Abstract: An apparatus for providing coherent detection of a series of data bursts without a preamble by providing a reference waveform which has the same frequency as the carrier for a received TDMA/MPSK transmission, but having an arbitrary phase with respect to such signals. The reference waveform is used for pseudo-coherent demodulation. Symbol timing is available at all times by using symbol synchronization obtained from the previous bursts of the same transmission. The symbol timing is used to sample two quadrature outputs of the pseudo-coherent demodulator once per MPSK symbol interval. Storage of a sufficient number of demodulator samples allows a carrier phase estimate to be obtained from the message burst itself. The carrier phase estimate is then used for coherent detection on a delayed sequence of the demodulator samples.

Abstract: New techniques are disclosed which utilize soft detection to improve the performance of FEC codes. For an FEC code with minimum Hamming distance, the decoder utilizes hard-detected code symbols to obtain d independent estimates of each information bit, including the detected information symbol itself and d-1 orthogonal parity checks on the information bit. Soft detection is employed for the purpose of computing reliability measures for the d estimates. The reliability numbers serve as weighting coefficients, with the bit decision determined on a weighted majority basis. In one preferred technique, Reliability Majority Decoding (RMD), the reliability of each estimate is determined from some number I of the least reliable of the m hard-detected code symbols contributing to the estimate. For I=1, a simplified logic circuit can be used to determine estimate reliability, while for I=m each symbol is assigned a number of demerits inversely proportional to reliability.

Abstract: A Frequency Shift Offset Quadrature (FSOQ) signal which is a constant envelope frequency-shift keyed version of an offset quaternary phase shift keyed (O-QPSK) transmission is provided. A technique for generating an FSOQ signal in a manner similar to O-QPSK by modulating offset quadrature channels with selectable pulse shapes which yields the desired 3-tone continuous phase FSK approximation of O-QPSK is disclosed. Receiver synchronization circuits are also provided which enable FSOQ to be detected as a special case of O-QPSK, whereby the detection performance of synchronous coherent QPSK, rather than the relatively inferior performance of FSK detection, is obtained. FSOQ provides a constant envelope and requires less bandwidth than minimum shift keying (MSK).

Abstract: A synchronization preamble for each TDMA burst consists of an alternating sequence of the two MSK tones on every other MSK bit interval. Such a preamble does not exhibit random polarity reversals at the two MSK frequencies as occurs during the message burst. The receiving circuit therefore directly processes the preamble symbols to extract carrier phase and symbol synchronization without conventionally squaring the IF to remove polarity modulation. A 6 dB increase in synchronization SNR is thereby provided since the requisite synchronizer filtering need only be effected at the fundamental frequencies F.sub.L and F.sub.H rather than at the their second harmonics 2F.sub.L and 2F.sub.H as is required when squaring is employed.

Abstract: In a TDMA burst preamble, the phase is varied between .+-..pi./4 radians, and carrier synchronization is achieved by averaging the phase over a given interval. Symbol timing synchronization is achieved by filtering the upper sideband of a conventional delay-and-multiply circuit and demodulating the filtered upper sideband with a coherent carrier available by the end of the preamble. After the preamble, modulation removal is used to obtain the unmodulated carrier.

Abstract: In a method and apparatus for convolutionally encoding a finite length digital data bit stream, the information samples arriving at the output of each encoder shift register are fed back to its input to simulate an infinite periodic data bit stream. Recirculation of the received information and parity symbols is also used for decoding.