Reduced MIPS pulse shaping filter

Abstract

In a communication system in which an incoming signal is pulse shaped prior to detecting the information bearing point of the signal, processing overhead can be significantly reduced by only processing the sample points corresponding to the information bearing point and two points bounding the information bearing point. During signal acquisition, the signal is over sampled and the pulse shaping filter processes every sample point, or every Nth sample point in the case of down sampling. Once the sample point corresponding to the information bearing point is determined, the pulse shaping filter can be instructed to only process the sampling corresponding to the information bearing point and two neighboring samples. The system and method is adaptive, in that the samples that are processed can shift as the information bearing point shifts relative to the sample points. This is accomplished by re-synchronizing the timing of the received slot.

1. A method of synchronizing with a transmitted signal, said method comprising:
 receiving said transmitted signal, wherein said signal comprises a periodic information bearing point, at an information rate and wherein an information period is the inverse of said information rate;  
 sampling said signal at a sampling rate greater than said information rate;  
 down sampling from said sampling rate wherein more than N sample points are sampled per information period to a filtering rate of N sample points per information period prior to step of filtering each of N sample points;  
 during a first predetermined number of information periods, filtering each of N sample points for each information bearing point with a pulse-shaping filter and outputting a filtered signal;  
 measuring a variable of said filtered signal at each of said N sample points, wherein said variable is independent of information content in said signal;  
 determining the location of said information bearing point in said signal based on said information content-independent variable;  
 identifying one of the N sample points that most closely corresponds to said information bearing point in said signal;  
 synchronizing processing of said signal with said information bearing point; and  
 during a second number of subsequent information periods, filtering a subset of the N sample points for each subsequent information period, the subset of the N sample points including the identified one of the N sample points.  
 
     
     
       2. The method of  claim 1  wherein the subset of N sample points consists of the identified one of the N sample points,  and each adjacent sample point. 
     
     
       3. The method of  claim 1  wherein the subset of N sample points consists of the identified one of the N sample points and one proceeding and one subsequent sample point of the N sample points. 
     
     
       4. The method of  claim 1 , wherein said information bearing point occurs between two of said sample points and wherein the method further comprising,  comprises:
 interpolating between sample points to determine the information bearing point during the first predetermined number of information periods; and  
 interpolating between the subset of N sample points during the second number of subsequent information periods;  
 tracking the results of the step of interpolating between the subset of N sample points; and  
 updating the subset of N sample points in response to the tracking steps.  
 
     
     
       5. The method of  claim 1  wherein the subset of N sample points is updated when the results of the interpolation step identifies  identify the information bearing point as being more than halfway between two or the N sample points. 
     
     
       6. The method of  claim 1  wherein N equals thirteen sample points for each information bearing point. 
     
     
       7. The method of  claim 1  wherein said information bearing point corresponds to a quadrature amplitude modulated symbol. 
     
     
       8. The method of  claim 1 , wherein said pulse-shaping filter meets Nyquist criteria. 
     
     
       9. The method of  claim 1  wherein said first predetermined number of information periods is 1320 periods. 
     
     
       10. The method of  claim 1  wherein said second number of information periods is 1320 periods. 
     
     
       11. The method of  claim 1  wherein the second number of information periods is variable. 
     
     
       12. The method of  claim 4 , wherein the second number of information periods is determined from the frequency with which the updating step  occurs. 
     
     
       13. A telecommunication receiver capable of synchronizing with a received signal, said receiver comprising:
 an analog-to-digital converter receiving an information signal, wherein said signal comprises a periodic information bearing point at an information rate, and wherein an information period is the inverse of said information rate;  
 a pulse shaping filter coupled to receive a digital signal from said analog-to-digital converter and to receive a control signal from a synchronization unit, wherein said digital signal comprises N sample points per said information period, the pulse shaping filter configured to operate in a first mode in which each of N sample points per information period is filter and a second mode in which a subset of N sample points per information period is filtered; and  
 said synchronization unit coupled to receive a pulse shaped sampled signal from said filter, said synchronization unit comprising: 
 a detector determining values of a variable of said signal at said N sample points when said pulse shaping filter is operating in said first mode and of said subset of N sample points when said pulse shaping filter is operating in said second mode, wherein said variable is independent of information content in said signal;  
 an accumulator to accumulate said detected values for each of said sample points which occurs at the same relative sample location within each information period, wherein there are (sample rate)/(information rate) sample locations within each information period;  
 sample bins for storing said accumulated values for said sample locations; and  
 a comparator for comparing said accumulated values in said sample bins, wherein the location of said information bearing point in said signal is determined based on said accumulated information content-independent values;  
 an identifier for identifying the relative sample location most closely corresponding to the information bearing point; and  
 a control signal generator for generating a control signal to said pulse shaping filter, the control signal including the identify  identity of the identified relative sample location.  
 
