Showing papers on "Constellation diagram published in 2001"

Degrees of freedom in adaptive modulation: a unified view

Abstract: We examine adaptive modulation schemes for flat-fading channels where the data rate, transmit power, and instantaneous BER are varied to maximize spectral efficiency, subject to an average power and BER constraint. Both continuous-rate and discrete-rate adaptation are considered, as well as average and instantaneous BER constraints. We find the general form of power, BER and data rate adaptation that maximizes spectral efficiency for a large class of modulation techniques and fading distributions. The optimal adaptation of these parameters is to increase the power and data rate and decrease the BER as the channel quality improves. Surprisingly, little spectral efficiency is lost when the power or rate is constrained to be constant. Hence, the spectral efficiency of adaptive modulation is relatively insensitive to which degrees of freedom are adapted.

Hybrid arq method with signal constellation rearrangement

Abstract: A hybrid ARQ retransmission method in a communication system, wherein data packets being encoded with a forward error correction (FEC) technique prior to transmission, are retransmitted based on an automatic repeat request and subsequently soft-combined with previously received erroneous data packets either on a symbol-by-symbol or a bit-by-bit basis. The symbols of said erroneous data packets are modulated by employing a predetermined first signal constellation. The symbols of the retransmitted data packets are modulated by employing at least a predetermined second signal constellation. Each symbol bit has a mean bit reliability defined by the individual bit reliabilities over all symbols of the predetermined signal constellation. According to the invention, the predetermined first and the at least second signal constellation are selected such that the combined mean bit reliabilities for the respective bits of all transmissions are averaged out.

Turbo codes with non-uniform constellations

Abstract: This paper presents parallel concatenated turbo codes that employ a non-uniform constellation to achieve shaping gain. The output signal approximates the Gaussian distribution by using equally likely signals with unequal spacing (a non-uniform constellation). The small distance of points near the center of the constellation may lead to a small overall free distance and thus a high error floor for turbo codes. We avoid this situation by a two-step design procedure, that first creates an interleaver, and then identifies the constituent encoders that maximize the turbo code free distance. Simulation results for 4 bits/sec/Hz show that this use of shaping can offer an improvement of approximately 0.2 dB for turbo codes.

Cutoff rate and signal design for the quasi-static Rayleigh-fading space-time channel

Abstract: We consider the computational cutoff rate and its implications on signal design for the complex quasi-static Rayleigh flat-fading spatio-temporal channel under a peak-power constraint where neither transmitter nor receiver know the channel matrix. The cutoff rate has an integral representation which is an increasing function of the distance between pairs of complex signal matrices. When the analysis is restricted to finite-dimensional sets of signals, interesting characterizations of the optimal rate-achieving signal constellation can be obtained. For an arbitrary finite dimension, the rate-optimal constellation must admit an equalizer distribution, i.e., a positive set of signal probabilities which equalizes the average distance between signal matrices in the constellation. When the number N of receive antennas is large, the distance-optimal constellation is nearly rate-optimal. When the number of matrices in the constellation is less than the ratio of the number of time samples to the number of transmit antennas, the rate-optimal cutoff rate attaining constellation is a set of equiprobable mutually orthogonal unitary matrices. When the signal-to-noise ratio (SNR) is below a specified threshold, the matrices in the constellation are rank one and the cutoff rate is achieved by applying all transmit power to a single antenna and using orthogonal signaling. Finally, we derive recursive necessary conditions and sufficient conditions for a constellation to lie in the feasible set.

Systems and methods for communicating spread spectrum signals using variable signal constellations

Abstract: According to embodiments of the invention, a communications system includes an error correction encoder that error correction encodes a bitstream according to an error correction code. The system also includes a variable symbol generator that generates a symbol according to a selected one of a plurality of selectable signal constellations from a group of bits of the error correction encoded bitstream. The system further includes a spreader that spreads the symbol according to a spreading code, and a transmitter that transmits the spread symbol in a communications medium. Preferably, the plurality of selectable signal constellations includes at least two signal constellations of different order. In other embodiments, the error correction encoder includes a variable error correction encoder that encodes the bitstream according to a selected error correction code of a plurality of selectable error correction codes. In still other embodiments, the spreader includes a variable spreader that spreads the symbol according to a selected spreading code of a plurality of selectable orthogonal spreading codes including at least two spreading codes of different lengths. A controller may select the error correction code used by the variable error correction encoder, the signal constellation used by the variable symbol generator, and the spreading code used by the variable spreader to provide a desired information transmission rate for the bitstream. Related methods are also described.

