Abstract:
Acquisition of a waveform such as a continuous-phase modulation (CPM) waveform is described. In one embodiment of the invention, the invention is directed to a method and apparatus for acquiring a waveform as defined by MIL-STD-188-181B including the preamble of such a waveform at a performance level defined by the standard. The present invention provides solutions to at least four primary issues presented in acquiring a CPM waveform such as the MIL-STD-188-181B compliant waveform. These primary problems include searching for the preamble, determination of the symbol rate, determination of an initial carrier frequency error (Doppler), determination of an initial carrier phase, and determination of the start-of-message to establish an absolute time marker within the waveform.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to the field of communications, and more specifically to radio-frequency data communications. 
   Acquisition is an important and often difficult task for any modem. This is especially true for waveforms having higher data rates that employ complex modulations such as continuous-phase modulation (CPM). In ultra-high frequency (UHF) satellite communications (SATCOM) military waveform standards, several higher data rate waveforms are defined, for example MIL-STD-188-181B. The majority of these modes rely on CPM for both the waveform preamble and data. In all cases, the preamble uses a type of CPM called minimum-shift keying (MSK). The specification defines the MSK preamble, and further requires that the signal be acquired 95 percent of the time with 90 percent confidence at a bit-error rate of 10 −3 . 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a method and apparatus for acquiring a continuous-phase modulation (CPM) waveform. In one embodiment of the invention, the invention is directed to a method and apparatus for acquiring a waveform as defined by MIL-STD-188-181B including the preamble of such a waveform at a performance level defined by the standard. The present invention provides solutions to at least four primary problems presented in acquiring an MSK waveform such as the MIL-STD-188-181B compliant waveform preamble. These primary problems include searching for the preamble, determination of the symbol rate, determination of an initial carrier frequency error (Doppler), determination of an initial carrier phase, and determination of the start-of-message to establish an absolute time marker within the waveform. In one embodiment, the invention includes a means for performing a Fourier transform on a sampled waveform, a means for providing a power spectrum of the Fourier transformed waveform, a means for estimating a signal-to-noise ratio based upon the power spectrum, a means for determining whether the signal-to-noise ration is less than a threshold value, a means for accumulating the power spectrum when the signal-to-noise ratio is not less than the threshold value, and a means for estimating a symbol rate of the waveform based upon an accumulated power spectrum. In another embodiment, the invention includes a means for normalizing samples of a sampled waveform, a means for correlating the normalized samples with known start-of-message samples to provide a correlation output, a means for storing a magnitude value of the correlation output, a means for adjusting the magnitude value of the correlation output to reduce an effect of a sync pattern of the waveform on the magnitude value of the correlation output, a means for determining whether the adjusted magnitude value of the correlation output exceeds a threshold value, and a means for detecting a correlation peak wherein the start-of-message of the waveform is determined. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
       FIG. 1  is a diagram of the structure of a preamble of a minimum-shift keying continuous-phase modulation waveform capable of being acquired in accordance with the present invention; 
       FIG. 2  is a diagram of an acquisition system that is capable of implementing a method for acquiring a continuous-phase modulation waveform in accordance with the present invention; 
       FIG. 3  is a flow diagram of a method for detecting a preamble of a CPM waveform in accordance with the present invention; and 
       FIG. 4  is a flow diagram of a method for detecting a start-of-message (SOM) of a CPM waveform in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to one or more embodiments of the invention, an example of which is illustrated in the accompanying drawings. 
   Referring now to  FIG. 1 , a preamble of a continuous-phase modulation (CPM) waveform in accordance with the present invention will be discussed. Preamble  100  is part of a higher data rate waveform such as a waveform compliant with the MIL-STD-188-181B continuous-phase modulation (CPM) waveform. Although  FIG. 1  shows a preamble for a particular CPM waveform, the present invention may be adapted to other CPM waveforms or the like without departing from the scope of the invention and without providing substantial change thereto. Preamble  100  is identical to the MIL-STD-188-181B preamble. Preamble  100  begins, after a transmitter power-up section  110 , with an alternating bit pattern  112 , in one embodiment comprising 192 sync bits (e.g., a repeated pattern 11001100 . . . ) to be used for preamble and symbol rate detection. After alternating bit pattern  112  is a 16-bit start-of-message (SOM) 114 bit pattern, in one embodiment AC3B hexadecimal, to provide symbol timing and absolute time marking. Following the SOM  114  is header  116 . In the embodiment shown, header  116  comprises a 12-bit header divided into sub blocks  118 , repeated 3 times for a total of 36 bits, containing waveform information. The tail of preamble  100  consists of 6 flush bits  120  (010101 or 110101) intended to force 0 phase after preamble  100 . 
