The study of a BOCC m-ary spread spectrum scheme

M-ary spread spectrum technique has been found wide applications in wireless communications, but it needs too many orthogonal spreading codes, and its despreading/demodulation is quite complex computationally, which limit its wider applications. This paper proposes a novel scheme for Code Division Multiple Access （CDMA） communication systems based on M-ary spread spectrum, where only one prototype spreading code is assigned to each user and the codes for different users are orthogonal or quasi-orthogonal with each other. The M spreading codes of each user to represent K bits data are generated by circularly shifting the assigned code and reversing its polarity. The spreading codes generated like that are called as BiOrthogonal Cyclic Codes （BOCCs）. At the receiver of the system, a transform domain matched-filter implemented by means of Fast Fourier Transform （FFT） is employed to despread and demodulate the received signals, which has very low computational complexity. The results of simulation experiments and bit-error performance analysis show that the proposed scheme is practical and very useful in many cases.     

Fast symbol-level synchronization for TR-UWB signals using double hybrid DS-TH spreading

A Transmitted Reference （TR） Ultra WideBand （UWB） signal with double hybrid Direct Sequence and Time Hopping （DS-TH） spreading is proposed and its symbol-level synchronization scheme is designed and analyzed. The proposed signaling format can significantly reduce the search space while reducing the spectral lines and eliminating inter-frame interference and multi-user inter- ference. Detailed analysis of the proposed scheme is carried out. Synchronization performance of the proposed signaling format in multipath channel is derived and supported by computer simulation. The search step which is closely associated with the delay of data modulated pulses is analyzed. Using the TH code, the proposed signaling format and synchronization scheme especially works well in moderate and low data rate systems.     

A Low Complexity VCS Method for PAPR Reduction in Multicarrier Code Division Multiple Access

This paper investigates a peak to average power ratio (PAPR) reduction method in multicarrier code division multiple access (MC-CDMA) system. Variable code sets (VCS), a spreading codes selection scheme, can improve the PAPR property of the MC-CDMA signals, but this technique requires an exhaustive search over the combinations of spreading code sets. It is observed that when the number of active users increases, the search complexity will increase exponentially. Based on this fact, we propose a low complexity VCS (LC-VCS) method to reduce the computational complexity. The basic idea of LC-VCS is to derive new signals using the relationship between candidature signals. Simulation results show that the proposed approach can reduce PAPR with lower computational complexity. In addition, it can be blindly received without any side information.     

Improved FEC Code Based on Concatenated Code for Optical Transmission Systems

The improved three novel schemes of the super forward error correction(super-FEC) concatenated codes are proposed after the development trend of long-haul optical transmission systems and the defects of the existing FEC codes have been analyzed. The performance simulation of the Reed-Solomon(RS)+Bose-Chaudhuri-Hocguenghem(BCH) inner-outer serial concatenated code is implemented and the conceptions of encoding/decoding the parallel-concatenated code are presented. Furthermore, the simulation results for the RS(255,239)+RS(255,239) code and the RS(255,239)+RS(255,223) code show that the two consecutive concatenated codes are a superior coding scheme with such advantages as the better error correction, moderate redundancy and easy realization compared to the classic RS(255,239) code and other codes, and their signal to noise ratio gains are respectively 2～3dB more than that of the RS(255,239)code at the bit error rate of 1×10 -13 . Finally, the frame structure of the novel consecutive concatenated code is arranged to lay a firm foundation in designing its hardware.     

Construction and performance analysis of variable-weight optical orthogonal codes for asynchronous OCDMA systems

A construction scheme of variable-weight optical orthogonal codes （VW-OOCs） for asynchronous optical code division multiple access （OCDMA） system is proposed. According to the actual situation, the code family can be obtained by programming in Matlab with the given code weight and corresponding capacity. The formula of bit error rate （BER） is derived by taking account of the effects of shot noise, avalanche photodiode （APD） bulk, thermal noise and surface leakage currents. The OCDMA system with the VW-OOCs is designed and improved. The study shows that the VW-OOCs have excellent performance of BER. Despite of coming from the same code family or not, the codes with larger weight have lower BER compared with the other codes in the same conditions. By taking simulation, the conclusion is consistent with the analysis of BER in theory. And the ideal eye diagrams are obtained by the optical hard limiter.     

Asynchronous diversity receiving scheme and analysis on cooperative relay networks简

The transmission antennas of cooperative systems are spatially distributed on multiple nodes, so the received signal can be asynchronous due to propagation delays. A receiving scheme for cooperative relay networks is proposed, multiple asynchronous signals are reconstructed at the receiver by forward and backward interference cancellation, which can obtain gains of cooperative transmission diversity with obvious delay and with no requiring timing synchronization or orthogonal channelization between relays. Analysis and simulation show that the bit error rate（BER） of the proposed scheme is similar to Alamouti code, and the scheme has the diversity order of orthogonal transmission scheme accompanied by minimal BER losses. It is demonstrated that the performance can be further improved by adding an error correcting code（ECC）.     

