AU701478B2

AU701478B2 - Handover method for mobile communications

Info

Publication number: AU701478B2
Application number: AU33169/95A
Authority: AU
Inventors: Vinod Kumar; Fabrice De Seze
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1994-10-26
Filing date: 1995-10-11
Publication date: 1999-01-28
Anticipated expiration: 2015-10-11
Also published as: EP0710044A3; FR2726424A1; CA2161395A1; FR2726424B1; EP0710044A2; AU3316995A; FI955047A; FI955047A0

Description

E~ii P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990 ooo o o o o o r o re

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a o s r r r o r

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "HANDOVER METHOD FOR MOBILE COMMUNICATIONS" The following statement is a full description of this invention, including the best method of performing it known to us:i 2 This invention relates in general to a handover method applicable in a cellular mobile radio network. Such a handover consists of switching a call established with the mobile from a current channel associated with a first cell to a second channel associated with a second cell while the mobile is moving, in view of ensuring call continuity.

The invention is applicable to synchronised cellular mobile radio networks. A cellular network is said to be synchronised when base stations within the network are inter-synchronised with a time gap almost nil. The elementary time intervals carrying the channel transmitted and received by any two base stations in the radio network are inter-synchronised.

Furthermore, the invention aims at increasing the "security" of such a .o"o handover which is a delicate step in the management of a call established with a mobile, by preventing this call from major downgrading during handover.

*13 A handover method is known according to which a call established with a S' mobile is switched from a first channel forwarded by a first base station in a first cell, to a second channel which is forwarded by a second base station in a second cell and preferably identical to the said first channel. In the GSM context, the identity of the first and second channels is translated by the same time slots and same frequenciGs.

"2Q According to this document, a test is designed to check the availability of the said second channel in the second cell prior to handover. Furthermore, ii is established i that, temporarily prior to handover, the call is transmitted simultaneously in the first and second channels of the first and second cells respectively. This simultaneous col transmission by the two base station to the mobile aim at preventing accidental interruption of the call during the critical handover phase, when the mobile is located at a maximum distance from both base stations, therefore, when the power of the signals received by the mobile and coming from these two base stations is at its minimum.

In a synchronised network, knowing that the difference in synchronisation between two base stations is nil, the time gap between the reception by a mobile of the same data transmitted simultaneously over two channels by two base stations

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respectively, is limited to the difference in propagation times, or the difference of operating time between the mobile on the one hand, and each of the two base stations on the other hand. This gap is inhibited by equalisation filtering techniques in the mobile, if it remains lower than a set value, the two base stations then operate in diverse transmission for the mobile.

According to this known method, an increase in the success rate of intercell handovers over the same channels can only be obtained to the detriment of a reduced optimisation of the frequency re-utilisation cellular plan. Indeed, a high success rate of handovers over the same channels requires identical frequencies in neighbouring cells, and these frequencies cannot be used simultaneously in two neighbouring cells to prevent inter-channel interference.

For cells rel i, ly small in relation with the product of the mobile average speeds within these ceils by the average call duration, handovers are frequent and a imply that, for these handovers, a number of identical channels be reserved in neighbouring cells, this number being relatively large in relation with the total number of channels within each cell, hence, it reduces the spectrum efficiency of the frequency S. re-utilisation plans.

The present invention aims at overcoming the above mentioned drawback of the previous art, according to which there exists a conflict between a high success rate of intercell handovers over identical channels and the re-utilisation of a much reduced number of identical channels in adjacent or neighbouring cells.

To this end, according to the present invention, there is provided a handover method for a cellular mobile radio network including an intercell call handover stage whereby a call established with a mobile is handed over from a first channel associated with a first base station to a second channel which is associated with a second base station and identical to the said first channel, wherein said intercell call handover is preceded by the following stages: selection of the said first channel such that the second channel identical to it, be available within the second base station, and intracell call handover from the current channel to the said first channel selected, the said current and selected channels being both associated with the said I I-1. -r <4" C, 8-DEC-98 TUE 16:01 P, 04 00~9 o 04D o 0**Soo 0 *o p *r p 0 0p

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to S first base station.

