Showing papers on "Constellation diagram published in 2003"

Bit labeling for amplitude phase shift constellation used with low density parity check (LDPC) codes

Abstract: An approach is provided for bit labeling of a signal constellation. A transmitter generates encoded signals using, according to one embodiment, a structured parity check matrix of a Low Density Parity Check (LDPC) code. The transmitter includes an encoder for transforming an input message into a codeword represented by a plurality of set of bits. The transmitter includes logic for mapping non-sequentially (e.g., interleaving) one set of bits into a higher order constellation (Quadrature Phase Shift Keying (QPSK), 8-PSK, 16-APSK (Amplitude Phase Shift Keying), 32-APSK, etc.), wherein a symbol of the higher order constellation corresponding to the one set of bits is output based on the mapping.

Signal constellations for bit-interleaved coded modulation

Abstract: Bit-interleaved coded modulation (BICM) is a bandwidth-efficient coding technique consisting of serial concatenation of binary error-correcting coding, bit-by-bit interleaving, and high-order modulation. BICM is capable of achieving excellent error performance provided that powerful codes, such as for example turbo codes or low-density parity-check (LDPC) codes, are employed. We address the problem of finding the signal sets that are the most suitable ones for designing power-efficient BICM schemes over an additive white Gaussian noise (AWGN) channel. To this end, we exploit the expression of the BICM capacity limit, and evaluate it for several 8- and 16-ary constellations. The bit-error rate (BER) performance of some BICM schemes made up of turbo codes and various signal sets is also investigated by computer simulations so as to illustrate the theoretical results. We show that, for spectral efficiencies of practical interest, the most attractive signal sets are those for which Gray mapping is possible, provided that their symbol error rate performance is "sufficiently close" to the optimum. This explains why some constellations having a simple structure, such as 8-PSK and 16-QAM, perform very well when combined with a powerful code. At the same time, the constellations displaying optimal error performance without coding are, generally, not of interest for BICM.

Adaptive modulation for multi-antenna transmissions with channel mean feedback

Abstract: Adaptive modulation has the potential to increase the system throughput significantly by matching transmitter parameters to time-varying channel conditions. However, adaptive modulation assuming perfect channel state information (CSI) is sensitive to CSI imperfections induced by estimation errors and feedback delays. In this paper, we design adaptive modulation schemes for multi-antenna transmission based on partial CSI, that models the spatial fading channels as Gaussian random variables with non-zero mean and white covariance, conditioned on feedback information. Based on a two-directional beamformer, our proposed transmitter optimally adapts the beam directions, the power allocation between two beams, and the signal constellation, to maximize the transmission rate while maintaining a target bit error rate (BER). Numerical results demonstrate the rate improvement, and illustrate an interesting tradeoff that emerges between feedback quality and hardware complexity.

Rate-diversity tradeoff of space-time codes with fixed alphabet and optimal constructions for PSK modulation

Abstract: We show that for any (Q/spl times/M) space-time code S having a fixed, finite signal constellation, there is a tradeoff between the transmission rate R and the transmit diversity gain /spl nu/ achieved by the code. The tradeoff is characterized by R/spl les/Q-/spl nu/+1, where Q is the number of transmit antennas. When either binary phase-shift keying (BPSK) or quaternary phase-shift keying (QPSK) is used as the signal constellation, a systematic construction is presented to achieve the maximum possible rate for every possible value of transmit diversity gain.

