Decision feedback schemes for enhanced channel estimation for turbo decoding in Rayleigh fading channels

A method for coherently detecting and decoding turbo-coded binary phase shift keying (BPSK) signals transmitted over frequency-flat fading channels is discussed. Estimates of the complex channel gain and variance of the additive noise are derived first from known pilot symbols and an estimation filter. After each iteration of turbo decoding, the channel estimates are refined using information fed back from the decoder. Both hard-decision and soft-decision feedback are considered and compared with three baseline turbo-coded systems: (1) a BPSK system that has perfect channel estimates; (2) a system that uses differential phase shift keying and hence needs no estimates; and (3) a system that performs channel estimation using pilot symbols but has no feedback path from decoder to estimator. Performance can be further improved by borrowing channel estimates from the previously decoded frame. Simulation results show the influence of pilot symbol spacing, estimation filter size and type, and fade rate. Performance within 0.49 and 1.16 dB of turbo-coded BPSK with perfect coherent detection is observed at a bit-error rate of 10/sup -4/ for normalized fade rates of f/sub d/T/sub s/=0.005 and f/sub d/T/sub s/=0.02, respectively.

An iterative detection and decoding (IDD) technique has recently received much attention due to its capacity approaching performance and manageable complexity for (parallel) turbo-coded systems. The existing IDD receivers process indiscriminately the a priori information of the systematic bits and the parity bits generated by the turbo decoder in the front-end detection. In this paper, we first show that the parity bits of the parallel turbo codes are inherently less protected than the systematic bits, which leads to suboptimality for the IDD receivers. Based on this observation, we then propose an unequal-error-correcting-capability-aware technique to mitigate the associated performance loss. As an illustrative example, the minimum-mean-square-error (MMSE) filtering and soft interference cancelation (MMSE-SIC)-aided iterative receiver for parallel-turbo-coded multiple-input-multiple-output (MIMO) systems is employed in this paper. Numerical results manifest that the MMSE-SIC-aided iterative receiver based on the proposed paradigm achieves noticeable performance gains over the conventional unequal-error-correcting-capability-agnostic one even with reduced computational complexity. The extension of the proposed principle to other iterative algorithms for parallel-turbo-coded data transmission is straightforward.

An iterative receiver with soft decision feedback is derived to yield a maximum a posteriori (MAP) estimate of fast Rayleigh flat fading channels. The derivation is based on the expectation-maximization (EM) algorithm. Simulation results indicate that the derived receiver performs better than an iterative receiver with hard decision feedback, for both uncoded data and convolutionally coded data. The combination with turbo codes is also investigated. Three kinds of decision feedback are proposed and compared namely, re-encoder, soft decision feedback and quantizer. It is shown via simulation that soft decision feedback is the best while the quantizer has similar performance. The re-encoder, on the other hand has performance degradation.

An iterative detection and decoding (IDD) receiver which combines a soft-input soft-output (SISO) detector and a SISO double binary (DB) turbo decoder improves the bit-error rate performance. However, the amount of the extrinsic information sent from the DB turbo decoder to the detector is much different from the amount of the extrinsic information sent from the detector to the decoder. Herein, a parity-enhanced DB turbo decoder is designed to provide the bit-level extrinsic information of systematic and parity bits for the WiMAX-based turbo-coded IDD receiver. Compared to the conventional turbo decoder, the proposed decoder achieves ~0.3 dB coding gain with a 6.06% area overhead.

We provide a performance analysis of a new class of serial concatenated convolutional codes (SCCC) where the inner encoder can be punctured beyond the unitary rate. The puncturing of the inner encoder is not limited to inner coded bits, but extended to systematic bits. We derive analytical upper bounds to the error probability of this particular code structure and address suitable design guidelines for the inner code puncturing patterns. We show that the proportion of systematic and parity bits to be deleted strongly depends on the SNR region of interest. Furthermore, we show that puncturing of the inner code systematic bits should be interleaver dependent. Based on these considerations, we derive design guidelines to obtain well-performing rate-compatible SCCCs families. Throughout the paper, the performance of the proposed codes are compared with analytical bounds, and with the performance of PCCC and SCCC proposed in literature

The large-system performance of iterative multiuser decision-feedback detectors (DFDs) is studied for synchronous coded direct-sequence code-division multiple access. Both successive and parallel demodulation of users are considered. The filters are optimized according to the minimum mean-squared error criteria, assuming perfect feedback. We first consider Viterbi decoding with hard decision feedback, and compute union bounds on the large-system error rate. We then consider maximum a posteriori (MAP) decoding with soft decision feedback, and evaluate the error rate semianalytically by assuming the log-likelihood ratios computed by the MAP decoder are Gaussian random variables. Performance is studied numerically as a function of noise level, spectral efficiency, and code rate. Results show that soft decision feedback gives substantial gains relative to hard decision feedback. At moderate spectral efficiencies (users divided by bandwidth expansion less than 0.9), the iterative DFDs with soft decision feedback based on a posteriori probabilities can achieve near-single-user performance at an E/sub b//N/sub 0/ close to the large-system capacity bound.