Preferably, the intercell handover stage is followed in the said second base station by an intracell handover stage whereby the established call is switched from the said second channel to the next available channel making up a current channel within the said second base station.

Advantageously, the said first and second base stations simultaneously transmit the data pertaining to the said established call into the said first and second identical channels respectively.

According to a first alternative, simultaneous call data transmission by the first and second base stations occurs based on the measurement of the time gap between the data transmitted by the mobile over the first channel and the second channel respectively.

According to another alternative, simultaneous call data transmission by the first and second base stations occurs in response to the second base station receiving 15 a signal transmitted by the mobile over the second channel whose level is greater than a preset threshold, the second base station having been message-activated or electrically activated prior to receiving this signal.

The invention also provides a base station controller used to implement the method. The controller contains some means of receiving from the mobile, some 20 signal level measurement messages which are received by the mobile from the respective base stations to trigger the handover stages. The controller is characterised in that it contains a table listing the availability of channels identical in several base stations, so as to select the said first channel within the said first cell which is identical to the second channel available in the second cell during the said intracell hindover stage within the said first base station.

In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to the accompanying drawings, in which: Figure 1 is a schematic representation of the handover stages according to the invention, each cell being represented in the form of a Cartesian diagram of channels forwarded by the respective base station associated with the said cell; and Figure 2 represents three geographical areas, roughly hexagonal, associated

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with three respective cells, to explain the implementation of simultaneous transmission of a call by two base stations involved in a handover.

Figure 1 shows three groups of channels G(n) and G(n+l 1), respectively associated with three base stations BTS(n-1), BTS(n) and BTS(n+ The invention is described in the particular context of a cellular digital mobile radio network, such as GSM, and a channel designed to carry an established call is defined by a couple: Frequency/Time Interval IT). For simplicity reasons, we have only shown the upward, respectively downward, channels, for links established with the mobiles. The corresponding downward, respectively upward, channels can easily be deduced by frequency and time translation from the Cartesian diagrams representing the groups of channels G(n) and G(n+ The term "channel" is not limited to a frequency/time interval couple, and depending on other implementations, may consist only of a time interval or only of a frequency, or of a sequence of S Frequency/Time Interval couples when dealing with the frequency jump technique.

According to figure 1 implementation, given as a non limiting example, each 1 base station BTS(n-1), BTS(n) and BTS(n+ 1) is defined by a group of (8 x 6) upward channels associated with the Cartesian product of 6 frequencies and 8 time intervals ITO to IT7. Base station BTS(n-1) provides the six frequencies FO, F1, F2, F3, F4, Base station BTS(n) provides the six frequencies FO, F6, F7, F8, F9, F10 and base station BTS(n+1) provides the six frequencies FO, F11, F12, F1 3, F14, p Each base station contains at least one channel identical to another channel within a base station which defines a cell adjacent to the cell associated to the said base station. Within the context of the implementation described, the number of channels identical within the three base stations is equal to eight, and these eight channels are defined by the Cartesian product of a frequency FO common to the three stations by eight time intervals ITO, IT1, IT2, IT3, IT4, IT5, IT6 and IT7.

Arrows drawn in a continuous and in a dotted line show the stages of the method according to the invention.

Initially, the mobile is connected (CON stage) to a channel defined by the couple (F3, IT2) among a set of channels associated with the base station BTS(n-1), called first base station. As is known in GSM, the mobile periodically transmits, to this r 1- -7 8-DEC-98 TUE 16:01 6 first base station BTS(n-1) to which it is connected, some signal level measurement messages received from neighbouring base stations. These messages are transmitted over the SACCH channel (Slow Associated Control CHannel) to the radio interface between mobile and ase station, to be sent up to a mobile network infrastructure, being typically a base station controller (not shown).

