U.S. patent number RE36,017 [Application Number 08/136,760] was granted by the patent office on 1998-12-29 for cellular digital mobile radio system and method of transmitting information in a digital cellular mobile radio system.
This patent grant is currently assigned to Telefonaktiebolaget LM Ericsson. Invention is credited to Alex K. Raith, Jan E. Uddenfeldt.
United States Patent |
RE36,017 |
Uddenfeldt , et al. |
December 29, 1998 |
**Please see images for:
( Certificate of Correction ) ** |
Cellular digital mobile radio system and method of transmitting
information in a digital cellular mobile radio system
Abstract
The invention relates to a cellular digital mobile radio system
including base stations (B.sub.m, B.sub.n) and mobile stations
(MS.sub.1, MS.sub.2) with transmitters and receivers. The invention
also relates to a method of transmitting message information
digitally between mobile and base stations in such a system. In
accordance with the invention, at least two base station
transmitters (B.sub.ma, B.sub.mb, B.sub.na, B.sub.nb) at a given
transmitting distance from each other are assigned to each of
certain cells (C.sub.m, C.sub.n) within a restricted geographical
area. The base station transmitters which are assigned to the same
cell transmit digitally modulated radio signals within the same
frequency range at least partially simultaneously to the mobile
stations of the cell. The radio signals from different base station
transmitters associated with the same cell are digitally modulated
with the same message information to the mobile stations in the
cell. Different base station transmitters (9A, 9B) preferably
transmit the digitally modulated radio signals with the same
message information to a given mobile station with a given mutual
transmission time shift. Here, the transmission time shift is
selected individually for each mobile station, such that
corresponding digitally modulated radio signals with the same
message information to a given mobile station from different base
station transmitters arrive practically simultaneously at the
mobile station.
Inventors: |
Uddenfeldt; Jan E. (Vallingby,
SE), Raith; Alex K. (Kista, SE) |
Assignee: |
Telefonaktiebolaget LM Ericsson
(Stockholm, SE)
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Family
ID: |
20371525 |
Appl.
No.: |
08/136,760 |
Filed: |
October 15, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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315561 |
Feb 27, 1989 |
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Reissue of: |
579283 |
Sep 6, 1990 |
05088108 |
Feb 11, 1992 |
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Foreign Application Priority Data
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Feb 29, 1988 [SE] |
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8800698 |
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Current U.S.
Class: |
375/230; 375/267;
455/442 |
Current CPC
Class: |
H04B
7/2625 (20130101); H04H 20/67 (20130101); H04W
88/08 (20130101) |
Current International
Class: |
H04B
7/26 (20060101); H04Q 7/30 (20060101); H04B
007/04 (); H04B 007/26 () |
Field of
Search: |
;455/33.1,33.3,33.4,51.1,51.2,52.3,54.1,56.1,63,65,436,437,442,443,444,525
;379/59,60,63 ;342/350,367,368,463,464
;375/229,230,232,260,267,299,348 |
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[Referenced By]
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Primary Examiner: Kizou; Hassan
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis,
L.L.P.
Parent Case Text
This application is a continuation of application Ser. No. 315,561,
filed Feb. 27, 1989, now abandoned.
Claims
We claim:
1. A cellular mobile radio system comprising a plurality of mobile
stations which are movable within and between a plurality of cells
and a plurality of associated base stations assigned to said cells
for digital transmission of message information, each of said
plurality of base stations and mobile stations having associated
therewith a respective set of a transmitter and a receiver so that
said message information in the form of radio signals with digital
modulation can be communicated therebetween, said radio signals
being digitally modulated with said message information within
modulation time intervals, each of said receivers including means
for reconstructing the digital modulation from corresponding radio
signals received within a predetermined reception time interval;
said cells and base stations being associated with one another in a
manner such that at least two base station transmitters are
assigned to each of predetermined cells within a limited geographic
area, said at least two base station transmitters being disposed at
a predetermined transmitting distance from each other and each
operable to transmit segments of digitally modulated radio signals
virtually simultaneously and within the same frequency range with
identical message information being transmitted by the other of
said at least two base station transmitters to mobile stations
within a cell to which both of said transmitters are assigned, the
digitally modulated radio signals from said at least two base
station transmitters having modulation time intervals which are
shorter than the time required for radio signals to propagate a
distance which is as long as a greatest transmitting distance
between two base station transmitters assigned to one cell within
said geographic area, wherein
said reconstructing means associated with each of said receivers in
said plurality of mobile stations operate to reconstruct digital
modulation of corresponding radio signals received during a
reception time interval which is at least as long as a time
required for radio signals to propagate a distance which is as long
as said greatest transmitting distance.
2. A cellular mobile radio system as claimed in claim 1, further
comprising:
time measurement means for estimating mutual reception time
shifting in a mobile station between digitally modulated radio
signals transmitted to said mobile station from a base station
transmitter associated with one of said predetermined cells where
said mobile station is located, and corresponding digitally
modulated radio signals transmitted to said mobile station from
another base station transmitter associated with said one of said
predetermined cells.
3. A cellular mobile radio system as claimed in claim 2, wherein
said time measurement means includes arrival time comparison means
associated with said mobile station for comparing arrival times for
corresponding digitally modulated radio signals transmitted from
different base station transmitters associated with said one of
said predetermined cells.
