Showing papers on "Quadrature amplitude modulation published in 1989"

The multitone channel

Abstract: The maximum bit rate of multitone QAM (quadrature amplitude modulation) over a general linear channel is found. First, the overall bit rate for an AWGN channel with a two-level transfer function is maximized, using a multitone QAM system. The power distribution between the tones and the number of bits/symbol per tone is optimized for a given symbol error rate. Extending these results to the general channel, it is shown that the optimum power division for multitone signals is similar to the water-pouring solution of information theory. Furthermore, multitone QAM performance is about 9 dB worse than the channel capacity, independent of the channel characteristics. The multitone results throughout are compared to those of an equivalent single-tone linearly equalized system. The comparison shows that the multitone system is useful for some channels, e.g. those with deep nulls in the transfer function. The maximum bit error rate over a twisted-pair channel which is performance dominated by near-end crosstalk (NEXT) is also found. >

Multidimensional constellations. I. Introduction, figures of merit, and generalized cross constellations

Abstract: The authors discuss the major attributes desired in signal constellations, such as signal-to-noise ratio (SNR) efficiency, simplicity of mapping bits to points and vice versa, compatibility with coded modulation schemes, and compatibility with quadrature amplitude modulation (QAM). The capability of supporting a so-called opportunistic secondary channel, often used for internal control signaling, is considered. The gain in SNR efficiency of a multidimensional constellation (lattice code) consisting of the points from a lattice Lambda within a region R compared to a cubic constellation is shown to be approximately separable into the coding gain of Lambda and the shape gain of R, for large constellations. Similarly, the expansion of the associated constituent 2-D constellation is shown to be approximately separable into a constellation expansion ratio (CER) coding component CER/sub c/( Lambda ) and a shaping component CER/sub s/(R). The N sphere is the region R with the best shape gain, but N also has large constellation expansion. Bounds for the best possible shape gain versus CER/sub s/(R) or peak-to-average-power ratio (PAR) are given. Generalized cross constellations are discussed. These constellations yield a modest shape gain with very low CER/sub s/(R) or PAR, are easily implemented, are well suited for use with coded QAM modems, and can be readily adapted to support an opportunistic secondary channel. >

A pragmatic approach to trellis-coded modulation

Abstract: Since the early 1970s, for power-limited applications, the convolutional code constraint length K=7 and rate 1/2, optimum in the sense of maximum free distance and minimum number of bit errors caused by remerging paths at the free distance, has become the de facto standard for coded digital communication. This was reinforced when punctured versions of this code became the standard for rate 3/4 and 7/8 codes for moderately bandlimited channels. Methods are described for using the same K=7, rate 1/2 convolutional code with signal phase constellations of 8-PSK and 160PSK and quadrature amplitude constellations of 16-QASK, 64-QASK, and 256-QASK to achieve, respectively, 2 and 3, and 2, 4, and 6 b/s/Hz bandwidth efficiencies while providing power efficiency that in most cases is virtually equivalent to that of the best Ungerboeck codes for constraint length 7 or 64 states. This pragmatic approach to all coding applications permits the use of a single basic coder and decoder to achieve respectable coding (power) gains for bandwidth efficiencies from 1 b/s/Hz to 6 b/s/Hz. >

Analysis of predistortion, equalization, and ISI cancellation techniques in digital radio systems with nonlinear transmit amplifiers

Abstract: An analysis made of the performance of predistortion, equalization, and intersymbol interference (ISI) cancellation techniques in compensating for the transmit amplifier nonlinearity in digital microwave radio systems. The study is carried out using the 64 QAM and 256 QAM signal formats and two values of the roll-off factor in the Nyquist pulse shaping. The simulated compensation techniques include three types of predistortion, two ISI cancellers, and several nonlinear equalizers with or without decision feedback. A basic result is that decision-feedback equalizers do not offer any significant advantage over nonrecursive equalizers. It is also shown that ISI cancelers with a memoryless equalizer as first-stage decision device do not perform any better than nonlinear equalizers of similar complexity. Another contribution an improved fifth-order analog signal predistortion technique is analyzed. The gain that can be achieved using a modified 256 QAM signal constellation that is more robust to nonlinear distortion is quantified. >

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.

