An iterative soft interference cancellation and decoding technique to mitigate the effect of home-LAN on VDSL

An iterative QR-based soft feedback segment interference cancellation (QRSFSIC) detection and decoder algorithm for a Reed–Muller (RM) space-time turbo system is proposed in this paper. It forms the sufficient statistic for the minimum-mean-square error (MMSE) estimate according to QR decomposition-based soft feedback successive interference cancellation, stemmed from the a priori log-likelihood ratio (LLR) of encoded bits. Then, the signal originating from the symbols of the reliable segment, the symbol reliability metric, in terms of an a posteriori LLR of encoded bits which is larger than a certain threshold, is iteratively cancelled with the QRSFSIC in order to further obtain the residual signal for evaluating the symbols in the unreliable segment. This is done until the unreliable segment is empty, resulting in the extrinsic information for a RM turbo-coded bit with the greatest likelihood. Bridged by de-multiplexing and multiplexing, an iterative QRSFSIC detection is concatenated with an iterative trellis-based maximum a posteriori probability RM turbo decoder as if a principal Turbo detection and decoder is embedded with an iterative subordinate QRSFSIC detection and a RM turbo decoder, exchanging each other’s detection and decoding soft-decision information iteratively. These three stages let the proposed algorithm approach the upper bound of the diversity. The simulation results also show that the proposed scheme outperforms the other suboptimum detectors considered in this paper.

In this paper, a second stage for algorithms applying iterative soft decision interference cancellation (ISDIC) for channel equalization is proposed, which performs error search and correction. Analysis by simulations shows that matched filter (MF)-based ISDIC (MF ISDIC) with subsequent second stage can outperform more complex minimum-mean-square-error (MMSE) based ISDIC. Furthermore, the latter can be improved by the proposed second stage by up to 2 dB. It is shown that the ISDIC schemes with subsequent second stage can reach the MF bound up to a fraction of a decibel for all analyzed channels and 4-ary quadrature amplitude modulation (QAM) transmission, and up to 1 dB for 16-ary QAM transmission. In the work of Gerstacker , it has been reported that MF ISDIC performs very well for highly dispersive channels, e.g., with 60 symbol-spaced taps, but for channels of moderate dispersion, an error floor occurs. By utilizing performance bounds for delayed decision-feedback sequence estimation (DDFSE), we show that now for channels of moderate length, e.g., 20 symbol-spaced taps, even low-complexity MF ISDIC with the proposed subsequent second stage outperforms a DDFSE approach with states.

In this paper, we propose a low-complexity pseudo-de-quantization based (PDQ-based) iterative receiver in massive multi-user multi-input multi-output (MU-MIMO) systems with low resolution analog-to-digital converters (ADCs). First, a unified framework of massive MU-MIMO system with low resolution ADCs is presented. Then, a low-complexity iterative algorithm applying iterative soft decision interference cancellation (ISDIC) is introduced, where a soft information updating scheme is proposed for the PQD-based optimal detection and an iterative architecture of PDQ-based receiver is given. After that, the effect of quantization step size on the performance of PDQ-based iterative receiver is discussed and a new method is proposed to optimize the quantization step with given resolution ADCs. Finally, simulation results show that the PDQ-based iterative receiver can achieve significant improvement with iterative processing and provide a noticeable performance gain over the MMSE-ISDIC receiver with low resolution ADCs.

This paper studies the performance of iterative multiuser decoding, interference cancellation, and channel estimation techniques applicable to third generation WCDMA systems. The concept uses a posteriori probabilities of code symbols to enhance detection, decoding and channel estimation in an iterative fashion. The performance is analyzed in a multipath channel with simulations. It is seen that the proposed concept combats multiuser interference effectively and performs almost as well as single user systems.

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.

In this paper, we investigate the soft iterative decoding technique for ternary recoding systems with run-length-limited (RLL) constraints. We employ a simple binary-to-ternary RLL encoder following the LDPC (low density parity check) encoder. In the decoder, the iteratively passing of soft information between the LDPC decoder and a detector is used, where the detector is constructed for a combination of the RLL encoder, PLM (pulse length modulation) precoder and the partial response channel. We provide two different decoding algorithms. For one of the decoding algorithm, we are able to obtain bit-error-rate performance which is inferior to the comparable system without considering the RLL constraint for the high sign-to-noise ratio (SNR) regime and is better for the low-to-moderate SNR regime.

Based on the minimum mean square error (MMSE) detection with iterative soft interference cancellation (SoIC), we propose an adaptive MMSE (A-MMSE) algorithm which acts as an MMSE operator at the beginning of iteration and a maximum ratio combination (MRC) when the interference is nearly cancelled. In our algorithm, a modified metric matrix based on the reliability of soft information from the decoder output is multiplied by the interference part of channel correlation matrix to update the detection operator. The simulation results have shown that this A-MMSE iterative SoIC algorithm can achieve significant performance advantage over the traditional MMSE iterative SoIC algorithm.

