Soft Interference Cancellation Research Articles

Iterative decoding and demodulation for single-antenna vector OFDM systems

Vector orthogonal frequency-division multiplexing (OFDM) for single-antenna systems generalizes OFDM in that it converts an intersymbol interference (ISI) channel into multiple ISI-free vector channels and involves vector channel matrices instead of channel coefficients in its one-tap equalization (demodulation). Over frequency-selective fading channels, vector OFDM with increasing vector length implicitly offers more signal space diversity advantages over OFDM. However, the complexity of the brute force demodulator increases exponentially in terms of the vector size. In this paper, iterative demodulation and decoding for convolutionally coded vector OFDM systems are studied, and in particular, low-complexity demodulation schemes that employ linear cancellation and feature linear complexity in terms of the vector length are proposed and compared. Following the introduction of demodulators with ideal linear cancellation as benchmark, demodulators with hard and soft interference cancellation and their initialization options are studied. The demodulator with soft interference cancellation dynamically updates the symbol estimates and their instantaneous reliability measure based on the code bit probabilistic information passed from preceding iterations. It alleviates error propagation that is detrimental to the demodulator with hard interference cancellation and shows performance close to that of the brute force demodulator, with great overall complexity reduction. Extensive simulation results demonstrate how the choice among the presented schemes and the vector length affect the convergence rate and the achievable performance of the iterative receivers

Iterative Demodulation of M-Ary Orthogonal Signaling Formats in Coded DS-CDMA Systems with Soft Interference Cancellation and Channel Estimation

The system under study is a convolutionally coded and orthogonally modulated DS-CDMA system over time-varying frequency-selective Rayleigh fading channels in multiuser environments. Iterative soft demodulation and decoding using the Turbo principle can be applied to such a system to increase the system capacity and performance. To combat multiple access interference (MAI), we incorporate the interference cancellation (IC) and decision-directed channel estimation (CE) in the demodulator. However, both IC and CE are subject to performance degradation due to incorrect decisions. In order to prevent error propagation from the decision feedback, soft interference cancellation and channel estimation assisted demodulation is proposed in this paper. The performance of this strategy is evaluated numerically and proved to be superior to the hard decision-directed approach with a minor increase in comolexity.

Optimization of irregular repeat accumulate codes for MIMO systems with iterative receivers

This paper takes into account the design optimization of the random-like ensemble of irregular repeat accumulate (IRA) codes for multiple-input multiple-output (MIMO) communication systems employing iterative receivers. First, the density evolution-based procedure for optimizing the IRA code ensemble is presented. An approximation method based on linear programming is adopted to design an IRA code with the extrinsic information transfer (EXIT) chart matched to that of the soft MIMO demodulator. The authors then reveal the relationship between the IRA codes and the low-density parity-check (LDPC) codes. With a code ensemble mapping relationship between an IRA code and an LDPC code, a quasi-optimal IRA code can be obtained by transforming an optimal LDPC code designed for MIMO systems. Two types of soft MIMO detectors are treated, namely, the maximum a posteriori (MAP) detector and the soft interference canceller with linear MMSE filtering (SIC-MMSE). The results show that with the MAP receiver the designed IRA codes can perform within 1 dB from the ergodic capacities of the MIMO systems under consideration. The authors also treat the short-length IRA code design for block fading MIMO channels. They adopt design techniques for short-length LDPC codes to improve the performance of the short-length IRA code and to reduce the error floor.

Adaptive MMSE Algorithm Used in Turbo Iterative SoIC of V-Blast System

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.

Monte Carlo FEXT cancellers for DSL channels

Crosstalk coupling is a severe impairment to transmission on twisted-pair copper cables such as digital subscriber telephone loops (DSLs). In this paper, we investigate the problem of far-end crosstalk (FEXT) cancellation for DSL. We propose a new soft interference canceller for coordinated self-FEXT cancellation across multiple twisted-pairs. The proposed receiver treats the FEXT-impaired channel as a multiple-input multiple-output (MIMO) channel; it uses the sequential Monte Carlo (SMC) technique to exploit the sequential structure inherent in iterative cancellation techniques for MIMO decoding. On telephone loops with strong FEXT, the proposed algorithm shows 0.5-dB coding gain with respect to previously proposed techniques. Furthermore, being soft-input-soft-output in nature, the proposed SMC receiver can iteratively exchange extrinsic information with a MAP decoder to successively cancel FEXT, leading to a turbo FEXT canceller.

