一种新的重叠变换域抗窄带干扰技术

本文采用改进的重叠变换域中值滤波算法进行变换域抗干扰处理.该算法既考虑到直接序列扩频信号的特点,同时又利用重叠变换的特性对变换域系数进行了二次滤波.该算法节省处理时间,并且不需要有关干扰的先验知识,系统性能不会随干扰频率变化而变化,因而是一种很稳健的处理方法.     

On the architecture and performance of an FFT-based spread-spectrumdownlink RAKE receiver

This paper describes a spread-spectrum downlink RAKE receiver that computes data detection in the frequency domain. We assume a pilot signal is transmitted with data signals for channel sounding. The pilot signal does not degrade the receiver bit error rate (BER) performance because the receiver estimates the pilot signal and subtracts the estimated pilot signal from the received signal before data detection. A spreading code matched filter, a channel matched filter, and a sounding receiver are implemented by fast Fourier transform (FFT)-based matched filtering and integrated in a unified architecture. Monte Carlo simulation is used to evaluate the receiver BER performance in both a static channel and a mobile radio channel. Simulation results show that the RAKE receiver performs well in both kinds of channels     

Narrow-band interference excision in spread spectrum systems usinglapped transforms

In order to mitigate narrow-band interference in spread spectrum communications systems, novel communications receivers incorporating transform domain filtering techniques are designed. In this paper, lapped transforms are used to transform the received data signal to the transform domain wherein adaptive excision is performed. Transform domain detection algorithms, which yield bit decisions based on the remaining signal energy, are analyzed and, together with excision, are employed on a block-by-block basis to suppress single-tone and narrow-band Gaussian interference. System performance is analytically quantified in terms of the overall system bit-error rate (BER). Subsequent results are presented for a variety of channel conditions and compared to those obtained using excision algorithms based on orthonormal block transforms (Medley 1995). These results demonstrate the improved performance and increased robustness with respect to jammer frequency and bandwidth of lapped transform domain excision techniques relative to similar algorithms based on nonweighted block transforms     

Combining the discrete wavelet transform and mixed-domain filtering

A novel filtering method is proposed that combines the discrete orthogonal wavelet transform (DWT) with the mixed-domain (mixed-D) filtering method. The method uses the DWT to pre- and postprocess those dimensions of the signal that are transformed to the discrete-frequency domain by mixed-D filtering. Using the DWT in this manner provides a controlled mechanism to partition the spectrum of the input signal into subband signals, which then may be selectively filtered during the linear difference equation (LDE) step of the mixed-D algorithm. It is shown that, when the DWT is computed using filters with ideal high- and lowpass frequency responses, the LDE filters used in the mixed-D filtering stage are unchanged by the introduction of the DWT (although the frequency tuple associated with each LDE filter is altered). This indicates that the mixed-D filtering scheme can be easily used in subband coding systems. Results are given for the filtering of a three-dimensional (3-D) linear trajectory signal, representing a common application in video processing.     

联合小波变换与残留干扰白化的测距仪脉冲干扰抑制方法

针对测距仪（DME）脉冲信号对L频段数字航空通信系统1(L-DACS1) 接收机产生干扰的问题,提出一种联合小波变换与残留干扰白化的测距仪脉冲干扰抑制方法。首先通过小波变换将正交频分复用(OFDM)接收机接收的信号转换到小波域;然后利用有用信号与干扰信号小波系数的差别提取DME信号小波系数,重构DME信号,并在时域进行脉冲干扰消除;最后,接收机通过信号解交织器与逆正交变换器将残留的脉冲干扰转换为白噪声,避免了残留干扰信号造成的突发性解调错误。仿真结果显示:所提出的联合干扰抑制方法可有效消除DME信号干扰,在相同误比特率（10-2）条件下,本文方法较常规脉冲熄灭方法可获得5dB以上的性能改善,提高了L-DACS1系统链路传输的可靠性。      

Rejection of frequency sweeping signal in DS spread spectrumsystems using complex adaptive filters

It is already known that the performance of narrowband interference adaptive filtering in DS spread spectrum systems is additionally degraded in the presence of a frequency hopped jamming signal compared to the case with CW interference. Even so, the adaptive filter can be rather efficient because during the hopping interval, while the jamming signal parameters are constant, the filter adjusts its weights; and if the hopping rate is not too high, there are time periods when the interfering signal is suppressed. In order to have a better insight into the performance of these systems, we analyze the DS spread spectrum receiver behavior, when the jamming signal frequency is continuously changing (chirp signal). We find that this is a reasonable step for a jammer that is trying to be more efficient. In such a case, a two-sided filter structure shows worse overall performance than a prediction error structure. As the jamming frequency sweeping rate or bandwidth is increased, the system performance is degraded. The analysis provides an insight into how much the system performance will be degraded as a function of the interfering (frequency sweeping) signal parameters     

