Method of handling over a call between two relay stations of a cell of a digital cellular mobile radio system

A method is described of handing over calls between two relay stations, respectively a current relay station and a target relay station in the same cell of a digital cellular mobile radio system, the cell being associated with a base transceiver station comprising a plurality of relay stations including the current and target relay stations and geographically distributed within the cell. Each relay station comprises at least one antenna connected to at least one transmit/receive device. Each relay station has a plurality of traffic channels and a plurality of associated signaling channels. A call being set up for a mobile station is allocated, from the channels of the current relay station, at least one current traffic channel and at least one current associated signaling channel. The plurality of relay stations shares at least one particular traffic channel, constituting a shared traffic channel, which is reserved at least partly for handover. The method includes a first handover phase in which the shared traffic channel is a first target traffic channel. The first phase includes the following successive steps: sending by the mobile station of at least one test message to the plurality of relay stations via the shared traffic channel, and selection by the system of the target relay station from the plurality of relay stations, in accordance with at least one predetermined selection criterion based on at least one parameter associated with the quality of reception of the at least one test message. The method also includes a second handover phase with a traffic channel associated with the target relay station as a second target traffic channel.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The field of the invention is that of digital mobile radio systems. The 
invention applies in particular, although not exclusively, to cellular 
systems to the GSM 900 standard (Global System for Mobile Communications 
in the 900 MHz band), the DCS 1800 standard (Digital Cellular System, 1800 
MHz) or the PCS standard (Personal Communication System). 
To be more precise, the invention concerns call handover between two relay 
stations of a cell of a digital cellular mobile radio system. 
Conventionally, a cell is associated with a base transceiver station. The 
base transceiver station uses a signaling carrier specific to it and which 
supports a broadcast control channel (BCCH). A mobile station in the cell 
communicates with the system via the base transceiver station. This 
communication uses a traffic carrier allocated by the base transceiver 
station. The information corresponding to this allocation is transmitted 
on the signaling carrier. 
Conventionally, each carrier is time-shared, using the time-division 
multiple access (TDMA) technique. This divides the time axis into 
successive frames of predetermined duration. Each frame is in turn divided 
into a particular number of time slots. The recurrence of a particular 
time slot in each frame corresponds to a physical channel onto which a 
number of logical channels can be multiplexed. 
A mobile station communicates via a traffic channel (TCH), different from 
those allocated to other mobile stations in the same cell, and receives 
information concerning the system via at least one signaling channel 
associated with that traffic channel. 
The invention applies to cells in which the base transceiver station uses a 
plurality of relay stations geographically distributed within the cell. 
Each relay station includes an antenna. Each antenna is associated with at 
least one transmit/receive device (or TRX, to use the GSM terminology). 
Each transmit/receive device includes a downlink traffic channel carrier 
transmitter and an uplink traffic channel carrier receiver. 
The relay stations, and therefore the antennas, are geographically 
distributed within the associated cell to provide improved coverage and/or 
to handle a greater density of calls. The various relay stations are 
associated with a common signaling carrier. To this end, at least some of 
the relay stations include a downlink signaling channel carrier 
transmitter and an uplink signaling channel carrier receiver. 
All the relay stations can be connected to the next higher level in the 
hierarchy (which is a base station controller (BSC) in the GSM) via a 
concentration center. In one implementation, the BSC provides the 
concentration functions directly and in this case the relay stations are 
connected directly to the BSC. 
The problem arises of handing over calls between two relay stations of the 
cell. "Conventional" handover is operative between the base stations of 
two different cells. This "conventional" process is as follows: each base 
transceiver station transmits its own signaling carrier. The mobile 
station measures the signaling carriers of adjoining cells continuously. 
The results of such measurements are regularly transmitted via the current 
base transceiver station to the base station controller, which decides 
whether there is an opportunity for handover. When a call must be handed 
over, the controller decides the target base transceiver station and 
allocates a target traffic channel therein to the mobile station. This 
allocation is transmitted to the mobile station so that it can switch from 
the current traffic channel to the target traffic channel. 
