Time-varying Mobile Research Articles

A Routing Protocol for Considering the Time Variant Mobility Model in Delay Tolerant Network

Delay Tolerant Network (DTN) has been emerged to support the network connectivity of the disruptive networks. A variety of routing methods have been proposed to reduce the latency for message delivery. PROPHET was proposed as a probabilistic routing that utilizes history of encounters and transitivity of nodes, which is computed as contact probability. While PROPHET improves the performance of DTN due to contact probability, contact probability is just one parameter reflecting the mobility pattern of nodes, and further study on utilizing contacting information of mobility pattern is still an important problem. Hence, in this paper, we try to improve routing for DTN by using a novel metric other than contact probability as mobility information. We propose the routing protocol to use mean residual contact time that describes the contact period for a given pair of nodes. The simulation results show that using the mean residual contact time can improve the performance of routing protocols for DTN. In addition, we also show in what situations the proposed method provides more efficient data delivery service. We characterize these situations using a parameter called Variation Metric.

Automatic mobility status estimation in wireless self-organised networks

Wireless sensor networks and ubiquitous computing are rapidly increasing in popularity and diversity. For many applications of these systems the mobility status of devices forms part of the operating context on which self-organisation is based. This paper describes a novel technique by which wireless devices such as sensor nodes can deduce their own mobility status, based on analysis of patterns in their local neighbourhood. The Self-Detection of Mobility Status algorithm (SDMS) uses a reinforcement learning inspired mechanism to combine the indications from five mobility metrics. For many systems in which a neighbour table is maintained through regular status messages or other interaction, the technique incurs no additional communication overhead. The technique does not require that nodes have additional information such as absolute or relative locations, or neighbourhood node density. The work considers systems with heterogeneous time-variant mobility models, in which a subset of nodes follows a random walk mobility model, another subset follows a random waypoint mobility model (i.e. has intermittent movement), some nodes have group mobility and there is a static subset. We simulate these heterogeneous mobility systems and evaluate the performance of SDMS against a number of metrics and in a wide variety of system configurations.

Entrant Growth and the Network of Interfirm Mobility in the Foreign Exchange Industry

In this paper we examine how personnel flows from incumbents to entrants affect entrant growth after a significant break in the rules governing competition in an industry. We argue that the time-varying network of personnel mobility between incumbents measures how technical knowledge and social resources are distributed among them. Consequently, where the prior employers of personnel are located in the incumbent network should determine the quality of knowledge these hires bring to an entrant and thus their contribution to the entrant's growth. We analyze the time-varying mobility network among incumbents in terms of its global structure and in terms of the local structure surrounding each entering firm. Our analysis shows that the global structure converges over time to a stable core and periphery pattern, which can be considered emergent in Bearman's (1997) definition. Entrant employees with job histories in the core significantly decrease the entrant's growth rate, whereas employees from the periphery increase growth. Also, entrants that occupy structural holes in the local network grow faster. The industry studied is foreign exchange trading among commercial banks from 1973 to 1993.

A dual combining based decoding scheme for two-branched mobile STBC

In this paper, a dual combining based decoding scheme is proposed to enhance the detection performance of space-time block coding with two transmit antennas in a highly time-varying mobile channel. The proposed scheme analyzes the signal-to-noise ratio and signal-to interference plus noise ratio of decision variables for a zero-forcing combiner (ZFC) and a matched filter combiner (MFC), respectively. Based on the analysis, we derive a sophisticated decoding scheme which utilizes tentative detection symbols of the ZFC based decoder to cancel inter-symbol interference from the decision variables of the MFC. Computer simulations show that the proposed scheme outperforms the conventional decoding scheme with the ZFC or the MFC.

