Abstract: A synchronizing apparatus for a differential OFDM receiver that simultaneously adjust the radio frequency and sample clock frequency using a voltage controlled crystal oscillator to generate a common reference frequency. Timing errors are found by constellation rotation. Subcarrier signals are weighted by using complex multiplication to find the phase differentials and then the timing errors. The reference oscillator is adjusted using the timing errors. Slow frequency drift may be compensated using an integral of the timing error. Frequency offset is found using the time required for the timing offset to drift from one value to another.

Abstract: A method and system for demodulating received signals in radio communication systems. Pi/4-DQPSK modulated signals can be demodulated to provide additional quality measurements and to facilitate diversity combination or selection. For example, a carrier is modulated with digital data using Pi/4-DQPSK to convey two bits of data by changing the radio carrier phase from the value at the end of the last symbol through an angle of either .+-.45 degrees or .+-.135 degrees, these four possibilities representing the bit pairs 00, 01, 11 or 10. The transitions of the radio signal are filtered in the complex (I,Q) plane to limit the spectrum. At the receiver, the received signal is downconverted, filtered and amplified using a hard-limiting intermediate frequency (IF) amplifier. The IF amplifier also produces an approximately logarithmic indication of the signal strength before limiting. The hard-limited IF signal containing phase information is fed to a direct phase digitizer.

Abstract: At the transmit end of a secure communication system, an input data symbol is mapped to a corresponding vector in a two-dimensional phase plane. The vector is pseudorandomly phase-rotated in a first direction according to a unique pseudorandom number and quadrature-modulated on a carrier for transmission. At the receive end, a quadrature detector quasi-synchronously quadrature-detects the transmitted carrier with a local carrier to recover a vector, which is pseudorandomly phase-rotated according to a pseudorandom number identical to the unique pseudorandom number in a second direction opposite to the first direction. A quadrature demodulator detects a phase error of the local carrier with respect to the received carrier and provides quadrature-demodulation on the oppositely phase-rotated vector using the detected phase error.

Abstract: An in-phase component and a quadrature component of a digitized input DQPSK modulated signal are provided from interpolation filters, respectively. An instantaneous amplitude and an instantaneous phase of the input signal for each symbol clock are detected from the in-phase and quadrature components and are fed to adders. At the same time, that one of symbols "0, 0," "0, 1," "1, 0" and "1, 1" which corresponds to the instantaneous phase is detected by a symbol detector. An ideal reference signal generating part generates an ideal amplitude component and an ideal phase component corresponding to the detected symbol and provides them to a parameter calculating part. In an I-Q origin offset detecting part, an I-Q origin offset is calculated on the basis of the relationship between a triangle formed by the current detected vector corresponding to an ideal symbol and a vector detected one symbol clock before and a triangle formed by the said two vectors.

Abstract: This invention is a method for detecting received signal sequences of a communication system transmitting differentially encoded MPSK (Multiple Phase Shift Keying) signal sequences. This invention uses previously received signal samples and previously decided data phases to generate a phase reference for the current operation of detecting the received signal sample. The phase reference can be easily generated by a recursive form.

Abstract: The invention is related to a method and circuit arrangement for processing a received signal in a variable symbol rate system, such as a digital television system. In the method and arrangement according to the system, a received signal is sampled at a fixed sampling frequency (f.sub.f) that is higher than the symbol frequency of any one of the received signals. The resulting sample sequence is converted to another sample sequence the sampling frequency of which equals the symbol frequency (f.sub.i) of the received signal or its integer multiple. Then the samples are filtered (8) and signal value decisions are made (9) for the filtered samples. Conversion of the sampling frequency is advantageously performed using a so-called modified Farrow-type fractional delay filter (6) which is controlled using a control signal proportional to the delay of each sample.

Abstract: This invention is a method for the detecting received signal sequences of a communication system transmitting a differentially encoded MPSK (Multiple Phase Shift Keying) or a differentially encoded 2MAPSK (2M-level Amplitude/Phase Shift Keying) signal sequence. The operation of detecting the currently received signal sample is based on a signal reference which is recursively generated by two or more previously generated signal references.

Abstract: The circuit has two parallel channels for the processing of two components in phase (I) and in quadrature (Q). Each channel has filtering means (50(I)) and delay means (60(I)). The circuit also incorporates a multiplication circuit (70), an integration circuit (80) and a programming circuit (90). Several circuits of this type can be connected in cascade.

Abstract: A transmission system is disclosed which permits the receiving end to demodulate multi-valued modulated symbols successfully under fading conditions and reduces the amount of transmitted reference data to improve data transmission efficiency. At the transmitting end, a multiplexing section, a modulating section, and a transmitting section are provided. In transmitting an OFDM transmission frame, null symbols and reference symbols are placed in the beginning portion of the frame and QPSK symbols are placed in an information symbol data region in the frame with equal spacings in time and frequency. At the receiving end, a receiving section, a demodulation section, an equalizing section, and a demultiplexing section are provided. An error detector detects amplitude and phase errors of each carrier from the reference symbols, and a variation detector detects variations in the amplitude and phase of a received signal from the QPSK symbols.