Constellation-constrained capacity for trellis code multiple access systems

Abstract: Trellis code multiple access (TCMA) is a narrowband multiple access scheme. There is no bandwidth expansion, so K users are using the same bandwidth as one single user. The load (the number of bits per channel use) of the system is therefore much higher than the load in, for example, conventional CDMA systems. It is shown here that the maximum number of users in a TCMA system is usually modest. This fact is based on constellation-constrained capacity, ie, the maximum capacity of a system with a specific constellation of the transmitted symbols. The minimum required signal-to-noise ratio for reliable transmission is also given for different code rates.

Adaptive data rate using ARQ and nonuniform constellations

Abstract: We introduce an automatic repeat request (ARQ) scheme that uses nonuniform signal constellations in order to adapt the data rate to the received signal-to-noise ratio (SNR) in a fading channel without using channel state feedback or incremental redundancy. We have defined nonuniform phase shift key (PSK) and quadrature amplitude modulation (QAM) constellations with an arbitrary number of classes of bits. These constellations may also be of interest for further research into unequal error protection. We present simulations of the throughput in Rayleigh fading and lognormal shadowing, and discuss the cases where our proposed scheme may improve the throughput compared to standard ARQ using uniform signal constellations.

Degrees of freedom in adaptive modulation: a unified view

Abstract: We examine adaptive modulation schemes for flat-fading channels where the data rate, transmit power and instantaneous BER are varied to maximize spectral efficiency subject to an average power and BER constraint. Both continuous-rate and discrete-rate adaptation are considered, as well as average and instantaneous BER constraints. We find the general form of power BER, and data rate adaptation that maximizes spectral efficiency for a large class of modulation techniques and fading distributions. The optimal adaptation of these parameters is to increase the power and data rate and decrease the BER as the channel quality improves. Surprisingly, little spectral efficiency is lost when the power or rate is constrained to be constant. Hence, the spectral efficiency of adaptive modulation is relatively insensitive to which degrees of freedom are adapted.

Serial concatenated trellis coded modulation with inner rate-1 accumulate code

Abstract: In this paper we propose a serial concatenated trellis coded modulation system using an inner accumulate code and a Gray-labeled signal constellation. The simple inner code and the Gray labeling allow us to extend to higher-order constellations the coding theorems of Divsalar et al., (1998), and Pfister et al., (2000), stating that when the signal-to-noise ratio (SNR) exceeds a certain, system-specific threshold, the word error probability goes to zero as the blocklength goes to infinity. We also evaluate the performance for finite blocklengths using an improved union bound. Despite the simple inner code, the simulated performance in AWGN and Rayleigh fading is equal to the performance of more complex systems suggested in the literature.

System for transmitting information codes with multi-level modulation scheme and a modulation apparatus

Abstract: A transmission system for transmitting information codes by using a multi-level modulation scheme has a transmitter and a receiver. The transmitter has a divider circuit for dividing inputted N-bit information data stream into M (M is integer not less than 2) data stream; an error correction coding circuit for individually converting the M streams into error correcting code streams so as to deliver M error correcting code streams; a multi-level modulation circuit for making the correspondence between data streams of N bits constituted by the M error correcting code streams and a plurality of signal points arranged on a signal constellation and modulating two carriers with coordinate values of signal points on the signal constellation corresponding to the N-bit code stream; and a high frequency circuit unit for outputting a modulated signal in a radio frequency to a transmission antenna.

Joint optimization of VQ codebooks and QAM signal constellations for AWGN channels

Abstract: A joint design scheme has been proposed to optimize the source encoder and the modulation signal constellation based on the minimization of the end-to-end distortion including both the quantization error and channel distortion. The proposed scheme first optimizes the vector quantization (VQ) codebook for a fixed modulation signal set, and then the modulation signals for the fixed VQ codebook. These two steps are iteratively repeated until they reach a local optimum solution. It has been shown that the performance of the proposed system can be enhanced by employing a new efficient mapping scheme between codevectors and modulation signals. Simulation results show that a jointly optimized system based on the proposed algorithms outperforms the conventional system based on a conventional quadrature amplitude modulation signal set and the VQ codebook designed for a noiseless channel.

