SYSTEM FOR CONTINUOUS WAVE REJECTION

Abstract: A method is disclosed for receiving transmitted signals in the presence of CW interference in a communication system that determines the presence of a code in a received signal by comparing with a detector 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. The method includes obtaining a first input power value of the received signal at a first sample time and obtaining a second input power value of the received signal 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. 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. Therefore in the CDMA systems if the received signal contains a CW signal, then the strong correlation between the CW signal and the short code used for acquisition may result in a false alarm. To prevent this, the following algorithm is suggested to be used for the cancellation of CW, if there is a CW signal present in the received CDMA signal.

Description: SYSTEM FOR CONTINUOUS WAVE REJECTION BACKG ROUND OF THE INVENTION Field of the Invention 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. Description of Prior Art 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 communications 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. 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. 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. 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. 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. 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 acceptance threshold, a determination is made that a short code is present. When the likelihood ratio decreases 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. 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 OF THE INVENTION 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 in;orporating 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 power 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 forgoing 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 Figure 1 shows a schematic representation of the system for CW rejection of the present invention; and Figure 2 shows a graphical representation of the relationship between samples obtained within the system of Figure 1 and CW interference applied to the input of the system of Figure 1. Figure 3 shows a schematic representation of an alternate system for CW rejection. DETAILED DESCRIPTION OF THE INVENTION 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. ReferriLg now to Figure 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 each for simplicity, it will be understood by those of skill in the art that both the RAKE 16 and the 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. The RAKE 16 provides one or more complex samples which are each converted into a power magnitude value P^ by blocks 14a, ..., 14n. It should be noted here that blocks 14a, .... 14n and associated despreader equipment may be replicated N times for a system 10 having N despreaders/filters. The power of a sample at time i at the jth RAKE filter is obtained. The power of a sample at time i at the jth RAKE filter is denoted as P^, 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 14a 14n can produce a magnitude value of P; , either with a magnitude function or a squaring function. Power value P ; | 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 AlJ by block 20. Similarly, block 20 and associated despreader equipment may be replicated and is shown here as blocks 20a, ..., 20N for a system having N despreaders/filters. Output sample value P^ j represents a power determination corresponding to a symbol period. The relationship between a sample value P tJ obtained from RAKE 16 and the previous sample value in time P,.^ is random in the case where the input of RAKE despreader system 10 is random. However, the relationship between P and P, , 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 and P,., is sensitive to the amount of CW interference in the input of RAKE despreader system 10 which correlates with the short code. The relationship between a sample value P A lJ obtained within AUX RAKE 18 and the previous sample value in time P^, l0 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 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. Therefore, in accordance with the present invention, at each sample time i, a determination is made of the value of P. j -P, i _, at the output of RAKE 16. The value of P. J -P, i 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. The correlation (b r ) between successive input values P I(J and P, , is found by taking the difference of values P 10 and P, , 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. j -P, tJ 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: κ-ι = Σ p, ,-> - -i .; - I K . Equation 1 * =0 When the input signal of RAKE 16 is only background noise and the sample values P and P, , have a 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, , will not be correlated to each other in the case of a random relationship then when the successive values of P are correlated b R can provide a measure of the correlation of the samples P u obtained from RAKE 16. In a similar manner, during each sample period i, a determination is made of the difference value of P^ j -P^u N , at the output of AUX RAKE 18. The difference value of can t e obtained using delay 28 and summer 30 or any other methods known to those skilled in the art. The correlation between successive input values P Aij and is found by taking the difference of values P^ j and P^ 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 J- A, ; ι j and divides the sum by the number of terms added. The value of P A i -P i . ι can be averaged over a predetermined number of sample periods to form an average difference value b^ using low pass filter 32. The average difference value b^ 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: bAR I K. Equation 2 As shown in Figure 1, the absolute values of b R and b^ are calculated in block 34 and these absolute values are compared in 38 to the threshold R τ . This value, Offset^ 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^, to have a positive value, which, when added to the SPRT detection threshold, will increase the threshold by an amount 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 the 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^, is used to adjust downward a likelihood ratio of an SPRT. This would have the same effect as raising the detection threshold. Referring now to Figure 2. there is shown graphical representation 50 of the ratio R = 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^, 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 may be calculated, and a threshold value R τ between these two cases is established. Only when R is greater than threshold value R 7 is the SPRT or similar detection method detection threshold adjusted by Offset^. An alternate embodiment is shown in Figure 3. A plurality of RAKE correlators 50a, ..., 50N receive the CDMA signal containing the CW signal. The complex sample is converted into a power magnitude value P where i indicates the sample in time and j indicates the RAKE correlator 50a, ..., 50N. The maximum power sample MAX (P, 0 ) is ascertained at block 60 and that sample is removed at block 70. The average value of P lj; avg(P,), is obtained by averaging over N-l of P values. That is: Λ' -l avg (p, ) = \ / (N - l)__. P t Equation 3 Note that the maximum P value is not used, since it might contain the signal rather than CW interference. The calculation of avg (P,) is performed at block 80. For the jth RAKE filter, the absolute value of the difference between the power samples obtained at time i, P, J5 and the previous power samples obtained at time i-1 , P, lj5 is denoted as a y . First, a delay 55a. ..., 50N is applied to each P ι The absolute value a„ of the difference between the power samples P, and P, lo is determined at block 57a, ..., 57N. The maximum MAX (a, ,) is removed at block 58. The average value of a, 0 , avg(a,), is obtained by averaging over the same N- 1 RAKE filters 50a, ..., 50N at blocks 59 and 82. That is: Λf -l αvg (fl, ) = 1 1 (n - l) fl 1 Equation 4 Then, avg(P,) is compared to avg(a.) 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^ and used similarly to the embodiment of Figure 1 to the threshold 42. 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.

