This invention relates to a synchronization signal processing system for use in a mobile communication network which comprises a plurality of mobile service switching centers and a plurality of base transceiver stations and is operable in a time division fashion.
The mobile communication network has an overall service area which is divided into cells or radio zones assigned with the base transceiver stations, respectively, and in which a plurality of mobile stations are present, namely, either moving or staying standstill, at a time. Each mobile station may be either a portable telephone device carried by a user or a subscriber's terminal installed in an automobile or in a like mobile vehicle and is movable from a first zone of the cells to a second zone of the cells.
It is possible to understand that each mobile service switching center is connected to a plurality of fixed subscriber substations either directly or through at least one exchange office. Some of the mobile service switching centers are connected to the base transceiver stations. More particularly, each of such mobile service switching centers is connected to a certain number of base transceiver stations.
The mobile service switching centers are connected to one another by wired communication lines. The mobile service switching centers and the base transceiver stations may be connected through wired communication lines. Among the overall service area, some of the cells are often referred to collectively as a radio communication area when assigned to the base transceiver stations which are served by one of the mobile service switching centers.
Each base transceiver station is for transmitting and receiving radio message signals to and from at least one of the mobile stations that is currently present in the cell assigned with the base transceiver station under consideration. For use in time division multiple access (TDMA), the radio message signals are carried by a radio carrier signal of a radio frequency in a plurality of time slots. A predetermined number of such time slots are successively arranged in a frame in the manner known in the art.
When a particular station of the mobile stations moves between the first and the second zones assigned with first and second stations of the base transceiver stations, the first and the second stations use different radio frequencies and different time slots in transmitting and receiving the radio message signals to and from the particular station. The first and the second stations may be connected either to one or to two of the base transceiver stations. In either event, the particular station is inevitably subjected to a handover processing between the first and the second stations. It is therefore desirable to preliminarily synchronize the frames and the time slots in the base transceiver stations in order to reduce a time necessary for such a handover processing as a handover processing time.
In the manner which will later be described, a conventional synchronization signal processing system comprises an individual synchronization signal generating circuit in each mobile service switching center. When connected to such a mobile service switching center, the base transceiver station can generate synchronized frames and synchronized time slots for the mobile stations which are currently present in the radio communication area served by the base transceiver station under consideration.
A little more in detail, the synchronization signal generating circuit comprises first and second time division switches, each comprising controllable connection paths and producing a switch trouble signal when a trouble occurs therein. A controller device is cross connected to the first and the second time division switches and is supplied with the switch trouble signal to control the connection paths of one of the first and the second time division switches that is not producing the switch trouble signal and serves as an active switch with the other of the first and the second time division switches used as a standby switch. A synchronization signal generator is connected to the active switch to supply a synchronization signal to the connection paths of the active switch. Output trunk circuits are connected to the connection paths of the first and the second time division switches to supply the synchronization signal to at least one of the output trunk circuit from the connection paths controlled by the controller device to the base transceiver stations served by mobile service switching center in question.
It is liable that the synchronization signal generator is involved into a trouble. First and second synchronization signal generators are therefore cross connected to the first and the second time division switches. Alternatively, it is possible to understand that the first and the second synchronization signal generators are connected to the active switch. In either event, each synchronization signal generator produces a generator trouble signal when a trouble occurs therein. Supplied with the generator trouble signal, the controller device controls the connection paths of the active switch to supply the output trunk circuits with the synchronization signal generated by one of the first and the second synchronization signal generators that is not producing the generator trouble signal.
As a consequence, the conventional synchronization signal processing system can deal with troubles that may occur in the time division switches and/or in the synchronization signal generators. It is, however, impossible to keep the phase of the synchronization signal when the first and the second synchronization signal generators are switched from one to the other.