunct steps, or even a group divided by software. The gain control &amp; jamming detection unit  50  calculated a deviation X from h i  and H i  (S 110 ). As mentioned, the deviation X can be calculated by any proper known statistic formula. The gain control &amp; jamming detection unit  50  compares the deviation X with a reference threshold X n,a , which is determined according to the total sample number n of a period (sampling rate) of the ADC  40  and a desired accuracy a (S 112 ). Once the deviation X reaches or exceeds the threshold X n,a , the gain control &amp; jamming detection unit  50  determines that there is jamming (S 114 ). Otherwise, the gain control &amp; jamming detection unit  50  determines that the signal is jamming-free (S 116 ). 
         [0018]    While the preferred embodiment of the present invention have been illustrated and described in detail, various modifications and alterations can be made by persons skilled in this art. The embodiment of the present invention is therefore described in an illustrative but not restrictive sense. It is intended that the present invention should not be limited to the particular forms as illustrated, and that all modifications and alterations which maintain the spirit and realm of the present invention are within the scope as defined in the appended claims.