Showing papers on "Quadrature amplitude modulation published in 1997"

Bit-interleaved coded modulation

Abstract: It has been recognized by Zehavi (1992) that the performance of coded modulation over a Rayleigh fading channel can be improved by bit-wise interleaving at the encoder output, and by using an appropriate soft-decision metric as an input to a Viterbi (1990) decoder. The paper presents in a comprehensive fashion the theory underlying bit-interleaved coded modulation, provides tools for evaluating its performance, and gives guidelines for its design.

A new Volterra predistorter based on the indirect learning architecture

Abstract: Nonlinear compensation techniques are becoming increasingly important. We present a new Volterra-based predistorter, which utilizes the indirect learning architecture to circumvent a classical problem associated with predistorters, namely that the desired output is not known in advance. We utilize the indirect learning architecture and the recursive least square (RLS) algorithm. Specifically, we propose an indirect Volterra series model predistorter which is independent of a specific nonlinear model for the system to be compensated. Both 16-phase shift keying (PSK) and 16-quadrature amplitude modulation (QAM) are used to demonstrate the efficacy of the new approach.

A novel channel estimation method for OFDM mobile communication systems based on pilot signals and transform-domain processing

Abstract: A method of estimating the channel transfer function is presented for orthogonal frequency division multiplexing (OFDM) mobile communication systems working under time-variant radio channel conditions. The proposed method employs lowpass filtering in a transform domain so that intercarrier interference and additive white Gaussian noise components in the received pilot signals are significantly reduced. The cutoff frequency of the transform-domain filter is dynamically selected by tracking the received pilot signals. The channel transfer function for all the subcarriers is obtained by a high-resolution interpolation realized by zero-padding and DFT/IDFT. The proposed method is applicable for all linear modulation OFDM systems. It is demonstrated with a 16QAM-OFDM system which includes both amplitude and phase modulations.

Adaptive coded modulation for fading channels

Abstract: We propose a variable-power and variable-rate coded MQAM modulation technique for high-speed data transmission on fading channels. Coding gain is obtained by superimposing trellis codes designed for AWGN channels on the adaptive modulation, and we obtain the same coding gains as these codes exhibit in AWGN. We present analytical and simulation results which show a 3dB coding gain relative to uncoded adaptive modulation for a simple 4-state trellis code, and a 4 dB coding gain for an 8-state trellis code. More complex trellis codes achieve higher gains.

Digital compensation for wideband modulation of a phase locked loop frequency synthesizer

Abstract: A digital compensation filtering technique is provided that enables indirect phase locked loop modulation with a digital modulation data stream having a bandwidth that exceeds, perhaps by an order of magnitude, the bandwidth characteristic of the phase locked loop. A modulation data receiver is provided for receiving from a modulation source digital input modulation data having a bandwidth that exceeds the cutoff frequency characteristic of the phase locked loop frequency response. A digital processor is coupled to the modulation data receiver for digitally processing the input modulation data to amplify modulation data at frequencies higher than the phase locked loop cutoff frequency. This digital processor is connected to the phase locked loop frequency divider to modulate the divider based on the digitally-processed input modulation data, whereby a voltage controlled oscillator of the phase locked loop is controlled to produce a modulated output carrier signal having a modulation bandwidth that exceeds the phase locked loop cutoff frequency. The digital processing of the modulation data can be implemented by adapting a digital FIR Gaussian transmit filter such that its filter characteristic reflects the intended modulation data amplification as well as enables Gaussian Frequency Shift Keyed modulation. With this implementation, no additional componentry beyond the PLL system is needed to implement the digital modulation data processing provided by the invention.

Wavelet packet modulation for orthogonally multiplexed communication

Abstract: Utilizing multidimensional signaling techniques, a generalized multirate wavelet-based modulation format for orthogonally multiplexed communication systems is presented. Wavelet packet modulation (WPM) employs the basis functions from an arbitrary pruning of a dyadic tree structured filter bank as orthogonal pulse shapes for conventional quadrature amplitude modulation (QAM) symbols. This generalized framework affords an entire library of basis sets with increased flexibility in time-frequency (T-F) partitioning. The bandwidth efficiency and power spectral density figures of merit for the general signal are derived and shown to be that of standard QAM.