Soft-PIC multiuser detection in MC-CDMA uplink system

It is necessary for an MC-CDMA uplink receiver to employ MUD (multiuser detection) in a frequency selective fading channel. After analyzing the algorithm of PIC (parallel interference cancellation) MUD, a novel MUD scheme, Soft-PIC (soft parallel interference cancellation) is proposed. Based on the reliability of each detected user signal in the former stage, this Soft-PIC detection scheme substitutes a soft decision of the variable for the hard decision in PIC scheme. Compared with the PIC scheme, it can reconstruct the interference signals more accurately and eliminate MAI (multiple access interference) in a more efficient way. PIC is one of the most practical schemes in numerous multiuser detection technologies. However, Soft-PIC as an improved PIC scheme deserves further study.

LDPC Codes for Fading Gaussian Broadcast Channels

In this work, we study coding over a class of two-user broadcast channels (BCs) with additive white Gaussian noise and multiplicative fading known at the receivers only. Joint decoding of low-density parity-check (LDPC) codes is analyzed. The message update rule at the mapping node linking the users' codes is derived and is found to exhibit an interesting soft interference cancellation property. High performance codes are found using the differential evolution optimization technique and extrinsic information transfer analysis adapted to our multiuser setting. The optimized codes have rates very close to the boundary of the achievable region for binary constrained input for both faded and unfaded channels. Simulation results for moderate block lengths show that our codes operate within less than 1 dB of their respective threshold.

Comparison of vector detection algorithms for MIMO-OFDM

The aim of this paper is to compare detection algorithms for MIMO-OFDM transmissions by simulation. For a coded transmission, it turns out that the list sphere detector, which has been considered a favorite for many applications, is far more complex than other methods yielding the same BER performance. Moreover, iterative equalization techniques based on soft interference cancellation connected with decoding via the well-known turbo principle can approach the matched filter bound for the 4 × 4 MIMO channels used in the simulation. Specifically, a block Rayleigh fading channel and a time-invariant frequency-selective channel with and without spreading in frequency and space are considered.

Receiver Design for Spatially Layered Downlink MC-CDMA System

One of the most promising candidates for the air interface of the upcoming multiuser broadband communication systems is multicarrier code-division multiple-access (MC-CDMA). In the pursuit for higher data rates and reliability of communication in fading environments, a vast amount of spatial multiplexing and transmit diversity techniques have been recently proposed for the multi-antenna systems. In this paper, we study the receiver design for downlink (base station to mobile) multi-antenna MC-CDMA when layered space-time (LST) architectures and space-time coding techniques are utilized. Due to the inevitable complexity restrictions, sub-optimal chip and symbol-level minimum mean squared error (MMSE) based receiver front-ends are derived. For further improvements in performance, an iterative MMSE based detector with soft co-antenna interference cancellation is designed. Performance of the different algorithms is evaluated in spatially correlated and uncorrelated fading channels. The results demonstrated that the proposed multi-antenna schemes combining recently discovered space-time turbo coded modulation and group-wise LST architectures are able to provide significant improvements both in error rate performance and spectral efficiency over the conventional single-antenna system also in spatially correlated environments. By utilizing iterative processing at the receiver, the considered 4/spl times/4 system was found to perform within 1.5 dB from the channel outage capacity in a spatially uncorrelated frequency-selective fading channel.

