1. Field of the Invention
The present invention relates to a method and system of transmitting data in a cellular radio system, as well as relates to a corresponding subscriber station and central station.
2. Description of the Prior Art
In a mobile radio system, an area to be supplied is usually divided into a large number of radio cells. In each of these radio cells there is provided a base station via which the connections between a public telephone network and the subscriber stations of the individual subscribers, for example mobile stations, in the respective cell can be established. Such a mobile radio system is, for example, the GSM (Global System for Mobile Communication) standardized by the European Telecommunications Standards Institute (ETSI). Such a system is described, for example, in a brochure xe2x80x9cD 900 Mobile Communication Systemxe2x80x9d SYD from Siemens AG, 1992. A similar system is the DCS 1800/PCN.
In the mobile radio station illustrated in FIG. 5, a radio region is divided up, in accordance with the GSM standard, into a plurality of cells Z, of which only the cells Z1 to Z7 are illustrated in FIG. 5. In a cell Z, the telecommunications service from and to a mobile station MS is made available by means of a base station BTS (BTS=Base Transceiver Station) which is part of a central station. In FIG. 5, only the base stations BTS1 to BTS3 assigned to the cells Z1 to Z3 are illustrated. One or more base stations BTS are connected to a base control unit BTSC (BTSC=Base Station Controller), of which only the base control units BTSC1 and BTSC2 are illustrated in FIG. 5. The base control units BTSC perform the local functions of call switching, monitoring and maintenance. They include, in particular, control units BCE (Base Station Control Equipment). A plurality of base control units BTSC are connected to a switching device SSS (SSS=Switching Subsystem) which is itself connected to the public network PSTN (PSTN=Public Switched Telephone Network). The PSTN Public Network can be designed as an ISDN network, as a mobile radio network or as some other telephone or data network.
If a subscriber wishes to communicate with another subscriber by means of a mobile station MS in the cell Z1, a prescribed protocol is processed via the base station BTS1 with the base control unit BTSC1, which protocol is defined, for example, in the abovementioned GSM standard. When the connection between the mobile station MS and the base control unit BTSC1 has been established, a connection is established to the public network PSTN via the switching device SSS. From this public network PSTN, the other subscriber, who may be provided with a mobile station, is then reached.
In the GSM, the signals are usually transmitted using the so-called TDMA (Time Division Multiple Access) method in a frequency and time division multiple access. In this method, the data, which may also include digitalized voice, are transmitted via time slots of a TDMA frame.
During the transmission of the data, the mobile station observes the radio field conditions of the adjacent base stations so that, if appropriate, a handover to another base station can be performed on the basis of these measurements.
In order to obtain relatively high data rates during transmission between the mobile station and the network, a plurality of time slots of the TDMA frame may be used for the same connection. However, this shortens the time period in which a mobile station can observe the adjacent base stations. If, for example, all the time slots of a TDMA frame are used for a connection, it may be completely impossible to observe the adjacent base stations. In order, to ensure that a observation takes place, it would be conceivable to provide a second receiver to monitor the adjacent base stations in the mobile station, wherein the receiver would be used, to observe the adjacent base stations. However, such a solution has the disadvantage that it requires a relatively large expenditure and increases the volume of the mobile station.
EP 662 778 A2 discloses the Asynchronous Transfer Mode (ATM) for transmitting permanently prescribed data packets by mobile radio. The monitoring of neighboring cells in a cellular system is not addressed in this known process of transmission.
Furthermore, WO 91-2436 A1 describes the provision in each base station of a cellular mobile radio system of devices which detect the temporary pauses in the transmission between the base station and the mobile station and bring about signaling operations with the mobile station during these pauses.
The present invention is directed to a process and a system by which a connection to a plurality of time slots is established in a simple and cost-effective way and, nevertheless, the use of a second receiver in the mobile station is avoided.
Accordingly, in an embodiment of the present invention, a method is provided of transmitting data in a cellular radio system between a base station in a cell, which is one of a plurality of base stations in a corresponding plurality of cells, and a mobile station where in the transmission occurs in a plurality of time slots of a time division multiplex frame pursuant to a time division multiplex method, the method including the steps of: determining a first data rate time period; transmitting data at a first data rate over a first number of time slots for the duration of the first data rate time period; monitoring base stations in adjacent cells during the transmission of data at the first data rate; determining a second at a rate time period; transmitting data at a second data rate over a second number of time slots upon expiration of the first data rate time period and for the duration of the second data rate time period, the second number of time slots being less than the first number of time slots; radio-monitoring the base stations in adjacent cells during the transmission of data at the second data rate; and transmitting data at the first data rate upon expiration of the second data rate time period.
In an embodiment, the method further includes the steps of: successively transmitting data at the second data rate after the expiration of the first data rate time period; and successively radio-monitoring the base stations in adjacent cells during the transmission of data at the second data rate.
In an embodiment, the method further includes the step of determining the first number of time slots pursuant to an agreement between the base station and the mobile station.
In an embodiment, the method further includes the step of transmitting the agreement with a signaling system.
In an embodiment, the method further includes the steps of: determining transmission requirements for the second data rate time period; and using the mobile station to reduce the number of time slots from the first number of time slots to the second number of time slots.
In an embodiment of the present invention, a system is provided for transmitting data in a cellular radio system between a base station and a mobile station where in the transmission occurs in a plurality of time slots in a time division multiplex frame pursuant to a time division multiplex method, the system including: a base station in a cell which is one of a plurality of base stations in a corresponding plurality of cells, the base station transmitting data at a first data rate over a first number of time slots for the duration of a first data rate time period, transmitting data at a second data rate over a second number of time slots upon the expiration of the first data rate time period for the duration of a second data rate time period wherein the second number of time slots is less than the first number of time slots, and again transmitting data at the first data rate upon expiration of the second data rate time period; and at last one mobile station for radio-monitoring the base stations in adjacent cells during the transmission of data at the second data rate.
Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and the Drawings.