Abstract:
An input MSK digital data signal is correlated with MSK correlation waveforms to produce first and second correlation signals. These first and second correlation signals are operated on by an arithmetic unit to calculate many time and phase conditions of the MSK digital data signal at one time and to produce third, fourth and fifth correlation signals as a function of assumed bit times. These third, fourth and fifth correlation signals are operated on by a ratio calculator to produce a ratio signal proportional to the ratio of a minimum magnitude squared to a maximum magnitude squared for all carrier phase angles for each of the assumed bit times. A synchronization detected signal is produced when the value of the ratio signal is less than a given amplitude threshold.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to MSK digital data transmission systems and more particularly to a MSK digital data synchronization detector for such systems. 
     MSK is an abbreviation for &#34;minimum shift keying&#34; and may be defined as a signal where the transmitted wave is a phase continuous frequency shift waveform. For each unit time interval the instantaneous frequency is a constant being either a high frequency or a low frequency relative to a center frequency. The high frequency is such as to advance by one phase revolution relative to the carrier frequency in four unit time intervals. The low frequency is such as to fall behind by one phase revolution relative to the carrier frequency in four unit time intervals. 
     MSK digital data synchronization detectors are used to determine the presence or absence of a MSK digital data signal and to synchronize in time and phase to such a signal. A MSK signal is hard to detect because there is no amplitude modulation or particular preferred phase that can be detected. 
     One prior art MSK digital data synchronization detector employs a method of detection which requires searching in various carrier phase and bit times looking for a stable tracking procedure. Stable tracking behavior and low orthogonal signals at the sampling time indicate the presence of a MSK digital data signal. 
     One of the disadvantages of the above-mentioned prior art detector is that it requires a higher signal-to-noise ratio or multiple search at a given signal-to-noise ratio to acquire synchronization with the same probability of acquisition and false alarm. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a MSK digital data synchronization detector that overcomes the disadvantage of the above-mentioned prior art MSK digital data synchronization detector. 
     In accordance with the principles of the present invention, the MSK digital data signal detector disclosed synchronizes better than the above-mentioned prior art because it provides a search using a calculation which mathematically considers many conditions of the MSK digital data signal at one time rather than using a tracking loop which can cover only one phase and one bit time condition at one time. In the technique of the present invention a number of time calculations for each of two phase calculations per time sample are made and then a calculation is made of the phase dependence using the known phase progression properties of the MSK digital data signal. If a MSK digital data signal is present, this will be indicated by a phase dependence near the correct bit times. Once an indication of signal presence has shown up, there is also an indication of the correct synchronism (the correct bit time has a maximum phase dependence and the correct phase has the maximum phase value). This signal presence indication can be followed up to either verify the MSK digital data signal presence or to reject the presence of a MSK digital data signal. 
     A feature of the present invention is the provision of a minimum shift keying (MSK) digital data synchronization detector comprising: a first source of the digital data; first means coupled to the first source to produce first and second correlation signals; second means coupled to the first means, the second means being responsive to the first and second correlation signals to calculate many time and phase conditions of the digital data at one time to produce third, fourth and fifth correlation signals as a function of assumed bit times; and third means coupled to the second means, the third means being responsive to the third, fourth and fifth correlation signals to produce a ratio signal proportional to a ratio of a minimum magnitude squared to a maximum magnitude squared for all carrier phase angles for each of the assumed bit times and to produce a synchronization detected signal when the value of the ratio signal is less than a given amplitude threshold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 is a schematic block diagram of the MSK digital data synchronization detector in accordance with the principles of the present invention; 
     FIG. 1A is a schematic block diagram of the modification of the chopping correlator and integrator of my copending application Ser. No. 618,537, filed Oct. 1, 1975 to provide chopping correlator and integrator 1 of FIG. 1; 
     FIG. 2 is a schematic block diagram of the arithmetic unit of FIG. 1 in accordance with the principles of the present invention; and 
     FIG. 3 is a schematic block diagram of the ratio calculator of FIG. 1 in accordance with the principles of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the input MSK signal r(t) is coupled to chopping correlator and integrator 1 with the input MSK signal being correlated with standard MSK correlation waveforms, as provided by generator 2, at several assumed bit times. Correlator and integrator 1 is fully disclosed in my copending application Ser. No. 618,537, filed Oct. 1, 1975, whose disclosure is incorporated herein by reference. The correlator and integrator 1 includes six chopping correlators of the above-cited copending application as illustrated in FIG. 1 thereof each of which is implemented as illustrated in FIG. 2 thereof and represented in FIG. 1A of the present application by block 1a. Each of the correlators of the above-cited copending application includes an A/D (analog-to-digital) converter at the output thereof with eight parallel bit outputs. Block 1b of FIG. 1A of the present application includes six parallel-to-serial converters each coupled to a different one of the A/D converters of block 1a to convert the eight parallel bit outputs from its associated A/D converter to a serial output of eight bits. Block 1b represents the modification of the above-cited copending application to implement chopping correlator and integrator 1 of FIG. 1 of the present invention. 
     MSK correlation waveforms generator 2 may include sine and cosine generators whose outputs are coupled to a multiplier to provide the standard MSK correlation waveforms as defined in the following equations: 
     
