Evaluating performance of two-hop routing in DTN based on Home-MEG model

Performance analysis in delay tolerant networks (DTN) is often based on unrealistic mobility models. Recently, the Home-MEG model is proposed. This model is able to accurately reproduce the power law and exponential decay distribution of inter-contact times between mobile nodes which has been observed in many real motion traces. This letter proposes a theoretical framework to evaluate the performance of two-hop routing based on Home-MEG model for the first time. Simulation results show the accuracy of our theoretical model. Using the model, we explore the impact of some parameters (e.g. the number of relay nodes) on the routing performance.     

Outage performance of two-hop multi-antenna co-operative relaying in rayleigh fading channel

Performance of co-operative relaying employing infrastructure based fixed relays having multiple antennas has been investigated. A closed-form expression of outage probability for such a system has been derived when the relay performs selection combining and the destination performs MRC combining of signals. The effect of relay placement on the system performance has also been studied.     

On the performance of centralized DS-SS packet radio networks withrandom spreading code assignment

The paper presents a random spreading code assignment scheme for enhancing channel efficiency in centralized DS-SS packet radio networks which employ a multiple-capture receiver for each code channel. Compared to the common code case, this approach requires a modest increase in receiver complexity, but the number of distinct spreading codes being used is considerably less than the number of radios in the network. A general theoretical framework for evaluation of collision-free packet performance in each code channel is described, in which the possibility of collision-free transmission is conservatively estimated using a combinatorial method, and the effects of asynchronous multiple-access interference are characterized in terms of the primary and secondary user interferences. At the link level, the capture and throughput performances are evaluated for a proper set of codes, and compared with the results from the common code scheme. It is shown that the use of a random assignment scheme with more than one code results in a higher performance gain, and most of this gain can be achieved with just two distinct spreading codes     

On the performance of multicarrier DS-CDMA with imperfect power control and variable spreading factors

Multicarrier direct-sequence code-division multiple access (MC-DS-CDMA) becomes an attractive technique for the future fourth-generation (4G) wireless system because it can flexibly adapt transmission rates by changing both time and frequency spreading factors and possesses many physical-layer advantages in dispersive fading channels. However, power control errors (PCE) and the complete multiple access interference (MAI) from all the intersubcarriers may significantly degrade the performance of the MC-DS-CDMA system. In this paper, we propose an analytical method to evaluate the joint effects of the PCE and the complete MAI on the multirate MC-DS-CDMA system. From analysis and simulation, we obtain some important insights into the performance issues of the MC-DS-CDMA system. First, the effect of PCE can exacerbate the impact of the complete MAI on the MC-DS-CDMA system, or vice versa . For BER=10/sup -3/ in a considered case, the joint effect of the complete MAI and PCE further degrades the performance by 2.1 dB compared with the sum of the degradation from the complete MAI and the PCE individually. Second, increasing frequency or time-domain spreading gain can improve the performance of the MC-DS-CDMA system, but the system also becomes more sensitive to power control errors. Third, a larger PCE can possibly make the frequency-domain diversity diminish faster than the gain obtained from the time-domain spreading although an MC-DS-CDMA system with a larger frequency-domain spreading gain (M) is usually better than that with a larger time-domain spreading gain (G/sub o/). In our example, for the standard deviation of PCE (/spl sigma//sub e/) equal to 0 dB, the BERs with (M,G/sub o/)= (4, 16) and (16, 4) are 9.3/spl times/10/sup -4/ and 3.7/spl times/10/sup -5/, respectively, while for /spl sigma//sub e/=4 dB, the BER performances of the two cases are all in the order of 10/sup -3/.     

Fair content dissemination in participatory DTNs

Thanks to advances in the computing capabilities and added functionalities of modern mobile devices, creating and consuming digital media on the move has never been so easy and popular. Most of the DTN routing protocols proposed in the literature to enable content sharing have been exploiting users’ mobility patterns, in order to maximise the delivery probability, while minimising the overall network overhead (e.g., number of message replicas in the system, messages’ path length). Common to all these protocols has been the assumption that devices are willing to participate in the content distribution network; however, because of battery constraints, participation cannot be taken for granted, especially if the very same subset of devices are continuously selected as content carriers, simply because of their mobility properties. Indeed, we demonstrate that state-of-the-art DTN routing protocols distribute load in a highly unfair manner, with detrimental effects on delivery once the assumption of unconditional participation is lifted. To overcome this limitation, we propose a load-balancing mechanism whereby nodes maintain local estimates of network workload, and use them to direct traffic towards the least loaded portion of the network. We implement the mechanism on top of a source-based DTN routing protocol, and demonstrate, by means of simulation using a variety of real mobility traces, that high delivery is now achieved without compromising fairness.     

