Namgi Kim

Adaptive handoff algorithms for dynamic traffic load distribution in 4G mobile networks

In 4G mobile networks which is a kind of packet-based cellular systems, resources are shared among all users and the amount of available resources is determined by traffic load. If traffic load is concentrated in a cell, this cell become to the hotspot cell. The hotspot cell can cause to block or drop calls and consequently degrades the service quality even though available resources are remained in neighboring cells. Therefore, it is essential to distribute traffic load of the hotspot cell in order to effectively use remained resources and maintain the acceptable service quality. We propose adaptive handoff algorithms for dynamic traffic load distribution in the hotspot cell. In the simulation, we find that these algorithms can reduce call drop rate of new and handoff calls thus enhance the service quality

Experimental link channel characteristics in wireless body sensor systems

In the resource-constrained environment such as wireless body sensor networks (WBSNs), all sensor nodes operating with limited battery need the very-low-power wireless technologies. The representative approach to reduce energy consumption in WBSNs is transmission power control (TPC). The TPC technique is used to maintain acceptable packet delivery performance with the low power consumption. So, it has been widely researched as the important subject in WSNs. However, the TPC algorithms which have been researched in WSNs are not directly applicable in WBSNs which deploy sensor nodes in, on, or around a human body. This is because instantaneous channel condition is frequently changed due to sensor mobility resulting from human movements in WBSNs. Therefore, we need a sophisticated TPC technology to achieve reliable communication in WBSNs. To do this, in this paper, we investigate link layer channel characteristics in WBSNs. For understanding link characteristics in terms of power and placement, we conducted diverse experiments with real sensor devices. Through the analysis of experimental results, we knew that RSSI values are deeply depended on the transmission power and sensor placement in WBSNs.

Hybrid Transmission Power Control for Wireless Body Sensor Systems

In wireless body sensor network systems (WB-SNSs), the sensor nodes have very limited battery power because they are tiny, lightweight, and wearable or implantable. As a result, WB-SNSs require a very efficient transmission power control (TPC) algorithm for effectively reducing energy consumption and extending the lifetime of sensor nodes. To achieve this goal, we propose a novel TPC algorithm referred to as hybrid TPC. The hybrid TPC algorithm adaptively selects a conservative or an aggressive control mechanism depending on current channel conditions. The conservative control mechanism, which slowly changes transmission power level (TPL), is suitable in a dynamic environment. On the other hand, the aggressive control mechanism, which rapidly changes TPL, is ideal in a static environment. In order to evaluate the effectiveness of the hybrid TPC algorithm, we implemented various TPC algorithms and compared their performances against the hybrid TPC algorithm in different channel environments. The experimental results showed that the hybrid TPC algorithm outperformed other TPC algorithms in all channel environments.

Wireless packet fair queueing algorithms with link level retransmission

In order to provide quality of service to wireless networks, a number of wireless fair queueing algorithms have recently been proposed. They, however, require perfect channel prediction before transmission and rarely consider algorithms under the link layer. Instead of perfect channel prediction, most wireless systems adopt the Link Level Retransmission (LLR) algorithm within the link layer for recovering channel errors. However, the LLR algorithm does not work well with the previous prediction-based wireless fair queueing algorithms. Therefore, we propose a new wireless fair queueing algorithm, Wireless Fair Queueing with Retransmission (WFQ-R), which is well matched with the LLR algorithm and does not require channel prediction. In the WFQ-R algorithm, the share consumed by retransmission is regarded as a debt of the retransmitted flow to the other flows. So, the WFQ-R algorithm achieves wireless fairness with the LLR algorithm by penalizing flows that use wireless resources without permission in the link layer. Through analyses, we proved that the WFQ-R algorithm guarantees throughput and delay fairness. Through simulations, we showed that our WFQ-R algorithm maintains fairness adaptively. Furthermore, our WFQ-R algorithm is able to achieve flow separation and compensation.

