Abstract:
A system and method is herein described for the wide band acquisition of a high performance waveform in an environment with poor signal to noise ratio. The waveform acquired has a preamble with a plurality of frequency modulated Maximum Length Sequences and a period of pure carrier. In one embodiment, method provides an application specific integrated circuit (ASIC) for receiving the waveform, filter coefficients for use with the MLS&#39;s portion of the waveform, and filter coefficients for the pure carrier portion of the signal. Another embodiment may also include, for the MLS portion of the signal, detecting the signal through partial correlation, extracting waveform information, and estimating symbol timing. Yet another embodiment may further include, for the pure carrier portion of the signal, estimating the phase and frequency of the signal and providing those estimates to an ASIC.

Description:
BACKGROUND OF THE INVENTION 
   The subject matter of the application deals with acquisition of High Performance Waveforms (“HPW”). 
   In order for a receiver to glean the information from a transmitted signal, the carrier frequency and phase of the signal must be determined and or synchronized. Typical transmitter and receiver pairs have matched expected transmit and receive frequencies. Thus signals sent by a transmitter possess a carrier frequency known or expected by associated receivers. The receiver pass band must be wide enough to accommodate not only the information-bearing signal, but also any fluctuation in the carrier, due perhaps to Doppler shift or drift in the transmitters frequency reference. Such transmitter errors and propagation characteristics can cause a frequency offset to be present. 
   Doppler shift is a characteristic of propagation that commonly produces a frequency offset as perceived by the receiver from the nominally transmitted frequency caused by relative motion of the transmitter and receiver. Ignoring higher order effects, the value of the frequency offset )f, is given by Vf 0 /c, where V is the relative velocity, f 0  the nominal frequency and c the speed of light. 
   This requirement in the receiver pass band results in additional noise energy passing over and above the amount theoretically required by the bandwidth of the information. Thus the frequency offset can result in a deterioration of the signal to noise ratio (SNR) to a degree that the signal is not easily distinguishable from non-signal noise by the receiver. The frequency offset additionally can reduce the capability of the receiver to retrieve the information contained in the signal. 
   Complicated receivers that employ a carrier frequency tracking loop are able to keep a narrow passband filter centered about the carrier, thereby substantially reducing the detected noise energy and improving the received SNR. However, large frequency offsets can prevent the tracking loop from even obtaining the signal. 
   Another consequence of an offset that occurs from time to time, especially for high frequency offsets between transmitter and receiver, is a phenomenon known as a “click”, which leads to a high estimation error of the carrier phase and frequency. Through sampling of the signal, complex samples travel a complete circle around the origin of the imaginary plane. However, if the influence of noise is significant enough, the path of the samples might not include the origin, this phenomena resulting in the accumulation of the phase missing 2B, or a click. 
   The subject matter in this application addresses the detection of large frequency offset signals with a preamble of frequency modulated Maximum Length Sequences (MLS&#39;s) and a predetermined period of pure carrier along with probable frequency offsets to widen the acquisition bandwidth. The subject matter described herein also addresses the detection and correction of clicks. 
   Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel method of wide band acquisition of a high performance waveform, in an environment with poor signal to noise ratio. The waveform having a preamble with a plurality of frequency modulated Maximum length sequences and a period of pure carrier. In one embodiment, an inventive method provides an application specific integrated circuit (ASIC) for receiving the waveform, filter coefficients for use with the MLSs portion, and filter coefficients for the pure carrier portion of the signal. This method may also include for the MLS portion of the signal, detecting the signal through partial correlation, extraction of waveform information, and estimation of symbol timing. The method may further include for the pure carrier portion of the signal, estimating the phase and frequency and providing such estimates to the ASIC. 
   It is another object of the present invention to provide a novel system and method for detecting a communication signal in an environment with poor signal to noise ratio, the signal including a preamble with a plurality of frequency modulated MLS and a predetermined period of pure carrier. In another embodiment, the method involves receiving a candidate signal, sampling the candidate signal a plurality of times for each symbol, and calculating a weighted frequency. From the weighted frequencies a 1 st  and 2 nd  attribute one calculated and the attributes are compared to determine whether a communication signal has been detected. 
   It is yet another object of the present invention to provide a novel system and method for detecting a narrow band communication signal in an environment with poor signal to noise ratio, the signal including a preamble with a plurality of frequency modulated MLS and a predetermined period of pure carrier. A further embodiment of the present invention may be a system and/or method of applying the candidate signal to a first mixer tuned to the expected carrier frequency to obtain a first intermediate signal, and supplying the first intermediate signal to a central processing branch, comprising x number of upper parallel processing branches and y number of lower parallel processing branches. The processing branches, sample a plurality of times each symbol in the intermediate signal, calculate a weighted frequency, calculate a 1 st  and 2 nd  attribute and compare the attributes to determine whether a communication signal has been detected. 
