Abstract: If no uncorrectable errors are discovered in a received data packet, an acknowledgment is sent back to the sender. Conversely, if the data packet contains uncorrectable errors, no acknowledgment is sent, forcing a re-transmission of that data packet in accordance with an automatic repeat request (ARQ) procedure. In such cases, sets of unquantized signal values relating to received data packets having uncorrectable errors are stored. When two or more sets of unquantized signal values for the same received data packet (as re-transmitted) have been stored, the unquantized signal values are numerically combined on a bit-by-bit basis, and then quantized to regenerate a combined data packet. If no uncorrectable errors are discovered in that combined data packet, an acknowledgment is sent back to the sender. Otherwise, no acknowledgment is sent, forcing another data packet re-transmission.

Abstract: A robust automatic frequency control (AFC) loop for use in radio frequency systems performs a threshold test on the accumulated error adjustments for controlling a frequency oscillator. Based on this test, the loop adjusts the value contained in the error accumulator to allow the receiver to remain locked to a desired frequency carrier even in the presence of a strong adjacent channel interferer. The AFC loop is designed to be used in error systems where the predetermined fixed carrier spacing. Assuming that the initial frequency error is never exceeded by one-half the carrier spacing, the system can determine if the wrong channel has been locked to if the oscillator in the AFC loop is corrected by more than half the carrier spacing. Through use of a threshold test on the accumulative oscillator correction, the AFC loop can determine if the receiver has locked to an adjacent interfering channel.

Abstract: A mode of communication for asset tracking units involves communication between a central station and the individual tracking units, usually through a satellite link. This mode is the primary communication link for tracking the assets. A second mode is the local area network (LAN) or "mutter" mode, in which a subset of tracking units communicate with each other in a mobile LAN. This mode is used as the secondary communication mechanism to conserve power, since mutter mode communication requires much less energy for local communication between the tracking units as opposed to direct satellite communication with the central station. In addition, mutter mode leads to increased reliability of the asset tracking system by enabling tracking of units which have batteries too weak to support communication with the central station, but strong enough to support communication in mutter mode.

Abstract: A low complexity adaptive equalizer for use in U.S. digital cellular radios demodulates .pi./4-shifted differentially encoded quadrature phase shift keyed (DQPSK) encoding in the presence of intersymbol interference (ISI) with reduced decoding complexity by employing an estimated received constellation which takes into account channel changes over time and ISI. The complexity is reduced by tracking a reduced number of estimated reference symbol constellation points and taking advantage of the geometry to estimate the remaining symbol constellation points. Branch metrics are also determined with a reduced number of computations.

Abstract: A low complexity adaptive equalizer for use in U.S. digital cellular radios demodulates .pi./4-shifted differentially encoded quadrature phase shift keyed (DQPSK) encoding in the presence of intersymbol interference (ISI) with reduced decoding complexity by employing an estimated received constellation which takes into account channel changes over time and ISI. The decoding complexity is reduced by tracking a reduced number of estimated reference constellation points and taking advantage of the geometry to estimate the remaining symbol constellation points. A slot of symbols is decoded, employing unique filtering, in segments from the beginning in a forward direction and from the end in a reverse direction meeting at a fade point. This isolates a fade and allows the equalizer to perform more reliably.

Abstract: A method of estimating the quality of a communication channel from a differential phase angle between a received signal and the corresponding transmitted phase angle employs determining a signal to impairment ratio (SIR) as an indicator of channel state information (CSI). A maximum likelihood estimation procedure is employed to calculate this CSI metric as a function of the differential phase angle between the received signal and transmitted signal. An alternate embodiment employs a estimation that incorporates average SIR information for a Rayleigh fading channel. Since CSI is derived from the phase angle of the received signal, and does not require signal amplitude information, it is attractive for use with differential detectors, phase-locked loops (PLLs) and hard-limited signals. The CSI provided can be used for implementing post detection selection diversity, by selecting the signal from a plurality of antennae which has the best SIR.

Abstract: A digital time division multiple access (TDMA) radio communications system employs a digital information source for providing digital information, a transmitter for transmitting encoded digital information in a radio-frequency (RF) signal to a receiver which demodulates the encoded symbols into digital information to be utilized by an output device. The receiver, synchronizes, compensates for frequency drift, samples and divides the samples into halfslots of samples. The halfslots are subdivided into subslots numbered from 1 to N, where N represents the last received subslot. Subslots 1 and N are demodulated into digital information in a forward and reverse direction, respectively with metrics calculated. If the metrics indicate a signal with a larger signal-to-noise ratio from subslot N, subslot N-1 is demodulated in a reverse sense with another reverse metric calculated, and vice versa.

Abstract: A low complexity adaptive equalizer for use in U.S. digital cellular radios demodulates .pi./4-shifted differentially encoded quadrature phase shift keyed (DQPSK) encoding in the presence of intersymbol interference (ISI) with reduced decoding complexity by employing an estimated received constellation which takes into account channel changes over time and ISI. The decoding complexity is reduced by tracking a reduced number of estimated reference symbol constellation points and taking advantage of the geometry to estimate the remaining symbol constellation points. Reference symbol constellation points are updated directly to compensate for changes in the channel, instead of determining channel impulse response (CIR) coefficients, and convolving the CIR coefficients with received symbols to determine new reference symbol constellation points.

Abstract: A Time Division Multiple Access (TDMA) radio system achieves synchronization by performing a two-step synchronization. A simplified frame/slot synchronization is followed by a symbol synchronization of higher accuracy. This symbol timing is passed to a frequency offset unit which determines the amount of frequency drift between the transmitter and receiver and compensates for the frequency drift. This results in improved receiver performance for the TDMA digital radio system.