Hoki Baek

Beacon-Based Slotted ALOHA for Wireless Networks with Large Propagation Delay

In a wireless network with large propagation delay S-ALOHA(Slotted ALOHA) requires large guard time which causes lower normalized throughput. To reduce the large guard time in the ISA-ALOHA, a time alignment mechanism was proposed under the assumption of propagation delay estimation. In this letter, we propose a framed structure which is able to estimate propagation delay by employing coordinator beaconing. The framed structure consists of a time period for beaconing and a group of multiple time slots for random access. The proposed Beacon-based S-ALOHA(BS-ALOHA) can make packets generated during the time of beaconing evenly distributed over the random access period. Furthermore, we propose an analytical model considering overhead due to coordinator beaconing time and show that BS-ALOHA provides higher normalized throughput than both ALOHA and S-ALOHA employing the large guard time.

Performance analysis of Block ACK-Based Slotted ALOHA for wireless networks with long propagation delay☆

Abstract Recently, many variants of Slotted ALOHA (S-ALOHA) have been proposed to solve a problem of performance degradation in wireless networks with long propagation delay. However, they do not consider the effect of retransmission, which also largely degrades performance, and do not provide any analytical model that considers the effect. In this paper, we design a variant of S-ALOHA to support retransmission and derive analytical models that do consider its effect. The designed scheme has a framed structure that starts with a coordinators beacon. The beacon consists of a coordinators timestamp and Block ACKnowledgment (B-ACK). Using the timestamp, a node estimates propagation delay to the coordinator ( PDC ) in order to reduce guard time while transmitting a packet. Moreover, B-ACK is used to report the results of all transmissions attempted in the previous frame at once. As a result, the designed scheme can largely reduce the number of feedbacks and waste of guard time. Even if there is no analytical model that considers the long propagation delay and retransmission simultaneously, we choose the existing analytical models that consider a framed structure and B-ACK as reference models. However, they are not fully mathematical and partially use simulation results because of high computational complexity. Moreover, these models only analyze stability and throughput as performance metrics. On the other hand, our analytical models are fully mathematical models and can analyze all metrics, such as stability, throughput, and packet delay. We expect our analytical models to be a foundation for deriving fully mathematical models for variants of S-ALOHA using a framed structure and B-ACK.

Fast Time Synchronization Strategy for Mitigating Collision of Initial Entry Message over Link-16

TDL (Tactical Data Link) is a communication technology that can be a base technique to allow all platforms used in the battlefield to communicate with each other. To operate smoothly, synchronizing network time over TDL is necessary. Most TDLs use GPS (Global Positioning System) for time synchronization. GPS is vulnerable to jamming attacks because the power of the GPS signal is low. Link-16 has a GPS independent time synchronization method. This method is performed when new nodes attempt to join a certain network. Synchronized nodes in the network transmit IEM (Initial Entry Message) sent by broadcasting to the same slot. This causes a problem with collisions on the entry node side. If the entry node does not receive the IEM, it waits for another IEM in the next frame to synchronize with a reference node roughly. In this paper, we propose a method to acquire coarse synchronization with a reference node by overhearing messages sent by other nodes. Our proposed method can be used to acquire coarse synchronization instead of IEM when entry nodes cannot receive IEM because of the collision. Simulation results show that the proposed scheme provides lower time for synchronization than Link-16.

Enhanced MAC Protocol with New Packing for Imagery Transmission in Link-16

Link-16 is a widely used Tactical Data Link (TDL) system and a very low rate system, thus, it supports transmission of limited data such as text-based tactical messages and voice. Recently, in order to support large amounts of data such as imagery, there have been several studies focusing on increasing the data rate of Link-16 by modifying the waveform. However, efforts to reduce the effect of a large propagation delay are also required, as a time slot in Link-16 includes a large waste of guard time. In particular, the transmission of imagery, which requires a number of time slots, is significantly affected by the guard time. Moreover, Link-16 uses a donated slot for acknowledgment (ACK) of one message. Thus, it causes large overhead when the imagery requires many time slots. In this paper, we propose an enhanced MAC protocol with a new packing type to improve the performance of Link-16 by reducing the effect of a large propagation delay and the overhead of ACK. In the protocol, time slots are allocated on the basis of the proposed Available Time Slot (ATS) selection, not on the Recurrence Rate Number (RRN) of Link-16. Thus, there may be successive time slots in which the proposed packing type can be used for transmitting more bits by reusing the guard time. Moreover, the protocol provides retransmission on the basis of Block Negative ACK (BNACK). Thus, the number of time slots required for ACK can be reduced, and the delay for retransmission is greatly decreased. Simulation results show that the proposed protocol with a new packing type can improve the performance of Link-16.

