Jaesung Lim

Group Contention-Based OFDMA MAC Protocol for Multiple Access Interference-Free in WLAN Systems

In WLAN systems, the difference in propagation delay among stations (STAs) can exceed the predefined cyclic prefix length, leading to multiple access interference (MAI). To solve this MAI problem, it is necessary to use a longer cyclic prefix length for orthogonality between different subchannels; however, this sacrifices system efficiency due to the cyclic prefix overhead. In this paper, we propose a group contention-based OFDMA to solve the MAI problem with an adequate cyclic prefix length and support a larger number of STAs. The STAs within the cyclic prefix duration that are synchronized to an access point (AP), are included in the same contending group and compete with other STAs of the group in transmitting packets. The proposed scheme allows for the adoption of an adequate cyclic prefix length without MAI, and is able to reduce the cyclic prefix overhead. In addition, since no packet errors are induced by MAI in any of the STAs, the STAs can transmit simultaneously without redundant short interframe spaces (SIFSs) and other control packets. Through analysis and simulation, we show that for a large number of STAs, the proposed scheme achieves higher throughput than 802.11 protocols and a conventional CSMA combined with OFDMA.

EASR: an energy aware source routing with disjoint multipath selection for energy-efficient multihop wireless ad hoc networks

Wireless ad hoc networks usually consist of mobile battery operated computing devices that communicate over the wireless medium. These devices need to be energy conserving so that the battery life is maximized. The energy for transmission of a packet in the wireless channel remains quite significant and may turn out to be the highest energy consuming component of the device. So, an energy-efficient communication protocol can minimize maintenance and maximize system performance. We propose an Energy Aware Source Routing (EASR) which can be efficient from network long-term connectivity point of view. In this algorithm, multiple routing paths are selected. However, only one path will be used for data transmission at a certain time among multiple paths and each path has probability to be selected. In EASR, the routing paths will be discovered without overlapped. In addition, each path hardly overhears other data transmission. We define an overhearing ratio in order to reduce the overhearing energy waste among each selected path. And we show how establish energy efficient multiple paths by making use of overhearing ratio. Our simulation results show that our proposed scheme can achieve magnitude improvement of network lifetime and reasonable packet latency time.

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.

An idle timeslot reuse scheme for IEEE 802.15.3 high-rate wireless personal area networks

The IEEE 802.15.3 high-rate wireless personal area network (HR-WPAN) has been emerged in order to support quality of service (QoS) for real-time multimedia traffic and high data rate wireless connectivity. The IEEE 802.15.3 has introduced a type of time division multiple accesses (TDMA) which allocates the variable length of timeslot within a constant time interval. However, it is rather complex because the length of timeslot is changed during the connection of the variable bit rate (VBR) services. This problem makes the idle timeslot that it decreases throughput and delay performance in TDMA system. In this paper, we propose an idle timeslot reuse scheme to complement an inaccuracy of the length of timeslot and to maximize the channel utilization by adding a simple idle timeslot reusing procedure. Through the simulations, we quantify the performance of the idle timeslot reusing scheme in both environments of real-time multimedia traffic and high data rate traffic. Keywords-component; multiple access; wireless personal area network

Capacity Scaling and Diversity Order for Secure Cooperative Relaying With Untrustworthy Relays

This paper presents an investigation of the secrecy performance of amplify-and-forward (AF) distributed beamforming (DBF) and opportunistic relaying (OR) through K untrustworthy relays. The capacity scaling and achievable diversity order of a secure communication are derived for K → ∞ and in high signal-to-noise-ratio (SNR) regimes, respectively. The DBF is shown to convey confidential messages with a capacity scaling of 1/2 log K regardless of the trustworthiness of the relays, even without intended jamming (IJ), where 1/2 log K corresponds to the maximal capacity scaling of cooperative relaying in non-eavesdropping environments. However, using multiple untrustworthy relays worsens the secrecy performance of the OR, and as K increases, the secrecy capacity converges to zero. When secrecy outage performance with IJ is considered, the achievable diversity order for both the DBF and OR is limited to one. However, only the DBF exploits the benefits using multiple untrustworthy relays, and its secrecy outage probability decreases exponentially as K increases.

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.

R-CRDSA: Reservation-Contention Resolution Diversity Slotted ALOHA for Satellite Networks

In this letter, a reservation scheme with contention resolution diversity slotted ALOHA (CRDSA), called R-CRDSA, is proposed. Although CRDSA promises excellent performance compared to slotted ALOHA (SA), there are two barriers to be overcome for its use in satellite networks. The first is a low critical point of an offered load; i.e., at this point, as the offered load increases, the throughput will decrease. CRDSA has a lower critical point than that of SA. The second barrier is that CRDSA has a lower capacity efficiency compared to SA; i.e., on average, CRDSA requires more power or bandwidth than SA for transmission. R-CRDSA can overcome these barriers at the same cost as CRDSA, especially for multi-packet messages. The analytic performances are derived by the observation of the transition between two successive frames, and the average number of packets in a message. The simulation results show that the analytic derivations are valid, and they confirm that the barriers are resolved by R-CRDSA.

Adaptive rapid channel-hopping scheme mitigating smart jammer attacks in secure WLAN

As the military continues its transformation towards Network Centric Warfare, a quickly-deployable communication solution for a Tactical Operations Center is required. The U.S. Army uses Secure Wireless Local Area Network (SWLAN) with IEEE 802.11 standard waveforms to quickly set up Tactical Operations Center communications without cables. A SWLAN provides extremely secure mobile wireless data communication with COMSEC, TRANSEC, and monitoring systems to detect PHY and MAC layer misbehavior. However, the SWLAN cannot mitigate a smart jammer that transmits back-to-back packets on the detected channel after scanning all of the channels until a legitimate communication hops to a new channel. In this paper, we introduce an adaptive rapid channel-hopping scheme using Dwell Window and a Deception Mechanism to mitigate smart jammer attacks. The Dwell Window is a novel concept to adjust transmission time based on jammers capability. The Deception Mechanism is another novel concept to force the jammer to attack an inactive channel by deceiving the jammer. Numerical analysis and simulation results show that the proposed scheme is more effective than prior studies.

Cyclic shifted orthogonal complementary codes for multicarrier CDMA systems

In this letter, we present a set of cyclic shifted orthogonal complementary (cyclic-OC) codes for multicarrier CDMA systems. The conventional OC codes are not adequate for the system because they are able to support only the limited number of users. By applying the proposed cyclic-OC codes for the inter-symbol interference (ISI)-free multicarrier systems, the number of users can be extended to the size of code length. We validate the effectiveness of the proposed scheme through simulations

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.