Theoretic Capacity Research Articles

Studies on the Short Term Fairness Properties of Joint Scheduler

Since the utilization of multiuser diversity in wireless networks can increase the information theoretic capacity, much attention has been paid to packet schedulers exploiting multiuser diversity. However, it is known if there exists a tradeoff between the information theoretic capacity and fairness achieved by schedulers exploiting multiuser diversity. Therefore, the authors consider the fairness of the schedulers exploiting multiuser diversity as well as their information theoretic capacity. Among schedulers exploiting multiuser diversity, the one-bit feedback fair scheduler is considered an attractive choice due to its good balance between the capacity and fairness. This paper considers a joint scheduler which incorporates the weighted round-robin scheduling into the one-bit feedback fair scheduling. It is expected that by incorporating the round-robin scheduling into the one-bit feedback fair scheduling, the short term fairness of the scheduler is greatly improved. Considering the statistical time-access fairness index (STAFI) as a measure of short term fairness, the authors study the short term fairness properties provided by the joint scheduler. The effects of the threshold of the scheduler on the STAFI are investigated. The threshold of the scheduler strongly affects its short term fairness properties.

Electrochemical properties of Li 4Ti 5−2 xNi xMn xO 12 compounds synthesized by sol–gel process

Ni and Mn co-doped spinel Li 4Ti 5O 12 materials in the form of Li 4Ti 5−2 x Ni x Mn x O 12 ( x = 0.000, 0.050, 0.075, 0.100, 0.125) were prepared by sol–gel route and following calcination in an air atmosphere, and were used as anode materials of Li-ion battery. The structures, morphologies and electrochemical properties of the as-prepared materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), charge/discharge cycling, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that Ni and Mn co-doping did not change the electrochemical reaction mechanism of Li 4Ti 5O 12, but can clearly influence its morphology and particle size. Besides, the Li 4Ti 5O 12 samples co-doped with different molar fractions of Ni and Mn exhibited an enhanced electrochemical performance even at high current rate. It has been found that the spinel Li 4Ti 5−2 x Ni x Mn x O 12 delivered a high reversible capacity and showed excellent cycling performance, especially Li 4Ti 4.8Ni 0.1Mn 0.1O 12 exhibited a relatively good rate capability and excellent cycling stability. The initial discharge capacity of Li 4Ti 4.8Ni 0.1Mn 0.1O 12 sample was 172.41 mAh g −1 at a current of 0.5 C, which was nearly close to the theoretic specific capacity of LiTi 5O 12; furthermore, a high discharge specific capacity of 142.53 mAh g −1 was still kept after 100 cycles.

Nano/Micro Lithium Transitionmetal (Fe, Mn, Co and Ni) Silicate Cathode Materials for Lithium Ion Batteries

Lithium transitionmetal (Fe, Mn, Co, Ni) silicate cathode materials are new promising substituting cathode materials for lithium ion batteries. They had caught the researchers' eyes in the past several years. Nowadays, there are growing interests for silicate cathode materials in the field of lithium ion batteries. Among the silicate cathode materials, Li2FeSiO4 is the most promising cathode materials because of its high structure stability, high reversible capacity, high electronic conductivity and the abundant resource of iron and silicon. Although Li2MnSiO4 and Li2CoSiO4 have much higher theoretic specific capacity than Li2FeSiO4, they all have inferior electrochemical behaviours due to different reasons. There are only calculation results about Li2NiSiO4 till now. This brief critical review firstly discussed some papers about the first-principle calculation of Li2MSiO4 (M=Fe, Mn, Co Ni), and then collects and discusses relevant papers and recent patents about the fabrication, structure, particle size and electrochemical performance of nano/micro Li2MSiO4 (M=Fe, Mn, Co Ni) and their composites. Finally, the future challenges of Li2FeSiO4 are also discussed.