Differential space time block codes using nonconstant modulus constellations

Abstract: We propose differential space time block codes (STBC) using nonconstant modulus constellations, e.g., quadrature amplitude modulation (QAM), which cannot be utilized in the conventional differential STBC. Since QAM constellations have a larger minimum distance compared with the phase shift keying (PSK), the proposed method has the advantage of signal-to-noise ratio (SNR) gain compared with conventional differential STBC. The QAM signals are encoded in a manner similar to that of the conventional differential STBC. To decode nonconstant modulus signals, the received signals are normalized by the channel power estimated forgoing training symbols and then decoded with a conventional QAM decoder. Assuming the knowledge of the channel power at the receiver, the symbol error rate (SER) bound of the proposed method under independent Rayleigh fading assumption is derived, which shows better SER performance than the conventional differential STBC. When the transmission rate is more than 3 bits/channel use in time-varying channels, the simulation results demonstrate that the proposed method with the channel power estimation outperforms the conventional differential STBC. Specifically, the posed method using the channel power estimation obtains a 7.3 dB SNR gain at a transmission rate of 6 bits/channel use in slow fading channels. Although the performance gap between the proposed method and the conventional one decreases as the Doppler frequency increases, the proposed method still exhibits lower SER than the conventional one, provided the estimation interval L is chosen carefully.

Design of multidimensional signal constellations

Abstract: An algorithmic technique intended for signal constellation design for an N-dimensional Euclidean signal space is presented. Such signals are used for reliable and efficient digital communications on an additive white Gaussian noise channel. The minimum Euclidean distance between signals and the constellation-constrained capacity are used as performance measures. The basic idea is to spread out the signal vectors using an iterative method modelling the behaviour of equally charged particles in free space. Initially, the vectors are randomly distributed. The vectors reach equilibrium after a few iterations. Numerical calculations for N=2, 3, 4 and 5 dimensions and M=N+1, N+2, ...,20 signals show that the generated codes are nearly as good as previously known codes, and in some cases even better

Satellite communication system utilizing low density parity check codes

Abstract: An approach for reliably communicating over a satellite in support of a communication service including, for example, as direct broadcast satellite and data service, is disclosed. An input message is encoded, yielding a structured Low Density Parity Check (LDPC) coded message. The coded message is modulated according to a high order modulation scheme that has a signal constellation representing more than two symbols per signaling point—e.g., 8-PSK (Phase Shift Keying) and 16-QAM (Quadrature Amplitude Modulation). The system includes a transmitter configured to propagate the modulated signal over the satellite. The above approach is particularly applicable to bandwidth constrained communication systems requiring high data rates.

Differential space-time block codes from amicable orthogonal designs

Abstract: In this paper, we propose an amicable-orthogonal-design-based differential space-time block code (ADSTBC) for multiple antennas. Compared with the existing differential modulation designs, our scheme imposes no restrictions on underlying signal constellation, and therefore can improve the spectral efficiency by exploiting efficient modulation techniques such as QAM, APSK, etc. We then derive a non-coherent maximum-likelihood decoder (MLD) with linear complexity for flat Rayleigh fading channels. Particularly, when the QAM constellation is used in ADSTBC, MLD can be further simplified to independently detect the real and imaginary parts of each modulated data symbol, and accordingly the implement cost can be greatly reduced; moreover, the computational amount for detecting single modulated data symbol is kept constant in MLD while the order of the QAM constellation increases.

A new full-rate full-diversity orthogonal space-time block coding scheme

Abstract: In this work, we present a new space-time orthogonal coding scheme with full-rate and full-diversity. The proposed space-time coding scheme can be used on quaternary phase-shift keyed (QPSK) transceiver systems with four transmit antennas and any number of receivers. An additional feature is that the coded signals transmitted through all four transmit antennas do not experience any constellation expansion. The performance of the proposed coding scheme is studied in comparison with that of 1/2-rate full-diversity orthogonal space-time code, quasi-orthogonal code, as well as constellation-rotated quasi-orthogonal code. Our study shows that the proposed coding scheme offers full rate and outperforms the 1/2-rate orthogonal codes as well as full-rate quasi-orthogonal codes when the signal-to-noise increases. Compared to the constellation-rotated quasi-orthogonal codes (the improved QO scheme), the newly proposed code has the advantage of not expanding the signal constellation at each transmit antenna. The performance of the newly proposed code is comparable to that of the improved QO scheme.

