Constellation shaping for pragmatic turbo-coded modulation with high spectral efficiency

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 with pragmatic turbo-coded modulation (TCM) for simple and flexible implementation. A variable-rate turbo code is followed by a Huffmann code mapping onto nonequiprobable points in a quadrature amplitude modulation constellation. The rate of the turbo code is matched to the Huffmann code by variable puncturing, such that both the input bit rate and the output symbol rate are constant. It is shown that the new scheme provides shaping gains of 0.6 and 0.9 dB, at rates 2 and 3 b/dimension, respectively, compared with the equiprobable pragmatic TCM, and reach about 1 dB from the continuous input Gaussian channel capacity.     

Iterative demodulation and decoding of turbo-coded M-ary noncoherent orthogonal modulation

This paper considers bit-interleaved coded modulation (BICM) with a turbo channel code and M-ary orthogonal modulation. The BICM signal is iteratively demodulated and decoded in a noncoherent fashion. A soft demodulator suitable for noncoherent orthogonal modulation is presented, and the convergence of the iterative receiver is analyzed through extrinsic information transfer charts. The demodulator can work either with or without fading amplitude estimates. Extensive simulation results are presented for the well-known cdma-2000 turbo code, and the results are compared with the corresponding channel capacities, which are computed using a Monte Carlo technique. The results indicate gains of up to 1 dB relative to noniterative BICM can be achieved with the iterative receiver.     

Benchmarking Iterative Receivers for BICM Multi-Carrier Systems Against Capacity Bounds

In this paper, we analyze iterative receivers for bit-interleaved coded modulation (BICM) multi-carrier systems and compare them against theoretical capacity bounds for the channel, coded modulation, and BICM. We map the theoretical capacity bounds into bit-error rate (BER) versus average signal-to-noise ratio per bit plots to simplify the comparison between the theoretical capacity bounds and simulated BER curves. As BER simulations show, iterative receivers with code doping or spreading reach the turbo-cliff within 1 or 0.3dB of the independent Rayleigh fading channel capacity. While the iterative receiver with spreading is closer to the channel capacity than the one with code doping, the later one can eliminate the residual bit-errors after the turbo-cliff. We further present a combinatorial analysis of the distribution of the spread symbol constellation for Walsh-Hadamard spreading codes used in a BICM multi-carrier system to explain the above results.     

Capacity-approaching bandwidth-efficient coded modulation schemes based on low-density parity-check codes

We design multilevel coding (MLC) and bit-interleaved coded modulation (BICM) schemes based on low-density parity-check (LDPC) codes. The analysis and optimization of the LDPC component codes for the MLC and BICM schemes are complicated because, in general, the equivalent binary-input component channels are not necessarily symmetric. To overcome this obstacle, we deploy two different approaches: one based on independent and identically distributed (i.i.d.) channel adapters and the other based on coset codes. By incorporating i.i.d. channel adapters, we can force the symmetry of each binary-input component channel. By considering coset codes, we extend the concentration theorem based on previous work by Richardson et al. ( see ibid., vol.47, p.599-618, Feb. 2001) and Kavc/spl caron/ic/spl acute/ et al.(see ibid., vol.49, p.1636-52, July 2003) We also discuss the relation between the systems based on the two approaches and show that they indeed have the same expected decoder behavior. Next, we jointly optimize the code rates and degree distribution pairs of the LDPC component codes for the MLC scheme. The optimized irregular LDPC codes at each level of MLC with multistage decoding (MSD) are able to perform well at signal-to-noise ratios (SNR) very close to the capacity of the additive white Gaussian noise (AWGN) channel. We also show that the optimized BICM scheme can approach the parallel independent decoding (PID) capacity as closely as does the MLC/PID scheme. Simulations with very large codeword length verify the accuracy of the analytical results. Finally, we compare the simulated performance of these coded modulation schemes at finite codeword lengths, and consider the results from the perspective of a random coding exponent analysis.     

Adaptive bit-interleaved coded modulation

Adaptive coded modulation is a powerful method for achieving a high spectral efficiency over fading channels. Previously proposed adaptive schemes have employed set-partitioned trellis-coded modulation (TCM) and have adapted the number of uncoded bits on a given symbol based on the corresponding channel estimate. However, these adaptive TCM schemes do not perform well in systems where channel estimates are unreliable, since uncoded bits are not protected from unexpected finding. In this paper, adaptive bit-interleaved coded modulation (BICM) is introduced. Adaptive BICM schemes remove the need for parallel branches in the trellis-even when adapting the constellation size, thus making these schemes robust to errors made in the estimation of the current channel fading value. This motivates the design of adaptive BICM schemes, which will lead to adaptive systems that can support users with higher mobility than those considered in previous work. In such systems, numerical results demonstrate that the proposed schemes achieve a moderate bandwidth efficiency gain over previously proposed adaptive schemes and conventional (nonadaptive) schemes of similar complexity     

