U.S. patent number RE36,078 [Application Number 08/938,832] was granted by the patent office on 1999-02-02 for handover method for mobile radio system.
This patent grant is currently assigned to Telefonaktiebolaget LM Ericsson. Invention is credited to Alex K. Raith, Jan-Erik Uddenfeldt.
United States Patent |
RE36,078 |
Uddenfeldt , et al. |
February 2, 1999 |
Handover method for mobile radio system
Abstract
The invention relates to a method in mobile radio systems in
which the responsibility for transmitting message information to a
mobile station (MS.sub.1, MS.sub.2) is handed over from at least a
first base station transmitter (B.sub.ma, B.sub.mb) to at least a
second base station transmitter (B.sub.na, B.sub.nb). In accordance
with the invention, the same radio channel is used, if possible,
before and after the handover for transmitting message information
to the mobile station. If the same radio channel can be utilized,
the handover takes place without the mobile station being informed
beforehand of the handover by a special signal or order or the
like. In digital mobile radio systems with digital transmission of
message information by digital modulation of the radio signals, the
transmission is preferably started from a second base station
transmitter before the transmission is terminated from a first base
station transmitter. During a transmission time substantially the
same message information is transmitted to the mobile station from
both at least one first and one second base station
transmitter.
Inventors: |
Uddenfeldt; Jan-Erik (Hasselby,
SE), Raith; Alex K. (Durham, NC) |
Assignee: |
Telefonaktiebolaget LM Ericsson
(Stockholm, SE)
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Family
ID: |
20372619 |
Appl.
No.: |
08/938,832 |
Filed: |
September 26, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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836874 |
Feb 19, 1992 |
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365432 |
Jun 13, 1989 |
5109528 |
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Reissue of: |
071356 |
Jun 3, 1993 |
05327577 |
Jul 5, 1994 |
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Foreign Application Priority Data
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Jun 14, 1988 [SE] |
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8802229 |
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Current U.S.
Class: |
455/442; 455/524;
370/331 |
Current CPC
Class: |
H04W
36/08 (20130101); H04M 15/825 (20130101); H04W
36/0083 (20130101); H04W 36/18 (20130101); H04M
2215/7853 (20130101); H04M 2215/32 (20130101) |
Current International
Class: |
H04Q
7/38 (20060101); H04Q 007/20 () |
Field of
Search: |
;455/436,438,442,524,525,502,503,517,450,509 ;370/331 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 040 731 |
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Dec 1981 |
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EP |
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0 072 479 |
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Feb 1983 |
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EP |
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0 072 984 |
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Mar 1983 |
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EP |
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0274857 |
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Jul 1988 |
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EP |
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0274857 A1 |
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Jul 1988 |
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EP |
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2022425 |
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Nov 1971 |
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DE |
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3012141 |
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Oct 1981 |
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DE |
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0045882 |
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Jan 1970 |
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JP |
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57-73539 |
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May 1982 |
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JP |
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58-111443 |
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Jul 1983 |
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JP |
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58-159083 |
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Sep 1983 |
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JP |
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59-176935 |
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Oct 1984 |
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JP |
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59-212041 |
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Nov 1984 |
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JP |
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60-52131 |
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Mar 1985 |
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JP |
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60-42950 |
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Mar 1985 |
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JP |
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61-171269 |
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Aug 1986 |
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JP |
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62-132444 |
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Jun 1987 |
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JP |
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WO87/01897 |
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Mar 1987 |
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WO |
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WO90/13187 |
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Nov 1990 |
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WO |
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WO91/07020 |
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May 1991 |
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WO |
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|
Primary Examiner: Urban; Edward F.
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis,
L.L.P.
Parent Case Text
This application is a continuation of application Ser. No.
07/836,874, now abandoned, filed Feb. 19, 1992, which is a
continuation of application Ser. No. 07/365,432, filed Jun. 13,
1989, now U.S. Pat. No. 5,109,528.
Claims
I claim:
1. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting, from a first base station transmitter, radio signals
digitally modulated with message information to a mobile
station;
while transmitting the radio signals from the first base station
transmitter to the mobile station, beginning to transmit from a
second base station transmitter to the mobile station radio signals
digitally modulated with substantially the same message
information;
.Iadd.combining, in said mobile station, information transmitted by
said first base station transmitter and information transmitted by
said second base station transmitter to reconstruct said message
information; .Iaddend.and
terminating the transmission from the first base station
transmitter to the mobile station while continuing to transmit from
the second base station transmitter radio signals digitally
modulated with message information to the mobile station.
2. A method according to claim 1 comprising the step of:
transmitting, from a third base station transmitter to the mobile
station, radio signals digitally modulated with substantially the
same message information as the signals transmitted by the first or
second base station transmitters.
3. A method according to claim 1 further comprising the step
of:
transmitting, from a fourth base station transmitter to the mobile
station, radio signals digitally modulated with substantially the
same message information as the signals from the second base
station transmitter.
4. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting, from a first base station transmitter for a first
cell, radio signals digitally modulated with message information to
a mobile station;
while transmitting the radio signals from the first base station
transmitter to the mobile station, beginning to transmit, from a
second base station transmitter for a second cell to the mobile
station, radio signals digitally modulated with substantially the
same message information;
.Iadd.combining, in said mobile station, information transmitted by
said first base station transmitter and information transmitted by
said second base station transmitter to reconstruct said message
information; .Iaddend.and
terminating the transmission from the first base station
transmitter to the mobile station while continuing to transmit from
the second base station transmitter.
5. A method according to claim 4 further comprising the step
of:
transmitting, from a third base station transmitter to the mobile
station, radio signals digitally modulated with substantially the
same message information to as the signals transmitted by the first
or second base station transmitters.
6. A method according to claim 4 comprising the step of:
transmitting, from a fourth base station transmitter to the mobile
station, radio signals digitally modulated with substantially the
same message information as the signals from the second base
station transmitter.
7. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting, from a first base station transmitter to a mobile
station, radio signals digitally modulated with message information
to .[.the.]. .Iadd.a .Iaddend.mobile station;
transmitting, from a second base station transmitter .[.to the
mobile station.]., radio signals digitally modulated with
substantially the same message information to .[.the.]. .Iadd.a
.Iaddend.mobile station;
.Iadd.combining, in said mobile station, information transmitted by
said first base station transmitter and information transmitted by
said second base station transmitter to reconstruct said message
information; .Iaddend.and
before terminating the transmission from the first or second base
station transmitter of the digitally modulated radio signals to the
mobile station, beginning to transmit from a third base station
transmitter radio signals digitally modulated with substantially
the same message information as the signals from the first and
second base station transmitters.
