Showing papers on "QAM published in 1985"

Distributed trellis encoder

Abstract: In the transmitter of a data communication system using QAM, a plurality of trellis coders with delay units are used for forward error correction. The output of each encoder is modulated using QAM to generate sequential baud signal elements. The redundant data bits generated are distributed between several non-consecutive bauds. Likewise, at the receiver a plurality of distributed convolutional decoders are utilized to decode the received signal element. The distributed trellis decoder is self-synchronizing.

Analysis of Four Decision-Feedback Carrier Recovery Loops in the Presence of Intersymbol Interference

Abstract: The performance of four decision-feedback carrier recovery techniques is evaluated in the presence of additive noise and intersymbol interference (ISI). For QAM signal constellations, a closed-form expression is given for the phase jitter variance (PJV) of each loop, and the loop tracking performance is examined. The analytic results are then computed in the case of a 16 QAM digital radio system subjected to multipath fading. Two cases are considered: in the first case no countermeasure techniques are used against selective fading, while in the second case a three-coefficient decision-feedback equalizer (DFE) is used. Computer simulations using a pseudorandom sequence to estimate loop performance are also reported which support the theoretical results.

SQAM - A new superposed QAM modem technique

Abstract: A spectral and power efficient modulation techniquesuperposed quadrature amplitude modulation (SQAM-is introduced. In SQAM, the premodulation baseband signal is a double-interval ( 2T_{s} ) raised-cosine pulse superposed with weighted single-interval (T s ) raisedcosine pulses. Our results indicate that SQAM has spectral advantages over OQPSK, QBL, and MSK, and better P(e) performance than MSK, IJF-OQPSK (or SQORC), and TFM.

Self-synchronizing interleaver for trellis encoder used in wireline modems

Abstract: In the transmitter of a data communication system using QAM, a trellis coder with k-baud delay units is used for forward error correction. The output of the encoder is modulated using QAM to generate sequential baud signal elements. The redundant data bits generated are distributed among several non-consecutive bauds. At the receiver a plurality of distributed convolutional decoders are utilized to decode the received signal element. The distributed trellis decoder is self-synchronizing.

Comparison of High-Level Modulation Schemes for High-Capacity Digital Radio Systems

Abstract: This paper presents a comparative study of four 2n-state quadrature amplitude modulation techniques, namely, 16, 32, 64, and 128 QAM, in a digital radio system environment. The effects of filtering, interference, amplifier nonlinearities, and selective fading are investigated using computational and simulation methods. Increase of the carrier-to-noise ratio (CNR) at a fixed symbol error probability (10^{-3} ) is taken as a robustness criterion.

Pseudobinary and Pseudoquaternary Detection Processes for Linearly Distorted Multilevel QAM Signals

Abstract: The paper describes two new detection processes suitable for QAM signals that have 16 or more levels (possible data symbol values) and that have been subjected to linear distortion, causing severe intersymbol interference in the received signal. The detectors are developments of the pseudobinary systems previously described, and are designed to operate in the presence of severe intersymbol interference. Results of computer simulation tests are presented, comparing the tolerances to additive white Gaussian noise of various arrangements of each detector with those of more conventional detectors. The tests have been carried out on a model of a 19 200 bit/s synchronous serial data transmission system, operating with a 64-level QAM signal that is transmitted, in turn, over two different telephone circuits.

Multiple quadrature-phase amplitude modulating system capable of reducing a peak amplitude

Abstract: of the Disclosure: In a multiple QAM system in which a pair of quadrature-phase carrier signals is modulated by a first and a second input digital signal, each representative of N or less levels, into a quadrature-phase amplitude modulated signal having M signal points on a phase plane where M is equal to N2, a code converting unit (41) converts the digital signals according to a predetermined rule into a first and a second modulating signal for use in modulating the carrier signals with the M signal points arranged approximately at a circle of a diameter which is shorter than a diagonal of a square of a side corresponding to the N levels. In a counterpart demodulat-ing system, a pair of demodulated signals is derived from the amplitude modulated signal and converted into reproductions of the respective digital signals according to an inverse of the predetermined rule.

Line impairment display for digital modems

Abstract: A modem receiver used in a QAM or PSK data communication system is provided with various circuits generating error or monitoring signals indicative of the status of the system. These signals and the in-phase and quadrature components of the received signals are fed to an interface circuit which generates display signals suitable for driving a display unit such as an oscilloscope. These display signals define an image composed of the signal constellation of the received signals and various error or monitoring points.

Diversity system of selection type equipped with a code correction/switch circuit for receiving a psk or qam wave carrying differentially encoded signal

Abstract: A diversity system of selection type comprising N (equal to or more than 2) receiver means for receiving multi-phase PSK or multi-level QAM waves carrying a differentially modulo-M (equal to or more than 2) encoded digital signal and providing N sequences of demodulated digital signals by phase-detecting said waves with a recovered carrier wave, and selector means for selecting out of said N sequences of demodulated digital signals one with a low bit error rate, which further includes first means for aligning the selected one of said sequence of demodulated digital signals with other sequences of demodulated digital signals, thereby providing a demodulated digital signal sequence retaining said differentially encoded digital signal as the output of said selector means.

