Christopher C. Tan

On first-order Markov modeling for the Rayleigh fading channel

Previous models for the received signal amplitude of the flat-fading channel that use first-order finite-state Markov chains are examined. The stochastic properties of a proposed first-order model based on these models are examined. The limitations of using an information theoretic metric, which is sometimes used to justify a first-order Markov chain as a sufficient model for very slowly fading channels, are discussed. A simple method of qualitatively comparing autocorrelation functions is instead proposed. The usefulness of the first-order Markov chain in representing the flat-fading channel is examined by looking at two specific problems in wireless system applications that represent two disparate cases. The first case involves analysis over a short duration of time, relative to the inverse of the normalized Doppler frequency, while the second involves analysis over a long duration of time. Contrary to previous reports, the results indicate that first-order Markov chains are not generally suitable for very slowly fading channels. Rather, first-order Markov chains can be suitable for very slowly fading applications, which require analysis over only a short duration of time.

Infinite series representations of the bivariate Rayleigh and Nakagami-m distributions

Despite the usefulness of the bivariate (correlated) Nakagami-m cumulative distribution function (cdf) in communications systems analysis, an infinite series for the computation of this function is not known. In this letter, an infinite series representation of the bivariate cdf is derived. Bounds on the error resulting from truncation of the infinite series are also derived.

A "better than" Nyquist pulse

A novel ISI-free pulse is presented that has smaller maximum distortion, a more open receiver eye, and a smaller probability of error in the presence of symbol timing error than the Nyquist pulse for the same excess bandwidth.

First-order Markov modeling for the Rayleigh fading channel

Previous models for the received signal amplitude of the flat-fading channel that use first-order, finite state, Markov chains are examined. The stochastic properties of a proposed first-order model based on these models are examined. The limitations of using an information theoretic metric, which is sometimes used to justify a first-order Markov chain as a sufficient model for very slowly fading channels, are discussed. A simple method based on qualitatively comparing autocorrelation functions is instead proposed. Contrary to previous reports, the results indicate that first-order Markov chains are not generally suitable for very slowly fading channels. Rather, first-order Markov chains can be suitable for very slowly fading applications which require analysis over only a short duration of time.

Short-Chain Aliphatic Polysulfonates Inhibit the Entry of Plasmodium into Red Blood Cells

ABSTRACT Several steps in the pathogenesis of a Plasmodium falciparum infection depend on interactions of parasite surface proteins with negatively charged sugars on the surface of host cells such as sialate residues or glycosaminoglycans. For these reasons, our previous studies examining agents that interfere with heparan sulfate-protein binding during amyloidogenesis suggested that short-chain aliphatic polysulfonates may prove useful as antimalarial agents. A series of related polysulfonates were synthesized and assessed both in tissue culture with the asexual stages of P. falciparum in human red blood cells and in vivo by use of Plasmodium berghei infections in mice. Poly(vinylsulfonate sodium salt) (molecular weight range, 1,500 to 3,000) proved effective in interfering with P. falciparum merozoite entry into human red blood cells and significantly delaying the increase in the level of P. berghei parasitemia in mice. The concept that anionic molecules that mimic large polysaccharide structures may have antimalarial properties has been suggested and examined previously. Our results suggest that related anionic agents [poly(vinylsulfonate sodium salt)-like molecules] orders of magnitude smaller than those previously considered may prove useful in abrogating merozoite entry into erythrocytes and may potentially block sporozoite entry into liver cells. Structure-activity studies conducted to enhance these properties may provide compounds with scope for significant further analysis and development.

Transmission properties of conjugate-root pulses

A family of pulses has recently been reported by Xia that are intersymbol interference-free with and without matched filtering. Reportedly, such pulses are important for systems where both matched filter and unmatched receivers are used. The transmission characteristics of these pulses are compared to the more commonly used raised-cosine pulse for three different receiver scenarios.

Letter FFT Based Generation of Bandlimited Gaussian Noise Variates

Rapid generation of time series samples of stationary, zero-mean, correlated Gaussian noise will accelerate digital communication system simulations. In this letter, we show that a fast Fourier transform (FFT) based method is well suited to rapidly generate such noise samples. The FFT method requires O(N) memory elements and O(N log 2 N) floating-point operations to generate each sequence of N variates. Sequences that are bandlimited incur an aliasing error in the correlation function of the sequence, but for practical simulations we show this error is negligible. We quantify this error for arbitrary sampling rates and correlation functions.

Stem cell research points the way to the cell of origin for intracranial germ cell tumours

Germ cell tumours found in the brain (intracranial GCTs) are a very unusual class of tumour for two reasons. First, they include a very diverse range of histological subtypes classified together due to their proposed common cell of origin. Second, this proposed cell of origin, the germ cell progenitor, would not normally be found in the tissue where these tumours arise. This is in contrast to all other primary brain tumours, in which the cell of origin is believed to be a brain cell. Indeed, no other class of primary cancer arises from a cell from a distant organ. This theory for the origins of intracranial GCTs has been in place for many decades, but recent data arising from studies of induced pluripotency for regenerative medicine raise serious questions about this dogma. Here we review the cellular origins of intracranial GCTs in the light of these new data and reanalyse the existing data on the biology of this unusual class of tumours. Together, these considerations lead us to conclude that the evidence now falls in favour of a model in which these tumours arise from the transformation of endogenous brain cells. This theory should inform future studies of the aetiology of these tumours and so lead the way to animal models in which to study their development and potential biological therapeutics. Copyright

An FFT method for generating bandlimited Gaussian noise variates

Digital communication system simulation may require the generation of correlated Gaussian noise samples. In cases where the noise is bandlimited, as for raised cosine Nyquist signaling, generation of the correlated random noise samples using conventional approaches may be difficult or problematic. It is verified that an FFT based method is well suited for generating bandlimited Gaussian noise variates.

An investigation of transmission properties of Xia pulses

A family of pulses has been reported by Xia (see IEEE Trans. on Commun., vol.45, no.10, p.1157-8, 1997) that are intersymbol interference (ISI)-free with and without matched filtering. Reportedly, such pulses are important for systems where both matched filter and unmatched receivers are used. The transmission characteristics of these pulses are compared to the more commonly used raised cosine pulse for three different receiver scenarios. Comparisons between the two families of pulses are made with eye diagrams and by determining the average probability of bit error accounting for noise, intersymbol interference (ISI), and timing error.