8. A method according to claim 7 further comprising the step
of:
terminating the transmission from the first base station
transmitter to the mobile station while continuing to transmit from
the second and third base station transmitters radio signals
digitally modulated with substantially the same message information
to the mobile station.
9. A method according to claim 7 further comprising the step
of:
terminating the transmission from the second base station
transmitter while continuing to transmit from the third base
station transmitters radio signals digitally modulated with message
information to the mobile station.
10. A method according to claim 7 further comprising the steps
of:
before terminating the transmission of the digitally modulated
signals from the second base station transmitter, beginning to
transmit from a fourth base station transmitter to the mobile
station radio signals digitally modulated with substantially the
same message information as the signals from the second and third
base station transmitters; and
terminating the transmission from the second base station
transmitter while continuing to transmit from the third and fourth
base station transmitters radio signals digitally modulated with
substantially the same message information to the mobile
station.
11. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting, from a first base station transmitter, for a first
cell radio signals digitally modulated with message information to
a mobile station;
transmitting from a second base station transmitter radio signals
digitally modulated with substantially the same message information
to the mobile station; .[.and.].
before terminating the transmission from the first or second base
station transmitter to the mobile station, beginning to transmit
from a third base station transmitter for a second cell radio
signals digitally modulated with substantially the same message
information as the signals from the first and second base station
transmitters.Iadd.; and
combining, in said mobile station, information transmitted by at
least two of said first, second and third base station transmitters
to reconstruct said message information.Iaddend..
12. A method according to claim 11 further comprising the step
of:
terminating the transmission from the first base station
transmitter while continuing to transmit from the second and third
base station transmitters radio signals digitally modulated with
substantially the same message information to the mobile
station.
13. A method according to claim 11 further comprising the step
of:
terminating the transmission from the second base station
transmitter while continuing to transmit from the third base
station transmitter radio signals digitally modulated with message
information to the mobile station.
14. A method according to claim 11 further comprising the step
of:
before terminating the transmission from the second base station
transmitter, beginning to transmit from a fourth base station
transmitter to the mobile station radio signals digitally modulated
with substantially the same message information as the signals from
the second and third base station transmitters; and
terminating the transmission from the second base station
transmitter while continuing to transmit from the third and fourth
base station transmitters radio signals digitally modulated with
substantially the same message information to the mobile
station.
15. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting, from a first base station transmitter, radio signals
digitally modulated with message information to a mobile
station;
transmitting, from a second base station transmitter, radio signals
digitally modulated with substantially the same message information
to the mobile station;
before terminating the transmission from the first or second base
station transmitter to the mobile station, beginning to transmit
from a third base station transmitter radio signals digitally
modulated with substantially the same message information as the
signals from the first and second base station transmitters;
.[.and.].
.Iadd.combining, in said mobile station, information transmitted by
at least two of said first, second and third base station
transmitters to reconstruct said message information; and
.Iaddend.
terminating the transmission from the first base station
transmitter while continuing to transmit from the second and third
base station transmitters radio signals digitally modulated with
substantially the same message information to the mobile
station.
16. A method according to claim 15 further comprising the step
of:
terminating the transmission from the second base station
transmitter while continuing to transmit from the third base
station transmitter radio signals digitally modulated with message
information to the mobile station.
17. A method according to claim 15 further comprising the steps
of:
before terminating the transmission from the second base station
transmitter, beginning to transmit from a fourth base station
transmitter to the mobile station radio signals digitally modulated
with substantially the same message information as the signals from
the second and third base station transmitters; and
terminating the transmission from the second base station
transmitter while continuing to transmit from the third and fourth
base station transmitters radio signals digitally modulated with
substantially the same message information to the mobile
station.
18. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting, from a first base station transmitter for a first
cell, radio signals digitally modulated with message information to
a mobile station;
transmitting, from a second base station transmitter, radio signals
digitally modulated with substantially the same message information
to the mobile station;
before terminating the transmission from the first or second base
station transmitter to the mobile station, beginning to transmit
from a third base station transmitter for a second cell radio
signals digitally modulated with substantially the same message
information as the signals from the first and second base station
transmitters; .[.and.].
.Iadd.combining, in said mobile station, information transmitted by
at least two of said first, second and third base station
transmitters to reconstruct said message information; and
.Iaddend.
terminating the transmission from the first base station
transmitter while continuing to transmit from the second and third
base station transmitters radio signals digitally modulated with
substantially the same message information to the mobile
station.
19. A method according to claim 18 further comprising the step
of:
terminating the transmission from the second base station
transmitter while continuing to transmit from the third base
station transmitter radio signals digitally modulated with message
information to the mobile station.
20. A method according to claim 18 further comprising the steps
of:
before terminating the transmission from the second base station
transmitter, beginning to transmit from a fourth base station
transmitter to the mobile station radio signals digitally modulated
with substantially the same message information as the signals from
the second and third base station transmitters; and
terminating the transmission from the second base station
transmitter while continuing to transmit from the third and fourth
base station transmitters radio signals digitally modulated with
substantially the same message information to the mobile
station.
21. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting to a mobile station, from each of a first base station
transmitter, a second base station transmitter and a third base
station transmitter, radio signals digitally modulated with
substantially the same message information; .[.and.].
.Iadd.combining, in said mobile station, information transmitted by
at least two of said first, second and third base station
transmitters to reconstruct said message information; and
.Iaddend.
terminating the transmission of the digitally modulated signals
from the first base station transmitter to the mobile station while
continuing to transmit from the second and third base station
transmitters..Iadd.
22. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting, from a first base station transmitter, radio signals
digitally modulated with message information to a mobile
station;
while transmitting the radio signals from the first base station
transmitter to the mobile station, beginning to transmit from a
second base station transmitter to the mobile station radio signals
digitally modulated with substantially the same message
information;
receiving, at said mobile station, radio signals from both said
first and said second base station;
performing, at said mobile station, a correlation between said
received radio signals and a predetermined pattern;
combining, in said mobile station, information transmitted by said
first base station transmitter and information transmitted by said
second base station transmitter to reconstruct said message
information; and
terminating the transmission from the first base station
transmitter to the mobile station while continuing to transmit from
the second base station transmitter radio signals digitally
modulated with message information to the mobile
station..Iaddend..Iadd.