   Referring now to  FIG. 2 , a block diagram of an acquisition system in accordance with the present invention is shown. The acquisition system  200  shown in  FIG. 2  is the architecture for a radio-frequency (RF) modem capable of acquiring a CPM waveform. All of the elements of acquisition system  200  may be tangibly embodied as structure that implements the corresponding elements, where the structure includes an appropriate circuit, e.g., filter, amplifier, oscillator, etc., or other hardware structure. In one particular embodiment, the elements of acquisition system  200  are implemented in a digital processing system that is configured to implement the elements of acquisition system  200 . In such an embodiment, digital processing system may include a digital signal processor and associated hardware that is configured via software instructions to implement the elements of acquisition system  200 , and may also include hardware logic circuits, such as a logic gates, multiplexers, latches, registers, etc., configured to perform the functions of the elements of acquisition system  200 . Acquisition system  200  acquires a CPM waveform by acquiring the preamble of the CPM waveform such as preamble  100  shown in  FIG. 1 . The preamble acquisition functions implemented by acquisition system  200  are preamble search, symbol rate detection, Doppler estimation, start-of-message (SOM) detection, which provides initial symbol timing, and initial carrier phase estimation. In addition, header detection and decoding are also a part of preamble acquisition implemented by acquisition system  200 . A multiplexer  204  receives complex samples of a CPM waveform at input  202 , which divides the CPM waveform input into paths  254  and  210 . Path  254  is further divided into paths  226  and  242  by multiplexer  206 . 
   Path  210  in  FIG. 2  provides the operations performed during preamble, symbol rate, and Doppler detection in accordance with the present invention. In the embodiment shown, the sampling rate is 9600 Hz for 5 kHz bandwidth channels or 48000 Hz for 25 kHz bandwidth channels. Path  210  includes a samples buffer  212 , a complex FFT unit  214 , a power spectrum detector  216 , an accumulator  218 , and a spectrum analyzer  220  to provide symbol rate and Doppler information at outputs  222  and  224  respectively. Preamble, symbol rate and Doppler detection are based on analyzing the spectrum obtained by performing a 256-point complex FFT with complex FFT unit  214  and a 256-point power spectrum with power spectrum detector  216 . Complex FFT unit  214  performs the complex FFTs over the most recent 256 complex samples after every few new samples are received. 
   Preamble detection consists of estimating a signal-to-noise ratio (SNR) from the power spectrum provided by power spectrum detector  216  and comparing the SNR to a threshold SNR. The signal power is computed as the normalized sum of the three highest tone powers, where the tone power is defined as the sum of three spectral bins around a peak. The noise power is computed as the normalized sum of all power spectral bins excluding five bins around each of five highest spectral tones. The SNR is declared valid only if the power of each of the lower and upper tones, going into the signal power estimate, is within a predetermined threshold of the center tone. This helps to assure that there are at least three tones present in order to reduce false preamble detection on two or fewer tones. 
   FFTs, power spectra, and SNR estimates are also performed during symbol rate detection. Accumulator  218  accumulates consecutive power spectra prior to estimating the symbol rate. During the power spectra accumulation period, if any SNR estimate drops below a “preamble lost” SNR threshold, then the acquisition system  200  returns to preamble search. After accumulator  218  has accumulated a fixed number of power spectra, an estimate of symbol rate is made. Based on the estimated symbol rate and current SNR measurements, further accumulations may be performed. Power spectra accumulation and symbol rate estimation are performed until the symbol rate estimate at output  222  does not support further accumulation. 
   Symbol rate detection is based upon resolving the frequency separation between the preamble sync pattern spectral tones of alternating bit pattern  112  where the center five tones are spaced one-fourth the symbol rate apart. Symbol rate detection consists of comparing the frequency separation between the 3 highest spectral tones. Tone frequency estimation involves computing the frequency associated with a given spectral bin and adjusting it based on the difference in the powers of the two neighboring spectral bins. The symbol rate most closely matching the minimum measured tone frequency spacing is selected. During symbol rate detection, the frequency of the highest three tones is estimated. The center tone of the three is the best estimate of the 0 Hz tone with Doppler. The Doppler estimate is set to the center tone frequency. Once the symbol rate has been detected, using control  208 , the sampling rate is changed to 16 times the detected symbol rate, and the receive center frequency is tuned off by the measured Doppler in order to center the waveform spectrum at 0 Hz. 
