Chaegwon Lim

Improvements to seamless vertical handover between mobile WiMAX and 3GPP UTRAN through the evolved packet core

Recent mobile devices are integrated with multiple network interfaces. Users want their devices connected to the network anytime anywhere. It is highly feasible for a user to change connection to another network for users that leave the service area of its current serving network, where handover needs to be executed seamlessly such that ongoing service sessions are not interrupted. The handover operation not only requires switching the interfaces within a device but also involves seamless reconfiguration of the supporting networks. In this article, an improved IP-based vertical handover technology for mobile WiMAX , 3GPP legacy systems (i.e., Global System for Mobile communications and Universal Mobile Telecommunications System), and 3G Long Term Evolution is presented, which is based on existing optimized handover techniques between mobile WiMAX and 3GPP accesses. Formerly proposed 3GPP WiMAX optimized VHO solutions introduced new elements, such as the forward attachment function and access network discovery and selection function. The ANDSF supports the discovery of target access, and the FAF provides the functionality that authenticates the UE before the execution of VHO. However, the previous technique has limitations that result in data loss and abnormal disconnection to the source access. This article provides a solution by introducing an additional network element called the data forwarding function (DFF) that eliminates the data loss during VHO execution. In addition, the DFF resolves the problem of abrupt disconnection to the source network. The simulation results show that the proposed VHO technique is effective in minimizing data loss during VHO execution between mobile WiMAX and 3GPP networks. As the proposed solution of this article is an IP based handover solution, it can be similarly applied to other communication networks.

A coordinate-based approach for exploiting temporal-spatial diversity in wireless mesh networks

In this paper, we consider the problem of mitigating interference and improving network capacity in wireless mesh networks from the angle of temporal-spatial diversity. In a nutshell, while the achievable throughput on a multihop wireless path is limited by intra-flow interference, the overall capacity of a multihop wireless network can be increased by exploiting temporal-spatial diversity of concurrent transmissions that exist among a number of wireless links. Connections that are routed along multihop wireless paths can be scheduled to take place simultaneously if their transmissions do not interfere with each other (significantly).To make a case of exploiting the temporal-spatial diversity to improve network capacity, we focus on transporting downstream traffic at gateway nodes with Internet access. We propose to construct, based on measurements of received signal strengths, a virtual coordinate system that is used to determine the sets of paths along which transmissions can take place with the least inter-flow interference. Based on the sets of non-interfering paths, the gateway node then determines the order with which a gateway node schedules frames of different connections to be transmitted. Through extensive simulation (with real-life measurement traces on an operational, city-wide wireless community network), we show that the downstream throughput of a gateway node in a wireless mesh network can be improved by 10-35% under a variety of network topologies and traffic distributions. This, coupled with the fact that the proposed approach requires only minor code change in the gateway nodes and does not require any additional hardware, makes it a viable option to improving network capacity in existing wireless mesh networks.

Enhancements to FPMIPv6 for improved seamless vertical handover between LTE and heterogeneous access networks

Fast handovers for proxy mobile IPv6 (FPMIPv6) was created to reduce packet-delay that occurs during proxy mobile IPv6 (PMIPv6) handover. Based on vertical handover (VHO) experiments conducted between Long Term Evolution (LTE) and heterogeneous accesses over the evolved packet core (EPC) using FPMIPv6, it was recognized that consistently reliable seamless VHO operations were difficult to accomplish due to limitations in FPMIPv6. Noticeably, VHO performance degradation resulted from the serving network (SN) lacking information of the target network (TN) when the TN is a heterogeneous protocol domain, packet congestion and loss problems occurring on specific network gateway interfaces, and also from using long packet-forwarding paths. In this article, an enhanced FPMIPv6 technique is proposed to solve these problems and improve the VHO operation by using shorter data-paths and improved coordination of buffered packet-forwarding and TN switching, which results in a significantly reduced packet-delay.

SHARE: seamless handover architecture for 3G-WLAN roaming environment

For the transition from 3G communication systems to 4G communication systems, 3G-WLAN interworking systems can be a reference model for 4G communication systems. In this paper, we identify challenging problems in 3G-WLAN interworking systems and propose a loosely coupled architecture called SHARE. In SHARE, each WLAN hotspot access point (AP) is equipped with a 3G radio transmission module to generate radio signals for control channels of 3G networks in addition to a WLAN radio module. Consequently, base stations of the 3G networks share their control channels with hotspot APs. By monitoring these channels, mobile nodes can easily detect available WLAN hotspots without probe delay for handovers.

