Showing papers on "Continuous phase modulation published in 1989"

Simple coherent receivers for partial response continuous phase modulation

Abstract: By using a pulse-amplitude-modulation representation of binary continuous-phase-modulation signals, the authors develops a novel optimum Viterbi sequence detector and a near-optimum Viterbi receiver with low complexity. For modulation index 0.5, where a linear receiver can be used, a minimum-mean-squared-error linear receiver filter is derived. The performance of all of these is analyzed, using the Gaussian minimum-shift-keying signal (GMSK) for illustration. It is shown that a GMSK receiver consisting of two matched filters and a four-state Viterbi algorithm performs with less than 0.24-dB degradation compared with the optimal receiver. The linear receiver is optimum for all values of E/sub b//N/sub 0/ (bit-energy-to-noise one-sided spectral density ratio). A design method for its filter is given. The filter is equivalent to a cascade of a matched filter and a Wiener filter estimator. Both upper and lower bounds for the bit-error probability are calculated. Simulation results which confirm the analysis are given. >

Simple coherent receivers for partial response continuous phase modulation

Abstract: By using a pulse-amplitude-modulation representation of binary continuous-phase-modulation signals, the authors develops a novel optimum Viterbi sequence detector and a near-optimum Viterbi receiver with low complexity. For modulation index 0.5, where a linear receiver can be used, a minimum-mean-squared-error linear receiver filter is derived. The performance of all of these is analyzed, using the Gaussian minimum-shift-keying signal (GMSK) for illustration. It is shown that a GMSK receiver consisting of two matched filters and a four-state Viterbi algorithm performs with less than 0.24-dB degradation compared with the optimal receiver. The linear receiver is optimum for all values of E/sub b//N/sub 0/ (bit-energy-to-noise one-sided spectral density ratio). A design method for its filter is given. The filter is equivalent to a cascade of a matched filter and a Wiener filter estimator. Both upper and lower bounds for the bit-error probability are calculated. Simulation results which confirm the analysis are given. >

Frequency selectivity in amplitude-modulation detection

Abstract: For a broadband noise carrier, the modulation detection threshold for sinusoidal amplitude modulation (the test modulation) is measured in the presence of an additional modulation (the masker modulation). Two traditional approaches for revealing effects of frequency selectivity in the audiofrequency domain are shown to give comparable results in the modulation‐frequency domain: (1) a typically peaked modulation‐detection threshold pattern when the masker modulation is a fixed narrow band of noise, and (2) an effect of leveling off of the increase of the modulation‐detection threshold when, for a fixed test‐modulation frequency, the masker‐modulation bandwidth is widened beyond a certain ‘‘critical’’ bandwidth. It is argued that the present results on frequency selectivity in modulation detection underline the perceptual relevance of a spectral decomposition of a signal’s temporal envelope and provide a rationale for the application of modern concepts like the speech‐envelope spectrum or the modulation‐transfer function in relation to speech intelligibility.

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

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

Phase-only modulation using a twisted nematic liquid crystal television

Abstract: We used a commercially available liquid crystal television display unit as a spatial optical phase-only modulator. To do this, we removed the integral polarizers from the unit and double passed the light under modulation through it. We found that it was possible to obtain continuous phase modulation from 0 to PI with essentially no change in the state of polarization of the output light and absorption changes of <2.5%. We wrote computer-generated phase-only holograms on the LCTV and reconstructed them optically.