   Path  226  in  FIG. 2  implements the operations performed during the start-of-message (SOM) search. Path  226  includes a samples buffer  228 , a complex correlator  230 , a magnitude detector  232 , a decision logic block  234 , and an arctangent calculator  236  to provide symbol timing and initial carrier phase information at outputs  238  and  240  respectively. New complex samples are normalized to a magnitude of 1. After every few new complex samples, 256-point correlations are performed, in one embodiment 16 symbols times 16 samples per symbol, between the 256 most-recent normalized complex input samples and a stored normalized copy of the known SOM samples. The correlator output magnitude is computed and stored. An adjustment is made to the correlator output to perform preamble sync pattern cancellation as the sync pattern can produce large undesired correlation peaks. When the modified correlation magnitude exceeds a preset SOM threshold, then SOM detection is declared by decision logic block  234 , and a control signal  256  is provided to multiplexer  206 . Symbol timing information is provided at output  238 . Once the SOM threshold is exceeded, an extra symbol of samples, in one embodiment 16 samples, is passed through correlator  230  to assure that the actual peak location is detected. The input sample that produces the correlation peak is the best estimate on the last, e.g., sixteenth, sample of the last SOM symbol. The initial carrier phase at the start of the SOM is provided at output  240  and is computed as:
 
Phase=arctan( cor   —   Q/cor   —   I )
 
   Path  242  in  FIG. 2  implements the operations performed by acquisition system  200  during preamble header reception. Path  242  includes a demodulator  244 , a header buffer  246 , a 2-of-3 decision voter  248 , and a lookup table  250  to provide header information at output  252 . In the embodiment shown, the 36 coded header bits are minimum-shift keying (MSK) demodulated by demodulator  244  and stored in header buffer  246 . The demodulated header bits are 2-of-3 voter decoded by 2-of-3 decision voter  248  to generate the 12 header bits of preamble  100 . Lookup-table  250  used to extract and retrieve the header information from preamble  200 . Header information is provided at output  252 . 
   Referring now to  FIG. 3 , a flow diagram of a method for detecting a preamble of a CPM waveform in accordance with the present invention will be discussed. Although one order of the steps of method  300  is shown, the number or order of the steps of method  300  may be altered, including providing fewer or greater steps, or modifying any one or more of the steps, without providing any substantial change thereto. Method  300  is executed by acquisition system  200  of  FIG. 2  and incorporates the functions thereof. Method  300  initiates with the sampling of a CPM waveform at step  310 . A fast Fourier transform (FFT) is performed on the waveform at step  312 . The power spectrum of the waveform is determined at step  314  from the FFT performed at step  312 . An estimate of the signal-to-noise ratio (SNR) is executed at step  316 . The SNR calculated at step  316  is compared to a threshold SNR, and a determination is made at step  318  whether the calculated SNR is less than the threshold SNR. In the event the calculated SNR is less than the threshold SNR, method  300  continues to search of a preamble by continuing execution at step  310 . In the event the calculated SNR is not less than the threshold SNR, the power spectra of the waveform are accumulated at step  320 . The symbol rate of the waveform is estimated at step  322 , and the Doppler is estimated at step  324 . A determination is made at step  326  whether further accumulation of power spectra should continue, and in the event such a determination is made, method  300  continues execution at step  320  so that estimations of the symbol rate and the Doppler are updated at steps  322  and  324 , respectively. In the event no further accumulation is required, at step  328  the sampling rate is set based upon the symbol rate determined at step  322 . At step  330  the center frequency is tuned based upon the Doppler determined at step  324  to center the waveform spectrum at 0 Hz. 
   Referring now to  FIG. 4 , a flow diagram of a method for detecting a start-of-message (SOM) of a CPM waveform in accordance with the present invention. Method  400  is executed by acquisition system  200  of  FIG. 2  and incorporates the functions thereof. Although one order of the steps of method  400  is shown, the number or order of the steps of method  400  may be altered, including providing fewer or greater steps, or modifying any one or more of the steps, without providing any substantial change thereto. Method  400  initiates with the sampling of a CPM waveform at step  410 . The waveform samples are normalized at step  412 . The samples are correlated with known SOM samples at step  414 . The magnitude of the correlator output is stored at step  416 , and the output of the correlator is adjusted at step  418  to account for and to cancel or reduce the effects of the preamble alternating bit pattern  112 , which in one embodiment is a sync pattern. A determination is made at step  420  whether the correlation output is greater than a threshold value, and if it is not, method  400  continues execution at step  410 . When the correlation output is greater than a predetermined value, additional samples are correlated at step  422 , and a correlation peak is detected at step  424 . The carrier phase at the start of SOM  114  is computed at step  426 . 
   Although the invention has been described with a certain degree of particularity, it should be recognized that elements thereof may be altered by persons skilled in the art without departing from the spirit and scope of the invention. One of the embodiments of the invention can be implemented as sets of instructions resident in a main memory of one or more digital processing systems configured to implement the invention. Until required by the digital processing system, the set of instructions may be stored in another computer readable memory such as an auxiliary memory, for example in a hard disk drive or in a removable memory such as an optical disk for utilization in a CD-ROM drive, a floppy disk for utilization in a floppy disk drive, a floppy-optical disk for utilization in a floppy-optical drive, or a personal computer memory card for utilization in a personal computer card slot. Further, the set of instructions can be stored in the memory of another digital processing system and transmitted over a local area network or a wide area network, such as the Internet, when desired by the user. Additionally, the instructions may be transmitted over a network in the form of an applet, a program executed from within another application, or a servlet, an applet executed by a server, that is interpreted or compiled after transmission to the digital processing system rather than prior to transmission. One skilled in the art would appreciate that the physical storage of the sets of instructions or applets physically changes the medium upon which it is stored electrically, magnetically, chemically, physically, optically or holographically so that the medium carries computer readable information. 
   It is believed that the system and method for acquisition of a CPM waveform of the present invention and many of its attendant advantages will be understood by the forgoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages, the form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.