Showing papers on "Demodulation published in 1998"

Blind equalization and multiuser detection in dispersive CDMA channels

Abstract: The problem of blind demodulation of multiuser information symbols in a high-rate code-division multiple-access (CDMA) network in the presence of both multiple-access interference (MAI) and intersymbol interference (ISI) is considered. The dispersive CDMA channel is first cast into a multiple-input multiple-output (MIMO) signal model framework. By applying the theory of blind MIMO channel identification and equalization, it is then shown that under certain conditions the multiuser information symbols can be recovered without any prior knowledge of the channel or the users' signature waveforms (including the desired user's signature waveform), although the algorithmic complexity of such an approach is prohibitively high. However, in practice, the signature waveform of the user of interest is always available at the receiver. It is shown that by incorporating this knowledge, the impulse response of each user's dispersive channel can be identified using a subspace method. It is further shown that based on the identified signal subspace parameters and the channel response, two linear detectors that are capable of suppressing both MAI and ISI, i.e., a zero-forcing detector and a minimum-mean-square-error (MMSE) detector, can be constructed in closed form, at almost no extra computational cost. Data detection can then be furnished by applying these linear detectors (obtained blindly) to the received signal. The major contribution of this paper is the development of these subspace-based blind techniques for joint suppression of MAI and ISI in the dispersive CDMA channels.

A high-efficiency carrier estimator for OFDM communications

Abstract: While multipath induced phase rotations can be dealt with by differential encoding in orthogonal frequency division multiplexing (OFDM) communications, the loss of orthogonality due to the carrier offset must be compensated before discrete Fourier transform (DFT)-based demodulation can be performed. In this letter, we present a high-performance/low-complexity blind carrier offset estimation algorithm by exploiting intrinsic structure information of OFDM signals. The algorithm offers the accuracy of a super resolution subspace method, viz. MUSIC, without involving computationally intensive subspace decompositions.

Universal modem for digital video, audio and data communications

Abstract: A universal modem has a software-configurable modulator/demodulator which accommodates different modulation formats such as those associcated with terrestrial, cable, phone line, satellite and wireless communications to be transmitted and received through a single device in which the modem has reconfigurable logic to accommodate the format of the signals being received or transmitted. In one embodiment, the system tracks channel noise and changes modulation format at both the transmit and receive sides of the system through the use of a controller that controls both sides.

Bit-interleaved coded modulation with iterative decoding using soft feedback

Abstract: A new iterative decoding (ID) scheme is suggested for bit-interleaved coded modulation (BICM). The bit-metric generation at the demodulator is optimised with the soft-output feedback from the convolutional decoder. Computer simulation results indicate that BICM-ID, with a few iterations, significantly outperforms conventional trellis coded modulation and is therefore another promising technique to achieve large coding gain without bandwidth expansion.

High-frequency demodulation of multi-photon fluorescence in hyper-Rayleigh scattering

Abstract: A novel technique for suppression of the multi-photon fluorescence contribution in second-order nonlinear optical hyper-Rayleigh scattering experiments is described. The technique takes advantages of the demodulation and the phase shift in the frequency domain of the time-delayed (multi-photon) fluorescence in the time domain. We demonstrate the effectiveness of demodulation at high modulation frequencies of the fundamental laser beam by determining the molecular second-order nonlinear polarizability for a reference molecule under fluorescent conditions. The value that was obtained for crystal-violet in methanol with 9,10-diphenylanthracene added as a centrosymmetric fluorophore compares very well with the values that were previously obtained. The possibility of complete suppression of all fluorescence, based on phase-sensitive measurements in quadrature with the fluorescence, is also discussed.

