Abstract: Methods and apparatus for canceling co-channel interference in a receiving system using spatio-temporal whitening. In some embodiments, a spatio-temporal interference canceling method, and apparatus for carrying out the method are provided which effectively cancel co-channel interference despite frequency offset between the desired signal and the interferer in a TDMA type system. Real and imaginary component values of the total received signal are used for virtual diversity branches, and a vector-valued auto regressive model is used to characterize the interference. In other embodiments, spatio-temporal interference whitening is used to improve timing estimates used for synchronization. The two uses of spatio-temporal whitening can be combined in one receiver. The invention is typically implemented in one or more programmed digital signal processors or application specific integrated circuits (ASICS), embodied in a receiving system.

Abstract: Methods for secure OFDM communications include changing the length of OFDM symbols in a pseudo-random fashion by appending a totally random signal to some of the OFDM symbols. An adaptive cyclic prefix is provided for covert and spectrally efficient communication. A developed PN based random data addition provides further security by removing the chance of combining synchronization information over several OFDM symbols.

Abstract: In the present disclosure, a method/apparatus is provided that detects the impairment in the received signal (12) and employs the structure in the receiver (10) that is designed for that purpose. Preferably, the method/apparatus detects the interferer modulation and employs the structure in the receiver (10) that is designed for that purpose. The detection is preferably performed using a quality measure (16), e.g., the residual errors after channel estimation or SNR (signa-to-noise-ratio) estimates after channel estimation. The present invention involves preferably hypothesis tests, threshold schemes, or schemes where the threshold is adapted according to one of the measures. The operations in the receiver is preferably that is selects between a powerful interference rejection method and a conventional receiver or that is selects between a powerful interference rejection method and a less powerful interference rejection method, or a combination thereof.

Abstract: To receive a signal from a multiple-input-multiple-output (MIMO) communication channel, initial channel taps are generated based on an impulse response estimate of the MIMO communication channel. The received signal is pre-filtered using the initial channel taps to generate output channel taps and a corresponding output signal having an increased signal-to-noise ratio (SNR) and uncorrelated noise. The SNR is based on a ratio of the energy in a first subset of the output channel taps to the energy in a second subset of the output channel taps.

Abstract: A joint demodulator is configured to generate an estimated first frequency or first frequency error for the first signal and an estimated second frequency or second frequency error for the second signal. A first long-term automatic frequency control is responsive to the estimated first frequency or first frequency error, wherein the joint demodulator is responsive to the first long-term automatic frequency control. A second long-term automatic frequency control is responsive to the estimated second frequency or second frequency error, wherein the joint demodulator is responsive to the second long-term automatic frequency control. First and second local automatic frequency controls also may be included in the joint demodulator, wherein the first long-term automatic frequency control is responsive to the first local automatic frequency control and the second long-term automatic frequency control is responsive to the second local automatic frequency control.

Abstract: An adaptive radio receiver utilizes control functions derived from received signals in an interference-limited environment to decide whether to implement conventional single-user demodulation or joint demodulation. The decision to implement joint or conventional demodulation is based upon the values of estimates derived for Doppler spread, carrier-to-interference plus noise, dispersiveness and the likelihood of the presence of a dominant interferer.

Abstract: A mobile communications system receiver has improved synchronization and channel estimation for receiving a transmitted signal passing through a radio channel. A processor receives and samples the transmitted signal for a plurality of timing hypotheses. A channel estimator hypothesizes unknown symbols at a pilot cluster of the sampled signal and estimates the radio channel of the pilot cluster for each hypothesized symbol sequence. The estimator determines an error corresponding to each hypothesized symbol sequence using the sampled signal, the estimated radio channel and the hypothesized symbols and determines actual timing estimates and actual radio channel estimates by selecting the radio channel estimates associated with the hypothesized symbols and the timing and hypothesis that minimizes the error.

Abstract: Received signals are decoded based on a coupling between bits of the received signals introduced by the channel over which the signal is transmitted or by the transmitter which transmitted the signals. For example, in a higher-order modulation scheme, such as 8-PSK, individual bits grouped in a given symbol are generally coupled rather than fully independent. Accordingly, decoding information on a first one of the bits processed through a forward error correction decoder may be used to adjust the soft information from a demodulator for one or more of the other bits contained in the same symbol for use in processing those bits through the forward error correction decoder. Particularly where interleaving is utilized, a first bit may be processed through the decoder before the soft information on another of the bits is needed, thereby allowing an intermediate decision soft value for the first bit to be used in generating the input value to the decoder for the subsequent bit.