Rayleigh fading compensation method for 16QAM in digital land mobile radio channels

Abstract: The authors propose a novel Rayleigh fading compensation method for 16 quadrature amplitude modulation (16QAM). The optimum parameters for 16QAM with a transmission rate of 16 k symbol/s, the BER (bit error rate) performance, and the interference performance are determined by computer simulation. It is shown that second-order interpolation is suitable for the fading compensation method. In the case of 16 k-symbol/s transmission, the optimum frame length is 16 symbols. The degradation due to the proposed fading compensation method is about 2 dB. The BER performance under Rayleigh fading environments at f/sub d/ >

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. >

Multi-symbol detection of M-DPSK

Abstract: The authors discuss optimal and suboptimal detection methods for improving the error performance when M-ary DPSK (differential phase shift keying) modulation is employed. All procedures use a block of N+1 measurements to produce N data decisions. The motivation is to lessen the energy penalty associated with classical DPSK detection and to approach the performance of fully-coherent detection, but without complicated feedback phase-tracking circuitry and the associated phase-acquisition time. The authors formulate optimal (block) detection under the assumption of unknown carrier phase. Performance analysis and simulation show significant improvement with increasing blocklength, but complexity increases exponentially in blocklength. Suboptimal methods are then proposed which have far less complexity, but retain the improvement over standard DPSK. The primary applications of the present work are in fast-acquisition TDMA (time-division multiple-access) systems with PSK modulation and in frequency-hopping systems using DPSK. >

Approaches to blind equalization of signals with multiple modulus

Abstract: The constant modulus algorithm (CMA) and decision-directed (DD) equalizer are two ways to approach blind equalization of signals that are known to lie on a circle of fixed radius, but where specific values at any given time are unknown. In m-ary quadrature amplitude modulation, the signals lie on n circles of known radius. The authors present two possible approaches to the n-modulus problem, both in the spirit of feature reconstruction algorithms. The multiple-modulus algorithm uses a straightforward generalization of the CMA cost function to derive its update, whereas the decision-adjusted-modulus algorithm is a hybrid of the CMA and DD approaches. >

Ten-to-twenty gigabit-per-second modulation performance of 1.5- mu m distributed feedback lasers for frequency-shift-keying systems

Abstract: The frequency modulation (FM) and amplitude modulation (AM) responses of a 1.5- mu m distributed feedback (DFB) laser were measured to 15 GHz. At 9-mW output power, the magnitude of the FM response was flat out to 12 GHz, and there was a 15-25-ps delay between the FM and AM responses. Computer simulation techniques indicate that the measured FM response is adequate to produce good eye patterns for frequency-shift-keying modulation at data rates up to 20 Gb/s. The high-speed frequency-shift-keying modulation capability of this 1.5- mu m DFB laser was experimentally confirmed at 11 Gb/s. >

Communication of a voice signal via continuous quadrature amplitude modulator

Abstract: A method and apparatus for communicating a voice conversation over a single telephone channel using continuously variable level quadrature amplitude modulation establishes and maintains two totally independent communication paths, each near perfect in amplitude and phase with respect to frequency over a bandwidth suitable for communication of continuously variable amplitude pulses. These pulses may be representations of any suitable bandlimited analog signal. The method establishes almost flawless QAM channels (real, imaginary), then allows switching to continuously variable amplitude analog transmissions.

