Youngkyu Choi

Optimization of femtocell network configuration under interference constraints

Femto BS (Base Station) is emerging as a key technology to secure the coverage and capacity in indoor environments. However, since the existing macrocell network is overlaid on femtocell networks utilizing the same set of frequency channels, femtocell networks can originate severe co-channel interference to the macrocell network unless the femtocell network is carefully configured. Therefore, according to a desired network-wide objective, we optimize the femtocell network with constraints such that the service connectivity with a femto BS is secured in the target indoor area while the signal emitted out of the building, playing as interference to the outdoor users, should be controlled with an appropriate strength in order not to interrupt the communication between macro BS and outdoor users. Each optimization problem is formulated as a mixed integer programming, and as the results, we obtain not only the transmit power and operational frequency channel of each femto BS, but also the optimal femto BS-to-user association pair at each geographical position.

IEEE 802.11 MAC-Level FEC scheme with retransmission combining

In this paper, we evaluate and enhance the performance of a Forward Error Correction (FEC) scheme for IEEE 802.11 Medium Access Control (MAC). A novel retransmission combining technique is proposed to enhance the performance of the MAC-level FEC scheme.,We also identify the problem with the IEEE 802.11a physical (PHY) layer when it is used with the MAC-level FEC. A new PHY frame format, backward compatible with the original format, is proposed to resolve the problem. Finally, we analytically evaluate the error performance of the MAC-level FEC, and its enhanced performance via retransmission combining and new 802.11a PHY frame format in AWGN environment. Additionally, we present and discuss the results from simulations using TCP/UDP traffic in more realistic channel environments.

Service Charge and Energy-Aware Vertical Handoff in Integrated IEEE 802.16e/802.11 Networks

This paper considers two issues arising in an integrated IEEE 802.16e/802.11 network: (1) finding a possible network, which mobile station (MSTA) can switch to, and (2) making a decision whether to execute a vertical handoff (VHO). For this purpose, we propose that 802.16e Base Stations (BSs) periodically broadcast the information about the density of 802.11 access points (APs) within their cell coverage. Based on this information, we develop a novel model, which predicts the successful scan probability during a given scan time. Using this analytical model, we devise an energy-efficient scan policy (ESP) algorithm, which enables an MSTA to decide (1) whether to attempt to discover APs in the current 802.16e cell, and (2) if so, how to set the 802.11 active scan interval considering the energy consumption. For the VHO decision, we mainly consider the impact of the service charge. Especially, a practical service fee models, i.e., flat pricing for WLAN and partially-flat pricing for 802.16e network, are considered. Under this service charge plan, we need to control the usage of the 802.16e network to minimize the users payment. To this end, we propose a scheme, which intelligently postpones the delivery of the delay-tolerant traffic within a certain time limit combined with ESP algorithm.

A MAC/PHY cross-layer design for efficient ARQ protocols

In many wireless data systems, automatic repeat request (ARQ) and hybrid automatic repeat request (HARQ) work independently. In this paper, we introduce a new ARQ feedback scheme called Cross-Layer ARQ (CL-ARQ), where ARQ closely interoperates with HARQ by making use of the acknowledgement of HARQ internally. By doing so, CL-ARQ not only removes the overhead for ARQ acknowledgement messages, but also improves the average delivery delay and TCP throughput. Especially, when the entities in charge of HARQ and ARQ are located far apart geographically, CL-ARQ remarkably outperforms the legacy scheme.

Enhancement of a WLAN-based internet service in Korea

A wireless LAN (WLAN)-based Internet service, called NESPOT, of Korea Telecom (KT), the biggest telecommunication and Internet service company in Korea, has been operational since early 2002. As the numbers of subscribers and deployed access points (APs) increase, KT has been endeavoring to improve its service quality as well as the network management. In this paper, we introduce a joint effort between Seoul National University (SNU) and KT to achieve it. We have been addressing two major issues as part of the joint project thus far: (1) a unified WLAN management tool; and (2) real-time traffic support enhancement. We present our on-going efforts as well as some preliminary results. Some issues, which need further attention for the future NESPOT service enhancement, are also introduced.

