Maximum Achievable Capacity Research Articles

Rapid Sulfur-Melt Integration into Electrospun Carbon Nanofibers As Free-Standing, Binder-Free Cathodes in Lithium Sulfur Batteries

Lithium sulfur (Li-S) has stimulated intensive research efforts due to its relatively high specific energy density of about five times that of Li-ion (2600 vs. 500 Wh kg-1, respectively), making it a conceivable alternative for use in electric vehicles, portable electronics, etc. However, the electronically insulating nature of sulfur and diffusion of reaction intermediates (shuttling effect), has steered efforts to utilize conductive polymers, graphene-oxides, and carbon scaffolds1to encapsulate sulfur and reduce shuttling, yet still combine these materials with binders and additional conductive additives to fabricate the final cathode upon a current collector. Herein we report a rapid, single-step sulfur-melt technique to impregnate electrospun carbon nanofibers (CNF), producing entirely free-standing cathodes, without the use of any binder/additives or additional current collector. Free-standing CNFs are produced by electrospinning polyacrylonitrile solution and subsequently heating the electrospun fibers for carbonization of the polymer chains to form stable CNFs. Commercial sulfur is integrated into the CNF mat by rapidly heating sulfur above its melting temperature (~120°C) combined with applied pressure. This quick melting-technique produces the final electrode, free of binding agents and completely device-ready; no further vacuum filtration or processing is required. This novel cathode fabrication utilizes the facile technique of electrospinning for structurally stable fiber mats in conjunction with a straight-forward method of infusing active material (sulfur). These cathodes were directly used as electrodes in CR2032 coin cells for electrochemical evaluation. Cathodes with a sulfur loading of 1.0 mg cm-2 exhibited high initial discharge capacities were 1382, 1310, and 1140 mAh g-1 at C/10, C/5, and C/2 rates, respectively (1C=1675 mAh g-1). The high initial discharge capacities indicate the CNF structure facilitates high sulfur utilization. One cathode with 0.6 mg cm-2 (~25wt%) sulfur loading tested at C/10 rate exhibited a very high initial discharge capacity of 1625 mAh g-1 (97% of the theoretical maximum, 1675 mAh g-1) and 1000 mAh g-1 at 100 cycles. The initial discharge capacity of this cell relative to the weight of the entire electrode (active sulfur + inactive materials) was 406 mAh g-electrode-1. Cathodes with higher sulfur loading (~1 mg cm-2 & ~50wt%), tested at faster rates (C/5) also show initial discharge capacities greater than 400 mAh g-electrode-1. These capacities are well above that of typical Li-ion. For comparison, the maximum achievable capacity of LiCoO­2 cathode is ~140 mAh g-1, which on a per gram-electrode basis, reduces to approximately 98 mAh g-electrode-1 when the current collector/binder weight (~30-50% of electrode weight) are factored into performance.2 The cathode design described in this work reduces processing steps, eliminates the dead-weight of binders and current collectors, and shows promising, practicalelectrochemical performance for the future of energy storage devices beyond lithium ion. 1. X. Ji, K. T. Lee and L. F. Nazar, Nat Mater, 2009, 8, 500-506. 2. N. Li, Z. Chen, W. Ren, F. Li and H.-M. Cheng, Proceedings of the National Academy of Sciences of the United States of America, 2012, 109, 17360-17365. Figure Caption: (a) SEM micrograph of pristine electrospun carbon nanofibers (CNF); (b) Discharge capacity of a sulfur-CNF cathode relative to weight of active material (sulfur); (c) Discharge capacities of a S-CNF cathode and the achievable capacity of Li-ion (LiCoO2) on a per-electrode-weight basis. Figure 1

Performance improvement of FSO satellite downlink using aperture averaging and receiver spatial diversity

