Search

High-speed railway (HSR) has demonstrated tremendous growth worldwide, and is currently attaining a maximum velocity of 575 km/h. Such a high speed makes mobile wireless communications extremely challenging for HSR since the handover (HO) rate increases with speed, resulting in a high call dropping probability. By employing a dedicated distributed antenna system (DAS) along with the two-hop network architecture, in this paper, we propose a frequency switch (FSW) scheme to mitigate the persistent HO issue in conventional HSR wireless communication systems. The proposed scheme significantly alleviates interruption time associated with the traditional HO process, providing an effective scheme when the train moves across small cells covered by remote antenna units (RAUs) belonging to the same central unit (CU). In order to further reduce the HO command failure probability, an enhanced fast predictive HO mechanism is proposed by starting the HO process in advance, when the train moves from one RAU to the next one controlled by the adjacent CU. Analytical results show that compared to the traditional HO scheme, the proposed schemes can improve the system performance substantially by delivering ultra-reliable low-latency communications with a reduced HO/FSW rate. [ABSTRACT FROM AUTHOR]

In this paper, we study the network average power consumption minimization problem in a two-tier heterogeneous network by optimally tuning the activation ratio of micro base stations (BSs) under the quality of service (QoS) constraints of the network mean queueing delay and the network signal-to-interference ratio (SIR) coverage. With the consideration of dynamic packets arrivals, each BS can either be busy or be idle depending on its queueing status. The network performance is thus critically determined by the traffic intensity of each BS. With the assumption of universal frequency reuse, the average traffic intensity of each tier is characterized by a set of fixed-point equations, which can be solved by a proposed iterative method. By using the approximation that BSs of the same tier have the same SIR coverage, the cumulative distribution function of the traffic intensity of each tier is further obtained. On that basis, the network average power consumption per area, the network mean queueing delay, and the network SIR coverage are characterized. Numerical results demonstrate that if the idle power coefficient is below a certain threshold, then the optimal activation ratio equals the one to minimize the network average power consumption per area; otherwise, the optimal activation ratio can be obtained according to the QoS constraints. It is further shown that universal frequency reuse outperforms spectrum partitioning in terms of both the network average power consumption and the network SIR coverage in the considered scenario. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the optimal throughput performance for simultaneous wireless information and power transfer relaying network in the presence of direct link between the source and the destination. By considering the direct link, system performance can be improved when the channel condition is good from source to destination. The cooperative network consists of a source, an amplify-and-forward relay, and a destination, where the relay harvests energy from the source and then uses the harvested energy to forward information to the destination. At the destination node, the signals from the direct link and relay are combined by the maximum ratio combining (MRC) method. The time switching (TS) relaying protocol is applied to enable energy harvesting and information processing simultaneously. At the same time, we made progress on analyzing the throughput of information transmission on the basis of the MRC method with power switching (PS) relaying protocol. Additionally, adaptive-MRC protocol is proposed based on the TS and PS relaying protocols, which can choose better mode to achieve the communication. The numerical analyses provide practical insights into the effect of various system parameters, such as energy harvesting time splitting ratio, source transmission rate, distance between nodes, and noise power at the destination and the relay. [ABSTRACT FROM AUTHOR]

This paper investigates simultaneous wireless information and power transfer (SWIPT) in the ad hoc network, where access points (APs) with multiple antennas can transfer energy as well as information using a common spectrum resource. The receiving nodes decode the incoming information data (information decoding, ID) and/or harvest the RF energy (energy harvesting, EH), where two different receiving node architectures—time switching (TS) and power splitting (PS)—are considered. By using a stochastic geometry approach, we aim to analyze the transmission capacity and the harvested energy per unit area as performance measures of the SWIPT in the ad hoc network. From the analytical results, we can derive the optimal node densities of ID nodes and EH nodes for TS architectures and, furthermore, optimize the power splitting ratio jointly with the node density for the PS architectures such that the harvested energy per unit area is maximized when the target transmission capacity is given. The proposed node density optimization gives us a useful insight into the design of SWIPT-based ad hoc network, where we can show how densely the APs/nodes can be deployed in a given area and furthermore, find that the ad hoc network with PS receiving nodes outperforms that with TS receiving nodes for a large target transmission capacity, and vice versa. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel physical layer strategy to improve the secrecy performance of multiuser multiple-input single-output networks. In this strategy, orthogonal space-time block code (OSTBC) is employed at an A\text{A}-antenna base station and artificial noise (AN) is employed at an A\text{J} -antenna cooperative relay to enhance the security level of the network. Moreover, two opportunistic scheduling schemes, namely selection combining (SC) and scan-and-wait combining (SWC), are leveraged to select one legitimate user for data transmission. To evaluate the secrecy performance of the proposed OSTBC-SC-AN and OSTBC-SWC-AN schemes, we derive new exact closed-form expressions for the secrecy outage probability and the effective secrecy throughput. Using numerical results, we show that the OSTBC-SWC-AN scheme outperforms the OSTBC-SC-AN scheme when the switching threshold is carefully chosen, while the OSTBC-SC-AN scheme is independent of the switching threshold. We also show that increasing A\text{A} at a low switching threshold or decreasing A\rm{A} at a high switching threshold achieves secrecy enhancement for the OSTBC-SWC-AN scheme, while increasing A\rm{A} brings better secrecy performance for the OSTBC-SC-AN scheme. [ABSTRACT FROM AUTHOR]

