Showing papers on "Quadrature amplitude modulation published in 1983"

Digital communications over fading radio channels

Abstract: A major contribution to system outage in a terrestrial digital radio channel is deep fading of the frequency transfer characteristic, which in addition to causing a precipitous drop in received signal-to-noise ratio (s/n) also causes signal dispersion that can result in severe intersymbol interference. Because the temporal variation of the channel is slow compared to the signaling rate, the information theoretic channel capacity and the “Efficiency Index” in bits/cycle — a figure-of-merit we use for the communication techniques considered — can be viewed as random processes. Starting from an established mathematical model characterizing fading channels (derived from extensive measurements), we estimate the probability distribution of channel capacity and the distributions of efficiency indices for different communications techniques. The repertoire of communication methods considered involves quadrature amplitude modulation with adaptive linear and decision feedback equalization, and maximum likelihood sequence estimation. For specific outage objectives the maximum number of bits per cycle achievable by each technique is estimated. The sensitivity of the distributions to bit-error-rate objective and unfaded s/n is assessed. For certain desired operating points the efficacy of adaptive equalization is demonstrated. There are some operating points where adaptive equalization alone is not adequate and therefore space diversity should be considered. An estimate of the effect of frequency diversity is also included.

Signal distribution system

Abstract: A head-end for a signal distribution system with selection facility, includes a matrix (8) and a multiplexer (12) connected thereto for forming a multiplex signal. The head-end further comprises A/D converters (4-1 to Δ-N) for converting the wide-band input signals into digital signals prior to supply to the matrix. The multiplexer comprises two amplitude modulators (13, 14) for the mutual quadrature amplitude modulation of a carrier (17) by two digital signals, and a summing means (18) for adding a digital signal in the base band to the modulated signal. A signal receiver for one of the modulated signals comprises a synchronous amplitude demodulator (21) connected to a synchronized carrier source (20) and also comprises means for digital-to-analog conversion (24).

An Automatically Controlled Predistorter for Multilevel Quadrature Amplitude Modulation

Abstract: In digital microwave transmission, the nonlinear characteristics in a high power amplifier, such as a TWT (traveling-wave tube) inhibit efficient output use. This note introduces a new predistorter control technique, and assesses the nonlinear compensation capability of a third-order predistorter incorporating this technique. Concerning 16-QAM (quadrature amplitude modulation) a 10 dB reduction in out-of-band emission and larger than 8 dB C/N improvement with respect to symbol error rate can be achieved at 3 dB TWT average output power backoff.

Universal Carrier Recovery Loop for QASK and PSK Signal Sets

Abstract: A new carrier recovery loop which applies to quadrature amplitude shift keying (QASK) and phase shift keying (PSK) modulation schemes is presented. This loop is shown to provide significant advantages compared with those already used. Furthermore, the baseband processing of the demodulated signals allows an easy implementation, especially for high-speed digital modems. Both geometrical interpretation and theoretical justification using estimation criteria are provided. Finally, experimental results are reported.

QAM modulator circuit

Abstract: A method and means for quadrature amplitude modulation is described. The improved QAM modulator circuit utilizes a small read only memory to hold multiple versions of a signal constellation resulting in the elimination of hardware modulator circuits. In addition, the described device can be potentially any modulation scheme on an input signal which is sampled in time by modifying the contents of said ROM.

Demodulation of Amplitude Modulated Noise: A Mathematical Evaluation of a Demodulator for Pathological Tremor EMG's

Abstract: A mathematical description is given of amplitude modulated noise, simulating EMG signals of pathological tremors, and its demodulation into a tremor signal by means of a fuli wave rectifier. In this description these signals are considered in the frequency domain in the form of power spectra. The mathematically derived formulas are compared with the results of an experimental setup using a demodulator designed for use in clinical practice. The purpose of the experiments was to determine the signal-to-noise ratio of the tremor signal with respect to the demodulation noise, the latter being an inevitable consequence of demodulating the amplitude modulated carrying noise. The theoretically derived data are in acceptable agreement with the experimental results obtained in this way. As a result of the theoretical approach to the amplitude modulated noise, the signal-to-noise ratio of the demodulated signal is proportional to the bandwidth of the carrying noise of the amplitude modulation, and, for small modulation depths, is proportional to the square of the modulation depth.

Modulation Methods Related to Sine-Wave Crossings

Abstract: A bounded band-limited signal may be represented by its sine-wave crossings, i.e., by a set of points on the time axis at which it crosses a reference sinusoid. If, at these points, pulses of a standard shape are generated, we obtain a position modulated pulse train. This pulse train is analyzed in detail. Two other modulation methods related to sine-wave crossings are also discussed, namely, edge position modulation of a square wave and bipolar pulse duration modulation. In all cases, the modulated waves may be expressed as sums of an infinite number of sinusuidal carriers amplitude modulated by Chebyshev polynomials of modulating signals.

