Robust dual-quantisation multibit sigma-delta modulator

A new dual-quantisation sigma-delta modulator is proposed, which introduces an additional feedback path in the input of the second integrator. In this way, unlike other dual-quantisation architectures, larger signal-to-noise ratios can be obtained by means of aggressive noise-shaping, like in a conventional multibit modulator. The proposed modulator is also shown to be more robust against non-idealities than other dual-quantisation architectures.     

Design of multibit noise-shaping data converters

A synthesis methodology for selecting locations of thez-domain poles for noise-shaping coders that use multibit internal converters is presented. A key aspect of the proposed methodology is the use of the |L|₁ norm of the noise transfer function to guarantee stability rather than the power gain or |L|₂ norm which is commonly used in the design of 1-bit noise-shaping coders. Simulation verifies that the performance predicted by the new method is within a few dBs of the actual performance. In addition, two hand-designed loop filters from the literature are compared with designs generated by the proposed method.     

Vulnerability of multibit clock-controlled cascades to inversionattack

The inversion attack on nonlinear filter generators is adapted to deal with cascades of clock-controlled shift registers with multibit outputs. It is shown that under certain conditions such cascades may be vulnerable to these attacks     

Self-calibration techniques for a second-order multibit sigma-deltamodulator

Design techniques for self-calibration of the digital-to-analog converter DAC in a multibit sigma-delta modulator are described. When used in conjunction with dynamic element matching, self-calibration provides linearity performance suitable for digital audio applications. The dynamic element matching circuitry provides the mechanism of determining device mismatch for self-calibration. Practical circuit details and an effective calibration method are discussed. Test results from a l-μm CMOS test chip are presented. In this test system, a second-order loop with a 3-b quantizer achieves an 89-dB dynamic range and -91-dB harmonic distortion after calibration. In addition, a new method of detecting the presence of tones is described, using the entropy of the spectrum of the decimation filter output     

Tree structure for mismatch noise-shaping multibit DAC

A tree structure for the efficient implementation of segment selection in a multibit oversampled DAC is presented. The structure offers high-order noise-shaping of mismatch errors without the use of vector quantisation and so can be implemented in low silicon area     

A multibit sigma-delta ADC for multimode receivers

A 2.7-V sigma-delta modulator with a 6-bit quantizer is fabricated in a 0.18-/spl mu/m CMOS process. The modulator makes use of noise-shaped dynamic element matching (DEM) and quantizer offset chopping to attain high linearity over a wide bandwidth. The DEM algorithm is implemented in such a way as to minimize additional delay within the feedback loop of the modulator, thereby enabling the use of the highest resolution quantizer yet reported in a multibit sigma-delta analog-to-digital converter of this speed. The part achieves 95-dB peak spurious-free dynamic range and 77-dB signal-to-noise ratio over a 625-kHz bandwidth, and consumes 30 mW at a sampling frequency of 23 MHz. The part achieves 70-dB signal-to-noise ratio over a 1.92-MHz bandwidth and dissipates 50 mW when clocked at 46 MHz.     

Corporate attitudes and commitments to telecommuting

The increased use of telecommuting by information and knowledge workers in many fields has been fostered by the availability of enabling technologies. Notebook sized computers, easy-to-use software, access to information both private or public, and electronic mail capabilities available worldwide have made telecommuting possible. This paper examines some of the key factors involved in the telecommuting evolution. Coverage includes background on how telecommuting started and which current technologies lend themselves to telecommuting. Also, the article reviews the trends that are stimulating dramatic double-digit growth and describes corporate attitudes toward and experiences with telecommuting. Topics covered include corporate commitments, motivation, productivity, supervision and employee satisfaction     

Distributed random signal detection with multibit sensor decisions

Distributed detection of weak random signals in additive, possibly non-Gaussian, noise is considered for cases with multibit sensor decisions. Signal-to-noise ratios are assumed unknown and the signals at the different sensors may be statistically dependent. Analytical expressions are provided that describe the best way to fuse the quantized observations for cases with any given number of sensors. The best schemes for originally quantizing the observations at each sensor are also studied for the case of an asymptotically large number of sensors. These schemes are shown to minimize the mean-squared error between the best weak-signal test statistic based on unquantized observations and the best weak-signal test statistic based on quantized observations. Analytical expressions describing optimum sensor quantizers are provided. The approach used to obtain these expressions insures these sensor quantizers give good performance for cases with a finite number of sensors. A novel iterative technique to search for optimum sensor quantizers efficiently is described. Numerical solutions are presented, some of which involve cases where the best schemes for independent signal observations are shown to be suboptimum     

Addressing static and dynamic errors in unit element multibit DACs

Nonlinearity in multibit current-steering digital-to-analogue converters (DACs) originates from static mismatch and switching imperfections. Several techniques are presented to avoid this nonlinearity. When applied, the DACs are suitable for use in the feedback of continuous-time /spl Sigma//spl Delta/-converters.     