Bit-interleaved coded modulation with iterative decoding

A simple iterative decoding technique using hard-decision feedback is presented for bit-interleaved coded modulation (BICM). With an 8-state, rate-2/3 convolutional code, and 8-PSK modulation, the improvement over the conventional BICM scheme exceeds 1 dB for a fully-interleaved Rayleigh flat-fading channel and exceeds 1.5 dB for a channel with additive white Gaussian noise. This robust performance makes BICM with iterative decoding suitable for both types of channels     

Channel capacity of bit-interleaved coded modulation schemes using8-ary signal constellations

The object of the work described was to determine the 8-ary signal sets that maximise the coding gains achieved by power-efficient bit-interleaved coded modulation (BICM) schemes over an additive white Gaussian noise channel. To this end, the channel capacity limit of BICM for several 8-ary constellations has been evaluated. It is shown that the most suitable constellation for designing a BICM scheme depends on the desired spectral efficiency of the system     

Accumulate-Repeat-Accumulate Codes

In this paper, we propose an innovative channel coding scheme called accumulate-repeat-accumulate (ARA) codes. This class of codes can be viewed as serial turbo-like codes or as a subclass of low-density parity check (LDPC) codes, and they have a projected graph or protograph representation; this allows for high-speed iterative decoding implementation using belief propagation. An ARA code can be viewed as precoded repeat accumulate (RA) code with puncturing or as precoded irregular repeat accumulate (IRA) code, where simply an accumulator is chosen as the precoder. The amount of performance improvement due to the precoder will be called precoding gain. Using density evolution on their associated protographs, we find some rate-1/2 ARA codes, with a maximum variable node degree of 5 for which a minimum bit SNR as low as 0.08 dB from channel capacity threshold is achieved as the block size goes to infinity. Such a low threshold cannot be achieved by RA, IRA, or unstructured irregular LDPC codes with the same constraint on the maximum variable node degree. Furthermore, by puncturing the inner accumulator, we can construct families of higher rate ARA codes with thresholds that stay close to their respective channel capacity thresholds uniformly. Iterative decoding simulation results are provided and compared with turbo codes. In addition to iterative decoding analysis, we analyzed the performance of ARA codes with maximum-likelihood (ML) decoding. By obtaining the weight distribution of these codes and through existing tightest bounds we have shown that the ML SNR threshold of ARA codes also approaches very closely to that of random codes. These codes have better interleaving gain than turbo codes     

比特交织Turbo编码调制的8PSK符号映射

基于对Turbo编码器输出的系统比特和校验比特的不等错误保护(UEP),提出了一种新的比特交织Turbo编码调制(BITCM)的8PSK符号映射方案,即对于码率为1/3的Turbo码,校验比特映射到8PSK星座点中具有较好传输性能的比特位置上,而系统比特映射到较差传输性能的比特位置上。实现新映射方案的关键是比特交织器的设计。仿真结果表明,新的映射方案在AWGN信道下,误码率为10-3时,可获得大于0.2dB的信噪比增益,而这一增益的获得并没有牺牲频谱效率和增加系统的复杂性。     

Distribution of max-log metrics for QAM-based BICM in fading channels

In this letter we derive closed-form expressions for the probability density functions (PDFs) of the bits' reliability metrics (L-values) in bit-interleaved coded modulation (BICM) transmission over fully-interleaved fading channels. The expressions are valid for the relevant case of quadrature amplitude modulation (QAM) with Gray mapping when the metrics are calculated via the so-called max-log approximation. Using the developed expressions, the performance of coded BICM transmissions is efficiently evaluated, i.e., without resorting to otherwise required two-dimensional numerical integration. The BICM capacity for different fading channels and constellation sizes is also evaluated.     

Turbo‐coded APSK modulations design for satellite broadband communications

This paper investigates the design of power and spectrally efficient coded modulations based on amplitude phase shift keying (APSK) modulation with application to satellite broadband communications. APSK represents an attractive modulation format for digital transmission over nonlinear satellite channels due to its power and spectral efficiency combined with its inherent robustness against nonlinear distortion. For these reasons APSK has been very recently introduced in the new standard for satellite Digital Video Broadcasting named DVB‐S2. Assuming an ideal rectangular transmission pulse, for which no nonlinear inter‐symbol interference is present and perfect pre‐compensation of the nonlinearity, we optimize the APSK constellation. In addition to the minimum distance criterion, we introduce a new optimization based on the mutual information; this new method generates an optimum constellation for each spectral efficiency. To achieve power efficiency jointly with low bit error rate (BER) floor we adopt a powerful binary serially concatenated turbo‐code coupled with optimal APSK modulations through bit‐interleaved coded modulation. We derive tight approximations on the maximum‐likelihood decoding error probability, and results are compared with computer simulations. The proposed coded modulation scheme is shown to provide a considerable performance advantage compared to current standards for satellite multimedia and broadcasting systems. Copyright © 2006 John Wiley & Sons, Ltd.     