An Efficient Method for All-To-All Broadcast in Long Propagation Delay Wireless Networks

All-to-all broadcast refers to the process by which every node broadcasts its certain piece of information to all other nodes in the network. The all-to-all broadcasting time is very large due to the long propagation delay time in the long-delay environment like the airborne network and the underwater network. In this letter, we propose an efficient all-to-all broadcast method based on location information in long-delay single-hop wireless networks. We propose a matrix transformation and an algorithm to avoid conflictions in parallel transmissions and reduce all-to-all broadcasting time. The effectiveness of the proposed scheme is proved through simulations. KeywordsAll-to-all broadcast, wireless networks, propagation delay.

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.

Enhanced Positioning Algorithm of ARPS for Improving Accuracy and Expanding Service Coverage

The airborne relay-based positioning system (ARPS), which employs the relaying of navigation signals, was proposed as an alternative positioning system. However, the ARPS has limitations, such as relatively large vertical error and service restrictions, because firstly, the user position is estimated based on airborne relays that are located in one direction, and secondly, the positioning is processed using only relayed navigation signals. In this paper, we propose an enhanced positioning algorithm to improve the performance of the ARPS. The main idea of the enhanced algorithm is the adaptable use of either virtual or direct measurements of reference stations in the calculation process based on the structural features of the ARPS. Unlike the existing two-step algorithm for airborne relay and user positioning, the enhanced algorithm is divided into two cases based on whether the required number of navigation signals for user positioning is met. In the first case, where the number of signals is greater than four, the user first estimates the positions of the airborne relays and its own initial position. Then, the user position is re-estimated by integrating a virtual measurement of a reference station that is calculated using the initial estimated user position and known reference positions. To prevent performance degradation, the re-estimation is performed after determining its requirement through comparing the expected position errors. If the navigation signals are insufficient, such as when the user is outside of airborne relay coverage, the user position is estimated by additionally using direct signal measurements of the reference stations in place of absent relayed signals. The simulation results demonstrate that a higher accuracy level can be achieved because the user position is estimated based on the measurements of airborne relays and a ground station. Furthermore, the service coverage is expanded by using direct measurements of reference stations for user positioning.

Spectrum Sharing for Coexistence of Fixed Satellite Services and Frequency Hopping Tactical Data Link

The growth in wireless communication services together with increasing demand has resulted in the scarcity of spectrum resources. Many researchers have attempted to address the scarcity problem by focusing on spectrum sharing to allow fixed satellite services (FSS) to coexist with other fixed services or other satellite communication systems. However, most secondary users are commercial systems, whose characteristics differ from military systems such as tactical data links (TDLs). A TDL requires anti-jamming capability and multiple operations support; thus, most TDLs use multiple frequency hopping (FH) patterns, known as multinet. To the best of our knowledge, no research has focused on the coexistence of FSS and TDL to date. In this paper, we verify whether spectrum sharing will facilitate the coexistence of FSS and FH-TDL. First, we propose FH-TDL with a spectrum use rule and a new FH method to reduce the pulse collision probability. We then prove that interference from FH-TDL to FSS is permissible. Instead of analyzing interference from FSS to FH-TDL, we simply consider the worst case, i.e., that a signal experiencing interference by FSS is always subject to collisions. We analyze the pulse collision probability under the worst case by constructing the mathematical models rather than using simulation. The numerical results show that the use of appropriate values for the weight parameter enables the proposed FH method to improve the pulse collision probability of FH-TDL.

Dynamic relay node selection scheme for multi-hop time synchronization in Link-16

Link-16 is a widely used tactical data link system for battlefield networks. A key component of Link-16 is network time synchronization. Link-16 provides two types of time synchronization methods: global positioning system (GPS)-based time synchronization and GPS-less time synchronization. In particular, Link-16 requires relay nodes for multi-hop time synchronization. However, in Link-16, the relay nodes are selected statically before the network begins, which inhibits direct communication from incoming nodes; therefore, time synchronization fails. In this paper, we propose a new dynamic relay node selection scheme for multi-hop time synchronization in Link-16. Thus, all nodes can be dynamically identified as relay nodes for new incoming nodes. Consequently, if a new incoming node connects to any registered node, time synchronization can be achieved. Simulation results show that our proposed scheme outperforms Link-16, regardless of the number of round trip time slots or the failure probability.

A Distributed Dynamic Address Assignment for Tactical Mobile Ad-hoc Networks

In this paper, we propose a group distributed dynamic address assignment scheme suitable for tactical mobile ad hoc networks(MANET). Efficient address assignment is an important issue in the MANET because a node may frequently leave the current network and join another network owing to the mobility of the node. The conventional schemes do not consider the features of the tactical networks: existence of a leader node and network activity on a group basis. Thus, they may not be suitable for military operations. In our proposed scheme, called grouped units dynamic address assignment protocol(G-DAAP), a leader node maintains the address information for the members in the network and any of the nodes can exploit the information for the assignment or request of the IP address by a simple message exchange procedure. This leads to fast address assignment with small overheads. In addition, G-DAAP based on the modified IEEE 802.11e Enhanced Distributed Channel Access(EDCA) can assign addresses more quickly. We describe the delay performance of the G-DAAP and compare it with conventional schemes by numerical analysis and computer simulations. The results show that the G-DAAP significantly improves the delay performance as compared with the conventional schemes.