Packet fair queueing algorithms for wireless networks with link level retransmission

Recently, a number of fair queueing algorithms for wireless networks have been proposed. They, however, need perfect channel prediction before transmission and rarely consider a medium access control (MAC) algorithm. In the wireless world, the link level retransmission scheme is popularly used in the MAC layer for recovering channel errors. Therefore, we propose a new wireless fair queueing algorithm that works well with link level retransmission and does not require channel prediction. Through simulation, we showed that our algorithm guarantees throughput and fairness. Also, we found that our algorithm achieves flow separation and compensation.

A study of secure data transmissions in mobile cloud computing from the energy consumption side

For mobile cloud computing, one of the key issues is to minimize energy consumption in data communication. Although many studies have examined energy consumption of data transmissions, they are limited in that they have focused mainly on bitstream transmissions over existing 3G networks or Wi-Fi environments. Thus, the present paper explores energy efficiency of mobile devices when transferring data securely over various communication networks including high-speed 4G networks such as LTE and Wibro.

Seamless handoff scheme for 4G mobile systems based on IP and OFDM

The goal of 4G mobile systems is to provide enhanced services with high data rates, and integrated and converged services with IP-based networks. In order to satisfy the high data rate requirement and support multimedia efficiently, OFDM (orthogonal frequency division multiplexing) has been considered for the modulation and multiple access scheme in 4G systems. Therefore, we propose an efficient and practical seamless handoff scheme for 4G mobile systems based on IP and OFDM. The goal of the seamless handoff scheme is to minimize handoff delay in providing a seamless connection. The seamless handoff scheme obtains the physical channel for handoff in a contention-free manner with pre-synchronization and pre-forwarding IP contexts. As a result, it thoroughly decreases physical channel blocking delays as well as IP layer context-switching delays during handoff.

Different Characteristics of Radio Modules in Wireless Body Sensor Network Systems

Wireless body sensor network systems (WB-SNSs) can use diverse radio modules. However, previous studies did not consider different characteristics of radio modules, which deeply impact the performance of WB-SNSs. In this paper, we analyze the performance of WB-SNSs using the representative radio modules CC2420 and CC1000. In this environment, we collected log data from real sensor devices deployed on the human body. After log data collection, we first show that CC2420 and CC1000 have different radio characteristics from diverse views, such as the received signal strength indication (RSSI) average and deviation, transmission power levels, and body movement. Through the analysis, we also find that an efficient transmission power control (TPC) algorithm should consider these diverse factors due to different radio modules.

IEEE 802.11b WLAN performance with variable transmission rates: in view of high level throughput

Wireless networks have been rapidly integrated with the wired Internet and have been widely deployed. In particular, IEEE 802.11b WLAN is the most widespread wireless network today. The IEEE 802.11b WLAN supports multiple transmission rates and the rate is chosen in an adaptive manner by an auto rate control algorithm. This auto rate control algorithm deeply affects the total system performance of the IEEE 802.11b WLAN. In this paper, we examine the WLAN performance with regard to the auto rate control algorithm, especially the ARF scheme, which is the most popular auto rate control algorithm in 802.11b based WLAN products. The experimental results indicate that the ARF scheme works well in the face of signal noise due to node location. However, the ARF scheme severely degrades system performance when multiple nodes contend to obtain the wireless channel and the packet is lost due to signal collision.

Adjusting Control Packet Transmission Intervals in Low Power Sensor Systems

In order to construct an efficient wireless sensor system, it is necessary to increase the lifetime of its battery-operated sensor nodes. To this end, a wireless body sensor system adopts a transmission power control (TPC) mechanism. However, existing TPC mechanisms adjust the transmission power level (TPL) according to the received signal strength indication (RSSI) value of the most recently received data packet. Therefore, they do not effectively cope with dynamically changing wireless body channel environments. In particular, when a wireless channel is unstable, changes in the TPL should be avoided to prevent energy consumption due to unnecessary transmission and reception of control packets. Accordingly, this paper proposes a new TPC mechanism that adaptively changes the control packet transmission interval on the basis of the current channel condition. Further, actual sensors are used to experimentally verify that the proposed mechanism (1) performs well in all channel environments and (2) facilitates the construction of an efficient wireless body sensor system.