Abstract: A multiple access digital transmission system including a transmitter for transmitting at least one modulated data stream, and at least one receiver for receiving, via a multipath transmission channel, a transmitted signal which includes the modulated data streams. The receiver includes a resolver for resolving at least one path for at least one multipath received modulated data stream and a channel estimator for estimating channel path delays in the received modulated data streams. The receiver includes a generator for generating an interference reduced signal from the received signal, the interference reduced signal being fed to the channel estimator.

Abstract: The present invention provides a device and method in multi-access communications system for reliably receiving burst transmissions from users in the presence of narrow band interference. The adaptive interference suppresser incorporates adaptive elements that are updated using a predetermined adaptation algorithm. These adaptive elements are stored in memory at the end of the burst transmission for reuse in receiving a next burst transmission.

Abstract: An apparatus for the error free transmission of PCM/FM telemetry data. The pparatus comprises a first mixer for receiving an RF signal which has a first frequency and which is the carrier for the PCM/FM telemetry data. A variable frequency oscillator provides a local oscillator signal to the first mixer which is an offset for the RF signal. The first mixer then mixes the RF signal with the local oscillator signal resulting in the RF signal having a second frequency. This RF signal is supplied to a second mixer which mixes the RF signal with a fixed frequency signal provided by a fixed frequency oscillator. The RF signal provided by the second mixer now has a third frequency and is supplied to a bandpass filter. The frequency of the local oscillator signal is adjusted so that the offset to the RF signal provided by the first mixer is approximately one half of the intermediate frequency bandwidth of the bandpass filter.

Abstract: An apparatus for measuring multipath propagation characteristics receives a plurality of transmitted waves propagated over a plurality of different propagation paths and subjected to multipath fading over the propagation paths, and measures multipath propagation characteristics with respect to those propagation paths based on the received waves. The multipath propagation characteristics can be measured relative to a plurality of propagation path systems using the same frequency band. The apparatus for measuring multipath propagation characteristics comprises a plurality of transmitters located in different spatial positions, respectively, and a receiver for receiving a plurality of waves transmitted from the transmitters while the receiver is moving. The transmitters generate respective waves to be transmitted that each comprise a plurality of discrete spectral components which are arranged in the frequency domain such that they do not overlap each other in the same frequency band.

Abstract: An FM radio receiver, particularly for mobile receivers, reduces the effects of multipath distortion. It selects either an adaptive process (constant modulus algorithm or CMA) or a antenna diversity process depending on the circumstances of the instantaneous reception. The receiver reduces both the decline of the level of received signals with the diversity process and waveform distortion with the CMA process.

Abstract: An apparatus, and an associated method, for facilitating reception and transmission of signals in a communication system, such as a cellular communication system. An antenna having an antenna pattern adaptively responsive to propagation characteristics extending between communication stations of the communication system. The antenna pattern of the antenna is selected responsive to the propagation characteristics. When the propagation characteristics are indicative of a channel exhibiting only insignificant multipath components, the antenna pattern is selected to be highly directional, and when the propagation characteristics are indicative of a channel exhibiting significant multipath components, the antenna pattern is altered to become less directional.

Abstract: A method of automatically determining the reliability of any LOS radio link given certain parameters. Specifically, this method allows a communications network operator to specify and determine the path reliability (PR) for each backbone and extension link in a communications network for a given operating frequency, path length and climatic factor. For each link, the fade margin required (RFM) for a desired path reliability (PRD) is calculated. Then, the actual fade margin of the selected link is calculated using known radio parameters. Preferably, these known radio parameters are stored in a database to facilitate access to this information. Then, the corresponding value of the actual path reliability for the link is calculated using modifications of known statistical models which take into account climatic and geographic factors.

Abstract: A phase array spread spectrum system for maximizing signal strength of a spread-spectrum signal with multipath through the use of receiving means, delaying means, combining means, despreading means, generating means, storing means and comparing means. The receiving means receives a plurality of spread-spectrum signals and a plurality of phased versions of the plurality of spread-spectrum signals. The delaying means delays the received plurality of spread-spectrum signals with respect to the plurality of phased versions of the plurality of spread-spectrum signals by a plurality of delays. The combining means combines the delayed spread-spectrum signals and the plurality of phased versions of the plurality of spread-spectrum signals as a plurality of combined signals. The despreading means despreads the plurality of combined signals as a plurality of despread signals. The generating means generates a plurality of magnitude values from the plurality of despread signals.