Consideration has been given to transposing this "conventional" handover 
between base stations to handover between relay stations of the same base 
station (which is no longer "conventional"), by simply substituting the 
relay station for the base station. 
Unfortunately, such transposition is not easy, even impossible. In other 
words, the "conventional" handover process cannot be easily implemented 
with the distributed base transceiver station configuration (i.e. one 
comprising a plurality of relay stations) to which the present invention 
applies. 
Each relay station does not transmit a signal that is specific to it (and 
therefore enables it to be identified). Consequently, the mobile station 
is not in a position to communicate with the measurement system enabling 
the latter to decide whether there is an opportunity for handover, or, for 
even stronger reasons, to determine the best target relay station to which 
the call should be handed over. 
One objective of the invention is to overcome this major drawback of the 
prior art. 
To be more precise, one objective of the present invention is to provide a 
method of handing over calls between two relay stations of the same cell 
of a mobile radio system that is simple and inexpensive. 
Another objective of the invention is to provide a method of the above kind 
which optimizes the use of resources. 
Another objective is to provide a method of the above kind that does not 
require any additional hardware to be developed, either for the mobile 
stations or for the system. 
Another objective is to provide a method of the above kind which limits the 
number of handovers between relay stations required to obtain a 
predetermined reception quality. 
SUMMARY OF THE INVENTION 
The above objectives, together with others that will become apparent 
hereinafter, are achieved in accordance with the invention by a method of 
handing over calls between two relay stations, respectively a current 
relay station and a target relay station, in the same cell of a digital 
cellular mobile radio system, the cell being associated with a base 
transceiver station comprising plurality of relay stations, including the 
current and target relay stations, geographically distributed within the 
cell, each relay station comprising at least one antenna connected to at 
least one transmit/receive device, each relay station having a plurality 
of traffic channels and a plurality of associated signaling channels, the 
call being set up for a mobile station and being allocated, from the 
channels of the current relay station, at least one current traffic 
channel and at least one current associated signaling channel, wherein the 
plurality of relay stations shares at least one particular traffic 
channel, namely a shared traffic channel, which is reserved at least 
partly for handover, and wherein the method includes a first handover 
phase in which the shared traffic channel is a first target traffic 
channel and a second handover phase with a traffic channel associated with 
the target relay station as a second target traffic channel, the first 
phase including the following successive steps: sending by the mobile 
station of at least one test message to the plurality of relay stations 
via the shared traffic channel, and selection by the system of the target 
relay station from the plurality of relay stations, in accordance with at 
least one predetermined selection criterion based on at least one 
parameter associated with the quality of reception of the at least one 
test message. 
The general principle of the invention is therefore based on the 
introduction of an intermediate shared traffic channel into the process of 
handing over a call from a current relay station to a target relay 
station, its use being interleaved in time between that of the current 
traffic channel and that of the target traffic channel. In other words, 
the method of the invention is similar to a double handover mechanism 
which successively performs a first handover from the current traffic 
channel to the shared traffic channel and a second handover from the 
shared traffic channel to the target traffic channel. 
It is noteworthy that the mobile station can send test messages via the 
shared traffic channel, comprising one or more traffic messages to the 
various relay stations of the cell and a specific message such as the GSM 
"Handover-Access" message. 
No additional hardware development is necessary, either for the mobile 
stations or for the equipment of the system itself. 
Accordingly, the method of the invention selects the target relay station 
that is best receiving the mobile station. In the "conventional" procedure 
previously referred to, the base station that is received best by the 
mobile station is selected. The fact that it is the relay station (and no 
longer the mobile station) that performs the measurement, by virtue of the 
shared traffic channel, eliminates the problem of no transmission by each 
relay station of a specific signal (and therefore the impossibility of 
selecting the target relay station by simply listening to the signals 
transmitted by the various relay stations). 
Clearly, in the current relay station, the current traffic channel and the 
shared traffic channel can be carried either by the same carrier or by two 
separate carriers (each current transmit/receive device is associated with 
a different carrier). 
