Simple Channel Model Research Articles

Voltage dependence of the apparent affinity for external Na(+) of the backward-running sodium pump.

The steady-state voltage and [Na(+)](o) dependence of the electrogenic sodium pump was investigated in voltage-clamped internally dialyzed giant axons of the squid, Loligo pealei, under conditions that promote the backward-running mode (K(+)-free seawater; ATP- and Na(+)-free internal solution containing ADP and orthophosphate). The ratio of pump-mediated (42)K(+) efflux to reverse pump current, I(pump) (both defined by sensitivity to dihydrodigitoxigenin, H(2)DTG), scaled by Faraday's constant, was -1.5 +/- 0.4 (n = 5; expected ratio for 2 K(+)/3 Na(+) stoichiometry is -2.0). Steady-state reverse pump current-voltage (I(pump)-V) relationships were obtained either from the shifts in holding current after repeated exposures of an axon clamped at various V(m) to H(2)DTG or from the difference between membrane I-V relationships obtained by imposing V(m) staircases in the presence or absence of H(2)DTG. With the second method, we also investigated the influence of [Na(+)](o) (up to 800 mM, for which hypertonic solutions were used) on the steady-state reverse I(pump)-V relationship. The reverse I(pump)-V relationship is sigmoid, I(pump) saturating at large negative V(m), and each doubling of [Na(+)](o) causes a fixed (29 mV) rightward parallel shift along the voltage axis of this Boltzmann partition function (apparent valence z = 0.80). These characteristics mirror those of steady-state (22)Na(+) efflux during electroneutral Na(+)/Na(+) exchange, and follow without additional postulates from the same simple high field access channel model (Gadsby, D.C., R.F. Rakowski, and P. De Weer, 1993. Science. 260:100-103). This model predicts valence z = nlambda, where n (1.33 +/- 0.05) is the Hill coefficient of Na binding, and lambda (0.61 +/- 0.03) is the fraction of the membrane electric field traversed by Na ions reaching their binding site. More elaborate alternative models can accommodate all the steady-state features of the reverse pumping and electroneutral Na(+)/Na(+) exchange modes only with additional assumptions that render them less likely.

Analytical calculation of throughput of ALOHA based protocols in optical wireless data networks

An analytical framework for the calculation of the throughput performance of random-access protocols based on ALOHA is presented under blockage conditions in optical wireless data networks. A simple approximate Markovian channel model, which describes the channel variability, is proposed, Based on that, an analytical technique is developed for the calculation of the throughput of S-ALOHA paired with a general backoff-retransmission policy. It is shown that this combination can provide stabilised throughput performance under adverse blockage conditions. Calculation of the throughput for reservation ALOHA under the same channel conditions is described. The analytical techniques described are based on equilibrium-point analysis and their predictions are in very good agreement with Monte-Carlo-simulation results.

Shannon-theoretic approach to a Gaussian cellular multiple-access channel with fading

Shannon-theoretic limits on the achievable throughput for a simple infinite cellular multiple-access channel (MAC) model (Wyner 1994) in the presence of fading are presented. In this model, which is modified to account for flat fading, the received signal, at a given cell-site's antenna, is the sum of the faded signals transmitted from all users within that cell plus an attenuation factor /spl alpha//spl isin/[0,1] times the sum of the faded signals received from the adjacent cells, accompanied by Gaussian additive noise. This model serves as a tractable model providing considerable insight into complex and analytically intractable real-world cellular communications. Both linear and planar cellular arrays are considered with exactly K active users in each cell. We assume a hyper-receiver, jointly decoding all of the users, incorporating the received signals from all of the active cell-sites. The hyper-receiver is assumed to be aware of the codebooks and realizations of the fading processes of all the users in the system. In this work we consider the intracell time-division multiple-access (TDMA) and the wideband (WB) protocols. We focus on the maximum reliably transmitted equal rate. Bounds to this rate are found for the intracell TDMA protocol by incorporating information-theoretic inequalities and the Chebyshev-Markov moment theory as applied to the limiting distribution of the eigenvalues of a quadratic form of tridiagonal random matrices. We demonstrate our results for the special case where the amplitudes of the fading coefficients are drawn from a Rayleigh distribution, i.e., Rayleigh fading. For this special case, we observe the rather surprising result that fading may increase the maximum equal rate, for a certain range of /spl alpha/ as compared to the nonfaded case. In this setting, the WB strategy, which achieves the maximum reliable equal rate of the model, is proved to be superior to the TDMA scheme. An upper bound to the maximum equal rate of the WB scheme is also obtained. This bound is asymptotically tight when the number of users is large (K/spl Gt/1). The asymptotic bound shows that the maximum equal rate of the WB scheme in the presence of fading is higher than the rate which corresponds to the nonfaded case for any intercell interference factor /spl alpha//spl isin/[0,1] signal-to-noise ratio (SNR) values. This result is found to be independent of the statistics of the fading coefficients.

