Abstract: A receiver may reduce the effects of distortion when determining signal usability in the following manner. Upon receiving a signal, the receiver separates a desired component, an undesired component, and a distortion component, wherein the desired component includes the signal that was originally transmitted and the undesired component includes interference and noise. Having isolated the distortion component, it can be mitigated such that the usability of the signal received can be determined based on a ratio between the desired and undesired components.

Abstract: A receiver that receives a stream of information symbols may detect the usability of the stream of information symbols received as follows. Upon receiving the stream of information symbols, the receiver separates the stream of information symbols into a desired portion and an undesired portion wherein the desired portion is the originally transmitted stream of information symbols and the undesired portion is interference and noise. From the undesired portion and the desired portion, the usability can be determined based on a ratio of the two portions.

Abstract: A diversity receiver that receives diverse modulated signals may produce a usable signal from the received modulated signals in the following manner. Two modulated signals, each including a desired component and an undesired component, are received by the diversity receiver, wherein each desired component includes an originally transmitted signal and each undesired component includes noise and interference. The diversity receiver estimates each desired and undesired component and produces the usable signal based on the estimated desired components and the estimated undesired components.

Abstract: A receiver may reduce the effects of distortion when determining signal usability in the following manner. Upon receiving a signal, the receiver separates a desired component, an undesired component, and a distortion component, wherein the desired component includes the signal that was originally transmitted and the undesired component includes interference and noise. Having isolated the distortion component, it can be mitigated such that the usability of the signal received can be determined based on a ratio between the desired and undesired components.

Abstract: In the following manner, a receiver that receives quadrature modulated signals may determine desired components of the quadrature modulated signals by minimizing effects of transmitted distortion produced by gain, phase, or gain-phase imbalance. A quadrature modulated signal that includes a desired component, a fading component, and a transmitter distortion component is received by the receiver, wherein the transmitter distortion component includes a conjugate component and an imbalance component. The receiver estimates the fading component, the conjugate component, and the imbalance component and determines the desired component based on these estimates and the quadrature modulated signal.

Abstract: The ratio of peak power level to average power level in a power amplifier used in a QAM communication system transmitter can be reduced by preselecting magnitudes and phase angles of complex-valued pilot symbols used in multi-channel, N-level QAM.

Abstract: A transceiver compatible with both wide channel constant envelope 4 level FSK FM modulation and narrow channel .pi./4 differential QPSK linear modulation allows compatible interaction between modified constant envelope and non-constant envelope transmitters. All Nyquist filtering occurs in the transmitters, and none in the receiver.

Abstract: In a QAM communications system, a novel synchronizing sequence of symbols added to the information channel simplifies acquisition of timing and synchronization by a receiver. Such synchronization vectors provide signals for improved AFC control signal generation.

Abstract: In order to synchronize wideband signals in time while reducing the necessary amount of processing, a number of narrowband signals (203, 205, 207, and 209) are modulated (503) on their own frequency sub-carriers, such that the summation of these modulated signals remains within the bandwidth of the wideband signal (201). This composite signal is transmitted (507) to a receiver, which uses a matched filter bank (405) to recover timing data from each transmitted frequency sub-carrier.

Abstract: To compensate the audio distortion resulting from amplitude and phase imbalance in the quadrature oscillator (103) and mixers (105, 115) in zero-IF downconverters (203), a balancing action is required. An apparatus for distortion compensation compensates for the amplitude and phase imbalance to reduce audio distortion. This is accomplished by determining (309) two gain values, which are combined (217, 227, 229) with the in-phase and quadrature components of the zero-IF downconverter (203) to compensate for the imbalance.

Abstract: In a digital communication system in which pulse-shape filtered sync, pilot, and data symbols, arranged in successive time slots, modulate sub-channel carriers via sub-channel mixers to form sub-channel symbol streams for combination into a composite signal for transmission, deterministic portions of each time slot are rendered identical by determining phase difference of sub-channel carriers and rotating the phase of each sub-channel symbol stream by an amount equal to the phase difference, but with opposite sign.

