Showing papers on "Demodulation published in 1989"

Computational complexity of optimum multiuser detection

Abstract: Optimum centralized demodulation of the independent data streams transmitted simultaneously by several users through a Code Division Multiple-Access channel is considered. Each user sends an arbitrary assigned signal waveform, which is linearly modulated by symbols drawn from a finite alphabet. If the users are asynchronous, the optimum multiuser detector can be implemented by a Viterbi algorithm whose time-complexity is linear in the number of symbols transmitted by each user and exponential in the number of users. It is shown that the combinatorial problem of selecting the most likely transmitted data stream given the sufficient statistics (sequence of matched filter outputs), and the signal energies and cross-correlations is nondeterministic polynomial-time hard (NP-hard) in the number of users. And it remains so even if the users are restricted to be symbol-synchronous.

Modulation and demodulation system employing AM-PSK and FSK for communication system using digital signals

Abstract: A communication network having a first broadband communications channel or transmitting bus and a second broadband communications channel or receiving bus, wherein the first broadband communications channel is for passing information upstream to a head end and the second broadband communications channel is for passing information downstream from the head end to remote nodes. All nodes of the system are coupled to both the first communications channel and the second communications channel. According to the invention, modulation of signals applied to the first communication channel is optimized for burst communication from a plurality of sources to central control at the head end, and modulation applied to the second communications channel is optimized for continuous communication from the central source to a target node.

A systematic approach to carrier recovery and detection of digitally phase modulated signals of fading channels

Abstract: The problem of optimal carrier recovery and detection of digitally phase modulated signals on fading channels by using a nonstructured approach is presented, i.e. no constraint is placed on the receiver structure. First, the optimal receiver is derived for digitally phase-modulated signals when transmitted over a frequency-nonselective fading channel with memory. The memory results from the fact that usually the coherence time of the channel is larger than the symbol period. Symbols adjacent in time cannot be detected independently and therefore the well-known quadratic receiver is not optimal in this case. A maximum a posteriori (MAP) detector is derived and explicitly utilizes the channel memory for carrier recovery. The derivation shows that the optimal carrier recovery is, under certain conditions, a Kalman filter. Some attractive properties of this carrier recovery unit (including the absence of hang up) are discussed. Then the error rate of several digital modulation schemes is calculated taking the performance of the filter into account. The differences in susceptibility of the modulation schemes to carrier phase jitter are specified. >

Magnetic disk apparatus

Abstract: A method of forming servo information in a magnetic disk apparatus of a sector servo system. Servo information is recorded in advance at the beginning of each of the sectors, and includes a DC erase signal, a track address signal and a burst signal. The track address signal indicates on which track of the magnetic disk the sector is located, and is formed by expressing the track address in Gray code and interleaving a dummy code into the Gray code. Servo information is recorded using an NRZI system or an RLL coding method. Servo information in which the dummy code is interleaved is decoded after the dummy code is removed at the time of demodulation.

Efficient digital frequency division multiplexed signal receiver

Abstract: A digital intermediate frequency (IF) receiver for frequency division multiplexed (FDM) signals including analog circuitry for receiving FDM signals and an analog-to-digital (A/D) converter for converting the received signals to a sampled digital received signal. A digital complex mixer responsive to the digital output of the A/D converter translates the spectrum of the sampled digital received signal to center the desired FDM channel at zero frequency (DC). Digital low pass filtering isolates the desired channel centered at DC, and a digital complex mixer can be used to translate the isolated selected channel to a predetermined IF frequency. The in-phase portion of the digital IF centered selected channel or the DC centered complex envelope selected channel can then be provided to appropriate demodulation or decoder networks.

