There is disclosed frequency modulation (FM) or phase modulation (PM) communication with a receiver having a limiter, in spread-spectrum operation for security purposes. The spectrum is spread on transmission by superimposing a psuedo-noise code modulation on the intelligence modulation of the carrier. On reception the spectrum is collapsed by auto-correlation of the pseudo-noise code. The pseudo-noise spectrum is produced by a digital coder which causes phase modulation of the carrier at the transmitter and the spectrum is collapsed by an identical coder which remodulates the received signal at the receiver which is operated in precise synchronism with the coder at the transmitter. To maintain synchronism a reference oscillator signal which periodically varies the phase of the received carrier is injected into the coder network at the receiver; this reference oscillator signal is processsed so that it passes through the limiter of the receiver. The modulated carrier from the transmitter is received in parallel paths. In one path the received carrier is modulated directly by the code from the coder at the receiver; in the other path, the received carrier is modulated by the code from the coder, periodically delayed in synchronism with a reference oscillator. The resulting so modulated carriers are correlated and the sum of the independently correlated carriers is impressed on the receiver as a carrier which is modulated by the intelligence and in addition is phase modulated in synchronism with the reference oscillator. The phase modulation is not suppressed by the limiter of the receiver and operates through a phase detector, low pass filter and voltage controlled oscillator, which are elements of a tracking loop, to maintain the coder at the receiver in synchronism with the received coder. The spread-spectrum operation disclosed lends itself to embodiment in adapters for converting conventional clear-signal communication equipment constructed in the past to spread-spectrum operation.
REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No. 86,999, filed Oct. 30, 1970, (herein called parent application) which is itself a continuation of application Ser. No. 754,375 filed Aug. 21, 1968 and now abandoned.
A system comprising apparatuses and methods for encoding and decoding spread spectrum signals is disclosed. Signals are encoded by modulating numerical sequences selected from an orthonormal basis of numerical sequences to provide channel coding. The modulated sequences are then combined to form an encoded signal. Because of the orthonormal character of the sequences, the encoded signal may be easily decoded using a matched filter. A method for generating long sequences from the product of shorter sequences is also disclosed.
A detector (20) is provided for use in a communication receiver where a received spread spectrum data signal is detected using a locally generated reference signal to decode the data signal. The detector (20) includes first (22) and second channels (24) and circuitry (90) for applying the received encoded data signal to the first (22) and second channels (24). A local generator (50) is provided for generating the reference signal wherein the reference signal has polarity transitions. A demodulator (40) is included in the first channel (22) for generating a detected recovered data signal from the received data signal in response to the reference signal. Circuitry (52) is provided for detecting the polarity transitions in the reference signal and for generating a differential PN signal. Circuitry (42) is further provided in the second channel (24) for correlating the received data signal and the differential PN signal to thereby generate a recovered error signal. The recovered data signal and the recovered error signal are correlated by circuitry (58) to generate a control signal for application to the local generator (50) for locking the reference signal to a component of the received data signal.
A bidirectional digital signal communication system includes a plurality of terminal units in communication with a center unit. Each of the terminal units includes a signal generator for generating digital data spread with a PN (pseudo-noise) code, a modulator for modulating the digital data on a carrier, and a transmitter for transmitting the modulated digital signal to the center unit through a transmission line. The center unit includes a receiver for receiving the modulated digital signal, a PN code generator for locally generating a PN code which corresponds to the transmitted PN code, a multiplier supplied with outputs of the receiver and the PN code generator to produce a digital signal de-spread from the PN code, a demodulator connected to the multiplier to reproduce the digital data from the modulated digital signal, and a memory for storing the PN code when the signal level of the multiplier exceeds a predetermined value. The stored PN code is retrieved from the memory to enable rapid synchronization with the next transmission received from the terminal unit.
A modem in which the transmitter uses spectrum spreading techniques applied to sequentially supplied input bits, a first group thereof having one spread spectrum sequence characteristic and a second group thereof having a different spread spectrum sequence characteristic, the spread spectrum bits being modulated and transmitted. The receiver generates complex samples of the received modulated signal at a baseband frequency and uses a detector for providing signal samples of the complex samples which are time delayed relative to each other. A selected number of the time delayed samples are de-spread and demodulated and the de-spread and demodulated samples are then combined to form a demodulated receiver output signal.
A system for using phase modulated information carried by a pilot tone generated in the transmitter of a frequency hopping communications system to synchronize the receivers dehop schedule generator with the transmitter's hop schedule generator.
