Digital multi-carrier spread spectrum versus direct sequence spreadspectrum for resistance to jamming and multipath

We compare single user digital multi-carrier spread spectrum (MC-SS) modulation with direct sequence (DS) SS (with a modified implementation) in the presence of narrowband interference (NBI) and multipath fading. We derive closed-form expressions for the symbol error probability for both the linear MMSE receiver as well as the conventional matched-filter receiver under different scenarios: additive white Gaussian noise (AWGN) channel with NBI, multipath channel with or without NBI. We show that DS-SS can achieve the same performance as MC-SS if the spreading code is carefully designed to have perfect periodic autocorrelation function (PACF). On the other hand, MC-SS is more robust to narrowband interference and multipath fading than is DS-SS with the widely used spreading codes that do not possess perfect PACE. Our analysis reveals that the performance improvement of MC-SS is precisely due to the implicit construction of an equivalent spreading code having nonconstant amplitude but possessing perfect periodic autocorrelation     

MC-SS系统抗部分频带干扰性能分析

多载波扩频(MC-SS)技术结合了OFDM和扩频技术的优点,具有很强的抗干扰能力。MC-SS系统、MC-DS-SS系统和MT-SS系统是3种主要的多载波扩频系统。而部分频带干扰是一种常见的干扰类型。在介绍几种主要的多载波扩频系统的基础上,分析了多载波扩频系统的抗部分频带干扰的能力,给出了在干扰信息状态(JSI)已知和未知状态下的误码率。     

分集接收混合遗传算法多用户检测器在异步CDMA移动通信中的应用研究

本文研究了将混合遗传法应用于码分多址扩频通信的多用户检测中，从而消除多址接入干扰的方法。本文先给出了在异步瑞利衰落信道下解调扩频信号的分集接收混合遗传算法多用户检测器的原理框图，然后分析了用混合遗传算法进行多用户检测的理论依据和实际性能。仿真结果表明无论是抗多址干扰还是抑制远近效应，分集接收混合遗传算法多用户检测器都明显优于传统接收机，其性能接近分集接收最佳多用户检测器。     

A Low-PAPR Differential Frequency Shift Orthogonal Keying Transceiver for Multi-Carrier Spread Spectrum System over High Mobility Multipath Channels

A new differential transceiver with a frequency-shift orthogonal keying (FSOK) technique is proposed for the multi-carrier spread spectrum (MC-SS) system over high mobility multipath fading channels. The design of the transceiver involves the following stages. First, the data stream is mapped into MPSK-FSOK symbols and spreaded by the frequency-shift orthogonal sequences. Second, the differential block encoder is exploited to combat the mobile channels. The Chu sequence is adapted for initial differential encoding, making the post-IFFT transmit signals with a low peak-to-average power ratio. Next, for the receiver, the maximum ratio combining technique is used for the block-based differential frequency-domain equalizer, which can overcome the multipath fading channel effect without requiring channel estimation. Finally, an efficient maximum likelihood despreading and demapping scheme is used to detect the modulation symbols. Furthermore, the differential MC-SS transceiver can be easily re-configured for a MISO differential MC-SS system with high link quality. Simulation results show that, under high mobility multipath channels, the proposed SISO differential MC-SS system can outperform the conventional MC-SS system. The proposed MISO differential MC-SS system with space-time diversity gain and M-ary modulation gain also exhibits excellent performance.     

The Applicative Investigation of Adaptive BP Networks for Multi-user Detection in Asynchronous DS-CDMA Mobile Communications

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基于DLL的MC-SS系统同步新算法

构建了多载波扩频(Multi-carrier Spread Spectrum,MC-SS)系统的收发端模型,并对模型进行了简要地分析。针对MC-SS技术现有的符号同步方法,提出了一种基于扩频信号结构的MC-SS系统符号同步新算法。该算法利用MC-SS系统的时域结构,直接在不进行快速傅里叶变换(Fast Fourier Transform,FFT)变换下对时域信号进行捕获、跟踪。采用多次驻留的搜索、捕获算法得到符号同步的初始同步位置,然后采用延时锁定环(Delay-Locked Loop,DLL)进行相位的精密跟踪。通过仿真分析表明该同步方法同步精度高,易于实现。     

An adaptive non-AWGN SSMA receiver

This article deals with the issue of optimum detection of a known spread spectrum (SS) signal in nonwhite noise and/or narrowband interference. Prior to detection, the input signal is whitened according to the reciprocal of an interfering spectrum, I(z)=1/AG(z). Next, in force of the maximum likelihood (ML) principle, the replica of signal is carried through the same filter, I(z), to match it to a new signal shape. Results of simulation show a significant gain in comparison to conventional single-matched detection.     