Performance of adaptive equalization for indoor radio communications

Abstract: Adaptive equalization is used to ensure that the outage probability is less than 10/sup -3/ for a target bit error rate of 10/sup -4/ in buildings with RMS delay spread of up to 100 ns. A time-division multiple-access system with four-level quadrature amplitude modulation point-to-point links strikes the right balance between flexibility and complexity. It is shown that such a system can support rates of at least 1 Mb/s. >

Digital signal magnetic recording/reproducing apparatus using multi-level QAM modulation and maximum likelihood decoding

Abstract: A digital signal magnetic recording/reproducing apparatus converts a digital signal into multi-value digital signals and performs recording/reproducing of a multi-value quadrature amplitude-modulated signal. By using the multi-value quadrature amplitude modulation, it is possible to remove the DC component in the recording signal and improve the utilization efficiency of the recording frequency band. Further, using a system to memorize the frequency characteristic of the magnetic recording/reproducing unit and effect Viterbi decoding regarding the intersymbol interference caused by the frequency as a convolutional coding, it is possible to detect the level without necessitating frequency equalization. Accordingly, there arises no deterioration of the S/N ratio by frequency equalization. Furthermore, by providing an encoder for error control coding by increasing the number of multi-values and a corresponding decoder, the error rate can be improved against the deterioration of the S/N ratio. Further, by bias recording the multi-value quadrature amplitude modulated signal, non-linearity of the electromagnetic conversion system is relaxed, and deterioration of the S/N ratio by distortion is improved.

Impact of non-Gaussian impulsive noise on the performance of high-level QAM

Abstract: The impact of non-Gaussian impulse noise (Middleton's class A noise) combined with Gaussian thermal noise on the performance of high-level QAM (quadrature amplitude modulation) (e.g., 16, 64, and 256 QAM) systems is analyzed. The P(e) (probability of error) performance of the system is evaluated in terms of CNR (carrier-to-noise ratio), impulsive index (A) of the noise, and power ratio of the Gaussian noise to the impulsive noise ( Gamma '). It is shown that, in the high-level QAM, non-Gaussian impulsive noise degrades the system performance significantly, even at high CNR. It is also found that an upper bound on the error probability exists for Gamma ' >

The effects of delay spread on 2-PSK, 4-PSK, 8-PSK and 16-QAM in a portable radio environment

Abstract: Computer simulation results are presented of a study of delay spread on digital modulations with different constellations in a quasi-static multipath radio channel. Unfiltered 2-, 4-, and 8-PSK and 16-QAM with a rectangular signaling pulse are compared first, followed by 4- and 16-QAM with a raised-cosine Nyquist pulse. The bit error rate performances averaged over fading samples under the influence of the intersymbol interference caused by delay spread are compared for modulations of different levels. It is found that 4-level modulation is the most desired method for both performance and implementation in a quasistatic, frequency-selective fading radio channel. Both the spectral and the power efficiencies can be enhanced using Nyquist signaling pulses. >

Digital signal magnetic recording/reproducing apparatus

Abstract: A digital signal magnetic recording/reproducing apparatus converts a digital signal into a multi-value digital signal, followed by recording/reproducing the quadrature biphase modulated multi-value quadrature amplitude modulated signal. By using a multi-value quadrature amplitude modulation, a direct current component in the recorded signal can be eliminated, and the utilization efficiency of the recording frequency band of the digital signal magnetic recording/reproducing apparatus can be improved. Also, with respect to the amplitude non-linear characteristic which occurs in the magnetic recording/reproducing process, by providing a compensating circuit during the mapping before recording or after reproducing and demodulating, the deterioration of the S/N ratio of the reproduced and demodulated signal can be reduced to decrease the errors in the decoded data. Furthermore by bias-recording the multi-value quadrature amplitude modulation signal, the non-linearity of the electromagnetic conversion system is alleviated to decrease the deterioration of the S/N ratio due to distortion, thereby making the application of the modulation scheme more reliable.

