NZ248564A - Digital mobile telephony: transmitting timing advance data to mobile to assure synchonism after handover - Google Patents

Abstract

The mobile telephone time advance system hands over from a first base station (BTS1) to a second base station (BTS2) when coverage in the first base area is lost. A mobile telephone (MS) time division multiplexes information, and when a handover command is received a predetermined time advance (TA-BTS2) from the second station is applied reducing the handover time when signals are not received.

Description

24 85 8 Priority Date(s): \s!.3.ia3L Complete Specification Filed: 543.^.3.. Class: Publication Date: P.O. Journal No: 28 MAY 1996 • •••••••••••••«•••#•••••*•' ....I:SfcS?..^, IMff COPY NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "A METHOD OF TRANSMITTING TIMING ADVANCE DATA TO A MOBILE MOVING IN CELLS OF AN ASYNCHRONOUS-BTS GSM NETWORK" WE, ALCATEL AUSTRALIA LIMITED, O^CM °ooCO5 3C3 A Company of the State of New South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 p ( > A i _) * j h'\ This invention relates to transmitting timing data to mobiles moving in a GSM (Special Mobile Group) network when the mobiles pass from one cell to another. The mobiles are for example constituted by radiotelephones onboard vehicles and the timing data must allow a mobile, which is passing from one cell 5 to another, to delay for a given length of time the transmission of digital data which it has to transmit, so as to be in synchronisation with the base transmitter-receiver station of the new cell. In GSM terminology, such a procedure is commonly known as "handover" and the term will be used subsequently.

The following description uses terms commonly used in GSM terminology. For more information, the review "Digital Cellular Mobile Communication Seminar" of the seminar held on this subject from 16 to 18 October 1990 in Nice may be consulted.

Figure 1 represents the structure of a GSM-type network. A mobile, 15 marked MS and for example constituted by a radiotele-phone, is moving in a cell CI constituted by the geographical coverage of a base transmitter-receiver station, marked BTS1. Other ceils C2, C3 each include a base transmitter-receiver station BTS2, BTS3. Each of the stations BTS1 to BTS3 constitutes one of the components of the GSM network and include one or more 20 transmitter-receivers, each combined with an antenna and processing equipment. The geographical coverages partially overlap. The stations BTS1 to BTS3 are managed by a base station controller, marked BSC. The function of the BSC controller is to particularly manage the frequency channels of the BTS 2 24 t>364 stations. A BSC controller associated with a certain number of BTS stations constitutes a base station system (BSS). Other controllers may also be provided, each controlling a predetermined number of BTSstations, and each being connected to a switching station, marked MSC, which is the main structure of a 5 GSM network. A given MSC can thus control the operation of several BSS systems comprising a public land mobile network, commonly known as PLMN.

The operation of such a network is as follows: the mobile MS sends a train of digital data in the form of packets to the station BTS1 as long as it is situated in the cell C1 and the station BTS1 retransmits these trains to their 10 addressee through the switching station MSC. The addressee may be constituted by a mobile or fixed station.

Each packet of data, comprising for example speech data, is transmitted in a time interval of length 577 us, eight successive time intervals comprising a frame. Eight mobile stations MS may therefore communicate on the same radio 15 channel, that is to say with the same carrier wave frequency, due to time division multiple access (TDMA). Usually, tweo to four channels are allocated to each station BTS and 16 to 32 radio channels are therefore available for transmission (as for reception) in each cell.

One of the problems of the GSM system is the synchronisation of the 20 time intervals allocated to a mobile MS with regard to the master clock situated in the station BTS. In effect, it is necessary to take account of the time of propagation of radio waves between a mobile MS and its base station BTS since the mobiles and the station BTS on which thay depend each have an internal bit 3 24 8 5 64 clock which are specific to them. Since a time interval lasts 577 us, and knowing that a radio wave travels 300 m in 1 ps (3 x 10® m/s), the time base of the mobile MS has to take account of this time lag, of 1 us per 300 m distance separating the mobile MS from its station BTS, so as not to transmit data during the time interval allocated to 5 another mobile MS.

Figure 2 is a correlative timing diagram of signals transmitted by the station BTS and the mobile MS.

The station BTS, which manages the coverage in which the mobile MS is situated, continuously transmits a clock signal (tO) which is received by the mobile 10 MS. This signal is transmitted on a specific channel known as SCH (Synchronisation Channel) which transmits synchronisation data to the mobiles. The mobile divides the signal into time intervals of length 577 us and thus the times t0', tT,..., are obtained, the time t0' being allocated to a delay according to the distance between the mobile and the station BTS.