Abstract: A channel equalizer for a digital television receiver which filters an input signal to remove a noise component mixed therewith during transmission. The channel equalizer comprises an initial coefficient storage unit for outputting a predetermined coefficient to channel equalizer circuit when a certain channel is firstly selected. The predetermined coefficient is used as an initial filtering value for allowing the channel equalizer circuit to update a filtering coefficient so that it can converge on an optimum value. The stored filtering coefficient is output to the channel equalizer circuit when the firstly selected channel is again selected, so that it can be used as the initial filtering value.

Abstract: A receiver is disclosed for a DS/CDMA communication system wherein a symbol is spread with a predetermined spread code to produce a DS/CDMA signal which is transmitted on a radio frequency carrier. The receiver includes an interference canceller for cancelling signals from undesired sources introduced to a signal received from a desired source using a feedback signal. A phase variation canceller is provided for cancelling a phase variation that occurs in an interval between successive symbols of the received signal. Both cancellers are connected in series. A decision circuit compares an input signal from the interference canceller or from the phase variation canceller with a threshold and produces an output representing one of two discrete values depending on whether it is higher or lower than the threshold. A correction circuit derives a correction signal from a signal produced by the interference canceller and applies it to the interference canceller as the feedback signal.

Abstract: Method and apparatus for using a plurality of correlators to improve an estimate of direct signal arrival time by identifying features of a correlation function at and adjacent to the correlation peak. In a first embodiment, the errors in location of the center point of a correlation function R(.tau.), formed by the incoming composite signal and a stored copy of the expected signal, are assumed to be strongly correlated for narrow sample spacing and wide sample spacing of the correlation function. In a second embodiment, multipath signal strengths and phases are estimated, using multiple sampling of the correlation function R(.tau.). This approach assumes that path delays of the direct signal and of the multipath signals can be determined separately.

Abstract: The invention provides a means by which a radio system in a multipath propagation environment may improve the probability of successful reception of a message by the application of diversity. It is applicable to such applications as paging, cordless telephones, cellular telephones, and radio identification tags. The transmitter sends the message repeatedly, each repetition transmitted from a different antenna. The antennas are separated so that multipath fading of the signal arriving at the receiver from each transmit antenna is substantially uncorrelated with the multipath fading of the signal arriving at the receiver from the other transmit antennas. The receiver examines the radio channel for an incoming message for a time period long enough for it to potentially receive the message from each antenna.

Abstract: A system for mitigating the effect of fading in a digital communication system. Deep signal nulls occur when signals from multiple paths are precisely aligned in amplitude and phase canceling the signal at the receiving antenna and creating a fade. In an indoor system, moving either transmitter or receiver a short distance can shift a fade away from the frequency of interest. This invention provides an actual or perceived movement of an antenna such that the effects of fading can be averaged over a time interval. The signal processing capability of modern digital transceivers can correct errors due to the time varying fades using the present invention. Thus, a dual-antenna element using the system of this invention can provide diversity receive and transmit paths to maintain capacity. The present invention may also provide improved capacity to due enhanced stability in power control.

Abstract: The present invention provides a system for determining, and correcting for, channel-induced distortion in transmitted wideband carrier signals. During a synchronization, or sounding mode, known as the preamble mode, a wideband signal consisting of a number of tones, which may be those of geometric harmonic modulation (GHM), each having a phase of zero are transmitted to a receive unit. At the receive unit, the phases for each tone are extracted. Since each tone is at a different frequency, a snapshot of the change in phase over a number of frequencies can be determined. These phase changes are employed in determining the generalized phase distortion over frequency equation which is used to remove the phase distortion induced by the channel before decoding of the signal. The device then operates in its traffic mode thereby transmitting information to a receive unit which uses the phase distortion information to remove phase distortion induced by the channel and then decode a message signal.

