Kyuman Lee

Distributed dynamic slot assignment scheme for fast broadcast transmission in tactical ad hoc networks

In this paper, we propose a fast, dynamic slot assignment scheme to reduce timeslot access delay for newly arrived nodes in tactical ad hoc networks. As there is no central coordinator in distributed networks, a newly arrived node in conventional schemes must negotiate separately with each of its neighbors for timeslot allocation. However, it is not necessary to negotiate with all the neighboring nodes in a specific case. This can result in unnecessary join delay that reduces network efficiency and responsiveness. The proposed scheme simplifies the slot assignment process by using mini-slots to share control packets for short periods. Numerical analysis and extensive simulation show that our scheme can significantly reduce the timeslot access delay compared with other existing slot assignment schemes. In addition, we investigate the effect of the mini-slot overhead on the performance of the proposed scheme.

Airborne Relay-Based Regional Positioning System

Ground-based pseudolite systems have some limitations, such as low vertical accuracy, multipath effects and near-far problems. These problems are not significant in airborne-based pseudolite systems. However, the monitoring of pseudolite positions is required because of the mobility of the platforms on which the pseudolites are mounted, and this causes performance degradation. To address these pseudolite system limitations, we propose an airborne relay-based regional positioning system that consists of a master station, reference stations, airborne relays and a user. In the proposed system, navigation signals are generated from the reference stations located on the ground and are relayed via the airborne relays. Unlike in conventional airborne-based systems, the user in the proposed system sequentially estimates both the locations of airborne relays and his/her own position. Therefore, a delay due to monitoring does not occur, and the accuracy is not affected by the movement of airborne relays. We conducted several simulations to evaluate the performance of the proposed system. Based on the simulation results, we demonstrated that the proposed system guarantees a higher accuracy than airborne-based pseudolite systems, and it is feasible despite the existence of clock offsets among reference stations.

UAV-Based Localization Scheme for Battlefield Environments

Considering the emphasis on network-centric concepts of warfare in recent battlefield environments, the study of location information has become more important. This is because most military weapons use GNSSs (Global Navigation Satellite Systems) such as GPS and Galileo to track position information. Situations in which it is impossible to use these systems have been increased by many factors such as jamming attacks, reception failure, and politics. In this study, we propose a localization method that uses UAV (Unmanned Aerial Vehicle) and TDOA (Time Differential of Arrival) method to determine receiver position in situations in which using GNSS is locally impossible. We also consider the building of a slim-budget localization system independent of GNSS and how to operate it efficiently. The simulation results show that our scheme is applicable on battlefield, the feasibility is proved by the accuracy, the DOP (Dilution of Precise) and the coverage.

A Cooperative Relay Scheme for Tactical Multi-hop Wireless Networks

In this paper, we propose and analyze a cooperative relay scheme for a time division multiple access (TDMA) protocol based tactical multi-hop wireless networks. In the proposed scheme, a neighbor of a relay node can relay a packet on behalf of the relay node in order to improve the throughput, without delay performance degradation. The neighbor node, that is, the cooperative node, may not have its own packet to transmit, whereas it can have a packet of the relay owing to the broadcast nature of the wireless networks, even though it is not the intended receiver. As the cooperative node takes the responsibility for relaying the packet, the relay node that is scheduled to relay the packet can transmit another packet. Cooperative nodes for the relay are selected by a simple distributed mechanism during the period of exchange of the control packets in each frame. Furthermore, an unnecessary packet relay by multiple node selection in multi-hop networks is prevented by the acknowledgement from the receiver. We formulate an analytical model on the basis of a Markov chain for the proposed scheme, which can be implemented over Rayleigh fading channels. The model is validated through various computer simulations. The analysis and simulation results show that the proposed scheme can effectively improve the throughput of a relay node, while maintaining the delay performance.

Artificial Collision-Based Dynamic Slot Assignment for In-Band Control Networks

In this paper, we propose a novel dynamic slot-assignment scheme using artificial collisions called artificial collision-based dynamic slot assignment (AC-DSA) for multi-hop networks, which is based on the in-band control mechanism. In-band control-based networks have no exclusively reserved slots for newly arrived nodes; thus, the throughput can be improved compared with that of the out-of-band control-based networks. In in-band control based networks, a newly arrived node can use idle slots to transmit its control packet to request a slot assignment. However, if no idle slots are available, the node cannot transmit its control packet even though it has a higher priority than the existing nodes in the network, because a newly arrived node has to wait until idle slots are available in conventional schemes. To address this issue, AC-DSA uses the duration information of artificial packet collisions as a control message for slot assignment, which enables nodes with high priority to deprive a non-idle slot of nodes with low priority to guarantee quality of service requirements when no idle slots are available. We formulate an analytical model for the proposed slot-assignment scheme and compare it with the conventional one. Further, we demonstrate the feasibility and efficiency of the AC-DSA during high-traffic conditions considering the capture effect.

