Showing papers on "Quadrature amplitude modulation published in 1993"

A new bandwidth efficient transmit antenna modulation diversity scheme for linear digital modulation

Abstract: A system is proposed and analyzed, which uses several transmit antennas to achieve diversity in a flat fading mobile radio environment. The signals at different antennas carry the same digital information, but have different modulation parameters (modulation diversity). In contrast to other proposals, no bandwidth expansion is required. Modulation diversity for quadrature amplitude modulation (QAM) type modulation is achieved by using different partial response type base pulses. Simulation results confirm the expected gain. >

Rayleigh fading compensation for QAM in land mobile radio communications

Abstract: A pilot symbol-aided Rayleigh fading compensation is investigated for M-ary quadrature amplitude modulation (QAM) to achieve highly spectrally efficient land mobile communication systems. The optimum parameters for fading compensation, bit error rate (BER) performance against E/sub b//N/sub 0/ (energy per bit to the noise power spectrum density), adjacent channel interference, and cochannel interference for 16-QAM, 64-QAM, and 256-QAM, and the spectral efficiencies for these modulation schemes in Rayleigh fading environments are investigated by computer simulation. To further verify the effect of pilot symbol-aided fading compensation from a a practical point of view, a 16-QAM modem is implemented, laboratory experiments are executed, and the impact of the dynamic range limitation due to the resolution of the analog-to-digital (A/D) converters is evaluated, along with the imperfection of the analog circuits. It is demonstrated by computer simulation and laboratory experiments that the pilot symbol-aided fading compensation can sufficiently compensate for fast varying Rayleigh fading, and 16-QAM gives the highest spectral efficiency in the case of cellular systems. >

Carrier recovery system using acquisition and tracking modes and automatic carrier-to-noise estimation

Abstract: A carrier recovery system for use in illustratively a passband QAM (quadrature amplitude modulation) demodulator (10) and specifically such a system, including apparatus and accompanying methods, that employs separate acquisition and tracking modes and automatic carrier-to-noise estimation. In particular, a carrier recovery system (200) is operated in two modes: an acquisition mode to first attain an initial carrier lock, during which simplified reduced constellation slicing is used to determine a quadrant of each incoming symbol; and a tracking (or lock) mode, during which full slicing is used, to accurately track variations in frequency and phase that may occur to a carrier while the system remains locked. The mode changes based upon whether, e.g., the average error energy associated with fully sliced symbols is greater than or less than a pre-defined switching threshold value. The value of the switching threshold changes in a pre-determined manner with changes in noise content, particularly a carrier-to-noise ratio (CNR), of the received symbols. Two appropriately spaced switching threshold values, one for use in each mode, can be used to provide enhanced operational stability. An implementation of a QAM CNR estimator circuit (800) is also provided.

Performance analysis of QAM modulations applied to the LINC transmitter

Abstract: Future mobile radiocommunications systems will use linear modulations because they show a higher spectrum efficiency than classical FM modulations. Furthermore, in order to use these modulations in hand-portable equipment, power efficiency is also requested for the power amplifiers. To obtain both power and spectrum efficiency, a LINC transmitter can be considered. The authors present an analysis of the effect of different types of imbalances between the parallel signal paths in a LINC transmitter. The system degradations are described in terms of adjacent channel rejection, (U/sub R/). Classical raised cosine (Nyquist filtered) 4, 16, and 64 QAM modulation patterns are taken into account, and in all cases, upper bounds for adjacent channel rejection as function of the gain and phase imbalances as well as of the guard band between adjacent channels are presented. Moreover, the impact of these imbalances in the system performance, characterized by means of the signal-to-noise ratio (SNR) increment needed to maintain a fixed error rate, is also considered. The results show that gain and phase imbalance between both RF paths could be a serious limitation for the LINC transmitter performance. >