This paper proposes a simple, yet effective generalized sidelobe canceller (GSC)-based Capon multiuser receiver for the Alamouti space-time block coded (STBC) multiple-input multiple-output code-division multiple-access (MIMO CDMA) systems. The new receiver makes full use of the space diversity provided by the antenna array and the time diversity provided by the decision feedback (DF) from the outputs of both of GSCs employed along with the transmit diversity induced in the Alamouti STBC to render superior performance. With an ingenious DF scheme, refer to as the the cross DF (CDF), the desired signals will be blocked more thoroughly after the blocking matrix so that the interference suppression capability can be further enhanced. Performance analysis is also provided to show that that the minimum mean squared error (MMSE) of the receiver using the CDF is indeed lower than those with or without the classical DF scheme. Conducted simulations show that the new receiver can not only provide superior performance compared with previous works, but it can also render faster convergence characteristics.

이 논문에서는 시변 레일리 페이딩 채널에서 STBC(Space Time Block Code)의 신호 검출 방법을 연구한다. 하나의 블록 내의 두 심볼 구간 동안 채널 환경이 변하는 경우, 채널 행렬의 직교성이 성립하지 않고 두 심볼간에 간섭이 발생한다. 이러한 채널에서 수신 성능을 개선하기 위해서 joint ML 검출 방식이 제안되었으나, 계산복잡도가 증가한다. 이를 줄이기 위해서 판정 궤환(decision feedback) 방식이 제안되었으나, joint ML 방식에 비해 수신 성능이 떨어진다. 이에 이 논문에서는 joint ML 검출 방법을 사용하는 경우에 비해 수신기의 복잡도를 줄이는 동시에 기존의 판정 궤환 방식에 비해 성능이 개선된 이중 판정 궤환 방식을 이용한 새로운 STBC 검출 알고리즘을 제안한다. In this paper, we study STBC(Space Time Block Code) detection scheme in time varying Rayleigh fading channel. When the channel is varying during the time duration of STBC, the channel matrix of orthogonal STBC is not orthogonal. To get the optimum reception performance in this channel, joint ML detection scheme may be used, however this scheme requires high computation complexity. Decision feedback scheme is proposed to reduce the computation complexity with less reception performance. In this paper, we propose a novel STBC detection algorithm using double decision feedback which is less complex than the joint ML scheme and outperforms the conventional decision feedback scheme.

This chapter discusses fundamentals of information theory and channel coding (error correction coding ECC, Forward error correction FEC). First, key results of Shannon, including channel capacity and the relationship between transmit power and bandwidth, are discussed. Block codes multiply a block of data with an encoding matrix. For systematic codes, the resulting codeword consists of systematic bits and parity check bits. Decoding is achieved by multiplication with a parity check matrix and determination of the syndrome vector. Correction of errors within the correction sphere can be done, e.g., through lookup tables. Convolutional codes send the source data stream through shift registers and adders, a process described by a trellis diagram. Maximum-likelihood sequence estimation (Viterbi algorithm) determines the (transmit) bit sequence with best distance metric (Hamming or Euclidean) from the received signal. Methods for combining coding and higher-order modulation include trellis coded modulation (using set partitioning for code design) and bit-interleaved coded modulation (BICM). Turbo codes are close to capacity-achieving. They are based on transmitting differently interleaved convolutional codes; receivers use soft decoding of the constituent codes and iterative exchange of soft information (log-likelihood ratios). Low-density parity check codes (LDPCC) are decoded by belief propagation (message passing), where variable nodes and constraint nodes exchange beliefs about the codeword. In fading channels, (block or convolutional) interleaving is essential to provide diversity. From an information-theoretic point of view, in fading channels we can distinguish between the ergodic capacity and the outage capacity. If the channel state information is known at the transmitter, waterfilling is the optimum power allocation strategy. Automatic Repeat Request ARQ) and Hybrid ARQ (including chase combining or incremental redundancy) are described in the appendix.