23. A method of communication in a cellular mobile radio system
having a plurality of base station transmitters and mobile stations
comprising the steps of:
transmitting, from a first base station transmitter, radio signals
digitally modulated with message information to a mobile
station;
while transmitting the radio signals from the first base station
transmitter to the mobile station, beginning to transmit from a
second base station transmitter to the mobile station radio signals
digitally modulated with substantially the same message
information;
combining, in said mobile station, information transmitted by said
first base station transmitter and information transmitted by said
second base station transmitter using information received from a
strongest signal path as well as other signal paths received within
a reception window to reconstruct said message information; and
terminating the transmission from the first base station
transmitter to the mobile station while continuing to transmit from
the second base station transmitter radio signals digitally
modulated with message information to the mobile
station..Iaddend..Iadd.
24. The method of any one of claims 1, 4, 7, 11, 15, 18, and 21
wherein said step of combining further comprises the step of:
using an adaptive equalizer to reconstruct said message
information..Iaddend.
Description
TECHNICAL FIELD
The present invention relates to a mobile radio system which has a
plurality of radio channels for transmitting control information
and message information between base stations and mobile stations.
More specifically, the invention relates to a method in which the
responsibility for transmitting message information to a mobile
station is handed over from at least a first base station
transmitter to at least a second base station transmitter.
BACKGROUND ART
It is desirable that a mobile radio system has high traffic
handling capacity and a high degree of coverage. The traffic
handling capacity of a mobile radio system is dependent, inter
alia, upon the number of available radio channels and how
effectively these channels may be utilised. It is known to arrange
several base stations with small covering areas close to each other
in a mobile radio system. Available radio channels can then be
utilised in a more efficient way for handling peak traffic within a
limited geographical area than if base stations with large covering
areas are arranged far away from each other in the mobile radio
system. Arranging several base stations close to each other can
also increase the accessibility in a mobile radio system,
particularly if the covering areas of the base stations are
mutually overlapping. Two base station with such overlapping
covering areas cannot, however, normally use the same radio channel
for communication with different mobile stations.
When a mobile station moves from the area covered by one base
station to the area covered by another base station in a mobile
radio system, the responsibility for communication with the mobile
must be transferred from one base station to another. Such transfer
in conjunction with an established connection to the mobile station
is usually called "handover".
Handover can be a critical function in known traditional mobile
radio systems. There is a risk of the established connection being
temporarily interrupted or completely lost in conjunction with the
handover. There is usually required comparatively extensive
signalling in the mobile radio system in conjunction with changing
of radio channel at handover. The degree of coverage of a mobile
radio system depends, inter alia, on the presence of radio shadows
and how the covering areas of the individual base stations overlap
each other. The possibility of establishing new connections and
maintaining established connections to/from mobiles also depends on
the presence of reflections and interference.
For increasing the degree of coverage in a mobile radio system, it
is known to transmit substantially the same information to a mobile
from two or more base transmitters and to receive information from
a mobile at two or more base receivers. The publication "RF
PERFORMANCE OF MACROSCOPIC DIVERSITY IN UNIVERSAL PORTABLE RADIO
COMMUNICATIONS: FREQUENCY REUSE CONSIDERATIONS" by Richard C.
Bernhardt, from IEEE International Conference on Communications,
Jun. 22-25, 1986, Toronto, Canada describes different types of
cells and placing of transmitters and the result of simulations of
such a system. The publication "USER ACCESS IN PORTABLE RADIO
SYSTEMS IN THE NOISE LIMITED ENVIRONMENT" by Richard C. Bernhardt,
from ICC'87, Jun. 1987, Seattle, Wash., U.S.A., describes different
types of cell and placing of transmitters as well as comparisons
between different criteria and algorithms for the selection of base
transmitter.
U.S. patent application Ser. No. 07/315,561 describes a mobile
radio system and a method of transmitting message information where
at least two bases at least partially simultaneously transmit
substantially the same message information to a mobile. The object
with this is, inter alia, to enable the degree of coverage to be
made greater without the cells in a mobile radio system needing to
be reduced, which gives greater freedom in the selection of cell
pattern, and fewer handovers. The intention is that the mobile
radio system shall be given better possibilities of establishing
new connections and maintaining already established connections.
According to the patent application, these objects are achieved by
certain relationships between modulation of radio signals and
parameters of adaptive equalisers in the mobiles and the distance
between base transmitters which transmit the same message
information.
In cellular mobile radio systems available channels may be
allocated to cells and mobiles in various ways. In systems with
fixed channel allocation a base for a particular cell may only use
certain predetermined channels. Bases in adjacent cells use
different channels. Bases in different cells sufficiently distant
from each other may use same channels according to a channel re-use
or allocation plan. In systems with adaptive channel allocation
some or all channels are a common resource to all base stations,
which means that some or all channels may be used by any base as
long as the radio transmission conditions permit, i.e. certain
carrier to interference or noise level.
SUMMARY OF THE INVENTION
At least in certain mobile radio systems, it is a problem to make
the handover so that the risk of temporary interruption in a
connection or that the connection is lost in conjunction with
handover is sufficiently small. This problem is particularly great
with peak traffic. Another problem in connection with handover is
the comparatively large need for signalling in the mobile radio
system. The object of the invention is, inter alia, to ameliorate
these problems.
One object of the present invention is to achieve a handover where
the risk of temporary interruption or lost connections can be
sufficiently small.
Another object of the present invention is to achieve a handover
where the need of signalling via radio signals in the mobile radio
system may be comparatively small.
A still further object of the present invention is to achieve a
handover which is particularly suitable for mobile radio systems
where two or more base transmitters spaced from each other transmit
essentially similar message information to a mobile.
Yet another object of the invention is to achieve a handover which
is particularly suitable for mobile radio systems with a high
degree of coverage and large overlapping between the covering areas
of different base transmitters.
In a method in accordance with the invention the responsibility of
transferring message information to a mobile station, is handed
over from at least one first base transmitter to at least a second
base transmitter. Distinguishing for a method in accordance with
the invention, and particularly preferred embodiments thereof will
be seen from the independent and the dependent claims. Somewhat
simplified, it may be said that in a method in accordance with the
invention, the same radio channel is preferably used for
transmitting message information to the mobile before and after the
handover.
The handover preferably takes place without the mobile being
informed beforehand that the handover is going to take place.
According to a further preferred embodiment of the invention, the
radio transmission of message information from a second base
station transmitter to the mobile station is started before
transmission of message information to the mobile from a first base
station transmitter is terminated. Essentially the same message
information is thus transmitted during a transition time to the
mobile both from a first and a second base station transmitter.
In mobile radio systems where at least two base station
transmitters are simultaneously responsible for transmitting
message information to at least certain mobiles, it may be
preferred not to start or interrupt the transmission from several
base transmitters simultaneously in conjunction with the handover.