Abstract: Information is recovered from a composite signal including first and second component signals modulated according to respective first and second modulations, for example, 8-PSK and GMSK. The composite signal is processed based on predetermined characteristics of the first and second modulations, for example, signal rotations, to generate a channel model for the first component signal and a whitening filter for the second component signal. The composite signal is filtered according to the whitening filter to generate a filtered signal. A symbol hypothesis for a symbol of the first component signal is evaluated using a metric with respect to the filtered signal that is a function of the whitening filter and the channel model, for example, in a multidimensional Viterbi symbol estimation algorithm. The invention may be embodied as methods, apparatus and computer program products.

Abstract: A Doppler spread for a communications channel is measured by providing an estimate of the communications channel and generating an autocorrelation function for the estimate of the communications channel. One of a plurality of autocorrelation function hypotheses is selected to approximate the autocorrelation function for the estimate of the communications channel wherein each of the autocorrelation function hypotheses corresponds to a respective Doppler spread estimate hypothesis. One of the Doppler spread estimate hypotheses is selected corresponding to the selected autocorrelation function hypotheses as an estimate of the Doppler spread for the communications channel.

Abstract: A method for receiving a plurality of communications from a respective plurality of transmitters using a common carrier frequency includes receiving a plurality of information signals on a common carrier frequency corresponding to the plurality of communications from the plurality of transmitters, and generating first and second separated signals corresponding to respective first and second ones of the information signals so that the first separated signal includes a primary component corresponding to the first information signal and so that the second separated signal includes a primary component corresponding to the second information signal. The first separated signal is demodulated to provide an estimate of a first information sequence corresponding to the first information signal, and the estimate of the first information sequence is modulated to provide a modulated estimate of the first information sequence.

Abstract: Methods and apparatus for canceling co-channel interference in a receiving system using spatio-temporal whitening. In some embodiments, a spatio-temporal interference canceling method, and apparatus for carrying out the method are provided which effectively cancel co-channel interference despite frequency offset between the desired signal and the interferer in a TDMA type system. Real and imaginary component values of the total received signal are used for virtual diversity branches, and a vector-valued auto regressive model is used to characterize the interference. In other embodiments, spatio-temporal interference whitening is used to improve timing estimates used for synchronization. The two uses of spatio-temporal whitening can be combined in one receiver. The invention is typically implemented in one or more programmed digital signal processors or application specific integrated circuits (ASICS), embodied in a receiving system.

Abstract: Methods and systems are provided which generate hard and soft values from signal samples of a differentially encoded signal by estimating channel coefficients associated with the received signal and determining metrics for each possible symbol pair which indicate the probability of a possible coherent symbol pair being encoded in the signal samples using the estimated channel coefficients and the signal samples. Soft values associated with differential bits or differential symbols encoded in the samples of the signal are then generated based on the determined metrics.

Abstract: A communications system and method for compensating for an interferer signal in a received signal comprising a radio processor for filtering the received signal, and a demodulator coupled to the radio processor for demodulating the filtered received signal. A channel quality estimator coupled to the demodulator determines a channel characteristic based on instantaneous noise values of the received signal, for controlling the demodulation of the received signal to compensate for the interferer signal responsive to the channel characteristic. Further, where the channel characteristic is a first channel characteristic, the channel quality estimator determines a second channel characteristic based on instantaneous noise values of the received signal and controls demodulation of the received signal to compensate for the interferer signal responsive to the first and the second channel characteristics.

Abstract: Methods, communication apparatus, and computer program products process a signal by detecting an information field therein. A signal may be received across a plurality of time slot intervals with each time slot interval including a plurality of symbol positions so as to provide a sequence of symbols associated with the received signal. An average value of the symbols received in a symbol position across the plurality of time slot intervals is determined. A determination may be made whether a symbol position contains a fixed symbol based on the average symbol value determined for that symbol position. Alternatively, a determination may be made whether the received signal contains symbols corresponding to a predefined symbol sequence based on the average values respectively determined for the plurality of symbol positions.

Abstract: Methods and systems are provided which utilize pilots in an information sequence to periodically retrain a channel estimator. Thus, a channel tracker may be synchronized using a synchronization sequence and then periodically retrained using known pilot symbols. Furthermore, the utilization of pilots may allow for the detection of errors in previous channel estimates. When errors are detected, a new channel estimate may be used based on the retraining using the pilot symbols and, optionally, previous errors in symbol estimation may be corrected. Thus, by retraining based on pilot symbols, the propagation of errors may be reduced.

Abstract: Methods and apparatus for receiving adjacent channel signals wherein adjacent channel interference effects are minimized through joint demodulation of the adjacent channel signals. A channel associated with each signal and each corresponding frequency band is estimated and used to form joint branch metrics for joint sequence estimation. In an exemplary embodiment, a baseband processor receives baseband samples corresponding to at least one carrier frequency, and then jointly demodulates at least two information streams corresponding to different carrier frequencies in dependence upon the received baseband samples. In another embodiment, a joint channel estimator receives at least two baseband sample streams, each stream corresponding to a different frequency band, and jointly estimates medium responses for each of at least two information signals which were transmitted in different frequency bands.