Orthogonal frequency-division multiple access and its application to CATV networks

Abstract: We introduce the concept of orthogonal frequency-division multiple access (OFDMA) and investigate its application to send upstream information from subscriber premises to the cable head-end in cable TV (CATV) networks. The carrier and timing synchronizations needed to ensure orthogonality between different subscriber signals are achieved by sending common pilot signals from the head-end. Whereas N demodulators are required to accommodate N simultaneous users in conventional FDMA systems, OFDMA only requires a single demodulator followed by an N-point discrete Fourier transform (DFT). We analyze the performance of this technique and compare it to time-division multiple access (TDMA) which appears in several international standards. Our results indicate that OFDMA is an attractive multiple access technique which offers increased robustness to narrowband interference, impulse noise, and other signal impairments which characterize typical CATV networks.

Wireless lan with enhanced capture provision

Abstract: A receiver and a method for operating the receiver, for a station in a wireless local area network using a common wireless communication channel and employing a CSMA/CA (carrier sense multiple access with collision avoidance) protocol includes various modes. In normal mode, the receiver follows typical states in order to detect a message and demodulate data from the message properly. Meanwhile, a process implements a message-in-message (MIM) mode when an energy increase above a specified level is detected. While in the MIM mode, if a carrier is detected, the energy increase is caused by a new message; otherwise, the energy increase is caused by an interfering station. If the carrier is detected, the receiver begins retraining so that it can start receiving the new message as soon as the first message ends. If no carrier is detected, the receiver waits a specified time to detect a carrier or for the end of the first message, after which the receiver returns to the beginning of the normal mode. While in the normal mode, if a message is detected that is not addressed to the station, the receiver enters a hunt mode. While in the hunt mode, the receiver waits for the current message to finish. At the same time, a process implements retraining when both an energy increase above a specified level and a carrier are detected. If no carrier is detected within a specified time, or if the energy level decreases below the specified level, the receiver returns to the beginning of the normal mode.

Improved universal lan power line carrier repeater system and method

Abstract: An improved local area network (LAN) to power line carrier (PLC) interface and protocol using FSK is described. This system provides improved data symmetry, higher data rates, lower bit error rates, improved synchronization and alignment of data, as well as improved carrier detection. The system provides high speed frequency shift key (FSK) modulation over the power line to achieve high data rates. Performance may be further improved by using a novel combination of FSK modulation and differential shift key (DFSK) modulation to provide an improved local area network (LAN) to power line carrier (PLC) interface and protocol using FSK and DFSK. DFSK is described and shown to provide improvements in the modulation and demodulation of data transferred over digital networks.

Practical blind demodulators for high-order QAM signals

Abstract: This paper examines the problem of demodulating time dispersed digitally modulated signals with particular emphasis on two aspects: the all-digital implementation of such demodulators and the use of "blind" algorithms for initializing the demodulator in the absence of explicit training by the transmitter. The factors impacting the architecture of blind demodulators are discussed, followed by a description of several practical implementations.

Pilot interference cancellation for a coherent wireless code division multiple access receiver

Abstract: A Code-Division Multiple Access (CDMA) receiver removes the pilot signal from the received signal. The pilot signal is defined by its multipath parameters (amplitudes, phase shift and delays) and its signature sequence. Since this information is known at the user's receiver terminal (i.e., handset), the pilot signals of the interfering multipath components of the baseband received signal are detected and removed prior to demodulation of the desired multipath component. The pilot signal may be cancelled prior to or following the data accumulation stage. The pilot signal cancellation can be switched on and off depending on the detected path signal level.

Multifunctional fiber-optic microwave links based on remote heterodyne detection

Abstract: The multifunctionality of microwave links based on remote heterodyne detection (RHD) of signals from a dual-frequency laser transmitter is discussed and experimentally demonstrated in this paper. Typically, direct detection (DD) in conjunction with optical intensity modulation is used to implement fiber-optic microwave links. The resulting links are inherently transparent. As opposed to DD links, RHD links can perform radio-system functionalities such as modulation and frequency conversion in addition to transparency. All of these three functionalities are presented and experimentally demonstrated with an RHD link based on a dual-frequency laser transmitter with two offset phase-locked semiconductor lasers. In the modulating link, a 1-Gb/s baseband signal is QPSK modulated onto a 9-GHz RF carrier. The frequency converting link demonstrates up-conversion of a 100-Mb/s PSK signal from a 2-GHz carrier to a 9-GHz carrier with penalty-free transmission over 25 km of optical fiber. Finally, the transparent link transmits a standard FM video 7.6-GHz radio-link signal over 25 km of optical fiber without measurable distortion.

