Abstract: A circuit for processing related signal components, such as in-phase and quadrature signal components generated within a modulator, in which a first filter and a second filter are matched such that the gain of each such filter is nearly identical. A modulator is provided having matched filters in accordance with the present invention in which the gain "K" of each filter is equal to 1. By using a filter which has a gain of 1, the gain is removed from consideration when attempting to match the filter response of one filter to another. Furthermore, each filter is relatively simple to implement, having only four resistors, two capacitors, and two operational amplifiers.

Abstract: The present invention provides a system and methods for coding and decoding digital data to improve the subjective quality of a voice signal. According to the invention, a sampled voice waveform is compressed by an ADPCM algorithm resulting in a sequence of samples. The sequence of samples are formed into an array and coded into parity bits. Row and column checksums are preferably performed on the parity bits to form row and column vectors. The samples and row and column vectors are transmitted to a receiving unit preferably having a diversity capability. The receiver processes each diversity block by generating row and column vectors from the received samples and comparing the generated row and column vectors to the row and column vectors received. Then based on the comparison, the samples of the best diversity block are used to reconstruct the voice waveform on a sample-by-sample basis.

Abstract: A digital demodulator and method for demodulating digital data representing a phase shift keyed (PSK) signal are provided. The demodulator comprises a phase detector, automatic frequency controller, automatic timing recovery controller, data decoder, and unique word detector. According to the method of the present invention, a PSK signal is received and digitized to substantially remove the signal's amplitude characteristics. The phase detector receives an input of the digital data and based upon transitions in the data from a high state to low state and from a low state to a high state, provides phase estimates. The phase estimates are converted by the data decoder into binary data representing the symbols transmitted to form the PSK signal. A number of overlapping windows of digital data are used to determine phase estimates.

Abstract: A programmable digital modulator and methods of modulating digital data for transmission by a communication system according to operating parameters selected for various applications are provided. A two-chip system is utilized by a preferred embodiment of the invention. One chip comprises a PROM for storing impulse response data which would result from filtering the data to be transmitted. The second chip comprises a data interface for accepting input data, an address generator for generating an address of the PROM where the impulse response data is stored which corresponds to the data input to the chip and for causing the PROM to output the impulse response data stored at the address generated, and a data modulator for modulating a carrier signal with the impulse response data provided by the PROM.

Abstract: A method and apparatus used in the context of a digital cellular communication network for allowing a remote unit to determine the conditions under which a handoff should occur and the procedure to be followed. The method and apparatus of the present invention controls handoffs in a manner which causes the boundaries of a cell within a digital cellular network to remain relatively well defined and to very closely conform to the boundaries of a cell of an advanced mobile phone system with which the cellular digital packet data system is associated. The present invention also reduces the effort required by a remote unit within a cellular digital packet data communication system when attempting to change channels. Primary and Secondary Thresholds are defined in accordance with the present invention.

Abstract: A digital demodulator and method for demodulating digital data representing a phase shift keyed (PSK) signal are provided. The demodulator comprises a phase detector, automatic frequency controller, automatic timing recovery controller, data decoder, and unique word detector. According to the method of the present invention, a PSK signal is received and digitized to substantially remove the signal's amplitude characteristics. The phase detector receives an input of the digital data and based upon transitions in the data from a high state to low state and from a low state to a high state, provides phase estimates. The phase estimates are converted by the data decoder into binary data representing the symbols transmitted to form the PSK signal. A number of overlapping windows of digital data are used to determine phase estimates.

Abstract: Disclosed herein are methods and apparatus for detecting the presence of a CDPD carrier in a channel of a cellular system (although the invention is broadly applicable to detecting a specific kind of carrier associated with an "overlay" system, wherein an "overlay" system is one that shares the frequency allocation of another system on a non-cooperating basis). This specification describes a spectral estimation technique for detection of a CDPD carrier. In addition, this specification describes an MMSE (minimum mean-squared error) method that improves on the spectral estimation method.

