Showing papers on "Phase-shift keying published in 1988"

A decomposition approach to CPM

Abstract: It is shown that any continuous-phase-modulation (CPM) system can be decomposed into a continuous-phase encoder and a memoryless modulator in such a way that the former is a linear (modulo some integer P) time-invariant sequential circuit and the latter is also time invariant. This decomposition is exploited to obtain alternative realizations of the continuous-phase encoder (and hence of CPM) and also to obtain alternative forms of the optimum decoding algorithm. When P is a prime p so that the encoder is linear over the finite field GF(p), it is shown that cascading it with an outside convolutional encoder is equivalent to a single convolutional encoder. It is pointed out that the cascade of the modulator, the waveform channel (which it is assumed is characterized by additive white Gaussian noise), and the demodulator that operates over one symbol interval yield a discrete memoryless channel that can be studied without the distractions introduced by continuous-phase encoding. >

The design of trellis coded MPSK for fading channels: set partitioning for optimum code design

Abstract: A previous work on criteria for designing trellis-coded MPSK modulation to achieve minimum error probability performance on the Rician fading channel (see ibid., vol.36, no.9, p.1004-1012, Sep. 1988) is extended. It is demonstrated that allowing for multiple symbols per trellis branch, i.e., multiple trellis-coded modulation (MTCM), provides an additional degree of freedom for designing a code to meet the optimization on the fading channel. Diversities larger than those achievable with conventional trellis codes having the same number of trellis states are now attainable, it is under these conditions that MTCM achieves its full potential. >

Multiple trellis coded modulation (MTCM)

Abstract: The authors demonstrate a trellis coded modulation technique referred to as multiple trellis coded modulation (MTCM) wherein more than one channel symbol per trellis branch is transmitted. They have found simple two-state trellis codes for symmetric MPSK multiple phase-shift keying and AM modulations that can achieve 3-dB gain over uncoded modulation at very high signal-to-noise ratios without bandwidth expansion or reduction in information bit rate. The gain of these codes with respect to previously reported two-state trellis codes is between 1 and 2 dB at very high signal-to-noise ratios, depending on the number of bits per Hertz transmitted. These gains are achieved for those of the equivalent conventional trellis codes with the same number of states in the trellis diagram. The authors note that additional computations per branch are needed for the multiple trellis coding scheme. The concept can be extended to a higher number of states and other types of modulations. >

The phase of a vector perturbed by Gaussian noise and differentially coherent receivers

Abstract: Some results are presented regarding the asymptotic distribution of the phase of a vector perturbed by Gaussian noise. It is shown that for large signal-to-noise ratio, the asymptotic distribution of the phase is of the Tikhonov type. This framework is then used for the synthesis of differentially coherent receiver structures, one for M-ary phase-shift keying (MPSK) and the other for minimum-shift keying (MSK). The first structure bridges the performance gap between coherent and differentially coherent demodulation of MPSK. The MSK receiver uses matched filtering with differential demodulation. >

Trellis coded modulation for transmission over fading mobile satellite channel

Abstract: The combination of trellis coding and MPSK signaling with asymmetry (nonuniform spacing) to the signal set is disclosed with regard to its suitability for a fading mobile satellite communication channel. For MPSK signaling, introducing nonuniformity in the phase spacing between signal points provides an improvement in performance over that achievable with trellis codes symmetric MPSK signaling, all this without increasing the average or peak power, or changing the bandwidth constraints imposed on the system. Block interleaving may be used to reduce error and pilot tone(s) may be used for improving the error correction performance of the trellis decoder in the presence of channel fading.

New phase and frequency detectors for carrier recovery in PSK and QAM systems

Abstract: Phase and frequency detectors (PFDs) are presented that considerably extend the acquisition range of carrier-recovery loops in digital communication systems. Based on a simple modification of conventional phase detectors (PDs), the proposed detectors are applicable to a large variety of modulation schemes, including the popular PSK and QAM signal formats. Their application to QPSK and 16- and 64-QAM is extensively discussed, and simulated frequency-detector (FD) characteristics, as well as acquisition behavior of several PFDs, are reported for QPSK and 16 QAM. The results of an experimental evaluation using a 16-QAM laboratory modem are also reported which show that the detectors increase the acquisition range achievable by conventional PDs by more than one order of magnitude. In PSK, the improved acquisition performance is obtained with no penalty in steady-state phase jitter. In combined amplitude- and phase-shift keying, it generally leads to increased jitter, but this is easily avoided by incorporating a lock indicator and switching back to the original PD after lock is acquired. >