Beamformed ultrasonic imager with delta-sigma feedback control

Abstract: A method and apparatus are provided for reducing distortion in a dynamically delayed digital sample stream of an imaging system The method includes the steps of delta-sigma modulating an input analog signal of the imaging system at a frequency above the Nyquist frequency of the input analog signal to generate a digital sample stream and changing a length of the sample stream to delay a portion of the sample stream while maintaining synchronism between a delta-sigma modulator and a demodulator of the system

System and method for adaptive modification of modulated and coded schemes in a communication system

Abstract: A system and method is provided having an adaptive channel coder and modulator, a channel decoder and demodulator connected to the adaptive channel coder and modulator, and a radio link protocol frame and channel decision unit connected to the adaptive channel coder and modulator.

Rake receiver and finger management method for spread spectrum communication

Abstract: A RAKE receiver (112) includes a plurality of fingers (122, 124, 126, 128). Each finger includes a demodulator (402) for demodulating a ray of a multipath signal and a time tracking circuit (404) for controlling the time position of the finger in accordance with time position of the ray. A low delay-spread condition is detected and the positions of two adjacent fingers are controlled to prevent convergence of two or more fingers about a common time position. By maintaining finger timing separation, path diversity is exploited by the RAKE receiver even during the low delay-spread condition to improve receiver performance.

High data rate spread spectrum transceiver and associated methods

Abstract: A spread spectrum radio transceiver includes a high data rate baseband processor and a radio circuit connected thereto. The baseband processor preferably includes a modulator for spread spectrum phase shift keying (PSK) modulating information for transmission via the radio circuit. The modulator may include at least one modified Walsh code function encoder for encoding information according to a modified Walsh code for substantially reducing an average DC signal component to thereby enhance overall system performance when AC-coupling the received signal through at least one analog-to-digital converter to the demodulator. The demodulator is for spread spectrum PSK demodulating information received from the radio circuit. The modulator and demodulator are each preferably operable in one of a bi-phase PSK (BPSK) mode at a first data rate and a quadrature PSK (QPSK) mode at a second data rate. These formats may also be switched on-the-fly in the demodulator. Method aspects are also disclosed.

Finger assignment in a CDMA rake receiver

Abstract: A method and apparatus of assigning fingers in a rake receiver for incorporation in a receiver of a CDMA communications system is disclosed herein. The method includes measuring the multipath profile of a received spread-spectrum radio-frequency (RF) signal to obtain a sequence of measurements, identifying the best candidate path of the RF signal to demodulate by determining which measurement within the sequence has the highest signal strength, deriving a timing offset for the best candidate path as a function of the measurement for the best candidate path and its adjacent measurements, and assigning a finger processing element to the best candidate path using the derived timing offset. The measurements are taken using a searcher having a predetermined timing resolution (e.g., ½ chip), and the resolution of the timing offset is higher (e.g., ⅛ chip). The best candidate path to demodulate is identified using the measurement having the highest amplitude or correlation function. The function used to derive the timing offset is a quadratic passing through the measurement for the best candidate path and adjacent measurements. A correction factor can be applied to the estimated timing offset to correct for non-quadratic multipath profiles. The assignment can be made for an available finger, or a finger can be re-assigned where the best candidate path is better than a path currently being demodulated. This approach can also be used to update and instantaneously adjust the timing of fingers to track on-time samples of the received multipaths during operation of the rake receiver.