Performance of chip-interleaving DS SS system with nonlinearity under pulsed interference

Analytical bit error rate (BER) results of a chip-interleaving direct sequence (DS) spread spectrum (SS) system using nonlinearity in the presence of pulsed interference are presented. Several simple nonlinearities, such as hole puncher, soft clipper, and hard limiter nonlinearities, are studied. The BERs of a conventional DS SS system employing the same nonlinearities are also given. The proposed system is compared with the conventional DS SS system using numerical examples.     

Short-time Fourier transform receiver for nonstationaryinterference excision in direct sequence spread spectrum communications

A new adaptive excision approach for nonstationary interference excision in direct sequence spread spectrum (DS/SS) communications is introduced. The proposed excision approach is based on the attractive localization properties of the impulse responses of the multiple pole filters. These impulse responses have Gaussian-like shapes and decrease in bandwidth with higher pole multiplicities. When used as data windows, they field a large class of computationally efficient short-time Fourier transforms (STFTs). Localization measures can be applied to determine the optimum window that maximally concentrates the interference in the time-frequency (t.-f.) domain. Interference mitigation is then achieved by applying a binary excision mask to the corresponding STFT for each data bit. We show that the proposed interference excision method permits both data-dependent windowing and time-varying filtering and leads to improved BER performance of the DS/SS system. The paper also derives the general optimum receiver implementing the STFT-based interference excision system     

Multicarrier Orthogonal-CDMA for Fixed Wireless Access Applications

In this paper, we suggest applying multicarrier spread spectrum (MC-SS) techniques to fixed wireless access applications. Due to the fixed nature of subscribers in these applications, cross-polarization discrimination and directional subscriber antennas can be used to reduce interference in a multicellular system. Power control schemes are applied on both uplink and downlink. The suggested system has a high capacity and uses a simple multiaccess layer. It is robust against multipath effects and can provide service coverage not only to line-of-sight (LOS) but also to nearly LOS subscribers. Only a single carrier frequency is used in the entire network.     

室内无线网格编码直扩多址通信系统的性能分析

本文分析室内无线多径衰落信道中网络编码调制的直扩多址通信系统的性能，系统采用网络编码多相调制技术以改善系统性能，本文讨论了网格编码技术与系统性能之间的关系及载波恢复相位误差对系统性能的影响，本文给出了误码率的理论分析结果和计算机模拟结果，并与常规的未编码直扩系统性能对比。     

基于一种特定混沌映射的跳频序列

使用一种简单的分段线性映射构造混沌跳频序列。用于跳频通信中取代传统的伪随机序列（PN序列），分析了此混沌序列的周期性。平衡性，汉明相关特性和线性间隔，最后给出了混沌序列的神经网络实现。仿真和实验结果表明，此序列具有较好的统计特性。应用于跳频多址通信中，可获得较好的抗干扰。抗截获能力。     

Polarization filtering for narrowband interference suppression in ultra‐wideband communications

To suppress narrowband interference (NBI) in an ultra‐wideband (UWB) communications environment, a null phase‐shift polarization (NPSP) filter is proposed. The proposed NPSP filter is a combination of a linear polarization‐vector transformer (PVT), a conventional single notch polarization (SNP) filter, and an amplitude and phase compensator (APC). The NBI, which has polarized states different from those of the UWB, can be suppressed completely and the UWB signal can be recovered without distortion if the polarized states can be estimated exactly. Analytical and simulation results indicate that the signal‐to‐interference ratio (SIR) can be improved effectively after NPSP filtering. The proposed NPSP filter can be implemented in a time‐hopping spread spectrum (TH‐SS) or a direct‐sequence spread spectrum (DS‐SS) UWB system. Copyright © 2007 John Wiley & Sons, Ltd.     

