 |
Claims  |
|
|
We claim:
1. A method of exchanging information in a cellular mobile radio network
between a mobile station (MS), at least first and second base transceiver
stations (BTS1, BTS2) and a base station controller (BSC) of said first
and second base transceiver stations (BTS1, BTS2), said exchanges of
information between said mobile station (MS) and said first and second
base transceiver stations (BTS1, BTS2) being time-division multiple access
exchanges, said method comprising the steps of:
transmitting from said base station controller (BSC) to said mobile station
(MS), via said first base transceiver station (BTS1) with which said
mobile station is communicating, an instruction (HO CHN) telling said
mobile station (MS) to interrupt communication with said first base
transceiver station (BTS1) in order to send to said second base
transceiver station (BTS2) a first sequence of synchronization signals (HO
ACCESS) enabling said second base transceiver station (BTS2) to calculate
a timing advance (TA), said instruction (HO CHN) being transmitted when
said mobile station (MS) moves from a first cell (C1) defined by the
geographical coverage area of said first base transceiver station (BTS1)
to a second cell (C2) defined by the geographical coverage area of said
second base transceiver station (BTS2),
said instruction (HO CHN) supplied to said mobile station (MS) also
indicates that said mobile station (MS) must continue to communicate with
said first base transceiver station (BTS1) immediately after sending said
first sequence of synchronization signals (HO ACCESS) to said second base
transceiver station (BTS2),
transmitting said calculated timing advance (TA) from said second base
transceiver station (BTS2) to said base station controller (BSC), and
transmitting said calculated timing advance (TA) from said controller (BSC)
to said mobile station (MS) via said first base transceiver station
(BTS1), said mobile station (MS) communicating with said second base
transceiver station (BTS2) in accordance with said timing advance (TA).
2. A method according to claim 1, further comprising the step of
transmitting a second sequence of synchronization signals (HO ACCESS) from
said mobile station (MS) to said second base transceiver station (BTS2)
after said mobile station (MS) receives said calculated timing advance
(TA).
3. A method according to claim 2, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) in a control
signal (HO CMD (TA)) from one of said first and second base station
controllers (BSC, BSC1), with which said first base transceiver station
(BTS1) is associated, to said mobile station (MS), via said first base
transceiver station (BTS1).
4. A method according to claim 2, wherein said first and second base
transceiver stations (BTS1, BTS2) are asynchronous and the relative phase
of their clock signals is unknown.
5. A method according to claim 1, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said second
base transceiver station (BTS2) to one of said first and second
controllers (BSC, BSC2), with which said second base transceiver station
is associated, in a synchronization indication signal (HO DETECTION).
6. A method according to claim 5, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) in a control
signal (HO CMD (TA)) from one of said first and second base station
controllers (BSC, BSC1), with which said first base transceiver station
(BTS1) is associated, to said mobile station (MS), via said first base
transceiver station (BTS1).
7. A method according to claim 5, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, further comprising the
step of transmitting said calculating timing advance (TA) from said second
base station controller (BSC2) to said switching center (MSC) in a
synchronization indication signal (HO DETECTION).
8. A method according to claim 7, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said
switching center (MSC) to said first base station controller (BSC1) in a
synchronization indication signal (HO DETECTION).
9. A method according to claim 5, wherein said first and second base
transceiver stations (BTS1, BTS2) are asynchronous and the relative phase
of their clock signals is unknown.
10. A method according to claim 5, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said
switching center (MSC) to said first base station controller (BSC1) in a
synchronization indication signal (HO DETECTION).
11. A method according to claim 1, wherein said first and second base
transceiver stations (BTS1, BTS2) are asynchronous and the relative phase
of their clock signals is unknown.
12. A method according to claim 1, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) in a control
signal (HO CMD (TA)) from one of said first and second base station
controllers (BSC, BSC1) with which said first base transceiver station
(BTS1) is associated, to said mobile station (MS), via said first base
transceiver station (BTS1).
13. A method according to claim 12, wherein said first and second base
transceiver stations (BTS1, BTS2) are asynchronous and the relative phase
of their clock signals is unknown.