In this study, the bit error rate performance evaluation of a free space optical (FSO) satellite downlink has been carried out. Subsequently, improvement in performance using aperture averaging and receiver spatial diversity techniques have been explored. With a single large aperture receiver or an array of small aperture receivers, the scintillation index in the downlink can be reduced to a very small value. Moreover, a single receiver gives better performance than an array of receivers (employing equal gain combining) with the total area same as that of the single receiver. The expression of channel capacity with outage for the slow fading FSO channel has been derived using the log-normal and gamma-gamma channel models. Based on maximum acceptable outage probability, the minimum required signal-to-noise ratio is determined, from which the capacity with outage is calculated. It has been observed that the maximum achievable capacity per unit bandwidth increases and the corresponding outage probability reduces with an increase in the aperture diameter for a single receiver or with the size of the array for a receiver with spatial diversity.

Performance enhancement of free space optical satellite uplink with transmitter spatial diversity technique

In this paper, the performance analysis of a Free Space Optical (FSO) satellite link from Earth to Geostationary Earth Orbit (GEO) satellite is carried out by considering atmospheric turbulence effects. It is observed that spatial diversity is a feasible option at the transmitter side and not at the receiver side. Further, increasing aperture diameter improves the performance due to higher received power (and not because of aperture averaging) and the improvement with increase in aperture diameter is higher at lower ground turbulence levels. For transmitter spatial diversity, the arrangement of transmitters in order to minimize correlation between the received beams is discussed. Initially, we see that there is considerable improvement in link performance by increasing the number of transmitters and the law of diminishing returns sets in with further increase in the number of transmitters. The expression of capacity with outage is derived. It is observed from the results that the maximum achievable capacity increases and the corresponding outage probability reduces with an increase in the number of transmitters for the same average received signal to noise ratio.

Relay Selection for Security-Constrained Cooperative Communication in the Presence of Eavesdropper's Overhearing and Interference

This letter addresses the problem of secure communication in a cooperative network operating in the presence of an eavesdropper as well as co-channel interference. Two opportunistic relay selection techniques are exploited for achieving physical-layer based security. The first one aims at minimizing the amount of information leaked to the eavesdropper by selecting the relay which achieves the lowest capacity to the wiretap node. In the second scheme, the relay, which yields the maximum achievable capacity at the destination node, is chosen. A performance analysis, which generalizes several previous results by accounting for interference affecting the network nodes, is conducted for both schemes and considering both selection combining and maximal ratio combining, in terms of the probability of non-zero achievable secrecy capacity and the secrecy outage probability, and numerical results are provided along with comparisons.

TV white space in London, UK: availability and maximum achievable capacity

The UK regulator Ofcom is holding an extensive pilot of TV white space (TVWS) technology in the UK. The Centre for Telecommunications Research, (CTR), London, is leading one of the largest trials within this pilot. On the basis of the UK's TVWS framework and using the capabilities of the CTR regarding the pilot, how much white space is available in London and the capacity that can be achieved as an upper bound by aggregating all available white spaces are assessed.

Secrecy Cognitive Gain of Cognitive Radio Sensor Networks with Primary Outage Constraint

This paper studies the physical layer security issue for cognitive radio sensor networks (CRSNs) with primary outage constraint, for which a tractable framework is developed to analyze the secrecy performance for CRSNs related to the random channel characteristics, the spatial distributions of nodes, the nearest neighbour routing protocol, and the aggregate interference. Based on stochastic geometry technique, a novel random analysis method is proposed to derive the closed-form expression of secrecy transmission capacity and analyze the connection outage probability (COP) and secrecy outage probability (SOP) for both secondary (SR) and primary (PR) systems. Then, the concept of secrecy cognitive gain is produced to highlight the effect of SR systems with secrecy on the whole networks security. Comparing with a single PR system, the SR system is beneficial to enhance the secrecy level for overlaid CRSNs. In addition, the maximum achievable secrecy capacity is achieved through optimizing the density of SR systems satisfying outage constraints of PR and SR systems. Analytical results provide insights into regimes in which the SR system with secrecy is beneficial for the overlaid network secrecy.