 
     
     
       14. The receiver of  claim 13  wherein the pulse shaping filter further comprises: a down sampler receiving the digital signal from the analog-to-digital converter at a first sampling rate and outputting the digital signal at a second sampling rate, the second sampling rate being equivalent to N samples per information period. 
     
     
       15. The receiver of claim  13    14  wherein the first sampling rate is 260 kHz and the second sampling rate is 52 kHz and N is equal to 13. 
     
     
       16. The receiver of  claim 13  wherein the pulse shaping filter and the synchronization unit are embodied as routines performed by , further comprising a digital signal processor configured to implement the pulse shaping filter and the synchronization unit. 
     
     
       17. The receiver of  claim 13  wherein the subset of N sample points is the identified sample point corresponding to the identified relative sample location most closely corresponding to the information bearing point and one additional sample point on either side of the identified one relative sample location. 
     
     
       18. The receiver of  claim 17  wherein the additional sample point on either side of the identified sample point is the next adjacent sample point on either side of the identified sample point. 
     
     
       19. The receiver of  claim 13  wherein the pulse shaping filter operates in the first mode during signal acquisition and operates in the second mode for a predetermined number of information periods subsequent to signal acquisition. 
     
     
       20. A method, comprising:
   down sampling a sampled signal from an initial sampling rate wherein more than N sample points are sampled per information period, said initial sampling rate being greater than an information rate of a signal being sampled to obtain said sampled signal, to a filtering rate of N sample points per information period prior to filtering each of N sample points;        during a first predetermined number of information periods, filtering each of N sample points for each information bearing point with a pulse - shaping filter and outputting a filtered signal;        measuring a variable of said filtered signal at each of said N sample points, wherein said variable is independent of information content in said signal;        determining the location of said information bearing point in said signal based on said information content - independent variable;        identifying one of the N sample points that most closely corresponds to said information bearing point in said signal;        synchronizing processing of said signal with said information bearing point; and        during a second number of subsequent information periods, filtering a subset of the N sample points for each subsequent information period, the subset of the N sample points including the identified one of the N sample points.     
     
     
       21. The method of  claim 20 , wherein the subset of N sample points consists of the identified one of the N sample points and each adjacent sample point. 
     
     
       22. The method of  claim 20 , wherein the subset of N sample points consists of the identified one of the N sample points and one proceeding and one subsequent sample point of the N sample points. 
     
     
       23. The method of  claim 20 , wherein said information bearing point occurs between two of said sample points, and wherein the method further comprises:
   interpolating between sample points to determine the information bearing point during the first predetermined number of information periods;        interpolating between the subset of N sample points during the second number of subsequent information periods;        tracking the results of the step of interpolating between the subset of N sample points; and        updating the subset of N sample points in response to the tracking steps.     
     
     
       24. The method of  claim 23 , wherein the subset of N sample points is updated when the results of said interpolating identify the information bearing point as being more than halfway between two of the N sample points. 
     
     
       25. The method of  claim 20 , wherein said information bearing point corresponds to a quadrature amplitude modulated symbol. 
     
     
       26. The method of  claim 20 , wherein said pulse- shaping filter meets Nyquist criteria.   
     
     
       27. The method of  claim 20 , wherein the second number of information periods is variable. 
     
     
       28. The method of  claim 20 , wherein the second number of information periods is determined from a frequency with which said updating occurs. 
     
     
       29. An apparatus comprising:
   a pulse shaping filter to receive a digital signal corresponding to a sampled information - bearing received signal that includes a periodic information - bearing point having an information period, whose inverse is a corresponding information rate, and to receive a control signal from a synchronization unit, wherein said digital signal comprises N sample points per said information period, the pulse shaping filter configured to operate in a first mode in which each of N sample points per information period is to be filtered and a second mode in which a subset of N sample points per information period is to be filtered; and        said synchronization unit to receive a pulse shaped sampled signal from said filter, said synchronization unit comprising:      a detector to determine values of a variable of said signal at said N sample points when said pulse shaping filter is operating in said first mode and of said subset of N sample points when said pulse shaping filter is operating in said second mode, wherein said variable is independent of information content in said signal;        an accumulator to accumulate said detected values for each of said sample points which occurs at the same relative sample location within each information period;        sample bins to store said accumulated values for said sample locations; and        a comparator to compare said accumulated values in said sample bins, wherein the location of said information bearing point in said signal is to be determined based on said accumulated information content - independent values;        an identifier to identify the relative sample location most closely corresponding to the information bearing point; and        a control signal generator to generate said control signal, the control signal to include the identity of the identified relative sample location.       
     
     
       30. The apparatus of  claim 29 , wherein the pulse shaping filter further comprises:
   a down sampler to receive the digital signal at a first sampling rate and to output the digital signal at a second sampling rate, the second sampling rate being equivalent to N samples per information period.     
     
     
       31. The apparatus of  claim 29 , wherein the pulse shaping filter and the synchronization unit are embodied as routines to be executed by a processor. 
     
