Showing papers on "Continuous phase modulation published in 1995"

Decomposition of M-ary CPM signals into PAM waveforms

Abstract: It is widely known that minimum shift keying (MSK) may be seen as a PAM signaling scheme and that the same is true, albeit approximately, with MSK-like modulations. It is also known (perhaps not so widely) that any binary continuous phase-modulated (CPM) signal may be exactly decomposed into the sum of a few PAM waveforms. In this paper we show that this property extends to multilevel CPM signaling. Features of a PAM decomposition are discussed as a function of the alphabet size, the modulation index, and the frequency response of the system. It is found that, especially with signaling schemes with a long memory, the decomposition has so many terms that it becomes unmanageable. For these cases an approximation is proposed with a limited number of terms. >

Digital modulation method with controlled envelope variation

Abstract: Continuous phase modulated (CPM) signals, such as quaternary CPFSK signals, are filtered to limit their bandwidth while simultaneously minimizing the amount of amplitude modulation and intersymbol interference introduced by this filtering process. By designing a modulation filter such as a finite impulse response (FIR) filter to add only a nominal, controlled amount of amplitude modulation, high efficiency, nonlinear power amplifiers may be employed to transmit the filtered CPFSK signals. Although the filter adds a small amount of amplitude modulation and intersymbol interference, digital signal processor-based receivers satisfactorily demodulate this type of signal using sequence estimation algorithms such as the Viterbi algorithm. Thus, the filtering of CPM signals permits narrowing of the transmitted signal spectrum while managing the intersymbol interference resulting from the controlled amount of amplitude modulation added by filtering. Narrowed bandwidth along with high efficiency, nonlinear power amplifiers make the present invention particularly well suited to narrowband mobile radio communications.

Coded continuous phase modulation using ring convolutional codes

Abstract: Rate 1/2 convolutional codes over the ring of integers modulo M are combined with M-ary continuous phase modulation (CPM) schemes whose modulation indices are of the form h=1/M. An M-ary CPM scheme with h=1/M can be modeled by a continuous-phase encoder (CPE) followed by a memoryless modulator (MM), where the CPE is linear over the ring of integers modulo M. The fact that the convolutional code and the CPE are over the same algebra allows the state of the CPE to be fed back and used by the convolutional encoder. A modified Euclidean distance function that substantially simplifies the search for good codes has been derived and used to find new codes. Numerical results show that this approach consistently improves the performance as compared to coded schemes using binary convolutional codes with the same decoding complexity.

Quasi-synchronous detection and demodulation circuit and frequency discriminator used for the same

Abstract: The present invention relates to a quasi-synchronous detection and demodulation circuit with a contingent demodulation carrier phase removing function and to a frequency discriminator used in the above circuit and for detecting with a good accuracy the difference between the carrier frequency of a modulated wave and a reference carrier frequency for demodulation. In order to detect a normal frame pattern by a frame pattern detecting means, the quasi-synchronous detection and demodulation circuit compensates an output carrier phase issued from a carrier generating means to add to a phase rotation means and removes a contingent demodulated carrier phase. The frequency discriminator limits a frequency deviation to less than an upper value according to the accuracy of a reference carrier signal.

Superconducting high-resolution A/D converter based on phase modulation and multichannel timing arbitration

Abstract: We have developed a flux-quantizing A/D converter (ADC) based on RSFQ elements, employing a novel front end capable of generating high-linearity multibit differential code within a wide dynamic range (up to 16 bits). The front end operates as a phase modulator/demodulator and uses fractional-flux-quantum least significant bit (LSB). It runs at multi-GHz speed, enabling ADCs with large oversampling ratio and effective resolution in excess of 20 bits (after decimation filtering). We have designed, fabricated and tested several versions of a complete ADC using this new architecture and demonstrated its operation with dynamic range of 14 bits. We have also confirmed continuous phase modulation of the flux quantizer with a carrier frequency of 10 GHz. >

An 800-MHz quadrature digital synthesizer with ECL-compatible output drivers in 0.8 /spl mu/m CMOS

Abstract: Direct digital frequency synthesizers (DDFS) play an important role in modern digital communications and instrumentation. They offer many advantages including fast continuous-phase switching response, fine frequency resolution, large bandwidth, good spectral purity, and low phase noise. The architecture in this design uses a phase accumulator to address a ROM look-up table that stores the sine samples. The input word (frequency-control word) to the phase accumulator controls the frequency of the generated sine waveform. This chip has modulation capabilities that include frequency modulation and phase modulation. Frequency modulation is performed by directly modulating the frequency control word and phase modulation is obtained by adding a phase offset to the phase accumulator output before addressing the ROM look-up table. The 7.2/spl times/7.9 mm/sup 2/ 94,000 transistor chip dissipates 3 W at 5 V and 800 MHz.