The transmission of message information to the mobile is then
preferably started from one of the second base transmitters before
initiating transmission of message information from another of the
second base transmitters. In such a case, the transmission of
message information to the mobile from one of the first base
transmitters is preferably terminated before the transmission of
such information is terminated from another of the first base
transmitters. Initiation and termination of the transmission from
the base transmitters preferably takes place in an order such that
at least two base transmitters transmit essentially the same
message information to the mobiles the whole time.
In the cases where transmission from a second base transmitter must
take place on another radio channel than the transmission from a
first base transmitter, it is preferable that the radio channels
are selected such that during the transition time the mobile
station can receive the transmissions from the different base
transmitters essentially without interference from each other.
Designing the handover of responsibility for transmitting message
information to a mobile in accordance with the invention means
several advantages, which are accentuated in preferred
embodiments.
Perhaps the most important advantage is that in the cases where the
same radio channel is utilised by the bases before, during and
after the handover, the need for handover signaling via radio
signals can be made particularly small. If the mobile sends and
receives on the same channels before, during and after the
handover, it does not really need to know whether the
responsibility for communication has been handed over. In an
extreme case, there is thus no need of handover signalling at all
via radio signals in conjunction with handover, and all handover
signalling could take place in the stationary part of the mobile
radio system. Another important advantage is that the risk of
temporary interruption in an established connection call to a
mobile or that the connection is lost can be made small.
Another advantage is great flexibility with regard to distributing
tasks between the mobile and base in conjunction with the handover.
Preparations for and initiative to the handover can take place to a
greater or lesser extent in the mobile. It will then be possible to
form and distribute tasks in conjunction with preparation for,
initiation and execution of the handover such that the need of
signalling with respect to the handover will be comparatively
small.
A still further important advantage with the method in accordance
with the invention is that it is particularly suitable in a mobile
radio system where at least two base station transmitters are
simultaneously responsible for transmitting message information to
at least certain mobiles. In such a system the increasing degree of
coverage can be utilised to the full in handing over the
responsibility in accordance with the invention.
Further advantages ought to be understood by one skilled in the art
after having studied the description of preferred embodiments.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates cells and location of base station transmitters
in a mobile radio system where in certain areas two or three base
station transmitters simultaneously transmit message information to
at least certain mobiles.
FIG. 2 illustrates two mobiles and some stationary parts of a
mobile radio system according to FIG. 1.
BEST MODES FOR CARRYING OUT THE INVENTION
A method in accordance with the invention is particularly suitable
for a mobile radio system where at least two base station
transmitters are simultaneously responsible for transmitting
message information to at least certain mobile stations. It seems
therefore to be suitable to describe an embodiment of a method in
accordance with the invention in connection with such a system.
A cellular mobile radio system illustrated in FIG. 1 has mobile and
base stations with transmitters and receivers for radio signals.
Message information is transmitted digitally to and from the
mobiles by transmission and reception of radio signals with digital
modulation corresponding to the message information. The radio
signals are sent on one of a plurality of radio channels. Several
radio channels can share in time multiplex the same radio
frequency, and radio signals can be transmitted to and from several
mobiles in different time slots on the radio frequency, these slots
being associated with the radio channels.
The mobiles may move within and between the cells of the system.
The base transmitters are assigned to the cells so that there is at
least one base transmitter for each cell, for transmitting signals
to the mobile of the cell.
There is no completely fixed channel allocation for all cells and
base stations in the entire system. At least for some base stations
and cells there is allowed some freedom in the selection of radio
channels. Thus the channel allocation in the system is some kind of
dynamic or adaptive channel allocation meaning that at least some
channels are a common resource to at least some neighbour
bases/cells.
Somewhat simplified, there is illustrated in FIG. 1 the division of
an area into cells and the assignation of base station transmitters
to the cells in a mobile telephone system. For the sake of
simplicity in FIG. 1, all cells C1 to C24 are illustrated as
regular hexagons with sides L. In practice, the cells will probably
have different sizes and shapes. In addition, depending on traffic
conditions, it will often be suitable with overlapping in the
boundary areas between the cells. To a certain extent, the base
transmitters can then off-load each other by handling transmissions
to mobiles where such transmission should from a purely
geographically point of view, be performed by the base transmitters
of a contiguous cell.
For each cell C1-C24 there is an ordinary base transmitter
BS1-BS24. For contiguous cells these transmitters are
conventionally co-located in groups of three. For example, the base
transmitter BS1 for the cell C1 is co-located with the base
transmitter BS3 for the cell C3 and with the base transmitter BS5
for the cell C5. Correspondingly, the base transmitter BS14 for the
cell C14 is co-located with the base transmitter BS16 for the cell
C16 and with the base transmitter BS18 for the cell C18. These
co-located ordinary base station transmitters are situated in the
boundary regions between the cells to which they are assigned. For
example, the ordinary base transmitters BS2, BS4 and BS6 are
co-located in the boundary areas between the cells C2, C4 and
C6.
Further to the ordinary base transmitters BS1-BS24 the system
includes a number of extra base transmitters for certain of the
cells. Cells C6, C7, C10, C11, C13, C14, C19 and C20 each has one
extra base station transmitter. For each of the cells C15, C18 and
C22 there are two extra base transmitters. Of the extra base
transmitters XS6, XS7, XS10, XS11, XS13, XS15A, XS15B, XS18A,
XS18B, XS19, XS20 and XS22A are co-located in groups with three
extra base transmitters in each group in a similar manner as the
ordinary base transmitters. Accordingly, for example, the extra
base transmitter XS15B for the cell C15 is co-located with the
extra base transmitter XS19 for the cell C19 and the extra base
transmitter XS18A for the cell C18. On the other hand, neither the
extra base transmitter XS14 for the cell C14 nor the extra base
transmitter XS22B for the cell C22 are co-located with any other
base transmitter, but are situated approximately at the centre of
the cell with which they are associated.
An extra base transmitter does not need to differ technically from
an ordinary base transmitter. For a given cell an extra base
transmitter can thus have technical equipment of the same type as
an ordinary base transmitter for the same cell. In principle, it
can also function in the same way as the ordinary one. If there are
two identical base station transmitters for a given cell, either of
them may be respectively regarded as ordinary or extra in certain
cases.