Abstract: A system for improved echo cancellation for use in particular in television receivers. The system features a superior ghost cancellation reference signal which exhibits improved performance in noisy environments and which exhibits the flat and wide bandwidth necessary for effective channel characterization while exhibiting a higher and more evenly distributed amplitude versus time characteristic than that provided by known, non-cyclic ghost cancellation signals.

Abstract: The invention relates to a method and an arrangement for improving the communication between a base station and a number of mobile units belonging to it in a microcell system. In greater detail, the invention has the aim of reducing the fading which is a problem when the mobile units are moving slowly or are standing still. According to the invention, the problem is solved by introducing diversity on the downlink in the system. The antenna pattern of the base station is turned so that a wave pattern varying with time is produced with spatially separated fading points. The communication preferably utilizes time-division multiple access (TDMA), the turning of the antenna pattern being effected step by step between suitable time slots, preferably between TDMA frames. The turning of the antenna pattern is advantageously produced by controlling a sectoral antenna (7, 7').

Abstract: An equalisation arrangement compensates for multipath phase and amplitude distortion effects in a transmission channel by first measuring such distortion effects as they occur over time in a calibration phase, thereby obtaining phase and amplitude compensation factors, then applying these factors to a correcting circuit situated in the signal path of the receiver at the appropriate times during a subsequent data transmission phase. Calibration is achieved by feeding a received calibration signal to an amplifier, preferably a successive detection logarithmic amplifier (26), comparing the phase of the limited linear output of the logarithmic amplifier with a delayed version of itself (30, 28), and using the amplitude information (39) in the logarithmic output (72) of the logarithmic amplifier (26) and the phase-change information (35) resulting from the phase comparison to compute the phase and amplitude compensation coefficients.

Abstract: A .pi./4 delay spread detection and compensation arrangement effectively detects distortion due to multipath delay spread in a digital channel and compensates for this distortion with minimum circuit complexity. An improvement in the bit error rate performance for a differential detector in the receiver is achieved through use of a detected direct current (DC) component, which is proportional to the amount of multipath delay spread on the digital channel. This DC component is used in the differential detector for compensating for this delay spread. In addition, the differential detector also provides a channel quality measure of the level of delay spread on the digital channel for use by the receiver in selecting either the differential detector or an equalizer for decoding of the data on the digital channel.

Abstract: A receiver (100) that includes an antenna (101) employs a method and apparatus for producing diversity gain of a signal (106) received by the antenna (101), wherein the signal (106) is subject to a time-variant fading condition. Upon receiving the signal (106), the receiver (100) determines a signal quality metric of the signal (106) and compares the signal quality metric to a quality threshold. When the signal quality metric is below the quality threshold, the receiver (100) orients a predetermined antenna pattern (304) of the antenna (101) to improve the signal quality metric, thereby producing the diversity gain.

Abstract: Signals from multiple signal paths are received using a multi-element antenna and a beam-forming network. Signals from each of the antenna elements are sampled to form a sample vector. Several sample vectors are used to form an auto-covariance matrix. A singular value decomposition of the auto-covariance matrix is used to form three matrices. The first matrix is used to determine the number of signal paths and the second matrix is used to form several polynomials. The polynomial roots that are on or near the unit circle are used to determine points on the unit circle that are associated with each signal path. Each point on the unit circle is used to calculate weights for a beam-forming network that forms a receive beam for each signal path.

Abstract: A radio paging system includes one or more transmitters (12) for sending radio signals to pagers (14) within a paging service area (10). Receivers (22) are located within the service area to receive low power transmissions from the pagers. These receivers are coupled to omnidirectional antennas (24, 26) that are disposed in pairs to form a two branch diversity antenna system. Each pair of antennas is located in a cell that forms part of a four cell cluster (66), and each pair of antennas is located on an axis (42) running through its cell. The axes are selected to provide a high and uniform level of diversity gain throughout the cluster.

Abstract: A method and apparatus for shaping the impulse response of a non-minimum phase communications channel, by reversing the CIR without enhancing the noise level using non-causal allpass filters is disclosed. Unstable allpass filters are implemented as stable non-causal filters operating in reversed time to obtain a minimum phase response from a non-minimum phase CIR. The original CIR is estimated using adaptive algorithms, and the coefficients of the allpass filters are taken directly from the estimated CIR. The cascade of the channel and allpass filter has an impulse response that is the approximate time reversed version of the original CIR. A "block" allpass equalizer and an "optimal" allpass equalizer are used in a two-stage filter to increase system performance. Other multiple stage filter configurations are disclosed, which can include use of decision feedback equalizer (DFE) decoders as well as Viterbi decoders.

