Patent Publication Number: US-2009227250-A1

Title: Method for detecting a cpe alert signal of a telecommunication system by utilizing an energy ratio

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of US application Ser. No. 11/162/603, which was filed on Sep. 16, 2005, and is included herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to signal detection in a telecommunication system, and more particularly to a method for an adaptive detection of a CPE alert signal by utilizing an energy ratio. 
     2. Description of the Prior Art 
     In addition to conventional telecommunication systems, a call waiting system is used in the U.S.A., which has a standard TIA/EIA-777. The TIA (Telecommunications Industry Association) standards describe the requirements for Caller ID capable Customer Premises Equipment (CPE) under both on-hook (type-1) and off-hook (type-2) modes. Please refer to  FIG. 1 , which is a flowchart illustrating operation of a prior art call waiting mechanism. The telephone service provider  10  is used to provide communication services for a plurality of CPEs  12 ,  14 ,  16 . Assume that the CPE  12  is currently communicating with the CPE  14  on line  1  through the telephone service provider  10 . If the CPE  16  wants to communicate with the CPE  14  on line  2 , the telephone service provider  10  will activate the call waiting mechanism for notifying the CPE  14 . Firstly, the telephone service provider  10  temporarily blocks the speech sounds outputted from the CPE  12  from reaching the CPE  14 , and sends a CPE alert signal (CAS) to the CPE  14  during this period. 
     Please refer to  FIG. 2 , which is a diagram illustrating the composition of the CAS signal. As shown in  FIG. 2 , the CAS signal is composed of two DTMF (Dual Tone Multi-Frequency) signals S 1  and S 2 , where one DTMF signal S 1  has a frequency of about 2130 Hz, and the other DTMF signal S 2  has a frequency of approximately 2750 Hz. 
     After detecting the CAS signal transmitted from the telephone company  10 , the CPE  14  automatically stops outputting speech sounds for a short period of time, and transmits an ACK signal to the telephone service provider  10  in response to the CAS signal. After receiving the ACK signal outputted from the CPE  14 , the telephone service provider  10  starts transmitting information related to the CPE  16  (e.g., the phone number of the CPE  16 ) to the CPE  14 . Once the CPE  14  successfully receives the incoming information related to the CPE  16  the call waiting operation is finished. 
     Please note that, during the process of transmitting the CAS signal to the CPE  14 , although speech sounds outputted from the CPE  12  are blocked, the CPE  14  continues to output speech sounds. Both the CAS signal and the speech sounds share the same frequency band. The outgoing speech sounds will cause interference to the incoming CAS signal. As a result, the interference imposed upon the CAS signal means the prior art call waiting mechanism may fail to correctly detect the CAS signal. Therefore, detection of the CAS signal becomes an important issue for a telecommunications system. 
     SUMMARY OF THE INVENTION 
     It is therefore one of the objectives of the claimed invention to provide an adaptive method for detection of a CPE alert signal of a telecommunication system by utilizing an energy ratio, to solve the above problem. 
     The claimed invention discloses a method for detecting a Customer Premises Equipment (CPE) alert signal of a telecommunication system in a Customer Premises Equipment of the telecommunication system. The method comprises: detecting a first signal energy at a first frequency; detecting a second signal energy at a second frequency; comparing a total received signal energy with a summation of at least the first signal energy and the second signal energy to generate a comparison result; enabling a first detecting procedure or a second detecting procedure for detecting the CPE alert signal according to the comparison result; and transmitting an ACK signal to a telephone service provider of the telecommunication system when the CPE alert signal is detected. 
     This present invention can adopt different CAS detection criteria to meet different detection conditions, such as the above-mentioned non-speech condition and speech condition. Obviously, the adaptive method of the present invention is capable of optimizing the performance of detecting the CAS signal in a telecommunication system. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart illustrating operation of a prior art call waiting mechanism. 
         FIG. 2  is a diagram illustrating the composition of the CAS signal. 
         FIG. 3  is a flowchart illustrating operation of detecting a CAS signal according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As interference to the incoming CAS signal will occur when there are outgoing speech sounds from the CPE, the method of the present invention utilizes an adaptive detection system which first determines whether or not there are outgoing speech sounds, and then adaptively selects predetermined thresholds for determining whether or not a CAS signal is received. 
     Please refer to  FIG. 3 , which is a flowchart illustrating operation of detecting a CAS signal according to an embodiment of the present invention. The operation of detecting the CAS signal of a telecommunication system can be summarized by: detecting a first signal energy at a first frequency; detecting a second signal energy at a second frequency; comparing a total received signal energy with a summation of at least the first signal energy and the second signal energy to generate a comparison result; and enabling a first detecting procedure or a second detecting procedure for detecting the call alert signal according to a comparison result. 
     In step  300 , an energy ratio ER of the summation of the first signal energy A (i.e., the energy of the DTMF signal S 1  shown in  FIG. 2 ) and the second signal energy B (i.e., the energy of the DTMF signal S 2  shown in  FIG. 2 ) to the total received signal energy T is determined. A and B are determined by digital Fourier transforms, or by using narrow band filters. In a truly linear system, the total received signal energy T will be equal to the sum of the first signal energy A and the second signal energy B. When speech sounds are outputted from the CPE, however, the total received signal energy will be equal to the sum of A, B and the speech signal S. Therefore, the present invention uses an energy ratio ER (A+B/T) and compares this ratio with a threshold K to adaptively select thresholds for determining whether or not the CAS signal is received. The total received signal energy T is determined using an ADC sampling output data X(n) for a sample period L/F, where F is the sample rate. 
     