A modified constant modulus algorithm for adaptive channel equalization for QAM signals

Abstract: A modified constant modulus algorithm (MCMA) for adaptive equalization of the wireless indoor channel for QAM signals is presented. The algorithm minimizes an error cost function that includes both the amplitude and phase of the equalizer output. In addition to the amplitude-dependent term that is provided by the conventional constant modulus algorithm (CMA), the cost function includes a signal constellation matched error (CME) term. This term speeds up convergence and allows the equalizer to switch to decision directed (DD), or any soft-decision mode, faster than the CMA applied alone. The constellation-matched error term is constructed using polynomials with desirable properties. The MCMA is applied to a decision feedback equalizer and shown to provide improved performance over dual mode techniques.

128-ary signal constellations suitable for non-linear amplification

Abstract: A communications system ( 10 ) which utilizes an 128-ary QAM signal constellation suitable for non-linear applications. The communications system includes a modulator ( 18 ) for utilizing the 128-ary constellation to implement the modulation. The 128-ary constellation is a circular constellation which provides a simplified amplitude predistortion by utilizing the subject 128-ary constellations, enabling more efficient communications can then be achieved through a peak-power-limited non-linear channel ( 16 ). Such non-linear channels ( 16 ) are more power efficient at creating RF energy from DC energy.

A new approach based on "soft statistics" to the nonlinear blind-deconvolution of unknown data channels

Abstract: In this paper, we present a new nonlinear receiver for the blind deconvolution of intersymbol interference (ISI) impaired data. The proposed receiver achieves fast identification of an unknown transmission channel using only one channel estimator and requiring the computation of only the second-order conditional statistics of the baud-rate sampled received signal and the knowledge of the transmitted constellation. The main novelty of the proposed approach is that the receiver accomplishes fast channel-identification by using soft-statistics. In particular, the receiver consists of a symbol-by-symbol maximum a posteriori (SbS-MAP) detector that feeds a nonlinear Kalman-like channel estimator with the soft statistics constituted by the a posteriori probabilities (APPs) of the state sequence of the ISI channel. Several numerical results confirm that the proposed blind detector achieves the identification of nonminimum phase channels with deep spectral notches within 300 symbols, even at low signal-to-noise ratios (SNRs). Furthermore, an attractive feature of the proposed blind channel estimator is that it directly estimates the discrete-time impulse response of the unknown channel so that, in principle, any equalization technique for known channels may be performed after channel identification has been achieved.

GCMAC-based predistortion for digital modulations

Abstract: The subject of this paper is the compensation for nonlinearities in digital communication systems by means of predistortion. In this work, we apply the generalized cerebellar model articulation controller (GCMAC) to simplify and accelerate the predistorter convergence. The range of analyzed predistorters includes: 1) a symbol-rate data predistorter that, for a given time span, achieves a similar level of compensation provided by present techniques, but with faster convergence; 2) a fractionally spaced data predistorter that controls, at the same time, the signal constellation and the transmitted spectrum; 3) a decision-feedback scheme that compensates for remote nonlinearities; and 4) a digital signal data predistorter. The performance of the proposed data and signal predistorters is evaluated using typical linear and nonlinear modulated transmitted signals such as QAM and GMSK.

Multilevel coded modulations based on asymmetric constellations

Abstract: This paper deals with multilevel coded modulations (MLC) based on ASK and PSK constellations for the AWGN channel. Under multistage decoding (MSD) and when block partitioning is applied to an asymmetric constellation, we show that the capacity of the modulation can be attained using constituent codes with equal signalling rates at each level. For finite code length N, performance is derived analytically by evaluating Gallager's coding exponent.