Abstract: A first transmitter (103) reduces an interference in a time division duplex (TDD) communication system (100). The first transmitter (103) alternates from a transmit state in which a first information signal (130) is transmitted to a standby state in which a residual signal (132) is transmitted. The first transmitter (103) includes a transmitter section (142) and a controller (141). The transmitter section (142 transmits the first information signal (130) and the residual signal (132). The transmitter section (142) includes a power amplifier (144) to amplify the first information signal (130) and the residual signal (132) to a power level; and an exciter (143) to control a carrier frequency of the first information signal (130) and the residual signal (132). The controller (141) is coupled to the power amplifier (144) and the exciter (143), for adjusting the power level and setting the carrier frequency.

Abstract: Methods and systems for jointly demodulating CDMA signals are presented. The disclosed methods and systems provide, among other advantages, for accurate demodulation of such signals in multipath, time-dispersion environments. Single signal or multiple signal demodulation, with or without taking intersymbol interference into account can be accomplished according to the present invention.

Abstract: The present invention discloses a wireless LAN, a peer-to-peer wireless LAN, a wireless transceiver and a method of transmitting data, all of which are capable of operating at frequencies in excess of 10 GHz and in multipath transmission environments. This is achieved by a combination of techniques which enable adequate performance in the presence of multipath transmission paths where the reciprocal of the information bit rate of the transmission is short relative to the time delay differences between significant ones of the multipath transmission paths. In the LANs the mobile transceivers are each connected to, and powered by, a corresponding portable electronic device with computational ability.

Abstract: A system for combining AM and FM transmissions. In-band, On-channel, FM Digital Audio Broadcast (IBOC FM-DAB) allows simultaneous transmission of DAB and FM over existing FM allocations without interfering with conventional analog FM signals. The utility of existing FM spectrum allocations is therefore enhanced.

Abstract: Each path is identified by a delay which is applied to at least one spreading sequence for determining a correlation between the received signal ans said spreading sequence. For each selected delay, a reception energy, contained in the CDMA signal consisting of a correlation between the received signal and a reference spreading sequence, is estimated. Delays to be tested are randomly chosen with a choice probability having a maximum for a delay substantially equal to the smallest selected delay. The delays thus chosen are tested by estimating the reception energy associated thereto. When the reception energy for a tested delay is greater than at least one of the reception energies associated to the previously selected delays, the tested delay is substituted for the previously selected delay having the smallest reception energy.

Abstract: A method and apparatus in a data communication receiver (100) for diversity reception of a radio signal including a predictably repetitive, predetermined data bit pattern (304) comprises a processor (114) controlling (600) an antenna switch (106) to select between a first antenna feed (102) and a second antenna feed (104) as a momentary source of the radio signal during transmissions of the predetermined data bit pattern (304). The radio signal received from the momentary source is monitored by a data receiver (110) during the transmission of the predetermined bit pattern (304) to derive the data therefrom, and at least one bit error count is determined (604, 610) by the processor (114). After completion of the predetermined bit pattern (304), an antenna feed (102, 104) for the radio signal is selected (616, 620) in response to the at least one bit error count.

Abstract: A communication system (100) is provided for transmitting data to mobile receivers utilizing a subcarrier within a commercial FM channel of a radio station (55). The data transmitted is first encoded in encoder (112), utilizing a forward error correction code. The sequence of the encoded data is altered in interleaver (116), subdivided into a plurality of subframes, in framing and synchronization circuit (120), which also adds channel state bits to each subframe. The framed data is modulated onto the subcarrier in the differential quadrature phased shift keying modulator (130), the output of which is coupled to the FM modulator (52) of radio station transmitter (50). The transmitted radio frequency signals may be received by a vehicle antenna (12) for coupling to the vehicle's FM receiver (80). The modulated subcarrier is recovered from the FM demodulator (84) of the receiver (80), the modulated subcarrier being demodulated to recover the encoded digital data therefrom.

