The invention relates to information transmission systems in which the band is spread at the transmitting end by means of a pseudo-noise sequence and is returned to normal at the receiving end by a similar sequence. In such systems it is essential that the frequency of the various oscillators used for the frequency conversions should be kept constant, or alternatively that the oscillators at the receiving end should be synchronized with the oscillators at the transmitting end. In accordance with the present invention the oscillators at the transmitting end are synchronized by the clock frequency of the code generator and at the receiving end this clock frequency is extracted from the received signal and used to synchronize the oscillators in the various conversions. One of the oscillators at the receiving end may be a Gunn oscillator which is synchronized by applying the output of the pulse generator to its synchronizing input terminal through a frequency multiplier. In another arrangement the frequency of the Gunn oscillator is controlled by comparing the phase of the output with the phase of the output of a frequency multiplier supplied by the pulse generator. In yet another arrangement the outputs of the Gunn oscillator and the frequency multiplier are mixed to form a difference signal which is compared in phase with the outputs of a low frequency reference oscillator.
An emitter assembly of low-power electromagnetic signals with random or pseudo-random frequency variation is located inside an electromagnetic shield to be monitored. These signals are detected by at least one synchronous detection receiver located outside the shield, a second transmission channel, preferably constituted by an optic fiber, connecting the emitter and receiver assemblies to enable synchronous detection of signals transmitted via an electromagnetic channel through the shield. The amplitude of the electromagnetic signals received by the detector is significant of the quality of the shielding.
A transmitter and receiver for direct sequence spread-spectrum (SS) communications is described. The audio or other analog information to be communicated controls the clock rate of a pseudorandom sequence (PRS) to produce clock rate encoded direct sequence spread spectrum signals. The signals are decoded by a feedback loop including a voltage controlled oscillator which clocks a local PRS generator. A phase detector responds to the clock rate encoded received PRS signal and to the local PRS signal to produce a control signal. The control signal is filtered and applied to the VCO to control the local PRS clock rate. The decoder loop forces the VCO rate to track the clock rate of the encoded signal, and the decoder loop VCO control signal reproduces the analog information. A demodulator for a clock rate encoded SS signal upconverted by a carrier includes a pair of mixers, a first of which receives the signal to be demodulated and the local PRS sequence from the decoder. When the local and received sequence are phase coincident, the first mixer produces the carrier. The second mixer receives the signal to be demodulated and the carrier from the first mixer and generates therefrom demodulated baseband clock rate encoded SS signals for application to the decoder.
A high frequency code division multiple access spread spectrum (CDMA SS) communications system includes multiple transmitters and their associated receivers, and a pilot carrier source. In order to reduce errors in the receiver IF frequency due to unavoidable differences between the desired transmitted carrier and receiver local oscillator (LO), the transmitted carrier ferquency is related to the pilot frequency. The receiver phase-locks the pilot carrier to the local oscillator, thereby providing an IF data signal with reduced frequency error. The IF data signal is multiplied by sine and cosine demodulating signals related to the local oscillator frequency to produce inphase (I) and quadrature (Q) components of the received data signal. The I and Q signals are applied to code matched filters, which produce filtered or decoded signals the amplitudes of which are related to the phase error between the demodulating signals and the IF carrier. A logic arrangement responds to the decoded signals to generate a control signal which step-corrects the demodulating signal to the correct phase. Correction can occur within one or a few bit intervals, thereby allowing burst mode communications while maintaining high throughput for the multiple users of the communication system.
A method for RF communication between transceivers in a radio frequency identification system that improves range, decreases multipath errors and reduces the effect of outside RF source interference by employing spread spectrum techniques. By pulse amplitude modulating a spread spectrum carrier before transmission, the receiver can be designed for simple AM detection, suppressing the spread spectrum carrier and recovering the original data pulse code waveform. The data pulse code waveform has been further encrypted by a direct sequence pseudo-random pulse code. This additional conditioning prevents the original carrier frequency components from appearing in the broadcast power spectra and provides the basis for the clock and transmit carrier of the transceiver aboard an RFID tag. Other advantages include high resolution ranging, hiding transmissions from eavesdroppers, and selective addressing.
Apparatus and method are disclosed for synchronziing the operation of the frequency sensitive devices of two or more transceiver units in a secure, wireless communication system. A fixed frequency pilot signal is generated in a first unit of the system and transmitted to the other units. The encoding noise signal locally generated in each unit is timed by clock pulses derived from the locally available pilot signal. If synchronism between the respective noise signals is lost, the pilot signal is disabled. This causes a reset pulse to be generated in each unit. The action resets each noise generator and restores synchronous operation.