联合两级滤波与压缩感知的测距仪干扰抑制方法

针对测距仪(Distance Measure Equipment , DME)脉冲信号对L频段数字航空通信系统1(L-band Digital Aeronautical Communication System 1 , L-DACS1)接收机产生干扰的问题,提出一种联合两级滤波与压缩感知的DME干扰抑制方法。首先,接收机通过高通滤波器估计出DME干扰,并在时域将其从接收信号中消除;然后,通过低通滤波器进一步滤除残留的DME干扰和噪声;最后,针对经过两级滤波后仍残留的DME干扰,基于其在时域中的稀疏特性应用压缩感知算法实现重构并消除。数值仿真表明,所提方法可获得的DME干扰功率衰减值约41dB,有效克服了DME脉冲干扰对L-DACS1系统接收机的影响。      

GST时频滤波SAR射频干扰抑制算法

空间中存在的射频干扰(Radio Frequency Interference,RFI)会污染合成孔径雷达(Synthetic Aperture Radar,SAR)的回波数据,进而影响成像质量以及基于图像的应用.本文针对RFI的特点,提出了一种基于广义S变换(Generalized S Transform,GST)时...     

基于部分数据的叠加序列慢时变信道估计

针对OFDMA通信系统,提出了一种基于部分数据的叠加序列慢时变信道估计算法,并在接收端给出了数据恢复的方法。时变信道采用复指数基扩展模型来描述,对OFDMA系统的导频序列进行了精心设计。提出在频域减去一个基于部分数据的序列,从而使发送数据经过反傅里叶变换后,特定位上的数据能量变低,进而大大减少了数据在信道估计时产生的误差。由于在发射端信号已经产生了畸变,在接收端采用特定的方法对数据进行补偿,消除了这种影响。实验仿真证明该方法在一定程度上提高信道估计的性能。     

LFM信号的分数阶傅里叶域自适应滤波算法研究

对于线性调频信号(LFM)的滤波,采用处理平稳信号的方法对其滤波往往得不到很好的效果。本文利用了线性调频信号在分数傅里叶变换域上具有很好的时频聚焦性的特点,来实现信号在分数阶傅里叶域的自适应滤波,自适应滤波算法采用改进的步长LMS方法,对传统的LMS算法做出了改进,算法中步长处理中引入了一个限制因子,可以较好地解决算法收敛速度和稳态失调量之间的矛盾。仿真结果表明,此算法在处理分数阶域的LFM信号滤波比传统的LMS算法有较好的滤波效果。      

DSSS中基于重叠变换的自适应干扰抑制算法及仿真分析

提出了一种基于重叠变换的自适应干扰抑制算法,利用调制重叠变换（MLT）把接收机信号映射到变换域进行抗干扰处理,它可以有效地把干扰能量集中在有限的变换域子带（Transform Bins）中,与基于DFT、DCT等块变换相比有更好的滤波效果。由于可以使用基于重叠变换的快速算法,因此它结构简单,节省计算量。仿真结果表明,基于MLT的变换域处理方案可以有效地改善系统性能。     

Suppression of narrowband jammers in a spread-spectrum receiverusing transform-domain adaptive filtering

The author investigates the use of transform-domain adaptive filtering for the suppression of narrowband jammers in a direct-sequence (DS) spread-spectrum receiver. An input signal consisting of the DS signal, white Gaussian noise, and a narrowband jammer is transformed into the frequency domain where adaptive filtering acts to suppress the jammer. Weight leakage is used to allow jammer suppression while preserving the desired DS signal. Bit error rate (BER) results obtained by computer simulation are presented to illustrate performance for a single-tone jammer and for a jammer consisting of a second DS signal with a lower chip rate. These results are compared to those for a transform-domain exciser. Expressions for the converged tap weights are derived and the results are verified via the simulation. Finally, the convergence properties of the adaptive suppressor are illustrated     

多分辨重叠变换在扩频通信系统抗干扰中的应用

 本文利用多分辨重叠变换(HLT)把接收机信号映射到变换域进行抗干扰处理.与重叠变换(LT)不同,HLT对信号频谱的非均匀划分可以有效地把干扰能量集中在有限的变换域子带(Transform Bins)中,因此对抑制随机脉冲干扰非常有效.与小波变换(WT)相比,HLT对信号频谱的划分更加灵活.由于可以使用基于重叠变换的快速算法,在滤波性能相当的情况下,HLT比小波变换节省计算量.仿真结果表明,其于HLT的变换域处理方案可以有效地抑制非时变窄带干扰和时变宽带干扰,明显地改善系统性能.     

CO-OFDM系统中一种新颖相位噪声抑制算法

针对相干光正交频分复用(CO-OFDM)系统中相位噪声造成的严重影响,提出了一种采用线性插值和卡尔曼滤波相结合的相位噪声抑制算法.该算法在第一阶中对接收端的时域信号进行线性组合,之后利用线性插值算法对相位噪声进行初步抑制,在第二阶中采用卡尔曼滤波技术来抑制残余的相位噪声.分析和仿真表明:提出的二阶算法能够有效地抑制相位噪声对OFDM符号的影响,在相位噪声线宽较大时明显地降低错误平层,利用所提出的二阶算法可使误码率达到10-7以下,有效地提高了系统的性能.     