14. A method of exchanging information in a cellular mobile radio network
between a mobile station (MS), at least first and second base transceiver
stations (BTS1, BTS2), first and second base station controllers (BSC1,
BSC2) respectively controlling said at least first and second base
transceiver stations (BTS1, BTS2), and a mobile services switching center
(MSC) controlling said first and second base station controllers (BSC1,
BSC2), said exchanges of information between said mobile station and said
at least first and second base transceiver stations (BTS1, BTS2) being
time-division multiple access exchanges, said method comprising the steps
of:
transmitting an instruction (HO CHN) from said first base station
controller (BSC1) to said mobile station (MS) telling said mobile station
(MS) to interrupt communication with said first base transceiver station
(BTS1) with which said mobile station (MS) is communicating in order to
send a first sequence of synchronization signals (HO ACCESS) to said
second base transceiver station (BTS2) enabling said second base
transceiver station (BTS2) to calculate a timing advance (TA), said
instruction (HO CHN) being transmitted when said mobile station (MS) moves
from a first cell (C1) defined by the geographical coverage area of said
first base transceiver station (BTS1) to a second cell (C2) defined by the
geographical coverage area of said second base transceiver station (BTS2),
said instruction (HO CHN) supplied to said mobile station (MS) also
indicates that said mobile station (MS) must continue to communicate with
said first base transceiver station (BTS1) immediately after sending said
first sequence of synchronization signals (HO ACCESS) to said second base
transceiver station (BTS2),
transmitting said calculated timing advance (TA) from said second base
transceiver station (BTS2) to said second base station controller (BSC2),
transmitting said calculated timing advance (TA) from said second
controller (BSC2) to said mobile services switching center (MSC),
transmitting said calculated timing advance (TA) from said mobile services
switching center (MSC) to said first base station controller (BSC1), and
transmitting said calculated timing advance (TA) from said first base
station controller (BSC1) to said mobile station (MS) via to said first
base transceiver station (BTS1), said mobile station (MS) communicating
With said second base transceiver station (BTS2) in accordance with said
timing advance (TA).
15. A method according to claim 14, further comprising the step of sending
a second sequence of synchronization signals (HO ACCESS) from said mobile
station (MS) to said second base transceiver station (BTS2) after said
mobile station (MS) receives said calculated timing advance (TA).
16. A method according to claim 15, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) in a control
signal (HO CMD (TA)) from one of said first and second base station
controllers (BSC, BSC1), with which said first base transceiver station
(BTS1) is associated, to said mobile station (MS), via said first base
transceiver station (BTS1).
17. A method according to claim 15, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, further comprising the
step of transmitting said calculating timing advance (TA) from said second
base station controller (BSC2) to said switching center (MSC) in a
synchronization indication signal (HO DETECTION).
18. A method according to claim 17, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said
switching center (MSC) to said first base station controller (BSC1) in a
synchronization indication signal (HO DETECTION).
19. A method according to claim 15, wherein said first and second base
transceiver stations (BTS1, BTS2) are asynchronous and the relative phase
of their clock signals is unknown.
20. A method according to claim 15, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said
switching center (MSC) to said first base station controller (BSC1) in a
synchronization indication signal (HO DETECTION).
21. A method according to claim 14, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculating timing advance (TA) from said second
base station controller (BSC2) to said switching center (MSC) in a
synchronization indication signal (HO DETECTION).
22. A method according to claim 21, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) in a control
signal (HO CMD (TA)) from one of said first and second base station
controllers (BSC, BSC1), with which said first base transceiver station
(BTS1) is associated, to said mobile station (MS), via said first base
transceiver station (BTS1).
23. A method according to claim 21, wherein said first and second base
transceiver stations (BTS1, BTS2) are asynchronous and the relative phase
of their clock signals is unknown.
24. A method according to claim 21, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said
switching center (MSC) to said first base station controller (BSC1) in a
synchronization indication signal (HO DETECTION).
25. A method according to claim 14, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said
switching center (MSC) to said first base station controller (BSC1) in a
synchronization indication signal (HO DETECTION).
26. A method according to claim 25, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) in a control
signal (HO CMD (TA)) from one of said first and second base station
controllers (BSC, BSC1), with which said first base transceiver station
(BTS1) is associated, to said mobile station (MS), via said first base
transceiver station (BTS1).
