Chun-Su Park

Selective Channel Scanning for Fast Handoff in Wireless LAN using Neighbor Graph

Handoff at the link layer 2 (L2) consists of three phases: scanning, authentication, and reassociation. Among the three phases, scanning is dominant in terms of time delay. Thus, in this paper, we propose an improved scanning mechanism to minimize the disconnected time while the wireless station (STA) changes the associated access points (APs). According to IEEE 802.11 standard, the STA has to scan all channels in the scanning phase. In this paper, based on the neighbor graph (NG), we introduce a selective channel scanning method for fast handoff in which the STA scans only channels selected by the NG. Experimental results show that the proposed method reduces the scanning delay drastically.

Channel Adaptive Error Resilience Scheme for Video Transmission over Mobile WiMAX

Video transmission over mobile worldwide interoperability for microwave access (WiMAX) can be serverly degraded due to the effect of fading and handoff. In this paper, we propose a channel adaptive error resilience scheme for video transmission over mobile WiMAX. When the channel condition begins to trigger handoff, the current frame is stored in the long-term memory for the forward error correction, and the following frames are encoded by using double motion vectors (MVs) in the sense of multi-hypothesis motion compensation. Even if a whole frame is lost, we can reconstruct the following frames using the stored frame in the long-term memory. However, the error propagation still remains in this forward error resilience method. To refresh the erroneous frames to the decoder, the encoder utilizes the channel adaptive refreshing (CAR). In the CAR, the channel rate is first predicted using channel parameter, a carrier to interference and noise ratio (CINR), and the encoder adaptively determines the number of blocks to be encoded in the intra mode based on the feedback information. Performance evaluations are presented to demonstrate the effectiveness of the proposed method.

FMO bitrate overhead reducing for High Definition Video streaming over packet networks

The transmission of High Definition Video (HDV) has to confront large size video packets due to the high resolution of video frames. The usage of Flexible Macroblock Ordering (FMO) to control the size of video packets for different network types causes bitrate overhead. In this paper, we propose an improved FMO algorithm that can efficiently reduce the bitrate overhead. In the proposed algorithm, all MBs are grouped in SGs based on the similarity of the transformation coefficients. Experimental results show that our algorithm can reduce the bitrate overhead significantly as compared with conventional FMOs.

QoS Provision Using Dual RF Modules in Wireless LAN

With the rapid growth of emerging demand and deployment of wireless LAN (WLAN), much of traffic including multimedia traffic is forced to travel over WLAN. Since the legacy IEEE 802.11 standard is unable to provide adequate quality of service (QoS) for multimedia traffic, IEEE 802.11e group developed medium access protocol (MAC) improvements to support QoS sensitive applications and to make more efficient use of the wireless channel. Although this QoS enhancement is great improvement for the legacy standard in terms of capability and efficiency of the protocol, it does not address the method to enhance QoS when a wireless station (STA) changes its associated access point (AP) due to its mobility. Thus, in this paper, we propose an effective QoS provision method that can guarantee QoS requirements in WLAN when the STA performs handoff and is turned on. In order to guarantee QoS requirements, the proposed method uses APs with dual radio frequency (RF) modules. By adding to AP an RF module which can only receive signals (SNIFFER), the AP can eavesdrop channels of its neighboring APs selected by the modified neighbor graph (NG). Experimental results show that the proposed mechanisms can guarantee QoS for multimedia traffic without ping-pong phenomenon by reducing the link layer (L2) handoff delay drastically and distributing the system loads.

Selective inter-layer residual prediction for SVC-based mobile devices

The scalable video coding (SVC) standard adopts the inter-layer residual prediction (ILRP) algorithm to encode the residual signal of the enhancement layer. The ILRP reduces the number of bits for the residual signal at the expense of excessive encoding time. In this paper, we propose a fast mode decision algorithm for SVC-based mobile devices. In this algorithm, the ILRP is selectively applied to the coding modes depending on their Lagrangian rate-distortion costs. Experimental results show that the proposed algorithm can reduce the computational complexity with negligible video quality degradation and bitrate increments.

Fast Handoff Algorithm Using Access Points with Dual RF Modules

With the spread of portable computers, mobile users rapidly increase and have growing interests in wireless LAN (WLAN). However, when a mobile node (MN) moves, handoff can frequently occur. The frequent handoff makes fast and seamless mobility difficult to achieve. Generally the handoff delay is so long that a provider can not support realtime services sufficiently. In this paper, to address the link layer (L2) handoff delay problem, we propose a fast handoff algorithm using access points (APs) with dual radio frequency (RF) modules. The proposed handoff algorithm is based on the modified neighbor graph (NG). Experimental results show that the proposed method reduces the L2 handoff delay drastically. Furthermore, the handoff delay in the network layer (L3) can be also reduced simultaneously, since the proposed algorithm can support L2 triggers.