Seokjoo Shin

A novel MAC scheme for prioritized services in IEEE 802.11a wireless LAN

We propose a novel medium access control (MAC) employing two fundamental accesses for priority services-a distributed co-ordination function with short contention-window (DCF/SC) and DCF. Three kinds of prioritized services are applied for the enhanced QoS of multimedia services in a wireless local area network (WLAN). The proposed MAC scheme is shown to process all real-time traffics, MAC and satisfy the maximum tolerable latency and jitter bounds under the test environments recommended by IEEE 802.11 Task Group-E.

QoS aware power allocation for combined guaranteed performance and best effort users in OFDMA systems

One of the key issues in design of an orthogonal frequency division multiple access (OFDMA) based system is subcarrier and power allocation. Assuming knowledge of the instantaneous channel gains for all users, we propose a resource allocation scheme for an OFDMA based system to simultaneously provide two services, guaranteed performance (GP) and best effort (BE), with different quality-of-service (QoS). Subcarrier and power allocation are carried out sequentially to reduce the complexity, and an optimal power allocation procedure is derived. Furthermore, we present a reduced complexity suboptimal power allocation algorithm.

COMIC: Intelligent Contention Window Control for Distributed Medium Access

In this letter, a scheme for Collision Mitigation with Intelligent Contention Window Control (COMIC) is proposed for backoff based collision resolution algorithm. COMIC intelligently mitigates collisions by probabilistically maximizing the selection likelihood of relatively less collision-probable contention slots over the backoffed contention window. A unified Markovian model for the Distribution Coordination Function (DCF) that incorporates COMIC, DCFcomic, is formulated for the performance analysis. The performance results show that DCFcomic outperforms the conventional DCF in both throughput and average packet delay due to the significant reduction in packet collisions.

Performance analysis of random access channel in OFDMA systems

The random access channel (RACH) in OFDMA systems is an uplink contention-based transport channel that is mainly used for subscriber stations to make a resource request to base stations. In this paper we focus on analyzing the performance of RACH in OFDMA systems such that the successful transmission probability, correctly detectable probability and throughput of RACH are analyzed. We also choose an access mechanism with binary exponential backoff delay procedure similar to that in IEEE 802.11. Based on the mechanism, we derive the delay and the blocking probability of RACH in OFDMA systems.

ML Approaching MIMO Detection Based on Orthogonal Projection

A detection algorithm for spatially multiplexed multiple input multiple output (MIMO) systems is proposed. The receiver first estimates the MIMO channel and rearranges the layers according to the measured SNRs. To determine the candidate vectors, an orthogonal projection combined with the M-algorithm is used. Without performing the exhaustive full search of the maximum likelihood (ML) method, the proposed algorithm can reach the performance which is closely akin to the ML method. The computational complexity markedly decreases: 0.66% over the ML method in terms of the number of real multiplications.

Fast broadcast at the intersection in VANET

Vehicular Ad-hoc Network(VANET) enables a wireless communication between vehicles to improve drivers safety. For the safety, a message including safety-related information is broadcast between vehicles and forwarded for farther vehicles. VANET poses challenges that broadcast attempts from many vehicles cause collisions between packets and result in packet drops. Many researches have concentrated on removing collisions in those excessive broadcasts. These approaches lead one vehicle to be selected as a rebroadcast vehicle. However, it makes difficult to forward the safety message to all directions at intersections. In this paper, we suggest an efficient way to broadcast a safety message to all directions at an intersection with a short delay and without a collision. Our simulation results show a coverage improvement of as much as 70% and a delay reduction of as much as 100msec.

Performance of the packet data transmission using the other-cell-interference factor in DS/CDMA downlink

Since the interference is related to the capacity and performance of DS/CDMA systems, it is necessary to investigate other-cell-interference characteristics according to the location of a user. In this paper, we specifically analyze the ratio of the intercell interference to the total power received from the own cell in downlink, defined as the other-cell-interference factor, as a function of normalized distance. Additionally, the maximum transmission rate under uniform user distribution throughout the cell is simulated when TDX mode packet scheduling is applied. The results show that a maximum transmission rate up to 7.6 Mbit/s for the privileged user could be supported when the bandwidth is 1.2288 MHz, orthogonality factor is 0.9 and the normalized distance is 0.1.

A Modified IEEE 802.15.4 Superframe Structure for Guaranteed Emergency Handling in Wireless Body Area Network

Wireless Body Area Network (WBAN) is a special purpose personal area network with health-care and other value-added applications. Recently, patient monitoring applications using WBAN have received significant attention. Patient monitoring applications are intended to report any sign of emergency in the monitored patient to the corresponding health personal so that necessary actions can be taken before it is too late. Therefore, the underlying protocol suite in WBAN should ensure that the emergency events will be timely reported. However, WBAN based on IEEE 802.15.4 might not be able to do so. Hence, in this paper, we devise the emergency handling capability in IEEE 802.15.4 MAC. We propose a modified superframe structure for supporting immediate emergency reporting and reliable data transmission. A fraction of inactive period is used opportunistically for three new periods in such a way that temporary switching between the new periods and inactive period occurs only for handling emergency events. We also suggest two different random channel access mechanisms for reporting emergencies. Through the numerical results, we show that the proposed scheme achieves improved latency and throughput in the WBAN environment with compare to IEEE 802.15.4.

Carrier sense multiple access with improvised collision avoidance and short-term fairness

In this paper, we present a simple method to simultaneously enhance collision avoidance efficiency and short-term fairness of a most popular contention based medium access control protocol, carrier sense multiple access with collision avoidance. The key idea here is to adaptively tune the shape of contention slot selection distribution over the temporal contention window during ongoing collision resolution process which, in the legacy scheme, used to be flat throughout. The tuning mechanism is such designed that it not only maximizes the selection likelihood of relatively less collision prone contention slots over the contention window but also compensates the idle delay that the contending stations have suffered in their recent access attempt. Through rigorous numerical and simulation based analysis, the proposed scheme is shown to enhance the performance of a IEEE 802.11 based distributed wireless network in terms of network throughput efficiency and packet transmission delay while allowing individual stations to share the channel fairly even in short time scale.

QoS Provisioning in Wireless Body Area Networks: A Review on MAC Aspects

Wireless Body Area Networks (WBANs) deal with variety of healthcare services with diverse Quality of Service (QoS) requirements. However, QoS handling is a challenging problem in such networks. In general, QoS related problems can be addressed from different layers in the networking protocol suite. Design of an efficient QoS aware Medium Access Control (MAC) protocol can address this problem in MAC layer. This paper analyzes the QoS requirements of WBAN, identifies the requisites of QoS handling system, and outlines the trends that are being followed for its advancement with focus on QoS issues at MAC layer. We review some prior works, compare them, and analyze the current research concerned with problem of providing QoS in WBAN. We also explore some open issues and discuss them.