27. A method according to claim 25, wherein said first and second base
transceiver stations (BTS1, BTS2) are asynchronous and the relative phase
of their clock signals is unknown.
28. A method according to claim 14, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said second
base transceiver station (BTS2) to one of said first and second
controllers (BSC, BSC2), with which said second base transceiver station
is associated, in a synchronization indication signal (HO DETECTION).
29. A method according to claim 28, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said
switching center (MSC) to said first base station controller (BSC1) in a
synchronization indication signal (HO DETECTION).
30. A method according to claim 28, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, further comprising the
step of transmitting said calculating timing advance (TA) from said second
base station controller (BSC2) to said switching center (MSC) in a
synchronization indication signal (HO DETECTION).
31. A method according to claim 30, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) from said
switching center (MSC) to said first base station controller (BSC1) in a
synchronization indication signal (HO DETECTION).
32. A method according to claim 14, wherein said cellular mobile radio
network is a GSM cellular mobile radio network, and further comprising the
step of transmitting said calculated timing advance (TA) in a control
signal (HO CMD (TA)) from one of said first and second base station
controllers (BSC, BSC1), with which said first base transceiver station
(BTS1) is associated, to said mobile station (MS), via said first base
transceiver station (BTS1).
33. A method according to claim 14, wherein said first and second base
transceiver stations (BTS1, BTS2) are asynchronous and the relative phase
of their clock signals is unknown.
34. A mobile station (MS) adapted to communicate by time-division multiple
access with at least first and second base transceiver stations
(BTS1,BTS2) of a cellular mobile radio network, said mobile station (MS)
comprising:
signal transceiver means (60, 65) for receiving a timing advance (TA) from
said first base transceiver station (BTS1) when said mobile station moves
from a first cell (C1), defined by the geographical coverage area of said
first base transceiver station (BTS1) with which said mobile station (MS)
is communicating, to a second cell (C2), defined by the geographical
coverage area of said second base transceiver station (BTS2),
means (61), coupled to said signal transceiver means (60,65), for detecting
an instruction (HO CHN) transmitted to said mobile station (MS) by said
first base transceiver station (BTS1) and which instructs said mobile
station (MS) to interrupt communication with said first base transceiver
station (BTS1) and to send a first sequence of synchronization signals (HO
ACCESS) to said second base transceiver station (BTS2) to enable said
second base transceiver station (BTS2) to calculate a timing advance (TA),
said instruction (HO CHN) also instructing said mobile station (MS) to
continue communicating with said first base transceiver station (BTS1)
immediately after sending said first sequence of synchronization signals
(HO ACCESS) to said second base transceiver station (BTS2), and
means (62), coupled to said instruction detecting means (61), for detecting
a control signal (HO CMD (TA)) transmitted to said mobile station (MS) by
said first base transceiver station (BTS1) and containing said timing
advance (TA), said control signal instructing said mobile station (MS) to
communicate with said second base transceiver station (BTS2) in accordance
with said timing advance (TA).
35. A mobile station according to claim 34, further comprising means (63),
coupled to said control signal detecting means (62), for deciding to send
a second sequence of synchronization signals (HO ACCESS) to said second
base transceiver station (BTS2) when said deciding means receives said
control signal (HO CMD (TA)) containing said timing advance (TA) to enable
said mobile station (MS) to communicate with said second base transceiver
station (BTS2).
36. A base station controller (BSC) for controlling at least first and
second base transceiver stations (BTS1,BTS2) of a cellular mobile radio
network having at least one mobile station, said base station controller
(BSC) comprising:
means for receiving a request signal from said first base transceiver
station (BTS1); and
means for sending, in accordance with said request signal, an instruction
to said first base transceiver station (BTS1) with which said mobile
station (MS) is communicating, said first base transceiver station (BTS1)
retransmitting this instruction to said mobile station (MS), said
instruction instructing said mobile station (MS) to interrupt
communication with said first base transceiver station (BTS1) in order to
send a sequence of synchronization messages (HO ACCESS) to said second
base transceiver station (BTS2) to enable said second base transceiver
station (BTS2) to calculate a timing advance (TA), in accordance with
which said mobile station (MS) is to communicate with said second base
transceiver station (BTS2), and to continue communicating with said first
base transceiver station (BTS1) immediately after sending said sequence of
synchronization messages to said second base transceiver station (BTS2).