Robust Filter Design for Tracking of Time-varying Mobile Radio Channels

In this paper, we deal with the problem of tracking smoothly time-varying parameters of linear regression channel models occurring in mobile radio communications. The system under consideration is subjected to time-varying coefficient uncertainties with bounded covariances and noises with inexplicit statistical characteristics. The objective of the tracking problem is to find the filter parameters such that the tracking performance, defined as the least mean square of the tracking error, is optimized. The key to the addressed problem is that the tracking problem is transformed into a filtering one and using a projection formula. A polynomial approach based solution for the tracking will be developed. The robust filter design proposed in this paper may have slightly higher computational complexity than the standard optimal filter designed for systems without random coefficient uncertainties. However, the tracking performance is improved.

Genetic algorithm-based MMSE receiver for MC-CDMA systems transmitting over time-varying mobile channels

A novel multi-user detector, based on a genetic algorithm-assisted per-carrier MMSE criterion, is proposed for MC-CDMA systems transmitting over time-varying multipath fading channels. The analysed multi-user detector outperforms state-of-the-art adaptive receivers based on deterministic gradient algorithms, particularly for an increasing number of users.

Spectrally Efficient Communication over Time-Varying Frequency-Selective Mobile Channels: Variable-Size Burst Construction and Adaptive Modulation

Methods for providing good spectral efficiency, without disadvantaging the delivered quality of service (QoS), in time-varying fading channels are presented. The key idea is to allocate system resources according to the encountered channel. Two approaches are examined: variable-size burst construction, and adaptive modulation. The first approach adapts the burst size according to the channel rate of change. In doing so, the available training symbols are efficiently utilized. The second adaptation approach tracks the operating channel quality, so that the most efficient modulation mode can be invoked while guaranteeing a target QoS. It is shown that these two methods can be effectively combined in a common framework for improving system efficiency, while guaranteeing good QoS. The proposed framework is especially applicable to multistate channels, in which at least one state can be considered sufficiently slowly varying. For such environments, the obtained simulation results demonstrate improved system performance and spectral efficiency.

SR ARQ for Adaptive Modulation Systems Combined With Selection Transmit Diversity

Adaptive automatic repeat request (ARQ) schemes are quite effective for throughput enhancement in time-varying mobile channel environments. In this paper, both throughput and packet error rate are analyzed for a selective-repeat ARQ scheme based on a constant-power variable-rate adaptive M-QAM system combined with selection transmit diversity over multiple-input multiple-output Markovian-Nakagami channels. In this analysis, the impact of using outdated and/or imperfect channel state information on the performance of the system is considered.

Impact of Channel Prediction on Adaptive Coded Modulation Performance in Rayleigh Fading

Adaptive coded modulation (ACM) is a promising tool for increasing the spectral efficiency of time-varying mobile channels while maintaining a predictable bit-error rate (BER). An important restriction in systems with such a transmission scheme is that the transmitter needs to have accurate channel-state information (CSI). Earlier analysis of ACM systems usually assumes that the transmitter has perfect knowledge of the channel or that the CSI is accurate but outdated. In this paper, we investigate the effects of predicting the CSI using a linear fading-envelope predictor in order to enhance the performance of an ACM system. For the case in which multidimensional trellis codes are used on Rayleigh-fading channels, we obtain approximative closed-form expressions for BER and average spectral efficiency. Numerical examples are given for the case of Jakes correlation profile and maximum a posteriori-optimal predictor coefficients.

Totally blind APP channel estimation for mobile OFDM systems

A new two-dimensional blind channel estimation scheme for coherent detection of orthogonal frequency-division multiplexing (OFDM) signals in a mobile environment is presented. The channel estimation is based on the a posteriori probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols, thus maximizing the spectral efficiency of the underlying OFDM system. The phase ambiguity problem is solved by using a 4-QAM (quadrature amplitude modulation) scheme with asymmetrical arrangement. The results clearly indicate that totally blind channel estimation is possible for virtually any realistic time-variant mobile channel.