Direct conversion FM receiver

Abstract: A direct conversion FM receiver that comprises a quadrature mixer, an A to D converter, a digital demodulator, a digital phase lock loop, and frequency offset setting means is disclosed. The quadrature mixer receives FM signals and mixes them with a local oscillator signal to produce an analog in-phase baseband signal and an analog quadrature baseband signal. The analog in-phase baseband signal and the analog quadrature baseband signal are converted into digital signals by the A to D converter. The digitally converted baseband and quadrature signals are digitally demodulated and filtered by the digital demodulator such that modulation information is produced. The modulation information is used, at least in part, to control the phase lock loop means. In addition, the phase lock loop is offset by the frequency offsetting means such that the desensing components of the analog devices are minimized, thus, substantially reducing the need for tight component tolerance of DC sensing components and the need for manufacturing adjustments of DC desensing components.

Range imaging laser radar

Abstract: A laser source is operated continuously and modulated periodically (typicy sinusoidally). A receiver imposes another periodic modulation on the received optical signal, the modulated signal being detected by an array of detectors of the integrating type. Range to the target determined by measuring the phase shift of the intensity modulation on the received optical beam relative to a reference. The receiver comprises a photoemitter for converting the reflected, periodically modulated, return beam to an accordingly modulated electron stream. The electron stream is modulated by a local demodulation signal source and subsequently converted back to a photon stream by a detector. A charge coupled device (CCD) array then averages and samples the photon stream to provide an electrical signal in accordance with the photon stream.

Automatic modulation recognition of digitally modulated signals

Abstract: A modulation recognizer that automatically reports modulation types of constant-envelope modulated signals is developed using zero-crossing techniques. The zero-crossing sampler, as a signal conditioner, has the advantage of providing accurate phase transition information over a wide dynamic frequency range. Signal parameters, such as zero-crossing variance, carrier-to-noise ratio (CNR), and carrier frequency, are estimated. Phase-difference and zero-crossing-interval histograms play the roles of features for modulation recognition. The classifier performance is given in the form of a confusion matrix. The obtained simulation results demonstrate that reasonable average probability of correct classification is achievable at CNR=15 dB and higher. >

An alternative approach to reduced-complexity CPM-receivers

Abstract: By separating the two complexity problems connected with optimum coherent CPM (continuous phase modulation) receivers, namely, the number of linear filters and the number of memory states required, a straightforward procedure for reducing the complexity is proposed and its usefulness is evaluated. A concise analysis of the inherent trellis encoder associated with CPM is given. It is then demonstrated that for almost all schemes of interest in practice, it is quite sufficient for the receiver to implement only four or six linear filters which represent proper reference signals. For a reduction in the number of memory states, decision-feedback sequence estimation together with minimization of the unprocessed intersymbol interference is proposed. Modifications of this procedure allow a state reduction without losses or with only negligible losses because only error events with large distances are affected. Combinations of these methods make possible an almost continuous tradeoff between receiver complexity and SNR (signal/noise ratio) losses. Several examples are considered for which evaluations of minimum Euclidean distances and simulation results are given. >

Signal discrimination system

Abstract: The invention provides a system for discriminating between modulated backscattered signals based upon transmission-to-reception time delay. The system employs a carrier, phase-modulated with a modulating signal, having characteristics which provide unity output when correlated with itself with zero time shift and a substantially lower output level when correlated with itself with a significant time shift. The system works with a backscatter-modulator located a finite distance from the transmitter for receiving and modulating the backscatter of the carrier. A receiver/detector receives and detects the modulated backscattered phase-modulated signal, the detector being phase-sensitive and having as a reference signal the transmitted phase-modulating carrier signal, and having as an input signal the phase modulated backscattered carrier. The detector provides an output signal whose averaged amplitude is substantially dependent upon the degree of phase correlation between the phase of the modulation returned input signal and the reference signal. Finally, the output signal from the detector is selectively processed only when its averaged amplitude is above a threshold level. In a preferred embodiment, the transmitted signal is a spread spectrum signal.

Homodyne down-converter with digital Hilbert transform filtering

Abstract: A homodyne down-converter, for use in an IF signal demodulator and the like, includes an analog-to-digital converter (ADC) which receives the IF analog signal for conversion to a digital data stream by sampling at a sampling rate frequency substantially equal to four times the IF signal frequency. A digital mixer controllably inverts the sampled data to convert the data to baseband, before a circuit removes the effects of DC offset in the analog IF signal applied to the ADC. A discrete Hilbert Transform filter is used for generating streams of sequential in-phase I' and quadrature-phase Q' data words, which are resampled to temporally align the two data word streams at a new data rate, thus effectively removing sample offset without the need for separate misalignment correction circuitry.

