Showing papers on "Demodulation published in 1997"

Bit-interleaved coded modulation

Abstract: It has been recognized by Zehavi (1992) that the performance of coded modulation over a Rayleigh fading channel can be improved by bit-wise interleaving at the encoder output, and by using an appropriate soft-decision metric as an input to a Viterbi (1990) decoder. The paper presents in a comprehensive fashion the theory underlying bit-interleaved coded modulation, provides tools for evaluating its performance, and gives guidelines for its design.

The role of synchronization in digital communications using chaos. I . Fundamentals of digital communications

Abstract: In a digital communications system, data is transmitted from one location to another by mapping bit sequences to symbols, and symbols to sample functions of analog waveforms. The analog waveform passes through a bandlimited (possibly time-varying) analog channel, where the signal is distorted and noise is added. In a conventional system the analog sample functions sent through the channel are weighted sums of one or more sinusoids; in a chaotic communications system, the sample functions are segments of chaotic waveforms. At the receiver, the symbol may be recovered by means of coherent detection, where all possible sample functions are known, or by noncoherent detection, where one or more characteristics of the sample functions are estimated. In a coherent receiver, synchronization is the most commonly used technique for recovering the sample functions from the received waveform. These sample functions are then used as reference signals for a correlator. Synchronization-based receivers have advantages over noncoherent ones in terms of noise performance and bandwidth efficiency. These advantages are lost if synchronization cannot be maintained, for example, under poor propagation conditions. In these circumstances, communication without synchronization may be preferable. The main aim of this paper is to provide a unified approach for the analysis and comparison of conventional and chaotic communications systems. In Part I, the operation of sinusoidal communications techniques is surveyed in order to clarify the role of synchronization and to classify possible demodulation methods for chaotic communications.

Multimode digital modem

Abstract: A modem operating selectively in the voice frequency and higher frequency bands which supports multiple line codes. A DSP is used to implement different existing ADSL line codes on the same hardware platform. The modem negotiates in real time for a desired line transmission rate to accommodate line condition and service cost requirements which may be implemented at the beginning of each communication session by exchange of tones between modems. A four step MDSL modem initialization process provides line code and rate compatibility. The handshake protocol and receiver algorithm for CAP based MDSL modems allows reliable modem synchronization over severely amplitude distorted channels and makes use of a short length sequence to train a synchronizing equalizer at the receiver. The algorithm and corresponding training sequence to train the transmitter filter are provided. After training to this sequence, a matched filter or correlator detects the inverted sync sequence. Detection of the inverted sequence signals commencement of normal reference training of the demodulation equalizers. An internal state machine in an MDSL modem records and monitors line status and notifies state change to other MDSL and host processor. The protocol for exchanging line connection management messages is a simplified LCP for MDSL. In a DMT system, a transmitter filter reduces the length of effective channel impulse response. Iimplementation of the filter combines time domain convolution and frequency domain multiplication to reduce needed computation power. The filter coefficients update may occur through a feedback channel.

Linear transmitter using predistortion

Abstract: A linear transmitter (101) using predistortion includes a modulator (103), a predistorter (107), a digital quadrature modulator (111), an upconverter (113), a power amplifier (115), and an antenna (117). In addition, the transmitter (101) has a feedback loop including a coupler (119), a downconverter (123), a digital quadrature demodulator (125), and a trainer (131). The digital data to be transmitted is provided into the modulator (103), which converts the digital data into in-phase and quadrature component signals. The in-phase and quadrature component signals are then provided to the predistorter (107), which "predistorts" the component signals prior to amplification. The digital quadrature modulator (111) converts the component signals into a single analog signal. The upconverter (113) upconverts this signal from the predistorter (107) into the desired frequency of transmission, which is provided to the power amplifier (115) and the antenna (117) for amplification and broadcast. The coupler (119) provides a portion of the amplified signal to the analog downconverter (123), which lowers the frequency of this signal to a range that is easily processed. The signal is then provided to the digital quadrature demodulator (125), which outputs the in-phase and quadrature component signals of the signal. These in-phase and quadrature component signals are provided to the trainer (131) which analyzes them with the output signals from the modulator. The trainer (131) compares these signals and updates the predistorter (107) so that the digital quadrature demodulator (125) output signals are substantially equivalent to the modulator (103) output signals.