Similarly, in the target relay station, the target traffic channel and the 
shared traffic channel can be carried either by the same carrier or by two 
separate carriers. 
In some cases, the target relay station is in fact the current relay 
station (for example, if the current relay station is the "least worst" in 
terms of reception quality). In this case, no call handover between relay 
stations is done. 
The first handover phase is advantageously preceded by a step in which the 
system and/or the mobile station detects deterioration in the quality of 
the call according to a predetermined detection criterion. 
The invention does not impose any constraint as to the detection criterion 
used. One example is the receive power falling below a predetermined 
threshold. 
The digital cellular mobile radio system is preferably of the time-division 
multiple access type. 
Clearly, however, the principle of the invention, which is based on 
introducing a shared traffic channel, can be applied to other types of 
mobile radio system, for example (but not exclusively) frequency-division 
multiple access (FDMA) or code-division multiple access (CDMA) systems. 
All the relay stations of said cell are preferably synchronous at frame 
level and the shared traffic channel is defined with identical features in 
all the relay stations. 
In other words, the resources corresponding to a shared channel are used 
and defined identically by all the relay stations. 
Reservation of the shared traffic channel for handover is advantageously of 
the strict reservation type or the partial reservation type, using a 
predetermined priority management strategy. 
In the strict reservation case (i.e. for handover only), it is advantageous 
for each relay station to include at least two transmit/receive devices to 
increase the number of traffic channels available and thereby to limit the 
handicap represented by strict reservation of traffic channels. 
Partial reservation of a shared channel means that the shared channel is 
sometimes used also as a "normal" channel (i.e. one that is not shared and 
is dedicated to a given mobile station, for example). Managing the 
priority of use of any such shared channel entails providing a higher 
priority level for use as a handover channel than for use as a normal 
(traffic or signaling) channel, for example. 
Handover is preferably asynchronous or synchronous. 
In asynchronous handover, the mobile station needs timing advance 
information (to use the GSM terminology) so that it can synchronize to and 
communicate with the target relay station. The timing advance information 
is transmitted to the mobile station with the PHYSICAL-INFO response 
message. 
In synchronous handover, the clocks of the various relay stations are, by 
hypothesis, synchronous. Accordingly, when a mobile station is handed over 
from one relay station to another, it is not necessary to provide it with 
new timing advance information, because the timing advance can be deduced 
immediately from that previously being used. 
The first handover phase is advantageously followed by the system sending 
the current relay station a message telling it to release the at least one 
current traffic channel. 
This optimizes re-use of the current relay station channels. 
The second handover phase is advantageously followed by the system sending 
the plurality of relay stations a message telling them to release the at 
least one shared traffic channel. 
In this way, in the case of strict reservation, the shared traffic channel 
is available for handing over other mobile stations. In the case of 
partial reservation, the shared traffic channel is left free for another 
mobile station, or possibly the same mobile station, to use it as a 
"normal" traffic channel. 
The at least one parameter related to the reception quality of the at least 
one test message and on which the predetermined section criterion is based 
is preferably the receive power, the signal to noise ratio, the raw bit 
error rate or the error rate after decoding. 
Clearly the above list is not limiting on the invention. Also, the 
selection can be performed in accordance with more than one of the above 
parameters. Finally, it is feasible to combine a number of parameters each 
weighted by a weighting coefficient (which can itself be predetermined in 
accordance with another parameter). 
The cell advantageously corresponds to indoor and/or outdoor coverage. 
Indoor coverage corresponds to coverage inside a building, for example. 
Outdoor coverage corresponds to a predetermined area around the building, 
for example. 
Other features and advantages of the invention will become apparent on 
reading the following description of one preferred embodiment of the 
invention given by way of illustrative and non-limiting example only and 
with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The remainder of the description concerns a time-division multiple access 
(TDMA) GSM mobile radio system. It is nevertheless clear that the 
invention applies to any digital cellular mobile radio system. 
