AU743541B2

AU743541B2 - A control method enabling signalling information to be received by a radio terminal communicating in TDMA mode with a fixed station

Info

Publication number: AU743541B2
Application number: AU23647/99A
Authority: AU
Inventors: Ulrich Barth; Jean-Francois Criqui; Philippe Desblancs; Xavier Dugast; Jean-Francois Rubon; Christoph Sperber
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel Lucent SAS
Priority date: 1998-04-20
Filing date: 1999-04-07
Publication date: 2002-01-31
Anticipated expiration: 2019-04-07
Also published as: EP0978959A1; JP2000078649A; FR2777730A1; HU9900982D0; HUP9900982A2; AU2364799A; FR2777730B1; HUP9900982A3

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: A control method enabling signalling information to be received by a radio terminal communicating in TDMA mode with a fixed station 9 The following statement is a full description of this invention, including the best method of performing it known to us: FHP.SYD rE\N ATPnfl3S2fnq9n7017 A CONTROL METHOD ENABLING SIGNALLING INFORMATION TO BE RECEIVED BY A RADIO TERMINAL COMMUNICATING IN TDMA MODE WITH A FIXED STATION The field of the invention is that of transmitting digital data between a terminal, which may be a mobile terminal, and fixed stations forming part of a cellular communications network. More precisely, the invention relates to a method enabling a terminal that is communicating in TDMA mode with a fixed station to receive signalling information from said fixed station and also from other fixed stations in the cellular network.

The description below relates to the context of a terminal suitable for communicating with fixed stations making up a small private cellular network. By way of example, the fixed stations may be distributed in the offices of a building and connected to the public network via a PABX, and the user of a terminal must be capable of communicating with a third party without being interrupted while moving about within the building. Such call continuity is ensured by procedures for transferring calls between the fixed stations, referred to as "handover" procedures.

Figure 1 shows a multiframe transmitted~in such a cellular network.

The multiframe of Figure i, given overall reference comprises 52 consecutive frames referenced TO to T51.

By way of example, each frame comprises eight time slots ITO to IT7. Guard times are provided between the frames, and after frame T51, there follows a new multiframe.

Frames TO to T24 and T26 to T50 are traffic frames designed solely for conveying traffic data between terminals and a given fixed station. This transfer of traffic data takes place in the up direction and in the down direction, a given terminal having time slots allocated thereto for the duration of a call, so as to receive traffic data during one time slot and to transmit traffic data during another time slot, where said time slots may be, but are not necessarily, portions of the same frame.

Frames T25 and T51 are signalling frames that are reserved in each terminal for receiving signalling data broadcast by the various fixed stations of the network.

This signalling information serves in particular to enable terminals to decide whether it is necessary to perform a handover procedure in order to be able to continue a current call with the fixed station providing best reception. By way of example, such a decision may be made on the basis of a criterion relating to the power at which the signalling information is received. For the 1 duration of a call being made by the terminal, each fixed station of the network is thus allocated a time slot in each of the frames T25 and T51. The fixed stations are synchronized with one another, and each terminal that is making a call must attempt to receive all of the signalling information transmitted by the various fixed 20 stations.

Figure 2 shows the frames T24 and T25 of the multiframe of Figure 1 in greater detail.

As mentioned above, the time slots ITO to IT7 of traffic frame T24 convey traffic data in the up direction 25 and/or the down direction, while signalling frame T25 is reserved for receiving signalling information transmitted by the various fixed stations of the cellular network.

This signalling information is preferably transmitted on a single frequency, for reasons of simplicity.

The problem with a communications network of this type is that the terminals must be capable not only of transmitting and receiving traffic data in the time slots that have been allocated to them, but they must also be capable of receiving the signalling data broadcast by the fixed stations, if not in all of the multiframes, then at least from time to time, in order to be able to request handover procedures. Unfortunately, this requires local oscillators in the terminals that are capable of changing frequency quickly. By way of example, the terminal which has been allocated time slot IT7 must be capable, in a length of time corresponding to the guard time provided between frames, of changing its transmission or reception frequency so as to be capable of properly receiving the signalling data conveyed in time slot ITO of frame This problem arises whenever the local oscillator is not fast enough to switch from the transmission or reception frequency for traffic data to the reception frequency for signalling information. Under such circumstances, the change of frequency must last for no longer than one time slot (assuming that the guard time is zero), and the same problem can arise at the terminal which has been allocated time slot IT6.

The same problem arises when changing from the signalling data. reception frequency in frame T25 to the transmission and/or reception frequency for traffic data in frame T26. Local oscillators capable of changing frequency that fast (in a time of less than 777 microseconds) either do not exist on the market or else are prohibitively expensive.

If a terminal is not ready to receive the signalling information broadcast by all of the fixed stations, there exists a risk of the terminal requesting a connection (a handover procedure) to a fixed station that is not going to provide the best available quality of communication, or that it will request such a handover procedure when there is no need for it.

One solution to this problem would be to use two local oscillators, one for use when transmitting and/or receiving traffic data, and the other remaining locked on the frequency at which signalling information is transmitted, with switching between the two local oscillators being performed during the above-mentioned guard time or during a period in which the terminal is neither receiving nor transmitting. The drawback of such a solution is that it is expensive since each terminal must be provided with two local oscillators.