Predictive distance-based mobility management for multidimensional pcs networks

This paper presents a mobile tracking scheme that exploits the predictability of user mobility patterns in wireless PCS networks. In this scheme, a mobile's future location is predicted by the network, based on the information gathered from the mobile's recent report of location and velocity. When a call is made, the network pages the destination mobile around the predicted location. A mobile makes the same location prediction as the network does; it inspects its own location periodically and reports the new location when the distance between the predicted and the actual locations exceeds a threshold. To more realistically represent the various degrees of velocity correlation in time, a Gauss-Markov mobility model is used. For practical systems where the mobility pattern varies over time, we propose a dynamic Gauss-Markov parameter estimator that provides the mobility parameters to the prediction algorithm. Based on the Gauss-Markov model, we describe an analytical framework to evaluate the cost of mobility management for the proposed scheme. We also present an approximation method that reduces the computational complexity of the cost evaluation for multidimensional systems. We then compare the cost of predictive mobility management against that of the regular, nonpredictive distance-based scheme, for both the case with ideal Gauss-Markov mobility pattern and the case with time-varying mobility pattern.

Design and Simulation of a Distributed Dynamic Clustering Algorithm for Multimode Routing in Wireless Ad Hoc Networks

This article presents analysis of the design and simulation of a new class of environmentally aware clustering algorithms for wireless ad hoc networks. The clustering algorithm establishes an adaptive self-organizing arrangement that enables multimode routing strategies. Multimode operation is needed to enhance scalability and robustness. In the present case, clustering is based on a criterion that enforces an upper bound on the probability of path failure within a cluster over time. The result is an adaptive hybrid routing strategy that dynamically balances routing overhead against routing optimality. The clustering algorithm unifies the routing strategies according to localized and time-varying mobility characteristics. A simulation model was developed to validate the effectiveness of the clustering algorithm and demonstrate the multimode routing behavior. Results show that the algorithm adapts effectively to node mobility and achieves relatively consistent overhead regardless of network size and mobility.

Blind decision feedback equalization of time-varying channels with DPSK inputs

This paper presents a new method to the blind equalization of a class of linear time-varying mobile channels for differentially encoded channel input signals. Through reparameterization, the equivalent discrete system can be modeled as a single-input multiple-output system. Unlike some existing methods, this new algorithm does not require prior knowledge on the cyclic frequency of the channel response. The algorithm is simple and relies only on a decision feedback structure and the differential encoding nature of the channel input signals. The convergence analysis of the decision feedback system is presented along with simulation examples.

An Efficient ARQ Protocol for Adaptive Error Control over Time-Varying Channels

This paper investigates a simple and efficient stop-and-wait (SW) automatic repeat-request (ARQ) scheme with adaptive error control. In this scheme, the channel state information (CSI) is extracted by monitoring the contiguous positive acknowledgment (ACK) or negative acknowledgment (NAK) messages. Exploiting this CSI, we adapt the coding strategy to the changes in the channel condition, and thus improve the throughput efficiency. In order to facilitate the throughput analysis and parameters optimization, we model the adaptive system with a Markov chain. Using this analytical model and assuming a static channel, an exact throughput expression for the adaptive ARQ protocol is derived and suboptimal adaptive system parameters are obtained. These design parameters are applied to the adaptive system in a typical time-varying mobile radio channel characterized by Rayleigh multipath fading on top of lognormal shadowing. The throughput performance of the proposed adaptive SW-ARQ scheme in such a time-varying fading channel is evaluated by computer simulation. For slow fading channels, the proposed adaptive system can track the channel variations very well, hence much throughput improvement can be achieved over conventional nonadaptive SW-ARQ schemes for almost all SNR values considered. The simulation results also confirm the applicability of the adaptive system parameters, so-obtained by the throughput analysis in static channel, to a time-varying mobile radio channel.

On ARQ scheme with adaptive error control

Adaptive automatic repeat request (ARQ) schemes are quite effective for throughput enhancement in time-varying mobile channel environments. An ARQ scheme with adaptive error-correcting codes is considered. A robust channel sensing algorithm by making use of XOR-ing is proposed for link adaptation. With the proposed channel estimation algorithm, an adaptive scheme that adapts its error-correcting codes according to channel conditions is presented. The efficiency of the proposed adaptive ARQ scheme is evaluated in terms of throughput performance by computer simulation for a time-varying mobile channel characterized by Rayleigh fading and log-normal shadowing. As an enhancement in counteracting long deep fades, the incorporation of frequency diversity into an adaptive ARQ scheme is also investigated and the simulation results show that this scheme is quite efficient in counteracting long deep fades.

