The invention relates to a cell handover method employed in a cellular radio system comprising at least one indoor subscriber terminal and cells with operating area of each being confined to the interior of a building, and at least one outdoor cell whose operating area extends from the exterior to the interior of the building, and in which cellular radio system the cell serving the subscriber terminal will be changed.
In cellular radio systems, the cell size may vary, for instance, depending on the number of subscriber terminals within the cell area. For instance, in an urban environment, where subscriber terminals are typically densely located in a relatively small area, so-called macro cells, micro cells and pico cells are employed which provide the necessary radio coverage over every serviceable area. The radius of the micro cell coverage area may range from 0.5 to 3 km, for instance. The radius of the pico cell coverage area may range from a few metres to a few tens of metres. For instance, from the view-point of network construction, an office environment can well be compared with the urban environment.
In cellular radio systems, the network monitors at all times the location of the subscriber terminal in the network. In order that monitoring would be possible, it is naturally necessary that the subscriber terminal is turned on. The network is typically divided into traffic areas, and some network element, for instance a base station controller, keeps a record of the subscriber terminal movements in different traffic areas. When the subscriber terminal is turned on, it registers to a network base station which transmits a signalling message on the basis of the registration to the base station controller. Thus the base station controller can switch an incoming call or message to a specific subscriber terminal irrespective of its location. If the subscriber terminal moves to the borders of the base station service area or outside the service area, the subscriber terminal has to reregister. If the subscriber terminal moves from one base station service area to another, a base station handover is performed. In the above-described situation, a channel and a cell are generally handed over.
In a small-cell area, there are areas and spots between cells where reception of the base station is poor. If the subscriber terminal is located in areas of that kind, problems may arise in call setup. Poor reception may cause interruption of an established connection, for instance. Problems caused by poor service areas are corrected by means of so-called umbrella cells. The umbrella cell is generally a high-power cell as compared with the underlying cells within its coverage area. The umbrella cell can be, for instance, a macro cell or a micro cell, whereby it provides a coverage area for a pico cell which serves as a small cell. In general, the calls are directed to the umbrella cell when all channels of the small-cell base station are busy or the subscriber is outside the small-cell coverage area.
In typical cellular radio systems used in office environments, pico cells are provided in predetermined locations in an office building. The cells are preferably located to cover as well as possible all spaces and floors of the building. The macro cells and micro cells surrounding the office building may, however, cause interference to data transmission inside the office building, since transmission powers used outside the office building are typically higher than those used inside. In practice this means that the subscriber terminal located inside the office building may perform a handover to a base station outside the building, and consequently the base station load of the cell outside the building may grow excessively. If the transmission power used indoors is increased in order to try to solve the problem, the overall interference in the cellular radio network increases.
When the subscriber terminal is in idle mode, it typically switches to the base station from which it receives the strongest signal. This means that the subscriber terminal which is located in a pico cell and which is in idle mode can select a cell outside the building, if the outdoor cell is also dominating inside the building.
In the cellular radio networks, specific C1 parameters are used, which determine the serving cell of the subscriber terminal, when the subscriber terminal is in idle mode. If the subscriber terminal is in dedicated mode, the cell can be forced to provide service to the subscriber terminal by means of the base station parameters. After the C1 parameter, the cellular radio networks further employ C2 parameters, by means of which the serving cell is determined to the subscriber terminal.
Currently, offices employ more and more internal information networks, so-called intranet networks, which in principle provide services that are similar to those of the internet networks. The intranet networks are implemented, for instance, by means of computers connected to a LAN network. In the future, the intranet networks operate more and more wirelessly. These wireless office systems are also called WIO systems (Wireless Intranet Office). In the new systems, it is possible for a mobile telephone, for instance, to establish a wireless connection to a transceiver located in a computer, wherefrom the connection signal passes through the LAN network to some other mobile telephone, for instance. The computer system comprising a transceiver that is able to establish a connection to a telephone is called an Intranet Mobile Cluster.
When the Intranet Mobile Cluster is installed in the office, the above-mentioned problems with radio resource management increase. In prior art systems, if the telephone that established a connection to a WIO system computer performs a cell handover to a cell outside the building, the load of the outdoor cell increases due to cell handovers. Also the telephone in idle mode inside the building loads the outdoor cell, when the telephone registers to a base station providing the cell outside the building.
The object of the invention is to provide a method and equipment implementing the method so as to solve the above problems. This is achieved with a method of the type presented in the preamble characterized in that the radio system is configurated for cell handover in such a way that cells are grouped into cell groups with a gateway cell provided around each group, and with one gateway cell provided at the entrance of the building, the gateway cell surrounding the cell is entered on the neighbor list of each cell, other gateway cells and an outdoor cell are entered on the neighbor list of the gateway cell provided at the entrance, and the gateway cells are entered on the neighbor list of the outdoor cell.
The invention further relates to a cellular radio system comprising at least one subscriber terminal inside the building and cells with operating area of each being confined to the interior of the building, and at least one outdoor cell whose operating area extends from the exterior to the interior of the building, and in which cellular radio system the subscriber terminal changes the serving cell if necessary.
The cellular radio network is characterized in that the radio system is configurated for cell handover in such a way that the cellular radio system comprises gateway cells, each surrounding a cell group consisting of cells, with one gateway cell provided at the entrance of the building, each cell comprising on the neighbor list the gateway cell surrounding the cell, the gateway cell at the entrance comprising other gateway cells and an outdoor cell on its neighbor list, and the outdoor cell comprises the gateway cells on its neighbor list.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on determining the neighbor lists of the cells such that the cell handover from inside the building to the outdoor cell must be carried out through a specific indoor cell, even though the subscriber terminal were in the operating area of the outdoor cell while inside the building.
Several advantages are achieved with the method and system of the invention. The method can reduce the load of the outdoor cell, since cell handovers to the outdoor cell decrease. The system of the invention can be readily applied to systems currently in use.