Research on the transmission performance of turbo codes in DDO-OFDM system

With the rapid development of optical elements with large capacity and high speed,the network architecture is of great importance in determing the performance of wavelength division multiplexing passive optical network(WDM-PON).This paper proposes a switching structure based on the tunable wavelength converter(TWC)and the arrayed-waveguide grating(AWG)for WDM-PON,in order to provide the function of opitcal virtual private network(OVPN).Using the tunable wavelength converter technology,this switch structure is designed and works between the optical line terminal(OLT)and optical network units(ONUs)in the WDM-PON system.Moreover,the wavelength assignment of upstream/downstream can be realized and direct communication between ONUs is also allowed by privite wavelength channel.Simulation results show that the proposed TWC and AWG based switching structure is able to achieve OVPN function and to gain better performances in terms of bite error rate(BER)and time delay.     

A new combination of adaptive channel estimation methods and TORC equalizer in MC‐CDMA systems

Adaptive systems identification has been widely studied, but most studies have focused on the convergence of these methods. Applications of equalization systems have also received much attention. This paper presents a new combination of adaptive Broadband Radio Access Networks (BRAN) channel identification algorithms for multicarrier code division multiple access (MC‐CDMA) systems downlink equalization. In fifth‐generation (5G) wireless communications, MC‐CDMA is expected to support the associated networks. The BRAN E channel parameters, representing an outdoor scenario normalized for MC‐CDMA systems, are identified using a recursive least mean pth power algorithm with logarithmic transformation (RlogLMP). For validity and test aim, this algorithm is compared with the existing recursive least square (RLS) and least mean square (LMS) algorithms. Moreover, we use the estimated coefficients in the adaptive equalization problem. We give a review of the threshold orthogonality restoring combining (TORC) equalizer, which is coupled with the presented algorithms to counteract channel fading, as evaluated by the bit error rate (BER). Our performance results show that the RlogLMP algorithm can estimate the measured BRAN E channel with good efficiency for various values of the signal‐to‐noise ratio (SNR), as compared with the classical algorithms RLS and LMS. In adaptive equalization problems, the achieved results demonstrate that two thresholds ρ_TH in the TORC equalizer minimize the performance degradation, in terms of the BER, of the MC‐CDMA system under multipath channel fading with very good accuracy, especially if the coefficients are estimated with the specific case of the power p in the RlogLMP algorithm.     

Performance analysis of uplink spatial multiplexing MIMO MT‐CDMA over multipath fading channels

In this paper, we consider multiple‐input multiple‐output (MIMO) multi‐tone code division multiple access (MT‐CDMA) uplink transmission over multipath fading channels. The zero‐forcing vertical Bell Laboratories layered space‐time architecture (ZF V‐BLAST) algorithm and maximum ratio combining scheme are applied at the receiver. The average bit error rate (BER) expression is derived provided that the number of receive antennas is not less than that of transmit antennas. The BER expression is verified by simulations. Numerical results show that the numbers of transmit and receive antennas have significant effects on the BER performance of the considered system. Spatial and path diversity show different capabilities to improve the BER performance. The MIMO MT‐CDMA system based on the ZF V‐BLAST algorithm is capable of achieving a better BER performance and a higher capacity than the conventional MT‐CDMA system. Copyright © 2011 John Wiley & Sons, Ltd.     

AWGN信道下Turbo码解码算法的选择

在加性高斯白噪声（AWGN）信道下,采用最大后验概率(MAP)算法的Turbo码解码是误比特率最低的算法。为了降低运算量实现快速解码,Log-MAP算法、Max-Log-Map算法和线性Max-Log-Map算法分别对MAP算法进行了不同程度的简化。本文简单介绍了基于MAP算法的Turbo码解码原理,从纠正函数的角度出发归纳和比较了三种MAP类简化算法,通过纠正函数从理论上对算法性能以及对信噪比估计误差的敏感度进行了分析,对分析结果进行了仿真验证。综合解码性能和运算量,提出了Turbo码解码的算法选择方案,以及实用,简易的Turbo码解码参数设置建议。      