Abstract: An analog circuit for multiplying a first input signal with a second input signal. The inventive analog circuit is capable of linear operation with a low voltage power source. A first pair of transistors is coupled as a first differential pair, and a second pair of transistors is coupled as a second differential pair. The first differential pair is coupled to the second differential pair in a manner that is similar to the corrections made between a first and second differential pair of a conventional Gilbert mixer. However, the emitter degeneration resistors of the present invention are not coupled to the collectors of a third differential pair, as is the case in conventional Gilbert mixers. Rather, the degeneration resistors of the present invention are coupled directly to the negative power supply terminal.

Abstract: A cellular digital packet data (CDPD) system includes a mobile data base station (MDBS) configured to interface easily with an advanced mobile phone system (AMPS). The elements of the MDBS are arranged on modules and the functionality of the MDBS is divided among the modules to facilitate flexibility, compactness and quick expansion of the MDBS. To further facilitate compactness, the MDBS includes a transceiver/modem board which uses a single chip to carry out delta-sigma modulation. In order to maintain the integrity of CDPD transmission, a channel hopping scheme is used based upon avoidance of AMPS channel use. A distinctive protocol is used to encapsulate control/query/response data for transmission throughout the CDPD system. This protocol also facilitates easy control of each MDBS throughout the system.

Abstract: The present invention provides a frequency tracking and compensation system and methods for minimizing the frequency offset between remote and local units of a communications system. The system of the present invention resides in the local unit and comprises a downconverter, a digital transceiver, a processor, a reference oscillator and a frequency synthesizer. The downconverter mixes a signal received from the remote unit with a local oscillator (LO) signal generated by the frequency synthesizer. The frequency of the LO signal is based on the frequency of a reference signal generated by the reference oscillator. The system of the present invention utilizes the digital transceiver to determine a frequency offset of the received signal without comparing the received signal to the reference signal. The processor then utilizes the frequency offset to adjust the frequency of the reference signal.

Abstract: Methods and apparatus for improving the perceived quality of ADPCM encoded signals by magnitude limiting samples of the ADPCM encoded signals prior to the decoding of samples of the ADPCM encoded signals, preferably, a sample of the ADPCM encoded signal is magnitude limited only if a receive error is detected for the sample.

Abstract: A cellular digital packet data (CDPD) system includes a mobile data base station (MDBS) configured to interface easily with an advanced mobile phone system (AMPS). The elements of the MDBS are arranged on modules and the functionality of the MDBS is divided among the modules to facilitate flexibility, compactness and quick expansion of the MDBS. To further facilitate compactness, the MDBS includes a transceiver/modem board which uses a single chip to carry out delta-sigma modulation. In order to maintain the integrity of CDPD transmission, a channel hopping scheme is used based upon avoidance of AMPS channel use. A distinctive protocol is used to encapsulate control/query/response data for transmission throughout the CDPD system. This protocol also facilitates easy control of each MDBS throughout the system.

Abstract: Methods and apparatus for frequency synthesization are shown for generating an output signal of desired frequency. The frequency synthesizer includes an oscillator for generating an output signal at the desired frequency in response to a control signal. A first detector compares the output signal to a reference signal and generates a first difference signal representative of the differences, preferably in phase and frequency, between the output and the reference signals. A second detector compares the output signal to the reference signal and generates a second difference signal. A controller generates the control signal in response to either the first or second difference signal. A selector member selects between the first and second detectors to provide either the first or second control signal to the controller in response to a selection signal.

Abstract: A unique word detector for use in detecting a unique word received by a demodulator in one of a plurality of windows offset in time is provided. The unique word detector includes a correlator for receiving successive sequences of binary data related to one of the windows. The correlator compares each of the successive sequences with the unique word to generate a measure of correlation between the sequences of binary data and the unique word. The unique word detector also includes a window detector interfaced with the correlator for comparing each of the measures of correlation to a predetermined threshold to determine whether the unique word has been received. A method of detecting the unique word in one of the windows is also provided.