However, the invention can also apply to other communication techniques, in 
particular, although not exclusively, frequency-division multiple access 
(FDMA) or code-division multiple access (CDMA). 
FIG. 1 shows a GSM mobile radio system in which the method in accordance 
with the invention can be used for handing over calls between a current 
relay station and a target relay station. 
The explanation considers a cell 11 covering a building 101 with three 
corridors 18, 19 and 110. The cell 11 is associated with a distributed 
base transceiver station including a concentration center 16 and three 
relay stations 12 through 14, one in each corridor. 
In the conventional way, each of the relay stations 12, 13, 14 includes an 
antenna 12.sub.1, 13.sub.1, and 14.sub.1, possibly a first 
transmit/receive device 12.sub.2, 13.sub.2, 14.sub.2 associated with the 
signaling carrier of the cell (all the relay stations of the cell transmit 
the same signaling carrier), and at least one second transmit/receive 
device 12.sub.3, 13.sub.3, 14.sub.3, each transmit/receive device being 
associated with a separate uplink or downlink traffic carrier. Each relay 
station 12, 13, 14 therefore uses one or more traffic carriers carrying a 
plurality of traffic channels TCH and a plurality of signaling channels 
associated with the traffic channels TCH. 
In one variant, the signaling carrier BCCH (which is unique to the cell) is 
transmitted by only some relay stations, or even by none of them if the 
base transceiver station includes means separate from the relay stations 
dedicated to this transmission alone. 
The base transceiver station uses a slow (i.e. frame by frame) frequency 
hopping technique, and so the frequency re-use factor is equal to 1. 
Accordingly, it remains only to plan the single frequency (or carrier) 
carrying the signaling. Coverage and/or call density are therefore 
considerably improved. 
The relay stations 12, 13, 14 are separated by a distance in the range from 
100 m to 200 m, for example. The transit time of a message between the 
mobile station 111 and its current relay station 14 can therefore be 
ignored. For explanatory purposes, it is assumed that a mobile station 111 
moves and enters one of the corridors (for example corridor 19) at 
position 112. In this example it is therefore necessary to handover the 
call between the current relay station 14 and a target relay station 12. 
For a call that has been set up, the mobile radio system allocates to the 
mobile station 111, from the channels of the current relay station 14 and 
those of the target relay station, at least one current traffic channel 
and at least one current associated signaling channel. 
The relay stations 12, 13, 14 are synchronous at frame level and each 
channel of each relay station 12, 13, 14 is defined with a particular set 
of features (frequency hopping law, TSC, SFN, etc). 
All the relay stations 12, 13, 14 of the cell 11 are connected to the relay 
station concentration center 16 via the Abis interface 15. This interface 
serves as an intermediary in communications between the relay stations 12 
through 14 and the base station controller (BSC) 17 that controls a 
plurality of base stations of the GSM mobile radio system. In one variant 
the concentration functions are handled by the BSC directly and the 
distributed base transceiver station does not have any concentration 
center as an independent physical entity. 
In accordance with an essential feature of the invention, the relay 
stations 12 through 14 of the cell share at least one traffic channel, 
called the shared traffic channel, and possibly at least one signaling 
channel associated with the shared traffic channel, called the shared 
associated signaling channel. The shared channels are reserved for call 
handover, as explained in detail hereinafter. 
The shared traffic channel, and possibly the shared associated signaling 
channel, are each defined with identical features in all of the relay 
stations 12 through 14. The identical features include the frequency 
hopping law, TSC, SFN, etc. 
The shared associated signaling channel can correspond to at least one slow 
associated control channel (SACCH) and/or at least one fast associated 
control channel (FACCH). 
Reservation of the shared channels for handover can be either strict or 
partial. In the former case, the shared channels are allocated to call 
handover only whereas in the second case they can be allocated to other 
functions (in particular traffic and/or signaling). In the former case it 
is advantageous for the relay station 14 to include one or more additional 
transmit/receive device(s) 14.sub.4 to increase the number of traffic 
channels available and thereby reduce the risk of blocking. 
