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Claims  |
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I claim:
1. A method of communication and handoff in a cellular mobile radio system
comprising a plurality of base stations, a plurality of mobile stations
and a plurality of radio channels, at least two of the radio channels
being used for digital communication channels by transmission of digitally
modulated radio signals on the radio channels, at least two of the digital
communication channels sharing a common radio channel in time division
multiplexing, at least two of the radio channels being used for control
channels by transmission of radio signals modulated with control
information on the radio channels, the method comprising the steps of:
transmitting, to a particular mobile station from a base station
responsible for the communication with the particular mobile station at
setup of a connection on a digital communication channel and at handoff of
a connection to a digital communication channel, information on a
particular time slot identifier code and a plurality of selected radio
channels;
transmitting, from the particular mobile station, the particular time slot
identifier code with radio signals in each time slot of the digital
communication channel of the connection during the connection;
measuring, at the particular mobile station, the strength of radio signals
received on the selected radio channels and the signal strength on the
digital communication channel used for the connection of the mobile
station;
transmitting repeatedly from the particular mobile station to the
responsible base station results of measurements of signal strength at the
particular mobile station;
measuring, at the base station responsible for the communication with the
particular mobile station, the signal strength on the digital
communication channel used for the connection of the mobile station;
processing and analyzing the results of the measurements done at the
responsible base station and the received results from the particular
mobile station and comparing with criteria for handoff;
determining a target base station assumed suitable for the communication
with the particular mobile station when, according to the processing and
analyzing and comparing, a handoff needs to be done;
supplying the target base station with information on the radio channel and
the time slot identifier code of the digital communication channel of the
established connection of the particular mobile station and requesting the
target base station to measure signal strength on the digital
communication channel;
using the supplied information on the radio channel and the time slot
identifier code at the target base station for finding the digital
communication channel of the established connection of the particular
mobile station and measuring signal strength on the digital communication
channel;
reporting from the target base station the measurement of signal strength
on the digital communication channel;
deciding whether the handoff shall be performed in view of a report from
the target base station and informing the target base station on the
particular mobile station when the handoff shall be performed and what
radio channel and time slot identifier code is to be used by the
particular mobile station for the established connection after the
handoff;
after receiving at the particular mobile station information that handoff
shall be performed, using the time slot identifier code at the particular
mobile station to find the correct time slots and transmitting from the
particular mobile station radio signals comprising the time slot
identifier code on the radio channel; and
after receiving at the target base station information that a handoff shall
be performed and information on the radio channel and the time slot
identifier to be used by the particular mobil station, checking at the
target base station for the appearance of radio signals comprising the
time slot identifier code on the radio channel.
2. A method according to claim 1 comprising the further steps of:
measuring bit error rates on the established connection on the digital
communication channel both at the responsible base station and at the
particular mobile station; and
using the results of the bit error rate measurements to assist in
determining when a handoff needs to be done.
3. A method according to claim 2 wherein at connection setup and handoff
the particular mobile station is informed of a digital voice color code to
be transmitted with radio signals from the particular mobile station, the
digital voice color code being selected independent of the time slot
identifier code;
transmitting the same digital voice color code with radio signals from at
least two mobile stations served by the same responsible base station;
transmitting different digital voice color codes from at least two mobile
stations using the same radio channel and time slot identifier but served
by different responsible base stations;
informing the target base station which digital voice color code is being
used by the particular mobile station; and
using the digital voice color code at the target base station when
measuring signal strength on the radio channel of the digital
communication of the particular mobile station before handoff is
performed.
4. A method according to claim 1 comprising the further steps of:
informing the particular mobile station at connection setup and handoff of
a digital voice color code to be transmitted with radio signals from the
particular mobile station;
transmitting the same digital voice color code from at least two mobile
stations served by the same responsible base station;
transmitting different digital voice color codes from at least two mobile
stations using the same radio channel and time slot identifier but served
by different responsible base stations;
informing the target base station which digital voice color code is being
used by the particular mobile station; and
after receiving at the target base station information that a handoff shall
be performed and information on the digital voice color code to be used by
the particular mobile station, checking at the target base station for the
appearance of radio signals comprising both the digital voice color code
and the time slot identifier on the radio channel.