Asynchronous digital correlator and demodulators including a correlator of this type

Abstract: A demodulator to demodulate a complex signal formed by two carriers, in quadrature, the spectrum of which is spread by a pseudo-random signal, and which are modulated by an M-ary, Walsh, code comprises: a chain of registers to store 256 bits of a pseudo-random signal; a multiplexer to temporally multiplex the processing of the two carriers; a chain of correlation macro-cells having a systolic structure enabling temporal multiplexing and having intermediate outputs providing for partial correlation values on 16 chips; a device for the computation of linear combinations of the functions of partial correlation, to compute two functions of correlation corresponding to two carriers for each M-ary code; a demultiplexing and computation device to compute the module of the function of correlation for each of the Walsh codes which may modulate the signal to be demodulated; a device for the selection of the greatest value of the correlation functions and for the selection of the corresponding piece of data.

Pulse width modulation amplifier circuit

Abstract: A balanced transformer-less pulse width modulation amplifier circuit has two filter coils wound on the same core with the same polarity relative to the output. The arrangement reduces the possibility of magnetic saturation of the core and permits the cutoff frequency to be within the audio range.

Direct conversion FM receiver

Abstract: A direct conversion receiver for demodulating an FM signal converted frequency down converts the signal to quadrature related baseband signals (I,Q). These signals are applied by way of respective d.c. blocking capacitors (16, 18) and differentiating circuits (26, 28) to a differential arctan demodulator (20). The action of the differentiating circuits (26, 28) is to remove any d.c. component in the I and Q signals and to perform a frequency shaping which enables the demodulator (20) to recover correctly the modulating information. Measures are disclosed to overcome the effects of 180 degree phase jumps which will occur when the direction of rotation of the phasor reverses and of frequency dependent amplitude scaling introduced into the differentiated signals (I', Q').

Full duplex transponder system

Abstract: A full duplex transponder system includes an interrogation apparatus and a transponder apparatus. The transponder apparatus includes a reception antenna for receiving an interrogation signal wave whose carrier wave is phase- or frequency-modulated with an interrogation signal, a demodulator for demodulating the interrogation signal from the interrogation signal wave received by the antenna, a rectifier for rectifying the carrier wave of the interrogation signal wave received by the antenna and outputting a DC power used as a drive power source of the transponder apparatus, a multiplier for generating a predetermined harmonic component from the carrier wave of the interrogation signal wave received by the antenna, a modulator for amplitude-modulating the harmonic component with a response signal, and a transmission antenna for transmitting a response signal wave output from the modulator. The interrogation apparatus includes an oscillator for generating the carrier wave, a modulator for phase- or frequency-modulating the carrier wave generated by the oscillator with an interrogation signal, a transmission antenna for transmitting an interrogation signal wave output from the modulator, a reception antenna for receiving the response signal wave output from the transmission antenna of the transponder apparatus, and a demodulator for demodulating the response signal from the response signal wave received by the reception antenna.

Adaptive carrier recovery systems for digital data communications receivers

Abstract: Adaptive or predictive carrier recovery systems, which are essential in high-performance quadrature-amplitude-modulated (QAM) data communications systems to correct for phase jitter and frequency offset, are considered. Analytical and experimental results are presented for two structures that implement a predictive carrier recovery system. These systems, which adapt their structure to match the spectral properties of the impairments, avoid the conflict between a wide bandwidth (to track fast jitter) and a narrow bandwidth (to minimize output noise) inherent in most carrier recovery loops. Such a system increases the likelihood that very bandwidth-efficient modems (e.g., 7 b/s/Hz for 19.2 kb/s voiceband modem applications) can provide reliable transmission in the presence of severe phase jitter and frequency offset. In particular, the predictive carrier recovery systems can track sinusoidal jitter present at more than one frequency as well as generalized time-varying phase jitter processes. Both finite-impulse-response (FIR) and infinite-impulse-response (IIR) adaptive phase tracking systems are considered. Prior limitations on adaptive IIR filters are overcome by designing a structure that is guaranteed to be stable and to possess only a global minimum as the filter coefficients converge to their desired values. >