Performance Enhancement of SC/S-MMSE Turbo Receiver for MIMO-SDM/OFDM Transmission

This paper aims to improve the performance of the soft canceller followed by simplified minimum mean-square error (SC/S-MMSE) turbo receiver [11] for multiple-input and multiple-output space-division multiplexing/orthogonal frequency division multiplexing (MIMO-SDM/OFDM) transmission; it performs iterative parallel soft interference cancellation and MMSE filtering, and stream-wise soft-input and soft-output decoding. For this aim, we newly introduce two detection techniques: 1) serial interference cancellation, and 2) cyclic redundancy check (CRC)-assisted interference cancellation and MMSE filter tap computation. Various computer simulations are conducted to evaluate the performance enhancement obtained via the use of the two detection techniques. The computer simulation results show that this paper's proposed serial SC/S-MMSE turbo receiver with CRC achieves frame error rate (FER) performance gain over existing MIMO receivers (MMSE receiver, V-BLAST receiver, parallel SC/MMSE-matched filter (MF) turbo receiver, and parallel SC/S-MMSE turbo receiver) for QPSK, 16QAM and 64QAM modulation while keeping the comparable complexity order.

Optimization of LDPC-coded turbo CDMA systems

We consider the analysis and design of low-density parity-check (LDPC) codes for turbo multiuser detection in multipath code division multiple access (CDMA) channels. We develop techniques for computing the probability density function (pdf) of the extrinsic messages at the output of the soft-input soft-output (SISO) multiuser detectors as a function of the pdf of input extrinsic messages, user spreading codes, channel impulse responses, and signal-to-noise ratios. Of particular interest is the soft interference cancellation plus minimum mean square error (SIC-MMSE) multiuser detector, for which the pdf of the extrinsic messages can be characterized analytically. For the case of additive white Gaussian noise (AWGN) channels, the extrinsic messages can be well approximated as symmetric Gaussian distributed. For the case of asynchronous multipath fading channels, the extrinsic messages can be approximated by a mixture of symmetric Gaussian distributions. We show that the expectation-maximization (EM) algorithm can be used to compute the parameters of this mixture. Using these techniques, we are able to accurately compute the thresholds for LDPC codes and design good irregular LDPC codes. Simulation results are in good agreement with the computed thresholds, and the designed irregular LDPC codes outperform regular ones significantly.

A linear front end for iterative soft interference cancellation and decoding in coded CDMA

In this paper, after a description of the principles of iterative (turbo) multiuser detection for CDMA systems, a new structure, based on a linear user separator, is introduced and analyzed. The new user separator processes the matched filter outputs according to a minimum mean-square-error (MMSE) criterion for the first few iterations, while it bypasses the MMSE filter when the interferers' bits are known with some reliability. This new receiver is tested by simulation and compared with other receivers previously introduced in literature. Also, a way of studying and designing iterative multiuser receivers is given by adapting the method of EXtrinsic Information Transfer (EXIT) charts. With the EXIT charts, the receiver performance (in an information-theoretic sense) can be evaluated as it evolves along the iterations. Thus, the receiver threshold can be reliably estimated.

Estimating the PDF of the SIC-MMSE equalizer output and its applications in designing LDPC codes with turbo equalization

We consider the analysis and design of low-density parity check (LDPC) codes for intersymbol interference (ISI) channels when used with soft interference cancellation plus linear minimum mean-square error filtering (SIC-MMSE) turbo equalization. We discuss techniques to compute the probability density function (pdf) of the extrinsic information at the output of the SIC-MMSE equalizer as a function of pdf of the input extrinsic information, channel impulse response, and the signal-to-noise ratio. For static ISI channels, we show that the output pdf can be modeled as symmetric Gaussian, and show that the mean can be evaluated without simulating the equalizer. For channels with long memory, we propose to use the unscented transform technique to compute the mean, which significantly reduces the computation required. Finally, for fading channels, we model the pdf by a mixture of symmetric Gaussian densities. Using these techniques, we are able to fairly accurately compute the thresholds for LDPC codes and design good irregular LDPC codes. Simulation results are in good agreement with the computed thresholds and the designed irregular LDPC codes outperform regular ones significantly.