   It is still another object of the present invention to provide a novel system for detecting a communication signal with a large frequency offset in an environment with a poor signal to noise ratio, the signal with a preamble including a plurality of MLS sequences and a pure carrier signal. A still further inventive system includes a first mixer and first filter, a central processing branch, comprising x number of positive processing branches and y number of negative processing branches for processing an intermediate signal. Each processing branch may include a limiter for removing a DC offset, a correlator and a logic circuit. The system may also include an adjustable first filter and an adjustable first mixer one or both of which may be adjusted with output of logic circuits. 
   These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a representation of the preamble structure. 
       FIG. 2  is a flowchart for acquisition of a high performance waveform according to an embodiment of the present invention. 
       FIG. 3  is a detection algorithm for an embodiment of the present invention. 
       FIG. 4  is a schematic representation for computation of attributes according to an embodiment of the present invention. 
       FIG. 5  is a flowchart for symbol timing estimation. 
       FIG. 6  is a flowchart for carrier phase and frequency estimation according to an embodiment of the present invention. 
       FIG. 7  is a flowchart for click detection and correction according to an embodiment of the present invention. 
       FIG. 8  is a schematic representation of an apparatus for signal detection according to an embodiment of the present invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Acquisition of the HPW waveform is achieved via a preamble, which is located in the first part of each burst. The preamble is used to provide burst detection, symbol timing estimation carrier phase and frequency estimation and also provides information regarding the data waveform. The preamble consists of a plurality of consecutive MLS&#39;s followed by a period of pure carrier. In a particular embodiment shown in  FIG. 1 , the preamble  100  has three MLS&#39;s  101 ,  102  and  103  with 31 symbols each at a 8 kbps rate, frequency modulated with a 2.5 kHz frequency deviation followed by a period of pure carrier  110 . 
   Each of the three MLS&#39;s can either contain the following symbols: 
   MLS(k)=(−1,1,−1,1,1,1,−1,1,1,−1,−1,−1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,−1,1,−1,−1,−1,−1,1) or an inverted version containing the following symbols: 
     MLS (k)=(1,−1,1,−1,−1,−1,1,−1,−1,1,1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,1,−1,1, 1,1,1,−1). 
   Each of the MLS in the present embodiment has a duration of 31/8000 seconds. The pure carrier portion has a duration of 1536/120000 seconds, making the total duration of the preamble 23.7 msec. Other durations apart from the example above are envisioned and should not be construed to be preempted by exclusion herein. 
   A receiver includes an associated Application Specific Integrated Circuit (ASIC) to receive the high performance waveform. After the end of receiving a previous burst, the receiver is in a detection mode, the elements of such operation is shown in  FIG. 2 . The filter coefficients appropriate for the MLS part of the preamble are loaded or activated in the ASIC as shown in block  201 . The filter coefficients and the associated filter function as those known in the art, in a particular embodiment a 127 tap square root Nyquist filter with a roll off factor of ∀=0.79 is used. A candidate communication signal is applied to the filter and a detection algorithm  202  is run to determine if a communication signal has been detected. The detection algorithm  202  partially correlates the MLS portion of the signal in the process of detection. A particular detection algorithm  202  is described latter herein in greater detail but does not preclude other detection algorithms. 
   Upon detection of a burst (communication signal), information regarding the data waveform and header is extracted from the signal, as shown in block  203  and the receiver switches into a hunting mode for the peak correlation in order to estimate the symbol timing in block  204 . After symbol timing is accomplished, new filter coefficients for the pure carrier portion are loaded into the ASIC as shown in block  205 . The new filter coefficients function similarly to the MLS filter coefficients except they act upon the pure carrier portion. The filter for the pure carrier portion in the present embodiment uses a 127-tap low pass filter. The carrier phase and frequency are estimated in thereafter in Block  206 . This estimation is done as soon as the last sample of the pure carrier is available. The ASIC must then load an associated set of filter coefficients for use with the header and the data that follow the preamble  100  (not shown). 
   A particular detection algorithm  300  is shown in  FIG. 3 . A candidate signal is received in the ASIC where the signal is sampled a plurality of times per symbol in block  301 . The samples of the received signal are inphase and quadrature samples that are for example available at a 64 kbps. Each of these samples are used in the detection algorithm. 
   In Block  302  weighted frequencies are calculated from successive samples of the symbols. The current n I-Q sample represented as z[n]=Re {z[n]}+i·Im {z[n]} is used together with the previous sample z[n−1] to calculate the weighted phase difference, which is a function of the frequency. The phase of the sample z[n] which are close to the origin in the complex plane is much more sensitive to Additive White Guassian Noise (AWGN) then the phase samples with larger magnitudes, therefore rather or in addition to calculating the frequency with the help of a discriminator, the weighted frequency is derived using the equation:
 
