Performance of binary block codes at low signal-to-noise ratios

The performance of general binary block codes on an unquantized additive white Gaussian noise (AWGN) channel at low signal-to-noise ratios is considered. Expressions are derived for both the block error and the bit error probabilities near the point where the bit signal-to-noise ratio is zero. These expressions depend on the global geometric structure of the code, although the minimum distance still seems to play a crucial role. Examples of codes such as orthogonal codes, biorthogonal codes, the (24,12) extended Golay code, and the (15,6) expurgated BCH code are discussed. The asymptotic coding gain at low signal-to-noise ratios is also studied     

基于Turbo码迭代的低信噪比载波同步技术

针对低信噪比短突发通信系统的载波恢复问题,研究了一种基于迭代思想的载波相位估计算法.该算法首先利用导频进行初始的载波相位估计,然后再利用Turbo译码器输出的软信息进行载波相位细估计,进而实现有效的载波同步.仿真结果表明,该算法仅利用11个QPSK导频符号就能校正较大范围的相位偏移,进而达到理想的误比特性能.     

Some analytical and experimental phase-locked loop results for low signal-to-noise ratios

This paper is concerned with the nonlinear behavior of the second-order phase-locked loop (PLL) in the presence of noise. The loop filter is of the proportional-plus-integral control type. This filter corresponds to the one generally employed for carrier tracking purposes in the implementation of phase-coherent communication systems. The paper is composed essentially of two parts: the first part presents analytical results which pertain to the probability distribution of the phase-error. Since these analytical results are approximations, valid only for certain regions of signal-to-noise ratio, they are complemented by experimental results obtained from simulation of the PLL system in the laboratory. The experimental techniques used to measure the statistical properties of the loop behavior and the corresponding results comprise the second part of the paper. Approximate analytical expressions for the distribution of the system phase-error are first obtained by using the Fokker-Planck apparatus and, secondly, by assuming that the PLL behaves as a very narrow band-pass filter. The range of signal-to-noise ratios for which these approximations are valid is obtained by graphically comparing the analytical expressions to experimentally derived phase-error distributions. In addition, measurements relative to the variance of the phase-error are compared to those predicted by the linear PLL theory and the variance as computed from the approximate solutions. Finally, experimental results relative to the probability distribution of the time intervals between cycle-slipping events are given for signal-to-noise ratios in a range where the linear PLL theory does not apply. In particular, the maximum length of time the loop may be expected to remain in-lock is illustrated graphically as a function of signal-to-noise ratio in the loop bandwidth.     

Pseudowhitening of weather Radar signals to improve spectral moment and polarimetric variable estimates at low signal-to-noise ratios

Pseudowhitening of oversampled signals in range is proposed as a method to improve the performance of spectral moment and polarimetric variable estimators on weather surveillance radars. In an attempt to overcome the noise sensitivity of the whitening transformation, a solution based on the minimum mean-square-error criterion is considered first; however, this transformation is less practical than whitening because it requires knowledge of the signal-to-noise ratio at every range location. Pseudowhitening techniques are introduced as practical solutions that achieve a suboptimal compromise between variance reduction and noise sensitivity. Based on regularization methods for the solution of ill-conditioned problems, two pseudowhitening schemes are proposed: the clipped singular value decomposition transformation and the sharpening filter. By comparing their statistical performance with theoretical minimum bounds, it is shown that pseudowhitening-based estimators are almost optimal under practical conditions. Estimators based on pseudowhitening techniques avoid the pitfalls of their whitening-transformation-based counterparts and lead to more accurate radar products and/or rapid data acquisition for a much wider range of signal-to-noise ratios.     

A novel non-data-aided symbol timing recovery technique for OFDM systems

A new highly efficient non-data-aided technique to recover symbol timing of orthogonal frequency-division multiplexing systems is proposed. The algorithm in the proposed work exploits the interference that results due to the loss of orthogonality between subcarriers, where the second-order statistics of the resulting interference is proportional to the offset from the optimum sampling point. The presented technique does not require prior fine carrier synchronization, and it is capable of extracting symbol timing at low E/sub s//N/sub 0/ values with large carrier frequency offsets (CFOs). The system performance was investigated in multipath fading channels with large CFOs and additive white Gaussian noise.     

