Abstract:
A method including correlating a code having a first offset with a signal to produce a first correlation result; correlating the code having a second offset with the signal to produce a second correlation result; determining a cost function using the first correlation result and the second correlation result; and adjusting the first offset and the second offset in dependence upon the cost function, wherein the cost function uses a first weighting for the first correlation result and a second, different weighting for the second correlation result.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/593,944, filed 12 Nov. 2009, which is the National Stage of International Application Serial Number: PCT/IB2007/02080, filed 2 Apr. 2007, which are incorporated herein in their entirety. 
    
    
     FIELD 
     Embodiments of the present invention relate to correlating a code with a signal. Some embodiments relate to correlating a spreading code and a received spread spectrum communications signal that may be subject to multi-path effects. 
     BACKGROUND 
     Spread spectrum communication techniques spread information using a pseudorandom spreading code and then transmit the spread signal. A receiver receives the spread signal and correlates the received signal against the same pseudorandom spreading code. A peak in the correlation indicates the time of arrival of the spread signal and the correlation result at that code offset provides the original information. 
     A problem can arise when a spread signal takes multiple different paths to the receiver by, for example, bouncing off the ground or surrounding topography or buildings. The signal from one path is noise in respect of the signal from another path. 
     It would be desirable to provide a receiver that operates effectively in the presence of multi-path spread spectrum signals. 
     SUMMARY 
     According to some embodiments of the invention there is provided a method comprising: correlating a code having a first offset with a signal to produce a first correlation result; correlating the code having a second offset with the signal to produce a second correlation result; determining a cost function using the first correlation result and the second correlation result; and adjusting the first offset and the second offset in dependence upon the cost function, wherein the cost function uses a first weighting for the first correlation result and a second, different weighting for the second correlation result. 
     According to some embodiments of the invention there is provided an apparatus comprising: a code generator configured for generating a code having a first offset and the code with a second offset; a first correlator configured for correlating the code having the first offset with a signal to produce a first correlation result; a second correlator configured for correlating the code having the second offset with the signal to produce a second correlation result; circuitry configured for determining a cost function using the first correlation result and the second correlation result and configured for controlling adjustment of the first offset and the second offset in dependence upon the cost function, wherein the cost function uses a first weighting for the first correlation result and a second, different weighting for the second correlation result. 
     According to some embodiments of the invention there is provided a computer program product which when loaded into a processor enables: determining a cost function using a first correlation result, formed by correlation of a code having a first offset with a signal, and a second correlation result, formed by correlation of the code having a second offset with the signal; controlling adjustment of the first offset and the second offset in dependence upon the cost function, wherein the cost function uses a first weighting for the first correlation result and a second, different weighting for the second correlation result. 
     According to some embodiments of the invention there is provided a method comprising: a) correlating a code having a first offset with a signal to produce a first correlation result; b) correlating the code having a second offset with the signal to produce a second correlation result; c) determining a cost function using the first correlation result and the second correlation result; and d) adjusting the first offset and the second offset in dependence upon the cost function to maintain a predetermined differential relationship between the first correlation result and the second correlation result. 
     According to some embodiments of the invention there is provided an apparatus comprising: a code generator configured for generating a code having a first offset and the code with a second offset; a first correlator configured for correlating the code having the first offset with a signal to produce a first correlation result; a second correlator configured for correlating the code having the second offset with the signal to produce a second correlation result; circuitry configured for determining a cost function using the first correlation result and the second correlation result and for configured controlling adjustment of the first offset and the second offset in dependence upon the cost function to maintain a predetermined differential relationship between the first correlation result and the second correlation result. 
     According to some embodiments of the invention there is provided a computer program product which when loaded into a processor enables: determining a cost function using a first correlation result, formed by correlation of a code having a first offset with a signal, and a second correlation result, formed by correlation of the code having a second offset with the signal; controlling adjustment of the first offset and the second offset in dependence upon the cost function to maintain a predetermined differential relationship between the first correlation result and the second correlation result. 
     The correlation that occurs using the first code with the first offset is an early correlation when it occurs with a code offset that is before that of the punctual signal and the correlation that occurs using the second code with the second offset is a late correlation because it occurs with a code offset that is after that of the punctual signal. By using values for the first offset and the second offset that are asymmetrically spaced on either side of that for the punctual signal so that the first offset is greater than the second offset, the decision window for the late correlation is reduced compared to that for the early correlation. The correlation process is therefore less affected by multi-path effects because the late correlation window is smaller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention reference will now be made by way of example only to the accompanying drawings in which: 
         FIG. 1  schematically illustrates a method for controlling correlation of a signal s and a code c; 
         FIG. 2  illustrates  30  the auto-correlation function of the correlation processes; 
         FIG. 3  schematically illustrates an apparatus that is suitable for performing the method illustrated in  FIG. 1 ; and 
         FIG. 4  schematically illustrates one embodiment of circuitry used in the apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a method  10  for controlling correlation of a signal s and a code c. 
     The signal s is typically a received spread spectrum communication signal that has been spread using a pseudorandom spreading code c. The received spread spectrum communication signal is then de-spread by correlation with the same pseudorandom spreading code c. At least two different correlations occur, with different offsets of the pseudorandom spreading code c. 
     Referring to  FIG. 1 , at step  12  the signal s is received. 
     Next at step  14 , the code c having a first offset φ 1  is correlated with the signal s to produce 15 a first correlation result A. At step  16 , the same code c having a second offset φ 2  is correlated with the signal s to produce 17 a second correlation result B. The second offset, φ 2  is delayed (late) relative to the first offset φ 1  (early). 
       FIG. 2  illustrates one auto-correlation function R  30  that could be used for the correlation steps  14 ,  16 . This particular auto-correlation can be represented as:
 