Controlling clipping probability in DMT transmission

Abstract: A discrete multitone signal has a much higher peak to average ratio than a comparable single tone modulation signal. Therefore a DMT transmitter has to either have a significantly more expensive analog front end (both in terms of the DAC, the analog filter and line driver) or it has to control the amount of clipping. We describe two general techniques for clipping control and present performance comparisons to previous techniques.

The multimodulus blind equalization algorithm

Abstract: This paper presents a new blind equalization algorithm called the multimodulus algorithm (MMA). This algorithm is particularly well suited to applications using two-dimensional transmission schemes such as carrierless AM/PM (CAP) and quadrature amplitude modulation (QAM). The paper also discusses some of the advantages provided by MMA when compared to traditional blind equalization algorithms, such as the reduced constellation algorithm (RCA) and the constant modulus algorithm (CMA).

Optimization and performance evaluation of multicarrier transmission

Abstract: Optimization of the performance of multicarrier transmission over a linear dispersive channel is presented. The optimum data and power assignment to the subcarriers are derived for both the conventional error probability criterion, and a new criterion based on the normalized mean-square error. The assignments and algorithms hold for channels where performance is degraded by additive noise, intersymbol and interchannel interference. Lower bounds on throughput are derived and are used to compare multicarrier performance with conventional single-carrier quadrature amplitude modulation (QAM) with both linear and decision feedback equalization. It is seen that multicarrier transmission can provide a significant improvement at low and intermediate channel signal-to-noise ratios. As an example, the optimization is applied to the high-speed digital subscriber loop, and multicarrier transmission is demonstrated to be superior to decision feedback equalized single-carrier QAM.

Introducing erasures in decision-feedback equalization to reduce error propagation

Abstract: A simple modification of the decision feedback equalizer (DFE) slicer is proposed to reduce the effect of error propagation. A comparison of the performance of the modified DFE and conventional DFE is made for specific channels. On these channels, the modified DFE performs only marginally better than the conventional DFE in terms of average error probability, but may offer some advantages in terms of error probability conditioned on specific input sequences and in terms of the distribution of error burst lengths. Some examples are given, concerning binary PAM and multilevel quadrature amplitude modulation (M-QAM) systems.

Optimum and suboptimum frame synchronization for pilot-symbol-assisted modulation

Abstract: Pilot-symbol-assisted modulation (PSAM) is a method to reduce the effects of fading in mobile communications by periodically inserting known symbols in the data stream. The receiver uses these pilot symbols to derive its amplitude and phase reference. One aspect of this procedure which has not received much attention in the literature is the method used by the receiver to locate the pilot symbols. This paper uses optimum frame synchronization techniques to develop two synchronizers for PSAM systems; one is based on a standard maximum likelihood (ML) estimation formulation, and the other is a sequential testing algorithm. Both methods use a simple quadratic correlation filter with an energy correction factor. Simulation results and a theoretical analysis are presented.

Symbol rate estimation by the wavelet transform

Abstract: Demodulation of a digital modulated waveform requires the symbol rate of a received signal. This parameter may not be known a priori and may need to be estimated in the receiver. This paper studies the use of wavelet transform to estimate the symbol rate of an M-ary phase shift keying (PSK) signal. The idea is to use wavelet transform to locate the transients produced from phase changes. The separation between transients gives a symbol rate estimate. Previous work uses the transform coefficients in a single scale to estimate symbol rate. This paper improves the performance of the estimator by combining the coefficients at several scales before estimation. The accuracy of the estimator is shown to be within 3-6dB of the CRLB, when the sampling rate is four times the carrier frequency and the carrier-to-noise ratio is greater than 7 dB.

Blind carrier phase acquisition for QAM constellations

Abstract: A constant need for ever-increasing throughputs through fixed bandwidths, fueled by several high-speed applications (such as digital TV), has pushed system designers toward more throughput-efficient modulation schemes. Because of their relatively good performance, large quadrature amplitude modulation (QAM) constellations are being used in many of these applications. One of the problems associated with the use of large QAM constellations is that of carrier acquisition, which, for efficiency reasons, must often be done without the use of a preamble. The problem is further complicated for cross constellations, for which the high signal-to-noise ratio (SNR) corner points used by some simple carrier phase estimators are not available. We derive simple algorithms for carrier phase acquisition that can be used for both square and cross constellations, and compare their performance to those of the maximum-likelihood (ML), the fourth-power estimator, and a modified fourth-power estimator, obtained by considering a reduced constellation. The introduced algorithms convert the problem of carrier phase estimation into one of estimating the mode of an underlying distribution. An expression for this underlying distribution is also obtained. The results obtained indicate that the introduced algorithms significantly outperform the fourth-power estimator for moderate to high SNRs, especially when cross constellations are used.