Transmission of subband-coded images via mobile channels

Abstract: The feasibility of a mobile video handset is investigated for Rayleigh-fading channels, where transmissions must be confined to the channel's coherence bandwidth to avoid the deployment of complex high-power-consumption channel equalizers. This necessitates the utilization of a low-bit-rate image codec error-protected by embedded low-complexity BCH codecs and spectrally efficient 16-level quadrature amplitude modulation (16 QAM). Motion-compensated nonuniform seven-band subband coding with subband-specific scanning, adaptive quantization, runlength coding, and adaptive buffering to equalize bit-rate fluctuations offer good objective and subjective image quality at moderate complexity and a bit-rate of 55 kb/s. Using a twin-class embedded-BCH error protection as well as pilot symbol 16 QAM and diversity assisted, the 22 kBd candidate system yields unimpaired image quality for average channel signal-to-noise ratios (SNRs) in excess of about 16-18 dB, when the mobile speed is 4 miles/h. >

Adaptively self-correcting modulation system and method

Abstract: A self-adjusting quadrature modulator and modulation method improve the accuracy with which digital data is impressed on an RF carrier, which is particularly useful for systems having Viterbi, echo-integrating demodulators and for systems using subtractive CDMA techniques. The method and apparatus involve receiving one's own transmission with a suitable modulation assessment receiver and determining the modulation error relative to the theoretically perfect transmission expected by the receiver. The measured error is used to adjust the modulation to minimize the error. The modulation assessment receiver may use log-polar signal processing to measure phase and (log)amplitude instead of cartesian I and Q components, and then convert to cartesian form. In addition, correction factors determined by the modulation assessment receiver can be directly identified with certain transform components produced by the Fast Walsh Transform.

Multi-tone transmission for asymmetric digital subscriber lines (ADSL)

Abstract: Multi-tone transmission is a potentially viable technique for application on asymmetric digital subscriber lines (ADSLs). The results of a preliminary study on the performance of an ideal discrete multi-tone (DMT) signaling technique for a 1.6-Mb/s ASDL are presented. The performance margins using single-tone 16-point quadrature amplitude modulation (QAM) with ideal decision feedback equalization (DFE) are presented. The feedforward filter of the DFE is a quarter-baud spaced fractionally spaced equalizer (FSE). The interference is assumed to be the sum of far-end crosstalk (FEXT) and additive white Gaussian noise (AWGN). The subscriber loops are assumed to be in a plant environment where the maximum resistance does not exceed 1300 /spl Omega/. The projected performance of an ideal DMT with a sampling rate of 1.024 MHz and 256-subchannel segmentation offers potential margin enhancement up to 3 dB over a quarter baud-spaced FSE-based 16-QAM signaling. >

Joint source and channel coding using a non-linear receiver

Abstract: The joint optimization of the source and channel coders in a system consisting of a vector quantizer (VQ) whose output indices are mapped directly into points in the modulation signal space is considered. A decoder based on a nonlinear estimator is used to reconstruct the source signal. An iterative algorithm is introduced which jointly optimizes the VQ and the modulation signal set using as optimality criterion the minimum mean-square error (MSE) between the original and the reconstructed signals. It is shown that a jointly optimized system based on average channel characteristics significantly outperforms (by up to 5 dB) a reference system based on a VQ designed for the given source and a standard quadrature amplitude modulation (QAM) modulation signal set. >

Performance of multi-level QAM with post-detection maximal ratio combining space diversity for digital land-mobile radio communications

Abstract: A new type of post-detection maximal ratio combining for space diversity receivers in digital land-mobile communications is proposed. The combining scheme is designed for multilevel quadrature amplitude modulation (QAM) and employs a pilot-symbol-aided fading estimator. It is realized at baseband using a digital signal processing (DSP) technique to simplify the receiver hardware. Computer simulation and laboratory experiments confirm that the proposed diversity scheme achieves a performance very close to that of ideal maximal ratio combining. >

Trellis coding with multidimensional QAM signal sets

Abstract: Trellis coding using multidimensional quadrature amplitude modulation (QAM) signal sets is investigated. Finite-size 2D signal sets are presented that have minimum average energy, are 90 degrees rotationally symmetric, and have from 16 to 1024 points. The best trellis codes using the finite 16-QAM signal set with two, four, six, and eight dimensions are found by computer search (the multidimensional (multi-D) signal set is constructed from the 2-D signal set). The best moderate complexity trellis codes for infinite lattices with two, four six, and eight dimensions are also found. The minimum free squared Euclidean distance and number of nearest neighbors for these codes were used as the selection criteria. Many of the multi-D codes are fully rotationally invariant and give asymptotic coding gains up to 6.0 dB. From the infinite lattice codes, the best codes for transmitting J, J+1/4, J+1/3, J+1/2, J+2/3, and J+3/4 b/sym (J an integer) are presented. >