 f[n]=Re{z[n]}·Im{z[n− 1]}− Im{z[n]}·Re{z[n− 1]}
 
This equation stems from:
 
                   f   ⁡     (   t   )       =         1     2   ⁢   π       ⁢       ⅆ     Φ   ⁡     (   t   )           ⅆ   t         =       1     2   ⁢   π       ⁢       ⅆ     arctan   ⁡     (       y   ⁡     (   t   )         x   ⁡     (   t   )         )           ⅆ   t                       =         1     2   ⁢   π       ⁢     1     1   +       (       y   ⁡     (   t   )         x   ⁡     (   t   )         )     2         ⁢             ⅆ     y   ⁡     (   t   )           ⅆ   t       ⁢   x   ⁢     (   t   )       -       y   ⁡     (   t   )       ⁢       ⅆ     x   ⁡     (   t   )           ⅆ   t               x   2     ⁡     (   t   )           ⇒     f   ⁡     [   k   ]                     =         1     2   ⁢   π   ⁢           ⁢     T   S         ⁢           y   ⁡     [   k   ]       ⁢     x   ⁡     [     k   -   1     ]         -       x   ⁡     [   k   ]       ⁢     y   ⁡     [     k   -   1     ]             |     z   ⁡     [   k   ]       ⁢     |   2           =       1     2   ⁢   π   ⁢           ⁢     T   S         ·       f   ∼     ⁡     [   k   ]       ·     1     |     z   ⁡     [   k   ]       ⁢     |   2                         
Hence, phase samples, which originate from samples with larger magnitudes, are amplified whereas phase samples from samples with smaller magnitudes are attenuated. Since the current I-Q sample z[n] is used in the subsequent calculation of the weighted frequency f[n+1], the sample is stored in a shift register or other appropriate storage device.
 
   Block  303  represents shifting the storage register and appending the new frequency sample previously calculated. In the present embodiment with 3 MLS&#39;s of 31 symbols with a sample rate of 64 kbps at the output of the ASIC and a symbol rate of 8 k there are 744 samples for the complete MLS part. The overall length of the shift register for storage of the samples is at least 746, allowing two or more previously calculated frequency samples to be stored for the peak correlation. When a new frequency sample has been calculated the register is shifted and the new frequency sample is appended. 
   In order to make a decision whether a burst or communication signal has been received, the receiver computes for each of the three MLSs  101 ,  102  and  103 , the partial energies, the partial correlation and partial averages every time a new frequency sample is available as indicated in block  304 . The mechanism for obtaining these attributes, partial energy, partial average and partial correlation is shown in  FIG. 4 . These quantities are partial in that out of the available samples per symbol, in the present embodiment 8, only one is used for each computation. The frequency samples  402  represent the symbols  403  of the MLSs  404 . The new frequency sample is shown as  401  in  FIG. 4 . 
   The partial energy (E) represents the sum of the square frequency samples as shown: 
   
