AU2065600A - Base transceiver station for cellular mobile radio system and system for synchronizing such base transceiver stations - Google Patents

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Societe Anonyme dite Alcatel Cit Actual Inventor(s): PIERRE DUPUY, CORINNE CHERPANTIER, MAX DOBROSIELSKI Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: BASE TRANSCEIVER STATION FOR CELLULAR MOBILE RADIO SYSTEM AND SYSTEM FOR SYNCHRONIZING SUCH BASE TRANSCEIVER STATIONS Our Ref 610485 POF Code: 1501/76619 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): 1 6ooeq BASE TRANSCEIVER STATION FOR CELLULAR MOBILE RADIO SYSTEM AND SYSTEM FOR SYNCHRONIZING SUCH BASE TRANSCEIVER

STATIONS

BACKGROUND OF THE INVENTION Field of the invention The present invention concerns cellular mobile radio systems.

The present invention is applicable to cellular mobile radio systems including cells of low capacity (for example microcells), for example.

*"*."Accordingly, in a time division multiple access system such as the GSM system, for example, the present invention applies to cells with a capacity limited to eight channels, i.e. seven dedicated channels plus one common channel, for example.

Description of the prior art The architecture of a fixed station, or base transceiver station, for a cell of this kind in the GSM 6 system is shown in figure 1 and the principle of sending by a base transceiver station of this kind is shown in figure 2.

In the GSM system the time division multiple access temporal structure is made up of repetitive frames each including eight time slots IT 0 through IT 7 In addition to traffic data relating to said dedicated channels, a base transceiver station in the GSM system must send control data relating to said common channel in a particular time slot (in this instance the first time slot IT 0 of a frame structure transmitted on a frequency called the BCCH frequency.

The base transceiver station shown in figure 1 therefore includes a transmitter E 0 sending a frequency f 0 (constituting the BCCH frequency) receiving the data to be sent in the various time slots of said time structure, this data being originated by baseband processing means T (encoding and conversion to the GSM system transmission format).

This data includes, in this instance: data forming a common channel CHO conveyed by the time slot IT 0 as shown in figure 2, data forming dedicated channels CH 1 through CH 7 respectively conveyed by the time slots IT 1 through IT 7 as shown in figure 2.

Mobile radio systems use the technique known as frequency hopping which improves transmission quality in the presence of the phenomenon of fading, for example.

This technique requires a minimum set of spectral resources, however; for example, it is not applicable to the base transceive stations of the type shown in figure.

I, since these base transceiver stations have only one carrier frequency.

Document WO 91/13502 discloses pooling frequencies allocated to different cells of the same re-use pattern *of a cellular system for frequency hopping so that the frequency used during a time slot in one cell of the reuse pattern is different from those used in the same time slot in the other cells of the re-use pattern.

In the above document, it is assumed that no overlapping occurs that could produce interference.

However, this application proceeds from the position that this assumption cannot be made, i.e. that the various base transceiver stations of the system cannot be perfectly synchronized, but nevertheless aims to avoid any such risk of interference.

SUMMARY OF THE INVENTION Accordingly, the present invention provides a base transceiver s-ation for time division multiple access type cellular mobile radio systems in which frequencies are shared between cells of the same re-use pattern and which use frequency hopping so that the frequency used at any time in one of the cells of a reuse pattern is different from those used at the same time in the other cells of that pattern, including means for commanding the sending of one of the frequencies shared in this way during a part of the time division multiple access frame and for commanding the stopping of all sending during the other part of the frame, said other part of the frame having a duration at least equal to the relative synchronization error between base transceiver stations of the system.

An aim of the present invention is to alleviate the previously stated problem of modifying as little as o• "-possible the architecture of the base transceiver stations compared to their current architecture as -shown.

in figure 1, without losing capacity relative to that current architecture.

Accordingly, in accordance with another feature of the invention, said part of the frame having a duration at least equal to the relative synchronization error between base transceiver stations is the first time slot

(IT

0 of that frame.

Moreover, in the application to the GSM system referred to above, in which said frame part of duration at least equal to the relative synchronization error between base transceiver stations consists of the first time slot (IT 0 of that frame, some synchronization of the base transceiver stations of the system ,is nevertheless needed; for example, a maximal synchronization error equal to half a time slot might be tolerable.

-4- A preferred embodiment of the present invention will now be described according to the accompanying drawings wherein:- Figure 1, described above, shows the prior art architecture of a base transceiver station previously referred to.

Figure 2 shows the principle of sending by a base transceiver station as shown in figure 1.

Figure 3 shows the principle of sending by base transceiver stations of the invention.

Figure 4 is a block diagram of a base transceiver station of the invention.

Figure 5 shows the general architecture of a cellular mobile radio system such as the GSM system, for example.

Figure 6 is a block diagram showing the means employed in a mobile station in one embodiment of a base transceiver station synchronization system of the invention.