A 0.7-3 GHz GaAs QPSK/QAM direct modulator

Abstract: A single chip I/Q direct modulator for use in digital radio links is presented. This device translates directly the phase and quadrature baseband signals to a microwave frequency that can be chosen between 700 MHz and 3 GHz avoiding the use of IF circuits. It is able to generate any type of digital modulation as QPSK, n-PSK, n-QAM, GMSK, etc., with band-limited spectrum. The bandwidth of the I/Q modulating signals is more than 500 MHz allowing the use of the circuit even in the highest capacity systems. The device has 120 components in a 2.2*2.4 mm chip and has been fabricated using 0.5 mu m GaAs MESFET process. >

Quadrature amplitude modulator including a digital amplitude modulator as a component thereof

Abstract: The invention provides various embodiments of a balanced modulator, each of which involves one or more individual amplitude modulators which, individually, do not provide suppressed-carrier outputs. A first embodiment employs a single amplitude modulator whose carrier input is selectively delayed by a half wavelength, depending on the polarity of the input modulating signal, and whose amplitude input receives the absolute value of the modulating input signal. A second embodiment involves summation of the outputs of two amplitude modulators whose carrier inputs are responsive to carrier signals offset by a half carrier wavelength and whose modulating signal inputs are responsive only to signals of respectively opposite polarity. Further, quadrature amplitude modulation (QAM) modulators are provided by a combination of the above-described modulators. Moreover, another embodiment of the QAM modulator involves use of an amplitude modulator operating on polar coordinate representations of the input modulating signal. The invention is useful in single sideband modulation, up-conversion, down-conversion, synchronous detection, product detection, and the like.

A neural network approach to data predistortion with memory in digital radio systems

Abstract: The authors present an algorithm to determine the coefficients of a general data-predistorter with memory for compensation of high-power amplifier (HPA) nonlinearities in digital microwave radio systems. This technique is based on modeling the predistorter-modulator-HPA system as a neural network with memory. It is shown that, by extending the optimization algorithm of back-propagation to complex signals and with neurons modeled as finite-impulse-response (FIR) filters, the proposed algorithm determines automatically the predistorter with the objective that the overall transmitter behaves as a linear system with a prescribed pulse shape. The novelty with respect to previous techniques is that in the present scheme a control on the spectrum of the signal after the HPA is exercised. This minimizes the interference between adjacent channels. The algorithm has been tested successfully in several radio systems employing quadrature amplitude modulation (QAM) signal formats, and some examples of its application are reported. >

An example of a multi-resolution digital terrestrial TV modem

Abstract: An example of a modem for two-resolution digital terrestrial transmission, providing two programs in a single 8 MHz channel is described. For each program, it is desired to transmit 5 Mb/s to portable receivers and additionally 5 Mb/s to fixed receivers. In the example, orthogonal frequency division multiplexing (OFDM) is used with guard intervals as in digital audio broadcasting. The two-resolution transmission is achieved by using a two-resolution quadrature amplitude modulation (QAM) signal constellation. Reed-Solomon coding gives burst error correction capability to combat the frequency selective nature of the transmission channel. The two-resolution system can easily be reconfigured into a compatible system, providing high-definition television to fixed receivers. >

The effects of constellation density on trellis-coded modulation in fading channels

Abstract: Dense constellations such as 16-QAM (quadrature amplitude modulation) have not seen much use in mobile communication because of their greater peak-to-average power ratio and their seemingly greater sensitivity to noise and channel interference. It is demonstrated that dense constellations can actually improve performance. Using a completely analytical method, three constellations are compared with the same net throughput of 2 bits/symbol: uncoded QPSK (quadrature phase-shift keying), rate 2/3 TCM (trellis-coded modulation) 8-PSK and rate 1/2 TCM 16-QAM. Comparison on the basis of average power puts TCM 16-QAM 5-dB ahead of TCM 8-PSK (phase-shift keying). Even comparison on the basis of peak power gives TCM 16-QAM a 2.44-dB advantage over TCM 8-PSK. QPSK is much poorer than either. >

