Maximum Achievable Rate Research Articles

Resource allocation in relay-assisted MIMO MAC systems with statistical CSI

In this work, the issue of non-cooperative resource allocation in the uplink of a relay-assisted MIMO MAC (multiple input multiple output multiple access channel) system with statistical CSI (channel state information) is considered. The mobile transmitters pursue individual achievable ergodic rate maximization, whereas the relay aims at optimizing the global performance of the system. The problem is formulated as a Stackelberg game with the relay as the leader, and the multiple access users as the followers. Moreover, necessary and sufficient conditions for beamforming optimality at the relay are derived, which simplifies the resource allocation process. Finally, numerical results corroborate the theoretical findings.

Probabilistic Limited Feedback Precoding in MIMO Interference Channels

In this letter, we consider a probabilistic limited feedback precoding in MIMO interference channels. The probabilistic use of precoders in the codebook is modeled as a mixed strategy in a nonzero-sum game. The sum rate of mixed strategy Nash equilibrium (NE) and the maximum achievable sum rate of the probabilistic limited feedback precoding are derived in a two-user MIMO interference channel. For a distributed realization of probabilistic limited feedback precoding, we also propose a distributed limited feedback precoding scheme for use in K-user MIMO interference channels.

Reliable Communication over Non-Binary Insertion/Deletion Channels

We consider the problem of reliable communication over non-binary insertion/deletion channels where symbols are randomly deleted from or inserted in the received sequence and all symbols are corrupted by additive white Gaussian noise. To this end, we utilize the inherent redundancy achievable in non-binary symbol sets by first expanding the symbol set and then allocating part of the bits associated with each symbol to watermark symbols. The watermark sequence, known at the receiver, is then used by a forward-backward algorithm to provide soft information for an outer code which decodes the transmitted sequence. Through numerical results and discussions, we evaluate the performance of the proposed solution and show that it leads to significant system ability to detect and correct insertions/deletions. We also provide estimates of the maximum achievable information rates of the system, compare them with the available bounds, and construct practical codes capable of approaching these limits.

Efficient Spectrum Sharing in the Presence of Multiple Narrowband Interference

In this paper, we study the spectrum usage efficiency by applying wideband methods and systems to the existing analog systems and applications. The essential motivation of this work is to define the prospective coexistence between analog FM and digital Spread Spectrum systems in an efficient way sharing the same frequency band. The potential overlaid Spread Spectrum (SS) system can spectrally coincide within the existing narrowband Frequency Modulated (FM) broadcasting system upon several limitations, originating a key motivation for the use of the FM radio frequency band in many applications, encompassing wireless personal and sensors networks. The performance of the SS system due to the overlaying analog FM system, consisting of multiple narrowband FM stations, is investigated in order to derive the relevant bit error probability and maximum achievable data rates. The SS system uses direct sequence (DS) spreading, through maximal length pseudorandom sequences with long spreading codes. The SS signal is evaluated throughout theoretical and simulation-based performance analysis, for various types of spreading scenarios, for different carrier frequency offset ({\Delta}f) and signal-to-interference ratios, in order to derive valuable results for future developing and planning of an overlay scenario.

Angular momentum transport and turbulence in laboratory models of Keplerian flows

We present angular momentum transport (torque) measurements in two recent experimental studies of the turbulent flow between independently rotating cylinders. In addition to these studies, we reanalyze prior torque measurements to expand the range of control parameters for the experimental Taylor-Couette flows. We find that the torque may be described as a product of functions that depend only on the Reynolds number, which describes the turbulent driving intensity, and the rotation number, which characterizes the effects of global rotation. For a given Reynolds number, the global angular momentum transport for Keplerian-like flow profiles is approximately 14% of the maximum achievable transport rate. We estimate that this level of transport would produce an accretion rate of $\dot{M}/\dot{M_0} \sim 10^{-3}$ in astrophysical disks. We argue that this level of transport from hydrodynamics alone could be significant.