Decreasing social contagion effects in diffusion cascades: Modeling message spreading on social media

Modeling retweeting behaviors is important for understanding and predicting how information spreads on social media platforms. The present study contributes to the literature by examining the decreasing social contagion and increasing homophily effects with the depth of diffusion cascades. To test the hypotheses, the study proposes a matching-on-followers method by combining choice and cascade models. More specifically, the study examines the impacts of interaction frequency, multiple exposures, and interest similarity between parent users and potential retweeters on the likelihood of retweeting. The study also incorporates the depth of diffusion cascades and network structures into the model. By using a random sample of original tweets, their retweets, and potential retweeters (N = 87,139), the study found that cascade depth is negatively associated with social contagion effects (interaction and multiple exposures) and positively associated with the effect of interest similarity on message sharing. These results indicate that influence-based and homophily-driven diffusion operate differently in cascades with different diffusion structures.     

Detection performance of chaotic spreading LPI waveforms

Low probability of intercept (LPI) performance of a direct-sequence (DS) spread-spectrum (SS) system with chaotic spreading sequences is investigated in this paper. Several intercept receivers, including energy detectors, synchronous and asynchronous, coherent and noncoherent structures, which are typically used to detect binary DS SS signals, are examined here to detect the presence of chaotic DS SS signals. A simple detection approach using a binary correlating function to detect nonbinary chaotic sequences is proposed. The expressions of detection probabilities of chaotic spreading signals using those intercept receivers are derived. Comparisons between systems using chaotic and binary sequences are given in terms of the LPI performance, and the performance improvement with chaotic spreading sequences is observed.     

Effects of current spreading on the performance of GaN-basedlight-emitting diodes

Effects of current spreading on the performance of multiple quantum well (MQW) GaN/InGaN light-emitting diodes (LEDs) were investigated. For the theoretical prediction of device performance, we developed a model using two device-design parameters, which consist of the applied current density and the effective length for the lateral current path. A comparison of the theoretical and experimental results clearly showed that the reliability characteristics and the optical efficiency of device are heavily dependent on the applied current density. In addition, the effective length for the lateral current path was found to have a profound effect on the uniform current spreading. Based on these findings, an ideal geometrical design of the highly efficient LED is proposed     

Design of optimal relay location in two-hop cellular systems

Relay stations are usually used to enhance the signal strength for the users near cell boundary, thereby extending the cell coverage. However, transmission through a relay station needs two transmission phases. The first phase is from base station to relay station, and the second one is from relay station to mobile station. Thus, using relay station may decrease system capacity due to two-phase transmission time. As a result, whether or not data are transmitted by one-hop or two-hop phases should be determined according to both signal strength and throughput. In this paper, we investigate the optimal relay location aiming to maximize system capacity. We consider two relay selection rules for determining whether two-hop transmission will be used: signal strength-oriented and throughput-oriented selection rules. We find that the signal strength-oriented two-hop transmission may yield even lower system capacity than the one-hop transmission. In the throughput-oriented scheme, the two-hop transmission can achieve higher system capacity than the one-hop transmission. By simulations, we determine the optimal relay location and show the coverage enhancement by the relaying network. Extensive simulations are performed to investigate the impacts of relay transmission power and the number of relay stations on system capacity and optimal relay location. The simulation results reveal important insights into designing a relaying network with high system capacity.     

On the calculation of spreading resistance correction factors

It is shown that the calculations of spreading resistance correction factors for graded structures can be readily carried out by using a simple recurrence formula for the integration factor that occurs in Schumann and Gardner's multilayer theory. The number of layers that can be used in the practical application of this theory has hitherto been limited by the computer core size requirement, because the earlier method of calculating the integration factor requires the inversion of a 2N × 2N matrix for an N-layer approximation. The use of the recurrence formula effectively removes this constraint. In terms of computation time, the recurrence-formula method is also very efficient. The economy thus achieved both in computation time and in core size requirement makes it possible now to make spreading resistance correction a routine matter, without having to resort to such measures as Hu's interpolation and space partitioning scheme.     