The extra base transmitter or transmitters for a given cell
transmit radio signals which are substantially the same as those
sent by the ordinary base transmitter of the cell. The signals are
digitally modulated with selective digital message information to
the individual mobiles in the cell. The radio signals may also be
digitally modulated with general control information common to all
mobiles concerned. A mobile in a cell for which there is one or
more extra base transmitter can therefore receive, at least in
certain cases, corresponding radio signals from more than one base
transmitter approximately simultaneously within the same frequency
range. Depending on the mutual, relative positions in the cell of
the mobiles and bases as well as the transmission times and
propagation paths of the radio signals from the base transmitters
to the mobile stations, corresponding signals from different base
transmitters can be received without, or with a given time shift at
the mobile. The greater the distance between the base transmitters
associated with the cell, the greater in general can be the time
shift. When the ordinary base station transmitters and the extra
base station transmitters are situated according to FIG. 1, the
distance between two base transmitters for the same cell varies
between L and 2L, i.e. between the side and diameter of the regular
hexagons. If, for the sake of simplicity, reflections are ignored
and the assumption made that the base transmitters transmit without
mutual time shifting, the time reception shift at the mobile
station could then attain a maximum of 2L/c, where c is the
propagation rate of the radio signals.
In FIG. 2 there are illustrated parts of a mobile radio system in
accordance with FIG. 1. A mobile radio exchange MSC is connected
via cables L.sub.1, L.sub.2, . . . L.sub.m, L.sub.n to a plurality
of bases of which two, B.sub.m and B.sub.n are illustrated in FIG.
2.
The base B.sub.m has a central unit connected via cables L.sub.ma
and L.sub.mb to two transceiver units B.sub.ma and B.sub.mb
situated at a distance from the central unit. The central unit of
the base B.sub.m includes a central line and control unit 1,
transmission time shifting means 2A and 2B, one for each of the
transceivers, reception time shifting means 3A and 3B, one for each
of the transceivers and line units 4A and 4B, one for each of the
transceivers.
Both transceivers in the base B.sub.m are alike. Each such
transceiver contains a line and control unit 5A or 5B, transmitter
means 6A or 6B, receiver means 7A or 7B, a transmission-reception
filter 8A or 8B and an antenna 9A or 9B.
The base B.sub.n differs partly from base B.sub.m, primarily due to
its central line and control unit 10 being situated in
juxtaposition with one of its transceivers B.sub.na. Accordingly,
no cable with associated line units corresponding to L.sub.m,
L.sub.mb, 4A-5B is needed for the transceiver B.sub.na, but only
for the other transceiver B.sub.nb. In addition, no transmission or
reception time shifting means is included in any central unit in
B.sub.n, but the corresponding means 2A, 2B, 3A and 3B are
respectively included in transceivers B.sub.na and B.sub.nb.
The mobiles MS.sub.1 and MS.sub.2 are mutually alike. Each mobile
includes sound sensing means 11, encoding means 12, transmitting
means 13, transmit-receive switch 14, reception means 15, equaliser
and decoding means 16, sound reproducing means 17, control means 18
and means 19 for feeding in and out or presentation of digital
information.
Apart from the bases having two transceiver units at a distance
from each other, and having controllable transmission and reception
time shifting means, the mobile radio system in FIG. 2 functions in
most respects in a way well-known in mobile radio systems. No
complete description of how the system functions in different
respects should therefore be necessary for one skilled in the art,
and it should only be necessary to describe what is unique or
unusual in the mobile radio system according to FIG. 2. One not
skilled in the art of mobile radio systems is referred to the
technical litterature.
Message information e.g. speech or data that the mobile radio
exchange forwards towards a mobile in the cell C.sub.m, e.g. the
MS.sub.1, is transmitted from the mobile radio exchange via the
cable L.sub.m to the line and control unit 1. From here the
information is transferred via the transmission time shifting means
2A, line unit 4A, cable L.sub.ma and line and control unit 5A to
the transmitting means 6A. The transmitting means 6A transmits, via
the transmission-reception filter 8A and antenna 9A, radio signals
with digital modulation in correspondance with the message
information from the mobile radio exchange.
The message information from the mobile radio exchange is also
transferred from the line and control unit 1 via the transmission
time shifting means 2B, line unit 4B, cable L.sub.mb and line and
control unit 5B to the transmission means 6B in the transceiver
B.sub.mb. The transmission means 6B transmits, via the transmission
reception filter 8B and antenna 9B, radio signals with digital
modulation in correspondance with the information from the mobile
radio exchange.
Depending on the delay in transferring the message information to
the transmission means 6A and the corresponding delay in
transferring to the transmission means 6B the radio signals can be
transmitted from the antenna 9A of the transceiver unit B.sub.ma
substantially without time shifting, or time shifted in relation to
the transmission of corresponding radio signals from the antenna 9B
of the other transceiver B.sub.mb.
The radio signals from the antenna 9A i B.sub.ma arrive at a given
mobile in the cell C.sub.m, e.g. the mobile MS.sub.1, with or
without time shift in relation to corresponding radio signals from
the antenna 9B in B.sub.mb. The possible time shift on arrival at
the mobile depends partly on possible time shifting at transmission
from the antennas and partly on possible difference in propagation
time for the radio waves from the antennas. The transmission time
shifting means 2A and 2B have a variable delay and can be
controlled by the line and control unit 1, such that the signals
are transmitted from the antenna 9A in B.sub.ma time-shifted more
or less before or after corresponding signals from the antenna 9B
in B.sub.mb. In the preferred embodiment according to FIG. 2, the
line and control unit 1 controls the variable delays in the
transmission time shifting means 2A and 2B so that the differences
in delay in the cables L.sub.ma and L.sub.mb as well as the
differences in the radio signal propagation times are counteracted.
This may also be expressed by saying that the line and control unit
controls the variable delays in the transmission time shifting
means 2A and 2B, such that the time shift of the radio waves on
arrival at the mobile is decreased compared with the situation
where the means 2A and 2B have the same fixed delay. It could be
thought that the ideal case were that the line and control unit
controlled the delays in the time shifting means 2A and 2B so that
the digitally modulated signals transmitted from the antenna in
B.sub.ma arrived at the antenna in MS.sub.1, exactly simultaneously
and in phase with corresponding signals transmitted from the
antenna in B.sub.mb. In practice, this is neither striven for nor
normally achieved. Reflections occur as the radio signals are
propagated between the antennas, and the mobile has an adaptive
equaliser. It is therefore not necessary for the signals from the
different transceiver units to arrive exactly simultaneously to the
mobile. On the contrary, there is preferably sought a small time
shift to achieve diversity against Rayleigh fading. One not skilled
in this art and who is desirious of obtaining further information
can find it in the technical litterature and in publications such
as: "Radio Test Performance of a Narrowband TDMA System-DMS 90",
J-E Stjernvall, B. Hedberg, K. Raith, T. Backstrom and R.
Lofdahl.
