Abstract:
A system and method is provided for improving the operating range for UWB communication systems by increasing the margin between true correlation peaks and false correlation peaks in a received UWB signal. A received signal is first correlated with a UWB kernel before being correlated with the PRN code. The UWB kernel is a very short UWB pulse having the same pulse shape as the UWB signal pulses. By pre-correlating the received signal with the UWB kernel at least once prior to correlating with the PRN code, the margin between true correlation peaks and false correlation peaks is substantially improved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 60/794,736 filed on Apr. 25, 2006. 
    
    
     FIELD OF INVENTION 
     The invention relates generally to Ultra Wide Band (UWB) receivers and, more particularly, to improving separation of correlation peaks from noise in a received UWB signal. 
     BACKGROUND 
     An Ultra Wide Band (UWB) signal occupies a wide frequency range due to its narrow pulse width. Various signal shapes can be used in order to meet frequency band restrictions and transmitter/receiver design requirements. The UWB signals are normally difficult to distinguish from noise. Various modulation schemes are used to detect and track a UWB signal and distinguish the signal from noise in a UWB receiver. 
     One such technique is to multiply the UWB pulse train by a synchronized, unique pseudo-random noise (PRN) code in the UWB transmitter. By correlating a received signal with the same code in a UWB receiver, the UWB signal train can be detected and recovered from noise. The signal output from a correlator includes a series of peaks in which some are “true” cross-correlation peaks representing a transmitted pulse, and others are “false” cross-correlation peaks representing signal noise. The “true” cross-correlation peaks are higher than the false peaks by some margin. The margin is determined by the length of the PRN code, the orthogonality properties of the code and how much noise and interference is present in the incoming signal. The operating range for UWB systems can be limited because the “safety margin,” that is, the margin between the true and false correlation peaks, quickly vanishes with distance due to propagation loss and multi-path loss. 
     SUMMARY 
     The present invention provides a system and method for improving the operating range for UWB communication systems by increasing the safety margin between true correlation peaks and false correlation peaks in a received UWB signal. A received signal is first correlated with a “UWB kernel” before being correlated with the PRN code. The UWB kernel is a very short UWB pulse having the same pulse shape as the UWB signal pulses. By “pre-correlating” the received signal with the UWB kernel at least once prior to correlating the signal with the PRN code, the safety margin between true correlation peaks and false correlation peaks is substantially improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention description below refers to the accompanying drawings, of which: 
         FIG. 1  is a schematic diagram of a UWB transmitter including a UWB antenna; 
         FIG. 2  is a graph of an illustrative PRN code sequence; 
         FIG. 3  is a graph of UWB signal amplitude versus time for an illustrative UWB signal; 
         FIG. 4  is a graph of a UWB pulse train modulated with a PRN sequence; 
         FIG. 5  is a graph of the power spectral density of a UWB pulse train modulated with a PRN sequence; 
         FIG. 6  is a graph of the power spectral density of illustrative uniformly spaced UWB pulses; 
         FIG. 7  is a graph of a UWB pulse train with added noise; 
         FIG. 8  is a graph of the power spectral density of a UWB pulse train with added noise; 
         FIG. 9  is a graph of cross-correlation peaks of a PRN sequence with a noise-free UWB pulse train; 
         FIG. 10  is a graph of cross-correlation peaks of a PRN sequence with a UWB pulse train in the presence of 10 dB noise; 
         FIG. 11  is a graph of an illustrative UWB kernal; 
         FIG. 12  is a graph of cross-correlation peaks of a PRN sequence with a UWB pulse train in the presence of 10 dB noise pre-correlated once with a UWB kernal; 
         FIG. 13  is a graph of cross-correlation peaks of a PRN sequence with a UWB pulse train in the presence of 10 dB noise pre-correlated twice with a UWB kernal 
         FIG. 14  is a graph of cross-correlation peaks of a PRN sequence with a UWB pulse train in the presence of 15 dB noise; 
         FIG. 15  is a graph of cross-correlation peaks of a PRN sequence with a UWB pulse train in the presence of 15 dB noise pre-correlated once with a UWB kernal; 
         FIG. 16  is a graph of cross-correlation peaks of a PRN sequence with a UWB pulse train in the presence of 15 dB noise pre-correlated twice with a UWB kernal; and 
         FIG. 17  is a functional block diagram of an illustrative UWB receiver and 
         FIG. 18  is a functional block diagram of an alternative embodiment of the UWB receiver. 