Upon the initiative of this base station controller, and based on the level measurement messages received, a handover decision is made. This handover consists of switching the call established with the mobile from the current channel (F3, IT2) associated with the first base station BTS(n-1) to the next channel (F7, associated with the second base station BTS(n), the latter being identified based on level measurement messages as providing optimum reception level for the mobile.

According to the invention and referring to figure 1, this handover consists of three successive stages S1, S2 and SO. The first stage (S1) consists of switching the established call intracell. According to stage S1, the established call is switched from 15 the current channel (F3, IT2) to the first channel (FO, IT3) which is identical within, or common to, the two base stations BTS(n-1) and BTS(n) among all channels [FO x (ITO, I IT1, IT7)], and which is also available in the first and second base stations. To this end, it is reminded that the frequency FO is common to both base stations BTS(n-1) and BTS(n). This intracell handover stage is preceded by a preliminary stage whereby 20 the base station controller selects a channel. The controller, contains a table listing the availability of channels common to, or identical within, the set of base stations it controls, this table being updated at the beginning and end of each handover.

Thanks to this table, the first channel (FO, IT3) to which the call is handed over intracell, from the current channel (F3, IT2), is selected according to the selection stage such that the second channel which is identical to it, be available in the second base station. It should be noted that this intracell handover stage does not lead to the conventional drawbacks encountered during intercell handover with regard to mobile synchronisation, since the mobile is already synchronised with base station BTS(n-1).

Indeed, the two respective channels (F3, IT2) and (FO, IT3) from which and to which the call is handed over belong the same base station BTS(n-1).

.P 06 8-DEC-98 TUE 16:02 6 7 Following this first stage the invention proposes a second stage (52) of the type described in the patent application mentioned in the foreword. According to this second stage there is an intercell handover of the established call from the first channel (FO, IT3) associated with the first base station BTS(n-1) to the second chcnnel (FO, IT3) identical to the said first channel and associated with the second base station BTS(n).

As an advantage, and in view of reducing the risk of interrupting the established call, there can be simultaneous call data transmission by the first and second base stations BTS(n-1) and BTS(n) respectively to the first and second identical channels (FO, IT3). This simultaneous transmission covers timewise, at least a part of the intercell handover stage, or second stage and is activated after the intracell handover stage (S1) in the first base station. The invention proposes two alternatives to activate the simultaneous transmission of the established call by the two base oo' stations BTS(n-1) and BTS(n). Typically, all cells within the mobile network are of 15 roughly identical dimensions. In practice, the difference between the respective sizes S of two cells is limited by a tolerance set by the equalisation filtering technique in the .mobile terminal. Equalisation tolerance is defined by the maximum operating time difference admissible between two identical signals received by the terminal. For an operating time difference smaller than the maximum admissible operating time 0* 20 difference, equalisation aims at absorbing the said operating time difference, thus Saccording to the prior art, solving the problem resulting from multipath propagation.

0 It is typically of 20 microseconds for GSM terminals.

We shall now describe the first alternative of the invention relating to the S" activation of simultaneous transmission of the established call by two base stations BTS(n-1) and BTS(n). Initially, upward call data, transmitted from the mobile to the base station controller, and downward call data, transmitted from the controller to the mobile, are forwarded over the first channel (FO, IT3) and transit via the first base station BTS(n-1), of rank Once this call is forwarded over the first channel (FO, IT3) of base station BTS(n-1), the base station controller, triggers the second base station BTS(n) of rank n to receive over the second channel (FO, IT3) which is identical to the first channel. As a result of the handover decision preceding the intracell RA4 handover stage the infrastructure determines which is the second base station rr u 8-DE.-88 TU "0 0