37. A base station controller (BSC) as claimed in claim 36, further
comprising:
means (80) for receiving said timing advance (TA) transmitted to said base
station controller (BSC) in a message (HO DETECTION (TA)) sent by said
second base transceiver station (BTS2) with which said mobile station (MS)
wishes to communicate, after at least one of said synchronization messages
(HO ACCESS) to enable said mobile station (MS) to communicate with said
second base transceiver station (BTS2) is sent by said mobile station (MS)
to said second base transceiver station (BTS2) when said mobile station
(MS) moves from a first cell (C1), defined by the geographical coverage
area of said first base transceiver station (BTS1) with which said mobile
station (MS) is communicating, to a second cell (C2), defined by the
geographical coverage area of said second base transceiver station (BTS2),
and
means (80) for sending a control signal (HO CMD (TA)), containing said
timing advance (TA) for said mobile station (MS), to said first base
transceiver station (BTS1), said control signal (HO CMD (TA)) instructing
said mobile station (MS) to communicate with said second base transceiver
station (BTS2) in accordance with said timing advance (TA).
38. A controller according to claim 37, further comprising means (83, 84)
for sending information (HO DETECT (TA)) containing said calculated timing
advance (TA) to a mobile services switching center (MSC) upon reception of
said message (HO DETECTION (TA)) sent by said second base transceiver
station (BTS2), said mobile services switching center (MSC) controlling
said second base transceiver station (BTS2) based on said information.
39. A system for exchanging information between a mobile station (MS), at
least first and second base transceiver stations (BTS1, BTS2), and a base
station controller (BSC) for said at least first and second base
transceiver stations (BTS1, BTS2), exchanges of said information between
said mobile station (MS) and said at least first and second base
transceiver stations (BTS1, BTS2) being time-division multiple access
exchanges, said mobile station (MS) comprising means (61) for detecting an
instruction (HO CHN) when said mobile station moves from a first cell (C1)
defined by the geographical coverage area of said first base transceiver
station (BTS1) with which said mobile station (MS) is communicating to a
second cell (C2) defined by the geographical coverage area of said second
base transceiver station (BTS2), said instruction (HO CHN) instructing
said mobile station (MS) to send a sequence of synchronization signals (HO
ACCESS) to said second base transceiver station (BTS2) which comprises
means (71) for calculating, based on receipt of said synchronization
signals (HO ACCESS), a timing advance (TA) to be supplied to said mobile
station (MS), said mobile station (MS) continuing to communicate with said
first base transceiver station (BTS1) immediately after sending said
sequence of synchronization signals (HO ACCESS) to said second base
transceiver station (BTS2),
said second base transceiver station (BTS2) further including means (70)
for sending said calculated timing advance (TA) to said controller (BSC),
said base station controller (BSC) comprising means (80) for sending said
calculated timing advance (TA) to said first base transceiver station
(BTS1),
and said first base transceiver station (BTS1) comprising means (70) for
forwarding said calculated timing advance (TA) to said mobile station
(MS), said mobile station (MS) communicating with said second base
transceiver station (BTS2) in accordance with said timing advance (TA).