4. A cellular mobile radio system as claimed in claim 2, wherein
said time measurement means includes arrival time comparison means
associated with one of said at least two base station transmitters
for comparing arrival times for digitally modulated radio signals
transmitted from a second mobile station in a second of said
predetermined cells and received at different base station
transceivers associated with said second of said predetermined
cells.
5. A cellular mobile radio system as claimed in claim 2, further
comprising:
transmission time shifting means for mutually time shifting
transmission times of said at least two base station transmitters
associated with said one of said predetermined cells such that said
at least two base station transmitters transmit corresponding
digitally modulated radio signals to said mobile station with
greater or less mutual transmission time shifting in response to
the estimated reception time shifting.
6. A method in a cellular mobile radio system for digitally
transmitting message information between a plurality of mobile
radio stations and a plurality of base stations associated with
respective cells or said cellular mobile radio system, said message
information being in the form of digitally modulated radio signals
that are transmitted and received by transmitters and receivers
respectively associated with each of said plurality of base and
mobile stations, said method comprising the steps of:
transmitting digitally modulated radio signals with identical
message information from at least two base stations associated with
the same cell, and located at a predetermined distance from one
another, to mobile stations within said cell, the digitally
modulated radio signals that are transmitted from one of said base
stations having certain segments which are transmitted virtually
simultaneously and within the same frequency range as digitally
modulated radio signals transmitted from the other of said two base
stations;
digitally modulating said radio signals at transmission of said
message information within modulation time intervals, said
modulation time intervals being shorter than the time necessary for
radio signals to propagate a distance which corresponds to the
greatest transmitting distance between two base stations associated
with a cell of said cellular mobile radio system; and
reconstructing the digital modulation from corresponding radio
signals that are received at a mobile station during a reception
time interval, said reception time interval being at least as long
as the time required for radio signals to propagate a distance
which corresponds to the greatest transmitting distance between
said two base stations associated with said cell.
7. A method as claimed in claim 6, wherein said digitally modulated
radio signals with identical message information are transmitted
from different base station transmitters having mutually variable
transmission time shifting capabilities to a predetermined mobile
station, and further including the step of selecting said mutually
variable transmission time shifting so as to counteract differences
in propagation time for said digitally modulated radio signals from
said different base station transmitters to said predetermined
mobile station.
8. A method as claimed in claim 7 further comprising the steps
of:
estimating an arrival time shift between said digitally modulated
radio signals with said identical message information from said
different base station transmitters at said predetermined mobile
station;
transmitting information relating to the estimated arrival time
shift from said predetermined mobile station to at least one base
station; and
utilizing the estimated arrival time shift for selecting
transmission time shifting for at least one base station
transmitter.
9. A method as claimed in claim 7, further comprising the steps
of:
estimating a reception time shift between reception by different
base station receivers of corresponding radio signals with
identical message information from said predetermined mobile
station; and
utilizing said reception time shift for selecting said transmission
time shift for corresponding base station transmitters.
10. A cellular mobile radio system for communicating message
information within a geographic area that is divided into
communication cells, comprising:
a plurality of base stations associated with said cells, some of
said cells having at least two base stations associated therewith
and located a distance from one another to transmit respective
radio signals into a cell, which signals are digitally
.[.encoded.]. .Iadd.modulated .Iaddend.with the same message
information and are transmitted at the same frequency and
substantially simultaneously with one another,
each base station including means for digitally .[.encoding.].
.Iadd.modulating .Iaddend.the radio signals with message
information, said .[.encoding.]. .Iadd.modulating .Iaddend.being
carried out with modulation time intervals which are .[.no longer
than.]. .Iadd.within a time interval related to .Iaddend.the time
required for .[.audio.]. .Iadd.radio .Iaddend.signals to propagate
a distance corresponding to the greatest transmitting distance
between .Iadd.said at least .Iaddend.two base stations associated
with .Iadd.said at least .Iaddend.one cell in said system; and
a plurality of mobile stations each having means for reconstructing
the digital .[.encoding.]. .Iadd.modulation .Iaddend.of plural
corresponding radio signals respectively received over the same
frequency range during a reception time interval from .[.the.].
.Iadd.said at least .Iaddend.two base stations associated with
.[.a.]. .Iadd.said at least .Iaddend.cell, which reception time
interval is at least as long as the time required for radio signals
to propagate a distance corresponding to the greatest transmitting
distance between .Iadd.said at least .Iaddend.two base stations
associated with .[.a.]. .Iadd.said at least one .Iaddend.cell.
.Iadd.11. The cellular mobile system of claim 10, wherein said time
interval is a few times greater than said propagation time.
