Turbo Receiver Research Articles

Efficient computation of symbol statistics from bit a priori information in turbo receivers

In this paper, an efficient computational scheme is proposed to calculate the symbol mean and variance from bit a priori information, when a so-called multilinear mapping is employed. The multilinear mapping is exploited to reduce the number of the terms needed for the calculation of the symbol mean and variance.

Advanced Receiver Design for Quadrature OFDMA Systems

Quadrature orthogonal frequency division multiple access (Q-OFDMA) systems have been recently proposed to reduce the peak-to-average power ratio (PAPR) and complexity, and improve carrier frequency offset (CFO) robustness and frequency diversity for the conventional OFDMA systems. However, Q-OFDMA receiver obtains frequency diversity at the cost of noise enhancement, which results in Q-OFDMA systems achieving better performance than OFDMA only in the higher signal-to-noise ratio (SNR) range. In this paper, we investigate various detection techniques such as linear zero forcing (ZF) equalization, minimum mean square error (MMSE) equalization, decision feedback equalization (DFE), and turbo joint channel estimation and detection, for Q-OFDMA systems to mitigate the noise enhancement effect and improve the bit error ratio (BER) performance. It is shown that advanced detections, for example, DFE and turbo receiver, can significantly improve the performance of Q-OFDMA.

Iterative Group-Blind Multiuser Detection and Decoding With Signal Rank Estimation for Coded CDMA Systems

In this paper, a turbo receiver structure is proposed for the uplink of coded code-division multiple-access (CDMA) systems in the presence of unknown users. The proposed receiver consists of two stages following each other. The first stage performs soft interference cancellation and group-blind linear minimum mean square error (MMSE) filtering, and the second stage performs channel decoding. The proposed group-blind linear MMSE filter suppresses the residual multiple-access interference (MAI) from known users based on the spreading sequences and the channel characteristics of these users while suppressing the interference from other unknown users using a subspace-based blind method. The proposed receiver is suitable for suppressing intercell interference in heavily loaded CDMA systems. Since the knowledge of the number of unknown users is crucial for the proposed receiver structure, a novel estimator is also proposed to estimate the number of unknown users in the system by exploiting the statistical properties of the received signal. Simulation results demonstrate that the proposed estimator can provide the number of unknown users with high accuracy; in addition, the proposed group-blind receiver integrated with the new estimator can significantly outperform the conventional turbo multiuser detector in the presence of unknown users.

A Bayesian MMSE Turbo Receiver for Coded MIMO Systems

In conventional interference cancellation (IC)-based turbo receivers, extrinsic information (EXT) from the soft-input-soft-output (SISO) decoders is used to compute the statistical mean (SM) of the interfering signals. In this paper, we present a class of Bayesian minimum mean-square-error (MMSE) turbo receivers for coded multiple-input-multiple-output (MIMO) systems. Instead of using the EXT to estimate the prior SM, we use it in the posterior Bayesian MMSE estimation of the interfering signals. The estimation accuracy is, therefore, improved, leading to better bit-error-rate (BER) performance from our proposed Bayesian receivers. For the cases that we have studied, the proposed receiver with two iterations has a lower BER than the conventional one with five iterations, thus significantly reducing the receiver processing delay and the implementation complexity. Its superior performance is also demonstrated by the higher detector output mutual information and fewer iterations to achieve convergence, which is obtained from the extrinsic information transfer (EXIT) chart analysis.

Performance of Turbo Interference Cancellation Receivers in Space-Time Block Coded DS-CDMA Systems

We investigate the performance of turbo interference cancellation receivers in the space time block coded (STBC) direct-sequence code division multiple access (DS-CDMA) system. Depending on the concatenation scheme used, we divide these receivers into the partitioned approach (PA) and the iterative approach (IA) receivers. The performance of both the PA and IA receivers is evaluated in Rayleigh fading channels for the uplink scenario. Numerical results show that the MMSE front-end turbo space-time iterative approach receiver (IA) effectively combats the mixture of MAI and intersymbol interference (ISI). To further investigate the possible achievable data rates in the turbo interference cancellation receivers, we introduce the puncturing of the turbo code through the use of rate compatible punctured turbo codes (RCPTCs). Simulation results suggest that combining interference cancellation, turbo decoding, STBC, and RCPTC can significantly improve the achievable data rates for a synchronous DS-CDMA system for the uplink in Rayleigh flat fading channels.