An optimal spectrum handoff scheme for cognitive radio networks (CRNs) is presented in this paper. This scheme has two novel features: 1) Hybrid rule-based priority queuing model: To overcome the limitations of preemptive resume priority and nonpreemptive resume priority (PRP/NPRP) queuing models, a hybrid queuing model with discretion rule is proposed to characterize the spectrum access priority among secondary users (SUs). This hybrid queuing model is then used to calculate the channel waiting time during spectrum handoff; and 2) Multiteacher apprentice learning: Unlike existing CRN cognition engine designs that focus on spectrum adaptation through SU self-learning (i.e., an SU learns how to adapt to the dynamic CRN environment by itself), we propose the concept of multiteacher knowledge transfer, wherein the multiple SUs that already have mature spectrum adaptation strategies share their knowledge with an inexperienced SU. Our simulation results show that the proposed new designs improve the spectrum handoff accuracy for the complex CRN environments. [ABSTRACT FROM PUBLISHER]

In this paper, a new mathematical approach for the analysis and optimization of cellular-enabled low-energy mobile devices with simultaneous wireless information and power transfer capabilities is introduced. The proposed methodology relies on modeling the locations of the base stations as points of a spatial Poisson point process, and it leverages stochastic geometry for system-level analysis. The tradeoffs emerging from simultaneous wireless information and power transfer transmission are characterized through the concept of “feasibility regions” and are quantified through the joint cumulative distribution function of harvested power and rate. To gain insight on the achievable performance, in addition, an upper bound is proposed, and its accuracy is discussed. The system model encompasses a realistic channel model that accounts for line-of-sight (LOS) and non-LOS (NLOS) links, different cell association criteria, practical receivers based on time switching and power splitting schemes, and directional beamforming. The analysis shows that optimal values for the time switching and power splitting ratios exist and that directional beamforming and network densification are capable of enhancing system performance. More specifically, high directional antennas lead cellular networks to operate in the noise-limited regime, which is proved to provide optimal performance, and because of the existence of LOS and NLOS links, an optimal deployment density for the base stations is proven to exist for typical system setups. [ABSTRACT FROM PUBLISHER]

In this paper, we characterize the achievable (linear) degrees of freedom (DoFs) of the K-user single-input single-output (SISO) interference channel with blind interference alignment (BIA) using staggered antenna switching. In this scheme, each transmitter is equipped with one conventional antenna, and each receiver is equipped with one reconfigurable (multimode) antenna. Assuming that the channel is known to the receivers only, we show that by using linear BIA with staggered antenna switching at the receiver, a sum DoF of \text{2}K/(K+\text{2}) is achievable. This result implies that in using BIA, we can double the DoF achieved by orthogonal multiple-access schemes for a large number of users. Moreover, we propose a systematic algorithm to generate the transmit beamforming vectors and the reconfigurable antenna switching patterns, showing that it can achieve the \text{2}K/(K+\text{2})$ sum DoF for any K$. Finally, we apply this algorithm to the four-user SISO interference channel and demonstrate that \text{4}/\text{3} sum DoF is indeed achievable. [ABSTRACT FROM PUBLISHER]