In principle, there are at least two conceivable methods of
determining how the line and control unit 1 shall control the delay
in the transmission time shifting means 2A and 2B. One method is to
estimate in the fixed part of the mobile radio system the time
shift between the mobiles radio signals at one of the transceivers
B.sub.ma and corresponding radio signals at the other transceiver
B.sub.mb. There is thus obtained an estimation of the differences
in propagation time to the mobile, these differences depending on
the position of the mobile. Remaining differences in delay are
related to the fixed part of the mobile radio system, e.g.
differences in length of the cables L.sub.ma and L.sub.mb and are
not dependent on the position of the mobile. In the embodiment
according to FIG. 2, this method can be applied in practice such
that the delays in the reception time shifting means 3A and 3B are
adjusted so that information received from the mobile MS.sub.1 at
B.sub.ma arrives at the line and control unit 1 simultaneously as
corresponding information received at B.sub.mb from the mobile
station MS.sub.1 arrives at the line and control unit 1. The delays
in the transmission time shifting means 2A and 2B are subsequently
adjusted in correspondence with the optimum delays in the reception
time shifting means 3A and 3B.
The other method is to estimate in the mobile the difference in
arrival time or time shift between the digitally modulated radio
signals from one transceiver B.sub.ma and the corresponding digital
signals from the other transceiver B.sub.mb. Some kind of encoding
of the radio signals is required for this, which indicates from
which transceiver they are transmitted. In TDMA systems it is known
to transmit special synchronizing words. These can be utilised if
they are formed or supplemented with additional individual wards so
that two base transmitters for the same cell do not only have
identical synchronising words. Alternatively, special synchronising
words can be transmitted from the base station transmitters solely
to enable the mobile to estimate the differences in arrival times
or time shifts. The mobile transmits information about the
estimated arrival time difference or time shift via radio signals
to the fixed part of the system, where it is utilised for
controlling the transmission time shifting means 2A and 2B. The
line and control unit 1 then receives, via the respective line
units 5A, 4A and 5B, 4B, information about estimated arrival time
difference from the mobile in the same way as the line and control
unit obtains message information from the mobile.
It is conceivable per se, but hardly to be preferred, to combine
both methods for controlling the transmission time shift in a
mobile radio system according to FIG. 2.
Measuring the difference in arrival time or time shifting for
corresponding radio signals can be performed in a conventional way,
e.g. with the aid of correlation. In the cases where the radio
signals conventionally contain predetermined synchronising patterns
(words), the time difference between the appearance of these in
different signals can be measured using conventional methods. A
mobile control means 18 and/or a base line and control unit 1 or
10, possible in combination with the transceivers' line units 5A
and 5B can then include time measurement means for estimating
reception time shifting, or arrival time comparison means for
comparing arrival times.
When so required, a base preferably conventionally utilises the
same transmitter means and antenna for transmitting, in time
multiplex within the same frequency range, radio signals digitally
modulated with message information to different mobiles associated
with the same cell. Radio signals with message information to a
given mobile are then transmitted from different base transmitters
with a possible transmission time shift which is specially adjusted
with regard to the position of this particular mobile. The case can
arise where a base in a mobile radio system needs to transmit a
radio signal with information other than message information
intended for reception by several or all of the mobiles in the
cell, e.g. information as to the identity of the base/cell. Such
signals are preferably transmitted simultaneously without mutual
time shifting, from the transceivers B.sub.ma, B.sub.mb and
B.sub.na, B.sub.nb of the base in a mobile radio system according
to FIG. 2. The transmission time shifting means are then controlled
to a balancing state where the delay of information from the line
and control unit 1 to the antenna in one transceiver B.sub.ma is
equally as great as the delay of information from the line and
control unit 1 to the antenna in the other transceiver B.sub.mb.
The corresponding situation can apply when a base "listens" in
unoccupied combinations of time slot and frequency range for set up
calls from mobiles in unknown positions relative to the
transceivers of the base. The reception time shifting means 3A and
3B can then be controlled to a balancing state where the delay of
the message information from the antenna in one transceiver
B.sub.na to the line and control unit 9 is equally as great as the
delay of information from the antenna in the other transceiver
B.sub.nb to the line and control unit 9.
The mobiles MS.sub.1 and MS.sub.2 have adaptive equalisers, whereby
the digital modulation during a modulation time interval in the
radio signals transmitted from a base transmitter can be
reconstructed from signals received during a reception time
interval. In known cellular, digital, mobile radio systems with
only one base transmitter per cell, the reception time interval of
the equalisers is dimensioned according as the dispersion on the
radio channel, i.e. expected time shifts between corresponding
signals from a single base transmitter due to reflections. Because
of the equaliser, not only the radio signal having the greatest
amplitude or arriving first to the mobile station is utilised for
reconstructing the digital modulation, but also other corresponding
radio signals arriving with a time shift within the extent of the
equaliser's reception time interval can be utilised. The mobiles in
a system according to FIGS. 1 and 2 preferably have equalisers
which are dimensioned such that the reception time interval of the
mobile in reconstruction of the digital modulation is greater than
the time it takes for signals to propagate a distance as long as
the greatest distance between two base transmitters associated with
the same cell within a restricted geographical area. With the base
transmitters placed according to FIG. 1, the mobile equalisers
would thus be dimensioned for a reception interval in
reconstruction which is greater than 2 L/c. However, there can be
dispersion, and reflections can extend the propagation time from a
base transmitter to a mobile more than the extension of the
propagation time from another base transmitter for the same cells.
The reception time interval of the mobiles is therefore preferably
substantially greater than the time it takes for radio signals to
propagate a distance which is just as great as the greatest
distance between two base transmitters associated with the same
cell within the geographical area in question.
In mobile radio systems according to FIGS. 1 and 2, the modulation
time interval of the digital modulation of the signals can be of
the same order of magnitude as the time it takes for radio signals
to propagate a distance just as long as the greatest transmitting
distance between two base station transmitters serving the same
cell within a limited area. Although the use of extra base
transmitters in combination with equalisers described affords
greater advantages the smaller the modulation time interval is in
relation to this propagation time, and the described combined use
maybe has its greatest importance when the modulation time interval
has the same size as or is less than the mentioned propagation
time, the combined use of equalisers and extras base transmitters
can mean substantial advantages even when the modulation time
interval is some few times greater than the mentioned propagation
time.