5. A method according to claim 2 wherein at least one of the selected radio
channels is a radio channel used as a control channel by a base station
different from the responsible base station.
6. A method according to claim 4 comprising the further steps of:
at the particular mobile station coding the digital information to be
transmitted in a binary error detecting code;
adding modulo two to the coded binary information a binary coded digital
voice color code;
transmitting the modulo two sum with the digitally modulated radio signals
from the particular mobile station;
at the target base station demodulating the received radio signals to
obtain binary coded information;
adding modulo two the binary coded digital voice color code to the binary
coded information obtained from the received signals;
decoding the modulo two sum at the target base according to the error
detecting code to detect possible errors; and
using, at the target base station, the absence of errors in the modulo two
sum as an indication of the appearance of the digital voice color code in
the radio signals received.
7. A method of communication and handoff in a cellular mobile radio system
comprising a plurality of base stations, a plurality of mobile stations
and a plurality of radio channels, at least two of the radio channels
being used for digital communication channels by transmission of digitally
modulated radio signals on the radio channels, at least two of the digital
communication channels sharing a common radio channel in time division
multiplexing, at least two of the radio channels being used for control
channels by transmission of radio signals modulated with control
information on the radio channels, the method comprising the steps of:
transmitting, to a particular mobile station from a base station
responsible for the communication with the particular mobile station at
setup of a connection on a digital communication channel and at handoff of
a connection to a digital communication channel, information on a
particular time slot identifier code and a plurality of selected radio
channels;
transmitting, from the particular mobile station, the particular time slot
identifier code with the radio signals in each time slot of the digital
communication channel of the connection during the connection;
measuring at the particular mobile station the strength of radio signals
received on the selected radio channels and the signal strength on the
digital communication channel used for the connection of the mobile
station;
transmitting repeatedly from the particular mobile station to the
responsible base station results of measurements of signal strength at the
particular mobile station;
measuring, at the base station responsible for the communication with the
particular mobile station, the signal strength of the digital
communication channel used for the connection of the mobile station;
processing and analyzing the results of the measurements done at the
responsible base station and the received results from the particular
mobile station and comparing with criteria for handoff;
determining a target base station assumed suitable for the communication
with the particular mobile station when, according to the processing and
analyzing and comparing, a handoff needs to be done;
supplying the target base station with information on the radio channel and
the time slot identifier code of the digital communication channel of the
established connection of the particular mobile station and requesting the
target base station to measure signal strength on the digital
communication channel;
using the supplied information on the radio channel and the time slot
identifier code at the target base station for finding the digital
communication channel of the established connection of the particular
mobile station and measuring signal strength on the digital communication
channel;
reporting from the target base station the measurement of signal strength
on the digital communication channel;
deciding whether the handoff shall be performed in view of a report from
the target base station and informing the target base station and the
particular mobile station when the handoff shall be performed and what
radio channel and time slot identifier code is to be used by the
particular mobile station for the established connection after the
handoff;
at least, after receiving at the target base station information that a
handoff shall be performed, transmitting from the target base station
radio signals comprising the time slot identifier code on a radio channel;
after receiving at the particular mobile station information that handoff
shall be performed using the time slot identifier code transmitted in
radio signals from the target base station, received at the particular
mobile station, to find the correct time slots for the digital
communication channel;
transmitting from the particular mobile station radio signals comprising
the time slot identifier code on the radio channel and in the time slots
found with the aid of the time slot identifier; and
after receiving at the target base station information that a handoff shall
be performed and information on the radio channel and the time slot
identifier to be used by the particular mobile station checking at the
target base station the appearance of radio signals comprising the time
slot identifier code on the radio channel.