An object of the present invention is to remedy those drawbacks.

More precisely, one of the objects of the invention is to provide a method making it possible to ensure that a terminal which is communicating in TDMA mode with a fixed station of such a cellular network can transmit or receive traffic data while receiving all of the signalling data transmitted by the fixed stations in a predetermined duration.

According to a first aspect of the present invention there is provided a control method for enabling signalling information to be received by a radio terminal that is 15 communicating in TDMA mode with a fixed station, said signalling information being transmitted by fixed .e ,stations in time slots of a signalling frame that follows and/or precedes traffic frames, said fixed stations being mutually synchronized, the method consisting in changing 20 the position of said signalling information relative to the position of said traffic data as transmitted or received by said terminal in such a manner as to enable said terminal to transmit or receive said traffic data while also, over a predetermined length of time, receiving all of the signalling information transmitted by said fixed stations.

The predetermined duration depends in particular on the rank of the time interval allocated to the terminal and on the position-changing relationship used.

In one implementation, the change of position of the signalling information relative to the position of the traffic data as transmitted or received by the terminal, consists in allocating a different time slot to the terminal in the traffic frame.

In another implementation, the change of position of the signalling information relative to the position of the traffic data as transmitted or received by the terminal consists in changing the time slots allocated to the fixed stations for transmitting the signalling information.

Other characteristics and advantages of the invention will appear on reading the following description of two preferred implementations given by way of non-limiting illustration, and from the accompanying drawing, in which: Figure 1 shows a multiframe transmitted in a cellular network of the type under consideration; Figure 2 shows the frames T24 and T25 of the Figure 1 multiframe in greater detail; Figure 3 shows an operation of circularly oo permutating time slots reserved for traffic data; and Figure 4 shows an operation of circularly :'permutating time slots reserved for signalling S• information.

Figures 1 and 2 are described above with reference to the state of the art.

The principle of the invention lies in the fact that to enable each terminal in the process of making a call to receive the signalling information transmitted by the various fixed stations of the network, provision is made to change the position of the signalling information relative to the traffic data that is being transmitted or received by each terminal. "Changing position" is used to mean modifying the time offset that exists between two data time slots of frames T24 and T25 in Figure 2 (and/or and T26, and/or T50 and T51, and/or T51 and TO). The purpose of this modification is to increase this time offset over the relatively long term (in the abovementioned predetermined duration) so as to leave sufficient time for the local oscillator of the terminal to change the frequency to which it is tuned.

In a first implementation, as shown in Figure 3, the position of the signalling data is changed relative to the traffic data as transmitted or as received by the terminal by giving said terminal a different time slot in the traffic frame.

In a first example, the terminal may be given another time slot by performing circular permutation on the time slots allocated to various terminals. Figure 3 shows this operation of circular permutation.

At time tO, corresponding to a time multiframe n, frame T24 conveys at most eight items of traffic data in time slots ITO to IT7. At time tl, corresponding to the following multiframe (or to the following halfmultiframe, with frames T24 and T25 then corresponding to frames T50 and T51), permutation of the time slots causes 1the call located at the end of the frame, in time slot oooo IT7, to change place to the beginning of the frame, now 15 occupying time slot ITO. The terminal to which time slot IT7 was allocated now has not only the guard time between frames T24 and T25 to change the frequency to which it is tuned for the purpose of receiving the signalling information in frame T25, but also an additional six time 20 slots. It is thus certain that the terminal will be able to "listen" to all of the signalling information in frame T25. At following time t2, the last time slot of frame oe T24 will be IT5 and the first IT6, and so on.

By way of example, if a single circular permutation is performed per multiframe and if each terminal requires four time slots to change its reception frequency, then it is guaranteed that each terminal will be able to listen to all of the transmitted traffic information after eight circular permutations.

This operation of allocating another time slot to a terminal can also consist in reversing the order of the time slots (switching from the sequence ITO IT1 IT6 IT7 to the sequence IT7 IT6 IT1 ITO) or in providing any other arrangement of the time slots making it possible to ensure that each terminal will have had access to all of the signalling information. For example, it is possible to use a pseudo-random time slot hopping relationship.

These operations amount to performing time slot hopping. The same applies to the frequencies used for transmitting signalling information, with the frequencies used in a given frame for conveying traffic data preferably being identical. A frequency hopping relationship can be provided for changing the frequencies at which the traffic data is transmitted on each frame.

In a second implementation, shown in Figure 4, the change in the position of signalling information relative to traffic data that is transmitted or received by the terminal consists in changing the time slots allocated to the fixed stations for transmitting signalling S 15 information.

In this implementation, instead of allocating another time slot in the traffic frame to the terminal, it is the time slots allocated to the fixed stations for transmitting signalling information that are changed. In 20 this example, as before, a circular permutation of the time slots is performed between times tO and tl. This makes it possible to ensure that all of the terminals transmitting or receiving traffic data in frames T24 and T26 will have access to all of the signalling 25 information, within a determined length of time.

As before, other ways of changing time slot can be used (reversing the order of the time slots, using a pseudo-random sequence algorithm,