         sin w.sub.o t sin w.sub.f (t - (N + X)T)                   (1) 
    
     
         cos w.sub.o t sin w.sub.f (t - (N + X)T)                   (2) 
    
     where w o  is equal to the carrier frequency in radians, w f  is equal to the modulating frequency in radians, t is equal to time, T is equal to a unit time interval, N is equal to an integer 0, 1, 2, 3 . . . and X is equal to assumed bit times to align the correlation waveforms with the input MSK digital data signal. 
     A practical procedure for the operation of correlator and integrator 1 is to use two values of X; namely, 0 and 1/2, where w f  T=π/2. 
     Timing generator 3 provides timing signals to control the integrators, the multiplexers, the analog-to-digital converter and serial-to-parallel converter of correlator and integrator 1. Timing generator 3 may be implemented by a stabilized clock source coupled to a digital counter with the output of various stages of this digital counter being coupled to gating circuits to provide the desired timing signals to perform the desired time control of the above-mentioned elements of correlator and integrator 1. Timing generator 3 also provides appropriate timing signals for the arithmetic unit 4. 
     Correlator and integrator 1 provide two output signals; namely, a sine correlation signal and a cosine correlation signal which are defined in equations (3) and (4) as indicated below: ##EQU1## , where D s  is the sine correlation signal, (N + X) equals bit time and indicates the end of the integration, D c  is equal to the cosine correlation signal and the other terms of equations (3) and (4) are as previously defined. The D s , N + X and D c , N + X correlation signals are coupled to arithmetic unit 4 and using appropriate timing signals from timing generator 3 form correlation signals U p  (X), W p  (X) and V P  (X) as a function of assumed bit times X. These three correlation waveforms at the output of arithmetic unit 4 are defined by the following equations: ##EQU2## , where N of equations (3) and (4) has two values for each of equations (3) and (4). The first value of N is equal to 2K for the correlation signals D s  and D c  at the present data bit time. The second value of N is equal to (1 + 2K) for the correlation signals D s  and D c  shifted by one data bit time with respect to the present data bit time. This later value of N is generated by passing the correlation signals D s  and D c  through time delay devices 1c and 1d having a time delay equal to one data bit time. With these values for N a number of time calculations for each of two phase calculations per time sample can be made. In the above equations (5) - (7) P is equal to the present bit number and the end of the summation, L is equal to the number of bits preceding bit P and K is the summation index. Each sum of equations (5) - (7) represents the sum of (2L + 2) terms ending at the (X + 2P + 1) term for assumed bit times X. 
     The correlation waveforms at the output of arithmetic unit 4 are coupled to ratio calculator 5 to determine the ratio of a minimum magnitude squared to maximum magnitude squared for all carrier phase angles for each assumed bit time X. This ratio is given as: ##EQU3## where TH is equal to the amplitude threshold value of threshold device 6. By transforming equation (8) as follows implementation of ratio calculator can be determined. Multiplying both sides of equation (8) by 
     
         (U.sub.P (X) + W.sub.P (X) +√(U.sub.P (X) - W.sub.P (X)).sup.2 + 4 V.sub.P.sup.2 (X) 
    
     there is obtained 
     
         (U.sub.P (X) + W.sub.P (X)) - √(U.sub.P (X) - W.sub.P (X)).sup.2 + 4 V.sub.P.sup.2 (X) &lt;TH(U.sub.P (X) + W.sub.P (X)) + TH √(U.sub.P (X) - W.sub.P (X)).sup.2 + 4 V.sub.P.sup.2 (X)                (9) 
    
     reorganize equation (9) as follows: 
     