We track the density of extrinsic information in iterative turbo decoders by actual density evolution, and also approximate it by symmetric Gaussian density functions. The approximate model is verified by experimental measurements. We view the evolution of these density functions through an iterative decoder as a nonlinear dynamical system with feedback. Iterative decoding of turbo codes and of serially concatenated codes is analyzed by examining whether a signal-to-noise ratio (SNR) for the extrinsic information keeps growing with iterations. We define a for the iterative decoder, such that the turbo decoder will converge to the correct codeword if the noise figure is bounded by a number below zero dB. By decomposing the code's noise figure into individual curves of output SNR versus input SNR corresponding to the individual constituent codes, we gain many new insights into the performance of the iterative decoder for different constituents. Many mysteries of turbo codes are explained based on this analysis. For example, we show why certain codes converge better with iterative decoding than more powerful codes which are only suitable for maximum likelihood decoding. The roles of systematic bits and of recursive convolutional codes as constituents of turbo codes are crystallized. The analysis is generalized to serial concatenations of mixtures of complementary outer and inner constituent codes. Design examples are given to optimize mixture codes to achieve low iterative decoding thresholds on the signal-to-noise ratio of the channel.

We report a very efficient semianalytical approach for the performance evaluation of differential detection schemes for GMSK signals of the DECT standard. Precisely, for a given channel, the performance is determined by means of an analytical procedure which includes the saddlepoint approximation. We consider both static channels (with impulse response generated by the simulation program SIRCIM) and two-ray Rayleigh and log-normal fading channels. As a departure from previous works, our receiver includes an all-digital part after the analog differential detection scheme. The digital part includes: (1) a block for the estimation of both the optimum sampling phase and the nonlinear channel coefficients (by making use of the DECT training sequence), (2) a one-tap decision feedback (DF) equalizer, and (3) a block for the evaluation of the approximate optimum bias level (/spl gamma//sub e/) in the threshold detector. Both the DF equalizer coefficient and /spl gamma//sub e/ are based on the nonlinear channel coefficients estimate. For channels with a normalized delay spread up to 0.2, the use of the optimum threshold together with the DF equalizer permits a gain of about 2 dB at BER=10/sup -6/ with respect to a receiver without equalization and a zero-level decision threshold. In addition, we discover that, in indoor environments, the 2-bit GMSK detector performs roughly the same as the 1-bit detector. The threshold optimization is also effective in the presence of channels affected by fading. To support this statement, we report the performance of the 1-bit differential detection scheme combined with antenna selection diversity in the presence of a two-ray log-normal and Rayleigh fading channel.

By an extension of Gallager's bounding methods, exponential error bounds applicable to coding schemes involving erasures, variable-size lists, and decision feedback are obtained. The bounds are everywhere the tightest known.

In this paper, the extrinsic information transfer (EXIT) functions of randomly punctured systematic codes (RP-SCs) are analyzed. We consider the systematic encoder to consist of a non-systematic code (NC) and a parallel path for the systematic bits; puncturing of the systematic and the parity bits is then carried out separately. We show that the decoding model for measuring the EXIT characteristics of a RP-SC can be transformed into a decoding model of the underlying NC. For the special case where the a priori/ communication channels are modeled by binary erasure channels, it becomes obvious that the EXIT functions of any RP-SC can be analytically derived from the EXIT functions of the underlying NC. This theoretical result is verified by a comparison to simulated EXIT functions.

A bandwidth-efficient joint network-channel coding scheme based on distributed turbo codes is proposed for relay-based wireless communications systems. The proposed scheme provides a solution to increase the transmission rate and/or improve the error performance. To enhance the system performance over the adverse block fading channels, the relay not only delivers the additional parity bits as in the conventional turbo coded systems but also repeats a portion of systematic bits by using higher-order constellations. This increases the number of information bits having a diversity order of 2. As a result, the proposed scheme yields a 3-dB gain in the transmitted power compared to the conventional scheme for the case of symmetric channels. The gain is further increased when the relay has a better channel condition to the destination than the users.

In this paper, the performance of turbo codes in speech transmission over AWGN channels is presented. Turbo codes of short block sizes suitable for real time communication systems have drawn attention recently. Embedded channel coding is used which means permitting rate variability using a single encoder/decoder. Puncturing patterns that minimize the BER were implemented and tested. The puncturing of systematic bits is completely excluded and thus puncturing is only done for parity bits. A comparison between the performances of different puncturing patterns shows that the puncturing patterns outperform other patterns.