Dongmyoung Kim

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.

Performance measurement over Mobile WiMAX/IEEE 802.16e network

WiBro (Wireless Broadband) is a Korean version of Mobile WiMAX/IEEE 802.16e system, which is designed for mobile broadband wireless access. Being a subset of IEEE 802.16e, WiBro employs orthogonal frequency division multiple access (OFDMA) and time division duplexing (TDD) schemes operating at 2.3 GHz bands. In mid 2006, the world first commercial Mobile WiMAX service, based on WiBro specification, started in Seoul, Korea. In this paper, we analyze the performance of commercial WiBro networks through traffic measurements. Many experiments are conducted in the various environments. We analyze the link capacity when the user datagram protocol (UDP) packets from multiple users fully utilize wireless links. We also analyze goodput performance with transmission control protocol (TCP) as well as round trip time performance. The measured performances are compared with those of HSDPA (High-Speed Downlink Packet Access) which is a competing system of Mobile WiMAX. We found that the RTTs of WiBro and HSDPA are very large compared with conventional data networks, e.g., Ethernet or Wireless LANs. Furthermore, it is shown that the user-perceived performance is limited by such long round trip times when TCP is utilized. It motivates us to improve round trip time performance of WiBro system. Finally, we analyze the VoIP performance and the performance when the user moves around the whole city.

Automatic configuration of random access channel parameters in LTE systems

In 3G long term evolution (LTE) systems, the random access channel (RACH) is used for initial access, resource request, and handover. Since the random access delay is determined by the arrival rate of the random access preambles and the number of RACH subframes, we should configure the number of RACH subframes given the arrival rate in order to guarantee the delay performance. In this work, by carefully taking account of a tradeoff between the number of RACH subframes and the random access delay, we present an optimization formulation that minimizes the number of RACH subframes for a given delay requirement. Furthermore, since the arrival rate of the random access preambles is time varying in reality, we further propose an estimation scheme for the arrival rate by reflecting the periodicity and the correlation between recent and future arrival rates. Our simulation results show that the proposed scheme for tuning the RACH subframes gives very promising network performance under time-varying environments.

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.

Load balancing with Mobile Base Stations in Tactical Information Communication Networks

A Tactical Information Communication Network (TICN) enables communication among military entities during warfare. In a TICN, base stations (BSs) are required to move along with mobile devices in order to provide the communication wherever they are. This type of BS is referred to as Mobile Base Station (MBS). The backhaual of an MBS is composed of a wireless communication link so that the performance of a TICN can be limited by the unstable backhaul link capacity. In this paper, we propose load balancing algorithms considering the capacity of backhaul links in a TICN. Association control based load balancing algorithms and transmit power control based load balancing algorithms are proposed and analyzed. The simulation results demonstrate that the proposed algorithms improve the system performance of a TICN by reducing the outage rate significantly.

Self-Optimization of RACH Power Considering Multi-Cell Outage in 3GPP LTE Systems

Self-organizing network (SON), which is an essential technology of future radio networks, is proposed in the 3GPP Long Term Evolution (LTE) specification as a usage case. In this paper, we focus on user equipment (UE)s power control considering interference mitigation and retransmission constraints especially for physical random access channel (PRACH) of LTE. When a UE initially accesses an evolved-NodeB (eNB), it sends a preamble with an initial access level, and raises the power by a fixed step whenever the preamble was not detected by the eNB. We present an analysis and simulation to study the tradeoff relationship between inter-cell interference and retransmission according to the PRACH power setting, i.e., the initial target received power and power ramping step, assuming Rayleigh fading channels. Furthermore, we propose an algorithm tracing the optimal power setting that minimizes the resource wastage of the cell in consideration and its adjacent cells satisfying the constraint of retransmission and failure probability bound.

Measurement and Analysis of One-Way Delays over IEEE 802.16e/WiBro Network

One-way delay is an important metric to evaluate the performance of communication networks. In this paper, we analyze the one-way delay performance of the commercial WiBro networks based on the traffic measurement. In measurements, much large delay is observed in the uplink than downlink, and we find that such asymmetry due mainly to the impact of the uplink bandwidth request mechanism. Due to this large uplink delay, the performance of commercial IEEE 802.16e service can be severely degraded. We also discuss some possible solutions to ameliorate this problem.

Association Control Based Load Balancing for Tactical Information Communication Networks

In this paper, we propose a distributed load balancing using association control, especially for Tactical Information Communication Networks (TICNs). We develop a scheme for effectively balancing the load at the Base Stations (BSs), where a BSs load represents the amount of resources utilized by a BS, thus minimizing the blocking probability as well as the transmission failure probability. In order to make it more suitable to the military environment, we develop the proposed scheme in a completely distributed manner. Extensive simulations are performed and the proposed scheme is found to outperform the association schemes existing in the literature.

Load balancing in open access femtocell based two-tier cellular networks

Femtocell base station (BS) is a low-power, low-price BS based on cellular communication technology. It is expected to become a cost-effective solution for improving the communication performance of indoor users, whose traffic demands are large in general. There are mainly two access strategies for femtocell, i.e., closed and open access strategies. While it has been known that open access femtocells contribute more to enhancing the system-wide performance than closed access femtocells, the performance of open access femtocell owners can be potentially degraded when sharing their BSs with other macrocell users. To prevent this potential problem, we develop a load balancing scheme which maximizes the average throughput of macrocell users while guaranteeing some degree of benefits to femtocell owners. To achieve this goal, we jointly optimize the ratio of dedicated resources for femtocells as well as the femtocell service area via an analytic model, which proves that the optimization is a convex optimization problem in the typical environments. Our analysis and simulation results show that the proposed load balancing scheme significantly enhances the system-wide performance.

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.