Trellis coded noncoherent QAM: a new bandwidth and power efficient scheme

Abstract: The authors introduce the concept of noncoherently detected, multilevel/phase trellis coded schemes. They derive the optimal noncoherent decoding algorithm for trellis coded QAM (quadrative-amplitude-modulated) signals. Further, two reduced complexity decoder structures are proposed and analyzed. The first is based on a reduced state algorithm, whereas the second one uses a novel algorithm that combines both envelope and multiple differential detection. The two decoders have been evaluated for a 16-QAM signalling format by means of computer simulation. The results indicate that the proposed schemes have performance gains of up to 5 dB over equivalent conventionally detected noncoherent systems. >

Joint blind equalization, carrier recovery, and timing recovery for 64-QAM and 128-QAM signal constellations

Abstract: Two existing blind equalization tap update recursions suitable for 64-point and greater QAM signal constellations are studied, along with existing carrier and timing recovery techniques. The superior tap update recursion is determined to be the constant modulus algorithm. Carrier recovery requires a modified second-order decision-directed digital phase-locked loop. An all-digital implementation of band-edge timing recovery is used. With 14.4-kb/s outbound transmission using CCITT V.33 trellis-coded 128-QAM signals having 12.5% excess bandwidth, a prototype blind retrain procedure is developed to demonstrate feasibility of the new techniques for high-speed multipoint modems. A WE DSP32-based real-time digital signal processor was employed to successfully test the retrain over a set of severely impaired channels. >

Quadrature amplitude modulation with line synchronization pulse for video telephone

Abstract: The present invention assigns amplitudes of sine wave cycles in a data packet on I and Q channels of a QAM signal to color, gray or luminance values. Overlapping sine wave cycles on the I and Q channels in the data packet are assigned to the same or neighboring pixels. A synchronizing signal is sent at the beginning of each transmission of a packet. The synchronizing signal is a plurality of cycles at a maximum amplitude on the I and Q channels. The synchronizing signal serves two functions simultaneously. First, it is used to indicate the beginning of a new packet. Second, the phase of the synchronizing signal is used to initialize the demodulator phase detecting circuitry.

Performance analysis of digital radio links with nonlinear transmit amplifier and data predistorter with memory

Abstract: A method of introducing an adaptive digital predistortion circuit to compensate for the nonlinearity produced in digital radio systems by the high-power amplifier (HPA) is proposed. The method is attractive for its simplicity (two digital finite-impulse response (FIR) filters) and because of the inherent adaptability to any nonlinearity. The simplicity of the predistorter is worth noting; only two parameters ( alpha /sub 1/ and alpha /sub 3/) need to be estimated in place of tens of the third-order kernel coefficients, as proposed in previous schemes. The performance gain is about 3 dB for a 64 quadrature amplitude modulation (QAM) system without predistortion and 1 dB for a system with memoryless data predistortion. The corresponding values for the 256 QAM system are 4 dB and 1.5 dB, respectively. >

20 channel coherent FSK system using subcarrier multiplexing

Abstract: A 20-channel multioctave coherent frequency shift keying (FSK) system using subcarrier multiplexing techniques is described. The microwave subcarriers range from 2.1 to 5.9 GHz with a 200-MHz channel spacing. The receiver consists of a p-i-n photodiode, electrical isolator, low-noise amplifier, and bandpass filter to provide channel selection. The channels are modulated at 100 Mb/s, corresponding to one digitized video channel. Results for the optimal phase modulation index and degradation due to second- and third-order intermodulation distortion are discussed. For the worst-case channel, a receiver sensitivity of -27 dBm was achieved. >

Development of mobile communication system for RACS

Abstract: The development of a road/automobile communication system is discussed. An inductive radio system and a microwave system are potential candidates. The inductive radio system has four lanes*4 m of communication range with a bit error rate characteristic of 5*10/sup -5/ to 5*10/sup -6/ and a maximum data volume of 460 kb attainable at a beacon when an automobile is traveling at 100 km/h. The microwave system using the 2.5 GHz frequency band has five lanes*60 m of communication range and a maximum data volume of 400 kb with a transmission data rate of 512 kbps. By deploying the phase shift keying/reverse amplitude modulation method, the system was able to determine the position of automobiles very accurately and to detect their direction of motion. The microwave system has very high potential for future applications and will be implemented in Japan. >