Constellation mappings for two-dimensional signaling of nonuniform sources

Abstract: The design of two-dimensional constellation mappings for the transmission of binary nonuniform memoryless sources over additive white Gaussian noise channels using standard M-ary PSK and QAM modulation schemes is investigated. The main application of this problem is the incorporation of an adaptive mapping assignment in modem devices that employ fixed PSK/QAM modulation schemes for the transmission of heterogenous data (such as multimedia information) containing various levels of nonuniformity. In general, the optimal mapping depends on both the probability distribution of the input signals and the signal-to-noise ratio (SNR) in the channel, in addition to the geometry of the signal constellation. We show that constellation mappings which follow the objective of minimizing the average symbol energy and, given this, maximizing the decoding probability of the most likely signals, can yield symbol-error-rate and bit-error-rate performance that is substantially better than Gray encoding maps. Gains as high as 3.5 dB in SNR E/sub b//N/sub 0/ are obtained for highly nonuniform sources. Finally, we note that the mappings techniques result in nonzero mean constellations and briefly consider their performances when they are converted to zero mean constellations by shifting. In this case, we observe that the shifted zero-mean Gray map outperforms our shifted maps for small- to medium-sized constellations (M/spl les/32), but not for larger sizes.

Transmit diversity over quasi-static fading channels using multiple antennas and random signal mapping

Abstract: We introduce a scheme that achieves a diversity gain for coded systems under static fading conditions by using multiple antennas and random signal mapping. In a two-antenna system, the bit-error rate performance of the proposed scheme approaches that of Alamouti's scheme when the channel is perfectly known. In the presence of channel mismatch, the proposed scheme outperforms Alamouti's scheme significantly. It is shown that, as the number of transmit antennas N goes to infinity, the effective channel for the introduced scheme behaves as if it were perfectly interleaved (i.e., as if the fading was independent). When N is small, further performance gain can be achieved by expanding the original signal constellation.

Fast and reliable data carrier detection by a cable modem in a cable television plant

Abstract: Apparatus, methods, and computer program products for detecting or locating a viable data carrier in a downstream channel by a cable modem. Potential channels for data carriers can be eliminated quickly by going through a “screening” process, i.e., the first constellation diagram match, thereby preventing those potential channels from going through a significantly more time-consuming second constellation match. The determination of whether the signal in the potential channel is modulated according to the first particular modulation scheme is done rapidly and only potential channels containing a signal likely to be modulated according to the second particular modulation scheme are examined for the second determination step.

Buffer and channel adaptive modulation for transmission over fading channels

Abstract: We consider the problem of adaptive modulation for increasing the throughput of transmission over fading channels. In the model, packets arrive at a finite-length buffer according to a Poisson distribution and are mapped into M-ary quadrature amplitude modulated (MQAM) symbols for transmission over a correlated Rayleigh fading channel. We assume that buffer and channel state information are always available at the transmitter. Our objective is to vary the transmit power and the signal constellation size of the modulator according to both the buffer and channel states so that the system throughput is maximized under some average transmit power and bit error rate (BER) constraints. We formulate this optimization problem as a Markov decision process (MDP) and use dynamic programming techniques to obtain the solution. More importantly, we show that, under certain conditions, the optimal transmission rate increases when the channel gain decreases toward the outage threshold - the point below which communication is not possible. This is in contrast to the water-filling structure of the link adaptive policy that achieves capacity on fading channels.

Multi-Pulse Pulse-Position-Modulation Signaling for Optical Communication with Direct Detection

Abstract: Am odification of the traditional pulse-position modulation (PPM) scheme typically employed on the optical direct-detection channel is proposed that allows for significantly improved information throughput and bandwidth advantage. The scheme sends a multitude (K )o fpulses per symbol interval and as such provides a signal constellation whose size, for a given number of pulse slots (M), varies as M K (for large M) rather than linearly with M ,a si sthe case for conventional PPM. Maximu mp erformance improvement is obtained for deep-space optical communications applications, where narrow high-peak-power transmitted pulses offer significant advantages in terms of detection probabilities and background suppression capabilities at the receiver.