     
       
         
           
             E 
             i 
           
           = 
           
             
               ∑ 
               
                 n 
                 = 
                 M 
               
               
                 M 
                 + 
                 30 
               
             
             ⁢ 
             
                 
             
             ⁢ 
             
               
                 f 
                 2 
               
               ⁡ 
               
                 [ 
                 
                   k 
                   · 
                   n 
                 
                 ] 
               
             
           
         
       
     
   
   where k=number of samples per symbol, and i=1 to the number of MLS sequences and M=(i−1)·j+1 in the present embodiment k=8 and j=31. Therefore i partial energies E i  are calculated resulting in three partial energies E 1    410 , E 2    411 , and E 3    412 . The frequency sample is squared and multiplied in multipliers  413  and summed with the output of the other frequency sample multipliers  413  of the respective MLS by summers  414 . 
   The partial correlation (C) represents the sum of the product of the samples with the not inverted MLS, which is known in by the receiver. The partial correlation is obtained with the following equation: 
             C   i     =       ∑     n   =   M       M   +   30       ⁢           ⁢       f   ⁡     [     k   ·   n     ]       ·     MLS   ⁡     [     n   -   M     ]                 
Likewise three values are obtained for C 1    421 ; C 1    422 ; C 2 , and C 3    423 . Where the frequency samples  401  are multiplied by the MLS known values  450  by multiplier  423  and summed by summer  424 .
 
   For carrier offset correcting purposes, the partial average, which represents the sum of the samples is calculated for each MLS as follows: 
   
     
       
         
           
             A 
             i 
           
           = 
           
             
               ∑ 
               
                 n 
                 = 
                 M 
               
               
                 M 
                 + 
                 30 
               
             
             ⁢ 
             
                 
             
             ⁢ 
             
               f 
               ⁡ 
               
                 [ 
                 
                   k 
                   · 
                   n 
                 
                 ] 
               
             
           
         
       
     
   
   resulting in A 1   430 , A 2    431  and A 3    432 . Where the frequency samples  402  are summed by summer  434 . 
   The partial energies, partial correlation and partial averages represent signal attributes either alone or mathematical combinations of each other. The attributes derived from these elements are used in block  305  for the threshold comparison. 
   In a particular embodiment the partial energies, correlation and averages are used to determine a 1 st  attribute and a second attribute for each of the MLSs. The 1 st  attribute, a function of the partial energy, partial correlation and overall average is:
 
 E   i   −A· ( C   i ) 2  
 
The 2 nd  attribute, a function of the partial average is:
 
             1   j     ·       (     A   i     )     2           
for each of the MLS&#39;s the attributes are compared for compliance to the following inequality:
 
               E   i     -     A   ·       (     C   i     )     2         &lt;       1   j     ·       (     A   i     )     2             
If and only if the equality holds true in block  306  for each of the MLS is the threshold reached indicating the detection of a communication signal or burst. Other thresholds could also be applied and are not foreclosed by the description of the present embodiment.
 
