Showing papers on "Quadrature amplitude modulation published in 1984"

Efficient Modulation for Band-Limited Channels

Abstract: This paper attempts to present a comprehensive tutorial survey of the development of efficient modulation techniques for bandlimited channels, such as telephone channels. After a history of advances in commercial high-speed modems and a discussion of theoretical limits, it reviews efforts to optimize two-dimensional signal constellations and presents further elaborations of uncoded modulation. Its principal emphasis, however, is on coded modulation techniques, in which there is an explosion of current interest, both for research and for practical application. Both block-coded and trellis-coded modulation are covered, in a common framework. A few new techniques are presented.

Transmitter and receivers using resource sharing and coding for increased capacity

Abstract: The present invention relates to transmitters and receivers in a digital time division multiple access (TDMA) communication system wherein the resource sharing concept is generalized by fully exploiting the available clear air carrier-to-noise ratio (CNR) of a transmission link to achieve very high transmission capacity while maintaining low rain outage. Transmission is accomplished by using a first predetermined redundancy code and a first predetermined signal constellation for quadrature amplitude modulation during clear air conditions to permit a high rate of information transfer. When the fade depth exceeds the built-in fade margin, a second predetermined redundancy code and a second predetermined signal constellation is used while using time slots borrowed from a resource sharing reserved time slot pool to maintain the data rate at the fade site. Either one of the first redundancy code or signal constellation can equal either one of the second redundancy code or signal constellation.

Error control codes for QAM signalling

Abstract: A class of block codes is described which exploits the properties of quadrature amplitude modulation signal constellations whose points lie on a square grid. The simplest codes in the class have block lengths 4 and 8 and offer coding gains of 3 dB and 4.5 dB, respectively.

Transmission of digital data

Abstract: The transmission of digital data over a data link, particularly an H.F. radio link, can be corrupted by narrow-band, high level interference signals randomly distributed in frequency. The uncorrupted transmission of data can be made more possible by a frequency diversity digital data transmission system in which a signal channel is divided into a plurality of lesser bandwidth sub-channels, each of which is modulated across substantially its entire bandwidth by a multi-level, spectrally efficient modulation scheme, for example 8PSK, 16QAM, 64QAM. In a receiver the sub-channels are examined for narrow-band interference and the data signal is demodulated using a decision algorithm based on the sub-channel outputs, and an assessment of sub-channel reliability.

SBPSK: A Robust Bandwidth-Efficient Modulation for Hard-Limited Channels

Abstract: A family of constant-envelope modulation formats is described in which good spectral containment is achieved through the use of controlled phase trajectories. The phase trajectory of shaped BPSK (SBPSK) in particular is such that it can be considered a PSK/FSK hybrid. For one member of this family, 50% SBPSK, extensive analytical results are presented, including sensitivities to filter bandwidths, timing jitter, phase jitter, and frequency error. In addition, performance in the presence of multiple adjacent channel interferers in a hard-limited satellite channel is analyzed and performance comparisons to conventional modulations are presented.

Multidimensional Signal Constellations for Voiceband Data Transmission

Abstract: A significant improvement in noise immunity can be achieved for digital transmission over band-limited channels by the use of multidimensional signal constellations. Conventional 16-point QAM signaling, such as that used in many digital transmission systems, is a two-dimensional modulation scheme where in each signaling interval a group of N bits is used to determine the amplitudes of the in-phase and quadrature dimension or coordinate, i.e., N/2 bits are conveyed per dimension. In a 2M -dimensional QAM system, a group of MN bits is used to determine the in-phase and quadrature ampllitudes for M consecutive, symbol intervals where N/2 bits are still conveyed by each dimension. It is the purpose of this paper to describe the basic theory and implementation for a particular 2 bits/dimension four-dimensional (two-symbol interval) encoding which readily lends itself to simple encoding and decoding. For this encoding, theory predicts a 1.2 dB gain in noise margin over conventional 16-point (two-dimensional) QAM signaling. Experimental results agreed with the theoretical predictions, and have demonstrated an order of magnitude reduction in block error rate. Extension to eight-dimensional signaling offers a theoretical gain of 2.4 dB over conventional 16-point QAM.