Efficient construction of multibit tries for IP lookup

Srinivasan and Varghese (see ACM Trans. Comput. Syst., p.1-40, 1999) have proposed the use of multibit tries to represent routing tables used for Internet (IP) address lookups. They propose dynamic programming algorithms to determine the strides of optimal multibit fixed-stride and variable-stride tries. We improve on these algorithms by providing alternative dynamic programming formulations for both fixed- and variable-stride tries. While the asymptotic complexities of our algorithms are the same as those for the corresponding algorithms of Srinivasan and Varghese, experiments using real IPv4 routing table data indicate that our algorithms run considerably faster. Our fixed-stride trie algorithm is two to four times faster on a SUN workstation and 1.5 to three times faster on a Pentium IV PC. On a SUN workstation, our variable-stride trie algorithm is between two and 17 times faster than the corresponding algorithm of Srinivasan and Varghese; on a Pentium IV PC, our algorithm is between three and 47 times faster. An added feature of our variable-stride trie algorithm is the ability to insert and delete prefixes taking a fraction of the time needed to construct an optimal variable-stride trie "from scratch".     

Pipelined multibit oversampled digital-to-analog converters with capacitor averaging

A combination of pipelined architecture and dynamic element matching technique is applied to multibit oversampled D/A (digital to analog) converters. The approach translates the harmonic distortion components of the nonideal internal DAC (digital-to-analog converter) of the oversampled DAC to high-frequency components, which can then be filtered out by the analog low-pass filter for anti-imaging. Computer simulations have confirmed that with this approach a third-order oversampled DAC employing a 3-bit quantizer, a 3-bit pipelined internal DAC with a random mismatch of 0.1%, can achieve a 94-dB dynamic range with an oversampling ratio of 64 while eliminating the harmonic distortion.This work was supported by NSERC (Canada).     

Comparison of mismatch error shaping in multibit oversampledconverters

A number of mismatch error-shaping schemes for oversampled DACs are compared. Simulation results are presented to show that such a comparison should take account of the distribution of the mismatch errors, since the pattern of errors present affects different algorithms in different ways. A scheme for improving the error tolerance of one mismatch shaping architecture is presented     

The combinatorics of authentication and secrecy codes

This paper is a study of the combinatorics of unconditionally secure secrecy and authentication codes, under the assumption that each encoding rule is to be used for the transmission of some numberL of successive messages. We obtain bounds on the number of encoding rules required in order to obtain maximum levels of security. Some constructions are also given for codes which have the minimum number of encoding rules. These constructions use various types of combinatorial designs.     

关于密码体制的完善保密性

本文讨论了密码体制的完善保密性,在分解密钥空间的基础上,给出了般密码体制完善保密的一个特征和两个判定条件;证明了三个特殊的密码体制完善保密的充分和必要条件。这些结果推广了保密系统的通信理论,对密码体制的设计和理解提供了帮助。     

Conditionally-perfect secrecy and a provably-secure randomized cipher

Shannon's pessimistic theorem, which states that a cipher can be perfect only when the entropy of the secret key is at least as great as that of the plaintext, is relativized by the demonstration of a randomized cipher in which the secret key is short but the plaintext can be very long. This cipher is shown to be perfect with high probability. More precisely, the eavesdropper is unable to obtain any information about the plaintext when a certain security event occurs, and the probability of this event is shown to be arbitrarily close to one unless the eavesdropper performs an infeasible computation. This cipher exploits the assumed existence of a publicly-accessible string of random bits whose length is much greater than that of all the plaintext to be encrypted; this is a feature that our cipher has in common with the previously considered book ciphers. Two modifications of this cipher are discussed that may lead to practical provably-secure ciphers based on either of two assumptions that appear to be novel in cryptography, viz., the (sole) assumption that the enemy's memory capacity (but not his computing power) is restricted and the assumption that an explicit function is, in a specified sense, controllably-difficult to compute, but not necessarily one-way.A preliminary version of this paper was presented at Eurocrypt '90, May 21–24, Århus, Denmark, and has appeared in the proceedings, pp.361–373.     

Mismatch shaping for a current-mode multibit delta-sigma DAC

Mismatch shaping allows the use of multibit quantization in delta-sigma analog-to-digital converters and digital-to-analog converters (DAC's) since it noise-shapes the error caused by static element mismatch in a multibit DAC. In this paper, mismatch-shaping techniques for low-pass delta-sigma (ΔΣ) modulators are reviewed, and a mismatch-shaping technique for bandpass ΔΣ modulators is described. The dynamic error caused by frequent element switching is identified as a major source of error in a current-mode DAC with a continuous-time output. Modifying the mismatch-shaping algorithm to account for this effect yields a continuous-time ΔΣ DAC that is insensitive to both element mismatch and element switching dynamics. Experimental results confirm the effectiveness of the proposed techniques     

Key-minimal cryptosystems for unconditional secrecy

This paper is concerned with cryptosystems offering perfect or unconditional secrecy. For those perfect-secrecy systems which involve using keys just once, the theory is well established; however, this is not the case for those systems which involve using a key several times. This paper takes a rigorous approach to the definition of such systems, and exhibits some new families of examples of systems providing perfect secrecy for which the number of keys is minimal.