Optimization of the peak capacity per unit time

The peak capacity per unit time or chromatographic productivity, Pct, was calculated and optimized for four different sample mixtures (small molecules, peptides from protein digests, and mixtures of small and large intact proteins) analyzed on 4.6mm I.D. columns packed with three different brands of core–shell particles. The calculations were based on the column permeabilities and on their van Deemter curves measured under isocratic conditions. The 3D plots of the peak capacity are provided as a function of the gradient steepness and the column length at three values of the inlet pressure and two temperatures. The maximum peak capacities obtained per unit time were 103min−1 for small molecules, 111min−1 for peptides, 63min−1 for small proteins, and 22min−1 for large proteins. They are all achieved for the shortest column lengths run at the maximum achievable flow rate (5mL/min) and for an optimum gradient steepness, which had to be calculated. Remarkably, the best separation productivity of these columns cannot be achieved with modern UPLC instruments that deliver flow rates no larger than 5mL/min. With instruments able to deliver a 15mL/min flow rate and acquire data at 320Hz, 4.6mm×50mm columns packed with shell particles operated at 600bar and a temperature of 60°C, would give peak capacities per unit time of 256min−1 for small molecules, 237min−1 for peptides, 186min−1 for small proteins, and 59min−1 for large proteins.

Optimal rate irregular low-density parity-check codes in binary erasure channel

In this study the authors design the optimal rate capacity approaching irregular low-density parity-check code ensemble over binary erasure channel, by using practical semi-definite programming approach. The method does not use any relaxation or any approximate solution unlike previous works. The simulation results include two parts. First, we present some codes and their degree distribution functions that their rates are close to the capacity. Second, the maximum achievable rate behaviour of codes in our method is illustrated through some figures.

Rate-limiting mechanisms in high-temperature growth of catalyst-free InAs nanowires with large thermal stability

We identify the entire growth parameter space and rate-limiting mechanisms in non-catalytic InAs nanowires (NWs) grown by molecular beam epitaxy. Surprisingly huge growth temperature ranges are found with maximum temperatures close to ∼600 °C upon dramatic increase of V/III ratio, exceeding by far the typical growth temperature range for catalyst-assisted InAs NWs. Based on quantitative in situ line-of-sight quadrupole mass spectrometry, we determine the rate-limiting factors in high-temperature InAs NW growth by directly monitoring the critical desorption and thermal decomposition processes of InAs NWs. Both under dynamic (growth) and static (no growth, ultra-high vacuum) conditions the (111)-oriented InAs NWs evidence excellent thermal stability at elevated temperatures even under negligible supersaturation. The rate-limiting factor for InAs NW growth is hence dominated by In desorption from the substrate surface. Closer investigation of the group-III and group-V flux dependences on growth rate reveals two apparent growth regimes, an As-rich and an In-rich regime defined by the effective As/In flux ratio, and maximum achievable growth rates of > 6 µm h−1. The unique features of high-T growth and excellent thermal stability provide the opportunity for operation of InAs-based NW materials under caustic environment and further allow access to temperature regimes suitable for alloying non-catalytic InAs NWs with GaAs.

An Analytical Approach to the Calculation of EVM in Clipped Multi-Carrier Signals

Error vector magnitude (EVM) is an important figure of merit in many communication systems. In this work an analytical approach to the calculation of EVM in clipped multi- carrier signals is proposed. In contrast to previous work, the EVM in this work is calculated with no simplifying model (e.g. a Gaussian assumption). The expression is given in a form of a power series. Based on the derived expressions for EVM, bounds on the maximal achievable rate are derived.