.Iaddend..Iadd.12. The cellular mobile system of claim 10, wherein
said at least two base stations associated with a cell are both
located within said at least one cell. .Iaddend..Iadd.13. A
cellular mobile radio system for communicating message information
within a geographic area that is divided into communication cells,
comprising:
a plurality of base stations for transmitting radio signals into a
cell, which signals are digitally modulated with substantially the
same message information and are transmitted at the same frequency
and substantially simultaneously with one another,
each base station including means for digitally modulating the
radio signals with message information, said modulating being
carried out with modulation time intervals which are at most a few
times greater than a time required for radio signals to propagate a
distance corresponding to a diameter of said cell; and
a plurality of mobile stations each having means for reconstructing
the digital modulation of plural corresponding radio signals
respectively received over the same frequency during a reception
time interval from the plurality of base stations, said reception
time interval is at least as long as the time required for radio
signals to propagate a distance corresponding to said diameter of
said cell. .Iaddend..Iadd.14. A cellular mobile radio system for
communicating message information and having a plurality of cells,
comprising:
a first base station for transmitting a first signal having message
information into a cell,
a second base station for transmitting a second signal having said
message information into said cell,
a mobile station in said cell having means for receiving said first
and second signals, wherein said first and second signals are
received with a propagation delay therebetween,
wherein both of said first and second base stations include means
for modulating said first and second signals, respectively, with
said message information using a modulation time interval which is
no longer than a few multiples of said propagation delay, and
wherein said mobile station includes means for reconstructing said
first and second signals during a reception time interval which is
at least as
long as said propagation delay. .Iaddend..Iadd.15. The cellular
mobile radio system of claim 10, wherein the time interval is less
than to a few times greater than the time required for radio
signals to propagate the distance corresponding to the greatest
transmitting distance between two base stations associated with
said at least one cell in said system. .Iaddend..Iadd.16. The
cellular mobile radio system of claim 10, wherein said time
interval is no longer than the time required for radio signals to
propagate the distance corresponding to the greatest transmitting
distance between two base stations associated with said at least
one cell in said system. .Iaddend..Iadd.17. The cellular mobile
radio system of claim 10, wherein said time interval is a few times
greater than the time required for radio signals to propagate the
distance between two base stations associated with said at least
one cell in said system. .Iaddend..Iadd.18. A cellular mobile radio
system for communicating message information across an area of
coverage, comprising:
a plurality of cells, each of said plurality of cells representing
a geographic division of said area of coverage;
a first base station for transmitting a first signal including
message information into at least one of said plurality of cells,
said first base station including means for modulating a radio
carrier with said message information, said message information
being represented by a sequence of symbols;
a second base station for transmitting a second signal, including
substantially the same message information as transmitted by said
first base station, into said at least one of said plurality of
cells, said second base station including means for modulating said
radio carrier frequency with said substantially the same message
information; and
at least one mobile station located within said at least one of
said plurality of said cells wherein said first and said second
signals are received by said mobile station with a time shift
therebetween wherein said time shift arises from a difference in a
first radio propagation delay between said at least one mobile
station and said first base station and a second propagation delay
between said at least one mobile station and said second base
station, said at least one mobile station further including means
for recovering said message information from said first and said
second signals during a reception time interval which reception
time interval is greater than said time shift. .Iaddend..Iadd.19.
The system of claim 18, wherein said time shift is intentionally
introduced in the transmission of said first signal and said second
signal. .Iaddend..Iadd.20. The system of claim 19, wherein said
first base station and said second base station further include
means for shifting the transmission time of said first signal and
said second signal,
respectively. .Iaddend..Iadd.21. The cellular mobile radio system
of claim 1, wherein said reconstructing means includes an adaptive
equalizer. .Iaddend..Iadd.22. The cellular mobile radio system of
claim 10, wherein said reconstructing means includes an adaptive
equalizer. .Iaddend..Iadd.23. The cellular mobile radio system of
claim 13, wherein said reconstructing means includes an adaptive
equalizer. .Iaddend..Iadd.24. The cellular mobile radio system of
claim 14, wherein said reconstructing means includes an adaptive
equalizer. .Iaddend..Iadd.25. The cellular mobile system of claim
18, wherein said
recovering means includes an adaptive equalizer. .Iaddend..Iadd.26.
A cellular mobile radio system for communicating message
information within a geographic area that is divided into
communication cells, comprising:
a plurality of base stations associated with said cells, at least
one of said cells having at least two base stations associated
therewith and located a distance from one another to transmit
respective radio signals into said at least one cell, which signals
are digitally modulated with substantially the same message
information and are transmitted at the same frequency and
substantially simultaneously with one another,
each base station including a transmitter that digitally modulates
the radio signals with message information, said modulation being
carried out with modulation time intervals which are within a time
interval related to the time required for radio signals to
propagate a distance corresponding to the greatest transmitting
distance between said at least two base stations associated with
said at least one cell in said system; and
a plurality of mobile stations each having a receiver that
reconstructs the digital modulation of plural corresponding radio
signals respectively received over the same frequency range during
a reception time interval from said at least two base stations
associated with said at least one cell, which reception time
interval is at least as long as the time required for radio signals
to propagate a distance corresponding to the greatest transmitting
distance between said at least two base stations
associated with said at least one cell. .Iaddend..Iadd.27. The
cellular mobile system of claim 26, wherein said time interval is a
few times greater than said propagation time. .Iaddend..Iadd.28.