Achievable transmission capacity of cognitive radio networks with cooperative relaying

With the rapid development of cognitive radio technologies, spectrum sharing becomes a promising approach to improving the efficiency of spectrum utilization and mitigating the spectrum scarcity problem. Previous research on cognitive networks argues that secondary users can only work under a low-transmission power in an underlay spectrum sharing model, especially when the primary transmitter is far away from the primary receiver. Motivated by the idea of cooperative communications, in this paper, we propose a cooperative framework in which a primary transmitter, being aware of the existence of the secondary network, may select a secondary user that is not in transmitting or receiving mode to relay its traffic. The feasible relay location region and optimal power ratio between the primary network and the secondary network are derived in the underlay spectrum sharing model. Based on the optimal power ratio, we derive the maximum achievable transmission capacity of the secondary network under the outage constraints from both the primary and the secondary network with or without cooperative relaying. Numerical results indicate that secondary users can achieve a higher transmission capacity with cooperative relaying, and that the capacity gain of the cooperative network is significantly affected by the location of the relay and the network system parameters.

Digital Filter Multiple Access PONs With DSP-Enabled Software Reconfigurability

Digital filter multiple access (DFMA) passive optical networks (PONs) are, for the first time to our knowledge, proposed and extensively investigated, where digital signal processing (DSP)-enabled, software-reconfigurable, digital orthogonal filtering is employed in each individual optical network unit (ONU) and the optical line terminal to enable all ONUs to dynamically share the transmission medium under the control of the centralized software-defined controller and the transceiver-embedded DSP controllers. The DFMA PONs fully support software-defined networking with the network control further extended to the physical layer. As digital filtering is the fundamental process at the heart of the proposed DFMA PONs, the filtering-induced tradeoffs among upstream transmission capacity, filter design flexibility, and filter DSP complexity are examined, based on which optimum filter design guidelines are identified for various application scenarios. Furthermore, the performance characteristics of the DFMA PONs are also numerically explored in terms of maximum achievable upstream transmission capacity, differential ONU launch power dynamic range, and ONU count-dependent minimum received optical power.

Systematic Approach to Determination of Maximum Achievable Capture Capacity via Leaching and Carbonation Processes for Alkaline Steelmaking Wastes in a Rotating Packed Bed

Accelerated carbonation of basic oxygen furnace slag (BOFS) coupled with cold-rolling wastewater (CRW) was performed in a rotating packed bed (RPB) as a promising process for both CO2 fixation and wastewater treatment. The maximum achievable capture capacity (MACC) via leaching and carbonation processes for BOFS in an RPB was systematically determined throughout this study. The leaching behavior of various metal ions from the BOFS into the CRW was investigated by a kinetic model. In addition, quantitative X-ray diffraction (QXRD) using the Rietveld method was carried out to determine the process chemistry of carbonation of BOFS with CRW in an RPB. According to the QXRD results, the major mineral phases reacting with CO2 in BOFS were Ca(OH)2, Ca2(HSiO4)(OH), CaSiO3, and Ca2Fe1.04Al0.986O5. Meanwhile, the carbonation product was identified as calcite according to the observations of SEM, XEDS, and mappings. Furthermore, the MACC of the lab-scale RPB process was determined by balancing the carbonation conversion and energy consumption. In that case, the overall energy consumption, including grinding, pumping, stirring, and rotating processes, was estimated to be 707 kWh/t-CO2. It was thus concluded that CO2 capture by accelerated carbonation of BOFS could be effectively and efficiently performed by coutilizing with CRW in an RPB.