Direct-sequence spread spectrum narrowband interference rejectionusing property restoration

A new method of rejecting narrowband interference in direct-sequence spread spectrum (DS/SS) communications is presented. A typical approach is to reject the interference using a filter with large attenuation at the interference frequencies before despreading. The interference rejection method presented incorporates vector space projection techniques to suppress the correlated interference. Several signal characteristics are formulated which lead to constraint surfaces in the vector space of possible solutions. These constraint surfaces describe interference rejection solutions which introduce minimal distortion to the spread spectrum signal and simultaneously remove the interference. The constraint surfaces essentially correspond to spread spectrum signal estimates which, after interference rejection, conform to known characteristics of the transmitted spread spectrum signal. The formulation of the surfaces relies on prior knowledge about the spread spectrum signal correlation and spectral properties     

Performance degradation of analog FM system due to spread spectrum overlay

The investigation of spectrum overlay of a spread spectrum system on the existing narrowband FM broadcasting system is presented. The overlaid spread spectrum (SS) system is assumed to utilize direct sequence (DS) spreading, using maximal length pseudorandom sequences. We studied the performance degradation of the analog FM system due to the interference produced by the SS signal, through laboratory subjective and objective measurements, for various types of spreading scenarios and for different carrier frequency differences (/spl Delta/f). The RF protection ratios for the FM receivers are derived, along with SINAD measurements. Finally, these experimental results have been compared with the theoretical study of the FM receiver's audio frequency SNR output due to SS interference.     

Performance Analysis of Narrowband Interference Suppression in Orthogonal Code ModulationSpread Spectrum System

1IntroductionInaspreadspectrumsystem,theuser'ssignalisspreadtoawidebandsignalfortransmission,thenthereceivedsignalswillbedecorrelatedbacktotheoriginalone.Theprocessinggainresultedfromthisprocessprovidestheadvantagesofperformanceimprovementtocombatint...     

Direct sequence code acquisition in multiplicative and non-Gaussian noises

A new detector for direct sequence spread spectrum code acquisition in multiplicative and non-Gaussian noises is proposed in this article. Modelling the acquisition problem as a hypothesis-testing problem, a detector is derived for multiplicative and non-Gaussian noises, based on the locally optimum detection technique. Numerical results show that the proposed detector can offer substantial performance improvement over the conventional schemes in multiplicative and non-Gaussian noises.     

Cyclostationarity-Based Narrowband Interference Suppression for Heterogeneous Networks Using Neural Network

This paper proposes a narrowband interference (NBI) suppression algorithm for Direct Sequence-Code Division Multiple Access systems. The NBI is considered from heterogeneous networks, and predicted based on its cyclostationary characteristic using a nonlinear feed-forward neural network predictor which eliminates the nonlinearity of the spread spectrum (SS) signal in the NBI prediction. To further improve the suppression performance, this paper exploits the structure of the spreading code, and proposes an iterative code-aided algorithm to jointly estimate the NBI and the SS signal. Simulation results reveal that the proposed algorithm largely outperforms the conventional linear prediction filtering and linear-conjugate linear polyperiodically time-varying filtering methods in both the signal to interference plus noise ratio improvement and the bit error rates, when it operates in NBI-contaminated environments.     

Mobile direct-sequence spread-spectrum underwater acoustic communication based on improved differential energy detector

A Doppler estimation method based on the structural features of spread spectrum signal was proposed,which could achieve real-time Doppler estimation for received spread spectrum signals by energy detecting the result of matching correlation calculation of adjacent received spreading symbols.In addition,this method made full use of the spread spectrum processing gain and could do Doppler estimation under low SNR condition.Improved differential energy detector was further proposed,which can effectively deal with the rapid carrier phase fluctuation interference caused by Doppler effect and time-varying multi-path interference by detecting the output energy of two correlators.Simulation verified the robustness of improved differential energy detector algorithm and the Doppler estimation method based on spread spectrum signal.At-sea data shows that improved differential energy detector combined with the proposed Doppler estimation method can achieve low bit error rate communication for direct-sequence spread spectrum underwater acoustic communication with large time-varying Doppler interference when SNR is ?10 dB.     

A novel random wireless packet multiple access method using CDMA

Random packet CDMA, a novel packet-based multiple access scheme for connectionless, uncoordinated random channel access is proposed. Random packet CDMA, or RP-CDMA, utilizes a novel packet format which consists of a short header and a data portion. Each header is spread with a unique spreading code which is identical for all users and packets, while the data portion of each packet is spread by a randomly chosen spreading sequence. The receiver operates in two stages: header detection and data detection. For header detection a conventional spread spectrum receiver is sufficient. Headers are spread with a large enough processing gain to allow detection even in severe interference. The data portion is decoded with a sophisticated receiver, such as a multiuser detector, which allows for successful decoding of overlapping active packets. It is shown that the RP-CDMA system is detector capability limited and that it can significantly outperform spread ALOHA systems whose performance is limited by the channel collision mechanism. RP-CDMA also experiences a much smaller packet retransmission rate than conventional or spread ALOHA, and provides better spectral efficiencies.