Abstract: A system for mitigating the effect of fading in a digital communication system. Deep signal nulls occur when signals from multiple paths are precisely aligned in amplitude and phase canceling the signal at the receiving antenna and creating a fade. In an indoor system, moving either transmitter or receiver a short distance can shift a fade away from the frequency of interest. This invention provides an actual or perceived movement of an antenna such that the effects of fading can be averaged over a time interval. The signal processing capability of modern digital transceivers can correct errors due to the time varying fades using the present invention. Thus, a dual-antenna element using the system of this invention can provide diversity receive and transmit paths to maintain capacity. The present invention may also provide improved capacity to due enhanced stability in power control.

Abstract: A method for processing electrical signals includes the steps of: storing a plurality of first data signals, each representative of an instantaneous amplitude of a first input signal in a memory; selecting one of the first data signals in response to a second input signal; combining the selected one of the first data signals with a second data signal representative of a subsequent instantaneous amplitude of the first input signal, to produce a difference signal; producing a first output signal in response to the difference signal; and combining the difference signal and the selected first data signal to produce a modified data signal and for replacing the selected one of the first data signals with the modified data signal in the memory. The method can be performed by an equalizer in a circuit for compensating for amplitude variations in a radio frequency signal or by a comb notch filter.

Abstract: A frequency modulation (FM) stereo broadcast receiver detects the onset of multipath interference based on phase changes in the received stereo pilot signal. In a preferred embodiment, the phase of the received pilot signal is compared to the phase of a regenerated pilot from a phase-locked loop in a stereo decoder circuit. The severity of a multipath event is determined by the phase shift detected at the onset of the multipath event.

Abstract: A plurality of input signals are processed (75) using a Hadamard transform (52) to produce a combined Walsh covered parallel sequence which is sampled in time. The time samples are commutated (76) by a switch (53) or multiplexer to form a Walsh covered serial sequence. The Walsh covered serial sequence is then quadrature phase shift keyed (77) and transmitted (79).

Abstract: An interference wave removing apparatus removing interference wave between angle diversity branches is disclosed. This apparatus is suitable for microwave communication beyond line-of-sight. The apparatus comprises a first reception unit for main beam, a second reception unit for angle beam, a first correlator for generating a first correlation signal corresponding to the first reception unit, a second correlator for generating a second correlation signal corresponding to the second reception unit, a first multiplier for multiplying an output of the first reception unit and the first correlation signal, a second multiplier for multiplying an output of the second reception unit and the second correlation signal, a subtractor for subtracting an output of the first multiplier from an output of the second multiplier, a channel quality compensator for compensating for an output of the subtractor for the channel quality, and a delay element for delaying an output of the channel quality compensator.

Abstract: A spread spectrum receiver for use in spread spectrum communication systems is disclosed. The spread spectrum receiver of the present invention includes an analog-to-digital converter for receiving an analog spread spectrum signal, and for converting the received analog signal to a digital signal. The spread spectrum receiver also includes a digital frequency offset correction device, for modifying the digital signal in accordance with a frequency correction term signal to thereby correct for frequency offset in the received analog signal. The spread spectrum receiver also contains a Rake receiver. The spread spectrum receiver searches through the modified digital signal to identify M of the main and multipath return signals having the strongest signal strengths.

Abstract: Circuit for recovery of a plurality of multipath direct sequence spread spectrum signal components received over a time span, wherein the multipath signal components are components of a signal generated from digital data comprised of data bits at a data bit rate modulated by a pseudo-noise code, consisting of a sequence of chips having a chip rate greater than the data bit rate, wherein the sequence of chips is a code repetition period equal or greater than the received time span of the multipath signal components. A plurality of correlators is provided. Each correlator has a first input for coupling to the multipath signal components, a second input, and an output. A pseudo-noise code generator generates a duplicate of the pseudo-noise code. A time delayer is coupled between the second inputs of a mulitiplicity of the correlators and the pseudo-noise code generator for delaying the duplicate of the pseudo-noise code by a multiple of a time delay equal to the time of a chip duration or less.