Figure 7 is a block diagram showing the means employed in a subordinate base transceiver station in one a MP C:y DoCflmetSXMARIEXGANODEL 65832d.DOC embodiment of a base transceiver station synchronization system of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 3 shows the principle of sending, in accordance with the invention, by base transceiver stations in the same re-use pattern, for example two base transceiver stations BS and BS'.

The time structure used for sending by each of these base transceiver stations is shown. As in figure i, the time slots IT 0 through IT 7 of two consecutive frames are shown, by way of example.

Note that the base transceiver stations BS and BS' are not perfectly synchronized and that the relative synchronization error is in this instance in the order of 15 half a time slot.

The BCCH frequency on which each base transceiver station sends is also shown, in this instance f 0 for the base transceiver station BS and f' 0 for the base transceiver station BS'.

20 As shown in figure 3, the com-mon channel CH 0 for each base transceiver station is sent during time slots

IT

0 on the BCCH frequency of that base transceiver station and dummy bursts DB (padding data) are sent during the time slots IT 1 through IT 7 on the BCCH frequency of each base transceiver station.

Figure 3 also shows the frequency at which the dedicated channels CH 1 through CH 7 are sent by each base transceiver station.

In this example, for each base transceiver station the frequency for sending the dedicated channels, chosen from the set of frequencies shared by the various cells of the re-use pattern concerned, changes from frame to frame and for each frame remains unchanged during the time slots IT 1 through IT 7 no sending taking place during the time slots IT 0 For example, the sending frequency changes from fl to f 2 for the base transceiver station BS and from f 2 to f 3 for the base transceiver station BS' from one of the two frames shown to the other.

The fact that the two base transceiver stations concerned are not perfectly synchronized with each other does not generate interference here, as would have been the case for the frequency f 2 in the example shown, since no sending takes place during the time slot IT 0 Also, there is no loss of capacity for the base transceiver stations relative to the situation shown in figure 1, since the time slot IT 0 is lost to traffic in any event.

ooo.

The modifications required relative to the architecture shown in figure 1 are reduced since the baseband processing means T can be the same.

Figure 4 shows one example of the architecture of a base transceiver station of the invention.

Compared to that shown in figure i, this base S 20 transceiver station includes additional sending means which, in the example shown, consist in a transmitter E controlled by control means C.

"The commands that the transmitter E receives from the control means C include, in this example, a command to stop sending during the time slots IT 0 of the frame structure, as explained with reference to figure 3. More generally, to achieve the desired effect, namely to avoid interference, this command to stop sending would be executed for a time period at least equal to the relative synchronization error between the base transceiver stations of the system concerned.

Means CG are also provided for generating dummy bursts to be sent during time slots IT 1 through IT 7 of the frame structure carried by the BCCH frequency.

The data from the baseband processing means T and from the means G for generating dummy bursts is routed to the transmitters E 0 and E by switching means COM in accordance with the principle described above and shown in figure 3.

Although the foregoing description corresponds more particularly to the case of cells with a capacity limited to eight channels (seven dedicated channels plus one common channel), by way of example, the invention is not limited to an application of this kind.

There will now be described one embodiment of a S. base transceiver station synchronization system of the invention, starting with an outline description of the general architecture of a cellular mobile radio system •g such as the GSM system, for example, given with reference S 15 to figure Each cell is ezuipped with a transmitter-receiver station BTS 1 through BSTn called the base transceiver station.

Each base transceiver station can be connected by a radio link to a mobile station MS 1 through MSm for the purposes of a call involving that mobile station.

A set of base transceiver stations BTS 1 through BTSn is also connected by cable to a base transceiver station controller BSC and a set of base transceiver station controllers BSC is connected to a mobile services switching center MSC, the combination of the MSC and the BSC handling call management, for example management of the signalling relating to calls.

The base transceiver stations are rendered plesiochronous through the intermediary of the mobile services switching center MSC connected to the cable telephone network. The clocks of the base transceiver stations therefore have the same frequency but can have any relative phase.

Means are additionally provided to enable a 8 reference base transceiver station to impose its timing reference on another base transceiver station, called the subordinate base transceiver station, in order to compensate the phase difference.

In accordance with the invention, and as shown in figure 6, these means comprise: means 1 enabling a mobile station to listen to said reference base transceiver station and said subordinate base transceiver station, means 2 for deducing the time shift D 1 between those two base transceiver stations, as seen from the mobile station, means 3 for determining, on the occasion of a change of cell (or "handover") involving said reference S 15 base transceiver station and said subordinate base transceiver station, the propagation time difference D 2 between the mobile station and each of the base transceiver stations, means 4 for determining from the values D 1 and D 2 20 the time shift to be corrected in order to synchronize the subordinate base transceiver station to the reference base transceiver station.

S"The means 1 enabling a mobile station to listen to a base transceiver station (reference or subordinate) are conventionally provided in a system like the GSM system, for example, to enable the mobile station to synchronize to the base transceiver station. The means 1 are thus means routinely provided in a mobile station and will therefore not be described here.