On the optimal spreading sequence allocation in flat-fading channels

The sum capacity of optimally allocated spreading sequences with and without power control is studied for synchronous code-division multiple-access channels subject to frequency flat fading. Continuing upon the recently derived characterization of sum capacity for asymmetric received powers with no power control at the transmitter, we present an asymptotic analysis of the sum capacity in the wideband limit of a large number of users and evaluate possible gains compared with the random spread sequences. Next, we discuss group orthogonal optimal sequence allocation and analyze its potential to decrease the complexity of the receiver while achieving the sum capacity of optimally allocated equal-energy sequences. For the power controlled case, we give a general characterization of sum capacity and identify the optimal power-control law and sequence allocation that achieves this capacity. These results are also analyzed in the asymptotic limit of a large number of users.     

Variability of user performance in cellular DS-CDMA-long versusshort spreading sequences

The uplink performance in a cellular direct-sequence code-division multiple-access system using long and short spreading sequences is compared in terms of the distribution of the bit-error probability. Three different receiver types are considered: conventional; MMSE; and interference cancellation, both with and without forward-error correction. The short code system has a slightly higher performance variability among the user population than the corresponding long code system, which requires attention when designing a short code system. Code hopping as a technique to mitigate this is investigated     

The effect of advice message location on user performance

In 1993, a study began with a large commercial oil and gas software interpretation system to determine the feasibility of a general taxonomy of on-line help content and a corresponding taxonomy of human interface access methods to this content. The preliminary findings from this work were encouraging and indicate that a taxonomic approach makes it easy both for help providers to understand what information they need to supply, and for help users to find the help they need quickly. Part of this taxonomy of help content includes application messaging. Existing studies of on-line help messaging indicate that both user-initiated or system-initiated advice messaging can improve user efficiency by prompting users with information about what something on the interface is, what it does, or what to do once it has been activated. This study examines the placement of on-line help messages in a multiwindow software application on user performance. Subjects were automatically timed as they performed two sets of tasks: one where help messages always appeared at the bottom of an application's main window and one where help messages appeared at the bottom of the current window in focus     

两跳MIMO级联中继网络自由度研究

未来移动通信系统的发展和演进涉及的两个基本方面是提高系统的吞吐量和覆盖率,为了实现这两个目标,多输入多输出(MIMO)中继技术成为主流的解决办法。因此模拟现实情况,使用了TDD模式下的由源节点、中继节点、目的节点组成的两跳MIMO级联中继网络模型。采用改进的最大流-最小割定理方法得到中继网络的该模型自由度的上界;采用解码-转发与迫零信号处理得到了该模型自由度的下界。最后得到各对应情况下自由度下界与上界相等的结论,且其自由度均为天线数的函数。     

Wireless network coding in slotted aloha with two-hop unbalanced traffic

This paper deals with two representative unbalanced traffic cases for two-hop wireless relay access systems employing network coding and a slotted ALOHA protocol. Network coding is a recent and highly regarded technology for capacity enhancement with multiple unicast and multisource multicast networks. We have analyzed the performance of network coding on a two-hop wireless relay access system employing the slotted ALOHA under a balanced bidirectional traffic. The relay nodes will generally undergo this unbalanced multidirectional traffic but the impact of this unbalanced traffic on network coding has not been analyzed. This paper provides closed-form expressions for the throughput and packet delay for two-hop unbalanced bidirectional traffic cases both with and without network coding even if the buffers on nodes are unsaturated. The analytical results are mainly derived by solving queueing systems for the buffer behavior at the relay node. The results show that the transmission probability of the relay node is a design parameter that is crucial to maximizing the achievable throughput of wireless network coding in slotted ALOHA on two-hop unbalanced traffic cases. Furthermore, we show that the throughput is enhanced even if the traffic at the relay node is unbalanced.     

