Abstract:
A method for receiving transmitted signals in the presence of CW interference is used in a communication system that determines the presence of a code in a received signal. A detector threshold is compared calculations made in accordance with a sample of a received signal. A first input power value of the received signal is obtained at a first sample time and a second input power value of the received signal is obtained at a second sample time. The first and second power values are compared in order to provide an input sample comparison and the forgoing steps are repeated in order to provide a plurality of input sample comparisons, and the detector threshold is adjusted in accordance with the plurality of sample comparisons. The CW signal can be strongly correlated with a short code used for the acquisition purposes, while avoiding false alarms.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 09/497,413, filed Feb. 4, 2000 which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION  
       [0002]     This invention relates to the field of Code Division Multiple Access (CDMA) communication systems. More particularly, the present invention relates to a system for accurately detecting short codes in a communication environment which includes continuous wave (CW) interference.  
       BACKGROUND  
       [0003]     With the dramatic increase in the use of wireless telecommunication systems in the past decade, the limited portion of the RF spectrum available for use by such systems has become a critical resource. Wireless communication systems employing CDMA techniques provide an efficient use of the available spectrum by accommodating more users than more traditional time division multiple access (TDMA) and frequency division multiple access (FDMA) systems.  
         [0004]     In a CDMA system, the same portion of the frequency spectrum is used for communication by all subscriber units. Typically, for each geographical area, a single base station serves a plurality of subscriber units. The baseband data signal within each subscriber unit is multiplied by a pseudo-random code sequence, called the spreading code, which has a much higher transmission rate than the data. Thus, the data signal is spread over the entire available bandwidth. Individual subscriber unit communications are discriminated by assigning a unique spreading code to each communication link.  
         [0005]     At times it is also useful in a CDMA system to transmit codes which are of shorter length than the usual spreading code. Instead of using a single, extremely long spreading code, a much shorter code is used and repeated numerous times. The use of short codes provides an advantage over the use of longer codes because the short codes can be detected much more quickly. However, the use of short codes has an inherent drawback, since the short code is repeated many times, it is much less random than a long code. When short codes are used, known detection algorithms can have an increased number of false acquisitions in the presence of continuous wave (CW) interference since the repetitive short codes can correlate with CW interference.  
         [0006]     When there is correlation between short codes and CW interference a false acquisition occurs, an incorrect output from a short code detector in a base station can last for a time period equal to the remainder of a short code. For example, in a known prior art short code system, short codes having 195 chips, which are transmitted at a rate of 15 megahertz, repeat for a three millisecond period. At the end of the three millisecond period a new short code is transmitted in the same manner. In such a system it is possible for a detector output to lock up for the remainder of the three millisecond period in response to a false acquisition in the presence of CW interference.  
         [0007]     It is known in the art of mobile communication systems which employ CDMA for a base-station receiver to use various detection tests to determine the presence of short codes transmitted by a subscriber unit. One such test known in the art is a sequential probability ratio test (SPRT) detection algorithm. The problem of false detections in the presence of CW interference can occur in detection algorithms such as a SPRT detection algorithm, even though SPRT detection algorithms can be very effective at rejecting noise under other circumstances.  
         [0008]     In SPRT detection algorithms, a likelihood ratio is computed and adjusted after each input sample is taken. The repeated adjustments cause the likelihood ratio to increase when a short code is present and decrease when a short code is not present. When the likelihood ratio increases and crosses a predetermined rejection threshold, a determination is made that a short code is not present. When the likelihood ratio is between the acceptance and rejection thresholds further samples are taken and further adjustments are made to the likelihood ratio until one of the thresholds is crossed. Thus, the false detection problem can occur in a SPRT detection algorithm when the CW incorrectly causes the likelihood ratio to increase and cross over the acceptance threshold.  
         [0009]     It is desirable to provide method for preventing false acquisitions of short codes in the presence of CW interference that does not limit the number of codes available for use within the system.  
       SUMMARY  
       [0010]     A method is disclosed for receiving transmitted signals in the presence of CW interference in a communication system that determines the presence of a short code in a received signal by comparing the output of a detector with threshold calculations made in accordance with a sample of a received signal. Such systems include but are not limited to those incorporating a Sequential Probability Ratio Test detection algorithm. The method includes obtaining a first input power value of the received signal at a first sample time and obtaining a second input value of the received signal at a second sample time. The first and second power values are compared to provide an input sample comparison and the foregoing steps are repeated to provide a plurality of input sample comparisons. The detector threshold is adjusted in accordance with the plurality of sample comparisons. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a schematic representation of the system for CW rejection of the present invention; and  
         [0012]      FIG. 2  shows a graphical representation of the relationship between samples obtained within the system of  FIG. 1  and CW interference applied to the input of the system of  FIG. 1 .  
         [0013]      FIG. 3  shows a schematic representation of an alternate system for CW rejection. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify corresponding elements throughout.  