   After a signal has been detected, a search for the peak correlation value is started. At this time, all the available 744 frequency samples stored in the shift register are used and the correlations are performed with the filtered MLS response. The response has a length of 248 samples and is stored in a memory.  FIG. 5  shows the implementation of symbol timing estimation algorithm for symbol timing estimation in Block  205  the first loop  501  sets up a register with three full correlation values, as shown in block  502 , the correlation values are obtained in block  503  where Corr [0] is the correlation value, taken 2 time steps back form the time instant a signal was detected. This is why the shift register for the frequency samples as discussed earlier is at least 746. Analogously corr [1] contains the full correlation value one-time step back and corr [2] is the full correlation value at the time instant the signal is detected as implemented in block  504 . If corr[1] contains the peak value, the second loop in  FIG. 5  is not entered and the symbol timing is estimated in block  560 , otherwise the second loop calculates the next correlation value and appends it to the shifted register. The second loop  550  takes new signal samples for the ASIC block  551 , Calculates Frequency in block  552 , shifts the register and store the frequency sample in block  553 , performs correlation in block  554  ans shifts the register and appends new correlation sample in block  555 . Loop two  550  is executed until the peak correlation is found and then the estimation is started. 
   The full correlation is performed with all 744 frequency samples in the register and a filter MLS response of Length  248  stored in memory. Thus three full correlations values are calculated one for each of the MLS&#39;s as follows; 
             C   i     =       ∑     n   =   kM       (     k   ⁡     (     M   +   30     )       )       ⁢           ⁢       f   ⁡     [   n   ]       ·     filtered_MLS   ⁡     [     n   -     M   ·   k       ]                 
The value C=C 1   2 +C 2   2 +C 3   2  is stored in the correlation register. As soon as the peak correlation have been found and stored in corr[1], the offset )t to the peak correlation as a fraction of the sampling time T S  is computed via:
 
   
     
       
         
           
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               t 
             
             
               T 
               S 
             
           
           = 
           
             
               1 
               2 
             
             · 
             
               
                 
                   corr 
                   ⁡ 
                   
                     [ 
                     0 
                     ] 
                   
                 
                 - 
                 
                   corr 
                   ⁡ 
                   
                     [ 
                     2 
                     ] 
                   
                 
               
               
                 
                   corr 
                   ⁡ 
                   
                     [ 
                     0 
                     ] 
                   
                 
                 - 
                 
                   2 
                   · 
                   
                     corr 
                     ⁡ 
                     
                       [ 
                       1 
                       ] 
                     
                   
                 
                 + 
                 
                   corr 
                   ⁡ 
                   
                     [ 
                     2 
                     ] 
                   
                 
               
             
           
         
       
     
   
   The flow chart for estimating the carrier phase and frequency estimation of block  205  is shown in  FIG. 6 . As opposed to the MLS portion of the preamble, carrier phase and frequency estimation is done with a not weighted computation of the frequency samples. The frequency samples are computed from phase sample from the ASIC. Because the highly correlated nature of the pure carrier waveform, a smaller number of samples can be used. In the present embodiment only every 6 th  sample is used. These phase samples are accessed from the ASIC in block  601  and every 6 th  one is selected in decision block  602 . 
   The phase difference )M is calculated in block  603  by subtracting successive phase samples selected in block  602  as shown below:
 
ΔΦ=φ[ n]−φ[n−k] 
 
where k is the interval between the samples selected, in the present embodiment k=6.
 
   Since the phase is limited within the range of −B and B, and the phase difference )M can vary between −2B and 2B, the phase difference is “unwrapped” in block  604 . The process of unwrapping resolves this ambiguity. Phase differences larger than B are reduced by 2B and phase differences smaller then −B, are increased by 2B as demonstrated below; 
   
     
       
         
           
             Δ 
             ⁢ 
             
                 
             
             ⁢ 
             
               Φ 
               ∼ 
             
           
           = 
           
             { 
             
               
                 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Φ 
                       
                       + 
                       
                         2 
                         ⁢ 
                         π 
                       
                     
                     , 
                     
                       
                         if 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Φ 
                       
                       ≤ 
                       
                         - 
                         π 
                       
                     
                   
                 
               
               
                 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Φ 
                       
                       - 
                       
                         2 
                         ⁢ 
                         π 
                       
                     
                     , 
                     
                       
                         if 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Φ 
                       
                       &gt; 
                       π 
                     
                   
                 
               
               
                 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Φ 
                     
                     , 
                     otherwise 
                   
                 
               
             
             } 
           
         
       
     
   