A new probability decoding scheme based on genetic algorithm for FEC codes in optical transmission systems

Based on the genetic algorithm(GA),a new genetic probability decoding(GPD) scheme for forward error correction(FEC) codes in optical transmission systems is proposed.The GPD scheme can further offset the quantification error of the hard decision by making use of the channel interference probability and statistics information to restore the maximal likelihood transmission code word.The theoretical performance analysis and the simulation result show that the proposed GPD scheme has the advantages of lower decoding complexity,faster decoding speed and better decoding correction-error performance.Therefore,the proposed GPD algorithm is a better practical decoding algorithm.     

Tracking structure suitable for L6 signal

A tracking structure suitable for L6 signal of quasi-zenith satellite system (QZSS) was proposed in order to track the L6 signal without other frequency assistance. Moreover, the tracking structure does not change the receiver's hardware structure. The main difference between the proposed and the traditional tracking structure lies in the generation of local codes of E, P and L branches. The method of local code generation is designed in a two-stage manner. The first stage is the generation of the P branch local code with fast fourier transform (FFT). In the second stage, the local codes of the E and L branches are obtained with the code-chip interval. The tracking structure can track and decode L6 signal separately, and track code shift keying (CSK) modulated signal as well. The structure was verified using both simulation data, generated in different conditions, and actual data obtained from QZSS satellites respectively. The results show that the improved tracking loop is able to track L6 signal without other frequency assitance. Furthermore, the biterror ratio (BER) of L6 tracking algorithm is lower than that of L1C/A assist L6 algorithm, when the Doppler remains a constant and reed solomn (RS) encode are applied. To be more specific, with the proposed structure the BER decreased by 11.40%, 17.07%, 15.00%, 11.15%, 5.19% when carrier to noise ratio (CNR) is 36 -40 dB·Hz.     

Interleaving scheme for multicarrier CDMA system

In MultiCarrier Code-Division Multiple-Access (MC-CDMA) system,the received signals scattered in the frequency domain are combined to get frequency diversity gain.However,the frequency diversity gain is limited because of correlation between subcarriers.A novel interleaving scheme for MC-CDMA system is proposed in this paper.A circular shifting register is introduced into each subcarrier branch to decrease the correlation between subcarriers.By using interleaving,frequency diversity gain of system is increased.System structure and model with interleaver are discussed.In the case of multiple users,Parallel Interference Cancellation (PIC) technique is also introduced.Computer simulations demonstrate the performance of proposed scheme,and the performance comparison of MC-CDMA with interleaver and conventional MC-CDMA system is shown as well.     

Blind Decorrelating Detection Based on Particle Swarm Optimization under Spreading Code Mismatch

A way of resolving spreading code mismatches in blind multiuser detection with a particle swarm optimization(PSO) approach is proposed. It has been shown that the PSO algorithm incorporating the linear system of the decorrelating detector, which is termed as decorrelating PSO(DPSO), can significantly improve the bit error rate(BER) and the system capacity. As the code mismatch occurs, the output BER performance is vulnerable to degradation for DPSO. With a blind decorrelating scheme, the proposed blind DPSO(BDPSO) offers more robust capabilities over existing DPSO under code mismatch scenarios.     

THE STUDY OF AN M-ARY MC-CDMA SYSTEM BASED ON CYCLIC SPREADING CODES & PRE-EQUALIZATION

A novel fast despreading scheme for M-ary Multi-Carrier Code-Division Multiple Access (MC-CDMA) system is proposed based on cyclic spreading codes and pre-equalizer. In the transmitter, the M spreading codes of each user are generated by circularly shifting the prototype spreading code. A feedback pre-equalizer is employed to process the M-ary MCCDMA signal before transmitted. The received signal is multiplied by the Inverse Discrete Fourier Transform (IDFT) result of the mirror image code of the prototype spreading code, and then demodulated by Orthogonal Frequency-Division Multiplexing (OFDM) demodulator. Compared with the conventional M-ary MC-CDMA receiver, the proposed scheme increases bandwidth efficiency, meanwhile, it achieves M-ary despread spectrum and multi-carrier demodulation, which reduces computation complexity remarkably.     