Analysis of the statistical time-access fairness index of one-bit feedback fair scheduler

Recently various schedulers exploiting multiuser diversity inwireless networks have been proposed and studied.Although the utilization of multiuser diversitycan increase the information theoretic capacity,there exists a tradeoff between the capacityand fairness.Among schedulers exploiting multiuser diversity,the one-bit feedback fair scheduler is considered asan attractive choicefor the reduction of feedback overheads andthe ease of implementation.In this paper, we studythe short term fairness of the one-bit feedback fair scheduler.Since the short term fairness has a strong impacton the quality-of-service of each mobile station,it is important to examine the short term fairness propertiesof the scheduler.As a short term fairness index,we consider the statistical time-access fairness index (STAFI).We then develop two numerical methodsto estimate the STAFI of the scheduler.The first method calculates the exact value ofthe STAFI by using the inverse discrete FFTmethod.The second method estimates the asymptotic decay rate ofthe STAFI by using the theory of large deviations.Numerical results show thatthe threshold value of the one-bit feedback fair schedulergreatly affects its short term fairness properties.

Capacity analysis of a diffusion-based short-range molecular nano-communication channel

Simulation-based and information theoretic models for a diffusion-based short-range molecular communication channel between a nano-transmitter and a nano-receiver are constructed to analyze information rates between channel inputs and outputs when the inputs are independent and identically distributed (i.i.d.). The total number of molecules available for information transfer is assumed to be limited. It is also assumed that there is a maximum tolerable delay bound for the overall information transfer. Information rates are computed via simulation-based methods for different time slot lengths and transmitter–receiver distances. The rates obtained from simulations are then compared to those computed using information theoretic channel models which provide upper bounds for information rates. The results indicate that a 4-input–2-output discrete channel model provides a very good approximation to the nano-communication channel, particularly when the time slot lengths are large and the distance between the transmitter and the receiver is small. It is shown through an extensive set of simulations that the information theoretic channel capacity with i.i.d. inputs can be achieved when an encoder adjusts the relative frequency of binary zeros to be higher (between 50% and 70% for the scenarios considered) than binary ones, where a ‘zero’ corresponds to not releasing and a ‘one’ corresponds to releasing a molecule from the transmitter.

Information theoretic capacity of cellular multiple access channel with shadow fading

In this paper, we extend the well-known model for the Gaussian Cellular Multiple Access Channel originally presented by Wyner. The first extension to the model incorporates the distance-dependent path loss (maintaining a close relevance to path loss values in real world cellular systems) experienced by the users distributed in a planar cellular array. The density of base stations and hence the cell sizes are variable. In the context of a Hyper-receiver joint decoder, an expression for the information theoretic capacity is obtained assuming a large number of users in each cell. The model is further extended to incorporate the log-normal shadow fading variations, ensuring that the shadowing models are fairly comparable to the free space model. Using these fair models the effect of the shadow fading standard deviation on the information theoretic capacity of the cellular system is quantified. It is observed that a higher standard deviation results in lower capacity if the mean path loss is appropriately adjusted in order to model the mean loss due to the physical obstacles causing the shadow fading. The results validate that larger cell sizes and a higher standard deviation of shadowing (with appropriately adjusted mean path loss) results in lower spectral efficiency.

Li 2ZnTi 3O 8 nanorods: A new anode material for lithium-ion battery

Spinel Li 2ZnTi 3O 8 nanorods were first synthesized using titanate nanowires as a precursor. The synthesized nanorods are highly crystalline and used as an anode material in a rechargeable Li-ion battery. A large capacity of 220 mA h g − 1 was kept after 30 cycles at a current density of 0.1 A g − 1 , which is close to the theoretic capacity. The electrochemical measurements indicate that the anode material made of spinel Li 2ZnTi 3O 8 nanorods displayed a highly reversible capacity and excellent cycling stability.

Energy optimal transmission scheduling in wireless sensor networks

One of the main issues in the design of sensor networks is energy efficient communication of time-critical data. Energy wastage can be caused by failed packet transmission attempts at each node due to channel dynamics and interference. Therefore transmission control techniques that are unaware of the channel dynamics can lead to suboptimal channel use patterns. In this paper we propose a transmission controller that utilizes different "grades" of channel side information to schedule packet transmissions in an optimal way, while meeting a deadline constraint for all packets waiting in the transmission queue. The wireless channel is modeled as a finite-state Markov channel (FSMC). We are specifically interested in the case where the transmitter has low-grade channel side information that can be obtained based solely on the ACK/NAK sequence for the previous transmissions. Our scheduler is readily implementable and it is based on the dynamic programming solution to the finite-horizon transmission control problem. We also calculate the information theoretic capacity of the finite state Markov channel with feedback containing different grades of channel side information including that, obtained through the ACK/NAK sequence. We illustrate that our scheduler achieves a given throughput at a power level that is fairly close to the fundamental limit achievable over the channel.