Secure Communication in Minimal Connectivity Models

Problems of secure communication and computation have been studied extensively in network models. In this work we ask what is possible in the information-theoretic setting when the adversary is very strong (Byzantine) and the network connectivity is very low (minimum needed for crash-tolerance). We concentrate on a new model called ``multicast lines,'' and show a sizable gap between the connectivity required for perfect security and for almost perfect security. Our results also have implications to the commonly studied simple channel model and to general secure multiparty computation.

Analysis of diffuse indoor infrared data links

The authors present a simple channel model for diffuse infra-red indoor data communication systems. Based on the model, important trade-offs between the transmitted optical power, the bit-rate, and the receiver parameters are presented for the first time.

Channel assignment problem in cellular systems: a new model and a tabu search algorithm

The channel assignment in cellular systems has the task of planning the reuse of available frequencies in a spectrum efficient way. A classical approach to frequency assignment problems, when applied to the frequency planning of cellular networks, does not enable this task to be performed in an efficient way, since it does not consider the cumulative effect of interferers. We propose a new model for the channel assignment problem in narrow-band cellular networks, which accounts for the cumulative effect of interferers. In this model, the service area is partitioned into regions and the propagation characteristics are assigned by means of the levels received in each region by the considered base stations (BSs). The objective is to maximize the sum of traffic loads offered by regions in which the ratio between the received power and the sum of powers received from interfering transmissions is above a threshold value. We also present an algorithm, based on tabu search (TS) techniques, to solve this problem. This algorithm has been tested on some instances obtained by using a simple radio channel model and on a real world instance.

The Influence of Mesoscale Eddies on Coarsely Resolved Density: An Examination of Subgrid-Scale Parameterization

Coarse-resolution numerical models of ocean circulation rely on parameterizations of unresolved mesoscale eddy effects. In order to investigate the role of eddy-flux divergences in the density equation, the GFDL Modular Ocean Model (MOM) has been configured as a simple flat-bottomed channel model with sufficient resolution to represent mesoscale eddies. Eady-type baroclinic instability and a wind-forced channel have been considered. As an analog to the large-scale components addressed by low-resolution models, the influence of eddy fluxes on the zonal-mean density field was evaluated. Results show that eddy-flux divergences are larger than meanflux divergences. The effect of mesoscale eddies on the mean density field is often hypothesized to take an advective form that conserves mean density so that eddy effects are adiabatic in the zonal mean. However, in both of the examples studied a significant component of the mesoscale eddy effect on the zonal mean is diabatic and makes mean density nonconservative. The associated diapycnal fluxes result from zonally averaging terms representing processes that are locally adiabatic. Subgrid-scale parameterizations (such as eddy diffusion) represent the unresolved eddy-flux divergence as a function of the resolved density field. The authors computed optimal coefficients for a variety of parameterizations and evaluated their skill. When the model output is time-averaged, quasi-adiabatic parameterizations, such as the one proposed by Gent and McWilliams, are able to explain as much as 43% of the mean-squared eddy-flux divergence. However, for shorter averaging periods or instantaneous snapshots, even for the spatially averaged model fields, parameterization skill drops.

A linear receiver for direct-sequence spread-spectrum multiple-access systems with antenna arrays and blind adaptation

A linear receiver for direct-sequence spread-spectrum multiple-access communication systems under the aperiodic random sequence model is considered. The receiver consists of the conventional matched filter followed by a tapped delay line with the provision of incorporating the use of antenna arrays. It has the ability of suppressing multiple-access interference (MAI) and narrowband interference in some weighted proportions, as well as combining multipath components without explicit estimation of any channel conditions. Under some specific simplified channel models, the receiver reduces to the minimum variance distortionless response beamformer, the RAKE receiver, a notch filter, or an MAI suppressor. The interference rejection capability is made possible through a suitable choice of weights in the tapped delay line. The optimal weights can be obtained by straightforward but computationally complex eigenanalysis. In order to reduce the computational complexity, a simple blind adaptive algorithm is also developed.

Sensitivity to changes in overall level and spectral shape: an evaluation of a channel model.