Defining, designing, and evaluating digital communication systems

Abstract: The fundamental relationships in defining, designing, and evaluating digital communication system performance are reviewed. The concept of bandwidth-limited and power-limited systems and how such conditions influence the design when the choices are confined to M-ary phase-shift keying (MPSK) and M-ary frequency-shift keying (MFSK) modulation are examined. The definitions and computations involved in transforming from data bits to channel bits to symbols to chips are discussed. It is suggested that since most digital communication systems share these concepts, understanding them should enable one to evaluate other such systems in a similar way. >

Precoding technique for partial-response channels with applications to HDTV transmission

Abstract: An equivalent partial-response (PR) channel 1-Z/sup -k/ arises in the envisioned terrestrial over-the-air broadcasting of digital high-definition television (HDTV) signals when a comb filter is used by an HDTV receiver to reduce the NTSC cochannel interference. The design of signal constellations and their associated precoders for this PR channel is considered. Besides PAM and square QAM, it is shown that generalized square and hexagonal constellations can also be used. Coded modulation and graceful degradation in the received signal quality are discussed. The results extended to a more general PR channel. >

Error-control properties of minimum shift keying

Abstract: A phase representation of minimum shift keying (MSK), which is a general framework suitable for use with other continuous phase modulation (CPM) schemes, is discussed. The representation is used to show that MSK can be viewed as a coded-modulation scheme with an expanded signal set using Ungerboeck's set partitioning concept (see IEEE Trans. Inf. Theory, vol.IT-28, no.1, p.55-67, 1982). The properties which follow from this view of MSK are explored. The uniqueness of the representation of MSK as a coded-modulation scheme and the structural relationships between different versions of MSK are described. >

Optimum MMSE equalization for staggered modulation

Abstract: "Staggered modulation" techniques have become prevalent in digital radio transmission. These techniques offset or stagger the modulation of the in-phase component with respect to the quadrature component of the signal. Examples include minimum shift keying (MSK), Gaussian MSK, offset QPSK, and offset QAM. Conventional equalization designed for QPSK/QAM is suboptimal for staggered modulation because it places unnecessary restrictions on the equalizer output. We derive a new decision feedback equalizer for staggered modulation that takes advantage of the signal's offset modulation nature. Given the channel pulse response, we compute the optimum DFE that minimizes the mean squared error only in the dimension of the modulated symbol, alternating the equalization and decision boundary from symbol to symbol. We show that this new equalizer achieves significant performance gain over conventional DFE structures. >

Calculation of channel capacity for M-ary digital modulation signal sets

Abstract: A method for calculating channel capacity for M-ary digital modulation signal sets over an Additive White Gaussian Noise (AWGN) channel is presented. By representing the signal set and noise process as vectors in an N-dimensional signal space, the computation of channel capacity becomes a problem of integration in N dimensions. An attempt to approximate multiple integration is based on Cartesian products of Gauss-Hermite quadrature formulae. This approach allows channel capacity calculation at any value of signal to noise ratio (SNR). The required computing time is small for a small to moderate number of signals and number of dimensions.

On the performance of a hybrid frequency and phase shift keying modulation technique

Abstract: The performance of a hybrid modulation technique derived from binary frequency shift keying is presented. In the hybrid scheme, each frequency is allowed to be phase modulated with any of p discrete, equally spaced phase shifts. The spectral separation between frequencies is chosen to make the carriers orthogonal, thus generating a multidimensional modulation scheme. The performance of complete and expurgated phase codes is examined for the Gaussian and Rayleigh fading channels, and their spectral characteristics are determined. >

Identification of nonlinear channels in digital transmission systems

Abstract: Nonlinearity in digital transmission channels has long been an important problem in digital communications. Being able to identify the nonlinear characteristics of the channels can help in the design of the nonlinear equalizer. The authors consider identification of PSK and QAM nonlinear channels which can be expressed as a third-order complex-valued Volterra series. Based on higher-order moment analysis, they derive a simple algorithm to identify the Volterra kernels. In addition, it is shown that, for properly designed input sequences, the estimate obtained by the proposed method is equal to the optimum minimum mean square error solution. >

