Il Young Han

Room-temperature triangular-lattice two-dimensional photonic band gap lasers operating at 1.54 μm

Thermally and mechanically dependable two-dimensional (2D) photonic band gap lasers are proposed and realized at room temperature. The thin slab photonic band gap laser structure is sandwiched between air and a drilled aluminum oxide layer provided by wafer fusion techniques. In this thin slab structure, the optical confinement of photons is achieved by 2D triangular photonic lattice in horizontal plane and total internal reflection in vertical direction. Pulsed lasing action is observed at 1.54 μm by optical pumping with duty cycle up to 10%. Polarization characteristics of two-dimensional photonic band gap defect modes are also studied below and above the lasing threshold.

Performance analysis of DS/SSMA unslotted ALOHA system with variable length data traffic

We analyze the throughput of a direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA system with variable length data traffic. The system is analyzed for two cases: (1) systems without a channel load sensing protocol (CLSP) and (2) systems with a CLSP. The bit-error probability and the throughput are obtained as a function of the signal-to-noise ratio (SNR) during message transmission, considering the number of overlapped messages and the amount of time overlap. We assume that the generation of data messages is Poisson distributed and that the messages are divided into packets before transmission. The system is modeled as a Markov chain under the assumption that the number of packets in a message is geometrically distributed with a constant packet length. The throughput variance of the DS/SSMA unslotted ALOHA system with variable length data traffic is obtained as the Reed-Solomon code rate varies. Results show that a significant throughput improvement can be obtained by using an error-correcting code.

Group location tracking based on representative identity and virtual VLR for transportation systems

A location tracking scheme known as group location tracking (GLT) is proposed to reduce the location update cost for terrestrial transportation systems (TSs) in personal communication networks. When a TS moves into a new registration area, the GLT scheme, based on a representative identity of the TS and a virtual visitor location register, updates location information by a group location update request message instead of individual location update of users. Compared with the IS-41, the GLT scheme significantly reduces signaling load.

Group location management for transportation systems in wireless cellular systems

A location tracking scheme called as a group location tracking (GLT) is proposed to reduce the location update cost for terrestrial transportation systems (TSs) in personal communication networks. To provide user mobility in a TS, we introduce two new concepts, virtual visitor location register (VVLR) and representative identity (RID). A VVLR is used to provide current location of a TS and an RID of a TS is used to represent a TS and the group of all users on the TS. When a TS moves into a new registration area, the GLT scheme based on an RID of the TS and a VVLR updates location information by a group location update request message instead of individual location update request messages generated by each user in the TS. We analyze the performance of our proposed scheme in view of the signaling load and call delivery failure. Compared with the IS-41 location tracking scheme, the GLT scheme significantly reduces frequent location updates caused by the high mobility of a TS and a large number of users in a TS.

Distributed group location tracking based on several RIDs and virtual VLRs for transportation systems

To efficiently support mobility of users on transportation systems, we propose a distributed group location tracking (DGLT) scheme based on several virtual visitor location registers (VVLRs) and representative identities. We analyze the DGLT scheme in view of the database (DB) access cost and the signaling cost under a one-dimensional network architecture. Compared with the IS-41 scheme, the DGLT scheme using distributed group management has a lower signaling cost as well as a lower DB access cost. As the user mobility-increases or the number of users on a TS increases, the outperformance of the DGLT scheme increases. Generally, the DGLT scheme in which a corresponding home location register selects an optimum VVLR among several VVLRs has a lower signaling cost, although a DB access cost is increased slightly compared with the GLT scheme.

Resource allocation scheme for supporting high data-rate service in an integrated CDMA system

A new resource allocation scheme is proposed for data services in an integrated voice/data CDMA system. After delay-sensitive voice users are first allocated, residual resources are then allocated to data users. To allocate data users fairly, we apply WFQ (weighted fair queueing) to a CDMA system with consideration of a generalized processor sharing (GPS) fluid-flow model. In order to implement this fair resource allocation scheme, a virtual clock and a credit table method are used. The throughput and delay of the data service under burst error conditions are shown. Numerical results show that the throughput of each data user has the same characteristics under burst error conditions.