A novel receiver is proposed for CDMA links with recursive turbo coding, based on a soft-input soft-output multistage interference canceler (IC) and an iterative turbo decoder (TD). After performing IC and TD a first time, the coder state estimates available from the two soft-output Viterbi algorithms in the last TD iteration are exploited to calculate the data bits and the redundancy bits. These are employed for preliminary hard cancellation in the first iteration of a supplementary IC run, and then the TD is run again. An alternative solution with soft feedback is obtained by approximating the reliabilities of the redundancy bits with those of the coder state estimates, and using them for preliminary soft cancellation. Simulation results show the effectiveness of the proposed solutions.

This paper presents a comparative study of single-carrier (SC)-frequency division multiple access (FDMA) and OFDMA using turbo soft interference cancellation (SIC) from the viewpoint of throughput for multiple-input multiple-output (MIMO) spatial division multiplexing (SDM). In turbo SIC, interference from other transmission streams is generated based on the log-likelihood ratio (LLR) at the turbo decoder output and subtracted from the received signal iteratively associated with a linear minimum mean-square error (LMMSE) filter. Hence, we compare the throughput performance levels of methods for generating soft-symbol estimates of other streams based on the extrinsic LLR and the a posteriori LLR at the Max-Log-MAP decoder output. We show by computer simulation the difference in performance of turbo SIC based on the a posteriori LLR and extrinsic LLR from the viewpoints of the average block error rate and throughput. We also show that the OFDMA yields superior throughput performance compared to SC-FDMA with 16QAM and 64QAM schemes when using turbo SIC for 2-by-2 to 8-by-8 MIMO SDM.

For the Gaussian multiple access channel, we studied a joint channel decoder and multiple access interference canceller. For each user, the decoder output was used to generate a soft estimate of the user signal. The estimate was improved by iterating decoding and cancellation until convergence was reached. A final, hard decoding was then performed. Simulation results are presented.

Performance analysis of multicell multiuser MIMO MC DS/CDMA system with MMSE user-ordered SIC technique

In this paper, modeling and simulation of a receiver with interference cancellation technique for the downlink transmission of multiple input multiple output (MIMO) multicarrier code-division multiple access (MC-CDMA) based cognitive radio is investigated. Synchronous transmission of multiple users’ signals over frequency selective Rayleigh channel is considered. Linear MMSE frequency domain equalization is utilized to overcome channel effect. Orthogonal Variable Spreading Factor (OVSF) codes are used to support users of different data bit rates. An adaptive subcarrier deactivation method is proposed for deactivating a set of subcarriers used for downlink transmission by the cognitive radio base station that overlap with primary user’s transmission in order to avoid causing interference to the primary user. Orthogonality between the spreading codes is lost due to subcarrier deactivation which causes interference among secondary cognitive radio users. In this paper a modified Group-wise Successive Interference Cancellation (m-GSIC) technique is proposed to suppress the interference caused due to subcarrier deactivation. The simulation results show that the proposed interference cancellation technique results in better BER performance over conventional interference cancellation techniques

This paper presents an optimized turbo equalizer based on Laot's turbo equalization scheme by means of soft intersymbol interference (ISI) cancellation. Due to Berrou's turbo-principle, common turbo equalization schemes use the extrinsic information as a-priori input for the equalizer. In contrast to this, we propose the use of the full soft output channel decoder's information for the turbo interference canceller. The performance of this improved turbo equalizer by means of soft interference cancellation is evaluated in the environment of two channels, which introduce severe ISI. Simulation results show that the proposed turbo equalizer achieves a performance gain of up to 4 dB compared to Laot's original approach. The performance gain is achieved without any increase of computational complexity. Moreover, we apply a modified extrinsic information transfer (EXIT) chart to the proposed turbo equalizer and illustrate the convergence advantages in comparison to Laot's approach.

In this paper we derive a novel extrinsic information transfer (EXIT) function for a channel estimator that uses both pilot symbols and soft decoding information. We consider a CDMA system with forward error correction that employs iterative multiuser detection based on soft interference cancellation and single user decoding. The J function is useful in convergence analysis as it provides a mapping between mutual information and variance. J functions are derived for both the flat-fading Rayleigh channel and channel estimate, allowing the a priori inputs to the channel estimator, interference canceller and decoder to be modeled. Algorithms for obtaining the EXIT functions for the three receiver blocks are proposed. We then combine the EXIT functions to create an EXIT chart that can be used to predict the behaviour of the overall receiver. Simulations are run for a 3GPP-like receiver in a mobile environment with a vehicle speed of 100 km/h. It is found that under these conditions, the detector trajectory closely matches our proposed EXIT chart.

An iterative receiver structure is proposed for decoding multiuser information data in a turbo-coded DS-CDMA system. The receiver performs two successive soft-output decisions, achieved by a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in turbo decoding. A low-complexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft-interference cancellation and instantaneous linear MMSE filtering. Turbo multiuser detectors for both the simple synchronous CDMA channels and the more complex asynchronous multipath CDMA channels are developed.