EXIT chart analysis of turbo multiuser detection

We study the mutual information transfer characteristics of the soft interference cancellation (SIC) multiuser detectors (MUD) for coded code-division multiple-access (CDMA) systems in synchronous additive white Gaussian noise (AWGN) and asynchronous multipath fading channels. Based on the extrinsic information transfer (EXIT) chart technique, we compare the asymptotic (as the block length goes to infinity) performance and the convergence behavior of different joint detection schemes which are highly dependent upon different system parameters and channel conditions. A framework of using the EXIT chart to analyze the coding-spreading tradeoff is also addressed for turbo multiuser detection. Results show that the SIC-minimum mean square error (MMSE) MUD generally provides a higher spectral efficiency than the SIC-match filter (MF) MUD. Moreover, the impact of finite block length on the convergence behavior of turbo MUD is also analyzed using a consistent Gaussian approximation on the distribution of the output extrinsic information.

Simple iterative methods to exploit the signal-space diversity

Signal-space diversity is a power- and bandwidth-efficient diversity technique. To exploit the signal-space diversity, joint maximum-likelihood (ML) detection at the receiver is usually needed, where the complexity grows exponentially with the dimension of the lattice. In this letter, we propose a serial concatenated scheme and two simple iterative methods to exploit the signal-space diversity. The simple iterative methods are based on the idea of soft interference cancellation. The first iterative method is based on a vector Gaussian approximation, while the second one is based on a scalar Gaussian approximation. The complexity of the first iterative method grows cubically with the dimension of the lattice, and the simulations show that its performance approaches that of the optimal maximum a posteriori detection method. The complexity of the second iterative method grows linearly with the dimension of the lattice, and the simulations show that when the dimension of the lattice N=32, at bit-error rate =10/sup -5/, the performance gap between the Rayleigh fading channel and the Gaussian channel is only 0.3 dB.

Comparison of bit error rate and convergence of four different iterative receivers for wireless OFDM-CDM

In this paper we investigate four different detection methods for Orthogonal Frequency Division Multiplexing Code Division Multiplexing (OFDM-CDM). The first method applies the Maximum A Posteriori (MAP) algorithm. The second and the third method use detection algorithms which are based on the Minimum Mean Squared Error (MMSE) criterion. The second method uses the Soft Symbol Based (SSB)-MMSE algorithm whereas the third receiver applies the Soft Chip Based (SCB)-MMSE algorithm. The fourth detection method uses a hybrid detection scheme which applies the SCB-MMSE and the Soft Interference Cancellation (SIC) detection method. We concatenate each detection method with the outer channel decoder which allows for iterative decoding at the receiver side. This system can be considered as a serially concatenated iterative decoding scheme whereby the inner decoder is replaced by the respective detection method. For the comparison of the four different detection methods the Bit Error Rate as a function of the signal to noise ratio is used as a performance measure. In addition the convergence of the iterative decoding loop is studied with the Extrinsic Information Transfer (EXIT) chart.

Coding-Spreading Tradeoff in LDPC-Coded CDMA With Turbo Multiuser Detection

We consider the problem of coding-spreading tradeoff in code-division multiple-access (CDMA) systems employing multiuser joint decoding. In particular, we assume the users employing the capacity-achieving low-density parity-check (LDPC) codes and at the receiver, turbo multiuser detection is used to implement joint decoding. We solve the coding-spreading optimization based on the extrinsic information signal-to-noise ratio (SNR) evolution curves for the soft-input soft-output (SISO) multiuser detectors and the SISO LDPC decoders. Two types of SISO multiuser detectors are treated, namely, the soft interference cancellation minimum mean-square-error (SIC-MMSE) detector and the soft interference cancellation matched filter (SIC-MF) detector. Moreover, both single-cell and multicell scenarios are considered. For each of these cases, we are able to characterize the extrinsic information SNR analytically, for both finite-size systems and for the so-called large systems where asymptotic performance results must be evoked. Our analysis indicates that the SIC-MMSE-based system offers a significant gain in spectral efficiency compared with the SIC-MF counterpart, in both single-cell and multicell scenarios. This is in contrast to the single-user decoding case, where it has been shown that the MMSE detector offers little advantage over the conventional matched-filter in terms of capacity in multicell scenario. Moreover, the results on coding-spreading tradeoff for finite-size systems and large systems match very well.