When the mobile wishes to connect with a station BTS, it sends an access signal to the station BTS, generally known as Access Burst, which is characterised by a length less than a normal signal (known as Normal Burst), so that it cannot interfere with another mobile transmitting in the next time interval. On reception of this signal AB (time tO"), the station BTS measures the time TA separating the 20 reception of the transmission from the preceding clock signal (time tO). The time TA corresponds to twice the transmission time of a signal between the mobile MS and its station BTS. The station BTS then sends a signal to the mobile MS to indicate f *\ that it has to transmit its signal with an advance of -> o\ m 4 24 8 i o Q TA relative to its clock signal: the mobile can then transmit normal signals without risk of overlapping those transmitted by other mobiles.

In the case of handover, the Access Burst is known as Handover Access and is marked HA in Figure 2.

Therefore it is assured that the signals transmitted by the various mobiles on a given transmission channel definitely arrive in succession at the station BTS, without there being any overlap of the signals. It is therefore necessary to frequently perform a synchronisation operation on the mobiles, since their distance can change relative to the station BTS.

A well-known problem is that of managing the transition of the mobile MS from one cell to another. In Figure 1, the mobile MS receives not only signals from BTS1, but also from BTS2 and BTS3, and, when the received signal power from BTS1 becomes less than that of the signals received for example from BTS2, the BSC connects the mobile MS to BTS2 which will track the 15 transmission.

This situation is typical when the mobile MS moves away from BTS1 and approaches BTS2. It is then necessary to modify the timing data TA, commonly known as "Timing Advance" so that the mobile is synchronised with the station BTS2 in the new cell C2.

Three types of handover are known which ensure such synchro- nisation: synchronous handover, pseudo-synchronous handover and asynchronous nandover.

Synchronous handover consists in controlling the clocks of various 24 85 6 i, stations BTS in a given GSM system in such a way that their clock signals are synchronous. Thus, when a mobile passes from one cell to another, it is not necessary to provide it with a new timing advance, since this is immediately deduced from that which it used earlier. However, this solution assumes, in 5 order to be generalised, the synchronisation of all stations BTS and its implementation is therefore costly.

To overcome this problem, the pseudo-synchronous handover is used which synchronises a mobile with the clock of the station in the new cell by taking into account the time lag existing between the stations of the original and 10 new cell. However, this solution is complicated to implement and necessitates a learning phase for the BSS system.

The asynchronous handover is the most frequently encountered and the easiest to implement. Its principle is represented in Figure 3. It will be supposed that the mobile MS is leaving the cell C1 to enter the cell 2. Eight successive 15 transmission steps are necessary.

Step 1 is that during which the mobile MS sends a signal MEAS REP to the station BTS1, comparable to a cell change request, containing timing results which are used by the controller BSC to start a handover. This signal is sent by the mobile MS every 0.5s to the station BTS1. In Step 2, the station BTS1 20 transmits this data (signal MEAS RES) to the controller BSC which decides whether a handover is necessary. The decision criteria are notably the quality and power of the received signals. Other types of data are available at the MSC or BSC in order to decide whether a handover has to be made. We will assume 248564 that this situation has actually arisen. In Step 3, the controller BSC activates a channel in the station BTS2 (signal CHAN ACT) and the station BTS2 releases the allocation (signal CHAN ACT ACK). In Step 4, the controller BSC sends a handover command (signal HANDOVER CMD) to the station BTS1 which 5 immediately retransmits it transparently to the mobile MS. The handover procedure then begins at the mobile MS (Step 5) which sends successive HA signals (HANDOVER ACCESS) to BTS2 with a zero timing advance, that is to say without taking distance into account. This step has been previously illustrated in Figure 2. The new timing advance which the mobile MS has to use 10 is not actually known to the mobile MS and it is the station BTS2 which provides it (signal PHYS INFO containing, among others, the instrustion TA). In Step 6, the mobile MS sends a connection signal SABM to the station BTS2 while taking note of the new synchronisation. The station BTS2 informs the controller BSC of it (signal ESTABLISH INDICATION) and signals to the mobile 15 MS (signal UA) that it has been understood. In Step 7, the mobile MS sends a signal HANDOVER COMPLT to the station BTS2 to inform it that the handover procedure is completed and the station immediately retransmits the signal to the controller BSC. The controller BSC then informs the switching centre MSC (signal HANDOVER PERFORMED). In Step 8, the controller BSC is directed to 20 the station BTS1 (signal RF CHAN REL) to make available the time interval previously allotted to the mobile MS and the station BTS1 replies to it (signal RF CHAN REL ACK).

