Abstract:
A receiver for carrier frequency estimation via symbol rate estimation in the presence of carrier frequency error for use in signal acquisition and signal demodulation of spread-spectrum chips affected by Doppler shift in an advanced tactical data link. Received LPD signals with a very low signal-to-noise ratio are input to a receiver designed to tolerate carrier frequency error caused by Doppler shift. Extremely low signal-to-noise ratio and short dwell times due to spread spectrum modulation and frequency-hopping make direct estimation of carrier frequency impractical. A method and apparatus is disclosed to use the error is symbol rate, the nominal carrier frequency, and the nominal transmitted symbol rate to estimate carrier frequency error. This enables longer coherent integration times and improves LPD receiver performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is filed concurrently with commonly assigned U.S. patent application Ser. No. 11/416,619, entitled “Signal Acquisition with Efficient Doppler Search”, listing as inventors Carlos J. Chavez, Gunter B. Frank, and Robert J. Frank, and U.S. patent application Ser. No. 11/416,621 entitled “Architecture for Signal Acquisition with Cyclic Range Search”, listing as inventors Carlos J. Chavez, Gunter B. Frank, and Robert J. Frank. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates generally to the field of command, control, communications, computer, intelligence surveillance, and reconnaissance (C4ISR) hardware and software systems and components, and in particular using spread-spectrum communications. 
   2. Description of Related Art 
   TTNT (Tactical Targeting Networking Technology) is an advanced tactical data link currently under development by Rockwell Collins Government Systems and the Advanced Technology Center. Modes supporting Low Probability of Detection (LPD) are a highly desirable addition to existing TTNT functionality. The primary challenge for an LPD receiver is to operate at extremely low signal-to-noise ratio (SNR), often well below negative 20 dB. Because an LPD system must operate at extremely low SNR, the known sequence of chips used for signal acquisition must be very long (on the order of 1 million chips) in order to produce reasonable probabilities of detection and false alarm. 
   In addition, LPD performance is enhanced by frequency-hopping, which limits the amount of time an LPD signal dwells on any single carrier frequency. 
   Critical to LPD operation is the capability of a receiver to tolerate carrier frequency error caused by Doppler shift. Extremely low SNR and short dwell times make direct estimation of carrier frequency impractical. The present invention presents a method and apparatus whereby carrier frequency error can be estimated indirectly via estimation of the error in symbol rate. This enables longer coherent integration times and thus improves LPD receiver performance. 
   The majority of carrier frequency error observed by an LPD receiver in a tactical environment is a result of Doppler frequency shift. The error fe in carrier frequency caused by Doppler is a function of relative velocity v, nominal (transmitted) carrier frequency fc, and the speed of light c. 
   fe=±vfc/c 
   The same holds for the error Re in symbol rate caused by Doppler, where Rs is the nominal (transmitted) symbol rate. 
   Re=±vRs/c 
   It can be shown from the preceding expressions that the error fe in carrier frequency caused by Doppler can be expressed as a function of the error Re in symbol rate caused by Doppler.
 
{EQUATION 1}
 
 fe=Re*fc/Rs   (EQ. 1)
 
   Thus, if the error in symbol rate can be estimated, an estimate of the error in carrier frequency may be easily computed by multiplying the symbol rate error estimate by the ratio of nominal carrier frequency to nominal symbol rate. 
   Because LPD systems operate at extremely low SNR, transmission lengths are often extremely long. Error in the symbol rate caused by Doppler can result in a significant shift in symbol timing over the length of a message, which complicates signal acquisition. 
   In the present invention, certain terms are used as appreciated by a skilled artisan. Thus “chip” is often defined as “channel bit”. A spread spectrum system, such as used by the present invention, achieves its spectral spreading using one or more techniques such as direct sequence, forward error correction, and orthogonal channel coding. Regardless of the technique used, the bits produced by the spreading are often referred to as “chips”. These chips are modulated and sent over the channel. This distinguishes the bits created by the spreading technique (“chips”) from the information bits going into the spreading technique (“bits”). Note that spread spectrum chips are not required to be binary. “Chip rate” is the rate or frequency at which the chips are transmitted. In a spread spectrum system, the chip rate is much faster than the information bit rate, thus the spectral spreading. “Chip time” is the reciprocal of the chip rate, or the duration in time of a single chip. “Multiple chip times” refers to a period of time that is equal to more than one chip time. A “known sequence” is a sequence of chips (or bits, or symbols) of which an authorized receiver has prior knowledge. The known sequence is typically sent at the beginning of a transmission. The receiver performs a search for the known sequence in order to detect the presence of a desired signal and synchronize its signal processing to it. The process of detecting the presence of a desired signal is often referred to as the signal “acquisition”. Signal acquisition precedes signal demodulation. 