The cellular mobile system of claim 26, wherein said at least two
base stations associated with a cell are both located within said
at least one cell. .Iaddend..Iadd.29. A cellular mobile radio
system for communicating message information within a geographic
area that is divided into communication cells, comprising:
a plurality of base stations for transmitting radio signals into a
cell, which signals are digitally modulated with substantially the
same message information and are transmitted at the same frequency
and substantially simultaneously with one another,
each base station including a transmitter that digitally modulates
the radio signals with message information, said modulation being
carried out with modulation time intervals which are at most a few
times greater than a time required for radio signals to propagate a
distance corresponding to a diameter of said cell; and
a plurality of mobile stations each having a receiver that
reconstructs the digital modulation of plural corresponding radio
signals respectively received over the same frequency during a
reception time interval from the plurality of base stations, said
reception time interval is at least as long as the time required
for radio signals to propagate a distance
corresponding to said diameter of said cell. .Iaddend..Iadd.30. A
cellular mobile radio system for communicating message information
and having a plurality of cells, comprising:
a first base station for transmitting a first signal having message
information into a cell,
a second base station for transmitting a second signal having said
message information into said cell,
a mobile station in said cell having a receiver that receives said
first and second signals, wherein said first and second signals are
received with a propagation delay therebetween,
wherein both of said first and second base stations include a
transmitter that modulates said first and second signals,
respectively, with said message information using a modulation time
interval which is no longer than a few multiples of said
propagation delay, and
wherein said mobile station receiver reconstructs said first and
second signals during a reception time interval which is at least
as long as said propagation delay. .Iaddend..Iadd.31. The cellular
mobile radio system of claim 30, wherein said time interval is less
than to a few times greater than the time required for radio
signals to propagate the distance corresponding to the greatest
transmitting distance between two base stations associated with
said at least one cell in said system.
.Iaddend..Iadd.32. The cellular mobile radio system of claim 30,
wherein said time interval is no longer than the time required for
radio signals to propagate the distance corresponding to the
greatest transmitting distance between two base stations associated
with said at least one cell in said system. .Iaddend..Iadd.33. The
cellular mobile radio system of claim 30, wherein said time
interval is a few times greater than the time required for radio
signals to propagate the distance between two base stations
associated with said at least one cell in said system.
.Iaddend..Iadd.34. A cellular mobile radio system for communicating
message information across an area of coverage, comprising:
a plurality of cells, each of said plurality of cells representing
a geographic division of said area of coverage;
a first base station for transmitting a first signal including
message information into at least one of said plurality of cells,
said first base station including a transmitter that modulates a
radio carrier with said message information, said message
information being represented by a sequence of symbols;
a second base station for transmitting a second signal, including
substantially the same message information as transmitted by said
first base station, into said at least one of said plurality of
cells, said second base station including a transmitter that
modulates said radio carrier frequency with said substantially the
same message information; and
at least one mobile station located within said at least one of
said plurality of said cells wherein said first and said second
signals are received by said mobile station with a time shift
therebetween wherein said time shift arises from a difference in a
first radio propagation delay between said at least one mobile
station and said first base station and a second propagation delay
between said at least one mobile station and said second base
station said at least one mobile station further including a
receiver that recovers said message information from said first and
said second signals during a reception time interval which
reception time interval is greater than said time shift.
.Iaddend..Iadd.35. The system of claim 34, wherein said time shift
is intentionally introduced in the transmission of said first
signal and said second signal. .Iaddend..Iadd.36. The system of
claim 34, wherein said first base station and said second base
station further include a time measurement unit that shifts the
transmission time of said first signal
and said second signal, respectively. .Iaddend..Iadd.37. A cellular
mobile radio system for communicating message information across an
area of coverage, comprising:
a plurality of cells, each of said plurality of cells representing
a geographic division of said area of coverage;
a first base station for transmitting a first signal including
message information into at least one of said plurality of cells
said first base station including a transmitter that modulates a
radio carrier with said message information, said message
information being represented by a sequence of symbols;
a second base station for transmitting a second signal, including
substantially the same message information as transmitted by said
first base station, into said at least one of said plurality of
cells, said second base station including a transmitter that
modulates said radio carrier frequency with said substantially the
same message information; and
at least one mobile station located within said at least one of
said plurality of said cells, wherein said first and said second
signals are received by said mobile station with a time of arrival
difference therebetween;
said at least one mobile station further including a receiver that
combines said message information from said first and said second
signals received during a reception time interval which reception
time interval is greater
than said time of arrival difference. .Iaddend..Iadd.38. A cellular
mobile radio system for communicating message information and
having a plurality of cells, comprising:
a first base station for transmitting a first signal having message
information into a cell,
a second base station for transmitting a second signal having said
message information into said cell,
a mobile station in said cell having means for receiving said first
and second signals, wherein said first and second signals are
received with a propagation delay therebetween,
wherein both of said first and second base stations include means
for modulating said first and second signals, respectively, with
said message information using a modulation time interval which is
no longer than a few multiples of said propagation delay, and
wherein said mobile station includes means for combining
information from said first and second signals received during a
reception time interval which is at least as long as said
propagation delay. .Iaddend..Iadd.39. A cellular mobile radio
system for communicating message information across an area of
coverage, comprising:
a plurality of cells, each of said plurality of cells representing
a geographic division of said area of coverage;
a first base station for transmitting a first signal including
message information into at least one of said plurality of cells,
said first base station including a transmitter that modulates a
radio carrier with said message information, said message
information being represented by a sequence of symbols;
a second base station for transmitting a second signal, including
substantially the same message information as transmitted by said
first base station, into said at least one of said plurality of
cells, said second base station including a transmitter that
modulates said radio carrier frequency with said substantially the
same message information; and
at least one mobile station located within said at least one of
said plurality of said cells
wherein said first and said second signals are received by said
mobile station with a time shift therebetween wherein said time
shift arises from a difference in a first radio propagation delay
between said at least one mobile station and said first base
station and a second propagation delay between said at least one
mobile station and said second base station
said at least one mobile station further including a receiver that
combines said message information from said first and said second
signals received during a reception time interval which reception
time interval is greater than said time shift. .Iaddend.