     
    
    
     DETAILED DESCRIPTION 
     The safety margin between “true” correlation peak and “false” correlation peaks in a received UWB signal that is correlated with a PRN code can be improved by first cross-correlating the incoming signal with a very short UWB pulse, which is referred to herein as a “UWB Kernel.” The UWB kernal illustratively has the same shape as a transmitted UWB signal pulse. The cross-correlation with the UWB kernal is referred to herein as “pre-correlation.” 
     According to an illustrative embodiment, a receiver applies the UWB Kernel to the received signal N times to essentially “filter” the narrow UWB pulses from the incoming “noisy” signal before the receiver cross-correlates the signal with the PRN code. A significant improvement in signal to noise ratio has been found using the pre-correlation. Accordingly, relatively weak UWB signals can be detected in the presence of noise and interference and additional range may obtained using the pre-correlation technique. 
     The illustrative transmitter depicted in  FIG. 1  and receiver depicted in  FIG. 17  use a novel UWB dual sphere antenna configuration  102  which is described in a co-pending U.S. patent application Ser. No. 11/693,880 entitled Dual Sphere Ultra Wide Band Antenna. However other antennas may be used to transmit and receive the UWB signals. 
       FIG. 1  is a schematic diagram of a UWB transmitter  100  driving a dual sphere UWB antenna  102 . A continuous wave (CW) input signal provided by a generator  104  is modulated by a pseudo-random noise (PRN)/Data Modulator  108 . The input signal is fed through a divide by N module  106  to produce a modulation signal that has a frequency that is suitable for input to the PRN/Data Modulator  108 . A PRN code on line  110  and a data signal on line  112  are also input to the PRN/Data Modulator  108 . In response to the modulation signal, the PRN code  110  and the data  112 , the PRN/Data Modulator controls switching circuitry  114  to produce pulses of the continuous wave signal. The pulses are communicated to the antenna  102  via a transmission line  116 . 
     A sample of a PRN sequence  200 , is shown in  FIG. 2 . The PRN sequence  200  is illustratively applied to a 2.7 GHz signal, such as signal  300  shown in  FIG. 3  and the resultant PRN modulated UWB signal  400  is shown in  FIG. 4 . The signal shown in  FIG. 4  may be produced by the UWB transmitter  100  shown in functional block diagram form in  FIG. 1 . 
     The sending of the UWB pulses modulated with the PRN sequence causes the power spectral density to be more uniformly distributed over the frequency range, by preventing the build up of spectral lines.  FIG. 5  shows an estimated power spectral density  500  of the PRN modulated UWB signal  400 . This is in contrast to the power spectral density  600  of UWB pulses transmitted uniformly, as illustrated in  FIG. 6  which shows the grouping of spectral lines in the power spectral density of uniformly spaced UWB pulses. 
     The PRN modulated UWB signal  400  shown in  FIG. 4  does not include a noise component. When 10 dB of noise is added to the signal  400  the UWB signal is indistinguishable from the noise.  FIG. 7  illustrates a noisy signal  700  including a PRN modulated UWB pulse train 10 dB added noise.  FIG. 8  is a graph of the power spectral density  800  of the UWB pulse train with 10 dB added noise. 
     A common approach for signal detection using a PRN sequence is to cross-correlate the incoming signal with a copy of the PRN sequence. As shown in  FIG. 9 , the result of cross-correlating the noise-free PRN modulated UWB signal  400  (shown in  FIG. 4 ) with the PRN sequence  200  (shown in  FIG. 2 ) is a clear correlation peak  902  separated from false peaks  904  by a substantial margin. The ratio between the cross-correlation peak and the false peaks is about 14 dB. In other words, there is a 14 dB safety margin between the “true” correlation peak and “false” correlation peaks. 