Self-synchronizing de-interleaver for viterbi decoder used in wireline modems

Abstract: In the transmitter of a data communication system using QAM, a plurality of trellis coders with delay units are used for forward error correction. The output of each encoder is modulated using QAM to generate sequential baud signal elements. The redundant data bits generated are distributed among several non-consecutive bauds. At the receiver a distributed convolutional decoder individually addressed having sectioned memory elements is utilized to decode the received signal element. The distributed trellis decoder is self-synchronizing.

Combined trellis coding with asymmetric modulations

Abstract: The use of asymmetric signal constellations combined with optimized trellis coding to improve the performance of coded systems without increasing the average or peak power, or changing the bandwidth constraints of a system is discussed. The trellis code, asymmetric signal set, and Viterbi decoder of the system model are examined. The procedures for assigning signals to state transitions of the trellis code are described; the performance of the trellis coding system is evaluated. Examples of AM, QAM, and MPSK modulations with short memory trellis codes are presented.

Index decoder for digital modems

Abstract: In a QAM digital modem decoder section, the received signals (after equalization) are transposed to a preselected quadrant of the signal constellation to generate an index pointer. The index pointer and the actual quadrant are used to decode the QAM signals. The index pointer is also used to update the equalizer taps and to operate a phase tracking loop used to reduce or eliminate phase jitter.

Adaptive noise suppression for digital modems

Abstract: This invention provides a receiver for a QAM modem with a phase tracking loop for eliminating phase errors due to phase jitter and frequency offset. The instantaneous phase error of the received signal is monitored and if it is below a certain preselected value, indicating the presence of white noise, the gain of the phase tracking loop is reduced.

Single-sideband communication system

Abstract: A bandwidth reduction technique for use in digital systems wherein elements of a data signal modulate quadrature-related carriers. This modulation, referred to as quadrature amplitude modulation (QAM) or phase shift keying (PSK), generates a double-sideband signal which is transmitted in a variety of communications systems. In accordance with the present invention, the above-described double-sideband signal is filtered (103) to form a single-sideband signal prior to transmission. While this use of a single-sideband signal, in lieu of a double-sideband signal, effectively doubles the system capacity by permitting the use of two communications systems in the bandwidth previously occupied by one system, the filtering process (103) contaminates the data signal elements. To recover the data signal elements at the receiver, received signal elements are formed by extracting the carrier signals (107). Next, these received signal elements are altered by preselected quantities to form estimates of each data signal element (301, 302...307). A comparison of the formed estimates (318) against the set of permissible values for each data signal element then determines which estimate is correct.

QAM modulator and demodulator

Abstract: In a digital-to-analog converter, a digital input signal of x bits is subjected to digital-to-analog conversion with at least two of x bit positions recognized as a common bit position and converted into analog levels of a number which is different from 2 x where x is an integer At least one additional common bit position may be selected from the x bit positions except the common bit position The digital input signal may be pre-processed by the use of a logic circuit (15) prior to the digital-to-analog conversion so as to control the number of analog levels The digital-to-analog converter is applicable to a modulator which produces a quadrature amplitude modulated signal having a circular arrangement of signal points on a phase plane A demodulator comprises an analog-to-digital converter for converting the above-mentioned analog signal into a reproduction of the digital input signal

On the Availability of Multilevel CPFSK Systems with Modulation Pulse Shaping During Multipath Propagation

Abstract: The behavior of CPFSK systems with modulation pulse shaping and limiter-discriminator detection is tested in the prsence of multipath fading. With the aim of determining the eye-closure domain, a theoretical analysis is presented for a generic number of modulation levels. It is shown that, by suitably varying the maximum angular frequency deviation ω d , control of outage probability P out is possible. Moreover, the choice of duty-cycle in the modulation pulse \tau/T leads to a lower value of P out with respect to the conventional case \tau/T = 1 , if we fix, as happens in practice, an upper bound in the transmitted power. This promising characteristic deeply differentiates the CPFSK from the PSK, DCPSK, and QAM systems: CPFSK systems, with a fixed number of modulation levels, allow us to obtain decreasing values of P out at the cost of increasing transmission power by suitably varying the set of signal parameters, even in the absence of diversity techniques and/or adaptive equalization.