Paging area optimization based on interval estimation in wireless personal communication networks

We consider an optimum personal paging area configuration problem to improve the paging efficiency in PCS/cellular mobile networks. The approach is to set up the boundaries of a one-step paging area that contain the locations of a mobile user with a high probability and to adjust the boundaries to gain a coverage that is matched to the mobile user's time-varying mobility pattern. We formulate the problem as an interval estimation problem. The objective is to reduce the paging signaling cost by minimizing the size of the paging area constrained to certain confidence measure (probability of locating the user), based on a finite number of available location observations of the mobile user. Modeling user mobility as a Brownian motion with the drift stochastic process and by estimating the parameters of the location probability distribution of the mobility process, the effects of the mobility characteristics and the system design parameters on the optimum paging area are investigated. Results show: (1) the optimum paging area expands with the time elapsed after the last known location of the user; (2) it also increases with the length of a prediction interval and the location probability; (3) the relative change in the paging area size decreases with the increase in the number of location observations.

Programmable mobile networks

Existing mobile systems (e.g., mobile IP, mobile ATM and third generation cellular systems) lack the intrinsic architectural flexibility to deal with the complexity of supporting adaptive mobile applications in wireless and mobile environments. We believe that there is a need to develop alternative network architectures from the existing ones to deal with the demands placed on underlying mobile signalling, adaptation management and wireless transport systems in support of new mobile services, e.g. interactive multimedia and web access. In this paper we present the design, implementation and evaluation of mobiware, a middleware technology that enables the introduction of new services in mobile networks. Mobiware provides a toolkit that service providers can utilize to build services that can dynamically exploit the intrinsic scalable properties of mobile multimedia applications in response to time-varying mobile network conditions. Based on an open programmable networking paradigm, mobiware runs on mobile devices, wireless access points and mobile-capable switch/routers providing a set of open programmable interfaces and distributed objects for adaptive mobile networking. Mobiware is software-intensive and is built on CORBA and Java distributed object technologies. The source code for mobiware v1.0 is freely available (comet.columbia.edu/mobiware) for experimentation.

Adaptive multilevel code selection and MUI cancellation for DS/CDMA over fading channel

For transmission of speech or multimedia information in a time-varying mobile channel, fixed rate codes are normally used designed for average or worst channel conditions. However, fixed rate codes fail to explore the time-varying nature of the mobile channel. In their previous work the authors have shown that a multilevel code, when used in conjunction with multiuser interference (MUI) cancellation in cascade, yields better performance than a conventional coded modulation scheme applied to CDMA. They show that, when multilevel codes are changed with varying channel conditions (i.e. are chosen adaptively according to the channel state information (CSI)), throughput (average number of information bits per symbol duration) can be increased without too much performance degradation. Code selection criteria as well as some codes with appropriate thresholds are also presented.

The mobiware toolkit: programmable support for adaptive mobile networking

Existing mobile systems (e.g., mobile IP, mobile ATM, and third-generation cellular systems) lack the intrinsic architectural flexibility to deal with the complexity of supporting adaptive mobile applications in wireless and mobile environments. We believe that there is a need to develop alternative network architectures from the existing ones to deal with the tremendous demands placed on underlying mobile signaling, adaptation management, and wireless transport systems in support of new mobile services (e.g., interactive multimedia and Web access). We present the design, implementation, and evaluation of mobiware, a mobile middleware toolkit that enables adaptive mobile services to dynamically exploit the intrinsic scalable properties of mobile multimedia applications in response to time-varying mobile network conditions. The mobiware toolkit is software-intensive and is built on CORBA and Java distributed object technology. Based on an open programmable paradigm developed by the COMET Group, mobiware runs on mobile devices, wireless access points, and mobile-capable switch/routers providing a set of open programmable interfaces and algorithms for adaptive mobile networking.