       
         
           
             
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     where L is the decision length. 
     Also, A and B may be calculated by ADC output data X n , and Fourier transform. 
     
       
         
           
             
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     where f 1  is 2130 Hz. 
     
       
         
           
             
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     where f 2  is 2750 Hz. 
     For a simplified system, the following equations may be taken, 
     
       
         
           
             
               
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     where L is the decision length. 
     
       
         
           
             
               
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     where f 1  is 2130 Hz. 
     
       
         
           
             
               
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     where f 2  is 2750 Hz. 
     In the simplified system, the following method and related thresholds K, L 1 , L 2 , L 3 , L 4 , α 1  and α 2  can also be used. The simplified equation&#39;s A′, B′, and T′ are more easily calculated by “Cordic” processors, or equivalent narrow-band filters. 
     K can be a value approximately between 0.3 and 0.7. The particular selection of K will depend on false alarm rate requirements specified by individual CPE manufacturers. 
     The comparison between ER and K generates a comparison result, which is used for the adaptive selection of thresholds. If the energy ratio ER is greater than the threshold K (i.e. S component of T is small), step  310  is performed to detect the CPE alert signal; and if the energy ratio ER is not greater than the threshold K (i.e. S component of T is large), step  320  is performed to detect the CPE alert signal. 
     In step  310 , the frequency energy level FL and the two-frequency energy difference TW are examined. Step  310  first compares the first signal energy A with a first frequency energy level L 1  and then compares the second signal energy B with a second frequency energy level L 2 . L 1  and L 2  are according to the standards of TIA-777. L 1 =−36 dbm and L 2 =−36 dbm. This is because the TIA-777 standards require a signal level of −32 dbm to be received when the signal is quiet. If the first signal energy A is greater than the first frequency energy level L 1  and the second signal energy B is greater than the second frequency energy level L 2 , step  310  then determines whether the CAS signal occurs in the telecommunication system, by checking whether the signal ratio A/B of the first signal energy A to the second signal energy B falls in a predetermined range where the limits are 1/α 1  and α 1 . α 1  is also a value according to the TIA-777 standards, and is taken in this example as 8 db. When a signal is quiet, TIA-777 standards require a twist of 6 db to be received. If the first signal energy A is greater than the first frequency energy level L 1 , the second signal energy B is greater than the second frequency energy level L 2 , and the signal ratio A/B falls in the predetermined range, the method of the present invention determines that the CAS signal exists (step  330 ). In the following, the method of the present invention goes back to step  300  for monitoring the occurrence of a next CAS signal. 
     In this embodiment, if either the first signal energy A is not greater than the first frequency energy level L 1  or the second signal energy B is not greater than the second frequency energy level L 2 , the method of the present invention directly judges that there is no CAS signal, and then goes back to step  300  to continue monitoring the occurrence of the CAS signal. Please note that in this embodiment the first frequency energy level L 1  is equal to the second frequency energy level L 2 . 