Orthogonal amplitude modulation system transmitter

Abstract: PURPOSE:To generate an average power of non-modulation signal of a multi-value QAM system independently of a multi-number by providing a means generating the average power of (2 X2 )-set of non-modulation signals to a transmitter. CONSTITUTION:Four sets of binary digital signals S0-S3 and one set of clock signal CLK are inputted to a transmission logical circuit 1 in figure, where the logical operation such as waveform shaping, code conversion and additive operation is conducted and the signals are inputted to D-A converters 2 and 3 as two rows of parallel axis (for PCH) and two rows of vertical axis (for QCH) digital signals P, Q. The D-A converters 2, 3 input the digital signals P, Q and convert then respectively into 4(2 =4)-value of signals Sp, Sq. The four-value 2-row (4-value X 4-value) signals Sp, Sq are inputted to an orthogonal linear modulator 6, where the multi-value QAM modulation in a carrier frequency (f) is applied, the result is amplified to a prescribed level by an amplifier 7 and a desired 16QAM modulation signal Sm is obtained.

Space diversity system

Abstract: PURPOSE:To attain space diversity to a multi-value QAM wave with simple constitution by extracting a carrier of main and subordinate input signals, comparing output signals corresponding to the phase and controlling an endless phase shifter with a compared output. CONSTITUTION:A digital radio wave using the multi-value QAM system where a carrier is included in a transmission spectrum is received by the diversity system. A main input signal Min from one antenna and a signal of a subordinate input signal Sin from the other antenna through an endless phase shifter PS are inputted to a synthesizer H, where they are synthesized. The carrier included in the signals Min, Sin is extracted respectively by BPF1, BPF2 and the extracted signals are inputted respectively to phase locked loops PLL1, PLL2. The PLL1, PLL2 output respectively a signal in synchronizing with the phase of the extracted carrier and input the signal to a comparator COMP. The COMP compares both signals and outputs a compared output to an integration device I. The integration device I controls the phase shifter PS by using the integrated output signal so as to synthesize (H) the carrier of the signals Min, Sin while the phase is made in phase with each other.

Cable television system

Abstract: In cable television systems, the subscribers are connected via individual remote-controllable converters to a cable which transmits a large number of programmes. The subscriber selects the desired programme by suitable tuning of the individual converter tuner. The converters are equipped with conventional cable tuners for television sets in their reception section and have a UHF band in addition to the VHF band. If a second cable is provided in order to increase the range of programmes offered, the channels in the VHF transmission band (Band III) of the second cable are converted into the UHF band by means of a frequency converter which is connected upstream of the converter. The two frequency bands at the input of the converter tuner are still converted back into Band I within the converter.

QAM demodulator with distortion compensation

Abstract: A demodulator for demodulating a QAM signal which is usable in combination with an adaptive transversal equalizer. Controls over the demodulator and the equalizer are each converged to an optimum point so that the sampling points of two analog-to-digital converters of the demodulator are optimally controlled. For a limited occupied bandwidth of a carrier passband, a large amount of data may be transmitted with distortions due to delay and attenuation inherent in a transmission path compensated for.

On the Availability of Multilevel CPFSK Systems with Modulation Pulse Shaping ..

Abstract: With the aim of determining the eye-closure domain, a theoretical analysis is presented for a generic number of modulation levels. It is shown that, by suitably varying the maximum angular frequency deviation ud, control of outage probability Po,, is possible. Moreover, the choice of duty-cycle in the modulation pulse T/T < 1 leads to a lower value of Po., with respect to the conventional case T/T = 1, if we ti, as happens in practice, an upper bound in the transmitted power. This promising characteristic deeply differentiates the CPFSK from the PSK, DCPSK, and QAM systems: CPFSK systems, with a fixed number of modulation levels, allow us to obtain decreasing values of Po,,@ at the cost of increasing transmission power by suitably varying the set of signal parameters, even in the absence of diversity techniques and/or adaptive equalization.

Semi-compatible hdtv signals for transmission over two catv channels

Abstract: A technique for generating a Semi-Compatible High Definition Television (SC-HDTV) signal which can be sent over two present-day Cable Television (CATV) channels. In accordance with the present invention, a HDTV signal is formed comprising a line scan rate which is a multiple of the line scan rate of a current conventional TV signal. The SC-HDTV signal is formed therefrom by transmitting a first HDTV line as is (output of 22, Fig. 5) and a second HDTV line as a field differential signal (output of 25) transmitted via QAM on the IF or RF carrier of the first line signal at a line scan rate of a current conventional TV signal (40-41, 43-47, Fig. 7). Arrangements are also disclosed for converting the SC-HDTV signal back into either a HDTV (Fig. 9) or conventional TV signal (Fig. 10).

lodul Analysis of the Sensitivity of Digital .ation Techniques to Frequency-Selectj

Abstract: Abstruct-This paper investigates the sensitivity to frequency-selective fading ef different modulation techniques commonly used in digital radio systems. The modulation methods considered are phase-shift-keying (Mary PSK), nonoffset and offset quadrature amplitude modulation (M-ary QAM), and quadrature partial response signaling (QPRS). The performances of these methods during multipath fading are compared on the basis of the “signatures” calculated for idealized systems. Results are presented from which the relative outage probability was determined, assuming a propagation channel with frequency-selective Rician fading.