Automatic modulation recognition of digitally modulated signals

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

An alternative approach to reduced-complexity CPM-receivers

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

An all digital receiver architecture for bandwidth efficient transmission at high data rates

Abstract: Using the maximum-likelihood approach, algorithms for detection and synchronization are derived that are well suited for VLSI implementation. Special emphasis is placed on an all-digital implementation where carrier and clock synchronization do not require a feedback of signals to the analog part, which simplifies the analog front-end design (mixing oscillator and A/D converter sampling clock run at fixed frequency). An important advantage of the proposed algorithms is that a high clock rate is not required; only two-four times the symbol rate is needed, depending on amplitude quantization. Implementation aspects, e.g. architecture, and quantization, are considered. A prototype is described which was implemented to prove the feasibility of the concept and to evaluate the performance under practical conditions. >

Design of coded CPFSK modulation systems for bandwidth and energy efficiency

Abstract: Consideration is given to the problems related to the design of M-ary continuous-phase frequency-shift keying (CPFSK) systems with modulation index h=J/M, combined with eternal rate r binary convolution encoders. The following questions are raised and answered: (1) how should different encoder-modulator systems be compared and how can comparable systems be recognized from the system parameters, i.e. M, h, and r?; (2) what are the limits on the information rate per unit bandwidth, versus signal-to-noise ratio, when reliable transmission is required?; (3) how does one choose the system parameters M, h, and r when the overall system has to achieve a specified performance?; and (4) how does one design the external rate r binary convolutional encoder to put in front of the M-ary CPFSK modulation system with h=J/M? A simple approximation for the bandwidth of a CPFSK signal is given and shown to be sufficiently accurate for system design purposes. The design of the external convolutional encoder is carried out in a novel way that leads to fewer states in the combined encoder-modulator system and thus yields improved performance for a given demodulation-decoding complexity compared to previous approaches for the design of coded CPFSK systems. >

Method and apparatus for measuring phase accuracy and amplitude profile of a continuous-phase-modulated signal

Abstract: A method and apparatus for determining the phase and amplitude accuracy of continuous-phase-modulated signals is described. A modulated RF signal generated by a transmitter is down converted to a relatively low intermediate frequency which is filtered and sampled by a high sampling rate analog-to-digital convertor. A digital signal processor processes the digital signals to produce a measured amplitude function and a measured phase function corresponding to the modulated RF signal. From the measured amplitude and phase functions, an ideal phase function corresponding to the modulated RF signal is calculated and synthesized. The ideal phase function is compared to the measured phase function to determine the phase function from which the modulated RF signal phase error and frequency error are computed.

Noncoherent detection of convolutionally encoded continuous phase modulation

Abstract: The authors consider the error probability at large signal-to-noise ratios of rate-1/2 convolutionally encoded CPFSK (continuous-phase frequency-shift keying) with an optimum noncoherent detector on an additive white Gaussian noise channel. The performance is given in terms of a parameter called the minimum squared normalized equivalent Euclidean distance which plays the same role mathematically as the minimum squared normalized Euclidean distance used for coherent detectors. It is shown that by introducing convolutional coding the error performance is significantly improved. The authors propose a decoding algorithm for these convolutionally encoded CPM schemes which is based on a limited tree search algorithm and uses the maximum-likelihood decision rule for noncoherent detection. Computer simulations show that the degradation in error performance compared to the performance of the optimum coherent Viterbi detector is less than 1 dB with a relatively simple noncoherent detector on an additive white Gaussian noise channel for most of the schemes considered. >

Transmitter circuit for efficiently transmitting communication traffic via phase modulated carrier signals

Abstract: Transmitter for transmitting communication traffic via phase modulated carrier signals. The device provides for an efficient amplification of a phase modulated carrier signal using Class C amplifier (19) technology. An envelope function is applied to the carrier signal by modulating the Class C amplifier (19) with a related spectrum-suppressing amplitude envelope signal. Phase distortion accompanying the amplitude modulation of the Class C amplifier is compensated by generating a phase offset for the carrier signal. As each envelope function for suppressing the spectrum is generated, a corresponding phase offset may be applied to the carrier signal, avoiding the consequences of phase shift resulting from amplitude modulating the carrier signal in the Class C amplifier (19).