Underwater Acoustic Communications: Long-Term Test of Turbo Equalization in Shallow Water

Acoustic telemetry sea trials lasting for three months with experiment periods of 7 h have been carried out, using a turbo receiver. The communication channel is shallow; the depth is approximately 10 m, and the range is 850 m, showing variable multipath. The wind speed and the sound-speed profile (SSP) are measured during some of the experiments. The results show reliable communication over the entire period and performance improvement of the turbo receiver compared to a decision feedback equalizer (DFE) used as reference receiver. The noise in the experiment data suggests significant deviations from the Gaussian assumption, and analysis of turbo receiver performance under heavy-tailed noise is presented.

Iterative ML-based estimation of carrier frequency offset, channel impulse response and data in OFDM transmissions

This paper provides an approximate closed form solution to the problem of data-aided joint maximum likelihood estimation of the carrier frequency offset and of the channel impulse response in an Orthogonal Frequency Division Multiplexing transmission over a multipath fading channel. This results in a novel pilot-aided feedforward estimator, that can be employed for pilot-based receiver training. It is also shown how the novel joint estimation strategy can be exploited in an iterative (turbo) receiver structure to track the fast channel and frequency offset changes occurring during the transmission of information symbols. The performance of this structure is assessed by computer simulations and is compared with that provided by other estimation/detection strategies.

EM-based turbo receiver design for low-density parity-check-coded MIMO-OFDM systems with carrier-frequency offset

The design of expectation-maximisation (EM)-based turbo receivers for low-density parity-check-coded multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems with the presence of carrier-frequency offset (CFO) is studied. First, starting from the maximum-likelihood principle, a novel EM-based CFO estimator for MIMO-OFDM systems is devised. This estimator iteratively provides the CFO estimate with the aid of pilot symbols. It is also capable of accommodating any space-time-coded-OFDM transmission. Then this CFO estimator is incorporated into the initialisation step of the turbo receiver. Simulation results show the effectiveness of the receiver design in combating CFO over unknown frequency-selective fading channels.

Iterative Channel Estimation and Decoding of Turbo Coded SFBC-OFDM Systems

We consider the design of turbo receiver structures for space-frequency block coded orthogonal frequency division multiplexing (SFBC-OFDM) systems in the presence of unknown frequency and time selective fading channels. The turbo receiver structures for SFBC-OFDM systems under consideration consists of an iterative MAP expectation/maximization (EM) channel estimation algorithm, soft MMSE-SFBC decoder and a soft MAP outer-channel-code decoder. MAP-EM employs iterative channel estimation and it improves receiver performance by re-estimating the channel after each decoder iteration. Moreover, the MAP-EM approach considers the channel variations as random processes and applies the Karhunen-Loeve (KL) orthogonal series expansion. The optimal truncation property of the KL expansion can reduce computational load on the iterative estimation approach. The performance of the proposed approaches are studied in terms of mean square error and bit-error rate. Through computer simulations, the effect of a pilot spacing on the channel estimator performance and sensitivity of turbo receiver structures on channel estimation error are studied. Simulation results illustrate that receivers with turbo coding are very sensitive to channel estimation errors compared to receivers with convolutional codes. Moreover, superiority of the turbo coded SFBC-OFDM systems over the turbo coded STBC-OFDM systems is observed especially for high Doppler frequencies.

Turbo equalization of serially concatenated turbo codes using a predictive DFE-based receiver

This paper investigates the performance of various “turbo” receivers for serially concatenated turbo codes transmitted through intersymbol interference (ISI) channels. Both the inner and outer codes are assumed to be recursive systematic convolutional (RSC) codes. The optimum turbo receiver consists of an (inner) channel maximum a posteriori (MAP) decoder and a MAP decoder for the outer code. The channel MAP decoder operates on a “supertrellis” which incorporates the channel trellis and the trellis for the inner error-correcting code. This is referred to as the MAP receiver employing a SuperTrellis (STMAP). Since the complexity of the supertrellis in the STMAP receiver increases exponentially with the channel length, we propose a simpler but suboptimal receiver that employs the predictive decision feedback equalizer (PDFE). The key idea in this paper is to have the feedforward part of the PDFE outside the iterative loop and incorporate only the feedback part inside the loop. We refer to this receiver as the PDFE-STMAP. The complexity of the supertrellis in the PDFE-STMAP receiver depends on the inner code and the length of the feedback part. Investigations with Proakis B, Proakis C (both channels have spectral nulls with all zeros on the unit circle and hence cannot be converted to a minimum phase channel) and a minimum phase channel reveal that at most two feedback taps are sufficient to get the best performance. A reduced-state STMAP (RS-STMAP) receiver is also derived which employs a smaller supertrellis at the cost of performance.