Optimal detection of coded differentially encoded QAM and PSK signals with diversity reception in correlated fast Rician fading channels

Abstract: The optimal sequence estimator for digital signals received over Lambda different channels is derived. Each of these channels corrupts the transmitted signal by a mixture of additive white Gaussian noise (AWGN) and frequency-nonselective, correlated, fast Rician fading. By analysis it is shown that for the lth (1 >

Multichannel adaptive blind deconvolution using the complex cepstrum of higher order cross-spectra

Abstract: An adaptive cross-spectral blind deconvolution method, cross-TEA, for the case in which data are available from several sensors, is described. Unlike ordinary cross-spectra, higher order cross-spectra are separable, and this fact is used to identify and deconvolve each system. Cross-TEA was tested on four-channel QAM (quadrature amplitude modulation) signals, and the results are compared to those for TEA, an autospectral method. >

Detection of symbol rate of unknown digital communication signals

Abstract: A new detection method for the symbol rate of unknown PSK (phase shift keying) and QAM (quadrature amplitude modulation) passband input signals is presented. In the detector, the input signal envelope obtained from the analytic signal is used to extract a line spectrum corresponding to the symbol rate. Simulation results show that the detector using absolute-valued signal envelope can detect the symbol rate accurately even for the input signal having small excess bandwidth.

Simulation and implementation of US QAM-based HDTV channel decoder

Abstract: The authors present system design considerations and simulation results for a receiver prototype for QAM (quadrature amplitude modulation)-based digital HDTV (high-definition television) systems in the context of current standard deliberations for terrestrial broadcasting. Efforts were concentrated on design choices of the receiver sub-systems for adaptive equalization, and carrier and symbol timing recovery. System performance evaluation results that highlight the effect of the above are presented, and some hardware design examples are also given. >

Fractionally spaced blind equalization and estimation of FIR channels

Abstract: The author considers the problem of blind estimation and equalization of digital communication FIR (finite impulse response) channels using fractionally spaced samples. The problem is cast in a mathematical framework of parameter estimation for a vector stationary process with a single input (information sequence) and multiple outputs, by using a time-series representation of a cyclostationary process. The channel parameters are estimated by nonlinear batch optimization of a quadratic cumulant matching criterion involving second and fourth order cumulants of the received data. A new algorithm is proposed for (asymptotically) globally convergent, linear estimation of the FIR channel parameters where the FIR model order is not necessarily known. A simulation example using QAM (quadrature amplitude modulation) signals is presented. >

Jitter-reduced digital timing recovery for multilevel PAM and QAM systems

Abstract: In multilevel pulse amplitude modulation/quadrature amplitude modulation (PAM/QAM) systems, e.g., digital radio, there is a strong demand for a reliable and precise timing phase provided by the timing recovery circuit. A phase detector that is theoretically jitter free, even in the case of noncoherent demodulation, is presented. In practical applications, with reduced complexity, a jitter reduction on the order of >15 dB is achieved. Solutions to improve the behavior of the timing recovery in case of linear channel distortions are given. >

Coded modulation with feedback decoding trellis codes

Abstract: It has previously been shown that with trellis-coded modulation it is possible to achieve a significant coding gain of 3 to 6 dB over uncoded modulation without bandwidth expansion. However, the decoding effort grows rapidly with the coding gain. A different approach is given by the author. Based on a feedback decoding scheme, trellis codes of large constraint length are proposed which can be easily decoded. At similar decoder complexity, but at a larger decoding delay, they attain additional coding gain over conventional trellis codes. >

Reduced-complexity multi-stage blind clustering equaliser

Abstract: A multi-stage blind clustering algorithm is proposed for equalization of M-quadrature amplitude modulated (QAM) channels. A hierarchical decomposition divides the overall task of equalizing a high-order QAM channel into much simpler sub-tasks. Each sub-task can be accomplished fast and reliably using a blind clustering algorithm derived originally for 4-QAM signals. The constant modulus algorithm (CMA) is used as a benchmark to assess this multi-stage blind equalizer. It is demonstrated that the new blind adaptive algorithm achieves much faster convergence. This multi-stage clustering equalizer only requires slightly more computations than the very simple CMA and, like the latter, its computational complexity does not increase as the levels of digital symbols increase. >