8. A method according to claim 7 comprising the further steps of:
measuring the bit error rates on the established connection on the digital
communication channel both at the responsible base station and at the
particular mobile station; and
using the results of the bit error rate measurements to assist in
determining when a handoff needs to be done.
9. A method according to claim 7 wherein at connection setup and handoff
the particular mobile station is informed of a digital voice color code to
be transmitted with radio signals from the particular mobile station, the
digital voice color code being selected independent of the time slot
identifier code;
transmitting the same digital voice color code with radio signals from at
least two mobile stations served by the same responsible base station;
transmitting different digital voice color codes from at least two mobile
station using the same radio channel and time slot identifier but served
by different responsible base stations;
informing the target base station which digital voice color code is being
used by the particular mobile station; and
using the digital voice color code at the target base station when
measuring signal strength on the radio channel of the digital
communication of the particular mobile station before handoff is
performed.
10. A method according to claim 7 comprising the further steps of:
informing the particular mobile station at connection setup and handoff on
a digital voice color code to be transmitted with radio signals from the
particular mobile station;
transmitting the same digital voice color code from at least two mobile
stations served by the same responsible base station;
transmitting different digital voice color codes from at least two mobile
stations using the same radio channel and time slot identifier but served
by different responsible base stations;
informing the target base station which digital voice color code is being
used by the particular mobile station; and
after receiving at the target base station information that a handoff shall
be performed and information on the digital voice color code to be used by
the particular mobile station, checking at the target base station for the
appearance of radio signals comprising both the digital voice color code
and the time slot identifier on the radio channel.
11. A method according to claim 10 comprising the further steps of:
at the particular mobile station coding the digital information to be
transmitted in a binary error detecting code;
adding modulo two to the coded binary information a binary coded digital
voice color code;
transmitting the modulo two sum with the digitally modulated ratio signals
from the particular mobile station;
at the target base station demodulating the received radio signals to
obtain binary coded information;
adding modulo two the binary coded digital voice color code to the binary
coded information obtained from the received signals;
decoding the modulo two sum at the target base station according to the
error detecting code to detect possible errors; and
using, at the target base station, the absence of errors in the modulo two
sum as an indication of the appearance of the digital voice color code in
the radio signals received.
12. A method according to claim 7 wherein at last one of the selected radio
channels is a radio channel used as a control channel by a base station
different from the responsible base station.
13. A method of communication and handoff in a cellular mobile radio system
comprising a plurality of base stations, a plurality of mobile stations
and a plurality of radio channels, at least two of said radio channels
being used for time divisional multiplexed digital communication channels,
comprising the steps of:
transmitting to a particular mobile station from a base station responsible
for the communication with said particular mobile station at setup of a
connection on a digital communication channel and at handoff of a
connection to a digital communication channel, information identifying a
particular time slot identifier code, a particular digital voice color
code and a plurality of selected radio channels;
transmitting from said particular mobile station said particular time slot
identifier and digital voice color code with radio signals in each time
slot of said digital communication channel of the connection during the
connection;
measuring at said particular mobile station the signal strength of radio
signals received on said selected radio channels and the signal strength
on said digital communication channel used for the connection of said
mobile station;
transmitting repeatedly from said particular mobile station to said
responsible base station results of measurements of signal strength at
said particular mobile station;
processing and analyzing results of the measurements and comparing with
predetermined criteria for handoff;
determining a target base station assumed suitable for being responsible
for the communication with said particular mobile station when, according
to said processing and analyzing and comparing, a handoff needs to be
attempted;
supplying said target base station with information identifying a radio
channel, said particular time slot identifier code and said particular
digital voice color code of said digital communication channel of the
established connection of said particular mobile station;
using said information on said radio channel and said particular time slot
identifier code at said target base station for finding said digital
communication channel of the established connection of said particular
mobile station and checking for the appearance of said particular digital
voice color code;
reporting from said target base station results of said checking for the
appearance of said particular digital voice color code on said digital
communication channel of the established connection;
deciding whether the handoff will be performed in view of said results
reported from said target base station and informing said target base
station and said particular mobile station if the handoff will be
performed;
informing said particular mobile station on new radio channels, new time
slot identifier and digital voice color codes to be used by said
particular mobile station and target base station for the established
connection after handoff;
transmitting from said target base station radio signals comprising said
new time slot identifier code on said radio channel to be used by said
target base station for the connection after handoff, at least after
receiving at said target base station information that a handoff will be
performed;
using said new time slot identifier code at said mobile station to find a
correct time slot and transmitting from said particular mobile station
radio signals comprising said new time slot identifier code and digital
voice color code on said new radio channel, after receiving at said
particular mobile station information that handoff will be performed; and
checking at said target base station for the appearance of radio signals
comprising both said new time slot identifier code and said digital voice
color code on said new radio channel, after receiving at said target base
station information that a handoff will be performed and information on
which new radio channel and new time slot identifier code is to be used by
said particular mobile station.