Shaping gain by non-uniform QAM constellation with binary turbo coded modulation

Abstract: A non-uniform signal constellation can be used to obtain shaping gain. One typical way to design non-uniform constellation is make the output signal follow a Gaussian distribution by using equally like signals with unequal spacing. However, for binary turbo coded modulation (BTCM) with M-QAM system, this non-uniform constellation design criteria can be modified based on different impacts of various turbo encoded hits sequence on BER performance. In this paper, author points out that shaping gain for M-QAM with BTCM can be achieved integrated with optimization of mapping rule, performance of the proposed scheme outperforms over BTCM with uniform constellation, especially under low and moderate SNR region. Meanwhile, the effects of code rate, order of QAM constellation on shaping gain are investigated. Numeral results are provided to support the proposed scheme and analysis. The proposed scheme can be applied to high rate data service with hybrid ARQ, such as HSDPA system.

Iterative trellis search detection for asynchronous MIMO systems

Abstract: The paper presents a reduced-complexity soft-input soft-output detection scheme for asynchronous space-time bit-interleaved coded modulation (ST-BICM) MIMO systems. This scheme is referred to as asynchronous iterative trellis search (A-ITS) detection. The A-ITS detector is based in part on a reduced-state approximation of the BCJR algorithm, and also uses a special type of bit mapping called multilevel mapping to reduce complexity for high-order QAM modulation schemes. The complexity per bit of the detector is linear in the number of transmit antennas and roughly independent of the signal constellation size. The paper presents error performance results generated from computer simulations of an asynchronous iterative ("turbo") MIMO receiver employing the A-ITS detector. It is demonstrated that allowing transmissions from different antennas to be asynchronous can result in considerable performance improvement compared to synchronous MIMO systems.

Interchannel interference cancellation in wireless OFDM systems via Gauss-Seidel method

Abstract: In a mobile fading environment, the time variation of the wireless channels is a dominating factor that leads to the system performance loss. Multipath delay spread also lead to a loss of subchannel orthogonality in orthogonal frequency division multiplexing (OFDM) system, known as interchannel interference (ICI). In this paper, we first analyze the influences of ICI in mobile OFDM system. It is shown that ICI is only a function of the symbol energy (E/sub s/), the subchannel number (N) and the Doppler frequency (f/sub D/) in OFDM system, and is independent of signal constellation. To eliminate ICI, an equalization algorithm based on Gauss-Seidel method is then proposed. The main advantage of the iterative algorithm is to update the data vector x/sup (k)/ in each iterative step as soon as the part of the new data vector x/sup (k+1)/ have been calculated, therefore, the convergence of the algorithm can be accelerated effectively. The symbol error rate for a 16-QAM system is presented by means of simulation results to confirm the effectiveness of the proposed algorithm.

Coded modulation for partially coherent systems

Abstract: A signal constellation is optimized for trellis coded modulation in fast fading channels, where the receiver does not have perfect knowledge of the channel parameters. Specifically, the signal constellation is partitioned into 2n mutually exclusive subsets, each preferably defining two points. Points within each subset are separated from one another by a distance between conditional distributions, preferably a Kullback-Leibler (KL) distance. For a block m=k1+k2 of information bits input into a trellis coder 30, the k1 bits are trellis encoded into n bits (n>k1) and used to select a subset of the constellation. The k2 bit(s) is/are used to select a particular point within the subset. Because the inter-subset distance between points is a KL distance that is effectively greater than a Euclidean distance, error at the receiver is substantially reduced, especially at higher SNR. Using a KL distance ensures statistics of channel fading are inherent within the signal constellation.

Convex optimization theory applied to joint beamforming design in multicarrier MIMO channels

Abstract: This paper addresses the joint design of transmit and receive beamvectors for a multicarrier MIMO channel within the general and powerful framework of convex optimization theory. From this perspective, a great span of design criteria can be easily accommodated and efficiently solved even though closed-form expressions may not be available. Among other criteria, we consider the minimization of the average bit error rate (BER) and also of the maximum BER among all carriers for a given signal constellation. We show how to include additional constraints to control the peak-to-average ratio (PAR) in the system design.