Differential detection of GMSK using decision feedback

Abstract: Differential detector structures that use decision feedback to partially remove the effect of destructive intersymbol interference from minimum-shift keying (MSK) are introduced. In comparison to the conventional two-bit differential detection of GMSK (Gaussian MSK), the proposed structures provide about 4-dB E/sub b//N/sub 0/ improvement when the premodulation filter time-bandwidth product is 0.25. As the premodulation filter becomes narrower, the advantages of the proposed receivers are more pronounced. Thus the bit-error-rate performance of differential detection is brought closer to coherent detection while avoiding the problems associated with the carrier recovery. >

The performance of trellis coded multilevel DPSK on a fading mobile satellite channel

Abstract: The performance of a trellis-coded multilevel differential phase-shift keying (DPSK) modulation over a mobile satellite channel characterized by additive white Gaussian noise (AWGN) and slow Rician fading is presented. Both the optimum and Gaussian (suboptimum) decoding metrics are considered, with performance results given only for the latter. Analytical results (upper bounds on bit error rate) are obtained wherever possible and illustrated by several numerical examples. Also given are simulation results which are more indicative of the exact system performance. Comparisons with results previously obtained for coherent detection of the same coded modulations are presented. >

Phase coherent TDMA quadrature receiver for multipath fading channels

Abstract: A method and apparatus for phase-coherently demodulating a multipath-impaired time division multiple access QPSK data timeslot is disclosed. A quadrature separator (105, 107, 111) generates multipath-impaired intermediate signals which, during a predetermined synchronizing sequence for the timeslot, are applied to a pair of synchronizing correlators (303,305) to generate quadrature multipath profiles. These profiles then are used to modify subsequently received QPSK timeslot signals at the separator outputs to coherently construct multipath compensated I and Q channel data.

Performance of DS/SSMA communications in impulsive channels. II. Hard-limiting correlation receivers

Abstract: For pt.I see ibid., vol.COM-35, no.11, p.1179-88 (1987). In part I it was demonstrated that impulsive channel noise can be a serious detriment to the performance of direct-sequence spread-spectrum multiple-access (DS/SSMA) communications when conventional linear correlation reception is used. Here, a hard-limiting correlator as an alternative for reception of multiple-access transmission in impulsive channels is considered. For K asynchronous binary PSK DS/SSMA users sharing a linear channel corrupted by impulsive noise that is modeled at the output of the front-end filter of the receiver, techniques are developed for analyzing bit error probabilities of this hard-limiting receiver by exact computation for short spreading sequences, by approximation for longer spreading sequences, and by asymptotic limits for infinitely long spreading sequences. Performance is compared to that of the linear correlator under a variety of conditions, showing that hard-limiting correlation reception can offer substantial improvement over conventional systems in impulsive channels. However, the linear receiver is more effective against multiple-access noise only, and so a tradeoff emerges between rejection of impulsive noise and rejection of multiple-access interference. >

Carrier recovery circuit for offset QPSK demodulators

Abstract: A carrier recovery circuit comprises a voltage-controlled oscillator with a π/2 phase shifter coupled to it for generating carriers of quadrature phase relationship First and second phase comparators respectively detect phase differences between an offset QPSK modulated signal and the carriers of the quadrature phase relationship Signal from the first phase comparator is delayed by a 1/2 symbol duration and applied to one input of a quadri-phase detector having stable phase angles at π/4, (3/4)π, (5/4)π and (7/4)π radian and signal from the second phase comparator is applied to the other input of the quadri-phase detector A bit timing recovery field (1010 1010) of the second channel is detected from the output of the second phase comparator Signal from the quadri-phase detector is applied to a loop filter and thence to the voltage-controlled oscillator during the time when a bit timing recovery field (BTR) of the second channel is not still detected To stabilize the operation of the carrier recovery loop of the circuit, the output of the second phase comparator from which that BTR is detected is briefly applied to the loop filter in response to the detection of a BTR of the second channel, instead of the signal from the quadri-phase detector

Digital modulation classification: the BPSK versus QPSK case

Abstract: The authors report on a novel BPSK/QPSK classifier and compare it to the more traditional, ad-hoc techniques of-a-square-law classifier and phase-based classifiers (weighting on the phase histogram). The proposed classifier is derived by an approximation of the likelihood-ratio functionals of phase modulated digital signals in white Gaussian noise, called a quasi-log-likelihood-ratio (qLLR) rule. It is shown analytically that its performance is significantly better than that of intuitively designed phased-based rules or the conventional square-law classifier. >