Power amplifier characteristic-aware energy-efficient transmission strategy

The energy consumption in transmitting an information bit, i.e., energy-per-bit, has been known to decrease monotonically as the transmission time increases [1]. However, when considering the power amplifier (PA) characteristics, we learn that the energy-per-bit starts to increase as the transmission time becomes long over a certain threshold. This is caused by the fact that, in a wireless device, it is not the transmission power, which determines the energy consumed during transmissions, but the input power to the PA whose output power is used as the transmission power. Based on our new trade-off model between the energy-per-bit and transmission time, we revisit known energy-efficient scheduling algorithms. Finally, we evaluate the impact of the new trade-off model and the performance of algorithms via simulations.

Memory Access Optimized VLSI for 5000-Word Continuous Speech Recognition

We have developed a memory access reduced VLSI chip for 5,000 word speaker-independent continuous speech recognition. This chip employs a context-dependent HMM (hidden Markov model) based speech recognition algorithm, and contains parallel and pipelined hardware units for emission probability computation and Viterbi beam search. To maximize the performance, we adopted several memory access reduction techniques such as sub-vector clustering and multi-block processing for the emission probability computation. We also employed a custom DRAM controller for efficient access of consecutive data. Moreover, we analyzed the access pattern of data to minimize the internal SRAM size while maintaining high performance. The experimental results show that the implemented system performs speech recognition 2.4 and 1.8 times faster than real-time utilizing 32-bit DDR SDRAM and SDR SDRAM, respectively.

Energy-aware WLAN scanning in integrated IEEE 802.16e/802.11 networks

We consider an issue arising in a vertical handoff (VHO) between IEEE 802.16e and IEEE 802.11 networks, i.e., how efficiently the scanning operation can be controlled to find a target wireless local area network (WLAN) access point (AP). For this purpose, we propose that the 802.16e Base Stations (BSs) periodically broadcast the information about the density of 802.11 APs within their cell coverage. Based on this information, we develop a mathematical model which predicts the probability that any WLAN AP is found within a given scan time. The analytical model is validated via a comparison with the simulation results obtained for both random waypoint and city section mobility models. Based on the analytical model, we devise an energy-efficient scanning algorithm, which enables an mobile station (MSTA) to decide (1) whether to conduct a scan operation in the current 802.16e cell, and (2) if so, how to configure the inter-scan interval considering the energy consumption due to the scan operation. An intensive simulation study shows that our proposed scanning algorithm indeed works well as designed.

Iterative Algorithm for Compound Instruction Selection with Register Coalescing

A compound instruction, encoding several ALU or memory operations within an instruction word, has been regarded as an efficient way of improving performance. In the compiler for embedded processors, the code generation algorithm for compound instructions has been built by dealing mainly with instruction selection which is a crucial phase of code generation. In this paper, we propose an iterative code generation algorithm for minimizing the detrimental impact of register coalescing that is applied to the code with compound instructions generated earlier from the instruction selection phase.

CL-ARQ: efficient ARQ for two-layer retransmission schemes

In the wireless data systems, automatic repeatrequest (ARQ) is used in order to reliably deliver packets to the upper layer while hybrid ARQ (HARQ) intends to improve the reliability over the air through the combination with other transmission techniques, e.g., modulation and channel coding. While the two functions are complementary to each other, it has been typically recognized as if they work independently at their corresponding layer. However, we note that the retransmission via ARQ is correlated with whether HARQ eventually delivers a packet successfully within a retransmission limit. Capturing this observation, we in this paper introduce a new ARQ feedback scheme called Cross-Layer ARQ (CL-ARQ). CL-ARQ makes use of the acknowledgement of HARQ internally. CL-ARQ not only allows ARQ to operate without explicit acknowledgement, but also improves the reliability combined with existing ARQ protocols. Especially, when the entities in charge of HARQ and ARQ are located far apart geographically, we show that CL-ARQ remarkably outperforms other schemes. Additionally, CL-ARQ is shown to resolve the stall problem, which occurs when the link capacity can not be fully utilized due to the limitation of ARQ window size. We evaluate the performance of CL-ARQ via both analysis and simulations.