Enhancing data throughput and lower correlations quasi orthogonal functions for 3G CDMA systems

The forward link of the 3G code‐division multiple‐access (CDMA) system may become limited under the increasing of the number of users. The conventional channelization code, Walsh code, has not enough sizes for much possible users, therefore, the quasi orthogonal function (QOF), which can process optimal cross‐correlation with Walsh code, is considered. To obtain lower correlations and better performance in 3G CDMA system, we investigate a quasi orthogonal function on Jacket matrices in this paper. Secondly, to simplify the detector and improve the bit error rate (BER) performance, we provide a novel detection scheme for QOF CDMA system. Finally, we give a brief discussion about a simple recursive generation of the bent sequences for QOF mask function. Copyright © 2005 John Wiley & Sons, Ltd.     

Adaptive predistortion of COFDM signals for a mobile satellite channel

In this paper, we consider the optimization of the performance of QPSK and 16‐QAM coded orthogonal frequency division multiplexing (COFDM) signals over the non‐linear and mobile satellite channel. A high power amplifier and Rician flat fading channel produces non‐linear and linear distortions; an adaptive predistortion technique combined with turbo codes will reduce both types of distortion. The predistorter is based on a feedforward neural network, with the coefficients being derived using an extended Kalman filter (EKF). The conventional turbo code is used to mitigate Rician flat fading distortion and Gaussian noise. The performance over a non‐linear satellite channel indicates that QPSK COFDM followed by a predistorter provides a gain of about 1.7 dB at a BER of 3×10^?3 when compared to QPSK COFDM without the predistortion scheme and 16‐QAM COFDM provides a gain of 0.5 dB output back‐off and 1.2 dB signal to noise ratio at a BER of 3×10^?5 when compared with an adaptive predistorter based on the Harmmerstein model. We also investigate the influence of the guard time interval and Doppler frequency effect on the BER performance. When the guard interval increases from 0 to 0.125T samples and the normalized Doppler frequency is 0.001, there is a gain of 0.7 and 1 dB signal to noise ratio at a BER of 6×10^?4 for QPSK and 16‐QAM COFDM, respectively. Copyright © 2003 John Wiley & Sons, Ltd.     

Semi‐random LDPC codes for CDMA communication over non‐linear band‐limited satellite channels

This paper considers the application of low‐density parity check (LDPC) error correcting codes to code division multiple access (CDMA) systems over satellite links. The adapted LDPC codes are selected from a special class of semi‐random (SR) constructions characterized by low encoder complexity, and their performance is optimized by removing short cycles from the code bipartite graphs. Relative performance comparisons with turbo product codes (TPC) for rate 1/2 and short‐to‐moderate block sizes show some advantage for SR‐LDPC, both in terms of bit error rate and complexity requirements. CDMA systems using these SR‐LDPC codes and operating over non‐linear, band‐limited satellite links are analysed and their performance is investigated for a number of signal models and codes parameters. The numerical results show that SR‐LDPC codes can offer good capacity improvements in terms of supportable number of users at a given bit error performance. Copyright © 2006 John Wiley & Sons, Ltd.     

Power Optimization of Three Dimensional Turbo Code Using a Novel Modified Symbiotic Organism Search (MSOS) Algorithm

In modern communication system error-control coding scheme is used to elevate the immunity of noisy communication channel. Turbo code (TC) is considered as one of the significant channel coding schemes which approaches to the Shannon limit. An upgraded version of TC named as 3 dimensional turbo code (3D-TC) has been emerged as a challenging research area in recent past. Meanwhile, considerable improvement in bit error rate (BER) performance of the TC has been achieved by incorporating suitable optimization algorithms. Motivated by above research trends, a modified symbiotic organisms search (MSOS) algorithm has been proposed by changing the organism structure and selection criteria of a newly developed symbiotic organisms search (SOS) algorithm. Subsequently the proposed MSOS has been used to design an improved 3D-TC. Here an optimal power allocation scheme of a new class of 3 dimensional turbo encoder has been investigated to improve its BER characteristics mainly in high SNR regions. Furthermore, the BER performance of the proposed 3D-TC code has been compared with conventional 2D serially concatenated and parallel concatenated turbo code as well as conventional 3D-TC. Finally, the BER performance of the proposed MSOS optimized 3D-TC has been compared with the SOS optimized 3D-TC and harmony search optimized 3D-TC.     