Non-contact system for sensing and signalling by externally induced intra-body currents

Abstract: A wireless system includes one or more transmitters and one or more remote receivers that are respectively coupled through one or more users and room ground. The transmitters each produce low-frequency, low power signals that, through capacitive coupling, pass as displacement currents into and from the body of the user, which acts as a conductive node. A receiver that couples capacitively to the user responds to the displacement currents and reproduces the signals. The transmitter includes a signal generator and a pair of electrodes. The signal generator produces modulated signals that vary the voltage between the electrodes, a first one of which is closely coupled capacitively to the user's body such that the "quasi-electrostatic" field resulting from the electrode potential causes a displacement current to pass to the user's body. The second electrode is oriented so that its coupling to the room ground is stronger than that of the first electrode, such that room ground acts as a return path for the current from the receiver. The signal generator may modulate the information to be transmitted using, for example, a pseudorandom code, to produce spread spectrum signals. This increases noise immunity and allows multiple transmitters, each using a different modulation code, to operate at the same time. The receivers each include a detector/demodulator that acquires and tracks the signals from the one or more transmitters.

Method and hand-held apparatus for demodulating and viewing frequency modulated biomedical signals

Abstract: This invention is a method and apparatus for transmission of biomedical waveform data from a patient to an attending physician, wherein the waveform data audio signal (30) is frequency modulated for subsequent wire line or wireless transmission to a remote hand-held computer that functions to digitize (36), record (42), and demodulate (40) the frequency modulated signal for display (18) on the computer, for permanent print-out, or for further retransmission.

Dual-mode digital FM communication system

Abstract: A dual-mode digital communication system for communicating an information signal during operation in frequency-modulated (FM) and multiple-access modes is disclosed herein. The digital communication system includes a dual-mode transmitter for transmitting the information signal using an FM communication signal during FM mode operation, and for transmitting the information signal using a multiple-access communication signal during multiple-access mode operation. The communication system further includes a dual-mode receiver for receiving the FM communication signal during FM mode operation, and for receiving the multiple-access communication signal during multiple-access mode operation. Incorporated within the dual-mode receiver is a digital demodulator for recovering the information signal from the received FM signal during operation in the FM mode, and for recovering the information signal from the received multiple-access signal during multiple-access mode operation. In a preferred implementation the dual-mode transmitter is disposed to convert a first sequence of binary data within the information signal into a sampled modulation waveform, and to provide the FM communication signal by modulating a carrier signal based at least in part on the sampled modulation waveform. The dual-mode transmitter may also be configured to multiplex a second sequence of wideband message data with the sampled modulation waveform so as to form a composite FM modulation waveform.

A high efficiency carrier estimator for OFDM communications

Abstract: In OFDM communications, the loss of orthogonality due to carrier offset must be compensated before DFT-based demodulation can be performed. We present two blind carrier offset estimation algorithms by exploiting the intrinsic structure information of OFDM signals. Both algorithms offer the accuracy of super resolution subspace methods, viz., MUSIC and ESPRIT, without involving computationally intensive subspace decompositions. Adaptation rules for implementation of one of the proposed algorithm are provided. Also derived is the Cramer-Rao bound of the carrier offset estimates with which the performance of the new algorithms is evaluated.

Modulated backscatter location system

Abstract: In accordance with the present invention, a radio communications system is disclosed that can determine the location of an RFID Tag. The radio communication system includes at least one Interrogator for generating and transmitting a modulated radio signal to one or more Tags. The Interrogator is at a known location, and is in motion with respect to the Tag at a known velocity. One or more Tags of the system receive and demodulate the modulated radio signal, whichcontains a first information signal which specifies which Tag or Tags should respond using Backscatter Modulator means. The Tag generates a subcarrier signal, and backscatter modulates the reflection of the radio signal using the subcarrier signal, thereby forming a reflected signal. The Interrogator receives and demodulates the reflected signal. The Interrogator then determines the Tag's relative direction from the location and velocity of the Interrogator, and from the Doppler shift of the subcarrier signal. More than one of such measurements allow the location of the Tag to be determined.