Efficient utilization of wireless communication resources and increases in the capacity of the communication networks have become very crucial claims for next-generation cellular networks. Full-duplex (FD) is a very promising technique that allows for the efficient use of wireless communication resources, given that the self-interference level can be suppressed to an acceptable level. In this paper, we consider a simple FD communication system, consisting of one FD access point (AP) and two half-duplex (HD) mobile users, and investigate when it is more advantageous for the communication network to operate in FD or HD multiuser multiple-input multiple-output (MIMO) mode. Since FD transmission is degraded by self-interference while HD suffers from spatial correlation between MIMO antennas, which causes rate loss, we study the effect of both the self-interference cancelation parameter at the FD AP and the mutual distance between the HD users on FD performances and the effect of the spatial correlation coefficient on HD MIMO. Afterward, a switching criterion is proposed, which chooses the operation mode that maximizes the downlink channel capacity while maintaining the uplink channel capacity at a certain level. Subsequently, based on our study of the system's parameters that affect the performance of both FD and HD, theoretical thresholds for these parameters are derived. Finally, numerical analysis is presented, verifying the validity of the optimization problem solution and the derived thresholds. [ABSTRACT FROM PUBLISHER]

This paper studies the optimization of power control for the two-user interference channel in which the terminals are time-switched between the communication and energy-harvesting phases. The objective is to maximize the sum rate, subject to the minimum data and harvested energy constraints at the receivers, assuming a fixed time-switching coefficient. Our key contribution is a geometric approach that analyzes the feasible region governed by the constraints, which gives rise to the optimal power control solution. We assume that perfect channel state information is available at both transmitters to determine the solution. [ABSTRACT FROM PUBLISHER]

In this paper, we propose an adaptive medium access control (MAC) solution for a fully connected mobile ad hoc network (MANET), supporting homogeneous best-effort data traffic. The MAC scheme achieves consistently high network performance by adapting to the ever-varying network traffic load. Based on the detection of a current network load condition, nodes can make a switching decision between IEEE 802.11 distributed coordination function (DCF) and dynamic time-division multiple access (D-TDMA), when the network traffic load reaches a threshold, referred to as MAC switching point. The adaptive MAC solution determines the MAC switching point to maximize network performance. Approximate and closed-form performance analytical models for both MAC protocols are established, which facilitate the computation of MAC switching point in a tractable way. Extensive analytical and simulation results demonstrate that the adaptive MAC solution provides consistently maximal network performance in the presence of traffic load dynamics. [ABSTRACT FROM AUTHOR]

In pocket switched networks (PSNs), devices carried by humans form a network environment, which utilizes both human mobility and occasional connectivity to deliver messages among mobile devices. Mobile phones are widely used in our daily lives. Therefore, the PSN composed of human-carried mobile phones will soon become an ubiquitous network environment. In this paper, we propose a communication strategy that applies the WiFi hotspot mode of a mobile phone in the PSN to realize the device-to-device communications. However, due to the lack of energy supply, a phone in hotspot mode could rapidly consume energy and shorten its battery lifetime significantly. To maximize the message dissemination scope within the limited energy constraint of each phone in PSNs, an Energy-efficient Phone-to-phone Communication method based on WiFi Hotspot (EPCWH) is presented to schedule the phone's switching between hotspot mode and client mode. Simulations based on the synthetic random-waypoint mobility pattern and real traces are conducted in an opportunistic network environment; the results show that EPCWH achieves the best performance in terms of message dissemination and energy consumption among different switching strategies. [ABSTRACT FROM PUBLISHER]

To improve the energy efficiency of small-cell networks, in this paper, we analyze the operating procedure of home base stations, i.e., Home evolved Node B (HeNB), in femtocells, by introducing an MAP/PH/1/k queueing model. In this analytical model, the HeNB's power on/off is represented as the alternative service and vacation periods. In addition, the hybrid-access mode defined in Third-Generation Partnership Project (3GPP) Release 9 is considered, which involves both high-priority and low-priority users. Based on the analytical results, an adaptive service rate and vacation length (ASV) method is proposed to maximize the HeNB's energy efficiency while satisfying its quality-of-service (QoS) requirements, such as blocking probability and user waiting time. Simulation results demonstrate that the proposed ASV method is more effective in improving energy efficiency compared with its counterpart. [ABSTRACT FROM PUBLISHER]

In a cognitive radio network (CRN), a necessary condition for nodes to communicate with each other is that they have a rendezvous (switching to the same channel at the same time). Most of the existing rendezvous-guaranteed schemes have some undesired requirements, such as demanding role preassignment, a common control channel (CCC), or a synchronous environment. In this paper, we first define the complete rendezvous problem and then propose a staggered channel hopping scheme (SCH) to solve the complete rendezvous problem. The SCH utilizes the Chinese remainder theorem (CRT) to enable a node to have a rendezvous with any of its neighbors. We have proved the correctness of SCH through the CRT. We have also analyzed the performance of the SCH under some sample scenarios. Simulation results verify the superiority of the SCH in terms of time-to-rendezvous (TTR), standard deviation of TTR, and maximum TTR (MTTR). [ABSTRACT FROM PUBLISHER]