General differential space-time modulation

Abstract: Relying on amicable orthogonal design, we develop for multiple-antenna systems a general differential space-time block code (GDSTBC), which imposes no restrictions on underlying signal constellation compared with the existing differential space-time designs. This generalization potentially allows the spectral efficiency to be increased by carrying information not only on phases but also on amplitudes. We then derive a noncoherent decoder (NCD) for flat Rayleigh fading channels. We will show that NCD may recover data symbols with full antenna diversity and linear complexity at high signal-to-noise ratio. Particularly, while three kinds of conventional signal constellations (QAM, PSK and APSK) are used in GDSTBC, we derive simplified versions of NCDs which can effectively reduce the implementation cost.

An improved pragmatic turbo encoding scheme for high spectral efficiency using constellation shaping

Abstract: We propose a new turbo encoding scheme for high spectral efficiency with performance close to the Gaussian channel capacity. The scheme combines nonuniform signaling on a Gaussian channel, and pragmatic turbo coded modulation for simple and flexible implementation. A table is used to map equiprobable input symbols into nonequiprobable points in the QAM constellation. It is shown that the new scheme provides shaping gains of 0.6 dB and 0.93 dB, at rates 2 and 3 bits/dim respectively compared to the equiprobable pragmatic turbo coded modulation, and reach about 1 dB from the Gaussian channel capacity.

A new error function for fast phase recovery of QAM signals in CMA blind equalizers

Abstract: A modification to the well-known constant modulus algorithm (CMA) for blind channel equalization is proposed. The conventional CMA has rotational ambiguity and hence cannot achieve carrier phase recovery. It happens because the cost function is made up of the absolute value of the equalizer output and phase information is not used. In this paper, we slightly modify the cost function and provide a different error function that restores the phase of the carrier. Simulations studies are shown for the case of 16-QAM signal constellation.

Method and system for detecting symbol of modulated signal received via channel of wireless communications system

Abstract: PROBLEM TO BE SOLVED: To detect symbols of a modulated signal received via a plurality of channels of a wireless communications system. SOLUTION: A symbol transmitted via the channels is initially estimated based on a channel estimate from either pilot symbol or previously estimated symbol and then the channel estimate is updated. The next estimate of the symbol is computed by using updated channel information and maximizing the expectation of the log likelihood function. The next estimate is then quantized according to a signal constellation. The quantized estimate of the symbol is compared with the previous estimate of the symbol to determine if the previous estimate of the symbol and the quantized next estimate of the symbol have converged. Otherwise, the quantized next estimate of the symbol is made as the input for the next iteration, and the updating, optimizing, quantizing, and comparing are repeated until the estimate converges. COPYRIGHT: (C)2004,JPO

Finite-alphabet based blind carrier frequency offset estimation for differentially coded OFDM

Abstract: A novel blind carrier frequency offset estimator is proposed for differentially coded OFDM. Exploiting differential encoding and finiteness of signal constellation, our carrier-frequency offset estimation is carried out without requiring either channel knowledge or training. It is proved that under certain mild assumptions, the proposed estimator is capable of identifying the carrier-frequency offset with probability one. Different from some existing estimators, our proposed estimator does not rely on virtual subcarriers. More importantly, it is able to achieve reliable carrier frequency offset estimation by using only two received data blocks, which makes it very attractive especially for short-burst communications. The merits of our estimator are confirmed both by theoretical analysis and by simulations.

True bit level decoding of TTCM (Turbo Trellis Code Modulation )

Abstract: True bit level decoding of TTCM (Turbo Trellis Coded Modulation) of variable rates and signal constellations. A decoding approach is presented that allows for decoding on a bit level basis that allows for discrimination of the individual bits of a symbol. Whereas prior art approaches typically perform decoding on a symbol level basis, this decoding approach allows for an improved approach in which the hard decisions/best estimates may be made individually for each of the individual bits of an information symbol. In addition, the decoding approach allows for a reduction in the total number of calculations that need to be performed as well as the total number of values that need to be stored during the iterative decoding. The bit level decoding approach is also able to decode a signal whose code rate and/or signal constellation type (and mapping) may vary on a symbol by symbol basis.