 R ( t )=1 −t/T  for − T≦t≦T  
 
 R ( t )=0 for elsewhere.
 
     T is the chip interval of the code c. 
     This correlation function is a simplistic view of a typical correlation function which is used for clarity. It should be appreciated that a ‘real’ correlation function R may differ significantly from the simplistic function illustrated and its slopes may, for example, be non-linear. 
     The normalized values of the first correlation result A and the second correlation result B before feedback loop obtained lock is reached are schematically illustrated in the figure. The first correlation result A at feedback loop obtained lock is also illustrated and has a value a. The second correlation result B at feedback loop obtained lock is also illustrated and has a value b. The first offset of the code c used to obtain the first correlation result A has as a generality a value φ A   1  and at feedback loop obtained lock has a value φ a   1 . The second offset of the code c used to obtain the second correlation result B has as a generality a value φ B   2  and at feedback loop obtained lock has a value φ b   2 . It should be appreciated that the value of φ B   2 −φ A   1  is a constant, although it may be user programmable  90 . 
     A cost function F may be defined as:
 
 F =( A/a−B/b )
         where a≠b and where a is the value of the correlation function, at feedback loop obtained lock, at the first offset φ a   1  and b is the value of the correlation function, at feedback loop obtained lock, at the second offset φ b   2 .       

