Reverse Link Research Articles

Cross‐Layer optimization of the packet loss rate in mobile videoconferencing applications

Videoconferencing transmission over wireless channels presents relevant challenges in mobile scenarios at vehicular speeds. Previous contributions are focused on the optimization of the transmission of multimedia and delay‐sensitive applications over the forward link. In this paper, a new Quality of Service (QoS) parameter adaptation scheme is proposed. This scheme applies the Cross‐Layer Design technique on the reverse link of an 1xEV‐DO Revision 0 channel. As the wireless channel parameters and the vehicle speed have significant influence in the network layer packet loss rate, it is proposed that the data rate generated by the application adapts itself to the throughput offered by the lower layers as a function of such packet loss rate. Simulations of the proposed model show a significant reduction in losses caused by wireless channel impairments and vehicle mobility, resulting in an improvement in the performance of the mobile videoconferencing session.

TCP Vegas-P: Enhanced TCP Vegas congestion control algorithm based on congestion status differentiation over wireless heterogeneous network

In wireless network,channel interference and noise cause significant performance degradation of TCP.In order to solve the problems existing in the application of TCP Vegas,based on TCP New Vegas algorithm,combining the advantages of TCP Vegas A+ algorithm,an improved algorithm named TCP Vegas-P was proposed.It was improved to solve the premature end of the slow start phase and congestion avoidance in the reverse link congestion resulting in the throughput decline,and the fairness deterioration when it was accompanied by NewReno.The simulation results show that the improved algorithm in the wireless network can better estimate Round-Trip Time(RTT),and resolve the problems about Vegas,and can be well applied to wireless network environment.

Employment as a factor increasing the population well-being in a social market economy

The paper infers competiveness of a national economy in connection with the country’s capacity to provide sufficient and sustainable employment. The author argues the importance of reverse link that goes from sustainable employment and well-being in a country to competiveness of national economy. It is concluded that active social policy is important tool in providing impulses to competitiveness of economy,

Employment as a factor increasing the population well-being in a social market economy

The paper infers competiveness of a national economy in connection with the country’s capacity to provide sufficient and sustainable employment. The author argues the importance of reverse link that goes from sustainable employment and well-being in a country to competiveness of national economy. It is concluded that active social policy is important tool in providing impulses to competitiveness of economy,

On the Selberg integral of the k-divisor function and the 2k-th moment of the Riemann zeta-function

In the literature one can find links between the 2k-th moment of the Riemann zeta-function and averages involving dk(n), the divisor function generated by ?k(s). There are, in fact, two bounds: one for the 2k-th moment of ?(s) coming from a simple average of correlations of the dk; and the other, which is a more recent approach, for the Selberg integral involving dk(n), applying known bounds for the 2k-th moment of the zeta-function. Building on the former work, we apply an elementary approach (based on arithmetic averages) in order to get the reverse link to the second work; i.e., we obtain (conditional) bounds for the 2k-th moment of the zeta-function from the Selberg integral bounds involving dk(n).

Digital signal processor against field programmable gate array implementations of space–code correlator beamformer for smart antennas

Software radio implementations of beamformers on programmable processors such as digital signal processor (DSP) and field programmable gate array (FPGA) still remain as a challenge for the integration of smart antennas into existing wireless base stations for 3G systems. This study presents the comparison of DSP- and FPGA-based implementations of space–code correlator (SCC) beamformer, which is practical to use in CDMA2000 systems. Implementation methodology is demonstrated and results regarding beamforming accuracy, weight vector computation time (execution time) and resource utilisation are presented. The SCC algorithm is implemented on Texas Instruments (TI) TMS320C6713 floating-point digital signal processors (DSPs) and Xilinx's VirtexIV family FPGA. In signal modelling, CDMA2000 reverse link format is employed. The results show that beamformer weights can be obtained within less than 10 ms via implementation on c6713 DSP with direction-of-arrival (DOA) search resolution of Δthetas=2°, whereas it can be achieved within less than 25 µs on VirtexIV FPGA for five-element uniform linear array (ULA). These results demonstrate that FPGA implementation achieves weight vector computation in much smaller time (nearly 500 times) as compared to DSP implementation in this study.

Domestic vs external sovereign debt servicing: an empirical analysis

AbstractThis paper analyzes the incidence of domestic and external debt crises for a sample of 53 emerging economies between 1980 and 2005. Even though there is substantial time variation in the default rates during the period, sovereign default rates for domestic debts are typically lower than those for external debts. The incidence of both types of defaults is explained by means of the estimation of independent and simultaneous limited‐dependent variable models. The results show that while there is considerable evidence that external defaults trigger domestic defaults, evidence for the reverse link disappears when default propensities are estimated in a simultaneous equation model. Copyright © 2009 John Wiley & Sons, Ltd.

