True Channel Research Articles

Kalman-Filter Channel Estimator for OFDM System In Time-Varying Channel

Abstract: This paper presents a method of improving the least square channel estimator accuracy without increasing the pilot density. The Kalman filter process equation can be represented as Autoregressive model, and the least square channel estimate is seen as a noisy measurement of the true channel state, so the Kalman filter measurement equation can be represented as the least square estimated channel. The process equation and the measurement equation jointly form a state space model of the dynamic of the channel. Thus the Kalman filter can be used to estimate the state variable, i.e., the time varying channel. The Kalman filter based channel estimator leads to a significant gain in performance as compared to the least square channel estimator.

On the Continuity of the Secrecy Capacity of Compound and Arbitrarily Varying Wiretap Channels

The wiretap channel models secure communication between two users in the presence of an eavesdropper who must be kept ignorant of transmitted messages. The performance of such a system is usually characterized by its secrecy capacity which determines the maximum transmission rate of secure communication. In this paper, the issue of whether or not the secrecy capacity is a continuous function of the system parameters is examined. In particular, this is done for channel uncertainty modeled via compound channels and arbitrarily varying channels, in which the legitimate users know only that the true channel realization is from a pre-specified uncertainty set. In the former model, this realization remains constant for the entire duration of transmission, while in the latter the realization varies from channel use to channel use in an unknown and arbitrary manner. These models not only capture the case of channel uncertainty, but are also suitable for modeling scenarios in which a malicious adversary jams or otherwise influence the legitimate transmission. The secrecy capacity of the compound wiretap channel is shown to be robust in the sense that it is a continuous function of the uncertainty set. Thus, small variations in the uncertainty set lead to small variations in secrecy capacity. On the other hand, the deterministic secrecy capacity of the \emph{arbitrarily varying wiretap channel} is shown to be discontinuous in the uncertainty set meaning that small variations can lead to dramatic losses in capacity.

Secrecy Performance of Maximum Ratio Diversity With Channel Estimation Error

In this letter, we analyze the performance of maximum-ratio combining (MRC) system with channel estimation error for physical layer security. The quantified measure for channel estimation error is the correlation coefficient between the true fading channel coefficients and their estimates. Exact closed-form expressions for secrecy performance including the statistical distribution of the signal-to-noise ratio (SNR), the secrecy outage probability and the secrecy capacity are derived. The analytical results are confirmed by numerical simulations.

Ruling out pyridine dinucleotides as true TRPM2 channel activators reveals novel direct agonist ADP-ribose-2'-phosphate.

Transient receptor potential melastatin 2 (TRPM2), a Ca(2+)-permeable cation channel implicated in postischemic neuronal cell death, leukocyte activation, and insulin secretion, is activated by intracellular ADP ribose (ADPR). In addition, the pyridine dinucleotides nicotinamide-adenine-dinucleotide (NAD), nicotinic acid-adenine-dinucleotide (NAAD), and NAAD-2'-phosphate (NAADP) have been shown to activate TRPM2, or to enhance its activation by ADPR, when dialyzed into cells. The precise subset of nucleotides that act directly on the TRPM2 protein, however, is unknown. Here, we use a heterologously expressed, affinity-purified-specific ADPR hydrolase to purify commercial preparations of pyridine dinucleotides from substantial contaminations by ADPR or ADPR-2'-phosphate (ADPRP). Direct application of purified NAD, NAAD, or NAADP to the cytosolic face of TRPM2 channels in inside-out patches demonstrated that none of them stimulates gating, or affects channel activation by ADPR, indicating that none of these dinucleotides directly binds to TRPM2. Instead, our experiments identify for the first time ADPRP as a true direct TRPM2 agonist of potential biological interest.

Quickest Spectrum Sensing With Multiple Antennas in Rician and Correlated Rayleigh Channels

In this letter, we study the quickest spectrum sensing employing multiple receive antennas at the cognitive user when the received signal experiences Rician and correlated Rayleigh channels. For both channels, we derive the joint probability density functions required to form the optimal cumulative sum (CUSUM) detector. We explore the effects of the Rician K-factor or correlation on the performance of the quickest spectrum sensing. Mismatched channel conditions are also considered, and we show that the sensing performance at a particular correlation coefficient or the Rician K-factor heavily depends on the true channel and is relatively insensitive to the detector.