The study of an m -ary MC-CDMA system based on cyclic spreading codes & pre-equalization

A novel fast despreading scheme for M-ary Multi-Carrier Code-Division Multiple Access (MC-CDMA) system is proposed based on cyclic spreading codes and pre-equalizer. In the transmitter, the M spreading codes of each user are generated by circularly shifting the prototype spreading code. A feedback pre-equalizer is employed to process the M-ary MC-CDMA signal before transmitted. The received signal is multiplied by the Inverse Discrete Fourier Transform (IDFT) result of the mirror image code of the prototype spreading code, and then demodulated by Orthogonal Frequency-Division Multiplexing (OFDM) demodulator. Compared with the conventional M-ary MC-CDMA receiver, the proposed scheme increases bandwidth efficiency, meanwhile, it achieves M-ary despread spectrum and multi-carrier demodulation, which reduces computation complexity remarkably. Supported by the National Natural Science Foundation of China (No.60172029). Communication author: Chu Zhenyong, born in 1972, male, Ph.D. candidate, lecturer. Post Box 119, State Key Lab. of ISN, Xidian University, Xi’an 710071, China.     

M元多码MC-CDMA系统信号包络特性分析

针对扩频通信中的M元多码系统,结合OFDM技术特点,提出了一种新的M元多码MC-CDMA系统。在分析系统基本原理基础上,建立了系统信号模型,对系统发射信号包络特性进行了分析研究,推导出系统信号包络公式,讨论了系统多码分路数与系统包络以及系统PAPR的关系。对系统不同分路数情况下包络状态进行了仿真,结果表明M元多码MC-CDMA系统发射信号包络和PAPR受多码分路数以及扩频序列的影响。     

高速数据抗干扰传输的设计与仿真

文章提出了一种采用正交循环码的M—ary扩频多载波传输方案（M—ary／OFDM）,给出了方案的系统模型、信号格式及关键部分的设计。在发端,每个用户采用正交循环码实现M—ary扩频后经OFDM调制,在收端,经OFDM解凋后利用两个数字匹配滤波器来完成M元扩频信号的解扩,实现了高速数据的抗干扰传输。仿真结果表明,在无人机信道中该方案与传统M—ary／QPSK方案相比,具有较强的抗多径干扰及多普勒频移的能力。     

正交循环码M进制扩频接收机的研究

该文提出了一种基于正交循环码的M进制扩频接收机方案,发端将一条原型扩频码循环移位构成M个扩频码,实现M进制扩频,接收端利用时频变换域正交匹配滤波器实现了非相干解扩解调。该方案不仅减少了需要的扩频码数目,而且有效地降低了接收机的计算复杂度。该文对所提出的系统在单用户和多用户条件下的误码性能进行了理论分析和仿真,并比较了3种M进制解扩方法的运算量。结果表明:正交循环码M进制扩频系统的计算复杂度和误码特性均优于传统的M进制系统。     

用复合正交换频码进行调制的M—ary MC—CDMA方法

本文提出了一种用复合正交扩频码进行调制的M－ary MC-CDMA方法,在频率选择性慢衰落信道,用等增益合成和最大化率合成分析了系统下行链的性能,并与单扩频码系统进行了比较,最后给出了保护时隙对系统的影响。     

Blind and Semi-Blind Equalization of Downlink MC-CDMA System Exploiting Guard Interval Redundancy and Excess Codes

This paper introduces new blind time domain equalization (TEQ) techniques for downlink multicarrier code division multiple access (MC-CDMA) systems. This equalization exploits the guard interval (GI) redundancy together with the excess codes (EC) to restore the structural properties of MC-CDMA signal destroyed by multipath fading channels. Indeed, since in CDMA based wireless systems, the number of users per cell is significantly less than the spreading factor, a base station can set apart a subset of the codes, the excess codes that will not be used for spreading. It is shown that the joint use of the EC and GI redundancy improves significantly the equalization performance compared to the GI only or EC only cases. A second contribution consists in exploiting the previous information together with the one given by a pilot sequence in a semi blind scheme to further improve the performance. Finally, we propose a new adaptive algorithm for the estimation and tracking of the equalizer that has a faster convergence rate compared to existing algorithms of similar complexity order. Simulation based performance evaluations are given at the end of this paper to assess the effectiveness of the proposed methods in MC-CDMA.     

A Low Complexity VCS Method for PAPR Reduction in Multicarrier Code Division Multiple Access

This paper investigates a peak to average power ratio (PAPR) reduction method in multicarrier code division multiple access (MC-CDMA) system. Variable code sets (VCS), a spreading codes selection scheme, can improve the PAPR property of the MC-CDMA signals, but this technique requires an exhaustive search over the combinations of spreading code sets. It is observed that when the number of active users increases, the search complexity will increase exponentially. Based on this fact, we propose a low complexity VCS (LC-VCS) method to reduce the computational complexity. The basic idea of LC-VCS is to derive new signals using the relationship between candidature signals. Simulation results show that the proposed approach can reduce PAPR with lower computational complexity. In addition, it can be blindly received without any side information.