Description
TECHNICAL FIELD
The present invention relates to mobile radio systems. More
specifically the invention relates to a digital, cellular, mobile
radio system. The invention also relates to a method of
transmitting information digitally to and from mobile stations in a
cellular mobile radio system.
BACKGROUND ART
The mobile radio systems that were first taken into common use were
of analog type, i.e. message information was transmitted in analog
form to and from mobile stations by transmitting and receiving
analog-modulated radio signals. In such systems it is known to have
two or more base station transmitters at a distance from each other
simultaneously transmitting radio signals within the same frequency
range and modulated with the same message information to the mobile
stations. Such mobile radio systems are described in EP 0040731 and
EP 0072479, as well as in the two publications: NTG-Fachberichte,
Bewegliche Funkdienste, Vortrage der NTG-Fachtagung vom 25. bis
Nov. 27, 1985 in Munich, "GLEICHKANALFUNKSYSTEME FuR DIE
FREQUENZOKONISCHE VERSORGUNG GROSSER GEBIETE" Berndt Heynisch pp
41-46, VDE-VERLAG GmbH, Berlin, Elektrizitatswirtschaft, Jg.
80(1981), Heft 6, pp 187-198 "Quasis-synchroner
Gleichwelienfunk-ein GleichkanalfunkVer-fahren zur Erhohung der
Erreichbarkeit in Mobilfunknetzen.
In known systems or the kind in question here, it is known to
transmit message information from a central station or exchange to
the base station transmitters via either cables or radio signals.
It is also known to have equalizers in the fixed part of the mobile
radio system for equalizing differences in propagation time and
attenuation in transmission from the exchange to the base station
transmitters. The equalizers can be at the exchange and/or at the
base station transmitters. The object of the equalisers is that
irrespective of position in relation to the exchange the base
station transmitters shall transmit the radio signals
simultaneously, and modulated with the same message
information.
Digital mobile radio systems in which message information is
transmitted digitally to and from mobile stations by transmission
and reception of digitally modulated signals have been proposed in
U.S. Pat. No. 4,675,863 and "Digital Mobile Telephone System Using
TD/FDMA Scheme", Kota Kinoshita, Masaharu Hata and Kenkichi Hirade,
IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL VT-31, NO.4,
NOVEMBER 1982, pp 153-157.
It has been proposed to have adaptive equalizers in mobile radio
stations in digital radio systems, whereby multi-path propagation
of radio signals can be used to improve signal quality, instead of
the multipath propagation acting as noise. Among the publications
on adaptive equalizers in digital mobile radio systems can be
mentioned: "Multi-path Equalization for Digital Cellular Radio
Operation at 300 k. bi/s". K Raith, J-E Sjternvall and J
Uddenfeldt, 36th IEEE Vehicular Technology Conference, pp 268-272,
Dallas. Tex., U.S.A. May 1986. "Radio Test Performance of a
Narrowband TDMA System", J-E Stjernvall, B. Hedberg, and S Ekmark,
IEEE Vehicular Conference, Tampa, Fla., U.S.A., June 1987, RADIO
TEST PERFORMANCE OF A NARROWBAND TDMA SYSTEM-DMS 90, J-E Stjervall,
B. Hedberg, K Raith, T Baickstrbm and R Lofdahl.
SUMMARY OF THE INVENTION
In mobile radio systems there are problems due to reflections and
radio shadows from natural obstacles such as rocks and hills, as
well as structures such as buildings. These problems are especially
troublesome in transmitting information requiring great
accessibility/reliability and high transmission speed. In
particular the problems may become large in certain urban
environments where the propagation conditions for radio signals can
vary heavily within a small geographic area, while radio traffic is
intensive at the same time. Up to now attempts have been made to
solve these problems by having adaptive equalizers in the mobile
stations and small cells with specially selected positioning of the
base station transmitters. In areas with much traffic it is,
however, a desire to be able to select the size of the cells and
their positions in the mobile radio system cell plan in an optimum
way with respect to the traffic handling capacity of the system.
Reducing the cell size and selecting the positions of the small
cells to avoid radio shadows thus involves a complication. Another
complication resulting from the reduction or cell size to below
what is necessary for reasons of capacity is that the number of
handovers increases.
The object of the present invention is to solve the above-mentioned
problems and complications, and to provide a method and a cellular
digital mobile radio system which are also suitable for
transmitting information requiring great accessibility/reliability
and high transmission speed.
What is distinguishing for a method and a digital cellular mobile
radio system in accordance with the present invention, and
particularly preferred embodiments thereof is disclosed in the
independent and dependent claims. Somewhat simplified, it may be
said that according to the present invention at least two base
station transmitters are utilized for each of a plurality of cells,
these transmitters being at a distance from each other and at least
partially simultaneously transmitting radio signals within the same
frequency range digitally modulated with the same message
information to the mobile stations in the cell. The digital
modulation is changed with a modulation time interval which is
adapted to the greatest transmitting distance between two base
station transmitters serving the same cell in an area. The mobile
stations have adaptive equalizers for reconstructing the digital
modulation in the transmitted signals from the signals received
during a reception time interval, which is also adapted to the
greatest transmitting distance between two base station
transmitters serving the same cell in an area.
In a preferred embodiment of a method in accordance with the
present invention, the digitally modulated signals are transmitted
with the same message information to a given mobile station with a
given, mutual transmission time shift from the different base
stations. The transmission shift is then selected such that it
counteracts the difference in arrival time for the signals from the
different base station transmitters to this mobile station.
