New architecture for incoherent optical CDMA to achieve bipolarcapacity

A simple and efficient method for using bipolar codes in noncoherent optical code-division multiple access systems is proposed. The proposed system requires only incoherent optical delay lines and direct detection receivers. Practical architectures for the encoder/decoder are given. The Letter shows that combining Gold codes of period M=127 and Barker code of period K=4 into composite sequences of period N=508 allows 20 simultaneous active users for P_e=10 ^-9, while requiring only four nonprogrammable delay lines     

A novel approach for coding color quantized images

An approach to the lossy compression of color images with limited palette that does not require color quantization of the decoded image is presented. The algorithm is particularly suited for coding images using an image-dependent palette. The technique restricts the pixels of the decoded image to take values only in the original palette. Thus, the decoded image can be readily displayed without having to be quantized. For comparable quality and bit rates, the technique significantly reduces the decoder computational complexity.     

Block motion compensated coding of interlaced sequences using adaptively deinterlaced fields

Video coding algorithms using block motion compensation were first developed for progressively scanned sequences and as such, are not entirely suitable for interlaced sequences In this paper we present a new approach for block-based coding of interlaced sequences. This proposed algorithm processes the interlaced sequence as a sequence of even and odd fields by using the last decoded field, adaptively deinterlaced, for the motion compensated prediction of the current field. The deinterlacing is performed at the decoder and no extra information has to be sent to guide the adaptation. The algorithm is a simple and efficient alternative to algorithms using the last two decoded fields for the motion compensated prediction of the current field. The new approach can easily incorporate the use of fast search algorithms and allows the use of true half-pixel accuracy in the estimates of the vertical component of the motion vectors. In HDTV sequences tested, this algorithm achieves superior performance due to this half-pixel accuracy.     

Optical code-division-multiplexed systems based on spectralencoding of noncoherent sources

Presents a new category of optical CDMA systems which work based on spectral encoding. In such systems, that the authors refer to as frequency-encoded CDMA (FE-CDMA) systems, the coding is done in the frequency domain while in the usual CDMA systems the code multiplies the modulation signal in the time domain. They present a new type of FE-CDMA system, based on encoding noncoherent broadband sources. They discuss the advantages of the system compared to other optical CDMA systems and present its performance. They show that very efficient, low-cost, CDMA systems can be obtained with an aggregate throughput of many gigabits per second. Also, for this system, the spreading gain of CDMA is independent of the modulation bandwidth. Hence, the system can accommodate variable bit rates, naturally     

Performance evaluation of optical CDMA systems using non-coherentdetection and bipolar codes

The efficiency of an asynchronous optical CDMA (code-division multiple access) system is addressed. Two types of data modulations are considered: ASK and large-deviation CPFSK. Each user's narrow-band information is spread by the use of an electrooptic phase modulator. The sequences considered are bipolar Kasami and Gold codes. The receiver consists of an optical filter of bandwidth ≈1/T, to filter the interfering signals, followed by simple, noncoherent detection. We thereby avoid all the difficulties usually met with coherent detection. Moreover, this type of demodulation is particularly suitable for a hybrid WDMA/CDMA (wavelength-division multiple access/code division multiple access) system where the spread-spectrum technique allows multiple users to share the same wavelength, thereby increasing the capacity of the network. Results are given in terms of the number of interfering signals that can be tolerated for different code periods N, and for an acceptable P_e. Comparison is made relative to the Gaussian approximation on the distribution of the interfering signals     

Performance analysis of optical heterodyne PSK receivers in thepresence of phase noise and adjacent channel interference

A phase-shift-keying (PSK) optical heterodyne receiver using synchronous detection by means of a Costas phase-locked loop (PLL) is investigated. Taking into account the laser phase noise and adjacent channel interference (ACI), an expression of the phase error variance is derived and error probability calculation is performed. Plots of the error probability versus the number of photons per bit are presented as a function of the optical domain channel spacing (D) and for several linewidth-to-bit-rate ratios (δf/R_b ). Relative sensitivity penalties, based on the performance with and without ACI, are evaluated for several combinations of D and δf/R_b. It is shown that, if lasers with larger linewidths are used, the frequency separation between optical carriers has to be increased in order to allow the same relative sensitivity penalty     

The science network in Italian population research: An analysis according to the social network perspective

The scientific community organises its relationships into network patterns, where the nodes are individuals (scientists) and the links are acquaintance and common work, usually presented at workshops and conferences and/or published in books and scientific journals. A references review on Population Studies by Italian scientists is delivered every two years by the Demography Section of the Italian Statistical Society; the review is exhaustive for academic demographers. In this paper, the properties of the demographers' network in 1998-1999 are evaluated, with the aim of identifying factors which may influence collaborative relations among actors. The probability of cooperation between couples (dyads) of demographers is modelled, conditionally on observed characteristics of the dyad (sex, academic position, university affiliation). Main results suggest that “closeness”, defined in a wider sense and not simply as geographical proximity, plays a major role in determining actors' relationships.