It is conceivable to use different kinds of digital modulation in a
mobile radio system, whereby somewhat different relationships can
exist between information transmission rate and modulation time
intervals. In digital modulation of the radio signals involving
transmission of one symbol at a time of a sequence of uncorrelated
symbols, the modulation time interval will be the time during which
a single symbol is decisive for the digital modulation. For
example, if a sequence of binary symbols individually and one at a
time determines the modulation, the modulation time interval will
be the time during which one symbol determines the modulation. This
can also be expressed by saying that the modulation time interval
will be the inverted value of the transmission rate in bits. In
digital modulation of the signals involving two or more at a time
of a sequence of digital symbols being decisive for the modulation
during wholely or partly overlapping times, the modulation interval
can be the time when a preceeding, but not the nearest subsequent
symbol affects the modulation. For example, in digital modulation
according to SE 8102802-9, FIGS. 1-2, a symbol affects the phase of
a carrier wave during a time interval 3T. The nearest preceeding
symbol also affects the phase change of the carrier wave during a
first part 2T of the time interval 3T. The nearest subsequent
symbol also affects the change of phase of the carrier wave during
a last part of 2T of the time interval 3T. In this case the
modulation time interval will be T, which agrees with what is
called the symbol time interval in SE 8102802-9. In general, it can
be said that the modulation time interval is to be interpreted as
the interval in time between two successive changes in the
transmitted digital modulation.
Perhaps the most usual reason for handover in conventional mobile
radio systems is that a mobile moves from one area which is best
served by one base to another area which is best served by another
base. Of course, there may be other reasons, e.g. changed traffic
conditions or changed radio reception conditions.
When three or more bases can receive radio signals from a mobile,
the position of the latter can be estimated by comparison of the
radio propagation times. The estimation can be improved if more
than three bases are used. Knowledge of the position of the mobile
can be used by the stationary part of the mobile radio system for
selecting what bases are suitable for use as new bases when handing
over the responsibility. The mobile radio exchange or other
stationary part of the system assigns new bases the responsibility
in a manner such that communication can be maintained with good
quality as the mobile travels through the cell structure.
Knowledge of the position of the mobile can also be used as a
criterion for performing the handing over of responsibility. For
example, if the mobile moves out of the cell which is serving it at
the moment, this is an indication that a handover of responsibility
should soon take place, irrespective of whether the communication
quality is still good.
In different known mobile radio systems, the decision for handing
over can take place in more or less different ways. Different
criteria have been proposed for the decision as to if and where
handover is to take place. Collection of the basis for the handover
decision can take place in different ways. The distribution of
tasks and the signalling in connection with handover between
mobiles and the stationary part of the mobile radio system can
differ from system to system. All this is well known to one skilled
in the art. One not skilled in the art can obtain necessary
information from the technical litterature or United States patents
in the field of cellular mobile radio.
What is distinguishing for handing over the responsibility in
accordance with the present invention is primarily the actual
execution of handover. For the handover decision, at least in
certain cases, known parameters can be used such as the
signal-noise ratio for transmitting signals to and from the
mobiles. The signal-noise ratio or other parameters can be
measured, either solely in the stationary part of the mobile radio
system e.g. by the base central or control units or solely in the
mobiles by the reception and control means or in both mobiles and
the stationary part. In the handover decision, the traffic
conditions in different parts of the mobile radio system can be
given attention, substantially conventionally, e.g. if certain
bases tend to have too high traffic while adjacent bases have
unoccupied capacity for further traffic.
An embodiment of the excution of handing over responsibility for
the communication in accordance with the invention will now be
described in connection with FIG. 2. For the sake of simplicity, it
is assumed that handing over the responsibility will take place in
conjunction with a mobile moving from the position of MS.sub.1 in
FIG. 2 to the position of MS.sub.2 in FIG. 2.
Sometime before the handover, the mobile is in the position of
MS.sub.1 within the coverage area of the base station B.sub.m, but
outside the coverage area of the base station B.sub.n, i.e. the
radio transmission conditions for radio signals between the mobile
station and B.sub.ma or B.sub.mb are sufficiently good, but the
radio transmission conditions for radio signals between the mobile
and B.sub.na and B.sub.nb are too poor.
Sometime after the handover, the mobile is in the position of
MS.sub.2 within the coverage area for the base B.sub.n, but outside
the coverage area for the base B.sub.m, i.e. the radio transmission
conditions for radio signals between the mobile and B.sub.na and
B.sub.nb are sufficiently good, but the radio transmission
conditions for radio signals between the mobile and B.sub.ma and
B.sub.mb are too poor.
When the mobile has moved to position somewhere between the
positions of MS.sub.1 and MS.sub.2, a handover decision is made,
and this decision may be based on facts such as the radio
transmission conditions and traffic within the coverage areas of
the base stations. The decision is made by the mobile radio
exchange or possible some other stationary part of the mobile radio
system.
Before handing over the responsibility for communication with the
mobile, it is tested in the stationary part of the mobile radio
system whether the second base transmitter can use the same radio
channel after handover for transmitting message information to the
mobile as the radio channel the first base transmitter used before
handover. This may also be expressed by saying that the stationary
part tests whether a change of radio channel can be avoided on
handover.
This testing takes place according to predetermined rules based on
predetermined parameters. For example, if the base to which the
responsibility is to be handed over already utilises the radio
channel in question for communication with another mobile having
higher priority, a change of a radio channel must take place on
handover. On the other hand, if the base to which responsibility is
to be handed over does not utilise the radio channel in question,
and possible transmission on this radio channel would not interfere
with other traffic, there is no need for changing channels on
handover. Another reason for having to change channels is that the
channel in question has too much interference in the cell served by
the base to which responsibility is to be handed over. Other rules
or reasons for, and against, changing channels will be understood
by one skilled in this art.
After a decision as to handover and radio channel has been made,
one of the base transmitters B.sub.na or B.sub.nb begins to
transmit the same message information to the mobile as the base
transmitters B.sub.ma and B.sub.mb. This base transmitter is
preferably transmitting on the same channel as the base
transmitters B.sub.ma and B.sub.mb. In a TDMA mobile radio system,
this means that B.sub.na or B.sub.nb begin to transmit the same
message information as B.sub.ma and B.sub.mb on the same radio
frequency and in the same time slot as B.sub.ma and B.sub.mb. The
transmission times at B.sub.na and B.sub.nb of the radio signals is
preferably adjusted to corresponding transmission times for the
signals at B.sub.ma and B.sub.mb in accordance with what has been
described above. After transmission of the message information to
the mobile has been started from B.sub.na and B.sub.nb, the
transmission of corresponding message information to the mobiles
from one of the base transmitters B.sub.ma and B.sub.mb is
terminated. One of the base transmitters B.sub.ma or B.sub.mb then
transmits the same message information, e.g. speech or data, to the
mobile as one of the base transmitters B.sub.na and B.sub.nb.
Neither one of the two other base transmitters in FIG. 2 then
transmit corresponding information to the mobile. The one of the
base transmitters B.sub.ma and B.sub.mb which has the best radio
transmission conditions and the one of base transmitters B.sub.na
and B.sub.nb which has the best radio transmission conditions
preferably transmit to the mobile.