Closed-Loop Beam Alignment for Massive MIMO Channel Estimation

Training sequences are designed to probe wireless channels to obtain channel state information for block-fading channels. Optimal training sounds the channel using orthogonal beamforming vectors to find an estimate that optimizes some cost function, such as mean square error. As the number of transmit antennas increases, however, the training overhead becomes significant. This creates a need for alternative channel estimation schemes for increasingly large transmit arrays. In this work, we relax the orthogonal restriction on sounding vectors. The use of a feedback channel after each forward channel use during training enables closed-loop sounding vector design. A misalignment cost function is introduced, which provides a metric to sequentially design sounding vectors. In turn, the structure of the sounding vectors aligns the transmit beamformer with the true channel direction, thereby increasing beamforming gain. This beam alignment scheme for massive MIMO is shown to improve beamforming gain over conventional orthogonal training for a MISO channel.

First Experiment of Thoracic Stentgrafts with a Proximal Notch

was observed in 54.5% of the cases (n1⁄412) when they were perfused by the false channel, and in 7.9% of the cases (n1⁄43) when they were perfused by the true channel. Conclusion: The extensive deployment of a bare stent in this model of type B acute dissection allowed a re-expansion of the true channel in all the cases but involved a significant fall of the systolic pressure in the digestive and renal arteries in 54.5% of the cases when they originated from the false channel.

Patency of the Digestive and Renal Arteries after Extensive Deployment of a Bare Stent in the True Channel of a Human Model of Aortic Dissection of Type B

Patency of the Digestive and Renal Arteries after Extensive Deployment of a Bare Stent in the True Channel of a Human Model of Aortic Dissection of Type B

User Capacity Scaling Laws of Multi-user Fading Channels in the Presence of MMSE Channel Estimators

Considering development of delay-sensitive applications in wireless communications and cellular networks along with lack of system resources and environmental factors including fading and noise all together make it impossible for all users to be active and to achieve quality of service requirements simultaneously. To analyze wireless systems performance, user capacity is defined as the maximum number of users that can be activated at the same time. In order to obtain user capacity scaling laws, channel distribution information is required and channel gains can be estimated by different channel estimators such as minimum mean square error estimators. In this paper, estimated channel gains are substituted for true channel gains and effects of imperfect channel estimation errors on the user capacity scaling laws are analyzed. It is shown that the user capacity scales double logarithmically and there is a gap depending on channel estimators accuracy between the lower and upper bounds. Moreover, assuming estimated channels for different users are independent and identically distributed, it is shown that the user capacity scaling laws are asymptotically tight.

Effect of intracellular diffusion on current--voltage curves in potassium channels

We study the effect of intracellular ion diffusion on ionic currents permeating through the cell membrane. Ion flux across the cell membrane is mediated by specific channels, which have been widely studied in recent years with remarkable results: very precise measurements of the true current across a single channel are now available. Nevertheless, a complete understanding of this phenomenon is still lacking, though molecular dynamics and kinetic models have provided partial insights. In this paper we demonstrate, by analyzing the KcsA current-voltage currents via a suitable lattice model, that intracellular diffusion plays a crucial role in the permeation phenomenon. We believe that the interplay between the channel behavior and the ion diffusion in the cell is a key ingredient for a full explanation of the current-voltage curves.

Ensemble Properties of RVQ-Based Limited-Feedback Beamforming Codebooks

The ensemble properties of random vector quantization (RVQ) codebooks for limited-feedback beamforming in multiinput multioutput (MIMO) systems are studied with the metrics of interest being the received \ssr SNR loss and mutual information loss, both relative to a perfect channel state information (CSI) benchmark. The simplest case of unskewed codebooks is first studied in the correlated MIMO setting and these loss metrics are computed as a function of the number of bits of feedback ( B), transmit antenna dimension ( Nt), and spatial correlation. In particular, it is established that: 1) the loss metrics are a product of two components-a quantization component and a channel-dependent component; 2) the quantization component, which is also common to analysis of channels with i.i.d. fading, decays as B increases at the rate 2-B/(Nt-1); 3) the channel-dependent component reflects the condition number of the channel. Further, the precise connection between the received \ssr SNR loss and the squared singular values of the channel is shown to be a Schur-convex majorization relationship. Finally, the ensemble properties of skewed codebooks that are generated by skewing RVQ codebooks with an appropriately designed fixed skewing matrix are studied. Based on an estimate of the loss expression for skewed codebooks, it is established that the structure of a good skewing matrix is critically dependent on the condition numbers of the effective channel (product of the true channel and the skewing matrix) and the skewing matrix.