System and method for mitigating intermittent interruptions in an audio radio broadcast system

Abstract: A system (100) for mitigating intermittent interruptions in an audio radio broadcast system is provided wherein a primary radio signal and a redundant radio signal are transmitted from a transmitter subsystem (120) and received by a receiver subsystem (140). The output (112) of an audio source (110) is coupled to a modulator (160) for modulating a radio frequency signal (162) for coupling to a transmit antenna (172). A second output (114) of audio source (110) is coupled to a delay circuit (116), for adding a predetermined time delay thereto. The delayed audio source signal is coupled to a modulator (164) for modulating a second radio frequency signal (166) that is also coupled to the transmit antenna (172). The receiver subsystem (140) receives both the primary radio signal and the delayed redundant radio signal and couples each to a respective demodulator (180, 182). At least demodulator (180) includes a circuit (181) for determining the degradation in the primary radio signal and provides a quality measurement output signal (186) to a blend control circuit (190). The recovered primary audio signal from demodulator (180) is coupled to a second delay circuit (184), the time delay of second delay circuit (184) being substantially equal to the time delay of delay circuit (116). The audio output from delay circuit (184) and the redundant audio output from demodulator (182) are coupled to a blending subsystem (135), wherein each is combined with a weighting factor and then combined together to form a composite audio signal for coupling to the audio output circuit (150).

Linear amplification systems and methods using more than two constant length vectors

Abstract: An input signal of varying amplitude and varying phase is converted into more than two signals of constant amplitude and controlled phase. Each of the more than two signals of constant amplitude and controlled phase is then separately amplified in separate amplifiers. The separately amplified more than two signals of constant amplitude and controlled phase are then combined to produce an output signal that is an amplification of the input signal at the desired power level. When converting the input signal into more than two signals, the phase of each of the more than two signals of constant amplitude and controlled phase is controlled to produce the output signal that is an amplification of the input signal at the desired power level. According to another aspect, a signal of varying amplitude and varying phase is generated from a plurality of constant amplitude varying phase signals, the sum of which is the signal of varying amplitude and varying phase. An IQ waveform generator generates a cosine carrier modulation waveform I(t) and a sine carrier modulation waveform Q(t) from the signal of varying amplitude and varying phase. A function generator generates a complimentary waveform Q′(t) from the cosine carrier modulation waveform I(t) such that the sum of squares of I(t) and Q′(t) is constant. A first modulator modulates a cosine carrier signal with I(t) to obtain a first modulated cosine carrier. A second modulator modulates a sine carrier signal with Q′(t) to obtain a first modulated sine carrier. A circuit such as a butterfly circuit forms the sum and difference of the first modulated cosine carrier and the first modulated sine carrier to obtain the constant amplitude varying phase signals.

Subchannel modulation scheme for carrying management and control data outside the regular data channel

Abstract: A method and apparatus for implementing a subchannel for management and control or other supplemental data on a media shared with a high speed data link Several FDMA approaches are disclosed including AM, FM, PM, phase, QAM, QPSK etc modulation of a subcarrier having a frequency which is in the bandwidth provided by the shared media which is not used by the DC balanced high speed data stream A preferred species which is compatible with existing high speed data transmitters and receivers comprises a subchannel transmitter which frequency shift keys a subchannel carrier of about 1 mHz with the subchannel data The modulated subchannel carrier is summed by superposition with a DC balanced NRZ format gigabit data stream and coupled onto the media Injection power levels of the subchannel carrier are controlled so as to not cause excessive jitter in the high speed data receivers but so as to provide sufficient amplitude in the face of noise sources including random variations in transition density to be recovered by a subchannel receiver At the destination node, the combined signal is directly coupled to the input of the high speed data receiver, but is coupled to the input of the subchannel receiver through a low pass filter The subchannel receiver is narrowband and includes an FSK demodulator

Method and apparatus for carrying out high data rate and voice underwater communication