Contrasting performance of faster binary signaling with QAM

Abstract: In this paper we determine the performance of Faster Binary Signaling (FBS), an alternative method to Quadrature Amplitude Modulation (QAM) for achieving a high bit rate over an ideal, bandlimited, noisy channel. With this method, signaling is faster than the Nyquist rate. Consequently, there are fewer points in the signal constellation, resulting in a greater separation of the points when the average transmitter power is the same as for QAM. Thus, at the expense of introducing Intersymbol Interference (ISI), there is an apparent improvement in noise immunity. The ISI can be mitigated with maximum likelihood sequence detection. We explore the advisability of trading freedom from ISI for added noise immunity for the extreme case where the system with faster signaling uses a four-point constellation. The question of the efficacy of FBS has been difficult to approach, but FBS has loomed as a possibly strong competitor among alternatives to QAM. We show here how to analyze FBS, and we give examples involving FBS operating at up to five times the QAM rate. In the examples, FBS is revealed to be, at best, of marginal value even if one allows for implementation capabilities far beyond those of forthcoming processors.

Universal ultra-precision PSK modulator with time multiplexed modes of varying modulation types

Abstract: An apparatus and method is disclosed which provides PSK modulation utilizing a single balance mixer. This apparatus allows the selection of any type of PSK modulation for each burst of data transmitted through the apparatus. This apparatus further provides a signal having little or no amplitude modulation and reduces the spectrum occupancy to reduce cross channel interference.

16-QAM Modulator with PLL amplification devices

Abstract: A 16-quadrature amplitude modulation (QAM) modulator for radio links converts a train of data into signals having a phase selected from sixteen predetermined phase values and an amplitude selected from four values. The modulator divides the data train into two data sub-trains. The two data sub-trains are respectively applied to a pair of four-phase modulators that are fed by a carrier source. The 16-QAM modulator also comprises two phase locked loops each including a voltage controlled microwave oscillator and an associated differential phase detector. The differential phase detector responds to the output of the associated four-phase modulator and the output of the associated voltage controlled microwave oscillator. Predetermined parts of the output signals of the two phase locked loops are added.

Multipath Fading Models and Adaptive Equalizers in Microwave Digital Radio

Abstract: In the past few years, a great deal has been learned about the modeling and equalization of multipath fading in microwave digital radio systems. This paper adds a number of new results to this body of knowledge. We begin with a description of two published multipath fading models that were derived from the same collection of data, i.e., the threepath and polynomial models. We then derive outage probability characteristics for a digital radio link using M -level quadrature amplitude modulation, cosine rolloff spectral shaping, and some form of equalization. We obtain results for both models, several kinds of equalization, and various combinations of system parameters. We find that the theoretically optimal equalizer, which is nonlinear, is not measurably better in performance than the best of the linear equalizers, which is simpler. Also, the performance of a properly equalized receiver can be very accurately estimated by knowing just the probability distribution, over many fades, of the gain amplitude at a single frequency. Finally, we show the influences on performance of such parameters as cosine rolloff factor, symbol rate, and unfaded carrier-to-noise ratio.

CPM - An efficient constant amplitude modulation scheme

Abstract: There is an increasing demand for digitial radio transmission. The cost of a channel is proportional to the bandwidth. For many applications power is limited. For satellite applications constant amplitude modulation is advantageous because of non-linear amplification. In this paper a class of power and bandwidth efficient constant amplitude continuous phase modulation (CPM) schemes is presented. The signals are analysed in great detail and the tools for both error probability analysis and power spectral calculations are presented. Numerical data for raised cosine (RC) pulse shaping is presented. This paper is basically an overview with new results on the trellis description of CPM. The transmission is assumed to be ideally coherent over the Gaussian channel. In a following paper we will deal with transmission imperfections such as bandlimitation and hard limiting and their effect on CPM1.

Differential coding system and apparatus therefor

Abstract: A differential coding system applicable to a staggered quadrature amplitude modulation transmission system is disclosed. Even when ambiguities with respect to phase and time have developed in combination in the transmission system, the differential coding system performs differential coding with ease and, thereby, faithfully regenerates an original data code sequence.