Analysis and Optimization of Interleave-Division Multiple-Access Communication Systems

The recently proposed interleave-division multiple-access (IDMA) system is a flexible spread-spectrum air-interface technique featuring low receiver complexity and high spectral efficiency. In IDMA, each user's chip sequence is interleaved by a distinct chip-level interleaver. The receiver employs a simple chip-level iterative multiuser detector to decode the user's data. Here we focus on the analysis and optimization of such an IDMA system. We show that IDMA can be viewed as a limit case of the conventional CDMA employing repetition code and random spreading. Two analytical tools, namely, the large-system performance approximation and the EXIT chart technique are tailored in the context of IDMA chip-level iterative detector to facilitate the system performance analysis and optimization. The spectral efficiencies of a low-rate coded IDMA system with equal power setting in both single-cell and multi-cell scenarios are then analyzed, from which it is seen that the coded IDMA system with turbo receiver is a spectral efficient multiple-access scheme. Finally, we consider optimal power allocation among users in IDMA to maximize the spectral efficiency with finite-alphabet constellation. The differential evolution technique for nonlinear optimization is adopted to solve the power profile optimization problem. With optimized power profiles, the low-rate coded IDMA system can approach the optimal spectral efficiency with finite input constellations

An Interleave-Division-Multiplexing MISO System With Partial CSI at Transmitter

We treat the performance analysis and optimization of an interleave-division-multiplexing (IDM) multiple-input-single-output (MISO) system with partial channel state information (CSI) at the transmitters and an iterative (turbo) receiver. We propose a general methodology to analyze and evaluate the performance of such an IDM-MISO system. In particular, an SNR tracking method is proposed to calculate the output SNR of the soft-input soft-output IDM detector. The extrinsic mutual information transfer chart technique is employed to analyze the performance of the entire system. More specifically, the repetition decoder and the low-density parity check decoder are considered for the uncoded IDM system and the coded IDM system, respectively. Furthermore, based on the above analytical framework, we present the optimal transmit strategy for the IDM-MISO system with an imperfect CSI at the transmitter. Extensive simulations are performed to demonstrate the performance of the proposed IDM schemes under the MISO channels with different correlation statistics. Our results show that the proposed IDM-MISO scheme can outperform the existing space-time block code schemes in the low-rate transmission scenario

Complexity Reduction of Iterative Receivers Using Low-Rank Equalization

This paper covers the consideration of an iterative or turbo receiver where the nonlinear trellis-based detection of the interleaved and coded data bits is replaced by linear detection using the Wiener filter (WF), i.e., the optimal linear filter based on the mean-square error (MSE) criterion. The equalization of channels with multiple antennas at the receiver as well as frequency-selective transfer functions requires high-dimensional observation vectors which involve computationally intense detectors. We extend an optimal but computationally efficient algorithm, originally derived for single receive antenna systems, to single-input multiple-output (SIMO) channels. To further reduce computational complexity, we apply the suboptimal low-rank multistage WF (MSWF), i.e., the WF approximation in the low-dimensional Krylov subspace, and replace additionally second-order statistics of nonstationary random processes by their time-invariant averages. Complexity investigations reveal the enormous capability of the proposed algorithms to decrease computational effort. Moreover, the analysis based on extrinsic information transfer (EXIT) charts as well as Monte Carlo simulations show that compared with reduced-rank detection methods based on eigensubspaces, the reduced-rank MSWF behaves near optimum although the rank is drastically reduced to two or even one

Soft Iterative Channel Estimation With Subspace and Rank Tracking

This letter presents an adaptative soft-based method for channel estimation in turbo receivers. The proposed approach is based on the particular algebraic structure of multipath Rayleigh-fading channels, and it is suited for mobile systems where the multipath pattern (namely, the times of delay) changes slowly over the time. The method is implemented through a rank-and-subspace tracking algorithm that allows to adapt the estimate to the multipath variations and also to reduce the computational cost with respect to the batch implementation based on eigenvalue decomposition. A performance analysis, in terms of mean square error of the channel estimate and bit error rate, shows the advantages of the proposed technique in communications over time-varying wireless channels

Design of Multilayer Coded Modulation for Nonergodic Block-Fading Channels

Multilayer coded modulation (MLyC) in single-antenna and multiple-antenna wireless fading channels is investigated. While the MLyC has been known to achieve ergodic fading channel capacity, the problem of interest here is to design practical and efficient MLyC for nonergodic block-fading channels. We develop a design methodology which maximizes the information rate of MLyC, subject to an upper bound on the decoding error probability. Moreover, we show that the MLyC performs closer to channel capacity when the overall diversity order of the channel is higher and proper spatial interleaving is employed. The relationship between the MLyC scheme and other transmission techniques, such as multilevel coded modulation and bit-interleaved coded modulation (BICM), is also discussed. It is shown through simulation examples that in realistic block-fading channels, the MLyC with practical coded modulation outperforms the BICM without turbo receiver iteration. We propose the MLyC as a promising alternative to BICM, especially when turbo iterative processing is not desired, e.g., for progressive layered media transmission