Digital cellular system with linear modulation

Abstract: The authors describe a digital cellular system with linear modulation. They discuss two kinds of modulation schemes-QPSK (quadrative phase-shift keying) and pi /4 shift QPSK, and two kinds of detection schemes-coherent detection and limiter-discriminator detection. The features of these modulation/detection schemes are clarified. From the collective point of view, the pi /4 shift QPSK/limiter-discriminator detection is judged to be the most advantageous. Next, the authors show examples of the radio link design for a digital cellular system with linear modulation. The system could be constructed at a transmission power of 100-200 mW. Finally, the breadboard equipment configuration and the experimental results are shown. >

Radar system with a digital expander

Abstract: In a radar system, comprising a digital expander, two analog modulation signals are generated with two digital to analog converters and raised to an intermediate frequency range through a quadrature modulator. The amplitude and phase ripple of the output signal occurring in the quadrature modulator due to limited carrier and image frequency suppression are compensated through a multiple regulating circuit. To this end, if necessary calibration signals, of constant amplitude and different phase positions, are generated sequentially by means of the two digital to analog converters and the amplitude of their output signals measured in an amplitude detector. Setting values for the multiple regulating circuit are determined from the averaged amplitude measured values in a phase and amplitude correction unit which via setting elements set the requisite offset and phase and amplitude symmetry values until the deviations of the output signal in amplitude and phase disappear.

Performance of convolutional coded SQAM in hardlimited satellite channels

Abstract: The hardware implementation and experimental measurements of a differential encoded superposed quadrature amplitude modulation (SQAM) modem combined with 3/4-rate convolutional encoding and threshold decoding for use in bandwidth and power-limited satellite communications are presented. To avoid error propagation caused by differential encoding/decoding, and also to combat bursty channel noise, a data interleave/deinterleave scheme is integrated into the forward error-code (FEC) codec. Specific signal waveshaping is achieved digitally after FEC encoding to produce signals free of intersymbol interference (ISI) and timing jitter while providing a compact power spectrum. Experimentally measured results including signal waveforms, eye diagrams, power spectrum, and bit error rate (BER) performance in linear and hardlimited transmission channels are reported. The BER results show that the power efficiency of the modem is excellent. Spectral regrowth of the modulated signal is also retained at the minimum to ensure minimum interference to adjacent channels and thus maximize the channel utilization efficiency. >

Quadrature amplitude modulation system using digital filters

Abstract: A modulating device applicable to a digital radio communications system which is adaptable to a 16 or greater multi-level QAM modulation system. The device has two multi-input digital filters to implement the role of cosine roll-off filters. A logic circuit precedes the digital filters to logically manipulate n-bit data streams except for m bits (m

Improved data predistortion using intersymbol interpolation

Abstract: An efficient data predistortion technique is presented to compensate for transmitter high-power amplifier (HPA) nonlinearities in digital microwave radio systems employing quadrature amplitude modulation (QAM) signal formats. The proposed technique makes use of a specific transmitter pulse shaping that leads to discrete signal levels at time instants spaced by half a symbol interval. The predistortion circuit is designed so as to cancel nonlinear distortion at the HPA output at these instants. With respect to conventional data predistortion, which can only compensate for warping of the signal constellation, this technique also compensates for nonlinear intersymbol interference (ISI). It is shown that, using the 64 and 256 QAM signal formats, the presented technique leads to a considerably more efficient utilization of the available HPA power. >

Carrier recovery for blind equalization

Abstract: Carrier recovery techniques are presented that work with blind equalization of high-order QAM (quadrature amplitude modulation) signal constellations, specifically, the square uncoded 64-QAM and CCITT V.33 cross-trellis-coded 128-QAM constellations. The techniques were confirmed by laboratory experiments using a WE-DSP32-based real-time digital signal processor and analog-simulated voiceband telephone channels containing the common impairments of additive noise, linear distortion, frequency offset, phase jitter, and nonlinear distortion. >