FPGA Based Implementation and Comparison of Beamformers for CDMA2000

For the integration of smart antennas into third generation code division multiple access (CDMA) base stations, it still remains as a challenging task to implement smart antenna algorithms on programmable processors. In this paper, we study implementations of some CDMA compatible beamforming algorithms, namely least mean square (LMS), constant modulus (CM), and space code correlator (SCC) algorithms, using Xilinx's Virtex family FPGAs. This study exhibits feasibility of implementing even simple, practical, and computationally small algorithms based on today's most powerful FPGA technologies. 16 and 32 bits floating point implementations of the algorithms are investigated using both Virtex II and Virtex IV FPGAs. CDMA2000 reverse link baseband signal format is used in the signal modeling. Randomly changing fading and Direction-of-arrivals (DOAs) of multipaths are considered as a channel condition. The implementation results in terms of beamforming accuracy, FPGA resource utilization, weight vector computation time, and DOA estimation error are presented. Beamformer weight vectors using LMS and CM can be computed within less than 20 μs on Virtex II FPGA and 10 μs on Virtex IV FPGA, and using SCC it can be achieved within less than 22 μs on Virtex IV FPGA. These results show that FPGAs provide approximately 500 times faster speed in implementations than our previous work with DSPs.

Power Allocation for Maximizing the Minimum Rate With QoS Constraints

Max-min fair power allocation brings higher average throughput and better utilization of the resources than a work-conserving equal-sharing policy. In this paper, instead of achieving a common maximum sum-rate objective, an optimal power-allocation design for maximizing the minimum rate of the reverse link of code-division multiple-access (CDMA) systems with quality-of-service (QoS) constraints is investigated. To avoid the difficulty of directly solving a nonconvex optimization problem, we split the original problem into two successive steps. First, we try to derive the structure of the optimum solution via majorization theory. Then, based on this result, we propose a very efficient search method to find the max-min fair solution, which reduces the search space from a 1-D space into a finite set of points. Compared with existing methods, a much lighter computational complexity is required by our method.

Improved channel allocation for multicarrier CDMA with adaptive frequency hopping and multiuser detection

Multicarrier code-division multiple access (MCCDMA) system with adaptive frequency hopping (AFH) has attracted attention of researchers due to its excellent spectral efficiency. A suboptimal water-filling (WF) channel allocation algorithm was previously proposed for the reverse link of this system. To overcome the limitations of the WF algorithm in the presence of fading-induced near-far problem, a new allocation algorithm is proposed and demonstrated to improve performance when the conventional matched filter (MF) receiver is employed. Moreover, the allocation methods are extended to accommodate multiuser detectors (MUDs) at the receiver for MC-CDMA system with AFH. It is demonstrated that the combination of the improved allocation algorithm and the linear MUDs is very efficient in mitigating the fading and multi-access interference (MAI) for realistic mobile radio channels with correlated subcarriers, channel state information (CSI) mismatch, and imperfect power control. Numerical results show that the proposed adaptive transmission method has much greater system capacity than conventional non-adaptive MC direct-sequence (DS)-CDMA system.

Statistical Analysis of Broadband Wireless Links in Rural Areas

Third-generation (3G) cellular systems are often considered as a promising strategy for high-speed internet deployment in rural areas. However, bandwidth in 3G systems is a limited resource and can be quite different from the advertised bandwidth. In addition, in wireless applications such as voice over IP, video-conference, remote monitoring and telemetry (e.g., for healthcare applications) the Quality of Service (QoS) is critical. It is therefore important to understand how different factors affect the QoS requirements of real-time applications. Focusing on the reverse link (or uplink), we performed an assessment of the bandwidth and packet loss rate in a real network environment, using test data collected with two wireless data cards from different providers, in both rural and urban locations. The main contribution of this paper is the presentation of a formal statistical method – a Design of Experiments (DOE) analysis - that can be used to analyze the interactions between different variables, such as packet size, location, buffer size, and wireless provider. With a better understanding of the impact of such factors and their interactions, the end-user may be able to make the best selection of certain controllable parameters, in an effort to improve the QoS of the 3G connection. This is especially important for users located in rural areas, where cellular coverage is limited.