Abstract: An underwater apparatus for transmitting and receiving high rate data and voice communication including a transmitter, a receiver, and a Doppler frequency shift compensator. The transmitter includes a data source comprising digital data to be transmitted by the apparatus through the water, a serial-to-parallel data processor for splitting the serial digital data into n parallel data channels, a n-channel modulator for receiving the n parallel data channels and for modulating those channels with n pairs of ultrasonic carriers to produce a modulated signal. The transmitter also includes a hydrophone for receiving and transmitting the modulated signal. The receiver includes a hydrophone for receiving a modulated signal. The receiver also includes RF circuitry for amplifying and shaping the received modulated signal, a serial-to-parallel data processor for splitting the amplified and shaped data into n parallel channels, a n-channel demodulator for demodulating the shaped signal and for outputting n channels of digital data, and a parallel-to-serial data processor for receiving n parallel channels of data from the demodulator and for combining those channels into serial data. The Doppler compensator measures the frequency of at least one of two unmodulated signals transmitted as part of the modulated signal and compares the measured frequency with a predetermined frequency. Operation of the demodulator is adjusted to compensate for any deviation between the measured and predetermined frequency.

An asynchronous multiuser CDMA detector based on the Kalman filter

Abstract: We introduce a multiuser receiver based on the Kalman filter, which can be used for joint symbol detection and channel estimation. The proposed algorithm has the advantage of working even when the spreading codes used have a period larger than one symbol interval ("long codes"), unlike adaptive equalizer-type detectors. Simulation results which demonstrate the performance advantage of the proposed receiver over the conventional detector, the minimum mean squared error (MMSE) detector and a recursive least squares (RLS) multiuser detector are presented. A thorough comparison of the MMSE detector and the proposed detector is attempted because the Kalman filter also solves the MMSE parameter estimation problem, and it is concluded that, because the state space model assumed by the Kalman filter fits the code division multiple access (CDMA) system exactly, a multiuser detector based on the Kalman filter must necessarily perform better than a nonrecursive, finite-length MMSE detector. The computational complexity of the detector and its use in channel estimation are also studied.

Telemetry system with phase-locking noise suppressing receiver

Abstract: There is provided a telemetry system with a receiver which enables detection of pulsed high frequency data (hf) signals in a manner which suppresses noise which may be present within the high frequency bandwidth. The transmitter carrier, which may be subject to some instability, is encoded at a symbol rate which is generated at a lower frequency than the hf, but which is reliably stable. The receiver detects the phase of each received hf pulse, and generates a phase-locked signal which is synchronized to the transmitter symbol signal. The phase-locked signal in turn is used to demodulate the received pulsed signals. In a specific DSP embodiment, the phase-lock loop has a numerically controlled oscillator which has a center frequency which corresponds to the aliased phase difference between the hf carrier phase and the phase of each pulse, thereby obtaining a signal which carries information as to the exact symbol rate. This information is used either to calculate the timing of a narrow window which gates a pulse detector coincident with each next expected symbol time, or to trigger generation of a replica signal which is used in the detect circuit, thereby providing high resolution detection of the pulses and good noise rejection.

MMSE interference suppression for timing acquisition and demodulation in direct-sequence CDMA systems

Abstract: It has been shown that minimum-mean-squared-error (MMSE) demodulators are effective means of interference suppression in code division multiple-access (CDMA) systems. The MMSE demodulator can be implemented adaptively using an initial training sequence, followed by decision-directed adaptation. This requires that the symbol-level timing of the desired user be known prior to training. We remove this requirement by providing a method for timing acquisition in which the output of the acquisition process is a near-far-resistant demodulator which automatically accounts for the delays and amplitudes of both the desired signal and the interference without explicitly estimating these parameters. The only requirements are a training sequence for the desired user and a finite uncertainty regarding the symbol timing. The latter condition can be realized by using a periodic training sequence even if the absolute timing uncertainty is arbitrarily large.

Optical transmitter-receiver

Abstract: A modulated electrical signal Smod produced upon amplitude-modulating a subcarrier having a high frequency (for example, a millimeter-wave band) by a baseband signal SBB to be transmitted and a main carrier MC outputted from a light source are inputted to an external optical modulating portion in an optical transmitter. The external optical modulating portion amplitude-modulates the main carrier MC by the modulated electrical signal Smod, to output a double-modulated optical signal OSdmod to an optical filter portion. The optical filter portion passes only a component of one of sidebands included in the double-modulated optical signal OSdmod, and outputs the component to an optical fiber as an optical signal OS. An optical/electrical converting portion in an optical receiver optical/electrical-converts the optical signal OS transmitted through the optical fiber, to directly obtain a baseband signal SBB. Consequently, the optical receiver is constructed simply and at low cost without requiring a wideband optical/electrical converting element for optical/electrical-converting a high-frequency electrical signal and a high-frequency electrical component (a frequency converter, a demodulator, a semirigid cable or a waveguide) which is very high in cost or is difficult to process.