Reduced Complexity Implementation of Passband Adaptive Equalizers

Abstract: High-speed modems typically employ quadrature amplitude modulation (QAM) so that two signal paths, in-phase and quadrature, are simultaneously being used. The standard receiver structure has, in effect, four adaptive equalizers, one for each of the two signal paths and two to correct for cross coupling between signal paths. As a result, equalization normally requires 8N multiplications per baud interval, where N is the number of taps, to perform the needed signal processing including adaptive updating of the weights., In this correspondence, we present a new signal processing algorithm which requires only 6N multiplications per baud interval. For fractionally spaced equalization and adaptive cancellation, N is quite large so that this saving can be of practical value in improwng throughput capability of custom chips, or depending on the architecture, with programmable signal processor chips. Specifically, the algorithm is advantageous for implementations where multiplications require a longer execution time than additions.

Error-correction coding for multilevel transmission system

Abstract: A technique for improving efficiency of error-correction is disclosed for use in a Gray-coded multilevel transmission system and, specifically, in a quadrature amplitude modulation (QAM) transmission system. To this end, information bits in information bit symbols are converted, i.e., mapped, into error-correction bit symbols having fewer bits than the corresponding information bit symbols. This bit-converting technique permits use of encoders and decoders having a code rate lower than the overall transmission code rate.

A Maximum Likelihood Receiver for an Orthogonally Multiplexed QAM System

Abstract: A maximum likelihood receiver for an orthogonally multiplexed QAM (OQAM) system has been derived based on the mathematical structure of an OQAM signal. The resulting receiver provides the minimum variance estimates of carrier and timing phases under the disturbance caused by not only Gaussian noise but also random data. It has also been proven that the carrier and timing joint control system is asymptotically stable almost everywhere in a state space. The equilibrium points are where the parallel synchronization over all subchannels in the OQAM system is established. Simulations have been carried out in order to verify the maximum likelihood receiver performances, where 9600 bit/s data are transmitted over 750-2450 Hz voiceband with 8×8 staggered QAM signal constellations. It has been shown that sufficient eye opening is obtained under 10 Hz carrier frequency offset and 10-4timing frequency stability.

QAM Demodulator automatic quadrature loop using n-m LSB's

Abstract: A quadrature amplitude modulation (QAM) demodulator system has in-phase and quadrature channel detectors receiving local oscillator signals which must be in precise phase quadrature relationship with each other. The in-phase and quadrature channels each have analog-to-digital converters with n output bits, where n is greater than m, the number of bits required for the data output for each channel. The n-m less significant bits in one channel are used to derive a signal indicating the deviation from a true quadrature relationship between the local oscillator signals. The error signal controls an adjustment of the phase of one of the local oscillator signals, to bring about the precise quadrature relationship required for accurate demodulation.

Performance analysis of 16-ary QAM signalling through two-link nonlinear channels in additive Gaussian noise

Abstract: This paper considers the problem of evaluating the performance of a 16-ary quadrature amplitude modulation (QAM) signal transmitted through a bandpass nonlinearity (BPNL), exhibiting AM-AM and AM-PM conversions, and in the presence of additive gaussian noise (AGN) both preceding and following the nonlinearity. An expression for the error probability is derived and computed. Also, the effect of TWT back-off from saturation on 16-ary QAM system performance is studied. Finally, as an example, a comparative study of 16-ary CPSK and 16-ary QAM transmissions is undertaken with the BPNL operated at various levels of back-off from saturation.

A New 4 GHz 90 Mbps Digital Radio System Using 64-QAM Modulation.

Abstract: This report examines the potential for ground-based weather radar (meteorological radar) interference to digital microwave systems in the common carrier bands of 37004200 MHz and 5925-6425 MHz. Reported cases of interference to microwave common carrier systems from ground-based weather radar systems have increased due to the trend towards digital modulations. Because of this interference, NTIA, the FCC and the National Spectrum Managers Association formed an informal working group to investigate and docu,ment the potential problems. The existing and planned spectrum uses by ground-based weather radars and digital microwave systems are addressed as well as regulations and policy pertaining to their electromagnetic compatibility. Methods to mitigate the interference in both the radar transmitter and microwave receiver are also provided.

Adaptive transversal equalization of multipath propagation for 16-QAM, 90-Mb/s digital radio

Abstract: Adaptive transversal equalization is an effective and relatively new countermeasure for dispersive multipath propagation in terrestrial digital radio networks. In this paper we describe the design and performance of a five-tap baseband analog equalizer developed for a family of 16-QAM, 90-Mb/s radio systems. Laboratory measurements and field evaluation during a five-month fading season in Palmetto, Georgia, indicate that the use of this adaptive transversal equalizer can significantly reduce the need for costly space-diversity equipment.