QPSK, Staggered QPSK, and MSK--A Comparative Evaluation

Abstract: Bandwidth- and power-efficient quadrature carrier modulation techniques are identified, evaluated, and compared. The modulation techniques include quadrature phase shift keying (QPSK), staggered QPSK (SQPSK), and minimum (frequency) shift keying (MSK). High quality QPSK, SQPSK, and MSK transmitters and receivers were used in the evaluation, along with a computer simulation program which modeled the hardware. Both linear and nonlinear channels were included in the comparative investigation. The agreement between simulated and measured results is shown to be excellent. In the comparison of modulation techniques, the effects of channel filtering (band limiting), nonlinearities, detection techniques, and phase equalization are addressed. The application-oriented results are intended to be an aid in selecting modulation and detection implementations, and in predicting the performance of linear and nonlinear communication systems. For systems with imperfections, data are presented for determination of passive detection filters which can outperform "matched" integrate and dump (I&D) filters.

A Performance Study of NLA 64-State QAM

Abstract: The performance of a modulation technique which is both power and bandwidth efficient, termed nonlinearly amplified 64-state quadrature amplitude modulation (NLA 64-state QAM), is studied. To assess the effects of selective fading and/or system filter imperfections, computer simulated results are presented which show the sensitivity of the modulation technique to typical group delay and amplitude distortions. In addition to the above-mentioned results, the effects of modulator imperfections in the power level combining on the P e performance are evaluated.

SAW-Filters for Spectral Shaping in a 140 Mbit/Ss Digital Radio System Using 16 QAM

Abstract: Digital radio systems require precise spectral shaping filters in order to achieve low bit error rates (BER). We have developed SAW-IF-filters for a 140 Mbit/s system using 16 QAM. First, ideal filters were designed by linear programming methods. Then relevant second order effects were simulated and compensated for. Measurements of the filters on 128' rot-Y, LiNbO, exhibit excellent agreement with the design, passband amplitude deviations being less than kO.05 dB and group delay time ripples are less than21 ns. Degradation of carrier to noise ratio against the theoretical limit is only 0.2 dB at a BER of lo-'. cy response within small tolerances /2, 3, 4/. The frequency responses of real filter geometries are computed with two-dimensional models which account for the relevant effects of transducer impedance and diffraction /5/. There are various methods for the compensation of diffraction effects /6, 7/. In ref. 7 a universal method for the compensation of second order effects is proposed. seb for spectral shaping IF filters for QAM signals are defined. The optimum designs of the FIR filters were computed using methods of linear programming. Second order effects were compensated for by the universal method /7/. Finally, filters were fabricated which fulfill all requirements for low intersymbol interfer ence .

High speed data modem using multilevel encoding

Abstract: A 14.4 kilobit/second modem uses an encoding scheme in which groups of five bits are encoded as one of thirty-two (2 5 ) possible code groups. This is done by using quadrature amplitude modulation and a 6 by 6 space-state constellation which allows a maximum of thirty-six different points to be encoded. Since only thirty-two points are needed the four outer corner points of the constellation are not used. In order to achieve the desired 14.4 KBPS data rate the baud clock must run at 2880 Hz. However, this bandwidth is very close to the maximum bandwidth available on voice-grade telephone lines. Accordingly, data encoding and data recovery techniques must be used which maximize the probabilities of correctly receiving the encoded data signals. These techniques include (1) data scrambling/descrambling; (2) assigning groups of five bits to constellation points, including performing rotational and Gray encoding; (3) a baud clock recovery scheme at the receiver which is performed prior to partial response encoding; and (4) a start-up technique using a three-level partial response ideal reference sequence during initial training at the receiver. In addition, the invention uses passband equalization, class I partial response encoding accomplished by the passband equalizer, and independent inphase and quadrature Viterbi decoders at the receiver.

A communication system having improved differential phase shift keying modulation.

Abstract: A communication system (10) which generates an output signal of a selected primary frequency having digital data bits serially modulated thereon by utilizing a hybrid differential phase shift keyed (DPSK) modulation. The improved DPSK modulation is frequency modulation of a phase encoded signal. A controller (12) controls frequency generator means (14 and 16) which frequency shift the output signal between primary and secondary frequencies. The controller (12) and frequency generator means (14 and 16) provide a modulated output signal which is D.C. balanced bit by bit and has substantially reduced harmonic energy.