Frequency detectors for CPM signals

Abstract: Frequency tracking loops are used in digital links to control the carrier frequency of the received signals. A basic component in such loops is the frequency difference detector (FDD). This paper concentrates on a class of FDDs for use with binary continuous-phase modulation (CPM). Their derivation is based on two ideas: the maximum-likelihood principle for parameter estimation and Laurent's (1986) decomposition of a CPM signal into the sum of linearly modulated components. The proposed FDDs are suitable for digital implementation and their operation is independent of clock recovery. Analysis and simulation are employed to investigate tracking operation and acquisition capability of frequency control loops endowed with such FDDs. Numerical examples are provided for modulation schemes of practical interest such as MSK and Gaussian MSK. The loop acquisition range is found to be fairly large, about 0.5-1.0 times the bit rate. >

Method and apparatus for noncoherent reception and correlation of a continous phase modulated signal

Abstract: A technique for modulating and demodulating continuous phase modulation (CPM) spread spectrum signals and variations thereof. A transmitter encodes M data bits using a selected spread spectrum code, divides the spread spectrum code into a plurality of chip codes (such as even chips and odd chips), independently modulates the even and odd chips with orthogonal carrier signals using CPM or a related technique, and superposes the plurality of resultants for transmission. A receiver receives the superposed spread spectrum signal and divides it into a real signal and an imaginary signal. The real signal is connected to a real CPM correlator which demodulates its input into a real I signal and a real Q signal, separately correlates the real I signal with the odd chips of a chip code and the real Q signal with the even chips of the chip code, and combines the correlation signals into a real correlation signal. The imaginary signal is connected to an imaginary CPM correlator which demodulates its input into an imaginary I signal and an imaginary Q signal, separately correlates the imaginary I signal with the odd chips of the chip code and the imaginary Q signal with the inverse of the even chips of the chip code, and combines the correlation signals into an imaginary correlation signal. The real and imaginary correlation signals are combined into a unified correlation signal.

A new maximum-likelihood method for modulation classification

Abstract: We develop or new maximum-likelihood method for modulation classification of digital amplitude-phase modulations, applicable to any constellation-based modulation types in an AWGN environment. Under reasonable assumptions, the method is tractable to theoretical performance analysis, something lacking by many existing modulation classification methods. The classifier performance for a variety of modulations is analyzed and the theoretical results are compared with simulations.

Pilot-symbol aided continuous phase modulation system

Abstract: Apparatus for transmitting and receiving digital information over mobile communication channels. The apparatus includes a pilot symbol insertion device for periodically inserting data dependent pilot symbols into frames of digital data for subsequent channel estimation, a continuous phase modulator for modulating and transmitting over a mobile communication channel the frames of digital data and means for receiving and filtering the transmitted data. A channel estimator estimates the channel amplitude and phase distortion from the received pilot signal at different time instants. Means are provided for demodulating the received signal and for decoding the demodulated signal to recover the estimated transmitted data.

Modulation system for spread spectrum CDMA communication

Abstract: The OCDMA waveform of the present invention uses hi-phase PN modulation (BPSK PN chip modulation) in conjunction with MPSK or MQASK data modulation (QPSK data modulation is one preferred embodiment) to increase bandwidth efficiency. The number of orthogonal users that can be placed on a single carrier is equal to, at most, the length of the orthogonal binary sequence. The Radamacher-Walsh (RW) sequence chip rate must be 4" times the symbol rate (where n is a positive integer) since the symbol transitions must be synchronized to the RW period to guarantee orthogonality of the multiple users when data transitions are present. The symbol rate for QPSK modulation is one-half that for BPSK modulation. As a result, twice as many orthogonal functions are available for a given clock rate for QPSK as for BPSK modulation. That is, an OCDMA system with QPSK data can support twice as many users in a given bandwidth as an OCDMA system with BPSK data.

