Method for determining timing advance in a cellular radio communication system

The field of the invention is that of radio systems digital cellular with mobile, including but not limited to, those designed according to the GSM public mobile radio standard.

By GSM standard is meant here both the GSM standard 900 ( "Group Special Systems for Mobile communications in the band of 900 MHz ") that the DCS 1800 standard (" Digital Cellular System "operating in the 1800 MHz band).

More specifically, the invention relates to a method of determining a timing advance in a cellular radio system, and a transfer method and an intercellular localization method using requiring such information timing advance.

The following description uses the terms commonly used in the GSM terminology. For more information, one can refer to the magazine "Digital Cellular Mobile Communication Seminar "seminar held on it from 16 to 18 October 1990 in Nice.

In general, a digital cellular mobile radio system is implemented within a network of geographical cells traversed by stations mobile. A base station is associated with each cell and typically uses several carriers. A mobile station communicates via its base station current, that is to say from the base station associated with the cell in which it is located.

Conventionally, each carrier is segmented temporally into using a fixed pattern division multiplexing in time (TDMA). according to this scheme, the time axis is divided into successive frames of fixed duration. each of frames is itself divided into a fixed number of slots. The recurrence of a particular time slot in each frame is a channel physical which can be multiplexed multiple logical channels.

One of the problems is the GSM synchronization time intervals assigned to a mobile station relative to the master clock from its current base station. Indeed, to prevent a mobile station transmits data over an interval of time allocated to another mobile station, it is necessary to consider the time of propagation of radio waves between the mobile station and the current base station, thereof each having a bit clock which is their own.

When a mobile station wants to connect to a base station, she said sends an access signal ( "Access Burst"). On receipt of this signal, the base station calculates timing advance information (or TA for "Timing Advance" in language English) associated with the mobile station, and sends a message telling her she shall transmit its signals with an advance of TA with respect to its clock signal, of to avoid any overlap with signals from other mobile stations. By Subsequently, the timing advance TA is updated frequently to keep account a possible displacement of the mobile station relative to the base station.

Furthermore, it should manage the handover (or "handover" in language English), that is to say the passage of a mobile station from one cell to another. This means changing the timing advance information TA, so that the mobile station syncronise on the base station of the new cell.

We know several types of handover, whose characteristics are now briefly recalled.

Synchronous handover assumes that the clocks of various base stations are synchronous. Thus, when a mobile station moves from one cell to another, it is not no need to provide a new timing advance since it is immediately deducted from that used previously. This solution, because it need to synchronize all base stations, is very expensive.

The pseudo-synchronous handover can synchronize a mobile station base station to the clock of the new cell by taking into account the offset timing between the base stations of the old and the new cell. This solution is complex to implement and requires a learning phase for radio subsystem (BSS).

The asynchronous handover "vector" is such that when the mobile station moves a first to a second cell (associated respectively with a first and a second base station) a channel (target) of the second base station is allocated to the mobile station; the mobile station sends to the second base station on the target channel, a synchronization signal sequence (HANDOVER ACCESS); the second base station, upon receipt of the frame synchronizing signals, calculates the timing advance information and sends it in a message (PHYSICAL INFO), the mobile station; the mobile station synchronizes and communicates with the second station Basic noting the new timing advance; finally, when the procedure is completed, the first base station releases the previously allocated channel in the mobile station.

The asynchronous handover "classic" is the most common solution met and easiest to implement. However, it has the disadvantage major to induce a break in the communication too long to be masked by a speech extrapolation software. Indeed, the extent of the advance information time by the second base station has a relatively long period (about 40-80 ms), during which the mobile station may continue to transmit data.

The asynchronous handover "improved" is specifically intended to reduce the time which communication is cut off during a handover. For more details, we can refer to the French patent FR 92 10996, for a "transmission method a timing advance in a mobile operating in the cells of a network GSM BTS asynchronous ", included in this descripiton by reference.

The asynchronous handover "improved" differs from asynchronous handover "Vector" essentially in that the mobile station returns to its communication on the original channel, with the first base station immediately after sending the second base station the frame synchronizing signals. Then, and only when she received the new timing advance sent by the second base station, it will synchronize and communicate on the target channel, with the second base station. Thus, the failure of communication between the mobile station and the first base station is limited to the duration of emission of the signal sequence of synchronization, and may be masked by a speech extrapolation software. By Moreover, the transfer of communication from the first to the second base station is very fast and can also be hidden.

However, the asynchronous handover "improved" as implemented currently does not have an optimal use of resources.

Indeed, the target channel of the second base station is now allocated to the mobile station at the start of the handover procedure. So it is generally a traffic channel (TCH or for "Traffic CHannel" in English), that is to say, a useful resource. However, until used by the mobile station to synchronize and communicate with the second base station, this useful resource is monopolized only for the transmission of the signal sequence of synchronization.

Furthermore, it happens that the handover procedure is not carried to completion, the target channel then being released without receiving communication with the mobile station. Currently, in such a case, the useful resource of the target channel is monopolized unnecessarily.