Sub-linear capacity scaling for multi-path channel models

The theoretic capacity of a communication system constituted of several transmitting/receiving elements is determined by the singular values of its transfer matrix. Results based on an independent identically distributed channel model, representing an idealized rich propagation environment, state that the capacity is directly proportional to the number of antennas. Nevertheless there is growing experimental evidence that the capacity gain can be at best scaled at a sub-linear rate with the system size. In this paper, we show under appropriate assumptions on the transfer matrix of the system that the theoretic information-capacity of multi-antenna systems is upper bounded by a sub-linear function of the number of transmitting/receiving links. Copyright © 2009 John Wiley & Sons, Ltd.

Uplink capacity of a variable density cellular system with multicell processing

In this work we investigate the information theoretic capacity of the uplink of a cellular system. Assuming centralised processing for all base stations, we consider a power-law path loss model along with variable cell size (variable density of Base Stations) and we formulate an average path-loss approximation. Considering a realistic Rician flat fading environment, the analytical result for the per-cell capacity is derived for a large number of users distributed over each cell. We extend this general approach to model the uplink of sectorized cellular system. To this end, we assume that the user terminals are served by perfectly directional receiver antennas, dividing the cell coverage area into perfectly non-interfering sectors. We show how the capacity is increased (due to degrees of freedom gain) in comparison to the single receiving antenna system and we investigate the asymptotic behaviour when the number of sectors grows large. We further extend the analysis to find the capacity when the multiple antennas used for each Base Station are omnidirectional and uncorrelated (power gain on top of degrees of freedom gain). We validate the numerical solutions with Monte Carlo simulations for random fading realizations and we interpret the results for the real-world systems.

Linear processing and sum throughput in the multiuser MIMO downlink

We consider linear precoding and decoding in the downlink of a multiuser multiple-input, multiple-output (MIMO) system, wherein each user may receive more than one data stream. We propose several mean squared error (MSE) based criteria for joint transmit-receive optimization and establish a series of relationships linking these criteria to the signal-to-interference-plus-noise ratios of individual data streams and the information theoretic channel capacity under linear minimum MSE decoding. In particular, we show that achieving the maximum sum throughput is equivalent to minimizing the product of MSE matrix determinants (PDetMSE). Since the PDetMSE minimization problem does not admit a computationally efficient solution, a simplified scalar version of the problem is considered that minimizes the product of mean squared errors (PMSE). An iterative algorithm is proposed to solve the PMSE problem, and is shown to provide near-optimal performance with greatly reduced computational complexity. Our simulations compare the achievable sum rates under linear precoding strategies to the sum capacity for the broadcast channel.

Information theoretic capacity of Gaussian cellular multiple-access MIMO fading channel

Higher spectral efficiency can be achieved by exploiting the space dimension inherent to any wireless communication system using multiple receiver and multiple transmitter antennas (MIMO). There are several results that provide closed form solutions for acellular system with a single antenna at each base station and each user terminal. Results are also available for the single cell case with MIMO. A cellular system with multiple antennas at the transmitter and the receiver nodes has not been investigated to obtain a closed form solution for the capacity limit. The main information theoretic theorems are not directly applicable to this system because of the form of the channel matrix of such a system. In this paper we extend the well known Wyner's model to a MIMO cellular system. It is observed that the achievable rate is bound by an upper limit and lower limit corresponding to two extreme fading conditions: channel with Rayleigh fading and with no fading. The analytical results are verified using Monte Carlo simulations. The analysis provides the insight that for a cellular system, increasing the number of transmitting antennas is not beneficial to increase the achievable rate, and this is reflected in the results obtained.