Throughput analysis of two-hop wireless networks with network coding

A Two-hop Wireless Network (TWN) is the basic topology structure that provides network coding opportunity for improving throughput. Network coding on a homogeneous TWN, in which all the data flows have the same packet size and all the links have the same transmission rate, has been extensively investigated. In this paper, network coding on more practical heterogeneous TWNs, featured by various packet sizes and transmission rates, is studied. Based on the Markov model, the throughput of the proposed network coding scheme, together with the throughput gain, is derived, which matches the simulation results very well. Numerical analyses indicate that, encoding the packets with close size and close transmission rate and enlarging buffer size at the relay node help in improving the throughput gain.     

Secrecy enhancement in two-hop DF relaying system under hardware impairment

In this paper, we investigate joint jammer selection and network coding for secrecy enhancement under transceiver hardware impairment. We propose two protocols of joint jammer selection and network coding (SCCJ-OJS and SCCJ-MiJS) to improve secrecy outage and throughput performance compared with a protocol without cooperative jamming (SCC). For performance evaluation, we derive exact closed-form expressions for the secrecy outage probability (SOP) and throughput performance of the three protocols. Our analysis is substantiated via Monte Carlo simulation. The results show that the SCC protocol is nearly in an outage state when the eavesdroppers are close to the source and/or relays, while the SCCJ-OJS and SCCJ-MiJS protocols still improve the performance compared with the SCC protocol in this strict case. In addition, transceiver hardware impairments can degrade the eavesdropping channel: the performance of the SCC protocol increases linearly with hardware impairment level, whereas SCCJ-OJS and SCCJ-MiJS protocol performance is enhanced at a low level and experience a minor decrease at a high level of hardware impairment. Finally, the theoretical results match the simulation results well.     

无线移动自组网中的两跳邻接表维护机制

针对无线移动自组网中的节点终端掌握拓扑变化的问题,两跳的邻接表能更好地为稳定路径提供基础的支持,为了减少因拓扑改变带来的信标交换而导致报文流量增大,充分利用网络数据广播传输的特性,提出了一种两跳邻接表建立方案,并对其在不同的状态下做了详细的分析。     

Improving performance of DS-CDMA systems using chaotic complex Bernoulli spreading codes

The most important goal of spreading spectrum communication system is to protect communication signals against interference and exploitation of information by unintended listeners. In fact, low probability of detection and low probability of intercept are two important parameters to increase the performance of the system. In Direct Sequence Code Division Multiple Access (DS-CDMA) systems, these properties are achieved by multiplying the data information in spreading sequences. Chaotic sequences, with their particular properties, have numerous applications in constructing spreading codes. Using one-dimensional Bernoulli chaotic sequence as spreading code is proposed in literature previously. The main feature of this sequence is its negative auto-correlation at lag of 1, which with proper design, leads to increase in efficiency of the communication system based on these codes. On the other hand, employing the complex chaotic sequences as spreading sequence also has been discussed in several papers. In this paper, use of two-dimensional Bernoulli chaotic sequences is proposed as spreading codes. The performance of a multi-user synchronous and asynchronous DS-CDMA system will be evaluated by applying these sequences under Additive White Gaussian Noise (AWGN) and fading channel. Simulation results indicate improvement of the performance in comparison with conventional spreading codes like Gold codes as well as similar complex chaotic spreading sequences. Similar to one-dimensional Bernoulli chaotic sequences, the proposed sequences also have negative auto-correlation. Besides, construction of complex sequences with lower average cross-correlation is possible with the proposed method.     

Resource scheduling scheme with QoS support in two-hop relay-enhanced OFDMA systems

As the system performance is obviously improved by introducing the concept of relay into the traditional orthogonal frequency division multiple access(OFDMA)systems,resource scheduling in relay-enhanced OFDMA systems is worthy of being studied carefully.To solve the optimization problem of achieving the maximum throughput while satisfying the quality of service(QoS)and guaranteeing the fairness of users,a novel resource scheduling scheme with QoS support for the downlink of two-hop relay-enhanced OFDMA systems is proposed.The proposed scheme,which is considered both in the first time sub-slot between direct link users and relay stations,and the second time sub-slot among relay link users,takes QoS support into consideration,as well as the system throughput and the fairness for users.Simulation results show that the proposed scheme has good performance in maximizing system throughput and guaranteeing the performance in the service delay and the data loss rate.