After one of the base transmitters B.sub.ma or B.sub.mb has ceased
to transmit message information to the mobile, a further one of the
base transmitters B.sub.na or B.sub.nb begins to send message
information to the mobile. Both B.sub.na and b.sub.nb as well as
one of the other base transmitters B.sub.ma and B.sub.mb are then
transmitting the same message information to the mobile. All three
transmitters preferably transmit on the same radio channel, and in
a TDMA mobile radio system this means the same radio frequency and
same time slot. The transmission times of B.sub.na and B.sub.nb and
at the third base transmitter of the radio signals are preferably
adjusted to each other in agreement with what has been described
above.
After both B.sub.na and B.sub.nb have begun to transmit the same
message information to the mobile as the remaining base transmitter
of B.sub.ma and B.sub.mb, the transmission of message information
to the mobile is terminated from this ramaining base transmitter.
Both B.sub.na and B.sub.nb are then transmitting message
information to the mobile, whereas none of the base transmitters
B.sub.ma and B.sub.mb is transmitting such information to the
mobile. At this, the responsibility for communication between the
mobile has been handed over from the base transmitters B.sub.ma and
B.sub.mb to the base transmitters B.sub.na and B.sub.nb. This may
also be expressed by saying that responsibility for the
communication has been handed over from the base B.sub.m to the
base B.sub.n, i.e. an execution of handover is completed.
During the whole of the above described handover, at least two of
the base transmitters send the same message information to the
mobile. During certain times there are three base transmitters
sending the same message information. On the other hand, all base
transmitters of the affected stations never send the same
information to the mobile. The risk of accidental interruption in
the transmission of message information between the mobile and the
fixed part of the mobile radio system will thus be small.
Simultaneously, the receiving time shift at the mobile will be less
than if all base transmitters were to send the same message
information to the mobile. In addition, re-use of radio channels if
facilitated in the mobile radio system when all affected base
transmitters never send simultaneously on the same radio
channel.
If, for some reason, it is necessary to change radio channels in
conjunction with handing over the responsibility for communication
from the base B.sub.m to the base B.sub.n, it is to be prferred
that the channel which is to be used after handover is selected
such that during a transition time the mobile can receive the same
message information on both channels. In a TDMA system according to
FIG. 2, this means that a base transmitter, e.g. B.sub.na, begins
to transmit the same message information as the base transmitters
B.sub.ma and B.sub.mb, but in another times slot, so that the radio
signals from B.sub.na arrive at the mobile during other times other
than the radio signals from B.sub.ma and B.sub.mb. When the base
transmitter B.sub.nb begins to transmit message information to the
mobile, it transmits in the same time slot as B.sub.na. Due to
B.sub.na and B.sub.nb transmitting message information in a time
slot than the one used by B.sub.ma and B.sub.nb, the mobile can
receive, by modifying its radio receiver tuning, both the message
information transmitted by B.sub.na and/or B.sub.nb and the message
information transmitted by B.sub.ma and B.sub.mb. In this case,
however, the mobile must be given advance information as to the
handover.
If handing over the responsibility for communication is to take
place from three or more base transmitters to more than two base
transmitters, its execution will naturally be somewhat different
than what has been described in connection with FIG. 2, since the
transmission form three or more base transmitters is to be
terminated or started. Different orders for starting and
terminating transmission are then conceivable. Common for these
varients is, however, that at least two base transmitters always
transmit the same message information to the mobile. In addition,
the transmission of message information is always started from at
least one base, to which responsibility shall be handed over before
transmission of message information is terminated from all base
transmitters, from which the responsibility shall be handed over.
The order is preferably such that never all base transmitters
simultaneously transmit the same message information to the mobile
in conjunction with the handover.
A mobile radio system where at least two base transmitters are
simultaneously responsible for transmitting message information to
a mobile does not need to be formed according to the above, but can
differ to a greater or lesser extent from the system described
above. For example, it is conceivable to co-localise at least parts
of the equipment in an ordinary base transmitter with parts of the
equipment in the extra base transmitter for the same cell,
providing that the antennas are spaced from each other. In an
extreme case, it is conceivable, in principle, for all equipment
excepting the antennas to be co-localised to one place, i.e. in the
vicinity of one of the antennas, and that the antennas are fed by
signals at radio frequency via a cable from this place. By "base
transmitter" shall be accordingly understood at least a transmitter
antenna for radio signals and preferably more or less of the
remining means required in a base. In a base transmitter there are
preferably included at least means corresponding to the means
included in a transceiver unit B.sub.ma or B.sub.mb in FIG. 2.
To avoid misunderstanding, it is pointed out that none of the base
transmitters to which responsibility shall be handed over is a
slave transmitter to any base transmitter from which the
responsibility shall be handed over. From FIG. 2 it will be seen,
for example, that the base transmitters B.sub.na and B.sub.nb can
be controlled directly by the mobile radio exchange MSC and
independently of how the exchange controls the base transmitters
B.sub.ma and B.sub.mb.
A method in accordance with the invention is of course not
restricted to a given implementation of base transmitter, even if
it is assumed in the example above that base transmitters can start
and terminate transmission at different times.
In actual fact, a method in accordance with the invention is not
restricted to mobile radio systems with two or more base
transmitters per cell, and it is conceivable to apply embodiments
of a method in accordance with the invention in a mobile system,
the method involves, summarily explained, tha the same radio
channel is used both before and after handover, if possible, for
transmitting message information to the mobile. A decision on
handover is preferably made in the mobile radio exchange or other
stationary part of the system, and the handover takes place without
the mobile being formed beforehand. If the mobiles have equalizers
and the system transmits message information digitally by digital
modulation of radio signals, it is also preferably in this case as
well that the transmission of message information is started from
the base transmitter to which responsibility is to be handed over,
before transmission of the same information is terminated from the
base transmitter from which responsibility is to be handed over. In
this case also, the same message information is transmitted during
a transition period to the mobile from two base transmitters. If it
is not possible to utilise the same radio channel for some reason,
it is then preferably in such a system to select the different
channels such that during a transition period the mobile can
receive the transmissions from the different base transmitters
essentially without their mutually obstructing each other. In a
TDMA system with mobiles according to FIG. 2, this means that the
base transmitters transmit in different time slots. The advantages
with a method in accordance with the invention will then be
substantially less, however, than in a system according to FIGS. 1
and 2, where the same radio channel can be utilised.
A method in accordance with the invention is not limited to
described embodiments or mobile radio systems according to FIGS. 1
and 2, but can be applied and varied within the scope of the
claims.
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