Carrier and data recovery and demodulation system

Abstract: A modulated carrier is received on a node (10) and then the I- and the Q-channels extracted with a local carrier clock (16) that is free-running relative to the modulated carrier. The I- and the Q-channels are then input to a digital signal processor (26) wherein a composite phase signal is generated representing the phase difference between the modulated carrier and the local carrier clock (16). The composite phase is output from a look-up table in an EPROM (30) and then the free-running phase the local carrier clock (16) and the modulated carrier is discriminated from the composite phase. The free-running phase is then subtracted from the composite phase to yield the phase modulation on the carrier.

Decoding of severely filtered modulation codes using the (M, L) algorithm

Abstract: The problem of decoding data in the presence of infinite-duration intersymbol interference that is caused by severe channel filtering is considered. Filtered continuous phase modulations (CPM) are the particular object of study. A state-variable approach is used for defining the decoder tree. Maximum likelihood sequence estimation requires exhaustive tree searching, which is restricted by using the (M, L) algorithm since this approach does not require that the channel intersymbol interference be finite. After briefly describing the (M, L) algorithm, the authors motivate the problem of equalization of infinite-impulse-response channels by considering the performance of a discrete-time single-pole channel filtering a binary input sequence. The state variable description of a linear system is used to analyze the filtered modulation. The state of the filtered modulation for a given input modulation is used to define the tree structure of the filtered signal upon which the (M, L) algorithm operates. The minimum signal space distance results for several filtered CPM schemes are then summarized. Extensive simulation results are presented, and comparisons to the optimal performance are made. >

Coherent demodulation method for a digitally modulated signal having a continuous phase and a constant envelope

Abstract: The method ensures the coherent demodulation by digitally processing a continuous-phase modulated signal (for instance, of the GMSK type). The received signal is transposed in the baseband, converted into a digital signal and transferred to a signal processor. Each transmitted data packet has a known preamble sequence of N bits which allows for approximate estimation of the frame-synchronization and the bit-synchronization and also the initial phase and the residual frequency offset. The progressive refinement of the estimation is obtained with two interleaved digital loops: a slow loop for detecting the bit-synchronization and a fast loop effecting intermediate decisions over additional blocks of bits for the estimation of the initial phase and the residual frequency offset.

Analysis of two loops for carrier recovery in CPM with index 1/2

Abstract: An examination is made of the performance of a squaring loop and a fourth-power loop used for tracking the received carrier phase of binary continuous-phase modulated (CPM) signals with index 1/2. Noise-free analysis is performed first and a simple relationship between the phase error and the control voltage is obtained for each case, with the premodulation filter response as a parameter. Linearized analysis of the phase error variance is then conducted in the presence of additive white Gaussian noise. It is shown that the phase variance in both cases contains a self-noise component that sets a lower bound on the synchronizer's performance at very high signal-to-noise ratios. Numerical results are presented for a number of popular CPM signals. It is found that the squaring loop performs best with MSK (minimum shift keying), while the fourth-power loop performs best with duobinary MSK. >

Probability of error analysis of digital partial response continuous phase modulation with noncoherent detection in mobile radio channels

Abstract: An analysis is presented of noncoherent detection of constant-envelope digital partial-response continuous-phase modulation (PRCPM) in fast Rayleigh fading that characterizes land mobile radio channels. Closed-form expressions for the probability of error are derived for limiter discriminator detection, and both 1- and 2-bit differential detection. Numerical results are presented for cases of practical interest to researchers and designers of land mobile radio systems. The expressions derived for the probability of error are general and can be used for all PRCPM schemes. >

Continuous phase shift modulation system with improved spectrum control

Abstract: A continuous phase shift modulation system wherein, for each of the in-phase and quadrature components, adjacent half-cosine pulses of the same polarity are joined by a continuous transition modulation signal which maintains continuity of the waveform and at least its first derivative When this occurs, the other component signal is adjusted to maintain constant the vector sum of the component signals