   Click detection and correction is accomplished in block  605 . As noted earlier, high frequency offsets greater than 800 Hz between the transmitter and receiver for the present embodiment can lead unacceptable high estimation error in the carrier phase and frequency. The detection and correction algorithm is shown in  FIG. 7 . The click is detected and correct with the use of a running summer, which add all phase deference samples obtained and a threshold comparison. 
   The decision block  701  determines if the running sum is greater than zero, 
             Δ   ⁢           ⁢     Φ   ∼       &lt;       -     30   180       ⁢   π           
and if the phase was not corrected in the last 8 samples. Upon a positive decision of all three a click is detected and corrected in block  702  resulting in a corrected phase difference of Δ{circumflex over (Φ)}=Δ{tilde over (Φ)}+2π. If the result of block  701  is negative, decision block  703  determines if the sum is less than zero, if
 
             Δ   ⁢           ⁢     Φ   ~       &gt;       30   180     ⁢   π           
and if the phase was not corrected in the last 8 samples. A positive decision indicates a detection of a click and the phase difference is corrected in block  704  resulting in a corrected phase difference of Δ{circumflex over (Φ)}=Δ{tilde over (Φ)}=2π. If the decision of block  703  is negative the phase difference remains uncorrected. The uncorrected phase difference is added to the running sum in block  706 .
 
   The phase and frequency estimation of the pure carrier is then updated in block  606  of  FIG. 6 . The estimation can be implemented as a Finite Impulse Response (FIR) filter. The phase difference samples Δ{circumflex over (Φ)} after the click correction are summed up and the new summation value is applied to filters for slope and intercept estimation. The phase differences are summed according to Δ{circumflex over ({circumflex over (Φ)}[n]=Δ{circumflex over ({circumflex over (Φ)}[n−1]+Δ{circumflex over (Φ)} where the slope and intercept are determine from:
 
slope[ n ]+slope[ n− 1]+ a[n]·Δ{circumflex over ({circumflex over (Φ)}[n];  
 
int rcept[ n ]=int ercept[ n]+b[n]·Δ{circumflex over ({circumflex over (Φ)}[n];  
 
   Where the filter coefficients a[k] and b[k] are computed as follows: 
   
     
       
         
           
             
               
                 a 
                 ⁡ 
                 
                   [ 
                   k 
                   ] 
                 
               
               = 
               
                 6 
                 · 
                 
                   
                     
                       2 
                       · 
                       k 
                     
                     - 
                     N 
                     - 
                     1 
                   
                   
                     N 
                     · 
                     
                       ( 
                       
                         
                           N 
                           2 
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
             
             ; 
             and 
           
           , 
           
              
           
           ⁢ 
           
             
               b 
               ⁡ 
               
                 [ 
                 k 
                 ] 
               
             
             = 
             
               2 
               · 
               
                 
                   
                     2 
                     · 
                     N 
                   
                   + 
                   1 
                   - 
                   
                     3 
                     · 
                     k 
                   
                 
                 
                   N 
                   · 
                   
                     ( 
                     
                       N 
                       - 
                       1 
                     
                     ) 
                   
                 
               
             
           
         
       
     
   
   Where N represents the number of samples used for the estimation which in the present embodiment N=256. Block  607  determines all the frequency samples have been taken from the ASIC and the phase and frequency estimations are calculated in block  607 . 
   As b[N] is the least mean square estimate of the interception, and thus the phase offset between the transmitter and receiver. A[N], on the other hand, is the LMS estimate of the phase change between two samples, which were used for the estimation. Thus the frequency offset is therefore given as: 
   
     
       
         
           
             f 
             offset 
           
           = 
           
             
               
                 1 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   π 
                 
               
               ⁢ 
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Φ 
                 
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   t 
                 
               
             
             = 
             
               
                 
                   1 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       π 
                       · 
                       6 
                       · 
                       
                         T 
                         S 
                       
                     
                   
                 
                 · 
                 
                   slope 
                   ⁡ 
                   
                     [ 
                     N 
                     ] 
                   
                 
               
               = 
               
                 
                   
                     64 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     kHz 
                   
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       π 
                       · 
                       6 
                     
                   
                 