14. A method according to claim 13, further comprising the steps of:
measuring bit error rates on the established connection on said digital
communication channel at said responsible base station and at said
particular mobile station; and
utilizing results of said bit error rate measurements when determining when
a handoff should be attempted.
15. A method according to claim 13, further comprising the steps of:
selecting a digital voice color code independently of a time slot
identifier code;
transmitting the same digital voice color code with radio signals from at
least two mobile stations served by said responsible base station; and
transmitting different digital voice color codes from at least two mobile
stations using the same radio channel and time slot identifier code, but
being served by different base stations.
16. A method according to claim 13, further comprising the steps of:
measuring at said base station responsible for the connection with said
particular mobile station the signal strength on said digital
communication channel used for the connection of said particular mobile
station;
utilizing results of said signal strength measurements at said responsible
base station when determining whether a handoff should be attempted;
measuring at said target base station the signal strength on said digital
communication channel of the established connection of said particular
mobile station;
reporting from said target base station results of said measurements of
signal strength on said digital communication channel; and
utilizing results of said signal strength measurements at said target base
station when determining whether the handoff will be performed.
17. A method according to claim 16, further comprising the steps of:
coding digital information to be transmitted in a binary error detecting
code at said particular mobile station;
adding to said coded digital information a binary coded digital voice color
code modulo-two to provide a modulo-two sum;
transmitting the modulo-two sum with said radio signals from said
particular mobile station;
demodulating received radio signals to obtain binary coded information at
said target base station;
adding modulo two said binary coded digital voice color code to said coded
digital information obtained from said received signals;
decoding said modulo-two sum at said target base station according to said
binary error detecting code in order to detect possible error; and
utilizing at said target base station an absence of error in said
modulo-two sum as an indication as to the appearance of said digital voice
color code in said radio signal received.
18. A method according to claim 13, wherein at least one of said selected
radio channels is a radio channel used for a control channel by a
different base station than said responsible base station.