In each mobile station, there is preferably estimated the arrival
time shift between the digitally modulated radio signals with the
same message information from the different base station
transmitters. Information about the estimated arrival time shift at
the respective mobile station is transmitted from there to at least
one base station transmitter. This estimated arrival time shift is
utilized at the base station for selecting the transmission time
shift for at least one base station transmitter. A transmission
time shift is thus obtained individually for each affected mobile
station, and which is adjusted for this particular mobile
stations's position in relation to the base station transmitters.
The amount or transmissions time shift can thus vary from mobile
station to mobile station.
According to a somewhat different, preferred embodiment, there is
estimated in different base station receivers the reception time
shift between corresponding radio signals with the same message
information from the mobile station. This estimated reception time
shift is utilized for selecting the transmission time shift between
corresponding base station transmitters. A transmission time shift
can thus be individually obtained for each affected mobile station
such that it is adjusted to the position of this particular mobile
station in relation to the base station transmitters. The amount of
transmission time shift can thus vary from mobile station to mobile
station.
A preferred embodiment of a mobile, radio system in accordance with
the present invention has a time measurement unit for estimating
the mutual reception time shift in a mobile station between, on one
hand, digitally modulated radio signals transmitted to the mobile
station via a base station transmitter for the cell where this
mobile station is, and, on the other hand, corresponding digitally
modulated signals transmitted to this station via another base
station transmitter for the cell. In this embodiment the mobile
radio system has a transmission time shifting unit for mutually
time shifting the transmission times for base station transmitters
of the same cell so that they transmit corresponding digitally
modulated signals to the mobile station with greater or less mutual
transmission time shifting in response to estimated reception time
shift.
The time measurement unit preferably includes an arrival time
comparison unit in at least certain mobile stations, for comparing
the arrival times for corresponding digitally modulated signals
transmitted from different base station transmitters for the same
cell.
According to a somewhat different, preferred embodiment, the time
measurement unit includes an arrival time comparison unit in the
stationary part of the mobile radio system, for comparing the
arrival times of digitally modulated signals transmitted from a
mobile station in a cell and received at different base station
transceivers of the cell.
A method and a cellular digital mobile radio system in accordance
with the invention provide substantial advantages. The coverage
degree can be made greater, signifying better opportunities for
establishing new connections and maintaining those already
established. The coverage degree can be made greater without the
cell size needing to be reduced, which give greater freedom in the
selection of cell plan and a lesser number of handovers. In
preferred embodiments, the transmission time shift, which is
individually selectable for each mobile station, reduces the risk
of noise and interruption of calls in progress, when a mobile
station goes from receiving signals chiefly from one base station
transmitter to receiving signals chiefly from another base station
transmitter. When a mobile station simultaneously receives signals
from at least two base station transmitters of the same cell, the
variable transmission time shift enables the received signals
together to be more like the signals obtained from a single base
station transmitter in conjunction with reflections.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates cells and positioning of transmitters in base
stations in one embodiment of a mobile radio system in accordance
with the invention.
FIG. 2 illustrates parts or a mobile radio system in accordance
with the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A cellular mobile radio system in accordance with the present
invention has mobile stations and base stations with transmitters
and receivers for radio signals. Message information is transmitted
digitally to and from the mobile stations by transmission and
reception of signals digitally modulated in correspondance with the
message information. The radio signals are transmitted on one of a
plurality of radio channels. Signals can be transmitted in time
multiple to and from several mobile stations on the same radio
channel.
The mobile stations move within and between the cells of the
system. The base station transmitters are assigned to the cells so
that there is at least one base station transmitter for each cell,
for transmitting signals to the cell's mobile stations.
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 in accordance with the
present invention. 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 service conditions, it will often be
suitable with overlapping in the boundary areas between the cells.
To a certain extent, the base station transmitter can then off-load
each other by handling transmissions to mobile stations where such
transmissions should, from a purely geographic point of view, be
performed by the base stations of a contiguous cell.
For each cell C1-C24 there is an ordinary base station transmitter
BS1-BS24. For contiguous cells these transmitters are
conventionally co-located in groups of three. For example, the base
station transmitter BS1 for the cell C1 is co-located with the base
station transmitter BS3 for the cell C3 and the base station
transmitter BS5 for the cell C5. Correspondingly, the base station
transmitter BS14 for the cell C14 is co-located with the base
station transmitter BS16 for the cell C16 and with the base station
transmitter BS18 for the cell C18. These co-positioned ordinary
base station transmitters are situated in the boundary areas
between the cells to which they are assigned. For example, the
ordinary base station transmitters BS2, BS4 and BS6 are co-located
in the boundary areas between the cells C2, C4 and C6.
Further to the ordinary base station transmitters BS1-BS24, the
system includes a number of extra base station 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 station transmitters. The
extra base station transmitters XS6, XS7, XS10, XS11, XS13, XS15A,
XS15B, XS18A, XS18B, XS19, XS20 and XS22A are co-located in groups
with three extra base station transmitters in each group in a
similar manner as the ordinary base station transmitters.
