Convergence and errors in turbo-decoding

Turbo-codes are decoded using iterative decoding algorithms with somewhat elusive convergence properties. In this paper, the modes of convergence observed in an extensive series of turbo-code simulations are categorized by examining the bit convergence behavior for each frame. Based on these results, a new method for decoder termination based on average log-likelihood ratios is presented and compared with other methods. As an application, a selective repeat automatic repeat request (ARQ) system is considered     

The geometry of turbo-decoding dynamics

The spectacular performance offered by turbo codes sparked intense interest in them. A considerable amount of research has simplified, formalized, and extended the ideas inherent in the original turbo code construction. Nevertheless, the nature of the relatively simple ad hoc turbo-decoding algorithm has remained something of a mystery. We present a geometric interpretation of the turbo-decoding algorithm. The geometric perspective clearly indicates the relationship between turbo-decoding and maximum-likelihood decoding. Analysis of the geometry leads to new results concerning existence of fixed points, conditions for uniqueness, conditions for stability, and proximity to maximum-likelihood decoding     

Coded continuous phase modulation with low-complexity noncoherentreception

Coded continuous phase modulation based on a feedback-free modulator with noncoherent detection is discussed. Low-complexity receiver processing is achieved by using only two or three linear filters for demodulation and applying noncoherent sequence estimation with reduced-state Viterbi decoding and simple branch metric calculation. Overall, the proposed noncoherent receiver provides significant advantages over previously presented approaches     

A low-complexity max-log-MAP detector

A low-complexity soft-output Max-log-MAP (maximum a posteriori probability) detector for N2-QAM is exemplified for a 64-QAM 2×M multiple-input-multiple-output (MIMO) system, exhibiting the following novel features: (1) hierarchical formulation of the exhaustive metric calculation to reduce the number of candidate tests by a factor of N per received symbol, and (2) multiplier-free implementation of the exhaustive search with 8-fold parallelization. The computational complexity is reduced by a factor of about 250, resulting in a chip area of 0.031 mm2 using 65nm CMOS.     

具有低复杂度的 LDPC 已编码非相干酉空时调制系统设计简

提出一种克服无线信道瑞利衰落和高斯白噪声干扰的非相干编码调制MIMO系统方案。纠错码采用IEEE 802.16e中的非规则QC-LDPC码,非相干调制采用基于三角函数的酉空时调制(SC-USTM)。在接收端,推导出SC-USTM的最大后验概率（MAP）解调算法;为了降低复杂度,构造了SC-USTM的双解调器方案;为了改善双解调的性能,在置信传播（BP）迭代解码器和MAP解调器之间引入了迭代反馈机制。仿真实验表明LDPC已编码SC-USTM的MIMO系统比未编码USTM的MIMO系统在 误码率时,性能改善15~17 dB,并且整个系统具有较低的计算复杂度。     

A family of low-complexity blind equalizers

Two important topics in equalizer design are its complexity and its training. We present a family of blind equalizers which, by incorporating a decomposition finite-impulse response filtering technique, can reduce the complexity of the convolution operation therein by about one half. The prototype algorithm in this equalizer family employs the prevalent Godard cost function. Several simplified algorithms are proposed, including a sign algorithm which eliminates multiplications in coefficient adaptation and a few delayed versions. We also study the convergence properties of the algorithms. For the prototype algorithm, we show that, in the limit of an infinitely long equalizer and under mild conditions on signal constellations and channel characteristics, there are only two sets of local minima on the performance surface. One of the sets is undesirable and is characterized by a equalized channel response. The other corresponds to perfect equalization, which can be reached with proper equalizer initialization. For the simplified algorithms, corresponding cost functions may not exist. Some understanding of their convergence behaviors are obtained via examination of their adaptation equations. Simulation results are presented to demonstrate the performance of the algorithms.     

On low-complexity joint-detection techniques for TD-CDMA

This paper is focused on low-complexity joint-detection (JD) algorithms, where the computational complexity is reduced using banks of discrete Fourier transformations. Mainly, we have developed a fast hybrid zero-forcing block decision-feedback equalizer (H-ZF-BDFE) combining the Fourier-based JD scheme (IEEE J. Sel. Areas Commun., vol. 19, p. 1461, 2001) with the fast decision-feedback structure (F-ZF-BDFE) IEEE J. Sel. Areas Commun., vol. 19, p. 245, 2001. In a Rayleigh frequency-selective fading channel, the new structure yields an improvement in E/sub b//N/sub 0/ of almost 2 dB against the fast block linear equalizer (BLE) (IEEE J. Sel. Areas Commun., vol. 19, p. 1461, 2001) at a bit error rate (BER)=10/sup -3/. Moreover, it has the same performance as the F-ZF-BDFE, but the computational complexity for the signal processing is reduced by almost 50%.     

