Abstract:
A method for multiple access in a radiocommunication system which avoids forming a signaling multiframe with an inflexible turn of transmission. A fixed unit broadcasts over a pilot channel a predetermined number of virtual identities for signaling which form the signaling multiframe, so that when a remote unit wishes to transmit, it selects one of the virtual identities received. The time slots in the signaling multiframe are distributed among those remote units which have signaling messages to transmit to the fixed unit.

Description:
OBJECT OF THE INVENTION 
     The present invention relates to a method for allocating the radio and signalling resources, in general, of a multiple access radiocommunication system, which comprises a set of fixed units such that each of them has an coverage area or cell associated with it. Within each cell are located a plurality of remote units, which employ time division multiple access (TDMA) techniques for communicating with their fixed unit. 
     The multiple access method is of special, but not exclusive, application in a point-to-multipoint radiocommunication system, so that the allocation of the different time slots that constitute a frame, in the uplink direction of the communication, is carried out in a dynamic mode as a function of the traffic requirements of each of the remote units that constitute the radiocommunication system. 
     STATE OF THE ART 
     A radiocommunication system is divided into a plurality of cells, and each cell comprises a fixed unit normally connected by means of a cable network to a telephone transport network such as a public switched telephone network (PSTN). 
     Each fixed unit has associated a coverage area inside which it establishes communications via radio with those remote units located within its coverage area, by using time division multiple access (TDMA) techniques, that is, the frequency band is divided into time slots which are assigned to the sending and receiving of signals. Consequently, a number of communications can be transmitted simultaneously in a single frequency band. 
     In a conventional point-to-multipoint radiocommunication system, each remote unit transmits, in the uplink direction of the communication, in addition to the data bursts corresponding to the communications already established, signalling information relative to both the communications that are already established and to the new ones arising in the remote units. 
     This gives rise to the appearance within a TDMA frame of one or more time slots reserved for signalling, which have to be shared among all the remote units. 
     A commonly employed method consists in the cyclic allocation of usage times for these signalling slots in an inflexible manner as a function of the identity of each remote unit, in the form of a signalling multiframe. 
     In this way, each remote unit must wait for its signalling time to appear for sending a limited message, limited in length because the signalling channel is shared among all the remote units in an inflexible manner regardless of whether they require to signal or not. Thus, although a remote unit has no message to send, it has to occupy the allocated time, for example, with a stuffing message. 
     Similarly, if a remote unit has several messages to send, it must wait for successive turns in order to transmit the information in question, even although there are time slots in the same frame which are filled with stuffing information, that is there is an inflexible turn or order. 
     Consequently, a fixed unit will be able to communicate simultaneously with as many remote units as there are time slots for signalling in the signalling multiframe in the uplink direction of the communication. 
     As a consequence, it is necessary to provide a method whereby the number of remote units that can occupy the signalling multiframe is increased, without excessively increasing the number of time slots into which the signalling multiframe is divided, which would result in unacceptable delay times. 
     CHARACTERISATION OF THE INVENTION 
     The proposed multiple access method for a radiocommunication system overcomes the problems above indicated. 
     The radiocommunication system has its coverage area divided into a plurality of cells, so that within each cell there is at least one fixed unit that communicates with a plurality of remote units located inside the cell. 
     The access method avoids forming a signalling multiframe working with a fixed turn for transmitting. To this end, the fixed unit broadcasts over a pilot channel a predetermined number of virtual identities for signalling and that form the signalling multiframe, so that when a remote unit wishes to transmit information in the uplink direction, it selects one of the virtual identities received. 
     The fixed unit comprises a first controller means that establishes the number of virtual identities and, based on their occupancy level, increases or diminishes it. The occupancy level is a function of the traffic present at each moment. 
     Thus, the signalling multiframe is only used by those remote units that have messages to send. This method makes allocation of the signalling multiframe more flexible, reducing signalling delays. 
     In brief, the number of remote units that can communicate simultaneously with a fixed unit is a function of the number of time slots into which the signalling multiframe is divided, it not being possible to increase these indefinitely without increasing, in turn, the signalling delay. Thus, by use of the method proposed, it is possible to increase the number of remote units since the multiframe time slots are distributed among those units which have to transmit signalling to the fixed unit. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A more detailed explanation of the invention is provided in the following description, based on the attached figures, in which: 
         FIG. 1  shows a diagram of a point-to-multipoint radiocommunication system according to the invention, 
         FIG. 2  shows a block diagram of a fixed unit according to the invention, and 
         FIG. 3  shows a block diagram of a remote unit according to the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a preferred embodiment of the point-to-multipoint radiocommunication system, which comprises a set of fixed units  11 - 1  to  11 - n  such that each one has associated with it a cell or coverage area of the radiocommunication system. 
     Each fixed unit  11 - j  (where j=1, . . . , n) is connected via radio with a set of remote units  12 - 1  to  12 - m  located within its coverage area. To carry out the communications between the different remote units  12 - 1  to  12 - m  and the fixed unit  11 - j  use is made of time division multiple access (TDM/TDMA) techniques. 