Abstract: The invention includes the concepts of early-prompt delay tracking, multipath correction of early-prompt delay tracking from correlation shape and carrier phase multipath correction. In early-prompt delay tracking, since multipath is always delayed with respect to the direct signals, the system derives phase and pseudorange observables from earlier correlation lags. In multipath correction of early-prompt delay tracking from correlation shape, the system looks for relative variations of amplitude across the code correlation function that do not match the predicted multipath-free code cross-correlation shape. The system then uses deviations from the multipath-free shape to infer the magnitude of multipath, and to generate corrections pseudorange observables. In carrier phase multipath correction, the system looks for variations of phase among plural early and prompt lags.

Abstract: Method of demodulating a signal including a plurality of time-dispersed signal components such as resulting from receiving in a multipath environment. A plurality of correlators are provided. The period of expected time uncertainty is scanned to identify the highest energy signal components. Each correlator is assigned to process one of the highest energy signal components. Each of the correlator outputs is pase corrected to compensate for carrier phase differences. The correlator outputs are coherently combined to provide a data output.

Abstract: In a digital radio communications system, a transmit signal is delayed and sent on a first diversity channel and a nondelayed version of the signal sent on a second diversity channel. At the receiving site, first and second diversity channel signals are recovered and normalized into first and second constant-amplitude signals. Predominant components of the constant-amplitude signals are phase-and-amplitude aligned with each other by feedback loops including phase-and-amplitude alignment circuits, an adder and an AGC amplifier. The output of the AGC amplifier or the output of the subtracter is selected and applied to a matched filter to which a decision feedback equalizer is coupled.

Abstract: A spread-spectrum noise canceller (182) is provided. A received phase and a received amplitude for a first (216) and a second (236) component of a received spread-spectrum signal (200) is determined. The second component (236) is structurally similar to [a replica of] the first component (216), but differs by being received at a different time, being transmitted along a different path, or having a different phase [which was received at a different time]. In addition, the spread-spectrum signal (200) includes a first and a second known signal. A portion of a spread-spectrum noise signal in the received signal (200) is canceled by generating an estimated signal (270) by spreading (260) the second known signal at the second component received phase (204) with the first known signal at the first component received phase (224) and adjusting a gain (268) of an integrated form of the spread second known signal as a function of the received amplitudes of the first (216) and the second (236) components.

Abstract: Method of processing a direct sequence spread spectrum communication system signal wherein digital data comprised of data bits is modulated by a first pseudo-noise code comprised of a sequence of chips, the pseudo-noise chip rate greater than said date bit rate, each chip having a chip period, said sequence repeating at a code period, and the pseudo-noise modulated data modualates a carrier frequency resulting in the signal. Digital samples are made of the signal. A second pseudo-noise code replicates the first pseudo-noise code. A first correlation is made of the sampled signal and the second pseudo-noise code. A second correlation is made of the sampled signal and the second pseudo-noise code delayed by one chip period. A first correlated signal energy value represents the signal energy of the first correlation. A second correlated signal energy value represents the signal energy of the second correlation.

Abstract: Apparatus for indicating the end of an acquisition procedure between a transmitter and receiver in a direct sequence spread spectrum communications system. A transmitter generates a first acquisition code having a first acquisition code period and comprised of digital bits at a first bit rate. A circuit modulates the first digital acquisition code by a pseudo-noise code repeating at a pseudo-noise code period and consisting of a sequence of chips at a chip rate greater than the first bit rate. The first acquisition code period is an integer multiple of the pseudo-noise code period. A circuit modulates a carrier by the first digital acquisition code modulated by the pseudo-noise code. The carrier modulated by said first digital acquisition code modulated by said pseudo-noise code is transmitted for a first period of time. A circuit synchronizes a receiver to the chip rate and carrier frequency. A circuit synchronizes the receiver to the pseudo-noise code period.

Abstract: A digital audio broadcasting system that is capable of providing a large number of high quality stereophonic channels to mobile receivers in an environment with severe multipath delay and fading. Optimum combination of frequency and time diversity to guarantee robust performance in the mobile channel, with its multipath delay and frequency-selective fading effects. The system is based upon a dynamic single channel per carrier assignment of each stereo channel to many carriers. Intersymbol interference degradations caused by multipath delay are mitigated via an adaptive equalizer in the receiver. This dynamic single channel per carrier system preserves the simplicity inherent in the single channel per carrier assignment while it incorporates the ability to address frequency-selective fading by providing substantial frequency diversity.