A heterogeneous wireless network (HetNet), which combines multiple cooperating radio access technologies in an overlapping structure, is a communication system that has been recognized as an efficient way to meet the increasing traffic demand in broadband wireless networks. In this paper, we exploit the network cooperation in HetNets to propose two joint radio resource management (JRRM) schemes that improve energy savings while satisfying the system quality-of-service (QoS) performance requirements. First, we present an optimal QoS-aware energy-efficient JRRM scheme, which is formulated as a semi-Markov decision process model, and provide the optimal control policy for the HetNet under analysis. Second, we present an implementation-friendly QoS-aware energy-efficient JRRM scheme that utilizes a threshold on the macrocell radio resource occupancy to trigger the switching-on/off procedure of the base transceiver station resources, as well as a load-balancing procedure to minimize the service disruptions that may occur because of radio resource shortage and to reduce power consumption during the HetNet operation. This JRRM scheme is analyzed by means of a multidimensional continuous-time Markov chain model. Third, we devise an algorithm to determine the threshold setting in the implementation-friendly JRRM scheme according to a desirable power saving level that is prespecified by the mobile network operator. Numerical results show that the proposed schemes achieve substantial energy savings while keeping satisfactory performance levels. [ABSTRACT FROM AUTHOR]

In a cognitive radio network (CRN), two nodes have a rendezvous when they tune to the same channel simultaneously. Providing rendezvous guarantee between any pair of nodes is essential because a rendezvous is necessary for communication. Some existing works claim that rendezvous guarantee can be provided by using a dedicated common control channel to negotiate the channel being used. A serious problem of these mechanisms is that a globally available channel may not exist. Some channel hopping protocols try to provide a rendezvous guarantee without using a common control channel. However, these solutions may suffer from a poor number of rendezvous or uneven channel utilization. In this paper, we propose a novel distributed channel hopping protocol: Quorum and Latin square channel hopping (QLCH). The QLCH scheme efficiently provides rendezvous guarantee by utilizing the concept of quorum systems and Latin squares. The former is utilized to guarantee balanced rendezvous among nodes, whereas the latter is adopted to share the rendezvous among channels. QLCH efficiently provides fast rendezvous guarantee and can cooperate with other channel hopping protocols to enhance network throughput. Analytical and simulation results verify the improvement of QLCH. For example, when there are 30 flows using 11 channels in a CRN, the achieved time to rendezvous of QLCH is 9.84 times lower than that of the existing protocol SYNC-ETCH, whereas the achieved network throughput of QLCH is 362% higher than that of the existing protocol, i.e., Quorum-based channel hopping (L-QCH). [ABSTRACT FROM PUBLISHER]

Recently, the mobile relay technique has been discussed to support communication services for Long-Term Evolution Advanced (LTE-A) high-speed rail networks. In the network, there are several mobile relays in a train. User equipment (UE) devices connect to in-car mobile relays instead of outside evolved NodeBs (eNBs). Mobile relays establish backhaul links with the serving donor eNBs (DeNBs) and coordinate control and data flows for UE devices. Since the train travels in high speed, mobile relays suffer from frequent handovers between DeNBs. We observe that the defined handover procedures in LTE-A specifications may incur message overhead and lengthen handover time. In this paper, we propose an enhanced handover scheme, which contains two procedures. The first procedure is an enhanced measurement procedure, which can accelerate the measurement procedure when the mobile relay knows that the train is moving toward some neighbor DeNBs. The second procedure is a group in-network handover procedure, which can aggregate similar in network handover procedures in the core network. Performance evaluations indicate that our design can conserve both time and messages when handover. [ABSTRACT FROM PUBLISHER]

Although multiple channels are supported in the physical layer, the IEEE 802.11 MAC-layer mechanism is designed for a single channel. Exploiting multiple channels enhances spatial reuse and reduces transmission collisions and, thus, improves network throughput. Designing a multichannel MAC protocol is much more difficult than designing a single-channel MAC protocol. New challenges, such as the channel allocation problem and the missing receiver problem, must be overcome. Existing multichannel MAC protocols suffer from either higher hardware cost (because of applying multiple transceivers) or lower channel utilization (due to limited transmission opportunity). In this paper, a fully distributed channel-hopping solution, i.e., the cyclic-quorum-based multichannel MAC protocol, is proposed. We use the cyclic quorum in a novel way and the proposed protocol has several attractive features. First, only a single transceiver is needed for each node. Second, any sender is guaranteed to meet its receiver in a short time. Third, each node's channel-hopping sequence is derived from its node ID. This avoids exchanging control messages, such as each node's hopping sequence or available channel list. Fourth, multiple transmission pairs can accomplish handshaking simultaneously. The proposed protocol is simple and efficient. Simulation and real system implementation results verify that our mechanism is a promising multichannel MAC protocol for mobile ad hoc networks. [ABSTRACT FROM AUTHOR]

