A Homotopy 2-Groupoid of a Hausdorff Space

If X is a Hausdorff space we construct a 2-groupoid G ₂ X with the following properties. The underlying category of G ₂ X is the `path groupoid" of X whose objects are the points of X and whose morphisms are equivalence classes f, g of paths f, g in X under a relation of thin relative homotopy. The groupoid of 2-morphisms of G ₂ X is a quotient groupoid X / N X, where X is the groupoid whose objects are paths and whose morphisms are relative homotopy classes of homotopies between paths. N X is a normal subgroupoid of X determined by the thin relative homotopies. There is an isomorphism G ₂ X(f,f) ₂(X, f(0)) between the 2-endomorphism group of f and the second homotopy group of X based at the initial point of the path f. The 2-groupoids of function spaces yield a 2-groupoid enrichment of a (convenient) category of pointed spaces.We show how the 2-morphisms may be regarded as 2-tracks. We make precise how cubical diagrams inhabited by 2-tracks can be pasted.     

Uniformly Packed Codes and More Distance Regular Graphs from Crooked Functions

Let V and W be n-dimensional vector spaces over GF(2). A function Q : V W is called crooked (a notion introduced by Bending and Fon-Der-Flaass) if it satisfies the following three properties:

Multiple access interference cancellation in Manchester-coded synchronous optical PPM-CDMA network

A new interference cancellation technique for direct-detection optical code- division multiple-access (OCDMA) network employing pulse-position modulation (PPM) is proposed in this paper. The multiple access interference (MAI) estimation is achieved by pre-reserving one of optical spreading code sequences at the receiver based on the correlation property of padded modified prime codes (PMPC). The estimated interference is then cancelled out after photo-detection process. Additionally, the transmitted signal is Manchester-coded to further improve the system performance. Based on this proposed interference canceller in a shot-noise limited regime, we have obtained an expression for the upper bound of the bit-error probability (BEP) taking into account effects of both MAI and shot-noise. This BEP is compared with that of a PPM-OCDMA without cancellation. Finally, the receiver structure of the proposed optical network unit (ONU) is fairly simple to compare with the conventional cancellation schemes.     

钕铁硼在高温超导磁悬浮技术实验中的应用

将新材料钕铁硼作为磁导轨引入到高温超导磁悬浮技术实验中,介绍了高温超导悬浮技术实验的基本原理、仪器及钕铁硼对实验的改进.     

基于三种调制模式下的turbo码光无线通信系统分析

从分析大气对光无线通信系统的影响出发,研究了大气信道特性,建立了大气信道信噪比和能见度的关系.为降低系统误码率,提出了基于OOK、BPSK、BPPM三种调制模式的turbo码的编码方案,推导出三种不同方案对应的turbo码迭代MAP算法.应用建立的信噪比关系,分析和计算了基于三种不同方案的系统的误码率.结果表明,基于BPSK的turbo码系统比OOK和BPPM系统的信噪比降低约3dB,且在高的信噪比情况下,BPSK系统能显著降低系统的误码率.因此,基于BPSK的turbo码方案较适合光无线通信系统.     

Novel family of prime codes for synchronous optical CDMA

In this paper we have proposed a novel prime spreading sequence family hereby referred to as “new-Modified Prime Code (n-MPC)” for direct-detection synchronous optical code-division multiple-access (OCDMA) networks. The new code has been applied to Overlapping Pulse-Position Modulation (OPPM) CDMA system and the performance of system is evaluated. In addition, we have analyzed the system throughput and also introduced a new interference cancellation technique which significantly improves the bit error probability (BEP) of OPPM-OCDMA systems.     

A novel super-FEC code based on concatenated code for high-speed long-haul optical communication systems

The structures of the novel super forward error correction (Super-FEC) code type based on the concatenated code for high-speed long-haul optical communication systems are studied in this paper. The Reed-Solomon (RS) (255, 239) + Bose-Chaudhuri-Hocguenghem (BCH) (1023, 963) concatenated code is presented after the characteristics of the concatenated code and the two Super-FEC code type presented in ITU-T G.975.1 have theoretically been analyzed, the simulation result shows that this novel code type, compared with the RS (255, 239) + convolutional-self-orthogonal-code (CSOC) (k₀/n₀ = 6/7, J = 8) code in ITU-T G.975.1, has a lower redundancy and better error-correction capabilities, and its net coding gain (NCG) at the third iteration is 0.57 dB more than that of RS (255, 239) + CSOC (k₀/n₀ = 6/7, J = 8) code in ITU-T G.975.1 at the third iteration for the bit error rate (BER) of 10⁻¹². Therefore, the novel code type can better be used in long-haul, larger capacity and higher bit-rate optical communication systems. Furthermore, the design and implementation of the novel concatenated code type are also discussed.     

A tuned damping device for reducing noise from railway track

A promising means to reduce the component of railway rolling noise radiated by the track is to increase the damping of the rail. This increases the attenuation with distance of vibrations transmitted along the rail and thereby reduces the noise radiated. To achieve this, a tuned, damped mass-spring absorber system has been designed. To cover a wide range of frequencies, multiple tuning frequencies are used along with a material with a high damping loss factor. Suitable materials have been found from extensive tests on samples and prototypes of the damper have been built and tested, both in the laboratory and in the field. Results are very promising with reductions of the track component of noise of around 6 dB being measured.     

Study on response of CR-39 detector to light ions

The response of CR-39 detector has been obtained precisely for protons, deuterons, tritons, alpha-particles and lithiumions with energies of a few MeV/nucleon. The track etch rates were determined from the growth curves of pit radius and summarized as the response functions. The accuracy of this method is high enough to discriminate among hydrogen isotopes under a certain condition. The correlation between the response and LET with various cut-off energies has also been examined. It was found that LET is not an unique parameter describing the response function adequately for different ions by the same expression.