Digital cordless telephone systems

Abstract: A digital cordless telephone system which comprises a primary station (PS) controlled by a system controller (14 or 15) and a plurality of secondary stations (SS) capable of communicating with the primary station by way of a time division duplex radio link, the primary and/or secondary station having means to generate a beacon signal which is receivable by a secondary station outside the range of normal speech communication and the beacon signal comprising a low bit rate signal which is transmitted at a power comparable to the digitized speech signal. In one embodiment the beacon signal is recovered using a narrowband filter and a demodulator. In another embodiment direct sequence spread spectrum techniques are used to send the beacon signal and a correlator/matching circuit is used to recover the low bit rate data representative of the beacon signal.

Performance of interleaved trellis-coded differential 8-PSK modulation over fading channels

Abstract: The performance of trellis-coded differential octal phase-shift keying (coded 8-DPSK) with differentially coherent detection and soft-decision Viterbi decoding is investigated. A suitable receiver is presented whose signal processing is based on Nyquist signaling, requiring only one complex sample per modulation interval. Symbol synchronization and automatic frequency control are performed in a decision-directed way. Bit-error-rate (BER) performance over Gaussian, Rayleigh, and Rician channels is determined by means of computer simulations. The performance of coded 8-DPSK on the Gaussian channel is shown for a four-state convolutional trellis code. The unquantized outputs of up to three symbol detectors with delays of 1, 2, and 3 symbol periods are used for metric computation. The coding gain which includes losses due to timing and frequency synchronization errors is found to be 2.5 dB at BER=10/sup -5/ with respect to uncoded 4-DPSK. Much larger gains are achieved for fading channels if interleaving is applied. Using an eight-state trellis code the performance is determined on Rayleigh and Rician channels for various Doppler spreads and interleaver sizes. >

Spread spectrum communication system employing composite spreading codes with matched filter demodulator

Abstract: A hybrid despread and demodulation receiver for low symbol rate communications employs a passive (SAW) matched filter to remove a "short" coding portion of a composite spreading code that has been used to spread the data signal. The composite spreading sequence is formed by multiplying different length coding sequences, thereby obtaining an overall signal processing operator the duration or symbol span of which is sufficient to maintain a high signal processing gain, but is considerably longer than can be processed using a practical sized passive (e.g. SAW) filter design. The design of the receiver takes advantage of the fact that the relatively short sequence can be despread using a practical SAW structure and is comprised of a hybrid signal processor, the front end of which contains a compact SAW matched filter and the downstream end of which is implemented using analog processing components. The matched filter removes the relatively short spreading sequence from the received signal and feeds its output to a mixer, which combines the output of the matched filter with the longer coding sequence to complete the despreading process. The despread signal is then differentially coherently decoded and coupled to an integrate and dump circuit, which accumulates the energy in successive long code symbol intervals in order to determine the value of the respective data bits.

Direct microwave modulation and demodulation device

Abstract: A microwave circuit intended for either direct modulation or direct demodulation of a microwave link communications signal is characterized in that a local oscillator (8) is incorporated in the circuit. The local oscillator supplies two signals that are in phase opposition at 180 degrees. Each signal is sent to a phase shifter. The first phase shifter (9) supplies two signals that are in quadrature, at 0 and 90 degrees. The second phase shifter supplies two signals in quadrature at 180 and 270 degrees. The 0 and 180 degree signals are sent to a first mixer (2), while the 90 and 270 degree signals are sent to a second mixer (3). In the case of a modulator, the first and second mixers receive, respectively, in-phase and quadrature signals, and the outputs of the mixers are combined to form a modulated signal. In the case of a demodulator, a modulated signal is applied through a ÷2 divider to inputs of the two mixers, and in-phase and quadrature signals are obtained at respective mixer outputs.