Comparison of EM-Based Algorithms for MIMO Channel Estimation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Iterative channel estimation can improve the channel- state information (CSI) with respect to noniterative estimation. New iterative channel estimators based on the expectation-maximization (EM) algorithm are proposed in this paper. A first estimator, called the unbiased EM (UEM), is designed to unbias the EM estimates. A second estimator is then put forward, which is based on the expectation-conditional-maximization (ECM) algorithm, and its complexity is lower than that of the EM. An unbiased ECM (UECM) estimator is also proposed. Although the unbiasedness of the UEM and UECM estimators is not rigorously proved, the use of these names is explained in the paper. The new estimators are compared with well-known ones, such as the EM, the decision-directed (DD), and the data-aided (DA) estimators. Simulations are reported for a turbo receiver operating over frequency-selective multiple-input multiple-output channels. It is shown that the UEM channel estimator outperforms the EM, and that the ECM-based estimators are very close to the EM-based ones. </para>

A low-complexity sequential Monte Carlo algorithm for blind detection in MIMO systems

In statistical signal processing, the sequential Monte Carlo (SMC) method is powerful and can approach the theoretical optima. However, its computational complexity is usually very high, especially in multiple-input multiple-output (MIMO) systems. This paper presents a new low-complexity SMC (LC-SMC) algorithm for blind detection in MIMO systems, the main idea of which is to shrink the sampling space via channel estimation which is initialized using the first differentially modulated symbol and then updated using the Monte Carlo samples. Since the a posteriori probability of the transmitted symbols can be calculated separately by each transmit antenna, the proposed LC-SMC algorithm is not only computationally efficient, as compared to the original SMC whose complexity grows exponentially with the number of transmit antennas, but also makes blind turbo receiver more feasible for multilevel/phase modulations. Simulation results are presented to demonstrate the effectiveness of the LC-SMC algorithm

A multicarrier interleave-division uwb system

We propose a multicarrier interleave-division multiple-access scheme for ultra-wideband wireless communications. For the uplink, a chip-level interleaving method at the transmitter with a simple turbo receiver is proposed to effectively suppress the frequency-selective fading and multiple-access interference. Both hard and soft frequency notching methods are suggested to suppress the narrowband interference. For the downlink, a two-layer interleaving scheme is proposed, in which data from different users are separated in the frequency-domain while the different data streams for the same user are separated in the interleaver-domain. A joint power and subcarrier allocation algorithm is developed to exploit the multiuser diversity and thereby improve the system performance. The performance of the proposed system is evaluated by both theoretic analysis and simulations. It is seen that within a few iterations, the proposed system exhibits single-user performance even under over-loaded conditions

Interference suppressing for cellular MIMO system based on turbo receiver

Space-Time Coding (STC), which combines channel coding, modulation, and multiple transmit antennas, is a powerful scheme to achieve higher data rates and combat fading in wireless systems. In this paper, we propose a soft/cancellation turbo equalization scheme to suppress Co-Channel Interference (CCI) in STC systems. The simulation results show that the proposed method significantly improves the system’s ability of interference suppression, while preserving the space-time structure.

MMSE turbo equalisation for real‐valued symbols

AbstractThis paper deals with iterative or turbo receivers where the maximum a posteriori detector has been replaced by a widely linear filter minimising the mean square error, that is the widely linear Wiener Filter (WF). In communication systems where the transmitted symbols can be assumed to be realisations of a non‐circular random variable, widely linear processing outperforms the linear one. Besides the non‐circularity due to the a priori information fed back in the turbo scheme, we investigate real‐valued modulation alphabets which are non‐circular even if the a priori information is zero. We derive the widely linear WF for this case and observe that the number of floating point operations involved in its computation, is higher than the one of the linear WF. Nevertheless, they have the same order of computational complexity. Moreover, we reveal the relationship between the proposed widely linear equaliser and interference cancellation based on the available a priori information. Simulation results of a frequency‐selective turbo system show that despite of the increased computational complexity, the performance gain of widely linear processing compared to the linear one, is negligible. However, in systems without turbo feedback, the widely linear WF outperforms tremendously the linear one. Copyright © 2006 AEIT