   SUMMARY OF THE INVENTION 
   Accordingly, an aspect of the present invention is concerned with carrier frequency estimation via symbol rate estimation, at the receiver side of a spread spectrum TTNT tactical data link. 
   The present invention works to first estimate carrier frequency via symbol rate estimation in a LPD spread-spectrum signal in a TTNT tactical data link, then to track the carrier frequency via symbol time tracking. 
   The above described and many other features and attendant advantages of the present invention will become apparent from a consideration of the following detailed description when considered in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Detailed description of preferred embodiments of the invention will be made with reference to the accompanying drawings. Disclosed herein is a detailed description of the best presently known mode of carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention. The section titles and overall organization of the present detailed description are for the purpose of convenience only and are not intended to limit the present invention. 
       FIG. 1  is a block diagram for the initial estimation of carrier frequency via symbol rate estimation. 
       FIG. 2  is a block diagram for the tracking of carrier frequency via symbol time tracking. 
   

   It should be understood that one skilled in the art may, using the teachings of the present invention, vary embodiments shown in the drawings without departing from the spirit of the invention herein. In the figures, elements with like numbered reference numbers in different figures indicate the presence of previously defined identical elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The method and apparatus of the present invention may be hardware—such as a spread-spectrum receiver—that is, a circuit hardwire programmed to perform the signal acquisition functions outlined herein (e.g., an ASIC), hardware running firmware, or hardware running software, with the software existing in memory, and which may be written in any computer language (such as C, C++, Perl, Java or the like), and further, and/or in the alternative, the software may be run by a computer system having an operating system. The computer system typically has one or more processors, primary and secondary memory cooperating with the processor(s), which executes instructions stored in the memory, I/O means such as monitor, mouse and keyboard, and any necessary specialized hardware or firmware. Depending on the language used to construct and implement the software, the source code, object code and/or executables of the software may have any number of classes, functions, objects, variables, templates, lines of code, portions of code and constructs (collectively and generally, “a process step”, “step”, “block”, “functional module” or “software module”) to carry out the invention in successive stages as described and taught herein, and may be either a standalone software application, or employed inside of or called by another software application, or as firmware. The software process or software module may be constructed so that one portion of code in the application performs a plurality of functions, as for instance in Object Oriented programming (e.g., an overloaded process). The converse is also true, in that a plurality of portions of code could perform a plurality of functions, and still be functionally the same as a single portion of code. At any stage of the process step of the present invention, intermediate values, variables and data may be stored for later use by the program. In addition, the binary executable or source code data comprising the software of the present invention may reside on computer readable storage medium (e.g., a magnetic disk, which may be portable); memory (e.g., flash RAM); DVD or CD-ROM. 
   Turning attention to  FIG. 1 , there is shown a block diagram schematic of the method and apparatus of the present invention, for the initial estimation of carrier frequency via symbol rate estimation. The signal acquisition function of a LPD receiver must first estimate the carrier frequency before it can be tracked. This enables subsequent demodulation and recovery of the information contained within the signal. 
   Thus  FIG. 1  depicts initial estimation of carrier frequency via symbol rate estimation. In a typical receiver, signal acquisition must occur before signal demodulation can take place. The signal acquisition function must detect the presence of a desired signal and determine an estimate of the initial symbol timing. If the symbol rate error caused by Doppler shift is significant, as can be the case for an LPD system, the signal acquisition function must also determine an estimate of the initial symbol rate error. Once a desired signal is detected, the signal demodulation function uses the initial symbol time and initial symbol rate error estimates to synchronously recover the data encoded in the received signal. 
   With the addition of the carrier frequency error estimation block shown in  FIG. 1 , an LPD receiver can apply the technique described herein. The carrier frequency error estimation block accepts the initial symbol rate error estimate and multiplies it by the ratio between the carrier frequency and the symbol rate, which are also known inputs, as outlined in connection with equation EQ. 1 above. 
   The resulting output is an estimate of the initial carrier frequency error. The signal demodulation function can compensate for this carrier frequency error and thus extend the length of time over which coherent integration of the desired signal can be performed. Longer coherent integration times enable better communication system performance. 
   Thus, turning attention to  FIG. 1 , showing the initial carrier frequency estimation circuit  10 , a received signal  15  is received by the circuit comprising Signal Acquisition block  20 , which can be any conventional spread-spectrum signal acquisition block that produces an initial symbol time and initial symbol rate error signal, or, equivalently, the signal acquisition block may be done by the method and apparatus as taught by U.S. patent application Ser. No. 11/416,619, entitled “Signal Acquisition with Efficient Doppler Search”, listing as inventors Carlos J. Chavez, Gunter B. Frank, and Robert J. Frank, incorporated by reference herein. The Signal Acquisition block  20  outputs the initial signal timing, output initial symbol time  25  which is an estimate. There are well known methods in the art for estimating symbol timing. The specific implementation is not important to the present invention. 