Adaptive interleaver based on rate-compatible punctured convolutional codes

This letter focuses on the design of an adaptive Bit- Interleaved Coded Modulation (BICM) scheme for frequency selective slow fading channels where the transmitter has certain knowledge of the channel response. In particular, we consider the design of the interleaver stage for a specific convolutional code operating with OFDM modulation. The adaptive interleaver uses the puncturing tables of the Rate-Compatible Punctured Convolutional Codes (RCPC codes) to rearrange the bits as a function of the fading values and the specific constellation. The performance of different interleavers are compared, revealing that the adaptive RCPC-based interleaver produces larger Euclidean distances between the received codewords and reduces the packet error rate (PER), specially when the number of deep-faded subcarriers increases. Numerical results also evidence the importance of the interleaver choice when comparing the performance of different power allocation strategies.     

Enhanced Secure Error Correction Code Schemes in Time Reversal UWB Systems

In this paper, secure channel coding schemes based on turbo codes are suggested for time reversal ultra wideband (TR-UWB) systems. Turbo code has the capability of error correction near Shannon’s limit. Adding security to turbo code is an attractive idea since it could reduce the overall processing cost of providing secure coded data and enjoys the advantages of high-speed encryption and decryption with high security, smaller encoder and decoder size and greater efficiency. The proposed turbo code schemes are labeled as follows: secure puncturing rate, secure frame length, and secure interleaving. Using these scenarios, secure turbo code is defined in a way that the redundant information used for error correction is not pre-determined by the nature of the error correction part of the algorithm but it can be chosen arbitrarily out of the whole set of possible strings. The lower bound of bit error probability for secure turbo code schemes in AWGN and TR-UWB systems are evaluated. Analytical and simulation results show secure turbo code performance is very satisfying. Various crypto-analytical attacks are investigated against these schemes. Based on this analysis, secure turbo code structures changed during the encryption procedure to increase the complexity of linear and differential cryptanalysis. It is seen that the performance of conventional turbo code and random frame length with Poisson distribution are the same. Comparing these schemes shows, secure interleaving approach has the best performance and secure puncturing rate the worst, but the latter provides the most security. The enhanced security of UWB, due to rich multipath nature of UWB channel, could be exploited. Due to space-time focusing property of time reversal UWB, there is an environmental confidentiality (or spatial security), which is additional security for secure turbo code in this system. Using secure turbo code, it is possible to increase the transmission range of UWB systems.     

A coding and Modulation scheme for HDTV services in DMB-T

In this paper a channel coding and modulation scheme for providing HDTV broadcasting services is presented, which adopts BICM structure as inner code in connection with 64QAM constellation and OFDM transmission. A new mapping method is designed to maximize the minimum Euclidean distance between signal points divided by feedback bits, thus an iterative algorithm between demodulator and decoder can be performed to acquire performance improvement. Analysis, simulation and laboratory test results have proved that the proposed system has good SNR performance in both AWGN and multipath channels.     

Speech transmission using rate-compatible trellis codes andembedded source coding

This paper presents bandwidth-efficient speech transmission systems using rate-compatible channel coders and variable bitrate embedded source coders. Rate-compatible punctured convolutional codes (RCPC) are often used to provide unequal error protection (UEP) via progressive bit puncturing. RCPC codes are well suited for constellations for which Euclidean and Hamming distances are equivalent (BPSK and 4-PSK). This paper introduces rate-compatible punctured trellis codes (RCPT) where rate compatibility and UEP are provided via progressive puncturing of symbols in a trellis. RCPT codes constitute a special class of codes designed to maximize residual Euclidean distances (RED) after symbol puncturing. They can be designed for any constellation, allowing for higher throughput than when restricted to using 4-PSK. We apply RCPC and RCPT to two embedded source coders: a perceptual subband coder and the ITU embedded ADPCM G.727 standard. Different operating modes with distinct source/channel bit allocation and UEP are defined. Each mode is optimal for a certain range of AWGN channel SNRs. Performance results using an 8-PSK constellation clearly illustrate the wide range of channel conditions at which the adaptive scheme using RCPT can operate. For an 8-PSK constellation, RCPT codes are compared to RCPC with bit interleaved coded modulation codes (RCPC-BICM). We also compare performance to RCPC codes used with a 4-PSK constellation     

Pilot-symbol-assisted LDPC coded BICM over correlated Rayleigh fading channels

Pilot-symbol-assisted low-density parity-check (LDPC)-coded bit-interleaved coded modulation (BICM) is analyzed using the density evolution (DE) and the extrinsic information-transfer (EXIT) chart for correlated Rayleigh fading channels. The key parameter (the power correlation coefficient) is identified, and the threshold degradation is quantified. The optimal tradeoff of energy allocation between pilots and coded symbols is found to be sensitive to the normalized Doppler spread of the channel, the interpolation filter, the modulation scheme, and the pilot selection. In addition, a simple upper bound on the performance of any receiver that performs joint iterative decoding and channel estimation is derived. Extension to irregular code design is also discussed.     