Maximum Achievable Capacity in Airborne MIMO Communications with Arbitrary Alignments of Linear Transceiver Antenna Arrays

In this paper, the capacity of airborne multiple-input-multiple-output (MIMO) wireless communication systems with arbitrary alignments of linear transmit and receive antenna arrays is systematically analyzed and the maximum achievable capacity is determined. Based on a general three-dimensional (3D) airborne MIMO communication model, we are able to approximate the airborne MIMO capacity as a function of the transmit and receive antenna array geometry in the 3D space. The capacity approximation is asymptotically tight as the distance between the transmit and receive antenna arrays large compared to their size. Based on the asymptotically tight capacity approximation, we derive an upper bound as well as a lower bound of the airborne MIMO capacity. Interestingly, both the upper and lower bounds are achievable. We also derive a necessary and sufficient condition for airborne MIMO communication systems to achieve the capacity upper bound for any given 3D transceiver antenna array geometry. The necessary and sufficient condition allows us to properly select the system parameters and design airborne MIMO communication systems that reach the best possible performance in terms of system capacity. We prove that when the distance between the transmit and receive antenna arrays is within a certain range, there exists a set of system parameter values (e.g. antenna element separation) for which the capacity of the MIMO communication system achieves the theoretical upper bound and this capacity value is larger than the average capacity of the corresponding conventional MIMO communication system under Rayleigh fading. Finally, we prove that the airborne MIMO capacity converges to the capacity lower bound when the distance between the transmit and receive antenna arrays goes to infinity. Extensive numerical studies included in this paper illustrate and validate our theoretical developments.

Analysis and Design of Best-Effort Broadcasting Services in Cellular Networks

In this paper, we propose a novel best-effort broadcasting service (BEBS) scheme in cellular networks, which can be used for location-based advertising and real-time multimedia broadcasting. Unlike the conventional broadcasting service that aims to assure reliable data delivery to every user, the main objective of the proposed BEBS is to maximize the effective capacity at the cost of reliability. To this end, we introduce the notion of target reliability and formulate the problem of maximizing the capacity of BEBS under any given reliability constraint. By deriving an analytical model, we investigate the effect of target reliability on the achievable capacity. We further prove several key properties of BEBS with respect to the optimal value of target reliability and the maximum gain of the achievable capacity. To realize the maximum achievable capacity of BEBS, we propose two feedback control schemes that adjust the target reliability or modulation and coding scheme based on the estimate of relative gain of BEBS. We validate our analytical model by comparison with simulation. In addition, the simulation results confirm that the proposed schemes for BEBS significantly outperform conventional reliable broadcasting under various network configurations.

Study on the performance of different types of three-dimensional chromatographic systems

The maximum achievable performance of possible types of three-dimensional chromatographic systems (LC×LC×LC) has been investigated. The Pareto-optimization approach was applied to establish a trade-off between three main objectives (total peak capacity, analysis time and dilution of the sample) and Pareto-front values were obtained. The performances of xLC×xLC×xLC (three-dimensional separation in space), tLC×tLC×tLC (three-dimensional separation in time) and the hybrid xLC×xLC×tLC system were compared mutually and with two-dimensional chromatographic systems. It was found that xLC×xLC×xLC performs best in terms of maximum achievable peak capacity in shortest analysis time. Based on current thin-layer-chromatography performance it should be possible to obtain a peak capacity of 50,000 within 20min. If contemporary column-packing standards can be upheld the achievable limit is approximately 50% higher. However, in an xLC×xLC×xLC chromatographic system analytes remain in the separation domain after the analysis, which complicates the detection. Use of an xLC×xLC×tLC system with elution in the last dimension alleviates the detection problem. The maximum achievable peak capacity in the same analysis time is lower for xLC×xLC×tLC than for xLC×xLC×xLC. Using the same (reasonable) length of the separation domain (e.g. a cube 200×200×200mm) for both systems, it is possible to achieve peak capacities of 78,000 for xLC×xLC×tLC operated in the gradient mode, which is twice higher than for an xLC×xLC×xLC system. A three-dimensional (three-column) time-based tLC×tLC×tLC system does not greatly improve the performance of tLC×tLC in terms of (maximum) peak capacity and (minimum) analysis time. Dilution factors in tLC×tLC×tLC are very high. Decreasing the dilution has a detrimental influence on the peak capacity. The trade-off between these objectives is of crucial importance. The influence of several parameters (length of the separation domain, particle size, etc.) on the performance of chromatographic systems was investigated, optimal ranges were found.