Multiuser Detection of DS-CDMA Signals Using Partial Parallel Interference Cancellation in Satellite Communications

Multiuser detection (MUD) using parallel interference cancellation (PIC) technique provides a good complexity, latency, and performance compromise. This technique is suitable for satellite systems using either code-division multiple-access (CDMA) or a combination of time-division multiple-access (TDMA) and CDMA. We offer a new scheme that is a combination of soft and hard PIC detectors whose performance is superior to that of the other famous suboptimal detectors. In soft partial parallel interference cancellation (PPIC), in the first few stages, when the performance is still poor, the accurate knowledge of power and phase cannot be of much use. However, in the following stages, accurate power and phase estimation can improve the performance. This coincides with the time when the decisions are reliable enough to be used for parameter estimation. In our scheme, after a few stages of soft interference cancellation (IC), estimation of the parameters will start. Having these estimates, in the subsequent stages hard IC is performed. The complexity of this scheme grows linearly with the number of users. Moreover, this scheme is much faster than other receivers such as successive interference cancellation (SIC). PIC detectors are usually studied in equal-power case, i.e., a perfect power control scheme is assumed. In this paper, PIC detector in a near-far condition where user signals arrive at the receiver with different power levels is also investigated.

Soft multiuser demodulation and iterative decoding for FH/SSMA with a block turbo code

The number of users that can be supported by frequency-hopped, spread-spectrum multiple-access systems can be increased greatly by using multiuser demodulation and iterative decoding. In the receiver employed hard-decision multiuser demodulation followed by iterative decoding, users exchange decoded information with each other. Additional information from multiuser demodulation in the first decoding iteration is limited by the hard-decision output of the multiuser demodulator. The error-correction used was an errors-and-erasures Reed-Solomon (RS) decoder. We revisit hard-decision demodulation and conventional RS decoding. Hard-decision multiuser demodulation is modified to provide a soft output, which is then given to a nonbinary block turbo code with shortened RS codes as the constituent codes. An iterative multiuser decoding algorithm is developed to do soft multiuser interference cancellation. This soft receiver with soft demodulation and decoding is shown to be more resistant to multiuser interference and channel noise, especially at lower values of signal-to-noise ratio. The results show a great improvement in the ability of the system to support more users (more than three times in some cases), as compared with systems that erase all hits or employ hard-decision multiuser demodulation followed by RS code. We examine the proposed method for synchronous as well as asynchronous frequency-hopped systems in both AWGN and fading channels.

Iterative space-time soft interference cancellation for UMTS-FDD uplink

Two efficient pilot-aided iterative space-time interference cancellation receivers are studied in order to increase the uplink capacity of the Universal Mobile Telecommunication System (UMTS) in frequency-division duplex (FDD) mode. Both iterative schemes use low-complexity beamforming and path combining techniques associated with soft-input soft-output decoding to mitigate the multiple-access interference in space and time. The difference between the two techniques is in the way they deal with unknown channels: the addition of a space-time channel estimation in each iteration on the one hand and iterative adaptive beamforming and path combining on the other hand. Thanks to the iterative structure, the observation signal used for estimation or adaptation contains less interference from one iteration to the following, and soft estimates of coded bits are available for data-aided estimation or adaptation. A detailed complexity analysis shows that renewing beamforming and path combining in each iteration without a-priori knowledge of the channel has no significant impact on the overall complexity of one iteration. Simulations of true UMTS-FDD uplink communications over a wideband directional channel model reveal that near-single user performance can be obtained for very high system loads, whereas more conventional receivers, such as the interference canceller without beamforming and the two-dimensional RAKE receiver, fail in recovering the transmitted information.