For example, if the mobile is communicating with either of these base transceiver stations (for example the reference base transceiver station, in which case the mobile station is synchronized to that base transceiver station), it listens to the other base transceiver station, in this instance the subordinate base transceiver station, in order to pre-synchronize to the latter ready for a possible handover.

It is then possible to determine in the mobile station, in this instance by the means 2, the time shift

D

1 between these two base transceiver stations. As the implementation of the means 2 will be obvious to the person skilled in the art, it will not be described in detail here.

The time shift D 1 is nevertheless a time shift as seen from the mobile station, and it is therefore necessary to correct it for the propagation time :ego difference D 2 between the mobile station and each of the base transceiver stations.

In a TDMA mobile radio system like the GSM system, the propagation time difference is generally calculated on the occasion of a handover, to determine how the timing advance applied to the mobile station in this type *l of system must be modified. Thus the means 3 are also means routinely provided in a mobile station and will not 20 be described here.

o*oo Given the time shifts D 1 and D 2 subtracting means 4 determine the difference D 1

D

2 that constitutes the time shift to be corrected in order to synchronize the subordinate base transceiver station to the reference base transceiver station.

The difference D 1

D

2 is calculated in the mobile station, for example. This difference could be transmitted as it stands by the mobile station to the subordinate base transceiver station.

In a different embodiment now described with reference to figure 7, to avoid consuming too much transmission resource at the air interface between the mobile station and the subordinate base transceiver station the mobile station transmits to the subordinate base transceiver station BTS s only the fractional part of that difference, which in practise in a system like the GSM system requires only one signalling message Ml, rather than several.

In this embodiment, the reference base transceiver station BTSr transmits the local time t(BTSr) of the station BTSr to the subordinate base transceiver station BTSs at time t in a message M 2 This message M 2 reaches the subordinate base transceiver station BTS s at a time t(BTS s such that: t(BTSs) t(BTSr) Atl At 2 where At 1 is the transmission time between the stations BTSr and BTS s and At 2 represents the time shift

D

1

D

2 to be corrected in order for the base transceiver station BTSs to be synchronized to the base transceiver 15 station BTSr.

For example, if it is assumed that: t(BTSr) 200.1 s At 1 0.6 s At 2 150.8 s S: 20 then: t(BTSs) 200.1 0.6 150.8 351.5 s In the base transceiver station BTS s subtractor means 5 receiving the content t(BTSr) of the message M 2 and the local time t(BTSs) of the base transceiver station BTSs, supplied by a clock 6, deduce that it is advanced by approximately 351.5 200.1 151.4 s relative to the base transceiver station BTSr (in this example).

Additionally, the mobile station indicates to the base transceiver station BTSs in a message M 1 the fractional part PF(At 2 of At 2 which is 0.8 s (in this example).

If it is also assumed (as is in fact the case in a system like the GSM system) that the transmission time AtI is less than or equal to 0.9 s, then calculation 11 means 7 in the base transceiver station BTSs deduce the value At 2 which is equal to the largest number ending 0.8 less than 151.4, in this instance 150.8 s.

This value At 2 is applied to a control input of the clock 6, which enables the base transceiver station BTS s to be synchronized to the base transceiver station BTSr.

Accordingly, in the example given, the base transceiver station BTSs delays its clock by 150.8 s.

Note that although the synchronization of base transceiver stations in accordance with the invention is advantageously achieved by means of a synchronization system of this kind, such synchronization could equally well be obtained with a different synchronization system, might not be necessary.

C

Claims (5)

1. A base transceiver station for time division multiple access type cellular mobile radio systems in which frequencies are shared between cells of the same re-use pattern and which use frequency hopping so that the frequency used at any time in one of the cells of a re-use pattern is different from those used at the same time in the other cells of that pattern, including means for commanding the sending of one of the frequencies shared in this way during a part of the time division multiple access frame and for commanding the stopping of all sending during the other part of the frame, said other part of said frame having a duration at least equal to the relative synchronization error between base transceiver stations of the system.

2. A base transceiver station as claimed in claim 1 wherein said part of said frame having a duration at least equal to said relative synchronization error S- between base transceiver stations of the system is the first time slot of said frame.

3. A base transceiver station as claimed in claim 2 further including means for sending data forming a common, channel during said first time slot on a BCCH frequency transmitted continuously.

4. A base transceiver station as claimed in claim 3 further including means for sending dummy bursts during time slots other than said first time slot on said BCCH frequency. MP CAMy DocumentsUAARIE\GABNODEL\65832d.DOC

13- A base transceiver station for time division multiple access type cellular mobile radio systems substantially as herein described with reference to Figures 3 to 7 of the accompanying drawings. DATED: 2 March, 2000 SOCIETE ANONYME DITE: ALCATEL CIT By: PHILLIPS ORMONDE FITZPATRICK Patent Attorneys per: 000 *o sees MP W:VmarieGABN0DELM582d.d0C