Contact discovery is a fundamental premise of information exchange in delay-tolerant networks (DTNs). However, the intermittent connectivity nature of DTNs makes energy-efficient contact discovery a tough challenge. To save energy, a node should work in low-power mode most of the time since contacts with the other nodes may rarely happen. However, such a strategy will probably fail to detect many contacts with relatively short durations, leading to system performance degradation. In this paper, we consider a specific type of DTN, i.e., the pocket switched networks (PSN), in which the nodes' mobility pattern shows a strong social property. In a PSN, although the end-to-end path may be disconnected for the majority of time, several nodes periodically gather at certain hot spots and form well-connected clusters. Based on such mobility pattern, cooperation among nodes is utilized in our contact discovery design. The nodes that have already joined a cluster collaboratively wake up to discover new contacts. With local synchronization and complementary consideration of possible traffic congestion, both the theoretical analysis and simulation results show that our cooperative duty cycling (CDC) greatly reduces the energy consumption while achieving comparable data delivery performance. [ABSTRACT FROM PUBLISHER]

The femtocell is becoming a promising solution to face the explosive growth of mobile broadband usage in cellular networks. Although each femtocell only covers a small area, a massive deployment is expected to form networked femtocells in the near future. One immediate challenge is to provide seamless mobility support for networked femtocells with minimal support from mobile core networks. In this paper, we propose efficient local mobility management schemes for networked femtocells based on X2 traffic forwarding under the Third-Generation Partnership Project Long-Term Evolution Advanced (3GPP LTE-A) framework. Instead of implementing the path switch operation at core network entity for each handover, a local traffic forwarding chain is constructed to use the existing Internet backhaul and the local path between the local anchor femtocell and the target femtocell for ongoing session communications. Both analytical studies and simulation experiments are conducted to evaluate the proposed schemes and compare them with the original 3GPP scheme. The results indicate that the proposed schemes can significantly reduce the signaling cost and relieve the processing burden of mobile core networks with the reasonable distributed cost for local traffic forwarding. In addition, the proposed schemes can enable fast session recovery to adapt to the self-deployment nature of the femtocells. [ABSTRACT FROM PUBLISHER]

Effective data broadcasting is essential in vehicular networks not only for road-safety message dissemination but also to aid routing and cooperative driving applications through periodic beaconing and to spread network initialization advertisements that are mandatory to support infotainment applications. Broadcast data are neither acknowledged nor retransmitted in case of failure, which raises the possibility of frame loss due to channel errors and collisions with multiple simultaneous broadcasts. This paper aims at modeling periodic broadcasting on the control channel of IEEE Std. 802.11p vehicular networks with multichannel architecture. Unlike previous related work, the proposed novel analytical approach accounts for mutual influence among nodes, frequent periodic updates of broadcasted data, standard network advertisement procedures, and 802.11p prioritized channel access with multichannel-related phenomena under various link quality conditions. [ABSTRACT FROM PUBLISHER]

In this paper, the problem of mode switching in multiple-input–multiple-output (MIMO) broadcast channels utilizing the Per-User Unitary Rate Control (PU2RC) transmission scheme is considered. Since there is no mathematical expression for the achievable rate of the PU2RC systems, it is proposed to approximate them using proper bounds. It is shown that certain simple feedback distributions will lead to tight bounds on the achievable rates of the PU2RC systems. The proposed bounds are used for the calculation of mode-switching points and maximization of the system rate under different multiplexing order conditions. The asymptotic behavior of the preferred modes is also studied under high-signal-to-noise-ratio (SNR) and low-SNR conditions, and it is shown that the single-user transmission mode is the preferred mode for both regimes. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate the performance of networks built from single-channel devices that use wireless access in vehicular environment protocols. We consider several traffic combinations, each of which presents a mix of traffic classes, over control and service channels. Our results show that time switching between the channels causes synchronization of backoff processes, which increases the frame collision probability, in particular for small sizes of contention windows. We also evaluate the impact of the interruption of the backoff process by inactive channel time, which gives rise to a probability distribution with repeated tails and a coefficient of variation larger than 1. Our model can also be used to evaluate different sets of enhanced distributed channel access parameters and to select the channel duty cycle according to the policy of the network operator. [ABSTRACT FROM PUBLISHER]