Design and evaluation of a high data rate optical wireless system for the diffuse indoor channel using barker spreading codes and RAKE reception [optical wireless communications]

The performance of a high data rate optical wireless system (OWS) over the diffuse indoor infrared channel based on direct sequence spread spectrum (DSSS) techniques is highly sensitive to the properties of the aperiodic autocorrelation function and the spreading factor of the spreading sequence used. Ideally, the aperiodic autocorrelation function should have zero sidelobes in order to eliminate intersymbol interference caused by multipath propagation. In practice, such an ideal sequence does not exist in the binary field when data modulation is applied. For high data rates, a small spreading factor is desired in order to avoid an excessive system bandwidth. The family of Barker sequences is investigated in a DSSS-OWS. The Barker sequences are binary spreading sequences which exhibit small aperiodic autocorrelation values and short spreading factors. The system bit error rate (BER) performance is characterized on the diffuse indoor infrared channel when using RAKE reception. Results of BER against Eh/NQ are presented with data rate and spreading factor as parameters. The results demonstrate that of the seven Barker sequences existing, the length three Barker sequence {0 0 1} provides the best system design trade-off in respect of good BER performance and low implementation complexity.     

Code optimization for direct sequence spread spectrum and SAW-matched filter implementation

This paper introduces a new optimization algorithm for the minimization of the time sidelobes of the correlation function of a pseudonoise (PN) sequence by applying dynamic weighting to the sequence. The resulting optimized time sidelobe level sequences are to be used in direct sequence spread spectrum (DS-SS) systems with digital modulations such as BPSK, DPSK, QPSK, etc. The new optimization algorithm starts with a PN sequence. It first optimizes the correlation time sidelobes for the case where the consecutive data bits are identical (11 or 00). It then optimizes the correlation time sidelobes for the case of alternating consecutive data bits (10 or 01). The suppressed time sidelobe level sequences are derived by iterating these algorithms alternately starting from the initial PN sequence. The derived suppressed time sidelobe sequences show excellent correlation characteristics when compared to conventional PN sequences such as maximal length sequences, Gold sequences and Barker codes. Surface acoustic wave (SAW) devices were used to implement the optimized time sidelobe level sequences in a matched filter pair. The design of the apodized SAW-matched filters and their predicted second order effects are presented. The experimental results for the SAW-matched filters for the optimized time sidelobe level sequences derived from a Barker code were found to be in good agreement with the theoretical predictions from this new algorithm.     

Radial basis function neural network for pulse radar detection

A new approach using a radial basis function network (RBFN) for pulse compression is proposed. In the study, networks using 13-element Barker code, 35-element Barker code and 21-bit optimal sequences have been implemented. In training these networks, the RBFN-based learning algorithm was used. Simulation results show that RBFN approach has significant improvement in error convergence speed (very low training error), superior signal-to-sidelobe ratios, good noise rejection performance, improved misalignment performance, good range resolution ability and improved Doppler shift performance compared to other neural network approaches such as back-propagation, extended Kalman filter and autocorrelation function based learning algorithms. The proposed neural network approach provides a robust mean for pulse radar tracking     

Speckle suppression in scanning laser display

The theory of speckle noise in a scanning beam is presented. The general formulas for the calculation of speckle contrast, which apply to any scanning display, are obtained. It is shown that the main requirement for successful speckle suppression in a scanning display is a narrow autocorrelation peak and low sidelobe level in the autocorrelation function of the complex amplitude distribution across a scanning light beam. The simple formulas for speckle contrast for a beam with a narrow autocorrelation function peak were obtained. It was shown that application of a diffractive optical element (DOE) with a Barker code phase shape could use only natural display scanning motion for speckle suppression. DOE with a Barker code phase shape has a small size and may be deposited on the light modulator inside the depth of the focus of the reflected beam area, and therefore, it does not need an additional image plane and complicated relay optics.     

由交织技术构建新的最优跳频序列族

跳频技术具有抗干扰、抗截获、频带共享和码分多址等优点,在民用移动通信、军事无线电通信、现代雷达和声纳等电子系统中具有广泛的应用.寻找和设计最优跳频序列族是研究跳频通信技术的重要议题,因为跳频序列是跳频系统中不可或缺的重要组成部分.本文基于交织技术和中国剩余定理,构造了两类新的跳频序列族,导出了新序列族的各类汉明相关性质.研究结果表明,通过选择恰当的参数和已知跳频序列族,本文所提方法可递归得到大量的最优跳频序列族.     

