Hongjun Noh

TDMA-Based Cooperative MAC Protocol for Multi-Hop Relaying Networks

In this letter, we propose and analyze a time division multiple access (TDMA)-based cooperative medium access control (MAC) protocol for wireless multi-hop relaying networks. The fundamental drawback of a reservation-based MAC protocol is channel waste, i.e., the existence of idle slots. The proposed MAC, called cooperative relaying TDMA (CR-TDMA), allows a packet that is pre-assigned to a busy relay node to be reassigned to a neighbor node with an empty queue, i.e., a helper node. With the assistance of helper nodes, the relay node can process multiple packets during only one frame. Furthermore, CR-TDMA does not require additional overhead, such as mini-slots or additional packet header information for negotiations of cooperative communication. The numerical and simulation results show that CR-TDMA can significantly improve performance if helper nodes are available and can also guarantee equivalent or better performance than a conventional TDMA even in the worst cases.

Dynamic Cooperative Retransmission Scheme for TDMA Systems

In this letter, we propose and analyze a dynamic cooperative retransmission scheme for TDMA systems in multi-hop networks. Our scheme cooperatively retransmits a failed packet during a neighbor nodes idle slot, which is selected using a simple mechanism. If an idle slot is not available, cooperative retransmission is attempted during the source nodes time slot. The throughput of TDMA can be significantly improved by cooperative retransmission. We formulate an analytical model for the proposed scheme that can be implemented over Rayleigh fading channels. The results showed that the throughput of TDMA systems was improved significantly if a sufficient number of idle slots were available for cooperative retransmission, although an overhead was introduced.

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.

ANC-ALOHA: Analog Network Coding ALOHA for Satellite Networks

We propose a new random access protocol based on reservation ALOHA (R-ALOHA), called ANC-ALOHA, for applying analog network coding (ANC) to the slotted ALOHA channel of satellite networks (SATNETs). Although ANC promises excellent performance in SATNETs, ANC is not guaranteed in slotted ALOHA owing to the inherent randomness. R-ALOHA can be used to acquire a slot for ANC, but there are still two remaining problems to be overcome. First, the reservation maintenance of R-ALOHA does not work for ANC owing to the superposed signals. Second, a channel estimation scheme for ANC is needed in a slotted ALOHA channel. ANC-ALOHA exploits a keep-on sequence as the preamble and postamble of a burst for ANC to solve the above issues. The reduced payload size and delayed transmission are also proposed to assure the reception of the keep-on sequence. We derived an analytic model to verify the throughput of ANC-ALOHA, and validated it based on the simulation results. The results indicate that ANC-ALOHA significantly improves the channel throughput compared to R-ALOHA.

Performance Evaluation of Access Control for CRDSA and R-CRDSA under High Traffic Load

In this paper, we propose access control schemes for the contention resolution diversity slotted ALOHA (CRDSA) and the reservation-CRDSA (R-CRDSA) schemes to enhance throughput under high traffic loads. We also propose an estimation technique for the traffic load of CRDSA. In slotted ALOHA, many schemes have been proposed for the same purpose. Some of them adjust channel access probability according to the estimated traffic load. In this paper, these techniques are extended to CRDSA and R-CRDSA by considering the number of packet replicas and the status of the reservation channels. Simulation results demonstrate that the proposed scheme enhances throughput and channel efficiency under high traffic loads.

A cooperative TDMA MAC protocol using dynamic slot assignment scheme

In this paper, we propose and analyze a cooperative TDMA MAC protocol with dynamic slot assignment. Our protocol allows a source node to retransmit failed packet cooperatively during a neighbor nodes idle slot. The source node can select idle slots for a cooperative retransmission by a simple control packet exchange among neighbor nodes. The neighbor nodes that have the failed packet transmit during the selected slot at the same time. If there are no idle slots available, the cooperative retransmission is attempted during the source nodes time slot. The throughput of TDMA can be significantly improved by the proposed cooperative retransmission scheme at the expense of increased overhead. We formulate an analytic model for the proposed protocol for implementation over Rayleigh fading channels and validate it by simulation. The results show that the throughput of TDMA systems is improved significantly if there are a sufficient number of idle slots available for cooperative retransmission, even though overhead is introduced.

Partitioned cyclic code shift keying for JTIDS

In this paper, we propose partitioned cyclic code shift keying (PCCSK) for the Joint Tactical Information Distribution System (JTIDS) by combining a Hadamard matrix and a modified maximal length sequence (MMLS). The proposed approach adaptively increases the transmission data rate within a limited transmission range in the battlefield. By exploiting a new code set as a spreading code and by adopting code selection, the system is easily able to achieve higher spectral efficiency. For additional performance enhancement, we apply adaptive channel coding with PCCSK, and we derive the appropriate code pairs. Monte Carlo simulations are conducted to show the transmission ranges of the proposed scheme, and the results show that PCCSK with adaptive coding can support a higher data rate with a reasonable transmission range.

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.

Stability of reservation-contention resolution diversity slotted ALOHA for satellite networks

Random access (RA) schemes for satellite networks (SATNETs) have been attracting renewed interest, with an increase in bursty traffic in SATNETs and military satellite communications (MILSATCOM). In particular, reservationcontention resolution diversity slotted ALOHA (R-CRDSA), an enhancement of CRDSA, is the latest RA scheme. R-CRDSA can achieve maximum throughput with fewer replicas and greater stability than CRDSA. In this paper, the stability of R-CRDSA is investigated. The stability must be analyzed before an RA scheme is used; it is the most important property of an RA scheme because a RA schemes dynamics may make it unusable in a practical system. We formulate a model of the stability of the R-CRDSA scheme with which we can not only evaluate the stability of a system configuration but also predict the range of stable states. Therefore, this model can be used for the design of SATNETs and various service models planned for next-generation MILSATCOM.