At this stage, the mobile MS converses with the station BTS2 which has 7 allotted to it a time interval in a frame conveyed by a fixed carrier wave, as well as a timing advance.

The main disadvantage of asynchronous handover is that the measurement of the timing advance by the station in the cell in which the mobile is entering is relatively long and requires about 40 to 80 ms, during which the mobile is unable to continue to transmit data. Furthermore, the transmission of HA signals is imposed by the GSM and requires 5 ms per signal. Other waiting times contribute to delay the handover and frequently communication is cut for more than 100 ms. There are speech extrapolation software which mask a break in transmission, but these are only effective for a subjective time, dependent on the ear of the interlocutor. It is generally agreed that beyond 80 ms, the break in communication is audible to an interlocutor with a good ear while an interlocutor who does not pay particular attention to the speech signals which are transmitted to him may not notice for 200 ms that what he is hearing is not what would have reached him (since the signals are generated by the speech extrapolation software).

The same applies to the user of the mobile who cannot receive telephone-type data while his mobile is in the handover phase. This drawback manifests itself particularly in an urban environment where the cells are small-scale and where numerous cells may be crossed during a conversation. Some annoyance ensues with using such a system.

This invention has particularly for its objective the overcoming of these drawbacks. ? h p. Pi { \ w \ i f U V !J Aj.

More precisely, one of the objects of the invention is to provide a method of asynchronous handover, at lowest cost and the most widespread, enabling a reduction in the time during which communication is effectively cut when a handover is made.

According to the invention, there is provided a method of data interchange particularly between a mobile, at leapt two transmitter-receiver stations and at least one station controller included in an asynchronous-type GSM transmission network where the data interchange between the mobile and the stations are of the time division multiple access type, the method consisting 10 in providing a timing advance to the mobile when it passes from a first cell, corresponding to the geographic coverage of a first of the stations, to a second celt, corresponding to the geographic coverage of a second of the stations, and consisting in the transmission of a cell change command from the controller of the first station to the mobile in response to a cell change request expressed by 15 the mobile, this method being characterised in that the timing advance data has a predetermined value and is transmitted by the controller of the first station at the same time as the cell change command.

The result of this handover method is that it is not necessary for the second cell to transmit a measurement of timing advance TA to the mobile 20 before establishing the speech path.

Preferentially, the timing advance data corresponds to the transmission time of a radio signal between the second station and a mobile situated at the maximum range of the second cell. 9 24 8 5 64 According to another mode of design, the value of the timing delay data is where TAm corresponds to the maximum timing advance of the mobiles when they are connected to the second station and where TAm corresponds to the minimum timing advance of the mobiles.

The method may be applied to an internal or external handover procedure.

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 represents the structure of a GSM-type network; - Figure 2 is a correlative timing diagram of signals interchanged between a station and a mobile; - Figure 3 represents a handover procedure requested by a mobile which wants to be connected to the station of a cell; - Figure 4 represents an internal handover procedure according to the invention; - Figure 5 represents an external handover procedure according to the equal to: TAm + TAm 2 invention.

- A 8 MAR 1396 y' 24 85 04 According to the invention, when a mobile has to make a handover (signal HANDOVER CMD), the value of the timing advance which is transmitted to it corresponds to the distance separating it from the station which it is receiving the best.

This value is transmitted to it by the station BTS1 with which the mobile is communicating.

Figure 4 represents a handover procedure according to the invention. The handover is internal because the stations are controlled by the same controller.

In this procedure. Steps 1 to 3 are identical to those in Figure 3, that is to say the mobile MS sends timing results to the controller BSC which decides whether a handover procedure has to initiated. The controller then starts the activation of a BTS2 channel and the station carries out the channel allocation.