Non-coherent spread-spectrum continuous-phase modulation communication system

Abstract: A technique for modulating and demodulating CPM spread spectrum signals and variations of CPM spread spectrum signals. A spread spectrum transmitter includes a chip sequence generator for generating a chip sequence from a data stream, a switch for dividing said chip sequence into an odd chip sequence and an even chip sequence, and a modulator for generating and transmitting a continuous phase modulated signal from said odd chip sequence and said even chip sequence. A spread spectrum receiver comprises a plurality of non-coherent serial CPM correlators, each generating a correlation signal. In a preferred embodiment, the chip sequence generator of the transmitter comprises a table of symbol codes, each symbol code comprising a series of chips corresponding to a unique series of bits in said data stream, and each non-coherent serial CPM correlator is configured to detect one of the symbol codes. The spread spectrum receiver selects a data symbol based on said correlation signals output from the non-coherent serial CPM correlators. A spread spectrum preamble may be used to set initial timing for the non-coherent serial CPM correlators.

Method and apparatus for controlling the modulation index of continuous phase modulated (CPM) signals

Abstract: A transceiver uses feedback control to monitor a transmitted continuous phase modulation (CPM) waveform, such as a spread spectrum CPM waveform, and adjust the modulation index in response thereto. Codewords are selected from a table according to a data signal and applied to a pulse shaping network. The pulse shaping network outputs a CPM waveform to a prefilter. The prefilter is connected to an FM modulator, such as a voltage controlled oscillator. In one embodiment, a correlator in the transceiver monitors the transmitted waveform during times when the transceiver is not receiving signals, and provides a correlation signal to a feedback control circuit. The feedback control circuit dynamically adjusts the gain of the prefilter, and may comprise, e.g., a second order tracking loop filter. The feedback control signal thereby tracks the amplitude of the transmitted waveform and adjusts the output gain of the prefilter so as to maintain a constant signal envelope. A tau-dither algorithm may be employed as part of the tracking circuit. In an alternative embodiment, a gradient search is used to adjust the output amplitude of the transmitted signal, wherein the step size of the gradient search is based on the slope of the signal amplitude and varied dynamically over time. In another embodiment, a two bandwidth differencing circuit is used to adjust the output amplitude of the transmitted signal.

Performance evaluation of FSK and CPFSK optical communication systems: a stable and accurate method

Abstract: The modified moments method for evaluating the performance of coherent optical FSK and CPFSK systems is presented. Since the classical procedure becomes ill-conditioned as the order of the moments increases, we consider the construction of Gaussian quadrature rules (GQR) from the modified moments. The analysis accounts for the influences of IF bandwidth, transmitter and local oscillator laser phase noise, postdetection filters, and additive Gaussian noise. It is found that the proposed approach is a highly reliable and efficient method for calculating the error probability. A comparison with results obtained from the Gaussian quadrature rule, Gaussian approximation method, and analytical approximation formulas shows that this technique is very accurate. Analytical expressions are derived for FSK and CPFSK receivers which include polarization and phase diversity techniques. The use of numerical programming to avoid many unnecessary computations is discussed. This evaluation method can be used to account for the effects of crosstalk in multichannel systems and the influence of error-control codes. >

Modulation, demodulation and antenna coupling circuits used in IC card reading/writing apparatus, and method of supplying power to the IC card

Abstract: A modulator used in an IC card reader/writer includes a phase variation detector for detecting a varying point of the phase of a modulated signal, a signal processing circuit for modifying the modulated signal for a half period at the phase varying point to have a frequency and amplitude twice those of the modulated signal, a first Miller integrator for integrating the modified signal to produce a triangular wave signal, and a second Miller integrator for integrating the triangular wave signal to produce a sinusoidal wave signal having a continuous phase. A demodulator used in the reader/writer includes a sync control circuit which controls a carrier sync signal generation circuit thereby to control the phase of a carrier sync signal in response to the discrimination as to whether the carrier sync signal is in lead-phase or lag-phase relative to the signal modulated based on binary phase shift keying (BPSK). A control data signal and non-modulated carrier wave sent from the reader/writer to the IC card are converted into power for use by the IC card.

Detection of CPM signals in fast Rayleigh flat-fading using adaptive channel estimation

Abstract: Differential detection techniques, which are commonly used in fast fading environments, are characterized by an irreducible error rate that increases with fading rate. The main source of this error floor is the phase error introduced by the multiplicative fading process. The paper describes a detection technique for continuous phase modulation (CPM) that employs decision feedback carrier recovery and adaptive channel estimation. This receiver was evaluated by software simulation and the results show a substantial reduction of the error floor relative to that of differential detection. Furthermore, in additive white Gaussian noise (AWGN) channels, the adaptive nature of the receiver allows it to perform close to ideal coherent detection of differentially encoded phase shift keying (DE-CPSK). >

Optimum quantised low sidelobe phase tapers for arrays

Abstract: A genetic algorithm is used to find optimum quantised low sidelobe phase tapers for linear arrays of point sources. This method has the advantage that the optimum taper is found in quantised phase space rather than in continuous phase space.