                 · 
                 
                   slope 
                   ⁡ 
                   
                     [ 
                     256 
                     ] 
                   
                 
               
             
           
         
       
     
   
   The frequency offset, once quantified, is used to refine the expected receive frequency of the receiver for subsequent reception of the information bearing portion of the communication signal. This refinement can include changes to the VCO and/or the front-end filter. 
   In narrow band operation, the relative frequency offset is on the order of 5 times larger than during wide band operation. Such a frequency offset lends itself to signal detection with an additional embodiment utilizing parallel processing of the preamble. The processing substantially mimics the detection method described in  FIG. 3 . 
   In signal detection in multiple parallel processing branches, the candidate signal is mixed with a frequency selected with respect to the expected carrier frequency in order to obtain the desired intermediate signal at a desired intermediate frequency. The intermediate signal is applied to a central processing branch, as well as upper and lower processing branches. The number of upper and lower processing branches need not be limited. 
   In the upper and lower parallel processing branches a respective signal associated with a predetermined frequency offset is mixed with the intermediate signal resulting resulting in a respective intermediate signal for each of the processing branches. Each branch processes the intermediate signal as if the signal has a carrier frequency with a frequency offset associated with the particular processing branch. The frequency of the respective signal is selected to account for positive and negative frequency offset or deviation between the expected transmitted signal and the receive frequency. In an embodiment the upper processing branch is associated with a 800 kHz positive frequency offset and the lower processing branch is associated with a negative frequency offset of 800 kHz. In embodiments with a plurality of upper and lower branches the associated frequency offsets can be incremented between the expected receive frequency and the predetermined maximum frequency offsets. 
   In each branch, the detection algorithm of  FIG. 3  is followed. Upon detection of a communication signal in one of the processing branches the mixer is supplied feedback for adjusting its frequency appropriate for a received signal at the offset frequency associated with the respective processing branch. If multiple processing branches detect a signal is present. The SNR is used to determine the frequency closest to the incoming carrier. The frequency offset for the selected processing branch is used as the initial frequency estimate and the signal is considered to have been acquired. 
     FIG. 8  shows an embodiment of narrow band signal detection with plural parallel processing branches. The candidate signal is received at the receiver  801 . The candidate signal is mixed with a signal with a frequency associated with the expected received signal frequency T c  at mixer  802  supplied by a Voltage Controlled Oscillator (VCO)  803  or other Oscillating means. An intermediate signal with a intermediate frequency created by the mixed signals is filtered by a filter  804 . The filtered intermediate signal is then applied to each of the processing branches  810 ,  820 ,  830 ,  840  and  850  respectively. The filtered intermediate signal is then mixed in the upper branches  820  and  840 , and the lower branches  830  and  850  by respective mixers  821 ,  841 ,  831  and  851 . Each mixer combines the filtered intermediate signal with a frequency associated with the predetermined offset frequency for each branch, +T, +nT, −T and −mT respectively. The elements and operation of each of the branches function identically from this point on, as such other processing branch  820  is discussed in detail. 
   The respected intermediate signal resulting from the mixing is demodulated in frequency demodulator  822  and the DC offset is removed by limiter loop  823 . The limiter loop  823  also provides a fine tuning frequency differential * associated with the DC offset which fine tunes the predetermined frequency offset. The signal is then applied to a correlator  824  which correlates the MLS samples as detailed previously in regards to the detector algorithm. The output of the correlator  824  is applied to a logic circuit  825 , which compares signal attributes to threshold relationships from  826 . Upon reaching a positive threshold relationship the output of the logic circuit  825  releases the fine tuned frequency offset from a storage buffer  827  that in turn is used to adjusted the expected receive frequency at adder  804 , thus adjusting the output of the VCO  803 . The output of the buffer  822  is also used in narrowing the IF filter  804 . If a signal is not detected in the respective branch, then the information contained in the buffer  822  is not released as it logically becomes inconsequential in determined the frequency offset. As mentioned previously if multiple branches detect a signal an additional logic circuit  808  can be used to select the branch with the greatest SNR and release only the offset information from the selected buffer. 
   While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.