Phase detector and frequency demodulator including such a phase detector

Abstract: Phase detector having a first input for an angle-­modulated input carrier to be detected, and a second input for a reference carrier having a phase shift which is dependent on the angle modulation with respect to the input carrier, the input and reference carriers being at least multiplied in the phase detector, and a frequency demodulator including such a phase detector. In order to inhibit signal distortion and notably second-order interference products in the demodulation of an angle-modulated input carrier, the phase detector according to the invention is adapted to derive an in-­phase carrier A and a phase quadrature carrier B from the input carrier and to derive an in-phase carrier C and a phase quadrature carrier D from the reference carrier which has a phase which, at an average, is equal or opposite to that of the input carrier, while the phase detector is further adapted to supply from its output a signal which substantially corresponds to AC - BD.

Multi-H phase-coded modulations with asymmetric modulation indexes

Abstract: Multi-H phase-coded modulation (MHPM) is a bandwidth-efficient modulation scheme which offers substantial coding gain over conventional digital modulation schemes. MHPM with asymmetric modulation indices corresponding to the bipolar data +1 and -1 is considered, and numerical results for the minimum Euclidean distances are provided. It is shown that performance improvements on the error probability over conventional MHPM are gained with essentially the same bandwidth and a very slight modification in implementation. The upper bounds on the error probabilities as functions of observation intervals and received E/sub b//N/sub 0/ are also investigated in detail. It is concluded that the concept of asymmetric modulation indices for MHPM is attractive for bandwidth and power-efficient modulation. >

On the performance of spectrally efficient trellis coded FM modulation employing noncoherent FM demodulation

Abstract: The authors analyze the problem of noncoherent FM demodulation of trellis-coded continuous-phase M-ary FSK (frequency-shift keying). The FM demodulation process is divided into two parts, the first being the actual noncoherent FM demodulation and the second being trellis decoding of the data. Upper bounds on the bit error rate as well as the 99% energy bandwidth are determined for the codes under consideration. In particular, the authors consider the trellis codes with rates 1/2 and 2/3 and symmetric and asymmetric signal constellations. Upper bounds on the probability of error are obtained for the symmetric and the optimum asymmetric cases. The optimum asymmetry is one which minimizes the bit error probability. The performance of this system is compared to that of the standard continuous phase modulation techniques employing noncoherent detection. >

Phased frequency steered antenna array

Abstract: A broadband phased frequency antenna array uses frequency steering with phase-shift stabilization. Phased frequency steering allows wider intermediate bandwidth than available from frequency steered arrays, with fewer phase shifters than required by phase steered arrays. For a given instantaneous bandwidth (such as for FM-chirp or frequency agility operations), the phased frequency steered array provides a straightforward trade-off between sidelobe level and the number of phase shifters. The antenna includes a linear array (10) of phase-shift/time-delay modules (FIG. 1b), each including (a) a phase-shift element (PSE) with a phase shifter (PS), and (b) a number of time-delay elements (TDE) coupled through respective time-delay feeds (TDF) to the phase shifter. In accordance with conventional antenna pattern weighting, the phase shifters are concentrated in the center of the array (10), with the number of time-delay elements in a phase-shift/time-delay module increasing for modules located toward the edge of the array, producing the desired phase shifter "thinning". The phase shifter of each phase-shift/time-delay module is cooperatively set relative to a scan frequency to provide an appropriate phase-shift offset that aligns the phase front segments (S 0 -S 13 ), achieving a continuous phase slope across the phase front (FIG. 1c).

Method and device for the demodulation of signals with constant envelope and continuous phase angle modulation by a train of binary symbols tolerating frequency drifts

Abstract: The continuous phase angular demodulation method disclosed digitally processes the signal in baseband after having over-sampled it with reference to the bit period. The processing consists in routinely demodulating sub-sets of differential phases at the bit period T b shifted with respect to one another by fractions T b /q of this period, in correcting them, a priori, by the phase deviations associated with a set of pre-defined d frequency drifts and in computing, for the d.q sets thus obtained, a noise criterion. The set of demodulated bits chosen is the one that reduces this noise criterion to the minimum. The set of bits then enables computation of the phase variation emitted and, using this variation and the measured variation, the real frequency drift. Another demodulation taking this drift into account is done for the q sets of initial differential stages and the set minimizing the noise criterion is then chosen and fixes the synchronization bit by the sampling instants associated with it.