     In step  320 , the frequency energy level FL and the two-frequency energy difference TW are examined. Step  320  first compares the first signal energy A with a third frequency energy level L 3  and then compares the second signal energy B with a fourth frequency energy level L 4 . L 3  and L 4  are according to the standards of TIA-777. In this embodiment, L 3  is recommended to be −29 dbm and L 4  is recommended to be −29 dbm, although both values can be as large as −24 dbm. If the first signal energy A is greater than the third frequency energy level L 3  and the second signal energy B is greater than the fourth frequency energy level L 4 , step  320  further determines if the CAS signal occurs in the telecommunication system. In other words, step  320  checks if the signal ratio A/B of the first signal energy A to the second signal energy B falls in a predetermined range where the limits are 1/α 2  and α 2 . α 2  is also a value according to the TIA-777 standards, and is taken in this example as 6 db. If the first signal energy A is greater than the third frequency energy level L 3 , the second signal energy B is greater than the fourth frequency energy level L 4 , and the signal ratio A/B falls in the predetermined range, the method of the present invention determines that the CAS signal exists (step  330 ). In the following, the method of the present invention goes back to step  300  for monitoring the occurrence of a next CAS signal. 
     In this embodiment, if either the first signal energy A is not greater than the third frequency energy level L 3  or the second signal energy B is not greater than the fourth frequency energy level L 4 , the method of the present invention directly judges that there is no CAS signal, and then goes back to step  300  to continue monitoring the occurrence of the CAS signal. 
     Please note that in a preferred embodiment the first frequency energy level L 1  is equal to the second frequency energy level L 2 , and the third frequency energy level L 3  is equal to the fourth frequency energy level L 4 . In addition, the third frequency energy level L 3  is greater than the first frequency energy level L 1 , and the second signal ratio α 2  is greater than the first signal ratio α 1 . However, in other embodiments, the first frequency energy level L 1  is not limited to being equal to the second frequency energy level L 2 , and the third frequency energy level L 3  is not limited to being equal to the fourth frequency energy level L 4 . For example, in an alternative design, these frequency energy levels L 1 , L 2 , L 3 , L 4  are not identical to each other, but the third frequency energy level L 3  is still required to be greater than the first frequency energy level L 1 , and the fourth frequency energy level L 4  is still required to be greater than the second frequency energy level L 2 . 
     Using the method of the present invention, the CPE  14  shown in  FIG. 1  selects one criterion out of two available criteria according to the signal-to-noise ratio (SNR). That is, step  300  is used to measure the SNR corresponding to signals at frequencies 2130 Hz and 2750 HZ. If a user of the CPE  14  shown in  FIG. 1  does not talk when the telephone service provider  10  passes the CAS signal to the CPE  14 , the measured SNR is sure to be high. In other words, the energy ratio ER is greater than the threshold K. Therefore, the following step  310  using lower frequency energy levels L 1 , L 2  is activated to check the existence of the CAS signal. On the other hand, if a user of the CPE  14  shown in  FIG. 1  talks when the telephone service provider  10  passes the CAS signal to the CPE  14 , the measured SNR is sure to be low. In other words, the energy ratio ER is not greater than the threshold K. Therefore, the following step  320  using higher frequency energy levels L 3 , L 4  is activated to check the existence of the CAS signal. In addition, both steps  310 ,  320  further check the signal ratio A/B to improve the detection accuracy. 
     In contrast to the prior art, the present invention can adopt different CAS detection criteria to meet different detection conditions, such as the above-mentioned non-speech condition and speech condition. Obviously, the adaptive method of the present invention is capable of optimizing the performance of detecting the CAS signal in a telecommunication system. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.