Non-data-aided ML carrier frequency and phase synchronization in OFDM systems

Abstract: In this paper non-data-aided(NDA), maximum likelihood(ML) algorithms are derived for the carrier frequency and phase offset, separately, for OFDM systems employing M-PSK modulation scheme. NDA ML estimation algorithm for frequency offset estimation exploits the redundant information contained in the cyclic prefix preceeding the OFDM symbols, thus reducing the need for pilots. Its mean-squared performance is obtained analytically and compared with simulation results. It is observed that the resulting algorithm generates very accurate estimation even when the offset is high. It is also shown that the frequency estimator may be used in a tracking mode. The ML algorithm derived for the carrier phase estimation is also a non-data-aided(NDA) and maximizes the low SNR limit of the likelihood function averaged over M-PSK signal constellation. It is shown that for sufficiently small SNR the ML phase estimator obtained reduces to the familiar Mth order power synchronizer which belongs to the class of NDA feedforward carrier synchronizers introduced earlier in the literature, Its mean-squared performance is obtained analytically and compared with simulation results. We observe that the resulting algorithm generates very accurate estimation even when the phase offset is high, that the self noise is absent and the performance of the algorithm is basically the same as the Cramer-Rao bound for moderate to high SN R. Finally we note that the error variance derived for the mean-squared performance of this NDA ML synchronizer is an extension of the approximate variance formula appeared in Reference 20,equation(14) for M-PSK.

A noncoherent coded modulation for 16QAM

Abstract: We present a noncoherent coded 16QAM (NC-16QAM) scheme by modifying the trellis-coded 16QAM (TC-16QAM) scheme. Our simulation results show that the performance of the NC-16QAM with noncoherent detection is close to the one of the original TC-16QAM with coherent detection. The NC-16QAM is an extension of the NC-8PSK previously obtained by Wei and Lin (see IEEE Commun. Lett., vol.2. p.260-62, 1998). A noncoherent initial phase estimation algorithm is also proposed.

Progressive source coding for a power constrained Gaussian channel

Abstract: We consider the progressive transmission of a lossy source across a power constrained Gaussian channel using binary phase-shift keying modulation. Under the theoretical assumptions of infinite bandwidth, arbitrarily complex channel coding, and lossless transmission, we derive the optimal channel code rate and the optimal energy allocation per transmitted bit. Under the practical assumptions of a low complexity class of algebraic channel codes and progressive image coding, we numerically optimize the choice of channel code rate and the energy per bit allocation. This model provides an additional degree of freedom with respect to previously proposed schemes, and can achieve a higher performance for sources such as images. It also allows one to control bandwidth expansion or reduction.

Embedded signal constellations

Abstract: The invention enables a communication device to efficiently communicate large circular signal constellations by embedding a square signal constellation in each of the circular signal constellation points. By embedding a square signal constellation in the error free slicing region associated with each circular constellation signal point, the beneficial shape of the circular constellation is retained while significantly increasing the amount of information that can be transmitted in each constellation. The invention allows the communication of high bit-per-symbol signal constellations (on the order of 15 or more bits-per-symbol) without the added cost of external memory for large look-up tables or the cost and power consumption of line drivers required to communicate high peak factor square signal constellations.

Viterbi decoding with channel and location information

Abstract: A method for exploiting location information inherent in the location of points within a transmitted signal constellation. A common method for assigning weights to branch metrics in a Viterbi decoder is to assign Hamming weights which are summations of the number of places in which a received bit pattern differs from the bit pattern assigned to a branch metric. Signal strength information may be incorporated into the weight assigned to a branch metric in the Viterbi decoder. Additionally information-inherent in the location of the points within a constellation may be taken into account. Bit errors which require a larger deviation to occur are given higher weights, and bit errors which require less deviation to occur are given lower weights. By taking into account signal strength and location information up to 2 dB of coding gain can be realized.

Serial concatenated trellis coded modulation with inner rate-1 accumulate code

Abstract: We propose a serial concatenated trellis coded modulation system using one or more inner accumulate code(s) and a Gray-labeled signal constellation. We show the existence of a threshold, such that if the signal-to-noise ratio (SNR) exceeds this threshold, the bit error probability goes to zero as the blocklength goes to infinity. Tight numerical values for the thresholds for an iterative decoder are found by density evolution. Despite the simple inner code, the simulated performance in AWGN and Rayleigh fading is comparable to that of more complex systems suggested in the literature.