PSK and DPSK trellis codes for fast fading, shadowed mobile satellite communication channels

Abstract: The performance of 8-PSK and 8-DPSK trellis codes is presented for a class of fast fading, land mobile satellite communication channels. The fading model is Rician but, in addition, the line-of-sight path is subjected to a fast lognormal attenuation that represents tree shadowing. The fading parameters used in this study represent the degree of shadowing and are based on measured data. The primary application considered is for digital speech transmission and thus, bit error probabilities in the order of 10/sup -3/ are emphasized. Sensitivity of the bit error probability to amplitude fading, amplitude and phase fading, and decoding delay is presented. Performance is determined via digital computer simulation. Optimal four- and eight-state codes are determined and optimality is found to be dependent on the presence of lognormal shadowing. >

Sensitivity penalty in multichannel coherent optical communications

Abstract: The error rates and sensitivity penalties for multichannel coherent optical communications systems are evaluated for amplitude-shift keying (ASK), phase-shift keying (PSK), and frequency-shift keying (FSK) modulation, taking into account adjacent channel interference. Both time-domain and frequency-domain analysis are used, the latter being based on a Gaussian approximation. Both techniques yield similar results for sensitivity penalties below 1 dB. For FSK systems, larger values of the modulation index Delta do not necessarily lead to larger channel spacings. ASK and PSK systems both require larger channel spacings than FSK systems with Delta =1. The study was conducted for sources with linewidths narrow enough so that phase noise does not degrade the performance of receivers with matched filter demodulators. >

Polarisation phase-shift keying: a coherent transmission technique with differential heterodyne detection

Abstract: An original coherent transmission technique with polarisation modulation and (two-channel) differential heterodyne detection is demonstrated. In a preliminary experiment, sensitivities within 4 dB of the shot noise limit have been achieved. High immunity to laser phase noise is obtained. >

Performance of differentially coherent digital communications over frequency-selective fading channels

Abstract: The performance of binary differential phase-shift keyed (DPSK) communications over frequency-selective wide-sense-stationary uncorrelated-scattering fading channels is considered. A technique is described for obtaining bounds on the average error probability for DPSK in terms of one or two parameters obtained from multipath spread or frequency-correlation-function channel measurements. A method of approximating the performance of systems using various data-pulse shapes and systems where it is difficult to obtain a complete channel characterization is developed. Numerical examples and comparisons of the most interesting cases are given. >

Probability of error of M-ary PSK and DPSK on a Rayleigh fading channel

Abstract: M-ary phase-shift keying and differential phase-shift keying (DPSK) on a slow fading Rayleigh channel without diversity is investigated. Expressions for the distribution of the phase angle between a vector with Rayleigh amplitude distribution and a noiseless reference, and between two vectors both with Rayleigh amplitude distribution perturbed by Gaussian noise are obtained. >

Alternatives for on-board digital multicarrier demodulation

Abstract: In this paper alternatives for digital multicarrier demodulators (MCD) suitable for advanced digital satellite communications systems are presented. The MCD permits the direct on-board interfacing of FDMA and TDM communication links by digital signal processing techniques. Two main functions are implemented by a MCD: demultiplexing (DEMUX) and demodulation (DEMOD). We focus here only on a digital implementation of the MCD, looking at its advantages, flexibility, better performance and VLSI integrability. The DEMUX may be implemented in a number of ways: the analytic signal method, fast Fourier transform with polyphase network technique, or multistage methods. For all the implementation methods considered it is shown that a certain degree of integration of DEMUX and DEMOD functions is possible. To this end, in the proposed MCD schemes the receiver pulse-shaping filter has been integrated in the DEMUX structure, reducing the overall implementation complexity. It is shown that, for the per-channel structure based on the analytic signal method, a highly modular and flexible implementation can also be achieved. Coherent demodulation is used to reduce the signal-to-noise ratio required to achieve a specified bit error rate. The coherent demodulation is carried out by using the maximum likelihood (ML) estimation method. Two different approaches to receiver synchronization have been studied. For the first, the carrier phase and symbol timing estimates are independently derived by suitable techniques. The second approach makes use of the maximum a posteriori probability method to estimate both the carrier phase and symbol timing of the receivied signal. In particular, for this technique it is shown that, by a suitable choice of the architecture of the digital coherent receiver, the ML demodulator can be integrated in the joint carrier and clock recovery circuit, with no increase in the overall system complexity. The digital architecture of the proposed MCD can be adapted to different digital modulation techniques. However, here we consider only the application for QPSK signals, as this modulation scheme is the most promising for digital satellite communications. A theoretical analysis and computer simulation have been used to evaluate the performance degradation of the proposed MCD, including finite-arithmetic implementation effects.