Abstract: A method and apparatus for mitigating error in a received communications signal includes an error mitigation unit (245) which receives and performs mitigation based on an error indication (e.g., a CRC) and a quality indication (QI). The error mitigation unit may include a magnitude adjuster (220) for adjusting the input signal value (e.g., an ADPCM nibble) when the QI is less than a threshold, and a soft correction unit (240) for, when the QI is above the threshold, either soft correcting nibble magnitudes or muting selective nibbles based on further signal information such as differential phase.

Abstract: In a wireless telephone of a mobile communication system, which performs transmission in a series of bursts, in synchronism with a receive signal transmitted from a base station, each transmission burst being performed responsive to a transmission timing signal, a sync signal is detected from the receive signal, the transmission timing signal is generated on the basis of the sync signal, and each transmission burst is effected in accordance with the transmission timing signal. When the sync signal ceases to be detected, the transmission timing signal is autonomously generated for a certain time period after the sync signal is last detected. In another aspect of the invention, during reception in a first channel, the channel quality of another vacant channel is measured by transferring PLL data to a PLL prior to the slot in which the measurement is effected.

Abstract: The invention relates to an 8-PSK differential coder for trellis-coded modulation and to a corresponding pragmatic decoder, enabling 90.degree., 180.degree., and 270.degree. phase ambiguities to be lifted. The invention applies in particular to transmitting images at high rate by radio link.

Abstract: An apparatus and method for receiving spread spectrum signals, and, further, for decoding phase encoded information from such signals, requires correlation of input signals into real and imaginary components and determination of the phase angles of the received signal. A transmitter can divide an input signal into a plurality of data streams, independently modulate these streams, and then superpose the plurality of resultants for transmission. A receiver can receive the superposed signal and separate it into real and imaginary parts. In one embodiment, the receiver uses a plurality of spread spectrum codes to generate a plurality of real correlation signals and a plurality of imaginary correlation signals. A transmitter may also generate a differentially phase encoded signal. Phase encoding of a signal generally involves the imposition of known phase changes in the transmitted signal at selected intervals. Decoding of the phase changes at the receiver allows recognition of the phase encoded information.

Abstract: The phase of a received signal is detected by a phase detector at symbol intervals T relative to the phase of a local signal. The detected phase is input to delay circuits that are connected in series and each of which has the delay interval T. Phases .PSI..sub.n (where n=0, 1, . . . , N) with delays of 1 to N symbols are output to a metric calculating portion. The sum of a partial sequence {.DELTA..phi..sub.i ; i=n+1-q, n+2-q, . . . , n} of a N-symbol phase difference sequence candidate {.DELTA..phi..sub.n ; n=0, 1, . . . , N} is added to a detected phase .PSI..sub.n-q at a time point (n-q)T (where q=1, 2, . . . , N) so as to obtain an estimated value of the received signal phase .PSI..sub.n. The v-th power value of the absolute value of a difference .mu..sub.n (q) between the estimated value and the received signal phase is defined as a branch metric of q-symbol differential phase detection. .SIGMA..vertline..mu..sub.n (q).vertline..sup.v =.lambda..sub.

Abstract: Systems and methods according to the present invention provide a combination of demodulation and decoding, termed decodulation herein. By using knowledge of known symbols, decoding known symbols first, and then using the information obtained by decoding known symbols to decode unknown symbols, improved performance can be achieved. This technique can also be used to alleviate the conventional 3dB loss suffered systems using differentially coding and modulation as compared to coherent detection systems.

Abstract: A tuner selects one of channels at different locations in a frequency band used for transmitting VSB HDTV signals including symbol codes descriptive of digital signals. The tuner also includes mixers for performing plural conversion of the selected channel to a final intermediate-frequency signal, which is digitized by an analog-to-digital converter. A phase tracker, operative on narrow-bandpass filtered portions of the digitized final intermediate-frequency signal centering on its carrier frequency, suppresses an imaginary portion of the final intermediate-frequency signal, arising from multipath distortion or from phase incoherence in local oscillations used during frequency conversion(s).