Feedforward ML-based timing estimation with PSK signals

Abstract: We propose a new nondata-aided clock recovery scheme for phase-shift keying (PSK) modulated signals. Its derivation is based on maximum-likelihood (ML) methods and leads to a feedforward structure that can be easily implemented in digital form. The algorithm exhibits improvements with respect to other existing circuits, especially with small excess bandwidth factors. Its performance is assessed by simulation for quaternary PSK (QPSK) and 8PSK formats.

New Rayleigh fading channel estimator based on PSAM channel sounding technique

Abstract: A new channel estimator, termed as the sinc interpolator, based on the pilot symbol assisted modulation channel sounding is proposed for Rayleigh fading channels. This method uses sinc function to estimate channel state information. A Wiener filtering estimator known to have optimum performance needs a priori information of autocorrelation function of the channel gain, Doppler frequency and signal-to-noise ratio to optimize filter coefficients. But the proposed method does not need a priori information while its performance is similar to that of the Wiener filtering estimator.

Receiver capable of demodulating multiple digital modulation formats

Abstract: A receiver (100) for demodulating multiple digital modulation formats including vestigial sideband (VSB), quadrature amplitude modulation (QAM), and offset QAM (OQAM). The receiver includes a timing recovery circuit (126) that produces accurate timing information for each modulation format and a signal processor (54) for adaptive equalization and quantization of each modulation format. The signal processor includes an adaptive equalizer (900) which is a passband equalizer that performs blind equalization using a feed forward equalizer (902) and a decision feedback equalizer (910).

VPSK and VMSK modulation transmit digital audio and video at 15 bits/sec/Hz

Abstract: Bandwidth efficiencies that have not been possible in the past are now being achieved using variable PSK (VPSK) and variable MSK (VMSK) modulation without the loss of signal power that normally accompanies high bandwidth efficiency methods. Theoretically, these two methods do not lose any signal energy with increasing bandwidth compression. Efficiencies up to 15 bits/sec/Hz are now being achieved in usable hardware with C/N ratios better than that obtainable using FM, BPSK or QPSK. This paper explains how this is accomplished and gives a full mathematical analysis of the method. FM-SCA, VSAT, STL and RPU equipment are now undergoing beta testing prior to FCC Type Acceptance submission. Installation is expected on a large network during 1997.

Space-time coded modulation for high data rate wireless communications

Abstract: This paper presents the theory and practice of a new advanced modem technology suitable for high data rate wireless communications and presents its performance over a frequency-flat Rayleigh fading channel. The new technology is based on space-time coded modulation (STCM) with multiple transmit and/or multiple receive antennas and orthogonal pilot sequence insertion (O-PSI). In this approach data is encoded by a space-time channel encoder and the output of the encoder is split into N streams to be simultaneously transmitted using N transmit antennas. The transmitter inserts periodic orthogonal pilot sequences in each of the simultaneously transmitted bursts. The receiver uses those pilot sequences to estimate the fading channel. When combined with an appropriately designed interpolation filter, accurate channel state information (CSI) can be estimated for the decoding process. Simulation results of the proposed modem as applied to the IS-136 cellular standard are presented. We present the frame error rate (FER) performance as a function of the signal to noise ratio (SNR) and Doppler spread in the presence of timing and frequency offset errors. Simulation results show that, for example, for 10% FER, data rates up to 54 kbps per a 30 kHz channel can be supported at a SNR of 11.7 dB and a Doppler spread of 180 Hz using a 32-state 8-PSK space-time code with 2 transmit and 2 receive antennas. Simulation results for other cases are also provided.

Transmission system with cross-phase modulation compensation

Abstract: In a WDM transmission system carrying amplitude modulated traffic in which significant cross-phase modulation occurs, each of the individual channels is pre-chirped at the transmitter with replicas, or low-pass filtered replicas of the amplitude modulation applied to each of the other channels. Prechirping of each individual channel with a replica of the amplitude modulation applied to that channel may be added in order additionally to provide compensation for self-phase modulation.