Block diagonalisation-based multiuser multiple input multiple output-aided downlink relaying

A novel block diagonalisation (BD)-based Multiuser multiple input multiple output (MU-MIMO)-aided relaying scheme is proposed for downlink transmissions, which does not require any channel state information at the base station (BS) and decomposes a MU-MIMO-aided relaying system into several parallel single-user MIMO-assisted relaying schemes. Furthermore, based on the proposed algorithm the optimal linear processing matrix designed for the maximum achievable capacity is derived, which significantly outperforms the so-called naive weighting matrix.

Extensive Comparisons of Optical Fast-OFDM and Conventional Optical OFDM for Local and Access Networks

A performance comparison between a recently proposed novel technique known as fast orthogonal frequency-division multiplexing (FOFDM) and conventional orthogonal frequency-division multiplexing (OFDM) is undertaken over unamplified, intensity-modulated, and direct-detected directly modulated laser-based optical signals. Key transceiver parameters, such as the maximum achievable transmission capacity and the digital-to-analog/analog-to-digital converter (DAC/ADC) effects are explored thoroughly. It is shown that, similarly to conventional OFDM, the least complex and bandwidth efficient FOFDM can support up to ~20 Gb/s over 500 m worst-case multimode fiber (MMF) links having 3 dB effective bandwidths of ~200 MHz × km. For compensation of the DAC/ADC roll-off, a power-loading (PL) algorithm is adopted, leading to an FOFDM system improvement of ~4 dB. FOFDM and conventional OFDM give similar optimum DAC/ADC parameters over 500 m worst-case MMF, while over 50 km single-mode fiber a maximum deviation of only ~1 dB in clipping ratio is observed due to the imperfect chromatic dispersion compensation caused by one-tap equalizers.

Experimental demonstration of record high 19125Gb/s real-time end-to-end dual-band optical OFDM transmission over 25km SMF in a simple EML-based IMDD system

Record high 19.125 Gb/s real-time end-to-end dual-band optical OFDM (OOFDM) transmission is experimentally demonstrated, for the first time, in a simple electro-absorption modulated laser (EML)-based 25 km standard SMF system using intensity modulation and direct detection (IMDD). Adaptively modulated baseband (0-2GHz) and passband (6.125 ± 2GHz) OFDM RF sub-bands, supporting line rates of 10 Gb/s and 9.125 Gb/s respectively, are independently generated and detected with FPGA-based DSP clocked at only 100 MHz and DACs/ADCs operating at sampling speeds as low as 4GS/s. The two OFDM sub-bands are electrically frequency-division-multiplexed (FDM) for intensity modulation of a single optical carrier by an EML. To maximize and balance the signal transmission performance of each sub-band, on-line adaptive features and on-line performance monitoring is fully exploited to optimize key OOFDM transceiver and system parameters, which includes subcarrier characteristics within each individual OFDM sub-band, total and relative sub-band power as well as EML operating conditions. The achieved 19.125 Gb/s over 25 km SMF OOFDM transmission system has an optical power budget of 13.5 dB, and shows almost identical bit error rate (BER) performances for both the baseband and passband signals. In addition, experimental investigations also indicate that the maximum achievable transmission capacity of the present system is mainly determined by the EML frequency chirp-enhanced chromatic dispersion effect, and the passband BER performance is not affected by the two sub-band-induced intermixing effect, which, however, gives a 1.2dB optical power penalty to the baseband signal transmission.

Joint MMSE Equalization and Power Control for MIMO System under Multi-User Interference

The joint MMSE equalization and power control (JMEPC) algorithm, which jointly and iteratively adjusts the equalization weights and the power level of each user, is proposed for the MIMO system under multi-user interference. Two types of MMSE equalization, namely, the parallel MMSE equalization and the optimum successive decoding (OSD), are considered. In the JMEPC algorithm, we let the mean capacity among cochannel the users be the capacity target and make the resulting capacity approach the maximum achievable capacity through the iterations that can be finished in a distributed manner. Simulation results show that the JMEPC algorithm increases the capacity of the MIMO system and the parallel MMSE equalization with JMEPC can have larger capacity than the OSD without JMEPC.