A Novel Construction of Multi-Group Decodable Space-Time Block Codes

Complex Orthogonal Design (COD) codes are known to have the lowest detection complexity among Space-Time Block Codes (STBCs). However, the rate of square COD codes decreases exponentially with the number of transmit antennas. The Quasi-Orthogonal Design (QOD) codes emerged to provide a compromise between rate and complexity as they offer higher rates compared to COD codes at the expense of an increase of decoding complexity through partially relaxing the orthogonality conditions. The QOD codes were then generalized with the so called g-symbol and g-group decodable STBCs where the number of orthogonal groups of symbols is no longer restricted to two as in the QOD case. However, the adopted approach for the construction of such codes is based on sufficient but not necessary conditions which may limit the achievable rates for any number of orthogonal groups. In this paper, we limit ourselves to the case of Unitary Weight (UW)-g-group decodable STBCs for 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sup> transmit antennas where the weight matrices are required to be single thread matrices with non-zero entries ∈{± 1,± j} and address the problem of finding the highest achievable rate for any number of orthogonal groups. This special type of weight matrices guarantees full symbol-wise diversity and subsumes a wide range of existing codes in the literature. We show that in this case an exhaustive search can be applied to find the maximum achievable rates for UW-g-group decodable STBCs with g>;1. For this purpose, we extend our previously proposed approach for constructing UW-2-group decodable STBCs based on necessary and sufficient conditions to the case of UW-g-group decodable STBCs in a recursive manner.

BAD-Dependent Regulation of Fuel Metabolism and KATP Channel Activity Confers Resistance to Epileptic Seizures

Neuronal excitation can be substantially modulated by alterations in metabolism, as evident from the anticonvulsant effect of diets that reduce glucose utilization and promote ketone body metabolism. We provide genetic evidence that BAD, a protein with dual functions in apoptosis and glucose metabolism, imparts reciprocal effects on metabolism of glucose and ketone bodies in brain cells. These effects involve phosphoregulation of BAD and are independent of its apoptotic function. BAD modifications that reduce glucose metabolism produce a marked increase in the activity of metabolically sensitive K(ATP) channels in neurons, as well as resistance to behavioral and electrographic seizures in vivo. Seizure resistance is reversed by genetic ablation of the K(ATP) channel, implicating the BAD-K(ATP) axis in metabolic control of neuronal excitation and seizure responses.

Towards a queueing-based framework for in-network function computation

We seek to develop network algorithms for function computation in sensor networks. Specifically, we want dynamic joint aggregation, routing, and scheduling algorithms that have analytically provable performance benefits due to in-network computation as compared to simple data forwarding. To this end, we define a class of functions, the Fully-Multiplexible functions, which includes several functions such as parity, MAX, and kth-order statistics. For such functions we characterize the maximum achievable refresh rate of the network in terms of an underlying graph primitive, the min-mincut. In acyclic wireline networks we show that the maximum refresh rate is achievable by a simple algorithm that is dynamic, distributed, and only dependent on local information. In the case of wireless networks we provide a MaxWeight-like algorithm with dynamic flow-splitting, which is shown to be throughput-optimal.

Resource Allocation in Amplify-and-Forward Relay-Assisted DS/CDMA Systems

Non-cooperative resource allocation in amplify-and-forward relay-assisted DS/CDMA networks is tackled in this paper. The relay allocates its amplify-and-forward matrix for achievable sum-rate maximization, whereas the mobile transmitters selfishly allocate their spreading codes for individual achievable rate maximization. Convergence of the proposed algorithm is proved and its performance contrasted against a centralized approach. Moreover, the proposed approach is extended to the case in which the direct path between transmitters and receiver is taken into account.