   In  FIG. 1 , the Carrier Frequency Error Estimation block  35  accepts as input the initial symbol rate error  30  as output from the Signal Acquisition block  20 , and, using equation EQ. 1 above, multiples the initial symbol rate error estimate by the ratio between the carrier frequency (nominal, transmitted) and the symbol rate (nominal, transmitted). Both the nominal, transmitted carrier frequency and symbol rate are known to the receiver. The Carrier Frequency Error Estimation block  35  then outputs an initial carrier frequency error signal  40 . A signal demodulation function, provided by Signal Demodulation block  45 , as is known per se, can compensate for the carrier frequency using the carrier frequency error and thus extend the length of time over which coherent integration of the desired signal can be performed, and outputs the recovered data information, as recovered data signal  50 . Coherent integration and signal demodulation is known per se in the art. 
   Note that the signal demodulation block may use the initial estimates of symbol time, symbol rate error, and carrier frequency error to initialize a tracking loop, such as that described in connection with  FIG. 2 . Alternately, signal demodulation may be performed in an open loop manner without further estimation or tracking of symbol timing or carrier frequency. Carrier frequency tracking is not necessarily required by the present invention; an open loop implementation simply assumes that the initial carrier frequency estimate is of sufficient accuracy and that the carrier frequency will not change significantly over the length of the signal. 
   If, however, it is desired to track carrier frequency, the circuit of  FIG. 2  may be employed.  FIG. 2  depicts the tracking of carrier frequency via symbol time tracking loop circuit  60 . A representative symbol time tracking loop functions performs feedback as follows, but suitable modifications can be made by one of ordinary skill in the art using the teachings herein without departing from the scope of the invention. 
   The received signal  65  is sampled by the sampling block, Sampler block  70 . Generally, the actual sample timing will be in error from the ideal sample timing. The Symbol Detector block  80  accepts the sampled signal from the Sampler block  70 , and as output by a Phase Rotator  75 , which receives a carrier frequency error signal, as explained further below, and removes carrier frequency error. The Symbol Detector block  80  makes a decision as to which symbol was sent by the transmitter and received, as output signal symbol decision  85 . The Symbol Timing Estimation block  90  accepts the sampled signal, as output from the Phase Rotator block  75 , which removes carrier frequency error, and optionally the Symbol Timing Estimation block  90  may accept the output signal symbol decision  85 . “Symbol Timing” in refers to both symbol time (the best time to sample a particular symbol) and symbol rate, which have error signals produced downstream, at the output, of the Symbol Timing Estimation block  90 . From these inputs, any number of well established decision-directed or non-decision directed techniques, known per se in the art, can be used to estimate the error in symbol time, output by the Symbol Timing Estimation block  90  as signal symbol time error  95  in  FIG. 2 . The symbol time error signal  95  drives the Symbol Time Tracking block  100 , which produces symbol time corrections to be made by the sampler, Sampler block  70 , as shown with output signal symbol time correction  105  in  FIG. 2 . The Symbol Time Tracking block  100  typically consists of a classical control feedback loop filter, known per se in the art. Thus, the error in symbol time may be tracked to zero by the symbol time tracking loop circuit  60 . 
   If the symbol rate error caused by Doppler shift is significant, as can be the case for an LPD system, the symbol time tracking block must be capable of tracking the symbol rate error as well as the symbol time error. An estimate of the symbol rate error is a natural byproduct of a such Symbol Time Tracking block  100 , and output as symbol rate error signal  110 , as shown in  FIG. 2 . 
   Thus with the addition of the Carrier Frequency Error Estimation block  120  shown in  FIG. 2 , an LPD receiver can apply the technique described previously, in connection with equation EQ. 1. The Carrier Frequency Error Estimation block  120  accepts the symbol rate error estimate signal  110  and multiplies it by the ratio between the carrier frequency and the symbol rate, as taught by EQ. 1. The resulting output, signal carrier frequency error  125 , is an estimate of the carrier frequency error. The Phase Rotator block  75 , as shown in  FIG. 2 , may be used to remove the carrier frequency error by multiplying the received signal of carrier frequency error  125  output by the Carrier Frequency Error Estimation block  120  by a rotating complex phasor. Thus, the error in carrier frequency may be tracked to zero via symbol time tracking and the application of feedback to the sampled received signal  65 . This extends the length of time over which coherent integration of the desired signal can be performed. Longer coherent integration times enable better communication system performance. 
   Although the present invention has been described in terms of the preferred embodiments above, numerous modifications and/or additions to the above-described preferred embodiments would be readily apparent to one skilled in the art. 
   It is intended that the scope of the present invention extends to all such modifications and/or additions and that the scope of the present invention is limited solely by the claims set forth below.