Jointly Optimal Signature Sequences and Power Allocation for CDMA Systems

The problems of designing signature sequences and power allocation policy for code-division multiple access (CDMA) are important and have been the subject of intensive research in recent years. Taking the maximization of the system information theoretic sum capacity as the design criterion, most of the previous works only consider the optimizations of signature sequences and power allocation separately. In contrast, this letter presents a jointly optimal design of signature sequences and power allocation for a CDMA system with fixed, but unequal channel gains and under the sum power constraint. The proposed design is of closed-form, and applicable for the general case of correlated signals and colored noise. Numerical results verify the superiority of the proposed design over the existing ones.

On Resource Allocation in Fading Multiple-Access Channels–An Efficient Approximate Projection Approach

We consider the problem of rate and power allocation in a multiple-access channel. Our objective is to obtain rate and power allocation policies that maximize a general concave utility function of average transmission rates on the information theoretic capacity region of the multiple-access channel. Our policies does not require queue-length information. We consider several different scenarios. First, we address the utility maximization problem in a nonfading channel to obtain the optimal operating rates, and present an iterative gradient projection algorithm that uses approximate projection. By exploiting the polymatroid structure of the capacity region, we show that the approximate projection can be implemented in time polynomial in the number of users. Second, we consider resource allocation in a fading channel. Optimal rate and power allocation policies are presented for the case that power control is possible and channel statistics are available. For the case that transmission power is fixed and channel statistics are unknown, we propose a greedy rate allocation policy and provide bounds on the performance difference of this policy and the optimal policy in terms of channel variations and structure of the utility function. We present numerical results that demonstrate superior convergence rate performance for the greedy policy compared to queue-length based policies. In order to reduce the computational complexity of the greedy policy, we present approximate rate allocation policies which track the greedy policy within a certain neighborhood that is characterized in terms of the speed of fading.

Approximation algorithms for maximizing the information – theoretic sum capacity of reverse link CDMA systems

Since the introduction of multimedia services to CDMA systems, many researchers have been working on the maximization of the aggregate capacity of the reverse link. This problem looks for the optimal set of transmission powers of the stations subject to a set of constraints. One of the research directions in this field is to devise a practically realistic set of constraints and then to propose an algorithm for solving the resulting problem. Through a unified approach, introduced recently by the authors, a more general investigation of the problem, equipped with a wide range of constraints, is possible. Here, we go further and propose an approximation to reshape the objective function into a more conveniently workable one. Then, we analyze the three available formulations of the problem and show that integrating this approximation into the available algorithms has the benefit of reducing the computational cost. The paper includes the mathematics involved in the approximation and its integration into the algorithms. Also, we analyze examples to demonstrate the achievements of the proposed method.

Capacity and performance analysis of space-time block codes in Rayleigh fading channels

Multi-Antenna systems are expected to play very important role in future multimedia wireless communication systems. Such systems are predicted to provide tremendous improvement in spectrum utilization. In this paper we consider the capacity analysis of Multiple-Input Multiple-Output (MIMO) systems. Space-Time coding schemes are the practical signal design techniques to realize the information theoretic capacity limits of MIMO systems. Here the orthogonal space-time block codes are considered for the capacity and error probability analysis of MIMO systems as a case study. The numerical and simulation results obtained using MATLAB are presented for the Multi-antenna system channel capacity and bit-error probability in Rayleigh fading channels.

Capacity of Ultra Wide Band Wireless Ad Hoc Networks

Throughput capacity is a critical parameter for the design and evaluation of ad-hoc wireless networks. Assuming a fixed per node transmission capability of T bits per second at a fixed range, it has been shown [1] that the information theoretic uniform throughput capacity per node r(n) is Theta (T/radicn log n), a decreasing function of node density n. However we consider an alternate communication model, with each node constrained to a maximum transmit power P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and capable of utilizing W Hz of bandwidth. Under the limiting case W rarr infin, such as in ultra wide band (UWB) networks, we show that the uniform throughput per node is O ((n log n) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">alpha-1/2</sup> (upper bound) and Omega(n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(alpha-1)/2</sup> (log n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(alpha+1)/2</sup> ) (achievable lower bound). Thus we demonstrate that the throughput increases with node density n, in contrast to previously published results. The capacity problem is also considered from an optimization theoretic perspective. The novel information theoretic capacity bound is compared to the network optimization results, demonstrating its practical applicability to UWB networks. The dramatic effect of the UWB physical layer on the capacity justifies the promise of UWB as a physical layer technology for ad-hoc networks.

