Abstract:
Which permits to solve the problems relating to the interference produced among communications equipment of the same technology but belonging to different communications and which simultaneously use the same communications channel in time and frequency. It comprises modifying one of the parameters of the starting signal. The main advantage of the invention is that, in a simple way, it allows equipment to be correctly synchronised with the signals reaching it from its own network while ignoring signals of lesser power received from other networks using the same or different technology, without any need to carry out a complex and costly processing on reception.

Description:
OBJECT OF THE INVENTION 
       [0001]    As stated in the title of this descriptive specification, the present invention relates to a procedure for simultaneous transmission in time and frequency of multiple communications of data by means of OFDM modulations. The technical field of the invention is that of data communications in any transmission medium, and its main advantage is that it allows equipment to be capable of correctly receiving signals from its own network while ignoring signals of lesser power reaching it from other networks, without any need to carry out a post-processing on reception. Thanks to this, the inventive procedure permits the reutilisation of the channel in time and frequency for different communications by means of modifications in the starting symbols, in such a way that the rest of the communications are indistinguishable from the background noise for a given communication. The transmission capacity is thereby increased due to not having to use mechanisms for sharing the channel among the different communications. 
       PRIOR ART OF THE INVENTION 
       [0002]    In the majority of telecommunications systems, a situation can occur in which many different communications are transmitting at the same time via the same channel. This can happen voluntarily, in order to try to transmit a greater amount of information, or because there exist different data networks whose communications cross in the same moment. 
         [0003]    In the state of the art, the procedure for distinguishing the right communication consists of addressing the communications by means of incorporating a destination address in the information being transmitted. In order to use this method, the receiver needs to demodulate the signal so that it can then accept it or reject it, in the event that it does not contain the right address. Another possibility is to coordinate the networks using co-existence procedures for sharing out the time or range of frequencies of the communications channel among the different communications. 
         [0004]    One possible solution is described in patent WO 98/09381, titled “High Capacity Wireless Communication Using Spatial Subchannels” in which the communications channel is separated into a series of independent subchannels for each one of the communications. In that procedure, an array of antennas is used for carrying out the spatial separation of the subchannels. The present invention consists of a totally different procedure in which no additional processing is needed for achieving the co-existence of communications in a single channel. 
         [0005]    Another form of carrying out multiple access to the channel is to use code division multiplexing (CDMA). This method requires widening the transmission bandwidth, and so a much broader bandwidth is used than that needed for carrying out the communication. The procedure of the invention improves the efficiency since the bandwidth of the signal is not widened. 
         [0006]    In either of these methods of the state of the art, it is necessary to carry out processing on reception in order to recover the correct information from the equipment belonging to the same network. The inventive procedure avoids having to carry out synchronisation with the signals of lesser power arriving from other networks, and therefore there is no need to carry out any processing on reception, since the signals from other networks become merged with the background noise of the channel. 
         [0007]    It can be pointed out that the Spanish patent referring to “Access procedure to transmission means of multiple communications nodes on electric network” with number ES2221803, belonging to the same patent holder as the present invention, describes a procedure in which a transmission starting signal is used. The inventive procedure can be used on this signal in order to solve the problems of interference produced among communications nodes using the same technology but belonging to different networks using the same communications channel. 
       DESCRIPTION OF THE INVENTION 
       [0008]    In order to achieve the objectives and avoid the drawbacks mentioned above, the invention consists of a procedure for simultaneous transmission in time and frequency of multiple communications of data by means of OFDM modulations, where multiple communications of data are being carried out at the same time on the same channel and in which a starting signal is used in the transmission of the frame, characterised in that it comprises modifying the starting signal for transmission of each communication by means of altering at least one parameter constituting that signal in such a way that the rest of the communications coinciding in time and frequency with respect to a given communication are regarded by it as being noise. 
         [0009]    Each communication is characterised by the use of a common set of starting signals with certain distinct characteristics from the rest of the communication. 
         [0010]    There exist many different ways of modifying the characteristics of the starting signal which can be used to form the set of distinct communications, and the one chosen by the invention is the alteration of the parameters constituting said signal. Standing out among these alterations is the frequency shift of that signal, time shift, alteration of the symmetry characteristics of the signal, of the duration of each repetition of the base signals forming the stating signal, of the phase and amplitude ratios between the carriers of the starting signal or alteration of the number of repetitions of the base signals forming the starting signal. A combination of these alterations could also be used. 
         [0011]    In the particular case of using a frequency shift in order to modify the starting signal, said shift will be equal to a fraction of the separation between carriers of the OFDM modulation. 
         [0012]    Below, in order to facilitate a better understanding of this descriptive specification and forming an integral part thereof, some figures are attached in which, on an illustrative rather than limiting basis, the object of the invention has been represented. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES  
         [0013]    FIG.  1 .—Represents two master slave networks in which the communications within each network interfere with the communications of the other network. 
           [0014]    FIG.  2 .—Shows the alteration to the starting signal consisting of the frequency shift of the communication carriers using the inventive procedure. 
           [0015]    FIG.  3 .—Shows an example of the problem it is intended to avoid: confusing a starting signal received from another network with the starting signal of one&#39;s own network. 
           [0016]    FIG.  4 .—Represents an example of embodiment in which the alteration of the starting signal consists of modifying the duration of the repetitions of the base signals forming that signal. 
           [0017]    FIG.  5 .—Represents an example of embodiment in which the alteration of the starting signal consists of using a different number of repetitions of the base signals forming that signal. 
       