低信噪比条件下的调制方式识别

传统的调制方式识别方法通常假设接收信号是平稳随机过程,一般要在信噪比不低于10dB时才能取得较高的识别率。本文根据模拟调制信号的谱特性和谱相关特性,给出了低信噪比下的调制方式识别算法,理论分析和仿真结果表明,本文提出的算法,在低信噪比(3dB)条件下,仍然具有很高的识别率。     

Switched-capacitor filters driven with very low voltage clock signals

It is well known that the switches of a very low voltage (1.5V) switched-capacitor filter in a standard CMOS process must be driven with a clock signal higher than the power supply, often generated on-chip. The authors present, however, a technique to implement very low voltage switched-capacitor filters with switches driven at the same very low voltages.<>     

A new action potential detector using the MTEO and its effects on spike sorting systems at low signal-to-noise ratios

This paper considers neural signal processing applied to extracellular recordings, in particular, unsupervised action potential detection at a low signal-to-noise ratio. It adopts the basic framework of the multiresolution Teager energy operator (MTEO) detector, but presents important new results including a significantly improved MTEO detector with some mathematical analyses, a new alignment technique with its effects on the whole spike sorting system, and a variety of experimental results. Specifically, the new MTEO detector employs smoothing windows normalized by noise power derived from mathematical analyses and has an improved complexity by utilizing the sampling rate. Experimental results prove that this detector achieves higher detection ratios at a fixed false alarm ratio than the TEO detector and the discrete wavelet transform detector. We also propose a method that improves the action potential alignment performance. Observing that the extreme points of the MTEO output are more robust to the background noise than those of the action potentials, we use the MTEO output for action potential alignment. This brings not only noticeable improvement in alignment performance but also quite favorable influence over the classification performance. Accordingly, the proposed detector improves the performance of the whole spike sorting system. We verified the improvement using various modeled neural signals and some real neural recordings.     

Fast acquisition clock and data recovery circuit with low jitter

This paper presents a half-rate clock and data recovery circuit (CDR)that combines the fast acquisition of a phase selection (PS) delay-locked loop (DLL) with the low jitter of a phase-locked loop (PLL). The PLL acquisition time improves considerably with use of a phase frequency magnitude detector(PFMD) that feeds back an estimate of the magnitude of the frequency difference in addition to the sign. Measurements in 0.5/spl mu/m CMOS technology show operation up to 700 Mb/s, a 7% acquisition range, an initial acquisition time of 8 bit times with jitter of 30% bit time, and jitter of 16 ps after the PLL acquires lock. With a phase frequency detector (PFD), the PLL locks in about 700 ns from an initial frequency difference of 7%. Measurements using a PFMD show the 700 ns PLL acquisition time is reduced on average by about a factor of 5 to 140 ns from an initial 7% frequency difference. The power dissipation is 300mW.     

提高人眼波前传感器信噪比的实验研究

Hartmann-Shack(H-S)人眼波前传感器噪声主要是角膜的反射光。本文用模拟人眼对消噪声的两种方法进行了实验研究。首先,对使用偏振分光棱镜(PBS)的H-S系统,在系统出瞳不变,入瞳变化的条件下测量了对应的信噪比。实验证明：与采用薄半透半反镜相比,在光线斜入射消除PBS内部反射噪声的前提下,该方法可使信噪比提高到11：1。其次,对离轴入射法进行实验研究,获得200：1信噪比,最后对上述两种方法获得的光斑点阵进行了比较分析。     

Minimum required power for carrier recovery at optical frequencies

The purpose of this study is to evaluate the minimum optical power required to achieve carrier recovery with arms phase error of 10°. This value corresponds to a degradation of about half a decibel for a coherent communication system using BPSK modulation. Two basic carrier recovery configurations were selected for this study. One uses an injection-locked oscillator; the other utilizes a phase-locked loop. The local source for both circuits is a laser providing 4 mW at 1.5 μm and having a full 3-dB linewidth of 20 MHz. The results show that the carrier recovery requirement can be achieved with a phase-locked loop receiving an unmodulated signal of -50 dBm. The minimum required power increases to a least -26 dBm for an injection-locked oscillator. A reduction of the sources linewidth by a given factor will decrease the minimum required power by the same amount.     

A wide range,low power clock and data recovery scheme for RFID tags

An extremely low power clock and data recovery (CDR) circuit that covers a large input speed range is reported in this paper. The CDR recovers accurately-synchronized clock and data directly from the pulse position modulated (PPM) input, and the bit information embedded in time domain is converted into a voltage through charging integration on linear capacitors. The threshold voltage required for data recovery is accurately generated from the input bit period to produce the best noise margin for both bit 1 and bit 0. Furthermore, the current that is used for charge integration is dynamically set by a feedback loop to expand the input speed range. The prototype CDR circuit was implemented using a standard CMOS 0.25 μm technology. Results show that the designed prototype can cover a large input speed range from 0.5 to 500 kbit/s, while only consumes 11–75 μW of power, respectively.     