Multi-user Multi-stream Vector Perturbation Precoding

Vector perturbation (VP) is a prominent precoding technique attracted a lot of attention in recent years. Till now, however, various extended VP techniques are almost restricted to one antenna configuration of each user, termed as multi-user single-stream VP, which contradicts with the multi-antenna terminals of the next generation wireless systems. So, the popular block diagonal (BD) algorithm and VP was naturally combined and proposed to solve this problem. However, the so-called BD-VP completely suppresses multi-user interference at the expense of noise enhancement and results in performance degradation. To overcome the shortcoming of BD-VP, we propose three criteria based multi-user multi-stream VP (MUMS-VP) algorithms with different performance and complexity tradeoffs: zero-forcing expanded channel inversion (ZF ECI)-VP algorithm, minimum-total-mean-square-error criterion (MTMSE) based two MUMS-VP algorithms and maximum signal-to-leakage-and-noise-ratio (MSLNR) criterion based MUMS-VP algorithm. The general expression of achievable rates of MUMS-VP algorithms is derived. Analysis and simulation results show that the proposed ZF MUMS-VP is equivalent to BD-VP, while MTMSE MUMS-VP II has the maximum achievable sum rates among these algorithms. MTMSE MUMS-VP I, II and MSLNR MUMS-VP greatly outperform BD-VP in BER performance for both fixed modulation scenario and adaptive modulation scenario. Furthermore, all proposed algorithms have much lower complexity than BD-VP.

Link Budget Analysis for Terahertz Fixed Wireless Links

Due to the broad bandwidths, terahertz (THz)-waves offer the possibility for wireless transmission of high data rates. Especially, broadband wireless access over short ranges and fixed wireless links based on THz-waves are very promising. They can be incorporated as a bridge for optical networks or an alternative for the connection of wireless stations in difficult environments, to transmit next generation HDTV signals or for the broadband connection of servers in a data center, for instance. The frequency range between 300 and 900 GHz is very promising for these applications since the possible bandwidth is very high and first electronic circuits will become available on the market soon. However, contrary to wireless links in the lower GHz-bands, the free-space path-loss is quite high and the attenuation due to molecules in the air or water droplets can significantly decrease the transmittable data rates in this frequency range. Here the basic properties of THz-waves will be investigated and the maximum achievable data rates for fixed wireless THz-links will be derived. In order to keep the considerations as general as possible, the derivations are based on simple assumptions and equations. Additionally, conclusions for the applicability of THz-waves for fixed wireless links with distances up to 1 km will be given and the special requirements for these systems will be discussed. As we will show, high data rates can only be transmitted via these links if transmitter and receiver antennas with very high gains are used. This requires an adaptive control of mechanical fluctuations.

Low-Complexity Near-Optimum Multiple-Symbol Differential Detection of DAPSK Based on Iterative Amplitude/Phase Processing

Differentially encoded and noncoherently detected transceivers exhibit low complexity since they dispense with a complex channel estimation. In pursuit of high bandwidth efficiency, differential amplitude/phase (A/P)-shift keying (DAPSK) was devised using constellations of multiple concentric rings. To increase resilience against the typical high-Doppler-induced performance degradation of DAPSK and/or to enhance the maximum achievable error-free transmission rate for DAPSK-modulated systems, multiple-symbol differential detection (MSDD) may be invoked. However, the complexity of the maximum a posteriori (MAP) MSDD exponentially increases with the detection window size and hence may become excessive upon increasing the window size, particularly in the context of an iterative detection-aided channel-coded system. To circumvent this excessive complexity, we conceive a decomposed two-stage iterative A/P detection framework, where the challenge of having a nonconstant-modulus constellation is tackled with the aid of a specifically designed information exchange between the independent A/P detection stages, thus allowing the incorporation of reduced-complexity sphere detection (SD). Consequently, a near-MAP-MSDD performance can be achieved at significantly reduced complexity, which may be five orders of magnitude lower than that of the traditional MAP-MSDD in the 16-DAPSK scenario that was considered.