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5 0i 1, BTS(n). In response, the call continues to be forwarded over the first channel (FO, IT3) associated with the first base station BTS(n-1) and, furthermore, the upward call data transmitted by the mobile are received by the second base station BTS(n) which has been activated to receive. The distances separating the mobile from the first station BTS(n-1) and the second station BTS(n) being different, the upward call data received by the infrastructure via the first base station BTS over the first channel (FO, IT3), and the same upward call data received by the infrastructure via the second base station BTS(n) over the second channel (FO, IT3) identical to the first channel, have an operating time difference. This difference is equal to the difference of propagation times between the mobile on the one hand and the two base stations BTS(n-1) and BTS(n) on the other hand. The controller determines whether the operating time difference measured is lower than a preset threshold. Should the said operating time difference be lower than the preset threshold, the downward call data is simultaneously transmitted to the mobile over the said first and second identical 15 channels (FO, IT3) by the said first and second base stations, BTS(n-1) and BTS(n) respectively, transmission of the downward data over the channel being already effective. The power and quality of the signals received by the first and second base stations BTS(n-1) and BTS(n) over the first and second channels respectively, could also be measured and used to validate the simultaneous transmission stage. Hence, a technique of diversity during transmission is implemented by the base stations BTS(n-1) and BTS(n), thus guaranteeing better quality of the call received by the mobile.

Furthermore, this diversity during transmission is effective only for a given duration in order to prevent scrambling between the data transmitted over the current channel and the following channel. In practice, this given duration is such that the difference in respective propagation times between mobile and base station BTS(n-1) on the one hand, and mobile and base station BTS(n) on the other hand, during the said given duration, is not too high. Two alternatives are proposed for '.election of the given duration. According to a first alternative, the given duration is a preset fixed duration stored and defined in the infrastructure. According to a second alternative, this given duration expires as soon e r J YIR 9 as the operating time difference exceeds a preset threshold that may be identical to the preset threshold triggering simultaneous transmission.

According to a second alternative of the invention relating to the actuation of simultaneous transmission of the established call by two base stations BTS(n-1) and BTS(n), the second base station BTS(n) is also activated beforehand by a message, via the controller, as soon as the first channel (FO, IT3) is allocated to the mobile for the call established with the first base station after the intracell handover. In response to this activation, the second base station listens to the signal transmitted by the mobile over the second channel (FO, IT3) which is identical to the first channel, and transmits over this channel simultaneously with the first base station as soon as the level received over this channel is greater than the preset threshold. The level parameter selected to characterise the link may involve power, quality or other.

I For the above twvo alternatives, it could also be envisaged that intercell 1. handover stage S2 be triggered in response to the processing of at least one of the two signals respectively supplied by the two base station listening to the mobile in Sidentical channels (FO, IT3) in the two cells BTS(n-1) and BTS(n), these two signals corresponding to the same signal transmitted by the mobile. Typically, the processing involves power measurement processing or quality measurement processing.

The expert in the field will see that intracell handover stage S1 and intercell handover stage S2 carried out in accordance with the invention during handover can be activated successively without delay between each, as soon as the need for a handover is detected by the base station controller as a result of the processing of *level messages transmitted by the mobile which listens to channels within neighbouring cells. in this case, it is not envisaged to simultaneously transmit the call data over the two identical channels transmitted by the first and second base stations.

Knowing that the channels [FO x (ITO, IT7)] common to two neighbouring cells are reserved for handover, it is better not to maintain the cali within a common channel, such as (FO, IT3), after handover.

To this end, the invention envisages, according to the third handover stage (SO in figure io switch the call intracell within the second base station, from the Ssecond common channel (FO, IT3) to a channel available outside the set of channels *rrz3lr~ Z I i- i common to these neighbouring cells. This channel is itemised (F7, IT5) in figure 1.

Hence, the channels or resources common to at least two adjacent cells are not unnecessarily occupied by calls in progress, therefore remaining available to later perform other intracell handovers. As shown by stages Si', S2' and SO', the stages in accordance with the invention can be repeated each time, as the mobile successively changes from cell to cell, until the call is disconnected (DEC).