Designing low-complexity equalizers for wireless systems

The demand on wireless communications to provide high data rates, high mobility, and high quality of service poses more challenges for designers. To contend with deleterious channel fading effects, both the transmitter and the receiver must be designed appropriately to exploit the diversity embedded in the channels. From the perspective of receiver design, the ultimate goal is to achieve both low complexity and high performance. In this article, we first summarize the complexity and performance of low-complexity receivers, including linear equalizers and decision feedback equalizers, and then we reveal the fundamental condition when LEs and DFEs collect the same diversity as the maximum-likelihood equalizer. Recently, lattice reduction techniques were introduced to enhance the performance of low-complexity equalizers without increasing the complexity significantly. Thus, we also provide a comprehensive review of LR-aided low-complexity equalizers and analyze their performance. Furthermore, we describe the architecture and initial results of a very-large-scale-integration implementation of an LR algorithm.     

低复杂度的快速降尺寸视频转码算法

为了降低降尺寸视频转码的运算复杂度,提出一种基于支持向量机的快速转码模式决策算法。首先从输入的高分辨率视频码流中的编码信息里选取多维特征向量,并选择与模式特征匹配的核函数训练SVM分类器模型,建立高分辨率视频编码信息与降尺寸视频宏块编码模式之间的相关性;然后构建分层式SVM分类器对降尺寸视频中宏块模式进行阶梯式预测分类,以此缩减预测模式数量,提高转码效率。实验结果证明,算法可以节省高达67.31%的运算量,同时保证转码后视频的高质量。     

A low-complexity intracardiac electrogram compression algorithm

Implantable cardioverter defibrillators (ICD's) detect, diagnose and treat the potentially fatal heart arrhythmias known as bradycardia, ventricular tachycardia (VT), and ventricular fibrillation (VF) in cases where these arrhythmias are resistant to surgical and drug-based treatments by direct sensing and electrical stimulation of the heart muscle. Since the ICD is implanted, power consumption, reliability, and size are severe design constraints. This paper targets the problems associated with increasing the signal recording capabilities of an ICD. A data-compression algorithm is described which has been optimized for low power consumption and high reliability implementation. Reliance on a patient's morphology or that of a population of patients is avoided by adapting to the intracardiac electrogram (ICEG) amplitude and phase variations and by using adaptive scalar quantization. The algorithm is compared to alternative compression algorithms which are also patient independent using a subset of VT arrhythmias from a data base of 146 patients. At low distortion the algorithm is closest to the Shannon lower bound achieving an average of 3.5 b/sample at 5% root mean square distortion for a 250-Hz sample rate. At higher distortion vector quantization and Karhunen-Loeve Transform approaches are superior but at the cost of considerable additional computational complexity     

A low-complexity noncoherent iterative space-time demodulator

Turbo coded unitary space-time modulation (USTM) can provide large coding gain as compared to uncoded USTM. Because the noncoherent space-time maximum a posteriori demodulator is very complicated, in this letter, we propose a new low-complexity noncoherent iterative space-time demodulator for the USTM constructed from pilot symbol-assisted modulation. The proposed demodulator utilizes both hard and soft decisions from the turbo decoder to simplify the computational task as well as produce reliable soft outputs. Several examples demonstrate that this demodulator has both low complexity and good error performance.     

A low-complexity space-time OFDM multiuser system

Exploiting the Fourier basis structure both in the space and the time domains, we develop a low-complexity multiuser space-time coding scheme, multiuser (MU) angle-frequency coding scheme (MU-AFCS), to properly schedule the data streams of each user with respect to its corresponding angle-frequency channel structure for downlink wireless systems. With the proposed approach, a large amount of space resource left over by one user, in clustered multipath wireless channels, can be easily identified and used by the others without serious signal collision in the space domain. In doing so, low channel capacity resulting from poor channel structures in systems, allowing only single-user transmission at a time, can be greatly boosted. The key advantage of this approach is that only limited feedback of channel state information to the transmitter is required while multiuser macro-diversity is sufficiently exploited. In addition, the complexity of the proposed approach is much lower than that of the existing ones serving similar purposes. Through theoretical analyses and computer simulations, we demonstrate that the MU-AFCS can significantly increase the channel capacity as compared to the traditional orthogonal resource division MU multiple-input multiple-output (MIMO) systems.     

Iterative low-complexity receiver for the UMTS downlink

In this paper, an iterative low-complexity receiver is proposed for Code Division Multiple Access (cdma) systems with small spreading factors. Theumts (Universal Mobile Télecommunication System) radio interface based oncdma has been designed to offer a wide range of data rates using variable spreading factors. High data rate services are obtained by using small spreading factors. For such services, the spreading sequences have bad autocor-relation properties causing the degradation of the Rake receiver performance because of the InterSymbol Interférence (isi). In order to improve the receiver performance, we propose to add a Decision Feedback Sequence Estimation (dfse) equalizer at the Rake receiver output. Thedfse is a low complexity equalizer which is able to take into accounta priori probability ratios and to deliver a posteriori probability ratios on bits in order to exchange soft information with the channel decoder, so that the proposed receiver benefits from the turbo-processing gains. Channel estimation is also treated in an iterative fashion. The complete receiver is well suited to theumts downlink system as it drastically reduces theisi while keeping a reasonable computational complexity.     