       FIG. 10  illustrates the result of cross-correlating the noisy PRN modulated UWB signal  700  (shown in  FIG. 7 ) with the PRN sequence  200  (shown in  FIG. 2 ). The margin between the true correlation peak  1002  and a false correlation peak  1004  in the cross-correlated noisy signal is less than 3 dB. This decreased safety margin significantly diminishes the receiver range and performance. 
     One known method for increasing the safety margin is to utilize a much longer PRN sequence. However, increasing the PRN sequence length reduces the effective data rate and requires more complex transmitters and receivers. Instead, the inventive receiver performs a pre-correlation step in which a short UWB pulse, referred to herein as a UWB kernal, is correlated with the signal before the PRN sequence is used for cross-correlation.  FIG. 11  shows an illustrative UWB kernal  1100  which is a very short UWB pulse in the shape of the transmitted UWB pulses. The UWB kernal pre-correlation essentially acts to “filter” the UWB pulse train from the noise. 
     The result of correlating the UWB kernal  1100  with the noisy signal  700  shown in  FIG. 7  and then cross-correlating the signal with the PRN code  200  shown in  FIG. 2  is an improved safety margin between the true correlation peak  1202  and false peak  1204  shown in  FIG. 12 . The step of pre-correlating with the UWB kernal  1100  improves the safety margin from about 2.5 dB to about 7 dB. 
     An alternative embodiment of the inventive UWB receiver pre-correlates the received signal with a UWB kernal  1100  multiple times before cross-correlating the signal with the PRN code  200 . In the preferred embodiment, the UWB receiver pre-correlates the received signal  700  with the UWB kernal  1100  twice before cross-correlating the signal with the PRN code  200 . The results of applying the UWB kernal twice to the signal and cross-correlating the signal with the PRN code is shown in  FIG. 13 . Note that the safety margin has improved to 8.8 dB. While the UWB kernal may be applied additional times, at some point the application of the UWB kernal results in shaping the noise to look more like the UWB pulse, and thus, adversely affects the margin that was achieved in the previous pre-correlation step. 
     The effect of pre-correlating a received PRN modulated UWB signal that includes 15 dB of noise is illustrated with reference to  FIGS. 14-16 .  FIG. 14  illustrates the result of cross-correlating the noisy PRN modulated UWB signal with a PRN code in the receiver without performing a pre-correlation in accordance with the inventive method. A very small safety margin results between the true correlation peak  1402  and false correlation peak  1404 .  FIG. 15  illustrates the result of cross-correlating the same noisy UWB signal with the same PRN code in the receiver after once pre-correlating the received signal with a UWB kernal  1100 . The margin between true peak  1502  and false peak  1504  is substantially improved (note the different scales in the two figures).  FIG. 16  illustrates the result of cross-correlating the same noisy UWB signal with the same PRN code in the receiver after twice pre-correlating the received signal with a UWB kernal  1100 . The margin between true peak  1602  and false peak  1604  is substantially improved over the margin shown in  FIG. 15 . Applying the UWB kernal a third time did not raise the safety margin appreciatively. 
       FIG. 17  is a schematic diagram of a UWB receiver according to an illustrative embodiment of the invention. A UWB antenna  1702  receives a signal including noise and a PRN encoded UWB signal. A pre-correlator  1704  correlates the received signal “n” times with a UWB kernal  1706 . Although the embodiment illustrated in  FIG. 7  shows a single pre-correlator which performs n correlations, it should be understood that a series of n separate pre-correlators  1704  could also be used in series wherein each separate pre-correlator performs one correlation with a UWB kernal within the scope of the present invention. A correlator  1708  receives the pre-correlated signal from the pre-correlator(s)  1704  and correlates the pre-correlated signal with the PRN code  1710 . The correlator outputs correlation peaks with an improved safety margin so that the UWB receiver can accurately locate UWB pulses in the received signal. 
     Although embodiments are described herein with respect to PRN modulated pulses, persons having ordinary skill in the art should appreciate that the present invention can also be implemented wherein the UWB pulses are encoded using alternate modulation techniques. For example, it is envisioned that a pulse position modulated (PPM) UWB signal can be pre-correlated in a UWB receiver to improve the associated safety margin within the scope of the present invention. 
     While the invention has been described with reference to various illustrative embodiments, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.