Hanjin Lee

Optimal modulation and coding scheme selection in cellular networks with hybrid-ARQ error control

We propose an optimal modulation and coding scheme (MCS) selection criterion for maximizing user throughput in cellular networks. The proposed criterion adopts both the Chase combining and incremental redundancy based hybrid automatic repeat request (HARQ) mechanisms and it selects an MCS level that maximizes the expected throughput which is estimated by considering both the number of transmissions and successful decoding probability in HARQ operation. We also prove that the conventional MCS selection rule is not optimized with mathematical analysis. Through link-level and system-level simulations, we show that the proposed MCS selection criterion yields higher average cell throughput than the conventional MCS selection schemes for slowly varying channels.

Adaptive Hysteresis Using Mobility Correlation for Fast Handover

The value of hysteresis has a significant effect on the handover performance measured in terms of the handover initiation delay and the number of handovers. To determine the hysteresis value effectively, a mobility correlation among users is utilized. In this letter, a grouping algorithm and an adaptive hysteresis algorithm based on the mobility correlation are proposed. It is shown that the proposed algorithms are very efficient in reducing both the handover initiation delay and the number of handovers.

A pseudo-random asynchronous duty cycle MAC protocol in wireless sensor networks

We propose a pseudo-random asynchronous duty cycle MAC protocol in wireless sensor networks. The proposed protocol adopts a hash function to determine the next wake-up times. As the next wake-up times are known in advance, the sensor nodes do not need to remain active until their intended receivers wake up. Given an end-to-end delay requirement, the proposed protocol significantly decreases energy consumption by reducing the idle listening time.

MRMAC: Medium Reservation MAC Protocol for Reducing End-to-End Delay and Energy Consumption in Wireless Sensor Networks

This letter proposes a novel medium reservation MAC protocol that reduces both end-to-end delay and energy consumption for wireless sensor networks. MRMAC reduces end-to-end delay by informing the intended receiver of NPAT (next packet arrival time) and MRI (medium reservation information) to reserve the medium in advance. Through this medium reservation phase, following packets can be delivered with reduced delay in multihop streaming phase. A simulation shows that the result of the proposed protocol outperforms previous works.

Bandwidth Efficient Key Distribution for Secure Multicast in Dynamic Wireless Mesh Networks

In the near future, various multicast based services will be provided over wireless mesh networks. For secure multicast services, various tree based group key management schemes have been introduced until now. Traditional tree based approaches mainly focus on reducing the number of rekeying messages transmitted by the key distribution center. However, they do not consider the network bandwidth used for transmitting each rekeying message.We propose a bandwidth efficient key tree management scheme for dynamic wireless mesh networks where membership changes occur frequently. Simulation results show that our scheme effectively reduces the bandwidth consumption used for rekeying compared to existing key tree schemes.

Preamble allocation scheme using trajectory information for mobile relay stations

We propose a novel preamble allocation scheme for mobile relay stations (MRS) in order to reduce the occurrence of preamble collision. We calculate the probability of preamble collision based on trajectory information of MRSs. Then, we select a preamble considering not only the probability of preamble collision but also interference from other base stations (BS) and relay stations (RS). Simulation results show that the proposed scheme significantly reduces the occurrence of preamble collision.

A Distributed Deterministic and Resilient Replication Attack Detection Protocol in Wireless Sensor Networks

To detect replica nodes in a wireless sensor network, we propose a distributed, deterministic and resilient (DDR) replica detection protocol developed from a witness node based strategy. In DDR, while a location claim message of each node is sent towards the designated verification location in the network, the consistency of the messages is verified at intermediate nodes en route to its final destination. Compared with previous replication attack detection protocols, DDR achieves better computation and communication performance due to the use of symmetric key cryptography only and early replica detection.

An Effective Hotspot Cell Management Scheme Using Adaptive Handover Time in 4G Mobile Networks

We propose a hotspot cell management scheme using adaptive handover time in 4G mobile networks. 4G networks are expected to support various multimedia services over IP network and satisfy high spectral efficiency requirement with overcoming time and frequency selective fading due to the high speed of users. In cellular systems including 4G networks, hotspot cells can be occurred when available wireless resources at some location are not enough to sustain the needs of users. The hotspot cell can potentially lead to blocked and dropped calls and cause degradation of the service quality. In the meanwhile, a band of users enjoying multimedia services can move around in a 4G mobile network, which may generate heavy flows of traffic load in the network. This situation can generate the hotspot cell which has a short length of life at most a few minutes. In this paper, we propose a handover-based scheme which can effectively manage hotspot cells in 4G mobile networks. The proposed scheme adopts hard handoff mechanism and dynamically controls the time point of handover based on the amount of load of the current serving cell and the target cell. Adaptation of handover time according to the amount of load effectively and flexibly can manage the hotspot cell. And, through our hotspot cell management scheme, acceptable service quality can be supported as users maintain graceful connections with the network. In the simulation results, we find that our scheme makes smaller number of hotspot cells and supports higher service quality than compared schemes.

Adaptive resource reservation for efficient resource utilization in the wireless multimedia network

With the increase of wireless network and demand for the variety of services, the handoff resource reservation for the Quality of Service(QoS) will be significant issue in future wireless networks. But on the other hand, the efficiency of resource usage also needs to be considered for preventing waste of network resources due to the reservation. Therefore, this paper suggests an efficient resource reservation scheme to support QoS provision as well as the efficient resource utilization simultaneously. In the proposed scheme, handoff probability estimation with the filtering algorithm is used to calculate the optimal amount of resources to be reserved in the neighbor cells. So, the mobile units with the proposed scheme can predict the amount of resources required in the future and reserve resource effectively. The performance simulation shows that the proposed scheme is advantageous with the balance of QoS provision and resource utilization in the wireless network.

Improving Handover Quality in 4G Mobile Systems

In this paper, we propose a new handover algorithm to guarantee handover quality in 4G mobile systems. The proposed algorithm limits the handover interruption time by improving the HARQ retransmission latency of the first packet transmitted from new serving cell. Through the simulations, we proved that our algorithm meets the requirement of handover interruption time for TCP services with high rate.