The effect of quadrature modulator and demodulator errors on adaptive digital predistorters for amplifier linearization

Abstract: With the increasing importance of spectral efficiency in mobile communications, the power amplifier linearity has become a critical design issue for nonconstant envelope modulation. Imperfections in quadrature modulators and demodulators-gain and phase imbalance and DC offset-have a crippling effect on amplifier linearization circuits, a fact that has previously been noted experimentally. This paper is the first analysis of these effects on an adaptive predistorter. The primary result is an expression that provides an explicit tradeoff among intermodulation power, accuracy of the quadrature modulator and demodulator, and speed of adaptation. Another useful result is a simple and easily measured error figure for quadrature modulators and demodulators.

Communication device and method for interference suppression using adaptive equalization in a spread spectrum communication system

Abstract: A communication device such as a mobile station ( 410 ) for a spread spectrum communication system includes a receiver ( 100 ) having an adaptive equalizer ( 104 ) which suppresses interference on a received spread spectrum signal to produce an equalized signal ( 126 ). A pilot channel demodulator ( 110 ) demodulates the equalized signal to produce an estimate of the pilot channel ( 140 ). A summer ( 112 ) compares the pilot channel estimate and a predetermined data pattern to produce an error signal ( 124 ). A traffic channel demodulator ( 108 ) demodulates the equalized signal to produce one or more traffic channels.

Frequency estimation by demodulation of two complex signals

Abstract: This paper presents a method for frequency estimation in a power system by demodulation of two complex signals. In power system analysis, the /spl alpha//spl beta/-transform is used to convert three phase quantities to a complex quantity where the real part is the in-phase component and the imaginary part is the quadrature component. This complex signal is demodulated with a known complex phasor rotating in opposite direction to the input. The advantage of this method is that the demodulation does not introduce a double frequency component. For signals with high signal to noise ratio, the filtering demand for the double frequency component can often limit the speed of the frequency estimator. Hence, the method can improve fast frequency estimation of signals with good noise properties. The method loses its benefits for noisy signals, where the filter design is governed by the demand to filter harmonics and white noise. The method has been previously published, but not explored to its potential. The paper presents four examples to illustrate the strengths and weaknesses of the method.

Blind adaptive interference suppression for the near-far resistant acquisition and demodulation of direct-sequence CDMA signals

Abstract: Two key operations required of a receiver in a direct-sequence (DS) code division multiple access (CDMA) system are the timing acquisition of transmissions that are starting up or have lost synchronization, and the demodulation of transmissions that have been acquired. The reliability of both these operations is limited by multiple-access interference, especially for conventional matched filter-based methods, whose performance displays an interference floor and is vulnerable to the near-far problem. Recent work has shown that, provided timing information is available for a given transmission, it can be demodulated reliably using blind or training-sequence-based adaptive interference suppression techniques. These techniques are near-far resistant, unlike the matched filter demodulator, and do not require explicit knowledge of the interference parameters, unlike nonadaptive multiuser detectors. In this paper, we present a blind adaptive interference suppression technique for joint acquisition and demodulation, which has the unique feature that the output of the acquisition process is not simply the timing of the desired transmission, but a near-far resistant demodulator that implicitly accounts for knowledge of the timing and amplitudes of all transmissions to suppress the multiple-access interference. The only knowledge required by the scheme is that of the desired transmission's signature sequence, so that it is amenable to a decentralized implementation. On the other hand, it can be efficiently implemented as a centralized scheme in which the bulk of the computations for the adaptation are common to all transmissions that need to be acquired or demodulated.

Flow analysis system and method

Abstract: A non-invasive flow analysis system and method wherein a sensor, such as an acoustic sensor, is coupled to a conduit for transmitting a signal which varies depending on the characteristics of the flow in the conduit. The signal is amplified and there is a filter, responsive to the sensor signal, and tuned to pass a narrow band of frequencies proximate the resonant frequency of the sensor. A demodulator generates an amplitude envelope of the filtered signal and a number of flow indicator quantities are calculated based on variations in amplitude of the amplitude envelope. A neural network, or its equivalent, is then used to determine the flow rate of the flow in the conduit based on the flow indicator quantities.