40. A system for exchanging information in a cellular mobile radio network
between a mobile station (MS), a first base transceiver station (BTS1)
associated with a first base station controller (BSC1), and a second base
transceiver station (BTS2) associated with a second base station
controller (BTS2), said first and second base station controllers (BSC1,
BSC2) being controlled by a mobile services switching center (MSC), said
exchanges of said information between said mobile station (MS) and said
first and second base transceiver stations (BTS1, BTS2) being
time-division multiple access exchanges, said mobile station (MS)
comprising means (51) for detecting an instruction (HO CHN) from said
first base transceiver station (BTS1) when said mobile station (MS) moves
from a first cell (C1) defined by the geographical coverage area of said
first base transceiver station (BTS1) with which said mobile station (MS)
is communicating to a second cell (C2) defined by the geographical
coverage area of said second base transceiver station (BTS2), said
instruction (HO CHN) instructing said mobile station (MS) to send a
sequence of synchronization signals (HO ACCESS) to said second base
transceiver station (BTS2) which comprises means for calculating, based on
receipt of said sequence of synchronization signals (HO ACCESS), a timing
advance (TA) to be supplied to said mobile station (MS), said mobile
station (MS) continuing to communicate with said first base transceiver
station (BTS1) immediately after sending said sequence of synchronization
signals (HO ACCESS) to said second base transceiver station (BTS2), said
second base transceiver station (BTS2) further comprising means (70) for
sending said calculated timing advance (TA) to said switching center (MSC)
via said second base station controller (BSC2),
said switching center (MSC) comprising means (90) for sending said
calculated timing advance (TA) to said first base station controller
(BSC1),
said first base station controller (BSC1) comprising means (80) for sending
said calculated timing advance (TA) to said first base transceiver station
(BTS1), and said first base transceiver station (BTS1) comprising means
(70) for forwarding said calculated timing advance (TA) to said mobile
station (MS), said mobile station (MS) communicating with said second base
transceiver station (BTS2) in accordance with said timing advance (TA). |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns the transmission of timing advance data to mobile
stations in a cellular mobile radio network such as a GSM (Groupe Special
Mobile) network when mobile stations move from one cell to another. The
mobile stations are car phones, for example, and the timing advance data
must enable a mobile station moving from one cell to another to advance
the timing of its transmission of digital data so that it is synchronized
with the transceiver station of the new cell. The standard term for this
procedure in the GSM system is "handover" and this term is used
hereinafter.
2. Description of the Related Art
The following description uses the standard GSM terminology. For more
information on this reference may usefully be had to the proceedings of
the "Digital Cellular Mobile Communication Seminar" held in Nice from 16
through 18 Oct. 1990.
FIG. 1 shows the structure of a cellular mobile radio network such as a GSM
type network.
A mobile station MS such as a car phone, for example, moves inside a cell
C1 defined by the geographical coverage of a base transceiver station
BTS1. Other cells C2, C3 each comprise a respective base transceiver
station BTS2, BTS3. Each of the stations BTS1 through BTS3 is one
component of the GSM network and comprises one or more transceivers each
associated with an antenna and processing equipment. The cells overlap in
part so that there are no shadow areas. The stations BTS1 through BTS3 are
managed by a base station controller BSC. The functions of the BSC include
management of BTS frequency channels. A BSC associated with a number of
BTS constitutes a base station system BSS. Other controllers may also be
provided, each controlling a predetermined number of BTS and each being
connected to a mobile services switching center MSC which is the master
structure of a GSM network. A given MSC can therefore control the
operation of several BSS constituting a public land mobile network (PLMN).
A network of this kind operates as follows: the mobile station MS sends
streams of digital data in the form of packets to the base transceiver
station BTS1 while it is in the cell C1 and the station BTS1 forwards
these streams to the BSC which sends them to their destination via the
MSC. This destination may be another mobile station or a fixed station.
Each data packet contains speech data, for example, and is transmitted in a
577 .mu.s time slot, eight consecutive time slots constituting a frame.
Eight mobile stations can therefore communicate on the same radio channel,
i.e. using the same carrier frequency, using time-division multiple access
(TDMA). Between two and four channels are usually assigned to each BTS and
between 16 and 32 radio channels are therefore available for transmission
(and reception) in each cell.
A problem arising in the GSM system is that of synchronizing time slots
assigned to a mobile station to the master clock at the BTS. It is
necessary to allow for the propagation time of radio waves between a
mobile station and its BTS because the mobile stations and the BTS serving
them each have their own internal bit clock. As the duration of a time
slot is 577 .mu.s and as a radio wave travels 300 m in 1 .mu.s
(3.times.10.sup.8 m/s), the mobile station clock must allow for a time
shift of 1 .mu.s per 300 m of distance between it and the BTS in order to
avoid sending data during the time slot assigned to another mobile
station.
FIG. 2 is a correlative timing diagram showing signals sent by the base
transceiver station BTS1 and by the mobile station MS. It shows how the
appropriate timing advance is communicated to the mobile station.