Turbo Code with Modified LLR and Performance Analysis for Bluetooth Packets in Indoor Environments

Most of the restrictions in indoor environments do not exist for outdoor sites. The main limitations for indoors are the attenuation of walls and the multipath fading effects. The signal propagation is limited by the standard free space path loss in outdoor environments or free space. The loss in free space is usually equal to the range squared whereas the loss exponent for indoors is more like the 4th power. This study investigates the performance of turbo codes with modified Maximum-A-Posteriori (MAP) decoding algorithm for Bluetooth data packets in indoor environments over the frequency selective Rayleigh fading channels; which are also called as indoor wireless communication channels. In order to reduce the computational complexity of turbo decoders, MAP algorithm is modified in this study. Bit error rate (BER) versus energy of bit-to-noise ratio (E _b /N _o ) of modified MAP algorithms for Bluetooth data packets at 2.4 GHz industrial-scientific-medical (ISM) band are evaluated by means of computer simulations. Thus, modified logarithmic likelihood ratio (LLR) method can significantly reduce the computational complexity of the MAP algorithm. Furthermore, the performances of different types of forward error correction (FEC) coding for Bluetooth data packets are compared in the study.     

扩频体制低轨卫星通信信号捕获与跟踪系统设计

针对扩频体制下低轨卫星信号的捕获及跟踪等系统设计实现问题,根据卫星信号模型进行了捕获及跟踪算法设计,重点对基于快速傅里叶变换的快速捕获算法、锁频环、锁相环和码跟踪环路进行了设计,并进行了工程系统实现。试验验证表明,该系统可以实现码分多址体制低轨卫星信号捕获、跟踪处理,工作性能参数满足系统需求。     

Effects of low noise amplifier non‐linearities on blind adaptive beamforming performance in CDMA wireless systems

We report a new approach to analyse the effects of low noise amplifier (LNA) non‐linear distortions in the code division multiple access (CDMA) wireless communication systems using spatio‐temporal analysis and Volterra series theory. For this purpose, the bit error rate (BER) performance of three blind algorithms is studied based on post correlated model of received signal, and a time‐varying multiple vector channel model which is an extended form of the Gaussian wide sense stationary uncorrelated scattering (GWSSUS) channel. By using the Volterra series theory, an analytical expression for amplitude modulation to phase modulation (AM–PM) conversion is determined as a phase statement of LNA compression. In this approach, by combining the analytical expression for AM–PM conversion and CDMA blind beamforming techniques, we evaluate the AM–PM distortion effects on BER performance of a CDMA system originated from multiple non‐linear LNA blocks. Simulation results show that conditions are found to minimize AM–PM conversion introduced by multiple non‐linear blocks in the system leading to low BER. Copyright © 2006 John Wiley & Sons, Ltd.     

Simple MAP acceleration using pre-decoding for punctured turbo code

A simple algorithm that accelerates maximum a posteriori (MAP) decoding of a punctured turbo code is proposed. A turbo code consists of a data bit stream and a plurality of parity bit streams, some of which possibly had components removed by puncturing. The proposed method performs pre-decoding and re-encoding to find omitted parity symbols of the received sequence. This protocol provides a turbo-coded system of between rate-1/2 and rate-1/3. Simulation results show that the proposed algorithm reduces the number of iterations, which is equivalent to the decoding delay, to 50% for approaching a bit error rate (BER) of approximately 3.38 times 10^-3 at 1.0 dB.     