Symbol-by-symbol MAP demodulation of CPM and PSK signals on Rayleigh flat-fading channels

Abstract: Demodulation using the symbol-by-symbol maximum a posteriori probability (MAP) algorithm is presented. The algorithm is derived for the case of continuous phase modulation (CPM) signals transmitted over Rayleigh flat-fading channels, and a corresponding receiver structure is specified. It is shown that the MAP algorithm requires computing, for each trellis branch, the sum of the products of the weights of all paths through the trellis which pass through that branch, and that this generic computational problem can be solved efficiently by an approach that uses a forward and backward recursion through the trellis. Simulation results are presented which show both the hard and soft decision performance of the MAP receiver to be robust, even in the presence of fade rates of up to 30% of the symbol rate. The application of the receiver concept to phase-shift keying (PSK) signals is also discussed, and then evaluated via simulation. The concept of joint demodulation and decoding using iterative processing techniques is introduced. It is shown that the MAP receiver is well suited for iterative processing applications due to its use of a priori symbol probabilities and its production of optimal soft decisions. Simulation results for the reception of quaternary PSK (QPSK) show that the bit error rate (BER) performance of the iterative MAP receiver can approach that of a receiver operating with perfect knowledge of the fading process.

Spread spectrum signal demodulator

Abstract: A spread spectrum signal demodulator receives a received signal including a plurality of spread spectrum signals, and comprises at least one signal strength suppressing means, which reduces a correlation signal outputted from a first correlation detecting means or replica signals until the correlation signal has been traveled through a replica signal generating means, a replica signal subtracting means and a second correlation detecting means. According to the invention, it is possible to improve interference cancellation capacity with a computational effort substantially comparable to that in a prior art replica signal cancellation system and improve interference cancellation performance with increasing number of interference cancellers irrespective of the magnitude of the correlation. With a sufficient number of interference cancellers provided, it is possible to obtain the same effect as obtainable with prior art decorrelation in a real time processing irrespective of the correlation magnitude.

Testing facility for a broadband communications system

Abstract: A test system for evaluating the operating state of a headend of a broadband communications network for communicating telephony signals between a telephony system and subscribers of communications services. Modulators at the headend transmit telephony signals in the forward band of the broadband communications network. The forward telephony channels are demodulated and demultiplexed by a plurality of subscriber terminals into individual telephony signals directed to an addressed subscriber. Signals returning from subscribers are digitized into standard telephony signals onto the reverse band of the broadband communications network, and demodulated by demodulators of the headend into a standard telephony signal that is interfaced to the telephony network. The test system evaluates the operating state of a selected demodulator of the headend by modulating a carrier with a predetermined data pattern to generate an upstream test signal. The upstream test signal is transmitted to the selected demodulator, which responds by outputting a detected upstream test signal. The test system compares the detected upstream test signal to the data pattern to generate error measurements that support an evaluation of the operating state of the selected demodulator. For an ingress test, the upstream signals present on the selected upstream channel are combined with the upstream test signal, and this signal combination is analyzed to determine the status of the selected upstream channel and the selected demodulator. The test system evaluates the operating state of a selected modulator of the headend in response to receiving a downstream test signal generated by the selected modulator. The test system outputs a detected downstream test signal in response to the downstream test signal. In turn, the test system conducts error measurements in response to the detected downstream test signal that support an evaluation of the operating state of the selected modulator.

Transmodulated broadcast delivery system for use in multiple dwelling units

Abstract: A system for redistributing a broadband audio-visual-data signal to a multiplicity of receiver units within a multiple dwelling unit (MDU) includes a main receiving antenna that receives a broadband video/audio/data signal having a number of individual program multiplex signals therein and a transmodulator device that transmodulates the individual program multiplex signals associated with the broadband signal from a first modulation scheme to a second modulation scheme to reduce the bandwidth of the broadband signal The transmodulated signals are broadcast over a cable network, along with terrestrial signals, to individual receiver units at the MDU The receiver units demodulate the transmodulated and/or terrestrial signals and provide user-specified channels to television sets for display

Frequency offset compensation of pilot symbol assisted modulation in frequency flat fading

Abstract: Pilot symbol assisted modulation (PSAM) is a practical technique in mobile digital communications since it can provide high performance in fading with large constellations and it has a simple implementation. Maintaining high performance with PSAM requires an accurate estimate of the transmitted carrier frequency. This paper examines model-based frequency estimation for mobile digital communications with PSAM. Maximum-likelihood estimators (MLE), which include the model for the fading, are derived and compared with those in an additive white Gaussian noise (AWGN) channel, and performance hounds are computed. Reduced complexity frequency estimators based on the MLE are derived and the performance is quantified by Monte-Carlo simulation.