Rapid reference acquisition and phase error compensation for radio transmission of data

Abstract: The method and apparatus for rapid phase acquisition and random radio channel phase error compensation for a TDMA QPSK receiver is disclosed. Received quadrature signals having a known synchronizing sequence transmitted on one of the vectors is correlated to a local replica of the sequence. The output correlation detection signals are held for the duration of the TDMA timeslot and multiplied by the received quadrature signals to produce two pairs of products. The mutually exclusive product pairs are summed to recover data having the random phase error compensated.

Tdma radio system employing bpsk synchronisation for qpsk signals subject to random phase variation and multipath fading

Abstract: A time division multiple access (TDMA) radio system conveying a quadrature phase shift keying (QPSK) modulated data signal is disclosed. In order to overcome multipath distortion and phase variation introduced by the radio channel, a synchronizing poriton of a TDMA timeslot is transmitted in only one vector of the QPSK modulation. The system utilizes the one vector synchronizing portion to develop a channel profile estimate which is applied to the received message portion of the QPSK data signal to compensate for the multipath distortion and phase variation during one TDMA timeslot.

Fourier-Bessel error performance analysis and evaluation of M-ary QAM schemes in an impulsive noise environment

Abstract: The authors derive closed-form expressions for the BER (bit error rate) performance of coherent M-ary quadrature amplitude modulated (QAM) system in the presence of an additive combination of Gaussian and highly impulsive noise by using a Fourier-Bessel series expansion method. The model used for the impulse noise, which closely models atmospheric and/or ignition noise, consists of a stream of Poisson impulses with areas distributed according to a bilateral Rayleigh probability distribution function. Although the methodology is general enough to accommodate any M-ary QAM scheme, they present analytical as well as some computer simulated results for 16-, 64-, and 256-QAM systems. An almost insignificant increase of the truncation error with the increase of the modulation levels verifies that the proposed method of analysis can be a useful tool in evaluating the performance of very large-alphabet M-ary QAM systems, e.g. M>or=256. >

Generalized minimum shift-keying modulation techniques

Abstract: The simultaneous data demodulation and phase tracking of an MSK signal using the Viterbi algorithm is described, and two variations of MSK modulation are studied. The MSK with overlay is a dual-rate modulation techniques in which low-rate and high-rate data are superimposed on an MSK signal. Here the demodulator uses the Viterbi algorithm to estimate both the low-rate and high-rate data simultaneously. The MSK with pseudorandom sequence spreading combats intentional or unintentional jamming. A simplified receiver for these spread-spectrum MSK signals is found that takes into consideration the effect of random phase perturbations. The performance of these demodulators is evaluated using transfer-function bounds for the bit error probability. For demodulation of the spread spectrum MSK signal, a simplified receiver is derived, and its performance in the presence of continuous jamming is evaluated. >

Modulation techniques for digital cellular systems

Abstract: A modulation technique is described that uses the properties of linear modulation techniques to achieve high spectral efficiency. When combined with high-quality low-bit-rate speech coding and channel coding, linear modulation provides a substantial increase in system capacity. A QDPSK system provides high-quality speech using a 10-kHz bandwidth, with better S/I protection ratios than the current analog system. A digital cellular system which uses QDPSK is discussed. Bit error rate and spectral efficiency data are presented. >

Adaptive generation of decision functions for classification of digitally modulated signals

Abstract: The performance of an adaptive technique for the generation of decision functions used in classification of several types of digitally modulated signals-binary amplitude-shift keying (BASK), binary phase-shift keying (BPSK), quaternary phase-shift keying (QPSK), and binary frequency-shift keying (BFSK)-is discussed. A computer simulation is performed to measure the properties of the classifier which is trained with one signal from each class. The method correctly identifies all the signals considered during this experiment. The most interesting result from a small number of trials is considered to be that a novel feature, useful for the identification of the number of phase states of BPSK and QPSK signals, is seen to provide adequate information identification at SNRs down to 5 dB. This feature is the existence of narrowband energy near M times the carrier frequency for M-ary PSK signals. >

Carrier recovery phase-locked loop

Abstract: A carrier recovery phase-locked loop for recovering a carrier from a digitized quadrature phase shift keying modulated wave arriving on time division basis comprises a PLL circuit, first unit, responsive to the received signal, for preparing a phase comparator's characteristic adapted for a binary phase shift keying modulated wave, second unit, also responsive to the received signal, for preparing a phase comparator's characteristic adapted for the quadrature phase shift keying modulated wave and switching unit for selectively supplying output signals from the first and second units to the PLL circuit. During reception of a preamble, the carrier is recovered using the binary phase comparator's characteristic through the PLL circuit.