Implementation and performance of a turbo/MAP decoder

The implementation and performance of a turbo/MAP decoder are described. A serial block MAP decoder operating in the logarithm domain is used to obtain a very-high-performance turbo decoder. Programmable gate arrays and EPROMs allow the decoder to be programmed for almost any code from four to 512 states, rate 1/3 to rate 1/7 (higher rates are achieved with puncturing) and interleaver block sizes to 65,536 bits. Seven decoding stages were implemented in parallel. For rate 1/3 and 1/7 16-state codes with an interleaver size of 65,536 bits and operating at up to 356 kbit/s the codec achieved an E_b/N₀ of 0⋅32 and −0⋅30 dB respectively for a BER of 10⁻⁵. BERs down to 10⁻⁷ were also achieved for a small increase in E_b/N₀. An efficient implementation of a continuous MAP decoder is also presented, along with a synchronization technique for turbo decoders. © 1998 John Wiley & Sons, Ltd.     

Estimation of the error performance objectives for satellite communication systems

We consider the physical layer error performance parameters and design criteria for digital satellite systems established by ITU‐R Recommendation S.1062, where the performance objectives are given in terms of the bit error rate (BER) divided by the average number of errors within a cluster. It is well known that errors on satellite links employing forward error correction (FEC) schemes tend to occur in clusters. The resulting block error rate is the same as if it was caused by randomly occurring bit errors with an error‐event ratio of BER/α, where α is the average number of errors within a cluster. The factor, α, accounts for the burstiness of the errors and also represents the ratio between the BER and the error‐event ratio. This paper proposes theoretical methods to estimate the factor, α. Using the weight distributions of the FEC codes, we derive a set of expressions for the factor, α, as well as their compact lower bounds. We present lower bounds for various FEC schemes including binary BCH codes, block turbo codes, convolutional codes, and turbo codes. The simulation results show that the proposed lower bounds are good estimates in the high signal‐to‐noise ratio region. Copyright © 2008 John Wiley & Sons, Ltd.     

Memory-Reduced Maximum A Posteriori Probability Decoding for High-Throughput Parallel Turbo Decoders

Wireless communication standards make use of parallel turbo decoder for higher data rate at the cost of large hardware resources. This paper presents a memory-reduced back-trace technique, which is based on a new method of estimating backward-recursion factors, for the maximum a posteriori probability (MAP) decoding. Mathematical reformulations of branch-metric equations are performed to reduce the memory requirement of branch metrics for each trellis stage. Subsequently, an architecture of MAP decoder and its scheduling based on the proposed back trace as well as branch-metric reformulation are presented in this work. Comparative analysis of bit-error-rate (BER) performances in additive white Gaussian noise channel environment for MAP as well as parallel turbo decoders are carried out. It has shown that a MAP decoder with a code rate of 1/2 and a parallel turbo decoder with a code rate of 1/3 have achieved coding gains of 1.28 dB at a BER of 10\(^{-5}\) and of 0.4 dB at a BER of 10\(^{-4}\), respectively. In order to meet high-data-rate benchmarks of recently deployed wireless communication standards, very large scale integration implementations of parallel turbo decoder with 8–64 MAP decoders have been reported. Thereby, savings of hardware resources by such parallel turbo decoders based on the suggested memory-reduced techniques are accounted in terms of complementary metal oxide semiconductor transistor count. It has shown that the parallel turbo decoder with 32 and 64 MAP decoders has shown hardware savings of 34 and 44 % respectively.     