Accordingly, for example, the extra base station transmitter XS15B
for the cell C15 is co-located with the extra base station
transmitter XS19 for the cell C19 and the extra base station
transmitter XS18A for the cell C18. On the other hand, neither the
extra base station transmitter XS14 for the cell C14 nor the extra
base station transmitter XS22B for the cell C22 are co-located with
any other base station transmitter, but are situated approximately
at the center of the cell with which they are associated.
An extra base station transmitter does not need to differ
technically from an ordinary base station transmitter. For a given
cell or extra base station transmitter can thus have technical
equipment of the same type as an ordinary base station 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, in certain cases either of them may
be respectively regarded as ordinary or extra.
The extra base station transmitter or transmitters for a given cell
transmit radio signals which are substantially the same as those
sent by the ordinary base station transmitter of the cell. The
radio signals are digitally modulated with digital message
information to the mobile stations in the cell. A mobile station in
a cell for which there is one or more extra base station
transmitters can therefore receive, at least in certain cases,
corresponding radio signals from more than one base station
transmitter approximately simultaneously within the same frequency
range (radio channel). Depending on the mutual, relative positions
in the cell of the mobile and base stations as well as the
transmission times and propagation paths of the radio signals from
the base station transmitters to the mobile stations, corresponding
radio signals from different base station transmitters can be
received without, or with a given time shift at the mobile station.
The greater the distance between the base station 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 station 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 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 speed or the radio signals. C is approximately 300000
km/s.
In FIG. 2 there are illustrated parts of a mobile radio system in
accordance present invention. 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 base stations, of which two, B.sub.m and B.sub.n are
illustrated in FIG. 2.
The base station 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 station B.sub.m includes a central line and
control unit 1, transmission time shifting (Tts) units 2A and 2B,
one for each of the transceivers, reception time shifting (Rts)
units 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 station B.sub.m are alike. Each such
transceiver contains a line and control unit (CU) 5A or 5B,
transmitter (T) units 6A or 6B, receiver (R) units 7A or 7B, a
transmission-reception filter (tRF) 8A or 8B and an antenna 9A or
9B.
The base station B.sub.n differs partly from station B.sub.m,
primarily due to its central line and control unit (CU) 10 being
situated in connection 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 to the other transceiver B.sub.nb. In addition, no
transmission or reception time shifting units are included in any
central unit in B.sub.n, but the corresponding units 2A, 2B, 3A and
3B are respectively included in transceivers B.sub.na and
B.sub.nb.
The mobile stations MS.sub.1 and MS.sub.2 are mutually alike. Each
mobile station includes sound sensing (SS) unit 11, encoding (EN)
unit 12, transmitting M unit 13, transmit-receive switch (TRS) 14,
reception (A) unit 15, equalizer and decoding (ED) unit 16, sound
reproducing (SR) unit 17, control unit (CU) 18 and a unit (ED) 19
for feeding in our out or presentation of digital information.
Apart from the base stations having two transceiver units at a
distance from each other, and having controllable transmission and
reception time shifting units, the mobile radio system in FIG. 2
functions in most respects in a way well-known in cellular 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 cellular mobile radio system according to
FIG. 2. One not skilled in the art of cellular mobile radio systems
is referred to the technical literature and to the publications
mentioned under the section "Background of the Invention".
Message information that the mobile radio exchange forwards to a
mobile station in the cell C.sub.m, e.g. the station MS.sub.1, is
transmitted from the mobile radio exchange via the cable L.sub.m to
the line and control unit 1. From the line and control unit 1 the
information is transferred via the transmission time shifting unit
2A, line unit 4A, cable L.sub.ma and line and control unit 5A to
the transmitting unit 6A. The transmitting unit 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 unit 2B, line unit 4B, cable L.sub.mb and line and
control unit 5B to the transmission unit 6B in the transceiver
B.sub.mb. The transmission unit 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 unit 6A and the corresponding delay in
transferring to the transmission unit 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 signals from the antenna 9A in B.sub.ma arrive at a given
mobile station in the cell C.sub.m, e.g. the mobile station
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 station 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 units 2A and 2B have a
variable delay and can be controlled by the line and control unit
1, such that the radio signals are transmitted from the antenna 9A
in B.sub.ma time-shifted more or less before or after corresponding
radio 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
units 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
variables delays in the transmission time shifting units 2A and 2B,
such that the time shift of the radio waves on arrival at the
mobile station is decreased compared if the units 2A and 2B had 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 radio
signals transmitted from the antenna in B.sub.ma arrived at the
antenna in MS.sub.1, exactly simultaneously and in phase with
corresponding radio signals transmitted from the antenna in
B.sub.mb. In practice, this is neither desired for nor achieved.
Reflections occur in the propagation of the radio signals between
the antennas, and the mobile station has an adaptive equalizer. It
is therefore not necessary for the signals from the different
transceiver units to arrive exactly simultaneously to the mobile
station. 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 publications mentioned in the section
"Background Art", e.g. "Radio Test Performance of a Narrow-band
TDMA System-DMS 90".
In principle, there are at least two conceivable methods of
determining how the line and control unit 1 may control the delay
in the transmission time shifting units 2A and 2B. One method is to
estimate in the fixed part of the mobile radio system the time
shift between the mobile station radio signals at one of the
transceivers B.sub.ma and the 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 station. In these
differences depending on the position of the mobile station.
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 station. In the embodiment according to FIG. 2, this method
can be applied in practice such that the delays in the reception
time shifting units 3A and 3B are adjusted so that information
received from the mobile station 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 units 2A and 2B are subsequently
adjusted in correspondence with the optimum delays in the reception
time shifting units 3A and 3B.