Room-temperature 2-D photonic bandgap single defect laser

Our photonic bandgap single defect laser structure is shown. For an active medium, six strain compensated InGaAsP quantum wells are employed because of the relatively small surface recombination velocity. To realize a mechanically-robust structure with good thermal properties, the InGaAsP layer was fused with the AlAs layer on top of a GaAs wafer. For optical confinement in the vertical direction, the top AlAs is wet-oxidized. After electron-beam lithography, the AlAs layer is etched by chemically-assisted ion-beam etching. The etched triangular lattice patterns form 2-D photonic bandgap cavities of various sizes having periodicity of 450 nm. The diameter of holes is 270 nm. The thickness of active layer sandwiched between air and the wet-oxidized AlAs layer is smaller than the 1/2 wavelength to obtain a single mode condition. The finished photonic bandgap cavity is optically pumped with a 980-nm diode laser beam focused to approximately 4 mm in diameter.

The effect of various wafer thicknesses on hot spot temperature

Hot spots on thin wafers of IC packages are becoming an important issue in thermal and electrical engineering. To investigate these hot spots, we developed a diode temperature sensor array (DTSA) that consists of an array of 32 times 32 diodes (1,024 diodes) in an 8 mm x 8 mm surface area. We used chemical-mechanical planarization to make DTSA chips with various thicknesses (21.5 mum, 31 mum, 42 mum, 100 mum, 200 mum, and 400 mum). In addition, we conducted the experiment with various heater power conditions (0.2 W, 0.3 W, 0.4 W, and 0.5 W). Finally, on the basis of the experimental results, we propose a correlation for the hot spot temperature as a function of the generated power and the chip thicknesses. This correlation can give an easy estimate of the hot spot temperature for flip-chip packaging when the wafer thickness and the generated power are given.

Study on the Transient Characteristics of the Sensor Tube of a Thermal Mass Flow Meter

Thermal mass flow meters (TMFMs) are most widely used for measuring mass flow rates in the semiconductor industry. A TMFM should have a short response time in order to measure the time-varying flow rate rapidly and accurately. Therefore, it is important to study transient heat transfer phenomena in the senor tube of a TMFM. Many models have been presented by previous investigators. But most of them focused on steady heat transfer phenomena, so it is impossible to analyze transient heat transfer phenomena in the sensor tube using previous models. Furthermore, it is impossible to predict the response time using results from previous research works. In the present work, a simple numerical model for transient heat transfer phenomena of the sensor tube of a TMFM is presented. The proposed model treats the fluid region and the tube region separately. Numerical solutions for the tube and fluid temperatures in a transient state are obtained using the proposed model and compared with experimental results to validate the proposed model. Based on numerical solutions, heat transfer mechanism in a transient state in the sensor tube is explained. A correlation for predicting the response time of a sensor tube is also presented. The functional form of the correlation is obtained using the scale analysis and coefficients appearing in the correlation are obtained by the proposed numerical model. The correlation is verified by experimental results. Using the proposed correlation, physical meaning and characteristics of the response time of the sensor tube are presented.Copyright

On performance analysis of CDMA unslotted ALOHA system with burst reservation

In this paper, we analyze the performance of a CDMA unslotted ALOHA system with burst reservation. A mobile terminal reserves the channel for the duration of its burst and transmits spread packets. The packets are transmitted in unslotted mode. As the packet transmission of the mobile terminal can be begun at any time, the level of multi-user interference fluctuates during the packet transmission. We, therefore, calculate the bit error probability and throughput by considering not only the number of overlapped packets but also the amount of time overlap. We assume that the generation of bursts is Poisson distributed, the number of packets transmitted at once by a mobile terminal is geometrically distributed, and the packet length is constant. By modeling the system as a Markov chain, we obtain the throughput of CDMA unslotted ALOHA system with burst reservation.