19. A method of communication and handoff in a cellular radio system
comprising a plurality of base stations, a plurality of mobile stations
and a plurality of radio channels, at least two of the radio channels
being used for time division multiple access communication channels, the
method comprising the steps of:
transmitting, from a base station responsible for the communication with a
particular mobile station at setup of a connection on communication
channel and at handoff of a connection to a communication channel to the
particular mobile station, information regarding a particular time slot
identifier code and a plurality of selected radio channels;
transmitting from the particular mobile station the particular time slot
identifier code with radio signals in each time slot to the digital
communication channel used by the particular mobile station for the
connection during the connection;
measuring at the particular mobile station the strength of radio signals
received on the selected radio channels and the signal strength on the
communication channel used for the connection of the particular mobile
station;
transmitting frequently from the particular mobile station to the
responsible base station results of the measurements of signal strength at
the particular mobile station;
measuring, at the base station responsible for the communication with the
particular mobile station, the signal strength on the communication
channel used for the connection of the particular mobile station;
processing and analyzing results of the measurement done at the responsible
base station and the received results from the particular mobile station
and comparing said results with predetermined criteria for handoff;
determining a target base station assumed suitable for the communication
with the particular mobile station when, according to the processing and
analyzing and comparing, a handover needs to be done;
supplying the target base station with information regarding the radio
channel and the time slot identifier code of the communication channel of
the established connection of the particular mobile station and requesting
the target base station to measure signal strength on the communication
channel;
using the supplied information regarding the radio channel and the time
slot identifier code at the target base station for finding the time slot
of the communication channel of the established connection of the
particular mobile station and measuring signal strength on the
communication channel;
reporting from the target base station the measurement of signal strength
on the communication channel;
deciding whether the handoff shall be performed in view of a report from
the target base station and informing the target base station and the
particular mobile station when the handover shall be performed and what
radio channel and time slot identifier code is to be used by the
particular mobile station for the established connection after the
handoff;
after receiving information that handoff shall be performed at the
particular mobile station, using the time slot identifier code to find the
correct time slots and transmitting from the particular mobile station
radio signals comprising the time slot identifier code on the radio
channel; and
after receiving at the target base station information that handoff shall
be performed and what new radio channel and time slot identifier is to be
used by the particular mobile station, checking at the target base station
for the appearance of radio signals comprising the time slot identifier
code on the radio channel.
20. A base station for communication with mobile stations in a cellular
mobile radio system having radio channels for time division multiple
access communication channels in the time slots in frames of the radio
channels comprising:
means for storing at least as many different time slot identifier codes as
the number of time slots in a multi slot frame on the radio channels;
means for transmitting bursts of radio signals in time slots in frames on
the radio channels;
means for incorporating in each burst transmitted from the base station one
of the stored time slot identification codes indicative of the time slot
of the frame of the channel where the burst is transmitted that
independent of the base station identity and radio channel where the burst
is transmitted;
means for receiving bursts of radio signals in time slots of frames on the
radio channels;
means for synchronizing the receiving means to received bursts by using the
time slot identifier codes of the received bursts;
means for identification of any of the predetermined time slot identifier
codes which appear in a received burst;
means for measuring the signal strength of a burst on a radio channel in a
time slot determined by a time slot identifier code;
means for storing a plurality of digital voice color codes independent of
the time slot identifier codes;
means for incorporating in each burst transmitted from said base station
one of said stored digital voice color codes independent of said time slot
identifier code in the burst and channel where the burst is transmitted;
and
means for identifying any predetermined digital voice color codes which
appear in a received burst.
21. A mobile station for communication with base stations in a cellular
mobile radio system having radio channels for time division multiple
access communication channels in time slots in frames of the radio
channels comprising:
means for storing at least as many different time slot identifier codes as
the number of time slots in a multi slot frame on the radio channels;
means for transmitting bursts of radio signals in time slot identifier
codes as the number of time slots in a multi slot frame on the radio
channels;
means for incorporating in each burst transmitted from the mobile station
one of the stored time slot identification codes indicative of the time
slot of the frame of the channel where the burst is transmitted but
independent of the mobile station identity and radio channel where the
burst is transmitted;
means for receiving bursts of radio signals in time slots of frames on the
radio channels;
means for synchronizing the receiving means to received bursts by using the
time slot identifier codes of the received burst;
means for identification of any of the predetermined time slot identifier
codes which appear in a received burst;
means for measuring the signal strength of a burst on a radio channel in a
time slot determined by a time slot identifier code;
means for storing a plurality of digital voice color codes independent of
said time slot identifier codes;
means for incorporating in each burst transmitted from said mobile station
one of said stored digital voice color codes independent of said time slot
identifier code in the burst and channel where the burst is transmitted;
and
means for identifying any predetermined digital voice color codes which
appear in a received burst. |
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Claims |
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Description |
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FIELD OF INVENTION
This invention relates to cellular mobile radio systems having radio
channels for transmitting digital or digitized information between base
and mobile stations by transmitting digitally modulated radio signals in
time multiplex. More precisely the invention relates to a handover method
whereby the responsibility for communication with a mobile having a
connection established on a radio channel is handed over from one
responsible base station to another target station. The invention also
relates to a base station and mobile station comprising means for
communication and performing handoff.