DPSK carrier acquisition and tracking arrangement

Abstract: A QDPSK communication receiver includes a frequency source, apparatus for translating a received QDPSK signal having inphase I and quadrature phase Q components to baseband and a baseband demodulator. For each symbol period of the baseband QDPSK signal, a signal (I diff ) representative of the inphase component of the phase different between immediately successive received symbols and signal (Q diff ) representative of quadrature phase component of the phase difference between immediately successive received symbols is formed. In order to synchronize the frequency of the receiver frequency source to carried frequency of the received QDPSK signal, a preamble interval is defined during which a sequence of identified symbols is received. The inphase difference component signals of the symbols of the preamble interval are compared with the quadrature difference component signals of the symbols of the preamble interval to generate a plurality of signals representative of a set of predetermined relationships between the inphase difference component signals and the quadrature phase difference component signals. The frequency of the receiver frequency source is adjusted responsive to the plurality of predetermined relationship signals.

A new quadrature sampling and processing approach

Abstract: A quadrature sampling and array signal processing technique that differs from earlier approaches in that is processes the data before the Hilbert transformation is presented. A fast Fourier transformation (FFT) technique that performs the discrete Fourier transformation (DFT) on the sampled data directly without Hilbert transformation is proposed for frequency-domain signal processing. For array signal processing, the proposed approach does not perform Hilbert transformation prior to signal combining. It features high processing speed, low distortion, and hardware simplicity. Error analyses, performance evaluation, and computer simulation results are included. >

Narrow-band interference rejection using real-time Fourier transforms

Abstract: An analysis is presented of a BPSK-PN (binary phase shift-keyed with pseudonoise) demodulator when present narrowband interference is attenuated using transform domain filtering. The signal of interest is a wideband direct-sequence spread-spectrum signal (DS-SS). The interference is a narrowband signal which may be of high power and which is either intentionally or unintentionally collocated in frequency and interfering with the signal of interest. The effect of windowing the input signal by a rectangular or Hamming windows as well as the use of an overlap and save scheme is presented. It is shown that the Hamming window far outperforms the rectangular window, and that the interference is for all purposes completely eliminated when the Hamming window is used in conjunction with the overlap and save scheme. The criterion used to evaluate the effectiveness of the different interference rejection techniques is the bit error rate (BER) as measured at the output of the demodulator. >

Phase ambiguity resolution for offset QPSK modulation systems

Abstract: A demodulator for OQPSK signals modulated with two unique words resolves eight possible cominbations of phase ambiguity which may product data error by first processing received I R and Q R data in an integrated carrier loop/symbol synchronizer using a digital Costas loop with matched filters for correcting four of eight possible phase lock errors, and then the remaining four using a phase ambiguity resolver which detects the unique words to not only reverse the received I R and Q R data channels, but to also invert (complement) the I R and/or Q R data, or to at least complement the I R and Q R data for systems using nontransparent codes that do not have rotation direction ambiguity.

Analysis of quadrature detectors using complex envelope notation

Abstract: Quadrature detection is a common technique for demodulating radio and microwave signals in which the signal is demodulated to form two orthogonal components. Complex envelope notation is a compact method of representing the modulation of a real signal in complex form. The relationship between these two concepts is discussed, and there follows an analysis of quadrature demodulation including the effects of gain mismatch, phase error, local oscillator breakthrough and DC offsets.< >

Method and system for a synchronized pseudo-random privacy modem

Abstract: A method and system for a synchronized pseudorandom privacy modem makes use of a pseudorandom signal generator to index into a table of N signal spaces thereby altering gain and phase modifiers so or to modify both the gain and phase of portions of the data signal being transmitted from a QAM modem and thus transmit an encrypted signal pattern to the other end of the communications channel. The modem transmitter circuit includes a data scrambler, and QAM signal point mapper, connected in series, as well as a pair of mixers having inputs from the pseudorandom signal generator and functioning to modify the gain and phase of portions of the QAM signal point mapper output signal before they are pulse amplitude modulated, filtered, converted to analog form, and transmitted over the communications channel. Complementary circuit elements in the receiver, which have been synchronized with those of the transmitter by baud rate counters at both locations, demodulate and decode the received encrypted signal.