A RELNAV enhancement for reducing cumulative position error in Link-16 without GRU

Position information is important for conducting battlefield missions. GPS generally enables allies to identify positions. RELNAV defined in Link-16 is used for JU in the air to estimate its position as an auxiliary navigation system of GPS. However, there is a critical problem in RELNAV. To operate accurately, RELNAV requires the presence of ground reference units that help to estimate the positions of JUs operated by RELNAV. However, guaranteeing the constant presence of ground reference units in a battlefield is difficult. If the presence of ground reference units is not assured, JUs operated by RELNAV have a critical problem in maintaining high positional quality. They depend on inertial navigation systems that can accumulate positional errors with time. In this environment, such errors will become large, and hence maintaining accurate operation of RELNAV is difficult. Therefore, we propose an alternative that can solve such problems. In the proposed scheme, we select new references that can play the role of GRU in RELNAV among allies conducting operations on the ground. The new references are responsible for estimating the positions of JUs. To do so optimally, a JU uses a reference selection algorithm that assures high quality of dilution of precision between the JU and the new references. Position errors of JUs decrease largely as a result of the presence of new references in the proposed scheme. RELNAV can therefore be conducted with higher positional accuracy of JUs than that of the existing RELNAV. We verify through some simulations that the feasibility and performance of the proposed scheme are better than those of the existing scheme.

Autonomous maneuvering of relay UAV for battlefields using TDOA localization

In this paper, we propose a scheme in which a communication relay unmanned aerial vehicle (UAV) estimates the distribution of the terminals and selects the optimal relay position for autonomous maneuvering. In order to accomplish mission success, warfighters on the battlefield must communicate with their commander all the time. Autonomous maneuvering of a relay UAV using the proposed scheme will support their communication and mission command effectively. In the first step of the proposed scheme, the UAV estimates the distribution of the terminals using the time difference of arrival of the received signal. Once the distribution of the terminals is estimated, in the second step, the optimal relay position and altitude are determined, and the UAV maneuvers itself to relay communications. By using this scheme, there is no need to exchange specific location information, and the optimal point can be provided from the beginning of flight. In addition, this scheme has the advantage of continuous relaying at minimum power. To verify the validity and parameters of the proposed scheme, various simulations are executed and indicate the possibility of utilization in the field.

Relay-Based Positioning in TDMA Networks

In this letter, we propose relay-based positioning in time division multiple access (TDMA) networks, called RPT. Although the concept of relay-based positioning is an improvement over the performance of relative navigation (RelNav), there are two problems to be solved when applying the relay concept to a TDMA network. First, measurement errors caused by mobility may degrade user accuracy because positioning references transmit navigation messages at their assigned times in TDMA, unlike conventional navigation systems, where all references continuously transmit. Second, a selection algorithm among airborne nodes is needed to relay messages to users. An RPT frame is designed to assign the slots for the broadcast and relay of navigation messages consecutively to minimize the effect of mobility. The selection algorithm determines the optimum set of airborne relays by selecting evenly distributed nodes depending on their distances to the master station. The simulation results indicate that RPT significantly improves user accuracy over RelNav.

Self-correction scheme by adjusting the navigation signal transmission time

Pseudolite-based systems and airborne relay-based positioning system (ARPS) have a limitation wherein the position error increases as the distance between the user and the area where the segments of the system are configured increases, owing to the low altitude of the navigation signal transmitters. To enhance the accuracy of the user, we propose a self-correction scheme based on the ARPS. In the proposed correction scheme, each reference station adjusts the transmission time of the navigation signal. This adjustment time is determined by calculating the average pseudorange differences between the true/estimated positions of the reference station and the estimated position of each airborne relay. Regardless of this adjustment, the user estimates the positions of the airborne relays and its own based on the original transmission time. Owing to this characteristic, the pseudorange errors that are included during the signal propagation are corrected and the accuracies of the airborne relays and the user are improved. Simulation results show that the proposed correction scheme reduces the error of the user, and the user in an enemy area can be provided with a relatively high accuracy service without the aid of correction messages.

Point Transformation Scheme to protect location data changing in real time

Location information is utilized for a variety of purposes, such as military operation, Internet of Things, big data, and application services. Problems can occur if location information is exposed, such as a higher likelihood of crime and invasion of privacy. In the battlefield, exposed location data can lead to significant losses because of the exposure of military secrets and operation information. Existing transformation schemes to protect location data have been studied. However, such schemes are not suitable for real-time processing systems that use location data because transformation processing requires too much time. In order to solve this problem, we propose a Point Transformation Scheme (PTS) for location data privacy. PTS has symmetry movement by selecting a direction based on previous location data. The proposed scheme makes complex transformation processes simple compared with conventional schemes. This way, processing time can be reduced to be suitable for real-time processing systems. In addition, location data of the proposed scheme have unique keys value and transformation formulas. This means that transformation data have original data attributes at all times. Therefore, PTS can be used to share location data by grouping different transformation data among authorized users. The simulation results show that PTS is a faster and more secure scheme for real-time processing systems than conventional schemes.