However, in Step 4, the controller BSC sends a specific order to the station BTS1, which is still in communication with the mobile MS, the specific order being the known HANDOVER CMD to which is added a timing advance TA of predetermined value. The complete data is noted below as "HANDOVER CMD (TA-BTS2)" and signifies that the mobile MS has been ordered to connect with the station BTS2 with the associated delay TA. The station BTS1 transmits the command HANDOVER CMD (TA-BTS2) transparently to the mobile MS which is then informed of the time period by which it will have to advance the transmission of its data. In Step 5, the mobile MS transmits at least one signal HANDOVER ACCESS for the attention of the station BTS2 (here four successive signals are sent to provide a reliable connection, as in the case of synchronous 11 0 L Q £ & f t. r O handover) so that the latter measures the actual TA which will be communicated to the mobile at the time of the next signal SACCH, without interfering with the transmission of speech.

The signal HANDOVER CMD used previously in the prior art has bits 5 which are available for the transmission of the delay TA.

Steps 6 and 7 are identical to those previously described in relation to Figure 3.

The controller BSC includes a table in which a timing advance of predetermined value is allocated to each station BTS. Thus, when any mobile 10 indicates to the controller that it is receiving synchronisation signals from a given second station, or even a station with a target cell, with greater power than those signals from the first station with which it is communicating, the controller BSC is in a position to provide it with the value of TA corresponding to the second station.

The value of TA corresponds for example to the time taken by a radio signal (being propagated at the speed of light), transmitted by a mobile being situated at the limit of the target cell, to reach the target cell. Each station is if} effect aware of the maximum range of its cell (marked R2 in Figure 1 for the cell C2) which depends on the traffic in the cell. The value of TA can also 20 take into account the available guard time in each station BTS, as will be described subsequently.

This invention particularly applies to the handover procedure carried out when the mobile MS attempts to establish communication with the station BTS 12 2 4 8 5 6 4 a in a reduced size cell. In effect, in accordance with the GSM, a mobile has to be able to correspond with a station BTS when the multi-path of the signals which it is transmitting is less than 16 us. This value corresponds to a maximum delay in a signal transmitted by the mobile to the station with regard to the time when 5 the signal should have theoretically reached the station BTS and it is necessary, to be able to correctly receive the signal, which is or is not affected by a delay possibly going up to this value, at the station BTS. Significant multi-paths occur particularly in mountainous environments but are almost non-existent in the urban environment, particularly if the terrain is flat. Buildings in fact do not 10 introduce much multi-path and have instead a tendency to weaken the signal or create interference. When the multi-path is low, for example of the order of 6 us, the station thus has a guard time of 10 us which authorises the mobile to not transmit its data at the exact instant which has been indicated to it by the time lag data TA. As a consequence, a relatively large error can be tolerated in 15 the TA transmitted by the station BTS to the mobile when the multi-path is low, that is to say in the urban environment where the cells have reduced size.

This error can be easily calculated by knowing the maximum multi-path which it is possible to get at each station BTS. By taking as an example a station BTS which measures a maximum multi-path of 6 us, a guard time of 10 20 us remains which allows approval of a distance error. The distance error corresponds to the difference between the actual distance of the mobile which is asking to be connected to the station and the distance corresponding to the TA allocated to the mobile. Now 10 us corresponds to a distance of 3000 m 13 2 4 8 5 6 4 and if it is assumed that the candidate cell has a range of 1500 m, the timing advance allocated to the mobile by its station BTS will correspond to the distance of 1500 m. However, since the mobile which is requesting that a handover be carried out can be situated, in the worst possible configuration, at 5 the foot of the station BTS with which it was communicating (it is in fact possible that a mobile is in the vicinity of a station but it cannot communicate with it on account of masking or local interference at the time, and it is then necessary to be able to make a successful handover to another cell), the distance error will be 1500 m (error in TA of 10 us) but will be transparent for 10 the station BTS which will, for all that, correctly receive the signal from the mobile due to the guard time.

Generally,the error allowed in TA is (16 - Tmt) where Tmt corresponds to the maximum multi-path time measured by the second station.

In the preceding configuration, the handover method according to the 15 invention is applicable to every cell in which the farthest limit of the station BTS is 1500 m (for a multi-path of 5 us), when a handover procedure requested by a mobile situated in the immediate vicinity of another station BTS has to be capable of being complied with.

By way of example, for a multi-path of 6 us, the approximation in the TA provided corresponds to a distance of 1500 m. In an urban network, if the cells have a maximum range of 1500 m, a handover will be successfully made within a range of 3000 m from the target station. In effect: 24 8 5 6 4 if the mobile is 1500 m from the target station, the TA provided corresponds exactly with the position of the mobile. if the mobile is in the immediate vicinity of the target station, the error made in the TA provided is 10 us fast, this error having no effect on the signals transmitted by the mobile due to the guard time., if the mobile is 3000 m from the target station, the error is equally 10 us slow and also having no effect on the transmitted signals.