On continuous phase modulation in cellular digital mobile radio systems

Abstract: This paper considers the application of constant-amplitude, partial-response, continuous-phase modulation with simple near-optimum receivers to cellular digital mobile radio systems. These smoothed modulation schemes have low spectral sidelobes and narrow main lobes. A combiner for time-division retransmission systems with space diversity is given for continuous-phase modulation. Cochannel interference and adjacent-channel interference are calculated for continuous-phase modulation in cellular systems with frequency reuse. The efficiency of 3RC and 4RC modulations with space diversity is evaluated for conventional cellular systems.

Effect of Intersymbol and Quadrature Channel Interference on Error Probability of 16-ary Offset Quadrature Amplitude Modulation with Sinusoidal and Rectangular Shaping

Abstract: In this paper we study the effect of intersymbol and quadrature channel interference on the average error probability of 16-ary offset quadrature amplitude modulation with rectangular or sinusoidal shaping functions. We present numerical results assuming that the transmitter and receiver filters are identical Butterworth filters of order N = 1,2,..., 6, and the decision rule is either fixed or adaptive.

Distortion correcting AM stereo receiver with non-flat AGC

Abstract: In a receiver for composite amplitude and angle modulated signals and having a non-flat automatic gain control characteristic, there is provided apparatus for providing distortion correction to the quadrature component produced by the angular modulation in accordance with the amplitude modulation component. In accordance with the invention, the output of an amplitude demodulator is modified to remove the carrier signal level dependent magnitude multiplier and develop a modification signal which is used to inversely amplitude modulate the quadrature signal component to provide distortion correction over a range of carrier signal levels. Such inverse modulation can be provided before or after quadrature demodulation of the received signal.

An Integrated Voice-Data Communication System for VHF Links

Abstract: In this work, a combined amplitude-phase modulation communication system is considered. Amplitude modulation is used to transmit a voice signal, and phase modulation is used for data transmission. Such a scheme was studied as a possible realization of a data channel between aircraft and ground station, which can be introduced without great modifications of the actual on-board equipment. The performance of the combined modulation system is evaluated through a computer simulation.

Design and Realization of a 16 QAM-Modem for a 140 Mb/s Digital Radio System Family

Abstract: A 16 QAM-modem for 140 Mb/s radio systems is described, with which a spectral efficiency of 3.5 bit/s/Hz is achieved. In the digital signal processing parts extensive use is made of ECL gate arrays. Spectrum shaping according to Nyquist's criterion is performed in the IF with the aid of SAW-bandpass filters (? = 0.5). Experimental results obtained with this modem are reported. The incorporation of an adaptive baseband equalizer employing a complex transversal filter is discussed.

High Linearity Microwave Power Amplifiers for Digital Radio Relay Systems using 16 QAM

Abstract: An experimental characterization of state of the art microwave power amplifiers for high capacity 16 QAM digital radio systems is given. The effects of amplifier nonlinearities on 16 QAM signals are qualitatively demonstrated with the aid of real time signal state diagrams. A quantitative description is obtained by measured degradation of bit error ratio characteristics as a function of output power level. The comparison of the power capability shows advantages for modern high linearity TWTs over GaAs FET amplifiers at slightly superior efficiency. Techniquss for fast and easy characterization of nonlinear amplifiers are derived. Finally, methods to improve amplifier linearity are discussed.

A Family of 16 QAM Radio Systems for Transmission of 140 MBIT/S

Abstract: The philosophy of the mechanical and electrical design of the radio equipment covering the frequency ranges 3.9, 6.7, 11.2 GHz is presented and details of some important subunits are discussed. Furthermore, measurement results of bit error rate depending on noise, linearity and selective fading are given.

Power spectral density of multilevel digital signals in nonlinear channels

Abstract: In digital microwave communication, a power amplifier is located at the transmitting side. Generally, such a power amplifier has a nonlinear input-output characteristic. However, the greatest problem with this kind of nonlinear transmission system is the spectrum width of the signals. Until now, a spectrum has been obtained by approximating the input-output characteristics of a power amplifier by third-order equations. When third-order equations are used, however, only one sidelobe appears in the spectrum. to obtain a more detailed spectrum, the input-output characteristics of the power amplifier must be approximated by equations of higher than third-order. This paper presents equations for the power spectral density of multilevel digital output signals of a transmission system. the system consists of a power amplifier having an input-output characteristic which can be approximated by a filter and fifth-order equations. Consequently, it has become possible to compute a power spectral density, for example, for quadrature amplitude modulation (QAM) having more than 32 levels, which was difficult to obtain by the simulation method. Examples show the results of computations for power spectral densities of 16- and 32-level QAMs.

