4-ary correlator, matched filter, and decoder for coherent, direct sequence, spread spectrum applications

#Spread spectrum variable data rate coherent phase-shift-keyed communication links offer a powerful trade-off between imagery data rate and jam resistance/low probability of detection (JR/LPD) performance. However, the implementation of such a data link requires considerations of waveform design and synchronization concepts beyond those required for a fixed data rate system. Almost invariably, analyses and studies indicate that the preferred data waveform is one which incorporates a coherent residual carrier component which is used for detection and tracking in the receiver. Such a waveform obviates the multiplicity of data rate dependent filters normally used in receivers which recover the carrier phase by non-linear methods (e.g., nth-power and Costas type designs).

#This invention employs coherent BPSK for both pseudorandom noise (PN) and data modulation because of its simplicity and low risk implementation at high rates. The long term balance of "one's" and "zero's" in conventional BPSK modulation ensures the (theoretical) absence of a discrete carrier component in the RF waveform; hence the term "suppressed" carrier. Coherent demodulation of this waveform (suppressed carrier), however, requires a carrier, and the carrier is typically derived from the waveform by non-linear operations in the receiver. Whether the non-linear operation is at IF (e.g. squaring loop) or baseband (e.g., Costas loop) an inevitable loss in signal-to-noise ratio (SNR) results. Under low input predetection SNR's, small signal suppression effects cause system performance to degrade faster than the input SNR; in the cases of the carrier and N code tracking loops, undesirable parameter changes (loop gain, damping and bandwidth) result. In order to minimize precipitous degradation in these loops, data rate dependent filters are required to maintain the predetection SNR as high as possible. In a multi-rate system, a fully optimized suppressed carrier receiver design would require as many different filter bandwidths (whether IF or baseband) as data rates and possibly several different IF frequencies to support all of the bandwidths.

#The penalty for not optimizing the performance for each data rate is, as implied above, non-graceful degradation. Loop bandwidths adapt to the rapidly reducing input SNR, quickly becoming too narrow to track the carrier or N code frequency dynamics, and tracking thresholds are reached just a few dB below the operating point for low error rate data recovery.

#For a better understanding of the nature and objects of the invention, reference should now be made to the following detailed specification and to the accompanying drawings in which:

#FIG. 1 illustrates the signal mapping of a two bit to 4 symbol word;

#FIG. 2 is a block diagram of the spread spectrum IF despread and system timing;

#The nature of the 4-ary waveform and its speed place the greatest demands on spread spectrum variable data rate coherent phase shift keyed (CPSK) demodulator and decoder functions, demands to which this invention responds. This being the case, the 4-ary signal must be described.

#The 4-ary signal is the result of a two bit to four symbol mapping as shown in FIG. 1. Since four two-bit words are possible, four four-symbol words are possible. Each of the 4-ary words is comprised of three logic "zero's" and one logic "one." This three-to-one imbalance provides a residual carrier whose signal strength is that of the modulated RF waveform minus 6 dB.

#The residual carrier of a 4-ary shared energy signal is not detectable as a discrete line spectrum in the transmitted waveform because in a direct sequence spread spectrum communication link it, too, is spread by the spreading waveform. The spreading waveform has a rate known as the chipping rate, and the spreading waveform has the spectral characteristics of pseudorandom noise (PN); thus, the spreading waveform is known as a N code. The N codes within the invention are generated and managed by a device known as a PN Generator 5. The chipping rate clock is the source of all timing located within the PN Generator. By exploiting the capabilities of epochs and subepochs designed into the spreading N waveform (not disclosed as part of this invention), the position of 4-ary words relative to the spreading waveform is established.