Optimal Linear Channel Prediction for LTE-A Uplink Under Channel Estimation Errors

This paper proposes an optimal design of the linear channel prediction scheme for LTE-A uplink transmission, taking into account the effect of channel estimation errors. The optimization criterion is to minimize mean square prediction errors, averaged over the distributions of true channel coefficients and estimation errors. We derive a closed-form solution of the optimal channel predictor and provide exact analysis for the achievable mean square error (MSE). The performance gain, in terms of the amount of reduction in MSE, as compared with the conventional channel predictor, is also analytically characterized. Simulation results are used to validate our MSE analyses and to confirm the performance advantage of the proposed solution.

Human ex-vivo model of Stanford type B aortic dissection

To report a new human ex vivo model of type B aortic dissection (TBAD) and to assess if the locations of the primary entry tear determine the patterns of dissection propagation. Twenty fresh human aortas were harvested. TBADs were surgically initiated 2 cm below the left subclavian artery at four different locations (lateral, n = 5; medial, n = 5; anterior, n = 5; posterior, n = 5). Aortas were thereafter connected to a bench-top pulsatile flow model to induce antegrade propagation of the dissection. Antegrade propagation of the dissection was achieved and reached at least the celiac trunk (CT) in all the cases. Dissection was propagated to the renal aorta in 16 (80%) and infrarenal aorta in seven cases (35%). Left renal artery with or without the CT originated more often from the false channel when primary entry tear was lateral. Right renal artery and the CT most often originated from the false channel when primary entry tear was medial. When the CT was the only one originating from the false channel, primary entry tear was more often anterior, whereas when it originated from the true channel, it was more often posterior. This human ex vivo model of TBAD is reproducible, since, in all the aortas, extended dissection was achieved and provides the first model of human aortic dissection with infrarenal aorta extension allowing future assessment of endovascular devices developed for human use. Furthermore, it allows clarification of the patterns of aortic dissection propagation and visceral and renal artery involvement according to the site of the primary entry tear.

Truthful Mechanisms for Secure Communication in Wireless Cooperative System

To ensure security in data transmission is one of the most important issues for wireless relay networks, and physical layer security is an attractive alternative solution to address this issue. In this paper, we consider a cooperative network, consisting of one source node, one destination node, one eavesdropper node, and a number of relay nodes. Specifically, the source may select several relays to help forward the signal to the corresponding destination to achieve the best security performance. However, the relays may have the incentive not to report their true private channel information in order to get more chances to be selected and gain more payoff from the source. We propose a Vickey-Clark-Grove (VCG) based mechanism and an Arrow-d'Aspremont-Gerard-Varet (AGV) based mechanism into the investigated relay network to solve this cheating problem. In these two different mechanisms, we design different "transfer payment" functions to the payoff of each selected relay and prove that each relay gets its maximum (expected) payoff when it truthfully reveals its private channel information to the source. And then, an optimal secrecy rate of the network can be achieved. After discussing and comparing the VCG and AGV mechanisms, we prove that the AGV mechanism can achieve all of the basic qualifications (incentive compatibility, individual rationality and budget balance) for our system. Moreover, we discuss the optimal quantity of relays that the source node should select. Simulation results verify efficiency and fairness of the VCG and AGV mechanisms, and consolidate these conclusions.

Retroperitoneal hematoma: An unexpected complication during intervention on an occluded superficial femoral artery via a retrograde popliteal artery approach

Peripheral arterial disease involvement of the superficial femoral artery (SFA) is common. Different endovascular techniques are used successfully for revascularization of this artery. A retrograde approach to chronic total occlusion (CTO) of the SFA through the ipsilateral popliteal artery has been used occasionally if an antegrade approach is not feasible or has failed. Some of the known complications encountered during this approach are arteriovenous fistula formation at the access site, occlusion of the popliteal artery if closure devices are used, and bleeding. There are no reports of perforation or bleeding of the SFA or the external iliac artery (EIA) during a popliteal approach, probably due to lack of flow in the occluded segment of the SFA. We report a case in which a retroperitoneal hematoma occurred due to retrograde blood flow through the established true channel in the proximal SFA and subsequently to the dissection plane with a wire tip perforation in the EIA, which was treated by stopping retrograde filling with prolonged balloon inflation in the distal SFA before the CTO.