Also known localization methods based on knowledge of several (at least two, and usually three) timing advance information for a same mobile station vis-a-vis a plurality of base stations. Indeed, knowing the speed of propagation of a radio wave (300m / us), each information timing advance provides an indication of distance from a base station data. By combining several of these distance specifications (usually by triangulation), it is possible to obtain a location information of the station mobile.

Conventionally, each of the timing advance information is obtained from the follows: a channel (target) of the base station concerned is allocated to the station mobile; the mobile station sends to the base station concerned, the target channel, a frame synchronizing signals; Finally, the base station concerned, the receiving the frame synchronizing signals, calculates the advance information time and sends it in a message, to the mobile station.

As currently implemented, these processes present location not an optimal use of resources. Indeed, to calculate each of each timing advance information, the target channel of the base station concerned is today a traffic channel (TCH or channel) or a standalone dedicated control channel (or SDCCH, for "Stand-alone Dedicated Control CHannel" in English), that is to say, a useful resource. In other words, location is supported at an cost radio in terms of useful resources, relatively high.

The invention particularly aims to overcome these various disadvantages of the state of the art.

More specifically, one of the objectives of the present invention is to provide a method of determining a timing advance in a system cellular radio, the method for optimizing the use of useful resources (and especially of TCH and SDCCH channels).

This object, and others which will appear later are achieved according the invention using a method for determining a timing advance in a cellular radio system, comprising the steps following:<ul><li>a mobile station interrupts its communication on a current channel, with first base station to send a second base station on a first target channel, a synchronization signal sequence allowing said second base station to calculate a timing advance associated with said mobile station vis-a-vis said second base station,</li><li>the mobile station resumes its communication on said current channel, with first base station immediately after sending to the second station base sequence of said synchronization signals,</li><li>said timing advance calculated by said second base station is transmitted via at least one controller, to said mobile station,</li></ul>characterized in that said first target channel is a common access channel in a plurality of mobile stations.

The general principle of the invention is therefore to provide, in the second station basic, common access channel for all mobile stations whose information timing advance to be calculated. In other words, each mobile station does sees not affect its own target channel but shares a target channel with other stations mobile.

Thus, it greatly reduces radio resources required for determining the second base station of the timing advance information from different stations mobile.

Preferably, said first target channel uses an uplink whose way down associated is also used by another channel.

In other words, using a hitherto unused channel. In this way, the determining the temporal information of the mobile station vis-à-vis the second base station monopolizing any useful resource (including either or channel TCH SDCCH).

Advantageously, said other channel is a broadcast channel.

Advantageously, said first target channel is inserted within a structure multiframe the same manner as the uplink channel of a standalone dedicated control channel.

Advantageously, each mobile station accessing said first target channel is seen assign a separate access reference.

In this way, the second base station knows that each mobile station him sends the first target channel.

Preferably, access to said first target channel is managed in a manner predetermined access.

This prevents collisions on the first target channel between signals from different mobile stations.

Preferably, said predetermined access method is a method sequential access-type "start time".

This method is currently used in GSM, in particular for the redefinition frequency, where it can indicate a mobile station at what moment it must use a new set of frequencies.

The invention also relates to a handover method type asynchronous in a cellular radio system, characterized in that comprises the method steps of determining a timing advance information as mentioned above,    and in that it further comprises a step in which said mobile station, in taking into account the timing advance information thus calculated and transmitted by said second base station synchronizes and communicates with said second station second base on a separate target channel said first target channel.

Thus, the handover method of the invention also constitutes an improvement of the asynchronous handover "enhanced" of the prior art, already discussed above. Indeed, it is in distinguished in that, for each mobile station, two target channels (and not just one target channel) of the second base station are successively used, namely on the one hand a first target channel common to the plurality of mobile stations and used only for the determination of the timing advance, and secondly a second target channel, unique to the mobile station and used by the latter only after the second base station sent him the timing advance.

In terms of useful resource, including TCH, the handover method of the invention therefore saves the useful resource of the second channel target, replacing it by the first target channel (which it is not a resource useful) throughout the first part of the handover procedure corresponding to the determination of the timing advance.

The invention also relates to a mobile station locating method in a cellular radio system, characterized in that it comprises, repeated at least once, the steps of the method of determining an advance information time as mentioned above, so as to obtain, in addition to the timing advance information associated with said mobile station vis-a-vis of the first base station, at least one timing advance information associated with said mobile station vis-a-vis at least one second base station,    and in that it further comprises a step of calculating an information location of said mobile station from a share of advance information temporal and secondly including location information of each of the first and second (s) station (s) affected by these basic advance information time.

Thus, the location method of the invention can be implemented without cost in useful resource, since the first target channel is not a useful resource.