Abstract: A polarization diversity radio communication system suitable for radio communication in indoor space is provided for transmitting from a transmitter a series of data sequentially from one of two or more transmission antennas of the transmitter, and for reproducing the data received at a receiver through a single reception antenna of the receiver when the series data are received from the transmitter. Thus, the transmitted data from any part of the indoor space can be received efficiently, and a high S/N ratio can be assured on the reception side.

Abstract: A method for processing electrical signals includes the steps of: storing a plurality of first digital data signals each representative of an instantaneous amplitude of a first input signal in a memory; selecting one of the first digital data signals in response to a second input signal; combining the selected one of the first digital data signals with a second digital data signal representative of a subsequent instantaneous amplitude of the first input signal, to produce a digital difference signal; producing a first output signal in response to the digital difference signal; and combining the digital difference signal and the selected first digital data signal to produce a modified digital data signal and for replacing the selected one of the first digital data signals with the modified digital data signal in the memory. The method can be performed by an equalizer in a circuit for compensating for amplitude variations in a radio frequency signal or by a comb notch filter.

Abstract: A frequency modulation (FM) stereo broadcast receiver detects the onset of multipath interference and initiates corrective action to minimize the audible effects of multipath. Under an unacceptable multipath interference condition, a multipath detection signal disables the FM RF mixer thereby preventing any further energy in the multipath event from entering succeeding sections of the receiver. This action reduces noise generated by multipath interference and allows the output to return to normal in a shorter time. In another aspect of the invention, a phase-locked loop employed in a stereo decoder circuit is latched during the multipath disturbance in order to avoid rapid changes in the phase of the regenerated pilot employed in the stereo decoder.

Abstract: A modified RAKE receiver, referred to as a WRAKE receiver, is used in CDMA communications. It is particularly adaptable to a subtractive CDMA system. The receiver includes a radio receiver for receiving and demodulating a composite signal of overlapping transmitted signals (spread signals), a sampler to sample the received signals, a selector for selecting different groups of samples, a correlator for correlating the samples with known despreading codes and a comparator to determine the symbol transmitted. The receiver also includes a combiner which combines either different groups of samples together or combines different correlation values together. The sample groups selected are channel independent and combine data received from different time shifts. Likewise, the sample groups for each channel and the correlation values are time-of-arrival independent.

Abstract: A repeater diversity spread spectrum communication system provides substantially fade free communications between a transmitter (1) and a receiver (7). A transmitted signal is relayed through a plurality of linear communications repeaters (3-6) that produce copies of the transmitted signal, the copies each arriving through an independently fading signal path. The transmitter and/or the receiver may be mounted upon a mobile platform, and the plurality of repeaters may be terrestrial, or may be provided in satellites in low earth orbit or in geosynchronous earth orbit. The receiver processes the received signal copies to equalize them to one another in delay, frequency, and phase, and then combines the multiple received and equalized signal copies to produce a composite signal having a greatly reduced fading depth.

Abstract: In a receiving circuit of a mobile telephone apparatus the incoming baseband signals are demodulated and equalized so as to compensate for deficiencies (multipath propagation) of the radio medium between base station and mobile telephone apparatus. It is not, however, always necessary to equalize the demodulated signals, for instance at low symbol rates or in the case of monopath propagation. According to the invention, time dispersion of the received signals is estimated prior to demodulation, and demodulation with equalization (3a) is effected when the time dispersion is established to be higher than a given value, whereas demodulation without equalization (3b) is effected when the time dispersion is lower than this value. This will enable the power consumed by the receiving circuit to be reduced, particularly in the listening mode of the apparatus.

Abstract: A television receiver comprising a system for improved echo cancellation. The system features a superior ghost cancellation reference signal which exhibits improved performance in noisy environments and which exhibits the flat and wide bandwidth necessary for effective channel characterization whiel exhibiting a higher and more evenly distributed amplitude versus time characteristic than that provided by known, non-cyclic ghost cancellation signals.