Abstract:
In known telecommunication systems comprising two separate radio systems each one using its own clock signal for defining its channels multiplexed in time, said radio systems cause too much interference with each other. By introducing a coupling between both radio systems for creating a predefined phase difference between their channels by adjusting at least one of said clock signals, the interference can be reduced, in particular in case said phase difference is continuously altered in time, by increasing or decreasing said phase difference all the time.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to a telecommunication system comprising a first radio system using a first clock signal for defining first channels multiplexed in time, and comprising a second radio system using a second clock signal for defining second channels multiplexed in time. 
     2. Discussion of Related Art 
     Such a telecommunication system is of common general knowledge and comprises said first radio system and said second radio system as in for example, well known GSM systems. The first GSM system uses said first clock signal for defining first communication channels multiplexed in time and situated in a first frequency band, and the second GSM system uses said second clock signal for defining second communication channels multiplexed in time and situated in a second frequency band being either equal to said first frequency band or being situated near said first frequency band. 
     This known telecommunication system is disadvantageous, inter alia, because of said radio systems causing too much interference with each other. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention, inter alia, to provide a telecommunication system as described in the technical field, in which said radio systems cause less interference with each other. 
     Thereto, the telecommunication system according to the invention is characterized in that the telecommunication system comprises a coupling between the first radio system and the second radio system for creating a predefined phase difference between at least one first channel and at least one second channel by adjusting at least one of said first and second clock signal. 
     Due to the presence of said coupling and the creation of said predefined phase difference as a consequence of said adjustment, the first location of said at least one first channel and the second location of said at least one second channel can be chosen (adjusted) such that interference is at a minimum level. 
     The invention is based on the insight that said locations of said channels affect their interference. 
     The problem of said radio systems causing too much interference with each other is thus solved by adjusting at least one of said first and second clock signals via said coupling. 
     It should be observed that it is of common general knowledge to reduce the interference in two radio systems by introducing a random phase for one of both clock signals. In this case there is no coupling between both radio systems for the creation of a predefined phase difference between both channels as a consequence of an adjustment of clock signals via said coupling. 
     A first embodiment of the telecommunication system according to the invention is characterized in that said coupling comprises a phase shifting device. 
     Such a phase shifting device, which receives one of said clock signals and in response generates the other clock signal having a shifted phase, could be located either between both radio systems or inside one of both radio systems. 
     A second embodiment of the telecommunication system according to the invention is characterised in that said phase shifting device alters a phase of at least one of said first and second clock signals with a constant value. 
     In this case, the phase shifting device receives one of said clock signals and adds or subtracts a constant value to or from the phase. 
     A third embodiment of the telecommunication system according to the invention is characterized in that said phase shifting device alters a phase of at least one of said first and second clock signals with a value which is continuously altered in time. 
     In this case, the phase shifting device receives one of said clock signals and adds or subtracts a value to or from the phase, which value is continuously increasing or decreasing. 
     The possibility of both radio systems each one having its own clock generator, whereby the coupling is only used for exchanging a control signal for creating said predefined phase difference between said channels by adjusting at least one of said clock signals, should of course not be excluded. 
     Said coupling between said first and second radio system could, of course, be either a wired one or a wireless one, whereby in the last case for example said second radio system monitors said first channels of said first radio system and derives said first clock signal from said monitored first channels and generates said second clock signal in response to the derived first clock signal by introducing said predefined phase difference. 
     The invention further relates to a radio system using a clock signal for defining channels multiplexed in time. 
     The radio system according to the invention is characterized in that the radio system comprises a coupling for a further clock signal belonging to a further radio system using said further clock signal for defining further channels multiplexed in time for creating a predefined phase difference between at least one channel and at least one further channel by adjusting at least one of said clock signal and said further clock signal. 
     A first embodiment of the radio system according to the invention is characterized in that said coupling comprises a phase shifting device. 
     A second embodiment of the radio system according to the invention is characterized in that said phase shifting device alters a phase of at least one of said clock signal and said further clock signal with a constant value. 
     A third embodiment of the radio system according to the invention is characterized in that said phase shifting device alters a phase of at least one of said clock signal and said further clock signal with a value which is continuously altered in time. 
     The invention yet further relates to a method for providing first radio communication via a first radio system using a first clock signal for defining first channels multiplexed in time, and for providing second radio communication via a second radio system using a second clock signal for defining second channels multiplexed in time. 
     The method according to the invention is characterized in that the method comprises the steps of 
     supplying at least one of said first and second clock signal respectively to at least one of said second and first radio system respectively, and 
     creating a predefined phase difference between at least one first channel and at least one second channel by adjusting at least one of said first and second clock signal. 
     A first embodiment of the method according to the invention is characterized in that said step of creating said phase difference comprises a substep of altering a phase of at least one of said first and second clock signal with a value. 