Blind equalization using cost function matched to the signal constellation

Abstract: The constant modulus algorithm (CMA) has been proved to be very effective when used to equalize non-minimum-phase channels blindly. However, the CMA suffers from a residual intersymbol interference (ISI) and slow convergence when applied to high order non-constant modulus (e.g. QAM) constellations and the equalizer is an FIR filter. We propose a method that is able to provide a lower ISI and a faster convergence rate than the CMA, based on cost functions specifically matched to the signal constellation. It is different from other CMA methods, where the matching between a certain number of cumulants between the equalizer output and the signal constellation is the driving criterion, the cost function used in this paper tries to force the equalizer output to belong to the signal constellation. The superiority of the proposed approach with respect to conventional approaches is verified by simulation.

Trellis coded QAM using rate compatible, punctured, convolutional codes

Abstract: The digital transmission system provides a selective protection by combining a convolutional coding (14), a punctured coding (16) and a digital quadrature amplitude modulation (15). The rate of the convolutional coding is N/N2, and the quadrature amplitude modulation uses a 22N-point constellation with N > 1. Application: digital transmission system, satellite television, cable television.

Digital linearizer for RF amplifiers

Abstract: Broadcast technology is at the beginning of a new era. It is characterized by the intensive use of the most advanced digital modulation formats (8VSB, QAM, OFDM) in combination with high power RF amplifiers. To date the linearity required for these digital formats has only been accomplished in cumbersome low efficiency class A amplifier or even more cumbersome feed-forward systems. A potentially more efficient and cost effective approach is the combination of nonlinear power amplifiers and a predistortion technique capable of compensating for the nonlinear amplifiers. Digital predistortion will provide a highly linear output and improved efficiency. Itelco has developed a digital adaptive base band predistorter to provide for improved performance and cost. The technique is independent of the modulation type, the output frequency, or the signal bandwidth. Furthermore the capability of automatic adaptive predistortion to compensate for the environment (temperature, power supply variations, aging, and even operation during replacement of a faulty module) is highly desirable.

Automatic gain control system that responds to baseband signal distortion

Abstract: An automatic gain control (AGC) system that monitors the distortion of the outer constellation points of a complex modulation format such as quadrature amplitude modulation (QAM) and adjusts both the RF and IF AGC gain to minimize such distortion. In particular, the receiver contains at least two stages of AGC gain. Typically, the RF AGC amplifier is contained in the tuner that is followed by an IF AGC amplifier. The IF amplifier is coupled to a mixer and an associated low pass filter which, in combination and when driven by a particular frequency, produces a baseband (or near baseband) signal, The baseband signal is digitized and processed by a gain control circuit. The gain control circuit produces IF and RF AGC signals that control the gain of the respective AGC stages.

Multilevel coding with independent decoding on levels for efficient communication on static and interleaved fading channels

Abstract: Error coded modulation is considered with focus on mobile radio. A new scheme, namely multilevel coded modulation with independent decoding on levels, and some attractive derivatives (coding based on multiple classes of levels) are introduced. A comparison of the new coded modulation schemes and well-known methods is performed by simulations on both static and interleaved Rayleigh fading channels. It is shown that the new schemes provide high power efficiency independent of the channel situation, and outperform the prior art methods in this context. The robustness to channel variations is the key feature which make the new scheme attractive for mobile radio systems.

All-optical gain control of in-line erbium-doped fiber amplifiers for hybrid analog/digital WDM systems

Abstract: Automatic gain control using an all-optical feedback loop in in-line erbium-doped fiber amplifiers (EDFA's) used in hybrid analog/digital wavelength division multiplexing (WDM) systems was studied. It is found that the signal level variation for the digital channels can be maintained within a range /spl les/3-dB between the presence and dropout of the analog channel when the narrowband feedback is centered at the amplified spontaneous emission (ASE) peak (/spl sim/1532 nm) with loop loss ranging between 13-22 dB. Robust transmission at 2.5 Gb/s without measurable power penalty was obtained for the digital channels when the EDFA was saturated by either the analog or the control lasing signal.