Joint space-time block decoding and viterbi decoding

Abstract: A space-time block decoder for a wireless communications system includes a demodulator that generates a demodulated symbol sequence by derotating a signal constellation of a received symbol sequence. A dimension demultiplexer communicates with the demodulator and generates in-phase and quadrature components of the demodulated symbol sequence. A branch metric computation module communicates with the dimension demultiplexer and generates branch metrics based on the in-phase and quadrature components. A Viterbi decoder communicates with the branch metric computation module and generates a user data sequence based on the branch metrics. The in-phase and quadrature components comprise Gray coded data that is bit-interleaved. The branch metric computation module implements bit-by-bit piecewise linear approximation to generate the branch metrics. A deinterleaver that communicates with the branch metric computation module generates deinterleaved metrics based on the branch metrics.

Multi-antenna adaptive modulation and transmit beamforming based on bandwidth-constrained feedback

Abstract: Adaptive modulation has the potential to increase system throughput considerably by matching transmission parameters to the time-varying channel conditions. Crucial to adaptive systems is the requirement of a feedback channel, that often carries only a limited number of bits. Under such a bandwidth-constrained feedback link, we aim to optimize a multi-antenna system based on transmit beamforming and adaptive modulation, where the transmit-power, the signal constellation, the beamforming direction, and the feedback strategy, are designed jointly. Our proposed nested iterative approach leads to an approximate yet practical solution. Simulation results demonstrate considerable improvement in transmission rate, as the number of feedback bits increases.

Modulation device, demodulation device, modulation/demodulation system, modulation method, demodulation method, modulation program and computer readable recording containing the modulation program demodulation program and computer- readable recording medium containing the demodulation program

Abstract: When a coding processing unit 12 of a multi-level modulation apparatus 1 maps REPETITION bits to transmission data, it is possible to allocate symbols including the REPETITION bits at signal points of four corners of a signal constellation by mapping the REPETITION bits to specific bit locations of the symbols as a result of segmenting the transmission data after mapping into plural symbols, and thus an error rate can be reduced.

An improved image processing-based method for disturbance classification in telecommunication networks

Abstract: This paper deals with the problem of the automatic classification of the disturbances affecting the quadrature amplitude modulation (QAM). This problem arises when a continuous monitoring system is needed for improving the quality of service in communication networks based on such a modulation scheme. In particular, a measurement method based on image processing is proposed. It automatically analyzes the constellation diagram and selects the disturbance actually present through a template-matching technique. The results of the first simulations, carried out on signals affected by different disturbances and signal-to-noise ratios (SNRs), are presented and discussed. Some experimental results are also given.

A general construction of space-time trellis codes for PSK signal sets

Abstract: The diversity order and coding gain are crucial for the performance of a multiple antenna communication system. It is known that space-time trellis codes (STTC) can be used to achieve these objectives. In particular, we can use STTCs to obtain large coding gains. Many attempts have been made to construct STTCs which achieve full-diversity and good coding gains, though a general method of construction does not exist. Delay diversity code (rate-1) is known to achieve full-diversity, for any number of transmit antennas and any signal set, but does not give a good coding gain. A product distance code based delay diversity scheme (Tarokh, V. et al., IEEE Trans. Inform. Theory, vol.44, p.744-65, 1998) enables one to improve the coding gain and construct STTCs for any given number of states using coding in conjunction with delay diversity; it was stated as an open problem. We achieve such a construction. We assume a shift register based model to construct an STTC for any state complexity. We derive a sufficient condition for this STTC to achieve full-diversity, based on the delay diversity scheme. This condition provides a framework to do coding in conjunction with delay diversity for any signal constellation. Using this condition, we provide a formal rate-1 STTC construction scheme for PSK signal sets, for any number of transmit antennas and any given number of states, which achieves full-diversity and gives a good coding gain.