QAM Demodulator carrier recovery loop using n-m LSB's

Abstract: A quadrature amplitude modulation (QAM) demodulator system derives a local oscillator signal which has the frequency and phase of the suppressed carrier of the QAM signal, so as to permit proper demodulation. In-phase and quadrature channels of the demodulator system each have an analog-to-digital converter with n output bits, where n is greater than m, the number of bits required for the data output for each channel. The n-m less significant bits are used to derive a signal indicating the error in phase alignment between the local oscillator signal and the QAM signal. In a phase locked loop, this error signal drives a voltage-controlled oscillator to phase lock with the suppressed carrier of the QAM signal. To acquire frequency and phase lock, a digital-to-analog converter in the loop is clocked each time the phase alignment of the VCO signal and QAM signal is close.

Differential phase shift keying demodulator

Abstract: A differential phase shift keying demodulator in which a signal modulated by differential phase shift keying is compared with the modulated signal delayed an integral number times as much as a period of the carrier wave of the modulated signal and shorter than a transmission period for one bit of the modulated signal.

Error probability of QASK with rectangular and circular symbol constellations in the presence of intersymbol quadrature channel and adjacent channel interference

Abstract: In the paper we study the effect of intersymbol, quadrature channel and adjacent channel interference (in addition to Gaussian noise) on the error probability of M-ary quadrature amplitude modulation (QAM) with M = 4, 16 symbols and phase shift keying (PSK) with M = 4, 8 and 16 symbols, which are special cases of quadrature amplitude shift keying (QASK) with a rectangular and circular constellation. The transfer function of the system for the main signal is a raised cosine with excess bandwidth s, and the receiver filter is matched to the transmitter filter. The interfering signal is of the same type as the main signal, but its carrier frequency differs from the carrier frequency of the main signal by the frequency fs. In the paper we also present the necessary formulas for the computation of error probability in the presence of frequency selective fading.

Necessary Bandwidth and Spectral Properties of Digital Modulation

Abstract: This report shows that for digital modulation systems, the necessary bandwidth, signal power, and bit error rate are all interrelated and tradeoffs among these factors are possible.

Method for digital quadrature amplitude modulation

Abstract: Spectrum-converting digital filters are employed for quadrature amplitude modulation. Given the method of the invention, the frequency of the two carrier oscillations residing at 90° relative to one another amounts to one-fourth the sampling frequency and, in addition, their phase positions relative to the sampling times is selected such that every second sample of both carrier oscillations is alternately zero.

Fast acquisition ringing filter MSK demodulator

Abstract: A clock and carrier recovery circuit, and a related method, for use in a minimum shift keying (MSK) receiver demodulator. The clock and carrier recovery circuit uses ringing filters to lock onto two frequency-doubled tone components of a received MSK signal. A signal having the same frequency as the clock is recovered by taking the difference of the outputs of the two filters. The output of each filter is then combined separately with a signal having a frequency to produce to signals, each having a frequency equal to twice the carrier frequency and having complementary amplitude modulation. These two signals are summed to cancel the amplitude modulation and the resulting sum signal, having constant amplitude and a frequency twice that of the carrier, is divided by two to recover the carrier signal.

CPM - The effect of filtering and hard limiting

Abstract: The attractive properties of the digital constant amplitude modulation scheme CPM (continuous phase modulation) have been demonstrated in a previous paper in this journal.1 This concerned both the power and spectral efficiency of the large family of CPM schemes over the Gaussian channel. In this paper the considered channel can be both bandlimited or band- and amplitude-limited at the same time. Analytical results as well as simulations are presented for a number of selected CPM schemes. In today's increasing demand for bandwidth, CPM is an attractive candidate for future satellite applications because of its spectral efficiency. In addition to this comes the fact that the transmitted signal has a constant amplitude and good detection performance when optimal coherent detection is performed.

Bandwidth Efficient Quadrature Overlapped Modulation

Abstract: This correspondence describes modified QORC (MQORC) modulation. The power spectral density of MQORC is shown to take on the form of the product of the QPSK and SFSK spectra, which attains good spectral characteristics. We show that the bandpass hard-limited staggered MQORC tends to keep its sidelobes suppressed, and therefore, its sidelobes drop off faster compared to staggered QORC.