         [0015]     Referring now to  FIG. 1 , there is shown a RAKE despreader system  10 . The RAKE despreader system  10  includes a RAKE  16  and an auxiliary (AUX) RAKE  18 . The RAKE  16  calculates correlation values between the input signal and a locally-generated short pseudorandom code (hereinafter “short code”). AUX RAKE  18  calculates correlation values between the input signal and a locally-generated long pseudorandom code (hereinafter “long code”). Although the RAKE  16  and AUX RAKE  18  are disclosed with one despreader output for simplicity, it will be understood by those of skill in the art that both the RAKE  16  and AUX RAKE  18  can be provided with a plurality of despreaders, each despreader providing an output for a different time sample in accordance with the present invention.  
         [0016]     The RAKE  16  provides one or more complex samples which are each converted into a power magnitude value P ij  by blocks  14   a , . . . ,  14   n . It should be noted that blocks  14   a , . . . ,  14   n  and associated despreader equipment may be replicated N times for a system  10  having N despreader/filters. The power of a sample at time i at the jth RAKE filter is obtained. The power of a sample a time i at the jth RAKE filter is denoted as P ij , where j=1, . . . , N and N is the total number of filters in the RAKE. It will be recognized by those skilled in the art that blocks  14   a , . . . , 14   n  can produce a magnitude value P ij  either with a magnitude function or a squaring function. Power value P ij  represents a power determination corresponding to a symbol period within a short code. A symbol period is the period required to transmit one information bit, where the bit has been spread by a pseudo random code. AUX RAKE  18  provides a complex sample which is converted in to a power value P A,ij  by block  20 . Similarly, block  20  and associated despreader equipment may be replicated and is shown here as blocks  20   a , . . . ,  20 N for a system having N despreaders/filters. Output sample value P A,ij  represents a power determination corresponding to a symbol period.  
         [0017]     The relationship between a sample value P ij  obtained from RAKE  16  and the previous sample value in time P i-1 j  is random in the case where the input of RAKE despreader system  10  is random. However, the relationship between P ij  and P i-1 j  is correlated when the input includes noise that is correlated with a short code being detected using RAKE despreader system  10 . Thus, the relationship between samples P ij  and P i-1 j  is sensitive to the amount of CW interference in the input of RAKE despreader system  10  which correlates with the short code.  
         [0018]     The relationship between a sample value P A,i-1 j  is random in the case where the input to RAKE despreader system  10  is random. However, AUX RAKE  18  is not correlated with a short code being detected using RAKE despreader system  10 . AUX RAKE  18  is not correlated with a short code being detected using RAKE despreader system  10 . AUX RAKE  18  uses a long pseudo-random code which does not correlate with CW interference. Therefore, the power of any two consecutive samples taken at the output of AUX RAKE  18  are not correlated to each other. Thus, AUX RAKE  18  provides an output substantially representative of background noise in the presence of CW interference. The relationship between the sample values obtained within RAKE  16  and the sample values obtained within AUX RAKE  18  can be used as a measure of the amount of CW interference in the input of RAKE despreader system  10 .  
         [0019]     Therefore, in accordance with the present invention, at each sample time i, a determination is made of the value of P ij −P i-1,j  at the output of RAKE  16 . The value of P ij −P i-1,j  can be determined using delay  22  and summer  24  of RAKE despreader system  10  or any other method known to those skilled in the art.  
         [0020]     The correlation (b r ) between successive input values P ij  and P 1-1 j  is found by taking the difference of values P ij  and P i-1 j  and passing this difference through low pass filter  26 . In one implementation, low pass filter  26  can be effected by an averaging routine which sums successive outcomes of P ij −p i-1,j  and divides the sum by the number of terms added. In such an implementation, where the predetermined number of sample periods used to determine b R  is K, the average difference value b R  can be expressed as:  
               b   R     =       (         ∑     k   =   0       K   -   1       ⁢     P     i   ,     j   -   k           -     P       i   -   1     ,     j   -   k           )     /     K   .               Equation   ⁢           ⁢   1             
 
         [0021]     When the input signal of RAKE  16  is only background noise and the sample values P ij  and P i-1 j  have random relationship with respect to each other, b R  can be expected to have a small value. Since the differences between successive values of P ij  obtained from RAKE  16 .  
         [0022]     In a similar manner, during each sample period i, a determination is made of the difference value of P A,i-1 j −P A,i-1 j  N, at the output of AUX RAKE  18 . The difference value of P A,i-1 j −P A,i-1 j  can be obtained using delay  28  and summer  30  or any other methods known to those skilled in the art.  