On Secrecy Rate Analysis of MIMO Wiretap Channels Driven by Finite-Alphabet Input

This work investigates the effect of finite-alphabet input constraint on the secrecy rate of a multi-antenna wiretap channel. Most existing works have characterized maximum achievable secrecy rate or secrecy capacity for single and multiple antenna systems based on Gaussian source signals and secrecy code. For practical considerations, we study the effect of finite discrete-constellation on the achievable secrecy rate of multiple-antenna wire-tap channels. Our proposed precoding scheme converts the underlying multi-antenna system into a bank of parallel channels. Based on this precoding strategy, we develop a decentralized power allocation algorithm based on dual decomposition to maximize the achievable secrecy rate. In addition, we analyze the achievable secrecy rate for finite-alphabet inputs in low and high SNR regions. Our results demonstrate substantial difference in secrecy rate between systems given finite-alphabet inputs and systems with Gaussian inputs.

Effective capacity of multiple antenna channels: correlation and keyhole

In this study, the authors derive the effective capacity limits for multiple antenna channels which quantify the maximum achievable rate with consideration of link-layer delay-bound violation probability. Both correlated multiple-input single-output and multiple-input multiple-output keyhole channels are studied. Based on the closed-form exact expressions for the effective capacity of both channels, the authors look into the asymptotic high and low signal-to-noise ratio regimes, and derive simple expressions to gain more insights. The impact of spatial correlation on effective capacity is also characterised with the aid of a majorisation theory result. It is revealed that antenna correlation reduces the effective capacity of the channels and a stringent quality-of-service requirement causes a severe reduction in the effective capacity but can be alleviated by increasing the number of antennas.

Link adaptation for energy-efficient uplink coordinated multi-point receptions

We investigate link adaptation methods for energy-efficient uplink coordinated multi-point receptions. A system model for practical cellular networks is introduced, in which only a subset of base stations participates in cooperative link adaptation and cooperative decoding for uplink transmissions. To cope with channel-state-information (CSI) mismatch incurred from the system model, link adaptation controllers implementing rate back-off from the maximum achievable rate calculated with the mismatched CSI is introduced. From analytical and simulation results, it is concluded that under a certain condition, the rate back-off does not help to improve energy efficiency, where, for example, the condition holds when the CSI errors are modeled as additive Gaussian random variables. Furthermore, energy efficiency of multi-user spatial-division-multiple-access uplink transmissions is studied in isolated cooperative cellular networks. In this scenario, an analytical expression for the optimal link adaptation achieving maximum energy efficiency is obtained.

Genome-Scale Consequences of Cofactor Balancing in Engineered Pentose Utilization Pathways in Saccharomyces cerevisiae

Biofuels derived from lignocellulosic biomass offer promising alternative renewable energy sources for transportation fuels. Significant effort has been made to engineer Saccharomyces cerevisiae to efficiently ferment pentose sugars such as D-xylose and L-arabinose into biofuels such as ethanol through heterologous expression of the fungal D-xylose and L-arabinose pathways. However, one of the major bottlenecks in these fungal pathways is that the cofactors are not balanced, which contributes to inefficient utilization of pentose sugars. We utilized a genome-scale model of S. cerevisiae to predict the maximal achievable growth rate for cofactor balanced and imbalanced D-xylose and L-arabinose utilization pathways. Dynamic flux balance analysis (DFBA) was used to simulate batch fermentation of glucose, D-xylose, and L-arabinose. The dynamic models and experimental results are in good agreement for the wild type and for the engineered D-xylose utilization pathway. Cofactor balancing the engineered D-xylose and L-arabinose utilization pathways simulated an increase in ethanol batch production of 24.7% while simultaneously reducing the predicted substrate utilization time by 70%. Furthermore, the effects of cofactor balancing the engineered pentose utilization pathways were evaluated throughout the genome-scale metabolic network. This work not only provides new insights to the global network effects of cofactor balancing but also provides useful guidelines for engineering a recombinant yeast strain with cofactor balanced engineered pathways that efficiently co-utilizes pentose and hexose sugars for biofuels production. Experimental switching of cofactor usage in enzymes has been demonstrated, but is a time-consuming effort. Therefore, systems biology models that can predict the likely outcome of such strain engineering efforts are highly useful for motivating which efforts are likely to be worth the significant time investment.