Efficient Multielement Ray Tracing With Site-Specific Comparisons Using Measured MIMO Channel Data

In this paper, an advanced site-specific image-based ray-tracing model is developed that enables multielement outdoor propagation analysis to be performed in dense urban environments. Sophisticated optimization techniques, such as preprocessing the environment database using object partitioning, visibility determination, diffraction image tree precalculation, and parallel processing are used to improve run-time efficiency. Wideband and multiple-input-multiple-output (MIMO) site-specific predictions (including derived parameters such as theoretic capacity and eigenstructure) are compared with outdoor site-specific measurements at 1.92 GHz. Results show strong levels of agreement, with a mean path-loss error of 2 dB and a mean normalized-capacity error of 1.5 b/s/Hz. Physical-layer packet-error rate (PER) results are generated and compared for a range of MIMO-orthogonal frequency-division-multiplexing (OFDM) schemes using measured and predicted multielement channel data. A mean E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> error (compared to PER results from measured channel data) of 4 and 1 dB is observed for spatial-multiplexing and space-time block-code schemes, respectively. Results indicate that the ray-tracing model successfully predicts key channel parameters (including MIMO channel structure) and thus enable the accurate prediction of PER and service coverage for emerging MIMO-OFDM networks such as 802.11n and 802.16e

Performance of UWB $N$-Orthogonal PPM in AWGN and Multipath Channels

This paper studies pulse-based M-ary ultrawideband communications analyzing both the symbol error rate (SER) and the information theoretic capacity C for single-link communications over both additive white Gaussian noise and multipath channels (taking into account random variations in both energy and correlation values). In particular, we consider M-ary N-orthogonal pulse-position-modulated (PPM) signals with numerical pulse-position optimization to exploit the negative values in the pulse correlation function. On one hand, the N-orthogonal PPM (N-OPPM) signals can accommodate a larger value of M than the nonoverlapping orthogonal PPM (OPPM) signals for a constant frame size and pulse width, allowing increasing the symbol transmission rate and/or the use of error correcting codes. However, N-OPPM requires a receiver with N correlators, while the receiver for OPPM requires one correlator. On the other hand, the pulse positions of the N-OPPM signals can be manipulated to shape the power spectrum density and decrease the level of the discrete components. However, this PPM manipulation changes the N-OPPM correlation properties of the signal set. At the same time, N-OPPM signals have equal or better performance than OPPM signals. More specifically, simulations show that for low values of M, the N-OPPM has lower SER and higher C than OPPM for the same signal-to-noise ratio (SNR), and that for larger values of M, it achieves similar SER and C. Hence, N-OPPM signals allow a tradeoff between transmission rate, signal performance, and receiver complexity

Joint Power Adaptation, Scheduling, and Routing for Ultra Wide Band Networks

A general cross-layer optimization problem, to maximize network efficiency (min-max power) of ad-hoc networks, is formulated including power adaptation, scheduling and routing functionalities. The non-convexity of the link capacity, high dimensionality of multi-hop routing and inter-layer interactions among the protocol layers, renders the problem hard. Conversion to an equivalent form, results in two clearly separable sub-problems, demonstrating the functionalities of the protocol layers. This decomposition allows the application of a simple shortest path based algorithm to the high-dimensional routing sub-problem. Further, in the case of UWB networks, the non-convex scheduling & power adaptation sub-problem can be effectively approximated and solved by applying a novel quadratic lower bound to the link capacity function. Using these algorithmic solutions to these sub-problems, an interior point solver generating solutions to the joint UWB network problem is developed. The various simulation results demonstrate interesting characteristics of the optimal routing and scheduling solutions, and provide benchmarks for UWB network design. Comparison with prior information theoretic capacity results, validates the importance of this cross-layer optimization framework