一种基于FPGA的中频数字接收机设计

提出了一种基于FPGA的中频数字接收机的设计方案。该方案首先采用FFT检测频率峰值，然后进行带通滤波、正交变换。对正交变换后的信号用CORDIC算法计算瞬时相位，然后利用相位差分方法对频率进一步精确估计。文中详细描述了设计思路和各模块的FPGA实现方法，并进行了硬件仿真。仿真结果表明，该方案具有实时性强、脉冲参数测挝精度高的优点。     

多频GNSS射频采样接收机设计与实现

针对多频GNSS接收机采用模拟射频前端存在的缺陷,研究基于直接射频采样技术的多频GNSS接收机设计与实现。根据当前公开导航频段,设计了一种可配置、可兼容的前端数字信号处理结构,将模数转换器件紧接在天线后端,直接在射频域对导航信号进行数字化,射频以下的所有处理功能全部采用软件模块来实现,使接收机通过参数设置就可以兼容多种导航系统。对采样频率选择、本振频率选择、抽取滤波等关键技术进行了分析,基于可编程片上系统平台给出一种射频采样GNSS接收机的实现方案,分析了其中各功能模块的实现框图与工作流程,通过测试给出接收机的性能指标。      

Performance enhancement of DSSS systems: two-dimensionalinterference suppression

We analyze a new interference suppression structure in direct-sequence spread-spectrum (DSSS) systems that is based on two dimensional (frequency- and time-domain) notch filtering. In the first step, the filter is realized in the form of variable-step (function of estimation error-power ν) least mean square algorithm. This modification will improve the system performance in the transient period by making this period shorter. Unfortunately, in the steady-state regime, the performance will be slightly degraded. This suggests a modification that will include an algorithm with ν=2 in the transient state and ν=1 in the steady-state operation. Further modification of the algorithm will use time notching of the code division multiple-access (CDMA) receiver in the period when estimation error is larger than a given threshold. A comprehensive analysis and comparison of these modifications are presented and discussed. These results can be used either for performance enhancement of DSSS systems in the presence of jamming or for CDMA overlay-type mobile communication networks. In the latter case, the interfering signal comes from another user employing narrow-band modulation. The analysis does not include fading so that the results can be used in the case when the narrow-band modulated signal is not significantly distorted by fading. An example is line-of-sight microwave communications, which happens to be one of the most feasible applications     

Performance of a Multicarrier CDMA System with Rayleigh Fading Multipath Channels

This paper examines the performance of a multicarrier CDMA system in a Rayleigh fading channel without a RAKE receiver. Since the multicarrier technique is a diversity reception technique used in the frequency domain, the signal is transmitted at different frequency bands and undergoes different fading. The superimposed signal of the multipath components after going through the mobile channel will be processed by an Fourier Transform to enter the frequency domain, where the time delays in various paths are converted to the phase shifts in the compound signal. In this paper, we use sounding bits to extract the characteristics of the mobile channel and to partly remove the multipath effect. From the simulation, it is found that the multicarrier technique is simpler than the technique used in the single carrier system with a RAKE receiver while producing a better performance than that of the RAKE receiver.     

Windowed overlapped frequency domain block filtering approach for direct sequence signal acquisition

This study applies a windowed frequency domain overlapped block filtering approach to acquire direct sequence signals. As a novel viewpoint, the windows not only allow pulse shaping without front-end pulse-shaping filter, but also increase the performance of the spectrum sensing unit, which can efficiently be implemented into this frequency domain receiver and may further be used for spectrum sensing in cognitive radios or narrowband interference cancellation in military radios. The proposed receiver is applicable for the initial time synchronization of different signals containing a preamble. These signals include single carrier, constant envelope single carrier, multicarrier, and even generalized multicarrier signals, making the proposed receiver structure a universal unit. Furthermore, the receiver can be used to perform filtering with long codes and compute the sliding correlation of an unknown periodic preamble. The receiver can further be modified to handle large Doppler shifts. We will also demonstrate herein the computational complexity and analysis of the acquisition performance in Rayleigh and Rician fading channels.     

Low Complexity Discrete Hartley Transform Precoded OFDM System over Frequency‐Selective Fading Channel

Orthogonal frequency‐division multiplexing (OFDM) suffers from spectral nulls of frequency‐selective fading channels. Linear precoded (LP‐) OFDM is an effective method that guarantees symbol detectability by spreading the frequency‐domain symbols over the whole spectrum. This paper proposes a computationally efficient and low‐cost implementation for discrete Hartley transform (DHT) precoded OFDM systems. Compared to conventional DHT‐OFDM systems, at the transmitter, both the DHT and the inverse discrete Fourier transform are replaced by a one‐level butterfly structure that involves only one addition per symbol to generate the time‐domain DHT‐OFDM signal. At the receiver, only the DHT is required to recover the distorted signal with a single‐tap equalizer in contrast to both the DHT and the DFT in the conventional DHT‐OFDM. Theoretical analysis of DHT‐OFDM with linear equalizers is presented and confirmed by numerical simulation. It is shown that the proposed DHT‐OFDM system achieves similar performance when compared to other LP‐OFDMs but exhibits a lower implementation complexity and peak‐to‐average power ratio.