New high performance SAW convolvers used in high bit rate and wideband spread spectrum CDMA communications system

New surface acoustic wave (SAW) convolver structures with high conversion efficiency and self-temperature compensation characteristics have been developed. Strong piezoelectric substrates, regardless of temperature coefficients of delay (TCD), can be used in these convolvers. New demodulation techniques using the developed SAW convolver for high bit rate and wideband spread spectrum code division multiple access (CDMA) communications have also been developed. I- and Q-channel demodulation data can be derived directly from binary phase shift keying (BPSK) or quadri-phase shift keying (QPSK) CDMA signals. In an experiment using a 128 degrees YX-LiNbO(3) substrate, CDMA signals of 9 Mbps (megabits per second) with 60 Mcps (megachips per second) spread by 13-chip Barker code and 11 Mbps with 140 Mcps spread by 25-chip Shiba's code were clearly demodulated, demonstrating the effectiveness of these techniques for use in future CDMA communications.     

一种新的通信抗干扰技术体制：预编码跳码扩谱

基于常规直扩体制的不足,笔者分析了跳码扩谱的必要性,提出和研究了一种新的预编码跳码扩谱通信抗干扰技术体制,重点讨论了预编码跳码扩谱的基本原理与自编码跳码扩谱的异同点、关键技术等,最后分析了其基本性能。该体制和性能已在实际中得到初步验证。     

Critical analysis of frequency selective surfaces embedded composite microwave absorber for frequency range 2–8 GHz

Radar wave absorbers are important for the reduction of radar cross section of the target for stealth applications. Earlier the radars were available in the frequency range 8–12 GHz (X-band) and 12–18 GHz (Ku-Band). Due to recent advancement in radar technology, radars are now available from 2 to 18 GHz frequency range. So there is an urgent need to develop such a material that can work as radar wave absorber in the lower frequency band of the microwave spectrum i.e., 2–8 GHz. For this purpose the selection of material is an important criterion as the radar wave absorption depends primarily upon the material characteristics i.e., complex permittivity and complex permeability. For lower frequency radar wave absorption, the material must also possess the conducting property along with dielectric and magnetic properties. Therefore, an attempt has been made to develop a radar wave absorbing nano-composite material by selecting constituent materials with such inherent properties that can work for the absorption of radar wave in the lower frequency range. It is observed that the developed composite give good absorption in the lower frequency range but with narrow radar wave absorption bandwidth (4–7 GHz). So we have explored the possibility of the efficient use of an advanced electromagnetic technique like frequency selective surface to enhance the radar wave absorption bandwidth in the lower frequency region of the microwave frequency spectrum and precaution has been taken such that complexity due to FSS can be avoided. It has been observed that the synthesised single layer absorber with single square loop, cross dipole and Jerusalem cross FSSs provides radar wave absorption bandwidth in the frequency range 2–8 GHz.     

BER performance analysis of compound chaotic sequence (CCS)-based NR-DCSK system under multipath fading channel

This paper presents the performance analysis of compound chaotic sequence (CCS)-based noise reduction differential chaos shift keying (NR-DCSK) system under multipath Rayleigh fading channel conditions. The special characteristics of chaotic sequences are their deterministic randomness behaviour that adds security and multipath immunity to the data when used as a carrier in communication systems. In this paper, the chaotic sequences are generated by combining the outputs of chaotic maps, such as logistic map, Chebyshev map, Bernoulli shift map, tent map, etc., leading to new complex sequences known as CCSs. This sequence possesses more randomness, overcomes severe interference levels encountered during transmission and provides higher multipath immunity compared with those of pseudo-noise (PN) codes. Since NR-DCSK is a spread spectrum technique, its performance in wireless multipath fading channels has important considerations. The CCS is used as a carrier in NR-DCSK systems, which leads to improved bit error rate (BER) performance. Comparisons of simulation results to theoretical BER expressions of additive white Gaussian noise (AWGN) and Rayleigh fading channels have been carried out to test the efficiency of the proposed CCS-based NR-DCSK system.     

Practical spread spectrum pulse compression for ultrasonic tissue imaging

Spread spectrum pulse compression is a signal processing algorithm that enhances critical system performance parameters such as signal-to-noise ratio, peak power requirements, minimum detectable signal, and total dynamic range. For this research, a digital, real-time, Barker coded, bi-phase modulator was designed and constructed, as well as a simple ultrasonic test tank containing both synthetic targets and excised goat's liver. Upon reception and demodulation of the spread spectrum ultrasonic echo, cross-correlation with a sidelobe suppression filter was performed. Due to limitations such as narrow bandwidth, and very short minimum ranges, a practical ultrasonic pulse compression system must be restricted to short code lengths. For 13 bit Barker code compression, the expected increase in signal-to-noise ratio of 11 dB was realized; at the same time greater than 30 dB of instantaneous dynamic range was maintained.     