Other features and advantages of the invention will become apparent from reading the following description of a preferred embodiment of the invention, given as an indicative and non-limiting example and the accompanying drawings, wherein:<ul><li>Figure 1 shows partially the structure of a network GSM radio;</li><li>2 shows a flowchart of the method according to the invention determining a timing advance;</li><li>3 shows a flowchart of the method according to the invention handover; and</li><li>4 shows a flowchart of a particular embodiment the method according to location invention.</li></ul>

In the following description, consider the case of a system of GSM radio. It is clear however that the invention is applicable to any type of digital cellular radio system.

is first reminded in relation with Figure 1, the structure of a network GSM. A mobile station MS operates in a cell C1 corresponding to the coverage geographical of a base station BTS1. Other cells C2, C3 are each associated with another base station BTS2, BTS3. The base stations BTS1 to BTS3 are managed by a base station controller BSC. The combination of a controller BSC and base stations of a number of base stations constitutes a system of base stations BSS. Several base stations BSS systems can be controlled by a switching center MSC, which is the master structure of a GSM network.

Will now be described in connection with the flowchart of Figure 2, the method according to the invention for determining a timing advance. In the example used for explanation purposes, we consider that the MS is in the cell C1 (it communicates with the base station BTS1) and try to determine its timing advance (or TA for "Timing Advance") vis-à-vis the station basic BTS2 of cell C2.

In step 1, the MS interrupts its communication on a channel current, with the base station BTS1. In step 2, the mobile station MS sends to the base station BTS2, a first target channel, a signal sequence of synchronization (type HAND OVER ACCESS). In step 3, immediately after sending to the base station BTS2 the frame synchronizing signals, the mobile station MS returns to its communication on the current channel with the base station BTS1. In step 4, performed simultaneously with step 3, the base station BTS2 calculated the timing advance TA associated with the mobile MS vis-a-vis the station base station BTS2. In step 5, the BTS2 base station transmits to the mobile station MS, via at least one BSC, the calculated timing advance TA. AT step 6, symmetrical to the step 5, the mobile station MS receives, via at least one controller BSC, the timing advance information TA calculated and transmitted by the base station BTS2. All of these steps is referenced 20.

According to an essential characteristic of the invention, the first target channel is a common access channel, that is to say can be used by all mobile stations whose TA timing advance information to be calculated by the base station BTS2.

This first target channel used eg uplink whose way associated downlink is also used by a broadcast channel (or channel CBCH, for "Cell Broadcast CHannel" in English). It fits for example in a multiframe structure in the same manner as the uplink of a dedicated control channel autonomous (or SDCCH, for "Stand-alone Dedicated Control CHannel" language English).

is also possible to predict the allocation of a separate access reference (type "Handover Reference") to each mobile station MS accessing the first target channel. Thus, in the application to the handover (explained in more detail below in connection with Figure 3), each access reference assigned to a mobile station allows for the link between on the one hand the messages (such as "HANDOVER COMMAND") sent by the system, including the base station BTS2 to the mobile station, and secondly the messages (such as "HANDOVER ACCESS") sent by the mobile station to the base station BTS2, the first target channel.

Optionally, access to the first target channel can be managed by a method predetermined access, such as for example the sequential access method type "Start time". As used herein, this method, known as the "starting time" in English language is to indicate to each mobile station MS the number of the frame to from which it can transmit on the first target channel without collision with emissions of other mobile stations.

As shown in the flowchart of Figure 3, the method according to the invention handover, which is the asynchronous type, include:<ul><li>the assembly 20 of steps 1 to 6 of the method determining information timing advance described above in relation to Figure 2; and</li><li>a step 30 during which the mobile station MS, taking into account the timing advance TA calculated and transmitted by the base station BTS2, synchronizes and communicates with the base station BTS2, a second target channel.</li></ul>

It is important to note that the second target channel, for example a TCH channel, is specific to the mobile station and perfectly distinct from the first target channel (which it is common to all mobile stations).

As shown in the flowchart of Figure 4, in one embodiment particular, the method according to the location of the invention comprises:<ul><li>the assembly 20 of steps 1 to 6 of the method determining information timing advance described above in relation to Figure 2;</li><li>a reiteration 20 ', but applied to the base station BTS3, of the assembly 20 Steps 1 to 6 of the method for determining an advance information temporal described above in connection with Figure 2. Thus, further information timing advance TA vis-à-vis BTS2, the other information is obtained timing advance TA 'vis-à-vis BTS3; and</li><li>a step 40 of calculating a mobile station MS location information from the timing advance information calculated and information of location of each of the base stations.</li></ul>

In the example shown, the calculation of step 40 is performed by a method any triangulation (well known to those skilled in the art), since it has three timing advance information, namely TA and TA 'vis-à-vis BTS2 and BTS3 respectively, and TA ", vis-a-vis BTS1. Indeed, the mobile station knows the timing advance vis-à-vis BTS1 since the latter is the station basic current memory cell.

It is clear that other types of calculations may be envisaged without departing from the scope of the invention. In other words, more than three timing advance information may be used, or even only two. In the latter case, it should be provide further decision means considering other information (such that measures the displacement of the mobile station using sensors, or map information).

above was presented two applications of the process according to the invention determining a timing advance information, namely the handover (see Fig.3) and location (see Figure 4). It is clear that other applications can be envisaged without within the scope of the invention.