Note that even though reservation of the shared traffic channel concerns 
all the relay stations, it is nevertheless possible to activate only some 
of the relay stations when implementing this double handover mechanism in 
accordance with the invention (as explained in detail hereinafter). In 
this case, the best relay station is selected from only the activated 
relay stations For example, only the relay stations nearest the current 
relay stations are activated (i.e. only the relay stations likely to be 
targets in handover). 
A first particular embodiment of the method in accordance with the 
invention of handing over calls between two relay stations is described 
next with reference to the FIG. 2 flowchart. 
As previously indicated, for explanatory purposes it is assumed that the 
aim is to transfer a call for mobile station 111 from the current relay 
station 14 to the target relay station 12. 
The method of the invention includes the following successive steps: 
The system detects (step 21) deterioration of call quality, in accordance 
with a predetermined detection criterion. In one variant, the 
deterioration is detected by the mobile station. In another variant it is 
detected by the mobile station and the system. The predetermined detection 
criterion corresponds to the comparison of the receive quality measured 
value with a required minimum threshold, for example, and observation that 
the measured value is below the minimum threshold. 
The system performs a first handover phase 22, with the shared traffic 
channel as the first target traffic channel. This first phase 22 
constitutes the first step of the double handover mechanism of the 
invention. The (final) target traffic channel has not been determined yet 
at this stage of the process. In the case of partial reservation, the 
shared channel(s) reserved for handover must be "reserved" after detecting 
deterioration of call quality. The first handover phase 22 includes the 
following successive steps: 
The mobile station sends one or more test messages to the plurality of 
relay stations and each relay station measures the quality of reception of 
the test messages (step 221). The test message(s) can include at least one 
specific message such as the "Handover-Access" message, and possibly one 
or more traffic messages. 
The system selects the target relay station from the plurality of relay 
stations in accordance with at least one predetermined selection criterion 
based on at least one parameter associated with the quality of reception 
of the test message(s) (step 222). The selected target relay station is 
advantageously that enabling best communication with the mobile station to 
which the current handover relates. In the worst case scenario, the 
current relay station may remain the "least worst" relay station for 
receiving messages from the mobile station, despite the deterioration in 
call quality. 
The system performs a second handover phase using a traffic channel 
associated with the target relay station as the second target traffic 
channel (step 23). 
In one variant, the first handover phase 22 can be followed immediately by 
a step of releasing the current channels (current traffic channel and 
current associated signaling channel(s)) used by the current relay 
station. 
In one variant, the second handover phase 25 can be immediately followed by 
a step of releasing the shared channel (shared traffic channel and shared 
associated signaling channel(s)) used by the current relay station. This 
is beneficial in particular if it is possible to use the shared channels 
as "normal" (traffic and associated signaling) channels, for example in 
the context of partial reservation of these shared channels. 
One particular embodiment of the handover method of the invention is 
described next with reference to the FIGS. 3A and 3B timing diagram. 
The following description concerns asynchronous handover. 
It is nevertheless clear that the invention applies equally to synchronous 
handover, and the skilled person will know how to switch easily from one 
to the other, without departing from the scope of the present invention. 
The fact that handover is asynchronous means in particular that the timing 
advance relating on the one hand to the relay stations 13, 14 that may be 
target stations and on the other hand to the target relay station 12 must 
be supplied to the mobile station. 
For simplicity, it is assumed in the remainder of the description that the 
transmit/receive device TRX.sub.i of the current relay station providing 
the current traffic channel and the current associated signaling channel 
is also that providing the shared traffic channel and the shared 
associated signaling channel. It is also assumed that the transmit/receive 
device TRX.sub.i of the target relay station providing the shared traffic 
channel and the shared associated signaling channel is also that providing 
the target traffic channel and the target associated signaling channel 
(see FIG. 4). 
Clearly, however, different transmit/receive devices can be used in the 
current relay station and in the target relay station to support the 
aforementioned channel. 