Cochannel and intersymbol interferences in QAM system

Abstract: The method of Ho and Yeh, with some modifications, is employed to analyse the performance of the QAM system in the presence of jammers and band-limiting. Results are presented for a 16-QAM system using the TDMA INTELSAT V communication system specifications.

Planning Considerations for High Capacity Digital Radio Relay Networks

Abstract: The paper deals with path and network design, considering the effects of multipath fading. A multipath propagation model is used, based on measurements of a 59 km path near Munich and on results reported in the literature. The impact of a simulated 2-and 3-path fading on the residual inband amplitude and delay dispersion of a 16 QAM 140 Mbit/s system has been studied including various methods of improvement and their optimum combination.

GaAs FET Linear Power Amplifiers for 16 QAM Signals at 6 and 11 GHz

Abstract: This paper describes design and realisation criteria for linear power amplifiers used in novel 16 QAM radio link systems The 64-71 GHz amplifiers use 6 cascaded stages and perform a 9 W mean output power with a 42 dB linear gain and -37 dBc 3rd order IM The 107-117 GHz version uses 8 cascaded stages to perform a 5 W mean output power with the same distortion value

Method for reducing the twelve-fold uncertainty of the carrier phase of a 16-QAM signal at the receiving end

Abstract: The invention is intended to reduce this uncertainty to only a four-fold uncertainty. For this purpose, a known phase-locked loop having a first frequency quadrupling stage (1) in the input and a second frequency quadrupling stage (2) in the feedback loop is used. The highest of the three amplitude steps occurring in the envelope curve of the 16-QAM signal is overproportionally amplified in the first frequency quadrupling stage (1). This results in a locking-in of the phase-locked loop at one of the four phase angles 45 DEG , 135 DEG , 225 DEG or 316 DEG . The overproportional amplification of the highest amplitude step is produced by utilising the characteristic of the first frequency quadrupling stage (1). In this arrangement, the highest amplitude step is amplified by about 6 dB in voltage compared with the middle amplitude step.

Method and arrangement for reducing at the transmitting end the twelve-fold uncertainty of the carrier phase of a 16-QAM signal occurring at the receiving end

Abstract: The invention achieves a reduction at the transmitting end of the twelve-fold uncertainty of the carrier phase of a 16-QAM signal, composed of a normal and quadrature signal, occurring at the receiving end. For this purpose, when the highest amplitude step (4) occurs simultaneously in the four-step normal and quadrature signal, both signals are amplified to a further fifth amplitude step (5) which marks the nominal value of the carrier phase with the amount DELTA u 2ROOT 2 as fourth amplitude step of the otherwise three-step 16-QAM signal after the quadrature modulation. In principle, each of the four corner points in the signal state diagram can mark the nominal value of the carrier phase as fourth amplitude step of the 16-QAM signal sent out. The corner point with the Gray-coded 4-bit word 1010 located in the first quadrant of the signal state diagram is preferably used. To carry out the method, the logic of the binary/quaternary converters for the normal and quadrature signal at the transmitting end, which are necessary in any case, is supplemented by an AND gate and two subsequent resistors (R1).

Method for converting a band-limited 16-QAM signal into an 8-QAM signal for the purpose of carrier recovery

Abstract: In the conversion of a band-limited 16-QAM signal into an 8-QAM signal, it is necessary to blank the middle amplitude level out of the 16-QAM signal. According to the invention, this is done firstly by approximating the envelope of the 16-QAM signal (35 MHz) to a square-wave signal by supplying the third (105 MHz) and fifth (175 MHZ) harmonics. The middle amplitude level is discriminated from the square-wave signal by the comparator using adjustable thresholds and corresponding logic combination. Thereafter, a 16-QAM signal is superimposed on the output signal of the comparator. The middle amplitude level thus receives a boosted average DC voltage. An overproportionally high downstream amplification of the middle amplitude level suppresses the remaining two amplitude levels. Finally, the compensation signal thus obtained is combined in antiphase with the input signal, and the middle amplitude level is thereby eliminated.

Simulation and analysis of digital frequency modulation.

Abstract: A numerical technique is developed tn this thesis to find the spectrum of a frequency modulated signal and to recover the transmitted signal from this spectrum. Analysis of frequency shift keying is done using this algorithm. Results obtained using computer simulation, show that restricting the bandwidth of a frequency shift keyed system has significant effects on the transition time and mean-square-error of the received data pulse. Pulse shapes suitable for telemetry purpose are examined and an optimum pulse satisfying the telemetry standards has been designed. In addition, this technique is used to evaluate some work done by other researchers related to narrow band frequency shift keying.