Abstract: A system and method for improving the performance of digital filters. The data bandwidth and spectral control are improved by adaptively selecting ramp-up and ramp-down symbols based on the actual filtered data. Thus, the energy in the truncated tail of the filter is minimized for a given filter design. The invention is of particular utility in systems that filters intermittent data streams, thus requiring the filter to energize and deenergize repeatedly.

Abstract: A method of apparatus for achieving and maintaining alignment of a data receiver to data frames in the presence of noise is taught. A statistical weighting process is employed to sample synchronization symbols that reoccur in data frames. The information is used to maintain alignment to the data frames by tolerating a greater number of data errors. A plurality of synchronization symbols are decoded with a weighted probability of error in decoding determined for each. The weighted probabilities are combined and the resultant value is compared to a threshold value. If the threshold is met, then alignment is achieved or maintained based upon the statistical process. Improved performance in lower SNR environments is achieved.

Abstract: A modulator, and demodulator, apparatus and method for use in a multiple sub-channel communication system is taught. A commutator is employed for fractionally sampling, or distributing, signals from, or to, a multiple channel polyphase filter. The filter is coupled with a discrete Fourier transform, or its inverse, such that the relationship between the base-band sampling rate of a plurality of sub-channel signals, the frequency spacing of the sub-channel signals, and the sampling rate of a composite signal can be related by any rational number, thereby freeing designers to optimize system design respecting channel spacing, bandwidth, and signaling rates. The advantages of the present invention are realized by adjusting the interpolation and decimation rates of the filter, and by adjusting the resolution and decimation rates of the transform.

Abstract: A method and apparatus that differentially encodes and decodes data symbols in dual domains is taught. Data packets are encoded, transmitted, and decoded during a plurality of symbol intervals on a plurality of sub-carriers. Encoding and decoding are accomplished across both the time and frequency domains such that the minimum number of carrier states are employed as reference only states that do not encode a symbol of data in and of themselves. A rule of adjacency is followed, both across time and frequency, so that decorrelation is minimized. Any modulation scheme that is applicable to differential encoding and decoding can be utilized. Communication systems that couple via radio waves, through metallic conductors, or over fiber optic paths can be employed.

Abstract: A system and method for information content-independent synchronization with a received signal. A variable of the signal (e.g., average energy or magnitude) which is related to the energy distribution, is measured over a period of time. The signal's information bearing point is found at the point within the information period of the signal with the highest averaged energy. The point may be found, for example, by detecting the sample point with highest average energy, or by correlating multiple sample points to a stored template. Interpolation may be performed to locate an information bearing point that is between sample points. Because the relative energy content of the signal is effectively independent of the specific information content, synchronization with the information bearing point is accomplished without requiring the insertion of any special information content or fixed content patterns into the signal.

Abstract: In a communication system in which an incoming signal is pulse shaped prior to detecting the information bearing point of the signal, processing overhead can be significantly reduced by only processing the sample points corresponding to the information bearing point and two points bounding the information bearing point. During signal acquisition, the signal is over sampled and the pulse shaping filter processes every sample point, or every Nth sample point in the case of down sampling. Once the sample point corresponding to the information bearing point is determined, the pulse shaping filter can be instructed to only process the sampling corresponding to the information bearing point and two neighboring samples. The system and method is adaptive, in that the samples that are processed can shift as the information bearing point shifts relative to the sample points. This is accomplished by re-synchronizing the timing of the received slot.

Abstract: Efficient usage of available spectrum is increased by logically dividing government licensed frequency channels into sub-channels, each of which can individually transmit a signal between a base unit and terminal. The sub-channels are each offset from the center of the frequency channel by a unique offset amount to avoid interference. Power control, sub-channel interference cancellation, and frequency control are employed to minimize the effects of out-of-band sub-channel signals on adjacent sub-channels. Any given sub-channel can be dynamically configured to transmit voice or data signals. Further spectral efficiency is realized using time division multiplexing on some or all of the sub-channels.

Abstract: A system and method of communicating additional data in a data communications system are disclosed. Selective application of a plurality of algorithms is advantageously applied to encoded data to yield a dual benefit of satisfying a primary function as well as communicating additional data. In the preferred embodiment, a plurality of error detecting algorithms are linked to a plurality of additional data so that the system can identify the type of data slot packet that is presently decoded without the need for additional data bits to make such determination.

Abstract: Channel induced distortion is cancelled from a QAM or similarly modulated signal that has both a signal component and a distortion component. Using digital signal processing, the information component of the received signal is removed by canceling magnitude variations arising from the information content. This is accomplished by normalizing the received signal to the magnitude value of one ring on the QAM constellation. Once the distortion component of the received signal is reconstructed, a compensation factor can be derived and applied to the received signal in order to remove the effects of the channel distortion prior to further processing and decoding of the received signal.

Abstract: Carrier wave frequency is quickly acquired without burdensome MIPs requirements by limiting the spectral analysis of the incoming channel to the expected band edges. The right and left band edges spectral energy is compared and clock frequency correction is made from the results of this comparison. A band pass filter, such as a Goertzel algorithm filter, is employed to quickly and with processing efficiency perform the spectral analysis at the band edges. Once the direction of the frequency error is derived, the correct frequency can be acquired through a feedback loop employing various iterative correction techniques, such as a gearshift approach in which certain predefined frequency corrections are imposed in decreasing steps until the desired carrier frequency has been acquired.

Abstract: An improved Nyquist filter can be used as a matched filter in a digital communications system. This filter is characterized in that the frequency domain response meets the Nyquist criteria and that the square root of the frequency domain response has a first derivative that is continuous at all points. In other embodiments, the square root of the frequency domain response continuous over all points for all higher order derivatives.