The myosin C-loop is an allosteric actin contact sensor in actomyosin.

Actin and myosin form the molecular motor in muscle. Myosin is the enzyme performing ATP hydrolysis under the allosteric control of actin such that actin binding initiates product release and force generation in the myosin power stroke. Non-equilibrium Monte Carlo molecular dynamics simulation of the power stroke suggested that a structured surface loop on myosin, the C-loop, is the actin contact sensor initiating actin activation of the myosin ATPase. Previous experimental work demonstrated C-loop binds actin and established the forward and reverse allosteric link between the C-loop and the myosin active site. Here, smooth muscle heavy meromyosin C-loop chimeras were constructed with skeletal (sCl) and cardiac (cCl) myosin C-loops substituted for the native sequence. In both cases, actin-activated ATPase inhibition is indicated mainly by the lower V(max). In vitro motility was also inhibited in the chimeras. Motility data were collected as a function of myosin surface density, with unregulated actin, and with skeletal and cardiac isoforms of tropomyosin-bound actin for the wild type, cCl, and sCl. Slow and fast subpopulations of myosin velocities in the wild-type species were discovered and represent geometrically unfavorable and favorable actomyosin interactions, respectively. Unfavorable interactions are detected at all surface densities tested. Favorable interactions are more probable at higher myosin surface densities. Cardiac tropomyosin-bound actin promotes the favorable actomyosin interactions by lowering the inhibiting geometrical constraint barriers with a structural effect on actin. Neither higher surface density nor cardiac tropomyosin-bound actin can accelerate motility velocity in cCl or sCl, suggesting the element initiating maximal myosin activation by actin resides in the C-loop.

Successive Interference Cancelers for Multimedia Multicode DS-CDMA Systems Over Frequency-Selective Fading Channels

Noncoherent and coherent multicode direct-sequence code-division multiple access (DS-CDMA) systems with successive interference cancellation (SIC) for multimedia reverse links over frequency-selective fading channels are studied. Followed by a RAKE receiver, the SIC scheme is applied for combating the multiple access interference. The bit error rate (BER) using the SIC technique over Nakagami-m fading channels is derived. Simulation results show that the multicode DS-CDMA system with SIC has demonstrated better performance than that without SIC under the multipath fading environment, while their corresponding numerical results from performance analyses are also provided for verifications. Furthermore, the coherent receiver could achieve a more satisfactory BER than the noncoherent counterpart at the expense of synchronization.

Development of an Interference Canceller in Satellite Communications Using a Multi-Level Modulation with Superposed Transmission

This paper proposes an architecture of an interference canceller for satellite communications with super-posed transmission, which is applicable not only to QPSK but also to 16QAM transmission to get higher satellite capacity. We implement it as an FPGA-based prototype and verify its performance. We propose here to use a new method to measure the satellite round-trip delay using an extended matched filter (EMF), which can work in low C/N conditions such as 0dB and under. Given this performance, our canceller can work in a network in which forward and reverse links have the same power level. The results of the laboratory tests for QPSK show that interference can be suppressed by about 30dB and that the BER degradation due to the canceller was small enough for operation.

An Analytical Model for the Reverse Link of WCDMA Systems With Repeaters in Nonuniform Traffic Distributions

The current extensive third generation network deployment has to cope with the existence of nonhomogeneous spatial traffic distributions, which are usually in the form of areas with high density of traffic. The concentration of users in some areas, particularly those far away from the base stations, can result in quality-of-service degradation due to high propagation losses. In this context, repeaters have turned out to be cost-effective nodes that are capable of mitigating these negative effects. This paper describes and models a multicellular/multiservice wideband code division multiple access system with repeaters in an environment with spatial nonuniformities. An analytical model is presented, and the expressions for the required transmission power, outage probability, and block error rate are derived for the reverse link. The implications of the noise amplification caused by the deployment of the repeaters as well as the variation of the propagation loss have been taken into account. Results focus on determining the maximum load factor for a given outage probability. In particular, special attention is paid to the effect of key parameters such as the gain between the base station and the repeaters, denoted as coupling gain, and the location of the different repeaters.