Digital carrier frequency estimation for multilevel CPM signals

Abstract: This paper analyzes a feedforward synchronization scheme for carrier frequency recovery in continuous phase modulation. The algorithm is non-data-aided and non-clock-aided and can be implemented in digital form. It operates with general multilevel partial response signaling. Because of its feedforward structure, its estimation time is independent of the frequency offset. Its estimation range is large, on the order of the symbol rate. Performance is investigated theoretically and is checked by simulation.

System and method for maintaining continuous phase during up/down conversion of near-zero hertz intermediate frequencies

Abstract: A digital rotating complex phasor generator (60) provides a constant magnitude continuous phase signal at either a positive or a negative frequency for translating the frequencies of a modulated signal from a near-zero intermediate frequency (IF) to baseband. The rotating complex phasor generator allows a digital receiver front-end to down-convert the received signal to an IF of substantially zero Hertz prior to analog to digital conversion, even in the presence of large Doppler frequency shifts and tuning uncertainties which may result in a residual IF carrier which has either a positive or a negative frequency.

Simplified coherent detection of CPM

Abstract: This proposal avoids the complex matched-filter bank required for optimal detection. Instead, two simple lowpass filters are employed, followed by a sampler and a Viterbi algorithm that accounts for ISI. For representative modulations it is shown that near-optimal performance can be achieved with just two samples per baud. >

Multi-bit correlation of continuous phase modulated signals

Abstract: A technique for demodulating CPM spread spectrum signals and variations thereof. A receiver receives a CPM spread spectrum signal, splits it into two signals, demodulates the signals (either coherently or non-coherently) and simultaneously attempts to correlate for a plurality of chip sequences (e.g., I and Q or even and odd), ultimately combining the results into a unified correlation signal. A plurality of A/D converters quantize the demodulated signals into multi-bit digital signals prior to correlation, and multi-bit correlators operate on the multi-bit digital signals to produce correlation signals that are combined to form a unified correlation signal for detection.

Method and apparatus for differential phase encoding and decoding in spread-spectrum communication systems with continuous-phase modulation

Abstract: A method and apparatus for phase encoding and decoding a CPM spread spectrum signal. A transmitter divides a data stream into a data symbol portion and an associated phase information portion. The data symbol portion is used to select one of a plurality of spread spectrum codes for transmission from a symbol table. The phase information portion is used to differentially phase encode the data symbol prior to transmission. The transmitter divides the phase encoded spread spectrum codes into a plurality of data streams (such as I and Q data streams), independently modulates the I and Q data streams using CPM or a related technique, and superposes the plurality of resultants for transmission. A receiver receives the superposed spread spectrum signal and simultaneously attempts to correlate for a plurality of chip sequences (such as I and Q chip sequences), and derives a real correlation signal and an imaginary correlation signal. For each received symbol, the receiver determines which of a plurality of phase sectors the phase angle lies in, and compares the difference between the present phase sector and the immediately preceding phase sector. For biphase encoding, if the difference in closer to 0°, then the receiver outputs a first bit, and if the difference is closer to 180°, the receiver outputs a second bit. In a particular embodiment, a plurality of correlators operate in parallel to carry out phase decoding of the received signal.

New techniques for multi-prototype waveform coding at 2.84 kb/s

Abstract: The paper describes new techniques for prototype waveform (PW) coding at coding rates as low as 2.84 kb/s. The algorithm produces good communications quality speech with significant improvements over previously reported PW coding schemes. In multi-prototype waveform (MPW) coding, prototypes are extracted at 2.5 ms intervals. No distinction is necessary between voiced and unvoiced speech since the normalised LP residual prototypes are coded as a combination of a noise vector and smoothly evolving pitch pulse. At low bit rates it is unnecessary to explicitly code the underlying pulse shape-the relative magnitude of the rapidly evolving noise vectors is sufficient to describe the level of periodicity in each prototype. Prototypes are quantised using either an open or closed-loop scheme with similar results. Quantised prototypes are interpolated by continuous phase interpolation of Fourier coefficients in the discrete frequency domain.