BACKGROUND OF THE INVENTION
In cellular mobile radio systems it is fundamental that a mobile with an
established connection on a radio channel shall be able to maintain the
established connection when moving from one cell served by one base
station to another cell served by another base station. It is also highly
desirous that a mobile with an established connection on a radio channel
shall be able to maintain the established connection when moving within
the same cell and the radio channel used is subject to increased
interference. The process by which a mobile can be able to maintain an
established connection when moving in cells in a cellular radio system is
generally called handoff.
In general a radio communication is only possible when the desired
information carrying radio signals have a sufficient signal strength at
the receiver and are sufficiently strong in relation to noise and
interfering radio signals at the receiver. The minimum strength of course
depends on the particular features of the system, e.g. the kind of
modulation and receiver used. In order to make sure an established
connection may continue on an intended radio channel between a mobile and
an intended base station a handoff process comprises some kind of
measurements on radio signals at the intended base and/or mobile station.
The first cellular mobile radio systems in public user were analog systems
for speech or other analog information. The systems comprised multiple
radio channels for transmitting analog information between base and mobile
stations by transmitting analogously modulated radio signals. In general
the first cellular mobile radio systems had comparably large cells. In
general the signal measurements in the handoff process in such systems
were performed by the base stations. One such system is the Nordic Mobile
Telephone system NMT 450. Another known cellular mobile radio system of
particular interest as a background to the present invention is the AMPS
mobile radio system in U.S.A.
Recently digital cellular mobile radio systems for public use have been
designed. Digital cellular mobile radio systems comprise digital radio
channels for transmitting digital or digitized analog information between
base and mobile stations by transmitting digitally modulated radio
signals. Digital cellular mobile radio systems may offer substantial
advantages over analog cellular mobile radio systems. To achieve these
advantages there are certain demands. In particular channel supervision
must be frequent and the handoff process must be rapid and be allowed to
be frequent in relation to old analog systems.
One digital mobile radio system intended to be a common system for many
european countries is the GSM system. In european countries already having
an analog cellular mobile system the new digital GSM system is intended to
be introduced as a new system independent of any old existing analog
system. The GSM system base and mobile stations has not been designed to
be compatible with old existing systems but to give optimum performance in
various aspects all by itself. Accordingly there has been a comparatively
great freedom of choice in technical matters when designing the GSM
system. In particular this is true with the handoff method in the GSM
system. In the GSM system measurements for the purpose of handoff are done
both by base and mobile station.
In contrast to introducing a new independent digital cellular mobile radio
system like the GSM system in an area with an existing analog cellular
system it has been proposed to introduce a digital cellular mobile radio
system designed for cooperation with the existing analog cellular mobile
radio system. To obtain digital radio channels within the frequency band
allotted to cellular mobile radio systems it is proposed to withdraw a
number of radio channels allotted to present analog mobile radio systems
and use them in digital cellular mobile radio system. Due to the proposed
design of the digital mobile radio system three or possibly six digital
radio channels may use in time division multiplex the frequency band of
one previous analog radio channel. Accordingly replacing some analog
channels by multiplex digital radio channels in time division may increase
the total number of radio channels in the joint analog and digital
systems.
The intention is to gradually introduce the digital system and with time
gradually increase the number of digital channels and decrease the number
of analog channels in the coexisting cellular systems. Analog mobiles
already in use shall be able to continue to use remaining analog channels.