Steps A through U are described successively hereinafter. 
In step A, the transmit/receive device TRX.sub.i of the current relay 
station transmits to the system, via the Abis interface, a measured value 
of the quality of reception of a traffic message on the current traffic 
channel C.sub.t,curr by the transmit/receive device TRX.sub.i. The 
measured value can be transmitted with the traffic message (transmitted 
originally by the mobile station MS) whose quality of reception it 
indicates. 
In one variant, the mobile station MS measures the quality of reception of 
a traffic message sent by the current relay station. The result of that 
measurement is then transmitted to the system from the mobile station and 
via the current relay station. 
After step A, it is assumed that the system detects deterioration in the 
quality of reception of the message sent by the mobile station MS. This 
detection constitutes step 21 (FIG. 2). 
The subsequent steps B through G constitute the first handover phase 22 of 
FIG. 2 of the double handover mechanism of the invention. 
In step B, the system transmits to the current relay station, via the Abis 
interface, a first handover request message HO-COMMAND 
("Handover-Command"). 
In step C, the transmit/receive device TRX.sub.i of the current relay 
station sends a first handover request message ("Handover-Command") to the 
mobile station via at least one current associated signaling channel 
C.sub.s,curr. The signaling channel is the FACCH, for example. By means of 
parameters specific to it, the message invites the mobile station MS to 
send at least a first "Handover-Access" test message, which is to be 
measured by all the relay stations, via the shared traffic channel 
C.sub.t,shar. 
In step C communication between the mobile station MS and the current relay 
station is via the current channels (current traffic channel C.sub.t,curr 
(and current associated signaling channel C.sub.s,curr) via the 
transmit/receive device TRX.sub.i of the current relay station. 
The subsequent steps D through E.sub.3 constitute step 221 from FIG. 2. 
In step D, the mobile station MS sends a first "Handover-Access" test 
message to all the relay stations via the shared traffic channel 
C.sub.t,shar. The shared traffic channel constitutes the first target 
channel of the double handover mechanism of the invention. All the relay 
stations of the cell then listen to the "Handover-Access" test message- 
The first test message may be followed by at least one second test 
message. 
In steps E.sub.1, E.sub.2 and E.sub.3, which are executed simultaneously, 
each relay station transmits via the Abis interface a measured value of 
the quality of reception of the first test message or the test message(s) 
(or quality information) on the shared traffic channel C.sub.t,shar. The 
measurement (or each measurement) may be accompanied by the first test 
message or the associated test message (i.e. the one on which the 
measurement was done). 
In step F, the system transmits a first "PHYSICAL-INFO" response message 
via the Abis interface to the transmit/receive device TRX.sub.i of the 
target relay station. The response message is intended in particular to 
indicate the information the mobile station needs to synchronize to the 
target relay station (power, timing advance, etc). The shared associated 
signaling channel C.sub.s,shar is a slow associated control channel 
(SACCH) or a fast associated control channel (FACCH), for example. 
In step G the transmit/receive device TRX.sub.i of the target relay station 
transmits the first response message to the mobile station. To this end 
the first response message is transmitted via the shared associated 
signaling channel C.sub.s,shar. The first response message is accompanied 
by the aforementioned synchronization information, in particular the 
timing advance (TA). 
In step H, the mobile station continues to communicate by sending at least 
one "TRAFFIC MESSAGE" via the shared traffic channel C.sub.t,shar to the 
target relay station. Note that it is nevertheless beneficial to limit the 
number of traffic messages that can use the shared traffic channel 
C.sub.t,shar. Other mobile stations must also be able to use it for their 
own handovers. 
In steps D through H communication between the mobile station MS and the 
relay station(s) involved is via the shared channels (shared traffic 
channel C.sub.t,shar and shared associated signaling channel C.sub.s,shar) 
via the transmit/receive device TRX concerned of each relay station 
concerned. 