相干检测中的能量守恒与信噪比分析

对光通信相干检测原理中两个容易引起误解的问题进行了深入分析。一是零差接收机的光耦合器输入输出能量不守恒现象,指出这是因为混频方案默认信号光和本振光频率和相位完全同步,提供了隐形的"锁相增益"。二是纠正了经典文献将外差接收机噪声带宽设为符号率一半的错误,证明了外差检测中频信号和基带信号的信噪比相对零差检测基带信号的信噪比分别低6dB和3dB。     

The clipping loss in correlation detectors for arbitrary input signal-to-noise ratios

The detection of stationary Gaussian signals in a background of stationary Gaussian noise by the analog multiplier correlator, the polarity coincidence correlator (PCC), and the sample polarity coincidence correlator (SPCC) is considered. It is assumed that signal and noise have identical normalized autocovariance functions, and they are not cross correlated with each other. The main contributions of this paper are the exact expressions for the output signal-to-noise ratio (SNR) for the correlators mentioned for all values of the input SNR. It is shown that there exists a critical value of the input SNR, such that, whenever this value is exceeded, the PCC output SNR exceeds that of the analog correlator. A sufficient condition for this gain in output SNR is obtained in terms of the input SNR. This result is illustrated for stationary Gauss-Markov processes.     

PLL-based BiCMOS on-chip clock generator for very high-speedmicroprocessor

Described is a phase-locked loop (PLL)-based BiCMOS on-chip clock generator (PCG), which is used to generate an internal clock synchronized to a reference clock from outside the chip. In order to obtain a very wide operation bandwidth, it is proposed that the PCG include a compensation circuit for voltage-controlled oscillator (VCO) operation. The compensation circuit varies the oscillation bandwidth of the VCO according to the reference clock frequency, preventing the expected oscillation frequency from being outside the oscillation bandwidth. The PCG is designed and fabricated with 1.0 μm BiCMOS technology, and it achieves an operation bandwidth of 3 to 90 MHz     

Architectures for multi-gigabit wire-linked clock and data recovery

Clock and data recovery (CDR) architectures used in high-speed wire-linked communication receivers are often shown as PLL or DLL based topologies. However, there are many other types of CDR architectures such as phase-interpolator, oversampling and injection locked based topologies. The best choice for the CDR topology will depend on the application and the specification requirements. This paper presents an overview and comparative study of the most commonly used CDR architectures. This analysis includes the circuit structures, design challenges, major performance limitations and primary applications. Finally, the tradeoffs among the various CDR architectures are summarized.     

High-performance detector read-out at very low temperatures

An infra-red detection system must be physically colder than the objects it needs to observe. As some of the objects to be observed by the European Space Agency's Infra-red Space Observatory (ISO) might be as cold as a few tens of degrees Kelvin, it is essential that the ISO's detectors should themselves operate at temperatures close to absolute zero. To avoid problems with electromagnetic interference, it is also necessary that detector read-out (i.e. first-stage signal amplification) be located as close as possible to the detectors. As conventional amplifiers cannot work at temperatures close to absolute zero, special techniques had to be used for the ISO's detectors     

Visual communication at very low data rates

A study is reported of the extraction of two-level cartoons from moving grey-level images for visual communication in the range 4.8-19.2 kbits/s. It is postulated that perceptually significant features of the human face and hands, at which cartoon lines should be drawn in the image, occur wherever surfaces in object space are approximately tangential to the line of sight of the camera or viewer. The implications of this postulate are analyzed for smooth lambertian reflectors seen against a background of the same luminance factor; it is shown that the combined effects of surface gradient and illumination lead to the formation of luminance valleys at the significant features. An experimental comparison of several detectors is described which confirmed the advantage of valley-based detection for deriving economical but recognizable cartoons.     

Procedure for determining diode parameters at very low forward voltage

A technique is proposed to extract the reverse saturation current parameter and ideality factor of semiconductor junctions from the low forward voltage region of the device’s characteristics. The method involves performing a mathematical operation on the experimental data that allows to calculate the parameters at values of forward current smaller than the reverse saturation current. The procedure was tested and its accuracy verified on synthetic I–V characteristics, with and without added simulated experimental error or noise. Good agreement is obtained between the parameters used in modeling and the extracted values. The procedure was also applied to experimentally measured I_B–V_BE characteristics of a real power BJT.