A partially coherent detector for continuous phase modulation

Abstract: A novel partially coherent detector for joint data detection and carrier phase estimation of continuous-phase-modulated (CPM) signal is presented. The algorithm differs from coherent Viterbi algorithm only in the metric which it maximizes over the possible transmitted data sequences. This metric is influenced both by the correlation of the hypothesized signal with the received one and the current carrier phase estimate. The system has been simulated and comparisons made with existing detection techniques on channels with additive white Gaussian noise and carrier phase jitter. Simulations of the detector are used to assess the relative susceptibility of different CPM (continuous phase modulation) schemes to phase jitter. The trend is that narrower band schemes are more sensitive to carrier phase inaccuracy than the less spectrally efficient ones. >

Arrangement for equalizing and demodulating binary continuous-phase angle-modulated data signals with modulation index 0.5

Abstract: A preprocessing unit is inserted between a baseband converter and an equalizer/detector for performing a signal processing which corresponds to a phase rotation of the CPM-signal in the baseband represented by in phase and quadrature components. The phase rotation progresses in discrete steps of 90° per bit interval of the binary data signal contained in the CPM-signal. The introduction of the preprocessing unit renders it possible to use a comparatively simple conventional equalizer/detector for binary pulse amplitude modulated signals (PAM-signals) for equalizing and detecting binary CPM-signals received in distorted form. When the equalizer/detector is adaptive, a further preprocessing unit is inserted between a channel estimator and adjusting inputs of the equalizer/detector.

Phase Only Liquid Crystal Light Modulator And Its Application In The Fourier Plane Of Optical Correlation Systems

Abstract: It has been shown theoretically that optical correlation systems which use phase only filters are capable of producing sharp correlation peaks and good discrimination between different input objects, whilst at the same time having high optical efficiency. In addition to this, the resolution required in the filter plane is very much lower than that required for an equivalent conventional Van der Lugt correlator using a matched filter which has been made holographically. Here, we describe a flexible phase only filter implemented using a special-purpose liquid crystal spatial light modulator, and present results which demonstrate how it can be applied in optical correlation systems. 1. INTRODUCTION There has recently been a great deal of interest in phase only filters as applied to optical correlation systems. This type of filter modifies the phase distribution of light passing through it, without altering the amplitude distribution. It has been shown by computer simulation that, when applied in the Fourier Plane of coherent optical,correlation systems, phase only^fliters are capable of giving very sharp correlation peaks and high optical efficiency .The work of Psaltis et. al. 3 and Flannery et. al. 45 ' has demonstrated very effectivelythat useful optical correlators can be constructed using phase only filters implemented with spatial light modulators which have limited pixel counts (typically 128 X 128 pixels).However, these authors used magneto-optic devices , which only allow binary phase modulation of the incoming light - i.e. the only phase modulation values allowed for each pixel are zero or PI. Whilst binary phase only filters can give good correlation performance in most situations, with some types of object (particularly self-symmetric, displaced, reference objects) they can give undesirably^ ghigh noise levels and even false correlation peaks in the output correlation plane ' . Even though the amount of information required to write binary phase only filters is lower, in many situations - particularly where safety ^considerations are involved - the true phase only filter is preferable. In previous work we have shown how it is possible to use a commercially available twisted nematic liquid crystal television panel as a continuous phase only modulator, but construction of a practical correlator using this technique would be very difficult since the phase modulation depth available is very limited, and the aberrations of the panel very high.In this paper, we describe a new special-purpose liquid crystal panel which can be used as a continuous phase only spatial light modulator, and it's application in an optical correlator system. The panel is not twisted nematic, and so phase modulation can be achieved in one pass with essentially very little change in the polarisation state of the light passing through it. The panel we used was designed to have a phase modulation depth in excess of 2PI, and low aberrations, and this enabled us to use it with high efficiency and with no aberration correction.