Quadrature frequency/phase modulation

Abstract: A digital modulation method based on quadrature-carrier multiplexing of two N-dimensional biorthogonal signal sets is proposed. An embodiment of the proposed method referred to as quadrature frequency/phase modulation (QFPM), and its continuous phase modification CP QFPM, are described. QFPM results when two NFSK/2PSK (combined frequency and phase shift keying) signals are quadrature-carrier multiplexed. The power efficiency of QFPM and CP QFPM on the AWGN (additive white Gaussian noise) channel is always superior when compared to that of quadrature-amplitude modulation (QAM), and their bandwidth efficiency is demonstrated to be under certain conditions comparable or better than that of QAM. Both schemes possess a self-synchronization capability. The demonstrated performance characteristics of QFPM and CP QFPM suggest that they may be competitive with respect to QAM on channels where little or no filtering is desired. >

Multi-level digital modulation performed directly at carrier frequency

Abstract: Direct modulation of the carrier signal has been shown to be an attractive means of reducing the complexity of the RF hardware required in a communications transceiver. To this end, a proof-of concept X-band vector modulator, realised using monolithic phase shifters and attenuators, has been used to demonstrate that multi-level digital modulation can be implemented at high data rates and without requiring significant control power. The measured constellations for QPSK, 32-PSK and 16 QAM modulation are presented. This technique is suitable for both microwave and millimetre wave appicatons.

Improved post-detection processing for limiter-discriminator detection of CPM in a Rayleigh, fast fading channel

Abstract: A simple, robust processing strategy, called the fading magnitude-integrate sample and dump (FM-ISD) processor, is proposed for use with limiter-discriminator detection of CPM signals in Rayleigh, fast fading channels. The FM-ISD processor is introduced as a simplification of an optimal estimator-correlator receiver. The performance is compared to a standard integrator processor and found to provide an improvement in performance for all values of signal-to-noise ratio. The FM-ISD processor is also shown to be robust to changes in modulation format, channel fading rate and pre-detection filter type. >

Method and apparatus for generating and detecting amplitude and phase modulated sensor signals

Abstract: A method of extracting a phase and amplitude modulation signal impressed on a carrier is disclosed. The method includes generating the carrier at a frequency which is a multiple of the bandwidth of the modulation signal. The multiple equals 4N+1, where N is an integer. The modulation signal is impressed on the carrier, generating a phase and amplitude modulated carrier, The modulated carrier is digitally sampled at a frequency of four times the bandwidth the modulation signal. The digital sampling is initiated substantially contemporaneous with initiation of generating the carrier. A 90-degree phase shift is applied to the digitally sampled carrier to generate a quadrature component of the digitally sampled carrier. A time delay is applied to the digitally sampled carrier to restore time coincidence between the quadrature component and said digitally sampled carrier, and the phase and amplitude modulation signal is extracted by commutation of the time-delayed digitally sampled carrier and the quadrature component of the digitally sampled carrier. In a particular embodiment of the invention, the phase and amplitude can be extracted from the extracted signal.

Co-channel demodulation for continuous-phase modulated signals

Abstract: Cochannel interference, which occurs when two or more signals share the same spectral and temporal channels, is a major problem in frequency- and time-division multiple access systems. We focus on the development and evaluation of a joint maximum likelihood algorithm for simultaneously detecting two cochannel continuous phase modulated (CPM) signals. Particular attention is given to GMSK, a class of CPM employed in the pan-European cellular communications system.

Chip timing recovery in digital modems for continuous-phase CDMA radio communications

Abstract: In this paper, we consider the architecture of an all-digital modem for band-limited spread-spectrum radio communication networks employing code-division multiple access with generalized MSK modulated signals. After a general description of the modem scheme, we focus on the problem of spreading code synchronization, presenting three different chip timing recovery loops and analyzing their relative performance in terms of RMS tracking error. >

Offset quadrature phase shift keyed modulation circuit

Abstract: An improved quadrature phase shift key modulator circuit of the type which includes a non-linear amplifier in the transmitter and includes a frequency select logic circuit for receiving the in phase and quadrature phase digital data to be modulated onto a carrier frequency signal. The output of the frequency select circuit produces select signals that are coupled to the input of a digital carrier frequency generator. The digital carrier frequency generator synthesizes and increases, decreases, or leaves unchanged the carrier frequency as a representation of the data occurring on the in phase and quadrature phase input lines. The output of the digital carrier frequency generator is smoothed and converted to an analog signal which has a constant vector power magnitude during phase change. The constant vector power when amplified in a non-linear amplifier of the transmitter is not susceptible to regeneration of side lobes of the carrier signal, thus, provides a more narrow bandwidth modulated carrier frequency signal.