Stochastic anti-collision scheme for dynamic framed ALOHA based RFID system considering capture effect

학위논문(석사)--서울대학교 대학원 :산업공학과,2007.Maste

Effect of dead layer on ferroelectric HZO films for low thermal budget

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Simple Bit Allocation Algorithms with BER-constraint for OFDM-based Systems

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Medium Access Control Protocols for Wireless Networks using Switchable Directional Antennas

DoctorIn conventional infrastructure wireless local area networks (WLANs), all stations including access points (APs) use omni-directional antennas. As the number of wireless devices, especially recent smart phones, increases explosively, the throughput performance may suffer from significant deterioration, since the degree of competition for transmission is getting higher and higher, which may result in frequent collisions. If whole service area of an AP is divided into many small pieces and only stations in one of the pieces are allowed to access medium at a time, the collision chance can be reduced and furthermore, concurrent transmissions may be possible. Motivated by this observation, we exploit a switchable directional antenna with four sectors to divide service area. The carrier sense multiple access/collision avoidance (CSMA/CA) protocol for WLAN does not work well with directional antennas. To take advantage of directional antennas for infrastructure wireless networks, a new MAC protocol is required, including the way to make AP association and data transmission. In this dissertation, we suggest medium access control (MAC) protocols for two different networks. One for an infrastructure WLAN consisting of an AP equipped with a switchable directional antenna (directional AP) and stations equipped with omni-directional antennas (omni-directional station) and the other for a wireless network consisting of a directional AP and stations equipped with switchable directional antennas (directional station). Furthermore, we extend it for wireless networks consisting of two directional APs and directional stations. The main contribution of this dissertation is that we consider an infrastructure WLAN using directional antennas and propose the corresponding MAC protocols. To replace conventional WLANs by the ones using directional antennas, we believe that we need to make additional efforts on developing well-designed protocols

Sensing and Transmission Parameter Determination for Cognitive Radio Networks

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Fast Simple Optimal Discrete Bit-Loading Algorithms for OFDM-based Systems

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Effect of dead layers on the ferroelectric property of ultrathin HfZrOx film

In this study, we investigate the effects of various electrodes on the ferroelectric properties of ultrathin HfZrOx (HZO) films. The ferroelectric polarization is totally suppressed in the HZO films with TiN and W bottom electrodes when the film thickness is below 5nm. These results can be attributed to the formation of a dead layer at the bottom electrode/HZO interface during the atomic layer deposition (ALD) and annealing processes. On the other hand, the HZO film with a Pt bottom electrode shows an excellent P-E loop with a very high switchable polarization (2P(r)) of 42.5 mu C/cm(2) even at a film thickness of 2.5nm. Through the short pulse switching technique, we confirm the formation of a thick dead layer in the HZO films with TiN and W electrodes, which inhibits the formation of the orthorhombic phase in these ultrathin HZO films. This implies that the ferroelectric property of ultrathin HZO films can be improved by choosing an appropriate electrode material capable of suppress ing the formation of a dead layer.11Nsciescopu