     From DE 32 24 922, Hoefgen (Standard Elektrik Lorenz AG) it is known to delay irregularly information to be transmitted from a subscribing station to a base station. From WO 91/13502 and corresponding U.S. Pat. No. 5,301,188 (Motorola) shared carrier frequency hopping is known. The invention of creating a predefined phase difference between at least one first channel of a first radio system and at least one second channel of a second radio system is known neither from DE 32 24 922 nor from WO 91/13502. All references including further references cited with respect to and/or inside said references are hereby incorporated by reference in their entirety into this patent application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in greater detail with the help of embodiments disclosed in the drawings, whereby 
     FIG. 1 discloses an overview of a telecommunication system according to the invention including two pairs of timing diagrams defining channels multiplexed in time, and 
     FIG. 2 discloses two radio systems according to the invention comprising two base stations which are mutually coupled via a coupling. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The telecommunication system according to the invention as disclosed in FIG. 1 comprises a Public Switched Telecommunication Network or PSTN, a first Mobile Switching Center or USC 1  of which a first side is coupled to said PSTN and of which a second side is coupled to a second Base Station or BS 2  for serving a second area II and to a fourth Base Station or BS 4  for serving a fourth area IV, and a second Mobile Switching Center or MSC 2  of which a first side is coupled to said PSTN and of which a second side is coupled to a first Base Station or BS 1  for serving a first area I and to a third Base Station or BS 3  for serving a third area III. The first base station BS 1  and the second base station BS 2  are mutually coupled via a first coupling, resulting in a first pair of timing diagrams defining first channels multiplexed in time of BS 1  and second channels multiplexed in time of BS 2 , said second channels being shifted in phase with 90 degrees with respect to said first channels, and the third base station BS 3  and the fourth base station BS 4  are mutually coupled via a second coupling, resulting in a second pair of timing diagrams defining third channels multiplexed in time of BS 3  and fourth channels multiplexed in time of BS 4 , each subsequent one of said fourth channels being shifted in phase with an increasing number of degrees with respect to the previous fourth channel and with respect to said third channels. 
     The two radio systems according to the invention as disclosed in FIG. 2 comprise said first base station BS 1  including a processor  10  for controlling a coder  12 , a modulator  11 , a demodulator  14  and a decoder  15 , and comprise said second base station BS 2  including a processor  20  for controlling a coder  22 , a modulator  21 , a demodulator  24  and a decoder  25 . An input of coder  12  is coupled to an input  17  of BS 1  for receiving signals from MSC 2 , an output of coder  12  is coupled to an input of modulator  11 , of which an output is coupled to an input of a coupler  13 . An antennae  16  of BS 1  is coupled to an in/output of coupler  13 , of which an output is coupled to an input of demodulator  14 . An output of demodulator  14  is coupled to an input of decoder  15 , of which an output is coupled to an output  18  of BS 1  for transmitting signals to MSC 2 . An input of coder  22  is coupled to an input  27  of BS 2  for receiving signals from MSC 1 , an output of coder  22  is coupled to an input of modulator  21 , of which an output is coupled to an input of a coupler  23 . An antennae  26  of BS 2  is coupled to an in/output of coupler  23 , of which an output is coupled to an input of demodulator  24 . An output of demodulator  24  is coupled to an input of decoder  25 , of which an output is coupled to an output  28  of BS 2  for transmitting signals to MSC 1 . Both processors  10  and  20  are mutually coupled via a first coupling comprising a phase shifting device  30 . 
     The telecommunication system according to the invention comprising BS 1  and BS 2  functions as follows. Processor  10  supplies a first clock signal to coder  12  and modulator  11  as a consequence of which said first channels multiplexed in time are situated in a first frequency band. Processor  20  supplies a second clock signal to coder  22  and modulator  21  as a consequence of which said second channels multiplexed in time are situated in a second frequency band. Due to the fact that said first frequency band and said second frequency band are either equal or close to one another, at least in the overlapping parts of service areas I and II there could be interference. By using said first coupling first between BS 1  and BS 2 , with said coupling first for example comprising phase shifting device  30 , at least one of said first and second clock signals can be shifted in phase with respect to the other, thereby reducing said interference. For example a phase shift having a constant value of 90 degrees will offer good results. Phase shifting device  30 , of course, is just an embodiment, because of another possibility being the transmission of a data signal from one of the processors to the other, with said data signal indicating the required phase shift to be realized inside a processor or between said processor and the corresponding decoder and modulator or inside said decoder and/or modulator. 
     The telecommunication system according to the invention comprising BS 3  and BS 4  with said second coupling that functions in a corresponding way, apart from the phase shift this time having a value which increases all the time. Another possibility which should not be excluded is said phase shift having a value which increases from 0 degrees to for example 180 degrees, and then decreases from said 180 degrees to for example 0 degrees or −180 degrees, and then increases again, etc. 
     As a consequence of this invention, for example within the range of a first radio system, a second (additional) radio system can be built, using (almost) the same frequency band, which is very advantageous. 
     According to an other embodiment, said coupling between said first and second radio system is a wireless one, whereby for example said second radio system monitors said first channels of said first radio system and derives said first clock signal from said monitored first channels and generates said second clock signal in response to the derived first clock signal by introducing said predefined phase difference. 
     In GSM systems having a time slot duration of 577 microsec., a drift of 50 ppm results in 576.97115 microsec., and a shift of one time slot at 20000 time slots corresponds with 11.54 sec. So, said phase shifting device could be realised by for example introducing a fixed or adjustable time delay. 
     It is of common general knowledge to couple base stations indirectly via an Operational Maintenance Center or OMC, and to couple them directly for synchronization purposes. In both cases, there is no coupling for creating a predefined phase difference between channels of both radio systems. Such a coupling for creating said predefined phase difference will preferably couple two base stations directly, but could also couple them indirectly via for example one or more mobile switching centers. 
     Of course, coders  12  and  22  could, apart from a coding function, also have further functions, for example a decoding function, and decoders  15  and  25  could, apart from a decoding function, also have further functions, for example a coding function. According to the narrowest scope the term ‘radio system’ will only comprise a base station or a combination of an antenna and a transmitter, but according to the broadest scope the term ‘radio system’ will comprise a set of base stations and one or more base station controllers and one or more Mobile Switching Centers or MSCs. Therefore, MSC 1  and MSC 2  disclosed in FIG. 1 could further be coupled directly instead of via the PSTN and/or could be integrated into one MSC. 
     Although the invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.