Measurement of radio channels using an elastic convolver and spreadspectrum modulation. I. Implementation

Using spread spectrum signals a measurement system has been built in order to measure the radio channel in steel works. It transmits a 511-chip long maximum length sequence with a chip rate of 44 MHz: binary phase shift keying modulated onto a 1.75 GHz carrier. A surface acoustic wave convolver yields the channel's impulse response with a time resolution of about 20 ns. The periodic autocorrelation performed within a measurement window removes aperiodic correlation sidelobes which limit the dynamic range of a matched-filter implementation. Additionally, absolute time delay measurements of all echoes were obtained using a synchronization cable connecting receiver and transmitter. Compared to other spread spectrum correlation techniques, the convolver makes possible a much simpler real-time measurement setup     

New class of 2D code and encoders for multiplexed measuring systems

A new class of 2D codes and encoders built by the self-supporting method, using the direct product of ternary pseudorandom sequences that consist of −1, 0, +1 and have an ideal periodic autocorrelation function is proposed and studied. Expressions are derived for the optimality criteria used in experimental design with allowance for the statistical characteristics of the distribution of the radiation sources and noise superimposed on the measurements. The point spread functions presented characterize the topographical properties of the multiplexed measuring system with an encoder based on a ternary sequence. Translated from Izmeritel'naya Tekhnika, No. 10, pp. 49–53, October, 1997.     

用于医学超声内窥成像的编码激励技术

为了提高医学超声内窥系统中图像的信噪比,将编码激励技术引入超声成像系统.仿真研究了编码长度和换能器相对带宽对成像信噪比（SNR）提升的影响.发现在采用长度较短的2-5位Barker编码激励时,成像信噪比的提升在编码长度为4位时达到峰值,且信噪比的提升随换能器相对带宽的增加而增加.基于超声内窥成像系统的编码激励实验表明,在峰值激励电压为25 V和换能器相对带宽为20%的情况下,采用编码激励技术能够获得1.85 dB的成像信噪比提升.     

远程水声扩频通信时变多普勒估计与码片同步联合处理算法

在低信噪比的远程浅海水声移动通信中,传统多普勒估计算法难以有效跟踪时变多普勒因子,针对该问题提出了一种基于时频联合搜索的新的时变多普勒跟踪算法。利用正交支路扩频码的扩频增益和互相关特性,结合迭代处理技术,搜索扩频码符号在不同码片相位下的多普勒频谱峰值,选择具有最大峰值的频谱对应的多普勒因子和码片相位,对信号进行多普勒补偿和码片同步。蒙特卡洛仿真表明,该算法能够在信噪比为-18 dB的情况下有效地对10 m.s~(-1)以内产生的时变多普勒进行逐符号跟踪补偿。算法经过海试测试,在远程浅海移动通信中成功完成了多普勒跟踪估计,通信误码率达到10~(-3)。     

Optimal and Suboptimal Quadriphase Sequences Derived from Maximal Length Sequences over Z _{{\bf 4}}

The paper presents families of quadriphase sequences derived from maximal length sequences over having good correlation properties. These are: (i) families of quadriphase sequences of period from maximal length sequences over ; each family consisting of sequences, (ii) families of quadriphase sequences of period from interleaved maximal length sequences over ; each family consisting of sequences. Such sequences are of interest in quadriphase modulated code division multiple access communication systems, where it is desirable to have large sets of sequences that possess low value of , the maximum magnitude of the periodic crosscorrelation and out of phase auto-correlation values. The sequences over are viewed as trace functions of appropriately chosen unit elements of Galois extension rings of . Quadriphase sequences are then obtained from sequences, by a quadriphase mapping, , from to roots of unity, given by, ; where and . Periodic correlation properties (correlation values and their distribution) of the quadriphase sequences are obtained by using an Abelian association scheme on the elements of the corresponding Galois extension ring of . The majority of the families of sequences derived are optimal with respect to the Welch lower bound on ; the rest being suboptimal with bounded by , where is the period\newpage of the sequences. However nearly half of the sequences in these families are balanced. Received July 18, 1995; revised version November 29, 1996     