Error probability analysis for continuous phase modulation with Viterbi detection on a Gaussian channel with multiple signal interference

Abstract: The effects are studied on the symbol error probability of the Viterbi detector for continuous phase modulations with constant amplitude, when the desired signal is received in multiple interfering signals and white Gaussian noise. The detection is assumed to be coherent and, furthermore, perfect timing in the detector is also assumed. An upper bound on the symbol error probability is derived for the Viterbi detector. The bound is based on the union bound technique, and it is shown to be tight for small error probability levels. It is found that the smoothed binary and all quaternary modulations sustain much larger interference power in adjacent channel interference than minimum-shift keying (MSK) does and, thus, the adjacent channels can be spaced closer in frequency for these modulations, In particular, the smoothed quaternary modulations seem to be significantly more efficient than MSK. In cochannel interference the difference in error performance between the schemes is relatively small. >

Method of coherently demodulating a continuous phase, digitally modulated signal with a constant envelope

Abstract: Le procede assure la demodulation coherente par traitement numerique d'un signal module en phase continue (par exemple de type GMSK). The method ensures the coherent demodulation by digitally processing a modulated signal in continuous phase (for example of GMSK). Le signal recu est transpose en bande de base, converti en numerique et transmis au processeur de signal. The received signal is transposed in the baseband, converted to digital and transmitted to the signal processor. Chaque paquet de l'information binaire transmise comporte une sequence preliminaire connue sur N bits permettant l'estimation approximative de synchro-trame et synchro-bit d'une part, de la phase initiale et de l'ecart residuel en frequence d'autre part. Each packet of the transmitted binary information includes a first known sequence of N bits for the rough estimation of frame-synchronization and bit synchronization on the one hand, the initial phase and the residual difference frequency in the other hand . L'affinement progressif desdites estimations est obtenu au moyen de deux boucles numeriques imbriquees : une boucle lente pour la detection de synchro-bit et une boucle rapide effectuant des decisions intermediaires sur des blocs additionnels de bits pour l'estimation de la phase initiale et de l'ecart residuel en frequence. The progressive refinement of said estimates is obtained by two interleaved digital loops: a slow loop for detecting bit synchronization and a fast loop effecting intermediate decisions over additional blocks of bits for the estimation of initial phase and the residual frequency deviation.

Trellis-coded 8PSK with embedded QPSK

Abstract: A coded eight-phase-shift-keying (C8PSK) method is described that is characterized by a quaternary phase-shift-keyed signal embedded in the modulated sequence. This method is a double-trellis-coded modification of the well-known C8PSK and is referred to as MC8PSK. Five coded bits are generated from four user bits in a single coding step and then mapped to an 8PSK symbol followed by a QPSK symbol. Given comparable decoder or receiver complexity, the coding gains of C8PSK and MC8PSK are comparable. Advantages of MC8PSK concern carrier synchronization. With C8PSK, problems with carrier phase tracking are encountered. With MC8PSK, these can be avoided, since carrier phase control can be based on the embedded QPSK. For some codes, error-free decoding is achievable in any locking condition of a QPSK-dependent PLL (phase locked loop), making synchronization and decoding especially simple. The partitioning of the channel symbol set, the associated bit mapping, and the code design are treated in order to describe MC8PSK. Aspects of receiver synchronization are discussed for C8PSK and for MC8PSK, focusing on QPSK phase-detection principles. The implementation of a four-state MC8PSK modem for an information rate of 2.048 Mb/s is described, and experimental results demonstrating the very robust carrier synchronization are presented. >