Self-test system and method for external programming device

Abstract: A communication system includes self-test means for automatically testing its performance. The communication system is adapted for use between an implantable device, such as a pacemaker, and an external device, such as the pacemaker's programmer. The communication system includes in the external device a transmitter circuit for transmitting an output signal through an antenna, and a receiver circuit for receiving an input signal through an antenna. A driver circuit controls the operation of the transmitter circuit. A microprocessor, connected to the driver circuit, controls the information content and timing of the transmitted output signal. A bandpass filter and AM demodulator process the input signal received through the receiver circuit. A multiplexer selectively allows one of a plurality of status signals from throughout the communication system within the external device, including the demodulated signal obtained from the AM demodulator, to be connected to the microprocessor. The self-test means includes means for having the microprocessor generate a test signal that is selectively injected into the transmitter circuit and/or the bandpass filter. The self-test means further includes a switch that selectively allows, as controlled by the microprocessor, either the output signal from the bandpass filter, or the output signal from the transmitter circuit, to be connected to the input of the AM demodulator. In this manner, the injected test signal, or the effects of the injected test signal, may be coupled back to the microprocessor via one of the status signals. The microprocessor then checks the status signals to ensure that a desired performance criteria is achieved. Individual tests may be selectively performed, or a prescribed sequence of several tests may be automatically performed.

Interrogation and remote control device; process for operating and using said device

Abstract: Said device (AFG) comprises two separate partial devices (A, B) interacting through antenna arrangements (1, 11). The first partial device (A) includes a high-frequency generator (4) for producing a carrier frequency, a high-frequency line (3) leading to the antenna (101) and a high-frequency demodulator (56). The latter includes a directional coupler (5) and a detector diode (6), both connected upstream from a discriminator and signal-processing unit (8). Power is supplied to said partial device (A) by a supply unit (58). The second partial device (B) includes a high-frequency direct current converter (14), a voltage-controlled resistance (16), a modulator (62) and a signal transmitter (19). Four-wire networks (102, 112) make the antenna arrangements (1, 11) self-resonant as regards carrier frequency; said arrangements are located in each other's near fields. The carrier frequency emitted by the first antenna (101) is received by the second antenna (111) and converted to direct current (i). The signal (S) produced by the signal transmitter (19) modulates the direct current (i) by means of a corresponding subcarrier frequency (f), resulting in an absorption modulation which retroacts on the first antenna (101) and on the high-frequency demodulator (56). The latter delivers the corresponding subcarrier frequency (f), from which the discriminator and signal-processing unit (8) derives an output signal (SS) corresponding to the signal (S) emitted by the signal transmitter (19). Said device can be used wherever signal transmitters which cannot be accessed by direct transmission, in particular measuring probes, are to be interrogated, for example in the case of signal transmitters implanted in living organisms.

Optically readable record carrier of the inscribable type, apparatus for manufacturing such a record carrier, and apparatuses for recording and/or reading information on/from such a record carrier

Abstract: A record carrier (1) comprises a disc-shaped carrier provided with a radiation-sensitive layer (6) and is formed with a servo track (4). The servo track exhibits a track modulation in the form of a radial wobble whose frequency is modulated with a position-information signal (Fig. 2). Also revealed is an apparatus (Fig. 8) for forming the track pattern during manufacture of the record carrier (1). When an information signal (Vi) is recorded on the record carrier (1) and the recorded signal is read by means of a recording and/or read apparatus (Fig. 4) the position-information signal (Fig. 2) is recovered by means of an FM demodulator device (60) from variations in the scanning beam (55) which are produced by the track modulation. Moreover, a clock signal for the purpose of scanning-velocity control is recovered from said variation in the scanning beam. Furthermore, embodiments of the record carrier which are very suitable for recording EFM-modulated signals are described.