High-performance photonic analogue-digital converter

Abstract: The authors demonstrate a photonic analogue-digital converter architecture based on temporal-spectral mapping. Linearily chirped optical pulses form a quasi-continuous carrier modulated by the analogue RF signal. The signal is separated into several distinct wavelength channels for parallel demodulation and quantisation. Measurements performed on 70 ps electrical signals indicate six bit resolution.

Contactless communication system for exchanging data

Abstract: The terminal includes a coil co-operating with a data transmitter/receiver. The portable object includes a remotely powered electric circuit including: a coil for picking up the modulated magnetic field from the terminal or for responding by producing modulated disturbance of the magnetic field; a converter including a rectifier stage and a filter stage transforming the magnetic field picked up by the coil into a DC power supply voltage; and a data transmitter/receiver, the receiver including a demodulator for demodulating the amplitude of the signal picked up by the coil, the amplitude demodulator operating on the signal output by the rectifier and filter stages. Each of the coils forms a portion of a tuned resonant circuit radiating the field in free space. The amplitude modulation is a low depth modulation, with a modulation ratio that is typically less than or equal to 50%.

Multi-frequency radio frequency transmitter with code learning capability

Abstract: A radio frequency transmitter for use in generating coded commands learned from received coded radio frequency signals. An transceiver circuit including a switching element and a tunable filter tuning element is coupled to a programmable controller, e.g, a microprocessor. The programmable controller operates the switching element of said transceiver circuit in either a first or a second mode for receiving or transmitting coded radio frequency signals, respectively via an antenna coupled to the tuning element. The switching element is operable in the first mode to demodulate received coded radio frequency signals, and the programmable controller learns the received coded radio frequency signals and stores coded commands in memory. In the second mode of operation, an oscillator is modulated by generated coded signals from the programmable controller using the stored coded commands from memory. The generation of plural coded radio frequency commands with the single radio frequency transmitter unit facilitates the learning, responsive to a received radio frequency signal, of an additional coded radio frequency command for additional door and gate operators.

Performance analysis of coherent DS-CDMA systems in a Nakagami fading channel with arbitrary parameters

Abstract: A new expression for the bit error rate (BER) of an asynchronous direct-sequence code-division multiple-access (DS-CDMA) system with coherent BPSK demodulation in a multipath Nakagami (1960) fading channel is derived. The analysis assumes an arbitrary number of independent, but nonidentical resolvable paths combined by the RAKE receiver. The results obtained show that in such systems, the effect of nonidentical fading on system performance cannot be ignored.

Improved demodulation scheme for fiber optic interferometers using an asymmetric 3/spl times/3 coupler

Abstract: Problems associated with the demodulation of the signal produced by a fiber optic interferometer which employs a 3/spl times/3 fiber coupler are investigated both theoretically and experimentally. It is shown that existing demodulation methods do not work well, especially when the 3/spl times/3 coupler is asymmetric, as is often the case. Modifications to improve the performance of a previously proposed method are suggested. Furthermore, an alternative demodulation circuit is presented which is simple and which works well even when the 3/spl times/3 coupler is asymmetric.

Apparatus and methods for modulation and demodulation of data

Abstract: A signal modulation method comprising receiving at least first and second synchronized incoming streams of complex symbols, thereby to define a plurality of incoming vectors each including at least first and second synchronized complex symbols, mapping each complex symbol into a signal component comprising a linear combination of an in-phase signal and a quadrature signal, the quadrature signal comprising a Hilbert transform of said in-phase signal, wherein all of the signal components are substantially mutually orthogonal and wherein the frequency spectrums of all signal components mapped from a single incoming stream are centered around a common frequency location which is unique to the single incoming stream and wherein the frequency spectrums of signal components mapped from different incoming streams having adjacent common frequency locations are partially overlapping and wherein signal components mapped from sequential incoming symbols partially overlap in time and combining all of the signal components into a representation of an output signal.