Quantized Multimode Precoding in Spatially Correlated Multiantenna Channels

Multimode precoding, where the number of independent data-streams is adapted optimally, can be used to maximize the achievable throughput in multi-antenna communication systems. Motivated by standardization efforts embraced by the industry, the focus of this work is on systematic precoder design with realistic assumptions on the spatial correlation, channel state information (CSI) at the transmitter and the receiver, and implementation complexity. For spatial correlation of the channel matrix, we assume a general channel model, based on physical principles, that has been verified by many recent measurement campaigns. We also assume a coherent receiver and knowledge of the spatial statistics at the transmitter along with the presence of an ideal, low-rate feedback link from the receiver to the transmitter. The reverse link is used for codebook-index feedback and the goal of this work is to construct precoder codebooks, adaptable in response to the statistical information, such that the achievable throughput is significantly enhanced over that of a fixed, non-adaptive, i.i.d. codebook design. We illustrate how a codebook of semiunitary precoder matrices localized around some fixed center on the Grassmann manifold can be skewed in response to the spatial correlation via low-complexity maps that can rotate and scale submanifolds on the Grassmann manifold. The skewed codebook in combination with a lowcomplexity statistical power allocation scheme is then shown to bridge the gap in performance between a perfect CSI benchmark and an i.i.d. codebook design.

Traffic Capacity of UMTS System in Reverse Link

UMTS can be connected to data networks. So Internet traffic services such as WWW browsing, email, ftp, SMTP, etc should be handled by UMTS. Each of the traffic services has some specific properties but all of them obey a basic ON/OFF model. For a traffic service there is some unused (OFF) times between their transmitted packets which we may use them for other services to increase the traffic capacity of the network. In this paper we propose a new method on the Internet traffic evaluation in an interference limited UMTS-WCDMA system. In this method first we calculate ON and OFF time durations of the above traffic services based on the model presented in 3GPP then their activity factors. Secondly we introduce new capacity curves such as The Number of Web browsing users/Email users/ftp users/Telnet users and Fax users versus voice users. These are applicable in traffic planning for wireless systems.

CD40: an upstream master switch for endothelial cell activation uncovered by RNAi-coupled transcriptional profiling

The CD40-CD154 dyad seems to play a prominent role fostering the immune-inflammatory response triggered by endothelial cell (EC)-T-cell communication. To delineate comprehensively the involvement of CD40 (TNFRSF5) in EC activation, we combined RNAi-mediated CD40 knockdown with comparative genome-wide transcriptional profiling of ECs interacting with (CD154+) T cells. We report the initiation of a profound stress response in ECs upon CD40-CD154 engagement through early up-regulation of, among others, the major proinflammatory NF-kappaB and MAPK/SAPK pathways and their associated transcription factors. Moreover, we have identified novel genes regulated through the CD40-CD154 interaction, and pathways previously unrecognized to be induced by CD40 signaling in ECs. Thus, we document a significant down-regulation of endothelial APLN by CD40-CD154 interaction, TNFalpha/IFNgamma exposure, and in immune-inflammatory pathologies, which could lead to hemodynamic dysfunction. Conversely, CD40-mediated up-regulation of the viral immune surveillance system, notably TLR3, IFIH1, RIG-I, and RNASEL, establishes a reverse link from adaptive to innate immunity in ECs. Moreover, systematic enrichment analysis substantiates endothelial CD40 involvement in the transcriptional regulation of gene networks associated with adhesion and motility, immunity, cell fate control, hemostasis, and metabolism. Our study also highlights the anti-inflammatory potential of RNAi-mediated CD40 inhibition, and the relevance of CD40 signaling for therapeutic intervention.

Sum capacity of the reverse link for multi-cell multi-user cellular systems

This letter studies the information-theoretic sum capacity of the reverse link for multi-cell, multi-user cellular systems subjected to a peak power constraint. It is proven that, for the optimal scheduling, there will be at most one user transmitting at part of the peak power within each cell. Therefore, we approximate the optimization scheduling problem to a combination optimization problem which can be solved by standard simulated annealing algorithm. Further, we propose a low complexity cell greedy scheduling algorithm which can achieve almost the same performance as simulated annealing algorithm.

Adaptive beam forming for time-variant channels with a multi-beam antenna

A multi-beam adaptive antenna is investigated by considering the adaptive beam forming performance for time-variant channels on the focus of mobile to base station or reverse link. This antenna is constructed by Fresnel zone phase-correcting plane (FZP) focusing element and uniformly spaced feeds array. An estimator for the beamforming weight vector in the presence of angle spreads is derived using a code filtering approach. Owing to the time-varying nature of the channels, a recursive method for computing and tracking the above optimal weight vector solution that is easy to implement is applied. The effects of angle spread, maximum Doppler frequency, and forgetting factor are studied. Simulation results showed that this type of adaptive antenna is capable of performing adaptive beam forming for time-variant channels effectively.