         (U.sub.P (X) + W.sub.P (X)) - TH (U.sub.P (X) + W.sub.P (X)) &lt; √(U.sub.P (X) - W.sub.P (X)).sup.2 + 4 V.sub.P.sup.2 (X) + TH √(U.sub.P (X) - W.sub.P (X)).sup.2 + 4 V.sub.P.sup.2 (X) (10) 
    
     reorganize equation (10) as follows: 
     
         (U.sub.P (X) + W.sub.P (X)) (1 - TH) &lt; (1 + TH) √(U.sub.P (X) - W.sub.P (X)).sup.2 + 4 V.sub.P.sup.2 (X)                  (11) 
    
     divide both sides of equation (11) by (1- TH) and obtain the following: ##EQU4## 
     Setting ##EQU5## a constant, equation (12) can be written as follows: 
     
         (U.sub.P (X) + W.sub.P (X)) &lt; C √(U.sub.P (X) - W.sub.P (X)).sup.2 + 4 V.sub.P.sup.2 (X)                                       (13) 
    
     squaring both sides of equation (13) there is obtained the following equation: 
     
         (U.sub.P (X) + W.sub.P (X)).sup.2 &lt; C.sup.2 ((U.sub.P (X) - W.sub.P (X)).sup.2 + 4 V.sub.P.sup.2 (X))                         (14) 
    
     reorganizing equation (14) there is obtained the following equation from which the implementation of ratio calculator 5 can be derived. 
     
         (U.sub.P (X) + W.sub.P (X)).sup.2 - C.sup.2 ((U.sub.P (X) - W.sub.P (X)).sup.2 - 4 V.sub.P.sup.2 (X)) &lt; 0                     (15) 
    
     the signal at the output of calculator 5 is the ratio signal and is tested for each assumed bit time against an amplitude threshold value in threshold device 6. The presence of a signal and, hence, synchronization is indicated when the value of the ratio signal is less than the amplitude threshold of device 6. If the ratio signal for several bit times are below the amplitude threshold value, a choice of best bit time is made by an estimate of the bit time or position with the largest ratio. A simple estimation procedure is to try a number of X values and pick the value with the largest ratio. An estimate of the proper phase angle can also be made as 1/2 tan.sup. -1  [2V p  (X)/(U P  (X) - W P  (X))]. 
     The digital data synchronization detector just described with reference to FIG. 1 can be used for detecting any signal which can be put into the form of sensitivity to phase as a function of assumed bit times. Two cases known are (1) straight MSK modulation and (2) the triangles formed by MSK modulated 7 bit Baudot characters. 
     Referring to FIG. 2 there is illustrated therein a schematic block diagram of the arithmetic unit 4 of FIG. 1. The implementation of unit 4 disclosed in FIG. 2 is derived from equations (5) - (7). This implementation as illustrated in FIG. 2 includes multipliers 7, 8, 9, 10, 11 and 12 having the indicated inputs thereto and the indicated outputs therefrom which conform to the terms of equations (5) - (7). The output signals from multipliers 7 and 8 are coupled to a summer 13 which is dumped at time P (the end of the summation) by a timing signal from generator 3 of FIG. 1 and generates the correlation signal U P  (X). The output from multipliers 9 and 10 are coupled to summer 14 which also is dumped at time P (by the timing signal from generator 3 of FIG. 1) to provide the correlation signal W P  (X). The output from multiplier 12 is coupled to summer 15 which also is dumped at time P (by the timing signal from generator 3 of FIG. 1) to produce the correlation signal V P  (X). 
     Referring to FIG. 3 there is disclosed therein one possible implementation of the ratio calculator 5 of FIG. 1, which is derived from equation (15) which resulted from transformation of the ratio set forth in equation (8). In accordance with equation (15) the ratio calculator 5 of FIG. 1 includes summers 16 and 17 having the indicated correlation signal inputs coupled thereto to provide the desired output signal as dictated by equation (15) and illustrated in FIG. 3. The output from summer 16 is coupled to multiplier 18 to provide the first term of equation (15). The output from summer 17 is coupled to multiplier 19 and then to multiplier 20 to produce the second term of equation (15). Multiplier 21 having the correlation signal input as illustrated and multiplier 22 cooperate to provide the third term of equation (15). The outputs from multipliers 18, 20 and 22 are coupled to a summer 23 which provides the ratio output signal which is coupled to threshold device 6 of FIG. 1. 
     It will be noted that summer 17 and summer 23 have certain ones of their input terminals labeled with a minus sign. This minus sign indicates that the input signal applied thereto is inverted prior to addition with the other input signals in the associated summers 17 and 23. 
     While I have described above the principles of my invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.