Abstract: Selectable demodulation means within a given receiver to demodulate signals potentially subjected to many different types of degradations as typically encountered in mobile communications systems, and to select the best result of provide significant performance improvement is disclosed. In accordance with the method, when an RF signal is received it is sampled and the samples are demodulated. Demodulation is performed using both coherent and non-coherent schemes with the preferred scheme selected according to the quality of performance of each demodulator. Coherent demodulation can provide better performance when the signal is subjected to significant path loss but little fading. Non-coherent demodulation can provide better performance when the signal is subjected to less path loss but significant fading. Details of the method are disclosed.

Abstract: Digital methods and apparatus for reverse link signal detection and recovery in a mobile data base station. In accordance with the method, the received signal is frequency downshifted by multiplying the received signal by inphase and quadrature components of a local carrier oscillator, and the resulting downshifted inphase and quadrature components of the signal vector is sampled and digitized at a rate of more than twice the expected bit rate. Difference vectors for digitized downshifted signal vectors one expected bit time apart are determined and examined for detection of a dotting sequence and for estimation of the carrier frequency error, which on detection of the dotting sequence, is used to initially correct the digitized downshifted signal vectors for the frequency error. Techniques for bit timing recovery, frequency error correction during data detection and detection of voice versus data, as well as other features of the invention, are disclosed.

Abstract: Open loop phase estimation methods and apparatus for coherent combining of signals on mobile channels using spatially diverse antennas are disclosed. In accordance with the method, the signals from each of two antennas are demodulated using inphase and quadrature components of a local carrier oscillator to provide RF demodulated inphase and quadrature components of the output of each antenna. These components are simultaneously sampled at a sample rate and digitized to provide the digitized components of the two signal vectors. A phase estimator vector having a phase equal to the difference in phase between the two signal vectors of each successive sample time is determined. An average phase estimator vector for a group of successive signal vectors is determined, and is used as the estimated phase difference between the signal vectors at the middle of the group of successive signal vectors to align the respective signal vectors prior to the same being combined into a single signal vector.

Abstract: Methods and apparatus for digital cordless telephone systems are disclosed for detecting and correcting errors in received signals. Such methods and apparatus include providing a table of predetermined syndromes, wherein each of the predetermined syndromes has an associated error correction pattern. The table is formed by selecting from all possible syndromes that could be predetermined from the segment length of received digital data, those predetermined syndromes having error patterns for correcting a predetermined number of errors within the segment and arranging the predetermined syndromes in the table in accordance with a predetermined order, such as ascending or descending numerical order. Thereafter, a current syndrome is calculated for the received digital data and the predetermined syndrome contained in the table matching the current syndrome is located by using a predetermined search technique.

Abstract: An RF circuit switch system having particular use in an antenna diversity scheme for a CDPD system relies upon a comparison between current signal values on a selected antenna and a predetermined value based upon previous signal values measured on the same antenna. This comparison determines whether switching will occur from one antenna to another. Normally, the predetermined value will be adjusted based upon present signal strength on the selected antenna. Switching is normally attempted when there is a drop of approximately 10 dB in the signal strength of the selected antenna. If the signal strength in the other antenna is found to be less than that of the original antenna upon switching, an automatic switch back is carried out.

Abstract: Open-loop phase estimation methods and apparatus for coherent demodulation on mobile channels is disclosed. A PSK RF modulated signal is received, RF demodulated to obtain inphase and quadrature components thereof, and the inphase and quadrature components sampled and digitized periodically to provide inphase and quadrature components of the signal vector. A phase estimator strips the data contribution from each vector, leaving only a phase error measurement related to the true phase error. That phase error indication is averaged over a fixed window of time to determine an average phase estimate applicable to the signal vector at the middle of the window. Resulting one hundred eighty degree ambiguities are resolved, and then the applicable signal vector is phase corrected by the average phase estimate and result of the ambiguity resolution, with the following demodulation being responsive to the phase of the phase corrected signal vector.