The method of the invention equally applies to large size target cells when the multi-path is small.

In an advantageous mode of implementation, the timing advance data TA corresponds to the maximum range of the target cell. When the cells have a range shorter or equal to the error made in TA (depending on the multi-path), it is possible for a mobile to successfully make a handover when it is situated in the immediate vicinity of the target station or at twice the distance corresponding to the error made at that time.

According to another mode of implementation of the method of the invention, the value of the timing advance data is the result of a calculation taking into account the actual configuration of the target cell. It is in fact possible to measure the timing advance of each mobile which is connected to the station in a cell, and to thus define a minimum timing advance as well as a maximum timing advance of the mobiles. A statistical calculation also may be performed from the measured timing advances. Thus, by defining TA as being the minimum timing advance of the mobiles when they are connecting to the 248564 16 new station and TAm as the maximum timing advance of the mobiles, the timing advance TA which will be allocated to each mobile wanting to be connected to the In having defined these terms, this invention can be considered to apply from the moment that the following relationship is satisfied: TA„ - TA„, < 2 x (16 - Tmt).

This invention applies not only to a handover made within the same BSS (internal handover), but also to the inter-BSS handover (external handover) as represented in Figure 5.

In Step 1, the controller BSC1 transmits a signal HO REQ to the switching station indicating that the mobile MS wants to be in communication with the station BTS2. In Step 2, the MSC transmits this request to BSC2 (signal HO REQUEST) which proceeds to allocate a channel from BTS2 (signal CHAN ACT). The station BTS2 replies by a signal CHAN ACT ACK and BSC2 transmits a signal HO REQ ACK to the switching station MSC. In Step 3, the switching station generates a handover command (signal HO CMD) intended for BSC! In Step 4, BSC1, which is the controller of the station with which the mobile MS is communicating, sends the previously specified signal HANDOVER CMD station of the cell will be equal to: TA TAm + TAm 2 24 85 3* (TA-BTS2) to MS. The mobile MS interrupts its communication in progress and generates for example four successive HO ACCESS (Step 5). The station BTS2 detects the first signal HO ACCESS 1 and sends a signal HO DETECTION to BSC2 which retransmits it to MSC. The remainder of the procedure (Steps 7 and 8) is identical to that in Figure 4.

Steps 4 and 5 of the invention correspond to those of a synchronous handover which does not include the provision of the timing advance TA by the addressee BTS. It can therefore be seen that the handover procedure of the invention produces a handover as efficient as an asynchronous handover, but in a GSM transmission network where the stations BST are asynchronous. The same performance as a synchronous handover is achieved, but the cost is greatly reduced since it is unnecessary to synchronise the network. 17

Claims (6)

24 8 5 64 What we claim is:

1. A method of data interchange between a mobile, at least two transmitter-receiver stations and at least one controller included in an asynchronous-type GSM transmission network where the said data interchanges 5 between the said mobile and the said stations are of a time division multiple access type, said method comprising the steps of providing to the said mobile a timing advance when the mobile passes from a first cell, corresponding to the geographic coverage of a first of the said stations, to a second cell, corresponding to the geographic coverage of a second of the said stations, and transmitting a cell 10 change command from the controller to the said mobile in reply to a cell change request expressed by the said mobile , wherein said timing advance data has a predetermined value and is transmitted to the said mobile by the said controller of the said first station at the same time as the said cell change command .

2. A method as claimed in Claim 1, wherein said timing advance data 15 corresponds to the transmission time of a radio signal between the said second station and a mobile situated at the maximum range of the said second cell.

3. A method as claimed in Claim 1, wherein said timing advance data is equal TAM + TAm where TAM corresponds to the maximum timing advance of the 20 mobile when connected to the said socond station and whe^jTA^ corresponds fo V A v the minimum timing advance of the mobile. Z. ^ f-8 MAR 1996 H 18 248564 19

4. A method as claimed in any one of Claims 1 to 3, applicable to an internal handover procedure.

5. A method as claimed in any one of Claims 1 to 3, applicable to an external handover procedure.

6. A method substantially as herein described will reference to Figures 1-5 of the accompanying drawings. ALCATEL AUSTRALIA LIMITED (A.C.N. 000 005 363) P.M. Conrick Authorised Agent P5/1/1703