In steps I.sub.1, I.sub.2 and I.sub.3, which are executed simultaneously, 
each target relay station transmits a measured value of the quality of 
reception of the traffic message (or quality information) that it has 
performed on the shared traffic channel C.sub.t,shar via the Abis 
interface. Each of the measured values can be accompanied by the 
corresponding traffic message. Because the shared traffic channel enables 
the system to communicate with the mobile station, the shared traffic 
channel may be regarded as a "real" traffic channel vis-a-vis the mobile 
station and/or the system reserved for the call. 
The system receives and analyzes the various measured values from the 
various relay stations (current relay station, target relay station to be 
determined and other relay stations). It then selects the target relay 
station according to a selection criterion. This selection constitutes 
step 222 of FIG. 2, which does not appear as such in FIGS. 3A and 3B 
currently being described. The selection criterion is based on the best 
value of the receive level measured by all the relay stations that have 
carried out the quality measurements, for example. Clearly, however, the 
selection criterion can be based on other parameters, also associated with 
the quality of reception of the test message(s), for example the signal to 
noise ratio, the raw bit error rate, the bit error rate after decoding, 
etc. 
In one variant, the selection can be performed between steps E.sub.3 and F. 
However, it is clearly preferable to perform one or more quality 
measurements on one or more traffic messages, serving as test messages, 
for more reliable selection of the target relay station. 
In step J the system transmits to the current relay station, via the Abis 
interface, a first "RELEASE" message telling it to release the current 
channels (current traffic channel C.sub.t,curr and the associated 
signaling channel C.sub.s,curr). 
In one variant this first message to release the current channels is 
transmitted immediately after step C of transmitting the first handover 
message "HO-COMMAND". 
The subsequent steps K through P constitute the second handover phase 23 
from FIG. 2 of the double handover mechanism of the invention. 
In step K, the system transmits to the target relay station, via the Abis 
interface, a second handover request message HO-COMMAND. In step B, a 
first handover request message HO-COMMAND was transmitted by the system to 
the current relay station to initiate the first handover phase of the 
double handover mechanism of the invention. 
In step L, the target relay station transmits the second handover request 
message "HO-COMMAND" to the mobile station MS via at least one shared 
associated signaling channel C.sub.s,shar. The message invites the mobile 
station MS to send a second test message "HO-ACCESS" via the target 
traffic channel C.sub.t,targ. The target traffic channel constitutes the 
second target traffic channel of the double handover mechanism of the 
invention. To this end, transmission of the second handover request 
message between the target relay station and the mobile station is 
performed on the FACCH, serving as the shared associated signaling channel 
C.sub.s,shar. 
In step M, the mobile station MS sends a second test message "HO-ACCESS" to 
the target relay station via the target traffic channel C.sub.t,targ. In 
step D, the mobile station MS sent a first test message "HO-ACCESS" to the 
current relay station via the current traffic channel C.sub.t,curr. 
In step N, the target relay station transmits the measured value of the 
quality of reception of the second test message (or quality information) 
via the Abis interface. This message may be accompanied by the 
corresponding second test message (on which the measurement was carried 
out). 
In step O, the system transmits a second response message "PHYSlCAL-INFO" 
to the target relay station via the Abis interface. In step F, the system 
transmitted a first response message "PHYSICAL-INFO" to the target relay 
station via the Abis interface. The second response message is also 
intended to indicate the information the mobile station needs to 
synchronize to the target relay station (power, timing advance, etc). To 
this end, the target associated signaling channel C.sub.s,targ is an SACCH 
(or FACCH), for example. 
In step P, the transmit/receive device TRX.sub.i of the target relay 
station transmits the second response message to the mobile station via 
the target associated signaling channel C.sub.s,targ, accompanied by 
synchronization information, in particular the timing advance (TA). 
In step Q, the second handover phase terminates and the mobile station 
communicates with the target relay station via the target traffic channel 
C.sub.t,targ. It therefore continues its call by sending traffic messages 
("TRAFFIC MESSAGE") via the new current traffic channel C.sub.t,current 
and the new current relay station. The target traffic channel and the 
target relay station have respectively become the new current traffic 
channel and the new current relay station. 