The base transceiver station BTS1 managing the cell in which the mobile
station MS is located sends a clock signal H.sub.0, H.sub.1, H.sub.2,
H.sub.4 regularly, at times T.sub.0, T.sub.1, T.sub.2, T.sub.3 and T.sub.4
on a synchronization channel SCH which is part of a broadcast control
channel BCCH for transmitting synchronization information to the mobile
stations. This clock signal is used when the mobile station must be logged
onto a cell of the GSM network, for example when it is switched on or in
the event of handover (see below).
The mobile station connects to the network for the first time after it is
switched on and can receive the clock signal only from the time MS.sub.ON
at which it is switched on.
Given that the mobile station is not usually located at the base
transceiver station BTS1, the first clock signal H.sub.1 that it receives
after time MS.sub.ON is shifted by a time T relative to the time T.sub.1
at which it is sent by the station BTS1. The signal H.sub.1 is therefore
received by the mobile station at time T.sub.1 +T.
At this time the mobile station requiring to connect to the base
transceiver station BTS1 sends to the latter on a signalling channel a
random access message (access burst). In the case of handover this message
is called the handover access message (HA). The duration of each handover
access is less than that of a burst constituting a normal signal (normal
burst), one containing speech data, for example, and accordingly cannot
interfere with signals sent by another mobile station in another time
slot.
On receiving this signal (at time T.sub.1 +TA) the base transceiver station
BTS1 can determine the timing advance TA between reception of this signal
and transmission of the clock signal H.sub.1. This is equal to twice the
time to transmit a signal between the mobile station and the base
transceiver station BTS1, i.e. twice the time T. The base transceiver
station BTS1 then sends a message to the mobile station over the access
grant channel AGCH to tell it that it must send its signals with a timing
advance TA relative to its clock signal: the mobile station can then send
normal signals without risk of them overlapping with those sent by other
mobile stations. This ensures that the signals sent by the various mobile
stations on a given transmission channel arrive in succession at the base
transceiver station BTS1.
This ensures that the signals sent by the various mobile stations on a
given transmission channel arrive in succession at the same BTS without
these signals overlapping. However, it is necessary to synchronize the
mobile stations frequently because their distance from the BTS serving
them may vary.
The problem of managing movement of the mobile station from one cell to
another is well known. In FIG. 1 the mobile station receives signals not
only from base transceiver station BTS1 but also from base transceiver
stations BTS2 and BTS3. If the power of the signals received from base
transceiver station BTS1 falls below that of signals received from base
transceiver station BTS2, for example, the BSC connects the mobile station
to base transceiver station BTS2 from which transmission then continues.
This is the typical situation when the mobile station moves away from base
transceiver station BTS1 and towards base transceiver station BTS2. It is
then necessary to modify the timing advance TA so that the mobile station
is synchronized to the base transceiver station BTS2 of the new cell C2.
There are three known types of handover for achieving such synchronization:
synchronous handover, pseudosynchronous handover and asynchronous
handover. Which type is used depends on whether the base stations are
respectively synchronized, have an internal clock at the same frequency
and of known phase, or have asynchronous clocks whose relative phase is
unknown.
Synchronous handover consists in controlling the clocks of the various BTS
of a given GSM system so that their clock signals are synchronized. It is
therefore unnecessary to supply a mobile station with a new timing advance
when it moves from one cell to another because the new timing advance is
deduced immediately from that previously used. To be generally adopted
this solution would require synchronization of all BTS, however, and would
therefore be costly to implement.
To alleviate this problem pseudosynchronous handover is used to synchronize
a mobile station to the clock of the BTS of the new cell allowing for the
time shifts between the clocks of the old and new BTS. This type of
handover is described in European patent application n.degree. 0 398 773
in the name of MATRA COMMUNICATION published on 22 Nov. 1990, for example.
This solution has the drawback that it is complex to implement and that
the BSS requires a learning phase.
Asynchronous handover is the simplest method to implement and therefore the
method most widely used. FIG. 3 shows the general principle. Consider the
case where the mobile station MS leaves cell C1 to enter cell C2. Eight
successive transmission steps are necessary.
Step 1 is that in which the mobile station MS sends the base transceiver
station BTS1 a message MEAS REP equivalent to a cell change request. This
standardized message is sent every 0.5 s. In step 2 the base transceiver
station BTS1 transmits this information (messa | | |