Automatic gain control circuit for multilevel duobinary AM/PSK modulated signals

Abstract: An automatic gain control circuit for multilevel duobinary AM/PSK (amplitude modulation/phase shift keying) signals allows multilevel modulated signals to be received accurately. The circuit includes an amplifier that is controlled such that during a preamble section the amplifier has a gain associated with the level of the inputted signal. During a data section the amplifier is controlled to maintain a substantially constant gain. The invention may be used in the reception of AM/PSK modulated signals.

Variable-rate sub-band speech coding and matched channel coding for mobile radio channels

Abstract: The effect of digital transmission errors on a family of variable-rate embedded subband speech coders has been analyzed. It is shown that there is a difference in error sensitivity of four orders of magnitude between the most sensitive and the least sensitive bits of the speech coder. As a result, a family of rate-compatible punctured convolutional codes with flexible unequal error protection capabilities has been matched to the speech coder. On a Rayleigh fading channel with 4-DPSK modulation, as much as 5-dB gain in channel signal-to-noise ratio can be obtained by using four levels of error protection compared to only two levels in more conventional designs. The gain is achieved at no extra bandwidth requirement and at a negligible complexity increase. Among the results, analysis and informal listening tests show that with a four-level unequal error protection scheme, transmission of 12-kb/s speech is possible with little degradation in quality over speech is possible with little degradation in quality over a 16-kb/s channel with an average bit error rate of 2*10/sup -2/ at a vehicle speed of 60 mph. >

Fiber-optic transmissions of microwave 8-phase-PSK and 16-ary quadrature-amplitude-modulated signals at the 1.3- mu m wavelength region

Abstract: Transmissions of a 6-GHz 8-phase phase-shift-keyed (8 phi -PSK) signal over a 12.5-km single-mode fiber with a 5-dB power margin and bit-error-rate (BER) of 10/sup -10/ and a 6-GHz 16-ary quadrature-amplitude-modulated (QAM) signal over the same distance with a 2-dB power margin and BER of 10/sup -12/ were demonstrated. The 8 phi -PSK digital modem operated at 78Mb/s and the 16-QAM digital modem operated at 90 Mb/s. A high-speed multimode InGaAsP laser diode and a high-speed p-i-n diode were used in both fiber-optic transmission systems. Floor characteristics and power penalties observed in the BER performances of both systems were found to be caused by the intensity noise of the laser diode, particularly the reflection-induced intensity noise. >

A lower bound on the minimum Euclidean distance of trellis-coded modulation schemes

Abstract: A lower bound on the minimum free Euclidean distance of trellis-coded modulation (TCM) is derived that guarantees the existence of good TCM codes of any complexity The bound is used to compare trellis codes combined with phase-shift keying, pulse amplitude modulation, and quadratic amplitude-shift keying modulation This random coding bound is the first lower bound on the free distance of trellis codes, is tighter than any upper bound for large constraint lengths, and predicts the asymptotic performance of TCM when the complexity of the code becomes large The bound can be used with any code rate and any modulation scheme and shows that the free distance increases linearly with the constraint length for large values of the constraint length >

A PSK group modem based on digital signal processing: Algorithm, hardware design, implementation and performance

Abstract: A PSK group modem which modulates and demodulates multiple PSK signals en bloc will play an important role in cost-effective satellite communications systems based on the narrow-band multicarrier PSK/FDMA technique. This paper describes the development of a PSK group modem, including its algorithm, hardware design and implementation. In the algorithm, PSK signals are efficiently filtered by digital signal processing applied to the digital transmultiplexor. Furthermore, in order to compensate for the difference between the operation timing of the PSK group modem and the symbol timing of each input/output data signal, digital signal processing based on a table-look-up method using ROM and a rate-conversion filter with adaptive weighting are proposed for the modulator and demodulator, respectively. An experimental four-phase PSK group modem has been developed, and experiments have been conducted under various conditions. The measured performance of the developed hardware is also demonstrated.