Communication server apparatus using frequency multiplexing and method

Abstract: A communication server (58) includes a plurality of pairs of frequency agile modulators (638) and demodulators (640) each set to operate at a unique frequency and associated with a twisted pair data line (54). The communication server (58) also includes a plurality of pairs of demodulators (644) and modulators (646) that each have a designatable operation frequency and are associated with one of a plurality of XDSL modems (648). A mixer (642) operates to combine signals from the frequency agile modulators (638) and provide the combined signal to the demodulators (644). The mixer (642) further operates to combine signals from the modulators (646) and provide the combined signal to the frequency agile demodulators (640). A detector (634) operates to detect a need for data service on a selected twisted pair data line (54). The controller (636) then operates to couple the selected twisted pair data line (54) to the selected XDSL modem (648) by designating the demodulator (644) and modulator (646) pair associated with the selected XDSL modem (648) to operate at the unique frequency of the frequency agile modulator (638) and demodulator (640) associated with the selected twisted pair data line (54).

Multiple mode capable radio receiver device

Abstract: A mobile handset is able to receive and transmit to base stations operating in compliance with different standards, for example CDMA, TDMA or AMPS. The mobile handset converts the received signal down to baseband and digitizes only a single channel at a time rather than the whole band. The channel chosen is the biggest of the various transmission systems used. For example, the channel widths for CDMA, TDMA and AMPS are, respectively 1.25 MHz, 30 KHz and 30 KHz. Accordingly to deal with these three systems a 1.25 MHz channel is digitized. Thereafter a digital signal processor determines which of the three systems is being used and appropriate processing and demodulation is carried out.

Design and analysis of transmitter diversity using intentional frequency offset for wireless communications

Abstract: Coded modulation (usually with interleaving) is used in fading channel communications to achieve a good error performance. The major benefit from using coded modulation in fading channels is achieved if each code symbol of a codeword (or coded sequence) suffers statistically different fading (preferably independent fading). However, in many applications of mobile communications (e.g., in a metropolitan environment), a low vehicle speed (and hence, a small Doppler spread, f/sub D/) is very common. With a small Doppler spread, ideal or close-to-ideal interleaving is no longer feasible and all code symbols of a codeword would suffer highly correlated fading especially in stationary fading (f/sub D//spl ap/0). Coded modulations will thus suffer seriously degraded performance. Previous performance analyses based on ideal interleaving are not accurate when a small Doppler spread is encountered and the much used union bound error probability analysis is loose for small Doppler spreads. To rectify this situation, this paper presents an improved performance analysis of coded modulations with correlated fading and pilot-symbol-assisted modulation (PSAM). Transmitter diversity can generate the necessary time-varying fading to maintain the effectiveness of a coded signaling scheme which this paper examines in detail using an intentional frequency offset between antennas. This work found that proper selections of the intentional frequency offset and interleaving depth can lead to good performance with traditional coded modulations (if enough antennas are used) using essentially the same simple demodulation structure as used in the traditional single-antenna PSAM.

Delta-sigma pwm dac for reduced switching

Abstract: A digital to analog (D/A) converter (306) for hearing aids and the like includes a delta-sigma modulator (204) and a duty cycle demodulator (208). The delta-sigma modulator converts medium rate, high resolution data (202) into medium rate, medium resolution data (206), and the duty cycle demodulator converts the medium rate, medium resolution data into low resolution, high rate data (210). The duty cycle demodulator utilizes a lookup function (220) to format the output data in a format having lowered transition rates, resulting in lower power use. The delta-sigma converter may include a correction factor (93) in at least one of its feedback loops to compensate for the characteristics of the output data from the duty cycle demodulator.

Frame synchronization method and apparatus for use in digital communication system utilizing OFDM method

Abstract: A frame synchronization method and apparatus for use in a digital communication system utilizing OFDM method are disclosed. The frame synchronization apparatus comprises a phase calculator for calculating phase values of TPS pilots within a symbol according to in-phase and quadrature-phase channel signals received from a transmitting side; a D-BPSK demodulator for performing D-BPSK demodulation for the phase values of TPS pilots supplied from the phase calculator and outputting the TPS pilots within the demodulated symbol; a control signal generator for comparing the demodulated TPS pilots with each other and outputting a control signal according to the compared result; and frame synchronization unit for confirming a sync word position according to the control signal supplied from the control signal generator and outputting a frame sync signal. Hence, frame synchronization can be achieved by using the synchronization word inverted at each frame in one TPS block without the need to increase the size of hardware.