Abstract: A method for implementing diversity reception to counteract effects of channel fading on a transmitted information signal. In diversity receive paths, estimates of complex channel gain are computed based upon pilot symbols inserted from time to time in the transmitted information symbol stream. Phase corrected and weighted samples from the diversity paths are summed prior to the decision process. The squared magnitudes of the diversity path channel gains are summed to provide the proper threshold adjustment.

Abstract: A convolutional decoder which assigns bit metrics to at least one bit of a symbol in a multilevel system. This decoder uses soft-decision Viterbi decoding with channel state information of a convolutionally-encoded communication transmitted using multilevel modulation.

Abstract: A digital radio receiver is described. The digital receiver of the present invention contemplates a digital radio receiver which operates on a received analog signal which has been converted to a digital form after preselection at the output of the antenna. The digital receiver of the present invention comprises a preselector, a high-speed analog-to-digital (A/D) converter, a digitally implemented intermediate-frequency (IF) selectivity section having an output signal at substantially baseband frequencies, and digital signal processor (DSP) circuit performing demodulation and audio filtering. The radio architecture of the present invention is programmably adaptable to virtually every known modulation scheme and is particularly suitable for implementation on integrated circuits.

Abstract: A digital receiver for GPS C/A-code signals is described. The GPS receiver of the present invention provides reception and tracking a plurality of satellites simultaneously, using four separate receiver channels. The GPS receiver of the present invention includes an analog front-end for selecting and frequency translating the received GPS signal. The GPS receiver further includes a highspeed digital signal processor for recovering the despread data of the GPS signal. The baseband signal is further processed by a general purpose digital signal processor for signal search, tracking, and data recovery operations, and a microprocessor provides overall receiver control, and interface with the operator of the GPS receiver.

Abstract: An improved method of doppler searching in a digital GPS receiver is described. According to the principles of the present invention, an N-point fourier transform is performed on samples of a down converted GPS signal representing the entire range of Doppler-shifted GPS carrier frequencies. The magnitude of the square of each of the output samples is calculated to produce N power quantities, corresponding to N search frequencies. The power quantities for each of the N search frequencies are then summed to produce N quantities proportional to average power. The maximum of the N-power quantities indicates the presence of the corresponding GPS signal.

Abstract: A digital FM demodulator is described. According to the principles of the present invention, a digital FM signal is demodulated by processing sampled input vectors of a quadrature FM digital signal centered approximately zero frequency. The sampled quadrature input vectors are quantized to lie within a predetermined magnitude range. The quantized vectors are coupled to a phase accumulator which generates a current coarse phase value related to input vectors of the quadrature FM signal. The output of the phase accumulator is also coupled to a vector rotation means which rotates the input vector to a predetermined quadrant to determine a fine phase value based on said rotated input signal vector. The fine and coarse values are then summed and output as a composite phase value, and digitally filtered by subtracting the value of the previous composite phase angle sample from the value of the current composite phase angle sample to produce a demodulated message sample.

Abstract: A method and means for improving the frequency resolution in a digital oscillator is described. According to the principles of the present invention, a digital oscillator may be comprised of a frequency latch, a phase accumulator, and a ROM based waveform generator. Improved frequency resolution is achieved in the digital oscillator, without increasing ROM size by quantizing the summed output of a dither generator and the phase accumulator before sending the resultant multi-bit signal to a ROM. The contents of the ROM are sequentially addressed with the resultant multi-bit signal. The output of the ROMs comprise digital words corresponding to a desired waveform envelope.

Abstract: An apparatus and method for determining the period and frequency of a signal intefering with a desired Loran C signal are provided. A sample of the interfering signal is generated at a time corresponding to the peak of the autocorrelation function of the interfering signal with respect to the predetermined pulse tracking reference point of the Loran C signal. Indicia of the period and frequency of the interferer is obtained from the timing of the sample generated at the autocorrelation function peak.

Abstract: A method and apparatus for attenuating an interfering signal corrupting a desired Loran C signal is provided. An interfering signal sample is determined which occurs at the peak of the autocorrelation function of the interfering signal such that the period of the interferer is known. The sample occurring at the peak of the autocorrelation function is added to or subtracted from the desired Loran C signal sample taken at a predetermined reference point during the pulses thereof such that the interfering signal is cancelled or substantially attenuated.