In step R, the new current relay station forwards a measured value of the 
quality of reception of the traffic message to the system via the Abis 
interface. This measured value can be transmitted with the associated 
traffic message. 
From step Q onward, communication between the mobile station MS and the new 
current relay station is via the new current channels, i.e. the target 
channels (target traffic channel C.sub.t,targ and target associated 
signaling channel C.sub.s,targ) via the transmit/receive device TRX.sub.i 
of the new current relay station, i.e. the target relay station. 
In step S, the system transmits a second "RELEASE" message to the current 
transmit/receive device TRX.sub.i of the current relay station, via the 
Abis interface, telling it to release the shared channels (shared traffic 
channel C.sub.t,shar and shared associated signaling channel 
C.sub.s,shar). The aim is, in particular, to make them available for other 
handovers associated with other mobile stations in the case of strict 
reservation or to leave them free for use as normal traffic and signaling 
channels in the case of partial reservation. 
In one variant, the second message to release the shared channels is 
transmitted immediately after step L of transmitting the first handover 
message "HO-COMMAND". 
In another variant, release of the current channels (current traffic 
channel C.sub.t,curr and associated current signaling channel 
C.sub.s,curr) simultaneously with step S can be considered. In this case, 
the first release message is sent in step S, not in step J. 
Management of collisions between the various messages sent on the shared 
channels (shared traffic channel(s) C.sub.t,shar and shared associated 
signaling channel(s) C.sub.s,shar) can optionally be provided. 
In step T, the mobile station continues to communicate by sending at least 
one "TRAFFIC MESSAGE" to the new current relay station via the new current 
traffic channel C.sub.t,current. 
In step U, the new current relay station transmits the measured value of 
the quality of reception of the preceding traffic message (performed by 
the new current relay station) via the Abis interface. As for step A, the 
measured value can be accompanied by the corresponding traffic message. 
Communication between the system and the mobile station can thereafter be 
subject to at least one other double handover mechanism in accordance with 
the invention. 
Clearly, if the current relay station is the "least worst" relay station of 
the cell in terms of the measured quality of reception, the target relay 
station and the current relay station are one and the same. However, in 
this case, the target transmit/receive device can be identical (so that 
there is no handover) or different (so that there is handover) from the 
current transmit/receive device. 
The double handover mechanism of the invention is described again with 
reference to the FIG. 4 diagram. 
Depending on the context of the invention (distributed base transceiver 
station with geographically distributed relay stations), each 
transmit/receive device TRX.sub.i (for the current relay station), 
TRX.sub.concerned (for each of the other relay stations) and TRX.sub.i 
(for the target relay station) is associated with a respective different 
carrier 41, 42, 43. Each carrier is structured in frames each divided into 
a particular number of time slots (conventionally eight time slots). Each 
traffic channel TCH No. 1 through TCH No. 7 of a given carrier is carried 
by a separate time slot. 
In the example the mobile station communicates initially with the 
transmit/receive device TRX.sub.i of the current relay station via the 
current traffic channel TCH No. 1 and the current associated signaling 
channel(s). 
It is then assumed that, following detection of deterioration in the 
quality of reception by the current relay station of a traffic message 
from the mobile station, handover to a target relay station yet to be 
determined is initiated. 
The method in accordance with the invention for handover between relay 
stations therefore consists in performing, in succession, a first handover 
from the current traffic channel (C.sub.t,curr) to the shared traffic 
channel (C.sub.t,shar), serving as the first target traffic channel (in 
other words, there is a change from the current transmit/receive device of 
the current relay station to the transmit/receive device concerned (i.e. 
carrying the shared channel(s) of each relay station)) and a second 
handover from the shared traffic channel (C.sub.t,shar) to the target 
traffic channel (C.sub.t,targ), serving as the second target traffic 
channel (in other words, there is a change from the transmit/receive 
device concerned of each relay station to the target transmit/receive 
device of the target relay station).