         [0023]     The correlation between successive input values P A,ij  and P A,i-1 j  is found by taking the difference of values P A,ij  and P A,i-1 j  and passing this value through low pass filter  32 . In one implementation, low pass filter  32  can be effected by an averaging routine which sums successive outcomes of P A,ij  and P A,i-1 j  and divides the sum by the number of terms added. The value of P A,i −P A,i-1  can be averaged over a predetermined number of sample periods to form an average difference value b AR  using low pass filter  32 . The average difference value b AR  provides a measure of the amount of background noise obtained by AUX RAKE  18  and, where the predetermined number of samples is equal to K, can be expressed as:  
               b   AR     =       (         ∑     k   =   0       K   -   1       ⁢     P     A   ,   i   ,     j   -   k           -     P     A   ,     i   -   1     ,     j   -   k           )     /     K   .               Equation   ⁢           ⁢   2             
 
         [0024]     As shown in  FIG. 1 , the absolute values of b R  and b AR  are calculated in block  34  and these absolute values are compared in  38  to the threshold R T . This value, Offset CW  is then used to adjust the detection threshold  42  in a detection algorithm  44  such as a Sequential Probability Ratio Test. The presence of CW interference in the input signal will cause Offset CW  to have a positive value, which, when added to the SPRT detection threshold, will increase the threshold by an amount of proportional to the amount of CW interference. Raising the detection threshold by an amount related to the degree of CW interference, ensures that CW interference will not cause a false detection of a short code. Those skilled in the art will recognize that in a system where the subscriber unit power is adjusted upward until the unit has been acquired by a base station, increasing the SPRT detector threshold in the presence of CW interference will result in the subscriber unit increasing its signal power until a legitimate short code can be acquired by the base station. In an alternative embodiment of the present invention, Offset CW  is used to adjust downward a likelihood ratio of an SPRT. This would have the same effect as raising the detection threshold.  
         [0025]     Referring now to  FIG. 2 , there is shown graphical representation  50  of the radio R=b R /b AR . Graphical representation  50  sets forth the relationship between the ratio R and the ratio of CW interference to background noise of the input signal applied to RAKE despreader system  10 . When no CW interference is present and b R =b AR , the ratio R reaches its minimum value of one. Under these conditions the false acquisition problems associated with codes having large imbalances do not occur. As CW interference increases with respect to background noise, the ratio R increases proportionally with the amount of CW interference. In another embodiment of the present invention, the ratio R increases proportionally with the amount of CW interference. In another embodiment of the present invention, the ratio R may be calculated, and a threshold value R T  between these two cases is established. Only when R may be calculated, and a threshold value R T  between these two cases is established. Only when R is greater than threshold R T  is the SPRT or similar detection method detection threshold adjusted by Offset CW .  
         [0026]     An alternate embodiment is shown in  FIG. 3 . A plurality of RAKE correlators  50   a , . . . , 50 N receive the CDMA signal containing the CW signal. The complex sample is converted into a power magnitude value P ij  where i indicates the sample in time and j indicates the RAKE correlator  50   a , . . . , 50 N. The maximum power sample MAX(P ij ) is ascertained at block  60  and that sample is removed at block  70 .  
         [0027]     The average value of P ij , avg(P i ), is obtained by averaging over N−1 of P ij  values. That is:  
               avg   ⁡     (     P   i     )       =       1   /     (     N   -   1     )       ⁢       ∑     j   =   1       N   -   1       ⁢     P     i   ,   j                   Equation   ⁢           ⁢   3             
 
         [0028]     Note that the maximum P ij  value is not used, since it might contain the signal rather than CW interference. The calculation of avg(P i ) is performed at block  80 .  
         [0029]     For the jth RAKE filter, the absolute value of the difference between the power samples obtained at time i, P ij , and the previous power samples obtained at time i−1, P i-1 j  is denoted as a ij . First, a delay  55   a , . . . , 50 N is applied to each P ij . The absolute value a i , of the difference between the power samples of P i  and P i-1 j  is determined at block  57   a , . . . , 57 N. The maximum MAX(A ij ) is removed at block  58 .  
         [0030]     The average value of a ij , avg(a i ) is obtained by averaging over the same N−1 RAKE filters  50   a , . . . , 50 N at blocks  59  and  82 . That is:  
               avg   ⁡     (     a   i     )       =       1   /     (     n   -   1     )       ⁢       ∑     j   =   1       N   -   1       ⁢     a     i   ,   j                   Equation   ⁢           ⁢   4             
 
         [0031]     Then, avg(P i ) is compared to avg(a i ) at summer  84  in order to find the offset term due to the CW interference present in the CDMA signal. This term is denoted by offset CW  and used similarly to the embodiment of  FIG. 1  to the threshold  42 .  
         [0032]     The previous description of the preferred embodiments is provided in order to enable those skilled in the art to make and use the present invention. The various modifications to the embodiments shown will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other embodiments without providing an inventive contribution. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features disclosed.  
         [0033]     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone (without the other features and elements of the preferred embodiments) or in various combinations with or without other features and elements of the present invention.  
         [0034]     Hereafter, a wireless transmit/receive unit (WTRU) includes but is not limited to a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, a base station includes but is not limited to a Node-B, site controller, access point or any other type of interfacing device in a wireless environment.