On power allocation for a cognitive radio network with hybrid spectrum sharing

In this study we propose a hybrid spectrum sharing scheme based on power control by combining Overlay with Underlay schemes, to improve radio spectrum efficiency. In the scheme, the secondary users dynamically switch their operational states between Overlay and Underlay according to the spectrum occupancy. Thus the dynamics of the primary network is first modeled with a discrete-state Markov process to find the time fraction of secondary users in the Overlay state and that in the Underlay state, which leads to the capacity model of the hybrid spectrum sharing system. Under the criterion of maximizing capacity, the power allocation of the cognitive network is researched and the optimum power allocation for secondary users is deduced. As a result, the maximum achievable capacity of the cognitive network is obtained. Simulations are given to prove the analysis further. Theoretical and simulated results indicate that hybrid spectrum sharing based on power control provides a higher capacity than single Overlay and Underlay systems for the cognitive network, i.e., hybrid spectrum sharing can further improve radio spectrum efficiency.

On the Secrecy Rate of Multi-Antenna Wiretap Channel under Finite-Alphabet Input

This work investigates the effect of limiting source signals to finite-alphabet on the secrecy rate of a multi-antenna wiretap system. Existing works have characterized maximum achievable secrecy rate or secrecy capacity for single- and multi-antenna systems based on Gaussian source signals and secrecy code. Despite the impracticality of Gaussian source, its compact closed-form expression of mutual information motivated broad use of Gaussian input assumption. For practical consideration, we study the effect of finite discrete-constellation on instantaneous and ergodic secrecy rate of multiple-antenna wire-tap channels. Our results demonstrate substantial difference between systems involving finite-alphabet inputs and systems with Gaussian inputs. Results from Gaussian inputs serve as upper-bound of secrecy rate for practical systems with finite alphabet inputs.

Access Strategies for Spectrum Sharing in Fading Environment: Overlay, Underlay, and Mixed

In this paper, we analyze the achievable capacity of the secondary service for overlay and underlay access strategies. We then propose a novel mixed access strategy in which in contrast to the underlay strategy, the secondary service transmits during the idle periods without considering the interference threshold constraint. In contrast to the overlay strategy, mixed strategy makes transmission during the busy periods with a probability p_a subject to satisfying the interference threshold constraint. Parameter p_a is a secondary service parameter, which can be adjusted based on the spectrum status. Moreover, we show that the secondary service can adjust p_a to select appropriate access strategy with the objective of maximizing the achieved capacity based on the interference at the secondary service receiver, I, imposed by the primary service transmitter. The proposed spectrum-sharing technique developed in this paper based on I significantly reduces the system complexity comparing to the system in which for spectrum sharing, the imposed interference at the primary receiver is required. We further suggest a simple power allocation scheme for the mixed strategy that its achieved capacity is very close to the maximum achievable capacity of the secondary service.

Achievable Capacity in Hybrid DS-CDMA/OFDM Spectrum-Sharing

In this paper, we consider DS-CDMA/OFDM spectrum sharing systems and obtain the achievable capacity of the secondary service under different subchannel selection policies in the fading environment. Subchannel selection policies are divided into two categories: uniform subchannel selection, and nonuniform subchannel selection. Uniform subchannel selection is preferred for cases where a priori knowledge on subchannels state information is not available at the secondary transmitter. For cases with available a priori knowledge on subchannels state information, we study various nonuniform subchannel selection policies. In each case, we obtain the optimum secondary service power allocation and the corresponding maximum achievable capacity. Then we present results on the scaling law of the opportunistic spectrum sharing in DS-CDMA/OFDM systems with multiple users. Numerical results show that the optimal subchannel selection is based on the minimum value of the subchannel gain between the secondary transmitter and the primary receiver.