On low-complexity space-time coding for quasi-static channels

We propose a new space-time coding scheme for the quasi-static multiple-antenna channel with perfect channel state information at the receiver and no channel state information at the transmitter. In our scheme, codewords produced by a trellis encoder are formatted into space-time codeword arrays such that decoding can be implemented efficiently by minimum mean-square error (MMSE) decision-feedback interference mitigation coupled with Viterbi decoding, through the use of per-survivor processing. We discuss the code design for the new scheme, and show that finding codes with optimal diversity is much easier than for conventional trellis space-time codes (STCs). We provide an upper bound on the word-error rate (WER) of our scheme which is both accurate and easy to evaluate. Then, we find upper and lower bounds on the information outage probability with discrete independent and identically distributed (i.i.d). inputs (as opposed to Gaussian inputs, as in most previous works) and we show that the MMSE front-end yields a large advantage over the whitened matched filter (i.e., zero-forcing) front-end. Finally, we provide a comprehensive performance/complexity comparison of our scheme with coded vertical Bell Labs layered space-time (V-BLAST) architecture and with the recently proposed threaded space-time codes. We also discuss the concatenation of our scheme with block space-time precoders, such as the linear dispersion codes.     

低复杂度2400b／s声码器

声码器因其极低的语音压缩编码速率而得到广泛应用。但是声码器自身的内在算法特性使其通常对系统的运算量和存储量要求很高,这就为其在某些场合的应用造成了困难。为了解决以上问题,提出了一种基于正弦激励线性预测（SELP）模型的2．4kb／s低复杂度高质量声码器算法。新提出的声码器在原有SELP声码器算法基础上采取了一系列有针对性的改进措施,在大幅缩减算法时间复杂度和空间复杂度的同时,保证了很高的合成语音质量。     

一种低复杂度的连续相位调制序列检测接收机

连续相位调制（CPM）以其较高的功率有效性和带宽利用率,在无线通信领域尤其是军事通信领域得到广泛应用。给出了一种连续相位调制的低复杂度序列检测接收机,通过对发送信号进行正交处理来减少接收机前端的匹配滤波器个数;在序列检测阶段通过状态融合减少维特比译码的状态数,然后结合判决反馈进行检测。分析和仿真表明,该算法与最优最大似然序列检测（MLSD）相比,能有效地简化运算复杂度而性能损失较小。     

A low-complexity image compression approach with single spatial prediction mode and transform

The well-known low-complexity JPEG and the newer JPEG-XR systems are based on block-based transform and simple transform-domain coefficient prediction algorithms. Higher complexity image compression algorithms, obtainable from intra-frame coding tools of video coders H.264 or HEVC, are based on multiple block-based spatial-domain prediction modes and transforms. This paper explores an alternative low-complexity image compression approach based on a single spatial-domain prediction mode and transform, which are designed based on a global image model. In our experiments, the proposed single-mode approach uses an average 20.5 % lower bit-rate than a standard low-complexity single-mode image coder that uses only conventional DC spatial prediction and 2-D DCT. It also does not suffer from blocking effects at low bit-rates.     

Onboard low-complexity compression of solar stereo images

We propose an adaptive distributed compression solution using particle filtering that tracks correlation, as well as performing disparity estimation, at the decoder side. The proposed algorithm is tested on the stereo solar images captured by the twin satellites system of NASA's Solar TErrestrial RElations Observatory (STEREO) project. Our experimental results show improved compression performance w.r.t. to a benchmark compression scheme, accurate correlation estimation by our proposed particle-based belief propagation algorithm, and significant peak signal-to-noise ratio improvement over traditional separate bit-plane decoding without dynamic correlation and disparity estimation.     

A low-complexity ML channel estimator for OFDM

Orthogonal frequency-division multiplexing with cyclic prefix enables low-cost frequency-domain mitigation of multipath distortion. However, to determine the equalizer coefficients, knowledge of the channel frequency response is required. While a straightforward approach is to measure the response to a known pilot symbol sequence, existing literature reports a significant performance gain when exploiting the frequency correlation properties of the channel. Expressing this correlation by the finite delay spread, we build a deterministic model parametrized by the channel impulse response and, based on this model, derive the maximum-likelihood channel estimator. In addition to being optimal (up to the modeling error), this estimator receives an elegant time-frequency interpretation. As a result, it has a significantly lower complexity than previously published methods.     

A low-complexity enhancement to suboptimal CDMA receivers

Conventional matched-filter detectors for code-division multiple-access (CDMA) systems suffer from multiple-access interference (MAI) caused by nonzero correlation between spreading codes at the receiver. A host of advanced detector structures have been proposed to reduce the effect of MAI and, hence, improve performance. However, most multiuser detectors suffer from their relatively complex implementations. A simple method is proposed to improve the performance of the conventional detector by detecting and correcting decision errors at its output without the use of forward error correcting (FEC) codes. The proposed post-detection error control method is shown to substantially improve the performance of the conventional detector, but has a much lower complexity than most other multiuser detectors.