A Parallel Concatenated Convolutional-Based Distributed Coded Cooperation Scheme for Relay Channels

In this paper, we investigate a coded cooperation diversity scheme suitable for L-relay channels operating in the soft-decode-and-forward (soft-DF) mode. The proposed scheme is based on parallel concatenated convolutional codes (PCCC). To improve the overall performance through diversity, the coded cooperation operates by sending the systematic and the first parity outputs via L?+?1 independent fading paths. Instead of using only a centralized turbo code system at the source node, we have proposed a DCC scheme, where the first recursive systematic coding is done at both source and relay nodes. At the destination, the received replicas are combined using the maximal ratio combining (MRC). The entire codeword, comprising the MRC sequence and the second parity part, is decoded via the maximum a-posteriori (MAP) algorithm and turbo decoding principle. We analyze the proposed scheme in terms of bit error rate (BER). In fact, we define the explicit upper bounds for error rate assuming Binary phase shift keying (BPSK) transmission for fully interleaved channels with channel state information (CSI). We use the Rayleigh fading channels with independent fading. Our study shows that the full diversity order is achieved when the source-relay link is more reliable than the other links. Otherwise, the diversity decreases. However, in both cases, it is shown that significant performance improvements are possible to achieve over non-cooperative coded systems. Theorical and simulation results are presented to demonstrate the efficacy of the proposed scheme.     

Performance analysis of variable rate: symbol-by-symbol adaptivebit interleaved coded modulation for Rayleigh fading channels

A challenging problem for a high-speed data service over wireless is to protect data over the error-prone fading channel in an effective way (high-bandwidth efficiency). We propose a bandwidth-efficient error correction scheme, namely variable rate adaptive bit interleaved coded modulation (ABICM), for the wireless mobile channel. The code rate and modulation level are varied according to the current channel state to exploit the time-varying nature of the wireless channel. Design challenges to achieve symbol-by-symbol adaptation are addressed. In particular, we address the criteria for choosing the family of component codes for the ABICM system. We propose a multilevel puncturing scheme that solves the problem of symbol-by-symbol adaptive puncturing and interleaving. The equivalent distance spectrum for variable rate symbol-by-symbol adaptive codes is introduced and analytical bounds on adaptive codes are derived that enable us to determine the optimal adaptation thresholds. Two operation modes, namely constant throughput and constant bit error rate (BER) controls, are proposed. It is found that there are significant gains relative to fixed-rate coding in terms of signal-to-noise ratio (SNR) and throughput. It is also found that the ABICM scheme is essentially not degraded when used with small interleaving depths. This makes the ABICM very suitable for real-time applications     

Reliable channel regions for good binary codes transmitted over parallel channels

We study the average error probability performance of binary linear code ensembles when each codeword is divided into J subcodewords with each being transmitted over one of J parallel channels. This model is widely accepted for a number of important practical channels and signaling schemes including block-fading channels, incremental redundancy retransmission schemes, and multicarrier communication techniques for frequency-selective channels. Our focus is on ensembles of good codes whose performance in a single channel model is characterized by a threshold behavior, e.g., turbo and low-density parity-check (LDPC) codes. For a given good code ensemble, we investigate reliable channel regions which ensure reliable communications over parallel channels under maximum-likelihood (ML) decoding. To construct reliable regions, we study a modifed 1961 Gallager bound for parallel channels. By allowing codeword bits to be randomly assigned to each component channel, the average parallel-channel Gallager bound is simplified to be a function of code weight enumerators and channel assignment rates. Special cases of this bound, average union-Bhattacharyya (UB), Shulman-Feder (SF), simplified-sphere (SS), and modified Shulman-Feder (MSF) parallel-channel bounds, allow for describing reliable channel regions using simple functions of channel and code spectrum parameters. Parameters describing the channel are the average parallel-channel Bhattacharyya noise parameter, the average channel mutual information, and parallel Gaussian channel signal-to-noise ratios (SNRs). Code parameters include the union-Bhattacharyya noise threshold and the weight spectrum distance to the random binary code ensemble. Reliable channel regions of repeat-accumulate (RA) codes for parallel binary erasure channels (BECs) and of turbo codes for parallel additive white Gaussian noise (AWGN) channels are numerically computed and compared with simulation results based on iterative decoding. In addition, an examp