A weighted RED for alleviating starvation problem in wireless mesh networks

In wireless mesh networks, the end-to-end throughput of a flow over a multiple-hop wireless link rapidly decreases as the hop-count between the source and destination nodes increases, and the flows that travel over a path of more than 4-5 hops from its source node eventually starve. To alleviate this unfairness, we propose a weighted random early detection (RED) mechanism that has a different dropping preference according to the hop-count information. When the network is congested, it forwards packets that come from a farther source node with a higher priority. Through extensive simulations, we show that the proposed queue management policy can improve the fairness performance effectively and alleviate the starvation problem in multihop wireless networks.

The methods to implementate self optimisation in LTE system

Reduction of operational efforts and complexity are key drivers for RAN Long Term Evolution (LTE). The mechanisms how to ease network performance analyses and problem findings, reduce efforts in maintaining the network becomes hot topic. By introducing Self-Organizing Network (SON) functions, operational efforts are expected to be greatly minimized. A self optimizing function shall increase network performance and quality reacting to dynamic processes in the network. In the heterogeneous deployment scenarios, the efforts to set up and optimize are significant and traditionally lead to lengthy periods of getting an optimum and stable system setup. This paper analyzed some methods to implement self optimization in LTE RAN system. Especially in the co-existence of eNB, home eNB (HeNB), 3G home Node B (HNB) etc., how to make the system work in optimized state was elaborated and new methods were proposed. In this paper, the concept of H(e)NB is used to cover both of HNB and HeNB.

Exploiting multi-flow diversity for mitigating intra-flow interference in wireless mesh networks

We consider the problem of improving network capacity in wireless mesh networks from the angle of multi-flow diversity. As consecutive packets on a multihop path can interfere with one another within their interference range, the number of concurrent transmissions is limited, thereby degrading throughput performance. This intra-flow interference can be alleviated by enhancing multi-flow diversity in wireless mesh networks. Hence, we propose the use of a fair queueing policy to evenly distribute the traffic load to achieve fair bandwidth allocation among the flows. Through ns-2 simulations we subsequently show that CHOKe can effectively reduce intra-flow interference and improve throughput performance.

Optimization approach for throughput analysis of multi-hop wireless networks

A critical performance metric of multi-hop wireless networks is the end-to-end throughput of a multi-hop wireless path. When the packets of a traffic flow originating from a source node are relayed by intermediate nodes (relay nodes), competition between the nodes on the multi-hop routing path incurs transmission delays from the carrier sensing mechanism, as well as packet drops and retransmissions due to hidden terminals. Owing to this complicated interaction between nodes, problems in the throughput performance analysis of multi-hop networks have not been fully resolved. In this paper, we propose an optimization approach to compute the throughput at each individual link in addition to the end-to-end throughput in an IEEE 802.11 DCF-based multi-hop wireless network. To this end, we first analyze a multi-hop chain topology with a single flow by taking into account transmission collisions from hidden terminals, and then formulate the optimization problem to obtain the end-to-end throughput performance of multi-hop networks. Through extensive ns-2 simulations, we then show that the proposed approach accurately predicts the throughput performance obtained by the simulations, within a discrepancy of less than 10-12 % in the worst case scenario under a variety of chain and cross topologies.

Sequential Coordination Function for Throughput and Fairness Enhancement of Wireless LANs

High throughput and fair resource sharing are two of the most important objectives in designing a medium access control (MAC) protocol. Currently, most MAC protocols including IEEE 802.11 DCF adopt a random access based approach in a distributed manner in order to coordinate the wireless channel accesses among competing stations. In this paper, we first identify that a random access–based MAC protocol may suffer from MAC protocol overhead such as a random backoff for data transmission and a collision among simultaneously transmitting stations. Then, we propose a new MAC protocol, called sequential coordination function (SCF), which coordinates every station to send a data frame sequentially one after another in a distributed manner. By defining a service period and a joining period, the SCF eliminates unnecessary contentions during the service period, and by explicitly determining the sequence of frame transmission for each stations, it reduces collision occurrences and ensures fairness among stations in the service period. The performance of SCF is investigated through intensive simulations, which show that the SCF achieves higher throughput and fairness performances than other existing MAC protocols in a wide range of the traffic load and the number of stations.