New digital mobiles shall be able to use new digital channels. New
dual-mode mobiles shall be able to use both remaining analog channels and
new digital channels. In such a system the handoff process for digital
radio channels can not simply be selected to be identical with any handoff
process for any digital system, e.g. the GSM system. The handoff process
for digital channels must not interfere with the handoff process of the
existing analog system and must not require amendments of signals in the
analog system. There are special advantages if some control channels or
signals from base stations may be used for various purposes in both the
analog and the digital system. The present invention aims at such a
handoff method. The invention also aims at a base station and a mobile
station comprising means for communication and performing such handover
for digital channels.
SUMMARY OF THE INVENTION
This invention aims at a method of communication and handoff method
particularly suitable for a cellular mobile radio system comprising base
stations, mobile stations, radio channels for transmitting analog
information between mobile stations and base stations by transmitting
analog modulated radio signals, radio channels for transmitting digital or
digitized information between mobile and base stations by transmitting
digitally modulated radio signals in time division multiplex, and control
channels for transmitting control information between base and mobile
stations.
One object of the present invention is to enable a handoff method for the
digital part of a joint digital analog cellular mobile radio system, the
handoff method not interfering with the analog part of the joint cellular
mobile radio system.
Another object of the present invention is to enable a handoff method for a
new digital cellular mobile radio system to be used in coexistance with an
old analog cellular mobile radio system, whereby the digital system may
use base station control channels or signals of the analog system without
requiring amendment of control signals in the analog system.
Yet another object of the present invention is to achieve a communication
and handoff method for digital radio channels in a cellular mobile radio
system where the burden of measurements of radio signal strength and
quality are shared by mobile and base stations.
Still another object of the present invention is to achieve a handoff
method for a cellular mobile radio system having both analog and digital
radio channels whereby dual-mode mobiles can easily change from a digital
to an analog radio channel in connection with handoff.
The invention also has the object of providing means for convenient handoff
for connections on digital communication channels sharing radio channels
in time division multiplex.
In brief a communication and handoff method according to the present
invention normally comprises the following steps for time division
multiple access communication channels:
At setup of a connection the base responsible for the communication with a
particular mobile station transmits to the mobile station, information on
a particular radio channel including a particular time slot identifier
code to be used for the connection and a plurality of selected radio
channels. After handoff the new base station responsible for the
communication transmits to the particular mobile station a new plurality
of selected radio channels.
The mobile station transmits the particular time slot code with radio
signals in each time slot of the digital communication of the connection
during the course of the connection.
The particular mobile station measures the strength of radio signals
received on the selected radio channels and the signal strength on the
digital communication channel used for the connection of the mobile
station.
The particular mobile station transmits regularly to the responsible base
station results of measurements of signal strength at the particular
mobile station.
The base station responsible for the communication with the particular
mobile station measures the signal strength on the digital communication
channel used for the connection of the mobile station.
The results of the measurements done at the responsible base station and
the received results from the particular mobile station are processed and
analyzed and compared with criteria for handoff.
A target base station assumed suitable for being responsible for the
communication with the particular mobile station is determined when,
according to the processing and analyzing and comparing, a handover ought
to be done. The target base station is supplied with information on radio
channel and time slot identifier code of the digital communication channel
of the established connection of the particular mobile station and is
requested to measure signal strength on the digital communication channel.
The supplied information on radio channel and time slot identifier code is
used at the target base station for finding the digital communication
channel of the established connection of the particular mobile station and
measuring signal strength on the digital communication channel.
The target base station reports the results of the measurements of signal
strength on the digital communication channel.
A decision is then made whether the handoff shall be performed in view of
the report from the target base station and the target base station and
the particular mobile station are informed when the handover shall be
performed and what radio channel and time slot identifier code is to be
used by the particular mobile station for the established connection after
the handoff.
After the particular mobile station has received information that handoff
shall be performed the time slot identifier code is used by the particular
mobile station to find the correct time slots and the particular mobile
station thereafter transmits radio signals comprising the time slot
identifier code on the radio channel.
After receiving information that a handoff shall be performed and
information on radio channel and time slot identifier to be used by the
particular mobile station the target base station checks the appearance of
radio signals comprising the time slot identifier code on the radio
channel.
Finally selecte | | |