Two experiments involving level and spectral shape discrimination which test an optimal channel model developed by Durlach et al. [J. Acoust. Soc Am. 80, 63-72 (1986)] are described. The model specifies how the auditory system compares and/or combines intensity information in different frequency channels. In the first experiment, psychometric functions were obtained for the discrimination of changes in level and discrimination of changes in spectral shape for an eight-tone complex sound. A variety of different base spectral shapes were tested. In some conditions, level randomization was introduced to reduce the reliability of across-interval changes in level. Increasing the amount of level variation degraded performance for the level discrimination task but had no effect on the shape discrimination task. In all conditions, sensitivity to changes in spectral shape was superior to sensitivity to changes in level. Consequently, two models of central noise are evaluated in an attempt to explain these results; one in which central noise acts prior to the formation of the likelihood ratio and one in which central noise degrades the likelihood ratio. The former model is more successful in accounting for the data. In a second experiment, the detectability of a level increment to one component of a multitone complex was measured. The frequency content of the complex was varied by systematically removing six components from a 23-component complex. Thresholds were measured for increments at three different signal frequencies. A common trend in the data was that when there was a spectral gap directly above the signal frequency, thresholds were lowest. This result differs from the predictions of a simple channel model, and contrasts with results presented by Green and Berg [Q. J. Exp. Psychol. 43A, 449-458 (1991)].

Performance comparison of space diversity and equalization techniques for indoor radio systems

By means of analytical and numerical methods, we derive the bit error rate (BER) of /spl pi//4-DQPSK systems in frequency-selective fading channels. For a theoretical analysis of the system, a simplified two-ray channel model has been used. However, both Rayleigh and lognormal distributions for the ray envelope have been considered. The system performance in the presence of antenna diversity and in combination with a new nonlinear equalizer has been evaluated. In particular, it is seen that in flat fading environments, space diversity may improve the performance by more than 10 dB at a BER=10/sup -3/. However, for channels with a large delay spread, nonlinear intersymbol interference (ISI) is the predominant disturbance, and the performance can only be enhanced by the nonlinear equalizer.

Conductivity noise in transmembrane ion channels due to ion concentration fluctuations via diffusion

A Green's function approach is developed from first principles to evaluate the power spectral density of conductance fluctuations caused by ion concentration fluctuations via diffusion in an electrolyte system. This is applied to simple geometric models of transmembrane ion channels to obtain an estimate of the magnitude of ion concentration fluctuation noise in the channel current. Pure polypeptide alamethicin forms stable ion channels with multiple conductance states in artificial phospholipid bilayers isolated onto tips of micropipettes with gigaohm seals. In the single-channel current recorded by voltage-clamp techniques, excess noise was found after the background instrumental noise and the intrinsic Johnson and shot noises were removed. The noise que to ion concentration fluctuations via diffusion was isolated by the dependence of the excess current noise on buffer ion concentration. The magnitude of the concentration fluctuation noise derived from experimental data lies within limits estimated using our simple geometric channel models. Variation of the noise magnitude for alamethicin channels in various conductance states agrees with theoretical prediction.

Simple RLCG model for category-5 cables

A simple RLCG channel model for category-5 cables is presented. Using this presented RLCG model, both frequency- and time-domain characteristics of a category-5 cable plant can be obtained by software.

Bits through queues

Considers the following simple communication channel model: an error-free bit pipe leading to a buffer modeled by a single-server queue whose packets or customers are single bits. If the service rate is /spl mu/ bits/sec, common wisdom would indicate that the Shannon capacity of this communication link is /spl mu/ bits/sec. As we show in this paper, that intuition is wrong: the answer is actually higher than /spl mu/ bits/sec. How could we possibly transmit information at a rate faster than the service rate? After all, overdriving the queue with an arrival rate higher than p will not do, as the queue will become unstable and its output rate will not be higher than /spl mu/. The capacity is higher than /spl mu/ because information can be encoded into the arrival epochs. A simpler and more fundamental problem, namely the Shannon capacity of the single-server queue, is also considered. >

On the Transport and Angular Momentum Balance of Channel Models of the Antarctic Circumpolar Current

Abstract Angular momentum balances are discussed, both in general as well as in the context of simple channel models of the Antarctic Circumpolar Current (ACC). Particular emphasis is placed on the close relationship between the angular momentum balance and the meridional circulation. It is found that topographic form drag is established very early in the integration, whereas interfacial form drag can take much longer to develop. Restrictions on the geostrophic portion of the meridional circulation imposed by zonally reconnecting potential vorticity contours in the upper ocean allow derivation of an estimate for the steady-state transport. The estimate assumes there to be little or no circumpolar flow at great depth, an assumption that stems from the belief that the band of zonally reconnecting geostrophic contours in the Southern Ocean does not extend to the ocean floor. The predicted transport is proportional to the strength of the stratification and compares favorably with numerical results in the lite...