    
    
     DESCRIPTION OF VARIOUS EXAMPLES OF EMBODIMENT OF THE INVENTION 
       [0018]    A description is now going to be given of various examples of embodiment of the invention, with reference to the numbering adopted in the figures. 
         [0019]      FIG. 1  shows a typical example in which the inventive procedure can be used for increasing the transmission capacity when there exist two communications from different networks using the same channel. In this example, two independent master-slave networks are represented, the first network comprising master equipment ( 1 ) and slave equipment ( 2 ) and the second network master equipment ( 4 ) and slave equipment ( 5 ). The proximity between the two networks means that the communications ( 3 ) made in the first network and the communications ( 6 ) made in the second network can be received by the opposite network, though presumably with less power than that sent by the equipment in the communications network itself. Due to using the same communications technology, the messages sent by the other network will be received, demodulated and finally rejected (due to not belonging to the equipment of the network itself) with all the cost that this entails. This problem can occur in communication ( 7 ) between masters and in communication ( 8 ) between slaves and even in crossed communication ( 9 ) and ( 10 ) between the masters and slaves of opposing networks. 
         [0020]    In  FIG. 3  an example can be seen of what happens when equipment ( 14 ) receives a starting signal for a frame from the other equipment in its own network ( 15 ) and from the other equipment of the other network ( 16 ) using similar technology. Although the starting signal ( 17 ) received from the other network arrives with less power than the starting signal ( 18 ) of the network itself, if the signal ( 17 ) from the other network is ahead of the starting signal ( 18 ) of the actual network, then the equipment will synchronise itself to the first one and open a reception window ( 19 ), which will cause the loss of part of the information ( 20 ) sent by the equipment from its own network. 
         [0021]    In general, the starting signal for frames has certain properties which the receiver looks for in the signals received via the channel in order to be able to receive the data being sent. These properties can be one or more of the following: symmetry of the signal, duration of each repetition, spectral position, phase/amplitude ratio between carriers, or the total duration of the signal. By minimally modifying the properties of the starting signal for the frame it is possible to prevent detection by the receivers looking exactly for that property, in such a way that various sets of possible tuned transmitters and receivers will be achieved which would be capable of continuing with their own communications while merging the communications from the other sets with the background noise. This constitutes the main advantage of the inventive procedure. Given that the communications from the other systems come to form part of the background noise and this can imply an increase in that noise, the performance of each individual system can find itself reduced, but, by being able to transmit several sets simultaneously, the aggregate performance of them all can be greater. 
         [0022]    Although there exist many different means for achieving this effect, by modifying the characteristics of the starting signal, in this specific example of embodiment, a frequency shift is performed between the carriers making up the starting signal for the two communications in such a way that the carriers have sufficient separation so that a receiver that is not synchronised with the correct position of the carriers is incapable of obtaining the information transmitted in the shifted carriers. 
         [0023]    In  FIG. 2 , the shift of carriers can be observed between the two systems in the example of embodiment. In this case, the shift is ½ that of the distance between carriers, though other fractions of this value could be used, such as ¼ or ¾ with similar results. In that figure the situation in frequency of the carriers ( 11 ) of the first network and the carriers ( 12 ) of the second network can be seen. Likewise, the figure shows the frequency shift ( 13 ) between the carriers ( 11 ) and ( 12 ) of the two networks. 
         [0024]    Another example of embodiment can be seen in  FIG. 4  in which the alteration of the starting signal consists of changing the duration of the repetitions of the base signal forming the starting signal. Represented in that  FIG. 4  are the duration ( 21 ) of each repetition in the first network and the duration ( 22 ) in the second network. On receiving a starting signal, if the duration ( 22 ) of each repetition is different from the duration ( 21 ) selected in the actual network, then the receiver will ignore that signal. The starting signal can also be altered by modifying the number of repetitions of the base signals forming the starting signal. This example of embodiment can be seen in  FIG. 5 , where one network functions with a starting signal having seven repetitions ( 23 ) and the other network functions with a starting signal having three repetitions ( 24 ). If the receiver does not hear the number of repetitions selected in its network, it ignores the starting signal and waits until it receives a starting signal with the right number of repetitions.