Abstract: In a reference signal adaptive estimation part 15, a received signal sample z.sub.n-2 at time (n-2) is phase rotated by M states .DELTA..phi..sub.n-1 at time (n-1), and the phase-rotated signal and a received signal sample z.sub.n-1 are used to calculate the following linearly predicted value of the received signal sample at time (n-1) that contains a fading variation at time n,z.sub.n-1 =(1+.lambda.)z.sub.n-1 -.lambda.z.sub.n-2 exp(j.DELTA..phi..sub.n-1).A square error between a signal phase-rotated .DELTA..phi..sub.n from the linearly predicted value z.sub.n-1 and a received signal sample z.sub.n is calculated as a branch metric in a branch metric calculating part 16 and this branch metric is used for decoding in a Viterbi decoding part 17. A prediction coefficient .lambda.

Abstract: A system for transmitting FM signals comprised of apparatus for receiving an input data signal, apparatus for precompensating the received data signal, apparatus for applying the precompensated data signal to a constant envelope modulator to provide a modulated signal, apparatus for applying the modulated signal to a power efficient non-linear amplifier and transmitting a signal resulting therefrom, apparatus for receiving the transmitted signal in an I-Q receiver, and apparatus for filtering the received transmitted signal for the precompensation, to obtain an output data signal representative of the input data signal.

Abstract: A process for transmitting digital signals, in which the digital data are coded differentially in symbols of a symbol period T.sub.s (2.1, 2.2), the symbols transmitted with the Q component being delayed by T.sub.s /2, relative to those of the I component, to achieve a time-staggering. A cross interference of the I and Q components, occurring on the receiver end and caused by the differential coding nd the time staggering, is resolved with a trellis decoder (10), for example. As a result, a differential offset QPSK process achieved on this basis combines the advantages of DQPSK and OQPSK procedures.

Abstract: A method and an apparatus is provided for decoding a communicated radio frequency signal containing a sequence of phase values. The method includes the steps of amplitude limiting the communicated signal and maximum likelihood decoding the amplitude limited signal.

Abstract: An in-phase component and a quadrature component of a digitized input DQPSK modulated signal are provided from interpolation filters, respectively. An instantaneous amplitude and an instantaneous phase of the input signal for each symbol clock are detected from the in-phase and quadrature components and are fed to adders. At the same time, that one of symbols "0, 0," "0, 1," "1, 0" and "1, 1" which corresponds to the instantaneous phase is detected by a symbol detector. An ideal reference signal generating part generates an ideal amplitude component and an ideal phase component corresponding to the detected symbol and provides them to a parameter calculating part. In an I-Q origin offset detecting part, an I-Q origin offset is calculated on the basis of the relationship between a triangle formed by the current detected vector corresponding to an ideal symbol and a vector detected one symbol clock before and a triangle formed by the said two vectors.

Abstract: A circuit and method for demodulating a .pi./4-DQPSK (differentially quadrature phase shift keying) modulated signal. A receiving .pi./4-DQPSK modulated signal is shifted to a base band and simultaneously separated to an I channel and Q channel. The separated I channel and Q channel signals are converted into digital data. The digitally converted data is stored in a memory and decoded to binary data by phase comparison or Viterbi algorithm.

Abstract: In a digital wireless communication system operating between a first unit and a second unit, the first unit transmits a digitally encoded RF signal at a first frequency in a plurality of a non-contiguous time slots to the second unit. The second unit receives the digitally encoded RF signals. The received RF digitally encoded signal is converted to an intermediate frequency (IF). An A to D converter samples the received IF digitally encoded signals and generates a plurality of discrete binary symbols during one of the plurality of non-contiguous time slots. A phase error signal is generated for each one of the plurality of discrete binary symbols. A frequency error signal is generated for the subsequent symbol in accordance with .DELTA.f(n+1)=.DELTA.f(n)+g.sub.1 (.crclbar.(n)-.crclbar.(n)). The conversion of the RF signal to intermediate frequency is controlled in response to the frequency error signal.

Abstract: A system for transmitting FM signals comprised of apparatus for receiving an input data signal, apparatus for precompensating the received data signal, apparatus for applying the precompensated data signal to a constant envelope modulator to provide a modulated signal, apparatus for applying the modulated signal to a power efficient non-linear amplifier and transmitting a signal resulting therefrom, apparatus for receiving the transmitted signal in an I-Q receiver, and apparatus for filtering the received transmitted signal for the precompensation, to obtain an output data signal representative of the input data signal.

Abstract: A coherent phase-shift keying (PSK) detector in a receiver generates an unmodulated carrier signal, without attempting to synchronize the unmodulated carrier signal in frequency or phase to the carrier employed at the PSK transmitter. The instantaneous phase of the received PSK signal is detected with reference to the unmodulated carrier signal to create an instantaneous phase signal. Phase rotation due to frequency offset between the two carrier signals is detected and removed from the instantaneous phase signal, then a remaining phase offset is detected and removed. Data are recovered from the resulting instantaneous phase signal.