     
       32. The apparatus of  claim 29 , wherein the subset of N sample points is the identified sample points corresponding to the identified relative sample location most closely corresponding to the information bearing point and one additional sample point on either side of the identified one relative sample location. 
     
     
       33. The apparatus of  claim 32 , wherein the additional sample point on either side of the identified sample point is the next adjacent sample point on either side of the identified sample point. 
     
     
       34. The apparatus of  claim 29 , wherein the pulse shaping filter operates in the first mode during signal acquisition and operates in the second mode for a predetermined number of information periods subsequent to signal acquisition. 
     
     
       35. An apparatus comprising:
   a pulse shaping filter to receive a digital signal corresponding to a sampled information - bearing received signal that includes a periodic information - bearing point having an information period and to receive a control signal from a synchronization unit, wherein said digital signal comprises N sample points per said information period, the pulse shaping filter configured to operate in a first mode in which each of N sample points per information period is to be filtered and in a second mode in which a subset of N sample points per information period is to be filtered; and        said synchronization unit to receive a pulse shaped sampled signal from said filter, said synchronization unit comprising:      a detector to determine values of a an information - content - independent quantity based on said signal at said N sample points when said pulse shaping filter is operating in said first mode and of said subset of N sample points when said pulse shaping filter is operating in said second mode;        an accumulator to accumulate said detected values for each of said sample points that occurs at the same relative sample location within each information period, to thus obtain accumulated values;        a comparator to compare said accumulated values, wherein the location of said information bearing point in said signal is to be determined based on said accumulated values; and        a control signal generator to generate said control signal, wherein said control signal is to identify the location of said information bearing point in said signal.       
     
     
       36. The apparatus of  claim 35 , wherein the pulse shaping filter further comprises:
   a down sampler to receive the digital signal at a first sampling rate and to output the digital signal at a second sampling rate, the second sampling rate being equivalent to N samples per information period.     
     
     
       37. The apparatus of  claim 35 , further comprising a digital signal processor configured to implement the pulse shaping filter and the synchronization unit. 
     
     
       38. The apparatus of  claim 35 , wherein the subset of N sample points is the identified sample point corresponding to the identified relative sample location most closely corresponding to the information bearing point and one additional sample point on either side of the identified one relative sample location. 
     
     
       39. The apparatus of  claim 38 , wherein the additional sample point on either side of the identified sample point is the next adjacent sample point on either side of the identified sample point. 
     
     
       40. The apparatus of  claim 35 , wherein the pulse shaping filter operates in the first mode during signal acquisition and operates in the second mode for a predetermined number of information periods subsequent to signal acquisition. 
     
     
       41. A processor- readable medium containing instructions that, when executed by a processor, cause the processor to implement a method comprising:      down sampling a sampled signal from an initial sampling rate wherein more than N sample points are sampled per information period, said initial sampling rate being greater than an information rate of a signal being sampled to obtain said sampled signal, to a filtering rate of N sample points per information period prior to filtering each of N sample points;        during a first predetermined number of information periods, filtering each of N sample points for each information bearing point with a pulse - shaping filter and outputting a filtered signal;        measuring an information - content - independent quantity based on said filtered signal at each of said N sample points;        determining the location of said information bearing point in said signal based on said quantity;        identifying one of the N sample points that most closely corresponds to said information bearing point in said signal;        synchronizing processing of said signal with said information bearing point; and        during a second number of subsequent information periods, filtering a subset of the N sample points for each subsequent information period, the subset of the N sample points including the identified one of the N sample points.     
     
     
       42. The medium of  claim 41 , wherein the subset of N sample points consists of the identified one of the N sample points and each adjacent sample point. 
     
     
       43. The medium of  claim 41 , wherein the subset of N sample points consists of the identified one of the N sample points and one proceeding and one subsequent sample point of the N sample points. 
     
     
       44. The medium of  claim 41 , wherein said information bearing point occurs between two of said sample points, and wherein the method further comprises:
   interpolating between sample points to determine the information bearing point during the first predetermined number of information periods;        interpolating between the subset of N sample points during the second number of subsequent information periods;        tracking the results of the step of interpolating between the subset of N sample points; and        updating the subset of N sample points in response to the tracking steps.     
     
     
       45. The medium of  claim 44 , wherein the subset of N sample points is updated when the results of said interpolating identify the information bearing point as being more than halfway between two of the N sample points. 
     
     
       46. The medium of  claim 41 , wherein said information bearing point corresponds to a quadrature amplitude modulated symbol. 
     
     
       47. The medium of  claim 41 , wherein said pulse- shaping filter meets Nyquist criteria.   
     
     
       48. The medium of  claim 41 , wherein the second number of information periods is variable. 
     
     
       49. The medium of  claim 44 , wherein the second number of information periods is determined from a frequency with which said updating occurs.