Current research in fourth-generation (4G) wireless systems is driven by the need to offer high data rate services such as multimedia and VoIP with high quality of-service at a reasonably low cost. The concept of relay station (RS) as an intermediate between between the base station (BS) and the user terminal(UT)is an architectural upgrade aiming to enhances the system performance in terms of capacity and coverage. As a complementary technology to the shortcomings of RS, Mobile relay station (MRS) has been introduced to provide a seamless connection service to the users even at high mobility, and thus envisioned to be of much advantage in the development of 4G wireless systems. An MRS, when mounted on top of a moving platform, like a bus or a train, is termed as a moving network. In order to ensure efficient application of these moving networks, both from business and technical standpoints, it is important to strategize when, where and to whom does the MRS serve. The 802.16j Technical Requirements for MRS suggests that for the on-board UTs, MRS handover process is allowed whereas in case of the off-board UTs, it is disallowed. The requirements, however, fall short on the details of how to identify on-board UTs. In this context, we consider the problem of distinguishing between on-board and off-board users in 4G wireless systems using the techniques of cellular systems, without the implementation of any outside technology, like external bill-checking system (e.g., RFID). This problem is further complicated by the recent categorization of MRS into two types by the IEEE 802.16j task group. One is the transparent MRS, which does not transmit preamble and MAP signal information to the user terminal (UT), thus making it impossible for the UT to sense its existence. The other is a high capability non-transparent MRS that generates and send its own preamble and MAP information and thus acts as a base station (BS) to the UT.
In this study, we introduce a novel idea of group join/leave for MRS mounted on moving networks. Based on this concept we design an efficient MAC-layer handover decision scheme for moving networks. Based on the MRS type, we exploit the downlink signal strength and the uplink signal strength of the MRS in order to distinguish between on-board and off-board UTs. The proposed scheme identifies the on-board users so that they can perform MRS handovers with the benefits of acquiring high data rate with better quality of service. In addition, our scheme recognizes the off-board users and disallows the handover of these external users to the MRS, thereby reducing the probability of unnecessary handover executions for these users which would otherwise eventually lead to the degradation of system throughput.
Moving network는 버스, 기차 등 차량 내부에 위치한 단말들을 위해 차량 위에 탑재된 MRS (Mobile Relay Station)가 서비스를 제공하면서 차량의 이동에 따라 함께 움직이는 것을 일컫는다. 버스, 기차 등 차량에 탑승한 단말들이 차량 위에 탑재된 MRS로부터 서비스를 받기 위해서는 단말의 차량 탑승 유무와 특정 시간 이상 MRS와 함께 그룹을 형성하여 움직인다고 판단되어야 한다. 하지만, 기존 셀룰러 시스템에는 단말의 차량 탑승 유무를 선별하여 MRS로부터 서비스를 제공받을 단말들의 그룹을 관리하는 기법이 제안되지 않았다. 단말이 차량에 탑승하였다면, 차량 위에 설치된 MRS로부터 지속적인 서비스를 제공받을 수 있는데, 이것은 MRS와 해당 단말들이 가까운 거리에 위치하여 그룹을 형성한다는 것을 의미한다. 도심지 (urban area) 환경에서 기지국으로부터의 신호 세기 또는 버스에 탑재된 MRS로부터의 신호 세기는 거리 외에 slow-fading 및 multi-path fading 등 fading에 의한 영향이 신호 세기를 결정하는 요인이 되므로, 거리에 기반한 그룹 판단 기법을 moving network에 그대로 적용하게 되면, 그룹 형성 유무를 정확히 판단하지 못하게 되어 MRS를 통해 효율적으로 서비스를 제공할 수 있는 단말을 선별하지 못하게 된다.
이 논문에서는moving network를 형성할 그룹에 참여 및 탈퇴하는 단말을 선별하기 위해 위치 정보와 신호 세기에 기반한 그룹 관리 알고리즘을 제안한고 이 moving network의 효과적 형성을 위한 그룹 참여 및 탈퇴 관련, Non-transparent MRS 와 transparent MRS를 구분하여 헌드오버 기법이다.