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 technique of demodulating a .pi./4-DQPSK composite carrier waveform using a non-coherent discriminator based receiver is presented. In particular, a means of recovering .pi./4-DQPSK modulated data symbols using a dual output discriminator in conjunction with a dual binary amplitude detection process in a discriminator based receiver is discussed. Means which improve the bit error rate of the receiver over the prior art are presented. Additionally, an amplitude detection means which readily provides synchronization of the detected data symbols is discussed which was not heretofore possible with the 4-level slicer of the prior art.

Abstract: A numerical value (Y-X).multidot.0.5 indicative of a phase angle .theta..sub.1 in the first quadrant is calculated on the assumption that a quadrant of an input vector signal (x+jy) is judged and the signal is rotated into the first quadrant by a rotational vector signal (a+jb) and the phase angle of 0 to .pi. is expressed by 0 to +2 and the phase angle of .pi. to 2.pi. is expressed by -2 to 0 as a prerequisite. Further, a numerical value c indicative of an offset phase angle .theta..sub.2 to return the vector signal into the original signal and the phase number indicative of each of the (N) divided phase regions of the phase plane is directly obtained as bit data.

Abstract: An automatic frequency control apparatus includes a frequency converter, an analog/digital converter, extractor, a storage unit, a controller, and a local oscillator. The storage unit stores unique word frequency modulation models representing time waveforms obtained by modulating, with a unique word, three frequencies which are a frequency equal to that of a desired intermediate-frequency signal, a frequency slightly higher than that of the desired intermediate-frequency signal, and a frequency slightly lower than that of the desired intermediate-frequency signal.

Abstract: A digital radio communications system employs a digital information source for providing digital information such as message bits, a transmitter for transmitting encoded digital information into data symbols in a radio-frequency (RF) signal to a plurality of antennae which sense the transmitted RF signal. A post detection measure of signal quality, the signal-to-impairment ratio, (SIR), is utilized by the receiver to perform post detection combining of signals received by a plurality of antennae. The antennae are coupled to a receiver which for each received signal: digitizes the signal, determines phase angles of the digitized signal, converts the signal to unit vectors, determines a signal-to-impairment ratio (SIR) estimates .gamma..sup.j of the digitized signals. The SIR estimate .gamma..sup.j is weighted by combining weight computation element and multiplied by each unit vector to provide an in-phase is component and a quadrature component for each signal.

Abstract: A DQPSK delay detection circuit is provided that can securely reproduce stable clock signal. An absolute value circuit ABS(14) calculates an absolute value of I signal. An absolute value circuit ABS(15) calculates an absolute value of Q signal. Subtraction circuit(16) generates a P signal according to the difference between the absolute values of I signal and Q signal. Zero-cross detection circuit(11) detects zero-cross timing of the P signal to input it as a timing signal to the DPLL(64). The zero-cross timing of the P signal can be detected even when the data pattern of I or Q signal makes it impossible to detect the zero-cross timing from I and Q signal. Because the zero-cross timing of the P signal has a variation less than that of the zero-cross timing determined from I or Q signal, it is becomes possible to reproduce stable clock signals and in turn reliability of data demodulation can be improved.

Abstract: A radio receiver directly digitizes the phase of an intermediate frequency (IF) signal with a desired resolution. The frequency of the reference oscillator in the direct phase digitizer is reduced when compared to the frequency previously required for the same resolution. The reduction in the reference oscillator frequency is accomplished by differentiating between IF zero-crossings that occur during the first half of a reference oscillator cycle and zero-crossings which occur during the second half of the reference oscillator cycle. The apparatus utilizes 2 zero-crossing detectors, the first zero-crossing detector is driven by a positive edge of the reference oscillator signal and the second zero-crossing detector is driven by a negative edge of the reference oscillator signal. Depending upon the alignment of the negative edge zero-crossing indicator and the positive edge zero-crossing indicator, the N-bit phase signal is modified or shifted by one-half a phase sector.

Abstract: A clock recovery circuit having a feedback system in a .pi./4 shift QPSK demodulator, comprising a signal state transition detector for detecting state transitions between consecutive symbols of the demodulated baseband signal which is formed from the detected baseband signal by .pi./4 reverse shifting. According to degree of the detected symbol state transition, the 1/2-symbol delayed baseband signal is shifted by the amount of .pi./8 in phase. According to direction of the detected symbol state transition, the .pi./8 phase shifted baseband signal is converted into an error signal in use for the feedback system. An oscillator generates a clock signal of a frequency controlled by the error signal such that the error signal is reduced in the feedback system.

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 receiver (30) in a digital communication system determines the phase error magnitude (60) and the magnitude error (54) for each received channel symbol and selects Reed-Solomon symbol erasures from the magnitude error and phase error magnitude values as a function of mobile unit speed (52) which is determined from demodulator data. The selected symbol erasures are implemented (78) in a Reed-Solomon decoder resulting in improved decoding performance.