The other method is to estimate in the mobile station the
difference in arrival time or time shift between the digitally
modulated radio signals from one transceiver B.sub.ma and the
corresponding digitally mobile radio 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 utilized if they are formed or
supplemented so that two base station transmitters for the same
cell do not only have identical synchronizing words. Alternatively,
radio signals digitally modulated with special synchronising words
can be transmitted from the base station transmitters solely to
enable the mobile station to estimate the differences in arrival
times or time shifts. The mobile station transmits information
about the estimated arrival time difference or time shift via radio
signals to the fixed part of the system, where it is utilized for
controlling the transmission time shifting units 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 station in the same way as the line and
control unit obtains message information from the mobile
station.
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 synchronizing patterns
(words), the time difference between these patterns (words) in
different signals can be measured using conventional methods. A
mobile station control unit 18 and/or a base station line and
control unit 1, 10 in a possible combination with the transceivers'
line units 5A and 5B can then comprise time measurement means for
estimating reception time shifting or the arrival time comparison
units for comparing arrival times.
When so required, a base station preferably utilizes conventionally
the same transmitter units and antenna for transmitting, in time
multiplex within the same frequency range on the same radio
channel, radio signals digitally modulated with message information
to different mobile stations associated with the same cell. The
radio signals with information to a given mobile station are then
transmitted from different base station transmitters with a
possible transmission time shift which is specially adjusted with
regard to the position of this particular mobile station. The case
can arise where a base station in a mobile radio system needs to
transmit a radio signal with information intended for reception by
several or all of the mobile stations in the cell, e.g. information
as to the identity of the base station/cell. Such signals are
preferably transmitted without mutual time shifting from the
transceivers B.sub.ma, B.sub.mb and B.sub.na, B.sub.nb of the base
stations in a mobile radio system according to FIG. 2. The
transmission time shifting units 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 station "listens" in unoccupied
combinations of time slot and frequency range channel for calls
from mobile stations of unknown positions relative to the
transceivers of the base stations. The reception time shifting
units 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 10 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 10.
The mobile stations MS.sub.1 and MS.sub.2 have adaptive equalizers,
whereby the digital modulation during a modulation time interval in
the radio signals transmitted from a base station transmitter can
be reconstructed from signals received during a reception time
interval. In known cellular digital mobile radio systems with only
one base station transmitter per cell, the reception time interval
of the equalizers is dimensioned according to the dispersion on the
radio channel, i.e. expected time shifts between corresponding
signals from a single base station transmitter due to multipath
propagation and reflections. Because of the equalizer, not only the
radio signal having the greatest amplitude or arriving first to the
mobile station is utilized for reconstructing the digital
modulation, but also other corresponding radio signals arriving
with a time shift within the extent of the equalizer's reception
time interval can be utilized. The mobile stations in a system in
accordance with the invention have equalizers which are dimensioned
such that the reception time interval of the mobile station 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 station transmitters associated
with the same cell within a restricted geographical area. With the
base station transmitters placed according to FIG. 1, and the
restricted area being the area composed of the cells C1 to C24, the
mobile station equalizers would thus be dimensioned for a reception
inverval of reconstruction which is greater than 2L/c. Taking into
account that there can be dispersion, and that reflections can
extend the propagation time from base station transmitter to a
mobile station more than the extension of the propagation time from
another base station transmitter for the same cell, the reception
time interval of the mobile stations is 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 station transmitters associated with the same cell within
the geographical area in question.
In prior art mobile radio systems it is known to have cells of
small size in city areas or densely populated areas with many calls
per hour and to have cells of large size in rural areas with few
calls per hour. Outside the restricted geographical area composed
of cells C1 to C24 in FIG. 1 there might be cells of greater size
than that or C1 to C24. Occasionally such cells may have plural
base stations transmitting the same information for overcoming
radio shadows from mountains. A mobile radio according to the
present invention may be used outside the restricted area and in
such a cell but the advantages conferred by the present invention
can not be expected if the transmission distance between the
transmitters assigned to such a cell is too great in relation to
the reception time interval.
In mobile radio systems in accordance with the present invention,
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 particular area.
Although the present invention affords greater advantages, the
smaller the modulation time interval is in relation to this
propagation time, and the present invention has its greatest
importance when the modulation time interval is less than the
mentioned propagation time, the present invention 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
interval. In digital modulation of the radio signals involving
transmission or 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 modulation. For example, if a
sequence of binary symbols individually and one at a time determine
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 or 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 and 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 or 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.
A method and a cellular digital mobile radio system in accordance
with the present invention is not restricted to the described
embodiments, and can be modified within the scope of the claims.
For example, it is conceivable to co-locate at least parts of the
equipment in an ordinary base station transmitter, with parts of
the equipment in the extra base station transmitter for the same
cell, providing that their antennas are spaced from each other. In
an extreme case it is conceivable in principle to have all
equipment except the antennas co-located in one place, e.g. in the
vicinity of one of the antennas, and that the antenna are fed with
radio frequency signals by cable from this position. By "base
station transmitter" is there fore meant at least a transmitter
antenna for radio signals and preferably more or fewer of the
remaining means required in a base station. In a base station
transmitter there are preferably included at least means
corresponding to 6A and 6B included in a transceiver unit B.sub.ma
or B.sub.mb in FIG. 2.
* * * * *