Ki-Young Han

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.

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.

Reducing Inter-Cell Handover Events based on Cell ID Information in Multi-hop Relay Systems

To reduce handover overhead such as signaling overhead and latency, optimal handover algorithm is needed for mobile multi-hop relay (MMR) systems. This article proposes a novel handover method that reduces inter-cell handover but increases intra-cell handover events, because inter-cell handover has more overhead in view of the system and gives more latency time to users rather compared to intra-cell handover. Therefore, in the proposed scheme, intra-cell handover is encouraged instead inter-cell handover using adaptive hysteresis level. Intra-cell handover performs based on higher hysteresis level than inter-cell handover. This paper also presents hierarchical relay station ID (RS ID) concept for IEEE 802.16j standard. And, additional method using MOB_NBR-ADV message is proposed in case that hierarchical ID could not be used. This proposed algorithm can decrease handover signaling overhead, latency, and unnecessary handovers called ping-pong effect as well. Also, the proposed algorithm is backward compatible with mobile station (MS). Performance analysis and simulation validate the efficiency of the proposed algorithm.

An Efficient ARQ mechanism in Multi-Hop Relay Systems Based on IEEE 802.16 OFDMA

A Mobile Multi-hop Relay (MMR) Task Group was created to achieve coverage extension and throughput enhancement by introducing a relay station (RS) in the IEEE 802.16 system. If the RS is introduced in the 802.16 system, the reliability of data transmission is worsen, since data transmission failures occur more frequently due to several wireless links establishing end-to-end connection and an ARQ mechanism used in the 802.16 system is not optimized for multi-hop environment. In this paper, we propose an ARQ mechanism focused on the 802.16 MMR system. The proposed ARQ mechanism introduces a new ARQ feedback message made by the RS for fast and efficient data transmission. The numerical analysis and simulation results show that transmission delay is reduced and transmission efficiency is enhanced by the aid of the RS with the proposed ARQ mechanism.

Capacity enhancement of uplink channel through spatial reuse in multihop cellular networks

Some multihop cellular networks use spatial resource reuses for concurrent data transmission in a cell. However, there are no fundamental resource reuse policies am ong multiple hops to analyze the capacity of multihop iellulae networks. So, in this letter, we propose a basic resource reuse scheme by applying a general resource reuse scheme that is used for cellular networks to multihop cellular networks. Then, we examine the capacity gains of the proposed resource reuse scheme through mathematical analysis based on Shannon capacity

Use of vestigial power management bit within Wi-Fi frame structure of access point for coexistence of Wi-Fi and WiMAX systems in shared bands

Coexistence of more than one wireless communication systems in a shared band such as 2.4GHz ISM band has been one of hot research topics. As an example, there are many research results related to the coexistence of Wi-Fi and Bluetooth systems in the ISM band. Recently, many schemes for coexistence of Wi-Fi and WiMAX systems in shared bands such as 2.4GHz ISM band have been proposed. In this paper, proposed scheme for the coexistence of Wi-Fi and WiMAX systems is TDO (Time Division Operation) using of vestigial power management bit within Wi-Fi frame structure.

Fast communication: Adaptive step-size widely linear linearly constrained constant modulus algorithm for DS-CDMA receivers in nonstationary interference environments

In this paper, we propose a new adaptive step-size widely linear constant modulus algorithm (CMA) in DS-CDMA systems especially for time-varying interference environment. The widely linear estimation enables CMA to produce better output signal to interference plus noise ratio (SINR) and the adaptive step-size tackles the time-varying interference environment effectively. Simulations have confirmed that the proposed algorithm performs better in a DS-CDMA system by employing a BPSK modulation than other algorithms that do not use widely linear processing.

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.

Non-handover Based Mobility Management in Hierarchically Structured Cellular Networks

Abstract—Hierarchical cell structure (HCS) is considered as a possible solution to support users in shadowing area or hot zone area. In HCS systems, both co-channel interference and mobility management are the most important problems. In this paper, we propose a new mobility management method that efficiently preserves the data rate of the mobile user passing small overlaid cell area. In the proposed method, handover is not used when the mobile user passes the overlaid cell area. Therefore, it is not affected by ping-pong and handover overhead. Instead, specific channel utilization is used to prevent co-channel interference and to guarantee quality of service in common channel. We show the procedure of the proposed mobility management method, and channel utilization in this method. We also evaluate the rate regions where the proposed method can support higher data rate than handover based method. System level simulation based on IEEE 802.16m system shows that the proposed method is more efficient in HCS system than handover-based method.

Circulator-based interference reduction method for WiMAX system and Wi-Fi system in shared band

Performance of a collocated system containing WiMAX and Wi-Fi systems in a single device is analyzed for two cases: Collaborative and Non-Collaborative. In order to reduce the mutual interference between Wi-Fi and WiMAX system in the collocated system, a circulator-based collocated system is proposed and its performance is analyzed with simulation.