Retroperitoneal hematoma: An unexpected complication during intervention on an occluded superficial femoral artery via a retrograde popliteal artery approach

Peripheral arterial disease involvement of the superficial femoral artery (SFA) is common. Different endovascular techniques are used successfully for revascularization of this artery. A retrograde approach to chronic total occlusion (CTO) of the SFA through the ipsilateral popliteal artery has been used occasionally if an antegrade approach is not feasible or has failed. Some of the known complications encountered during this approach are arteriovenous fistula formation at the access site, occlusion of the popliteal artery if closure devices are used, and bleeding. There are no reports of perforation or bleeding of the SFA or the external iliac artery (EIA) during a popliteal approach, probably due to lack of flow in the occluded segment of the SFA. We report a case in which a retroperitoneal hematoma occurred due to retrograde blood flow through the established true channel in the proximal SFA and subsequently to the dissection plane with a wire tip perforation in the EIA, which was treated by stopping retrograde filling with prolonged balloon inflation in the distal SFA before the CTO.

Improved Carrier Mobility in Few-Layer MoS2 Field-Effect Transistors with Ionic-Liquid Gating

We report the fabrication of ionic liquid (IL)-gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS2. Our transport measurements indicate that the electron mobility μ ≈ 60 cm(2) V(-1) s(-1) at 250 K in IL-gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from ≈ 100 cm(2) V(-1) s(-1) at 180 K to ≈ 220 cm(2) V(-1) s(-1) at 77 K in good agreement with the true channel mobility determined from four-terminal measurements, ambipolar behavior with a high ON/OFF ratio >10(7) (10(4)) for electrons (holes), and a near ideal subthreshold swing of ≈ 50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin IL dielectric layer.

Estimation of Primary User Parameters in Cognitive Radio Systems via Hidden Markov Model

For a cognitive radio (CR) system, we investigate the estimation problem in which a secondary user (SU) estimates the channel parameters such as the sojourn times on the active and the inactive states of the primary user (PU) as well as the PU signal strength on the basis of the sequence of the sensing results. By modeling the CR system as a hidden Markov model (HMM), the channel parameters are estimated by the standard expectation-maximization (EM) algorithm. We focus on mathematically analyzing the condition under which the EM algorithm can find the true channel parameters. For this, we apply the theory of the equivalence and the identifiability to the proposed HMM model for a CR system. Based on the identifiability analysis, we propose a parameter estimation algorithm for our problem by extending the EM algorithm. The numerical results show that the proposed algorithm successfully estimates the true channel parameters as long as the condition for finding the channel parameters is satisfied.

Robust Precoding With Limited Feedback Design Based on Precoding MSE for MU-MISO Systems

For the separation of the signals in the vector broadcast channel (BC), some information about the channel state is necessary at the transmitter. In many cases, this channel state information (CSI) must be fed back from the receivers to the transmitter. We jointly design the channel estimators and the quantizers at the receivers together with the precoder at the transmitter based on a precoder-centric criterion, i.e., the minimization of a mean square error (MSE) metric appropriate for the precoder design. This is in contrast to our previous works, where the quantizer design was based on a CSI MSE metric, i.e., based on the minimization of the MSE between the true channel and the channel recovered by the transmitter using a feedback channel. Interestingly, the estimators resulting from this joint formulation are independent of the used codebook. The codebook entries are the employed precoders. Therefore, each receiver feeds back the index of a set of precoders and the intersection of the sets gives the appropriate precoder. Since the quantizers of the different receivers have to work separately, the metric for the computation of the partition cells cannot be expressed as a simple squared error depending on the quantizer output. The proposed system based on a joint optimization clearly outperforms previous designs with separate optimization of feedback and precoding.

Partially-Blind Estimation of Reciprocal Channels for AF Two-Way Relay Networks Employing M-PSK Modulation

We consider the problem of channel estimation for amplify-and-forward two-way relays assuming channel reciprocity and M-PSK modulation. In an earlier work, a partially-blind maximum-likelihood estimator was derived by treating the data as deterministic unknowns. We prove that this estimator approaches the true channel with high probability at high signal-to-noise ratio (SNR) but is not consistent. We then propose an alternative estimator which is consistent and has similarly favorable high SNR performance. We also derive the Cramer-Rao bound on the variance of unbiased estimators.