Differentially amplitude and phase-encoded QAM for the correlated Rayleigh-fading channel with diversity reception

Abstract: This paper studies the differentially amplitude and phase-encoded (DAPE) quadrature amplitude modulation (QAM) transmission over correlated Rayleigh channels with diversity reception. Operating over two successive received symbols, the optimum and an asymptotic maximum-likelihood (AML) differentially coherent receiver are developed and compared with a conventional switched diversity combining (SDC) grid receiver. It is shown that the AML and SDC grid receivers are much simpler in complexity than the optimum receiver in that no channel side information is required in their realization. An exact expression of the bit-error probability (BEP) is obtained for the SDC grid receiver. Based on a union bound argument, a BEP upper bound for the AML receiver is also derived and verified by simulation. Numerical results on 16- and 64-point constellations show that the AML receiver exhibits an almost optimum performance and the SDC grid receiver with a small level of diversity is nearly optimum. It is also shown by simulation that the conventional equal-gain diversity receiver is almost optimal for demodulating a 16-point DAPE QAM signal.

Modulation classification by means of different orders statistical moments

Abstract: This paper deals with modulation classification of 4PSK (4 states phase shift keying) and 16QAM (16 states quadrature amplitude modulation) using a pattern recognition approach. The discriminating feature is build as an optimized combination of fourth and second order moments in order to maximize the probability of correct classification (P/sub /spl infin//). This system is an extension of the one proposed in Boitreau and Le Martret (1996) where the discriminating feature is fourth order cumulant slice. Theoretical and experimental performances are presented for different signal to noise ratios (SNR) and are compared to the one obtained in Boitreau and Le Martret. A P/sub /spl infin//=0.99 is reached for SNR=0 dB and only 500 transmitted symbols.

Approximate minimum bit-error rate equalization for binary signaling

Abstract: Although most linear and decision-feedback equalizers are designed to minimize a mean-squared error (MSE) performance metric, the equalizer that directly minimizes bit-error rate (BER) may significantly outperform the minimum-MSE equalizer, especially for binary antipodal signaling and its biorthogonal extensions, such as four quadrature-amplitude modulation. We show that the performance gain of the minimum-BER equalizer over the minimum-MSE equalizer is most pronounced when the number of equalizer coefficients is small relative to the severity of the intersymbol interference. We propose a simple stochastic gradient algorithm that approximately minimizes BER in the presence of linear intersymbol interference and white Gaussian noise. Computer simulations reveal that the proposed algorithm compares favorably to the popular least-mean-square algorithm in terms of both steady-state performance and complexity.

Frequency division multiplexed transmission of sub-band signals

Abstract: A method and apparatus for processing an input signal for transmission and/or storage, uses an analysis filter bank (21; 51) to decompose the signal into sub-band signals which are used to modulate a plurality of carriers. The carriers are combined into a single encoded signal for transmission/storage. The encoder/decoder is especially applicable to telecommunications systems and recording systems. The analysis filter bank may comprise a multiresolution filter, such as an octave band filter bank (40A/B/C/D...43A/B/C/D) implementing Discrete Wavelet Transform. The modulation may comprise double-sideband, single-sideband, quadrature amplitude modulation, and so on. Where the input signal is analog, the carriers may be modulated directly by the sub-band signals. Where the input signal is digital, however, the sub-band signals are interpolated, all to the same rate, and then used to modulate the carriers. The corresponding decoder (13) extracts the modulated carrier signals, demodulates them, decimates them (if applicable) and then synthesizes them to reconstruct the original input signal. One or more of the sub-bands, especially at the higher frequencies, may be discarded. Discrete wavelet transformation is applied to segments of the digital signal.

A new suboptimal pulse-width modulation technique for per-phase modulation and space vector modulation

Abstract: A new suboptimal pulse-width modulation (PWM) technique for power converter applications is presented in this paper. The technique is developed by considering both two-phase and three-phase modulation techniques. In comparison with the existing suboptimal PWM techniques, the new suboptimal technique provides online calculation, extension of maximum linear modulation index, and can be applied to both per-phase modulation and space vector modulation. For high switching frequency applications or low switching frequency applications with modulation index above a particular range, the new technique increases the frequency of the first dominant harmonic by 50% and minimises switching losses. Experimental and simulation results will be presented to confirm the theoretical analysis.