FM-DCSK: a novel method for chaotic communications

Abstract: In binary Differential Chaos Shift Keying (DCSK), each information bit is mapped to the correlation between two pieces of a chaotic waveform. The receiver determines the correlation (which is proportional to the energy per bit) in order to demodulate the received signal. Since a chaotic signal is not periodic, the energy per bit is not constant and can only be estimated, even in the noise-free case. This estimation has a non-zero variance that limits the attainable data rate. This problem can be avoided if the energy per bit is kept constant. In this paper, the DCSK technique is combined with frequency modulation in order to achieve two properties: the excellent noise performance of DCSK is maintained; in addition, the energy per bit is kept constant in order not to limit the data rate. A low-pass equivalent model that significantly speeds up the simulation of an FM-DCSK system is also developed. Finally the noise performance of the proposed FM-DCSK system is given.

Bidirectional trunk amplifier and cable modem for cable hybrid fiber and coax network which utilizes an upstream pilot signal

Abstract: A method of utilizing a bidirectional amplifier in a cable hybrid fiber and coax (HFC) and a cable modem utilizing an upstream signal. The invention provides an upstream pilot signal from the cable modem and automatically controls the gain of the signal so that the headend receives a constant level upstream pilot signal from each terminal. A cable modem for bidirectional services in the HFC network has a saw filter, a mixer & AGC, a LPF, a tuner, a gain controller, a downstream demodulator, an upstream modulator, a media access controller (MAC), a CPU, a programmable attenuator, a pilot signal generator and a switch connecting the tuner and the pilot signal generator under the control of the CPU. A bidirectional trunk amplifier for bidirectional services in the HFC network, has a directional filter (DF) which separates the upstream signals from the downstream signals, a pad, having characteristics approximately equivalent to the frequency characteristics of the coaxial cable, for compensating the attenuated value of the coaxial cable and matching the input level, an equalizer for compensating the frequency deviation occurring in the line, an amplifier for amplifying the input signal to a desired level, and a level controller for controlling the slope and the automatic gain.

Receiving a spread spectrum signal

Abstract: A receiver (Fig. 6) for use in a spread spectrum communication system includes the following components: an acquisition system configured to detect a transmitted spread spectrum signal by simultaneously correlating multiple search phases of a reference spreading signal against an output from a receiver channel; a demodulation (Figs. 8A-B) system configured to recover data embedded in the spread spectrum signal by simultaneously correlating the spread spectrum signal with multiple possible data phases of the reference spreading signal over consecutive data periods; and a bank of correlating devices configured for use both in the acquisition system and in the demodulation system.

Transmodulator with dynamically selectable channels

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 a selected subset of the individual program multiplex signals associated with the broadband signal from a first modulation scheme to a second modulation scheme The transmodulated signals are broadcast over a cable network, along with terrestrial signals, to individual receiver units at the MDU The receiver units send requests for user-selected ones of the individual program multiplex signals to the transmodulator device, demodulate the transmodulated and/or terrestrial signals and provide user-specified channels to television sets for display

Tuner with switched analog and digital modulators

Abstract: A television receiver includes a tuner for receiving either analog or digital signals. Separate analog and digital demodulators are selectively coupled to the tuner through an RF switch that is controlled by the signal from a sync detector in the output of the analog demodulator. The selected one of the demodulators develops an AGC signal that is coupled to the tuner through a current mirror. Operating potential for the demodulators is coupled through the RF switch so that the oscillator in the non-selected demodulator is disabled and precluded from interfering with the oscillator in the enabled demodulator.