Level crossing statistics for optical beam wander in a turbulent atmosphere with applications to ground-to-space laser communications

Level crossing statistics is applied to the complex problem of atmospheric turbulence-induced beam wander for laser propagation from ground to space. A comprehensive estimate of the single-axis wander angle temporal autocorrelation function and the corresponding power spectrum is used to develop, for the first time to our knowledge, analytic expressions for the mean angular level crossing rate and the mean duration of such crossings. These results are based on an extension and generalization of a previous seminal analysis of the beam wander variance by Klyatskin and Kon. In the geometrical optics limit, we obtain an expression for the beam wander variance that is valid for both an arbitrarily shaped initial beam profile and transmitting aperture. It is shown that beam wander can disrupt bidirectional ground-to-space laser communication systems whose small apertures do not require adaptive optics to deliver uniform beams at their intended target receivers in space. The magnitude and rate of beam wander is estimated for turbulence profiles enveloping some practical laser communication deployment options and suggesting what level of beam wander effects must be mitigated to demonstrate effective bidirectional laser communication systems.     

Time-domain estimation of the center frequency and spread of Doppler spectra in diagnostic ultrasound

Two sets of estimators of the centre frequency and mean-square bandwidth of a Doppler spectrum are analyzed. The first set is based on a linear combination of the complex autocorrelation of the Doppler signal for different lags. It is then shown that the argument (phase angle) of the complex autocorrelation function at unit sample lag is a close approximation to the mean Doppler shift, whereas its modulus (magnitude) gives information about the mean-square bandwidth of the spectrum. Approximate expressions for the bias and variance of the estimators are derived, valid for long averaging times. The performances of the estimators are compared for signals with rectangular spectra and different signal-to-noise ratios.     

Time-domain estimation of the center frequency and spread of Doppler spectra in diagnostic ultrasound

Two sets of estimators of the center frequency and mean square bandwidth of a Doppler spectrum are analyzed. The first set is based on a linear combination of the complex autocorrelation of the Doppler signal for different lags. It is shown that the argument (phase angle) of the complex autocorrelation function at unit sample lag is a close approximation to the mean Doppler shift, whereas its modulus (magnitude) gives information about the mean square bandwidth of the spectrum. Approximate expressions for the bias and variance of the estimators are derived, valid for long averaging times. The performances of the estimators are compared for signals with rectangular spectra and different signal-to-noise ratios.     

Study on the Detection of CFRP Material with Subsurface Defects Using Barker-Coded Thermal Wave Imaging (BC-TWI) as a Nondestructive Inspection (NDI) Tool

In this paper, a Barker-coded thermal wave imaging approach is reported on the detection of carbon fiber reinforced polymer (CFRP) laminate with subsurface defects, using an integrated Barker code sequence and sinusoidal carrier-modulated laser as the excitation source. Artificial flat bottom holes as subsurface defects are prepared for the experimental investigation. Cross-correlation (CC) algorithm is applied for extracting characteristics of thermal wave signal and forming the corresponding peak delay time and phase images. The effects of Barker code sequence length and carrier-modulated frequency are investigated, which are both most important factors on the detectability of BC-TWI method. The results of the experiments show 5-bit Barker code and 0.1 Hz carrier frequency are the most suitable selection for enhancing inspection capability and obtaining the highest image SNR for a given CFRP laminate material. Furthermore, a comparative experiment is carried out between BC-TWI and lock-in thermography (LIT) method by taking the defect contrast and SNR into account. The results indicate that the BC-TWI CC phase image has higher contrast and SNR than the LIT phase image.     

Clutter rejection filters in color flow imaging: a theoretical approach

A general class of linear clutter rejection filters is described, covering the commonly used filter types including FIR/IIR filters with linear initialization, as well as regression filters, where the clutter component is estimated by least square curve fitting. The filter can be described by a complex valued matrix, and a frequency response is defined. However, in contrast to a time invariant filter, the general linear filter may create frequency components which are not present in the input signal. This produces bias in the velocity and velocity spread estimates. It is shown that the clutter filter effect on the autocorrelation estimates can be described by a frequency domain transfer function, but unlike time invariant filters, the transfer function is different for each temporal lag of the autocorrelation function. Using a two dimensional (axial and temporal dimension) model of the received signal, the bias in velocity and velocity spread is quantified, both for the autocorrelation algorithm and the time shift cross-correlation estimator. Theoretical expressions, as well as numerical examples are given.