Performance of DS/CDMA-based equatorial LEO satellite system usingturbo-codes

Improving the bit error rate (BER) performance at low elevation angles is a crucial determining factor for the capacity of any low earth orbit (LEO) satellite system. In previous work, it has been demonstrated that the BER performance of a DS/CDMA-based equatorial LEO satellite system on a downlink can be improved significantly by using satellite diversity. The authors address the issue of improving BER at low elevation angles by using turbo codes     

Multiuser receiver scheme for full‐rate space–time coded CDMA system with imperfect estimation

By introducing a full‐rate space–time coding (STC) scheme, a synchronous CDMA (code division multiple access) system with full‐rate STC is given, and the corresponding uplink performance is investigated in Rayleigh fading channel with imperfect estimation. Considering that existing STC‐CDMA system has high decoding complexity, low‐complexity multiuser receiver schemes are developed for perfect and imperfect estimations, respectively. The schemes can make full use of the complex orthogonality of STC to reduce the high decoding complexity of the existing scheme, and have linear decoding complexity compared with the existing scheme with exponential decoding complexity. Moreover, the proposed schemes can achieve almost the same performance as the existing scheme. Compared with full‐diversity STC‐CDMA, the given full‐rate STC‐CDMA can achieve full data rate, low complexity, and partial diversity, and form efficient spatial interleaving. Thus, the concatenation of channel coding can effectively compensate for the performance loss due to partial diversity. Simulation results show that the full‐rate STC‐CDMA has lower bit error rate (BER) than full‐diversity STC‐CDMA systems under the same system throughput and concatenation of channel code. Moreover, the system BER with imperfect estimation are worse than that with perfect estimation due to the estimation error, which implies that the developed multiuser receiver schemes are valid and reasonable. Copyright © 2010 John Wiley & Sons, Ltd.     

A Novel Detection Algorithm Efficient for Turbo Coded CDMA Signals in Detect and Forward Cooperative Channels

In this paper, a new detection algorithm is proposed for turbo coded Code Division Multiple Access (CDMA) signals in detect and forward cooperative channels. Use of user cooperation makes much improvement in the performance of CDMA systems. Due to the special structure of CDMA systems, cooperative schemes increase the sum and cutoff capacities of CDMA based wireless systems and improve the quality of user-partner link which enhances the overall performance of the system. In this paper, a new combining scheme is proposed that makes the receiver more robust against the decision errors in the partner link. This structure is simulated for punctured 1/2 rate 4 states turbo code in a channel with first order Markov time variation and different Rice factor variances. Through various simulations, it is shown when the channel estimates are available in the partner and receiver, the cooperation between users provides much diversity gain especially while using the new proposed combining algorithm.     

Channel coding for asynchronous fiberoptic CDMA communications

Several studies have explored the feasibility and systems performance aspects of a code division multiple access (CDMA) format for fiberoptic networks. Previously discussed CDMA architectures would either have to tolerate a high bit error rate or be forced to use long code sequences in networks with even a moderate number of simultaneous users. The use of long sequences lowers the maximum achievable bit rate or places unrealistic requirements on the system hardware. This paper examines some of the possible improvements to system performance that could be realized by combining the CDMA format with external error correcting codes (ECCs) or a PPM format. It is determined that ECCs can be highly effective in lowering the BER, and/or increasing the achievable information bit rate and number of network users. The results are sufficiently encouraging to conclude that one should seriously consider including ECCs in any practical fiberoptic CDMA network     

Application of Sequential Estimation Algorithm Based on Branch Metric to Frequency-Selective Channel Equalization

In this work, a sequential estimation algorithm based on branch metric is used as channel equalizer to combat intersymbol interference in frequency-selective wireless communication channels. The bit error rate (BER) and computational complexity of the algorithm are compared with those of the maximum likelihood sequence estimation (MLSE), the recursive least squares (RLS) algorithm, the Fano sequential algorithm, the stack sequential algorithm, list-type MAP equalizer, soft-output sequential algorithm (SOSA) and maximum-likelihood soft-decision sequential decoding algorithm (MLSDA). The BER results have shown that whilst the sequential estimation algorithm has a close performance to the MLSE using the Viterbi algorithm, its performance is better than the other algorithms. Beside, the sequential estimation algorithm is the best in terms of computational complexity among the algorithms mentioned above, so it performs the channel equalization faster. Especially in M-ary modulated systems, the equalization speed of the algorithm increases exponentially when compared to those of the other algorithms.