     The traffic generated by a remote unit  12 - i  (where i=1, . . . , m) is directed through the fixed unit  11 - j  to a telephone transport network such as a public switched telephone network (PSTN). 
     According to the TDM/TDMA technique, a carrier frequency is divided into time slots that are grouped into frames, there being frames for both transmission directions. Within the TDMA frame there are time slots reserved for signalling between the fixed unit  11 - j  and the remote units  12 - 1  to  12 - m , which form a signalling multiframe. 
     When a remote unit  12 - i  wishes to transmit information relative to a communication, it inserts its data bursts into an allocated time slot of the frame. The time slot allocation is carried out by a first controller means  111  belonging to the fixed unit  11 - j  (see  FIG. 2 ). 
     According to the time slot allocation method proposed, the first controller means  111  incorporates an algorithm that determines a first predetermined number of virtual identities for signalling and implements it on the basis of the traffic conditions of the radiocommunication system. The virtual identities proposed form the signalling multiframe and are independent of the true identities of each remote unit  12 - i , which shall be used for other purposes (management). 
     The virtual identities created are supplied to a first radio transmitter  112  of the fixed unit  11 - j  to be broadcast over a pilot channel, being received in the remote unit  12 - i  by means of a second radio receiver  123 . 
     Thus, when a subscriber connected to a remote unit  12 - i  wishes to transmit a signalling message, for example in order to set up a communication, the remote unit  12 - i  selects one of the virtual identities available, independently of its true identity, by employing a second controller means  121  (see  FIG. 3 ). 
     The second controller means  121  receives from the second radio receiver  123  the proposed identities and makes the selection of one of them on the basis of traffic requirements and on the nature of the information to be transmitted, be this voice, data, video, television or other digital signal. The duration of traffic bursts is therefore variable. 
     The virtual identity selected shall indicate the order of transmission within the signalling multiframe. Thus, the remote unit  12 - i  shall start to transmit, by means of a second radio transmitter  122 , signalling messages in the time slot allocated within an uplink frame. The number of virtual identities conditions the duration of the signalling multiframe, there being a maximum duration permitted. 
     A first radio receiver  113  of the fixed unit  11 - j  receives the signalling message and feeds it to the first controller means  111 , which analyses it and records the virtual identity occupied. 
     The occupancy of this virtual identity is transmitted over the broadcast channel in order to prevent it being used by another remote unit  12 - k , consequently, the signalling multiframe becomes reduced and shall try to occupy another virtual identity of the signalling multiframe. 
     When a signalling process is over, the virtual identity is once again made available for selection for another communication, and so on successively. The signalling multiframe is dynamic; that is, it is organised as a function of the number of virtual identities proposed at any given moment. 
     If the virtual identities proposed by the fixed unit  11 - j  are gradually taken up, the first controller means  111  increases the number of proposed virtual identities within the signalling multiframe. Consequently, the duration of the signalling multiframe increases and the spacing between bursts corresponding to a given communication also increases. As the signalling processes are gradually concluded, the first controller means  111  reduces the number of virtual identities proposed within the signalling multiframe. 
     During system start-up, and while a given traffic situation exists, the first controller means  111 , by default, establishes a reduced number of virtual identities for selection by the remote units  12 - 1  to  12 - m . For example, if the virtual identities  1  to  4  are put into play, a four time slots multiframe is obtained. 
     Thus, in the case where there is only one remote unit  12 - i  with a call set-up process in course, it will choose one of the identities in play and will make use of the corresponding time slot. Since there are only four time slots, the multiframe duration is short, and the bit flow that the remote unit  12 - i  enjoys is high. 
     The multiframe duration (time between consecutive bursts transmitted from the same remote unit  12 - i ), gradually increases as virtual identities are added in order to permit signalling from more remote units  12 - 1  to  12 - m , and, in addition, the duration of the burst from each of the remote units  12 - 1  to  12 - m  diminishes. 
     As the radio resources of the system are released, the fixed unit  11 - j  puts the released virtual identities into play again, to be assigned to new calls and thereby avoid having to introduce more virtual identities than those necessary, which would lengthen the duration of the multiframe needlessly. 
     The fixed unit  11 - j  transmits, over the broadcast channel, the virtual identities that are presently occupied, with an input acknowledgement message. The remote unit  12 - i  that detects this acknowledgement message, shall continue to use its virtual identity and the corresponding time slot, while the rest shall interpret this as a warning of virtual identity and time slot occupancy. 
     In the event that another remote unit  12 - k  wishes to initiate a call set-up process, it shall select another virtual identity available. If two remote units  12 - i  and  12 - k  coincide in the use of the same virtual identity, and consequently of the same time slot, the fixed unit  11 - j  detects this collision and broadcasts a “not confirmed” message for the right to employ said virtual identity and time slot. The remote units  12 - i  and  12 - k  that tried to gain access, decline to use said virtual identity and time slot, open a time-out period and opt for the virtual identities proposed at that moment.