Performance analysis of slotted aloha over time‐varying channels

AbstractThis paper considers a slotted ALOHA network which consists of a central base station and many mobile terminals. In classical performance analyses of random access networks, a channel between the station and a terminal is assumed to be always in a good state. However, this is not the case in mobile radio networks since the channel suffers from many factors such as fading, shadowing, and so on. Moreover, the channel condition changes with the terminal location.In this paper, a simple channel model is used to consider the effects of time‐varying channels in a random access network. The channel model assumed in this paper has three states: shadowing, good, and bad. The transitions between the channel states are described by a Markov chain. The dynamic performance of a slotted ALOHA network over such time‐varying channels is evaluated by means of the equilibrium point analysis. Then the influence of the time‐varying channel on the throughput performance of a slotted ALOHA network is studied.

Analytical outage evaluation of TDMA local radio systems with coding and diversity

A completely analytical approach is presented in order to evaluate outage probability of TDMA (time-division multiple access) local radio systems using coding and diversity to combat frequency selective fading. First, starting from a multiray model for multipath propagation, performance sensitivity is tested by suitably simplifying the channel model. An analytical expression is obtained of the probability that BER may exceed a fixed value in the presence of a simplified channel model. A suitable upper bound formulation is given to express the same probability in the presence of coding and diversity techniques. Finally, some application examples are proposed both with reference to slow frequency hopping and space diversity systems. >

Nonlinear effects of transition broadening

Nonlinear broadening of closely spaced transitions has been studied using micromagnetics simulations. Results show a large increase in the width of a transition due to the demagnetizing field effect of earlier transitions. Based on this observation, a simple recording channel model is developed to investigate the effects of nonlinear transition broadening on detection performance. This analysis shows that the conventional peak detector is much less sensitive to the detrimental effects of nonlinearity than the more advanced partial response maximum likelihood (PRML) scheme, mainly due to the extra transition spacing provided by the minimum run-length-limited constraint. This result suggests that in order to preserve the improved signal detection capability of advanced signal processing schemes like PRML, reduction of nonlinearity is essential in the media design. >

A maximum likelihood detector for nonlinear magnetic recording

Extremely high density magnetic recording is not well modeled by linear superposition. A model for nonlinearities in thick particulate media is described, and a simple maximum likelihood detector to this channel is derived. The authors first review the nonlinear recording model of A. Armstrong et al., (1991) and then describe the architecture of the maximum likelihood sequences estimator for the channel. It is observed that further simplifications to the detector can be made when the channel can be described in terms of partial response signals. Some simulation results obtained for a simple channel model are reviewed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Bounds on the capacity of the bit-shift magnetic recording channel

A simple statistical channel model is suggested to account for single position bit shifts that have been identified as one of the major impairments in magnetic recording. The authors investigate bounds on the capacity of this channel where the channel inputs are the (d,k) codes commonly used in magnetic recordings. For d>or=2, this channel is conveniently described in terms of phrase lengths where a bit shift causes a phrase either to shrink or to expand. The inherent correlation present in consecutive shift-affected phrases introduces memory into the channel model. Sequences of nondecreasing lower bounds and nonincreasing upper bounds on the capacity are evaluated and investigated for a variety of parameters. Lower bounds on the zero-error capacity are also studied. The channel model is extended to a concatenated scheme of a bit shift channel connected in tandem with the binary symmetric channel, thus capturing both major error generating mechanisms-the bit shifts as well as randomly generated errors. Lower and upper bounds on the capacity of this concatenated channel model with (d,k) input sequences are derived and discussed. >

Temperature- and concentration-dependence of urea permeability of human erythrocytes determined by NMR

The temperature- and concentration-dependence of [ 13C]urea self-exchange across the human red cell membrane has been determined by NMR measurements of T 1 (spin-lattice) relaxation times. T 1 for intracellular label is 17 s, which is much longer than the urea exchange time across the cell membrane (about 0.5 s). T 1 for urea in extracellular solution is quenched with 17 mM of impermeable Mn 2+ in less than 2 ms. Hence the observed T 1 (corrected for intracellular decay) is a measure of urea exchange across the cell membrane. The method is tested by showing both PCMBS and increasing concentrations of urea lengthen T 1. Urea exchange permeability, defined as P urea = flux/ conc, can be described by P urea = V max / (K 1 2 + conc ) . Studies of temperature-dependence showed that activation energies were strongly dependent on both temperature and concentration. However, this apparently anomalous behavior was resolved into two well-behaved functions, K 1 2 and V max, with linear Arrhenius plots and apparent ‘activation energies’ of 15.5 and 12.4 kcal/mol, respectively. These were used to construct an equation for calculating P urea at any concentration and temperature. Assuming a simple channel model with single binding, K 1 2 becomes the dissociation equilibrium constant for the site with ΔH° = 15.5 kcal/mol and ΔS° = 51.8 cal/(mol·deg); dissociation is entropically driven.