     This function is zero valued at feedback loop obtained lock. 
     The relative values of a and b may be user programmable  92 . The value a+b need not be a constant and may vary. The values of a and b may vary independently. 
     For example, the value a and/or the value of b and/or the value of a+b may be varied dynamically. In one embodiment, the size of the correlation window (a+b) is varied and/or the ratio of b/a is varied using a feedback loop to obtain optimal reception. This may result in a smaller correlation window being used in urban environments for example. 
     Referring back to  FIG. 1 , after obtaining the first correlation result A and the second correlation result B, at step  18  the cost function F is determined using the first correlation result A and the second correlation result B. 
     The value of the cost function F is used to adjust the first offset φ 1  and the second offset φ 2  and the adjusted values are fedback via feedback loop  27  and the method  10  is repeated. The feedback loop  27  may be a delay locked loop (DLL) or a phase locked loop (PLL). If the cost function F is +ve, then the first offset φ 1  and the second offset φ 2  are both reduced by an amount dependent upon the magnitude of F. If the cost function F is −ve, then the first offset φ 1  and the second offset φ 2  are both increased by an amount dependent upon the magnitude of F. The difference between φ 1  and φ 2  therefore remains constant. Referring back to  FIG. 2 , this has the effect of moving A and B towards a and b, i.e. the method  10  finds a feedback loop obtained lock via a negative feedback. 
     When feedback loop obtained lock has been achieved, the method  10  maintains a predetermined relationship between the first correlation result A and the second correlation result B, i.e. A/B=a/b where a/b&lt;1. 
       FIG. 3  schematically illustrates an apparatus  40  that is suitable for performing the method  10 . The apparatus  40  comprises a code generator  42  for generating a code  44  with a first offset and a code  46  with a second offset, a first correlator  50  for producing the first correlation result A, a second correlator  52  for producing the second correlation result B and circuitry  60  for controlling the code generator  42 . The first correlator  50  and the second correlator  52  receive the signal s  51 . 
     The first correlator  50  performs step  14  of the method  10 . The second correlator  52  performs step  16  of the method  10 . The circuitry  60  performs step  18  of the method  10 . The code generator performs step  26  of the method  10 . 
     The circuitry  60  comprises a first multiplier  20  that applies a first weighting (1/a) to the first correlation result A to produce a first weighted correlation result  21 . The circuitry  60  comprises a second multiplier  22  that applies a second weighting (1/b) to the second correlation result B to produce a second weighted correlation result  23 . 
     The circuitry  60  subtracts one of the weighted first and second correlation results from the other of the first and second weighted correlation results to produce the cost function value  25 . In this implementation, the second weighted correlation result  23  (B/b) is subtracted from the first weighted correlation result  21  (A/a) as described in relation to block  24  of the method shown in  FIG. 1 . 
     The cost function value  25  controls the code generator  42  to adjust the first offset and the second offset in dependence upon the cost function value  25  as described in relation to step  26  of the method  10 . 
     The apparatus  40  may in some embodiments be a module that is sold for integration into or attachment to an electronic device. The apparatus  40  may in some embodiments be an electronic device. An electronic device may be, for example, a mobile cellular telephone, a personal wireless communication device, a satellite positioning receiver etc. 
     The circuitry  60  of the apparatus  40  may be implemented in software, firmware or hardware.  FIG. 4  schematically illustrates a software implementation. 
       FIG. 4  schematically illustrates a computer comprising a processor or controller  70  and a memory  72 . The memory  72  stores computer program instructions  74  that control the operation of the circuitry  40  when loaded into the processor  70 . The computer program instructions  74  provide the logic and routines that enables the electronic device to perform at least step  18  of method  10 . 
     The computer program instructions may arrive as a computer program product at the circuitry  40  via an electromagnetic carrier signal or be copied from a physical entity  76  such as a computer program product, a memory device or a record medium such as a CD-ROM or DVD. 
     The computer program product determines the cost function F using the first correlation result A (formed by correlation of a code having a first offset with a signal) and the second correlation result B (formed by correlation of the code having a second offset with the signal) and controls adjustment of the first offset and the second offset in dependence upon the cost function F. At feedback loop obtained lock the computer program product controls adjustment of the first offset and the second offset in dependence upon the cost function F to maintain a predetermined relationship between the first correlation result A and the second correlation result B, i.e. A/B=a/b. 
     Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, although a particular cost function F has been described it will be appreciated that different costs functions may be used, in particular, differently expressed but mathematically equivalent cost functions may be used and reference to ‘cost function’ or ‘cost function F’ should be interpreted accordingly. 
     Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.