Figure 2 shows three cells C(n) and C(n+1) respectively associated to base stations BTS(n-1), BTS(n) and BTS(n+ and schematically represented in the form of an hexagon. A continuous line represents the path followed by the mobile during the call, while the two segments (dl and d2) within this continuous line (T) represent two sections of the path of the mobile during which the respective base station couples BTS(n-1) BTS(n) and BTS(n), BTS(n+1) simultaneously transmit the 0" established cal! data. These simultaneous transmissions occur when the mobile is 9 J5 located roughly at the boundary of two adjacent cells, corresponding to an area where call reliability is poor. Hence, simultaneous transmission of the call by two base stations reduces the risk of call cut off according to the principle of diversity during transmission.

The expert in the field will be aware that modifications may be made within the context of the invention. In particular, the mobile could be listened to not only by a single second station, in addition to the first station, but by several second stations which would have previously been message activated or electrically activated. In this case, the intercell handover stage is triggered in response to the processing of signals respectively generated by the first base station and several second base stations, and all corresponding to the same signal transmitted by the mobile, this stage aiming at selecting one of these second cells to which the established call is to be handed over.

Claims

1. A method of performing handover from one cell to another in a cellular radiocommunications network for mobiles, said method including an inter-celi call- switching step for switching a call established with a mobile from a first associated with a first base station to a second channel which is associated with a second base station and which has identical mobile radio characteristics to said first channel; wherein said said inter-cell call-switching step is preceded by the following steps: a selection step in which said first channel is selected such that the second channel which is identical to it is free in the second base station; and an intra-cell call-switching step in which said call is switched from a current channel to the selected first channel, said current channel and said selected channel both being associated with said first base station.

2. A method as claimed in clain 1, wherein said inter-cell call-switching step is followed, in said second base station, by an intra-cell switching step in which an 15 established call is switched from said second channel to a free following channel which constitutes a current channel in said second base station.

3. A methoo as claimed in 1 or 2, wherein simultaneous data-transmission in which said first base station and said second base station rimultaneously transmit data from said established call, in respective ones of the identical first and second channels. 20

4. A method as claimed in claim 3, wherein said simultaneous data-transmission, in which data from the established call is transmitted simultaneously by the first base station and by the second base station, is performed as a function of a measurement of the timing difference between data transmitted by the mobile respectively in the first channel and in the second channel.

5. A method as claimed in claim 3, wherein said simultaneous data-transmission, in which data from the established call is transmitted simultaneously by the first base station and by the second base station, is performed in response to the second base station receiving a level of a signal transmitted by the mobile in the second channel, which level is higher than a predetermined threshold, said second base station being activated electrically or by means of a message prior to receiving said level.

6. A base station controler for implementing the method as cla" .ied in any preceding claims, said controller including means for receiving signal level f> 6-DZO-g8 T .16.O 9 12 measurement messages from a mobile, which messages are received by said mobile from respective base stations, so as to trigger steps for performing handover from one cell to another, wherein said controller includes a table giving the state of availability of channels that are ;dentical in a plurality of base stations, so as to select said first hannel in a first cell on which said handover is being performed, which first channel i, .dentical to the free second channel in a second cell, during said intra-cell call- switching step in said first base station.

7. A method substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings. DATED THIS EIGHTH DAY OF DECEMBER 1998 ALCATEL N.V. 1 *0 0* .0 1 f I ABSTRACT The method includes an intercell call handover stage (S2) whereby a call established with a mobile is handed over from a first channel (FO, IT3) associated with a first base station (BTS(n-1)) to a second channel (FO, IT3) which is associated with a second base station (BTS(n)) and is identical to the said first channel. According to the method the intercell call handover stage (S2) is preceded by the following stages: selection of the first channel (FO, IT3) such that the second channel (FO, IT3) identical to it, be available within the second base station and S intracell call handover (S1) from the current channel (F3, IT2) to the first channel selected (FO, IT3), the current and selected channels being both associated with the first base station (BTS(n-1)). Figure to be published: Figure 1. e4 a I I 5

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