Bit interleaved coded modulation (bicm) mapping

Abstract: Described is a transmission system for transmitting a multilevel signal (XK) from a transmitter (10) to a receiver (20). The transmitter (10) comprises a mapper (16) for mapping an input signal (ik) according to a signal constellation onto the multilevel signal (Xk). The receiver (20) comprises a demapper (22) for demapping the received multilevel signal (Yk) according to the signal constellation. The signal constellation comprises a number of signal points with corresponding labels. The signal constellation is constructed such that Da⊃Df, with Da being the minimum of the Euclidian distances between all pairs of signal points whose corresponding labels differ in a single position, and with Df being the minimum of the Euclidean distances between all pairs of signal points. By using this signal constellation a significantly lower error rate can be achieved than by using a prior-art signal constellation, in an iteratively decoded BICM (Bit Interleaved Coded Modulation) system.

Performance of a rotated QPSK based system in a fading channel subject to estimation errors

Abstract: This paper presents a performance analysis of a modified QPSK scheme for transmission over fading channels. Interleaving and rotation of the signal constellation are applied with the objective of improving its performance. It is shown that the proposed system outperforms the original scheme even in the presence of channel estimation errors.

TCM decoding device and method

Abstract: A branch metric calculating device using two convolutional codes has two decoders for decoding the two convolutional codes and a branch metric calculation unit. The branch metric calculation unit determines a branch metric I by calculating the distance with respect to real number components between a signal point corresponding to an input signal and each of the other signal points on a signal constellation given according to the characteristics of an encoder for encoding outputs of the two decoders and determines a branch metric Q by calculating the distance with respect to imaginary number components between the input signal point and each of the other signal points.

Voice-band modem: A device to transmit data over telephone networks

Abstract: Over the last 40 years, there has been continuous evolution in the design of voice-band modems-starting at a data rate of 300 bits per second in the late 1950s, a rate of 33,600 bits per second has been achieved in 1995. Realising such high data rates over the voice band of 3400 Hz is a remarkable feat, made possible by combining sophisticated techniques from three disciplines, communication theory, signal processing and information theory. In this article, we briefly describe certain advanced ideas, which led to data rates very close to the channel capacity limit, established by Shannon.

Signal constellation design for optical intensity modulated channels

Abstract: We define lattice codes for the optical intensity direct-detection channel. These codes obey a non-negativity constraint and are shaped to minimize average optical power. Expressions for the coding and shaping gain of such codes over a rectangular PAM baseline are presented. Over short distances, we show that lattice codes provide significant rate gains for free-space optical links.

A two-stage decoder for pragmatic trellis-coded M-PSK modulation using a symbol transformation

Abstract: A two-stage decoding procedure for pragmatic trellis-coded modulation (TCM) is introduced. It applies a transformation from the received I-channel and Q-channel samples onto points in a two-dimensional (2-D) signal space that contains a coset constellation. For pragmatic TCM over M-PSK signal sets with /spl nu/ coded bits per symbol, /spl nu/=1, 2, the signal points in the coset constellations represent cosets of a B/QPSK signal subset-associated with the coded bits-in the original M-PSK signal constellation. A conventional Viterbi decoder operates on the transformed symbols to estimate the coded bits. After reencoding these bits, the uncoded bits are estimated in a second stage, on a symbol-by-symbol basis, with decisions based on the location of the received symbols. In addition to requiring no changes in the Viterbi decoder core, it is shown that the proposed method results in savings of up to 40% in the memory required to store (or in the size of the logic required to compute) metrics and transformed symbols.

Digital transmission system transmits phase information differentially and amplitude information non-differentially

Abstract: At a transmitter (S) a combination of differential pre-coding of phase and the absolute amplitude and differential phase shift keying (ADPSK) of the signal constellation is used. The information to be transmitted is assigned to the phase changes and the actual amplitudes of the transmission symbols. Information relating to phase is transmitted differentially, and information relating to amplitude is transmitted non-differentially. In the receiver (E) differential demodulation is performed in accordance with an adequate estimation of the amplitude of the channel state. Independent claims are included for a transmitter and for a receiver.

Universal redundant precoders for guaranteed recovery of block transmissions through unknown FIR channels

Abstract: Without channel state information available, universal designs of wireless OFDM transmitters and receivers offer performance that is robust to the underlying channel fades and the adopted signal constellation. We show that judiciously designed precoders with redundancy twice as long as the channel order are necessary and sufficient for guaranteed constellation- and channel-irrespective symbol recovery as well as for blind channel identifiability. Simulated comparisons of the novel precoders with zero-padded and cyclic-prefixed block transmissions delineate their relative merits.