Claims: What is claimed is: 1. A method for receiving transmitted signals in a communication system having a detector threshold for comparing with the detector threshold calculations made in accordance with a sample of a received signal in order to determine the presence of a CW interference in the received signal, comprising the steps of: (a) obtaining a first input sample of the received signal at a first sample time; (b) obtaining a second input sample of the received signal at a second sample time; (c) comparing the first and second samples in order to provide an input sample comparison; (d) repeating steps (a) - (c) in order to provide a plurality of input sample comparisons; and (e) adjusting the detector threshold in accordance with the plurality of sample comparisons. 2. The method for receiving transmitted signals of claim 1 , comprising the step of determining whether the first and second input samples are representative of a CW signal. 3. The method for receiving transmitted signals of claim 2. comprising the step of adjusting the detector threshold in accordance with the CW signal determination. 4. The method for receiving transmitted signals of claim 2, comprising the step of determining an average of the plurality of input sample comparisons. 5. The method for receiving transmitted signals of claim 4, comprising the step of adjusting the detector threshold in accordance with the CW signal. 6. The method for receiving transmitted signals of claim 5, wherein the detector threshold is an upper threshold comprising the step of adjusting the detector threshold upwards in accordance with the CW signal. 7. The method for receiving transmitted signals of claim 1, wherein the step of comparing the first and second input samples comprises performing a subtraction operation upon the first and second input samples. 8. The method for receiving transmitted signals of claim 7, wherein the step of comparing the first and second input samples comprises averaging the first and second input samples. 9. A method of receiving transmitted signals in the presence of CW interference comprising: sampling a received signal to generate a plurality of signal samples; converting each signal sample into a respective power value; selecting and eliminating a maximum power value from the plurality of power values; averaging the remaining power values to generate an average power value; determining an average difference between each power value and the power value of an adjacent sample; comparing the average power value to the average difference to generate an offset signal for adjusting a detection threshold. 10. A noise rejection subsystem for use in a communications system comprising: a first filter for sampling a received signal and generating a power value for each sample; a second filter for determining a difference value between power values of adjacent samples; an averaging device which determines an average power value; a second averaging device which determines an average difference value; and, a comparator for comparing a ratio of the average power value per the average difference value to a threshold and generating an offset signal for adjusting the threshold. 11. The noise rejection subsystem of claim 10 further comprising a maximum value device for selecting a maximum power value from the samples. 12. The noise rejection subsystem of claim 11 wherein the averaging device is configured to subtract the selected maximum power value prior determining the average power value. 13. The noise rejection subsystem of claim 10 further comprising a second maximum value device for selecting a maximum difference value from the samples. 14. The noise rejection subsystem of claim 13 wherein the second averaging device is configured to subtract the selected maximum difference value prior determining the average difference value.

Inventors: DICK STEPHEN G, SEZGIN NADIR, DICK, STEPHEN, G., SEZGIN, NADIR
Applicants: DICK STEPHEN G, INTERDIGITAL TECH CORP, SEZGIN NADIR, DICK, STEPHEN, G., INTERDIGITAL TECHNOLOGY CORPORATION, SEZGIN, NADIR
Application year: 2000
Main classification: H04B1
IPC codes: H04B1, H04Q7